High-grade compressed heat storage system and method for using same

Abstract

The present invention relates to a cryogenic energy storage system for utilizing and storing high-grade heat of compression. The system includes a liquefaction subsystem (100) and a thermal energy storage device (300). The liquefaction subsystem (100) includes a first compressor (102), first, second, and third heat exchangers (104, 116, 112), and a second compressor (114). The first and second heat exchangers (104, 116) are configured to transfer high-grade heat of compression from the first and second compressors (102, 114), respectively, to the thermal energy storage device (300). The third heat exchanger (112) is configured to recover low-grade heat of compression from the second compressor (114) back to the second compressor (114) so ​​that the second compressor (114) can produce high-grade heat of compression. A further system uses a compressor (124) and an expander (122). Further systems include a power recovery subsystem 400. The present disclosure also relates to a method for storing heat of compression in a cryogenic energy storage system.

Description

[Technical field]

[0001] The present invention relates to a cryogenic energy storage system and method of operation thereof, and in particular to heat-of-compression storage in a subsystem thereof. [Background technology]

[0002] Electricity transmission and distribution networks (or grids) must balance the generation of electricity with the demand from consumers. This is usually achieved by adjusting the generation side (supply side) by switching power plants on and off and operating some at reduced load. Most existing thermal and nuclear power plants are most efficient when operating continuously at full load, but they become less efficient when balancing the supply side in this way. The expected introduction of large intermittent renewable generation capacity into the network, such as wind turbines and solar collectors, creates uncertainty in the availability of some of the generation sources, further complicating grid balancing. Means to store energy during periods of low demand and use it during subsequent periods of high demand or when intermittent output from generators is low, would be of great advantage in balancing the grid and ensuring security of supply.

[0003] Many new energy storage methods have appeared on the market, such as cryogenic energy storage, pumped hydroelectricity, compressed air, chemical batteries, among others. Such storage devices have three operational phases: charging, storage, and discharging. During periods of low demand, surplus energy from the grid is used to charge the storage device. Energy is stored by the storage device in a respective medium, such as a cryogenic fluid, the reservoir of a hydroelectric dam, or the internal chemical potential of a battery. During periods of high demand, this energy is released back to the grid, ensuring that the demand is met.

[0004] For an energy storage device to be commercially viable, the following factors must be taken into account: capital cost per MW (power capacity) and MWh (energy capacity), round trip cycle efficiency (RTE), and lifetime in terms of the number of charge / discharge cycles that can be expected given the initial investment and its environmental impact (national regulations on carbon dioxide emissions and possible use or production of hazardous chemicals). For widespread utility-scale applications, the energy storage device must be deployable wherever it is needed in the electric network. In other words, it must have a small footprint and its operating principle should not require specific geographical constraints, as applies to hydroelectric systems (e.g. reservoir space) or compressed air energy storage devices (e.g. underground caverns).

[0005] Cryogenic Energy Storage (CES) technology, which uses a refrigerant (cryogen) such as liquid air, offers many advantages over other available power storage technologies: CES systems typically have high energy density due to the physical properties of liquid air, are easy to deploy (using relatively small storage tanks that are not geographically restricted), are environmentally friendly (as their operating principle does not involve the use or production of hazardous materials or the generation of carbon emissions as found in most batteries), and are relatively inexpensive (as they can utilize equipment long in use in the liquid natural gas industry).

[0006] CES technology liquefies air from the outside environment, stores it at low pressure, and then vaporizes it and can use it to drive a turbine to generate electricity. The charge or liquefaction phase of a CES system uses low-cost electricity during periods of low demand (off-peak periods) or periods of intermittent excess supply from renewable generators to liquefy air in a liquefaction unit or subsystem. The liquefaction process typically uses two compressors. The air stream is first compressed to about 15-20 barA in a main air compressor (MAC), then the air is purified in an air purification unit (APU) and compressed to about 55-65 barA in a second recycle air compressor (RAC). The stream can then enter a cooling and liquefaction system, commonly referred to as a cold box. The cold box can typically be realized as an insulated (usually metal) box filled with high-performance insulation such as perlite. The low-temperature heat exchange process occurs in the cold box through components such as multi-pass heat exchangers, phase separators, and expansion turbine stages of the hot and cold turbines. The gas is cooled in the cryobox until it is substantially condensed to a liquid. The liquefaction product is separated from the vapor phase and sent from the cryobox to a cryogenic storage tank where it is stored for later use. The vapor phase is passed back through a multi-pass heat exchanger and then sent to a recycle air compressor where it may be further liquefied to supplement the air flow upstream of the compressor. The liquefied product, such as liquefied air or refrigerant, is stored in the storage tank during the storage phase. During the discharge phase, the liquid product is discharged to a power recovery unit or subsystem where it is compressed, evaporated, and heated to drive an expansion turbine to generate electricity. The discharge or power recovery phase is often performed during peak periods when electricity costs are high. CES technology relies on the thermodynamic energy potential between cryogenic liquid air and gaseous air at or above ambient temperature. The CES system can simultaneously perform various operations from the charge and discharge phases, where the CES system also simultaneously liquefies, stores, and generates electricity from air.

[0007] The pressurized gas stream to be liquefied exiting the liquefaction unit and the pressurized refrigerant exiting the power recovery unit are typically referred to as the "liquefaction unit process stream" and the "power recovery unit working fluid", respectively.

[0008] The round trip efficiency of a CES system is defined as the ratio of the net electrical energy output of the power recovery unit to the net electrical energy input of the liquefaction unit. In order to maintain a high round trip efficiency in a CES system, the heat generated as a result of the electrical energy input must be captured and reused; any heat not captured and used will eventually be dissipated to the environment, potentially negatively impacting the round trip efficiency.

[0009] For example, in a liquefaction process, a process stream is compressed, cooled, and expanded again. This process is repeated, with each compression step increasing the temperature of the process stream, and each cooling and expansion step decreasing the temperature significantly until the process stream is fully liquefied. The term "heat of compression" refers to the high thermal energy embedded in the compressed fluid. In other words, "heat of compression" refers to the increase in sensible energy experienced by the fluid as a result of compression. When heat is described herein as being "transferred" from one location, fluid, or component to another location, fluid, or component, this is a process in which high thermal energy is transferred between the two locations, fluids, or components. The transfer method can include any of the typical methods such as conduction, convection, or radiation, and will be apparent to one of ordinary skill in the art.

[0010] The heat of compression can be recycled for use in a power recovery process. During the power recovery process, liquid air is pumped from a cryogenic storage tank as the working fluid and then evaporated, heated, and expanded through a series of heat exchangers and expansion turbines. As the working fluid is heated and expanded through the expansion turbine, it acts against the turbine causing it to spin. The turbine is coupled to a generator, which then produces electrical energy that can be sent to the power grid.

[0011] The heat of compression captured during the liquefaction phase can be used to raise the temperature of the working fluid in the power recovery unit before expansion. The increased temperature of the working fluid increases the power output delivered by the power recovery unit during the power recovery phase, which leads to an increase in the round-trip efficiency of the CES system. This is because the hotter the expanding gas is, the more kinetic energy the expansion turbine can lose because it can spin faster or with more torque, thereby producing more mechanical energy.

[0012] A CES system may employ a subsystem designed to capture the heat of compression of the process stream produced in the liquefaction unit during the liquefaction phase, then store it in a thermal energy storage device (TESD), and then release it to the working fluid of the power recovery unit during the power recovery phase. Such a subsystem is known as a compression heat recycle system.

[0013] Known compression heat recycle systems typically involve recycling the heat of compression from a compressor and transferring it to an expansion turbine via a TESD. The compressor, expansion turbine, and TESD are thermally coupled via a heat exchanger and a heat transfer fluid. For example, a process stream compressed by the compressor may pass through a heat exchanger, and a heat transfer fluid may flow in the opposite direction through the heat exchanger as a separate stream (this type of heat exchanger is known as a counterflow heat exchanger). In the heat exchanger, the heat of compression is transferred from the process stream to the heat transfer fluid and then to the TESD. The heat is then transferred from the heat transfer fluid to a storage medium in the TESD for long-term storage. In cases where the heat transfer fluid is used as a storage medium, the heat transfer fluid is stored directly in the TESD. To distribute the heat to the working fluid of the power recovery unit, the heat is extracted from the storage medium of the TESD and transferred to the heat transfer fluid, which is sent to a heat exchanger of the power recovery unit upstream of the expansion turbine. When a heat transfer fluid is used as the storage medium, it is passed directly from the TESD to the power recovery unit's heat exchanger upstream of the expansion turbine. In the power recovery unit's heat exchanger, heat is transferred to the working fluid before being transferred to the expansion turbine. The purpose of a thermal energy storage device (TESD) is to capture, store, and release thermal energy in a controlled manner.

[0014] The heat of compression generated during compression of a fluid is characterized by its grade as well as its quantity. It can be said that the grade and quantity of heat contained in a given fluid processed by a compressor is a function of the mass flow rate processed by the compressor, the inlet temperature or pressure of the compressor, the overall pressure ratio of the compressor, and the efficiency of the compressor. Above ambient temperature, the grade of hot heat energy also increases with increasing temperature. Conversely, below ambient temperature, the grade of cold heat energy increases with decreasing temperature. Typically, heat energy contained in a fluid between ambient temperature and the solidification temperature or lower limit temperature of the storage medium of a high-grade TESD is considered to be low-grade high-temperature heat energy, also referred to herein as low-grade heat energy, e.g., in the case of molten salt as the storage medium of a high-grade TESD, these temperatures can be 140-150°C and 190°C, respectively. In contrast, energy contained in a fluid above this temperature is considered to be high-grade high-temperature heat energy, also referred to herein as high-grade heat of compression. These values ​​can vary depending on the design of the system using this heat of compression. A fluid is considered to contain low-grade heat when heat is extracted from a fluid that contains high-grade heat of compression such that the temperature of the fluid falls below the high / low grade temperature threshold. The opposite is true when heating a fluid. The lower temperature limit includes a safety margin between the freezing temperature of the storage medium required to ensure that crystallization of the storage medium does not occur during operation of the CES system. Freezing or crystallization of the storage medium can damage the heat exchangers and other components in the CES system, and freezing of the heat exchange medium requires the CES system to be taken offline for an extended period of time to replace or restore the heat exchangers. This downtime can last from weeks to months, depending on the severity of the molten salt solidification.

[0015] The main air compressor of a CES system typically has a higher pressure ratio than the recycle air compressor. The pressure rise across the main air compressor is typically higher than the recycle air compressor because the vapor phase from the phase separator supplements the inlet flow to the recycle air compressor. This increases the mass flow rate upstream of the recycle air compressor. To achieve a similar pressure ratio (and therefore outlet temperature) as the main air compressor at this higher mass flow rate would make the recycle air compressor prohibitively expensive. For example, in a typical main air compressor, air is compressed from about 1 barA at 15-20°C to 15-20 barA at 350-450°C. Meanwhile, in a typical recycle air compressor, air is compressed from about 15-20 barA at 20-25°C to 50-60 barA at 150-200°C. The temperature of the process stream leaving the main air compressor is in the range of 350-450°C from which both high-grade compression heat and low-grade heat can be extracted. However, the temperature of the process stream leaving the recycle air compressor is significantly lower than that of the process stream leaving the main air compressor, <200°C, so only low-grade heat can be extracted from it.

[0016] Considering that both high-grade and low-grade heat of compression can be extracted from the fluid compressed by the main air compressor, whereas only low-grade heat can be extracted from the fluid compressed by the recycle air compressor, known compression heat recycle systems typically employ two types of TESDs to store high-grade and low-grade heat to avoid the loss of the heat of compression to the environment and the resulting negative impact on round-trip efficiency. Two types of TESDs are primarily used because each grade has a different optimal medium for storing thermal energy and different storage requirements.

[0017] There are several different types of TESDs, which typically differ in their internal architecture and efficiency in storing high- or low-grade thermal energy. Some TESDs, commonly known as "packed beds," are filled with a stationary solid phase through which a thermal energy transfer fluid circulates to charge the TESD with thermal energy or release thermal energy to provide thermal energy where it is needed. The stationary solid phase can be made of a porous solid medium or a packed bed of solid particles that can retain thermal energy. The packed bed matrix may include randomly stacked particles made of a sensible heat material (e.g., quartz), made of a latent heat phase change material, or a combination thereof. The packed bed matrix may include non-randomly stacked particles made of a sensible heat material (e.g., metal oxide beads), made of a latent heat phase change material, or a combination thereof. The packed bed matrix may include fused particles (e.g., ceramic). A more elaborate packed-bed TESD, disclosed in WO2012020233A2, aims to provide a flexible system that can accommodate asymmetric charging and discharging while maintaining pressure drop at an acceptable level and minimizing the net effect by increasing the flow rate of thermal energy transfer fluid towards the charging and discharging ends of the TESD. Other TESDs are filled with a stationary liquid phase through which at least one heat exchange coil passes to allow the passage of thermal energy transfer fluid. Other TESDs, commonly known as thermoclines, consist of a vessel containing two density-dependent regions of a single thermal energy transfer fluid at different temperatures, the regions being stacked on top of each other (due to density differences). One version of a thermocline consists of two separate vessels, each containing the same thermal energy transfer fluid at two different temperatures (i.e., a warm tank and a cold tank).

