GB2532281A

GB2532281A - A waste heat recovery system combined with compressed air energy storage

Info

Publication number: GB2532281A
Application number: GB1420343.4A
Authority: GB
Inventors: Willoughby Essex Coney Michael; Wazni Karim
Original assignee: DEMETAIR SYSTEMS
Current assignee: DEMETAIR SYSTEMS
Priority date: 2014-11-17
Filing date: 2014-11-17
Publication date: 2016-05-18
Also published as: GB201420343D0; WO2016079485A1

Abstract

A system of waste heat recovery and compressed air energy storage is provided wherein the system comprises at least two compression stages 3, 7 powered by a supply of electrical energy, and an intercooler 5 or aftercooler 17 for removing heat from a compressed air flow, and a compressed air energy storage vessel (CAES) 27. Compressed air stored in the CAES is stored at near ambient temperature. The compressed air is extracted from the CAES and heated in at least two stages provided by air heaters (64, 70, figure 2). The air heaters use waste heat at a temperature of at least 400°C. The compressed air, after heating, is expanded (73) to produce power. Preferably the waste heat is a provided by a gas turbine exhaust. Heat may be recovered from the intercooler or aftercooler by way of a heat transfer fluid such as water or thermic oil. Heat may be transferred between the intercooler or aftercooler to a boiler (43) of an organic Rankin cycle.

Description

A Waste Heat Recovery System Combined with Compressed Air Energy Storage

DESCRIPTION

BACKGROUND

[0001] It is well known that it is difficult and expensive to store large amounts of electrical energy, which means that most electric power must be generated at the same time that it is required by the customer. This is particularly inconvenient for most forms of renewable energy, such as solar and wind energy, since the output of these plants is dependent on natural phenomena, not on customer requirements.

[0002] A considerable amount of work has been and is still being done to try to develop better and cheaper methods of energy storage in order to try to overcome this limitation on the supply and use of electrical energy. Pumped storage of water is currently the main method of storage of electric energy in commercial operation, but this method is constrained by the need for geographic suitability. Batteries are of course also used commercially for storage of electrical energy, but this technology generally becomes uneconomic for storage of large amounts of energy for periods of several hours.

[0003] Various forms of compressed air energy storage (CAES) have been proposed and two plants have been built, both of which use a gas turbine to heat the compressed air before it is expanded to produce power. Some other types of CAES plant involve the storage of heat as well as compressed air, although none has been built to date. Energy recovery is performed using the stored heat. This type may be termed adiabatic CAES, since the aim is to perform the compression and expansion without heat loss or gain to the overall system.

[0004] Another type of CAES involves the quasi-isothermal compression of air using water sprays or liquid foam and heat rejection at low temperature. During energy recovery, the compressed air is expanded quasi-isothermally, again using water sprays of foam. We can use the term isothermal CAES to describe this type of system.

[0005] To achieve technical and commercial success, any proposed CAES system has to meet certain demanding objectives. One of these is to maximise the so-called round-trip efficiency which refers to the fraction of the stored energy which is actually recovered and turned back into useful electric energy when it is needed. Another key objective is to minimise the total capital cost of the plant. In some systems it is the cost of air storage which is the dominant factor, in other systems it may be the cost of heat storage or the cost of the compressors, expanders, heat exchangers and other plant items. In some situations, the operating cost may be a very important factor, for example if the recovery of stored energy involves the burning of additional amounts of high grade fuel.

[0006] The requirement for energy storage is generally linked to a particular application, a particular location, a particular power and energy storage capacity and a particular response time. In some situations, a pure energy storage system with no additional fuel consumption may be appropriate. In other situations, the ability to produce more power than was originally stored is very attractive, even though this requires consumption of fuel.

BRIEF DESCRIPTION

[0007] The invention described here is applicable to situations where waste heat is available at a temperature greater than 400°C. It is also most suited to sensible heat sources such as contained in hot gases.

[0008] The invention is not constrained to any particular source of the hot waste heat and may be applied to a wide range of industrial processes and to power generating equipment such as diesel engines and gas turbines.

[0009] The invention is also applicable to available waste heat in situations where a gas containing waste heat is part of a closed circuit, such that the gas is not released to the atmosphere. This situation could arise in some industrial processes.

