The present invention relates to a heat transfer circuit and to a valve for use therein.

Heat transfer circuits are widely used in a variety of arrangements for transferring heat between heat sources and heat sinks. When multiple heat sources and/or heat sinks are used within a single heat transfer circuit, valves are regularly used to switch the flow between the multiple heat sources and/or heat sinks so that one or more of the heat sources and/or heat sinks can be bypassed when no useful heat can be absorbed from the heat source or no useful heat can be dissipated into the respective heat sink. Such valves can be electronically controlled or be thermal self-actuating valves. The latter are directed to be actuated to open or close when the temperature in the heat transfer circuit, or in the heat source or the heat sink, reaches a pre-set temperature. In many applications, for example in renewable energy applications such as solar thermal circuits and in recovery of waste heat from refrigeration systems, the temperature of the heat sources and/or heat sinks are variable, and switching should preferably occur based on a temperature difference, instead of at a pre-set temperature.

Heat transfer circuits using self-actuating valves based on temperature difference are known in the prior art, for example in EP2573392, but those self-actuating valves have only one inlet and therefore feature an externally protruding sensor that is used to make a temperature comparison. The sensor needs to be located within a heat sink or heat source and this tends to restrict the application thereof in some heat transfer circuits. It is therefore desirable to use valves having multiple inlets that enable a temperature comparison to be made internally within the valve. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art in any country. SUMMARY

According to one aspect, there is provided a heat transfer circuit comprising:

a thermal reservoir having a reservoir entry and a reservoir exit, wherein the thermal reservoir is configured to be traversed by a fluid arranged to exchange heat with the thermal reservoir;

a valve provided between the reservoir entry and the reservoir exit, the valve being arranged to compare temperatures of fluid entering the reservoir entry and fluid exiting the reservoir exit,

wherein the heat transfer circuit is configured to selectively direct the fluid to traverse the thermal reservoir or to bypass the thermal reservoir, dependent on the compared temperatures.

The heat transfer circuit may comprise multiple thermal reservoirs and one or more of the valves, wherein the or each valve is associated with at least one of the thermal reservoirs. One or more of the thermal reservoirs may not be associated with one of the valves. In some embodiments two or more of the thermal reservoirs associated with a respective valve may be arranged in parallel with one another.

The valve may comprise a first valve inlet, a second valve inlet and a valve outlet, and wherein the first valve inlet is arranged to receive fluid entering the reservoir entry and the second valve inlet is arranged to receive fluid exiting the reservoir exit. The valve may define a bypass passage extending between the first valve inlet and the valve outlet, whereby fluid flowing from the first valve inlet to the valve outlet will bypass the thermal reservoir. The valve may define a traverse passage extending between the second valve inlet and the valve outlet, whereby fluid flowing from the second valve inlet to the valve outlet will traverse the thermal reservoir. The valve may define a flow-through passage for conveying the fluid to the thermal reservoir, wherein the first valve inlet is located in the flow-through passage so as to define a T-junction within the valve.

The valve may comprise a thermal actuator for comparing the temperatures of fluid entering the first valve inlet and fluid entering the second valve inlet. The thermal actuator may comprise a hollow body containing a working fluid, wherein a change in weight distribution of the working fluid inside the hollow body causes movement of the thermal actuator under gravity.

The thermal actuator may be joined to valve member being arranged to close off either the first valve inlet and open the second valve inlet or to close off the second valve inlet and open the first valve inlet. The valve member may be arranged to permit a residual stream of fluid to flow through the closed off first or second valve inlet respectively so that the thermal actuator can perform the temperature comparison. The residual stream may have a flow rate of < 10% or < 0.1 % of its full bore flow rate.

The valve member may be arranged to completely close off the first or second valve inlet respectively. A biasing weight may be attached to the actuator to bias the actuator towards a desired valve inlet to assist in achieving such complete closing thereof. The biasing weight may be arranged to introduce an offset of a temperature difference that must be overcome before the actuator will move. The offset of the temperature difference may be between 0-2 Kelvin. The first valve inlet may be substantially smaller than the second valve inlet. The first valve inlet may have a cross-sectional area being less than 20% of a cross-sectional area of the first inlet.

