Improvements in cooling systems

Abstract

A system 10 comprises a cryogenic refrigeration system 60 for cooling the contents of a container 100 which is connected to a source 80 of cryogenic fluid. A heat pump refrigeration system 70 contains a refrigerant and is connected to an electrical source 510. The heat pump refrigeration system comprises a condenser 170 and a heat exchange system 90 which exchanges heat between the refrigerant and the contents of the container. A sub-cooler 120 with a cryogenic fluid inlet 122, a cryogenic fluid outlet 124, a refrigerant inlet 126, and a refrigerant outlet 128 is connected between a refrigerant outlet 178 of the condenser and a refrigerant inlet 93 of the heat exchange system and exchanges heat between the refrigerant and the cryogenic fluid. A compressor 300 may be driven by a motor 500 and the electrical source may be a battery 522, solar array 524, or a rectifier 532. The condenser may exchange heat between the refrigerant and ambient air. The system may include a pre-condenser 160 to exchange heat between the refrigerant and the cryogenic fluid and direct cryogenic fluid to an exhaust 150. A vehicle 600 including the system is also claimed.

Description

Field The present invention relates to a refrigeration system for cooling a chamber. More particularly, the present invention relates to an integrated refrigeration system connectable to a source of cryogenic fluid and a source of electrical energy. Background The majority of vehicle transport refrigeration systems in use today are powered by an internal combustion engine. For example, the refrigeration unit may run 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 cycle. It is a known feature of refrigeration systems that elevating the peak cycle temperature will increase the work required, as the larger difference in temperature requires higher compression ratios. In other words, it is more difficult to cool a container when the ambient temperature is higher. The working fluid for a refrigeration system may be stored at a lower temperature before heat is transferred to the working fluid. For example, cryogenic working fluid may be stored at very low temperatures, meaning temperatures at which gases such as air, nitrogen, oxygen and natural gas are in a liquid phase at atmospheric pressure. Thus, the storage temperature for cryogenic working fluids 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. 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) ATT 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). In any installation efficiency is important, however, with transport moving to electric power supplied by a battery, efficiency is even more important. With a conventional TRU (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 refrigerant to air heat exchanger) and the COP of the system deteriorates as the cycle continues with time due to effects such as condenser fouling. 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. The present invention seeks to address these and other disadvantages encountered in the prior art by providing an improved refrigeration system. Summary of Invention According to a first aspect, there is provided a system comprising a cryogenic refrigeration system for cooling the contents of a container, connectable to a source of cryogenic fluid (CF). The system further comprises a heat pump refrigeration system connectable to a source of electrical energy and containing a refrigerant (RF), wherein the heat pump refrigeration system comprises a condenser having an RF inlet and an RF outlet. The system further comprises a heat exchange system through which said refrigerant (RF) pass to exchange thermal energy with the contents of the container, wherein said heat exchange system includes an RF inlet for receiving refrigerant. The system further comprises a sub-cooler for exchanging heat between said refrigerant (RF) and said cryogenic fluid (CF), and connected between the RF outlet of the condenser and the RF inlet of the heat exchange system. The sub-cooler includes a cryogenic fluid inlet, a cryogenic fluid outlet connected to said cryogenic fluid inlet and further includes a refrigerant inlet for receiving refrigerant (RF) and a refrigerant outlet , connected to said refrigerant inlet, for directing refrigerant (RF) from said sub-cooler. For the purpose of brevity, the term cryogenic fluid is abbreviated to CF, and the term refrigerant is abbreviated to RF. The term working fluid (abbreviated to WF) may also be used to refer to the cryogenic fluid (CF). Advantageously, systems according to the current invention can reject more heat than systems of the prior art, in particular when ambient temperature is high and / or the compartment temperature is low. Refrigerant cooling capacity is improved over systems of the prior art. System stability is also improved as the feed liquid to the refrigerant expansion valve can be maintained as fully liquid. Optionally, the heat pump refrigeration system may be driven by an electric motor. Advantageously, allowing use of renewable energy sources or the power supply of a battery electric vehicle. Optionally, the system may comprise the source of electrical energy including a direct current source, and the direct current source may comprise