[00010] The invention described here is very suitable for retrofitting to existing equipment, if sufficient space is available. The required modifications to the existing plant essentially consist of inserting heat recovery heat exchangers in the flow path of the waste gases. This will have some effect on the back-pressure of the system, but it does not involve any other significant modification to the original process.

[00011] The source of waste heat must be available when it is required to recover the stored energy of the compressed air. This means that the source of waste heat must either be in operation continuously or it must be brought into operation at the same time as the compressed air energy recovery process.

[00012] Peaking gas turbines are widely used to provide power at times of high demand. The capital costs of these turbines is low but the running costs are high, since the thermal efficiency of simple cycle gas turbines is much less than combined cycle plants, in which the waste heat is recovered by a steam turbine. Consequently most of these simple cycle gas turbines run relatively few hours in a year and the economics of such plant is very dependent on having sufficient running hours or if not, receiving adequate capacity payments for availability. The timing of the operation of the peaking plant is most likely to coincide with the time at which stored energy recovery is required.

[00013] The addition of a CAES plant which makes use of the waste heat of such turbines to provide energy storage and recovery could be very beneficial to the utilisation of peaking gas turbines. The energy storage capability would enhance the power output and significantly reduce the running cost of the plant, so that it could be called into operation more frequently and receive higher revenues from additional grid services it could provide, such as frequency regulation and flexible capacity. It would also allow the peaking plant to operate with a better efficiency at part load. This would be accomplished by running the peaking plant at this nominal load while at the same time running the compressor to store excess power so that the net power delivered to the grid could be much smaller than the minimum part load capability of a stand-alone peaking plant. This also has advantages relative to air emissions in particular CO emissions, which tend to increase when a peaking plant is run below its nominal capacity.

[00014] The present invention is a compressed air energy storage (CAES) scheme in which air is compressed to a high pressure by multistage intercooled compression, then stored at high pressure, without storage of heat. The energy is recovered by a multistage process of heating the compressed air with the high temperature waste heat and then expanding it. A preferred embodiment of the system has recovery of the heat of multistage compression by means of an organic Rankine cycle (ORC). The same organic Rankine cycle is also used during energy recovery to recover low temperature waste heat, which is available at the outlet of the heat exchanger, which transfers high temperature waste heat to the compressed air before each expansion stage.

[00015] Typically there are four compression stages, which is a good compromise between cost and performance for the situation where the compression heat is recovered and the air is compressed to about 200 bar. In this case, the temperature of the air after each compression stage may be about 220°C. The power produced by the ORC serves to reduce the net work of compression. The ORC power output may be used internally within the system to help to power the air compressors or it may be exported.

[00016] The number of expansion stages is not necessarily the same as the number of compression stages.

[00017] The compressed air which is extracted from the compressed air energy store is preheated in a recuperator. The heat source for the recuperator is the expanded air exiting from the last expansion stage.

[00018] The preheated air is heated by the gas turbine exhaust or other source of waste heat to a temperature of at least 400°C and possibly more than 500°C, which is much higher than the typical temperature of about 220°C of the air at the end of each compression stage. The hot air is then expanded through a certain pressure range and then reheated and expanded again in each successive stage. However, the system is designed such that the temperature of the air after each expansion stage is comparable with the temperature of the air after each compression stage. The purpose of this arrangement is that the same organic Rankine cycle system may be used in the expansion process as in the compression process.

[00019] Since the air expansion system uses high temperature heat, which is at least 400°C at the hot end and preferably above 200°C at the cold end, this provides a substantial differential between the mean air expansion temperature and the mean air compression temperature.

[00020] This temperature differential combined with the application of the organic Rankine cycle under similar conditions both during the compression and during the expansion allows the system to produce significantly more electrical energy during the energy recovery phase than is consumed in the energy storage phase. In other words, the apparent round trip efficiency from an external point of view would seem to exceed 100%.

[00021] This situation is in contrast to that of either isothermal or adiabatic compressed air energy storage systems, in which it is difficult to achieve a round trip efficiency higher than 60%.

[00022] The organic Rankine cycle is very suitable for the conversion of sensible heat at low temperatures to useful power, but it is not very suitable for the utilisation of high temperature heat, because suitable organic fluids are not available.

[00023] On the other hand, the air compression and expansion cycle, which may be termed a Brayton cycle, is inefficient at low temperatures, but is very suitable for the conversion of sensible heat at high temperatures.