[0018] High-grade TESDs are typically constructed with molten salt or packed beds as the storage medium and are best suited to store high-grade heat of compression. These allow the use of pressurized thermal oil or the molten salt itself as the heat transfer fluid. Molten salt high-grade TESDs are typically two tank (hot molten salt tank and cold molten salt tank) systems. In a CES system using molten salt high-grade TESD and thermal oil as the heat transfer fluid, the high-grade heat of compression generated in the compressor can be captured in a heat exchanger downstream of the compressor and transferred to the cold molten salt tank via the thermal oil. The thermal oil passes through the molten salt heat exchanger to transfer the heat of compression to the molten salt and then returns to the heat exchanger downstream of the compressor to capture more heat, while the heated molten salt is stored in the hot molten salt tank. During the power recovery cycle, the hot molten salt is pumped to another molten salt heat exchanger where the high-grade heat is transferred to the thermal oil. The (relatively) low temperature molten salt is then sent to a low temperature molten salt tank in preparation for further heat transfer from the hot thermal oil exiting the heat exchanger downstream of the compressor. Meanwhile, the thermal oil is sent to a heat exchanger in the power recovery unit upstream of the expansion turbine to transfer heat to the working fluid of the power recovery unit. When molten salt is used as a heat transfer fluid, it is sent directly to and withdrawn from the high or low temperature molten salt tank. Although molten salt is an efficient high grade storage medium, it cannot be used for low grade applications because it has a lower temperature limit of about 190°C. The reason for this is that the freezing temperature of typical molten salts is less than about 140-150°C. Some types of molten salts have lower freezing temperatures, but still higher than the upper limit for low grade applications. This lower temperature limit of the molten salt is typically selected to provide sufficient time for the operator of the CES system to respond to the temperature drop of the molten salt and adjust the operating parameters of the CES system to prevent the molten salt from freezing. The freezing of the molten salt must be avoided at all costs.

[0019] The low-grade TESD of the CES system is usually configured as a thermocline-based system with a single stratified tank, preferably containing water or a mixture of water and glycol, which is ideal for storing low-grade heat. This type of low-grade TESD operation is based on the thermal stratification process. Stratification is a natural process in which fluids of different temperatures and densities, in this case water, separate, i.e., hot water floats and stays on top of cold water. In the case of the low-grade TESD of the CES system, as air is compressed during the filling operation and low-grade heat is extracted from the process stream, the water in the stratified tank acts as a heat transfer fluid and is sent to the heat exchanger corresponding to the compressor and heated. The water is then introduced to the top of the stratified low-grade TESD tank. The hot water stays at the top of the tank and the cold water stays at the bottom. During the discharge process, when the working fluid of the power recovery unit needs heat, the hot water is extracted and pumped to the heat exchanger in the power recovery unit, where it transfers its heat to the working fluid. In the process, the water is cooled and returned to the bottom of the stratified low-grade TESD tank. In stratified low-grade TESD, the tank is always full, but the boundary between the two stratified fluids moves up and down depending on the filling / discharging phases. The thermocline (the temperature gradient between the warm and cold parts of the fluid) can be up to a metre thick.

[0020] WO2019158921A1 (herein incorporated by reference) discloses several CES systems including a low grade TESD 16 and a high grade TESD 12, which allow for the transfer of different grades of heat from both the main and recycle air compressors to the working fluid of the power recovery unit 33. FIG. 4A shows a system in which the high grade heat of compression of the process steam from the main air compressor 10 is captured by the heat exchanger 11 and stored in the high grade TESD 12. The low grade heat of compression of the process steam from the recycle air compressor 10 is captured by the heat exchanger 11 and stored in the low grade TESD 12. FIGS. 5A-5F show how TESDs can be combined to transfer different grades of thermal energy to the working fluid of the power recovery unit. FIG. 7A shows a further CES system including an additional heat exchanger 150 configured to capture the low grade heat of compression of the process steam from the main air compressor 10. This heat of compression is then transferred to the low grade TESD 16. Configurations of the power recovery unit for transferring different grades of heat are shown in Figures 8A-8B.

[0021] These systems address the practical implementation of a compression heat recycling system in a CES system, where various grades and amounts of compression heat released and captured during the liquefaction phase are then utilized to enhance the power output provided by the power recovery unit during the power recovery phase. Utilizing the stored compression heat to increase the temperature of the working fluid before expansion by pre-stage heating or inter-stage reheating increases the power output of the power recovery unit, leading to improved round-trip efficiency of the CES system. Furthermore, storing different grades of heat separately allows them to be added separately to different parts of the process stream. It has been found that adding different grades of heat at different locations in the power recovery system results in particularly efficient power recovery. Furthermore, it has been noted that storing different grades of heat separately from each other has many advantages, such as efficient storage and efficient power recovery when adding the stored heat to the process stream.

[0022] The inventors have recognized that while there are some benefits to capturing the heat of compression in various thermal energy reservoirs, there may be additional benefits to removing the low-grade TESD. This is because the low-grade TESD creates technical challenges, most importantly thermal mixing at the stratified boundary. Over time, thermal diffusion at the boundary of the two stratified fluids contained within the low-grade TESD can erode the heat / density gradient and "blur" the boundary. Eventually, further diffusion will completely mix the stratified fluids, which tend toward thermal equilibrium, at which point the entire low-grade TESD vessel will have a single temperature. Thus, the inventors have recognized that removing the low-grade TESD may alleviate this problem.

[0023] FIG. 14 of WO2013034908A2 (hereby incorporated by reference) shows a CES system in which the compression heat recycle system is composed of two separate closed loops sharing a single TESD 1002. The first closed loop indirectly receives the compression heat generated by two compressors (main air compressor 401 and recycle air compressor 403) and stores this high temperature thermal energy in the single TESD 1002. The recycle air compressor 403 is located downstream of the main air compressor 401 and the air purification unit 402. On page 23, paragraph 1 of WO2013034908A2 states that the temperature of the thermal energy captured from the compressor is 60-90°C. The document does not mention the use of a high grade TESD or the storage of various grades of high temperature thermal energy from the compressor. Therefore, it is considered that the system captures and stores only low grade thermal energy. As mentioned before, the melting points of typical molten salts are much higher, around 140-150 °C. Therefore, high-grade TESDs, which include a storage medium such as molten salt, are not applicable to this system. The document does not appear to provide solutions to the technical challenges associated with low-grade TESDs.

[0024] Furthermore, air purification units (e.g., 402 in WO2015181553A2) are used to extract pollutants and undesirable compounds from the air stream so that a "purified" air stream is generated for the process. These typically operate using an adsorption process in which contaminants such as gases, water molecules, hydrocarbon particles, or other undesirable species are adsorbed onto the surface of an adsorbent material. During the purification process, pressure and temperature need to be specifically controlled to ensure efficient and optimal purification, and such a system providing this control is disclosed in WO2015181553A2 (incorporated herein by reference in its entirety). As previously mentioned, the output temperature of the main compressor of a CES system can be very high. Thus, in WO2013034908A2, the high-grade heat energy contained in the air output from the main compressor needs to be released into the atmosphere to avoid interrupting the process of the air purification unit 402, which would negatively affect the round trip efficiency. Furthermore, without cooling the air from the main air compressor, the moisture in the air cannot be easily removed by the air purification unit 402. To ensure sufficient water removal, the air purification unit 402 would need to be significantly larger, which would result in a large capital investment.

[0025] With reference to WO2013034908A2, WO2019158921A1 (incorporated herein by reference) discusses the problem of storing different grades of thermal energy in a single TESD and states that there are three ways to deal with two heats of compression of different amounts and grades (stored in a single TESD). 1. The heat of compression can mix in the conduits that transfer the thermal energy from the compressor to the TESD. 2. By using at least one cooler, the temperature of one compression heat can be adjusted to the temperature of the other compression heat; or 3. By using at least one cooler for each compressor, the temperature of both compressors can be adjusted to reach the same target temperature.

[0026] WO2019158921A1 (herein incorporated by reference) correctly points out that all three of these methods result in an undesirable loss of high-temperature thermal energy grades. Cooling and mixing different grades of heat of compression results in an overall reduction in the temperature of the storage medium of the thermal energy storage device. This means that less heat is added to the working fluid of the power recovery unit, which may lead to a reduction in round-trip efficiency. However, the solution proposed by WO2019158921A1 is to use two TESDs of different grades.

[0027] Accordingly, the inventors have realized that there is a need for an improved compression heat recycling system that stores and uses different grades of compression heat during the liquefaction phase of a single high-grade TESD, essentially eliminating the low-grade TESD, and avoiding the undesirable loss of high temperature thermal energy grades while mitigating the technical challenges associated with the low-grade TESD discussed above. Summary of the Invention

[0028] Aspects of the present invention relate to cryogenic energy storage systems and methods as set out in the accompanying claims.

[0029] The inventors have improved upon known compression heat recycling systems by developing a system that stores and uses the heat of compression generated by both the main and recycle air compressors during the liquefaction phase in a single high-grade TESD. This system essentially allows for the removal of low-grade TESD, avoiding the reduction in temperature and therefore the reduction in grade of the heat of compression. The disclosed system can dump heat from the high-grade TESD to the working fluid of a power recovery unit to increase power output during the power recovery phase.

[0030] Accordingly, in a first aspect, the present invention provides a cryogenic energy storage system, the cryogenic energy storage system comprising: A liquefaction subsystem, the liquefaction subsystem comprising: A first compressor, A first heat exchanger, A second compressor, A second heat exchanger, A third heat exchanger, and a liquefaction subsystem including a first arrangement of conduits having an upstream end and a downstream end and configured to pass a process stream from the first compressor through the first heat exchanger, the second compressor, the second heat exchanger, and a third heat exchanger; a thermal energy storage device configured to store high grade heat; a second configuration of conduits configured to form a closed circuit and to convey a heat transfer fluid between the thermal energy storage device, the first heat exchanger, and the second heat exchanger; a first heat exchanger positioned along the first arrangement of conduits downstream of the first compressor and configured to transfer at least a portion of a high-grade heat of compression of the process stream from the first compressor to a thermal energy storage device via a heat transfer fluid; a second heat exchanger positioned along the first arrangement of conduits downstream of the second compressor and configured to transfer at least a portion of a high-grade heat of compression of the process stream from the second compressor to a thermal energy storage device via a heat transfer fluid; The third heat exchanger is positioned along the first configuration of conduits downstream from the second heat exchanger and is configured to transfer at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the second compressor, thereby returning at least a portion of the low-grade heat of compression to the second compressor.

[0031] By returning at least a portion of the low-grade heat of compression from the second heat exchanger to the second compressor, the third heat exchanger enables the second compressor to compress the process stream to a much higher temperature, thus producing high-grade heat of compression. This high-grade heat of compression can then be stored in a thermal energy storage device via a heat transfer fluid and the second heat exchanger. Thus, both high-grade and low-grade heat can be captured and stored in a single high-grade thermal energy storage device.

[0032] In some embodiments, the liquefaction subsystem may further include an air purification unit positioned along the conduits of the first configuration downstream of the first heat exchanger, the air purification unit may be configured to purify the process stream.

[0033] The liquefaction subsystem may further include a first air conditioning unit positioned along the first configuration of conduits between the air purification unit and the first heat exchanger, and the first air conditioning unit may be configured to substantially or completely remove heat of compression of the process stream from the first compressor.

[0034] The liquefaction subsystem may further include a fourth heat exchanger positioned along the first arrangement of conduits between the first heat exchanger and the air purification unit and configured to transfer at least a portion of the low-grade heat of compression of the process stream from the first heat exchanger to the process stream downstream of the air purification unit. Transferring a portion of the low-grade heat of compression to the process stream downstream of the air purification unit enables the process stream to be compressed to a higher temperature in the second compressor, which produces high-grade heat of compression that can be stored in a single high-grade thermal energy storage device. Thus, the heat of compression from both compressors may be captured and stored in the single high-grade thermal energy storage device.

[0035] A third heat exchanger configured to transfer at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the second compressor can include transferring at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the process stream upstream of the second compressor.

[0036] The liquefaction subsystem may further include a third configuration of conduits having an upstream end configured to be coupled to the cold box and a downstream end, and the third configuration of conduits may be configured to pass a return flow from the cold box to replenish the process flow upstream of the second compressor.

[0037] The third heat exchanger configured to transfer at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the second compressor can include transferring at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the return stream prior to replenishing the process stream. By transferring the low-grade heat of compression to the return stream, the third heat exchanger can heat the return stream to be similar to the heat of the process stream. This avoids mixing of streams at different temperatures, which can reduce the energy potential of the process stream.

[0038] A third heat exchanger configured to transfer at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the second compressor can include transferring at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the process stream after the process stream has been replenished with the return stream.

[0039] In a second aspect, the present invention provides a cryogenic energy storage system, the cryogenic energy storage system comprising: 1. A liquefaction subsystem comprising: A first compressor, A first heat exchanger, A second compressor, A second heat exchanger, and a liquefaction subsystem having an upstream end and a downstream end and including a first arrangement of conduits configured to pass the process stream through a first compressor, a first heat exchanger, a second compressor, and a second heat exchanger; a thermal energy storage device configured to store high grade heat; A first expander; A third compressor; and a second arrangement of conduits configured to form a closed circuit and to convey a heat transfer fluid among the thermal energy storage device, the first heat exchanger, the first expander, the second heat exchanger, and the third compressor; a first heat exchanger positioned along the first arrangement of conduits downstream of the first compressor and configured to transfer at least a portion of a high-grade heat of compression of the process stream from the first compressor to a thermal energy storage device via a heat transfer fluid; a second heat exchanger is positioned along the first arrangement of conduits downstream of the second compressor and configured to transfer at least a portion of a low-grade heat of compression of the process stream from the second compressor to the third compressor via a heat transfer fluid; a third compressor positioned along the second arrangement of conduits downstream of the second heat exchanger and configured to compress a heat transfer fluid and transfer at least a portion of a high-grade heat of compression of the heat transfer fluid from the third compressor to a thermal energy storage device; A first expander is positioned along the second arrangement of conduits downstream from the thermal energy storage device and is configured to expand the heat transfer fluid to extract work therefrom.

