Abstract:
A liquid air energy storage system comprises an air liquefier, a liquid air storage facility for storing the liquefied air, and a power recovery unit coupled to the liquid air storage facility. The air liquefier comprises an air input, an adsorption air purification unit for purifying the input air, and a cold box for liquefying the purified air. The power recovery unit comprise a pump for pressurizing the liquefied air from the liquid air storage facility; an evaporator for transforming the high-pressure liquefied air into high-pressure gaseous air; an expansion turbine capable of being driven by the high-pressure gaseous air; a generator for generating electricity from the expansion turbine; and an exhaust for exhausting low-pressure gaseous air from the expansion turbine. The exhaust is coupled to the adsorption air purification unit such that at least a portion of the low-pressure gaseous air exhausted from the expansion turbine is usable to regenerate the adsorption air purification unit.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to liquid air energy storage systems, adsorption air purification devices, and methods for their use. In particular, the present invention relates to the integration of air purification devices into liquid air energy storage systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    Air Purification Units (APUs) are used to extract contaminants and undesirable compounds (henceforth “contaminants”) from an air stream such that a “purified” air stream is produced for a process. These typically operate using an adsorption process whereby the contaminants, which may be gases, water molecules, hydrocarbon particles or any other undesired species, are adsorbed onto the surface of an adsorbent material. The adsorbent material is carefully selected to preferentially adsorb the contaminants that the process designer wishes to remove. There are two types of adsorption: physical and chemical. 
         [0003]    APUs are well known in the art of air liquefaction. They are used to produce a clean, dry stream of air to be liquefied—notably avoiding fouling of the process as contaminants freeze, and ensuring a pure liquid air product. Typically, the APU of an air liquefier is designed to remove carbon dioxide, moisture and hydrocarbons. 
         [0004]    An APU typically consists of a vessel containing a particulate bed of adsorbent material through which the process stream flows. Since the adsorbent capacity of an adsorbent material is finite, APUs operate in two principal phases: adsorption and regeneration (otherwise known as desorption). Adsorption is an exothermic process, releasing heat. Desorption requires the addition of heat. 
         [0005]    Two of the main process parameters affecting adsorption are pressure and temperature, which may be manipulated to alter the equilibrium between the fluid and the adsorbent. In a physical adsorption process, adsorption increases at higher pressure and decreases at higher temperature. In a chemical adsorption process, the relationship with temperature is often more complex. For simplicity, the following description concentrates on physical adsorption but the principles of the present invention may equally be applied to chemical adsorption processes. Equally, the following description concentrates on the use of APUs for air liquefaction; however, a person skilled in the art will recognise that the principles of the present invention apply to any similar application. 
         [0006]    During the adsorption process cycle, pressure and temperature are controlled such that the adsorbent material adsorbs during the adsorption phase and desorbs during the regeneration phase. In what is known in the art as a Pressure Swing Adsorption process, pressure is controlled so that it is high during the adsorption phase and low during the regeneration phase. In what is known in the art as a Temperature Swing Adsorption process, temperature is controlled so that it is low during adsorption and high during regeneration. 
         [0007]    In a combined cycle, during the adsorption phase, the pressure of the process air stream is high and the temperature is low so that contaminants are adsorbed onto the surface of the adsorbent material. During the regeneration phase, a lower-pressure, higher temperature regeneration gas stream (which may be air or otherwise) is flowed through the bed. As a result, the equilibrium between the gas stream and the adsorbent material is changed such that contaminants are desorbed from the adsorbent material into the gas stream. The regeneration gas stream is then typically exhausted to atmosphere in order to remove the contaminants from the system. The regeneration phase is usually followed by a cooling phase where the adsorbent bed is cooled, using a cooler stream of gas, to a lower temperature before recommencing the adsorption phase. The lower the temperature of the bed, the more efficient the adsorption. 
         [0008]    Since the adsorption and regeneration phases are both necessary, in order to achieve a continuous flow of purified air to the process, APUs in air separation plants predominantly consist of two vessels, of which one adsorbs while the other is regenerated and then cooled. Once the effective capacity of the adsorbing vessel is reached (saturation), the flow paths are swapped using a system of valves so that the regenerated vessel becomes the adsorbing vessel and the “full” vessel begins regeneration. 
         [0009]    The phenomenon of adsorption exhibits a number of characteristics which the designer must account for when designing an adsorption system. 
         [0010]    As an adsorbent bed adsorbs contaminants from the process stream, a concentration front moves through the vessel. Upstream of this front, the adsorbent material is saturated with contaminant and downstream of this front the adsorbent material is “fresh”. In reality, this front is not a discontinuity but a concentration gradient between saturated adsorbent upstream and fresh adsorbent downstream. The zone occupied by this gradient is often referred to as the “mass transfer zone” as this is where mass is transferred from the fluid to the adsorbent during adsorption and from the adsorbent to the fluid during regeneration. 
         [0011]    The mass transfer zone will traverse the length of the adsorbent bed at a velocity often referred to as the wave velocity. This determines the time required for the mass transfer zone to traverse the adsorbent bed, and therefore the amount of time to complete the adsorption phase or the regeneration phase. 
         [0012]    The length and velocity of the mass transfer zone depends on a number of process parameters, including, for example, the adsorbent used, the size of the adsorbent particles and the velocity of the flow. The shape and velocity of the mass transfer zone generally differ between the adsorption and regeneration phases. Moreover, the shape and velocity of the mass transfer zone may change with time during the cycle. 
         [0013]    During the adsorption process, the flow through the adsorption vessel must be stopped before an unacceptable concentration of contaminants arrives at the outflow, when the leading edge of the mass transfer zone arrives at the end of the vessel. In the region of the mass transfer zone, the adsorbent is not fully saturated and the full capacity of the bed has not been used. While the process designer may be able to control the shape and speed of the mass transfer zone to a certain extent, the mass transfer zone will inevitably occupy a portion of the length of the vessel. The shorter the vessel, the larger the relative portion occupied by the mass transfer zone. It is therefore desirable to design the adsorption vessel with sufficient length such that the area occupied by the mass transfer zone is proportionally small and a minimum of adsorbent remains unsaturated at the end of the adsorption phase. 
         [0014]    This problem is not generally encountered during the regeneration phase since the regeneration stream is exhausted to atmosphere and one is not concerned with the concentration of contaminants. 
         [0015]    A technique for improving the utilisation of the adsorbent bed, which is known in the art of adsorption processes (as described, for example, in Wankat, Phillip C. (1986). Large-Scale Adsorption and Chromatography, Volumes 1-2) but is not disclosed for use in any particular application other than waste water treatment, consists of two columns in series whereby the mass transfer zone may be entirely “pushed” out of one column and into the other so as to fully utilise the bed from the first column. An exemplary implementation of this system in the art comprises three identical columns, of which one regenerates while the remaining two adsorb. The two adsorbing columns are arranged in series in the flow such that the mass transfer zone may overrun from the first column into the second, allowing the first column to be fully saturated. During this time the third column is regenerated. Once the first column is saturated, the third regenerated column is connected in series with the second to capture the mass transfer zone as it exits the second column. Meanwhile the first column is regenerated. By continuing these steps in a cyclical fashion, continuous adsorption may be performed while utilising the full capacity of the adsorbent beds; the mass transfer zone is effectively consistently “pushed” into the newly regenerated vessel.  FIG. 1  illustrates the two phases described above. 
         [0016]    Another key consideration for the designer of a system is pressure drop—a higher pressure drop equates to more wasted energy. The flow of air through a particulate bed of adsorbent experiences a pressure drop that is primarily a function of the size of the adsorbent particles, the length of the bed and the superficial flow velocity. 