[00024] The envisaged storage pressure for the proposed system may be about 200 bar, which is much higher than is normally considered appropriate for an underground salt cavern.

[00025] Underground storage is still a possible option if for example the cavity is drilled out of rock and is at a sufficient depth which can support the high pressure. The rock may need to be lined with a suitable liner in order to seal it.

[00026] The liner may be designed to take part of the pressure load with the surrounding rock also bearing part of the pressure load.

[00027] Above ground pressure vessels can be made from steel, but an alternative option may be to use composite pressure vessels.

[00028] The air compression and air expansion may be performed by turbo-machinery or by adiabatic reciprocating compressor/expanders. Alternatively other types of compressor/expander, such as screw machines, or rota ry vane machines may be used.

[00029] In the case of reciprocating compressors/expanders the same machines may be used for both compression and expansion, but with different valve timing in each case. The reciprocating compressor/expanders may be modified commercial diesel engines.

[00030] The pressure of the air storage system may be maintained at a constant level by means of a hydraulic pump-turbine which pumps water at high pressure into the air storage system during the energy recovery phase and releases the water in the opposite direction through the pump-turbine during the energy storage phase.

PRIOR ART

[00031] US patent number 8726629 by Coney describes a compressed air energy storage system in which an organic Rankine cycle may be used to recover low temperature energy after expansion. In the system described in US patent 8726629, the compressed air which is extracted from storage is expanded in one or more high pressure expanders before it is admitted to the combustion chamber of a modified gas turbine, in which the blading of the gas turbine has been removed. The partially expanded compressed air is admitted to the combustion chamber at the same temperature and pressure as would apply in the unmodified gas turbine. Fuel is burned in the compressed air and the combustion products are expanded through the turbine as in the unmodified gas turbine. The combustion products then flow to one or more recuperators, which are used to preheat the compressed air. Finally the exhaust gases may be released to the atmosphere or they may be passed to an organic Rankine cycle if the exhaust temperature is sufficient to justify this.

[00032] US patent 8726629 describes an invention which involves the combustion of fuel in the compressed air taken from storage. It also requires the use of a modified gas turbine, and is therefore not applicable to the recovery of waste heat from an unmodified gas turbine or from some other source of waste heat such as a diesel engine or an industrial process.

[00033] US patent 8726629 does not describe the use of an organic Rankine cycle in combination with the air compression system.

[00034] US patent number 6305158 by Nakhamkin describes a compressed air energy storage system which utilises the exhaust heat of a gas turbine to preheat the compressed air which is taken out of storage before it is injected into the combustion chamber of the same gas turbine in such a way that the added air supplements the flow from the gas turbine compressor. The injection of the additional air increases the output of the turbine without increasing the load on the gas turbine compressor.

[00035] US patent 6305158 does not describe the application of an organic Rankine cycle to recover part of the heat of compression or to recover low temperature heat from the exhaust of the gas turbine. Also US patent 6305158 does not describe the use of a separate air expansion system with a multistage expansion system. Furthermore, US patent 6305158 involves some modification to an existing gas turbine, since it requires the facility to inject compressed air into the gas turbine combustor. It is also clear that US patent 6305158 describes an invention which cannot be applied to other types of waste heat source, unlike the present invention.

[00036] Other prior art relating to compressed air energy storage can be found in which fuel is combusted with the compressed air which is extracted from storage and the combustion gases are expanded through a turbine. Two energy storage plants of this type have been built and are now in operation. An essential difference is that the present invention does not involve combustion of fuel in the compressed air which is extracted from storage. Another important difference is that the present invention uses an organic Rankine cycle to recover the heat of compression and residual heat after the air expansion.

[00037] A further type of compressed air energy storage system involves the quasi-isothermal compression of air by using water sprays or a liquid foam to cool the air during the actual compression process. Energy recovery is performed by a quasi-isothermal expansion using water sprays or a liquid foam to maintain the temperature. Although waste heat can be used in this case to heat the water sprays or liquid foam, the temperature of the water or foam is limited to less than 400°C.

DETAILED DESCRIPTION OF THE ENERGY STORAGE SYSTEM

[00038] There are other possible embodiments of the invention which will be described below, but the present detailed description focuses on a particular preferred system which is shown in Figures 1 and 2. Figure 1 shows the configuration used during the energy storage phase when air is being compressed and stored. Figure 2 shows the configuration used during the energy recovery phase when the air is gradually released from storage and is heated and expanded. Many of the components are common to both diagrams.

