CA2716776A1

CA2716776A1 - Adsorption-enhanced compressed air energy storage

Info

Publication number: CA2716776A1
Application number: CA2716776A
Authority: CA
Inventors: Timothy F. Havel
Original assignee: ENERGY COMPRESSION LLC
Current assignee: ENERGY COMPRESSION LLC
Priority date: 2008-03-14
Filing date: 2009-03-16
Publication date: 2009-09-17
Also published as: CN101970833A; EP2262993A4; RU2010139758A; GB2470337A; GB201015869D0; AU2009223725A1; EP2262993A2; JP2011518270A; WO2009114205A2; IL207938A0; BRPI0909360A2; WO2009114205A3

Abstract

In an embodiment of the present disclosure, an energy storage device is presented. The energy storage device includes a porous material that adsorbs air and a compressor. The compressor converts mechanical energy into pressurized air and heat, and the pressurized air is cooled and adsorbed by the porous material.
The energy storage device also includes a tank used to store the pressurized and adsorbed air and a motor. The motor is driven to recover the energy stored as compressed and adsorbed air by allowing the air to desorb and expand while driving the motor.

Description

Adsorption-Enhanced Compressed Air Energy Storage CROSS-REFERENCE TO RELATED APPLICATIONS
This patent application claims priority to U.S. Provisional Application Serial No.
61/036,587, filed in the U.S. Patent and Trademark Office on March 14, 2008, by Timothy F.
Havel, the entire contents of which is being incorporated herein in its entirety.

BACKGROUND
1. Technical Field The present disclosure relates to the field of energy storage. In particular, the present disclosure is directed to an energy storage device that includes a pressure chamber containing a porous material that adsorbs air.

2. Description of the Related Art Compressed air energy storage is commonly known by its acronym "CAES." In some CAES devices, the air compressor is driven by an electric motor, and subsequently used to drive an air motor or turbine connected to an electromagnetic generator, thereby forming the functional equivalent of an electrochemical battery. If the charge-discharge cycle is carried out slowly enough to be approximately isothermal, meaning that the heat generated by compression dissipates without raising the temperature of the air appreciably during compression, and the heat drawn in from the environment likewise keeps the air from cooling appreciably during expansion, this form of electricity storage can have good efficiency.

CAES systems can also be engineered to have higher reliability, lower maintenance and longer operating lifetimes than chemical batteries, and their cost can be comparable to battery-based systems providing that an inexpensive means of storing the compressed air is available.
Unfortunately, the high cost, weight and large size of manufactured pressure vessels in which to store the air, such as steel tanks, prevents CAES devices from competing with batteries in all of their usual applications.

To date CAES has been used for three commercial purposes. The first and most widespread use is not as a means of energy storage per se, but to power pneumatic tools and machines in shops and factories. Pneumatic tools have higher weight-to-power ratios than electrically powered tools, and the small electric motors in such tools also tend to be inefficient compared to the larger motors that drive air compressors. The compressed air is stored in a tank big enough to serve as a buffer and ensure that the pressure in the system stays constant. The overall efficiency of these systems is limited by the fact that they discard the heat of compression and do not reheat the air during its rapid expansion. This inefficiency is limited by using modest pressures, usually less than ten atmospheres, which also reduces the capital costs of such CAES
systems.

The second use of CAES is for temporary backup power to keep essential machinery running in the event of a power failure, for example in computer data centers or hospitals. In such cases floor space is at a premium, necessitating the use of pressures of a hundred or more atmospheres to attain a relatively high energy density, but the cost of the high-pressure steel storage tanks for the compressed air is justified by the high reliability of the system and the high power it can immediately deliver in the event of a power failure. Subsequently a longer-term backup system like a diesel generator can be brought online if need be.
Although the same functionality could be obtained from electrochemical batteries, a battery system that could deliver enough power would also have to store more energy than was needed while waiting for the long-term backup system to come online, making batteries a relatively expensive solution. A CAES
system also requires less maintenance, has a longer lifetime, and does not have the disposal costs associated with environmentally hazardous chemicals. Other such short-term backup power solutions include supercapacitors and flywheels, which are likewise relatively costly.

The third commercial use to which CAES has been put is to lower the cost of generating and/or distributing electric power by utility companies. This can be done in several ways, the most common of which is to enhance central generation capacity. Large central power plants such as coal and nuclear are expensive to stop and start, while smaller plants such as gas-fired turbines are readily turned off and on but are comparatively expensive to operate. Hence, if the energy from large plants can be stored when demand is low and used to produce electricity when demand is high, the need to install and operate small peak-load plants can be reduced, thereby also reducing the average or "levelized" cost of producing electricity.

SUMMARY OF THE INVENTION

In an embodiment of the present disclosure, an energy storage device is presented.
The energy storage device includes a porous material that adsorbs air and a compressor. The compressor converts mechanical energy into pressurized air and heat, and the pressurized air is cooled and adsorbed by the porous material. The energy storage device also includes a tank used to store the pressurized and adsorbed air and a motor. The motor is driven to recover the energy stored as compressed and adsorbed air by allowing the air to desorb and expand while driving the motor.

In another embodiment of the present disclosure, another energy storage device is presented. The energy storage device includes a porous material, where a suitable fluid has been adsorbed. The device also includes a compressor that converts mechanical energy into pressurized air and heat and a barrier. The pressurized air is cooled by allowing the heat to flow through the barrier, the heat is transported to the porous material to which a fluid has been adsorbed, and the heat raises the temperature of the porous material, causing the fluid to desorb from it.

In yet another embodiment, another energy storage device is presented. The energy storage device includes a porous material that adsorbs air and a thermal energy storage system that stores heat. The device further includes a compressor that converts mechanical energy into pressurized air and heat. The pressurized air is cooled and adsorbed by the porous material and the temperature of the porous material and surrounding air is controlled by allowing the heat to flow through a barrier that prevents the pressurized and adsorbed air from escaping. The heat is directed to the thermal energy system and is stored there. Further, the device includes a tank that stores the pressurized and adsorbed air, and the energy it contains is recovered when needed by allowing the air to desorb and expand.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention and the extrapolated experimental data that this embodiment is based upon are illustrated in Figures 1 through 11.

Figure 1 plots adsorption isotherms for the principal constituents of air;

Figure 2 plots the ratio of the number of nitrogen to the number of oxygen molecules versus nitrogen pressure where the ratio of nitrogen to oxygen pressures has a fixed value of 4.0;

Figure 3 is a schematic diagram of mass and energy flow in an adsorption-enhanced compressed air energy storage embodiment, showing these flows during the first half of the charging process;
Figure 4 is a schematic diagram of mass and energy flow in an adsorption-enhanced compressed air energy storage embodiment, showing these flows during the second half of the charging process;

Figure 5 is a schematic diagram of mass and energy flow in an adsorption-enhanced compressed air energy storage embodiment, showing these flows during the first half of the discharging process;

Figure 6 is a schematic diagram of mass and energy flow in an adsorption-enhanced compressed air energy storage embodiment, showing these flows during the second half of the discharging process;

Figure 7 is a process flow diagram which illustrates in greater detail how an adsorption-enhanced compressed air energy storage embodiment operates during the first half of the charging process;
Figure 8 is a process flow diagram which illustrates in greater detail how an adsorption-enhanced compressed air energy storage embodiment operates during the second half of the discharging process.

Figure 9 is a three-dimensional drawing of an array of air adsorption cylinders in a temperature-control chamber;

Figure 10 is a three-dimensional drawing of the adsorption heat pump that is primed and used to upgrade stored heat during the first half of the charging and second half of the discharging processes, respectively; and Figure 11 is a three-dimensional drawing of the mixer-ejector air turbine used to recover the energy stored as compressed air, adsorbed air, and heat during the discharging process.
DETAILED DESCRIPTION OF THE INVENTION

The present disclosure provides two novel uses for the physical process of adsorption in porous materials, both of which greatly improve the economics of compressed air energy storage (CAES). Further the present disclosure provides several improvements to devices that store energy in the form of compressed air, and that may also store some of the energy in the form of sensible or latent heat.

In order to make the use of CAES for central generation capacity cost effective, the compressed air is presently stored in underground geological reservoirs such as natural aquifers or man-made depleted gas or oil wells, rather than in manufactured tanks. The economics is further improved by using the compressed air to turbo-charge a gas-fired turbine, thereby saving the turbine from having to expend energy compressing the air itself. This allows the energy stored in the compressed air to be recovered while at the same time generating additional energy from natural gas. Although the pressures required for turbo-charging are fairly high, of order 50 or so atmospheres, turbocharging allows the stored energy to be delivered at a high power level and recovered with an overall efficiency of about 70%.