         [0017]    Larger particles result in a lower pressure drop but less effective adsorption. Pressure drop may also be reduced by limiting the length of bed or reducing the velocity of the flow through it. 
         [0018]    While the velocity of the flow is important for pressure drop, it is most important to maintain low velocity in order to remain below the fluidisation velocity of the adsorbent particles. The fluidisation velocity is the velocity at which the adsorbent particles begin to move due to the forces exerted on them by the moving fluid. This can cause layers of different adsorbent types to mix and may result in contamination of the wider process with adsorbent leaving the APU. 
         [0019]    In order to maintain low velocity, it is well understood in the art that for a vessel of length L and diameter d containing a given quantity of adsorbent, velocity may be reduced by selecting a smaller length to diameter ratio (L/d). This has the effect of increasing the cross-sectional flow area, resulting in lower flow velocity. Furthermore, the shorter vessel length will contribute to a lower pressure drop. 
         [0020]    However, cost considerations lead the designer to limit the diameter of the vessel. Furthermore, if the vessel diameter is too great, the flow may not be well distributed within it and dead zones may exist around the circumference at the extremities where little or no mass transfer occurs between the fluid and the adsorbent. 
         [0021]    There is also a motivation to maintain sufficient length in the vessel so that the mass transfer zone does not occupy a large proportion of the length of the adsorbent bed. 
         [0022]    While it is desirable for the above reasons to limit flow velocity, it is preferable to keep the flow velocity high enough so that axial dispersion is not a dominant mass transfer mechanism, as axial dispersion tends to reduce the efficiency of the adsorption process by elongating the mass transfer zone. 
         [0023]    It is therefore known in the art that there is a trade-off in the design of the APU with regard to the different requirements of the process and the cost of building the system. 
         [0024]    A further consideration in the design of an APU is the source of the regeneration stream. In state-of-the-art air liquefiers, the regeneration gas stream is primarily sourced from the clean input air stream, a portion of which is diverted, expanded to a lower pressure, heated and used to regenerate the regenerating vessel. 
         [0025]      FIG. 2  shows a simplified example of a state-of-the-art regeneration scheme, wherein a stream of feed air from ambient is drawn into compressor  100  where it is compressed. The air stream flows through adsorption vessel  111  where contaminants are removed by adsorption. Now consisting of clean, dry air, the air stream is split into a process air stream and a regeneration air stream. The process air stream is supplied to a cold box  120  which forms part of an air liquefier. The regeneration air stream is let down to lower pressure in valve  201  and flows through heating device  101  where heat is added to raise the temperature to the required regeneration temperature. The required regeneration temperature at the outflow of heating device  101  depends notably on the adsorbent material(s) used and the desired concentration to be achieved. The warmed regeneration air stream is then flowed through the regeneration vessel  112 , where contaminants are desorbed into the regeneration air stream and evacuated with it to atmosphere. Once regeneration vessel  112  is regenerated, heating device  101  is turned off and the now cooler regeneration stream is used to cool the adsorbent in regeneration vessel  112 . The regeneration and cooling nominally last the same time as the adsorption process. 
         [0026]    However, it will be appreciated that the above method requires the feed air compressor  100  to be over-sized in order to provide the extra flow rate required for regeneration, which is ultimately wasted to atmosphere and not liquefied. 
         [0027]    Alternatively, where another gas stream is available, this may be used to regenerate the APU. EP2510294 describes an air separation plant wherein air is liquefied in an air liquefier and separated into its component parts in a cryogenic distillation column. A portion of the resulting pure nitrogen is used to regenerate the APU before being vented to atmosphere while the oxygen component is a final product stream. This method is used only where there is not sufficient demand for the nitrogen product. 
         [0028]    In state of the art APUs, the flow of the regeneration stream is continuously available as long as there is a process stream to be purified. In such cases, the APUs, and the systems in which they are used, operate well. However, several problems have been met when using APUs in cryogenic energy storage systems such as liquid air energy storage (LAES) systems. Such systems are known to provide an effective means of storing energy on a large scale to balance consumer demand for electricity with electricity generating capacity, and to smooth out levels of intermittent supply from, for example, renewable energy sources. 
         [0029]    WO2007/096656 and WO2013/034908 disclose cryogenic energy storage systems which exploit the temperature and phase differential between low temperature liquid air and ambient air, or waste heat, to store energy at periods of low demand and/or excess production, allowing this stored energy to be released later to generate electricity during periods of high demand and/or constrained output. The systems comprise a means for liquefying air during periods of low electricity demand, a means for storing the liquid air produced, and a series of expansion turbines (or a series of stages of an expansion turbine) for expanding the liquid air. The expansion turbine(s) are connected to a generator to generate electricity when required to meet shortfalls between supply and demand. 
         [0030]    An advantage of LAES over other energy storage technologies is that the liquefaction of air may be decoupled from power recovery such that the rates of charge and discharge, and the quantity of energy stored as liquid air are all independent (i.e. the respective stages of operation take place separately; that is singly and usually consecutively, rather than concomitantly). The differing charge and discharge rates are referred to as asymmetric operation and allow, for example, slow charging overnight and rapid discharge of the stored energy over only a few hours of peak electricity demand during in the day. This is known as ‘asymmetric operation’ and charge times may be several times longer than discharge times. 
         [0031]    In a conventional air liquefaction plant, a stream of clean, dry air is required for liquefaction, and an APU must be employed. 
         [0032]    In a state-of-the-art LAES system, during the charging phase when air is being liquefied, a regeneration scheme typical of traditional air liquefaction plants is used, as shown in  FIG. 2 . In such a system, a portion of the process air is diverted via a heating device to the regeneration vessel and then vented to ambient to remove the contaminants from the system. Thus, it is necessary to sacrifice a portion of the input air as no waste gas streams are generally available for regeneration, and this is undesirable. 
         [0033]    One of the key parameters of a commercially viable energy storage system is the round-trip efficiency, which represents the portion of the energy input to the system that is recovered following storage. It is desirable to minimise the energy required to produce liquid air in the liquefier and maximise the energy extracted from the air in the power recovery unit. 
         [0034]    In order to optimise the round-trip efficiency of LAES systems, there is a need to reduce the power required for the regeneration of the APU, and thus avoid sacrificing a portion of the input air. 
         [0035]    Furthermore, there is a need to better adapt an APU to operate efficiently within the constraints imposed by the asymmetric operation of LAES (i.e. without the two-phase continuous adsorption cycles described above, wherein the regeneration stream is available for the same duration as the adsorption phase). 
       SUMMARY OF THE INVENTION 
       [0036]    The present inventors have realised that the energy required for the regeneration of the APU may be reduced by utilising the clean, dry air available at the exhaust of the power recovery unit during the LAES discharge phase to regenerate the adsorbent material in the APU. 
         [0037]    Accordingly, in a first aspect, the present invention provides a liquid air energy storage system, comprising:
       an air liquefier comprising:
           an air input having compression means configured to compress input air and cooling means configured to cool the input air;   an adsorption air purification unit coupled to the air input and configured to purify the compressed and cooled input air from the air input; and   a cold box coupled to the adsorption air purification unit and configured to liquefy the purified air from the adsorption air purification unit;   
           a liquid air storage facility for storing the liquefied air from the air liquefier;   a power recovery unit coupled to the liquid air storage facility comprising:
           a pump configured to pressurize the liquefied air from the liquid air storage facility into a high-pressure liquefied air;   an evaporator configured to transform the high-pressure liquefied air from the pump into high-pressure gaseous air;   an expansion turbine coupled to the evaporator and capable of being driven by the high-pressure gaseous air from the evaporator;   a generator coupled to the expansion turbine and configured to generate electricity from the expansion turbine; and   an exhaust coupled to the expansion turbine for exhausting low-pressure gaseous air from the expansion turbine to an air output; characterised in that:   
           the exhaust is coupled to the adsorption air purification unit such that at least a portion of the low-pressure gaseous air exhausted from the expansion turbine is usable to regenerate the adsorption air purification unit.       