[00039] Referring to Figure 1, during the energy storage phase, atmospheric air at 1 is drawn into a first compressor stage 3 and compressed. This air exits from the compressor 3 and flows through pipe 4 to the intercooler 5, where it is cooled by a suitable heat transfer fluid. The cooled air exits the intercooler 5 via pipe 6 and flows to another compressor 7, where it is further compressed. The air exits the compressor 7 via pipe 8 and is cooled by the heat transfer fluid in intercooler 9 before flowing through pipe 10 to the third compressor stage 11. The air is compressed further in compressor 11 and then exits via pipe 12 to the third intercooler 13, where it is again cooled by the heat transfer fluid. The compressed air exits the third intercooler via pipe 14 and flows to the last compressor stage 15. The air exits the final compressor stage via pipe 16, then flows to an aftercooler 17, where is cooled again by the same heat transfer fluid.

[00040] The air typically enters each successive intercooler and the aftercooler at a temperature which may be in the range 200-280°C and exits each cooler at a temperature which may be near to 50°C.

[00041] It is advantageous to cool the compressed air as much as possible before it enters the compressed air energy store, so the air leaving the aftercooler 17 flows via pipe 18 to the cooler 19. The air entering cooler 19 may be cooled by a fan 20 blowing atmospheric air 21, which exits the cooler at 22. Alternatively the cooler 19 may use water circulated from a river, lake or the sea, with or without a cooling tower. The cold compressed air exits the air cooler via pipe 23. The cold compressed air will contain some liquid water which is condensed during the compression and cooling process. The condensed water is drained off via pipe 24 to a vessel 25, where it may be stored.

[00042] The compressed air minus condensed water flows via pipe 26 to the high pressure air storage vessel 27. Figure 1 shows a constant pressure air storage system in which water flows into or out of vessel 27 by means of a hydraulic pump-turbine 30. During the storage phase which is shown in Figure 1, compressed air flows into 27 and water flows out via pipe 29 to the pump-turbine 30. This generates some power which to some extent offsets the power consumption of the air compressors.

[00043] Typically, the power generated in the ORC and the power generated by the constant pressure system each amount to about 10% of the compression power. Thus there is total power generated which is about 20% of the compression power.

[00044] The power generated by the ORC and by the hydraulic pumping system during compression may be exported or it may be used internally within the air compression system. For example, the ORC power and the hydraulic pump-turbine power could be used to power part of the compression system.

[00045] The heat transfer fluid which circulates through the intercoolers 5, 9, 13 and the aftercooler 17 may be a commercial thermic oil such as Dowtherm A. This particular heat transfer fluid may be operated at temperatures up to 257°C in an unpressurised system. The maximum operating temperature of Dowtherm A is about 400°C, but the pressure needs to be maintained above 11 bar abs to prevent boiling. In the present application, it should not be necessary to pressurise Dowtherm A, if this is used as the heat transfer fluid.

[00046] Alternatively water may be used as the heat transfer fluid, but in this case, the water would certainly have to be pressurised to prevent boiling.

[00047] Figure 1 shows a pump 31 which circulates the heat transfer fluid via a pipe 32 and a manifold 53. The heat transfer fluid flows from the manifold 53 in parallel paths 33, 34, 35 and 36 to each intercooler 13, 9, 5 and the aftercooler 17.

[00048] After passing through the intercoolers and the aftercooler, the heat transfer fluid flows to an outlet manifold 41 via pipes 37, 38, 39 and 40. Then the heat transfer fluid flows via pipe 42 to the boiler 43, which evaporates the organic working fluid of the organic Rankine cycle. After giving up its heat, the heat transfer fluid exits the boiler 43 via pipe 44 and returns to the circulating pump 31.

[00049] The organic working fluid may be a hydrocarbon such as butane or isopentane or it may be a refrigerant, which is chosen so that it meets the regulatory requirements concerning the depletion of ozone in the atmosphere. The choice of working fluid depends on the intended working temperatures of the cycle.

