METHOD AND APPARATUS FOR PRE-HEATING A PROCESS GAS

Description

FIELD

The present invention relates to cryogenic energy storage systems and methods for operating the same, and particularly to energy recovery methods and subsystems thereof.

BACKGROUND

Electricity transmission and distribution networks (or grids) must balance the generation of electricity with the demand from consumers. This is normally achieved by modulating the generation side (supply side) by turning power stations on and off, and running some at reduced load. As most existing thermal and nuclear power stations are most efficient when run continuously at full load, there is an efficiency penalty in balancing the supply side in this way. The expected introduction of significant intermittent renewable generation capacity, such as wind turbines and solar collectors, to the networks will further complicate the balancing of the grids, by creating uncertainty in the availability of parts of the generation sources. An efficient means of storing energy generated during periods of low demand for later use during periods of high demand, or during low output from intermittent generators, would be of major benefit in balancing the grid and providing security of supply.

There are many emerging and established methods of energy storage in the market, including cryogenic energy storage, pumped hydroelectric, compressed air and chemical batteries among others. Power storage devices such as these have three phases of operation: charge, store and discharge. During periods of low demand, excess energy from the grid is used to charge the power storage devices. The energy is stored by the power storage device in its respective medium, such as cryogenic fluids, hydroelectric dam reservoirs or a battery's internal chemical potential. During periods of high demand this energy is released back into the grid to ensure it meets demand.

Cryogenic energy storage (CES) technology using a cryogen such as liquid air offers many advantages over the other available power storage technologies. A cryogen is a liquified gas. CES systems are typically energy dense due to the physical properties of a cryogen, highly locatable (because they use relatively small storage tanks that are geographically unconstrained), environmentally friendly (because their working principle does not involve the use or production of hazardous material, such as those found in most batteries, or the generation of carbon emissions) and relatively inexpensive (since a CES system utilises equipment that have been in use over many years in the liquid natural gas industry).

CES technology liquefies air from the external environment or another suitable gas and stores it at low pressure which can then be vapourised and used to power turbines to generate electricity. In the charge or liquefaction phase of a CES system, low cost electricity at periods of low demand (off-peak period) or of excess supply from intermittent renewable generators power is used to compress, cool, purify and liquify a gas such as ambient air in a liquefaction unit or subsystem. In the liquefaction process, two compressors are commonly used. A stream of air is first compressed to approximately 15-20 barA in a main air compressor (MAC), the air is then purified in an air purification unit (APU) and compressed in a second recycle air compressor (RAC) to approximately 55-65 barA. The stream can then enter a cooling and liquefaction system commonly referred to as a cold box. A cold box may generally be embodied as an insulated (typically metal) box, filled with high performance insulation materials such as perlite. Low temperature heat exchange processes occur within the cold box via components such as multi pass heat exchangers, a phase separator, and at least one expansion turbine stage and/or valve. The gas is cooled in the cold box until it substantially condenses to liquid. The liquid product is separated from the gas and passed from the cold box to a cryogenic storage tank where it is stored for later use. The gas may be returned back through the multi-pass heat exchangers to cool the original incoming air stream and supplement the stream of air upstream of the recycle air compressor to be further liquefied. The liquid product, such as liquefied air or cryogen, is stored in a storage tank during the storage phase. During the discharge or energy recovery phase, the liquid product is released into an energy recovery unit or subsystem where it is pressurised, vapourised and heated to drive one or more expansion stages as part of one or more expansion turbines to drive a generator and produce electricity. The discharge or energy recovery phase is often performed during the peak period where the electricity costs are high. CES technology relies on the thermodynamic energy potential between liquid air at cryogenic temperatures and gaseous air at ambient temperature and above. CES systems may perform various operations from the charging and discharging phase simultaneously.

The pressurised stream of gas to be liquefied present in the liquefaction subsystem and the pressurised cryogen or liquified gas present in the energy recovery subsystem are typically designated a ‘process stream of the liquefaction subsystem’ and a ‘working fluid of the energy recovery subsystem’, respectively.

The round-trip efficiency of a CES system is simply defined as being the ratio of the net electrical energy output of the system measured over the full discharge period to the net electrical energy input of the system measured over the time to charge. In CES systems, heat produced as a result of the electrical energy input should be captured and re-used to maintain a high round-trip efficiency, any heat not captured and used may eventually dissipate into the environment, negatively impacting the round-trip efficiency.

For example, in the liquefaction process, the process stream is compressed, cooled and then expanded again. This process is repeated, each compression step increases the temperature of the process stream, and each cooling and expansion step significantly reduces the temperature, until the process stream has been fully liquefied. The term ‘heat of compression’ refers to the hot thermal energy embedded in a fluid that has been compressed. In other words, ‘heat of compression’ refers to the increase in temperature experienced by a fluid as a result of compression. Herein when heat is described as “transferred” from one location, fluid or component to another, this is the process where the thermal energy is transferred between the two locations, fluids or components. The method of transfer may comprise any of the typical methods, such as conduction, convection or radiation, or any combination thereof, and will be obvious to the skilled reader.

CES systems may use an energy capture subsystem designed to capture the heat of compression of the process stream generated in the liquefaction subsystem during the liquefaction phase, then store it in one or more thermal energy storage devices (TESDs). Such a CES system may release this captured heat to the working fluid in the energy recovery subsystem during the energy recovery phase. These subsystems are also known collectively as heat-of-compression recycle systems or energy recycle subsystems.