 
         [0050]    By utilising the low-pressure gaseous air exhausted from the expansion turbine to regenerate the adsorption air purification unit, the round-trip efficiency of LAES systems is improved compared with systems that sacrifice a portion of the input air or use an alternative supply. 
         [0051]    The liquid air energy storage system preferably comprises valve means coupled to the adsorption air purification unit and configured to switch between a first state in which it enables the adsorption air purification unit to receive air from the air input and the cold box to receive purified air from the adsorption air purification unit; and a second state in which it enables the adsorption air purification unit to receive low-pressure gaseous air exhausted from the expansion turbine and the adsorption air purification unit to pass the low-pressure gaseous air to the air output. 
         [0052]    Thus, the APU of the LAES system is operable either to purify the input air (i.e. during the charging phase of the LAES) or to regenerate (i.e. during the discharging phase of the LAES) and is switchable between these states. 
         [0053]    Preferably, the system is operable in each of:
       a charging phase, wherein air is received at the air input, purified in the air purification unit, liquefied in the air liquefier and passed to the liquid air storage facility;   a storage phase, wherein liquid air is stored in the liquid air storage facility; and   a discharging phase, wherein liquefied air is passed from the liquid air storage facility to the power recovery unit, transformed into high pressure gaseous air, used to drive the expansion turbine, and exhausted through the exhaust.       
 
         [0057]    In that case, the valve means is in its first state whilst the system is in its charging phase and is in its second state whilst the system is in its discharging phase. 
         [0058]    Preferably, the system is operable in each of the charging phase, storage phase and discharging phase singly and consecutively. In other words, the respective stages of operation take place separately; that is singly and usually consecutively (although this need not necessarily be the case), rather than concomitantly, as with continuous systems. 
         [0059]    Preferably, the system comprises a heating device between the adsorption air purification unit and the exhaust to heat the exhaust stream used for regenerating the APU. 
         [0060]    The heating device may be either an electrical heater or a heat exchanger coupled to a source of heat, preferably waste heat. For instance, the heating device may be a heat exchanger which exchanges heat with another part of the LAES system, or with a co-located process such as a co-located process that generates waste heat. Using a source of waste heat instead of an electric heater has the advantage that the energy cost of regenerating the APU may be reduced. 
         [0061]    In some embodiments, the system comprises a thermal store configured to receive heat from the compression means of the air input and/or the cold box of the air liquefier during the charging phase. This enables the heat generated in those components to be stored and used elsewhere in the system rather than wasted, further improving the efficiency. 
         [0062]    For instance, the thermal store may be configured to transfer heat to the evaporator or to inter-stage reheat heat exchangers in the power recovery unit, and/or to the aforementioned heating device, during the discharging phase. 
         [0063]    In some embodiments, the system comprises a cold store configured to transfer cold energy to the cold box of the liquefier during the charging phase. This enables the cold generated in the power recovery components to be stored and used elsewhere in the system rather than wasted, further improving the efficiency. 
         [0064]    For instance, the cold store may be configured to receive cold energy from the evaporator of the power recovery unit during the discharging phase. 
         [0065]    It will be appreciated that the use of the terms ‘cold’, ‘cold energy’ and ‘cold store’ are analogous to ‘heat’, ‘thermal energy’ and ‘thermal store’ except for temperatures below ambient. This terminology is conventional in cryogenic arts, and is well understood by a skilled person. 
         [0066]    In one embodiment, the exhaust is configured to pass a first portion of the low-pressure gaseous air to the adsorption air purification unit and pass the remaining low-pressure gaseous air to the air output. Thus, the amount of low-pressure gaseous air diverted through the APU may be controlled, and the remainder may be bypassed. This enables the back-pressure experienced at the turbines to be limited to only what is necessary for regenerating the APU while also ensuring that the flow velocities within the APU are acceptable; in particular, below the fluidisation velocity. 
         [0067]    The air output may be vented to atmosphere. Alternatively, the air output may be coupled to the cold store via the evaporator such that the low-pressure gaseous air exhausted from the expansion turbine is cooled and then transfers cold to the cold store, so that it may be later used to further improve efficiency. 
         [0068]    Methods of operating a liquid air energy storage system corresponding to the systems mentioned above are also provided and claimed, and have corresponding advantages. 
         [0069]    It will be appreciated that with the asymmetric operation of an LAES, the exhaust stream from the power recovery turbine often lasts for considerably shorter periods than the flow of input air purified by the APU. There is therefore also a need to provide for faster regeneration of the adsorbent bed without exceeding fluidisation velocity and while minimising the pressure drop across the APU. 
         [0070]    Accordingly, in a second aspect, the present invention provides an adsorption air purification device for use in a liquid air energy storage system, the device comprising:
       first and second fluid openings;   at least two adsorbent vessels connected between the first and second fluid openings;   an arrangement of conduits and valves configured to direct a first stream of air to pass through a combination of one or more of the adsorbent vessels to remove contaminants from the first stream of air, and to direct a second stream of air to pass through the combination of one of more of the adsorbent vessels to regenerate the adsorbent vessels; and   a controller configured to open and close said valves;   wherein the arrangement of conduits and valves is such that the controller may open and close said valves so as to selectively direct:
           a) the first stream of air to pass between the first and second fluid openings through at least a first adsorbent vessel in series; and   b) the second stream of air to pass between the first and second fluid openings through at least the first and a second adsorbent vessel in parallel.   
               
 
         [0078]    By passing one stream through one or more of the vessels in series whilst passing another stream through that vessel (with another) or those vessels in parallel, the time taken for regeneration of the vessels may be reduced whilst maintaining optimum flow characteristics and without exceeding fluidisation velocity. In other words, the APU is more flexible, and can be tailored to accommodate the constraints of a liquid air energy storage system in ways not achievable by systems of the prior art. 
         [0079]    Preferably, the arrangement of conduits and valves is such that the controller may open and close said valves so as to selectively direct:
       a) the first stream of air to pass between the first and second fluid openings through two or more of the adsorbent vessels in series; and   b) the second stream of air to pass between the first and second fluid openings through the said two or more of the adsorbent vessels in parallel.       
 
         [0082]    The APU may comprise at least two adsorbent vessels; or at least three adsorbent vessels; or at least four adsorbent vessels; or at least six adsorbent vessels; or at least eight adsorbent vessels; or at least twelve adsorbent vessels. Irrespective of the number of vessels, the controller may still be configured to open and close said valves so as to selectively direct the first stream of air to pass through at least a first adsorbent vessel in series; and through at least the first and a second adsorbent vessel in parallel. 
         [0083]    The arrangement of conduits and valves may be such that the controller may open and close the valves so as to direct the first stream of air to pass through two or more of the adsorbent vessels in series, or three or more of the adsorbent vessels in series, or four or more of the adsorbent vessels in series, or six or more of the adsorbent vessels in series, or all of the adsorbent vessels in series. Again, irrespective of the number of vessels through which the first stream passes in series, the APU may comprise any number of adsorbent vessels (providing, of course, it is as least as many as the number through which the second stream passes in parallel); and the controller may still be configured to open and close said valves so as to selectively direct the second stream of air to pass through two or more of the adsorbent vessels in parallel. 