a battery, solar array or rectifier for adapting a mains alternating supply. Optionally, cryogenic fluid (CF) passes through the heat exchange system to exchange thermal energy with the contents of the container, and the heat exchange system may include a CF inlet for receiving cryogenic fluid (CF) from the source of CF, and the CF inlet may be connected to a CF outlet for passing CF therefrom. Advantageously, when both the CF and the RF exchange thermal energy with the contents of the container, there is improved cooling capacity of the system and efficiency is improved. Optionally, the cryogenic fluid inlet of the sub-cooler may be connected to CF outlet of the heat exchange system for receiving cryogenic fluid therefrom. Advantageously, when the CF is received by the sub-cooler and CF doesn’t pass through the container, there is improved safety as there is no need for gas monitoring. This system is also simpler and may be cheaper to manufacture. Further advantageously, the volume or mass of cryogenic fluid required for such a system is much reduced when compared to a system that passes cryogenic fluid through a heat exchange system, whilst still providing a substantial amount of the overall benefit. Cryogenic fluid is a significant cost in the running of such a system accordingly a reduction in the required amount for a lower reduction in performance is desirable. Optionally, the cryogenic fluid inlet of the sub-cooler may be connectable to the source of cryogenic fluid for receiving cryogenic fluid therefrom. Optionally, there is provided the source of cryogenic fluid. Optionally, the condenser may comprise an air condenser for exchanging heat between the refrigerant (RF) and ambient air. Optionally, there is provided a pre-condenser for exchanging heat between the refrigerant (RF) and the cryogenic fluid (CF), including a CF fluid inlet, connected to the cryogenic fluid outlet of the sub-cooler and for receiving cryogenic fluid therefrom. Advantageously, the pre-condenser allows the system to reject heat to the cryogenic fluid that has already passed through the sub-cooler dispite the higher temperature of the CF as the RF temperature is raised after the compressor. Accordingly, the system may reject more heat and / or the heat rejection requirement of an air-based condenser is reduced. This improves the system COP and lowers the required compression ratio at a given ambient and compartment temperature. Optionally, the pre-condenser may comprise a CF outlet connected to an exhaust for transferring cryogenic fluid (CF) out of the cryogenic refrigeration system. Optionally, the pre-condenser comprises a RF outlet connected to the RF inlet of the condenser and for transferring refrigerant (RF) from said pre-condenser to the condenser. Optionally, the heat exchange system includes a RF outlet connected to the RF inlet of the heat exchange system. Optionally, there is provided a compressor including a refrigerant inlet connected to the RF outlet of the heat exchange system and a refrigerant outlet connected to a refrigerant inlet of the pre- condenser. Optionally, the compressor may be coupled to the motor and driven thereby. Optionally, there is provided 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. Optionally, there is provided 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. Advantageously, air moving systems improve heat transfer between the contents of the compartment and the CF and / or the RF. Optionally, the heat exchange system may include a CF heat exchanger and / or a RF heat exchanger. Optionally, the container may be a cold chamber for storing items to be refrigerated. Optionally, container may be a refrigeration container for mounting on a vehicle. According to an aspect, there is provided a vehicle including a system of the present disclosure. Optionally, the motor may comprise a source of motive power for the vehicle. Brief Description of Drawings Embodiments will now be described, by way of example only and with reference to the accompanying drawings having I ike-reference numerals, in which: Figure 1 shows a schematic representation of an integrated refrigeration system in which the heat exchange system receives cryogenic fluid, according to some implementations of the present invention; Figure 2 shows a schematic representation of an integrated refrigeration system in which the sub-cooler receives cryogenic fluid from a source, according to some implementations of the present invention; Figure 3 shows a graph showing the COP of a traditionally powered refrigeration unit reliant on heat exchange with ambient air, and the COP of the integrated cryogenic and electrical refrigeration unit according to the current invention. Specific Description In overview, and without limitation, the application discloses an integrated cryogenic refrigeration system 60 with a heat pump refrigeration system 70 in which the heat pump refrigeration system 60 is connectable to a source of electrical energy, for example an electric motor 500. The heat pump refrigeration system 70 may also be referred to as a mobile vapor compression cycle system. These systems are integrated via a sub-cooler heat exchanger 120 for exchanging heat between a refrigerant (RF) and cryogenic fluid (CF). Integrated cryogenic