SINGLE FLUID CIRCUIT HEAT TRANSFER SYSTEM FIELD OF THE INVENTIQN This invention relates to heat transfer systems and more particularly to a heat transfer system incorporating a single fluid circuit which may be used in conjunction with heat transfer devices having a cold side and a hot side. The heat transfer device may generate heat flow as is the case with heat pump or use heat flow as is the case with power generators. For the sake of convenience, the invention will be described in relation to heat transfer systems adapted for thermo electric cooling but it is to be understood that the invention is not limited thereto. BACKGROUND ART Conventional heat transfer systems incorporate dual fluid circuits with one fluid circuit for the cold side of the system and another fluid circuit for the hot side of the system. However, in the case of refrigeration, where the temperature difference between the refrigerated space and the ambient is high, the efficiency of a heat pump is severely impaired by that temperature difference. A common solution (as shown in Fig. 2) is to cascade the thermo electric heat pump modules by mounting one on top of the other, thus reducing the temperature differential of each module. If two modules are used, the temperature differential is approximately half the total. Up to six cascade stages are commonly used. A disadvantage of cascading is that the inefficiency of preceding stages results in a large amount of useless heat that has to be pumped through at each stage, limiting the usefulness of this approach to relatively low heat loads. There is a need for improved efficiency with heat transfer systems generally and in particular with thermo electric cooling and it has been found that this can be achieved by a single fluid circuit heat transfer system according to the invention which differs from the normal fluid circuits used in cooling in that it has a common hot and cold side fluid circuit. Although it would seem counter intuitive to pipe the hot and cold fluids circuits together, the single fluid circuit heat transfer system provides system performance benefits by reducing the temperature difference across the heat transfer devices which leads to much higher efficiencies. SUMMARY OF THE INVENTION A heat transfer system according to the invention comprises a heat transfer device having a hot side and a cold side and a heat transfer fluid flow line in heat transfer relationship with the cold side and the hot side of the heat transfer device. In a preferred embodiment of the invention, there is provided a plurality of parallel heat transfer devices and the heat transfer fluid flow line is arranged in heat transfer communication with all the cold sides and all the hot sides of the heat transfer devices. DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of a conventional prior art dual fluid circuit heat transfer system incorporating parallel heat pumps, Fig. 2 is a schematic diagram of a conventional prior art dual fluid circuit heat transfer system incorporating heat pumps in series, Fig. 3 is a schematic diagram of a single fluid heat transfer system according to one embodiment of the invention, Fig. 4 is a graphical representation of temperature (T) vs heat pump (Q) for the single fluid circuit heat transfer system shown in Fig. 3, Fig. 5 is a graphical representation of heat pumped (Q) vs temperature difference (dT) for the single fluid circuit heat transfer system shown in Fig. 3 and for a conventional dual flow circuit heat transfer system, Fig. 6 is a schematic diagram of a single fluid circuit heat transfer system according to a second embodiment of the invention, Fig. 7 is a schematic diagram of a single fluid circuit heat transfer system according to a third embodiment of the invention, Fig. 8 is a schematic diagram of a single fluid circuit heat transfer system according to a fourth embodiment of the invention which does not have a heat exchanger at the cold end, and, Fig. 9 is a schematic diagram of a single fluid circuit heat transfer system according to a fifth embodiment of the invention which does not have a heat exchanger at the cold end or the hot end. The conventional dual fluid circuit heat transfer system 10 shown in Fig. 1 incorporates a plurality of parallel heat pumps 11 each of which has a cold side 12 and a hot side 13. A first fluid circuit 14 is connected between the cold sides 12 of the heat pumps 11 and a cold heat exchanger 15 which, in this instance, is located in a refrigerated space 16. A second fluid circuit 17 is connected between the hot sides 13 of the heat pumps 11 and a hot heat exchanger 18 located externally of the refrigerated space 16. The flow of heat is indicated by arrows A, B and C. At arrow A, heat Q1 is transferred from the refrigerated space 16 to fluid circuit 14. At arrow B, the heat gain Q2 from the heat pump is transferred to fluid circuit 17 and at arrow C the total heat of Q1 plus Q2 is pumped out of the heat exchanger 18. The dual fluid circuit heat transfer system shown in Fig. 2 is substantially similar to that shown in Fig. 1 except that the heat pumps 11 are arranged in