GB2237372A

GB2237372A - Air conditioning systems

Info

Publication number: GB2237372A
Application number: GB8922830A
Authority: GB
Inventors: Michael John Atkinson; Peter Hugh Birch
Original assignee: Aisin Seiki Co Ltd
Current assignee: Aisin Corp
Priority date: 1989-10-10
Filing date: 1989-10-10
Publication date: 1991-05-01
Anticipated expiration: 2009-10-10
Also published as: GB2237372B; GB8922830D0

Abstract

A compact, exhaust gas driven air cycle air conditioning system comprising a compressor (12), turbine (16), two heat exchangers (14, 18) and an exhaust turbine (40) is controlled by means of an exhaust gas bypass arrangement (42) and supplies either hot or cold (as shown) conditioned air, as required, to a load air space (26). Switching between cooling and heating modes is implemented using two-way valves (22, 32) such that the same components may be used, and in all embodiments described the conditioning air is supplied from a dry, closed loop. The system may operate in reverse flow i.e. the working air enters the turbine (40) first. Moisture removal devices (34, 34') may be incorporated to avoid heat exchanger icing and also control humidity. In addition, de-icing systems arc included in the reverse flow embodiments because of their greater susceptibility to heat exchanger icing due to lower cycle temperatures. The temperature at which air is delivered to the load may be controlled by diverting amounts of hot or cold air, as appropriate, to be mixed with the conditioning air. A high speed motor may be included for use as an alternative to the exhaust gas drive under conditions of low exhaust gas energy. <IMAGE>

Description

TITLE OF THE INVENTION Exhaust Driven Air Cycle Air Conditioner.

BACKGROUND OF THE INVENTION Field of the Invention The invention relates to the provision of air conditioning in applications where a source of exhaust gas is readily available, for instance in automotive applications. More specifically, it provides an exhaust gas driven, compact air conditioning system which uses air as the refrigerant medium and in a preferred form is capable of providing both heating and cooling.

Description of the Prior Art Motor driven air cycle air conditioners have been proposed for some years as an alternative to the well-established vapour-compression air conditioners used in residential, or similar, applications (for example, US Patent No.429S518 and European Patent No.EP 45144-A2). This is because they have several advantages over vapour-compression systems, which are known to have a number of disadvantages.

virstly, because the evaporators of vapour-compression devices operate at around 50C the heating capacity of these devices is seriously impaired at low ambient temperatures, since they rely on heat transfer from ambient to the evaporator. Secondly, in cold weather ice forms on the evaporator heat transfer surfaces thereby increasing pressure losses and also reducing evaporator effectiveness.

Thirdly, vapour-compression devices use chlorofluorocarbons (CFCs) as the refrigerant which presents manufacturing and maintenance problems with regard to refrigerant leakage, problems which are substantially reduced when air is used as the refrigerant. In addition, CFCs are known to have a harmful effect on the Earth's ozone layer and it is thought that they may also be contributing to global warming caused by the so-called greenhouse-effect.

Despite the advantages, air cycle air conditioning systems have yet to make a significant impact on the markets currently served by CFC based air conditioners. This is largely due to their relatively poor coefficient of performance (C.O.P.) but also because air cycle designs have previously been based on conventional electric motors as prime movers and are therefore relatively low speed1 and hence large, devices. 'Prior art air cycle air conditioners do often incorporate regenerative heat exchangers for claimed improvements in C.O.P. but the improvement is minimal when the increased losses due to the presence of the heat exchanger(s) are taken into account. Such an improvement may also be seen as expendable when offset against the attendant increases in size, complexity and cost of the conditioning system.

A recognised way of significantly improving the C.O.P. of air cycle devices is to operate in reverse flow mode, i.e.

the refrigerant air flows through the turbine, rather than the compressor, prior to the heat exchanger. However, this approach is not common in the prior art, probably because the reduced cycle temperatures can create icing problems in the heat exchangers.

In prior art air cycle air conditioners the air delivery temperature is dictated by the ambient and operating conditions, which may be undesirable in some applications from the point of view of safety and/or comfort. Such air conditioners would therefore be improved by incorporating some means of air delivery temperature control.