[0040] By transferring low-grade heat of compression from the process stream downstream of the second compressor to a heat transfer fluid and compressing that fluid in a third compressor, the heat transfer fluid can include high-grade heat of compression that can be stored in a high-grade thermal energy storage device. This allows substantially all of the heat generated by the second compressor to be captured and stored in a single high-grade thermal energy storage device while maintaining a low compression temperature in the second compressor. The system further allows for the extraction of work from the heat transfer fluid.

[0041] In some embodiments, the liquefaction subsystem may further include a third heat exchanger positioned along the first configuration of conduits downstream of the first heat exchanger. The third heat exchanger may be configured to transfer at least a portion of the low-grade heat of compression of the process stream from the first heat exchanger to the third compressor via a heat transfer fluid. Transferring the low-grade heat of compression from the first compressor allows for capture of substantially all of the heat generated by the first compressor.

[0042] The liquefaction subsystem may further include an air purification unit positioned along the first arrangement of conduits upstream of the second compressor, the first air purification unit configured to purify the process stream.

[0043] The liquefaction subsystem may further include a first air conditioning unit positioned along the first configuration of conduits upstream of the air purification unit, and the first air conditioning unit may be configured to substantially or completely remove heat of compression of the process stream from the first compressor.

[0044] The liquefaction subsystem may further include a third configuration of conduits having an upstream end configured to be coupled to the cold box and a downstream end, and the third configuration of conduits may be configured to pass a return flow from the cold box to replenish the process flow upstream of the second compressor.

[0045] The liquefaction subsystem may further include a cold box positioned at a downstream end of the first configuration of conduits and configured to at least partially liquefy the process stream to form a liquefied product.

[0046] The liquefaction subsystem may further include a second air conditioning unit positioned along the first configuration of conduits upstream of the cold box, and the second air conditioning unit may be configured to substantially or completely remove heat of compression of the process stream from the second compressor.

[0047] The cold box may be configured to route a return stream including at least a portion of any non-liquefied process stream to a third configuration of conduits.

[0048] The cold box may be configured to route at least a portion of the liquefaction product to a cryogenic storage tank.

[0049] In some embodiments, the heat transfer fluid may be a first heat transfer fluid, and the cryogenic energy storage system may further include a power recovery subsystem; The power recovery subsystem comprises: pump, Evaporator, At least one heater; an expansion stage corresponding to each of the at least one heater; a fourth arrangement of conduits having an upstream end and a downstream end and configured to pass working fluid from the pump through the evaporator, each of the at least one heater and a corresponding expansion stage, each heater positioned upstream of its corresponding heater; and a fifth arrangement of conduits configured to form a closed circuit and to convey a second heat transfer fluid between the thermal energy storage device and each of the at least one heater; Each heater is configured to transfer at least a portion of the high-grade heat of compression from the thermal energy storage device to the working fluid via the second heat transfer fluid; Each expansion stage is configured to expand a working fluid and extract work therefrom.

[0050] The power recovery subsystem may further include a preheater positioned along the fourth configuration of conduits upstream of the first heater, the preheater may be configured to receive at least a portion of the exhaust air from the final expansion stage and transfer at least a portion of the heat from the final expansion stage exhaust to the working fluid upstream of the first heater. By preheating the working fluid using the exhaust air, the preheater may prevent the working fluid from solidifying a heat transfer fluid in the heater, preventing potential clogging.

[0051] The pump may be configured to pump the liquefied product from the cryogenic storage tank and pressurize it to form the working fluid.

[0052] In some embodiments, the first and second heat transfer fluids may be the same fluid, and the conduits of the second arrangement and the conduits of the fifth arrangement may be fluidly connected and share the same heat transfer fluid.

[0053] According to a further aspect of the present invention there is provided a method for storing heat of compression in a cryogenic energy storage system, the method comprising: providing a liquefaction subsystem including a first compressor and a second compressor; providing a thermal energy storage device configured to store high grade heat; compressing a process stream in a first compressor; capturing at least a portion of the high grade heat of compression of the process stream from the first compressor and storing it in a thermal energy storage device via a heat transfer fluid; compressing the process stream in a second compressor; capturing at least a portion of the high grade heat of compression of the process stream from the second compressor and storing it in a thermal energy storage device via a heat transfer fluid; capturing at least a portion of the low grade heat of compression of the process stream from the second compressor and returning it to the second compressor.

[0054] By returning at least a portion of the low-grade heat of compression from the second compressor to the second compressor, the process stream can be compressed to a much higher temperature, thereby enabling the second compressor to generate high-grade heat of compression. This high-grade heat of compression can then be stored in a thermal energy storage device via a heat transfer fluid. Thus, both high-grade and low-grade heat can be captured and stored in a single high-grade thermal energy storage device.

[0055] The method may include purifying the process stream after capturing at least a portion of the high grade heat of compression of the process stream from the first compressor.

[0056] The method may include conditioning the process stream to substantially or completely remove the heat of compression of the process stream from the first compressor prior to purifying the process stream.

[0057] The method may include capturing at least a portion of the high-grade heat of compression of the process stream from the first compressor, followed by capturing at least a portion of the low-grade heat of compression of the process stream from the first compressor and transferring the heat of compression to the process stream after purifying the process stream. Transferring the portion of the low-grade heat of compression to the process stream after purifying the process stream allows the process stream to be compressed to a higher temperature in a second compressor, which in turn allows the second compressor to generate high-grade heat of compression, which can be stored in a single high-grade thermal energy storage device. Thus, both high-grade and low-grade heat can be captured and stored in a single high-grade thermal energy storage device.

[0058] Returning at least a portion of the low-grade heat of compression of the process stream from the second compressor to the second compressor may include transferring at least a portion of the low-grade heat of compression of the process stream from the second compressor to the process stream prior to compressing the process stream in the second compressor.

[0059] The method may include supplementing the process stream with a return stream prior to compressing the process stream in the second compressor.

[0060] Returning at least a portion of the low-grade heat of compression of the process stream from the second compressor to the second compressor may include transferring at least a portion of the low-grade heat of compression of the process stream from the second compressor to the process stream after the process stream has been replenished by the return stream.

[0061] Returning at least a portion of the low-grade heat of compression of the process stream from the second compressor to the second compressor may include transferring at least a portion of the low-grade heat of compression of the process stream from the second compressor to the return stream prior to replenishing the process stream.

[0062] The method may include conditioning the process stream to substantially or completely remove the heat of compression of the process stream from the second compressor after capturing at least a portion of the low grade heat of compression of the process stream from the second compressor.

[0063] According to a further aspect of the present invention there is provided a method for storing heat of compression in a cryogenic energy storage system, the method comprising: providing a liquefaction subsystem including a first compressor, a second compressor, a third compressor, and an expander; providing a thermal energy storage device configured to store high grade thermal energy; compressing a process stream in a first compressor; capturing at least a portion of the high grade heat of compression of the process stream from the first compressor and storing it in a thermal energy storage device; compressing the process stream in a second compressor; capturing at least a portion of the low grade heat of compression of the process stream from the second compressor and transferring it to the third compressor via a heat transfer fluid; compressing a heat transfer fluid in a third compressor; capturing at least a portion of the high-grade heat of compression of the heat transfer fluid from the third compressor and storing it in a thermal energy storage device; and expanding the heat transfer fluid in an expander and extracting work therefrom.

[0064] By capturing low-grade heat of compression from the process stream downstream of the heat transfer fluid and compressing the fluid in a third compressor, the heat transfer fluid can contain high-grade heat of compression, which can be stored in a high-grade thermal energy storage device. This allows substantially all of the heat generated by the second compressor to be captured and stored in a single high-grade thermal energy storage device while maintaining a low compression temperature in the second compressor. The system further allows for work to be extracted from the heat transfer fluid.

[0065] The method may include capturing at least a portion of a high-grade heat of compression of the process stream from the first compressor, after capturing at least a portion of a low-grade heat of compression of the process stream from the first compressor and transferring it to a third compressor via a heat transfer fluid.

[0066] The method may include purifying the process stream prior to compressing it in the second compressor.

[0067] The method may include conditioning the process stream to substantially or completely remove the heat of compression of the process stream from the first compressor prior to purifying the process stream.

[0068] The method may include conditioning the process stream to substantially or completely remove the heat of compression of the process stream from the second compressor after capturing at least a portion of the low grade heat of compression of the process stream from the second compressor.

[0069] The method may include supplementing the process stream with a return stream prior to compressing the process stream in the second compressor.

[0070] The method may include at least partially liquefying the process stream to form a liquefied product.

[0071] The method may include sending at least a portion of the liquefaction product to a cryogenic storage tank.

[0072] The method may include passing at least a portion of any non-liquefied process stream as a return stream to replenish the process stream.

[0073] This method is heating a working fluid using at least a portion of the high degree of heat of compression stored in the thermal energy storage device; and expanding the working fluid in an expansion stage and extracting work therefrom.

[0074] The method may further include pre-heating the working fluid using the exhaust stream from the expansion stage prior to heating the working fluid.

[0075] The method may further include pumping the liquefied product from the cryogenic storage tank and pressurizing it to form the working fluid.

[0076] It is understood that the present invention is not limited to the embodiments, aspects, or examples described herein, and various modifications and improvements are possible without departing from the concepts disclosed herein. Except where incompatible, any of the features described herein can be used separately or in combination with any of the other features. The present disclosure includes all combinations and subcombinations of one or more features described herein. [Brief description of the drawings]

[0077] [Figure 1] FIG. 1 is a general schematic diagram of a cryogenic energy storage system (CES). [Diagram 2] FIG. 1 is a detailed schematic diagram of a CES system, specifically a liquefaction subsystem utilizing a recuperator. [Diagram 3] FIG. 2 is a detailed schematic diagram of a CES system, and in particular an alternative liquefaction subsystem. [Figure 4] 1 is a detailed schematic diagram of a CES system, and in particular a further alternative liquefaction subsystem; [Diagram 5] 1 is a detailed schematic diagram of a CES system, and in particular a further alternative liquefaction subsystem; [Figure 6] FIG. 1 is a detailed schematic diagram of a CES system, specifically a liquefaction subsystem that utilizes an expander and a compressor. [Figure 7] FIG. 2 is a detailed schematic diagram of the CES system, and in particular the power recovery subsystem. [Figure 8] FIG. 2 is a detailed schematic diagram of a CES system, and in particular an alternative power recovery subsystem. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0078] The physical values ​​of pressure, temperature, mass flow, etc. used in the following description are intended to illustrate the invention, and those skilled in the art will appreciate that there are a wide range of possible values ​​for these physical quantities depending on the particular design of the CES system and the requirements of the user.

[0079] In this description and the accompanying drawings, the same or similar elements are designated with the same reference numbers.

[0080] FIG. 1 is a schematic diagram of a cryogenic energy storage (CES) system including a liquefaction subsystem 100, a cryogenic storage tank 200, a power recovery subsection 400, and a high-grade thermal energy storage device (TESD) 300.

[0081] During the charging stage of the CES system, the liquefaction subsystem 100 is configured to receive and liquefy ambient air through a series of compression and cooling levels. The flow of air or liquid air passing through the liquefaction subsystem 100 is referred to herein as the process stream of the liquefaction subsystem 100. The liquid air produced by the liquefaction subsystem 100, also referred to herein as refrigerant, may then be routed to the cryogenic storage tank 200 through a first refrigerant conduit 710 configuration of refrigerant conduits 700. The liquefaction subsystem 100 may employ any of the configurations shown in Figures 2-6 or described below.

[0082] The high-grade TESD 300 is configured to receive high-grade heat of compression of the process stream from the liquefaction subsystem 100 via a heat transfer fluid and store it in a storage medium. The heat transfer fluid passes from the high-grade TESD 300 to the liquefaction subsystem 100 where it contains the high-grade heat of compression generated as a result of compression of the process stream in the liquefaction subsystem 100. The heat transfer fluid then passes through the high-grade TESD 300 where the high-grade heat is transferred to the storage medium of the high-grade TESD 300.

[0083] During the fill phase of the CES system, the refrigerant exiting the liquefaction subsystem 100 is stored in the cryogenic storage tank 200 where it can be stored for long periods of time under low pressure. It is understood that the cryogenic storage tank 200 can be formed from multiple cryogenic storage tanks with a common connection to the liquefaction subsystem 100 and / or the power recovery subsystem 400 and a common header in fluid communication therewith. As is common in the safety design of all cryogenic energy storage systems, the pressure within the cryogenic storage tank 200 described herein can be prevented from rising above a design value by a pressure relief valve (not shown).

