Apparatus and Method for storing and providing energy

This invention relates to an apparatus and a method for storing and providing energy.

Consumer demand for electrical energy changes from minute to minute with peak demands typically occurring at approximately the same times every weekday.

Major power stations are designed to operate at a particular base output which cannot be changed rapidly.

A number of systems are used in order to satisfy transient demands above the base output. These include high level lakes in mountainous regions. During periods of low demand surplus, and therefor inexpensive, electrical energy is used to pump water up into these high level lakes.

At periods of peak demand the water is released through turbines situated well below the high level lakes.

Whilst this works well in mountainous regions it is not possible to use such systems in flat areas. Also transmission losses become unacceptable as the distance from the electricity generator to the point of use increases.

Another solution includes the use of gas fired steam turbines connected to generators. However, these typically take at least 30 minutes to bring on line.

Other solutions include aircraft engines connected to generators. These can be started and brought up to full operating load within a few minutes. However, they are expensive both in terms of capital and maintenance costs.

The present invention seeks to provide an alternative which, in its preferred embodiments, can be skid mounted and even fit in an ISO container.

The present invention provides an apparatus for storing and providing energy, which apparatus comprises

(i) means for compressing gas;

(ii) means for storing said compressed gas; and

(iii) means for generating energy by the expansion of said compressed gas.

The means for compressing gas could comprise a single pressure vessel. However, it preferably comprises at least two pressure vessels arranged in either series or in parallel. In the case where there are two or more pressure vessels arranged in series the pressure vessels are preferably of progressively decreasing volume although they could be of the same volume if desired.

The pressure vessels are preferably arranged at progressively increasing altitudes although they could also be of the same altitude if desired.

Advantageously, the, at least some of or each pressure vessel is provided with an atomising nozzle so that, in use, liquid can be introduced into said pressure vessel in the form of droplets. As gas is compressed it becomes hot and the surface area of the droplets facilitates heat transfer from the hot gas into the liquid. The apparatus preferably includes at least one electrically driven pump capable of pumping liquid to an elevated pressure to compress said gas. The elevated pressure depends on the overall design of the apparatus but would typically be between 80 and 250 bara.

The means for storing the compressed gas preferably comprises one or more gas storage cylinders. Such gas cylinders are preferably each provided with individual valves and are preferably connected to a common manifold which is provided with a valve to allow compressed gas to be introduced into said cylinders and a valve to allow compressed gas to leave said cylinders. The gas cylinders are preferably arranged in bundles which are arranged in racks. The bundles are individually replaceable to minimise down time in the event that a storage cylinder shows signs of potential failure.

If desired the storage cylinders may be provided with a separate and distinct valve through which any moisture which might collect in the storage cylinder can be drained. The means for generating energy most preferably comprises an

expander/generator set connected to said gas storage cylinder(s) and operable, in use, by expansion of gas from said gas storage cylinder(s) to generate electrical energy. As an alternative the gas from the storage cylinder(s) could be used to drive a screw compressor attached to a generator, a Roots compressor, a lobe compressor or even pressurise liquid to drive a pump having a shaft connected to a generator.

However, the expander/generator arrangement is currently much preferred.

If the gas is compressed in direct contact with liquid (usually water preferably containing corrosion inhibitors) the compressed gas will be moist. When compressed gas is expanded the relative humidity decreases. However, the temperature normally falls significantly. If the temperature is allowed to fall sufficiently water ice and possibly carbon dioxide ice will form and this can result in severe damage to an expander.

To address this problem the compressed gas may be dried, and preferably most/all the carbon dioxide removed prior to storage. This means that the compressed gas can simply be expanded through an expander with minimal risk of ice formation.

This does however involve the use of driers, typically molecular sieve driers, which add cost and volume to the apparatus.

An alternative approach proposed by the applicant is to store the compressed gas with its associated water content and carry out the expansion in such a manner that a damaging quantity of ice does not form during expansion.

To achieve this the temperature of the gas during expansion has to be maintained sufficiently high at each point in the expander so that ice does not form under the prevailing moisture content of the gas and its temperature and pressure.

This approach requires heat, which is a by-product of the compression process.

For this approach the apparatus preferably comprises a tank which, in use, can contain water heated by heat generated during the compression of said gas and which is connected to said means for storing said compressed gas so that said compressed gas can be heated prior to entering the expander of said expander/generator set. It should be explained that in the context for which preferred embodiments of the present invention are intended heat loss from the hot liquid between compression and expansion cycles should not create any problems. However, the apparatus may be thermally insulated if desired. In the event that the temperature of the liquid were to drop excessively then the liquid would either have to be heated externally or the compressed gas vented to atmosphere without passing through the expander.

The expander is preferably a multistage expander and heat exchangers are provided which enable partially expanded gas to be reheated between said stages.

The tank can form at least part of said heat exchanger.

One or more of the pressure vessels in the compression stages can be used to heat the incoming compressed gas and/or reheat gas between compression stages.

Preferably the apparatus includes a number of heat exchangers corresponding to the number of stages in the multistage expander, said heat exchangers each being within said tank, and each connected to the outlet of a respective stage.

The tank can receive heat during compression in the form of hot liquid from one or more of the compression stages.

If desired the tank can be capable of withstanding elevated pressure. This can be particularly advantageous as the boiling point of liquids, for example water increases as pressure increases.

If desired, the means for compressing gas can comprise a multistage compression chain of which the first compression chain comprises a mechanical compressor, for example a screw compressor arranged to compress the gas from typically 1 1 to 41 bara.