and electrical refrigeration systems operate by vaporising a cryogenic liquid, known as a working fluid (WF) such as liquid air, nitrogen, oxygen or liquid natural gas, etc. in an enclosed space. They use the resulting pressurised gas to further help a traditional refrigeration system powered by an electrical power source to reject heat. As a result, they are especially efficient and suitable for refrigerating a cool chamber in high ambienttemperature environments. This effect arises because significant cooling potential still remains in the cryogenic cooling fluid after it has passed through the main heat exchanger 90 (also referred to as an evaporator) in the cold chamber 100 of a refrigeration system, especially when at lower compartment temperatures. This “waste cold” can be used to further sub-cool the refrigerant in the refrigeration line after it leaves the condenser and before entering the evaporator or heat exchanger 90 in the cold chamber 100. This improves the cooling power and the COP of the refrigeration circuit, leading to a high overall system efficiency. This application additionally discloses use of a pre-condenser 160 connected along the refrigerant circuit, and preferably between the compressor 300 and the condenser 170 The pre-condenser transfers any remaining cold energy potential to further boost the performance of a given condenser. This feature improves efficiency further by maximizing the cooling potential from the cryogenic fluid, and further reduces the heat rejection requirement for the condenser. The latter is particularly important at high ambient temperatures. For a given airflow, a lower condenser pressure is required, as the refrigerant entering the condenser is cooler and less heat rejection is required to the cooling medium (such as air) to achieve condensation. This has the additional effect of improving system COP, and lowering the required compression ratio at a given ambient and compartment temperature. Advantageously, the integrated system of the present disclosure allows for an increase in refrigerant subcooling. As will be described elsewhere in the disclosure, the overall cooling capacity is markedly improved compared to a traditional system both due to the use of a subcooler and a pre-condenser to reduce the required work of the compressor and a reduction in heat rejection required to the cooling medium. The system provides increased refrigerant heat rejection allowing for a corresponding increase in refrigerant cooling capacity at a given refrigerant flow rate and pressure. It also improves the system range for a given on-board power source, such as an electric motor driving the compressor. As a result, this system is particularly advantageous in high ambient temperature environments. It is also cheaper and more efficient, to both manufacture and run compared to legacy systems such as those that rely only on electrical refrigeration powered by a battery. Referring firstly to Figure 1, a system 10 of an embodiment of the present invention is shown. The system 10 includes a CF path 12 comprising a plurality of CF lines 13 through which CF can pass, and a RF circuit 16 comprising a plurality of RF lines 17 through which RF can pass. The CF path 12 and RF circuit 16 serve a container 100 for chilling the contents of said container 100. The CF path 12 is connectable to a source of cryogenic fluid 80, and in some embodiments the source 80 may form part of the system. The CF path 12 may include a cryogenic fluid storage tank 84 as a source of cryogenic fluid 80, and a cryogenic pump 40 for moving CF through the system 1. The CF path 12 includes a heat exchange system 90 for cooling the contents of a container 100 or cold chamber 100, a sub-cooler 120 for transferring thermal energy between the CF and the RF in the RF path 16. The heat exchange system 90 may also be referred to as an evaporator. The CF path 12 may in some optional embodiments further include a pre-condenser 160 for exchanging heat between the RF and the CF. The pre-condenser may also be referred to as a pre-cooler 160. System 10 includes a plurality of CF lines 13 for fluidly connecting the CF path in the sequence given. In Figure 1 the CF pump 40 is not shown, however it will be understood that the CF pump 40 could be included at any suitable location in the CF circuit 12 as means for pressuring the CF source 80 or moving CF along the CF path 12. This includes, for example, locating pump 40 between the CF source 80 and the heat exchange system 90. The RF path 16 of the system 10 may include a compressor 300 connectable to a source of electrical energy 510. The compressor may be driven by an electric motor 500. The heat pump refrigeration system 70 is connectable to and driven by electrical energy, as will be discussed elsewhere in this disclosure. In some optional embodiments, the RF path 16 further includes a pre-condenser. The pre-condenser is located before or upstream of the condenser 170 and preferably between the compressor 300 and the condenser 170. The RF path 16 also includes the condenser 170 for condensing RF, preferably an air condenser for exchanging heat between the RF and ambient air. Optionally, the pre-condenser and air condenser may be integrated. The RF path further includes the sub-cooler 