A carbon-neutral diabatic compressed air energy storage system

Abstract

Compressed air energy storage (CAES) using high temperature thermal energy stored in a solid or liquid thermal mass and methods of transferring the thermal energy from the TES to the air expansion system for the purpose of combined thermal and mechanical energy recovery.

Description

APPLICATION FOR PATENTINVENTORS:STEVEN F. SCIAMANNAASHOK KRISHNATITLE:A CARBON-NEUTRAL DIABATIC COMPRESSED AIR ENERGY STORAGE SYSTEMATTORNEY DOCKET NO.: 70091-8Attorney Docket No.: 70091-8PCT International Patent ApplicationSPECIFICATIONCROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to and the benefit of co-pending US Provisional Patent Application Serial No. 63 / 751,972 filed on January 31, 2025, titled “A CARBON-NEUTRAL DIABATIC COMPRESSED AIR ENERGY STORAGE SYSTEM.” This reference is incorporated in its entirety.FIELD

[0002] The present disclosure generally relates to compressed air energy storage (CAES) using high temperature thermal energy stored in a solid or liquid thermal mass and methods of transferring the thermal energy from the TES to the air expansion system for the purpose of combined thermal and mechanical energy recovery.BACKGROUND

[0003] The short- and long-term duration storage and later dispatch of electrical energy produced by carbon-neutral (C-N), intermittent sources (e.g. wind and solar-PV) is an enabling requirement for the growth of those energy sources. Energy storage and delivery at the grid scale are necessary to provide a stable supply of C-N power throughout a day and for longer duration energy demand events such as cloudy and windless days. Pumped-storage hydroelectricity is often the preferred solution where the natural topography is suitable. Another approach is Compressed Air Energy Storage (CAES) where air is compressed adiabatically causing it to get hot, then subsequently cooled, as necessary followed by repeated compression and cooling steps then ultimately storing it in a high-pressure underground reservoir. When the sun stops shining or the wind speed slows or the power demand surges, then pressurized air isAtorney Docket No.: 70091-8PCT International Patent Applicationreleased for recovery using power-generating turbines or expanders. A conventional expansion turbine is a near adiabatic process causing the expanding gas to get quite cold which limits the energy recovery from the compressed air.

[0004] In a conventional Diabatic CAES (DC AES) facility, the compression heat is discarded to the environment. During the expansion step, natural gas is added to the high pressure air prior to expansion where the mixture is combusted and heat released at a very high temperature (>800 °C) to increase energy recovery. In this case about 40% or more of the DCAES energy output is associated with burning the natural gas. The use of fossil- derived natural gas does not achieve the societal objective of the recovered energy being C-N. Bio-gas could be used to achieve C-N operation for a conventional DCAES system. However, the supply of bio-gas is typically insufficient and location-dependent making its use problematic in a DCAES system.

[0005] The round-trip efficiency (RTE) and capital cost are key metrics of any energy storage and recovery system. The RTE of DCAES can be improved by saving the adiabatic compression heat for later use during the adiabatic expansion process in a configuration described as Adiabatic CAES (ACAES). The temperature of the heat produced from adiabatic gas compression heat is limited by material constraints of the compressor which is normally <200°C. The temperature level achieved of the stored heat is less as a result of the temperature driving force needed for indirect heat transfer to the storage material.

[0006] Methods for storing large quantities of thermal energy recovered from the adiabatic compression of air requires the use of multiple pressure vessels. Some ACAES schemes store this heat as hot pressurized water in multiple pressurized vessels for later use during air expansion. Other ACAES schemes store large amounts of compression heat using a pressure vessel containing a porous bed of high heat capacity solids (i.e. thermal mass), through which the hot air passes after compression. A pressure vessel and thermal mass are needed for each stage of adiabatic compression. Large vessels containing a packed bed of a thermal mass material, suitable for operation at elevatedAtorney Docket No.: 70091-8PCT International Patent Applicationpressure and temperature which have piping configured for routing compressed air from / to both the compression and expansions systems, contribute to increased system complexity and cost. In either scheme, storing large amounts of compression heat at this temperature level is costly, and using it during later air expansion necessitates the use of multiple expanders (> 3) with interstage heating.

[0007] This temperature level is much less than the temperature of heat utilized during expansion in natural gas fired DCAES system. The higher the temperature of heat used during expansion, the fewer number of expanders that are required resulting in a lower capital cost for expensive rotating machinery, despite higher cost of an individual higher temperature expander.