The compression of gas in the mechanical compressor will generally significant heat and the apparatus preferably comprises means which, in use, will transfer heat (directly or indirectly) generated by said mechanical compression to liquid in said tank.

The mechanical compressor will typically be oil cooled and, in such a case it is anticipated that the hot oil will be used to indirectly heat the liquid in the tank in a heat exchanger.

A pump may be provided which, in use, pumps water from said tank, through said heat exchanger where it is heated and returns it back to said tank. The present invention also provides a method for storing and providing energy, which method comprises the steps of;

(i) compressing gas;

(ii) storing said compressed gas; and

(iii) generating energy by the expansion of said compressed gas. Preferably said gas is air.

The gas may be compressed using any conventional compression technique, for example a multi stage piston pump. However, with a view to obtaining maximum reliability the gas is preferably compressed by direct contact with liquid.

Preferably the liquid is provided in the form of droplets.

The liquid, which is typically water, can conveniently be supplied by an electrically driven pump and can contain additives to inhibit corrosion.

When gas is compressed it normally becomes quite hot. However, if the surface area of the droplets is adequate heat from the compressed gas will be absorbed into the water. Depending on the volume of liquid available the temperature of the liquid will progressively rise during the compression cycle. The maximum temperature of the liquid will depend on the liquid user. Thus, for example, if the liquid was water held under a suitable pressure the temperature could be as high as 130 to 180 degrees C.

The gas may be compressed in a single stage. However, it is preferably compressed in several stages in a compression train.

In one embodiment the gas is compressed in three stages.

The compression ratio in each stage can be the same or different.

Conveniently, the gas is compressed with a compression ratio of 5: 1 in the first stage and 4: 1 in the second and third stages. This means that gas entering the compression stage at atmospheric pressure (1 bara) leaves at 80 bara. The pressure vessels in each stage could be the same volume. However, each pressure vessel is preferably smaller in volume than the previous vessel to reflect the progressively decreasing volume of the compressed gas.

The pressure vessels are preferably arranged at progressively increasing altitudes to facilitate the transfer of compressed gas from one pressure vessel to the next.

Compressed gas from one pressure vessel is preferably conveyed to the next pressure vessel in a pipe which contains a non-return valve, for example a flapper valve or a reed valve such as made by Hoerbiger. The pressure vessels can conveniently comprise standard cylinders similar to those used for storing industrial gases. If desired such cylinders can be internally coated to inhibit corrosion from moisture.

Depending on the nature of the expansion cycle the compressed gas can be dried before being stored in the cylinders. Carbon dioxide may also be removed during the drying stage. If the compressed air is sufficiently dry and carbon dioxide free then it can be expanded through an expander with minimal risk of damage to the expander from ice.

Removal of carbon dioxide could be achieved in a separate vessel from the drier but can conveniently be removed in the same vessel using a suitable molecular sieve.

Preferably, the method includes the step of expanding the gas stored in the gas storage cylinders through an expander/generator set to generate electricity.

Advantageously, the method includes using heat generated to heat the compressed gas prior to entering the expander of the expander/generator step. This step may be carried out regardless of whether the compressed air has been dried.

Preferably, the heating can take place in a tank. If the compressed air is dry then the heat exchange should be indirect. If the compressed air has not been dried then the heat exchange can be indirect or direct - although in the latter case the tank would have to be designed to withstand the pressure of the compressed gas.

Additionally or alternatively the heating could take place in one or more of the pressure vessels. The expander can be a multistage expander and the heat is used to reheat the gas between at least some (and preferably all) of the stages.

The gas can conveniently be reheated in separate and distinct heat exchangers in the tank.

As mentioned previously the tank can be operated at elevated pressure, for example from 5 to 10 bara.

If desired the steps of initially raising the pressure of the gas, for example from 1 bara to 41 bara, preferably 10 bara to 41 bara, may be carried out using a mechanical compressor, for example a screw compressor.

Conveniently, the heat generated in the mechanical compressor is used to heat the liquid in the tank.

The gas may be compressed in pressure vessels acting in series or in parallel,

In a preferred embodiment the gas is compressed in a compression train in, preferably, five sequential stages the first of which is by said mechanical compressor and the remaining stages, preferably four, of which involve compressing said gas by direct contact with liquid.

Conveniently each of the remaining, preferably four, sequential stages comprises two pressure vessels which, during compression, operate alternately so that one of said two pressure vessels is receiving gas from the first or previous stage while the other is compressing gas to be transferred to the next stage or to storage, In this embodiment the expander preferably has four stages and the gas entering each stage is heated by liquid in a respective one of said pressure vessels in a respective one of said four sequential stages. In another embodiment the gas is compressed in the mechanical compressor and the output, typically at between 1 1 bara and 41 bara, is compressed in several, preferably four stages in parallel. Each stage preferably comprises two pressure vessels working alternately so that whilst one is being charged the other is

pressurising the gas.

The manner of control depends on the requirements of the user. At one extreme the gas could simply be expanded through the expander starting at the pressure at which the gas is stored and continuing until the gas is near exhausted. However, it is envisaged that this type of power supply will rarely be required and a steady supply will be required for a fixed duration. For this purpose the inlet pressure of the gas to the expander is preferably kept constant. This can be achieved by a standard pressure control valve. However, most expander have an inlet section which includes adjustable vanes which can be varied to keep the inlet pressure to the expander substantially constant. Thus, for example, the gas could be stored at 150 bara but the adjustable vanes varied as the pressure of the compressed gas falls to maintain a constant pressure of 150 bara. When the pressure in the storage cylinders falls to 150 bara then the expansion cycle is stopped.