When stored, a cryogen such as liquid air occupies a lower volume compared to its gaseous state. During the energy recovery phase, the cryogen is pumped from a cryogenic storage tank for use as a working fluid and then vapourised, heated and expanded by passing it through a series of interleaved or alternating heat exchangers and expansion stages. Vaporising the working fluid creates a very low temperature gas with a much greater volume than the cryogen which expands further as it warms. As the working fluid is heated then expanded through an expansion turbine it does work against the turbine to drive the turbine's rotation which reduces the temperature and pressure of the working fluid. The turbine may be coupled to a motive energy user such as a generator which in turn produces electrical energy which can be exported to the grid. Increasing the temperature of the working fluid increases its pressure and correspondingly the energy available to drive the expansion turbine. This allows the turbine to extract more energy from the hotter working fluid and to rotate faster and/or with more torque, and thus the power output provided by the energy recovery subsystem during the energy recovery phase also increases, which leads to an improvement of the round-trip efficiency of the CES system. The heat of compression rejected during the liquefaction phase and captured by the energy capture sub-system can be used to increase the temperature of the working fluid in the energy recovery sub-system prior to its expansion.

In the energy recycle subsystem, the heat of compression is transferred from the process stream of the liquefaction subsystem to at least one TESD in the energy recycle subsystem. Heat exchangers located down-stream of the compressors in the liquefaction sub-system transfer heat from the process stream to a heat transfer fluid (HTF) which passes it to said one or more TESDs. The heat of compression is transferred from said at least one TESD to the working fluid of the energy recovery subsystem by the heat transfer fluid (HTF), or by a second separate HTF, using heat exchangers interleaved with the expansion stages. Heat transfer fluids used in TESDs vary depending on the temperature or quality of the thermal energy captured by the TESD. For example, water, water-glycol mixes, and thermal oil may be used at lower or ambient temperature applications. Molten salts such as molten nitrate salts may be used for higher temperature applications. Molten salts have the advantage of high thermal heat capacity, high boiling point, good stability, low toxicity, they are not flammable and low cost. Higher quality thermal energy has a higher temperature.

Energy recycle subsystems designed to store high quality thermal energy often use HTF with a high melting point that is frequently well above ambient temperatures to store and/or transfer the heat of compression. Some fluids may be used as a HTF and a thermal storage medium within the same energy recycle subsystem. Molten salts are one such fluid. The temperature at which molten salts freeze depends on the blend and type of salt used. Blends and types of salt relevant to CES systems typically freeze at temperatures of approximately 130° C. and have a safe working temperature above approximately 150° C. The safe working temperature is the temperature above which no crystals will form in the salt and includes a margin of safety that is acceptable to the operators or engineers running the CES system. The high freezing point of the HTF poses a challenge as recently vaporised cryogen or working fluid can be at temperatures substantially below the freezing temperature of the HTF, risking freezing or crystalising the HTF in heat exchangers.

In a known energy recovery subsystem, a pump moves working fluid from the cryogenic storage tank through an evaporator to an expansion turbine. To increase the temperature of the working fluid and improve efficiency at the turbine, the working fluid can be passed through a gas to molten salt heat exchanger located between the evaporator and the expansion turbine. The working fluid passing from the evaporator is approximately at ambient temperature, or 20° C. Since the freezing temperature of the molten salt is higher than the process air stream discharged from the evaporator, over time there is a risk that the molten salt may freeze in this heat exchanger. System start up presents an even higher risk of the molten salt freezing in this heat exchanger as the system or heat exchangers are not yet up to temperature.

Freezing of molten salt causes production losses and may lead to catastrophic equipment failure. In the event of HTF such as molten salt freezing, the whole CES system may need to be brought offline for maintenance and checking. The conduits and heat exchangers thorough which working fluid and HTF pass are often at high pressure and thus present a safety hazard with the smallest damage. Defrosting heat exchangers of the scale seen in a commercial CES plant is challenging and may require replacement of the heat exchanger altogether as it is difficult to assess damage and thus safety. The timescale for this maintenance may be weeks, or even months.

SUMMARY OF INVENTION

Aspects and/or embodiments of the current invention seek to address the issue of freezing or crystalising heat transfer fluid (HTF) and thus provide a more robust energy recovery subsystem in which the danger of HTF freezing or crystalising in the conduits or heat exchangers of the HTF flow path is reduced.

According to a first aspect, there is provided an energy recovery subsystem for a cryogenic energy storage system (CES) comprising: a pump for moving a working fluid, an evaporator; one or more heat exchangers including a first heat exchanger for passing heat energy from a heat transfer fluid to the working fluid;

• • one or more expansion stages for extracting work from the working fluid; a recuperator for passing heat energy from the working fluid to the working fluid; a first arrangement of conduits for the passing of working fluid; a second arrangement of conduits connectable to a thermal energy storage device (TESD) and connected to the first heat exchanger for passing heat transfer fluid therethrough.

The first arrangement of conduits having an upstream end connectable to a source of working fluid and a downstream end. The first arrangement of conduits connecting in sequence from the upstream end the evaporator, the recuperator, the first heat exchanger, the recuperator for a second time and the one or more expansion stages.

Advantageously, the recuperator passes heat from the working fluid that has already passed through the first heat exchanger to the working fluid upstream of the first heat exchanger, thereby increasing the temperature of the working fluid entering the first heat exchanger and reducing the chance of HTF freezing in the first heat exchanger.