[0008] Advances in Thermal Energy Storage (TES) technology enable high temperature heat generated directly from electrical energy, preferably renewable, can be stored at a lower capital cost per unit of energy than for lower temperature heat. For example, heat generated from adiabatic gas compression. As the RTE increases so does the capital cost to achieve it. Wind & solar energy are primarily stored during low-cost periods. Therefore, a DCAES system with a lower capital cost lower than an ACAES system which has a higher RTE can result in the DCAES system as the economically preferred alternative.SUMMARY OF THE INVENTION

[0009] The methods of this invention provide for the expansion of air in a C-N DCAES system using high temperature thermal energy stored in a solid or liquid thermal mass and methods of transferring the thermal energy from the TES to the air expansion system for the purpose of combined thermal and mechanical energy recovery.Atorney Docket No.: 70091-8PCT International Patent ApplicationBRIEF DESCRIPTION OF THE DRAWINGS

[0010] The detailed description will be better understood in conjunction with the accompanying drawings as follows:

[0011] Figure 1 depicts a table with suitable molten salt mixtures

[0012] Figure 2 depicts a table showing the impact on multiple expanders on round trip efficiency

[0013] Figure 3 depicts an overall flow diagram of an embodiment of the C-N DCAES System.

[0014] Figure 4 depicts a flow diagram of an embodiment showing the C-N DCAES expansion train with heat transfer from TES.

[0015] Figure 5 is a chart showing the relationship between the overall RTE and the air expander inlet temperature for configurations with one or more expanders.

[0016] The embodiments of the present disclosure are detailed below with reference to the listed Figures.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] Before explaining the present disclosure in detail, it is to be understood that the disclosure is not limited to the specifics of particular embodiments as described and that it can be practiced, constructed, or carried out in various ways.

[0018] While embodiments of the disclosure have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the disclosure. The embodiments described herein are exemplary only, and are not intended to be limiting.

[0019] Specific structural and functional details disclosed herein are not to be interpreted asAttorney Docket No.: 70091-8PCT International Patent Applicationlimiting, but merely as a basis of the claims and as a representative basis for teaching persons having ordinary skill in the art to variously employ the present embodiments. Many variations and modifications of embodiments disclosed herein are possible and are within the scope of the present disclosure.

[0020] Where numerical ranges or limitations are expressly stated, such express ranges or limitations should be understood to include iterative ranges or limitations of like magnitude falling within the expressly stated ranges or limitations.

[0021] The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and / or the specification may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.”

[0022] The word “about” means plus or minus 5% of the stated number.

[0023] The use of the term "optionally" with respect to any element of a claim is intended to mean that the subject element is required, or alternatively, is not required. Both alternatives are intended to be within the scope of the claim. Use of broader terms such as comprises, includes, having, etc. should be understood to provide support for narrower terms such as consisting of, consisting essentially of, comprised substantially of, and the like.

[0024] When methods are disclosed or discussed, the order of the steps is not intended to be limiting, but merely exemplary unless otherwise stated.

[0025] Accordingly, the scope of protection is not limited by the description herein, but is only limited by the claims which follow, encompassing all equivalents of the subject matter of the claims. Each and every claim is hereby incorporated into the specification as an embodiment of the present disclosure. Thus, the claims are a further description and are an addition to the embodiments of the present disclosure.

[0026] The inclusion or discussion of a reference is not an admission that it is prior art to the present disclosure, especially any reference that may have a publication date after theAtorney Docket No.: 70091-8PCT International Patent Applicationpriority date of this application. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated by reference, to the extent they provide background knowledge; or exemplary, procedural or other details supplementary to those set forth herein.

[0027] The embodiments of the present disclosure generally relate to expansion of air in a C- N DCAES system using high temperature thermal energy stored in a solid or liquid thermal mass and methods of transferring the thermal energy from the TES to the air expansion system for the purpose of combined thermal and mechanical energy recovery.

[0028] This disclosure provides a method for a C-N DCAES system where high-pressure air is released from storage in an underground reservoir where it is heated prior to an expansion step using heat transferred from a high temperature Thermal Energy Storage (TES) system.

[0029] The present system offers many advantages and degrees of novelty over the present state of the art. While many current systems bum natural gas to heat gasses being expanded (to prevent freezing), the present disclosure makes use of long duration stored heat. This is a “green” methodology which does not produce carbon dioxide.

[0030] Because some generated electricity comes from the stored thermal energy used to heat the compressed gas, the presently disclosed method and system are more efficient than existing techniques. The present disclosure also does not require the storage or utilization of hot water or natural gas. More efficient procedures result in a greater than forty -five percent round trip efficiency for the energy stored in the compressed gas and the thermal energy storage.