[0084] In the discharge phase of the CES system, the refrigerant is routed from the cryogenic storage tank 200 through the second refrigerant conduit 720 configuration of the refrigerant conduit 700 to the power recovery subsystem 400. The refrigerant, liquid air, or air passing through the power recovery subsystem 400 is referred to herein as the working fluid. The power recovery subsystem 400 is configured to pressurize, vaporize, heat, and expand the working fluid. Mechanical energy recovered from the expanding working fluid can be converted to electrical energy. Exhaust from the power recovery subsystem 400 can be vented to the atmosphere through a vent 500 or recycled to another system co-located with the present CES system or a portion thereof (e.g., any portion of the power recovery subsystem, the liquefaction subsystem, or other portions of the broader CES system). The power recovery subsystem 400 can employ any of the configurations shown in FIG. 7 or FIG. 8 or described below.

[0085] The CES system can be configured in combination with any of the liquefaction subsystem 100 configurations shown in Figures 2-6 or described below, and any of the power recovery subsystem 400 configurations shown in Figures 7 or 8 or described below.

[0086] In the discharge phase, the CES system is configured to transfer the stored high-grade compressed heat from the high-grade TESD 300 to the power recovery subsystem 400 via the heat transfer fluid. The heat transfer fluid containing the high-grade thermal energy is sent from the high-grade TESD 300 to the power recovery subsystem 400. In the power recovery subsystem 400, the high-grade compressed heat contained in the heat transfer fluid is transferred to the working fluid before the working fluid expands, which improves the power output of the power recovery subsystem during the discharge phase. This leads to an improvement in the round trip efficiency of the CES system. The cold heat transfer fluid is then returned to the high-grade TESD 300 for reheating.

[0087] The heat transfer fluid used to transfer high grade heat from the liquefaction subsystem 100 to the high grade TESD 300 can pass through a configuration of heat transfer conduits 610 forming a closed circuit configured to route the heat transfer fluid between the liquefaction subsystem 100 and the high grade TESD 300. The heat transfer fluid configured to transfer high grade heat from the high grade TESD 300 to the power recovery subsystem 400 can pass through a further configuration of heat transfer conduits 620 forming a closed circuit configured to route the heat transfer fluid between the high grade TESD 300 and the power recovery subsystem 400. Alternatively, the above-mentioned heat transfer fluids can be the same heat transfer fluid and the above-mentioned configurations of heat transfer conduits can be fluidly connected to form a single combined configuration of heat transfer conduits 600 connecting the liquefaction subsystem 100, the power recovery subsystem 400, and the high grade TESD 300. In this case, the combined configuration of conduits 600 can share the same heat transfer fluid and route the heat transfer fluid between the liquefaction subsystem 100, the high grade TESD 300, and the power recovery subsystem 400. The heat transfer fluid circulating through the liquefaction subsystem 100, the high-grade TESD 300, and the power recovery subsystem 400 may be a gas, such as air or compressed air, or a liquid, such as water or a mixture of water and glycol, or preferably thermal oil or a mixture of thermal oils (synthetic, natural, mineral), or a molten salt.

[0088] Although "liquid air," "air," or "refrigerant" is used throughout this description, the present invention is not limited to the production and use of liquid air or air alone, but may also be used to produce and use other cryogenic fluids, such as liquid nitrogen, liquid oxygen, liquid natural gas (LNG), etc.

[0089] 2 illustrates a CES system including a liquefaction subsystem 100 and a high-grade TESD 300. The liquefaction subsystem 100 is configured to convert a flow of ambient air into liquid air or refrigerant for storage in a cryogenic storage tank. The liquefaction subsystem 100 can include a main air compressor (MAC) 102, a main air compressor-high grade heat exchanger (MAC HG HX) 104, a main air compressor-recuperator-heat exchanger (MAC Recuperator) 106, a main air compressor-air conditioner (MAC AC) 108, an air purification unit (APU) 110, a recycled air compressor-recuperator-heat exchanger (RAC Recuperator) 112, a recycled air compressor (RAC) 114, a recycled air compressor-high grade heat exchanger (RAC HG HX) 116, a recycled air compressor-air conditioner (RAC AC) 118, and a cryobox 120.

[0090] The liquefaction subsystem 100 may further include a first liquefaction configuration of conduits 101 having an upstream end A and a downstream end B and configured to route the liquefaction subsystem 100 process stream from the MAC 102 through the MAC HG HX 104, the MAC recuperator 106, the MAC AC 108, the APU 110, the second MAC recuperator 106, the RAC recuperator 112, the RAC 114, the RAC HG HX 116, the second RAC recuperator 112, the RAC AC 118 to the cold box 120. This first configuration of conduits is represented by line AB in FIG. 2 with the upstream end at point A and the downstream end at point B.

[0091] The conduits 101 of the first configuration of the embodiment of FIG. 2 include liquefaction conduits fluidly connected as follows: a first conduit 131 connecting the MAC 102 to the MAC HG HX 104; a second conduit 132 connecting the MAC HG HX 104 and the MAC recuperator 106; a third conduit 133 connecting the MAC recuperator 106 to the MAC AC 108; a fourth conduit 134 connecting the MAC AC 108 to the APU 110; a fifth conduit 135 connecting the APU 110 to the MAC recuperator 106; a sixth conduit 136 connecting the MAC recuperator 106 to the RAC recuperator 112; a seventh conduit 137 connecting the RAC recuperator 112 to the RAC 114; an eighth conduit 138 connecting the RAC 114 to the RAC HG HX 116; a ninth conduit 139 connecting the RAC HG HX 116 to the RAC recuperator 112; a tenth conduit 140 connecting to the AC 118 and an eleventh conduit 141 connecting the RAC AC 118 to the cold box 120 .

[0092] The CES system may further include a second configuration of heat transfer conduits 610, 600 configured to form a closed circuit and route heat transfer fluid between the high grade TESD300, MAC HG HX104, and RAC HG HX116. The configuration of the heat transfer conduits is shown as closed circuit C in FIG.

[0093] The second configuration of heat transfer conduits 610, 600 of the embodiment of Figure 2 includes heat transfer conduits fluidly connected as follows: a first conduit 161 connecting the high grade TESD 300 to the RAC HG HX 116, a second conduit 162 connecting a branch F of the first conduit 161 to the MAC HG HX 104, a third conduit 163 connecting the RAC HG HX 116 and the high grade TESD 300, and a fourth conduit 164 connecting the MAC HG HX 104 to a branch G of the third conduit. It will be appreciated that the second conduit 162 can directly connect the high grade TESD 300 and the MAC HG HX 104, and the fourth conduit 164 can directly connect the MAC HG HX 104 to the high grade TESD 300 instead of the branches F and G, respectively.

[0094] The liquefaction subsystem 100 may further include a third configuration of liquefaction conduits 150 having an upstream end configured to be coupled to the cold box 120 and a downstream end and configured to route a second process stream, referred to herein as a return process stream or return stream, from the cold box 120 at the upstream end to replenish the process streams of the first configuration of conduits upstream of the RAC recuperator 112 at the downstream end. The third configuration of conduits is illustrated in Figure 2 by line DE, with the upstream end at point D and the downstream end at point E. The third configuration of conduits in the embodiment of Figure 2 includes conduit 150 connecting the cold boxes 120 to branch E of the sixth liquefaction conduit 136.

[0095] During the fill phase of the CES system, the MAC 102 compresses ambient air (i.e., air present in the atmosphere around the CES system) from ambient pressure to a first pressure, which may be between approximately 2 bar and tens of bar. Compressing the ambient air heats it significantly, and its temperature may increase from ambient to approximately 400° C. or more depending on the ambient conditions, due to the inclusion of heat of compression. At such temperatures, the process stream includes high grade heat of compression.

[0096] Downstream of the MAC 102 and along the first configuration of conduits, the MAC HG HX 104 receives the high temperature compressed process stream from the MAC 102. The MAC HG HX 104 also receives a flow of a heat transfer fluid from the second configuration of conduits. The MAC HG HX 104 is configured to transfer at least a portion of the high grade heat of compression of the process stream to the heat transfer fluid. As a result, the process stream may be cooled to about 180-220° C., such that the process stream may still include residual low grade heat of compression.

[0097] The MAC recuperator 106 is configured to receive a process stream at two different locations and transfer low-grade heat of compression of the process stream at the first location to the process stream at the second location, essentially forming a low-grade heat of compression recuperation loop in which heat is transferred from one location to another. The two portions of the process stream passing through the MAC recuperator 106 may pass through two independent fluid paths within the MAC recuperator 106 to prevent mixing.

[0098] Downstream of the MAC HG HX 104, along a first configuration of conduits, the MAC recuperator 106 receives the process stream from the MAC HG HX 104. The MAC recuperator 106 also receives the process stream downstream of the APU 110 (described below). The MAC recuperator 106 is configured to transfer at least a portion of the low-grade heat of compression from the process stream exiting the MAC HG HX 104 to the process stream exiting the APU 110. The process stream exiting the MAC HG HX 104 can be cooled from about 210° C. to 60° C. using an outlet flow to the APU 110. A portion of the process stream exiting the MAC HG HX 104 and cooled in the MAC recuperator 106 is then sent to the MAC AC 108, while a portion of the process stream exiting the APU 110 and heated in the MAC recuperator 106 is then sent to the RAC recuperator 112 (described below).

[0099] Downstream of the MAC recuperator 106, along a first configuration of conduits, the MAC AC 108 is configured to receive the process stream from the MAC recuperator 106 and substantially or completely remove the final heat of compression from the process stream. The cooling provided by the MAC AC 108 determines the final temperature of the process stream before it is sent to the APU 110, and therefore determines the inlet temperature of the APU 110. The MAC AC 108 may cool the process stream to approximately ambient temperature. The MAC AC 108 may also remove a significant portion of moisture from the process stream by condensation.

[0100] Downstream of the MAC AC 108, along a first configuration of conduits, the APU 110 receives process streams from the MAC AC 108, including water and CO that would otherwise solidify and clog the liquefaction subsystem during operation. 2 The APU 110 is configured to purify the process stream by removing CO from the process stream. The APU 110 can purify the process stream through molecular adsorption, temperature swing adsorption (TSA), pressure swing adsorption (PSA), or a combination of both. 2 Certain adsorbents that preferentially remove nitrogen and water may be used within the APU. The process stream may exit the APU 110 at about ambient temperature or slightly higher due to the fact that exothermic processes may occur within the APU 110.

[0101] As discussed above, the process stream then passes again through the MAC recuperator 106. Downstream of the APU 110, along a first configuration of conduits, the MAC recuperator 106 receives the process stream from the APU 110 and also receives the process stream from the MAC HG HX 104. The cool process stream exiting the APU 110 may be at about ambient temperature, which is used in the MAC recuperator 106 to cool the hot process stream exiting the MAC HG HX 104, which may be about 190-230° C. The process stream exiting the APU 110 may be heated in the MAC recuperator 106 to about 180-225° C. using the outlet stream from the MAC HG HX 104. A portion of the process stream exiting the APU 110 and heated in the MAC recuperator 106 is then sent to the RAC recuperator 112. The MAC AC 108 or the APU 110, or a combination of both, may include an additional cooling system, such as a chiller or chiller system, to further assist in removing water from the process stream.

[0102] Downstream of the MAC recuperator 106, along a first configuration of conduits, the RAC recuperator 112 receives the process stream exiting the MAC recuperator 106. The RAC recuperator 112 also receives the process stream exiting the RAC HG HX 116 (described below). Similar to the MAC recuperator 106, the RAC recuperator 112 is configured to receive the process stream at two different locations, capture heat from the process stream at a first location, and transfer it to another portion of the process stream at a second location. In essence, it forms a heat recuperation loop for the process stream where heat is transferred from one location to another. The two portions of the process stream passing through the RAC recuperator 112 may pass through two independent fluid paths within the RAC recuperator 112 to prevent mixing.

[0103] As explained in the Background section above, in a typical CES system, compression of the process stream in a recycle air compressor generates a heat of compression. However, the outlet temperature of such a recycle air compressor may have a temperature of about 190° C., and therefore the heat of compression of the output stream is typically only of low grade.

[0104] The RAC recuperator 112 is configured to transfer at least a portion of the low-grade heat of compression from the process stream exiting the RAC HG HX 116 (described below) to the process stream exiting the MAC recuperator 106 and flowing to the RAC 114. Thus, the process stream exiting the MAC recuperator 106 can be heated to 190° C. or higher using the outlet stream of the RAC HG HX 116. As a result, the inlet temperature of the RAC 114 is much higher than typical recycled air, which can cause the discharge temperature of the RAC 114 to be significantly higher, consistent with the discharge temperature of the MAC 102 being greater than about 400° C. The process stream exiting the MAC recuperator 106 and heated in the RAC recuperator 112 is then sent to the RAC 114 (described below), while the process stream exiting the RAC HG HX 116 and cooled in the RAC recuperator 112 is then sent to the RAC AC 118 (described below).

[0105] Downstream of the RAC recuperator 112 and along a first configuration of conduits, the RAC 114 receives the process stream exiting the RAC recuperator 112. During the aforementioned stages, the pressure of the process stream may drop slightly due to leaks or other environmental factors. However, the RAC 114 may compress the process stream from approximately the first pressure (taking into account any pressure drop) to a second pressure, which may be about tens of bars. As the process stream is compressed, its temperature increases significantly because heat of compression is generated within the stream. The increase in the inlet temperature of the RAC 114 results in an outlet temperature of greater than about 400-440°C. Compared to the outlet process stream of a typical recycle air compressor, the outlet process stream of the RAC 114 may contain high-grade compression heat energy.