Optionally the recuperator having a first input, a first output, a second input and a second output; wherein the first input of the recuperator is connected by the first arrangement of conduits downstream of the evaporator; the first output of the recuperator is connected to the first heat exchanger by the first arrangement of conduits, the second input of the recuperator is connected to the first heat exchanger by the first arrangement of conduits; and the second output of the recuperator is connected to the first arrangement of conduits upstream of the one or more expansion stages.

Optionally, the one or more heat exchangers further including a second heat exchanger for passing thermal energy from the heat transfer fluid to the working fluid, said second heat exchanger connected to the second arrangement of conduits for passing heat transfer fluid there through and connected along the first arrangement of conduits between the recuperator and a first expansion stage of the one or more expansion stages for passing working fluid therethrough.

Passing through the recuperator after first heat exchanger lowers the temperature of the working fluid, the second heat exchanger increases the temp of the working fluid before entering the one or more expansion stages increasing the temperature of the working fluid once more to improve efficiency of the first expansion stage.

Optionally the energy recovery subsystem wherein the one or more expansion stages (30) include one or more second expansion stages for extracting work from working fluid; and the one or more heat exchangers include one or more third heat exchangers for passing heat energy from the heat transfer fluid to the working fluid. Optionally, wherein each third heat exchanger receives working fluid from one of the one or more expansion stages located upstream along the first arrangement of conduits and is connected by the first arrangement of conduits to a further of the one or more expansion stages downstream of each of said one or more third heat exchangers. Said further of the one or more expansion stages may be down stream of each of the one or more third heat exchangers and may be before a second expansion stage.

Passing the working fluid through the third heat exchanger before the working fluid enters each of the one or more second expansion stages increases the temperature of the working fluid and increases the work that can be extracted from the working fluid in each second expansion stage.

Optionally, the energy recovery subsystem further including a final expansion stage for extracting work from working fluid and connected by the first arrangement of conduits downstream of all other expansion stages along the first arrangement of conduits and wherein said final expansion stage is further connected by the first arrangement of conduits to an outlet for releasing used working fluid from the energy recovery subsystem at the downstream end of the first arrangement of conduits. Preferably the final or third expansion stage is connected downstream of the previous expansion stage without passing through a heat exchanger of the one or more heat exchangers.

Thereby, extracting any remaining energy from the working fluid before it is released from the energy recovery system. It is likely that this will be a lower pressure expansion stage than the first and second expansion stages.

Optionally, the one or more expansion stages turn a common shaft connectable to a power sink for example an electrical generator.

Optionally, the energy recovery subsystem having a second recuperator for transferring heat energy from the working fluid to the working fluid; and a fourth heat exchanger for transferring heat energy from heat transfer fluid to working fluid; where in the second recuperator is connected by the first arrangement of conduits downstream of the first recuperator, and the first arrangement of conduits connects in sequence the second recuperator, the fourth heat exchanger, the second recuperator for a second time, upstream of the one or more expansion stages.

Optionally, wherein the second recuperator has a first input, a first output, a second input and a second output; wherein the first input of the second recuperator is connected by the first arrangement of conduits to first recuperator; the first output of the second recuperator is connected to the fourth heat exchanger by the first arrangement of conduits, the fourth heat exchanger is connected to the second input of the second recuperator by the first arrangement of conduits; and the second output of the second recuperator is connected by the first arrangement of conduits upstream of the one or more expansion stages.

Optionally, wherein the first recuperator and optionally the second recuperator are gas to gas heat exchangers.

Optionally, the first heat exchanger is a liquid to gas heat exchanger. Further optionally the second heat exchanger is a liquid to gas heat exchanger. Optionally the third heat exchanger is a liquid to gas heat exchanger.

Optionally, the working fluid is one or more of air, nitrogen, oxygen, hydrogen, or carbon dioxide.

Optionally, the source of working fluid is a cryogen storage tank.

Optionally, the one or more heat exchangers are connected by the second arrangement of conduits to the TESD in parallel. Thus ensuring that each heat exchanger benefits from high temperature HTF that has not yet been used.

Optionally, the second arrangement of conduits includes a first sub-set of conduits and a second subset of conduits separate from the first subset of conduits; the first subset and the second subset each for passing heat exchange fluid and connectable to a TESD; wherein the first subset of conduits is connected to the first heat exchanger and optionally to the fourth heat exchanger for passing HTF there through and the second subset is connected to the second heat exchanger and or the third heat exchanger for passing HTF there through.

Separate circuits for the first and optionally fourth heat exchangers mean that the flow rate of HTF through said heat exchangers associated with the recuperators can be varied independently of the remaining heat exchangers. This can be particularly useful during the warm up phase when there is most danger of a freezing event.

Optionally, the energy recovery subsystem of any previous claim further including a preheater for heating working fluid and located along the first arrangement of conduits upstream of the first recuperator.

Optionally, the preheater for heating working fluid may be located along the first arrangement of conduits downstream of the first recuperator.

This arrangement of the pre-heater downstream of the recuperator is particularly advantageous as it reduces the risk of molten salt freezing in the second heat exchanger 22 because the heat from the first heat exchanger is subsequently captured and recuperated back into the air upstream of the first heat exchanger by the recuperator.

Optionally, including a TESD wherein the second arrangement of conduits is connected to the TESD for supplying HTF therefrom.

Optionally, the second arrangement of conduits comprises a circuit for supplying the one or more heat exchangers with heat transfer fluid and returning said heat transfer fluid to said TESD after use.