120 and the heat exchange system 90. The vapor compression cycle of the present disclosure will contain a compressor, condenser, expansion valve and evaporator in a similar way to conventional existing systems, as would be understood by the skilled person. Valves and pumps may be provided as required. The source of electrical energy for the electric motor 500 may be an external power source 510. In some optional embodiments, the source of electrical energy 510 may comprise a direct current source 520 such as a battery 522 or solar array 524. Alternatively, or additionally, the heat pump refrigeration system 70 may be connectable to an alternating power supply 530. Optionally, the electric motor 500 may comprise a rectifier 532 for adapting a mains alternating power supply 530. Alternating current compressor systems may be particularly suitable for a 3-phase compressor system. An advantage associated with using an electrical power source is that no combustion is needed to generate power. There are therefore fewer emissions compared to refrigeration units powered by hydrocarbon fuels. This makes the current invention particularly suitable for use as a portable system fitted to an electrically propelled vehicle. Relative to systems powered by cryogenic engines, an electrically powered system consumes cryogenic fluid at a lower flow-rate and pressure. Cryogenic fluid is costly and requires space to store, the system of the current invention is therefore smaller and cheaper in terms of both manufacturing and running costs. 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, an 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 sub-cooler 120 includes a CF inlet 122 and a CF outlet 124 a RF inlet 126 and a RF outlet 126. The pre-condenser 160 includes a CF inlet 162 for receiving CF from the CF outlet 124 of the sub-cooler 120. The pre-condenser may comprise a CF outlet 164 out of which CF can flow out of the system via an exhaust 150 for exhausting spent cryogenic fluid. As described elsewhere, the heat exchange system 90 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. In some implementations, the heat exchange system 90 may include 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. It may also include 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 may further include 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 96 may optionally include an evaporator 97 for expanding and evaporating the RF in order to reduce its temperature and remove thermal energy 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 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, and a fourth CF line 134 is fluidly connected between the CF outlet 124 of the subcooler 120 and the CF inlet 162 of the pre-condenser 160. A fifth CF line 135 may be connected between the CF outlet 164 of the pre-condenser and an Exhaust 150. The Exhaust 150 may include a CF capture device, a work expander or may vent to atmosphere. 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 pre-condenser 160, a second RF line 172 fluidly connected between the RF outlet 168 of the pre-condenser 160 and the RF inlet 176 of the condenser 170, a third RF line 173 fluidly connected between the RF outlet 178 of the condenser 170 and the RF inlet 126 of the sub-cooler 120, a fourth RF line 174 connected between the RF outlet 128 of the sub-cooler 120 and the RF inlet 93 of the heat exchange system 90 and a fifth RF line 175 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 removes heat from the contents of the container 100 by way of the heat exchange system 90. The contents of the container 100 may be moved by the air circulation means 400. 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. Figure 2 shows an alternative embodiment of the present invention comprising a system 20. The system 20 of figure 2 includes many of the same components as the system 10 of figure 1 and these components are referred to and labelled with the same reference numerals. As in Figure 1, the system 20 includes a CF path 12 comprising a plurality of CF lines 13 through which CF can pass, and a RF circuit 16 comprising a plurality of RF lines 17 through which RF can pass. In the embodiment depicted in Figure 2, the CF path 12 does not serve container 100, however the RF circuit 16 does serve the container 100 and is used to chill the contents of said container 100. The CF path 12 is connectable to a source of cryogenic fluid 80, and in some embodiments the source 80 may form part of the system. The CF path 12 may optionally include a cryogenic fluid storage tank 84 as a source of cryogenic fluid 80, and a cryogenic pump 40 for moving CF through the system 1. The CF path 12 includes a sub-cooler 120 for transferring thermal energy between the CF and the RF in the RF path 16. The CF path 12 may in some optional embodiments further include a pre-condenser 160 for exchanging heat between the RF and the CF. The pre-condenser 160 may also be referred to as a pre-cooler. System 20 includes a plurality of CF lines 13 for fluidly connecting the CF path in the sequence given. In Figure 2 the CF pump 40 is shown, however it will be understood that the CF pump 40 is an optional feature and in some systems simply pressure from the source of CF 80 will be sufficient