An alternative to this would be to heat the compressed gas to progressively higher temperatures as the pressure in the storage cylinders dropped. However, this would require complex engineering, particularly as the heat available is diminishing as the expansion cycle progresses.

In a preferred embodiment the method includes the step of expanding sufficient gas to generate 1.1 MW of electricity at a substantially constant rate for a period of 1 hour. The gas can be compressed at any convenient time when energy prices are low and be stored in the cylinders until required.

When energy is required, for example to supplement the supply in the National grid the energy contained in the compressed gas in the cylinders needs to be unleashed. Whilst this can most conveniently be achieved by work expanding the compressed gas through an expander connected to a generator as indicated above. However, in an alternative (but not preferred) embodiment the expansion of the gas is used to pressure liquid which is passed through a pump and/or turbine connected to a generator.

Preferably, if a pump is used, the pump and generator are the same pump and motor used to pressurise the liquid during the compression stage.

It will be appreciated that in this embodiment the pump and generator are the only major items requiring significant maintenance. Having said that both components are widely used in the oil industry and rugged and highly reliable units are available.

In the preferred embodiment remotely controlled on/off valves are employed and highly rugged and reliable units are also readily available.

It is envisaged that an apparatus in accordance with the present invention designed for providing 1 .1 MW of electrical energy for 1 hour will fit in an ISO container which means that it can simply be transported on a standard container lorry.

It will be appreciated that the commercial viability of apparatus in accordance with the present invention lies in the difference between the cost of compression using low price energy and the income from selling high price electricity at times of peak demand. A technical problem is to find a source of inexpensive electrical energy.

In recent years there has been considerable investment in renewable energy and wind farms are now commonplace. The problem with wind farms is that they cannot be relied upon to provide a base level of energy as they are dependent on the strength of the wind. As a result there are times when every blade on a windfarm can be seen rotating. However, there are also times when perhaps only 1 in 10 of the blades will be rotating, the remaining 9 blades being feathered and held fast against rotation by brakes because the energy they could generate is not wanted on the national grid. A typical turbine on an onshore windfarm can generate between 1.5 and 6 MW of energy and when the blades are not rotating the energy in the wind is lost. Offshore turbines can even generate up to 10 MW of power. Alternatively there are times when the wind speed is low and or erratic and although the blades will rotate the quality of the power produced is insufficient to meet the requirements of the National Grid.

It is proposed that in times when this energy is not required by the national grid the wind turbines should be utilised and their output used for compressing gas in accordance with the present invention. This utilises otherwise unharnessed wind energy to produce compressed gas at minimal cost. The compressed gas can be used to supplement the output of the wind turbine in times of low wind or to provide independent electrical energy at times of maximum demand. The present invention can also be used in conjunction with solar power panels, more particularly but not exclusively, in those countries where bountiful solar power is available all day but power is required after dusk.

For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by example, to the accompanying drawings in which:

Figure 1 shows a flowsheet of a first embodiment of an apparatus in accordance with the present invention;

Figure 2 shows a simplified flowsheet of a compression train for use in a second embodiment of an apparatus in accordance with the present invention;

Figure 3 is a simplified flow sheet showing a first embodiment of an expansion train using the apparatus used in the compression train shown in Figure 2;

Figure 4 is a simplified flow sheet showing a second embodiment of an expansion train using the apparatus used in the compression train shown in Figure 2 and

Figure 5 is a simplified view of an alternative compression chain.

Referring to Figure 1 of the drawing, there is shown an apparatus for storing and providing energy.

The apparatus, which is generally identified by reference numeral 100, comprises a three stage compression section which comprises a first pressure vessel 102, a second pressure vessel 104 and a third pressure vessel 106.

A pipe 108 connects an air reservoir 1 10 to the first pressure vessel 102 via a filter 1 1 1 and a non-return valve 1 12.

A pipe 1 14 connects the first pressure vessel 102 to the second pressure vessel 104 via a non-return valve 116. The second pressure vessel 104 is at a higher elevation than the first pressure vessel 102 so that the pipe 1 14 extends upwardly. The non-return valve 1 16 is situated immediately adjacent the second pressure vessel 104.

A pipe 118 connects the second pressure vessel 104 to the third pressure vessel

106 via a non-return valve 120. The third pressure vessel 106 is at a higher elevation than the second pressure vessel 104 so that the pipe 1 18 extends upwardly. The nonreturn valve 120 is situated immediately adjacent the third pressure vessel 106.

A pipe 122 connects the third pressure vessel to a bank of gas storage cylinders 124 via a non-return valve 126 and a pair of molecular sieves 128 containing adsorbent designed to remove water and carbon dioxide from gas passing therethrough. Two pumps 130 and 132 are connected to a water reservoir 134 by a pipe 136 and to a header pipe 138 connected to the first pressure vessel 102, second pressure vessel 104 and third pressure vessel 106 by pipe 140, pipe 142 and pipe 144 respectively.

Pipes 140, 142 and 144 are provided with remotely controllable on/off valves 146,

148 and 150 respectively and the free end each pipe 140,142 and 144 is provided with an atomising spray nozzle (not shown).

