WO2008022407A1

WO2008022407A1 - A system and method for producing work

Info

Publication number: WO2008022407A1
Application number: PCT/AU2007/001227
Authority: WO
Inventors: Patrick Glynn; Colin Buckland
Original assignee: Commonwealth Scientific And Industrial Research Organisation; Perpetuity Management Pty Ltd
Priority date: 2006-08-25
Filing date: 2007-08-24
Publication date: 2008-02-28

Abstract

A system for producing work by expanding a working material, the working material comprising at least one component each component transitioning between liquid and gas phases in the system, the system comprising: a compressor for compressing the working material; a heater for heating the working material; an expander for expanding the working material to produce the work; and a recuperator comprising a condensing side and a boiling side for transferring at least some of the energy of the working material from the outlet of the expander in the condensing side to the working material at the outlet of the compressor in the boiling side, wherein a substantial portion of the energy transferred in the recuperator is at least a portion of the latent heat of the working fluid from the outlet of the expander.

Description

A SYSTEM AND METHOD FOR PRODUCING WORK

Field of the Invention

The present invention relates to a system for producing work by expanding a working material . The present invention also relates to a method for producing work by expanding a working material.

Background of the Invention A heat engine is a system arranged to convert thermal energy to mechanical work, thus enabling the production of power and/or refrigeration. The heat engine does this by transferring energy from a high temperature heat source (T_H) to a low temperature heat sink (T_L) . The efficiency of any heat engine is understood to be determined by, amongst other factors, the difference in temperature between the heat source and the heat sink. The efficiency of various heat engines currently in use range from 3% to about 60%. Most automotive engines have an efficiency of approximately 25% and supercritical coal- fired power stations have an efficiency of approximately 35-41%.

Because the efficiency of any heat engine is understood to be dependent on the temperature gradient between the heat source and heat sink, many attempts have been made to increase heat engine efficiencies by increasing this temperature gradient. It is generally understood that in order to increase the temperature gradient in a heat engine, then the temperature of the heat source has to be raised, because the temperature of the heat sink is limited by the atmospheric temperature of the Earth.

Theoretically, the most efficient heat engine is defined by the Carnot cycle and comprises a boiler, a turbine, a condenser and a pump. Under the Carnot cycle, the working fluid undergoes reversible isothermal heating from the high temperature reservoir in the boiler. reversible adiabatic expansion of the working fluid with a reduction in temperature from the high temperature (T_H) to the low temperature (T_L) , reversible isothermal cooling of the working fluid to the low temperature reservoir in the condenser, and reversible adiabatic compression of the working fluid with an increase in temperature from T_L to T_H in the pump. The thermal efficiency (ηra) of a heat engine operating according to the Carnot cycle is defined by the equation:

In practice, however, it is not possible to operate a heat engine according to the ideal Carnot cycle because none of the process steps are truly "reversible" . A reversible process is an ideal process that once having taken place can be reversed and in doing so leave no change in either the system or its surroundings. A number of factors are responsible for the processes in the Carnot cycle being irreversible, including friction losses in the system. An alternative, but not as efficient, cycle for operating a heat engine is the Rankine cycle. The ideal Rankine cycle involves reversible adiabatic compression from a low pressure to a high pressure by the pump, constant pressure (isobaric) heat transfer from the high temperature heat source in the boiler, reversible adiabatic expansion from the high pressure to the low pressure in the turbine, and a constant pressure (isobaric) transfer of heat from the working fluid to the low temperature heat sink in the condenser. The Rankine cycle differs from the Carnot cycle primarily in that complete condensation of the working fluid from a vapour to a liquid in the condenser occurs in the Rankine cycle. The reason for doing this is that whilst it reduces the efficiency of the heat engine, in practice, it is difficult for a pump to handle a mixture of liquid and vapour as is the case in the Carnot cycle. A further difference is that if the working fluid is heated to a superheated vapour in the boiler, in the Carnot cycle all the heat transfer is at a constant temperature and hence during this process the pressure must be reduced. This means that the heat must be transferred to the vapour as it undergoes an expansion process (which is difficult to carry out in practice) , as opposed to the Rankine cycle in which the vapour is superheated at a constant pressure. The isobaric heat transfer process in the Rankine cycle is easier to achieve in practice than the isothermal process in the Carnot cycle .

Most common power generation plants, including coal fired power generation plants operate according to the Rankine cycle. In practice, however, heat engines operating according to the Rankine cycle have a lesser efficiency than the maximum theoretical efficiency (ie. the efficiency of the ideal Rankine cycle) for similar reasons to those outlined for the Carnot cycle above. Variations on the Rankine cycle, in order to increase the efficiency of the heat engine, have been considered. Two such variations include the Rankine cycle with reheat and the regenerative Rankine cycle. In the Rankine cycle with reheat, the heat engine comprises two turbines in series. Working fluid as a vapour from the boiler at high pressure enters the first turbine where it is expanded to a lower pressure. The reduced pressure vapour exiting the first turbine re-enters the boiler where it is reheated before passing through the second turbine which operates at lower pressures. One advantage of this system is that reheating of the working fluid between the turbines prevents the working fluid from condensing from a vapour to a liquid during expansion in the turbines which could result in significant damage to the turbine . The regenerative Rankine cycle involves preheating of the working fluid prior to its entry to the boiler by splitting a small portion of steam from an intermediary stage in the turbine and mixing it with the liquid working fluid after it has been cooled in the condenser in a "feed water heater⁰ which is located at an intermediary pumping stage prior to the inlet of the working fluid to the boiler.

A variety of different working fluids have been proposed for and/or are in use in the Rankine cycle other than water, including molten metals. Differing working fluids have been selected for low temperature applications of the Rankine cycle such as in geothermal power generation where hydrocarbons (such as propane, butane or mixtures thereof for example) , carbon dioxide or refrigerants have been used. However, in low temperature geothermal applications, the overall cycle efficiencies achieved in practice are low (between 7 - 15%) . Similar efficiencies are achieved in solar thermal power generation systems.

Many other attempts have been made to increase the efficiency of real heat engines, such as the combined Brayton-Rankine cycle or C06AS cycle, which involves using the hot exhaust gas from a gas combustion heat engine operating according to the Brayton cycle as the heat source for the boiler of a second heat engine operating according to the Rankine cycle. However, the efficiencies of all real heat engines remain significantly limited, and improvements which increase the efficency of power and refrigeration production are still sought.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a system for producing work by expanding a working material, the working material comprising at least one component each component transitioning between liquid and gas phases in the system, the system comprising: a compressor for compressing the working material ; a heater for heating the working material; an expander for expanding the working material to produce the work; and a recuperator comprising a condensing side and a boiling side for transferring at least some of the energy of the working material from the outlet of the expander in the condensing side to the working material at the outlet of the compressor in the boiling side, wherein a substantial portion of the energy transferred in the recuperator is at least a portion of the latent heat of the working fluid from the outlet of the expander.