A somewhat different approach to using CAES for utility purposes, which has yet to be commercially deployed, is known as "advanced adiabatic CAES." In AA-CAES, the heat extracted from the air during compression is stored and used to reheat the air during expansion as it powers an air motor or turbine. In principle, this allows both the energy stored as heat and stored as compressed air to be recovered, so the efficiency of AA-CAES can approach 100% in principle. In practice, it is difficult to store and recover the heat of compression without significant losses especially at high power levels. In all the proposed embodiments of AA-CAES to date, the air is again to be stored in underground reservoirs at high pressure, and the heat is to be stored in sensible rather than latent form, usually at temperatures well above 200 C.

Energy storage has the potential to reduce the operating costs of electric utilities in several other ways as well, although none have yet come into widespread use.
These include transmission capacity deferral and congestion reduction, various ancillary services, bulk electricity price arbitrage, and load shifting or leveling at the end-user level. In the future, however, the most important such service is likely to be renewable capacity firming.
Renewable energy sources such as wind and solar tend to be intermittent, so that their capacity varies in time and is often not sufficient to satisfy the demand for electricity. If the energy can be stored at times when capacity exceeds demand and used to produce electricity when demand exceeds capacity, these renewable energy sources will become much more cost effective.

The main drawback of existing CAES systems for central capacity generation is that suitable underground reservoirs are neither common nor transportable. A
modular system that could be assembled anywhere and scaled to the size of the power plant there would, if cost effective, be much more useful for central generation capacity as well as renewable capacity firming. In addition, if it were possible to deliver inexpensive, self-contained CAES systems to well-chosen locations on the grid, nearer to substations or end users, CAES
could provide some or all of the other cost reduction services mentioned above. The main reason that such CAES
systems are not presently cost-effective is, once again, the high cost of manufactured storage tanks for compressed air. It should be noted that, to a first-order approximation, the cost of the tank is independent of the pressure at which the air is stored, since raising the pressure allows the tank to be made smaller but requires its walls to become proportionately thicker, and vice versa.

One approach to making CAES systems more economical, which has not received much attention, is to take advantage of the fact that the compression and expansion of air is a facile means of pumping heat from one place to another. This means that a CAES system could easily be developed to provide combined heat, cooling and power to end users. If such a CAES system were installed in a home or business where time of day electricity pricing is available, for example, it could be charged during the night when the electricity is relatively inexpensive while simultaneously providing heat to the building, and the electricity it produced used or sold back to the grid during peak daytime hours while also providing air conditioning.
During the winter, when the cooling was not needed, a passive solar collector could be used to heat water, and this hot water used to provide heat for the air during expansion, increasing the power output significantly with only a modest increase in cost. The economics of such a system would depend on many factors including the utility tariffs, the prevailing climate, and of course the cost of the air storage tank.

It is well known that the storage of gases and of heat can be accomplished by adsorption in suitable porous materials such as activated carbon, silica gel or zeolites.
Gases are more easily stored in the presence of such a material because the adsorbed phase is much denser than the free gas, thus reducing the volume of the tank required to store a given mass of the gas at a given pressure, or equivalently the pressure required at a given volume.
In addition, it is well known that heat may be stored in latent form using adsorbent materials because the process of desorption consumes heat. The heat may subsequently be regenerated by allowing the adsorbate (e.g. water vapor) to be re-adsorbed by the adsorbent. Additionally, the heat released upon condensation of the desorbed vapor may be stored in sensible form, and recovered by using it to promote the evaporation of the condensate and then allowing the resulting vapor to re-adsorb. Such a device is commonly known as an adsorption refrigerator or heat pump.
Nevertheless there have been no attempts to use the process of adsorption in any of these ways to make CAES systems less expensive, more efficient or transportable, better suited to combined heat-and-power applications, and / or safer to deploy.

The present invention improves upon the economics of compressed air energy storage in four interrelated ways. The first is the use of an adsorbent for air in order to reduce the pressure in and/or volume of the vessel needed to store a given quantity of energy in the form of compressed air. The second is the desorption of water or some other suitable fluid, possibly combined with storage of the low-grade sensible heat released upon condensation of the vapor thereby produced, as a means of storing the heat of compression so as to make AA-CAES more economical. The third is to store the heat generated by adsorption of the air and to recover this energy at a later time by using it, possibly along with the heat of compression, to raise the temperature of the adsorbent material and/or the compressed air as it expands.
The fourth is a new thermodynamic cycle for CAES, in which the temperature of the compressed air is varied so as to keep the pressure of the stored air approximately constant over the charge/discharge cycle.
This "temperature-swing" cycle is especially advantageous when an adsorbent for air is utilized, as just described, and it is also applicable when the heat of compression and/or adsorption is stored for subsequent use, for example by means of an adsorbent for water or some other suitable fluid. The use of a temperature-swing cycle in adsorption-based gas separation processes is well established (see, for example, USPTO Pub. No. 2006/0230930).

It should be noted that energy can be stored by compressing gases other than air, and that a regenerative braking system has been proposed that utilizes adsorbent materials to enhance this process (US Patent No. 7,152,932). This has the advantage that other gases may be more compressible and also more strongly taken up by common adsorbents than is air, allowing energy to be stored more densely than could be done when using air as the working .fluid. The main difference between this kind of system and those under consideration here is that the use of any fluid other than air necessitates a closed system in which the fluid can be recycled and reused. In contrast, air can be taken freely from the environment and released again without environmental consequences. This leads to an open system which is much more economical for large-scale energy storage at the end user, electric substation or power plant level. The present invention considers only the use of adsorbents for air in large-scale, stationary energy storage applications, the desorption of water or some other suitable fluid as a means of storing the heat of compression and / or adsorption of the air, and CAES systems that use a temperature-swing cycle. None of these processes are suitable for small-scale, mobile applications such as regenerative braking.

Although several kinds of porous materials are known that adsorb the nitrogen and oxygen constituents of air to some degree, an adsorption-enhanced CAES
embodiment of the present invention utilizes a zeolite material for this purpose. At modest pressures and ambient temperatures, zeolites adsorb nitrogen more strongly than oxygen, and so have been extensively utilized to separate the oxygen and nitrogen constituents of air for industrial and medical purposes. Nevertheless, there have been few detailed studies of the adsorption of air to zeolites or other porous materials at the relatively high pressures of interest for CAES. In particular, the temperature-pressure boundary at which the air in zeolites liquefies has not been mapped out in any detail. This process, also called capillary condensation, is not normally observed at temperatures well above the critical point of the adsorbate gas, or about -140 C in the case of air. Such a low temperature would be difficult to achieve in a cost-effective adsorption-enhanced CAES device.

If a new porous material were discovered or could be developed in which capillary condensation occurred at much higher temperatures, probably through a process of chemisorption rather than physisorption as in all the presently known adsorbents for air, it could greatly enhance the first new use of porous materials in the present invention. The operation of such a device would be complicated somewhat by the slow kinetics and hysteresis associated with capillary condensation, but the resulting AE-CAES systems would have a much higher energy density, and the quantity of material required would also be greatly reduced with a corresponding reduction in the cost per unit energy stored. Classes of porous materials presently under development that may prove suitable for this purpose include mesoporous organosilicas and metal-organic frameworks.

Thus a new use of adsorption in porous materials provided by the present disclosure is as a means of reducing the volume of the tank needed to store a given mass of air at a given pressure and temperature, or alternatively, of reducing the thickness of the walls of the tank or the strength of the materials of which it is made, by reducing the pressure needed to store a given mass of air in a given volume and at a given temperature. Either of these two alternatives may be achieved by placing a suitable porous material inside the pressure chamber that holds the compressed air, where by suitable we mean that this porous material adsorbs a greater volume of air than the material itself occupies at the temperature and pressure of the compressed air in the chamber. Such porous materials exist by virtue of the fact that, at equilibrium with the temperature and pressure fixed at suitable values, air molecules in an adsorbed state have greatly reduced mobility and a much higher density than those in the gaseous air around them.

Likewise, another new use of adsorption in porous materials is as a means of storing the heat generated by the process of compressing the air, and/or the heat generated by the process of adsorption of the air as in the first new use above. This second new use is achieved by placing a porous material to which water or some other suitable fluid is adsorbed in thermal contact with, but outside of, the air compressor and/or pressure chamber. The porous material of the second new use need not be the same kind of material as that of the first new use.
The heat increases the temperature of this porous material and so promotes desorption of the water or other fluid from it. At the molecular level, this process converts kinetic energy into potential energy, which may then be stored indefinitely by preventing the vapor produced by desorption from coming back into contact with the porous material and being re-adsorbed. This may be described by saying that the heat has been stored in latent form. The transfer of heat from the compressed air to the porous material of the second new use reduces the temperature of the compressed air, thereby also reducing the work needed to further compress it, as well as the size or strength of the tank in which it is stored. Similarly, the cooling of the porous material of the first new use, which is concomitant upon transferring the heat of adsorption from it, increases the amount of air that it adsorbs at any given pressure.

