FIELD OF THE INVENTION

The present invention relates to cryogenic engine systems used to cool a chamber and relates particularly to cryogenic engine systems using a liquid cryogenic fuel and relates still more particularly to apparatus and methods for improving efficiency of such engines when used in tandem with a heat pump refrigeration system to cool a chamber.

BACKGROUND OF THE INVENTION

Cryogenic engine systems operate by vaporising a cryogenic liquid or fluid (CF), known as a working fluid (WF) such as liquid air, nitrogen, oxygen or liquid natural gas, etc. in an enclosed space and using the resulting pressurised gas to do work by turning a turbine or pushing a piston. The quantity of work extracted from the WF is governed by how much the WF expands between storage and exhaust which is governed by the difference in temperature between the cryogenic fuel supply and the exhaust. It is a known feature of cryogenic engine systems that elevating the peak cycle temperature will increase their work output as this increases said difference in temperature. In fact, because of the low cycle starting temperature, very high conversion efficiencies of heat into shaft power may be achieved. The efficiency of a cryogenic engine is not static and can be reduced through a usage cycle as the temperature of the cylinder and the WF supply path is reduced due to the passage of cryogenic fluid through the cryogenic engine system drawing heat energy from the components of the Engine and thus reducing the peak cycle temperature.

The present invention is a development of the cryogenic engine system described in US 6,983,598 which addresses said reduction in efficiency caused by the reduction in peak cycle temperatures through a usage cycle. This engine includes one or more cylinders and a piston in each cylinder and employs a source of working fluid, normally comprising a gas derived from a liquid cryogenic source, which is introduced into a chamber of the engine in combination with a heat exchange fluid (HEF) which transfers heat to the working fluid (WF) such as to cause a higher degree of expansion of the working fluid (WF) within the chamber than would otherwise be possible and thereby improving both output and therefore efficiency. The expansion of the working fluid (WF) is used to drive the piston which in turn drives an output shaft such as to produce useful work as motive force or shaft horsepower. The engine includes inlet and outlet valves for each of a number of cylinders and these are controlled such as to ensure both working fluid and HEF are supplied to the cylinder before the inlet valves are closed. Equally an expansion turbine may be used to extract useful work from the expansion of the WF.

In order to transfer heat to the working fluid, the HEF comes into thermal contact with the working fluid. The HEF is generally mixed with the working fluid and then recovered. The cryogenic engine may additionally comprise a heat exchanger for transferring heat to the working fluid. The working fluid and the HEF may be introduced to the expander separately, where they become mixed, and/or the HEF may come into thermal contact with the working fluid before the working fluid is introduced to the expander.

The working fluid may be stored at very low temperatures before heat is transferred to the working fluid. By “very low temperatures” is meant temperatures at which gases such as air, nitrogen, oxygen and natural gas are in a liquid phase at atmospheric pressure. Thus the storage temperature is always less than about -150 degrees Celsius. However, once heat has been transferred to the working fluid, the working fluid is at a temperature above the storage temperature, usually significantly above the storage temperature, and most usually at or near to ambient temperature, which is in a range of from about +5 to about +25 degrees Celsius, although it may be at a temperature below 0 degrees Celsius. For refrigeration- related applications, the working fluid is usually in a range from about 0 to about +30 degrees Celsius and for waste-heat recovery applications, in a range of from about +60 to about +100 degrees Celsius.

The working fluid may be a liquefied gas as it is introduced to the expander, and the expander may then vaporize the working fluid, or the working fluid may already be in the vapour phase but under pressure or in a supercritical state before it is introduced to the expander. By a “supercritical state” is meant that the working fluid may be at a temperature and pressure above its critical point in the fluid’s phase diagram, where distinct liquid and gas phases do not exist. Thus the expansion may involve a phase change of the working fluid from liquid to vapour, or if the working fluid is already in the vapour phase and under pressure or in a supercritical state before being introduced to the expander, it need not involve a phase change.