to move CF around the system. Systems in which the CF pump 40 is omitted have reduced complexity, energy consumption and therefore cost. It would also be understood that the CF pump 40 could be included at any suitable location in the CF circuit 12 as means for pressuring the CF source 80. This includes, for example, locating pump 40 between the CF source 80 and the sub-cooler 120. Accordingly, system 20 differs from the system 10 shown in figure 1 in that the CF is passed along the CF path 12 from the source of CF 80 to the sub-cooler without passing through the cold chamber 100. Preferably, the CF path 12 directly connects the source of CF 80 with the sub-cooler 120 This arrangement is simpler, and therefore has a lower manufacturing cost. It also provides a beneficial increase in COP and therefore energy consumption, whilst reducing the consumption of costly cryogenic fluid. Correspondingly, there is reduced storage capacity required for CF for a given time period, or extending the time period. The arrangement is also safer as CF doesn’t pass through the compartment, which will be discussed in more detail elsewhere in this disclosure. The RF path 16 of system 20 is the same as that of system 10 depicted in Figure 1. The heat exchange system 90 of system 20 includes an 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 received from the source of CF 80. The sub-cooler 120 is preferably a fluid to fluid, RF to CF heat exchanger. The sub-cooler 120 includes a CF inlet 122 connected to a CF outlet 124, and a RF inlet 126 connected to a RF outlet 126. The CF inlet 122 of the sub-cooler 120 receives CF from the source of CF 80. The pre-condenser 160 includes a CF inlet 162 for receiving CF from the CF outlet 124 of the sub-cooler 120. The pre-condenser may comprise a CF outlet 164 out of which CF may flow out of the system via an exhaust 150 for exhausting spent cryogenic fluid. The heat exchange system 190 is for cooling the contents of the compartment 100 and includes a RF inlet 93 and a RF outlet 94. In this embodiment, there is no flow of CF through the heat exchange system. In some implementations, the heat exchange system 90 may include 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 may further include an air moving system 400 for moving air over or through the RF heat exchanger 96 for improving heat transfer between the contents of the compartment 100 and the RF. Advantageously, there is no need for the CF to enter the heat exchange system 90. The CF path 12 may pass directly from the source of CF 80 to the CF inlet 122 of the sub-cooler 120, and there is no need for the CF to enter the heat exchange system 90. Although this reduces the cooling capacity, it means that there is no need to manufacture a bespoke heat exchanger. Further advantageously there is no need for gas monitoring which would be required due to the potential hazard of having cryogenic fluid enter the cooling chamber 100. For example, there is a reduced risk of oxygen deficiency which may result in an asphyxiation risk, or oxygen enrichment which may increase the risk of fire, and there is therefore no need to provide oxygen monitors. Accordingly, the components of system 20 may be easily retrofitted to existing cooling systems allowing them to function at higher ambient temperatures or improve efficiency. As in Figure 1, the RF heat exchanger 96 may optionally include an evaporator 97 for expanding and evaporating the RF in order to reduce its temperature and remove more thermal energy from the compartment 100. 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 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 sixth CF line 136 fluidly connected between the CF pump 40 and the sub-cooler 120, a fourth CF line 134 is fluidly connected between the CF outlet 124 of the sub-cooler 120 and the CF inlet 162 of the pre-condenser 160 and a fifth CF line is connected between the CF outlet 164 of the condenser precooler 160 and the exhaust 150. It will be understood that the exhaust 150 may include a CF capture device 152. The plurality of RF lines 17 of the RF circuit 16 include a first RF line 171, a second RF line 172, a third RF line 173, a fourth RF line 174, and a fifth RF line 175 as described with reference to Figure 1. Figure 3 shows the COP of a standalone standard closed loop vapour cycle refrigeration system, such as those commonly used for mobile refrigeration units on a vehicle. The COP of a traditional TRU is depicted as black diamonds. Figure 3 also shows the COP achieved by a system according to the present invention, depicted as grey crosses. A COP of 1 or above is possible through the full working range of the refrigeration system when an integrated cryogenic and electrical energy system is provided. By contrast, the COP of the traditional system drops rapidly as the temperature of the compartment drops to a normal working region which can be expected to be 0 to 5C for refrigeration and -15C to -20C for frozen goods. Thus at 0 to 5C, COP is under 0.8 and at -15C to -20C the COP drops further to 0.5. Figure 3 demonstrates the benefits of the embodiments described herein over a normal heat pump or vapour cycle refrigeration system. These benefits arise from the amount of heat the system can reject, in particular when