In order to recover the stored latent heat in sensible form, the vapor produced by desorption of the fluid must be available for re-adsorption when needed.
Unfortunately, the large volume occupied by the vapor makes it difficult to store in that form, and compressing or condensing it releases a smaller but still significant amount energy in the form of sensible heat. It is nevertheless possible to store this sensible heat, and to subsequently use the process of expansion of the vapor or evaporation of the liquid to harvest this heat and so regenerate the vapor. The advantage of doing this, instead of storing the heat generated by compression and /
or adsorption of the air directly in sensible form, lies in the fact that in the former case the sensible heat is contained in a material at a lower temperature that can be more easily insulated against losses. While such low-grade heat is normally difficult to harvest, i.e. to convey to where it is needed, the process of expansion or evaporation serves to refrigerate this material and so pump the heat from it much more rapidly and efficiently than could otherwise be done. This could also, in principle, be done directly by using the compressed air as a refrigerant, but it is difficult to both transfer large quantities of low-grade heat from a solid or liquid material into the expanding air and at the same time to capture the mechanical energy generated. It also takes energy to convert low-grade heat to the high-grade heat needed to facilitate the rapid expansion and / or promote desorption of the air.

Regardless of how the vapor needed is obtained, the latent heat may be recovered, along with the energy stored as compressed and / or adsorbed air, in mechanical form by placing the porous material of the second new use in thermal contact with the air motor or turbine and at the same time allowing the water or other fluid vapor to re-adsorb to it. The sensible heat generated as the water or other fluid re-adsorbs is conducted or otherwise transferred to the compressed air as it expands in the air motor or turbine, raising its temperature and pressure so that it does more useful work. At the same time this transfer of heat cools the porous material of the second new use and so further promotes the spontaneous re-adsorption of water or some other suitable fluid to it. Similarly, the transfer of heat from this porous material to the porous material of the first new use promotes the desorption of air from it at the pressure in the chamber, and this compressed air may then be converted back to mechanical energy via the air motor or turbine as just described.

When porous materials are incorporated into a CAES device for either of these two new uses, we shall refer to the resulting process as adsorption-enhanced CAES, or AE-CAES, and to the energy storage device itself as an AE-CAES device or AE-CAES system.

The invention further provides a new use for the industrial process of temperature-swing adsorption, which has been widely employed as a means of separating mixtures of gases. This new use is actually applicable whether or not a porous material has been incorporated into a CAES device for the first new use of the physical process of adsorption. In this process, the temperature of the air and of the porous material to which air is adsorbed, if the device incorporates a porous material for the first new use, is lowered when charging the CAES device with energy, and raised again when discharging it, all the while pumping air in or allowing air to escape from the pressure chamber at a rate that keeps the pressure of the compressed air therein approximately constant.

A constant air pressure will simplify the construction and operation of any CAES device, but more important for the purposes of the present invention is the fact that the temperature-swing process is a convenient means of increasing the amount of air stored and released by any given quantity of porous material as in the first new use. It does this because the quantity of a gas adsorbed by the vast majority of known porous materials decreases as the temperature thereof is raised, and vice versa. It follows that if the minimum temperature, attained when the AE-CAES device is in its charged state, is low enough to ensure that the porous material is largely saturated by air at the working pressure of the device, while the maximum temperature, attained when the AE-CAES device is in its discharged state, is high enough to ensure that most of the air is desorbed from the material at the working pressure of the device, then one will obtain a greater benefit from the chosen porous material of the first new use than if a pressure-swing cycle had been utilized, at least without the costly and energy consuming expedient of going to subatmospheric pressures. This includes a pressure-swing cycle with either a constant temperature, or with the spontaneous temperature variation of the pressure-swing cycle which reaches its minimum temperature in the discharged state and its maximum in the charged state.

Although this temperature-swing cycle was motivated by the benefit it offers in AE-CAES, it has its advantages over the pressure-swing cycle that has been used by all CAES devices heretofore reduced to practice. To show this, we will now estimate the maximum energy density that can be achieved by temperature-swing CAES in the absence of an adsorbent for air. We will further not specify how the heat of compression or the additional heat taken from the compressed air in order to implement the temperature swing is stored, but for the sake of simplicity assume that it is perfectly stored and fully recovered with only a negligible addition to the overall volume of the system. Similarly we will neglect the volume needed by the compressor, air motor or turbine, and all the other external apparatus, in accord with our intent to estimate the simplest possible upper bound on the energy density. The CAES device may then be characterized by the following parameters:

= Vsys: The volume of air in the system.
= Psys: The pressure at which we maintain the air in the system by controlling its temperature.
= Tats: The temperature of the air in the system when it is in its discharged (minimum energy) state.
= Tom: The temperature of the air in the system when it is in its charged state (Tchg < Td;.)-10 = Pat,,,: The pressure against which the system does work while being discharged (here taken to be 100,000 Pascal, approximately one atmosphere).

We will further let ndis and n,hg be the number of moles of air in the system (i.e. multiples of Avogadro's number of air molecules) in its discharged and charged states, Vatm be the volume of air at pressure Palm needed to charge the system, and VM be the molar volume of air at Patm (=
24.8 liters at Pat,,, = 100,000 bar and 300 K). The ideal gas law and the definition of molar volume then imply that the following relations among these variables hold:

Psys Vsys = ndis R Tdis Psys Vsys = nchg R T.hg Vatm = (nchg - ndis) VM

(where R = 8.314472 J / (mol K) is the ideal gas constant in units of (Joule per mole) per Kelvin).
On using the first two of these equations to eliminate ndis and nchg from the third, we obtain:

Vatm = Psys Vsys (1 / Tchg - 1 / Tdis) VM / R
Finally, the "injection" work done on the system during charging is:
Wsys = (Psys - Patm) Vatm Together, these last two equations show that the energy density is proportional to the difference in the inverse absolute temperatures between the charged and discharged states, and that it grows quadratically with its operating pressure Psys. For the specific case of Psys = 10 x Patm, Tchg = 300 K and Tdis = 500 K, we obtain:

Vatm = 3.975 x Vsys [M3 = cubic meters]
Wsys = 3.578 x Vsys [MJ = megajoules]

This works out to 994 WHr / M3 (watt-hours per cubic meter); the corresponding figure for Psys =
20 x Patm, Tchg = 300 K and Tdis = 500 K is 4196 WHr / M3, in accord with the approximately quadratic dependence on the operating pressure Psys. The fact that air does not behave exactly as an ideal gas particularly when compressed means, that all these numbers are only approximate, but this quadratic scaling relation will nevertheless hold in practice.

In contrast, the energy density for advanced adiabatic CAES based on the usual pressure-swing cycle at constant temperature with perfect heat recovery is:

Wsys = ((Psys / Patm)T - t - 1) Pam Vat. / (y - 1) (where y = 1.4 is the adiabatic index of air). This yields 1050 WHr / M3 for Psys = 10 x Patm and 3214 WHr / M3 for Psys = 20 x Pam, which shows that at higher pressures the quadratic dependence on working pressure obtained with temperature-swing CAES gives it the potential to surpass pressure-swing CAES in terms of energy density.

For each of the two new uses of the physical process of adsorption given above, a variety of porous materials are available by which useful embodiments of the invention may be constructed. In an AE-CAES embodiment that will now be described in detail, the first new use is implemented by a zeolite known as NaX. This is a widely available Faujasite-type zeolite containing sodium ions, which is commonly sold under the commercial name of 13X.

Dry air is about 78% nitrogen, 21 % oxygen and 1 % argon by mole fraction.
Like most naturally and/or commercially available zeolites, NaX adsorbs nitrogen more strongly than oxygen or argon, i.e. on a molar basis it adsorbs more nitrogen than oxygen or argon when placed under these pure gases at a given pressure and temperature -- at least at the relatively low pressures usually considered for the purpose of purifying oxygen or nitrogen.
Furthermore, oxygen and argon are largely adsorbed at chemically identical sites on the NaX pore walls and also have similar adsorption isotherms, while nitrogen is largely adsorbed at distinct sites which do not overlap with those of oxygen and argon. Because of these facts, we may simplify our analysis by treating the argon fraction of air as if it were oxygen in the following without making any errors large enough to invalidate the principles that an AE-CAES embodiment is intended to exemplify.
Furthermore, the above observations together with experimental data presented by E. A. Ustinov (Russ. J. Chem. 81, 246, 2007) show that we may assume that the amount of nitrogen adsorbed is independent of the amount of oxygen (and argon) adsorbed, and vice versa.