Typically, some sensible heat is also transferred in the recuperator. In an embodiment, the working material has as a physical property a critical point where saturated liquid transitions to saturated vapour, wherein above the critical pressure and temperature the working material is a gas, and wherein the working material has a saturated liquid vapour envelope below the critical point that separates the saturated liquid and vapour phases.

In an embodiment, the working material has a pressure expansion ratio of between two to thirty, preferably at least four, preferably ten or less. In an embodiment, the working material as an expanded gas can be condensed by ambient conditions or mechanically without exceeding the useful work produced by the system.

The at least one component of the working material may comprise any one or more organic compounds

(such as nitrogen, oxygen, hydrogen, helium for example) , hydro-carbon compounds (such as methane, ethane, propane, butane, pentane with or without flame retardants for example) , industrial and commercial refrigerants (such as chloro-fluoro-carbons, hydro-chloro-fluoro-carbons, hydro- fluoro-carbons for example) , inorganic compounds (such as ammonia, water or carbon dioxide for example) or metals (such as potassium, sodium or mercury for example) , or a specifically designed substance or a mixture of any two or more of these components .

In the embodiment where the working material comprises two or more components, it is preferable that the components exhibit similar condensing physical properties at the lowest pressure in the system.

In an embodiment, the expander comprises any suitable expander such as a turbine, a positive displacement rotary expander, a linear expander, a reciprocating engine, a scroll expander or a helical screw expander for example.

In other embodiments, the expander comprises multiple turbines, rotary expanders, linear expanders, reciprocating engines, scroll expanders or helical screw expanders, connected in parallel or in series and with or without interstage reheat. In some embodiments, the different expander types may be used in combination.

Where the expander is a turbine, the turbine may have variable pitch blades.

In an embodiment, the expander is connected to a load which may be a mechanical or electrical load.

The working material is preferably a supercritical gas at the inlet to the expander. In an embodiment, the temperature of the working material at the outlet of the expander is greater than the critical temperature (T_c) of the working material when the maximum pressure in the system is at or around the critical pressure (P_c) or greater than the corresponding saturation vapour temperature (T_Bat) for the maximum pressure when this pressure is below the critical pressure

⁽Pc⁾ •

In an embodiment, the temperature of the working material at the outlet from the expander is above the boiling temperature of the working material.

In an embodiment, the working material at the outlet of the expander is in the gas phase. In another embodiment, the expander is designed to pass out a portion of the working material at a pressure below the maximum pressure in the system or at several different pressures for use of this portion of the working material to provide an energy input to another system.

The portion of the working material passed out of the expander may or may not be recovered for re-use in the system. In the embodiment where the working material is recovered, it is mixed at the appropriate pressure with the working material exiting the expander or elsewhere in the system.

In an embodiment, the recuperator converts at least some of the working material from the outlet of the expander from a gas to a saturated liquid. Preferably, the remainder of the working material is a saturated gas .

In an embodiment, the recuperator converts at least some of the working material from the outlet of the compressor from a liquid to a saturated gas. Preferably, the remainder of the working material is a saturated liquid.

In an embodiment, the recuperator is in the form of a shell and tube heat exchanger. In an embodiment, the recuperator is in the form of a falling film condensor.

In an embodiment, the working material on the condensing side of the recuperator substantially condenses as it loses energy to the working material on the boiling side.

In an embodiment, the working material on the boiling side of the recuperator substantially boils as it receives energy from the working material on the condensing side. In an embodiment, the condensing side of the recuperator is the shell of a shell and tube heat exchanger and the boiling side is the tubes of a shell and tube heat exchanger.

In an embodiment, the condensing side of the recuperator is arranged to receive working material from the outlet of the expander. In an embodiment, not all of the working material condenses on the condensing side of the recuperator.

In an embodiment, the working material is cooled on the condensing side of the recuperator to the condensing side approach temperature. Throughout the specification, the "condensing side approach temperature" is understood to mean the temperature required to condense the working material at the pressure on the condensing side of the recuperator and is preferably less than the saturation temperature of the working material at that pressure. This temperature differential from the saturation temperature may be dependent on the working material, but in an embodiment is at least 2⁰C during normal operation of the system.

In an embodiment, the working material is boiled on the boiling side of the recuperator to the boiling side approach temperature.

Throughout the specification, the "boiling side approach temperature" is understood to mean the temperature required to boil the working material at the pressure on the boiling side of the recuperator and is preferably greater than the saturation temperature of the working material at that pressure. This temperature differential from the saturation temperature may be dependent on the working material, but in an embodiment is at least 2⁰C during normal operation of the system.

In an embodiment, the condensing side of the recuperator has a liquid fraction outlet and a gas fraction outlet.

In an embodiment, the system may comprise multiple recuperators connected in parallel or series.

In an embodiment, the recuperator boiling side comprises several stages of tubes, each tube stage having an increasing surface area for enabling the expansion of the working material with a low pressure drop in the boiling side of the recuperator.

In other embodiments, the recuperator boiling side may comprise a double or multiple pass tube bundle.

In an embodiment, the inside surfaces of the recuperator tubes are finned in any suitable manner, such as with cross sectional fins or spiral wrapped extended fin surfaces for example. In an embodiment, the recuperator tubes have an enhanced fluted tube profile for increasing the cross sectional area for heat transfer to the working material.

In an embodiment, the outer surfaces of the recuperator tubes are finned. In an embodiment, the recuperator condensing side comprises flow guidance baffles for directing the working material to a heat transfer surface (s) with the boiling side.

In an embodiment, the recuperator condensing side comprises at least one condensate collector for collection and removal of condensate.

Preferably, the at least one collector is located downstream of the working material flow inlet.

In an embodiment, the at least one collector is arranged to direct the collected condensate to a liquid collection basin of the recuperator.

In an embodiment, the system comprises a condenser for condensing the gas fraction of the working material from the condensing side of the recuperator. In an embodiment, the condenser is a shell and tube heat exchanger.

However, in other embodiments, particularly for certain types of working material, the condenser may be another form of heat exchanger such as an air cooled radiator or finned multiple pass or serpentine cooling coils enclosed in a plenum with a condensate basin, for example . In an embodiment, the condenser comprises a condensing side and a cooling side.

In an embodiment, the condensing side is the shell of a shell and tube heat exchanger and the cooling side is the tubes of the heat exchanger.

In an embodiment, the working material on the condensing side of the condenser substantially condenses as it is cooled by a cooling medium on the cooling side.

In an embodiment, the cooling medium on the cooling side of the condenser is heated by the working material on the condensing side.

The cooling medium used in the condenser may be any suitable medium such as a gas or a liquid including water, brine, eutectic, air, a refrigerant, an organic or inorganic compound or a metal for example.