         [0084]    In various of the above described arrangements, the arrangement of conduits and valves may be such that the controller may open and close said valves so as to direct the second stream of air to pass through the said three or more of the adsorbent vessels in parallel, or the said four or more of the adsorbent vessels in parallel, or the said six or more of the adsorbent vessels in parallel, or all of the adsorbent vessels in parallel. Again, irrespective of the number of vessels through which the second stream passes in parallel, the APU may comprise any number of adsorbent vessels (providing it is at least two and, of course, as least as many as the number through which the first stream passes in series); and the controller may still be configured to open and close said valves so as to selectively direct the first stream of air to pass through one or more of the adsorbent vessels in series. 
         [0085]    Preferably, the arrangement of conduits and valves is such that the controller may open and close said valves so as to direct the first stream of air to pass between the first and second fluid openings through only a first subset of the at least two adsorbent vessels in series; and subsequently through only a second subset of the at least two adsorbent vessels in series, wherein the first and second subsets have at least one adsorbent vessel in common. By ‘a subset’, it is meant at least one vessel but not all vessels. For instance, if the APU has five vessels, a subset could be any one, two, three or four vessels. In some embodiments, each of the first and second subsets are subsets of at least two vessels. By utilising only a subset of the vessels at a time the length is reduced, thereby minimising the pressure drop compared with utilising all vessels. Nevertheless, by cycling through all of the vessels split into two or more subsets, the total volume of adsorbent required can be kept to a minimum by reducing the length of unused bed. 
         [0086]    For instance, in one embodiment, the at least two adsorbent vessels connected between the first and second fluid openings includes first, second and third adsorbent vessels, and wherein the arrangement of conduits and valves is such that the controller may open and close said valves so as to direct the first stream of air to pass:
       a) between the first and second fluid openings through only the first and second adsorbent vessels in series; and subsequently   b) between the first and second fluid openings through only the second and third adsorbent vessels in series.       
 
         [0089]    In another embodiment, the at least two adsorbent vessels connected between the first and second fluid openings includes a fourth adsorbent vessel, and wherein the arrangement of conduits and valves is such that the controller may, subsequent to step (b), open and close said valves so as to direct the first stream of air to pass between the first and second fluid openings through only the third and fourth adsorbent vessels in series. 
         [0090]    It will be appreciated that any number of vessels could be connected between the first and second fluid openings, and utilised in the cycle in the same manner. 
         [0091]    In one exemplary embodiment, the at least two adsorbent vessels connected between the first and second fluid openings includes first and second adsorbent vessels, wherein the arrangement of conduits and valves is such that the controller may open and close said valves so as to direct the first stream of air to pass:
       a) between the first and second fluid openings through only the first adsorbent vessel; and subsequently   b) between the first and second fluid openings through only the second adsorbent vessel.       
 
         [0094]    In an alternative exemplary embodiment, the at least two adsorbent vessels connected between the first and second fluid openings includes first, second and third adsorbent vessels, wherein the arrangement of conduits and valves is such that the controller may open and close said valves so as to direct the first stream of air to pass:
       a) between the first and second fluid openings through only the first adsorbent vessel; and subsequently   b) between the first and second fluid openings through only the second adsorbent vessel; and subsequently   c) between the first and second fluid openings through only the third adsorbent vessel.       
 
         [0098]    The advantage of this configuration is that it is simpler, requiring fewer valves and less pipework, whilst still providing many of the advantages described above. 
         [0099]    Preferably, the arrangement of conduits and valves is such that the controller may open and close said valves so as to direct the second stream of air to pass between the first and second fluid openings through only a first subset of the at least two adsorbent vessels in parallel; and subsequently through only a second subset of the at least two adsorbent vessels in parallel. Again, by ‘a subset’, it is meant at least one vessel but not all vessels. For instance, if the APU has six vessels, a subset could be any one, two, three, four or five vessels. By utilising only a subset of the vessels the flow rate of the portion of the exhaust used to regenerate may be minimised while achieving the same velocity within the vessels. Nevertheless, by cycling through all of the vessels split into two or more subsets, the total volume of adsorbent can be maximised. In some embodiments, each of the first and second subsets are subsets of at least two vessels. 
         [0100]    Preferably, the first and second subsets mentioned in the preceding paragraph have no adsorbent vessels in common. In the case where there are six vessels, for example, the first and second subsets could be four and two vessels, or three and three vessels, or three subsets of two vessels respectively. 
         [0101]    The present invention also provides a liquid air energy storage system, comprising:
       an air liquefier comprising an air input having compression means and cooling means, an adsorption air purification device as set out above, and a cold box;   a liquid air storage facility; and   a power recovery unit comprising a pump, an evaporator, an expansion turbine, a generator and an exhaust;   wherein the system is selectively operable in each of:
           a charging phase, wherein air is received at the air input, purified in the air purification unit, liquefied in the air liquefier and passed to the liquid air storage facility;   a storage phase, wherein liquid air is stored in the liquid air storage facility; and   a discharging phase, wherein liquefied air is passed from the liquid air storage facility to the power recovery unit, transformed into high pressure gaseous air, used to drive the expansion turbine, and exhausted through the exhaust;   
           wherein the arrangement of conduits and valves in the adsorption air purification device is arranged such that the first stream of air is provided by the air received at the air input and the second stream of air is provided by at least a portion of the air exhausted through the exhaust; and   wherein the controller is configured to open and close said valves so as to selectively direct:
           a) the air received at the air input to pass between the first and second fluid openings through at least a first adsorbent vessel in series, during the charging phase; and   b) the at least a portion of the air exhausted through the exhaust to pass between the first and second openings through at least the first and a second adsorbent vessel in parallel during the discharging phase.   
               
 
         [0113]    Preferably, the controller is configured to open and close said valves so as to selectively direct:
       a) the air received at the air input to pass between the first and second fluid openings through two or more of the adsorbent vessels in series, during the charging phase; and   b) the at least a portion of the air exhausted through the exhaust to pass between the first and second openings through the said two or more of the adsorbent vessels in parallel during the discharging phase.       
 
         [0116]    LAES systems may operate for different durations at different times; for example, according to the time of the week, month or year. It will be recognised that valves and pipework may be provided such that the configuration of the APU may be altered within the same system, in accordance with the principles laid out hereinabove. For example, in a weekly cycle where the charge phases are shorter on weekdays compared to weekends and conversely discharging phases are longer on weekdays than weekends, the APU may comprise multiple vessels, of which all are saturated by the end of the weekend and which are progressively regenerated over the course of the week days. 
         [0117]    Whilst the optional arrangements and methods mentioned above pertaining to adsorption and desorption of the APU have been set out in the context of a liquid air energy storage system, they are novel arrangements and methods in their own right. Accordingly, protection may be sought for one or more of the novel arrangements and methods for adsorption (i.e. passing a fluid through two or more adsorbent vessels in series) and/or one or more of the novel arrangements and methods for desorption (i.e. passing a fluid through two or more adsorbent vessels in parallel). Whilst the APUs described herein have a particular benefit in LAES systems, nothing about such LAES systems is essential to the principles of operation of the APUs described herein. 
         [0118]    For instance, the present invention also provides an adsorption air purification device comprising:
       first and second fluid openings;   at least four adsorbent vessels connected between the first and second fluid openings;   an arrangement of conduits and valves; and   a controller configured to open and close said valves;   wherein the arrangement of conduits and valves is such that the controller may open and close said valves so as to direct the stream of air to pass between the first and second fluid openings through only a subset of the at least four adsorbent vessels in series.       
 
         [0124]    A corresponding method is also provided. 
         [0125]    The present invention also provides an adsorption air purification device comprising:
       first and second fluid openings;   at least four adsorbent vessels connected between the first and second fluid openings;   an arrangement of conduits and valves; and   a controller configured to open and close said valves;   wherein the arrangement of conduits and valves is such that the controller may open and close said valves so as to direct the stream of air to pass between the first and second fluid openings through only a subset of the at least four adsorbent vessels in parallel.       