[0031] Advances in TES technology enable the efficient conversion of electrical energy to high temperature heat via resistive heating. The resulting thermal energy is stored as sensible heat in a solid material for long durations. For the purposes of this disclosure, time lengths of greater than ten hours shall be considered long duration storage. SuchAttorney Docket No.: 70091-8PCT International Patent ApplicationTES systems can efficiently and economically store large amounts of thermal energy at temperatures > 500 °C, often for longer than a day.

[0032] Alternatively electrical energy can be converted to thermal energy and stored directly in other suitable materials such as phase change materials or as sensible heat in a liquid such as a molten salt. For the purposes of this disclosure, phase change materials shall be defined as substances that absorb, store, and release latent heat at relatively constant temperatures during phase transitions. Often, phase change materials store or release heat by melting or solidifying respectively.

[0033] If the electrical source is renewable then the goal of the overall system being carbon- neutral is also satisfied. Thermal energy at this high temperature level reduces the number of expanders required to < 3. Figure 3 shows the DCAES system of the present invention.

[0034] In Figure 3, input air 110 enters a compression system 120 for pressurization and storage. Heat 122 from the compressed gas 124 can be beneficially used and / or potentially stored in thermal energy storage 160. Compressed gas 124 can be placed in a gas storage 130, for example within a subsurface saline aquifer. When determined by persons having ordinary skill in the art, the stored pressurized gas 140 can be extracted from gas storage 130 and sent to an expansion system 150. Expansion system 150 can recover energy and send electrical power 180 to the grid. Heat 170 from thermal energy storage 160 can be added to the stored pressurized gas 140. Electrical energy 154 can be converted to high temperature heat and stored in thermal energy storage 160. Air 152 can be discharged by the expansion system.

[0035] Depending on how the thermal energy is extracted from the TES system and on the physical layout of the TES relative to the air-expansion systems, it may be preferable to convey the high pressure gas to the TES for heat recovery or alternatively transport the heat from the TES to the air expansion system using a heat transfer fluid (HTF).

[0036] Depending on the TES system, the heat transfer between the TES and air-expansionAtorney Docket No.: 70091-8PCT International Patent Applicationsystems is accomplished with either a gas-gas or a gas-liquid heat exchanger or both. In one case where heat is extracted from the TES system using a low-pressure gas (e g. air) and where the air-expansion and TES systems are geographically close, then heat transfer can be accomplished via gas-to-gas heat exchange. In the situation where the reservoir air storage pressure, the TES temperature, and the expansion-ratio characteristics of the expander are such that only one expander and no final heat recovery are needed, this method of gas-gas heat transfer may have economic advantage.

[0037] Another case is described by a large energy storage facility where there is a significant distance separating the TES and air-expansion systems, potentially resulting in excessive air pressure-drop and heat loss. In this case moving large volumes of hot, high-pressure air may not be cost-effective. Methods of this invention transfer the thermal energy from the TES system to the air expansion system using superheated steam or a heat transfer fluid (HTF) in the form of a low vapor pressure liquid that is stable at the operating temperature levels of the TES expansion process.

[0038] An example of such a HTF is the molten salt mixtures typically used in concentrated solar-thermal (CST) facilities which are transported over large distances and insulated to achieve acceptable thermal energy losses. In part, the molten salt mixture is selected so that its melting point and upper temperature limit are within the operating temperature range of the TES and air expansion systems. Additional factors are corrosivity, toxicity and cost. Examples of suitable molten salt mixtures are shown in Table 1 in Figure 1. However, others that satisfy the operating constraints and other considerations are also suitable.

[0039] The expansion system 150 can have one or more expanders depending on the temperature level of the heat delivered from the TES system and the allowable expansion ratios of the air expander.

[0040] Figure 4 shows the expansion portion (150 in Figure 3) of a C-N DC AES system utilizing two adiabatic expanders 250 and 252. High-pressure air 280 is supplied fromAttorney Docket No.: 70091-8PCT International Patent Applicationthe subsurface reservoir storage 270 to a heat exchanger 260 (e g. a counter-current recuperator) which preheats the incoming high-pressure air 280 from the subsurface reservoir storage 270 using the hotter, low (near ambient) pressure air exiting the second adiabatic expander 252.

[0041] In this embodiment, the high pressure, preheated air is further heated via a first heat exchanger 222 to the target temperature prior to entering the first adiabatic expander 250. The first heat exchanger 222 can receive a heat transfer fluid (HTF) 220 conveying heat from the thermal energy storage 160 using the high-temperature HTF 220. The air discharge from the first expander 250 is reduced in both temperature and pressure as a result of the adiabatic expansion step.

[0042] This air is reheated, via a second heat exchanger 224 using another portion of the high- temperature HTF 220, to a target temperature determined by persons having ordinary skill in the art for the second adiabatic expander 252. If more expanders are utilized, then persons having ordinary skill in the art can repeat this sequence the air pressure leaving the last expander is just sufficient to accommodate the pressure drop in the heat exchanger 260 before final discharge from the system at near atmospheric pressure to the environment.