[0106] Downstream of the RAC 114, along the first configuration of conduits, the RAC HG HX 116 receives the process stream exiting the RAC 114. The RAC HG HX 116 also receives a flow of a heat transfer fluid from the second configuration of conduits. The RAC HG HX 116 is configured to transfer at least a portion of the high-grade heat of compression from the process stream to the heat transfer fluid. As a result, the process stream may be cooled to about 210° C. and may still contain residual low-grade heat of compression.

[0107] As described above, downstream of the RAC HG HX 116 along the first configuration of conduits, the RAC recuperator 112 receives the process stream exiting the RAC HG HX 116 and the process stream exiting the MAC recuperator 106. The RAC recuperator 112 is configured to transfer at least a portion of the low-grade heat of compression from the process stream exiting the RAC HG HX 116 to the process stream exiting the MAC recuperator 106. The process stream exiting the RAC HG HX 116 and cooled in the RAC recuperator 112 is then sent to the RAC AC 118.

[0108] Downstream from the RAC recuperator 112 along the first configuration of conduits, the RAC AC 118 receives the process stream exiting the RAC recuperator 112. The RAC AC 118 is configured to remove residual heat of compression from the process stream before sending the process stream to the cold box 120. The RAC AC 118 may cool the process stream to approximately ambient temperature.

[0109] Downstream of the RAC AC 118, along a first configuration of conduits, the cold box 120 can receive the process stream exiting the RAC AC 118. As described in the Background section above, the cold box 120 is configured to cool the process stream until it is substantially condensed to a liquid. The liquid product is separated from the vapor phase and at least a portion is sent to the cryogenic storage tank 200 for storage. At least a portion of the vapor phase (i.e., any non-liquefied process stream) can be used to cool the process stream entering the cold box 120 to aid in liquefaction. This is done by passing the vapor phase through the cold box 120 in a countercurrent direction relative to the process stream entering the cold box 120. A return stream is formed from the vapor phase after passing through the cold box 120 and passes through a third configuration of conduits to replenish the process stream upstream of the RAC recuperator 112.

[0110] The return stream from the cold box 120 may be at ambient temperature and may have a much larger mass flow rate than the process stream flowing from the outlet to the MAC recuperator 106. Thus, the return stream may have a large impact on the temperature at the inlet of the RAC 114. Due to the use of an expansion turbine in the cold box 120 to liquefy the process stream, the mass flow rate of the return stream is typically greater than the process stream exiting the MAC recuperator 106. For example, the ratio of the mass flow rate of the return stream to the process stream before the return stream is combined may be between about 11:7 and 13:5. While the process stream exiting the MAC recuperator 106 is fully heated, the return stream exiting the cold box 120 may not yet be heated. Thus, the RAC recuperator 112 may recover at least a portion of the low-grade heat of compression from the RAC 114 and return it to the inlet of the RAC 114, thus increasing the outlet temperature of the RAC 114, and extract high-grade heat of compression from the outlet of the RAC 114. This also prevents (counters) cooling of the process stream after it has been replenished by return flow from the cold box 120 .

[0111] The high-grade TESD 300 is configured to store high-grade heat of compression contained in the process stream as a result of compression of the process stream in the MAC 102 and RAC 114. The high-grade heat is transferred from the process stream to the high-grade TESD 300 by the MAC HG HX 104, the RAC HG HX 116, and the heat transfer fluid in the second configuration of conduits. The high-grade TESD 300 may be a packed bed TESD, a stationary liquid phase based TESD, or preferably a two-reservoir TESD as described in the Background section above. The heat transfer fluid may be a gas, such as air or compressed air, or a liquid, such as water, glycol, a mixture of water and glycol, thermal oil, a mixture of thermal oil, or preferably a molten salt.

[0112] The process stream of the liquefaction subsystem 100 is compressed in the MAC 102 so that the process stream contains high grade heat of compression. The process stream then passes through the MAC HG HX 104 where at least a portion of the high grade heat of compression is transferred to the heat transfer fluid in the second configuration of conduits. The heat transfer fluid is then pumped through the second configuration of conduits to the high grade TESD 300 where the high grade heat is transferred from the heat transfer fluid to a storage medium. The cold heat transfer fluid is then recirculated back to the MAC HG HX 104 or RAC HG HX 116 to repeat the heat transfer process.

[0113] In a similar process, when the process stream of the liquefaction subsystem 100 is compressed in the RAC 114, the process stream contains high-grade heat of compression due to heat recovery of the process stream by the RAC recuperator 112, as described above. The process stream then passes through the RAC HG HX 116, where at least a portion of the high-grade heat of compression is transferred to a heat transfer fluid in the second configuration of conduits. The heat transfer fluid is then pumped through the second configuration of conduits to the high-grade TESD 300. In the high-grade TESD 300, the high-grade heat is transferred to a storage medium. The cold heat transfer fluid is then pumped back to the MAC HG HX 104 or the RAC HG HX 116, and the heat transfer process is repeated. The storage medium of the high-grade TESD 300 may preferably be a molten salt.

[0114] As mentioned above, the process stream flowing from the MAC recuperator 106 to the RAC recuperator 112 is supplemented by the return stream from the cold box 120 before entering the RAC recuperator 112. Thus, the mass flow rate through the RAC recuperator 112 can be much greater than the mass flow rate through the MAC recuperator 106. For example, the ratio of the mass flow rate through the RAC recuperator 112 to the mass flow rate through the MAC recuperator 106 may be between about 17:7 and 19:5. Generally, in a heat exchanger, the heat transfer rate is proportional to the mass flow rate. Increasing the mass flow rate of the fluid through the heat exchanger allows more molecules to interact with the inner surface of the heat exchanger, allowing more heat to be conducted through its surfaces, and allowing the heat exchanger to transfer more thermal energy. The increased mass flow rate through the RAC recuperator 112 means that the RAC recuperator 112 can have significantly more heat available for exchange than the MAC recuperator 106, and therefore can play a larger role in heating the process stream before it enters the RAC 114. Thus, the MAC recuperator 106 may be removed leaving only the RAC recuperator 112 .

[0115] Figure 3 shows a CES system that is identical to that of Figure 2, except that it lacks the MAC recuperator 106. This modification increases the simplification of the liquefaction subsystem and can reduce the capital expenditures to build such a system.

[0116] The CES system includes a liquefaction subsystem 100 and a high-grade TESD 300. The liquefaction subsystem 100 may similarly include a main air compressor (MAC) 102, a main air compressor and high-grade heat exchanger (MAC HG HX) 104, a main air compressor and air conditioner (MAC AC) 108, an air purification unit (APU) 110, a recycled air compressor and recuperator and heat exchanger (RAC Recuperator) 112, a recycled air compressor (RAC) 114, a recycled air compressor and high-grade heat exchanger (RAC HG HX) 116, a recycled air compressor and air conditioner (RAC AC) 118, a cold box 120, conduits of a first configuration, conduits of a second configuration, and conduits of a third configuration, in a similar configuration to the CES system of FIG. However, in the present CES system, the MAC recuperator 106 has been removed and the process stream leaving the MAC HG HX 104 is routed to a MAC AC 108 and the process stream leaving the APU 110 is routed to a RAC recuperator 112 .

[0117] The CES system of FIG. 3 includes the same first configuration of liquefaction ducts as FIG. 2, except that the second duct 132 and the third duct 133 are replaced by a twelfth duct 142 that directly connects the MAC HG HX 104 to the APU 110, and the fifth duct 135 and the sixth duct 136 are replaced by a thirteenth duct 143 that directly connects the APU 110 to the RAC recuperator 112.

[0118] The MAC HG HX 104 is configured to transfer high-grade heat of compression contained in the process stream as a result of compression in the MAC 102 to the heat transfer fluid in the second configuration of conduits. The process stream is then sent directly to the MAC AC 108. However, the process stream received by the MAC AC 106 may still contain residual low-grade heat of compression. It is understood that the APU 110 operates optimally when the process stream is at about ambient temperature. Thus, the MAC AC 108 may be configured to dump excess heat into the atmosphere to cool the process stream to about ambient temperature. Alternatively, the MAC AC 108 may be configured to transfer at least a portion of the residual low-grade heat of compression to another system co-located as part of the CES system or the CES system (e.g., a power recovery subsystem, a liquefaction subsystem, or other parts of a broader CES system). As a further alternative, the MAC AC 108 may transfer at least a portion of the residual low-grade heat of compression to an external process or system requiring additional heat (e.g., a heating plant, a boiler, an air conditioner, etc.).

[0119] The APU 110 receives process streams from the MAC AC 108 and processes water and CO 2 The process stream is then sent from the APU 110 to the RAC recuperator 112. The process stream leaving the APU 110 may be at about ambient temperature and may not include residual low grade heat of compression from the MAC 102 because the process stream does not pass through the MAC recuperator 106.

[0120] Downstream from the APU 110 and along a first configuration of conduits, a RAC recuperator 112 receives the process stream from the APU 110 and the process stream from the RAC HG HX 116. The RAC recuperator 112 is configured to transfer at least a portion of the low-grade heat of compression from the process stream from the RAC HG HX 116 (described below) to the process stream from the APU 110. The process stream from the APU 110 is heated in the RAC recuperator 112 and then sent to the RAC 114, while the process stream from the RAC HG HX 116 is cooled in the RAC recuperator 112 and then sent to the RAC AC 118. The remaining components of the CES system perform similar operations to the remaining components of the CES system shown in FIG. 2 and described above.

[0121] The return stream exiting the cold box 120 joins the process stream downstream of the APU 110. As noted above, the return stream may be at about ambient temperature. The process stream exiting the APU 110 may also be at about ambient temperature. Thus, the RAC recuperator 112 may be configured to transfer sufficient thermal energy to the process stream after being supplemented by the return stream to ensure that the process stream contains high-grade thermal energy after compression in the RAC 114.

[0122] As mentioned above, the return stream from the cold box 120 may have a significantly higher mass flow rate than the process stream exiting the MAC recuperator 106 of the CES system of FIG. 2 or the APU 110 of the CES system of FIG. 3. Furthermore, the return stream may be at approximately ambient temperature. However, particularly in the CES system of FIG. 2, the process stream exiting the MAC recuperator may be at a higher temperature, approximately 190° C. Thus, when the process stream is supplemented by the return stream, there is a risk of significant thermal mixing of the streams at different temperatures. This may reduce the energy potential of the process stream.

[0123] 4 illustrates a CES system that is identical to that illustrated in FIG. 2 and described above, except that the RAC recuperator 112 is configured to transfer at least a portion of the low-grade heat of compression from the process stream exiting the RAC HG HX 116 to the return stream from the cold box 120 prior to replenishing the process stream exiting the MAC recuperator 106 to the RAC 114. This configuration enables the system to heat the return stream from the cold box 120 to approximately the temperature of the process stream, thereby avoiding thermal mixing of streams at different temperatures and the problems discussed above.

[0124] The CES system of Figure 4 includes the same first configuration of liquefaction conduits as Figure 2, except that the sixth conduit 136 connecting the MAC recuperator 106 to the RAC recuperator 112 is replaced with a fourteenth conduit 144 connecting the MAC recuperator 106 to the RAC 114. The fourteenth liquefaction conduit 144 includes a branch E that connects to the liquefaction conduits of the third configuration. The third configuration of liquefaction conduits 150 includes a first return conduit 151 connecting the refrigerated box 120 to the RAC recuperator 112 and a second return conduit 152 connecting the RAC recuperator 112 to branch E.

[0125] The CES system includes a liquefaction subsystem 100 and a high grade TESD 300. The liquefaction subsystem 100 may similarly include a main air compressor (MAC) 102, a main air compressor and high grade heat exchanger (MAC HG HX) 104, a main air compressor and recuperator and heat exchanger (MAC Recuperator) 106, a main air compressor and air conditioner (MAC AC) 108, an air purification unit (APU) 110, a recycled air compressor and recuperator and heat exchanger (RAC Recuperator) 112, a recycled air compressor (RAC) 114, a recycled air compressor and high grade heat exchanger (RAC HG HX) 116, a recycled air compressor and air conditioner (RAC AC) 118, a cold box 120, a first configuration of conduits, a second arrangement, and a third configuration of conduits similar to the CES system shown in FIG.

[0126] The process stream from the MAC recuperator 106, which may include low-grade heat of compression, is sent to the RAC 114. Before being sent to the RAC 114, the process stream is supplemented by a return stream from the cold box 120. The process stream is then compressed in the RAC 114 and sent to the RAC HG HX 116, where the high-grade heat of compression is transferred to the high-grade TESD 300 via the heat transfer fluid in the second configuration of conduits. The process stream is then sent to the RAC recuperator 112.

[0127] The RAC recuperator 112 is configured to receive the process stream exiting the RAC HG HX 116 and the return stream from the cold box 120 before replenishing the process stream exiting the MAC recuperator 106 to the RAC 114. The RAC recuperator 112 is configured to transfer at least a portion of the low-grade heat of compression of the process stream exiting the RAC HG HX 116 to the return stream. The return stream from the cold box 120 may be heated in the RAC recuperator 112 to an approximate temperature of the process stream flowing from the MAC recuperator 106 to the RAC 114. The process stream flowing from the RAC HG HX 116 to the RAC recuperator 112 is cooled in the RAC recuperator 112 and then sent to the RAC AC 118 and follows the same subsequent process as shown in FIG. 2 and described in connection with the CES system described above. The return stream flowing from the cold box 120 to the RAC recuperator 112 is heated in the RAC recuperator 112 and then routed to a first configuration of conduits upstream of the RAC 114 where it replenishes the RAC 114 with the process stream exiting the MAC recuperator 106 .