Complete isotherms for nitrogen, oxygen (and argon) adsorption to NaX have been measured at pressures of up to about 4 atmospheres and at four widely separated temperatures between -70 and 50 C (see G. W. Miller, AIChE Symp. Ser. 83, 28, 1987). The values of the parameters in the Sips and Langmuir isotherm equations, as determined by fitting these data, were also given in that paper, and may be used to extrapolate these measurements to higher pressures.

Figure 1 plots adsorption isotherms for the principal constituents of air, namely nitrogen and oxygen, with the commercially available zeolite widely known as NaX or 13X, at four different temperatures and at pressures of up to 20 atmospheres. The isotherms for nitrogen, obtained from the Sips isotherm formula, are plotted with solid lines, while those for oxygen are obtained from the Langmuir isotherm, a special case of the Sips, and are plotted with dashed lines. The plots shown thus extrapolate Miller's data to the higher pressures needed for a cost-effective adsorption-enhanced compressed air energy storage device.

Figure 2 plots the ratio of the number of nitrogen molecules to the number of oxygen molecules adsorbed to NaX against pressure at the same four temperatures as in Figure 1, where the pressure of oxygen at each point on the plot is 25% that of nitrogen and hence approximately equal to the partial pressure of oxygen in air at 125% of the nitrogen pressure.
These ratios are calculated using the extrapolated isotherms shown in Figure 1. The dashed horizontal line shows where this ratio has the value 4.0, so that the ratio adsorbed is approximately equal to the ratio of the partial pressures of nitrogen and oxygen in air. The corresponding pressure at a temperature of -40 C, indicated by the dashed vertical line, is expected to be a reasonably cost-effective nitrogen partial pressure for an embodiment of adsorption-enhanced compressed air energy storage based on a temperature-swing cycle with a minimum temperature of -40 C. This is because going to higher pressures or lower temperatures would increase the amount of air adsorbed at a lower rate than had been achieved at lower pressures and higher temperatures, so that the cost-benefit ratio obtained from the use of the NaX adsorbent would become less favorable.

Figure 3 through 8 show schematic diagrams of the complete AE-CAES (adsorption-enhanced compressed air energy storage) embodiment. These diagrams are graphic versions of the well-known process flow diagrams and the associated symbols for the common mechanical, fluidic and electrical components of chemical and materials processing systems, which are widely used by the engineering community. Process flow diagrams are not intended as blue-prints for a specific design, but rather to allow one skilled in the art of chemical and materials processing to design a system that can reproduce a specific process using such standard components. The diagrams thus provide a suitable means of describing the invention, which provides processes by which CAES systems may be enhanced using adsorption in porous materials, rather than a specific device or design. In those parts of the embodiment in which the components employed are not perfectly standard, more detailed drawings are given, and these have been enlarged in Figures 9 through 11.

Figures 3 through 6 give high-level views of the principal mass and energy fluxes through an exemplary embodiment of an AE-CAES system at four points in its charge -discharge cycle.
Figure 3 shows these fluxes at the beginning of the charging process, when the pressurized NaX
bed 1 is near 100 C and so has the minimum quantity of air adsorbed to it, while the unpressurized NaX bed 41 is largely saturated with water. Figure 4 shows how the fluxes are altered about halfway through the charging process, when the temperature of the pressurized NaX bed 1 has fallen to the prevailing ambient air temperature and the unpressurized NaX bed 41 is has lost most of its water. Figure 5 shows the fluxes at the beginning of the discharging process, when the pressurized NaX bed 1 is at -40 C and so has the maximum amount of air adsorbed to it, while the unpressurized NaX bed 41 is still hot and dry.
Figure 6 shows how these fluxes are altered about halfway through the discharging process, when the temperature of the pressurized NaX bed 1 is approaching the ambient air temperature and water vapor is now being carried into the unpressurized NaX bed 41 to produce the heat needed for complete discharge.

Figure 7 shows a more detailed view of an AE-CAES embodiment in the beginning of the process of being charged with energy (cf. Fig. 3), when the unpressurized NaX
bed 41 of the adsorption heat pump is being heated to drive off the adsorbed water. Figure 8 shows the same embodiment following the halfway point of the discharging process (cf. Fig.
6), when water vapor is being passed through the unpressurized NaX bed 41 to generate the high temperatures needed for full discharge. Figure 9 shows a cutaway enlargement of a compressed air storage module, which contains cylinders 2 packed with zeolite pellets 1, within the condensation /
vaporization chamber 4 used to control the temperature. Figure 10 shows an enlargement of the adsorption heat pump 40 containing the zeolite bed 41 used to store the heat generated by the compression and adsorption of air, including the baffles 42 used to ensure that the atmospheric air, which carries water vapor out of it during charging, roughly reverses the flow of the air, which carries water vapor into it during discharging, for maximum efficiency. Figure 11 shows an enlargement of the mixer / ejector air turbine, including the components labeled 53, 54 and 55, used to efficiently convert both the energy stored as pressure and as heat back into mechanical energy during the discharging process.

The foregoing assumptions, together with extrapolations graphed in Fig. 1, imply that at -40 C and 10 atmospheres the ratio of the quantities of nitrogen to oxygen adsorbed will be about 4 (Fig. 2). Since this is also about the ratio of the partial pressures of nitrogen to oxygen in air and NaX is largely saturated by nitrogen at this temperature and 8 atmospheres, the amount of air adsorbed should not increase greatly at higher pressures or lower temperatures. An AE-CAES embodiment thus utilizes a working pressure of 10 atmospheres and a minimum temperature, obtained when the device is fully charged with energy, of -40 C.

Similarly, the approximations and the extrapolations shown in Fig. 1 imply that at 10 atmospheres and 24 C, about 34.5% of the nitrogen and 74.5% oxygen adsorbed at -40 C has been desorbed, while at 50 C these percentages are 53.5% and 82.5%
respectively. Thus if one goes up to 100 C at 10 atmospheres, at least 75% of nitrogen and essentially all of the oxygen will have been desorped. This in turn implies that at least 80% of the total air that is adsorbed at -40 C will be desorped at 100 C. Because going beyond 100 C would make the device more complicated and expensive, an AE-CAES embodiment utilizes a maximum temperature, attained when the device is fully discharged, of 100 C, which as just argued implies a duty cycle of at least 80% in an AE-CAES embodiment.

Under dry air at -40 C and 10 atmospheres, our approximations and the extrapolated isotherms further indicate that NaX will have adsorbed 4.24 and 1.14 moles of nitrogen and oxygen, respectively, per kilogram of anhydrous crystalline NaX. With a molar volume for ambient air of 24.8 liters and a density for crystalline NaX of 1.53 Kgr / L
(Kgr / L = Kilogram /
Liter), this implies about 204 L of ambient air will be adsorbed per liter of NaX under these conditions. This is about 160 L of air at -40 C and one atmosphere, or 16.0 L
for air at this temperature and 10 atmospheres.

Rather than working with a microcrystalline powder, however, it is necessary to form the NaX into pellets that will allow air to flow readily through the zeolite beds used in the device, by means of a thermally conducting binder that will also enable rapid heat transfer through the beds.
Typically these pellets are about 20% by volume of the binder, and can be packed with a density of about 80% by volume, thus reducing the volume of air adsorbed at the working pressure and minimum temperature to about 0.83 x 16.0 = 10.25 L per liter of NaX pellets.
Taking the 20% void fraction into account, at equilibrium the total quantity of air in a tank packed with a bed of NaX
pellets and filled with air at -40 C and 10 atmospheres will thus be 10.45 times the amount that could be stored in the same tank at the same temperature and pressure.
Together with the 80%
duty cycle conservatively estimated above, this gives us an 8.35 fold reduction in the amount of structural material needed to make a tank that can store and release a given quantity of air at the working pressure and minimum temperature of an AE-CAES embodiment.

The improvement in cost per unit energy stored when NaX is used to adsorb the air, however, is considerably larger. The foregoing calculations show that when fully charged each cubic meter of the NaX pellet bed in an AE-CAES embodiment will store about 133 cubic meters of ambient air. Assuming as we did in our previous analysis of the temperature-swing cycle that we perfectly store and recover the heat while operating the device, but assuming once again an 80% duty cycle, the injection work needed to compress this much air to 10 atmospheres comes out to 94.8 MJ / M3, or 26.6 kilowatt-hours in each cubic meter of the bed. We previously found that 100% efficient temperature-swing CAES operating at 10 atmospheres between temperatures of 27 and 227 C achieved an energy density of 994 watt-hours per cubic meter, and a similar calculation shows that it gets 1201 watt-hours per cubic meter when operated at the same pressure between -40 and 100 C. These numbers in turn are comparable to what would be achieved by 100% efficient isothermal pressure-swing AA-CAES from ambient pressure to 10 atmospheres . The volumetric energy density of the zeolite pellet bed in an AE-CAES
embodiment is in fact about half that of typical lead acid batteries. The efficiency with which this energy can be recovered in practice is discussed in what follows.