The bottoms of first pressure vessel 102, second pressure vessel 104 and third pressure vessel 106 are connected to a return line 152 via pipes 154, 156 and 158 respectively.

Pipe 154, 156 and 158 are provided with on/off valves 160, 162 and 164 each of which are remotely controllable.

Pipe 156 is also provided with a pressure regulating valve 166 set marginally below 5 bara.

Pipe 158 is provided with a pressure regulating valve set at marginally below 20 bara.

The return line 152 opens into a reservoir 170 which is open to the atmosphere.

Each gas storage cylinder in the bank of gas storage cylinders 124 is provided with a cylinder valve connected to a common manifold which can be connected to either the pipe 122 or to an expander/generator set 174 via a flow control valve 176.

The expander/generator set 174 has an exhaust outlet which is connected to a duct 178 which leads to a heat exchanger 180 in the reservoir 170.

The heat exchanger 180 has an outlet which is connected to the reservoir 1 10.

The operation of the apparatus 100 will now be described.

COMPRESSION

When the apparatus 100 is first started the gas storage cylinders 124 will be empty.

Pumps 130 and 132, which are driven by electric motors, are started and water from water reservoir 134 is pumped into the header 138 which is maintained at a pressure of about 80 bara. First pressure vessel 102, second pressure vessel 104 and third pressure vessel 106 are filed with water by opening on/off valves 146, 148 and 150 which are then closed. (During this time on/off valve 160 is closed and on/off valves 162 and 164 open.

At the start of the first compression cycle on/off valves 146, 148, 150 and 160 will be closed and on/off valves 162 and 164 open.

On/off valve 160 is opened. Air enters the first pressure vessel 102 as water leaves through pipe 154 and passes through return line 152 to the water reservoir 134.

Once the first pressure vessel 102 is full of air on/off valve 160 is closed.

On/off valve 146 is then opened to allow water at 80 bara to pass through pipe 140 to the atomiser. The water leaves the atomiser in the form of tiny droplets.

As the droplets enter the first pressure vessel 102 the air is progressively compressed. During compression the air becomes warmer. However, much of the heat is transferred to the droplets because of their very large combined surface area. Because of the high specific heat of the water the temperature rise in the compressed air and the water is relatively small with the result that the compression is nearly isothermal in this embodiment taking into account the overall volume of water in the system.. The droplets coalesce at the bottom of the first pressure vessel 102.

The compressed air passes through pipe 1 14 and non-return valve 1 16 into the second pressure vessel 104. Since the pressure regulating valve 166 is set to just under 5 bara the compressed air, which is at 5 bara enters the second pressure vessel and displaces the water through pipe 156 into the return line 152.

This continues until substantially all the compressed air from the first pressure vessel 102 is in the second pressure vessel 104. Pipe 104 is inclined upwardly towards the second pressure vessel 104. The non-return valve 1 16 is also positioned closely adjacent the second pressure for this purpose.

At this time on/off valve 146 is closed and on/off valve 160 is opened to allow the water in the first pressure vessel 102 to drain and be refilled with air ready for compression.

On/off valve 162 is then closed and on/off valve 148 opened to allow water at 80 bara to pass through pipe 142 to the atomiser. The water leaves the atomiser in the form of tiny droplets. As the droplets enter the second pressure vessel 104 the air is progressively compressed. During compression the air becomes warmer. However, much of the heat is transferred to the droplets because of their very large combined surface area. Because of the high specific heat of the water the temperature rise in the compressed air and the water is relatively small with the result that the compression is nearly isothermal. The droplets coalesce at the bottom of the first pressure vessel 104.

The compressed air passes through pipe 1 18 and non-return valve 120 into the third pressure vessel 106. Since the pressure regulating valve 168 is set to just under 20 bara the compressed air, which is at 20 bara enters the third pressure vessel 106 and displaces the water through pipe 158 into the return line 152.

This continues until substantially all the compressed air from the second pressure vessel 104 is in the third pressure vessel 106. Pipe 1 18 is inclined upwardly towards the third pressure vessel 106. The non-return valve 120 is also positioned closely adjacent the third pressure vessel 106 for this purpose.

On/off valve 164 is then closed and on/off valve 150 opened to allow water at 80 bara to pass through pipe 144 to the atomiser. The water leaves the atomiser in the form of tiny droplets.

As the droplets enter the third pressure vessel 106 the air is progressively compressed. During compression the air becomes warmer. However, much of the heat is transferred to the droplets because of their very large combined surface area. Because of the high specific heat of the water the temperature rise in the compressed air and the water is relatively small with the result that the compression is nearly isothermal. The droplets coalesce at the bottom of the third pressure vessel 106.

The compressed air passes through pipe 122, non-return valve 126 and molecular sieves 128 into the gas storage cylinders 124. The molecular sieves 128 remove water vapour and carbon dioxide.

This continues until substantially all the compressed air from the third pressure vessel 106 is in the gas storage cylinders.

During this procedure the air in the first pressure vessel 102 is being compressed and the compressed air is entering the second pressure vessel 104.

The above sequence of operations is continued until the gas cylinders 124 have been charged to the required pressure - typically 70 bara. During this time the water in the water reservoir will have warmed a little above ambient depending on the volume of water in the system.

ENERGY GENERATION

Once the gas storage cylinders are charged to the required pressure the apparatus is ready for action.

When there is a demand for electricity it is simply a matter of opening the flow control valve 176 as required.