[00050] The organic working fluid is circulated by means of pump 52, which supplies condensed liquid to the boiler 43 via pipe 45. The organic fluid is completely evaporated in the boiler so that dry organic vapour flows via pipe 46 to the organic turbine 47, which drives a generator 48. Unlike water, organic fluids do not become wet after expansion in a turbine, so dry organic vapour exits the turbine 47 via pipe 49 and flows to the condenser 50. The condenser is cooled by external cooling means, which may use river or sea water or it may use atmospheric air as the coolant. The organic vapour is condensed to liquid and returns to the circulating pump 52 via pipe 51.

[00051] The organic Rankine cycle may include a regenerator (not shown) which transfers heat from the vapour flowing in pipe 49 to the condensed liquid flowing in pipe 45, but this may not be advantageous in the present application.

[00052] The hydraulic pump-turbine 30 may be used as a fast response system to modify the input or output of the energy storage system as a short-term temporary measure. This may be useful as a means of stabilising a grid supply in a similar way to that which may be done with hydraulic pumped storage systems. The hydraulic pump-turbine could be adjusted to run at a lower or higher flow rate or it could be stopped altogether. Over a longer period of time the pressure of the air reservoir may drift from its normal setting, but the fast response of the hydraulic pump-turbine could give valuable time for other generating plant to be run up or run down as necessary. The drift of pressure in the air storage system can then be corrected.

[00053] The hydraulic system consumes or generates power in opposition to the air storage system, which means that the power rating and performance of the overall system at a fixed pressure condition is degraded relative to that which could be obtained without such a hydraulic system. Without the hydraulic system however, it is not possible to realize a system that could operate continuously at a fixed pressure, unless the air storage volume was effectively infinite, or the air was stored deep underwater at a fixed hydrostatic pressure. In practice the pressure of a finite air storage volume would vary over a wide range, for example from a maximum of 200 bar down to about 50 bar.

DETAILED DESCRIPTION OF THE ENERGY RECOVERY SYSTEM

[00054] The energy recovery system is shown diagrammatically in Figure 2. Some of the components are used for both the energy storage and for energy recovery, in which case they appear both in Figure 1 and in Figure 2. The main items in common in both diagrams are the air storage system, which includes the constant pressure system, and the organic Rankine cycle, including the organic boiler and the circulating pump for the heat transfer fluid. Depending on the type of compressors and expanders, it may be possible to use some or all of these for both purposes, but for the purpose of the present discussion it is assumed that the compressors and expanders are separate components.

[00055] For the purpose of the present description it is assumed that the source of high temperature waste heat is a gas turbine, but other sources of heat could be used instead, as described above.

[00056] Figure 1 shows a gas turbine consisting of a compressor 61, gas turbine combustor 62 and turbine 63. The gas turbine drives a generator 60. The gas turbine and generator may be pre-existing, such that the proposed CAES system is a retrofit. Similarly the CAES system could be a retrofit to another existing source of waste heat.

[00057] At the exhaust end of the gas turbine Figure 2 shows a heat recovery heat exchanger which consists of two parts 64 and 65. Exhaust heat at the highest temperature is used in a gas-to-air heat exchanger 64, which consists of parallel flow paths, which correspond to the number of expansion stages in the air expansion system. Figure 2 shows three expansion stages, but for some applications a different number of expansion stages might be preferable.

[00058] The second part 65 of the heat recovery heat exchanger is a gas-to-liquid heat exchanger which supplies heat to the organic Rankine cycle via the same heat transfer fluid which was used in compression.

[00059] After giving up nearly all the exhaust heat to the compressed air and to the heat transfer fluid of the organic Rankine cycle, the exhaust gas exits to the atmosphere through an exhaust duct 83.

[00060] In operation, compressed air is taken from the storage vessel 27, which is maintained at constant pressure by pumping water from the low pressure reservoir 28 into the high pressure air storage vessel by means of the hydraulic pump-turbine 30. The compressed air flows via pipes 26 and 69 to the air-to-air recuperator 70.

[00061] Condensed water which was stored in vessel 25 during the compression phase may be re-injected into the compressed air by means of a small pump 84 so that it also flows along pipe 69 to the air-to-air recuperator 70.

[00062] If sufficient waste heat is available, additional water may be injected into vessel 25 to increase the thermal capacity of the air during the expansion process and thereby increase the power output.

[00063] The compressed air with some liquid water is pre-heated in recuperator 70 using fully expanded air which may be at a temperature of about 250°C. The compressed air then flows via pipe 71 to the high pressure air tubing of heat exchanger 64 and is heated to near the maximum available temperature of the waste heat. The hot compressed air then flows via pipe 72 to the high pressure expander 73 and is expanded to a lower pressure and temperature once again of about 250°C.