The reason that the use of an adsorbent material improves the cost per unit energy stored over three times more than it decreases the cost of the tank per se is because increasing the temperature of the adsorbent during discharge causes a much larger fraction of the air to be released than would be the case if the same quantity of air had simply been stored in a larger tank and its temperature raised by the same amount with no adsorbent present.
Indeed, the duty cycle for a temperature-swing CAES device operated over the same temperature range and at the same pressure, but with no adsorbent present, is only 32.5% instead of the 80% that we conservatively estimated above for an AE-CAES embodiment. Once detailed measurements of the amount of air adsorbed by NaX have been made over the full range of economically viable temperatures and pressures, so that the cost-benefit ratio of the device can be fully optimized, it is likely that the cost per unit energy stored will be considerably further improved.

Before moving on to discuss the rest of an AE-CAES embodiment, we will estimate the heat released by the adsorption of air to the NaX bed, as well as the amount of heat that must be taken from it simply to lower its temperature by 140 C. Miller (loc. cit.) has estimated that the heat of adsorption of nitrogen to NaX over the range of loadings utilized in an embodiment is 18.87 KJ / (mol K), while that of oxygen is about 13.09 KJ / (mol K). It follows that the energy released on adsorbing 4.24 moles of nitrogen and 1.14 moles of oxygen is 94.9 KJ (KJ = Kilo-Joules). Taking into account the reductions due to our use of a packed bed of NaX pellets and assuming an 80% duty cycle as before, this comes out to about 48.6 MJ (Mega-Joules) or 13.5 KWHr / M3 (Kilo-Watt-Hours per cubic Meter). This is about half the amount of energy that could be stored and recovered per cubic meter, and (as we shall see) considerably exceeds the heat generated by compressing the air. Although E. A. Ustinov (loc. cit.) found a slightly lower heat of adsorption oxygen to NaX and also some fall off in that of nitrogen at 10 atmospheres, it is clear that the most of the heat of adsorption must be stored and recovered in any reasonable efficient embodiment of AE-CAES.

The heat of adsorption, however, will be considerably smaller than the sensible heat needed to cool and reheat the NaX bed itself over the 140 C temperature swing.
The specific heat capacity of the bed will vary with the how the pellets are prepared and to some extent with temperature, but is typically of order 1 KJ / (Kgr K), which together with the above assumptions concerning the pellets' packing density implies a volumetric heat capacity of about 1 MJ / (M3 K).
Multiplying this by 140 and converting to kilowatt-hours gives 38.9, which is about 50% larger than the energy to be stored and recovered per cubic meter. Fortunately, as we shall see, the relatively high-grade heat needed to raise the temperature of the NaX bed from ambient to 100 C
is easily recovered, and it is of course not necessary to keep the temperature high once the air has been removed from the pressure chamber and the valve leading into it has been closed.
Similarly, the relatively low-grade heat that must be removed to take the temperature of the bed from ambient down to -40 C does not need to be stored and recovered, since that heat can readily be obtained from the environment while discharging the device. We now turn to the mechanisms used in an AE-CAES embodiment to accomplish all of the above tasks.

Referring now to the schematic diagrams shown in Fig. 7 and 8, we first point out that the parallel dashed lines separated by white space which cut the diagrams in two are meant to indicate that the scale of the device is somewhat arbitrary, and will be determined in practice largely by how it is transported to its site and utilized. Purely for the sake of discussion, however, we will often use one megawatt-hour as amount of energy stored per module in what follows.
This would require about 40 M3 of NaX pellets (horizontal-vertical cross-hatching in the diagrams), which with packaging and the temperature-control apparatus would lead to a compressed air storage system that would fill about two standard 67.5 M3 shipping containers.

As may be seen in the drawing of Fig. 9, the NaX zeolite pellets 1 of an AE-CAES
embodiment are packed into cylinders 2, with a perforated hollow tube 3 extending from a hole at the bottom of each cylinder all the way to the other end of the cylinder. This tube allows the compressed air (right-upwards slanted hatching in the diagrams) to pass rapidly from the vent at the bottom of the cylinder through its entire length when charging the AE-CAES
device, and back out again when discharging it. As a result, the length of the cylinders is not critical, but their diameters should be small enough to allow the rapid diffusion of air from the holes in the tube 3 through the NaX bed 1 to the surface of the cylinder 2, as well as the rapid diffusion of the heat generated as the air is adsorbed.

Primarily because they are mass produced and hence available for a low cost, an AE-CAES embodiment uses cylinders similar to, but longer than, the aluminum cans in which beverages like Coca Cola are commonly packaged. Aluminum is more costly than steel, but is more easily formed into such cylinders, more corrosion resistant and has a higher thermal conductivity, although slightly thicker walls than those of typical aluminum cans will be needed in order to contain ten atmospheres of pressure. As such, the diameter of the cylinders 2 in an embodiment will be 6.0 centimeters, while the perforated tubes 3 down their centers need be no more than 0.5 centimeters in inner diameter and are made from steel in order to provide structural support to the packed cylinders. The distance through which air and heat must diffuse in order to reach the surface of the cylinders is thus only about 2.75 centimeters. Of course neither the exact dimensions of the cylinders, the material of which they are made, nor even a cylindrical form for the pressure vessels that contain the bed of pellets of NaX or other porous material is essential to the invention.

The cylinders 2 in turn are contained in a chamber with thermally insulated walls 4 that can withstand modest pressures and be evacuated over the temperature swing of an AE-CAES
embodiment. This chamber serves to contain a heat transfer fluid, which in turn is used to control the temperature of the compressed air and NaX bed 1 inside the cylinders 2 and so implement the temperature-swing cycle utilized. Neither the geometry of the chamber nor the way in which the cylinders 2 are arranged within it are critical, but for the sake of economy the packing should be as dense as possible while allowing the heat transfer fluid to flow freely around the cylinders.
In Fig. 9 a temperature-control chamber 1.25 M in diameter is shown, which contains 108 cylinders each 1.0 M long and arranged on a square grid with its points 0.1 M
apart, for a total of about 0.25 M3 of NaX bed per chamber. One hundred sixty such chambers would be needed to store a megawatt-hour of energy.

In an AE-CAES embodiment, the fluid that carries heat to and from the chamber with walls 4 is methanol. This is a liquid at ambient pressures and -40 C, the lowest temperature reached over the temperature-swing cycle, while it is a gas at ambient pressures and 100 C, the highest temperature reached. It also has a high heat of vaporization, averaging about 36 kJ /
mole over this temperature range, and its exact boiling point can be set to any value between -40 and 100 C by controlling the pressure in the chamber with walls 4.
Specifically, the boiling point of methanol at a pressure of one atmosphere is 64.7 C, and if we assume that its heat of vaporization does not depend on pressure, we may use the Clausius-Clapyron equation to show that its boiling point will be 100 C at 3.6 atmospheres and -40 C at 231.5 Pascal (about 0.2% of an atmosphere). These modest temperatures and pressures allow the walls 4 of the chamber to be made out of an inexpensive fiberglass composite formed from a heat-resistant phenolic resin or epoxy, which will also provide some of the requisite thermal insulation. Of course other embodiments are possible in which fluids besides methanol are utilized to transfer the heat, and /
or other materials are used for the walls 4 of the chamber.

When charging an AE-CAES embodiment, liquid methanol (heavier left-downwards slanted hatching) is sucked from a hermetically sealed and thermally insulated tank 15 through the control valve 10 and sprayed at a programmed rate from nozzles 8 in the top of the chamber 4 with walls 4, as indicated in Fig. 7. A portion of this methanol vaporizes and exits the chamber through vents 9 interspersed with the nozzles while the remaining liquid methanol, now at its boiling point for the pressure in the chamber, flows down the sides of the cylinders 2 and boils off of them as it does so, thereby cooling them along with the NaX beds 1 which they contain. The additional methanol vapor (lighter left-downwards slanted hatching) generated by this process rises and exits the chamber through the vents 9 as before, while any liquid methanol that makes it to the bottom of the chamber flows into a drain 6 in the bottom and thence back to a small sealed holding tank 7 for reuse.

In contrast, when discharging an AE-CAES embodiment, the valve 10 is closed, another control valve 11 opened, and the methanol in the storage tank 15 is heated by the passage of hot water (heavier diagonal cross-hatching in the diagrams) through a heat exchanger 16 inside the tank. The resulting methanol vapor exits the tank 15 through a vent 14 in its top and flows through a pipe that leads to a network of perforated tubes 5 at the bottom of the chamber with walls 4. The methanol vapor then rises and condenses on the surfaces of the cylinders 2, transferring its heat of vaporization to them at the temperature determined by the prevailing pressure in the chamber. This in turn increases the temperature of the NaX bed 1 towards its desired value, while the condensed liquid methanol again flows out of the chamber through the drain 6 and into the holding tank 7. A simple positive-displacement pump 12 then returns it to the tank 15 via the now-open valve 13 for reuse, as indicated in Fig. 8.