According to a second aspect of the invention a cryogenic energy storage system is provided comprising the energy recovery subsystem described above.

Optionally, the CES system comprising: a liquefaction subsystem for storing energy in the form of a cryogen and connected to, a cryogen storage tank for storing and supplying the cryogen, a heat energy capture subsystem for capturing heat of compression created by the liquefaction subsystem and connected to; a TESD for storing said captured heat energy; and the energy recovery subsystem connected to the cryogen storage tank and the TESD for releasing energy from said cryogen.

According to another aspect of the invention, a method of operating an energy recovery subsystem is provided. The method comprises receiving, at an upstream end of a first arrangement of conduits, a working fluid. It further comprises passing, by a pump, the working fluid along the first arrangement of conduits through an evaporator, then through a recuperator, then through a first heat exchanger, then through the recuperator for a second time, and then through one or more expansion stages for recovering energy from the working fluid. The method further comprises receiving heat transfer fluid (HTF) at a second arrangement of conduits. The method further comprises passing the heat transfer fluid along the second arrangement of conduits through the first heat exchanger for exchanging heat energy between the working fluid and the heat transfer fluid.

Optionally, the method may further comprise passing the working fluid along the first arrangement of conduits through a second heat exchanger between the recuperator and the first expansion stage of the one or more expansion stages; and passing the heat transfer fluid along the second arrangement of conduits through the second heat exchanger.

Optionally, the one or more expansion stages may include one or more second expansion stages, and the one or more heat exchangers may include one or more third heat exchangers. Optionally, the method may further comprise passing the working fluid along the first arrangement of conduits from one of the one or more expansion stages through one of the one or more third heat exchangers, and then a further of the one or more expansion stages.

Optionally, the method may further comprise passing the working fluid along the first arrangement of conduits through a final expansion stage downstream of all other expansion stages, then through an exhaust for venting used working fluid from the energy recovery subsystem at a downstream end of the first arrangement of conduits.

Optionally, the method may further comprise passing the working fluid along the first arrangement of conduits through a second recuperator downstream of the first recuperator, then through a fourth heat exchanger, then through the second recuperator for a second time, upstream along the first arrangement of conduits of the one or more expansion stages.

Optionally, the working fluid is received from a cryogenic storage tank.

Optionally, the heat transfer fluid is passed along the second arrangement of conduits from the TESD to each of the one or more heat exchangers in parallel.

Optionally, the second arrangement of conduits comprises a first subset of conduits and a second subset of conduits separate from the first subset of conduits. Optionally, the method may further comprise passing heat exchange fluid along the first subset through the first heat exchanger and optionally through the fourth heat exchanger; and passing heat exchange fluid along the second subset through the second heat exchanger and/or the third heat exchanger. Preferably, the heat exchange fluid passed along the first subset of conduits does not pass through the second and/or the third heat exchanger. Further preferably, the heat exchange fluid passed along the second subset of conduits does not pass through the first and/or the fourth heat exchanger.

Optionally, the working fluid is passed through a pre-heater upstream of the recuperator.

Optionally, the working fluid is passed through a pre-heater downstream of the recuperator. Optionally, along the second arrangement of conduits comprises a circuit and the method may further comprise passing the heat transfer fluid along the circuit through the one or more heat exchangers and returning said heat transfer fluid to the TESD after use.

Optionally, the heat transfer fluid is received from a thermal energy storage device (TESD).

BRIEF DESCRIPTION OF DRAWINGS

Embodiments will now be described, by way of example only and with reference to the accompanying drawings having like-reference numerals, in which:

FIG. 1 shows a cryogenic energy storage system including a recovery subsystem according to the current invention;

FIG. 2 shows a variation of the system of FIG. 1 wherein the energy recovery subsystem further includes a second recuperator and associated heat exchanger;

FIG. 3 shows a variation of the system of FIG. 1 wherein the energy recovery subsystem includes separate subsets of conduits feeding heat exchange fluid from a thermal energy storage device to the heat exchanger associated with the recuperator two that feeding heat exchange fluid to the remainder of the heat exchangers;

FIG. 4A shows a variation of the system of FIG. 1 with the addition of a pre-heater located upstream in the flow of working fluid to the recuperator;

FIG. 4B shows a variation of the system of FIG. 1 with the addition of a pre-heater located downstream in the flow of working fluid to the recuperator; and

FIG. 5 shows a variation of the system of FIG. 1 without a heat exchanger between the recuperator and the first expansion stage in the process flow of working fluid.

SPECIFIC DESCRIPTION

Referring to FIG. 1, a first embodiment will now be described. FIG. 1 shows a cryogenic energy storage (CES) system 1 having an energy capture subsystem 2, a liquefaction subsystem 4 and an energy recovery subsystem 10.

The liquefaction subsystem 4 is for storing energy as potential energy in the form of a cryogenic gas. The liquefaction subsystem 4 receives air or another suitable gas and compresses and cools that gas in order to turn it into a cryogenic liquid known as a cryogen which is then fed to a cryogenic gas storage means 8a such as a cryogenic storage tank 8b. The CES systems 1 and/or any subsystem 2, 4, 10 described herein may be operable to use any suitable gas that can be liquefied for use as a cryogen for energy storage purposes. For example, the working fluid 5 may be one or more of air, nitrogen, oxygen, hydrogen, or carbon dioxide. The CES systems 1 and/or any subsystem 2, 4, 10 may also be operable to use a combination of such gases and may therefore comprise the necessary gas separation apparatus 80 and/or liquid separation apparatus 82 and separate storage apparatus necessary to use and process different gases or liquids.