The dry and carbon dioxide free compressed air from the gas storage cylinders 124 can be expanded through the expander/generator set 174 without fear of water ice/dry ice formation. The cold expanded air leaving the outlet of the expander/generator set passes through duct 178 into heat exchanger 180 in water reservoir 134. The cold expanded air cools the water in the water reservoir 134 whilst the expanded air returns to the reservoir 1 10 at 5 to 10 degrees Celsius below the temperature of the water in the reservoir. The water can conveniently contain glycol or other additives to reduce its freezing point.

It will be appreciated that electrical energy can be made available extremely rapidly and this is critical in situations where there is an unexpected failure in the National Grid or an unexpected surge in demand. Electricity can be supplied until the pressure in the gas storage cylinders falls to a base level - perhaps 10 bars g at which time the apparatus must be disconnected from the National Grid.

Because of the uncertainties associated with the duration and magnitude of demand by the National Grid and similar uncertainties surrounding the availability of inexpensive energy there can be no perfect design. At best a design can be optimised on a combination of known historic data and future predicted needs. The presently preferred apparatus is capable of generating 1.1 MW of energy for a period of 1 hour once all its gas storage cylinder 124 are fully charged. However, there is no reason why the apparatus could not be designed for higher outputs, for example for 5MW for an hour, or for different durations, for example 30 minutes to 3 hours.

Whilst the embodiment described uses air and water - which are currently considered the best options - it is conceivable that other liquids and gases might also be utilised, it is also envisioned that additives such as anti-corrosive agents might conveniently be added to the water. Whilst droplets of water are required for compression this is not required for simply filling the pressure vessels with water and a separate valved line bypassing the atomiser may be provided if desired.

Whilst it is presently envisaged that the compressed air will be stored at about 70 bara it could be stored at considerably higher pressure if desired, for example up to 1024 bara subject to appropriately designed gas storage cylinders and additional safety considerations.

Referring now to Figure 2 there is shown a compression train for a second embodiment of an apparatus in accordance with the present invention.

The compression train, which is generally identified by reference numeral 200 comprises five compression stages arranged in series which are designed to compress air from atmospheric pressure to 250 bara.

The first stage is generally identified by reference numeral 201 , the second stage by reference numeral 202, the third stage by reference numeral 203, the fourth stage by reference numeral 204 and the fifth stage by reference numeral 205.

The first stage 201 comprises a filter 206 mounted upstream of a mechanical compressor in the form of screw compressor 208 which is connected to a heat exchanger 210.

A pump 212 is connected the heat exchanger 210 by a feed pipe 214 and a return pipe 216 connects the heat exchanger 210 to a tank 218.

In use the screw compressor 208 draws air through the filter 206 which prevents leaves, insects and other solid debris entering the compression train. A three stage filter which progressively removes debris larger than 2 microns is currently preferred.

The screw compressor 208 compresses the air to 11 bara. The hot air leaving the screw compressor is passed through heat exchanger 210 where it is cooled to 100 degrees C by indirect heat exchange with water which is pumped through the heat exchanger 210 by pump 212 via feed pipe 214. The water leaves the heat exchanger at 90 degrees C via return pipe 218 and enters tank 218. The volume of tank 218 is chosen so that the end of the compression stage ie: when the storage cylinders 500 are fully charged to 250 bara the water in the tank 218 is at about 90 degrees C.

The tank 218 is insulated to reduce heat loss. The screw compressor 208 may be oil cooled. In this case the air would leave the screw compressor 208 at about 100 degrees C and the heat in the oil would be transferred to water in tank 218 by a separate and distinct heating circuit which would normally work in parallel with the heat exchanger 210.

The compressed air leaves the first stage 201 via pipe 220 and enters the second stage 202 at 1 1 bara.

The second stage 202 comprises two pressure vessels - a first pressure vessel 222 and a second pressure vessel 224.

The discharge of a pump 226 is connected to the first pressure vessel 222 via a feed pipe 228 which divides into a bottom feed pipe 230 and a spray header feed pipe 232 which is connected to a spray header inside the pressure vessel 222.

A return pipe 234 connects the bottom of the first pressure vessel 222 to a return line 236 which is connected to the inlet of the pump 226.

An inlet pipe 238 connects the pipe 220 to the top of the first pressure vessel 222 and an outlet pipe 240 extends upwardly from the top of the first pressure vessel 222.

In use the first pressure vessel 222 is filled with water. Compressed air at 1 1 bara from pipe 220 passes through a non-return valve 242 and enters the first pressure vessel 222 through inlet pipe 238. A valve 239 in the return pipe 234 is opened and the compressed air displaces water from the first pressure vessel 222 through return pipe 234 to return line 236. A pressure regulator 241 in return pipe 234 only allows the water to flow through return pipe 234 when the pressure is slightly less than 1 1 bara.

When the first pressure vessel 222 is nearly full of compressed air the valve 239 in return pipe 234 is closed.

At this stage a valve 243 in feed pipe 228 is opened to allow water from pump

226, which has an outlet pressure of 250 bara, to flow into the first pressure vessel 222 via both the bottom feed pipe 230 and the spray header 233. If desired additional valves (not shown) may be provided in the bottom feed pipe 230 and in the spray header feed pipe 232 to vary the ratio of the water which enters the first pressure vessel through the bottom feed pipe 230 and the spray header 233 or to direct the entire flow to either the bottom feed pipe 230 or the spray header 233. As the water enters the first pressure vessel 222 the air is compressed and leaves the first pressure vessel at 70 bara through outlet pipe 240 and passes through non-return valve 242 until the first pressure vessel 222 is full of water at which stage the valve 243 in the feed pipe 228 is closed leaving the first pressure vessel 222 full of water, at which stage the cycle can be repeated.