SUMMARY OF THE INVENTION The invention is intended as an alternative to vapour-compression air conditioning systems commonly used in automotive, or similar, applications. The invention provides an air cycle air conditioning system for cooling a load air space, comprising: an exhaust turbine drivable by a supply of exhaust gases from an engine; an air compressor drivingly connected to an expansion turbine, the power requirements of the compressor being provided at least in part by the exhaust turbine; and an expansion air heat exchanger for receiving expanded air from the expansion turbine and delivering it to the compressor for recompression, the expanded air in the expansion air heat exchanger being in heat exchange relationship with a fan-assisted flow of conditioning air to the load air space for effecting cooling of the load air space.

Advantageously an air conditioner according to the invention can be designed to utilise the same components to deliver cooling or heating to the load air space, depending on the requirements, and which uses air as the refrigerant fluid. Switching between heating and cooling modes is facilitated using a number of two-way valves.

Preferably the expansion turbine and compressor are mounted on the same shaft. Also mounted on said shaft is the exhaust turbine which is driven by exhaust gas from an appropriate source and thus provides the power input to the expansion turbine and compressor arrangement. It is a particular feature of the invention that the rotating components are preferably designed to operate at high rotational speed in order to produce a compact and light-weight unit.

A number of embodiments are described beginning with the most basic system for heating or cooling a load, or conditioned, air space; regenerative heat exchangers have been excluded from all of the embodiments in order to minimise their size, cost and complexity. Reverse flow heating and cooling embodiments are shown since they offer improvements in terms of the coefficient of performance.

The potential problem of heat exchanger icing in the reverse flow embodiments is minimised by appropriatge use of moisture removers. Despite this, gradual icing of the -heat exchanger passages may occur, and of particular interest in the reverse flow embodiments is the possible incorporation of de-icing facilities. Additional embodiments allow control over the conditioning air delivery temperature for both cooling and heating modes in reverse flow and normal flow configurations.

Finally, embodiments are included which incorporate a very high speed1 compact electric motor of the type disclosed in British Patent Application No.2217118A in addition to the aforementioned exhaust turbine. These embodiments are arranged such that the motor is automatically selected to provide the required power input when there is insufficient energy available in the exhaust gas to operate the air conditioner at or near the design point; switching between the two drive means is achieved using appropriate shaft couplings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a schematic diagram of an exhaust driven air cycle air conditioning system according to the invention for cooling a load air space using air in a dry, closed loop.

FIG.2 is a block diagram of the preferred control system for obtaining the required power input to the air conditioning system of FIG.1.

FIG.3 is a schematic diagram of the air cycle air conditioning system of FIG.1 in heating mode.

FIG.4 is a schematic diagram of an exhaust driven, reverse flow air cycle air conditioning system according to the invention for cooling a load air space using air in a dry, closed loop.

FIG.S is a schematic diagram of the reverse flow air cycle air conditioning system of FIG.4 in heating mode.

FIG.6 is a schematic diagram of the reverse flow air cycle air conditioning system of FIG.4 with the additon of a cooling mode heat exchanger de-icing facility associated with the expansion air heat exchanger.

FIG.7 is a schematic diagram of the reverse flow air cycle air conditioning system of FIG.S with the addition of a heating mode heat exchanger de-icing facility associated with the expansion air heat exchanger.

FIG.8 is a schematic diagram of the air conditioning system of FIG.1 modified to allow control over the air delivery temperature.

FIG.9 is a block diagram of the preferred control system necessary to obtain the desired air delivery temperature from the air conditioning system of FIG.8.

FIG.10 is a schematic diagram of the air conditioning system of FIG.1 modified to incorporate a high speed motor, in addition to the exhaust gas drive1 for use under conditions of low exhaust gas energy.

DETAILED DESCRIPTION OF THE INVENTION In the following description those components common to the various embodiments are referred to, and shown in the relevant figures, using the same reference numerals.

Figure 1 is a schematic diagram of an air cycle air conditioning system 10 according to the invention in cooling mode. The working air flows in a closed loop comprising: a compressor 12 which raises the temperature and pressure of the air; a compression air heat exchanger 14 in which heat is rejected to an ambient air stream; a turbine 16 in which the compressed and cooled air is expanded back to its original pressure; an expansion air heat exchanger 18 in which heat is absorbed from the conditioning air stream; two two-way valves 20 and 22 which are adjusted such that they maintain the working air closed loop; and a moisture removal device 24 positioned between the turbine 16 and the valve 22 to prevent ice from forming in the heat exchanger 18.