In other embodiments, the condenser is arranged to provide cooling of the working material by using ambient conditions such as the atmosphere or the ocean or another large heat sink such as a river and/or by using mechanical refrigeration and/or by providing an energy input to another process.

In an embodiment, the condenser is arranged to receive the working material as a saturated vapour from the gas fraction outlet of the condensing side of the recuperator.

In an embodiment, the condenser is arranged to cool the gas fraction of the working material from the condensing side of the recuperator to the working material's saturated liquid temperature. In another embodiment, the condenser is arranged to cool the gas fraction of the working material from the condensing side of the recuperator to below the condensing side approach temperature of the recuperator.

In an embodiment, the condenser comprises a liquid collection basin on the condensing side.

In an embodiment, a secondary system is arranged to remove from the condenser the energy required to condense the gas working material in the condenser and transfer this energy to an external heat sink or source such as ambient conditions, air, cooling water from a cooling tower or the ocean, or an external refrigeration system or an external process or another power cycle for example.

The system may comprise multiple condensers connected in parallel and/or in series.

In an embodiment, the system also comprises at least one cooler for ensuring complete condensation of the working material from the condensing side of the recuperator.

In an embodiment, the at least one cooler is arranged to cool at least a portion of the working material from the condensing side of the recuperator.

In another embodiment, the at least one cooler is arranged to cool all of the working material from the condensing side of the recuperator.

In an embodiment, the at least one cooler is also for cooling the or a portion of the working material to a sufficiently low temperature to act against vaporisation of the working material during its subsequent compression in the compressor.

In an embodiment, the at least one cooler cools the or a portion of the working material to a temperature below the condensing side approach temperature of the recuperator.

In an embodiment, the at least one cooler is a shell and tube heat exchanger, a counter flow flat plate heat exchanger, a tube bundle or coil in a tank or any other suitable heat exchanger for transfer of heat from the working material to a cooling medium.

In an embodiment, the cooling medium in the at least one cooler is any suitable cooling medium such as water, brine, eutectic, a gas, or a refrigerant for example.

In an embodiment, a secondary system is arranged to remove the energy gained by the cooling medium in the at least one cooler and transfer this energy to an external heat sink or source such as ambient conditions, air, cooling water from a cooling tower, an external refrigeration system, an external process or power cycle for example.

In an embodiment, the system comprises a feed tank for feeding the working material to the compressor.

In an embodiment, the feed tank is also for receiving the working material from the condensing side of the recuperator.

In an embodiment, the feed tank receives the gas fraction of the working material from the condensing side of the recuperator via the condenser. In an embodiment, the working material from the condenser is mixed with the remaining working material from the condensing side of the recuperator in the feed tank.

In an embodiment, the pressures of the working material mixed in the feed tank are approximately equalised prior to mixing.

In an embodiment, the system is arranged to reduce the pressure of the gas working material from the condensing side of the recuperator either before or within the condenser to approximately the same pressure as the working material with which it is to be mixed.

In an embodiment, the system is arranged to reduce the pressure of the liquid working material from the condensing side of the recuperator either within or after the recuperator to approximately the same pressure as the working material with which it is to be mixed.

In an embodiment, the system is a closed system having substantially no mass inputs or outputs during operation of the system, other than replacement of incidental losses.

In an embodiment, the system comprises a make up feed, preferably to the feed tank, of the working fluid for replacing any incidental losses. Incidental losses may result from leaks, maintenance, or high pressure or high temperature releases for example.

In another embodiment, the system is an open system, wherein the gas fraction of the working material from the condensing side of the recuperator is not recovered. In this embodiment, the feed tank is arranged to receive the liquid fraction of the working material from the condensing side of the recuperator and comprises a substantial make up feed of working material.

In an embodiment, the feed tank is pressurised to a pressure greater than the ambient pressure.

In another embodiment, the feed tank comprises a vapour recycling system for recovering any gas working material from the feed tank and returning it to the inlet side of the condenser.

In an embodiment, the vapour recycling system comprises a compressor for compressing the recovered gas working material from the feed tank. In an embodiment, the volume of working material held in the feed tank is sufficiently large enough to maintain the lowest temperature in the system substantially constant.

In an embodiment, the or one of the at least one cooler is located in the feed tank.

In this embodiment, the or one of the at least one cooler is arranged to cool the mixture of working material from the liquid fraction outlet of the condensing side of the recuperator and the working material from the condenser.

In another embodiment, the or one of the at least one cooler is arranged to cool the liquid working material from the liquid fraction outlet of the condensing side of the recuperator. In this embodiment, the cooled liquid working material from the cooler may be used to ensure complete condensation of the working material from the gas fraction outlet of the condensing side of the recuperator, preferably, by further cooling the working material from the condenser.

In another embodiment, the or one of the at least one cooler is arranged to cool the working material from the outlet of the feed tank.

In another embodiment, the temperature of the working material from the liquid fraction outlet of the condensing side of the recuperator is cooled to below the condensing side approach temperature of the recuperator by mixing with working material from the condenser which has been supercooled.

In an embodiment, the compressor comprises a pump. Preferably, the compressor provides substantially adiabatic compression of the working material.

Typically, the pressure of the working material at the inlet to the expander is approximately equal to the pressure at the outlet of the compressor, less any losses in the system in between.

In an embodiment, the compressor is arranged to receive a single feed of liquid working material from the feed tank.

In an embodiment, the pressure of the working material at the outlet of the compressor is at or around the critical pressure (P_c) .

In other embodiments, the pressure of the working material at the outlet of the compressor is less than the critical pressure (P_c) if the system follows the load on the expander or if the system is designed to operate at an expander inlet temperature below the critical pressure

⁽Pc⁾.

In an embodiment, the temperature of the working material at the outlet of the compressor is less than the saturation temperature of the working material for the outlet compressor pressure.

In an embodiment, the temperature of the working material at the outlet of the compressor is less than the working material saturation temperature for the working material at the outlet of the expander.

In an embodiment, the compressor may comprise multiple compressors connected in parallel and/or in series.

In an embodiment, the compressor comprises a variable speed drive and/or a modulating bypass control valve for achieving variable system mass flow rates and following the load on the expander.

In an embodiment, the boiling side of the recuperator is arranged to receive working material from the outlet of the compressor.

In an embodiment, the heat source for the heater may be from combustion, nuclear power, geo thermal, solar thermal, waste heat from another process, stored thermal heat, electrical heat and/or ambient conditions.lt is to be understood that the heater may have multiple heat sources of the same or different type. The ambient conditions could be atmospheric conditions or could be ocean conditions if the system is used in a submarine vessel, for example.

In an embodiment, when the heater is arranged for direct heating of the working fluid, the working fluid is passed in direct contact with a heat transfer surface such as of a furnace, solar collector, waste heat source heat exchanger, or resistive electrical element for example.

In an embodiment, the heater is arranged to receive the working material from the outlet of the boiling side of the recuperator.