Ideally, the heat transferred to the working fluid by the HEF is equal to the heat which would otherwise be lost by the working fluid during its expansion, so that the expansion of the working fluid is isothermal. This is in contrast to a steam engine and to an internal or external combustion engine, for example, all of which operate by ideally adiabatic expansion of a working fluid to do work.

The majority of vehicle transport refrigeration systems in use today are powered by an internal combustion engine running on diesel fuel, either directly with an auxiliary generator mounted on the refrigerated trailer, or indirectly by taking power from the tractor engine unit mechanically or electrically via an alternator. Cooling is then attained through using that power to drive a standard closed loop refrigeration system.

Typically, both the power take-off and refrigeration unit are over specified for the level of cooling typically required to maintain the compartment temperature in transit. This is for a number of reasons: i) The refrigeration unit must be capable of cooling down the container after the doors have been opened; ii) The insulation performance of such cold compartments degrades by 3 - 5 % per year, increasing the cooling power required through the lifecycle; and iii) APT mandate that the refrigeration unit must be able to extract heat at 1.35 to 1.75 times the heat transfer through the container wall at a 30 °C ambient temperature.

The result of this is that the refrigeration units on mobile vehicles spend much of their operational lives running at an inefficient point. The consequence of this is that coefficients of performance (COP) of mobile refrigeration units are typically quite low compared to other cooling equipment (e.g. approximately 0.5 for frozen compartments at -20 °C to 1.5-1.75 for compartments refrigerated to 3 °C at an ambient temperature of +30c).

With a conventional transport refrigeration unit powered by diesel or battery, the system is limited by the amount of heat it can reject across the condenser as the heat is rejected to ambient air (via a refrigerate to air heat exchanger) and the COP of the system deteriorates as the cycle continues as the compartment temperature decreases.

Currently, it is estimated that approximately 0.05% of total greenhouse gas emissions in the UK come from the refrigeration equipment used for food transportation. This is a small proportion but represents a significant quantity. Consequently, there is a need to reduce emissions from refrigerated transport units. The inefficient use of hydrocarbon fuels for these refrigeration units is also disadvantageous and so a method of reducing their consumption in this application is required.

WO2014/076508 describes a cryogenic engine system connected to and powering a refrigeration system in order to address this issue. The cryogenic engine system and the refrigeration system are thermally coupled with each other via a heat exchange system in a mutually beneficial manner such that working fluid in the cryogenic engine system acts as a heat sink for removing heat from the refrigerated cold chamber and heat generated by the refrigeration system is used to warm the working fluid prior to use in the cryogenic engine system. Therefore, improving the efficiency of both systems.

It will be understood that whilst the current invention is particularly suited to mobile vehicle transport refrigeration systems the efficiency gains may improve any refrigeration system coupled to a cryogenic engine system.

The current invention is directed to improving the efficiency of a cryogenic engine system, specifically one linked to or including a refrigeration system. Significant cooling potential still remains in the cryogenic cooling fluid after it has passed through the main heat exchanger (evaporator) in the cold chamber of the refrigeration system especially when at lower compartment temperatures. This waste cold is used to further sub-cool the refrigerant in the refrigeration line after it leaves the condenser and before entering the evaporator or heat exchanger in the cold chamber. This decouples the capacity of the refrigeration circuit from the amount of heat that can be rejected in the condenser alone. Therefore, improving the cooling power and the COP of the refrigeration circuit, leading to a high overall system efficiency. This has the added benefit of warming the cryogenic fuel before it enters the cryogenic engine and maintaining an increased peak cycle temperature, thus improving the engine efficiency and output, specially at lower compartment temperatures.

SUMMARY OF THE INVENTION

The system of the present invention aims to provide a method of operating a cryogenic engine 20 and refrigeration system that eliminates the above-mentioned problems.

