Sorption-enhanced compressed air storage

Abstract

The present invention discloses a sorbent-enhanced process of storing and releasing compressed gas. This process comprises the steps of providing an adsorbent solid in an interior of a pressurizable container, compressing gas, preferably air, to provide a compressed gas. The process further comprises supplying said compressed gas to the adsorbent in said pressurizable container and allowing said gas to adsorb to the adsorbent under pressure. It also comprises storing said compressed gas in the pressurizable container and releasing said compressed gas and adsorbed gas from the pressurizable container to drive a mechanical device. The present invention further discloses a plant for storing and releasing compressed gas, a process of storing gas at pressure, and an adsorbent material composition.

Description

Sorption-enhanced compressed air storageTECHNICAL FIELD

[0001] The invention relates generally to the storage of energy, for example the storage of electrical or pneumatic energy that may be in excess of an immediate demand. For example, the storage preferably concerns conversion of the excess energy into potential energy in a pressurized gas, and reconversion of the stored energy to an alternative energy form, such as electricity or pneumatic energy, as required.

[0002] In preferred examples the storage preferably concerns conversion of electrical energy to potential energy in pressurized gas, and re-conversion of the stored energy to electrical energy as may be required. It relates particularly to a system and method for storing electrical energy in the form of potential energy in pressurized gas.

[0003] The invention may also relate to components and materials for use in such energy storage and release systems, for example advantageous storage media and storage media reservoirs, release mechanisms, and energy conversion components.

[0004] The discussed technology may relate generally to Compressed Air Energy Storage (CAES), specifically to sorption-enhanced CAES technology.BACKGROUND

[0005] Electricity generators having minimal output levels at which the unit can be maintained in service, such as nuclear power plants, and electricity generators having highly variable and low predictability, intermittent output levels, such as wind, tidal, wave and solar power etc., often generate electrical output in excess of needs during off-peak usage time, and / or may provide inadequate electrical output during peak usage times. To make use of the excessive or fluctuating electrical energy production, storage systems have been proposed for (temporary) storage of electrical energy (in various forms) during excess generation or low off-peak usage times, for later release in times of higher demand (i.e., the electrical energy supply can be time-shifted through storage).

[0006] Effective electrical energy storage may in that way assist in reducing overall energy wastage; and facilitate the adoption of wind, solar, wave or tidal energy systems as a replacement of traditional fossil fuel based electricity generation by acting as a buffer to reduce the unpredictability and intermittency of their output. An aim is to achieve flexibleand reliable energy supply (mechanical, thermal, or electrical) despite the unpredictable and intermittent nature of inherently unpredictable / intermittent energy sources.

[0007] Unfortunately, current energy storage solutions are not fully suited for grid-scale (electrical energy supply networks, such as national energy grids) application due to use of unsustainable materials (e.g. limited supplies of materials and limited recyclability of materials used in energy storage such as batteries), short lifetimes (e.g. less than 10,000 cycles), poor scalability (e.g., safety hazards or limited economies of scale beyond 1 MWh of storing capacity), and geographical limitations (e.g. pumped hydro stations and compressed air energy storage).

[0008] Several attempts have been made to achieve sustainable energy storage solutions in forms including chemical, gravitational potential, electrical potential, elevated temperature, latent heat, thermochemical heat, and kinetic. Examples have been reported in scientific and commercial literature. An energy storage technology known in the art is Compressed Air Energy Storage (CAES).

[0009] The empty space between particles in air and gases allows the air or gases (hereinafter also referred to simply as “gas”) to be compressed and confined in a pressurized container, which effectively stores a potential energy (also referred to as “energy potential”) in the compressed gas with respect to the gas’s initial state, or in relation to an ambient pressure, such as atmospheric pressure. This stored energy potential can be used as an energy buffer and decouples the pressurization oaf the gas from the eventual conversion of gas pressure to another form of energy into different points in time. For example, the energy potential when released can be used to generate electricity using an expander-generator or to provide a suitable fluid power , or any other type of mechanical power, converted from the potential energy of the pressurized gas toa pneumatic tool such as a painting nozzle, air blower gun, pneumatic actuated valves, vortex tubes which can separate a pneumatic energy of a compressed gas into hot and cold streams with different thermal energy levels, or any other pneumatic tool which can function as wrenches, grinders, hammers, sanders, drills, nail / staple guns, ratchets, screwdrivers, actuators, and more use the flow of air or gas as needed.

[0010] CAES is a method of energy storage that utilizes compressed air to store and release energy. In preferred forms it can be embodied as large-scale, grid-level energy storage technology designed to balance electricity supply and demand, particularly in systems with significant intermittent renewable energy sources, such as wind and solar. It may also beemployed at smaller scales, for example, at a factory site to provide electricity or fluid power, or any other type of mechanical power to various devices, or at a domestic level.

[0011] Both when using CAES systems for powering an expander-generator to generate electricity, as well as when using it to provide pressure energy to a pneumatic tool, a large buffer (i.e. capacity) is advantageous. In the former (electricity) example, a larger buffer may allow more energy to be stored and later converted to electricity on demand. In the latter example, a larger buffer may increase the fluidic capacitance in a pneumatic installation and can facilitate higher peak demand while maintaining low peak power usage from the compressor. In the latter example, the storage can be essentially an efficient compressed gas container compressed by excess energy on the grid.

[0012] In known CAES systems, increasing the buffer size means that a greater mass of air is stored in the volume of a container, for example a gas cylinder or any type of gas tank. This is traditionally solved by increasing the storage volume, increasing storage pressures, and / or by phase change of the gas into a liquid state under significantly high pressure or significantly low temperature. Storing gas in a liquid state results in a high mass density compared to the gas form. However, for many gases (e.g., nitrogen, oxygen, and carbon dioxide), this adds significant complexity due to a critical point that is far removed from atmospheric conditions. For example, very high pressures or very low temperatures may be required, which can increase complexity and raise risks associated with a storage system. Alternative solutions for increasing the buffer size may either require more space to contain a larger volume or more material and complex handling to bear the load of higher pressures. These disadvantages have limited the application of CAES systems and have resulted in the increased size of pneumatic installations due to the increase of container tank sizes. In addition, manufacturing gas containers which can endure a sufficiently high pressure and / or low temperature can be costly because it requires more robust materials, larger thicknesses, and appropriate design compared to manufacturing gas containers for a moderate pressure and / or temperature. When designing such gas container, one needs to take into account the regulations which the gas container needs to conform with. An example of such regulation is directive 2014 / 68 / EU of the European Parliament and of the Counsel known as “Pressure Equipment Directive”. Higher the pressure, hence the risk, the more requirements the gas containers need to fulfil according to such regulation, which in turn results in a higher design cost.

[0013] Sorption (adsorption and desorption) surfaces can be used to increase the total mass density of gas storage for a given volume. In such methods, adsorbents, such as nanoporous materials, may be used to provide a framework of pores that provide (very) high surface areas per volume with many adsorption sites. In such solid frameworks, intermolecular forces between gas molecules and solids can be leveraged in addition to the pressure energy that is stored by the storage container itself. As a result, a gas container having such solid frameworks can store more air / gas than a gas container having a same pressure but without the solid frameworks. In other words, inclusion of adsorbents in a pressure container’s inner volume may assist in storage of a mass of gas material at a lower pressure, as compared to if the same mass of gas where stored in the inner volume without the adsorbents. The lower pressure represents a lower mechanical load on the vessel / container walls, while maintaining or allowing an increase in the mass of gas contained per unit of volume. In turn, this method may allow a reduction in the robustness e.g. the required amount of material usage, for the storage container while being capable of storing the same amount of gas. Likewise, this allows more gas to be stored in the same storage container. A larger buffer may therefore be achievable without needing to resort to a large volumes, extreme pressures, phase change, or complex control.

[0014] CAES systems may principally work with any compressible gas, however, in some cases it is advantageous to make use of atmospheric air, predominantly due to its ready availability, as opposed to speciality or purified gases that require special storage and are expensive.