series. Thus, there is only one cold side 12 and one hot side 13. As indicated above and shown in Figs. 1 and 2, the conventional dual fluid circuit heat transfer system has one fluid circuit for the cold side and another fluid circuit for the hot side. Such a system will work with many heat pumps as shown in Figs. 1 and 2 or with only one heat pump. A single fluid circuit heat transfer system 20 according to one embodiment of the invention is shown schematically in Fig. 3. In this instance, the system 20 includes a plurality of heat pumps 21 each of which has a cold side 22 and a hot side 23. Each heat pumps 21 is thermally isolated from adjacent heat pumps 21. The single fluid circuit 24 is in heat transfer communication with the cold sides 22 and the fluid is delivered to a cold heat exchanger 25 located in a refrigerated space 26. The fluid leaving the cold heat exchanger 25 is in heat transfer communication with the hot sides 23 of the heat pumps 21 and is delivered to the hot heat exchanger 27 located externally of the refrigerating space 26. At point 30 below the hot heat exchanger 27, the fluid has been cooled to temperature T1 which is close to the ambient temperature by the heat exchanger 27. The fluid then flows over the cold side 22 of the first heat pump 22 and is further cooled as it flows over the cold sides 22 of the remaining two heat pumps 21. As the fluid enters the cold heat exchanger 25 at point 31 , it is at temperature T2 which is below the temperature of the refrigerated space 26. The fluid then absorbs heat (Q) form the refrigerated space 26 through the cold heat exchanger 25 as indicated by arrow D and at point 31 exits the refrigerated space 6 at temperature T3 which is just under the temperature of the refrigerated space 26. The fluid then enters the bank of heat pumps 22 at the hot side 23 of the first heat pump 21 and then runs through the hot sides 23 of the remaining heat pumps 21. The heat removed from the fluid as it passes over the cold side of the heat pumps is returned to the fluid plus the heat generated from power in the heat pumps (Pp) indicated by arrow E. Finally at point 33 the fluid, at temperature T4 flows to the hot heat exchanger 27 where the heat removed from the refrigerating space (Q) and the heat gained from the heat pumps (Pp) is absorbed by the ambient air as indicated by arrow F. The temperature of the hot side of each heat pump is increased as the fluid absorbs heat as shown in Fig. 4. Because of this arrangement, the heat due to inefficiencies in earlier parts of the circuit does not have to be pumped serially through each succeeding stage. This contrasts with the cascade system of the prior art where every stage has to pump more heat than Q and enables much larger effective cooling capacities for thermoelectric systems in particular. Rather than use heat pumps to pump heat directly from a refrigerated space, the heat pumps 21 are used to create a temperature gradient within the fluid circuit by pumping heat from one side of the circuit to the other. This cools the fluid to below the temperature of the refrigerated space, so it can absorb heat and then reheats the fluid to above ambient temperature where it can release the heat absorbed from the refrigerated space. If the temperature difference across the cold heat exchanger 25 is low, then the temperature difference from the hot to cold side of the heat pumps 21 is also low. This has the effect of putting the heat pumps 21 in a condition of low dT that significantly increases the Coefficient of Performance (COP) of the heat pumps. However the price for this increase in COP is the amount of heat that the heat pumps are required to pump. Using the single fluid circuit system, designers can choose the temperature difference the heat pumps will pump against by varying the mass flow rate. By increasing the fluid mass flow rate (see Fig. 5), the temperature difference across the cold heat exchanger will be reduced (for the same heat load Qc), however the heat pumped from one side of the fluid circuit to the other will be increased. Thus by changing the fluid mass flow rate, the temperature difference across the heat pump(s) can be reduced at the cost of a higher heat pumping requirement. A system with a 200C temperature difference from the cold heat exchanger to the hot heat exchanger can have a 5°C (or any other temperature) average temperature difference over the heat pumps. The price for this reduced heat pump dT is an increased amount of heat that needs to be pumped. The trade off between dT and heat pumped can be seen in Fig. 5. A dual fluid circuit has a fixed heat pump temperature difference and required heat pumping Q, which is determined by the system heat load Qc and system temperatures. Thus a larger degree of flexibility is available with the Single Fluid circuit than with the Dual Fluid circuit. The single fluid circuit can be used in other configurations to obtain cooling at two or more different temperatures as shown in Figs. 6 and 7. the single fluid circuit refrigeration system 60 shown in Fig. 6 includes a freezer heat exchanger 61 , a refrigerator