[00064] After the high pressure expansion the air exits the high pressure expander 73 via pipe 74 and re-enters the heat exchanger 64, passing through a set of tubing designed for an intermediate pressure. The air is re-heated to the maximum available temperature and exits the heat exchanger 64 via pipe 75. The reheated air is then expanded in the expander 76 and exits via pipe 77. The air is then reheated again to the maximum available temperature in another set of heat exchanger tubing and then flows via pipe 78 to the last expander 79. Finally the expanded air flows to the air-to-air recuperator 70 via pipe 81 and from there the cooled low pressure air escapes to the atmosphere via duct 82.

[00065] The air expanders 73, 76 and 79 drive a generator 80. This generator could be a motor-generator which can operate both as the motor 2 shown in Figure 1 and as the generator 80 shown in Figure 2.

[00066] The operation of the organic Rankine cycle is essentially the same as described in connection with the energy storage phase. The main difference is that in this case the heat transfer liquid is circulated through the gas-to-liquid heat exchanger 65 shown in Figure 2, instead of through the intercoolers and aftercooler shown in Figure 1.

[00067] The optimum design of the various components depends on the nature of the waste heat source, in particular the mass flow rate and the temperature. The temperature may affect the optimum number of stages in the compressed air expansion and the choice of working fluid in the organic Rankine cycle. The mass flow rate clearly affects the size of the compressed air system and that of the organic Rankine cycle.

[00068] The number of air expansion stages may not be the same as the number of compression stages. In expansion, the available waste heat from the gas turbine or other source is shared fairly equally between the different expansion stages. The thermal capacity of the air is similar to that of gas turbine exhaust gas. The exhaust gas flows in parallel paths through the different stages, while the air flow path is in series. Thus if there are fewer expansion stages, then a larger air flow is required to match the available waste heat. The air flow during the compression phase is not constrained so it can be chosen to match the expansion air flow or not, depending on the planned utilisation of the CAES system. However, the numbers of air compression and expansion stages and the magnitude of air mass flow in compression relative to that in expansion does have an impact on the design of the organic Rankine cycle.

[00069] It is desirable to achieve a reasonable match between the conditions seen by the ORC in expansion relative to that seen in compression. However, the optimum cost and performance may be achieved by building in some flexibility in the ORC regarding the operating temperature and the flow rates both of the heat transfer fluid and of the organic fluid. For example, it may be advantageous to have two ORC turbines to maintain high efficiency at two different flow rates.

SYSTEM OPERATION FOR PART LOAD CAPABILITY ENHANCEMENT

[00070] Peaking gas turbines are sometimes forced to shut down at low power levels because of limits on emissions. If the compressed air store is depleted, the proposed system may be operated without energy recovery to increase the output power range of a gas turbine peaking plant from the typical range of 35-100% of its nameplate capacity to 0-100% of its capacity. This would result in higher capacity revenues for the plant operator.

[00071] This mode of operation is accomplished by running the peaking plant in its normal operational range while at the same time using the CAFS compressor train to store excess power so as to achieve the desired net power output, as seen from the grid.

SYSTEM OPERATION WITHOUT COMPRESSED AIR STORAGE

[00072] The proposed system can also be operated as a heat recovery system without storing or recovering compressed air, as illustrated in Figure 3. This figure shows the compression system, which is the same as shown in Figure 1. The air storage system with air cooler, which appear in Figure 1, are not shown in Figure 3 since they are not used in this situation. Figure 3 includes the air expansion system and the organic Rankine cycle, which are also shown in Figure 2. Consistent numbering of the components is used between Figures 1, 2 and 3 in order to assist the description.

[00073] Figure 3 also shows a pipe 86 which supplies compressed air directly from the aftercooler 17 of the compression system to the recuperator 70 of the air expansion system.

[00074] Figure 3 also shows that the pump 31 supplies heat transfer fluid both to the manifold 53 of the air compression system via pipe 32 and to the low temperature heat recovery heat exchanger 65 via pipe 66. The two flows operate in parallel. Similarly the hot heat transfer fluid returns from the outlet manifold 41 of the air compression system via pipe 42 and from the low temperature heat recovery heat exchanger via pipe 67.