[0015] There are many classes of adsorbent material that have affinity for air (e.g. nitrogen and oxygen), and previous attempts have been made to achieve adsorbent enhanced CAES systems.

[0016] For example, porous zeolite materials (crystalline aluminosilicate minerals with a large variety of unique porous structures) have been popularly employed and found numerous applications in the field of gas and air storage, as well as in the field of selective filtering of gases. Their high surface area and intricate network of channels are seen as making them good candidates for adsorbing and storing gases. Zeolites act as molecular sieves, selectively adsorbing molecules based on their size and polarity.

[0017] There remains, however, a desire to improve upon sorption enhanced CAES systems. For example, it may be desirable to improve capacities and / or efficiencies of sorptionenhanced CAES systems while maintaining or lowering pressures and / or temperatures swings involved in the capture, storage and release of gases. For example, the present invention may have an aim to reduce the buffer size in CAES systems by introducing improved techniques for sorption-enhanced CAES. The present invention may also or alternatively seek to improve upon the sorption characteristics of sorption enhanced CAES, for example in comparison to current sorption capabilities.

[0018] US2013219892A1 discloses a compressed air energy storage module including an integrated thermal energy storage and recovery apparatus. It discloses a tank filled with a particulate material that stores thermal energy and adsorbs air.

[0019] WO2015022633A1 discloses a vehicle comprising a sorption store for keeping a stock of fuel.

[0020] US2010133280A1 discloses a gas pressure vessel having comprising a framework component comprising at least one porous metal organic framework comprising at least one bidentate organic compound coordinated to at least one metal ion.SUMMARY

[0021] To address the above discussed drawbacks of the prior art, there is proposed, according to a first aspect of the disclosure a sorbent-enhanced process of storing and releasing compressed gas. This process comprises the steps of providing an adsorbent solid (framework) in an interior of a pressurizable container, compressing gas, preferably air, to provide a compressed gas. The process further comprises supplying said compressed gas to the adsorbent in said pressurizable container and allowing said gas to adsorb to the adsorbent under pressure. It also comprises storing said compressed gas in the pressurizable container and releasing said compressed gas and adsorbed gas from the pressurizable container to drive a mechanical device. The mechanical device can be driven by the released gas to ultimately convert the potential energy of the pressurized gas into an electrical, thermal, or mechanical energy (such as pneumatic energy or kinetic energy), fluid power, or any other type of mechanical power, or any other form of energy conversion known in the field of compressed air energy storage (CAES) technology.

[0022] In embodiments, it is preferred that the gas contained in the pressurizable container is non-flammable and not suitable to be used as a fuel. In more preferred embodiments, the gas is suitable for driving pneumatic tools in a non-combustive manner.

[0023] In an embodiment of the above aspect, the adsorbent comprises a metal-organic framework.

[0024] In another embodiment of the above aspect, the metal-organic framework comprises a metal-organic framework (MOF) selected from Al-Fum (aluminium fumarate), HKUST-1 (Cu-BTC), MOF-177 (IRMOF-1), MIL-101 (Cr), ZIF-8, MOF-5, UiO-66, and / mixtures thereof, preferably wherein the MOF is selected from Aluminium Fumarate, HKUST-1, and / or MOF-177, most preferably comprising aluminium fumarate.

[0025] In another embodiment of the above aspect, the metal-organic framework has air adsorption capacity such that the density of adsorbed air is at least about 2 times more than that of free air at the same ambient pressure and volume at a pressure in the range of 5-100 bars, preferably between 5-30 bars, and at a temperature in the range of 5-85 °C, preferably between 15-30 °C.

[0026] In another embodiment of the above aspect, the metal-organic framework has a water uptake of less than 0.25 g / g at a relative humidity of less than 25% at a temperature of 5-85 °C; preferably less than 0.05 g / g at a relative humidity of less than 25% at a temperature of 5- 85 °C.

[0027] In another embodiment of the above aspect, the gas is cooled using a cooler, heat exchanger, an intercooler, and / or a precooler before being supplied to the adsorbent.

[0028] In another embodiment of the above aspect, the metal-organic framework has a pore volume / porosity of at least 0.4-2.5 cm3 / g, preferably 0.6-1.5 cm3 / g, more preferably 0.8-1.3 cm3 / g.

[0029] In another embodiment of the above aspect, the metal-organic framework has a surface area of at least 700 m2 / g, more preferably at least 2000 m2 / g, more preferably at least 3000 m2 / g.

[0030] In another embodiment of the above aspect, the adsorbent comprises Aluminium Fumarate and / or aluminium particles, and / or a combination of different MOFs.

[0031] In another embodiment of the above aspect, the adsorbent comprises pellets, a powder, cartridge, and / or granulate.

[0032] In another embodiment of the above aspect, the adsorbent in the form of pellets-1comprises a thermally conductive binder, preferably wherein the binder has a higher thermal conductivity than the adsorbent.

[0033] In another embodiment of the above aspect, the thermally conductive binder comprises aluminium, graphene, and / or carbon nanotubes.

[0034] In another embodiment of the above aspect, supplying gas to the adsorbent comprises suppling the gas at a pressure of 5-100 bars.

[0035] In another embodiment of the above aspect, releasing adsorbed gas to a mechanical device comprises releasing the adsorbed gas at a pressure of 2 bars or higher.

[0036] In another embodiment of the above aspect, the mechanical device is a work recovering expander such as air motor, turbine, or a thermodynamic converter such as a vortex tube, or an expansion valve.

[0037] In another embodiment of the above aspect, releasing adsorbed gas to a mechanical device comprises desorbing adsorbed gas from the adsorbent.

[0038] In another embodiment of the above aspect, desorbing the gas from the adsorbent comprises thermal desorption, for example by temperature swing or electromagnetic radiation, and / or mechanical desorption, that is by mechanical perturbation, for example by application of pressure waves using air or another gas, by pressure swing and / or by ultrasound, techniques.

[0039] In another embodiment of the above aspect, said gas comprises one of nitrogen, oxygen, carbon dioxide, air, and / or mixtures thereof.

[0040] To address any of the drawbacks of the prior art, a second aspect of this disclosure provides a plant for storing and releasing compressed gas, comprising: a compressor for compressing gas, preferably air, a storage unit in gaseous communication with the compressor, said storage unit comprising a pressurizable container with an inner volume containing an adsorbent for adsorbing compressed gas, and a mechanical device in gaseous communication with the storage unit, the mechanical device arranged to receive released compressed gas from the storage unit and generate mechanical energy therefrom.

[0041] In an embodiment of the second aspect, the gas from the compressor is cooled using an intercooler or an aftercooler prior to the storage unit, and the heat extracted as a result is stored in a thermal storage unit.

[0042] In another embodiment of the second aspect, the storage unit comprises a metal tank, or polymer, such as aramid, preferably with external reinforcements such as carbon fiber, a metal tank with substantially straight walls, for example cuboid, or a cylindrically-shaped tank.

[0043] In another embodiment of the second aspect, the storage unit further comprises a device for exchanging heat, an interheater, heat trace, and / or a preheater.

[0044] In another embodiment of the second aspect, the device for exchanging heat is arranged to circulate cold or hot fluids during charging and discharging of heat respectively.

[0045] In another embodiment of the second aspect, the mechanical device is a work recovering expander such as an air motor, turbine, or a thermodynamic converter such as a vortex tube or an expansion valve.

[0046] In another embodiment of the second aspect, the plant further comprises a thermal storage unit for storing thermal energy.

[0047] In another embodiment of the second aspect, the heat exchanger, interheater and / or the preheater uses heat extracted from the thermal storage.

[0048] In another embodiment of the second aspect, the plant is arranged to use excess electrical and / or thermal energy to drive the compressor and / or to add heat to the thermal storage unit.