In an embodiment, the heater is arranged to heat the working material to above the critical temperature.

In an embodiment, the system may comprise multiple heaters, connected in series and/or parallel and supplied from one or more heat sources. In an embodiment, the expander is arranged to receive the working material from the outlet of the heater. In an embodiment, the system comprises a drainage mechanism for draining and/or decanting the working material from units and connecting pipework of the system for maintenance, servicing and appropriate storage of the drained or decanted working material.

In another embodiment, where a metal is the working material, the drainage mechanism comprises a storage vessel, transfer pumps and a heat input to the storage vessel, connecting pipework, and system units for removing and recharging of the system with the working material .

According to a second aspect of the present invention, there is provided a method for producing work by expanding a working material, the method comprising the steps of: providing the working material comprising at least one component, each component transitioning between liquid and gas phases in the method; compressing the working material in a compressor; heating the working material in a heater; expanding the working material to produce the work in an expander; and transferring in a recuperator comprising a condensing side and a boiling side at least some of the energy of the working material after it has been expanded, in the condensing side, to the working material prior to it being heated in the heater, in the boiling side, wherein a substantial portion of the energy transferred is at least a portion of the latent heat of the working material after it has been expanded in the expander.

Preferably, the method is a closed cycle method also comprising the step of repeating the steps of the method after at least some of the energy of the working material has been transferred in the recuperator. In an embodiment, the working material after the step of expanding is in the gas phase.

In an embodiment, the temperature of the working material after the step of expanding is above the boiling temperature of the working material.

In an embodiment, the temperature of the working material after the step of expanding is greater than the critical temperature (T_c) of the working material when the maximum pressure in the method is at or around the critical pressure (P_c) or greater than the corresponding saturation vapour temperature (T_aat) for the maximum pressure when this pressure is below the critical pressure (P_c) .

In an embodiment, the step of transferring at least some of the energy in the recuperator converts at least some of the working material after it has been expanded from a gas to a saturated liquid. Preferably, the remainder of the working material is a saturated gas.

In an embodiment, the step of transferring at least some of the energy in the recuperator converts at least some of the working material prior to it being heated in the heater from a liquid to a saturated gas. Preferably, the remainder of the working material is a saturated liquid.

In an embodiment, the working material after the step of expanding is received in the condensing side of the recuperator. In an embodiment, the working material prior to it being heated in the heater exits the boiling side of the recuperator .

In an embodiment, the working material after it has been compressed is received in the boiling side of the recuperator.

In an embodiment, the method also comprises the step of separating the gas fraction of the working material in the condensing side of the recuperator from the liquid fraction of the working material. In an embodiment, the method also comprises the step of flowing the gas fraction of the working material through a gas fraction outlet from the condensing side of the recuperator and flowing a liquid fraction of the working material through a liquid fraction outlet of the condensing side of the recuperator.

In an embodiment, the method also comprises the step of condensing the gas fraction of the working material from the condensing side of the recuperator in a condenser.

In an embodiment, the step of condensing comprises using a cooling medium to cool the working material in the condenser.

In another embodiment, the step of condensing comprises using ambient conditions such as the atmosphere or the ocean or another large heat sink such as a river and/or using mechanical refrigeration and/or by providing an input to another process to cool the working material in the condenser.

In an embodiment, the step of condensing comprises cooling the gas fraction of the working material from the condensing side of the recuperator to the working material saturated liquid temperature.

In another embodiment, the step of condensing comprises cooling the gas fraction of the working material from the condensing side of the recuperator to below the condensing side approach temperature of the recuperator. In an embodiment, the method also comprises the step of feeding the working material to the compressor from a feed tank.

In an embodiment, the method also comprises the step of receiving the working material from the condensing side of the recuperator in the feed tank.

In an embodiment, the step of receiving the working material in the feed tank comprises mixing the working material from the liquid fraction outlet of the condensing side of the recuperator with the working material from the condenser.

In an embodiment, the method also comprises the step of equalising the pressures of the working material to be mixed in the feed tank prior to the step of mixing. In an embodiment, the method also comprises the step of recycling any gas in the feed tank to the inlet of the condenser. In an embodiment, the method also comprises the step of cooling at least a portion of the working material from the condensing side of the recuperator in at least one cooler.

In another embodiment, the method also comprises the step of cooling all of the working material from the condensing side of the recuperator in at least one cooler.

In an embodiment, the step of cooling comprises cooling the or a portion of the working material to a sufficiently low temperature to act against vaporisation of the working material during the step of compressing.

In an embodiment, the step of cooling comprises cooling the or a portion of the working material to a temperature below the condensing side approach temperature of the recuperator. In an embodiment, the step of cooling comprises using a cooling medium to cool the working material in the at least one cooler.

In an embodiment, the step of cooling comprises cooling the mixture of working material from the liquid fraction outlet of the condensing side of the recuperator and the working material from the condenser in the feed tank.

In another embodiment, the step of cooling comprises cooling the working material from the feed tank prior to the step of compressing.

In an embodiment, the step of compressing comprises compressing the working material to at or around the critical pressure (P_c) .

In an embodiment, the step of compressing comprises compressing the working material whereby the temperature of the working material is less than the saturation temperature of the working material for the outlet compressor pressure.

In an embodiment, the step of heating comprises superheating the working material to above the critical temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic of a system for producing work by expanding a working material according to an embodiment of the invention;

Figure 2 is a schematic of a system for producing work by expanding a working material according to another embodiment of the invention;

Figure 3 is a schematic of a solar thermal power generation application of the system for producing work of Figure 1;

Figure 4 is a schematic of a geo-thermal power generation application of the system for producing work of Figure 1; and

Figure 5 is a schematic view of a HYSYS^® model of a system for producing work according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Referring firstly to Figure 1, a system 10 for producing work by expanding a working material according to an embodiment of the present invention is shown. The working material used in the system comprises at least one component, each component transitioning between liquid and gas phases in the system 10.

The system 10 comprises a compressor 11 for compressing the working material, a heater 12 for superheating the working material and an expander 13 for expanding the working material to produce the work. The system 10 also comprises a recuperator 14 for transferring at least some of the energy of the working material from the outlet of the expander to the working material at the outlet of the compressor. A substantial portion of the energy transferred in the recuperator 14 is at least a portion of the latent heat of the working fluid from the outlet of the expander.

Latent heat is the energy associated with the change of state of a material, ie. when transitioning between the liquid and gas states. It is noted that usually at least some of the energy transferred in the recuperator 14 is sensible heat. Advantageously, transfer of the latent heat enables the recovery of a significant amount of energy which will be otherwise lost from the system and therefore greatly improves the efficiency of the system 10. The latent heat recovery in the recuperator 14 of the system 10 is particularly advantageous in low temperature applications such as solar thermal and geo- thermal power generation where the efficiency in such applications is not readily improved by the conventional method of increasing the top temperature of the system (ie. the temperature at the outlet of the heater) .