the system is cooling, because the system cannot absorb more heat than it is able to reject. In some preferred implementations, the heat rejection may come from a nitrogen outlet of the main heat exchanger (evaporator) in the cold chamber of the refrigeration system, which can be -35C. The liquid nitrogen evaporator allows for improved system cooling via the expansion of cryogenic fluid from a liquid to a gas. This allows for improved system cooling capacity at all ambient temperatures compared to a traditional system. Additionally, due to the heat rejection across the condenser, and optionally the pre-condenser, the system’s compressor works less hard because a lower temperature refrigerate means lower pressure. The compressor is driven by an electric motor, which advantageously does not produce emissions as it provides energy without the combustion of hydrocarbons. In some implementations, the source of electrical energy may be a battery or a solar array. This means the system can reject more heat compared to systems that rely on ambient air for heat rejection. The temperature gradient for heat transfer is higher. Embodiments of the present invention are therefore more suitable for operating in high ambient-temperature environments. It is further advantageous in that the relative condensing pressure for a given ambient and airflow will be lower. As a result, the system is also able to absorb more heat from the compartment. Since the compressor is working less hard, the overall result is the system uses less energy relative to a conventional system to produce refrigeration. In other words, it is a more efficient refrigeration system. Some types of compressors have improved volumetric efficiency at lower compression ratios, this helps further increase system capacity. As COP is calculated by-----c°°ling Power----- resu|ts jn a SyStem COP much higher Total system power supplied than a conventional system’s COP because more cooling is achieved while the compressor mechanical work reduces. Advantageously, the presence of the sub-cooler also ensures that all the refrigerant going to the evaporator 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 disruption will cause an impact in superheat downstream of the evaporator and the suction line of the compressor. In other words, the subcooler ensures that the feed liquid to the refrigerant expansion value is maintained as fully liquid. This improves the system stability. As the CF is stored at temperatures far below ambient temperature, the sub-cooler helps to decouple the refrigeration system from the effects of a higher ambient temperature on a vapor compressor cycle’s performance. Further advantageously, the presence of a pre-condenser means that the CF and the RF will interact prior to the air-based condenser. Due to the elevated temperature of the RF after the compressor the CF that has been warmed in the sub-cooler can still advantageously receive heat from the RF. Note that the CF leaving the sub-cooler will remain cooler than ambient temperature if ambient temperature is elevated. The pre-condenser is a heat-exchanger for extracting the maximum cooling potential from the CF. This allows for a reduction in the heat rejection requirement of an air-based condenser in which the RF exchanges thermal energy with ambient air. For a given fixed airflow and condenser area, this allows for a lower condensing pressure to be achieved, as less heat rejection is required for the cooling medium (air) to achieve a lower temperature. This will have the additional effect of improving system COP and lowering the required compression ratio at a given ambient and compartment temperature. As the RF is at a high temperature prior to entering the pre-condenser from the compressor (for example, the RF may be around 70C), it maximizes the CF cooling potential. This facilitates a temperature range from cryogenic temperatures (via a state change) to 60C. There is a surprising increase in overall cooling capacity achieved by the integration of a subcooler driven by an electric motor that is further improved through use of a pre-condenser. The required work of the compression system is markedly decreased, which is reflected as an improvement in the refrigeration system COP. This improves the system range for a given onboard power source. It also helps to raise the maximum operating point for single compressor systems operating in higher ambient-temperature environments. This system is smaller and cheaper as a result of operating at a much lower pressure than legacy systems. 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 5 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. It will be understood that the above description of specific embodiments is by way of example 10 only and is not intended to limit the scope of the present disclosure. Many modifications of the described embodiments, some of which are now described, are envisaged and intended to be within the scope of the present disclosure. For example, the heat exchanger 90 may be adapted such that the CF either does or does not interact with compartment 100. In some implementations, the CF may instead exchange heat directly with the RF via the sub-cooler is 120, and therefore the CF does not flow via the heat exchanger.