[0049] In another embodiment of the second aspect, the plant is arranged to release compressed gas from the storage tank filled with the adsorbent and to remove heat from the thermal storage unit to be converted into mechanical energy, and possibly further converted to deliver electrical and / or thermal energy to external sinks.

[0050] To address any of the drawbacks of the prior art, a third aspect of this disclosure provides a process of storing gas at pressure comprising the steps: compressing a gas, preferably to 5-100 bar, supplying said compressed gas to a sealable container in which an adsorbent material is provided, and allowing said compressed gas to adsorb to said adsorbent.

[0051] In an embodiment of the third aspect, the process further comprises at least one of the steps of: storing said gas for a finite period of time and / or releasing said adsorbed gas to drive a turbine to generate electricity.

[0052] In another embodiment of the third aspect, the compressed gas in the sealable container is heated using a heat exchanger, an interheater, and / or a preheater provided before or inside the storage unit.

[0053] To address any of the drawbacks of the prior art, a third aspect of this disclosure provides an adsorbent material composition, comprising: a metal-organic framework (MOF) adsorbent and a thermally conductive material.

[0054] In an embodiment of the third aspect, the MOF is selected from Al-Fum (aluminium fumarate), HKUST-1 (Cu-BTC), MOF-177 (IRMOF-1), MIL-101 (Cr), ZIF-8, MOF-5, UiO- 66, and / mixtures thereof, preferably wherein the MOF is selected from Aluminium Fumarate, HKUST-1, and / or MOF-177, most preferably comprising aluminium fumarate.

[0055] In another embodiment of the third aspect, the thermally conductive material is a binder material having a higher thermal conductivity than the MOF, and binding particles of MOF material together.

[0056] In another embodiment of the third aspect, the adsorbent material provided in the form of pellets with MOF adsorbent and a thermally conductive binder.

[0057] In another embodiment of the third aspect, the MOF is mixed or blended with a solid material that has higher thermal conductivity.

[0058] In another embodiment of the third aspect, the thermally conductive material comprises aluminium.BRIEF DESCRIPTION OF DRAWINGS

[0059] Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0060] FIG. 1 shows a plant and process for storing and releasing compressed air;

[0061] FIG. 2 shows water uptake characteristics of aluminium Fumarate and Zeolite-13X;

[0062] FIG. 3 shows water adsorption and desorption characteristics of aluminium Fumarate and Zeolite- 13X;

[0063] FIG. 4 shows the pore size distribution of aluminium fumarate.DETAILED DESCRIPTION

[0064] Hereinafter, certain embodiments will be described in further detail. It should be appreciated, however, that these embodiments are not to be construed as limiting the scope of protection for the present disclosure.

[0065] CAES involves the steps of energy storage and energy release. During times of excess electricity production (e.g., when (renewable) energy sources are generating more power than the demand requires), it may be advantageous to store the surplus electricity. When electricity demand increases or (renewable) energy generation decreases, stored surplus energy can thenbe released. Although many methods for energy storage and release exist, the most significant advantage of CAES systems is its potential for large-scale and long-duration energy storage. CAES is particularly well-suited for providing grid-level energy storage solutions.

[0066] CAES may also be sized for small scale energy storage, for example domestic scale energy storage.

[0067] In its simplest form, energy storage in CAES systems involves using surplus electrical energy to drive a compressor that compresses air. This compressed air is then stored inside a container, underground cavern, depleted natural gas field, or specially constructed aboveground vessels. After storing, this compressed air can be released and used to drive a mechanical unit that converts the generated mechanical energy back into electrical energy.

[0068] As discussed before, inclusion of adsorbents in the storage volume of a compression vessel or gas canister, can increase the available buffer size in that container that is used for storing compressed air, i.e. the mass of gas held in a given volume at a given temperature and pressure can be increased as compared to a volume that does not include the sorbent. To maximize the effectiveness of sorption in compressed air storage, the intermolecular interaction between the compressed air (adsorbate) and the adsorbent (adsorbent) is preferably optimized. To achieve this, the adsorbent preferentially has a high affinity with the compressed air (consisting mainly of nitrogen and oxygen).

[0069] It has been identified that zeolites known for use in CAES systems are effective in adsorption of nitrogen and oxygen, and are principally useful. However, it has been identified that the lifetime of zeolites in repeated cycles of adsorption and desorption of atmospheric air may be limited, as is the ability of zeolites to desorb gases under mild temperature conditions.

[0070] Following investigation, yet without wishing to be bound by theory, it has been identified that adsorption sites in known zeolites may preferentially adsorb water (vapor) instead of nitrogen and oxygen, and that desorption of water from the zeolite surface requires high energy input to heat the zeolite in order to drive off water.

[0071] In this manner it has been identified that a downside of current CAES systems using zeolite sorbents is that their longevity is limited or that there are high energy demands for reconditioning the zeolites. As explained, it is believed that the zeolite water uptake-offtake heavily impacts their overall performance. Zeolites commonly have high affinity for water, even at low pressures, which results in water vapor present in atmospheric air preferentiallyoccupying the adsorption sites at ambient pressures and blocking the sites available for the adsorption of air. This may be of particular concern for practically usable CAES systems making use of atmospheric air as the compressible gas, because atmospheric air typically contains at least some humidity. Although this concern might be addressed by drying air to low humidity levels prior to compression, such steps would be energy consuming and so detrimental to efficiency. Similarly, the use of specialist, purified gases could address this concern, but are typically not (cost) efficient.

[0072] One might also partially mitigate the issue by employing pressure-swing desorption, which optimizes the desorption process by controlling the pressure. However, one would need to reach very low pressures for removal of water from occupied sites. This commonly poses a problem, given that the stored air in CAES systems should ultimately be released at a minimum allowable pressure (e.g., for conversion to mechanical energy with an expander or air motor). Alternatively, one could resort to temperature-swing desorption, which optimizes the desorption process by controlling the temperature. However, this also commonly poses a problem, as temperatures required for complete water desorption can be as high as 200 °C for common zeolites. This, in turn, results in more energy consumption and a decrease in storage- and release efficiency.

[0073] To maximize the effectiveness of sorption in CAES, the gas selectivity of the adsorbent is preferably attuned to the gases used as the storage gas. However, it is possible that contaminants show a higher affinity to the adsorbent, causing them to fill up pores that are afterwards no longer available for adsorption of the intended gas (nitrogen and oxygen). This is not necessarily a problem, as long as the contaminants are able to contribute to the energy conversions that follow desorption. However, if these contaminants are not (fully) desorbed after storing, then the pores in which they are accommodated will remain occupied in concurrent cycles. This further reduces the sorption capacity of the adsorbent after every cycle and is not preferable. Therefore, the interaction between the adsorbent and any possible contaminants needs to be mitigated: everything that is adsorbed, is preferably easily desorbed as well.

[0074] A high affinity between compressed air and the adsorbent means that this interaction preferably has a high heat of adsorption, which is a measure of the strength of their interaction. In characterizing materials, this is expressed in the isosteric heat measured from analysing sorption at different temperatures. Moreover, to improve the reversible sorptionprocess, sorption should only involve physisorption. This means that stronger covalent or ionic bonds are not utilized, but weaker inter-molecular interaction that can be more easily reversed. As such, the main phenomenon that results in high isosteric heat, is a combination of gas / solid with a highly attractive Van der Waals force. A high Van der Waals force can minimize needs or levels of external force required for the gas to fill up the pores in the adsorbent, resulting in more adsorption capacity at lower gas storage pressures. High isosteric heat for the sorbent material is preferred.

[0075] However, similar to a condensation / evaporation balance of a liquid level, the equilibrium point of sorption is always determined by a balance between adsorption / desorption. This means that high adsorption is coupled with low desorption. Therefore, the operating conditions preferably change when either adsorption or desorption is desired, shifting the equilibrium point to where it is needed. In the case of compressed air storage for eventual conversion into mechanical energy, the desorption conditions may usefully occur at mild pressure levels. If, for example, a vacuum pressure (e.g. below ambient, atmospheric pressures) is needed for desorption, the gas is less suited for CAES as it would require re-pressurization which counters the aim of providing an efficient energy buffer.