According to a first aspect, there is provided a system comprising: a cryogenic engine , utilising a cryogenic working fluid (WF); a cryogenic refrigeration system for cooling the contents of a container , including a source of cryogenic fluid (CF), a heat pump refrigeration system containing a refrigerant (RF); and a heat exchange system through which said cryogenic fluid (CF) and said refrigerant (RF) pass to exchange thermal energy with the contents of the container; wherein said heat exchange system includes a CF outlet for receiving cryogenic fluid (CF) passed therethrough and wherein said cryogenic engine includes a CF inlet for receiving cryogenic fluid (CF) from the CF outlet of the heat exchange system; and including a sub-cooler connected between the CF outlet of the heat exchange system and the CF inlet of the cryogenic engine for exchanging heat between said refrigerant (RF) and said cryogenic fluid (CF). Advantageously, using remaining cold energy from the CF, which would otherwise be unused, to cool the RF whilst increasing the efficiency of the cryogenic engine by increasing the energy of the WF.

Optionally, the cryogenic engine utilising a heat exchange fluid (HEF).

Preferably, wherein the sub-cooler includes a cryogenic fluid inlet connected to a cryogenic fluid outlet and further includes a refrigerant inlet for receiving refrigerant (RF) connected to a refrigerant outlet for directing refrigerant (RF) from said sub-cooler. Further preferably, wherein the cryogenic fluid inlet is for receiving cryogenic fluid (CF) from the CF outlet of the heat exchanger and the cryogenic fluid outlet is for passing cryogenic fluid (CF) to the CF inlet of the cryogenic engine.

Preferably, wherein the heat exchange system includes a refrigerant inlet for receiving refrigerant (RF) from a source thereof and a refrigerant outlet for expelling refrigerant therefrom and wherein said refrigerant inlet is connected to the refrigerant outlet of the sub cooler for receiving refrigerant (RF) therefrom.

Preferably, the cryogenic engine (20), utilises a heat exchange fluid (HEF); includes a heat exchanger for exchanging heat between the refrigerant (RF) and the heat exchange fluid (HEF) before the HEF enters the engine , thereby to extract heat from the refrigerant (RF).

Preferably, including a source of HEF and wherein said heat exchanger receives heat exchange fluid (HEF) from the source thereof and includes a condenser for receiving refrigerant (RF) from the sub-cooler and condensing the refrigerant (RF) by heat exchange with the HEF.

Preferably, wherein the source of HEF has an outlet and wherein said heat exchanger includes a HEF inlet and a HEF outlet and a refrigerant (RF) inlet for feeding said condenser and refrigerant (RF) outlet fed by said condenser and wherein said HEF inlet of the heat exchanger is connected to the HEF outlet of said source thereof and said refrigerant fluid inlet of the heat exchanger is connected to the refrigerant outlet of the heat exchange system for receiving refrigerant therefrom.

Preferably, including a first heat exchange fluid (HEF) pump between the source of heat exchange fluid (HEF) and the heat exchanger.

Preferably, further including second heat exchange fluid (HEF) pump between the heat exchanger and the inlet of the cryogenic engine for pumping said heat exchange fluid (HEF) to said engine. Preferably, further including a separator for receiving mixed cryogenic fluid (CF) and heat exchange fluid (HEF) expelled from the cryogenic engine and for separating them and further including an exhaust (192) for exhausting spent cryogenic fluid (CF) and a HEF return port for returning separated heat exchange fluid (HEF) to said source thereof.

Preferably, further including an air moving system for moving air from within the container through the heat exchanger system to exchange thermal energy between the contents of the container and with refrigerant (RF) within the heat exchange system.

Preferably, further including an air moving system for moving air from within the container through the heat exchange system to exchange heat with cryogenic fluid (CF) within the heat exchange system.

Preferably, wherein the heat exchange system includes a CF heat exchanger and/or a RF heat exchanger.

Preferably, wherein CF heat exchanger of the heat exchange system is an air to CF heat exchanger and/or the RF heat exchanger is an air to RF heat exchanger.

Preferably, including a compressor for compressing refrigerant (RF) before it enters the heat exchanger.

Preferably, wherein said compressor is coupled to the cryogenic engine for being driven thereby.

Preferably, wherein the container is a cold chamber for storing items to be refrigerated.

Preferably, wherein said container is a refrigeration container on a vehicle.