During the compression stage the air becomes hot and heat is transferred into the water.

The compressed air at 70 bara passes into the first pressure vessel 322 of the third stage 203 from which it displaces water through return pipe 334 in which valve 339 is open and pressure regulator 341 is set to open at slightly less than 70 bara.

The plumbing associated with the second pressure vessel 224 mirrors the first pressure vessel 222 . However, in use, the operation of the second pressure vessel 224 is displaced from the operation of the first pressure vessel 22 by 180 degrees. In particular, as the air in the first pressure vessel 222 is being compressed to 70 bara, compressed air at 1 1 bara is being introduced into the second pressure vessel 224.

Similarly, as the air in the second pressure vessel 224 is being compressed to 70 bara, air at 1 1 bara is being introduced into the first pressure vessel 222.

The plumbing associated with the third stage 203, fourth stage 204 and fifth stage 205 mirrors that of the first stage 201.

In use the air is compressed to 130 bara in the third stage 203, 190 bara in the fourth stage 204 and 250 bara in the fifth stage 205.

The pressure regulators associated with the respective outlet pipes are accordingly set to open just below 70 bara, 130 bara and 190 bara whilst the respective pressure vessels are being charged with compressed air from the previous stage.

In this connection the pressure in the storage cylinders 500 will normally range from 250 bara at the start at an expansion cycle and 150 bara at the end of the expansion cycle. It will be appreciated that the pressure regulators associated with the pressure vessels could be adjusted as the pressure in the storage cylinders 500 increases from 150 bara to 250 bara to reduce overall compression energy.

The second stage 202, third stage 203, fourth stage 204 and fifth stage 205 can be operated with identical cycle times. Alternatively, by adjusting the compression in each stage or the relative size of the pressure vessels in the different stages, the stages could be operated with different cycle times. Thus, for example, it would be possible for the third stage to cycle once for every two cycles of the second stage by making the volume of the pressure vessels in the third stage a little larger than those in the second stage.

In any event, at the end of the compression cackle the compressed air in the storage cylinders 500 is at 250 bara.

It will be noted that in this embodiment the compressed air has not been dried and, whilst this is desirable, the space requirements and the cost of molecular sieve driers at this point of the design has been eliminated.

Throughout the compression cycle, which will typically take around three hours and corresponds with relatively low cost energy in the early hours of the morning, the water in the tank and the pressure vessels and the associated plumbing will have been becoming progressively hotter. For the purposes of this embodiment the highest practical temperature is desirable. Bearing in mind that towards the end of the compression phase the lowest pressure at the output of the first stage 201 and in the second stage 202, third stage 203, fourth stage 204 and fifth stage 205 is 10 bara, the water can be heated to well above 100 degrees C without boiling. (The boiling point of water at 5 bara is about 145 degrees C). Whilst it is envisaged that the volume of water in the pressure vessels and the associated plumbing will suffice, a small pressurised storage tank may be provided for emergency use or if a lower temperature is desired.

At the end of the compression stage there will thus be thermal energy in tank 218 (at about 90 degrees C) and in the pressure vessels (at about 120 degrees C).

One of the problems associated with providing emergency energy is that, by definition, there can be absolutely no guarantee as to when it will be needed. In particular, if there is an unexpected power failure somewhere in the national grid power can be required immediately even at times when there is normally low demand. Similarly, periods of normally peak demand can be shifted by a few hours, for example when there is a major event on the television.

Unlike the situation where dry compressed air is expanded care has to be taken when expanding moist air. In particular, whilst many present day expanders can cope with a feed which is saturated with moisture (the relative humidity drops as the air expands, balanced against the increase in relative humidity as the temperature of the air falls) the formation of ice presents a major problem which can result in turbine blades being broken.

To address this problem this embodiment proposes to use the heat stored in the water to raise the temperature of the compressed air sufficiently to prevent ice formation. This is preferably achieved by using a multi stage expander with intermediate re-heaters between the stages.

Referring now to Figure 3, when it is desired to recover energy from the compressed air in the storage cylinders 500 the compressed air is directed to a heating coil 502 in the tank 218 via feed pipe 504.

The compressed air initially leaves the tank 218 at about 80 degrees C although this falls as the expansion cycle progresses. The warmed compressed air is then bubbled through first pressure vessel 222 via bottom feed pipe 230 and a diffuser (valve 243 is closed).. (An indirect heating coil could also be used), The compressed air leaves the first pressure vessel 22 via outlet pipe 240 and is fed to the inlet of an expander 506 via a feed pipe 504.

The expander 506, which is connected to a generator, has four stages, ie first stage 508 , second stage 510, third stage 512 and fourth stage 514.

A variable vane guide 507 is situated at the entrance to the first stage 508 of the expander 506. The variable vane guide 507 is continually adjusted so that the pressure of the gas entering the first stage 508 remains at a constant 150 bara throughout the complete expansion cycle which terminates when the pressure of the compressed gas in the storage cylinders 500 falls to 150 bara.

The compressed air is expanded to 42.9 bara in the first stage 508 and is then returned by pipe 516 to a heating coil 518 in tank 218 from which it emerges initially at about 80 degrees C before being further heated by liquid in the first pressure vessel 322 of the third stage 203 of the compression chain. Again the heating in the first pressure vessel 322 can be direct (as shown) or indirect (for example by a coil).