[0076] The equilibrium point of sorption is affected by molecular statistics. There is an X number of pores available on the solid for sorption and a Y number of gas molecules that can possibly fit into the pores. To increase the chance for a gas molecule adsorbing, the number of possible interactions between the gas and solid may be increased. This may be achieved by increasing pressure. Additionally, high vibrations of molecules can make adsorption more difficult as they can overcome the Van der Waals force. This means that lower temperatures (and low system vibrations in general) will contribute to easier adsorption as well. For desorption, the opposite conditions are preferred; low pressure and high temperature.

[0077] These conditions, however, only affect the steady-state conditions of CAES. Preferably, the addition of an adsorbent does not affect the ability to charge or discharge the gas storage with any desired mass flow rate. This means that sorption kinetics may be considered as well. More extreme pressure and temperature swings can be used to improve this but are not always desirable because they may require additional complexity or energy input to achieve.

[0078] By employing temperature changes during the charging (adsorption) and discharging (desorption) stages, the sorption process can be optimized for a better balance between the adsorption capacity and the desorption energy requirement. Typically, one requires lower temperatures during adsorption and higher temperatures during desorption. Heating and cooling of the contents of the storage unit can be achieved by utilizing thermal storage in parallel to the compressed air storage. The heat source that is necessary for this temperature increase can be recovered from the compression heat, which coincides with the desirable temperature swing (i.e., cooling down for adsorption, heating up for desorption). This might be achieved by utilizing thermal storage in parallel to gas storage.

[0079] However, the second law of thermodynamics ensures that the temperature that can be reached before expansion cannot exceed the temperature that is created during compression (without any external heating source involved). This issue may be addressed by either (i) using adiabatic compression with the least number of stages possible, generating a temperature from compression heat that is as high as possible. This maximizes the exergetic value of the thermal storage or (ii) using quasi-isothermal compression or multi-stage compression with many stages and inter- and / or aftercoolers. Practically, option (ii) seems most realistic because many compressors are limited in their capacity to exert high temperatures to avoid damage to their sealing materials and lubricating oils. However, for energy storage applications (e.g., CAES), option (i) is more beneficial as it can leverage the high energy density of thermal storage materials at increased temperatures, as well as faster responses from higher desorption rates that are invoked from the more extreme temperature swing.

[0080] Temperature swing can be achieved by heating the gas, the adsorbent or both with a heat exchanger. This can be done before (upstream) or inside the gas storage tank. The time in which either of these elements can reach a desired temperature is dependent on the heat transfer coefficient of the heat exchanger, as well as the conductivity of the sorbent material.

[0081] Generally, a higher heat transfer coefficient can be achieved by increasing the contact area over which heat is transferred and / or by utilizing some form of mass flow alongside this area (such as a fan or stirring apparatus).

[0082] Alternatively or in addition, the heat transfer coefficient of the adsorbent material may be increased. A manner to increase overall heat transfer rate inside the gas storage tank is to mix, blend or comprise the adsorption material with a (more) thermally conductive material.For example, the adsorbent material may be combined with a binder material that has a higher thermal conductivity. For example, the material may be provided in the form of pellets with adsorbent and heat conductive binder. That is a binder having a higher thermal conductivity heat transfer coefficient than the adsorbent material.

[0083] Alternatively or in addition, the adsorbent can be mixed with a different solid material that has high thermal conductivity. For example, aluminium (or other highly heat conductive and generally inert material) particles or chips may be dispersed through adsorbent material.

[0084] It may also be that a heat exchanger is provided in the gas storage tank (e.g., with a coil or parallel tubes, with or without fins), the temperature swing can be applied with a thermal fluid flowing through the heat exchanger. The heat exchanger may be employed to capture sensible heat that is released upon gas adsorption, and / or to provide sensible heat for gas desorption.

[0085] One way to apply temperature swing before compressed air is stored in the storage unit, is to use a cooler after the air has been compressed. Such a cooler is arranged to cool the gas before it enters the gas storage tank that contains the adsorbent. In this way, the cooler will also indirectly cool the adsorbent and the bulk temperature of the storage unit will reach that of approximately a weighted average between the temperature of the compressed air and the adsorbent.

[0086] Similarly, finding an optimal pressure swing mechanism can also greatly enhance the efficiency of sorption enhanced CAES. It enables the adsorption and desorption processes to be efficiently controlled, ensuring a proper balance between the adsorption capacity and the energy required for desorption. Pressure swing can be achieved by compressing the gas at a higher pressure before storage, followed by an expansion at lower pressure after storage. However, as mentioned before, this may result in more energy consumption per unit mass of gas than what can be recovered during the energy conversion after storage. The pressure after compression needs to be at least the value for the storage pressure. The storage pressure in a sorption-enhanced gas storage container should be one that maximizes stored gas mass inside this container. This can only be within a pressure range in which the equivalent gas density (i.e., adsorbed molecules and non-adsorbed molecules combined, per unit storage volume) is higher than the gas density that would be in same container without the adsorbent. Otherwise, the added storage capacity introduced by the adsorbent cannot overcome its own added volume occupation, making the addition counter-effective.

[0087] Another property that may affect gas / solid interaction that can be optimized to improve sorption kinetics is shape (surface area and / or pore volume) of the adsorbent and size of the gas molecules. For faster kinetics, gas molecules are desirably able to easily travel (or diffuse) through the material’s framework and reach the pores, and the pore size desirably closely resembles the size of the gas molecule such that it can fit inside optimally and Van der Waals forces can attract the molecule from multiple angles.

[0088] In addition to temperature- and pressure swing, other measures can also be used that increase molecular vibrations with macro forces (e.g., collisions with other molecules, pressure waves or electromagnetic perturbations). These forces may be adjusted to the combination of the adsorbent and adsorbed gas(es). In case of collisions with other molecules, they preferably will not be adsorbed in place of the molecules that were ‘knocked out’ of the occupied site. The likelihood of adsorption should therefore be minimized in this scenario. In the case of pressure waves and electromagnetic perturbations, their magnitude and frequency may be attuned to the adsorbed gas molecules. For example, it may be most beneficial to perturb the gas molecules at their resonance frequency.

[0089] Forced collisions with other molecules can be achieved by injecting a pressurized gas into the gas storage tank, effectively knocking adsorbed molecules from the adsorption sites. When more molecules are adsorbed, the chance of collision is higher and thereby increasing the effect of this perturbation. Ideally, this pressurized gas is the same as the adsorbed gas such that separation is not necessary, and can be stored in a second gas storage container.

[0090] FIG. 1 shows a plant 100 for storing and releasing compressed air. As an input, plant 100 may comprise air supply 101. Air supply 101 mainly comprises oxygen and nitrogen, and may be supplied by atmospheric air or an external canister of air. Air supply 101 may further comprise a (plurality of) filter(s), for example a chemical filter, particulate filter, HEPA filter, coalescing filter, activated carbon filter, adsorption filter, electrostatic filter, and / or a filter using UV-light. The air may be (partially) dried to reduce or remove water vapor, i.e. there may be a step of reducing humidity of the air to a desirable level.

[0091] Plant 100 further comprises compressor 102, which is arranged to convert mechanical energy into compressed air 104. Compressor 102 takes air supply 101 and compresses it into compressed air 104. The compressor 102 may be driven by (excess) electricity. Compressor 102 may be any type, for example a reciprocating compressor, (e.g., piston compressor), rotary screw compressor, scroll compressor, centrifugal compressor, axial compressor,diaphragm compressor, and / or a vane compressor. After compressing using compressor 102, compressed air 104 preferably has a pressure at which the air density is still less or equal than the equivalent density of the air inside the storage unit 105 containing the adsorbent at equilibrium. Preferably, this is ranging from 5-100 bar, preferably between 5-30 bar. All pressures described herein are absolute, meaning pressures that are relative to the zero pressure in the empty, air-free space of the universe.