[0128] Thus, the return stream from the cold box 120 contains at least a portion of the low grade heat of compression in the RAC recuperator 112. Because the process stream flowing from the MAC recuperator 106 to the RAC 114 contains a portion of the low grade heat of compression in the MAC recuperator 106, when these streams combine, they may not experience significant mixing at different temperatures. Furthermore, because both streams are heated, when combined and compressed in the RAC 114, the process stream contains high grade heat of compression. This allows the system to transfer at least a portion of this high grade heat to the high grade TESD 300 via the RAC HG HX 116 and the sheet transfer fluid of the second configuration of conduits.

[0129] 4, the return flow from the cold box 120 may have a significantly higher mass flow rate than the process flow exiting the MAC 102. Thus, the RAC recuperator 112 transfers significantly more energy than the RAC recuperator 112, and as a result plays a larger role in determining the input temperature of the RAC 114 and the amount of high-grade compression heat generated within the RAC 114. Therefore, the MAC recuperator 106 may be removed to leave only the RAC recuperator 112.

[0130] Figure 5 shows a CES system that is identical to that of Figure 4, except that it lacks the MAC recuperator 106. This modification increases the simplification of the liquefaction subsystem and may reduce the capital expenditure to build such a system.

[0131] The CES system includes a liquefaction subsystem 100 and a high-grade TESD 300. The liquefaction subsystem 100 may similarly include a main air compressor (MAC) 102, a main air compressor and high-grade heat exchanger (MAC HG HX) 104, a main air compressor and air conditioner (MAC AC) 108, an air purification unit (APU) 110, a recycled air compressor and recuperator and heat exchanger (RAC Recuperator) 112, a recycled air compressor (RAC) 114, a recycled air compressor and high-grade heat exchanger (RAC HG HX) 116, a recycled air compressor and air conditioner (RAC AC) 118, a cold box 120, a first configuration of conduits, a second configuration of conduits, and a third configuration of conduits, all of which are similar to the CES system of FIG. However, the MAC recuperator 106 has been removed, with the process stream exiting the MAC HG HX 104 being routed directly to the MAC AC 108 and the process stream exiting the APU 110 being routed directly to the RAC recuperator 112 .

[0132] The CES system of FIG. 5 includes the same first configuration of liquefaction ducts as FIG. 4, except that the second duct 132 connecting the MAC HG HX 104 and the MAC recuperator 106 and the third duct 133 connecting the MAC recuperator 106 to the MAC AC 108 are replaced by a twelfth duct 142 directly connecting the MAC HG HX 104 to the APU 110, and the fifth duct 135 connecting the APU 110 to the MAC recuperator 106 and the sixth duct 136 connecting the MAC recuperator 106 to the RAC recuperator 112 are replaced by a thirteenth duct 143 directly connecting the APU 110 to the RAC recuperator 112.

[0133] The process stream exiting the MAC HG HX 104 includes the residual low-grade heat of compression and is then sent to the MAC AC 108. The MAC AC 108 may be configured to dump the excess heat to the atmosphere to cool the process stream to approximately ambient temperature. Alternatively, the MAC AC 108 may be configured to transfer at least a portion of the residual low-grade heat of compression to another system co-located with a portion of the CES system (e.g., a power recovery subsystem, a liquefaction subsystem, or other part of a broader CES system). As a further alternative, the MAC AC 108 may transfer at least a portion of the residual low-grade heat of compression to an external process or system requiring additional heat (e.g., a heating plant, boiler, air conditioner, etc.).

[0134] The APU 110 receives process streams from the MAC AC 108 and processes water and CO 2 The APU 110 is configured to purify the process stream by removing heat from the MAC 102. The process stream is then sent from the APU 110 to the RAC 114. The process stream exiting the APU 110 may be at near ambient temperature because it does not pass through the MAC recuperator 106 and does not receive low grade heat from the MAC 102. The process stream exiting the APU 110 is supplemented by a return flow from the cold box 120 before being sent to the RAC 114. The remaining components of this CES system perform operations similar to those of the CES system of FIG. 4, as described above.

[0135] Downstream of the RAC HG HX 116 and along a first configuration of conduits, the RAC recuperator 112 receives the process stream exiting the RAC HG HX 116 and the return stream exiting the cold box 120. The RAC recuperator 112 is configured to transfer at least a portion of the low-grade heat of compression of the process stream downstream of the RAC HG HX 116 to the return stream from the cold box 120. The return stream exiting the cold box 120 and the process stream exiting the APU 110 may be at approximately ambient temperature. Thus, the RAC recuperator 112 may be configured to transfer sufficient heat energy to the return stream before replenishing the process stream to ensure that the process stream contains high-grade compression heat energy after compression in the RAC 114.

[0136] 6 illustrates a CES system including a liquefaction subsystem 100 and a high-grade TESD 300. The liquefaction subsystem 100 is configured to convert a flow of ambient air into liquid air or refrigerant for storage in a cryogenic storage tank 200. The liquefaction subsystem 100 can utilize a compressor and an expander to raise or lower the temperature of a heat transfer fluid so that low-grade heat of compression from a process stream can be directly captured and stored as high-grade heat in the high-grade TESD 300.

[0137] The liquefaction subsystem 100 may include a main air compressor (MAC) 102, a main air compressor-high grade heat exchanger (MAC HG HX) 104, a main air compressor-low grade heat exchanger (MAC LG HX) 105, a main air compressor-air conditioner (MAC AC) 108, an air purification unit (APU) 110, a recycle air compressor (RAC) 114, a recycle air compressor-low grade heat exchanger (RAC LG HX) 117, a recycle air compressor-air conditioner (RAC AC) 118, and a cold box 120. The CES system may further include a heat transfer fluid expander (HTF Expander) 122 and a heat transfer fluid compressor (HTF Compressor) 124. The HTF Expander 122 and the HTF Compressor 124 may be configured within the liquefaction subsystem 100 or may be separate from the liquefaction subsystem 100. The CES system may further include a cryogenic storage tank 200.

[0138] The liquefaction subsystem 100 may further include a first configuration of conduits having an upstream end and a downstream end and configured to route the process stream of the liquefaction subsystem 100 from the MAC 102 through the MAC HG HX 104, the MAC LG HX 105, the MAC AC 108, the APU 110, the RAC 114, the RAC LG HX 117, the RAC AC 118, and to the cold box 120. The first configuration of conduits is represented by line AB in FIG. 6, where the upstream end is at point A and the downstream end is at point B. The CES system may further include a second configuration of conduits configured to form a closed circuit and route the heat transfer fluid between the high grade TESD300, the MAC HG HX 104, the HTF expander 122, the MAC LG HX 105, the RAC LG HX 117, and the HTF compressor 124. The second configuration of conduits is represented by closed circuit C in FIG. 6. Liquefaction subsystem 100 may further include a third configuration of conduits having an upstream end and a downstream end and configured to route a second process stream (referred to herein as a return stream) from cold box 120 to replenish the process stream of the first configuration of conduits upstream of RAC 114. The third configuration of conduits is indicated by line DE in FIG. 2 with its upstream end at point D and its downstream end at point E.

[0139] The conduits 101 of the first configuration of the CES of FIG. 6 include liquefaction conduits fluidly connected as follows: a first conduit 131 connected to the MAC 102 and the MAC HG HX 104, a fifteenth conduit 145 connected to the MAC HG HX 104 and the MAC LG HX 105, a sixteenth conduit 146 connected to the MAC LG HX 105 and the MAC AC 108, a fourth conduit connected to the MAC AC 108 and the APU 110, a seventeenth conduit connected from the APU 110 to the RAC 114 and including a branch E, an eighteenth conduit connecting the RAC 114 to the RAC LG HX 117, a nineteenth conduit 149 connected to the RAC LG HX 117 and the RAC AC 118, and a twelfth conduit 142 connected to the RAC AC 118 and the cold box 120.

[0140] The heat transfer conduits 610 of the second configuration of the CES of FIG. 6 include heat transfer conduits fluidly connected as follows: a fifth conduit 165 including a branch F and connected to the high grade TESD 300 and the HTF expander 122; a sixth conduit 166 connected to the HTF expander 122 and a branch H of a seventh conduit 167; a seventh conduit 167 connected to the MAC LG HX 105 and the RAC LG HX 117 and including a branch H; an eighth conduit 168 connected to the MAC LG HX 105 and a branch J of a ninth conduit 169; a ninth conduit 169 connected to the RAC LG HX 117 and including a branch J of the ninth conduit 169; 2-5, it will be appreciated that the connections of the second conduit 162 and the fourth conduit 164 can be directly connected to the high grade TESD 300 instead of branches F and G, respectively.

[0141] During the discharge phase of the CES system, the MAC 102 compresses ambient air (i.e., air present in the atmosphere around the CES system) from ambient pressure to a first pressure, which may be between approximately 2 bar and tens of bar. As the ambient air is compressed, it heats up significantly and contains heat of compression, so its temperature may increase from ambient to approximately 400° C. or more, depending on the ambient conditions. At such temperatures, the process stream contains high grade heat of compression.

[0142] Downstream of the MAC 102 along the first configuration of conduits, the MAC HG HX 104 receives the high temperature compressed process stream from the MAC 102. The MAC HG HX 104 also receives a flow of heat transfer fluid from the second configuration of conduits. The MAC HG HX 104 is configured to transfer at least a portion of the high grade heat of compression of the process stream to the heat transfer fluid. As a result, the process stream may be cooled to about 180-220° C., such that the process stream may still include residual low grade heat of compression.

[0143] Downstream of the MAC HG HX 104 along the first arrangement of conduits, the MAC LG HX 105 receives the process stream from the MAC HG HX 104. The MAC LG HX 105 also receives a flow of a heat transfer fluid from a second arrangement of conduits downstream of the HTF expander 122 (discussed below). The MAC LG HX 105 is configured to transfer at least a portion of the residual low-grade heat of compression of the process stream to a heat transfer fluid in a second arrangement of conduits upstream of the HTF compressor 124 (discussed below). The heat transfer fluid, including the low-grade heat of compression from the MAC 102, is then routed through the second arrangement of conduits to the HTF compressor 124 (discussed below).

[0144] Downstream of the MAC LG HX105 along the first configuration of conduits, the MAC AC108 is configured to receive the process stream from the MAC LG HX105 and remove the final heat of compression from the process stream. The cooling provided by the MAC AC108 determines the final temperature of the process stream before it is sent to the APU110 and therefore determines the inlet temperature of the APU110. The MAC AC108 may cool the process stream to approximately ambient temperature. The MAC AC108 may also remove a significant portion of moisture from the process stream by condensation.

[0145] Downstream of the MAC AC 108 and along a first configuration of conduits, the APU 110 receives process streams from the MAC AC 108, including water and CO that would otherwise solidify and clog the liquefaction subsystem during operation. 2 The APU 110 is configured to purify the process stream by removing CO from the process stream. The APU 110 can purify the process stream through molecular adsorption, temperature swing adsorption (TSA), pressure swing adsorption (PSA), or a combination of both. 2 Certain adsorbents that preferentially remove nitrogen and water may be used within the APU. The process stream may exit the APU 110 at about ambient temperature or slightly higher due to the fact that exothermic processes may occur within the APU 110.

[0146] Downstream of the APU 110, the process flow is supplemented by a return flow from the cold box 120, similar to the CES systems of Figures 2-5. The return flow from the cold box 120 may be at a moderately low temperature, near ambient temperature. Furthermore, the return flow from the cold box 120 typically has a much larger mass flow rate than the process flow from the outlet to the APU 110.

[0147] Downstream of the APU 110, along the conduits of the first configuration, the RAC 114 receives the process stream exiting the APU 110 after being supplemented by the return stream. During the aforementioned stages, the pressure of the process stream may slightly decrease, but the RAC 114 may compress the process stream from approximately the first pressure (taking into account the pressure drop) to a second pressure, which may be approximately tens of bars. As the process stream is compressed, its temperature increases significantly due to the generation of heat of compression. Compared to the CES system of Figures 2-5, the inlet temperature of the RAC 114 may be approximately ambient temperature, and the outlet temperature may be less than approximately 190°C because the pressure ratio of the RAC 114 is lower than the MAC 102. Thus, the process stream exiting the RAC 114 may contain only low-grade heat of compression.

[0148] Downstream of the RAC 114 and along a first configuration of conduits, the RAC LG HX 117 receives the process stream from the RAC 114. The RAC LG HX 117 also receives a flow of a heat transfer fluid from a second configuration of conduits downstream of the HTF expander 122 (described below). The RAC LG HX 117 is configured to transfer at least a portion of the low-grade heat of compression of the process stream to the heat transfer fluid in the second configuration of conduits upstream of the HTF compressor 124. The heat transfer fluid with the low-grade heat of compression from the RAC 114 is then sent through the second configuration of conduits to the HTF compressor 124 (described below).