Preferably, including a vehicle including said cryogenic engine and container.

According to a second aspect of the current invention the above system is provided further wherein the cryogenic engine comprises a source of motive power for a vehicle.

The benefits of the system of the current invention over a normal heat pump or vapour cycle refrigeration system is the amount of heat the system can reject is greater than systems of the prior art (the system cannot absorb more heat than it can reject), in particular when ambient temperature is high and/or the compartment temperature is low.

The increase in heat rejection is provided by the CF from the outlet of the heat exchange system acting on the contents of the compartment. The CF exiting the heat exchange system can still be as low as -35 °C or lower still.

This means the system can reject more heat than through the condenser alone as it is not restricted to rejecting heat to ambient air. Thus the temperature gradient for heat transfer is higher.

As a result, the system is also able to absorb more heat from the compartment.

Additionally, due to the increased heat rejection across the sub-cooler and condenser, the system’s compressor works less hard requiring less energy to power. A lower temperature refrigerate means lower pressure gain is required.

Since the compressor is working less hard, the overall result is the system uses less energy relative to a conventional system to provide refrigeration at a similar level.

COP is calculated by - ^Coohng Power - Therefore the system COP of the current

Compressor Mechnical Work invention is much higher than a conventional system’s COP because more cooling is achieved while the compressor mechanical work reduces.

The presence of the sub-cooler also ensures that all the refrigerant going to the evaporator of the heat exchange system has been liquefied and avoids flash-gas from entering the expansion valve prior to the evaporator, because flash-gas entering the expansion valve will lead to a disruptive behaviour of the expansion valve. This can cause refrigerant to be superheated downstream of the heat exchange system in the refrigerant feed line to the compressor.

BREIF DESCRIPTION OF DRAWINGS

The present invention will now be more particularly described with reference to the accompanying drawings, in which:

Figure 1 , is a schematic representation of a cryogenic engine and refrigeration system of the prior art;

Figure 2, is a schematic representation of a cryogenic engine and refrigeration system of the current invention;

Figure 3, is a graph showing the COP of a traditionally powered refrigeration unit reliant on heat exchange with ambient air; and Figure 4, is a graph showing the COP of the cryogenic engine and refrigeration unit according to the current invention.

For the purposes of brevity, the term heat exchange fluid is hereafter abbreviated to HEF, the term cryogenic fluid is abbreviated to CF, the term working fluid is abbreviated to WF and the term refrigerant is abbreviated to RF. The working fluid (WF) and cryogenic fluid (CF) are one and the same. The CF is used as the WF in the cryogenic engine 20 of the system 10. The CF referred to below may include at least one of liquid nitrogen, liquid air, liquefied natural gas, carbon dioxide, oxygen, argon, compressed air, compressed nitrogen or compressed natural gas. The HEF may include one or more incompressible or near incompressible liquids such as, for example, water, antifreeze or mixtures thereof.

A cryogenic engine 20 includes an expander 30 for generating motive energy from WF. The expander 30 may comprise one or more cylinders 31 each having a reciprocating piston 32 or a turbine 33 for creating motive force from expanding WF.

Referring firstly to figure 1 , a system 1 of the prior art is shown. The prior art system 1 , includes a CF path 12 comprising a plurality of CF lines 13 through which CF can pass, a HEF path 14 comprising a plurality of HEF lines 15 through which HEF can pass and a RF circuit 16 comprising a plurality of RF lines 17 through which RF can pass.

The CF path 12 includes a cryogenic fluid storage tank 84 as a source of cryogenic fluid 80, a cryogenic pump 180 for moving CF through the system 1 , a heat exchange system 90 for cooling the contents of a cold chamber 100, a preheater 2 for transferring thermal energy from HEF to CF, a cryogenic engine 20 for generating motive power from the CF, and a separator 190 for separating HEF from used CF or WF. The system 1 includes a plurality of CF lines 13 for fluidly connecting the CF path.