[00075] For the purpose of operating purely as a heat recovery system without CAES, it is necessary to size the organic Rankine cycle so that it can operate with both the compression and the expansion heat recovery at the same time. This requires additional pumping capacity for the ORC fluid and for the heat transfer fluid. Also the ORC boiler needs to be sized accordingly. In addition it would probably be necessary to have two parallel ORC turbines. One of the turbines could be sized for the compression duty and the other for the expansion duty, with valves controlling the flow to either one or to both simultaneously. When the system is run as a pure heat recovery system, both ORC turbines could be used.

[00076] The performance of the system in pure heat recovery mode without storage of compressed air is improved slightly relative to the performance with compressed air storage since the losses in the pump-turbine of the hydraulic pumping system are avoided.

ALTERNATIVE EMBODIMENTS OF THE INVENTION

[00077] An alternative embodiment of the invention is to use an air pressure store without a hydraulic pressure compensating system. For example, the pressure in the air storage system might be allowed to vary from a maximum of 200 bar down to about 50 bar, such that the system pressure varies by a factor of 4. In this case, it is necessary to have much more flexibility in the individual components and in the complete system to cope with the varying pressure, particularly if it is desired to maintain a constant power input during energy storage and a constant power output during energy recovery. This implies that the air mass flow should actually increase as the pressure falls and decrease as the pressure rises.

[00078] The need for flexibility in the individual components can be reduced by reducing the variation in the system pressure. For example the system pressure may be allowed to vary between 200 bar and 100 bar. The disadvantage is that a significantly larger air storage volume is required for the same amount of stored energy.

[00079] The flexibility of the system can be significantly enhanced if the motor 2 in Figure 1 and generator 80 in Figure 2 have a variable frequency drive such that the speed of the compressors and expanders can be varied. This allows the air mass flow and the power input or output to be varied with little variation in efficiency.

[00080] If turbo-compressors or expanders are used guide vanes may be used to increase the operational flexibility of these components by altering the angle at which the air flow meets the rotating blades of the turbo-compressors or turbines. This allows the turbo-compressors and turbo-expanders to operate more efficiently over a wider range of flow conditions.

[00081] Another possible embodiment of the invention is not to use an organic Rankine cycle for the recovery of low temperature compression heat. In this case the compression system becomes the same as a conventional intercooled compression system and it may be appropriate to increase the number of compression stages and to cool the air in the intercoolers and aftercooler with water, with heat rejection directly to the environment.

[00082] A further possible embodiment is not to use an organic Rankine cycle for the recovery of residual low temperature heat of the gases supplying the waste heat. In this case, the gases would simply be allowed to exhaust to the atmosphere or to flow to their intended destination without removal of the low temperature waste heat.

[00083] If an organic Rankine cycle is not used in compression or in expansion there would be a significant reduction in the round-trip efficiency. However, there may be some situations where the saving in capital cost would justify the omission.

[00084] A further possible embodiment is to use a different system for low temperature heat recovery in compression and expansion, which is not an organic Rankine cycle. For example, a working fluid which is not considered to be organic could be used, such as carbon dioxide or ammonia. Such a cycle could possibly be operated in a transcritical or supercritical regime. Alternatively, a cycle such as a Trilateral Cycle may be used in which a two-phase mixture is expanded through a turbine, instead of a vapour. Another possibility is an ionic or electronic process which does not require a working fluid.

[00085] A further possible embodiment of the invention is to use the low temperature heat of compression and/or expansion for a heating purpose, rather than to generate power. For example, the low temperature heat could be used for space heating or water heating in a commercial, domestic or industrial premises or it could be used for an industrial process, such as drying.