The heat transfer circuit may be configured to direct the fluid to traverse the thermal reservoir if the fluid entering the reservoir entry is cooler than fluid exiting the reservoir exit. The thermal reservoir may comprise a heat source, such as a solar thermal collector.

The heat transfer circuit may be configured to direct the fluid to traverse the thermal reservoir if the fluid entering the reservoir entry is warmer than fluid exiting the reservoir exit. The thermal reservoir may comprise a heat sink such as a heat accumulator.

The heat transfer circuit may comprise a pump for pumping the fluid, the pump being joined to an electricity supply and having a thermal switch arranged to switch the pump on or off, wherein the thermal switch is exposed to the fluid and arranged to interrupt the electricity supplied to the pump when the temperature of the fluid becomes too high. The thermal switch may be a bimetal switch. In one embodiment the heat transfer circuit is used with a heat accumulator. In another embodiment the heat transfer circuit is used with a solar thermal collector.

The heat transfer circuit may be used with multiple heat accumulators arranged in parallel. The multiple heat accumulators may be hot water storage tanks. According to another aspect, there is provided a valve comprising:

a housing defining a chamber;

a first valve inlet leading into the chamber;

a second valve inlet leading into the chamber;

an outlet leading from the chamber;

a valve member movably supported in the chamber and being arranged to selectively substantially close off either the first valve inlet or the second valve inlet; and a thermal actuator being arranged to cause movement of the valve member, wherein the thermal actuator is configured to compare temperatures of fluid entering the first valve inlet and fluid entering the second valve inlet. The thermal actuator may comprise a hollow body containing a working fluid, wherein a change in weight distribution of the working fluid inside the hollow body causes movement of the thermal actuator under gravity.

The valve member may be arranged to permit a residual stream of fluid to flow through the closed off first or second valve inlet respectively to contact the thermal actuator. The residual stream may have a flow rate of < 10% or < 0.1 % of its full bore flow rate.

The valve member may be arranged to completely close off the first or second valve inlet respectively during steady state conditions to prevent fluid to flow through the respective valve inlet. The valve member may be arranged to partially open the completely closed off valve inlet in response to a change in the steady state condition, thereby permitting a residual stream of fluid to flow through the respective valve inlet to contact the thermal actuator.

The valve member may be pivotally joined to the housing at a pivot. A biasing weight may be attached to the actuator to bias the actuator towards a desired valve inlet. The valve member may comprise a first plug arranged to engage with the first valve inlet and a second plug arranged to engage with the second valve inlet.

The housing may define a flow-through passage for conveying the fluid passed the first valve inlet towards the second valve inlet, wherein the first valve inlet is located in the flow-through passage so as to define a T-junction within the housing.

The first valve inlet may be substantially smaller than the second valve inlet. The first valve inlet may have a cross-sectional area being less than 20% of a cross-sectional area of the first inlet.

In one embodiment the valve is used in a heat transfer circuit comprising a heat accumulator. In another embodiment the valve is used in a heat transfer circuit comprising a solar thermal collector.

BRIEF DESCRIPTION OF THE FIGURES

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 is a schematic flow diagram of a first heat transfer circuit having three thermal reservoirs arranged in series, one of which can be selectively bypassed during use;

Figure 2 is a schematic flow diagram of a solar heating system representing a practical application of the first heat transfer circuit of Figure 1 ;

Figure 3 is a schematic flow diagram of a combination solar heating system representing a further practical application of the first heat transfer circuit of Figure 1 ;

Figure 4a is top view of a valve for use in the heat transfer circuits of Figures 1 and 3, wherein the valve is configured to receive two fluid streams and is arranged to promote through flow of a warmer of the fluid streams;

Figure 4b is a sectional end view of the valve shown in Figure 4 seen along arrows B-B;

Figure 4c is a sectional end view of the valve shown in Figure 4 seen along arrows C-C;

Figure 4d is a sectional end view of the valve shown in Figure 4 seen along arrows D-D; Figure 5a is top view of a valve for use in the heat transfer circuits of Figures 1 to 3, wherein the valve is configured to receive two fluid streams and is arranged to promote through flow of a cooler of the fluid streams;