 
         [0131]    A corresponding method is also provided. 
         [0132]    In the present description, the term ‘vessel’ is used to mean a single pressure vessel that provides a volume for containing a certain amount of adsorbent, wherein that volume has an input and an output which can be switched into or out of communication with the remainder of the system. As will be appreciated, the volume of any given vessel need not be provided as a single container or reservoir and may instead be provide by a vessel made up of two or more interconnected containers or reservoirs that behave to all intents and purposes as a single larger vessel and share a common input and output for communicating with the rest of the system. In a vessel that is made up of two or more interconnected containers or reservoirs, it will be observed that if there is flow through one of the containers or reservoirs, there is flow through the other(s). 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0133]    Preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
           [0134]      FIG. 1  is a schematic of a state of the art APU; 
           [0135]      FIG. 2  is a schematic of a state-of-the-art regeneration scheme in an exemplary liquefier; 
           [0136]      FIG. 3  is a schematic of a first embodiment of a liquid air energy storage system according to the first aspect of the invention; 
           [0137]      FIG. 4  is a schematic of a second embodiment of a liquid air energy storage system according to the first aspect of the invention; 
           [0138]      FIG. 5  is a schematic of an alternative arrangement of the expansion stages and the APU of  FIG. 4 ; 
           [0139]      FIG. 6  is a schematic of a third embodiment of a liquid air energy storage system according to the first aspect of the invention; 
           [0140]      FIG. 7  is a schematic of a fourth embodiment of a liquid air energy storage system according to the first aspect of the invention; 
           [0141]      FIG. 8  is a schematic of an adsorption process in an APU according to a first embodiment of the second aspect of the invention; 
           [0142]      FIG. 9  is a schematic of a desorption process in the APU of  FIG. 8 ; 
           [0143]      FIG. 10  is a schematic of an adsorption process in an APU according to a second embodiment of the second aspect of the invention; 
           [0144]      FIG. 11  is a schematic of a desorption process in the APU of  FIG. 10 ; 
           [0145]      FIG. 12  is a schematic of an alternative adsorption process in the APU of  FIG. 10 ; 
           [0146]      FIG. 13  is a schematic of an alternative adsorption process in the APU of  FIG. 10 ; and 
           [0147]      FIG. 14  is a schematic of an alternative desorption process in the APU of  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION 
       [0148]    The pressures, temperatures and flow rate used in the following description are intended to illustrate the invention. A person skilled in the art will recognise that a wide range of possible values exist depending on the design of the adsorption process. 
         [0149]    A first embodiment of the first aspect of the invention is shown in  FIG. 3 . According to this embodiment, during the charging phase the APU is fluidly connected to the air liquefier such that contaminants are removed from the input air stream prior to the gas-liquid phase change. During the discharging phase, the APU is fluidly connected to the power recovery unit such that the clean, dry air exhausted from the expansion means flows through the adsorption vessels and desorbs the contaminants (i.e. regenerates the adsorbent material) before being vented to ambient, evacuating the contaminants from the system. Cooling of the bed may be achieved passively or actively between charge and discharge phases or actively as part of the discharge phase. 
         [0150]    As shown in  FIG. 3 , the LAES system comprises a compressor  100 , a cold box  120 , a cryogenic store  130 , a cryogenic pump  140 , an evaporator  150 , expansion turbines  161  to  164  (although any number of turbines may optionally be provided), a heating device  101 , and an APU  110 . Together, the compressor  100 , APU  110  and cold box  120  are elements of an air liquefier, as understood by a skilled person. During the charging phase, the APU  110  is situated in the liquefaction stream (shown by the continuous line), downstream of the compressor  100  and upstream of the cold box  120 . During the discharging phase, the APU is situated in the power recovery stream (shown by the dashed line), downstream of the heating device  101  and upstream of the ambient outflow. Q represents the addition of heat. 
         [0151]    Heating device  101  may comprise an electrical heater or alternatively a heat exchanger for exchanging waste heat from a heat source such as a co-located process (for instance, a power plant), or from within the LAES process (for instance, the heat of compression). The addition of cold and/or heat energy from a co-located process is described in WO2012020234. 
         [0152]    The embodiment shown in  FIG. 3  may additionally comprise a thermal store (not shown). During the charging phase, the thermal store may recover heat from any of the compressors including compressor  100  and the compression devices located in the cold box arrangement  120  of the air liquefier. The recovered heat may be stored in the thermal store. During the discharging phase, heat stored in the thermal store may be recycled to reheat the air stream between expansion stages  161  to  164  and/or the heating device  101  during the discharge phase. 
         [0153]    The embodiment shown in  FIG. 3  may additionally comprise a cold store (not shown). During the discharge phase, the cold store may recover cold from the evaporator  150 . The recovered cold may be stored in the cold store. During the charging, cold stored in the cold store may be recycled to provide cooling to the cold box arrangement  120  of the air liquefier. 
         [0154]    A second embodiment of the first aspect of the invention is shown in  FIG. 4 . The second embodiment is identical to the first, except for the following. In the embodiment of  FIG. 4 , a portion of the exhaust from the expansion device  164  may be diverted through the APU  110  (as described in connection with  FIG. 3 ) while the remainder of the exhaust from the expansion device  164  bypasses the APU  110 . Optionally, the exhaust is vented directly to atmosphere, but in accordance with the second embodiment it can be used in the LAES system. 
         [0155]    As shown in  FIG. 4 , the exhaust from the final expansion stage  164  during the discharge phase is split into a first and a second portion. The first portion is heated in heating device  101  and used to regenerate the APU  110  before being exhausted to ambient. The second portion of the exhaust is directed to evaporator  150  where it is cooled by the evaporating air passing from pump  140  to expansion means  161 . The second portion of the exhaust stream then flows through cold store  121  where it is warmed before being rejected to atmosphere. Cold store  121  stores the cold recovered from evaporator  150  and later supplies it to the cold box arrangement  120  during the charge phase. 
         [0156]    Cooling of the APU may be achieved through passive cooling if there is sufficient time between discharge and charge phases for the adsorbent to return to the required temperature. Alternatively, where there remains liquid air in the cryogenic storage tank, boil-off gas due to heat ingress into said tanks may be used to cool the APU. 
         [0157]    Alternatively, the APU may be actively cooled using the exhaust stream from the power recovery stages, simply by turning off (in the case of an electric heater) or bypassing (in the case of a heat exchanger) heating device  101 . Where cooler temperatures are required, the regeneration stream may be further cooled using a blast cooler. Alternatively, colder temperatures may be achieved at the exhaust stream by performing the last stage of expansion without reheat, or by recovering low-grade cold from the stream exiting the evaporator  150 . 
         [0158]      FIG. 5  shows an alternative arrangement of the expansion stages and the APU wherein, during the discharge phase, following the penultimate expansion stage  163 , the air stream is split into a first and a second portion. At the start of the regeneration phase, both the first and second portions are reheated (Q1 and Q2 respectively) and expanded through expansion devices  165  and  166  respectively. The first portion is exhausted to ambient. The second portion is then further heated in heating device  101  and used to regenerate APU  110 . Following regeneration, the reheat Q2 is removed and heating device  101  is turned off or bypassed. The second portion is expanded directly through expansion stage  166  to arrive at a lower temperature. The now lower temperature second portion is used to cool APU  110  to a lower temperature. Using a lower temperature stream, cooling may be also performed faster. 