[0092] Compressed air 104 may be stored in storage unit 105. Storage unit 105 is arranged to store compressed air 104 by encapsulating the compressed air 104 in a container for example a sealable container, canister, metal tank, inflatable polymer (such as aramid with external reinforcements made of carbon fiber), a metal tank with substantially straight walls, for example cuboid, or a cylindrically-shaped tank, an underground cavern, a depleted natural gas field and / or any other container capable of withstanding pressure exerted by compressed air 104. Storing, supplying, and / or adsorbing of compressed air to storage unit 105 may further comprise controlling pressure and / or temperature to maintain a certain level or profile.

[0093] Storage unit 105 may further be arranged to store adsorbent 125. Plant 100 comprises adsorbent 125 arranged for adsorbing compressed air. Adsorbent 125 may comprise a nanoporous material, zeolite (such as zeolite 13X), activated carbon, silica (aero)gel, carbon nanotube, a porous polymer, preferably a metal-organic framework (MOF), such as Al-Fum (aluminium fumarate), HKUST-1 (Cu-BTC), MOF-177 (IRMOF-1), MIL-101 (Cr), ZIF-8, MOF-5, UiO-66. Aside from having affinity to air, the abovementioned MOFs are particularly suitable to be used as adsorbents in CAES systems employing humid gases, such as atmospheric air, because of their ability to improve upon one or more of the following concerns: (i) that other gaseous compounds such as water vapor compete with nitrogen or oxygen and occupy the adsorption sites at low pressures (which makes pressure swing desorption difficult) or at high temperatures (which makes temperature swing desorption difficult), i.e., selective affinity towards air becomes important, (ii) manufacturing of adsorbents at large scale depends on the availability of its constituents (such as ligands) and synthesis routes to provide large yields at low costs, and (iii) lack of stability of adsorbents under high mechanical and thermal stress. The MOFs discussed above address one or more of these constraints along with being an excellent adsorbent of air. For example, as compared to zeolites, it is known that MOFs, such as aluminium fumarate, are less susceptible tosaturation with water molecules at lower pressures and desorbs water molecules at a relatively faster rate at lower temperatures.

[0094] Further advantages of using MOFs, like Al-Fum, over zeolites, may include: i) low cost associated with the use of a potentially renewable dicarboxylic linker and an abundant metallic cation, e.g., aluminium; ii) good water stability, mechanical stability and thermal stability; iii) an environmentally friendly synthesis route, which involves only water and simple aluminium salts, and the chemical linker may be obtained from renewable biomass; iv) large scale production, with a record space-time yield of 3,600 kgm3 / day; and v) a large pore volume (0.65 cm3 / g) and surface area (1100 m2 / g). Al-Fum, in particular, has excellent tunability of properties such as porosity, surface area, thermal conductivity, etc. The combination of these qualities along with the excellent affinity towards nitrogen (major constituent of air) make MOFs, particularly Al-Fum, good for air adsorption in compressed air energy storage systems.

[0095] Adsorbent 125 may comprise a powder, cartridge, and / or granulate. Granulate is understood as being chunks or pieces of adsorbent with their largest spatial dimension no larger than 3 cm, preferably 2 cm, most preferably 3 cm and larger than at least 50 microns such that the adsorbent is blocked by a compressed air filter. Alternatively, adsorbent 125 may comprise such forms with a thermally conductive binder. When using Al-Fum as an adsorbent, adsorbent 125 may comprise a mixture of Aluminium Fumarate and Aluminium, and / or a combination of different MOFs. When using a MOF as adsorbent, adsorbent 125 preferably has air adsorption capacity such that the density of adsorbed air is at least 2 times more than that of free air at the same pressure in a pressure range of 5-100 bars (preferably between 5-30 bars) and in a temperature range of 5-85 °C (preferably between 15-30 °C). The adsorbent should also have a low water uptake at low pressure (less than 30 cm3 / g up to 5 bars), and high desorption kinetics for water (complete desorption in less than 20 minutes at relative humidity of 40% and at temperature of 65 °C). Storage unit 105 is further arranged to store adsorbed air, which is compressed air that has been adsorbed by adsorbent 125.

[0096] A temperature of the compressed air 104 and adsorbent 125 inside storage unit 105 may be controlled by in-storage heat exchanger 106, which may comprise a coil, or parallel tubes (with or without fins) for exchanging heat between the contents of storage unit 105 and the thermal fluid inside heat exchanger 106. Heat exchanger 106 or any other device forexchanging heat is arranged to circulate cold or hot thermal fluids during charging and discharging of heat respectively.

[0097] The contents of storage unit 105 may be stored indefinitely or until there is need for energy output, which can be in the form of mechanical energy or further conversion to electricity. If there is a need for energy output, adsorbed air inside storage 105 may be desorbed and / or compressed air 107 may be released from storage unit 105. Releasing, desupplying, and / or desorbing of compressed air from storage unit 105 may further comprise controlling pressure and / or temperature to maintain a certain level or profile. Released compressed air 107 preferably has a pressure of at least 2 bar.

[0098] Plant 100 may further comprise heat exchanger 108, arranged to control the temperature of compressed air 108. Once controlled by heat exchanger 108, compressed air 109 preferably has a temperature of around room temperature (10-30 °C).

[0099] Plant 100 may further comprise mechanical device 110, which is arranged to convert compressed air into mechanical energy. Mechanical device 110 may comprise an air motor, turbine, expander, a thermodynamic converter such as a vortex tube, an expansion valve, and / or any other pneumatic actuator capable of converting compressed air into mechanical energy.

[0100] As described before, storing, supplying, and / or adsorbing of compressed air to storage unit 105 and releasing, de-supplying, and / or desorbing of compressed air from storage unit 105 may further comprise controlling pressure (by mechanical perturbations resulting from pressure waves using air or another gas, by pressure swing, and / or by ultrasound) and / or temperature (temperature swing or electromagnetic radiation) to maintain a certain level or profile.

[0101] Controlling pressure may be achieved by using acoustophoresis, which refers to the use of ultrasound waves to manipulate particles within storage unit 105. While it may not directly control pressure uniformly, it can influence the movement and distribution of particles or fluids, which can affect pressure locally. As such, pressure waves can be formed that assist in the releasing, de-supplying, and / or desorbing of compressed air from storage unit 105. To achieve this, storage unit 105 may further comprise means for transmitting ultrasound, such as a transducer, an energy source, and / or a frequency control unit.

[0102] Controlling of storing- and / or release pressure may be achieved by controlling the flow rate at which compressed air is stored and / or released, respectively. Controlling oftemperature during storing, supplying, and / or adsorbing of compressed air to storage unit 105 and releasing, de-supplying, and / or desorbing of compressed air from storage unit 105 may be achieved by operating heat exchanger 106. Heat exchanger 106 may be arranged to heat and / or cool storage unit 105 using inflow 124.

[0103] Plant 100 may further comprise thermal storage unit 117, which may comprise one or more installations of a canister, metal tank, inflatable polymer (such as aramid with external reinforcements made of carbon fiber), a metal tank with substantially straight walls, for example cuboid, or a cylindrically-shaped tank, an underground cavern, a depleted natural gas field and / or any other insulated container capable of storing thermal energy generated by plant 100. Thermal storage unit 117 may comprise one or more heat exchangers, such as first and second heat exchangers 116 and 121, which may be housed inside a single compartment of thermal storage unit 117 or multiple different ones. Given that plant 100 may comprise a plurality of compressor(s) and requires compressed air to flow and be stored at a specific temperature, thermal storage unit 117 may be arranged to store and release heat at certain positions in plant 100.