The energy capture subsystem 2 is for capturing the heat of compression released during the liquefaction process in the liquefaction subsystem 4. The energy capture subsystem 2 passes the heat recovered from the liquefaction subsystem 4 to a thermal energy storage device 3 (TESD) via a thermal energy transfer fluid 6. Thermal energy transfer fluid 6 may also be referred to as heat transfer fluid 6 (HTF).

The CES system 1 also includes an energy recovery subsystem 10 for recovering energy from the stored cryogen which is used as a working fluid 5 in the energy recovery subsystem 10. The energy recovery subsystem 10 includes a first arrangement of conduits 100 for passing working fluid 5 and a second arrangement of conduits 200 connectable to a thermal energy storage device (TESD) 3 and for passing heat transfer fluid (HTF) 6. The first arrangement of conduits 100 has an upstream end 120 fluidly connected to a downstream end 121 for passing working fluid 5 therebetween. The downstream end 121 of the first arrangement of conduits 100 is connected to an outlet 70 for releasing spent working fluid 5 from the energy recovery subsystem 10. That outlet 70 may be connected to the atmosphere or to another system which may capture working fluid 5 and/or put the working fluid 5 to further use. The working fluid 5 used by the energy recovery sub-system 10 is received from a source of working fluid 8. The source of working fluid may be the cryogenic gas storage means 8a fed by the liquefaction sub system 4 described above.

The energy recovery subsystem 10 further includes a pump 12 for moving working fluid 5 along the first arrangement of conduits 100, an evaporator 14 for evaporating liquid working fluid 5, a recuperator 40 for passing thermal energy between the working fluid 5 to the working fluid 5 at another point along the first arrangement of conduits 100, one or more heat exchangers 20 for passing thermal energy between a heat transfer fluid 6 and the working fluid 5 and one or more expansion stages 30 for extracting energy from the working fluid 5.

The one or more expansion stages 30 are connected to an output shaft 36 which is connected to a power sink 60 such as an electrical generator 61. If there are a plurality of expansion stages 30 they may share a common output shaft 36. A power sink 60 may be a generator 61 or another machine requiring a rotational source of motive power 62.

The evaporator 14, the recuperator 40, a first heat exchanger 21 of the one or more heat exchangers 20, the recuperator 40 for a second time and the one or more expansion turbines 30 are fluidly connected by the first arrangement of conduits 100 in this sequence from the upstream end 120 of the first arrangement of conduits 100 for passing working fluid 5 therethrough. The recuperator 40 includes a first input 42, a first output 44, a second input 46 and a second output 48. The first input may be connected by the first arrangement of conduits 100 to receive working fluid from the evaporator 14, the first output 44 connected by the first arrangement of conduits 100 to pass working fluid to the first heat exchanger 21, the second input 46 connected by the first arrangement of conduits 100 to receive working fluid 5 from the first heat exchanger 21 and the second output 48 connected to the first arrangement of conduits upstream of the one or more heat exchangers 20. In the embodiment of FIG. 1 the second output 48 is connected to pass working fluid to a second heat exchanger 22.

The pump 12 is also connected to the arrangement of conduits 100 for moving the working fluid 5 therethrough. In FIG. 1 the pump 12 is shown connected between the upstream end 120 of the first arrangement of conduits 100 and the evaporator 14. It will be noted that the pump 12 can be located anywhere suitable for moving working fluid 5 along the first arrangement of conduits 100.

FIG. 1 shows a second heat exchanger 22 located along the first arrangement of conduits 100 between the recuperator 40 and the one or more expansion stages 30. The second heat exchanger 22 increases the temperature of the working fluid 5 after it has been reduced by passing through the recuperator 40, in order to improve the efficiency of a first expansion stages 31 of the one or more expansion stages 30.

Whilst a single expansion stage 30 is possible it is preferable to include a plurality of expansion stages 30. FIG. 1 includes a plurality of expansion stages 30 including a first expansion stage 31 connected downstream of the recuperator 40 for receiving working fluid 5 therefrom, two second expansion stages 32 and a third expansion stage 33 connected in sequence by the first arrangement of conduits 100 downstream of the first expansion stage 31.

The one or more second expansion stages 32 each receive working fluid from a third heat exchanger 23 connected along the first arrangement of conduits 100 located between said second expansion stage 32 and the next expansion stage 30 upstream along the first arrangement of conduits 100. FIG. 1 shows two second expansion stages 32 fed by two third heat exchangers 23, however, it will be understood that one or more second expansion stages 32 and or third heat exchangers 23 may be included in an energy recovery subsystem 10, wherein the second expansion stages 32 are interleaved with the third heat exchangers 23 along the first arrangement of conduits 100.

The third expansion stage 33 is connected to the first arrangement of conduits 100 downstream of another of the one or more expansion stages 30 without the working fluid passing through a heat exchanger 20 therebetween. One or more third expansion stages 33 can be included. FIG. 1 shows a third expansion stage as the final expansion stage 33 connected by the first arrangement of conduits 100 downstream of all other of the one or more expansion stages 30. Said final expansion stage 33 is further connected by the first arrangement of conduits 100 to an output 70 for releasing used working fluid 5 from the energy recovery subsystem 10 at the downstream end 121 of the first arrangement of conduits 100. The third expansion stage 33 connected by the first arrangement of conduits 100 downstream of all other of the one or more expansion stages 30 and preferably receives working fluid 5 from the to the last of the second expansion stages 32 without passing through a heat exchanger 20. Alternatively, a final one of the one or more expansion stages 30 may be connected by the first arrangement of conduits 100 to the output 70 for releasing used working fluid 5 from the energy recovery subsystem 10 at the downstream end 121 of the first arrangement of conduits 100.