         [0159]    The expansion device  166  through which the second portion of the air stream passes may also have a slightly higher discharge pressure than the expansion device  165  through which the first portion of the air stream passes in order to allow a higher pressure drop across the APU  110  during regeneration. Exemplary discharge pressures for the expansion device  165  through which the first portion of the air stream passes are between 1.1 and 1.3 bar abs , preferably 1.2; and exemplary discharge pressures for the expansion device  166  through which the second portion of the air stream passes are between 1.4 and 2.1 bar abs . Preferably the discharge pressure of the expansion device  166  through which the second portion of the air stream passes is between 100 and 1000 mbar higher than the discharge pressure of the expansion device  165  through which the first portion of the air stream passes, preferably between 100 and 500 mbar higher. 
         [0160]    Indeed, the arrangement in  FIG. 5  may even be employed to provide a higher pressure drop across the APU without removing reheat Q2 and/or without turning off or bypassing heating device  101 . Preferably, in such an arrangement, a different means for cooling is provided, such as that described in relation to the embodiment shown in  FIG. 7  below. 
         [0161]    A first preferred operation of the LAES device will now be described with reference to the third embodiment of the first aspect of the invention, as shown in  FIG. 6 . The third embodiment is identical to the first embodiment shown in  FIG. 3 , in combination with the alternative arrangement shown in  FIG. 5 . 
         [0162]    In the present example, for the purposes of illustration liquid air from cryogenic storage device  130  is pumped to 120 bar. A skilled person will recognise that the specific pumping pressure is not essential to the invention but may be selected based on the specific design of any given LAES system. Typical pumping pressure range from 60 to 200 bar and more preferably 100 to 140 bar. Furthermore, the inter-stage reheat temperature is 150 deg C. Equally, the specific reheat temperature is not essential to the invention but may be selected based on the specific design of any given LAES system. The reheat temperature into the final expansion stage has an influence on the temperature of the exhaust stream, which in turn is one of the aspects informing the design choices for the APU according to the present invention; as will be apparent to a skilled person based on the following examples. 
         [0163]    The LAES system shown in  FIG. 6 , operates with a 12-hour charge phase and a 4-hour discharge phase. During the charge phase, air is drawn in from ambient at a flow rate of 10 kg/s, compressed in compression means 100 to 10 bar, cooled to ambient temperature in cooler  102 , and purified in APU  110 . The resulting clean, dry air is sent to the cold box  120  to be liquefied and stored in cryogenic storage device  130 . 
         [0164]    During the discharge phase, liquid air from cryogenic storage device  130  is pumped to 120 bar absolute at a flow rate of 30 kg/s, evaporated in evaporator  150  with the addition of heat (Q) to become clean, dry, high-pressure gaseous air at 120 bar and approximately ambient temperature. 
         [0165]    This air stream is superheated to around 150° C. and expanded in expansion stage  161  to approximately 40 bar, emerging from expansion stage  161  at approximately 60° C. The air stream is reheated to 150° C. and expanded in expansion stage  162  to approximately 10 bar, emerging from expansion stage  162  at approximately 45° C. The air stream is again reheated to 150° C. and then expanded in expansion stage  163  to approximately 5 bar, emerging from expansion stage  163  at approximately 95° C. The air stream is then split into a first and a second portion. 
         [0166]    The first portion is reheated to 150° C. (Q1) and then expanded in expansion stage  165  to approximately 1.2 bar, emerging from expansion stage  165  at approximately 45° C., and then exhausted to ambient. 
         [0167]    At the start of the discharge phase, the second portion is reheated to 150° C. (Q2) and then expanded in expansion stage  166  to approximately 1.2 bar, emerging from expansion stage  166  at approximately 45° C. It is then further heated in heating device  101  to approximately 200° C. and flowed through APU  110  to regenerate the adsorbent material of said APU. 
         [0168]    Once APU  110  has been regenerated, heat source Q2 is removed and heating device  101  is turned off or bypassed. The first portion enters expansion device  166  at approximately 5 bar and 95° C. and is expanded to around 1.2 bar, emerging from expansion device  166  at approximately 1° C. The now cooler second portion flows through the APU and cools the adsorbent bed. 
         [0169]    It will be understood that in a commercial LAES, the charging phase may last for many hours and require a large volume of air to be purified. A sufficient volume of adsorbent material is required to purify the entire quantity of air to be liquefied, as no regeneration occurs during this phase. 
         [0170]    Thus, for a liquefaction process running for many hours, a large bed of adsorbent is required and a larger pressure drop may be expected across the adsorbent bed during both adsorption and regeneration. 
         [0171]    Furthermore, it will be understood that the charge and discharge phases of LAES are asymmetric; for example, the charging phase may last several times as long as the discharge phase or longer. Therefore flexibility is required in the APU to enable shorter regeneration times (to cope with the comparatively short discharge phase) while minimising pressure drop and maintaining the correct flow conditions for adsorption and regeneration. 
         [0172]    A fourth embodiment of the first aspect of the invention is shown in  FIG. 7 . The third embodiment is identical to the second embodiment shown in  FIG. 4 , except for the following. 
         [0173]    In the embodiment in  FIG. 7 , the second portion of the exhaust from the final expansion stage  164  is vented directly to atmosphere rather than being sent to evaporator, as with the embodiment in  FIG. 4 . However, as with the embodiment in  FIG. 4 , the first portion of the exhaust from the final expansion stage  164  is vented separately to atmosphere after leaving the APU. 
         [0174]    In the embodiment in  FIG. 7 , cold from the evaporator  150  is conveyed to the cold store  121  in a separate thermal transfer loop, rather than being exhausted to atmosphere, as with the embodiment in  FIG. 4 . 
         [0175]    In addition, the embodiment in  FIG. 7  further comprises a first valve means 1 and a second valve means 2 upstream and downstream of heating device  101 , respectively; and a heat exchanger  151  with a flow path from the first valve means 1, through the heat exchanger  151  to the second valve means 2. 
         [0176]    Valve means 1 and 2 may be operated so that:
       during a first period of regeneration, the first portion of the exhaust from the final expansion stage  164  passes through heating device  101 , where it is heated, and is then directed to the APU to regenerate the adsorbent; and   during a second period of regeneration, the first portion of the exhaust from the final expansion stage  164  bypasses heating device  101  and passes through heat exchanger  151 , where it is cooled, and then directed to the APU to cool the adsorbent.       
 
         [0179]    Heat exchanger  151  is situated downstream of the evaporator  150 . At the outflow of the evaporator  150 , the air flow to the expansion stages may be moderately cold. For example it may be available at 15 deg C. Thus when the second portion of the exhaust from the final expansion stage  164  passes through heat exchanger  151  during the second period of regeneration, it may therein be cooled to moderately low temperature, for example 20 deg C., which is sufficient for cooling of the APU. The heat in the second portion of the exhaust from expansion stage  164  serves to preheat the air entering into expansion stage  161 , thus recovering residual heat. 
         [0180]    In a variation of the fourth embodiment (not shown), all the exhaust from expansion stage  164  may pass through heat exchanger  151  before being split into the first and second portions, such that a greater quantity of the residual heat available in the exhaust is recycled. 
         [0181]    A second preferred operation of the LAES device will now be described with reference to the fourth embodiment of first aspect of the invention, as shown in  FIG. 7 . 
         [0182]    The LAES system shown in  FIG. 7 , operates with a different charging and discharging regime on different days. On weekdays, the system operates with an 8-hour charge phase and a 4-hour discharge phase. On weekends, the system operates with a 12-hour charge phase and a 2-hour discharge phase. During the charge phase, air is drawn in from ambient at a flow rate of approximately 15 kg/s, compressed in compression means 100 to 10 bar, cooled to ambient temperature in cooler  102 , and purified in APU  110 . The resulting clean, dry air is sent to the cold box  120  to be liquefied and stored in cryogenic storage device  130 . 