The conditioning air is drawn from the conditioned, or load, air space 26 by means of an electrically powered fan 28, and passes through the expansion air heat exchanger 18 via the fan 28 and a two-way valve 30. Heat is rejected to the working air stream within the heat exchanger 18 and the cooled conditioning air returns to the conditioned air space 26 via another two-way valve 32. Humidity is controlled in the conditioning air closed loop by means of a moisture removal device 34 positioned between the fan 28 and heat exchanger 18. The ambient air stream is fed through the low pressure side of the compression air heat exchanger 14 by means of a second electrically powered fan 36 and via valve 32. Heat is absorbed from the working air stream within the heat exchanger 14 and the ambient air is then ducted back to ambient via valve 30.

A second moisture removal device 34, between the conditioned air space 26 and the two-way valve 30, is not used in the cooling mode of the system but is described below in connection with the system in heating mode.

In passing through the turbine 16 the working air does work which is transferred via a shaft 38 to provide part of the power requirement of the compressor 12. The remaining power needed to drive the compressor 12 is obtained from an exhaust gas source, for instance a car exhaust, which is used to drive an exhaust turbine 40 mounted on the same shaft 38. The exhaust turbine 40 is designed to operate at high rotational speed1 thereby reducing the size of the rotating components for a given duty, and hence resulting in a compact air conditioner 10. The amount of exhaust gas passing through the exhaust turbine 40 is controlled by means of a bypass duct 42, bypass control valve 44, valve controller 46, valve actuator 48 and pressure sensor SO such that operation of the air conditioner 10 is maintained at or near design conditions.

Figure 2 is a block diagram of the preferred control system for achieving this. The design operating condition is factory-set in terms of a desired compressor outlet pressure which is compared, by a negative feedback summer 52, with the pressure measured by pressure sensor 50 positioned in the compressor outlet duct 54. The summer 52 adjusts the bypass control valve 44 by means of the actuator 48 thereby altering the amount of exhaust gas allowed to bypass the exhaust turbine 40. In this way the power input to the compressor 12 is adjusted until the compressor outlet pressure lies within a factory-set pressure band defined in the feedback circuit 56 by a high/low threshold crossing detector 58. Once the measured pressure lies within this band a switch 60 is opened to nullify the feedback and hence maintain the current setting of the control valve 44.When the measured pressure subsequently moves out of the band, for instance due to a sufficient change in car engine condition, the switch 60 is again closed and adjustment of the valve 44 resumes.

The control system described in the preceding paragraph is incorporated in all of the embodiments described hereafter. It will be appreciated by those skilled in the art that a parameter other than compressor outlet pressure, for instance rotational speed or compression ratio, may be used as a measure of the operating condition without departing from the scope of the invention as described. It will also be appreciated that control can be facilitated at less expense using a simple mechanical link between the bypass valve 44 and sensor 50 such as those commonly used in automotive turbochargers. However, the valve setting is then limited to only two discrete settings1 e.g. fully open and fully closed.

As described for Figure 1, the conditioning air circulates in a closed loop. If it is a requirement that fresh air be supplied to the conditioned air space 26 rather than the recirculated air of the closed loop, then it will be appreciated by those skilled in the art that it is a simple matter for ambient air to be ducted into the fan 28 as conditioning air, and for the air leaving the conditioned air space 26 to be ducted back to ambient. However, operation in this way substantially reduces humidity control. The expansion air heat exchanger 18 ensures that the conditioning air is isolated from the working air in the event that the working air becomes contaminated due to ineffective sealing or seal failure between the exhaust turbine 40 and compressor 12.

Figure 3 is a schematic diagram of the air conditioning system of Figure 1 switched to operate in heating mode.

Switching to heating mode is achieved simply by altering the settings of the valves 20,22,30 and 32 as appropriate.