The reheated gas is introduced into the second stage 510 of the expander 506 where it is expanded to 12.2 bara.

The air leaving the second stage 510 of the expander 506 is passed through a pipe 520 to a heating coil 522 in tank 218 where it is heated initially to 80 degrees C before being further heated in the first pressure vessel 422 of the fourth stage 422 of the compression train either by being bubbled through the water via a diffuser or passed through a heating coil in the first pressure vessel 422.

The re-heated compressed air is introduced into the third stage 512 of the expander 506 where it is expanded to 3.5 bara. The air leaving the third stage 512 of the expander 506 is passed through pipe 524 before being passed through a heating coil 526 in the tank 218.

The air leaves the tank 218 initially at 80 degrees C and is passed through the first pressure vessel in the fifth stage 205 of the compressor train. It is then expanded through the fourth stage 514 of the expander before being vented to atmosphere.

The storage cylinders 500 are vented until they reach a pressure of approximately 150 bara at which time the expansion cycle is terminated.

The exact manner in which expansion is carried out depends to some extent on the quality of the output required.

Thus, where a steady output is required, the pressure of the gas at the inlet of the expander 506 is maintained at a steady pressure of 150 bara throughout the expansion cycle as described. Alternatively the temperature of the compressed air from the storage cylinders 500 could be increased as the pressure in the storage cylinders 500 falls to produce a substantially constant upstream pressure to the expander 506.. This would have to be fine tuned to allow for the decrease in the water temperature as the expansion process continues.

Our calculations show that in certain circumstances it might be possible to dispense with the services of the heat in tank 218 completely. In such an embodiment the required heat for heating and re-heating the compressed gas would come entirely from the heat stored in the hot water pressure vessels. It is anticipated that in such an embodiment the water would be under pressure and at the hottest realistic temperature feasible - perhaps up to 130 degrees C (water at 3.5 bara boils at about 140 degrees C).

Figure 4 shows an alternative embodiment. In this embodiment the water in tank 218, which has an operating pressure of 250 bara, is pressurised to 10 bara during the compression cycle and the water in the tank 218 is at 130 degrees C.

During expansion compressed gas from the storage cylinders 500, initially at 250 bara is bubbled through the water which it leaves through pipe 528 before being bubbled through hot water in the first pressure vessel 222 of the second stage 202 of the compression chain. Initially this does not increase the temperature of the compressed air although it does as the temperature in the tank 218 drops. The compressed air could be passed through a heating coil in tank 218 which would obviate the need for the tank 218 to have to be constructed to operate at 250 bara. A non-return valve 227 is mounted downstream of pump 226 to inhibit compressed air entering the pump 226. The valve 243 in the feed pipe 228 to the first pressure vessel is closed as are the corresponding valves in the feed pipes to all the remaining pressure vessels.

After passing through the variable vane guide 507 the hot compressed air is passed to the first stage 508 of the expander 506 at 150 bara where it is expanded and which it leaves at 42.9 bara before being reheated in the first pressure vessel 322 of the third stage 203 of the compression chain. The reheated air is then introduced into the second stage 510 of the expander 506 where it is expanded to 12.2 bara before being reheated in the first pressure vessel of the fourth stage 204 of the compression train before being introduced into the third stage 512 of the expander 506 where it is expanded to 3.5 bara. The air is then reheated in the first pressure vessel 522 of the fifth stage 205 of the compression train before being introduced into the fourth stage 514 of the expander 506. The expanded air is vented to atmosphere.

As in the previous embodiment the gas can be bubbled through the first pressure vessels 222, 322, 422 and 522 or passed through coils in the same.

Considerable care must be taken to prevent ice forming in the expander 506 and temperature and pressure sensors are provided in and adjacent the expander 506 to cause it to automatically close down if the formation of ice appears probable.

Referring now to Figure 5 there is shown an alternative compression chain. The compression chain differs from that shown in Figure 2 in that there is a mechanical compression chain 201 followed by four compression stages 202, 203, 204 and 205 which work in parallel rather than in series.

Referring now to Figure 5 there is shown an alternative compression train to the arrangement shown in Figure 2.

The compression train, which is generally identified by reference numeral 1200 comprises five compression stages, viz a mechanical compression stage followed by four compression stages arranged in parallel. The first stage is generally identified by reference numeral 1201 , the second stage by reference numeral 1202, the third stage by reference numeral 1203, the fourth stage by reference numeral 1204 and the fifth stage by reference numeral 1205.

The first stage 1201 comprises a filter 1206 mounted upstream of a mechanical compressor in the form of screw compressor 1208 which is connected to a heat exchanger 1210.

A pump 1212 is connected the heat exchanger 1210 by a feed pipe 1214 and a return pipe 1216 connects the heat exchanger 1210 to a tank 1218.

In use the screw compressor 1208 draws air through the filter 1206 which prevents leaves, insects and other solid debris entering the compression train.

The screw compressor 1208 compresses the air to 40 bara. The hot air leaving the screw compressor is passed through heat exchanger 1210 where it is cooled to 100 degrees C by indirect heat exchange with water which is pumped through the heat exchanger 1210 by pump 1212 via feed pipe 1214. The water leaves the heat exchanger at 90 degrees C via return pipe 1216 and enters tank 1218. The volume of tank 1218 is chosen so that the end of the compression stage ie: when the storage cylinders 1500 are fully charged to 250 bara the water in the tank 1218 is at about 90 degrees C.