[0104] One such position where heat may be released to and / or stored is storage unit 105. Storage unit 105 may comprise heat exchanger 106, which is connected to second heat exchanger 121, connected through inflow 124, and located inside thermal storage unit 117. Heat stored inside thermal storage unit 117 may be exchanged to second heat exchanger 121, which then exchanges its heat to heat-exchanger 106, located inside storage unit 105, through inflow 124. In turn, heat exchanger 106 may be arranged to release heat to storage unit 105. Conversely, heat exchanger 106 may be arranged to exchange heat that is generated inside storage unit 105 to second heat exchanger 121 through outflow 113. Before outflow 113 reaches second heat exchanger 121, outflow 113 (which is either liquid or gaseous) may be pressurized by circulation device 120. Heat generated by circulation device 120 may additionally be stored inside thermal storage 117 using one or more heat exchangers.Whereas inflow 124 provide heat to storage unit 105, the temperature of inflow 124 may also be lower than that of (the contents ol) storage unit 105, essentially cooling (the contents ol) storage unit 105.

[0105] Another source of heat inside plant 100 is generated by compressors, pumps, motors or generators, such as used in devices 102, 110, 114, and / or 120. This heat may be reused in plant 100 by storing it inside thermal storage unit 117 and releasing it to heat exchanger 106located in storage unit 105. Compressor 102, which is arranged to convert mechanical energy into compressed air 104, may be coupled to heat exchanger 103. Heat generated by compressor 102 may be exchanged to heat exchanger 103, which transports the heat to first heat exchanger 116 through outflow 118. Similarly to second heat exchanger 121, first heat exchanger 116 is located inside thermal storage unit 117. Heat exchanged to first heat exchanger 116 through outflow 118 may be released and stored to thermal storage unit 117. Stored heat inside thermal storage unit 117 may then be used to, for example, heat (the contents ol) storage unit 105 to achieve temperature swing during desorption. Once first heat exchanger 116 has released heat to thermal storage unit 117, new heat may be captured at compressor 102 through inflow 115. Inflow 115 may first be pressurized by circulation device 114 to obtained inflow 112. Inflow 112 then captures heat exerted by compressor 102 using heat exchanger 103 and transports it back to first heat exchanger 116.

[0106] Given that low temperatures are generally required for adsorption, compressor 102 may further comprise an intercooler and / or aftercooler to decrease the temperature of compressed air 104. Heat extracted using e.g. either an intercooler or aftercooler may be stored in thermal storage unit 117 or any other storage unit for storing thermal energy. The cold source to provide this cooling energy may come from a low-temperature region inside thermal storage unit 117, which can be regenerated by any pre-, inter- or after-heater that is positioned upstream, inside, or downstream of the expansion device 110. Alternatively, an external cold source could provide additional cooling before inflow 112 to further reduce temperature of compressed air 104. Another option to further decrease the temperature of compressed air 104 is by introduction of an externally powered chiller upstream of storage tank 105.

[0107] Some types of mechanical devices 110 cannot operate on compressed air outside a certain range of temperature, i.e. an inlet temperature that is too low may cause ice formation inside the device due to the temperature drop associated with expansion. This is why plant 100 may further comprise heat exchanger 108, arranged to further control the temperature of compressed air 107. To control the temperature of compressed air 107, heat exchanger 108 uses inflow 122 from second heat exchanger 121, which is located inside thermal storage unit 117. In turn, heat exchanger 108 uses outflow 123 to release thermal energy to second heat exchanger 121, which in turn releases and / or stores thermal energy to thermal storage unit 117.

[0108] Further in illustrated plant 100, there is provided a process 199 for storing and releasing compressed air, comprising the steps of i) providing an adsorbent 125, ii) supplying air 104 to the adsorbent 125, and iii) releasing adsorbed air to a mechanical device 110. Instead of using compressed air, plant 100 and process 199 may instead use gases selected from nitrogen, oxygen, carbon dioxide, and / or mixtures thereof. Additionally, each of these gases, including air, may comprise some amount of water vapor.

[0109] FIG. 2 shows water uptake characteristics of aluminium Fumarate and Zeolite-13X. As shown in FIG. 2, Zeolite 13X has high affinity for water even at low pressures, which results in water vapour filling up the pores at ambient pressures and blocking the sites available for the adsorption of air.

[0110] Thus, it has been identified that although Zeolite 13X is a good adsorbent for air and has been used in the prior art, its water uptake-offtake can lead to performance problems. Inadvertently, during desorption one needs to reach very low pressures for water removal from the occupied sites. Given that the stored air is preferably to be used at a minimum pressure for reasons of operation simplicity, for example for conversion to mechanical energy with an expander or air motor, this may detrimentally lead to use of temperature swing for complete water desorption, which can be at least as high as 200 °C for Zeolite 13X. This, in turn, results in more energy consumption and a decrease in storage- and release efficiency. As also shown in FIG. 2, some MOFs such as Al-Fum, were found to have much less affinity to water vapour at lower pressures.

[0111] FIG. 3(a) shows water adsorption characteristics of aluminium Fumarate and Zeolite- 13X for relative humidities (RH) of 20%, 30%, and 40%. Prior to adsorption, the sorbent materials were fully dehydrated. Looking at FIG. 3(a), at low RH, Al-Fum adsorbs significantly low (almost negligible) water mass when compared to zeolite 13X. Furthermore, the adsorption kinetics (or rate of adsorption) is much slower for Al-Fum than for Zeolite- 13X. Therefore, it is considered that more sites are likely available on the surface of Al-Fum for air adsorption even from kinetic arguments.

[0112] FIG. 3(b) shows water desorption characteristics of aluminium Fumarate and Zeolite- 13X for relative humidities of 20%, 30%, and 40% and temperatures of 65 °C, 85 °C, and 120 °C under 0% RH. Prior to desorption, the sorbent materials were saturated at 30 °C and 40% RH. Zeolite-13X clearly exhibits the slower desorption kinetics. Even at 120 °C, all the water molecules adsorbed were not desorbed or removed after 100 minutes.

[0113] In a process involving adsorption of air (especially atmospheric air), such as may be used in the illustrated process 199, the supplied air for compression may be regulated to have a relative humidity at the conditions of the supply position of less than 40%, preferably less than 35%, less than 30%, less than 25%, less than 20%, most preferably less than 20%. The process may involve a step of drying incoming air, e.g. by heating or desiccation, prior to adsorption.

[0114] FIG. 4 shows the pore size distribution of aluminium fumarate. As can be observed, Al-Fum is largely microporous and slightly mesoporous. The pore size distribution of a metal-organic frameworks in general is largely microporous (pore size up to 2 nm) and some mesopores (pore size up from 2 nm up to 50 nm). This is particularly advantageous for adsorbing air molecules because: i) micropores, with their small diameters, provide a large surface area per unit volume. They are highly efficient in adsorbing smaller molecules like nitrogen gas. An adsorbent with micropores ensures that a significant portion of the surface area is available for adsorption, thus enhancing the overall adsorption capacity, and ii) mesopores allow for efficient diffusion of gas molecules deeper into the adsorbent's structure during adsorption and for the same reason quick desorption, leading to a well-balanced adsorption process.

[0115] Surface area of adsorbents may be measured using the Brunauer-Emmett-Teller (BET) method, specifically using ISO 9277:2022. Pore volume, and pore size distribution may be measured according to ISO 15901-2:2022. Water adsorption may be measured by thermal gravimetric analysis (TGA) or dynamic vapour sorption (DVS) techniques.

[0116] For measuring water uptake, prior to the adsorption measurement in the thermal gravimetric analysis (TGA), the materials can be activated under dry nitrogen flow at 150°C for the metal-organic frameworks and 300°C for zeolite 13X until the sample mass is stable for at least one hour. After activation, the samples may be cooled down to 30°C under dry nitrogen flow. Immediately after reaching 30°C, the adsorption measurement is started. The temperature during adsorption is kept constant at 30°C and the relative humidity can be adjusted by changing the respective ratio of dry to humidified nitrogen flow. The change in mass as measured by the microbalance in TGA is the mass of water vapour adsorbed.