As before, the working air passes through valve 20, the compressor 12, the high pressure side of the compression air heat exchanger 14, the turbine 16 and valve 22 but in this case the air is taken from ambient and flows in an open loop, being ducted back to ambient on passing through valve 22. In this way any ice which develops due to the low temperature at turbine exit is expelled from the system, and the moisture removal device 24 is therefore not utilized. The conditioning air is driven around a closed loop by fan 36. The air is drawn from the conditioned air space 26 and passes1 via valve 30, through the low pressure side of the compression air heat exchanger 14 where heat is absorbed from the compressed working air stream. The heated conditioning air then returns to the conditioned air space 26 via the fan 36 and valve 32.Humidity is controlled by means of the moisture removal device 34' positioned between the conditioned air space 26 and valve 30. Heat exchanger 18, fan 28 and moisture removal device 34. are not utilised in this mode since the conditioning air is isolated from the working air by heat exchanger 14.

As in the cooling mode, if fresh air rather than recirculated air is required for conditioning, it is a simple matter for conditioning air to be ducted in from ambient and subsequently returned to ambient in an open loop. However, humidity control is again substantially reduced.

In certain applications cooling will be the primary duty of the air conditioner 101 for instance in the automotive industry where heating can be provided from the engine heat loss using well established methods. In such cases it will be appreciated that the air conditioner 10 may be modified to operate in cooling mode only. thereby simplifying the embodiment of Figure 1, by eliminating the two-way valves 20, 22, 30 and 32, and the moisture removal device 34'.

Figures 4 and 5 show an embodiment of the air cycle air conditioning system 10 in which the majority of the components are the same as for Figures 1 and 3 but in which the flow direction of the working air is reversed. This has the effect of significantly improving the coefficient of performance of the system 10. In Figures 4 and S the heat exchanger 14 is the expansion air heat exchanger and the heat exchanger 18 is the compression air heat exchanger.

Figure 4 is a schematic diagram of the reverse flow air conditioner 10 in cooling mode. As with the embodiment of Figure 1 the working air flows in a circuit comprising a compressor 12, compression air heat exchanger 14 and turbine 16 but in this case it is open loop with air being drawn from, and returned to, ambient. As mentioned, the working air enters the turbine 16 from ambient via valve 22 and a moisture removal device 24. In the turbine 16 it is expanded to sub-atmospheric pressure and is thus cooled.

It then passes through the expansion air heat exchanger 14, where it absorbs heat from the conditioning air, and on to the compressor 12 to be compressed back to ambient pressure and finally ducted back to ambient via valve 20. The moisture remover 24 is required to prevent ice from forming in the heat exchanger 14.

The conditioning air is drawn from the conditioned air space 26 by a fan 36 and passes, via valve 32 and said fan, through the ambient pressure side of the expansion air heat exchanger 14 thereby rejecting heat to the working air. It then returns to the conditioned air space 26 via a moisture removal device 34 and a valve 30. The moisture remover 34 allows humidity control of the conditioning air closed loop. As with the embodiments of Figures 1 and 3, ambient air may be used as the conditioning air in an open loop but humidity control is again restricted. Heat exchanger 18 and fan 28 are not utilised in this mode since the conditioning air is isolated from the working air by heat exchanger 14.

Figure 5 is a schematic diagram of the reverse flow air conditioner 10 of Figure 4 in heating mode. The device is switched to heating mode by altering the settings of the two-way valves 20, 22, 30 and 32 as appropriate. The working air flows in a closed loop comprising: the turbine 16 in which it is expanded to sub-atmospheric pressure and thus cooled; the expansion air heat exchanger 14 in which heat is absorbed from an ambient air stream. the compressor 12 which compresses the air back to its original pressure and thus heats the air, the compression air heat exchanger 18 in which heat is rejected to the conditioning air stream; two two-way valves 20 and 22 which are adjusted such that they maintain the working air closed loop; and a moisture removal device 24' positioned between the turbine 16 and the heat exchanger 14 to prevent ice from forming in the heat exchanger 14.The moisture removal device 24 between the two-way valve 22 and the turbine 16 is not used in the heating mode.

The conditioning air also circulates in a closed loop, in this case driven by an electrically powered fan 28. The air leaves the conditioned air space 26 and passes through a moisture removal device 34', for humidity control, and a valve 32 before entering heat exchanger 18 where it absorbs heat from the working air stream. The heated conditioning air then returns to the conditioned air space 26 via fan 28 and valve 30. The ambient air stream is fed through the essentially ambient pressure side of the expansion air heat exchanger 14 by means of a second electrically powered fan 36 and via valve 32. Heat is rejected to the working air stream within the expansion air heat exchanger 14 and the ambient air is then ducted back to ambient via valve 30.