Claims

CLAIMS1. A system of waste heat recovery and compressed air energy storage in which o the air is compressed to a high pressure in at least two separate stages of compression using compressors powered by an external supply of electrical power o heat is removed from the air in one or more intermediate stages by means of one or more intercoolers and also by means of an aftercooler when the multistage compression is completed o the compressed air is stored at near ambient temperature in pressure vessels above ground or in an underground cavity o and compressed air is extracted from storage and heated in at least two stages by air heaters using waste heat at a temperature of at least 400°C and then expanded to produce power.
2. A system of waste heat recovery and compressed air energy storage as in Claim 1, in which heat is recovered from the aftercooler and intercoolers by means of a heat transfer fluid such as water or a thermic oil.
3. A system of waste heat recovery and compressed air energy storage as in Claim 2, in which the heat transfer fluid transfers heat from the intercoolers and aftercooler to the boiler of an organic Rankine cycle.
4. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which the air expansion system and its associated air heaters operates entirely in a high temperature range, with a temperature after each expansion of at least 200°C.
5. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which a heat transfer fluid such as water or thermic oil is used to extract low temperature heat from the hot waste gases after the high temperature heat has been extracted.
6. A system of waste heat recovery and compressed air energy storage as in Claim 5 in which the low temperature waste heat is transferred to the boiler of an organic Rankine cycle.
7. A system of waste heat recovery and compressed air energy storage as in Claim 3 and in Claim 6 in which the same organic Rankine cycle is used to recover the heat of compression and the residual heat of the waste gases.
8. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which a recuperator is used to transfer heat from the fully expanded compressed air to the cold compressed air which is extracted from compressed air storage.
9. A system of waste heat recovery and compressed air energy storage as in Claim 1 which can be operated as a pure heat recovery system without storing or recovering compressed air from storage.
10. A system of waste heat recovery and compressed air energy storage as in Claim 9 in which an organic Rankine cycle is capable of utilising the heat of compression and the residual heat of the waste gases either when only one is available or when both are available at the same time.
11. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which condensed water is removed from the compressed air before storage and is reinjected into the compressed air during energy recovery.
12. A system of waste heat recovery and compressed air energy storage as in Claim 11 in which water is added to the stored condensate and is injected into the compressed air during energy recovery for the purpose of increasing the capacity to absorb heat from the waste heat source.
13. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which the compressed air is stored in a purpose-made underground cavity in a suitable rock formation.
14. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which the purpose-made underground cavity is lined with a liner which provides gas tightness in situations where the cavity may not be gas tight.
15. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which a gas tight pressure vessel is fitted inside the underground cavity and transfers some of the pressure load to the cavity around it, for the purpose of ensuring gas tightness or storing air at a higher pressure than the mechanical properties of the soil would allow, or for both purposes.
16. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which the compressed air is stored above ground in pressure vessels.
17. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which some or all of the compressors and expanders can be operated at variable speed by means of one or more electrical motor/generators with a variable frequency drive.
18. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which some or all of the compressors and expanders consist of reciprocating machines.
19. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which some or all of the compressors and expanders consist of turbo-machines.
20. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which some or all of the compressors and expanders consist of screw or other rotary machines.
21. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which one or more compressors are also used as expanders during the energy recovery stage.
22. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which the number of expansion stages is different from the number of compression stages.
23. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which the final stage of heat removal from the compressed air is performed by an air or water cooling system without heat recovery.
24. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which the stored compressed air is maintained at constant pressure by means of a hydraulic pump-turbine, which pumps water into the air storage vessel(s) during energy recovery and extracts water via the turbine during the energy storage phase.
25. A system of waste heat recovery and compressed air energy storage as in Claim 24 in which the hydraulic pump-turbine is used as a fast response system for stabilising an electrical supply grid.
26. A system of waste heat recovery and compressed air energy storage as in Claim 18 in which the reciprocating machines can be configured to operate either as compressors or as expanders, by means of changes to the operation of the valves.
27. A system of waste heat recovery and compressed air energy storage as in Claim 18 in which the reciprocating machines have adjustable timing on the intake valves and/or the discharge valves.
28. A system of waste heat recovery and compressed air energy storage as in Claim 19 in which some or all of the turbo-machines have variable inlet guide vanes.
29. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which the source of waste heat is air or a gas such as a combustion gas, which may be released to the atmosphere after heat is extracted.
30. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which the source of waste heat is a gas which is not released to the atmosphere but is part of a circulation path, which retains the cooled gas either for some industrial purpose or for environmental reasons.
31. A system of waste heat recovery and compressed air energy storage as in Claim 29 in which the source of waste heat is a peaking gas turbine, which may be kept in operation at a power level above the minimum level dictated by emission limits, even when the required net output is below that minimum level, by storing the excess electric power in the form of compressed air.
32. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which a heat recovery cycle, which is not an organic Rankine cycle, is used to recover the low temperature waste heat produced during compression or during expansion or both.
33. A system of waste heat recovery and compressed air energy storage as in Claim 1 in which the low temperature heat produced during compression or expansion or both is used for a heating purpose.