The working material used in the system has as a physical property, a critical point where saturated liquid transitions to saturated vapour. Above the critical pressure and temperature the working material is preferably a gas. Furthermore, the working material preferably has a saturated liquid vapour envelope below the critical point that separates saturated liquid and vapour phases. The working material also has as a physical property a pressure expansion ratio between 2 to 30, preferably at least 4, preferably 10 or less. Preferably, the working material as an expanded gas can be condensed by ambient conditions or mechanically without exceeding the useful work produced by the system. This enables the system 10 to provide a useful work output whilst operating as a closed system.

The at least one component of the working material comprises any one or more organic compounds (such as nitrogen, oxygen, hydrogen, helium for example) , hydrocarbon compounds (such as methane, ethane, propane, butane, pentane with or without flame retardants for example) , industrial and commercial refrigerants (such as chloro-fluoro-carbons, hydro-chloro-fluoro-carbons, hydro- fluoro-carbons for example) , inorganic compounds (such as ammonia, water or carbon dioxide for example) or metals (such as potassium, sodium, or mercury for example) , or a specifically designed substance or a mixture of any two or more of these components. Where the working material comprises two or more components, it is preferable that the components exhibit similar condensing physical properties at the lowest pressure in the system. This avoids the need to waste energy in the system lowering the temperature of the working material to condense one of the components when the one or more other components condense at a significantly higher temperature.

The expander 13 comprises any suitable expander such as a turbine, a positive displacement rotary expander, a linear expander, a reciprocating engine, a scroll expander or a helical screw expander for example. The expander may comprise multiple turbines, rotary expanders, linear expanders, reciprocating engines, scroll expanders or helical screw expanders, connected in parallel or in series and with or without interstage reheat. In some embodiments of the present invention, the different expander types may be used in combination. Where the expander is a turbine, the turbine may have variable pitch blades.

The temperature of the working material at the outlet of the expander 13 is greater than the critical temperature (T_c) of the working material when the maximum pressure in the system 10 is at or around the critical pressure (P_c) or greater than the corresponding saturation vapour temperature (T_βat) f°^r the maximum pressure when this pressure is below the critical pressure (P_c) . Furthermore, the temperature of the working material at the outlet from the expander is above the boiling temperature of the working material, so that the working material at this point in the system is in the gas phase. These conditions ensure boiling of the working material from the compressor 11 in the recuperator 13 to which the energy in the working material exiting the expander 13 is transferred. The second condition is generally also required during start up and shut down of the system, during which the pressure in the system is being either built up or turned down.

In a further variation to that shown in Figure 1, the expander is designed to pass out a portion of the working material at a pressure below the maximum pressure in the system or at several different pressures for use of this portion of the working material to provide an energy input to another system. The portion of the working material passed out of the expander in this variation may or may not be recovered for reuse in the system. Where the working material is recovered it may be mixed at the appropriate pressure with the working material at the outlet of the expander or elsewhere in the system.

The recuperator 14 comprises a condensing side 20 and a boiling side 21. On the condensing side 20, the recuperator 14 converts at least some of the working material from the outlet of the expander 13 from a gas to a saturated liquid. The remainder of the working material exits the condensing side 20 as a saturated gas. On the boiling side 21, the recuperator 14 converts at least some of the working material from the outlet of the compressor from a liquid to a saturated gas, with the remainder of the working material exiting the boiling side 21 as a saturated liquid. It is noted that whilst the system is shown having only one recuperator, it may comprise multiple recuperators connected in parallel and/or in series.

The recuperator 14 is generally in the form of a shell and tube heat exchanger, more specifically in the form of a falling film condensor. The condensing side 20 of the recuperator is the shell and the boiling side 21 is the tubes. More specifically, the recuperator boiling side 21 comprises several stages of tubes, each tube stage having an increasing surface area for enabling the expansion of the working material with a low pressure drop across the boiling side 21. In another embodiment, however, the recuperator boiling side 21 may comprise a double or multiple pass tube bundle. The inside surfaces of the recuperator tubes (ie. the boiling side 21) may be finned in any suitable manner, such as having cross sectional fins or spiral wrapped extended fin surfaces for example. The tubes of the recuperator boiling side 21 may also have an enhanced fluted tube profile for increasing the cross sectional area for heat transfer to the working material on the boiling side 21. The outer surfaces of the recuperator tubes may also be finned in any manner. Fins on the outer surfaces of the recuperator tubes not only provide improved heat transfer in the recuperator 14 but also assist in removing the condensed working material from the tube outer surfaces.

The recuperator condensing side 20 comprises flow guidance baffles for directing the working material to a heat transfer surface (s) with the boiling side. The recuperator condensing side 20 also comprises at least one condensate collector for collection and removal of condensate downstream of the working material flow inlet to the condensing side 20. The at least one collector is arranged to direct the collected condensate to a liquid collection basin in the condensing side 20 of the recuperator 14.

As not all of the working material condenses on the condensing side 20 of the recuperator 14, the condensing side 20 has a liquid fraction outlet 22 and a gas fraction outlet 23.

The system 10 also comprises a condenser 15 for condensing the gas fraction of the working material from the condensing side 20 of the recuperator 14. The condenser 15 is therefore arranged to receive working material from the gas fraction outlet 23 of the recuperator 14. It is noted that whilst Figure 1 shows the system comprising only one condenser, the system 10 may comprise multiple condensers connected in parallel and/or in series.

The condenser 15 is preferably a shell and tube heat exchanger. However, in other embodiments, particularly for certain types of working material, the condenser may be another form of heat exchanger such as an air cooled radiator or finned multiple pass or serpentine cooling coils enclosed in a plenum with a condensate basin for example.

The condenser 15 comprises a condensing side 24 and a cooling side 25. The condensing side 24 is the shell of a shell and tube heat exchanger and the cooling side 25 is the tubes of the heat exchanger. The working material on the condensing side of the condenser substantially condenses as it is cooled by a cooling medium on the cooling side 25. Conversely, the cooling medium on the cooling side 25 of the condenser is heated by the working material on the condensing side 24. The cooling medium used in the condenser 15 may be any suitable medium such as a gas or a liquid including water, brine, eutectic, air, a refrigerant, an organic or inorganic compound or a metal for example.

In a variation, the condenser is arranged to provide cooling of the working material by using ambient conditions such as the atmosphere or the ocean or another large heat sink such as a river and/or by using mechanical refrigeration and/or by providing an energy input to another process. The condenser 15 cools the gas fraction of the working material from the condensing side 20 of the recuperator 14 to the working material saturated liquid temperature and/or below the condensing side approach temperature of the recuperator 14.

The condenser 15 comprises a liquid collection basin on the condensing side 20. The working material as it condenses in the condensing side collects in the liquid collection basin and exits the condenser from the basin through a liquid outlet.