         [0183]    During the discharge phase, liquid air from cryogenic storage device  130  is pumped to 120 bar absolute at a flow rate of approximately 30 kg/s, evaporated in evaporator  150  with the addition of heat (Q) to become clean, dry, high-pressure gaseous air at 120 bar and approximately ambient temperature. 
         [0184]    This air stream is expanded in expansion stages  161 ,  162 ,  163  and  164 , and is reheated between each stage to 400 deg C. An exhaust stream emerges from the final stage of expansion  164  at approximately 250 deg C. 
         [0185]    In this embodiment, the process is such that:
       during adsorption, it requires approximately 4 hours for the mass transfer zone to traverse the length of each vessel for the full 15 kg/s flow; and   during regeneration, at a flow rate of 7.5 kg/s through a vessel (25% of the total exhaust mass flow rate), it requires 1 hour for the mass transfer zone to traverse the length of the vessel; and assuming a constant mass flow rate a further hour to cool the adsorbent bed.       
 
         [0188]    The exhaust stream is then split into a first and a second portion. The first portion is exhausted to atmosphere and the second portion is used to regenerate the APU. 
         [0189]    At the start of the discharge phase, the first portion is flowed through APU  110  to regenerate the adsorbent material of said APU. Heating device  101  is not utilised as the temperature of the stream is already sufficient. 
         [0190]    Once APU  110  has been regenerated, valve means 1 and 2 are operated such that the first portion is diverted to heat exchanger  151 , where it is cooled to approximately 20 deg C. The now cooler second portion flows through the APU and cools the adsorbent bed. 
         [0191]    It will be recognised that in the above system, the shorter charging duration on weekdays means that some of the adsorbent of the APU will not be used on weekdays. To illustrate this, and the usefulness of the possibility of adsorbing or regenerating only a subset of the vessels, two different regenerating regimes will now be described based on the second preferred operation of the LAES device with reference to the fourth embodiment of first aspect of the invention, as shown in  FIG. 7 . 
         [0192]    According to a first regime the APU comprises four vessels and flow may be directed through subsets of these vessels in series or parallel, according to the above teachings. During the discharge phase, the second portion of the exhaust stream is substantially fixed at 50% of the total exhaust flow—approximately 15 kg/s. It is therefore possible to regenerate 2 vessels in 2 hours and 4 vessels in 4 hours. 
         [0193]    At the beginning of the weekend, all four vessels are regenerated. At the end of the first 12-hour charging phase, three vessels are saturated and one vessel remains clean. At the end of the first 2-hour discharging phase, one vessel is saturated and three vessels are clean. Following the second 12-hour discharge, all four vessels are saturated. Following the second 2-hour discharge phase, two vessels are saturated and two vessels are clean. 
         [0194]    At the beginning of the weekdays, two vessels are saturated. Following the first 8-hours charging phase, all four vessels are saturated. After the first 4-hour discharge, all four vessels are regenerated. For the remaining weekdays, the APU cycles between two clean vessels at the end of the 8-hour charging phase and four clean vessels at the end of the 4-hour discharging phase. 
         [0195]    According to a second regime the APU comprises three vessels and flow may be directed through subsets of these vessels in series or parallel, according to the above teachings. During the discharge phase, the second portion of the exhaust stream may vary between 25% and 75% of the total exhaust flow—approximately 7.5 kg/s to 22.5 kg/s. It is therefore possible to regenerate up to 3 vessels in 2 hours and 6 vessels in 4 hours. 
         [0196]    At the beginning of the weekend, all three vessels are regenerated. At the end of the first 12-hour charging phase, all three vessels are saturated. During the first 2-hour discharging phase, the second portion of the exhaust comprises 75% of the total exhaust flow. At the end of the first 2-hour discharging phase, all three vessels are clean. Following the second 12-hour discharge, all three vessels are saturated. During the second 2-hour discharging phase, the second portion of the exhaust also comprises 75% of the total exhaust flow. Following the second 2-hour discharge phase, all three vessels are clean. 
         [0197]    At the beginning of the weekdays, all vessels are clean. Following the first 8-hours charging phase, two vessels are saturated. During the first 4-hour discharging phase, the second portion of the exhaust comprises 25% of the total exhaust flow. After the first 4-hour discharge, two vessels are regenerated. For the remaining weekdays, the APU cycles between one clean vessel at the end of the 8-hour charging phase and three clean vessels at the end of the 4-hour discharging phase. 
         [0198]    The advantage of said second regime is that it allows the number of vessels and the quantity of adsorbent to be reduced. The use of variable regeneration flow does however imply more complexity in the system. 
         [0199]    A person skilled in the art will understand how to connect the vessels in parallel subsets in order to achieve the above regimes for regeneration during variable length discharge phases. 
         [0200]    A third preferred operation of the fourth embodiment of the LAES device, as shown in  FIG. 7 , is identical to the second except the air stream is heated to 275 deg C. between stages  161 ,  162 ,  163  and  164 . The exhaust stream is at 140 deg C. In this case, at the start of the discharge phase, heating device  101  is used to increase the temperature of the first portion to 250 deg C. before it is used to regenerate the APU. The heat required for this may be derived from the same source as is used to reheat the air steam between stages  161 ,  162 ,  163  and  164 . 
         [0201]    A first embodiment of the second aspect of the invention is shown in  FIGS. 8 and 9 . In this embodiment, the APU comprises multiple adsorbent vessels connected by a multitude of conduits and valves such that they may be variously switched into parallel and series configurations. More specifically, different arrangements are used for adsorption and regeneration. 
         [0202]    It should be noted that the arrows shown in  FIGS. 8 to 14  are schematic and for understanding the underlying principles of the invention. They do not imply any physical direction through the vessels. 
         [0203]      FIG. 8  shows the adsorption phase of the APU of the first embodiment of the second aspect of the invention. As can be seen, the adsorbent material of the APU is divided into four vessels. Adsorption takes place as follows. At first, the first and second vessels are fluidly connected in series, such that the fluid stream flows from a first fluid opening in the APU (i.e. a fluid input) through the first and second vessels to a second fluid opening in the APU (i.e. a fluid output). At this time, the remaining vessels (i.e. third and fourth) are fluidly unconnected from the fluid stream. 
         [0204]    The mass transfer zone traverses the first and second vessels at a given velocity. When the mass transfer zone has traversed the first vessel and entered the second vessel, the first vessel is disconnected from the process stream and the third vessel is connected in series with the second. Now, the fluid stream flows from the first fluid opening, through the second and third vessels to the second fluid opening. 
         [0205]    When the mass transfer zone has traversed the second vessel and entered the third vessel, the second vessel is disconnected from the process stream and the fourth vessel is connected in series with the third. Now, the fluid stream flows from the first fluid opening, through the third and fourth vessels to the second fluid opening. 
         [0206]    It should be noted that if it takes time T for the mass transfer zone to traverse one vessel, it takes approximately time  4 T to traverse all 4 vessels. 
         [0207]    The vessels which are outside of the mass transfer zone are fluidly unconnected from the process stream, thus reducing the pressure drop across the APU. This scheme allows the pressure drop experienced by the process stream to be reduced even for a large quantity of adsorbent material, while maintaining the required flow conditions within the vessel. 
         [0208]    If the contaminant concentration at the outflow of the APU is important, it is preferable to stop the flow when the mass transfer zone is situated at the extremity of the fourth vessel. This region of the vessel will not be fully saturated. However, the first three vessels will be fully saturated over their entire length. 
         [0209]      FIG. 9  shows the desorption phase of the APU of the first embodiment of the second aspect of the invention. Here, the same arrangement of four vessels is shown, but the four vessels are fluidly connected in a parallel configuration. 