In the reverse flow embodiments sub-zero temperatures are encountered at the expansion air heat exchanger 14 for a wide range of ambient and operating conditions. A third moisture removal device 62 is therefore positioned in the ambient air stream prior to the valve 32 in order to avoid ice formation in said heat exchanger 14.

As with the previous embodiments, ambient air may be used as the conditioning air in an open loop but humidity control is again restricted. As mentioned previously, in certain applications cooling will be the primary duty of the air conditioner 10, e.g. in automotive systems. In such cases it will be appreciated that the air conditioner 10 may be modified to operate in cooling mode only, thereby simplifying the embodiment of Figure 4 by eliminating the compression air heat exchanger 18, the fan 28 and the two-way valves 20, 22, 30 and 32.

It has been mentioned that in both the cooling and heating reverse flow embodiments, sub-zero temperatures are encountered at the heat exchanger 14 for a wide range of ambient and operating conditions. Therefore, despite the presence of moisture removers, it is likely that gradual icing of the'heat transfer surfaces of the heat exchanger 14 will occur, thus impairing its performance.

Figure 6 shows the reverse flow cooling mode embodiment of Figure 4 modified to incorporate a de-icing facility and thereby alleviate this problem. Under normal circumstances operation of the air conditioner 10 is as described for Figure 4. In this case, however, the pressure drop across the working air side of the expansion air heat exchanger 14 is measured by a pressure difference sensor 64 which communicates with the de-icing control electronics 66: the pressure drop is measured on the working air side since this will be more susceptible to icing1 being an open loop.

If, due to the formation of ice, the pressure drop increases to a factory-set level then the control electronics 66 switch the main cycle off and cause the settings of valves 30, 32 and 68 to be altered such that air is driven, by the fan 36, from a source of hot air, through the heat exchanger 14, via valves 68 and 32 and the fan 36, and on to ambient via valve 30. In this way the hot air de-ices the expansion air heat exchanger 14 and the resultant water leaves said heat exchanger via a drain 70.

The source of hot air will depend on the application, for instance air from around the exhaust manifold might be used in an automotive application. Operation in de-icing mode continues for a factory-set time period sufficient to complete de-icing, after which the control electronics 66 switch the system back to normal mode.

Figure 7 shows the reverse flow heating mode embodiment of Figure 5 also modified to incorporate a de-icing facility.

The modifications and operation of the facility are essentially the same as those described for Figure 6.

However, in this case it is only necessary for valve 68 to be altered when switching to de-icing mode, and the pressure drop is measured across the ambient air side of the heat exchanger 14 since in this embodiment it will be most susceptible to icing.

l Figure 8 shows the embodiment of Figure 1 modified to allow a greater degree of control over the conditioned air. The addition of a control valve 72, valve controller 74, valve actuator 76, temperature sensor 78 and extra duct 80 allows a controlled amount of air to be diverted from the hot stream leaving the ambient pressure side of the heat exchanger 14 and mixed, via the duct 80, with the cold air being delivered to the conditioned air space 26. In this way the temperature at which the conditioning air is delivered can be adjusted.

Figure 9 is a block diagram of the preferred additional control system for implementing the air delivery temperature control described for the embodiment of Figure 8. The desired air delivery temperature may be input by the user but would normally be factory-set. A negative feedback summer 82 compares the set temperature with that measured by the temperature sensor 78 positioned in the air delivery duct 84, and adjusts the valve 72 by means of the actuator 76 thereby altering the amount of hot air diverted through the duct 80. The air delivery temperature is adjusted in this way until it lies within a temperature band defined in the feedback circuit 86 by a high/low threshold crossing detector 88. Once the . measured temperature lies within this band a switch 90 is opened to nullify the feedback and hence maintain the current setting of the control valve 72.If the temperature subsequently moves out of the band, for instance due to a sufficient change in car engine conditions, then the switch 90 is again closed and adjustment of the valve 72 resumes.

It will be clear to those skilled in the art that control of the air delivery temperature in this way may be applied to all of the air ' conditioner embodiments described previously, with slight modifications determined by the appropriate point from which hot or cold air should be diverted.

Under certain conditions, for instance during idling in automotive applications, it is possible that the energy available in the exhaust gas will be insufficient to drive the air conditioner 10 at or near the design point.