While charging an AE-CAES embodiment, the pressure in the chamber with walls 4 is reduced via a compressor 19 into which the methanol vapor flows from the vents 9 through the valve 18, as indicated in Fig. 7. It exits the compressor 19 at a high pressure and temperature, and flows into a heat exchanger 21 in a thermally insulated tank 20, where it is cooled by a stream of water at ambient pressure to a temperature of about 100 C. The methanol vapor then passes through the pressure-reducing valve 24, which allows it to expand ,further cool and largely condense, and from there back through the open valve 17 to the storage tank 15 for reuse. In this way, the heat generated by adsorption of the air to the NaX bed 1 is transferred to the water or steam (diagonal cross-hatching in the diagrams) passing through the tank 20. Many kinds of compressors could be used for 19, with the exact choice to be determined mainly on economic grounds, in accord with the following technical considerations.

For efficient heat transfer to boiling water, the compressed methanol vapor should have a temperature well above that, say 150 C. With an adiabatic index for methanol of 1.3 it follows that early in the charging process, when the methanol vapor enters the compressor 19 with a pressure of 3.6 atmospheres and a temperature of 100 C, it will only need to increase the pressure by a factor of about 1.7, or to 6.2 atmospheres. Late in the charging process, however, as the pressure and temperature in the chamber with walls 4 fall to 231.5 Pascal and to -40 C, respectively, it would need to increase the methanol vapor pressure by a factor of almost 13.3, resulting in a pressure that is still only 0.03 atmosphere. The Carnot limit on the coefficient of performance of this cooling system is infinite at the beginning when the temperature in the chamber with walls 4 is 100 C, but only 1.66 at the end of the charging process when it has fallen to -40 C. In accord with our earlier discussion of the large quantity of sensible heat that must also be removed from the NaX beds 1 during charging, once the theoretical coefficient of performance falls below about 3, which happens when the NaX bed temperature reaches 7 C, it will no longer be profitable to try to store this heat, nor the smaller amount of heat released by adsorption, in a form that can subsequently be used to generate high temperatures. This issue will be taken up again presently (cf. Figs. 3 and 4).

Before describing where the heat goes next, we first consider the process by which the air is compressed to ten atmospheres when charging an AE-CAES embodiment, and at the same time much of the heat of compression is removed from it. Due to their high efficiency, in the AE-CAES embodiment this is done by two standard centrifugal compressors 26 and 28 in tandem, each of which increases the pressure of the air by a factor of 3.16 (the square root of ten). An air filter and desiccator 25 is used to remove particulate matter and water vapor from the air prior to entering the first compressor 26. Using an adiabatic index for air of 1.4, it may be shown that each compression stage will increase the absolute temperature of the air by a factor of 1.39, or to about 141 C starting from ambient temperatures. With a heat capacity at constant volume for air of 20.77 J / (mol K), the heat of compression over the two stages is thus 54 watt hours per cubic meter of ambient air compressed to ten atmospheres, or 21.6% of the total energy to be stored.

The air is cooled as it exits the each of the two compressors 26 and 28. This is done using the pump 39 to drive a stream of cool water through the countercurrent heat exchangers 27 and 29 in the exits of the compressors 26 and 28, respectively. In this way the heat of compression preheats the water, which in turn is directed through a pipe to the nozzle 22 where, during the first half of the charging process (see Fig. 3), it is boiled by the compressed methanol vapor, as previously described. Later in the charging process, i.e. once the theoretical coefficient of performance of the methanol heat pump has fallen below 3 or so, the compression ratio of the compressor 19 is lowed so that the methanol vapor is raised to at most 100 C.
At the same time the rate of water flow through the air compressors 26 and 28 is increased so that it is not preheated as much, with the net result that now the water is not boiled but instead merely heated and recirculated (as indicated in Fig. 4). The compressed air itself is directed through the open valve 30 to the NaX beds 1, as indicated in Fig. 7. Any residual heat of compression remaining in it will subsequently be removed in the course of cooling the NaX beds 1 and wind up in the steam or water exiting the tank 20 as well. This steam or water thus contains most of the heat of compression and of adsorption of the air, as well as the sensible heat removed from the NaX
beds 1 to cool them.

During the first half of the charging process (Fig. 3), the high-grade heat contained in the steam exiting the tank 20 is used to prime an adsorption heat pump that uses NaX-water as its adsorbent-adsorbate pair. This open adsorption system is modeled after one recently demonstrated by Andreas Hauer in the Federal Republic of Germany, where it was used to reduce the cost of heating buildings by desorbing water from the NaX at night and using the re-adsorption of water vapor to upgrade waste heat during the day when the demand for heating is greater (see section 2 of chapter 25 by A. Hauer, pp. 409-27 in "Thermal Energy Storage for Sustainable Energy Consumption," NATO Sci. Ser. II: Math., Phys. and Chem., vol. 234, H. O.
Paksoy, ed., Springer, 2007). This open adsorption heat pump is simply a thermally insulated tank 40, constructed in an embodiment from a heat-resistant fiberglass composite as before, which is filled with NaX pellets 41 similar, but not necessarily identical in form, to those used to adsorb the air.

Thus an AE-CAES embodiment also utilizes the NaX zeolite for the second new use of adsorption in porous materials of the invention. It should nevertheless be emphasized that a great many other porous materials, such as silica gel, are available that can also be used to pump heat via the adsorption of water, or indeed any other suitable fluid. The water-NaX
adsorbate-adsorbent pair used here is chosen because, like the air-NaX pair, the adsorbate is inexpensive and environmentally benign, while the adsorbent is well understood, not prone to degradation with repeated use (when a suitable binder is used for the pellets;
see G. Storch, G.
Reichenauer, F. Scheffler and A. Hauer, Adsorption 14, 275, 2008), and commercially available.
A further advantage of the water-NaX system lies in the fact that the differential heat of adsorption of water vapor to NaX increases from a value close of that of the heat of vaporation of water, or 44 KJ / mole, to about twice that value as the amount of water adsorbed to the NaX falls from 30 to 0% by weight. This means that in addition to providing a means of upgrading heat to higher temperatures, the NaX bed 41 of the heat pump will also store a significant amount of heat in latent (as well as sensible) form, even after deducting the heat needed to evaporate water during discharge. Because the heat of adsorption of water vapor to NaX is so much larger than the heat of adsorption of air to NaX, the amount of NaX needed for this adsorption heat pump is less than one fourth that which is required to adsorb the air itself.

Once again during the first half of the charging process (Fig. 3), the steam from the tank 20 passes through vents 23 in its top to another compressor 31, which raises the steam's pressure by a factor of 2.8 and, since the adiabatic index of water is also about 1.3, its temperature to about 200 C. It then passes via the open valve 32 to a heat exchanger 36, where the steam is cooled by a countercurrent stream of atmospheric air which is blown over the heat exchanger by the fan 37, heating the air to a temperature of about 150 C in the process. The Carnot limit on the coefficient of performance for this heat pump is 7.5, which should be comparable to the average coefficient of performance of the methanol compressor 19 over the first half of the charging process. It should be noted that the energy needed by the compressors 19 and 31 also winds up as stored heat, and may subsequently be recovered thereby making up for losses elsewhere in the system; the energy needed to run the fan 37 is not significant by comparison.

The hot air from the heat exchanger 36 flows into the thermally insulated tank 40 and through the unpressurized bed of NaX zeolite pellets 41, which initially have about 30% of their weight in water adsorbed to them (see Fig. 10). The hot air raises the temperature of the NaX
pellets 41, causing this water to desorb from them in the form of water vapor and cooling the air in the process. This water vapor is carried by the air through the NaX-pellet-packed container 40 and exits from its other end in the form of moist air at a temperature of about 40 C. The steam used to heat the air entering the NaX bed 41 exits from the heat exchanger 36 through the pressure-reducing valve 38, whereupon it also cools down well below the normal boiling point of water and largely condenses. Because no heat transfer is ever complete, this water still holds a portion of the heat it contained entering the heat exchanger. The energy contained in this sensible heat is stored by returning the water to the surface of the reservoir 43 from which it originated.

Similarly, the warm moist air exiting from the NaX bed 41 passes over a condenser 47 through which water is passed via the action of the pump 44. This water flows from the cool bottom of the reservoir 43 through the condenser 47 and back through the open valve 50 to the warm surface of the reservoir 43. The heat of condensation is thereby likewise transferred to the surface water of the reservoir. The need to use the heat of condensation for efficiency's sake has been stressed by A. Hauer (loc. cit.), and the option to store it in a reservoir has also been claimed in a more recent patent (US 6,820,441). The condensed water itself collects in the basin 49, and may be discarded or added to the reservoir 43 once an AE-CAES
embodiment is fully charged.