The second arrangement of conduits 200 is connectable to a thermal energy storage device 3 and for passing heat exchange fluid 6 from said thermal energy storage device 3 through the one or more heat exchangers 20 in order to warm the working fluid 5. FIG. 1 shows the second arrangement of conduits 200 including a feed conduit 202 connectable to a TESD 3 and connected to each of the one or more heat exchangers 20 for passing HTF 6 thereto and a return conduit 204 for receiving HTF 6 after passing through the one or more heat exchangers 20. The one or more heat exchangers 20 are connected by the second arrangement of conduits 200 in parallel between the feed conduit 202 and the return conduit 204. FIG. 1 shows the second arrangement of conduits 200 as a circuit passing HTF 6 from the TESD 3 through each of the one or more heat exchangers 20 and returning the HTF 6 to the TESD 3.

In the figures the TESD 3 is shown as a single block, however, it will be understood that the TESD 3 may include one or more hot tanks 3a for high energy HTF 6 connected to the feed conduit 202 and one or more cool tanks 3b for used HTF 6 fed by the return conduit 204.

As shown in FIG. 1 the first arrangement of conduits 100 includes a first conduit 101 connectable to a source of working fluid 8 and extending between the upstream end 120 of the first arrangement of conduits 100 and the pump 12; a second conduit 102 extending between the pump 12 and the evaporator 14; a third conduit 103 extending between the evaporator 14 and the first input 42 of the recuperator 40; a fourth conduit 104 extending between the first output 44 of the recuperator 40 and the first heat exchanger 21; a fifth conduit 105 extending between the first heat exchange at 21 and the second input 46 of the recuperator 40; a sixth conduit 106 extending between the second output 48 of the recuperator 40 and the second heat exchanger 22; a seventh conduit 107 extending between the second heat exchanger 22 and the first expansion stage 31; an eighth conduit 108 extending between the first expansion stage 31 and the first of the third heat exchangers 23; a ninth conduit 109 extending between the first of the third heat exchangers 23 and the first of the second expansion stages 32; a tenth conduit 110 extending between the second expansion stage 32 and the second of the third heat exchangers; an eleventh conduit 111 extending between the second of the third heat exchangers 23 and the second of the second expansion stages 32; a twelfth conduit 112 extending between the second and last of the second expansion stages 32 and the third expansion stages 33; and a thirteenth conduit 113 extending between the third expansion stage and the downstream and 121 of the first arrangement of conduits 100 incorporating the outlet 70 for passing working fluid 5 there between.

FIG. 2 shows a variation of the CES system of FIG. 1 including the same features as FIG. 1 referred to by the same reference numbers with the addition of a second recuperator 50 and fourth heat exchanger 24 located downstream along the first arrangement of conduits 100 from the first recuperator 40 and upstream of the one or more expansion stages 30. The first recuperator 40, the second recuperator 50, the fourth heat exchanger 24, the second recuperator again 50 are connected along the first arrangement of conduits 100 in the aforementioned sequence for passing working fluid 5 therethrough.

The second recuperator 50 includes a first input 52, a first output 54, a second input 56 and a second output 58. The first input 52 may be connected by the first arrangement of conduits 100 downstream of the first recuperator 40, the first output 54 connected by the first arrangement of conduits 100 to pass working fluid to the fourth heat exchanger 24, the second input 56 connected by the first arrangement of conduits 100 to receive working fluid 5 from the fourth heat exchanger 24 and the second output 58 connected to the first arrangement of conduits 100 upstream of the one or more heat exchangers 30. In the embodiment of FIG. 2 the second output 58 is connected to pass working fluid to a second heat exchanger 22.

In the embodiment of FIG. 2 the sixth conduit 106 of the first arrangement of conduits 100 of the embodiment of FIG. 1 is replaced by a fourteenth conduit 114 extending between the second outlet 48 of the first recuperator 40 and the first input 52 of the second recuperator 50; a fifteenth conduit 115 extending between the first output 54 of the second recuperator 50 and the fourth heat exchanger 24; a sixteenth conduit 116 extending between the fourth heat exchanger 24 and the second input 56 of the second recuperator 50; and a seventeenth conduit 117 extending between the second output 58 of the second recuperator 50 and the second heat exchanger 22 for passing working fluid 5 therebetween. The second arrangement of conduits 200 is connected to the fourth heat exchanger 24 for the passing of HTF 6 thereto.

FIG. 3 shows a variation of the CES system of FIG. 1 including the same features as FIG. 1 having the same reference numerals. The arrangement of FIG. 3 differs from that shown in FIG. 1 in that the second arrangement of conduits 200 includes a first sub-set of conduits 210 and a second subset of conduits 220. Each of the first subset of conduits 210 and the second subset of conduits 220 having a feed 202, 212, 222 and a return 204, 214, 224. The first subset of conduits 210 is connectable to a TESD 3 and is connected to and for providing HTF 6 to the first heat exchanger 21. Thereby allowing independent control of the flow rate of HTF 6 through the first heat exchanger 21 which is fed by the recuperator 40. This can be especially useful during the start-up phase of the energy recuperation system 10. The second subset of conduits 220 is connectable to a TESD 3 and connected to the second heat exchanger 22 and/or the one or more third heat exchangers 23. Preferably, the second heat exchanger 22 and/or the one or more third heat exchangers 23 are connected in parallel.