Figure 10 shows the embodiment of Figure 1 modified in such a way as to overcome this problem. In addition to the exhaust turbine drive described for Figure 11 an electrically driven high speed motor 100 of the type described in British Patent Application No.2217118A is incorporated as an alternative means of power input for use under low exhaust energy conditions; the use of a high speed motor of this type significantly reduces the size of the rotating components relative to those of a similar device powered by a more conventional motor. A sensor 102 is used to monitor a suitable parameter chosen as a measure of the energy capacity of the exhaust gas, for instance engine rotational speed in automotive applications, and communicates with the control electronics 104.

While sufficient exhaust energy is available adequately to drive the air conditioner 101 the exhaust turbine 40 is coupled to the shaft 38 by means of the shaft coupling 106 and the system operates as described for Figure 1.

However, if the available energy falls below a factory-set minimum, as measured by the sensor 102, the control electronics 104 decouple the exhaust turbine 40 by deactivating shaft coupling 106, and couple the high speed motor 100 to the shaft 38 by activating shaft coupling 108; electrical power input to the high speed motor 100 is also initiated. The control electronics 104 maintain the high speed motor 100, and hence the turbine 16 and compressor 12, at constant design speed until such time as the energy capacity of the exhaust gas rises sufficiently for the control electronics 104 to switch the system back to exhaust turbine drive.

It will be clear to those skilled in the art that the drive arrangement described for Figure 10 may also be incorporated in all of the air conditioner embodiments described previously.

Claims

CLAIMS:

1. An air-cycle air conditioning system for cooling a load air space, comprising: an exhaust turbine drivable by a supply of exhaust gases from an engine; an air compressor drivingly connected to an expansion turbine, the power requiremepts of the compressor being provided at least in part by the exhaust turbine; and an expansion air heat exchanger for receiving expanded air from the expansion turbine and delivering it to the compressor for recompression, the expanded air in the expansion air heat exchanger being in heat exchange relationship with a fan-assisted flow of conditioning air to the load air space for effecting cooling of the load air space.

2. An air conditioning system according to claim 1, which is switchable between cooling and heating modes for cooling or for heating the load air space. further comprising a compression air heat exchanger for receiving compressed air from the compressor and delivering it to the expansion turbine, the compressed air in the compression air heat exchanger being in heat exchange relationship with a fan-assisted flow of conditioning air to the load air space when the system is in its heating mode for effecting heating of the load air space and the expanded air in the expansion air heat exchanger being in heat-exchange relationship with its fan-assisted flow of conditioning air to the load air space only when the system is in its cooling mode.

3. An air conditioning system according to claim 2, wherein the supply of air to the expansion turbine in both the cooling and heating modes is a supply of compressed air from the compressor via the compression air heat exchanger.

4. An air conditioning system according to claim 2, wherein the supply of air to the expansion turbine in the cooling mode is a supply of ambient air at ambient pressure. and the supply of air to the expansion turbine in the heating mode is a supply of compressed air from the compressor via the compression air heat exchanger.

5. An air conditioning system according to any of claims 2 to 4, wherein the fan-assisted flows of conditioning air are both at substantially ambient pressure.

6. An air conditioning system according to any of claims 2 to 5, comprising a first fan for driving the conditioning air through the expansion air heat exchanger in the heating mode and for driving ambient air through the expansion air heat exchanger in the cooling mode; and a second fan for driving the conditioning air through the compression air heat exchanger in the cooling mode and for driving ambient air through the compression air heat exchanger in the heating mode.

7. An air conditioning system according to claim 6, wherein the air flows from the fans to the heat exchangers are controlled by four two-way valves operable in unison to switch the system between its cooling and heating modes.

8. An air conditioning system according to claim 6 or claim 7, further comprising a moisture removal device positioned between the expansion turbine and the expansion air heat exchanger to control the humidity of the working air in the cooling and heating modes.

9. An air conditioning system according to claim 8, further comprising a moisture removal device positioned between the second fan and the expansion air heat exchanger to control the humidity of the conditioning air in the cooling mode.

10. An air conditioning system according to claim 8 or claim 9, further comprising a moisture removal device positioned between the load air space and the compression air heat exchanger to control the humidity of the conditioning air in the heating mode.