[0149] Downstream of the RAC LG HX 117 and along the first configuration of conduits, the RAC AC 118 receives the process stream exiting the RAC LG HX 117. The RAC AC 118 is configured to remove residual heat of compression from the process stream before sending the process stream to the cold box 120. The RAC AC 118 may cool the process stream to approximately ambient temperature.

[0150] Downstream of the RAC AC 118 along the first configuration of conduits, the cold box 120 receives the process stream exiting the RAC AC 118. As described in the Background section above, the cold box 120 is configured to cool the process stream until it is substantially condensed to a liquid. At least a portion of the liquid product is separated from the vapor phase and sent to a cryogenic storage tank 200 for later use. At least a portion of the vapor phase (i.e., any non-liquefied process stream) may be used to cool the process stream entering the cold box 120 to aid in liquefaction. This is done by passing the vapor phase through the cold box 120 in a countercurrent direction relative to the process stream entering the cold box 120. A return stream is formed from the vapor phase after passing through the cold box 120 and passes through the third configuration of conduits to replenish the process stream upstream of the RAC 114.

[0151] As described above, the MAC HG HX104 is configured to transfer at least a portion of the high-grade heat of compression from the process stream to the heat transfer fluid, which is then pumped through the second configuration of conduits to the high-grade TESD 300. Meanwhile, the MAC LG HX105 and the RAC LG HX117 are configured to transfer at least a portion of the residual low-grade heat of compression from the process stream to the heat transfer fluid, which is then pumped through the second configuration of conduits to the HTF compressor 124.

[0152] The HTF compressor 124 is configured to receive heat transfer fluid from both the MAC LG HX 105 and the RAC LG HX 117. The HTF compressor 124 then compresses the heat transfer fluid, which may increase its temperature to approximately that of the high grade TESD 300 and / or the temperature of the heat transfer fluid exiting the MAC HG HX 104. As a result of the additional heat of compression generated, the heat transfer fluid exiting the HTF compressor 124 may include high grade heat of compression. The heat transfer fluid is then pumped through a second configuration of conduits to the high grade TESD 300.

[0153] The high-grade TESD 300 may be configured to store the high-grade heat of compression contained in the heat transfer fluid in a storage medium. The storage medium of the high-grade TESD 300 is preferably a molten salt. The high-grade TESD 300 may be a packed bed TESD, a stationary liquid phase based TESD, or preferably a two-reservoir TESD as described in the background section above. The heat transfer fluid is preferably a gas or compressed gas. In the high-grade TESD 300, the high-grade heat of compression contained in the heat transfer fluid is transferred from the heat transfer fluid to the storage medium. The cold heat transfer fluid is then recirculated back to the MAC HG HX 104 and / or the HTF expander 122. As described above, the MAC HG HX 104 is configured to receive at least a portion of the cooled heat transfer fluid and transfer at least a portion of the high-grade heat of compression from the process stream resulting from compression of the process stream in the MAC 102 to the heat transfer fluid.

[0154] The HTF expander 122 is configured to receive at least a portion of the cold heat transfer fluid exiting the high grade TESD 300. The heat transfer fluid exiting the high grade TESD 300 may still have a relatively high temperature compared to the process stream exiting the MAC HG HX 104 or RAC 114. The HTF expander 122 therefore expands the heat transfer fluid, thereby cooling the heat transfer fluid and also allowing work to be extracted from the expanding heat transfer fluid. The HTF expander 122 may be mechanically coupled to a generator for generating electricity. The heat transfer fluid is then sent to the MAC LG HX 105 and RAC LG HX 117 to receive low grade heat of compression from the process stream as described above.

[0155] Generally, in a compressor, as the fluid inlet temperature increases, the internal kinetic energy of the fluid molecules increases, and therefore the work required to compress the fluid also increases. In the present CES system, the inlet fluid temperature of the RAC 114 is much lower than in the CES systems of Figures 2-5, and can be about ambient temperature rather than about 200°C. Thus, the RAC 114 of the present CES system is required to do less work to compress the process stream, which means that the RAC 114 of this system may last longer with a reduced workload and / or a less expensive compressor type or model can be procured for the RAC 114, reducing capital expenditures.

[0156] In a manner similar to the CES systems of Figures 3 and 5, the MAC LG HX105 can be removed (not shown). In this case, the MAC HG HX104 sends the process stream directly to the MAC AC108. The MAC AC108 can be configured to reject excess heat to the atmosphere to cool the process stream to approximately ambient temperature. Alternatively, the MAC AC108 can be configured to transfer at least a portion of the residual low-grade heat of compression to another system co-located with the CES system or a portion of the CES system (e.g., a power recovery subsystem, a liquefaction subsystem, or other portion of a broader CES system). As a further alternative, the MAC AC108 can transfer at least a portion of the residual low-grade heat of compression to an external process or system requiring additional heat (e.g., a heating plant, boiler, air conditioner, etc.).

[0157] 7 illustrates a CES system including a power recovery subsystem 400 and a high-grade TESD 300. The power recovery subsystem 400 is configured to evaporate, heat, and expand a working fluid to extract work therefrom. The power recovery subsystem 400 may include a cryogenic pump 402, an evaporator 404, at least one heater 406, and at least one expansion stage 408. The CES system may further include a vent 500 and a cryogenic storage tank 200.

[0158] The at least one heater is represented in FIG. 7 by several heaters 406a, 406n, and 406j, and the at least one expansion stage is represented by several expansion stages 408a, 408n, and 408j. The various heaters and expansion stages shown in FIG. 7 show possible fluid connections of the possible heaters and expansion stages. Heater 406a represents the first heater, heater 406n represents the middle heater, and heater 406j represents the last heater. Expansion stage 408a represents the first expansion stage, expansion stage 408n represents the middle expansion stage, and expansion stage 408j represents the last expansion stage. Each expansion stage of the at least one expansion stage is located downstream of the corresponding heater. The power recovery subsystem 400 preferably includes four heaters and respective downstream expansion stages.

[0159] The power recovery subsystem 400 may further include a first configuration of power recovery conduits 401 configured to route the working fluid of the power recovery unit 400 from the pump 402 through each of the evaporator 404 and at least one heater 406 and corresponding expansion stages 408 to a final expansion stage 408j. The first configuration of conduits is represented by line AB in FIG. 7, with an upstream end at point A and a downstream end at point B.

[0160] The power recovery conduit 401 in the first configuration may include the following fluidly connected power recovery conduits: a first conduit 431 connected to the pump 402 and the evaporator 404, a second conduit 432 connected to the evaporator 404 and at least one of the at least one heater 406n, one or more third conduits 433 connected between each of the at least one heater 406n and a corresponding expansion stage 408n, one or more fourth conduits 434 connected from each of the at least one expansion stage 408n to the next at least one heater 406n+1, if included, a fifth conduit 435 connected to the final expansion stage 408j and a vent 500. It will be understood that there may be any number of heaters 406n, and that the expansion stages 408n and j are not indicative of a maximum number of stages or a number of stages required.

[0161] The CES system may further include a second configuration of heat transfer conduits 620, 600 configured to form a closed circuit and route a heat transfer fluid between the high grade TESD 300 and each of the at least one heater 406. In FIG. 7, as in FIG. 2, the second configuration of conduits is designated as closed circuit C.

[0162] The second configuration of heat transfer conduits 620, 600 includes heat transfer conduits fluidly connected as follows: one or more first conduits 461 each connected to a high grade TESD 300 and at least one heater 406, and one or more second conduits 462 each connected to the at least one heater 406 and the high grade TESD 300.

[0163] During a discharge phase of the CES system, the cryogenic pump 402 is configured to extract a flow of liquid product, such as refrigerant or liquid air, from the storage tank 200 and pressurize the flow to form a working fluid. The pump 200 is configured to control the flow of refrigerant withdrawn from the storage tank 200.

[0164] Downstream from the pump 402 and along the first configuration of conduits, an evaporator 404 is configured to receive the working fluid from the pump 402 and substantially or completely evaporate the working fluid.

[0165] Downstream of the evaporator 404, along a first configuration of conduits, a first heater 406a of the at least one heater 406 receives the working fluid from the evaporator 404. The first heater 406a also receives a heat transfer fluid from the high-grade TESD 300 via a second configuration of conduits. The heat transfer fluid exiting the high-grade TESD 300 may include high-grade heat of compression originating from the liquefaction subsystem 100. The first heater 406a is configured to transfer at least a portion of the high-grade heat of compression from the heat transfer fluid to the working fluid.

[0166] Downstream of the first heater 406a and along the first configuration of conduits, a first expansion stage 408a of the at least one expansion stage 408 receives the working fluid from the first heater 406a. The first expansion stage 408a is configured to expand the working fluid and extract work therefrom. The first expansion stage 408 may be mechanically coupled to a generator (not shown) that converts mechanical energy to electrical energy.

[0167] Downstream of the first expansion stage 408a, in the case of multiple heaters and expansion stages, the second heater 406n receives the working fluid from the first expansion stage 408a, and the second expansion stage 408n receives the working fluid from the second heater 406n. The second heater 406n and expansion stage 408n can perform the same heating and expansion process of the working fluid as the first heater and first expansion stage described above. This is true for any number of heaters and corresponding expansion stages. That is, each heater 406 is configured to transfer high-grade heat from a heat transfer fluid to the working fluid, and each expansion stage 408 is configured to expand the working fluid to generate mechanical energy, which can be used to generate electrical energy. Each expansion stage 408 can be coupled to a common shaft that is ultimately mechanically coupled to a common generator, such that each expansion stage 408 can contribute to the electrical energy generated by the generator.

[0168] Downstream of the final expansion stage 408j (which may be the first expansion stage as described above), along the first configuration of conduits, the exhaust from the final expansion stage 408j may be discharged to the atmosphere through vent 500 or may be recirculated to another part of the CES system (e.g., a liquefaction subsystem, a TESD, etc.) or to another system co-located with the CES system (e.g., a refrigeration system, an air conditioning system, an air liquefaction system, etc.).

[0169] The high-grade TESD 300 may be configured to store in a storage medium the high-grade heat of compression generated within the process stream of the liquefaction subsystem 100 during the release phase, as described in connection with the CES system of Figures 2-5. The storage medium of the high-grade TESD 300 is preferably a molten salt. The high-grade TESD 300 may be a packed-bed TESD, a stationary liquid phase based TESD, or preferably a two-reservoir TESD as described in the Background section above. The heat transfer fluid of the second configuration of conduits may be a gas, such as air or compressed air, or a liquid, such as water, glycol, a mixture of water and glycol, thermal oil, a mixture of thermal oil, or preferably a molten salt. In the high-grade TESD 300, during the release phase, the high-grade heat of compression contained in the storage medium is transferred to the heat transfer fluid of the second configuration of conduits. The heat transfer fluid is then distributed to each heater 406, where the high-grade heat is transferred to the working fluid before the working fluid expands in each corresponding expansion stage 408.

[0170] In general, the power generated by an expansion turbine is a function of the pressure ratio across the turbine, the mass flow rate through the turbine, and the inlet temperature. In power recovery systems known in the art (e.g., WO 2019 / 158921 A1 described in the Background section above), some expansion stages of the power recovery subsystem are designated to generate power from a working fluid heated by a low-grade heat of compression, while some expansion stages are designated to generate power from a working fluid heated by a high-grade heat of compression. This means that at least some of the expansion stages operate at a lower temperature and therefore have a lower specific power.

[0171] In contrast, all of the expansion turbines 408 of the present CES system may be configured to generate power from a working fluid heated by high-grade heat of compression from the high-grade TESD 300. This means that the inlet temperature of the expansion turbines may be higher, the turbines may operate at higher temperatures, and more work may be extracted from the hotter working fluid, resulting in a higher specific power. This means that the present CES system operates at a higher average temperature, and therefore has a higher energy density, producing more energy per unit of working fluid. Furthermore, building and maintaining a CES plant with expansion stages operating at similar temperatures makes construction more efficient, maintenance easier, and reduces capital expenditures, as all expansion turbines can be sourced with the same, similar, or equivalent specifications. This can lead to a simplification of the system, which can lead to an overall reduction in the cost of the plant's components.

[0172] FIG. 8 illustrates a CES system that is identical to the CES system of FIG. 7, except that the power recovery subsystem 400 further includes a preheater 405 configured to transfer at least a portion of the heat from the exhaust of the final expansion turbine 408j back to the working fluid before the working fluid is heated by the first heater 406a.

[0173] 8 includes the same power recovery conduit 401 as the CES of FIG. 7 except that the fifth conduit 435 further includes branches K and L, and the second conduit 432 of FIG. 7 is replaced by a sixth conduit 436 connected to the evaporator 404 and the preheater 405, a seventh conduit 437 connected to the evaporator 404 and the first heater 406a, an eighth conduit 438 connected to branch K and the preheater 405, and a ninth conduit 439 connected to the preheater 406a and branch L. Preferably, branch L is downstream of branch K in the fifth conduit 435. It will be appreciated that any connection to branch K may also be connected to the final expansion stage 408j, and any connection to branch L may also be directly connected to the vent 500.

[0174] The CES system includes a power recovery subsystem 100 and a high-grade TESD 300. The power recovery subsystem 400 can similarly include a cryogenic pump 402, an evaporator 404, at least one heater 406, at least one expansion stage 408, a first configuration of conduits, and a second configuration of conduits, in a similar configuration to the CES system of Figure 7. The CES system can also further include a vent 500 and a cryogenic storage tank 200.