Claims

1. A system (10, 20) comprising:a cryogenic refrigeration system (60) for cooling the contents of a container (100), connectable to a source (80) of cryogenic fluid (CF);a heat pump refrigeration system (70) connectable to a source of electrical energy (510) and containing a refrigerant (RF);wherein the heat pump refrigeration system comprises a condenser (170) having an RF inlet (176) and an RF outlet (178);a heat exchange system (90) through which said refrigerant (RF) pass to exchange thermal energy with the contents of the container (100);wherein said heat exchange system (90) includes an RF inlet (93) for receiving refrigerant (RF); and includinga sub-cooler (120) for exchanging heat between said refrigerant (RF) and said cryogenic fluid (CF), connected between the RF outlet (178) of the condenser (170) and the RF inlet (93) of the heat exchange system (90),wherein the sub-cooler (120) includes a cryogenic fluid inlet (122), a cryogenic fluid outlet (124) connected to said cryogenic fluid inlet (122) and further includes a refrigerant inlet (126) for receiving refrigerant (RF) and a refrigerant outlet (128), connected to said refrigerant inlet (126), for directing refrigerant (RF) from said sub-cooler (120).

2. The system (10, 20) of claim 1, wherein the heat pump refrigeration system is driven by an electric motor (500).

3. The system (10, 20) of claim 2, further comprising the source of electrical energy including a direct current source (520), and optionally wherein the direct current source comprises a battery (522), solar array (524) or rectifier (526) for adapting a mains alternating supply.

4. The system (10, 20) of any preceding claim, wherein cryogenic fluid (CF) passes through the heat exchange system (90) to exchange thermal energy with the contents of the container (100), and wherein the heat exchange system (90) includes a CF inlet (91) for receiving cryogenic fluid (CF) from the source (80) of cryogenic fluid (CF), and wherein the CF inlet (91) is connected to a CF outlet (92) for passing CF therefrom.

5. The system (10, 20) of claim 4, wherein the cryogenic fluid inlet (122) of the sub-cooler (120) is connected to CF outlet (92) of the heat exchange system (90) for receiving cryogenic fluid therefrom.

6. The system (10, 20) of any of claims 1 to 3, wherein the cryogenic fluid inlet (122) of the sub-cooler (120) is connectable to the source of cryogenic fluid for receiving cryogenic fluid therefrom.

7. The system (10, 20) of any preceding claim, further comprising the source of cryogenic fluid.

8. The system (10, 20) of any preceding claim, wherein the condenser (170) comprises an air condenser for exchanging heat between the refrigerant (RF) and ambient air.

9. The system (10, 20) of any preceding claim, further comprising a pre-condenser (160) for exchanging heat between the refrigerant (RF) and the cryogenic fluid (CF), including a CF fluid inlet (22), connected to the cryogenic fluid outlet (124) of the sub-cooler (120) and for receiving cryogenic fluid therefrom.

10. The system (10, 20) of claim 9, wherein the pre-condenser (160) comprises a CF outlet (24) connected to an exhaust (15) for transferring cryogenic fluid (CF) out of the cryogenic refrigeration system (60).

11. The system (10, 20) of claim 9 or 10, wherein the pre-condenser (160) comprises a RF outlet (168) connected to the RF inlet (176) of the condenser (170) for transferring refrigerant (RF) from said pre-condenser (160) to the condenser (170).

12. The system (10, 20) of any preceding claim, wherein the heat exchange system (90) includes a RF outlet (94) connected to the RF inlet (93) of the heat exchange system (90).

13. The system (10, 20) of claim 12, further comprising a compressor (300) including a refrigerant inlet (301) connected to the RF outlet (94) of the heat exchange system (90) and a refrigerant outlet (302) connected to a refrigerant inlet (166) of the pre-condenser (160).

14. The system (10, 20) of claim 13, wherein the compressor (300) is coupled to the motor (500) and driven thereby.

15. The system (10, 20) of any preceding claim, further comprising an air moving system (400) for moving air from within the container (100) through the heat exchanger system (90) to improve exchange of thermal energy between the contents of the container (100) and with refrigerant (RF) within the heat exchange system (90).

16. The system (10, 20) of any of claims 4 to 15, further comprising an air moving system (400) for moving air from within the container (100) through the heat exchange system (90) to exchange heat with cryogenic fluid (CF) within the heat exchange system (90).

17. The system (10, 20) of any preceding claim, wherein the heat exchange system (90) includes a CF heat exchanger (95) and / or a RF heat exchanger (96).

18. A system (10, 20) as claimed in any one of claims 1 to 17 wherein the container (100) is a cold chamber (100) for storing items to be refrigerated.

19. A system (10, 20) as claimed in any one of claims 1 to 18 and wherein said container (100) is a refrigeration container for mounting on a vehicle (600).

20. A vehicle (600) including the system of claim 19.

21. A system (10, 20) according to claim 20, wherein the motor comprises a source of motive power for a vehicle (600).

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