Whilst FIG. 3 shows only one recuperator 40 associated with the first heat exchanger. It will be understood that the first subset of conduits 210 could also feed a second recuperator 50 associated with a fourth heat exchanger 24 as shown in FIG. 2. Preferably in such an arrangement the first heat exchanger 21 and the fourth heat exchanger 24 would be connected and supplied HTF 6 by the first subset of conduits 210 in parallel.

FIG. 4A shows a variation of the CES system 1 of FIG. 1 including the same features as FIG. 1 having the same reference numerals. The arrangement of FIG. 4A differs from that shown in FIG. 1 in that the first arrangement of conduits 100 of the energy recovery subsystem 10 includes a pre-heater 16 located along the first arrangement of conduits 100 between the evaporator 14 and the recuperator 40. In particular, preheater 16 is located between the evaporator 14 located upstream along the first arrangement of conduits 100 and the first input 42 of the recuperator 40 located downstream along the first arrangement of conduits 100. The third conduit 103 of the embodiment of FIG. 1 is replaced by an Eighteenth conduit 118 extending between the evaporator 14 and the preheater 16 and a nineteenth conduit 119 extending between the pre-heater 16 and the recuperator 40 for passing working fluid 5 therebetween.

FIG. 4B also shows a variation of the CES system 1 of FIG. 1 including the same features as FIG. 1 having the same reference numerals. As in FIG. 4A, the arrangement of FIG. 4B depicts the first arrangement of conduits 100 of the energy recovery subsystem 10 including a pre-heater 16 located along the first arrangement of conduits 100. It differs from FIG. 4A in that the pre-heater 16 is located along the first arrangement of conduits 100 between the second outlet 48 of the recuperator 40 located upstream along the first arrangement of conduits 100, and the second heat exchanger 22 located downstream along the first arrangement of conduits 100. The sixth conduit 106 of the embodiment of FIG. 1 is replaced by a twentieth conduit 106a extending between the second outlet 48 of the recuperator 40 and the pre-heater 16 and a twenty first conduit 106b extending between the pre-heater 16 and the second heat exchanger 22 for passing working fluid 5 therebetween.

Advantageously, this arrangement of the pre-heater 16 downstream of the recuperator 40 reduces the risk of molten salt freezing in the second heat exchanger 22. When air is passed through the first heat exchanger 21, it is heated by the molten salt, however this heat is subsequently captured and recuperated back into the air upstream of the first heat exchanger 21 by the recuperator 40. This reduces the temperature of the air leaving the recuperator and passing into the second heat exchanger 22, thereby increasing the risk of freezing the molten salt in the second heat exchanger 22. As discussed elsewhere in this disclosure, freezing of molten salt causes production losses and may lead to catastrophic equipment failure and is particularly marginal during system start up.

A pre-heater 16 provides heat from a heat source 18 that may be different to the TESD 3 to which the second arrangement of conduits 200 is connectable and used to increase the temperature of the working fluid 5 before entering the first heat exchanger 21. The heat source 18 may be contained within the pre-heater 16 or it may be remote to the preheater 16. If the heat source 18 is remote from the pre-heater 14 a second HTF may be used to transfer heat energy from the heat source 18 to the pre-heater 16. The second HTF may be different to and/or have a lower freezing temperature or crystallisation temperature than the first HTF 6 used in the second arrangement of conduits 200. The pre-heater 16 may be an electrical heater 16a or a combustion powered heater 16b or may receive waste heat energy from a source external to the energy recovery subsystem 10 such as waste heat from a co-located industrial system. The preheater 16 may be preferably used during a start-up phase of the energy recovery subsystem 10 during which time the temperature of HTF 6 reaching the first heat exchanger 21 may be lower.

FIG. 5 shows a variation on the CES system 1 of FIG. 1 wherein the one or more heat exchangers 20 of the energy recovery subsystem 10 includes no second heat exchanger 22. The first arrangement of conduits 100 include a twentieth conduit 122 extending between the recuperator 40 and the one or more expansion stages 30 for passing working fluid 5 therebetween in place of sixth conduit 106 and seventh conduit 107 of the arrangement of FIG. 1. This arrangement requires fewer heat exchangers 20 which may be beneficial, both by reducing component count and removing the pressure drop across the second heat exchanger 22, which may outweigh the disadvantage of providing cooler working fluid 5 to the first expansion stage 31 which may reduce efficiency.

It will be understood that whilst the figures show the energy recovery subsystem 10 as part of a CES system 1, such an energy recovery subsystem 10 can be fed by any source of working fluid 8 and the benefits of the claimed energy recovery subsystem 10 be realised. It will be further understood that the variations on FIG. 1 provided by the embodiments and arrangements of FIGS. 2 to 5 may be combined in any combination.

The recuperator 40, 50 may be a gas to gas heat exchanger for passing heat energy between the working fluid 5 and the working fluid 5 at different points along the first arrangement of conduits 100. Each of the one or more heat exchangers 20 may be a liquid to gas heat exchanger for passing heat energy between HTF 6 to the working fluid 5.