The system 10 also comprises a feed tank 16 for feeding the working material to the compressor 11. The feed tank 16 is also arranged to receive the working material from the condensing side 20 of the recuperator 14. In the embodiment shown in Figure 1, the feed tank 16 receives the working material from the liquid fraction outlet 22 of the condensing side 20 of the recuperator 14 and the working material from the gas fraction outlet 23 via the condenser 15 and mixes them together. Notably, the pressures of the working material mixed in the feed tank are approximately equalised prior to mixing. This is to act against vaporisation of the liquid working material when the two streams of the working material are mixed together, in particular the liquid fraction from the recuperator 14 which may be at the saturation temperature (Tsat) and therefore susceptible to vaporisation on any reduction of pressure or increase in temperature. More specifically, the pressure of the working material from the gas fraction outlet 23 of the condensing side 20 of the recuperator 14 is reduced in pressure either before or within the condenser 15 to approximately the same pressure as the working material with which it is to be mixed. Similarly, the system 10 is arranged to reduce the pressure of the working material from the liquid fraction outlet 22 of the condensing side 20 of the recuperator either within or after the recuperator 14 to approximately the same pressure as the working material with which it is to be mixed. The feed tank 16 may or may not be pressurised to a pressure greater than the ambient pressure. With the feed tank 16 mixing the working material from both the liquid and gas fraction outlets 22, 23 of the condensing side 20 of the recuperator 14, the system is a closed system having substantially no mass inputs or outputs during operation of the system, other than replacement of incidental losses. The system 10 does, however, comprise a make up feed 17, preferably to the feed tank 16, of the working fluid for replacing any incidental losses . Incidental losses may result from leaks, maintenance, or high pressure or high temperature releases for example.

In a variation to the embodiment shown in Figure 1, the system is an open system, wherein the gas fraction of the working material from the condensing side 20 of the recuperator 14 is not recovered. In this variation, it is not necessary to have the condenser and the feed tank 16 is simply arranged to receive the liquid fraction of the working material from the condensing side 20 of the recuperator 14 and mixes it with a substantial make up feed of working material.

In a further variation, the feed tank 16 comprises a vapour recycling system for recovering any gas working material from the feed tank 16 and returning it to the inlet side of the condenser 15 for condensation. The vapour recycling system may comprise a compressor for compressing the recovered gas working material from the feed tank 16 to the pressure of the working material at the gas fraction outlet 23 of the condensing side 20 of the recuperator 14. The volume of working material held in the feed tank 16 is sufficiently large enough to maintain the lowest temperature in the system 10 substantially constant. The volume of working material in the feed tank 16 is thus sufficiently large for fluctuations in the mass flow rate of the working material through the system 10 to not significantly affect the temperature of the working material in the feed tank 16. Advantageously, this enables the system 10 to be rapidly started up and shut down as well as enabling more efficient control of the system 10 during normal operation.

The system 10 also comprises a cooler 18 for ensuring complete condensation of working material from the condensing side 20 of the recuperator 14. In Figure 1, the cooler 18 is shown located at position A in the feed tank 16. However, the cooler 18 may be located at position B where it is arranged to cool the liquid working material from the liquid fraction outlet of the condensing side of the recuperator or at position C, where it is arranged to cool the working material form the outlet of the feed tank as outlined in Figure 1. Alternatively, the system 10 may comprise two coolers at two of positions A, B or C or a cooler at each of positions A, B and C.

The cooler 18 is arranged to cool at least a portion of the working material from the condensing side 20 of the recuperator 14. In positions A and B, it is noted that the cooler 18 is arranged to cool all of the working material from the condensing side 20 of the recuperator 14.

The cooler 18 is also for cooling the working material to a sufficiently lower temperature to act against vaporisation of the working material during its subsequent compression in the compressor 11.

Advantageously, this prevents any damage being caused to the compressor 11 due to the formation of gas in the compressor 11, as well as increasing the efficiency of the energy recovery in the recuperator 14 (and hence in the system 10 overall) by ensuring that the working material at the inlet to the boiling side 21 of the recuperator 14 is a liquid. The cooler 18 achieves these functions by cooling the or a portion of the working material to a temperature below the condensing side approach temperature of the recuperator 14.

The cooler 18 may be a shell and tube heat exchanger, a counter flow flat plate heat exchanger, a tube bundle or coil in a tank, or any other suitable heat exchanger for transfer of heat from the working material to a cooling medium. The cooling medium in the cooler 18 may be any suitable cooling medium such as water, brine, eutectic, a gas, or a refrigerant for example. In a variation, a secondary system is arranged to remove the energy gained by the cooling medium in the cooler 18 and transferred as energy to an external heat sink or source such as ambient conditions, air, cooling water from a cooling tower, an external refrigeration system, an external process or power cycle for example.

Where the cooler 18 is located at position B, the cooled liquid working material from the cooler may be used to ensure complete condensation of the working material from the gas fraction outlet 23 of the condensing side 20 of the recuperator 14. With the cooler located at position B, it may therefore not be necessary to have the condenser 15, with the cooled liquid material from the cooler providing all of the condensation of the working material from the gas fraction outlet 23. However, preferably, condensation of the working material from the gas fraction outlet 23 is achieved in this embodiment by a combination of cooling in the condenser and cooling by mixing with the cooled liquid working material from the cooler. In a further variation, the functions of the cooler are provided by the condenser, which supercools the working material from the gas fraction outlet of the recuperator. In this variation, a separate cooler may not be required in the system. The compressor 11 comprises a pump which preferably provides a substantially adiabatic compression of the working material. Typically, the pressure of the working material at the inlet to the expander 13 is approximately equal to the pressure of the outlet of the compressor 11, less any losses in the system in between. The compressor 11, may comprise multiple compressors connected in parallel and/or in series. The compressor 11 is arranged to receive a single feed of liquid working material from the feed tank 16. The working material is compressed in the compressor 11 to a pressure which is at or around the critical pressure (P_c) . Additionally, the temperature of the working material at the outlet of the compressor 11 is preferably less than the saturation temperature of the working material for the outlet compressor pressure (ie. less than T_c) . This is to act against any of the working material at the outlet of the compressor being in the gas phase, the advantages of which have been discussed above.

The compressor 11 may also comprise a variable speed drive and/or a modulating bypass control valve for achieving variable system mass flow rates and following the load on the expander 11.

The heater 12 provides a heat input to the working material from the boiling side 21 of the recuperator 14 from a heat source to heat the working material, preferably to above the working material's critical temperature. The heat source for the heater 12 may be from combustion, nuclear power, geothermal, solar thermal, waste heat from another process, stored thermal heat, electrical heat and/or ambient conditions. The ambient conditions could be atmospheric conditions or could be ocean conditions if the system is used in a submarine vessel, for example. Thus, the heater 12 may be arranged for either indirect or direct heating of the working fluid. When the heater 12 is arranged for direct heating of the working fluid, the working fluid is passed in direct contact with a heat transfer surface such as of a furnace, solar collector, waste heat source heat exchanger, or resistive or inductive electrical element for example.