The HEF path 14 includes a HEF source 110 comprising a HEF tank 111 , connected to, the pre-heater 2, connected to a condenser 170 connected to the HEF tank 111. The HEF thank 111 is also connected to the cryogenic engine 20 which is connected to the separator 190 which is connected to the HEF tank 111 for returning used HEF thereto. The connections of the HEF path are 14 are fluidly made by a plurality of HEF lines 15. The HEF circuit 14

The refrigerant circuit 16 includes a compressor 300 driven by the cryogenic engine 20 for compressing RF before entering, the condenser 170, connected to the heat exchange system 90 which is connected to the compressor 300 to close the RF circuit 16. The system 1 includes a plurality of RF lines 17 fluidly connecting the RF circuit 16 in the order given above. Referring to figure 2, a system 10 of the current invention is shown in which the same reference numbers are used for similar components to figure 1. The system 10, includes a CF path 12 comprising a plurality of CF lines 13 through which CF can pass, a HEF path 14 comprising a plurality of HEF lines 15 through which HEF can pass and a RF circuit 16 comprising a plurality of RF lines 17 through which RF can pass. The HEF path 14 may be a HEF circuit 14a if the HEF is recycled. The CF path 12, HEF path 14 and RF circuit 16 serve a cryogenic engine 20 and a container 100 for chilling the contents of said container 100.

The CF path 12 of the system 10 comprises a source of CF 80, a heat exchange system 90 for cooling the contents of a container 100 or cold chamber 100, a sub-cooler 120 for exchanging thermal energy between CF and RF in the RF path, a cryogenic engine 20 and optionally a separator 190 for separating used CF and used HEF if the HEF is to be recycled after passing through the cryogenic engine 20. The CF path 12 is fluidly connected by a plurality of CF lines 13 in the order given above. A CF pump 40 for moving CF fluid within the CF path 12 is also included in the CF path 12. In figure 2 the CF pump 40 is located between the CF source 80 and the heat exchange system 90 however, it will be understood that the CF pump 40 could be included at any suitable location in the CF circuit 12 and could be replaced by any means of moving CF such as a means of pressuring the CF source 80.

The HEF path 14 of the system 10 of the current invention includes a HEF source 110, a heat exchanger 160, the cryogenic engine 20 and the separator 190. Optionally the HEF path 14 may then return to the HEF source 110 if the HEF is being recycled. The HEF path 14 includes a plurality of HEF lines 15 that fluidly connect the HEF path 14 in the order given above. The HEF path 14 includes a first HEF pump 180 for moving HEF around the HEF path 14, this is shown in figure 2 between the HEF source 110 and the heat exchanger 160 however, may be located in any suitable position about the HEF path 14.

The RF path 16 of the system 10 includes a compressor 300 preferably driven by the cryogenic engine 20, the heat exchanger 160 including a condenser 170 for condensing RF and passing heat extracted from the RF to the HEF, the sub-cooler 120 and the heat exchange system 190.

The cryogenic engine 20 includes an expander 30 for generating motive energy from WF after it has passed through the heat exchange system 90 and the sub-cooler 120. The cryogenic engine 20 includes a CF/WF inlet 22, a HEF inlet 23 and an outlet 24 for expelling the used mixed HEF and WF after passing though the expander 30. The expander 30 may comprise one or more cylinders 31 each having a reciprocating piston 32 or a turbine 33 for creating motive force from expanding WF. If HEF is injected directly into the expander 30 of the cryogenic engine 20 a second HEF pump 26 may be required that is able to provide a higher pressure than the first HEF pump 180. The second HEF pump 25 may be driven by the cryogenic motor 20.

A source of CF 80 includes a CF outlet 82 and may comprise a CF tank 84 or a CF supply line 86. The heat exchange system 90 includes a CF inlet 91 , a CF outlet 92, a RF inlet 93 and an RF outlet 94. The sub-cooler 120 is a heat exchanger for transferring thermal energy between the refrigerant and the CF and preferably a fluid to fluid, RF to CF heat exchanger. The subcooler 120 includes a CF inlet 122 and a CF outlet 124 a HEF inlet 126 and a HEF outlet 126. The separator 190 may be a cyclonic separator 190 and includes an inlet 191 for receiving combined used WF and HEF from the cryogenic motor an exhaust outlet 192 for releasing used CF and a HEF outlet 194 for returning separated HEF to the HEF source.