The one or more expansion stages 30 can be arranged in the form of one turbine with multiple expansion stages or as a plurality of turbines with any combination of the one or more expansion stages 30 in each turbine. The figures show one or more expansion stages 30 including a first expansion stage 31, one or more second expansion stages 32 and a third expansion stage 33. The skilled person will understand that the one or more expansion stages 30 may comprise one or more of each of the first expansion stage 31, second expansion stage 32 and/or third expansion stage 33.

In use a working fluid 5 is received by the energy recovery system 10 from a source of working fluid 8 at the upstream end 120 of the first arrangement of conduits 100. In the CES system 1 shown in FIGS. 1 to 5 this working fluid 5 is a cryogen that has been provided by an energy capture subsystem 2 including a liquefaction subsystem 4 for creating said cryogen. A pump 12 moves the working fluid 5 along the first arrangement of conduits 100 passing the working fluid 5 through the evaporator 14 where the liquid cryogenic working fluid 5 is evaporated to become a gas. The working fluid 5 is then passed along the first arrangement of conduits 100 optionally through a preheater 16 and through a recuperator 40 then a first heat exchanger 21 and back through the recuperator 40. The recuperator 40 passes heat from the working fluid 5 that has been warmed by the first heat exchanger 21 to the working fluid 5 upstream along the first arrangement of conduits 5 to the first heat exchanger 21. Thereby increasing the temperature of the working fluid 5 before entering the first heat exchanger 21 and reducing the chances of crystals forming in the HTF 6 or freezing of the HTF in the first heat exchanger 21. The optional preheater 16 can further increase the temperature of the working fluid 5 before entering the first heat exchanger 21. Downstream along the first arrangement of conduits 100 the working fluid 5 is passed to one or more expansion stages 30 to extract work therefrom. The working fluid 5 is at lower temperature after exiting the recuperator 40 than when exiting the first heat exchanger 21 or recuperator heat exchanger 21. Optionally, the first arrangement of conduits 100 passes the working fluid 5 through a second heat exchanger 22 or a booster heat exchanger 22 to increase the temperature of the working fluid 5 before it enters the one or more expansion stages 30 and therefore increasing the efficiency and/or output of the first expansion stage 31 of the one or more expansion stages 30. If the one or more expansion stages 30 comprises two or more expansion stages 30 the first arrangement of conduits may pass working fluid 5 through one or more third heat exchangers 23 or interstage heat exchangers 23 before passing the working fluid 5 through a further expansion stage downstream of the third heat exchanger 23. Each third heat exchanger 23 increases the temperature of the working fluid 5 before entry to the next expansion stage along the first arrangement of conduits 100 thus increasing the pressure of the working fluid 5 and the work that can be extracted from the working fluid which in turn increases the efficiency of the second expansion stage 32. The first arrangement of conduits 100 may pass the working fluid 5 through a further third expansion stage 33 or final expansion stage 33 without passing the working fluid through a third heat exchanger 23. This third expansion stage may be referred to as a final expansion stage 33 and is for extracting any remaining work from the working fluid 5 before the working fluid 5 is released from the energy recovery system 10 at the downstream end 121 of the first arrangement of conduits 100. The one or more heat exchangers 30 are fed with HTF 6 by the second arrangement of conduits 200 for heating the working fluid 5. The one or more heat exchangers 20 are preferably connected in parallel such that each is fed hot HTF 6 from the TESD 3.

HTF 6 used in the second arrangement of conduits 200 has a freezing point above ambient temperature. The use of molten salts as HTF 6 in particular have been found to be highly efficient for both transferring and storing hot thermal energy. As stated above this presents the issue that if the energy recovery system 10 allows the HTF 6 to cool below the temperature that it starts to freeze, known as the crystallisation temperature or the temperature at which crystals of salt start to appear in the molten fluid. Crystals or a complete freeze of HTF 6 in a risk when the energy recovery subsystem 10 is running and in particular when the system is being started working fluid enters the upstream end of the first arrangement of conduits 100 as a cryogenic liquid which is at very low temperature and can easily freeze HTF 6 in the first or most upstream of the one or more heat exchanger's 20 in the energy recovery subsystem 10.

The first arrangement of conduits 100 and the second arrangement of conduits 200 fluidly connect the components connected thereto. Unless otherwise stated herein connected “in sequence” gives the order in which features given are passed working fluid or connected to the first arrangement of conduits 100, 200 relative to each other. It does not mean that components are connected directly up or down stream of each other. Further components such as valves, sensors, connecting pieces or other components may be connected along the arrangement of conduits 100, 200 in between components in a given sequence.

The second arrangement of conduits 200 and each of the subsets of conduits 210, 220 of the second arrangement of conduits 200 include feed conduits 202, 212, 222 and return conduits 204, 214, 224. The one or more heat exchangers 20 are connected across feed conduits 202 and return conduits 204 for supplying hot heat transfer fluid 6 thereto and removing used heat transfer fluid 6 therefrom respectively. The first arrangement of conduits 200 may form one or more circuits if the TESD 3 includes a single reservoir for HTF 6. However, the TESD 3 preferably includes a hot tank 3a for storing hot HTF 6 and a colder tank 3b for storing used HTF 3. Note, that even in the colder tank the temperature 6 is maintained at a safe working temperature substantially above the freezing temperature of the HTF.

Any system feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure.

Any feature in one aspect, variation or embodiment may be applied to other aspects variations or embodiments, in any appropriate combination. In particular, method aspects may be applied to system aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination.

It should also be appreciated that particular combinations of the various features described and defined in any aspects can be implemented and/or supplied and/or used independently.