11. An air conditioning system according to any preceding claim, further comprising an exhaust turbine bypass valve controlled by a control system for automatically adjusting the proportion of the total exhaust gases passing through the exhaust turbine to maintain a desired power input to the compressor.

12. An air conditioning system according to any preceding claim, further comprising a high speed electric motor for driving the compressor when the exhaust gas flow is insufficient to power the system. and means for disconnecting the exhaust turbine from the compressor in synchronism with actuation of the motor.

13. An air conditioning system according to any preceding claim, further comprising means for selectively adding a controlled flow of heated ambient air to cooled conditioning air delivered to the load air space, for temperature control of the load air space.

14. An air conditioning system according to any preceding claim, further including a de-icing control for the compression air heat exchanger. wherein the de-icing control comprises a pressure difference sensor for sensing differences between the entry and exit pressures of at least one of the air flows through the expansion air heat exchanger; a controller responsive to a pressure difference so sensed; a source of heated air; and at least one two-way valve controlled by the controller to direct the heated air in a fan-assisted air flow through the expansion air heat exchanger to melt any ice therein responsible for the pressure difference.

15. An air cycle air conditioning system for cooling or heating a load air space, said system comprising: a compressor for compressing working air supplied to it at ambient pressure; a first heat exchanger for rejecting heat from said compressed working air to an essentially ambient pressure ambient air stream in cooling mode, and to an essentially ambient pressure conditioning air stream in heating mode; a turbine for expanding said compressed and cooled working air back to ambient pressure and in so doing provide part of the power required to drive said compressor; a second heat exchanger for transferring heat to said expanded working air from an essentially ambient pressure conditioning air stream in cooling mode, and at the same time isolating said conditioning air stream from said working air stream; a first fan to drive said ambient air through said first heat exchanger in cooling mode, and to drive said conditioning air through said load and said first heat exchanger in heating mode; a second fan to drive said conditioning air through said second heat exchanger and said load in cooling mode; an exhaust turbine which is driven by a supply of exhaust gas thereby providing that part of said compressor power requirement not provided by said turbine; means for conducting said working air in a closed loop from said second heat exchanger through said compressor. first heat exchanger and turbine and back to said second heat exchanger in cooling mode; means for conducting said ambient air in an open loop from ambient through said first fan and first heat exchanger and back to ambient, also in cooling mode; means for conducting said conditioning air in a closed loop from said load through said second fan and second heat exchanger and back to said load, also in cooling mode; means for conducting said working air in an open loop from ambient through said compressor, first heat exchanger and turbine and back to ambient in heating mode; means for conducting said conditioning air in a closed loop from said load through said first heat exchanger and first fan and back to said load, also in heating mode.

16. An air cycle air conditioning system for cooling or heating a load air space. said system comprising: a turbine for expanding working air supplied to it at ambient pressure and in so doing provide. part of the power to drive a compressor; a first heat exchanger for transferring heat into said expanded working air from an essentially ambient pressure conditioning air stream in cooling mode, and from an essentially ambient pressure ambient air stream in heating mode, said compressor for compressing said expanded and heated working air back to ambient pressure; a second heat exchanger for rejecting heat from said working air to an essentially ambient pressure conditioning air stream in heating mode, and at the same time isolating said conditioning air stream from said working air stream; a first fan to drive said conditioning air through said first heat exchanger and said load in cooling mode, and to drive said ambient air through said first heat exchanger in heating mode; a second fan to drive said conditioning air through said load and said second heat exchanger in heating mode; an exhaust turbine which is driven by a supply of exhaust gas thereby providing that part of said compressor power requirement not provided by said turbine; means for conducting said working air in an open loop from ambient through said turbine, first heat exchanger and compressor and back to ambient in cooling mode; means for conducting said conditioning air in a closed loop from said load through said first fan and first heat exchanger and back to said load, also in cooling mode; means for conducting said working air in a closed loop from said second heat exchanger through said turbine, first heat exchanger and compressor and back to said second heat exchanger in heating mode; means for conducting said ambient air in an open loop from ambient through said first fan and first heat exchanger and back to ambient, also in heating mode; and means for conducting said conditioning air in a closed loop from said load through said second heat exchanger and second fan and back to said load, also in heating mode.

17. An air conditioning system substantially as described herein with reference to the drawings.