It is noted that the heator 12 may comprise multiple heaters connected in parallel and/or series, and having one or more heat sources of the same or different type. The system 10 may also comprise a drainage mechanism for draining and/or decanting the working material from units and connecting pipe work of the system 10 for maintenance, servicing and appropriate storage of the drained or decanted working material. Furthermore, where a metal is the working material, the drainage mechanism comprises a storage vessel, transfer pumps and a heat input to the storage vessel, connecting pipe work, and system units for removing and recharging the system with the working material.

Referring now to Figure 2, a system 110 for producing work by expanding a working material according to another embodiment of the invention is shown. The system 110 generally comprises two cycles IIOA and HOB which are each similar to the system 10 shown in Figure 1, arranged in parallel. The first cycle HOA provides a bottom cycle operating at a lower temperature than the second top cycle HOB. Advantageously, the combined two or more cycles provide improved efficiency and reduced water consumption as compared to a comparative Rankine power cycle using atmospheric cooling towers (for example) .

A notable feature of the system 110 is that the heater of the bottom cycle HOA is the cooling side of the condenser of the top cycle HOB. Although the working materials in the top and bottom cycles HOA, HOB may be the same, it is preferable to choose different working materials for the different conditions in the different cycles HOA, HOB. For example, the bottom cycle HOA may have a working material with a low temperature boiling point such as propane or butane for example and the top system HOA may have a working material with a higher temperature boiling point such as water or ammonia for example.

In a variation to the embodiment shown in Figure 2, the top cycle HOB may not have a recuperator as shown in Figure 2. This is because the latent heat of the working material in the top cycle HOB can be recovered in the heater of the bottom cycle IIOA (i.e. the condensor of the top cycle HOB) . Also, in further variations, the system 110 may comprise a third or more cycles connected in parallel to the two cycles HOA and HOB as shown in Figure 2 with or without recuperators in the top cycles and preferably using different working fluids in each cycle such as R134a, butane or pentane in a first (bottom) cycle, water in a second (middle) cycle and mecury in a third (top) cycle, for example. Referring now to Figure 3, a solar thermal power generation application of the system 10 from Figure 1 is shown. In the embodiment shown in Figure 3, the heater 12 receives a solar thermal heat input. To achieve this, the system comprises at least one, preferably numerous solar collectors 30 for collecting radiation from the sun. The solar collectors 30 may be in the form of flat plate collectors as shown in Figure 3 or may be in the form of any suitable solar collector such as flat plate collectors with or without extended reflective collection surfaces, evacuated tube collectors, solar heat pipes, solar ponds, parabolic trough concentrator collectors, circular trough concentrator collectors, fixed or tracking fresnel reflectors with parabolic or circular trough concentrator collectors or evacuated tube concentrator collectors, paraboloidal dish concentrator collectors, semi-spherical dish concentrator collectors, convex or fresnel lens concentrator collectors, paraboloidal or spherical Fresnel dish concentrator collectors, a compound parabolic concentrator collector, or a field or array of heliostat mirrors concentrating on a central receiver for example.

The solar collectors may incorporate sun position tracking either single axis or double axis to track solar altitude angle and or solar azimuth angle.

In the embodiment shown in Figure 3, the solar collectors 30 are arranged to provide indirect heating of the working material. A circulating pump 31 pumps a first heating medium through the solar collectors 30 either in parallel as shown in Figure 3 or in series. Furthermore, the piping may be arranged for direct return or reversed return (as shown) . The first heating medium is pumped by the circulating pump 31 from and to a storage tank 32 via the solar collectors 30. The storage tank 32 contains a reservoir of the first heating medium. The heating medium may be water, brine, oil, eutectic, a molten salt, a molten metal or any other suitable heating medium. In a variation, the storage tank may be a heat cell for storing heat and comprises a volume or multiple volumes of a phase change material which is heated by the first heating medium or heated directly by concentrated solar radiation. In another variation, multiple storage tanks may be used to enable high temperature storage from the solar collectors 30 and low temperature return from the system

10. In another variation, the first heating medium, heated by the solar collectors 30, may also be used to provide a heat input to another process such as for hot water, heating or refrigeration for example. A heat input to the working material in the heater 12 is provided by flowing a second heating medium through the storage tank 32 and then through the heater 12. In a variation, the first heating medium in the storage tank 32 may be taken from the storage tank 32 and flow through the heater 12 to heat the working material.

Auxiliary heaters may also be provided in the system shown in Figure 3 to provide a back up heat source during the times of low sun exposure.

In a further variation, the solar collectors 30 may be arranged to provide direct heating of the working material by flowing the working material through the solar collectors 30.

In another variation, the system 110 of Figure 2, comprising first and second cycles HOA, HOB, may be used in a solar thermal power generation application. This variation is particularly suitable for high temperature solar collection. The solar collectors for this variation may comprise any suitable collector such as a field or a array of heliostat mirrors concentrating on a central receiver, paraboloidal dish concentrator collectors, semi- spherical dish concentrator collectors, fresnel paraboloidal or semi-spherical dish concentrator collectors for example. Advantageously, this variation would enable the use of a working fluid with a low critical point in the bottom system IIOA (such as isobutane or refrigerant 134a) and a working fluid with a higher critical point in the top system HOB (such as ammonia or water) . Furthermore, the system 110 could comprise a third top cycle above the second cycle HOB, for attaining a higher efficiency.

Referring now to Figure 4, a geothermal power generation application of the system 10 of Figure 1 is shown. In this embodiment, the heat source for the heater is provided by hot fluid from a geothermal resource. In the embodiment shown in Figure 4, hot fluid flows from a production well 40 through the heater 12 to heat the working fluid therein. The now colder fluid from the production well 40 exits the heater 12 to be disposed of in a reinjection well 41.

EXAMPLE

A model of a system for producing work according to embodiments of the present invention was constructed in HYSYS^®. Figure 5 provides a schematic view of the model. The model was run for a number of different working materials, namely refrigerant 134A, isobutane, ammonia and an 80:20 mixture of isobutane and pentane. Parameters assumed for the model included: • a dryness fraction of 50% at the outlet of the condensing side of the recuperator

• a pump (compressor) efficiency of 80%

• an expander efficiency of 80%

• a heater efficiency of 90% • a mass flowrate of 1 kilogram per second

Table 1 below sets out the conditions at points A-F in the system as indicated on Figure 5.

Table 1

Table 2 below shows the net shaft (or brake) power outputs and efficiencies of the model system calculated for the different working materials.