[0117] In the following, numbered clauses define embodiments of the invention.Clause 1. A sorbent-enhanced process of storing and releasing compressed gas, comprising the steps of:i) providing an adsorbent solid in an interior of a pressurizable container; ii) compressing gas, preferably air, to provide a compressed gas, iii) supplying said compressed gas to the adsorbent in said pressurizable container and allowing said gas to adsorb to the adsorbent under pressure; iv) storing said compressed gas in the pressurizable container; and v) releasing said compressed gas and adsorbed gas from the pressurizable container to drive a mechanical device.Clause 2. The process of clause 1, wherein the adsorbent comprises a metalorganic framework.Clause 3. The process of clause 2, wherein the metal-organic framework comprises a metal-organic framework (MOF) selected from Al-Fum (aluminium fumarate), HKUST-1 (Cu-BTC), MOF-177 (IRMOF-1), MIL-101 (Cr), ZIF-8, MOF- 5, UiO-66, and / mixtures thereof, preferably wherein the MOF is selected from Aluminium Fumarate, HKUST-1, and / or MOF-177, most preferably comprising aluminium fumarate.Clause 4. The process of any of clauses 2-3, wherein the metal-organic framework has air adsorption capacity such that the density of adsorbed air is at least about 2 times more than that of free air at the same ambient pressure and volume at a pressure in the range of 5-100 bars, preferably between 5-30 bars, and at a temperature in the range of 5-85 °C, preferably between 15-30 °C.Clause 5. The process of any of clauses 2-4, wherein the metal-organic framework has a water uptake of less than 0.25 g / g at a relative humidity of less than 25% at a temperature of 5-85 °C; preferably less than 0.05 g / g at a relative humidity of less than 25% at a temperature of 5-85 °C.Clause 6. The process of any of clauses 2-5, wherein the gas is cooled using a cooler, heat exchanger, an intercooler, and / or a precooler before being supplied to the adsorbent.Clause 7. The process of any of clauses 2-6, wherein the metal-organic framework has a pore volume / porosity of at least 0.4-2.5 cm3 / g, preferably 0.6-1.5 cm3 / g, more preferably 0.8-1.3 cm3 / g.Clause 8. The process of any of clauses 2-7, wherein the metal-organic framework has a surface area of at least 700 m2 / g, more preferably at least 2000 m2 / g, more preferably at least 3000 m2 / g.Clause 9. The process of any of clauses 1-8, wherein the adsorbent comprises Aluminium Fumarate and / or aluminium particles, and / or a combination of different MOFs.Clause 10. The process of any of clauses 1-9, wherein the adsorbent comprises pellets, a powder, cartridge, and / or granulate.Clause 11. The process of clause 10, wherein the adsorbent in the form of pellets comprises a thermally conductive binder, preferably wherein the binder has a higher thermal conductivity than the adsorbent.Clause 12. The process of clause 11, wherein the thermally conductive binder comprises aluminium, graphene, and / or carbon nanotubes.Clause 13. The process of any of clauses 1-12, wherein supplying gas to the adsorbent comprises suppling the gas at a pressure of 5-100 bars.Clause 14. The process of any of clauses 1-13, wherein releasing adsorbed gas to a mechanical device comprises releasing the adsorbed gas at a pressure of 2 bars or higher.Clause 15. The process of any of clauses 1-14, wherein the mechanical device is a work recovering expander such as air motor, turbine, or a thermodynamic converter such as a vortex tube, or an expansion valve.Clause 16. The process of any of clauses 1-15, wherein releasing adsorbed gas to a mechanical device comprises desorbing adsorbed gas from the adsorbent.Clause 17. The process of clause 16, wherein desorbing the gas from the adsorbent comprises thermal desorption, for example by temperature swing or electromagnetic radiation, and / or mechanical desorption, that is by mechanicalperturbation, for example by application of pressure waves using air or another gas, by pressure swing and / or by ultrasound, techniques.Clause 18. The process of any of clauses 1-17, wherein said gas comprises one of nitrogen, oxygen, carbon dioxide, air, and / or mixtures thereof.Clause 19. A plant for storing and releasing compressed gas, comprising: i) a compressor for compressing gas, preferably air; ii) a storage unit in gaseous communication with the compressor, said storage unit comprising a pressurizable container with an inner volume containing an adsorbent for adsorbing compressed gas; and iii) a mechanical device in gaseous communication with the storage unit, the mechanical device arranged to receive released compressed gas from the storage unit and generate mechanical energy therefrom.Clause 20. The plant of clause 19, wherein the gas from the compressor is cooled using an intercooler or an aftercooler prior to the storage unit, and the heat extracted as a result is stored in a thermal storage unit.Clause 21. The plant of any of clauses 19-20, wherein the storage unit comprises a metal tank, or polymer, such as aramid, preferably with external reinforcements such as carbon fiber, a metal tank with substantially straight walls, for example cuboid, or a cylindrically-shaped tank.Clause 22. The plant of any of clauses 19-21, wherein the storage unit further comprises a device for exchanging heat, an interheater, heat trace, and / or a preheater.Clause 23. The plant of clause 22, wherein the device for exchanging heat is arranged to circulate cold or hot fluids during charging and discharging of heat respectively.Clause 24. The plant of any of clauses 19-23, wherein the mechanical device is a work recovering expander such as an air motor, turbine, or a thermodynamic converter such as a vortex tube or an expansion valve.Clause 25. The plant of any of clauses 19-24, wherein the plant further comprises a thermal storage unit for storing thermal energy.Clause 26. The plant of clause 25, wherein the heat exchanger, interheater and / or the preheater uses heat extracted from the thermal storage.Clause 27. The plant of any of clauses 19-26, wherein the plant is arranged to use excess electrical and / or thermal energy to drive the compressor and / or to add heat to the thermal storage unit.Clause 28. The plant of any of clauses 19-27, wherein the plant is arranged to release compressed gas from the storage tank filled with the adsorbent and to remove heat from the thermal storage unit to be converted into mechanical energy, and possibly further converted to deliver electrical and / or thermal energy to external sinks.Clause 29. A process of storing gas at pressure comprising: i) compressing a gas, preferably to 5-100 bar; ii) supplying said compressed gas to a sealable container in which an adsorbent material is provided; iii) allowing said compressed gas to adsorb to said adsorbent.Clause 30. The process of clause 29, further comprising at least one of the steps of: iv) storing said gas for a finite period of time; and v) releasing said adsorbed gas to drive a turbine to generate electricity.Clause 31. The process of any of clauses 29-30, wherein the compressed gas in the sealable container is heated using a heat exchanger, an interheater, and / or a preheater provided before or inside the storage unit.Clause 32. An adsorbent material composition, comprising: a metal-organic framework (MOF) adsorbent; and a thermally conductive material.Clause 33. The adsorbent material of clause 32, wherein the MOF is selected from Al-Fum (aluminium fumarate), HKUST-1 (Cu-BTC), MOF-177 (IRMOF-1), MIL- 101 (Cr), ZIF-8, MOF-5, UiO-66, and / mixtures thereof, preferably wherein the MOFis selected from Aluminium Fumarate, HKUST-1, and / or MOF-177, most preferably comprising aluminium fumarate.Clause 34. The adsorbent material of any of clauses 32-33, wherein the thermally conductive material is a binder material having a higher thermal conductivity than the MOF, and binding particles of MOF material together.Clause 35. The adsorbent material of any of clauses 32-34, wherein the adsorbent material provided in the form of pellets with MOF adsorbent and a thermally conductive binder.Clause 36. The adsorbent material of any of clauses 32-35, wherein the MOF is mixed or blended with a solid material that has higher thermal conductivity.Clause 37. The adsorbent material of clause 36, wherein the thermally conductive material comprise aluminium.

Claims

CLAIMS1. A sorbent-enhanced process of storing and releasing compressed gas, comprising the steps of: i) providing an adsorbent solid in an interior of a pressurizable container; ii) compressing gas, preferably air, to provide a compressed gas, iii) supplying said compressed gas to the adsorbent in said pressurizable container and allowing said gas to adsorb to the adsorbent under pressure; iv) storing said compressed gas in the pressurizable container; v) releasing said compressed gas and adsorbed gas from the pressurizable container; and vi) driving a mechanical device.