The HEF source 110 which may be a HEF tank 111 or a HEF supply line 113 includes a HEF outlet 112 and a HEF return port 114 for receiving used HEF from the separator 190. The heat exchanger 160 includes a HEF inlet 162, a HEF outlet 164, a RF inlet 166 and an RF outlet 168. The heat exchanger 160 also includes a condenser 170 connected between the RF inlet 166 and the RF outlet 168 for condensing gaseous RF and releasing heat from said RF to the HEF via the heat exchanger 160. The heat exchanger 160 may be referred to as a RF/HEF heat exchanger 160.

The heat exchange system 190 is for cooling the contents of the compartment 100 and includes a CF inlet 91 and a CF outlet 92, a RF inlet 93 and a RF outlet 94. The heat exchange system 90 also includes a CF heat exchanger 95 connected between the CF inlet 91 and CF outlet 92 for exchanging thermal energy between the contents of the compartment 100 and the CF and a RF heat exchanger 96 connected between the RF inlet 93 and the RF outlet 94 for exchanging thermal energy between the contents of the compartment 100 and the RF. The heat exchange system 190 further includes an air moving system 400 for moving air over or through the CF and RF heat exchangers 95, 96 for improving heat transfer between the contents of the compartment 100 and the CF and RF. The RF heat exchanger further includes an evaporator 97 for expanding and evaporating the RF in order to reduce its temperature and remove enable more thermal energy to be transferred from the compartment 100. The CF heat exchanger 95 and the RF heat exchanger 96 may be collocated, unitary or separate. It is preferable that the CF and RF heat exchangers are collocated in order that the air moving system 400 may be simplified. The heat exchange system 90 must be in fluid communication with the contents of the compartment 100, however, may be located in the compartment or external to the compartment and the contents of the compartment 100 passed through the heat exchange system 90 for example by way of ducting. The compartment 100 may be referred to as a cold chamber 100 and is preferably a refrigerated compartment 100 most preferably a mobile refrigerated compartment 100 of a vehicle 600 as the current invention is particularly suited to mobile applications.

The compressor 300 is for compressing the RF prior to entry to the condenser 170 and includes an RF inlet 301 and an RF outlet 302.

The plurality of CF lines 13 of the CF path 12 include a first CF line 131 fluidly connecting the outlet 82 of the CF source to the CF pump 40, a second CF line 132 fluidly connected between the CF pump 40 and the heat exchange system 90, a third CF line 133 is fluidly connected between the CF outlet 92 of the heat exchange system 90 and the CF inlet 122 of the subcooler 120, a fourth CF line 134 is fluidly connected between the CF outlet 124 of the Subcooler 120 and the CF inlet 22 of the cryogenic engine 20, a combined CF/HEF line 135 is fluidly connected between the outlet 24 of the cryogenic engine 20 and the inlet 191 of the separator 190.

The plurality of HEF lines 15 of the HEF path 14 include a first HEF line 151 fluidly connecting the outlet 112 of the HEF source 110 to the HEF pump 180, a second HEF line 152 fluidly connecting the first HEF pump 180 to the HEF inlet 162 of the heat exchanger 160, a third HEF line fluidly connected between the HEF outlet 164 of the heat exchanger 160 and the HEF inlet 23 of the cryogenic engine 20. The aforementioned combined CF/HEF line 135 is fluidly connected between the outlet 24 of the cryogenic engine 20 and the inlet 191 of the separator 190 and a fifth HEF line 155 is fluidly connected between the HEF outlet 194 of the separator 190 and the HEF return port 114 of the HEF source 110 for returning used HEF to the source. It will be understood that HEF may not be returned to the source 110 and may be lost or reused in another way.