In contrast, during the latter half of the charging period (Fig. 4), the fan 37 is turned off and the container 40 sealed so that moisture cannot prematurely re-adsorb to the NaX bed 41 it contains. Instead of steam at 200 C, hot water at well below its boiling point flows directly from the tank 20, where it has picked up heat from the hot compressed methanol vapor, through the now-open valve 35 which by-passes the now-passive compressor 31, and on to the surface of the reservoir 43 without further cooling. In this way the heat generated by the compression and adsorption of the air during the latter half of the charging period, as well as the remaining sensible heat in the NaX bed 1, also winds up in the reservoir 43. How this heat is subsequently recovered will be described below.

Once an AE-CAES embodiment has been fully charged, the majority of the mechanical energy put into it is stored largely in the form of adsorbed air in the NaX
pellet bed 1 within the cylinders 2. As previously noted, about 21.6% of this energy is also stored as heat, primarily in the water reservoir 43. At the same time a quantity of energy equal to about twice the stored energy has been taken out of the NaX bed 1 in the form of heat, the majority of which was sensible heat with a smaller but significant contribution from the heat generated by adsorption of the air. Most of this heat will likewise be stored as sensible heat in the water reservoir 43, although a significant amount will also be stored as both latent and sensible heat in the NaX bed 41 of the adsorption heat pump.

As long as the valves 30 and 56 are kept closed to trap the compressed and adsorbed air, essentially none of the energy stored in this form will be lost prior to discharge. Similarly, as long as the container 40 is kept sealed from moisture, none of the energy stored as latent heat in the NaX bed 41 will leak from it prior to discharge. As shown above, a considerably larger quantity of heat will be stored as sensible heat in the water reservoir 43, but the rate at which this heat leaks from the reservoir will not be large because the temperature difference between the water and the reservoir's environment will not be large (well under 100 C even in cold weather).
Another, less direct, form of loss would be from heat leaking into the chamber with walls 4, raising the temperature of the NaX beds 1 therein and forcing release of some of the compressed air to keep the pressure from rising beyond that which the cylinders 2 are able to withstand. Once again, however, the AE-CAES embodiment strives to keep these temperature differences low by using minimum and maximum temperatures symmetrically placed about 70 C below and above normal ambient temperatures. For such modest temperature gradients, standard low-cost insulation such as polyurethane foam should keep all of the loses due to sensible heat leakage down to an acceptable level over the anticipated storage period of a day or less.

When the time comes to recover the mechanical energy stored in an AE-CAES
embodiment, warm water from the surface of the reservoir is directed through the heat exchanger 16 by closing the valve 50 and opening the valve 51. At the same time the fan 37 is used to blow ambient air through the NaX bed 41 of the adsorption heat pump, where it picks up sensible heat from the bed but not much of the latent heat because it does not contain much moisture to re-adsorb. Some of this heat will be transferred to the water flowing through the heat exchanger 47 at the exit, whence it continues to the heat exchanger 16, but most of the heat will be carried along with the air into the exit chamber 48 at a still elevated temperature.
This warm air is directed via the duct 52 to an air turbine, which includes components 53, 54 and 55, by rearranging the baffling in the exit chamber 48, as indicated schematically in Figs. 7 and 8. It will be used there to keep the expanding compressed air from cooling, as will be described presently.

Meanwhile, the warm water flowing through the heat exchanger 16 boils the methanol in the storage tank 15, which is initially under a pressure of a fraction of an atmosphere. The resulting methanol vapor is then used to heat the cylinders 2 containing the NaX pellet beds 1 to which air is adsorbed, as previously described. This converts the adsorbed air to compressed air at a rate that is controlled by controlling the rate at which methanol vapor enters the chamber with walls 4. This compressed air is also directed as it is generated by desorption through the now-open valve 56 to the air turbine with components 53, 54 and 55, as shown in Fig. 8. The mass and energy fluxes during this first half of the discharging process are illustrated in Fig. 5.

Once about half the stored energy has been recovered and the temperature of the pressurized NaX bed 1 is approaching ambient temperatures, the valve 45 is opened to let warm water from the surface of the reservoir 43 pass through a vaporizer 46, which dispenses it as a mist over the heat exchanger 36. At the same time warm water from the reservoir 43 is driven by the pump 39 through the heat exchanger 36 via the open valve 34, and prevented from getting to the air compressors 26 and 28 by closing valves 32, 33 and 35, so as to keep the evaporating water from cooling the air around it. In this way the air from the fan 37 is saturated with water vapor prior to entering the unpressurized NaX bed 41, and heated by the process of adsorption of the water vapor as it passes through the unpressurized NaX bed. The mass and energy fluxes during this second half of the discharging process are illustrated in Fig. 6.
. Of course the use of a simple vaporizer such as 46 is not essential to the invention, and could easily be replaced by an impeller or ultrasonic humidifier if so desired.

A. Hauer (loc. cit.) has shown that the air will exit the far end of the adsorption heat pump container 40 at a temperature in excess of 100 C. As it does so, a portion of the heat it contains will be transferred via the heat exchanger 47 to the countercurrent stream of warm water from the surface of the reservoir 43, heating it gradually towards 100 C as the discharge process progresses. This will raise the temperature and pressure of the methanol vapor generated in the tank 15 to ever higher levels, thereby heating the NaX beds 1 in the cylinders 2 to 100 C at the end of the discharge process. At the same time the water passing through the heat exchanger 36 has been cooled and is returned to the bottom of the reservoir 43 to be used the next time the device is charged.

The efficiency of the AE-CAES embodiment is also improved by passing the air during discharge through the unpressurized NaX pellet bed 41 in approximately the reverse of the direction in which hot air was passed through it in order to desorb moisture from the unpressurized NaX bed during charging. This increases the efficiency because otherwise some of the sensible heat picked up by the air entering the bed during the first half of the discharge process, or generated by the adsorption of moisture from the air during the second half, will be lost to the cooler and / or less dry NaX bed before it reaches the far end.
This approximate reversal of the flow is accomplished by a system of internal baffles 42, depicted by heavy solid lines in the drawings, which are arranged so that during charging the air enters the near end through the center of the bed but exits the far end around the periphery, and then rearranged during discharging so that the air enters the periphery on the near end but exits through the center on the far end, as indicated schematically in Figs. 7 and 8 (see also Fig. 10). Of course other embodiments are possible in which the far end includes a second fan, enabling the air to take exactly the opposite path back through the NaX bed 41 while the roles of the heat exchangers 36 and 47 are swapped while discharging the device.

Finally, we describe how the warm air entering the exit chamber 48 and passing via the duct 52 is used to heat the expanding compressed air from the NaX bed 1 and thereby recover the heat of compression throughout both halves of the discharging process.
This air turbine, which includes the components labeled 53, 54 and 55 in Figs. 7 and 8, is designed so that the stream of compressed air entering it expands and accelerates through a venturi with twisted vanes running in parallel along its length (see Fig 11). This creates a vortex which generates a vacuum behind it, which in turn draws the warm air from the duct 52 through a larger-in-diameter annulus of static blades 54 slightly up-wind of the blades 53. This second vortex of warm air merges with the vortex of cold expanding air from the blades 53 and is rapidly and thoroughly mixed with it by this process. The principles behind this turbine are in fact similar to those of the mixer-ejector wind turbine designs recently disclosed in USPTO Pub. No.
2008/023957 Al, from which we have drawn inspiration. The now rapidly moving air vortex hits the blades of the air turbine rotor 55 and thereby converts the energy stored in the compressed air and a portion of the energy stored as heat into mechanical form for external use. Of course many other devices are available, such as reciprocating air motors, by which heat and compressed air may be converted into mechanical energy in various alternative embodiments, although these will generally not be as efficient as the mixer-ejector air turbine just described.

Assuming that the AE-CAES embodiment releases one megawatt-hour of energy at a constant rate over a six hour period and that the compressed air is heated back to ambient temperatures in the process, the compressed air must be released at flow rate of about 700 M3 per hour, measured at ambient temperature and pressure. The actual temperature of the compressed air will start out at -40 C and gradually rise to near 100 C over the six hour period, and air at -40 C is 1.6 times more dense than air at 100 C at any given pressure. It follows that the air at ten atmospheres must be released at a rate of 54 M3 per hour at the beginning of discharge period and 86 M3 per hour at the end. Under adiabatic conditions, this air would cool as it expands to -152 C at the beginning and -80 C at the end of the discharge period, which in turn would reduce the flow due to the release of compressed air to 283 and 454 M3 per hour respectively. To return air at those temperatures to ambient temperatures, it must be mixed with about 8.87 and 5.25 times the same mass of air at a temperature of 45 C, the approximate temperature of the air entering the air turbine through the duct 52. The required flow rate of 45 C
air through the duct thus varies from 6628 to 3920 M3 per hour over the six hour discharge period.