The tank 1218 is insulated to reduce heat loss.

The screw compressor 1208 may be oil cooled. In this case the air would leave the screw compressor 1208 at about 100 degrees C and the heat in the oil would be transferred to water in tank 1218 by a separate and distinct heating circuit.

The compressed air leaves the first stage 1201 via pipe 1220 and enters the second stage 202 at 1 1 bara.

The second stage 1202 comprises two pressure vessels - a first pressure vessel 1222 and a second pressure vessel 1224.

The discharge of a pump 1226 is connected to the first pressure vessel 1222 via a feed pipe 1228 which divides into a bottom feed pipe 1230 and a spray header feed pipe 1232 which is connected to a spray header inside the pressure vessel 1222.

A return pipe 1234 connects the bottom of the first pressure vessel 1222 to a return line 1236 which is connected to the inlet of the pump 1226. An inlet pipe 1238 connects the pipe 1220 to the top of the first pressure vessel 1222 and an outlet pipe 1240 extends upwardly from the top of the first pressure vessel 1222.

In use the first pressure vessel 1222 is filled with water. Compressed air at 40 bara from pipe 1220 passes through a non-return valve 1242 and enters the first pressure vessel 1222 through inlet pipe 1238. A valve 1239 in the return pipe 1234 is opened and the compressed air displaces water from the first pressure vessel 1222 through return pipe 1234 to return line 1236. A pressure regulator 1241 in return pipe 1234 only allows the water to flow through return pipe 1234 when the pressure is slightly less than 10 bara.

When the first pressure vessel 1222 is nearly full of compressed air the valve 1239 in return pipe 1234 is closed.

At this stage a valve 1243 in feed pipe 1228 is opened to allow water from pump 1226, which has an outlet pressure of 250 bara, to flow into the first pressure vessel 1222 via both the bottom feed pipe 1230 and the spray header 1232. If desired additional valves (not shown) may be provided in the bottom feed pipe 1230 and in the spray header feed pipe 1232 to vary the ratio of the water which enters the first pressure vessel through the bottom feed pipe 1230 and the spray header 1233 or to direct the entire flow to either the bottom feed pipe 1230 or the spray header 1233.

As the water enters the first pressure vessel 1222 the air is compressed and leaves the first pressure vessel at 250 bara through outlet pipe 1240 and passes through non-return valve 1242 until the first pressure vessel 1222 is full of water at which stage the valve 1243 in the feed pipe 1228 is closed leaving the first pressure vessel 1222 full of water, at which stage the cycle can be repeated.

During the compression stage the air becomes hot and heat is transferred into the water.

The compressed air passes directly into the storage cylinders 1500 which will progressively increase in pressure until they reach 250 bara. (Typically the pressure of the compressed air in the storage cylinders 1500 will drop from 250 bara to 150 bara during the expansion cycle so the pressure in the storage cylinders will be raised from 150 bara to 250 bara during the compression cycle. The plumbing associated with the second pressure vessel 1224 mirrors the first pressure vessel 1222 . However, in use, the operation of the second pressure vessel 1224 is displaced from the operation of the first pressure vessel 122 by 180 degrees. In particular, as the air in the first pressure vessel 1222 is being compressed to 250 bara, compressed air at 40 bara is being introduced into the second pressure vessel 1224.

Similarly, as the air in the second pressure vessel 1224 is being compressed to 250 bara, air at 40 bara is being introduced into the first pressure vessel 1222.

The plumbing associated with the third stage 1203, fourth stage 1204 and fifth stage 1205 mirrors that of the first stage 1201.

In use the second stage 1202, third stage 1203, fourth stage 1204 and fifth stage

1205 are all run in parallel, ie the first pressure vessels in each stage are operated in sync and the second pressure vessels in each stage are operated in sync.

The pressure regulators associated with the respective outlet pipes are accordingly all set to open just below 40 bara.

The second stage 1202, third stage 1203, fourth stage1204 and fifth stage 1205 are operated with identical cycle times.

In any event the compressed air is passed into storage cylinders 1500 until the pressure rises to 250 bara.

It will be noted that, as in the embodiment shown in Figure 2, the compressed air has not been dried and, whilst this is desirable, the space requirements and the cost of molecular sieve driers at this point of the design has been eliminated.

Although each pressure vessel is shown with a separate and distinct pressure regulator 1241 it is anticipated that it will be possible to replace all of these with a single pressure regulator mounted in the return line 1236 immediately upstream of the pump 1226.

Throughout the compression cycle, which will typically take around three hours and corresponds with relatively low cost energy in the early hours of the morning, the water in the tank and the pressure vessels and the associated plumbing will have been becoming progressively hotter. For the purposes of this embodiment the highest practical temperature is desirable. Bearing in mind that towards the end of the compression phase the lowest pressure at the output of the first stage 1201 and in the second stage 1202, third stage 1203, fourth stage 1204 and fifth stage 1205 is 40 bara, the water can be heated to well above 100 degrees C without boiling. (The boiling point of water at 40 bara is about 250 degrees C). Whilst it is envisaged that the volume of water in the pressure vessels and the associated plumbing will suffice, a small pressurised storage tank may be provided for emergency use or if a lower temperature is desired.

At the end of the compression stage there will thus be thermal energy in tank 1218 (at about 90 degrees C) and in the pressure vessels (at about 120 degrees C).