[0043] The inlet pressure and the allowable expansion pressure ratio for an expansion machine are key factors which determine the number of expanders that are required in series. When multiple expanders are used, operating them at the same expansion ratio typically provides the most efficient operation.

[0044] In the case where the storage air pressure is low enough such that expansion to near atmospheric pressure is within the expansion ratio limit of the expander machine, then it is possible to accomplish the entire expansion step using a single expander. However, in this case the feed air temperature must be heated to a sufficiently high temperature so that the expander exhaust air is above the moisture dewpoint temperature to prevent condensation during the adiabatic expansion step.Atorney Docket No.: 70091-8PCT International Patent Application

[0045] In this case a heat exchanger may be not needed as there is no useful heat to recover. As a general rule, the highest allowable air inlet temperature to the expanders increases the RTE of the overall system. Thermal energy contained in the final expander discharge is mostly recovered via the recuperator heat exchanger 260.

[0046] An example is based on the following conditions: air delivered from a subsurface reservoir at 75 atm and 35 °C; isentropic efficiency of adiabatic compressors and expanders is 85%; four compression stages; ambient air temperature is 35 °C ; the approach temperature in the recuperator is 20 °C; 8 hr. of charging time, 12 hr. of discharging time; 400-600 °C heat transported via a HTF supplied from the TES; an adiabatic air expansion system equipped with one to three expanders; all expander preheaters heat air to the same air inlet temperature; and for the configurations with multiple expander configurations, all have equal expansion ratios. Calculations from Aspen Plus simulations are shown in Table 2 in Figure 2.

[0047] Figure 3 also shows that the RTE increase due to adding a third expander is much smaller than the difference between one and two expanders. Additional expanders do reduce the amount of air that must be stored. However, the incremental cost of additional subsurface air storage is very small compared to the capital cost of an additional air expander. Figure 3 shows that heating air prior to expansion to the highest temperature allowable provides the highest RTE.

[0048] For the example case shown in Figure 5, there is a benefit to the RTE from having more than one expander especially if the heat from the TES is supplied at > 533 °C.However, a single expander capable of discharging air at or near ambient temperature still has a benefit from a reduced capital cost since there is no need for the recuperator. In the multiple expander configurations with a recuperator included, the upper operating temperature limit is set by either the maximum temperature allowable for the HTF and / or by the mechanical limitations of the expander. The maximum temperature contained within some TES systems can exceed the temperature limits of the air expansion system. Design practices are well known to those skilled in the art whichAttorney Docket No.: 70091-8PCT International Patent Applicationwill prevent equipment damage and unsafe conditions.

[0049] While the present disclosure emphasizes the embodiments, it should be understood that within the scope of the appended claims, the invention might be practiced other than as specifically described herein.Attorney Docket No.: 70091-8PCT International Patent Application

Claims

CLAIMSWhat is claimed is:

1. A system for storing and retrieving energy comprising:a. a compression system for compressing gas;b. a gas storage, wherein compressed air from the compression system is stored;c. a thermal energy storage for storing heat; andd. an expansion system for receiving compressed gas from the gas storage and receiving heat from the thermal energy storage to generate electricity;2. The system of claim 1, wherein the thermal energy storage comprises a heat storage medium which is a solid, a phase change material, or a fluid.

3. The system of claim 1, wherein there exists a gas-to-gas heat transfer from the thermal energy storage to the expansion system, or a liquid-to-gas heat transfer from the thermal energy storage to the expansion system.

4. The system of claim 1, wherein a heat transfer fluid is utilized to transfer heat from the thermal energy storage to the expansion system.

5. The system of claim 1, wherein a molten salt is utilized to transfer heat from the thermal energy storage to the expansion system.

6. The system of claim 1, wherein a superheated steam is utilized to transfer heat from the thermal energy storage to the expansion system.

7. The system of claim 1, wherein the expansion system comprises a plurality of expanders.

8. A method for storing and retrieving energy comprising:a. compressing a gas and storing a compressed gas to create a stored compressed gas;Attorney Docket No.: 70091-8PCT International Patent Applicationb. retrieving the stored compressed gas for expansion, creating a retrieved compressed gas;c. providing the retrieved compressed gas to an expansion system;d. providing a thermal energy storage;e. supplying heat to the expansion system from the thermal energy storage via a heat transfer fluid;f. heating the retrieved compressed gas;g. expanding the retrieved compressed gas after heating the retrieved compressed gas to create an expanded gas and energy;h. capturing the energy.Atorney Docket No.: 70091-8PCT International Patent Application

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