Using a 7000 kilogram NaX pellet bed, A. Hauer (loc. cit.) was able to heat an air flow of 6000 M3 per hour to between 120 and 100 C, also over a six hour period, which corresponds to about 120 kilowatts of heat. Because only 21.6% of the energy is stored as heat, it follows that about 0.216 x 1000 / 6 = 36 kilowatts of heat will be needed by the turbine during the assumed 6 hour discharge period for one megawatt hour. Early in the discharge process it will not be necessary to heat the methanol by very much, so the rate of non-humidified air flow through the NaX pellet bed 41 can kept relatively high, and water can be pumped through the heat exchanger 47 at a high speed. The resulting air will enter the duct 52 at a temperature somewhat below the 45 C assumed above, but its flow rate into the turbine will also be greater than the 6628 M3 per hour found above at 45 C. As the discharge progresses, the pump 44 is slowed so that by the end of the discharge period the temperature of the water exiting the heat exchanger 47 approaches that of the air passing over it, or 100 C. At the same time the rate of humidified air flow through the NaX pellet bed 41 is gradually slowed, so that near the end of the discharging process the temperature of the air entering the turbine through the duct 52 will be somewhat larger than 45 C while its flow rate will also be less than the 3920 M3 per hour estimated above at 45 C.

It should be clearly understood that all of the components of the AE-CAES
embodiment have previously been demonstrated in the published engineering literature, albeit for purposes quite different from those for which they are employed here. These components include the water-NaX adsorption heat pump, the NaX zeolite bed that stores compressed air in adsorbed form, and advanced air turbines based on mixer-ejector principles. The only significant additional engineering needed to put these components together into an operational AE-CAES device is to develop the control systems required to make them work together in synchrony, as outlined above. In particular, the pressure in the chamber with walls 4 and the rate at which methanol enters it during charging and discharging must be regulated so that compressed air is converted to and from adsorbed air at the same rate that it is produced by the compressors 26 and 28 or fed to the turbine including the components labeled 53, 54 and 55, respectively, thereby keeping the pressure of the gaseous air in the cylinders 2 approximately constant throughout. This task, although not trivial, is nevertheless a perfectly standard systems integration problem in chemical process engineering that can be accomplished by one skilled in that art.

It will be obvious to those skilled in mechanical and chemical engineering that there are numerous substitutes for the mechanical and fluid components of the AE-CAES
embodiment as well as for the materials it employs, all of which were chosen only the illustrate the advantages to be obtained through the use of adsorbents to facilitate the storage of compressed air and heat, along with the complementary temperature-swing cycle. Because the energy needed to run the pumps and compressors must be subtracted from the energy released in calculating the overall efficiency of an AE-CAES device, it is entirely possible that modest improvements to an embodiment could be attained by such substitutions, although they must still be subject to the Carnot limits given above. It should be noted, in particular, that we have refrained from saying where the motive force that drives the compressors 19, 26, 28 and 31 comes from, or what the mechanical force generated by the air turbine including components 53, 54 and 55 is used for.
Normally compressors are driven by electric motors, but at a coal or nuclear power plant it would be more economical to drive them directly, for example via a hydraulic system, from the steam turbines of the power plant than it would to convert the mechanical energy from the turbines into electricity and then back to mechanical energy in the compressors. The same, of course, is true of an AE-CAES device installed at a wind turbine farm. Similarly, it could under some circumstances be more economical to use the compressed air released while discharging an AE-CAES device to power pneumatic tools or machinery, rather than to generate electricity.

It will further be obvious to those skilled in the art of advanced adiabatic CAES or thermal energy storage more generally that an AE-CAES device, and/or a temperature-swing CAES
device, could also employ a variety of other established chemical processes without materially deviating from the intent of the inventors. For example, the water-NaX heat pump 40 and 41 of an embodiment could be based on other adsorbate-adsorbent pairs, the absorption of a gas in a liquid medium, or even be replaced by a wide variety of solid-liquid phase-change materials, which can also store heat in latent form. It is further possible to supplement or replace the heat storage subsystem entirely by waste heat recovery or thermal energy harvesting in a variety of ways. If, for example, an AE-CAES device were located at a power plant that produces heat as a by-product, such as a coal or nuclear power plant, then this heat could be used to reheat the expanding air and/or the adsorbent for air. Alternatively, a passive solar thermal collector could also readily generate the modest temperatures needed when discharging an AE-CAES device, installed for example at a wind turbine farm. The main point is that the heat utilized by any component of an AE-CAES device during discharge need not have been produced by the inverse process while charging it.

Given a suitable inexpensive source of heat, it would also be possible to use it to regenerate an adsorbent refrigeration system during the storage or discharge period, which could be utilized instead of the vapor-compression refrigeration system of an embodiment to cool the NaX bed while it adsorbed air during the charging period. In cases where such environmental heat sources are not always available at the time they are needed, the heat could be stored when available in either sensible or latent form along with the heat generated while charging the device, and used to make up for any energy loses due to incomplete heat transfer. It should also be possible to reduce the size of the temperature swing needed for a high duty cycle, and hence the amount of heat that must be taken from and returned to the adsorbent for air, by using some combination of a temperature and pressure swing instead of a pure temperature swing as in the above AE-CAES embodiment. The inventor is well aware that all of these variations could significantly improve the economics of building and / or operating an AE-CAES
device in many of its diverse potential applications.

Claims

1. An energy storage device, comprising the following:
a porous material that adsorbs air;

a compressor, wherein the compressor converts mechanical energy into pressurized air and heat, wherein the pressurized air is cooled and adsorbed by the porous material;

a tank used to store the pressurized and adsorbed air;

a motor, driven to recover the energy stored as compressed and adsorbed air by allowing the air to desorb and expand while driving the motor.

2. The energy storage device of claim 1, wherein the pressurized air is cooled and this cooled air is adsorbed by the porous material.

3. The energy storage device of claim 1, wherein the adsorption of the pressurized air by the porous material produces heat, while the temperature of the porous material and surrounding air is controlled so that the pressure remains substantially constant during the adsorption process.

4. The energy storage device of claim 1, wherein when the temperature of the adsorbent for air reaches its minimum value, the amount of energy stored as compressed and adsorbed air in the energy storage device is maximized.

5. The energy storage device of claim 1, wherein when the temperature of the adsorbent for air reaches its maximum value, the amount of energy stored as compressed and adsorbed air in the energy storage device reaches its minimum value.

6. The energy storage device of claim 1, wherein the motor is a turbine.

7. An energy storage device, comprising the following:

a porous material, wherein a suitable fluid has been adsorbed;

a compressor that converts mechanical energy into pressurized air and heat;

a barrier, wherein the pressurized air is cooled by allowing the heat to flow through the barrier; wherein the heat is transported to the porous material to which a fluid has been adsorbed;

wherein the heat raises the temperature of the porous material, causing the fluid to desorb from it;

8. The energy storage device of claim 7, wherein the heat has been stored in latent form when resulting vapor is released.

9. The energy storage device of claim 7, wherein the heat released upon condensation of the vapor is stored in a heat sink, and is recovered by using it to evaporate the suitable fluid.

10. The energy storage device of claim 8, wherein the resulting vapor is brought back in contact with and re-adsorbed by the porous material, the latent heat is returned to sensible form and transported back through the barrier, where it keeps the expanding air from cooling further.

11. The energy storage device of claim 7, wherein the heat is converted into a higher air pressure, and the energy in the pressurized air recovered, by using a motor.

12. The energy storage device of claim 11, wherein the motor is an air turbine based on mixer-ejector principles.

13. An energy storage device, comprising the following:
a porous material that adsorbs air;

a thermal energy storage system that stores heat;

a compressor that converts mechanical energy into pressurized air and heat;
wherein the pressurized air is cooled and adsorbed by the porous material;

wherein the temperature of the porous material and surrounding air is controlled by allowing the heat to flow through a barrier that prevents the pressurized and adsorbed air from escaping;

a thermal energy storage system, wherein the heat is directed to the thermal energy system and is stored there; and a tank that stores the pressurized and adsorbed air, and the energy it contains is recovered when needed by allowing the air to desorb and expand.

14. The energy storage device of claim 13, wherein the heat is stored in sensible form.

15. The energy storage device of claim 13, wherein the heat is stored in latent form.

16. The energy storage device of claim 13, wherein desorption and expansion cool the air and the porous material, causing sensible heat to flow from the thermal energy storage system back through the barrier.