2. The process of claim 1, wherein the adsorbent comprises a metal-organic framework.

3. The process of claim 2, wherein the metal-organic framework comprises aluminium fumarate.

4. The process of claim 2, wherein the metal-organic framework comprises a metalorganic framework (MOF) selected from HKUST-1 (Cu-BTC), MOF-177 (IRMOF-1), MIL- 101 (Cr), ZIF-8, MOF-5, UiO-66, and / mixtures thereof, preferably wherein the MOF is selected from HKUST-1, and / or MOF-177.

5. The process of any of claims 1-4, wherein process further comprises: providing the released gas directly to the mechanical device.

6. The process of any of claims 1-5, wherein the process further comprises: converting a potential energy of the compressed gas into electrical, thermal, or mechanical energy using the mechanical device.

7. The process of claim 6, wherein the gas is non-flammable.

8. The process of any of claims 2-7, wherein the metal-organic framework has air adsorption capacity such that the density of adsorbed air is at least about 2 times more than that of free air at the same ambient pressure and volume at a pressure in the range of 5-100 bars, preferably between 5-30 bars, and at a temperature in the range of 5-85 °C, preferably between 15-30 °C.

9. The process of any of claims 2-8, wherein the metal-organic framework has a water uptake of less than 0.25 g / g at a relative humidity of less than 25% at a temperature of 5-85 °C; preferably less than 0.05 g / g at a relative humidity of less than 25% at a temperature of 5- 85 °C.

10. The process of any of claims 2-9, wherein the gas is cooled using a cooler, heat exchanger, an intercooler, and / or a precooler before being supplied to the adsorbent.

11. The process of any of claims 2-10, wherein the metal-organic framework has a pore volume / porosity of at least 0.4-2.5 cm3 / g, preferably 0.6-1.5 cm3 / g, more preferably 0.8-1.3 cm3 / g.

12. The process of any of claims 2-11, wherein the metal-organic framework has a surface area of at least 700 m2 / g, more preferably at least 2000 m2 / g, more preferably at least 3000 m2 / g.

13. The process of any of claims 1-12, wherein the adsorbent comprises Aluminium Fumarate and / or aluminium particles, and / or a combination of different MOFs.

14. The process of any of claims 1-13, wherein the adsorbent comprises pellets, a powder, cartridge, and / or granulate.

15. The process of claim 14, wherein the adsorbent in the form of pellets comprises a thermally conductive binder, preferably wherein the binder has a higher thermal conductivity than the adsorbent.

16. The process of claim 15, wherein the thermally conductive binder comprises aluminium, graphene, and / or carbon nanotubes.

17. The process of any of claims 1-15, wherein supplying gas to the adsorbent comprises suppling the gas at a pressure of 5-100 bars.

18. The process of any of claims 1-16, wherein releasing adsorbed gas to a mechanical device comprises releasing the adsorbed gas at a pressure of 2 bars or higher.

19. The process of any of claims 1-18, wherein the mechanical device is a work recovering expander such as air motor, turbine, or a thermodynamic converter such as a vortex tube, or an expansion valve.

20. The process of any of claims 1-19, wherein releasing adsorbed gas to a mechanical device comprises desorbing adsorbed gas from the adsorbent.

21. The process of claim 20, wherein desorbing the gas from the adsorbent comprises thermal desorption, for example by temperature swing or electromagnetic radiation, and / or mechanical desorption, that is by mechanical perturbation, for example by application of pressure waves using air or another gas, by pressure swing and / or by ultrasound, techniques.

22. The process of any of claims 1-21, wherein said gas comprises one of nitrogen, oxygen, carbon dioxide, air, and / or mixtures thereof.

23. A plant for storing and releasing compressed gas, comprising: i) a compressor for compressing gas, preferably air; ii) a storage unit in gaseous communication with the compressor, said storage unit comprising a pressurizable container with an inner volume containing an adsorbent for adsorbing compressed gas; and iii) a mechanical device in gaseous communication with the storage unit, the mechanical device arranged to receive released compressed gas from thestorage unit and generate a mechanical, electrical, or thermal energy therefrom.

24. The plant of claim 23, wherein the adsorbent comprises a metal-organic framework.

25. The plant of claim 24, wherein the metal-organic framework comprises aluminium fumarate.

26. The plant of any of claims 23-25, wherein gas is non-flammable.

27. The plant of any of claims 23-26, wherein the gas from the compressor is cooled using an intercooler or an aftercooler prior to the storage unit, and the heat extracted as a result is stored in a thermal storage unit.

28. The plant of any of claims 23-27, wherein the storage unit comprises a metal tank, or polymer, such as aramid, preferably with external reinforcements such as carbon fiber, a metal tank with substantially straight walls, for example cuboid, or a cylindrically-shaped tank.

29. The plant of any of claims 23-28, wherein the storage unit further comprises a device for exchanging heat, an interheater, heat trace, and / or a preheater.

30. The plant of claim 29, wherein the device for exchanging heat is arranged to circulate cold or hot fluids during charging and discharging of heat respectively.

31. The plant of any of claims 23-30, wherein the mechanical device is a work recovering expander such as an air motor, turbine, or a thermodynamic converter such as a vortex tube or an expansion valve.

32. The plant of any of claims 23-31, wherein the plant further comprises a thermal storage unit for storing thermal energy.

33. The plant of claim 32, wherein the heat exchanger, interheater and / or the preheater uses heat extracted from the thermal storage.

34. The plant of any of claims 23-33, wherein the plant is arranged to use excess electrical and / or thermal energy to drive the compressor and / or to add heat to the thermal storage unit.

35. The plant of any of claims 23-34, wherein the plant is arranged to release compressed gas from the storage tank filled with the adsorbent and to remove heat from the thermal storage unit to be converted into mechanical energy, and possibly further converted to deliver electrical and / or thermal energy to external sinks.

36. A process of storing gas at pressure comprising: i) compressing a gas, preferably to 5-100 bar; ii) supplying said compressed gas to a sealable container in which an adsorbent material is provided; iii) allowing said compressed gas to adsorb to said adsorbent.

37. The process of claim 36, wherein the supplying step includes supplying the adsorbent material comprising a metal organic framework.

38. The process of claim 37, wherein the supplying step includes supplying the metal organic framework comprising aluminium fumarate.

39. The process of any of claims 26-28, wherein the process further comprises at least one of the steps of: iv) storing said gas for a finite period of time; and v) releasing said adsorbed gas to drive a turbine to generate electricity.

40. The process of any of claims 36-39, wherein the compressed gas in the sealable container is heated using a heat exchanger, an interheater, and / or a preheater provided before or inside the storage unit.

41. An adsorbent material composition, comprising:a metal-organic framework (MOF) adsorbent; and a thermally conductive material.

42. The adsorbent material of claim 41, wherein the metal-organic framework (MOF) adsorbent comprises aluminium fumarate.

43. The adsorbent material of any of claims 41-42, wherein the MOF is selected from HKUST-1 (Cu-BTC), MOF-177 (IRMOF-1), MIL-101 (Cr), ZIF-8, MOF-5, UiO-66, and / mixtures thereof, preferably wherein the MOF is selected from HKUST-1, and / or MOF-177.

44. The adsorbent material of any of claims 41-43, wherein the thermally conductive material is a binder material having a higher thermal conductivity than the MOF, and binding particles of MOF material together.

45. The adsorbent material of any of claims 41-44, wherein the adsorbent material provided in the form of pellets with MOF adsorbent and a thermally conductive binder.

46. The adsorbent material of any of claim 41-45, wherein the MOF is mixed or blended with a solid material that has higher thermal conductivity.

47. The adsorbent material of claim 46, wherein the thermally conductive material comprise aluminium.

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