         [0210]    Assuming that the flow velocity within each column is the same (quadruple the total volume flow rate) and the mass transfer zone traverses the vessels at the same velocity, it takes time T to traverse one vessel and the same time T to traverse all four vessels. Thus, all four vessels may be regenerated during the desorption phase in approximately one quarter of the time required to saturate them during in adsorption phase described previously in connection with  FIG. 8 , while maintaining the same the same flow conditions within each vessel. 
         [0211]    It will be understood that the four vessels may be switched from the configuration shown in  FIG. 8  to the configuration shown in  FIG. 9  by an appropriate series of valves at each end of each vessel. The valves may be controlled by a controller, as would be understood by a person skilled in the art. Thus, the controller may control the valves to open and close so as to selectively provide the configurations shown in  FIG. 8  and  FIG. 9 . 
         [0212]    Although this embodiment has been described in the context of a preferred adsorption/desorption processes, it will be understood that the process described with reference to  FIG. 8  (i.e. the series configuration) may be used for either adsorption or desorption where a minimal pressure drop is required, whilst the process described with reference to  FIG. 9  (i.e. the parallel configuration) may be used for either adsorption or desorption where relatively fast purification or regeneration process is required. 
         [0213]    It will also be appreciated that any number of vessels may be used in the APU. It will also be appreciated that any subset of those vessels may be connected to the process stream in series (for example one or three vessels may be fluidly connected in series rather than two, such that the fluid stream flows from the first fluid opening in the APU through the first, second and third vessels to the second fluid opening in the APU, and so on). It will also be appreciated that any subset of those vessels may be connected to the process stream in parallel. 
         [0214]    A skilled person will thus recognise that the arrangement of multiple vessels in series and parallel configurations according to the above teachings provides a means to adapt the APU to different durations of the adsorption and regeneration phases, as imposed by the charge and discharge phases of a LAES system. This may be achieved while minimising the pressure drop across the APU maintaining correct flow conditions. 
         [0215]    Furthermore, a skilled person will recognise that by switching between configurations such that one configuration is used during charging of the LAES system (adsorption phase of the APU) and another during discharging (regeneration of the APU), it is possible to optimise for the different requirements of each phase. 
         [0216]    A preferred operation of an APU according to the invention in the system shown in  FIG. 6  (i.e. having a 12-hour charge phase and a 4-hour discharge phase) will now be described with reference to the second embodiment of second aspect of the invention, as shown in  FIGS. 9 and 10 . In this embodiment, the APU comprises 3 equal-sized adsorption vessels with the adsorption bed equally distributed between them. In this embodiment, the process is such that:
       the flow velocity for regeneration is approximately the same as the flow velocity for adsorption in order to avoid fluidization of the adsorbent;   at said velocity, during adsorption, it requires approximately 4 hours for the mass transfer zone to traverse the length of each vessel; and   at said velocity, during regeneration it requires 2 hours for the mass transfer zone to traverse the length of each vessel; and assuming a constant mass flow rate a further 2 hours to cool the adsorbent bed.       
 
         [0220]      FIG. 10  shows the APU during a charge phase of an LAES in which the APU is provided (such as the LAES of any one of  FIGS. 3 to 5 , for example), when the APU operates in its absorption phase. During the charge/adsorption phase, compressed ambient air flows at 10 bar, ambient temperature and a flow rate of 10 kg/s from compression device (not shown) through the 3 vessels of the APU  110  arranged in series. The corresponding cross-sectional flow area is s, the mass flow is 10 kg/s, the superficial flow velocity is u and the adsorption time is 12 hours. 
         [0221]    In the above example, for the purposes of illustration the APU is described with an operating pressure of 10 bar during adsorption. The skilled person will recognise that the specific operating pressure essential to the invention but may be selected based on the specific design of any given APU. Typical operating values during the adsorption phase range between 4 and 12 bar and more preferably between 7 and 9 bar. However, in a pressure swing system or combined pressure/temperature swing system, it is highly preferable that the adsorption pressure be higher than the regeneration pressure in order to maximise the differential in adsorbent capacity between the adsorption and regeneration phases. 
         [0222]      FIG. 11  shows the APU during a discharge phase of the LAES, when the APU operates in its desorption phase. During the first 2 hours of the discharge phase, around 10% (3 kg/s) of the exhaust stream forms the first portion. This clean, dry air flows at around 1.2 bar absolute and 200° C. through heating device  101 , where it is heated, and then through the 3 vessels of APU  110  fluidly connected in parallel. The corresponding cross-sectional flow area is 3 s and the mass flow is 1 kg/s per vessel. 
         [0223]    At this lower pressure and higher temperature, the first portion of the exhaust stream is approximately 10 times less dense than the compressed ambient air stream during the charging phase. The corresponding superficial flow velocity is approximately u and the regeneration time is 2 hours per vessel and 2 hours in total. 
         [0224]    During the third and fourth hours of the discharge phase, reheat Q2 is removed and heating device  101  is turned off. The resulting lower temperature stream flows through the 3 vessels of APU  110  fluidly connected in parallel so as to cool the adsorbent material within them. 
         [0225]    A skilled person will recognise that the embodiment described above is particular advantageous over state of the art systems. For instance, if the vessels where arranged in series during regeneration, it would require 6 hours for the mass transfer zone to traverse the three vessels. However, the exhaust stream from the power recovery unit would only be available for 4 of the 6 hours, which would not be enough time. 
         [0226]    Furthermore, if the vessels were arranged in parallel during adsorption, there would be a larger portion of the adsorbent which was not fully saturated due to the existence of concentration gradients at the extremity of each vessel at the end of the adsorption phase. 
         [0227]      FIG. 12  shows an alternative adsorption process which is identical to the embodiments described above and shown in  FIGS. 10 and 11 , except the charge phase operates as follows. At first, the compressed ambient air stream flows at 10 bar and 18° C. at a flow rate of 10 kg/s from cooler  102  through the first two of the three vessels of APU  110  fluidly connected in series. When the first vessel reaches the saturation threshold, it is disconnected from the process stream and the third vessel is connected in series with the second. This charge/adsorption phase is particularly advantageous because length of the bed in the flow at any one time is reduced by a third, and so too is the associated pressure drop. 
         [0228]      FIG. 13  shows an alternative adsorption process which is identical to the embodiments described above and shown in  FIGS. 10 to 12 , except the charge phase operates as follows. At first, the compressed ambient air stream flows at 10 bar and 18° C. at a flow rate of 10 kg/s from cooler  102  through the first of the three vessels of APU  110 . When the first vessel reaches the saturation threshold, it is disconnected from the process stream while the second vessel is connected to the flow. When the second vessel reaches the saturation threshold, it is disconnected from the process stream while the third vessel is connected to the flow. This charge/adsorption phase is particularly advantageous because length of the bed in the flow at any one time is reduced by two thirds, as is the associated pressure drop. Moreover, fewer valves and less pipework may be used. 
         [0229]    In a further alternative arrangement, the system comprises six adsorption vessels of half the length as the embodiments described in  FIGS. 10 to 12 . The time required for the mass transfer zone to traverse the length of one vessel of this arrangement is 1 hour. During the first hour of discharge, the regeneration stream flows through three of the six vessels in parallel. Once these vessels are regenerated, the regeneration stream flows through the remaining three vessels. Finally (and with reference to the LAES of  FIG. 6 ) the cycle is repeated with reheat Q2 removed and heating device  101  turned off or bypassed to provide cooling to the vessels. The advantage of this arrangement is that effective length of the bed is reduced by half during the discharge phase and the pressure drop is reduced by approximately one half. The pressure differential across the expansion stages may be maximised and the work produced may thus be increased. 
         [0230]    A skilled person will appreciate that modifications may be made to the embodiments disclosed herein within the scope of the invention, which is defined by the appended claims.