The plurality of RF lines 17 of the RF circuit 16 include a first RF line 171 fluidly connected between the RF outlet 302 of the compressor 300 and the RF inlet 166 of the heat exchanger 160, a second RF line 172 fluidly connected between the RF outlet 168 of the heat exchanger 160 and the RF inlet 126 of the sub-cooler 120, a third RF line 173 connected between the RF outlet 128 of the sub-cooler 120 and the RF inlet 93 of the heat exchange system 90 and a fourth RF line fluidly connected between the RF outlet 94 of the heat exchange system 90 and the RF inlet 301 of the compressor 300.

In use the container 100 is cooled by a cryogenic refrigeration system 60 and a heat pump refrigeration system 70. Each refrigeration system 60, 70 removing heat from the contents of the container 100 by way of the heat exchange system 90. The contents of the container 100 is moved by the air circulation means 400 for maintaining a concentration gradient across the CF heat exchanger 95 and the RF heat exchanger 96. Though it will be understood that the contents of the container 100 may include solids as well as fluids and may be held in any fluid medium in any state, not only air. The CF refrigeration system works by pumping cryogenic fluid through the heat exchange system 90 where the CF absorbs thermal energy from the contents of the container 100.

The cryogenic refrigeration system 60 comprises the CF source 80 the CF pump 40 and the CF heat exchanger 95 of the heat exchange system 90.

The heat pump refrigeration system 70 comprises a standard evaporation and condensing refrigeration cycle including the compressor 300, the condenser 170 of the heat exchanger 160 and the evaporator 97 of the heat exchange system 90, with the addition of the sub-cooler 120.

The efficiency of a cryogenic engine 20 is improved if the peak cycle temperature can be increased and maintained when the CF is used as WF in the expander 30 of the cryogenic engine 20 to output work as a motive force. Preferably this motive force is used to power the compressor 300 for the heat pump refrigeration system 70. The power of a refrigeration system is dictated by how much heat can be rejected from the RF before entry to the heat exchange system 90. After passage through the heat exchange system 90 CF is still colder than the required temperature of the compartment and colder than the RF before its entry to the evaporator 97 of the heat exchange system 90. In the system 10 of the current invention a sub-cooler 120 is provided to transfer thermal energy from the RF after leaving the condenser to the CF which still has a very low temperature after passing though the heat exchange system 90. The RF is around +25C after passing through the condenser 170 and the CF is still at around -30C after passing through the heat exchange system 90 having been used to cool the contents of the chamber 100. On exiting the sub-cooler 120 the CF is around +20C and the RF is at -10C. Therefore, the RF has greater potential to cool the cold chamber as a greater difference is maintained between the evaporated RF and the contents of the container 100. After passing through the sub-cooler 120 the CF is passed to the cryogenic engine 20 where it is used as WF in the expander 30 to output work as motive force. Preferably that motive force is used to power the compressor 300 of the heat pump refrigeration system 70. Thus efficiency of the cryogenic motor 20 is improved as surplus cold has been transferred to the refrigerant before being used as WF in the cryogenic engine 20.

The efficiency of the system 10 is further improved though transferring the thermal energy released by condensing the compressed RF in the condenser 170 via the heat exchanger 160 to the HEF before use in the cryogenic engine 20.

Figure 3 shows the COP of a standard heat pump refrigeration cycle, such as those found on a diesel powered mobile refrigeration unit on a vehicle. At a high compartment temperature of 10°C an efficiency of 2.5 is possible. However the COP drops rapidly as the temperature of the compartment drops to a normal working region of a refrigerated compartment 100 which can be expected to be 0 - 5°C for refrigeration and -15°C - -20°C for frozen goods. Thus at 0- 5°C COP is under 0.75 and at -15°C - -20°C the COP drops further to 0.5.

Figure 4 shows a simulation of the improved efficiency which is achieved by the current system. A COP of 2 or above is possible through the full working range of the refrigeration system.

Figure 5 shows a simulation of the efficiency of the system of the current invention in relation to the temperature gradient of the compartment to ambient temperature. Again, it can be seen that the COP of the system is de-coupled from the ambient temperature.

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 may be applied to other aspects, 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.