heat exchanger 62, a hot side heat exchanger 63, a first bank of heat pumps 64 having a cold side 65 and a hot side 66 and a second bank of heat pumps 67 having a cold side 68 and a hot side 69. The single fluid circuit 60a extends from the hot side heat exchanger 63 to the cold side 65 of the first bank of heat pumps 64, through the refrigerator heat exchanger 62 to the cold side 68 of the second bank of heat pumps 67, through the freezer heat exchanger 61 and via the hot sides 69 and 66 of both banks of heat pumps 67 and 64 to the hot side heat exchanger 63. The single fluid circuit refrigeration system 70 shown in Fig. 7 includes a freezer heat exchanger 71 , a refrigerator heat exchanger 72, a hot side heat exchanger 73, a first bank of heat pumps 74 having a cold side 75 and a hot side 76 and a second bank of heat pumps 77 having a cold side 78 and a hot side 79. The single fluid circuit 70a extends from the hot side heat exchanger 73 to the cold side 75 of the first bank of heat pumps 74 and in one configuration through the refrigerator heat exchanger 72 and via the hot side 76 of the first bank of heat pumps 74 to the hot side heat exchanger 73. In another configuration of the refrigeration system shown in Fig. 7, the single fluid circuit 70a extends from the cold side 75 of the first bank of heat pumps 74 to the cold side 78 of the second bank of heat pumps 77, through the freezer heat exchanger 71 to the hot side 79 of the second bank of heat pumps 77 and the hot side 76 of the first bank of heat pumps 74 to the hot side heat exchanger 73. The refrigeration system 70 of Fig. 7 differs from that of Fig. 6 in that the single fluid circuit can operate in several modes such as through the refrigerator heat exchanger 72 and not through the freezer heat exchanger 71 , through the freezer heat exchanger 71 and not through the refrigerator heat exchanger 72 and also through both the freezer heat exchanger 71 and the refrigerator heat exchanger 72. In some cases, temperature change in the recirculated fluid is the objective and in this case there is no requirement for a heat exchanger at the cold end. The single fluid circuit heat transfer system 80 shown in Fig. 8 includes a hot side heat exchanger 81 and a bank of heat pumps 82 having a cold side 83 and a hot side 84. The single fluid circuit 85 extends from the hot side heat exchanger 81 to the cold side 83 of the bank of heat pumps 82 and via the hot side 84 of the bank of heat pumps 82 to the hot side heat exchanger 81. This configuration has improved efficiency because the hot side of the circuit does not have added head from a refrigerated space. The temperatures at the hot side of the heat pumps (and therefore the dT across them) will be lower leading to a higher co-efficient of performance. The heat flow in this configuration is shown by the arrows g and H. In another case, if the aim is only to induce temperature change in the fluid, it may make only a single pass through the circuit and then be discharged. The efficiency of this circuit will be even higher because there is no need for a hot side heat exchanger and the fluid temperature entering the system will always be at ambient, not slightly above ambient as in the case where a hot side heat exchanger is employed. An example of a circuit where this arrangement is advantageous is in dehumidification of air. The air dehumidification system 90 shown in Fig. 9 consists of a bank of heat pumps 91 having a cold side 92 and a hot side 93. The single fluid circuit 94 is not connected to any heat exchanger and extends from the inlet 95 to the cold side 92 of the bank of heat pumps 91 , through the hot side 93 of the heat pumps 91 to the outlet 95. Ambient air is the fluid recirculated and it enters the circuit at inlet 94 in Fig. 9. As the air passes along the cold side 92 the fluid is progressively cooled. At the end of its path along the cold side 92, the fluid is immediately recirculated along the hot side 93 and then discharged to atmosphere at the exit 95, instead of being passed through a hot heat exchanger. In order to achieve the maximum amount of dehumidification the cold side temperatures may be reduced to below O0C. Water will freeze onto the heat pump cold side 92 and may be recovered by reversing the polarity of the thermoelectric heat pumps 91. This will heat the ice collected on the cold side 92 and melt it. This water may be used in a range of applications such as for drinking or for commercial and industrial purposes where purified water is required. By reversing the direction of air flow through the cold side 92, the work done to melt the ice can be drawn from the air stream which is to be cooled. Thus the defrosting action does not need to be a parasitic draw from the amount of useful cooling carried out by the system. Using the principles outlined above, the single fluid circuit can be utilized to obtain similar or better efficiencies to "staged" cooling circuits but with much larger cooling powers. Various modifications may be made in details of design and circuit configuration without departing from the scope and ambit of the invention.