Table 2

In the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, ie. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

1. A system for producing work by expanding a working material, the working material comprising at least one component each component transitioning between liquid and gas phases in the system, the system comprising: a compressor for compressing the working material; a heater for heating the working material; an expander for expanding the working material to produce the work; and a recuperator comprising a condensing side and a boiling side for transferring at least some of the energy of the working material from the outlet of the expander in the condensing side to the working material at the outlet of the compressor in the boiling side, wherein a substantial portion of the energy transferred in the recuperator is at least a portion of the latent heat of the working fluid from the outlet of the expander.

2. A system as claimed in claim 1, wherein the temperature of the working material at the outlet of the expander is greater than the critical temperature (T_c) of the working material when the maximum pressure in the system is at or around the critical pressure (P_c) or greater than the corresponding saturation temperature (T_SAT) for the maximum pressure when this pressure is below the critical pressure (P_c) .

3. A system as claimed in either one of claims 1 or 2, wherein the recuperator is in the form of shell and cheap heat exchanger.

4. A system as claimed in any one of the preceding claims, wherein the recuperator is in the form of a falling film condenser.

5. A system as claimed in any one of the preceding claims, wherein the condensing side of the recuperator has a liquid fraction outlet and a gas fraction outlet.

6. A system as claimed in any one the preceding claims, the system comprising a condenser for condensing the gas fraction of working material from the condensing side of the recuperator.

7. A system as claimed in claim 6, wherein the condenser is arranged to cool the working material to the working material saturated liquid temperature.

8. A system as claimed in claim 6, wherein the condenser is arranged to cool the working material to below the condensing side approach temperature of the recuperator.

9. A system as claimed in any one of the preceding claims, wherein the system comprises a feed tank for feeding the working material to the compressor.

10. A system as claimed in claim 9 when dependent on any one of claims 6 to 8, wherein the working material from the condenser is mixed with the remaining working material from the condensing side of the recuperator in the feed tank.

11. A system as claimed in claim 10, wherein the pressures of the working material mixed in the feed tank are approximately equalised prior to mixing.

12. A system as claimed in any one of claims 9 to 11, wherein the volume of working material held in the feed tank is sufficiently large enough to maintain the lowest temperature in the system substantially constant.

13. A system as claimed in any one of the preceding claims, wherein the system also comprises at least one cooler for ensuring complete condensation of the working material from the condensing side of the recuperator.

14. A system as claimed in claim 13, wherein the at least one cooler is arranged to cool at least a portion of the working material from the condensing side of the recuperator.

15. A system as claimed in claim 13, wherein the at least one cooler is arranged to cool all of the working material from the condensing side of the recuperator.

16. A system as claimed in any one of claims 13 to

15, wherein the at least one cooler cools the or a portion of the working material to a temperature below the condensing side approach temperature of the recuperator.

17. A system as claimed in any one of claims 13 to 16 when dependent on any one of claims 9 to 12, wherein the or one of the at least one cooler is located in the feed tank.

18. A system as claimed in any one of claims 13 to 17 when dependent on any one of claims 9 to 12, wherein the or one of the at least one cooler is arranged to cool the working material from the outlet of the feed tank.

19. A system as claimed in any one of claims 13 to 18, wherein the or one of the at least one cooler is arranged to cool the liquid fraction of the working material from the condensing side of the recuperator.

20. A system as claimed in any one of the preceding claims, wherein the temperature of the working material at the outlet of the compressor is less than the saturation temperature of the working material for the outlet compressor pressure.

21. Λ system as claimed in any one of the preceding claims, wherein the compressor comprises a variable speed drive and/or a modulating bypass control valve for achieving variable system mass flow rates and following the load on the expander.

22. A method for producing work by expanding a working material, the method comprising the steps of: providing the working material comprising at least one component, each component transitioning between liquid and gas phases in the method; compressing the working material in a compressor; heating the working material in a heater; expanding the working material to produce the work in an expander; and transferring in a recuperator comprising a condensing side and a boiling side at least some of the energy of the working material after it has been expanded, in the condensing side, to the working material prior to it being heated in the heater, in the boiling side, wherein a substantial portion of the energy transferred is at least a portion of the latent heat of the working material after it has been expanded in the expander.

23. A method as claimed in claim 22, wherein the method is a closed cycle method also comprising the step of repeating the steps of the method after at least some of the energy of the working material has been transferred in the recuperator.

24. A method as claimed in either one of claims 22 or 23, wherein the temperature of the working material after the step of expanding is above the boiling temperature of the working material .

25. A method as claimed in any one of claims 22 to

24, wherein the temperature of the working material after the step of expanding is greater than the critical temperature (T_c) of the working material when the maximum pressure in the method is at or around the critical pressure (P_c) or greater than the corresponding saturation vapour temperature (T_sat) for the maximum pressure when this pressure is below the critical pressure (P_c) •

26. A method as claimed in any one of claims 22 to

25, wherein the method also comprises the step of separating the gas fraction of the working material in the condensing side of the recuperator from the liquid fraction of the working material.

27. A method as claimed in any one of claims 22 to

26, wherein the method also comprises the step of condensing the gas fraction of the working material from the condensing side of the recuperator in a condenser.

28. A method as claimed in claim 27, wherein the step of condensing comprises cooling the gas fraction of the working material from the condensing side of the recuperator to the working material saturated liquid temperature .

29. A method as claimed in claim 27, wherein the step of condensing comprises cooling the gas fraction of the working material from the condensing side of the recuperator to below the condensing side approach temperature of the recuperator.

30. A method as claimed in any one of claims 22 to 29, wherein the method also comprises the step of feeding the working material to the compressor from a feed tank.

31. A method as claimed in claim 30, wherein the method also comprises the step of receiving the working material from the condensing side of the recuperator in the feed tank.

32. A method as claimed in claim 31, when dependent on any one of claims 27 to 29, wherein the step of receiving the working material in the feed tank comprises mixing the working material from the liquid fraction outlet of the condensing side of the recuperator with the working material from the condenser.

33. A method as claimed in claim 32, wherein the method also comprises the step of equalising the pressures of the working material to be mixed in the feed tank prior to the step of mixing.

34. A method as claimed in any one of claims 22 to

33, wherein the method also comprises the step of cooling at least a portion of the working material from the condensing side of the recuperator in at least one cooler.

35. A method as claimed in claim 34, wherein the step of cooling comprises cooling the or a portion of the working material to a temperature below the condensing side approach temperature of the recuperator.

35. A method as claimed in any one of claims 22 to

34, wherein the step of compressing comprises compressing the working material to at or around the critical pressure

⁽Pc).

36. A method as claimed in any one of claims 22 to

35, wherein the step of compressing comprises compressing the working material whereby the temperature of the working material is less than the saturation temperature of the working material for the outlet compressor pressure .

37. A method as claimed in any one of claims 22 to 36, wherein the step of heating comprises superheating the working material to above the critical temperature.