[0175] Downstream of the evaporator 404, along the first arrangement of conduits, the preheater 405 receives the working fluid from the evaporator 404. The preheater also receives at least a portion of the exhaust gas from the final expansion turbine 408j. The preheater is configured to transfer at least a portion of the heat contained in the exhaust gas to the working fluid. The heated working fluid is then sent to the first heater 406a and follows the same subsequent process through the first arrangement of conduits as described in connection with the CES system of FIG. 7 above. The cooled portion of the exhaust gas can be exhausted to the atmosphere through a vent 500 or recirculated to another part of the CES system (e.g., liquefaction subsystem, TESD, etc.) or to another system co-located with the CES system (e.g., refrigeration system, air conditioning system, air liquefaction system, etc.).

[0176] By preheating the working fluid, the system can prevent the heat transfer fluid from freezing as a result of being cooled in the first heater 406a by the working fluid leaving the evaporator 404. For example, after evaporation in the evaporator 404, the working fluid may have a temperature of approximately ambient temperature, while the temperature of the heat transfer fluid flowing from high grade TESD 300 to the first heater 406a may be approximately 350-400°C depending on the fluid used. Without the preheater 405, some types of heat transfer fluids may freeze, such as some typical molten salts that have a freezing temperature below 190°C as mentioned above. If these types of molten salts acting as heat transfer fluids drop below 190°C in the first heater 406a, they risk freezing and clogging the conduits of the second arrangement. Furthermore, if cooled significantly or approaching its freezing temperature, the heat transfer fluid may become highly viscous and require additional work from the pumps pumping the heat transfer fluid through the conduits of the second arrangement. The working fluid may be heated from near ambient temperature to approximately 50-150°C before entering the first heater 406a, while the exhaust leaving the final expansion turbine 408j may be cooled from approximately 210°C to 125°C by the outlet to the evaporator 404.

[0177] The CES systems described above are considered "high energy density" systems because the heat of compression captured during liquefaction of the process fluid is hotter than in conventional CES systems since only high grade heat is captured, and therefore the specific work of the liquefaction subsystem 100 is higher. In the power recovery subsystem, the captured heat energy can be applied to the working fluid, resulting in a higher operating temperature of the expansion turbine. Thus, the power recovery subsystem can extract more energy from the expanding working fluid, which means that the specific work of the power recovery subsystem also increases since more energy is produced per unit of working fluid. Thus, more energy is released and charged in these systems. As a result, less refrigerant is required to generate MWh of power. This means that the cryogenic storage tanks, and therefore the CES system, can be significantly smaller on a volumetric basis for a given plant energy size (e.g., a 300 MWh plant). Furthermore, the charge time required to store a given amount of energy can also be reduced due to the reduced use of refrigerant.

[0178] Furthermore, as discussed above, the disclosed CES system can efficiently store the heat of compression from the MAC 102 and RAC 114 in a single high-grade TESD 300, meaning that no low-grade TESD is required, reducing plant complexity and the capital expenditure required to build the plant. Additionally, the elimination of the low-grade TESD avoids issues such as heat mixing at the stratified boundaries of thermocline-based low-grade TESD systems as discussed above, and thus the losses and inefficiencies associated with low-grade TESD. Capturing high-grade heat also allows the use of molten salts as a heat transfer fluid or storage medium, which is advantageous due to their availability, low cost, non-toxicity, non-flammability, high thermal stability, and low vapor pressure. The low vapor pressure means that the salts are liquid at atmospheric pressure, which allows for storage system designs that do not require pressurized tanks.

[0179] As mentioned above, the CES system of the present invention may include a combination of any of the liquefaction subsystem 100 configurations shown in or described in Figures 2-6 and any of the power recovery subsystem 400 configurations shown in or described in Figures 7 or 8. The CES system of the present invention may further be configured to simultaneously perform any operation from the charging phase to the discharging phase, or any operation from the entire charging phase process to the entire discharging phase. Furthermore, the heat transfer fluid of the liquefaction subsystem 100 and the power recovery subsystem 400 may be the same fluid and may pass through a common closed double loop configured to route the heat transfer fluid between the liquefaction subsystem 100, the high-grade TESD 300, and the power recovery subsystem 400. In this case, the conduits of the second configuration described in connection with Figures 2-6 may be fluidly coupled to the conduits of the second configuration of Figures 7 or 8 to form a closed double loop structure.

Claims

1. 1. A cryogenic energy storage system, comprising: A liquefaction subsystem (100), comprising: A first compressor (102), A first heat exchanger (104), A second compressor (114), A second heat exchanger (116), A third heat exchanger (112), and a liquefaction subsystem (100) having an upstream end (A) and a downstream end (B) including a first arrangement of conduits (AB) configured to pass a process stream from the first compressor (102) through the first heat exchanger (104), the second compressor (114), the second heat exchanger (116), and the third heat exchanger (112); a thermal energy storage device (300) configured to store high grade heat; a second arrangement of conduits (C) configured to form a closed circuit and to convey a heat transfer fluid between the thermal energy storage device (300), the first heat exchanger (104), and the second heat exchanger (116); the first heat exchanger (104) is positioned along the first arrangement of conduits (AB) downstream of the first compressor (102) and configured to transfer at least a portion of a high-grade heat of compression of the process stream from the first compressor to the thermal energy storage device (300) via the heat transfer fluid; the second heat exchanger (116) is positioned along the first arrangement of conduits (AB) downstream of the second compressor (114) and configured to transfer at least a portion of a high-grade heat of compression of the process stream from the second compressor to the thermal energy storage device (300) via the heat transfer fluid; the third heat exchanger (112) is positioned along the first arrangement of conduits (AB) downstream of the second heat exchanger (116) and configured to transfer at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the second compressor (114), thereby returning at least a portion of the low-grade heat of compression to the second compressor (114); Cryogenic energy storage systems.

2. 2. The cryogenic energy storage system of claim 1, wherein the liquefaction subsystem (100) further comprises an air purification unit positioned along the first arrangement of conduits (AB) downstream of the first heat exchanger (104), the air purification unit configured to purify the process stream.

3. 3. The cryogenic energy storage system of claim 2, wherein the liquefaction subsystem (100) further comprises a first air conditioning unit positioned along the first arrangement of conduits (AB) between the air purification unit and the first heat exchanger (104), the first air conditioning unit configured to substantially or completely remove heat of compression of the process stream from the first compressor (102).

4. 3. The cryogenic energy storage system of claim 2, wherein the liquefaction subsystem (100) further comprises a fourth heat exchanger positioned along the first arrangement of conduits (AB) between the first heat exchanger (104) and an air purification unit and configured to transfer at least a portion of a low-grade heat of compression of the process stream from the first heat exchanger to the process stream downstream of the air purification unit.

5. 5. The cryogenic energy storage system of claim 1, wherein the third heat exchanger (112) configured to transfer at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the second compressor (114) comprises transferring at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the process stream upstream of the second compressor (114).

6. 2. The cryogenic energy storage system of claim 1, wherein the liquefaction subsystem further comprises a third arrangement of conduits having an upstream end configured to be coupled to a cold box and a downstream end, the third arrangement of conduits configured to pass a return flow from the cold box to replenish the process flow upstream of the second compressor.

7. the third heat exchanger (112) configured to transfer at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the second compressor (114), including transferring at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the return stream prior to replenishing the process stream; 7. The cryogenic energy storage system of claim 6, optionally comprising: a third heat exchanger (112) configured to transfer at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the second compressor (114) after the process stream has been replenished by the return stream, the third heat exchanger (112) transferring at least a portion of the low-grade heat of compression of the process stream from the second heat exchanger to the process stream after the process stream has been replenished by the return stream.

8. 2. The cryogenic energy storage system of claim 1, wherein the liquefaction subsystem (100) further comprises a cold box positioned at a downstream end of the first arrangement of conduits (AB) and configured to at least partially liquefy the process stream to form a liquefied product.

9. 10. The cryogenic energy storage system of claim 8, wherein the liquefaction subsystem (100) further comprises a second air conditioning unit positioned along the first arrangement of conduits (AB) upstream of the cold box, the second air conditioning unit configured to substantially or completely remove heat of compression of the process stream from the second compressor (114).

10. 9. The cryogenic energy storage system of claim 8 when dependent on claim 6, wherein the cold box is configured to route the return stream including at least a portion of any non-liquefied process stream to the third configuration of conduits.

11. 10. The cryogenic energy storage system of claim 8, wherein the cold box is configured to deliver at least a portion of the liquefaction product to a cryogenic storage tank.

12. the heat transfer fluid is a first heat transfer fluid, and the cryogenic energy storage system further includes a power recovery subsystem; The power recovery subsystem comprises: pump, Evaporator, At least one heater; an expansion stage corresponding to each of the at least one heater; a fourth arrangement of conduits having an upstream end and a downstream end and configured to pass working fluid from the pump through the evaporator, each of the at least one heater and a corresponding expansion stage, each heater being positioned upstream of its corresponding expansion stage; and a fifth arrangement of conduits configured to form a closed circuit and to convey a second heat transfer fluid between the thermal energy storage device (300) and each of the at least one heater; each heater configured to transfer at least a portion of high grade heat of compression from said thermal energy storage device (300) to said working fluid via said second heat transfer fluid; Each expansion stage is configured to expand the working fluid and extract work therefrom.

10. The cryogenic energy storage system of claim 1.

13. the power recovery subsystem further includes a preheater positioned along the fourth arrangement of conduits upstream of the first heater; 13. The cryogenic energy storage system of claim 12, wherein the preheater is configured to receive at least a portion of an exhaust gas from a final expansion stage and to transfer at least a portion of heat from the exhaust gas of the final expansion stage to the working fluid upstream of the first heater.

14. 13. The cryogenic energy storage system of claim 12, wherein the pump is configured to pump a liquefied product from a cryogenic storage tank and pressurize it to form the working fluid.

15. 13. The cryogenic energy storage system of claim 12, wherein the first and second heat transfer fluids are the same fluid, and the second arrangement of conduits (C) and the fifth arrangement of conduits are fluidly connected and share the same heat transfer fluid.

16. 10. The cryogenic energy storage system of claim 1, wherein the heat transfer fluid comprises air, compressed air, water, glycol, a mixture of water and glycol, thermal oil, a mixture of thermal oils, or a molten salt.

17. 1. A method for storing heat of compression in a cryogenic energy storage system, the method comprising: providing a liquefaction subsystem (100) including a first compressor (102) and a second compressor (114); providing a thermal energy storage device (300) configured to store high grade heat; compressing a process stream in the first compressor (102); capturing at least a portion of the high grade heat of compression of the process stream from the first compressor and storing it in the thermal energy storage device (300) via a heat transfer fluid; compressing the process stream in the second compressor (114); capturing at least a portion of the high grade heat of compression of the process stream from the second compressor and storing it in the thermal energy storage device (300) via the heat transfer fluid; capturing at least a portion of the low grade heat of compression of the process stream from the second compressor and returning it to the second compressor (114). method.

18. 20. The method of claim 17, comprising purifying the process stream after capturing at least a portion of the high grade heat of compression of the process stream from the first compressor.

19. 20. The method of claim 18, further comprising conditioning the process stream to substantially or completely remove a heat of compression of the process stream from the first compressor (102) prior to purifying the process stream.

20. 20. The method of claim 18, comprising capturing at least a portion of the high grade heat of compression of the process stream from the first compressor after capturing at least a portion of the low grade heat of compression of the process stream from the first compressor and transferring that heat of compression to the process stream after purifying the process stream.

21. 18. The method of claim 17, wherein returning at least a portion of the low-grade heat of compression of the process stream from the second compressor to the second compressor (114) comprises transferring at least a portion of the low-grade heat of compression of the process stream from the second compressor to the process stream prior to compressing the process stream in the second compressor (114).

22. 20. The method of claim 17, further comprising supplementing the process stream with a return stream prior to compressing the process stream in the second compressor (114).

23. 23. The method of claim 22, wherein returning at least a portion of the low-grade heat of compression of the process stream from the second compressor to the second compressor comprises transferring at least a portion of the low-grade heat of compression of the process stream from the second compressor to the process stream after it has been replenished by the return stream.

24. 23. The method of claim 22, wherein returning at least a portion of the low-grade heat of compression of the process stream from the second compressor to the second compressor comprises transferring at least a portion of the low-grade heat of compression of the process stream from the second compressor to the return stream prior to replenishing the process stream.

25. 20. The method of claim 17, comprising conditioning the process stream to substantially or completely remove the heat of compression of the process stream from the second compressor after capturing at least a portion of the low grade heat of compression of the process stream from the second compressor.

26. 26. The method of any one of claims 17 to 25, wherein the method comprises at least partially liquefying the process stream to form a liquefied product.

27. 27. The method of claim 26, further comprising sending at least a portion of the liquefaction product to a cryogenic storage tank.

28. 27. The method of claim 26 when dependent on claim 21, the method including passing at least a portion of any non-liquefied process stream as the return stream to replenish the process stream.

29. The method comprises: heating a working fluid using at least a portion of the high-grade heat of compression stored in the thermal energy storage device (300); 20. The method of claim 17, further comprising expanding the working fluid in an expansion stage and extracting work therefrom.

30. 30. The method of claim 29, further comprising the step of preheating the working fluid using an exhaust stream from the expansion stage prior to heating the working fluid.

31. 31. The method of claim 30, further comprising pumping a liquefied product from a cryogenic storage tank and pressurizing it to form the working fluid.

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