Figure 5b is a sectional end view of the valve shown in Figure 5 seen along arrows B-B;

Figure 5c is a sectional end view of the valve shown in Figure 5 seen along arrows C-C;

Figure 5d is a sectional end view of the valve shown in Figure 5 seen along arrows D-D;

Figure 6 is a schematic flow diagram of a second heat transfer circuit having three thermal reservoirs, wherein two of the thermal reservoirs are arranged in parallel and both of which can be selectively bypassed during use;

Figure 7a is top view of a valve for use in the second heat transfer circuit of

Figure 6, wherein the valve is configured to receive two fluid streams and is arranged to promote through flow of a cooler of the fluid streams; and

Figure 7b is a sectional end view of the valve shown in Figure 7 seen along arrows D-D.

DETAILED DESCRIPTION

Generally the present disclosure relates to a heat transfer circuit having a valve that is arranged to receive two fluid inlet streams and is selectively able to promote flow of one of the inlet streams to an outlet while substantially blocking or retarding flow of the other inlet stream. The selection between the inlet streams is based on a comparative temperature difference between the inlet streams and for this purpose the valve contains a valve member joined to a thermal actuator that is arranged to be contacted by the two inlet streams. The valve actuator causes actuation of the valve member to selectively promote flow of either a warmer of the inlet streams or a cooler of the inlet streams.

Referring to Figure 1 of the drawings, there is shown a schematic flow diagram of a first heat transfer circuit 100, which is arranged to transfer heat between a first thermal reservoir 102, a second thermal reservoir 104 and a third thermal reservoir 1 12 by pumping fluid to circulate through the circuit 100. Accordingly the first thermal reservoir 102 functions as a heat source and the second thermal reservoir 104 functions as a heat sink. In this embodiment the third thermal reservoir 1 12 intermittently functions as a heat sink.

The fluid is pumped by pump 106.

Intermediate the first thermal reservoir 102 and the second thermal reservoir 104, in the flow direction of the fluid through the circuit 100, the circuit 100 comprises a T-junction 108 associated with a valve 1 10 that is arranged to selectively divert the fluid flowing from the first thermal reservoir 102 through the third thermal reservoir 1 12 before the fluid is returned to the valve 1 10. Thus valve 1 10 has a first inlet 1 10.1 and a second inlet 1 10.2, wherein the first inlet 1 10.1 is arranged to receive fluid flowing from the first thermal reservoir 102 and the second inlet 1 10.2 is arranged to receive fluid flowing from the third thermal reservoir 1 12. Valve 1 10 further has an outlet 1 10.3 arranged to direct fluid to flow to the second thermal reservoir 104. The third thermal reservoir 1 12 contains fluid that is either warmer or cooler than the fluid flowing from the first thermal reservoir 102, whereby valve 1 10 is configured to compare the temperature of the fluid flowing into the first inlet 1 10.1 with fluid flowing into the second inlet 1 10.2 and to promote the flow of the cooler fluid.

When the temperature of the fluid flowing into the first inlet 1 10.1 is warmer than the fluid flowing into the second inlet 1 10.2 the valve 1 10 is actuated to substantially close off the first inlet 1 10.1 so that the fluid flowing from the first thermal reservoir 102 is substantially diverted through T-junction 108 to flow through and traverse the third thermal reservoir 1 12. In this manner the third thermal reservoir 1 12 functions as an intermediate heat sink that lowers the temperature of the fluid from the first thermal reservoir 102 before it enters the second thermal reservoir 104.

Conversely, when the temperature of the fluid flowing into the first inlet 1 10.1 is cooler than the fluid flowing into the second inlet 1 10.2 the valve 1 10 is actuated to substantially or completely close off the second inlet 1 10.2 so that the fluid flowing from the first thermal reservoir 102 substantially bypasses the third thermal reservoir 1 12 and flows directly to the second thermal reservoir 104. In this manner unwanted heating of the fluid by the third thermal reservoir 1 12 is substantially or completely avoided. In this configuration described above the circuit 100 is arranged to transfer heat from a heat source, being first thermal reservoir 102, to either one or two heat sinks. When the fluid in the third thermal reservoir 1 12 is too warm so that it cannot function as a heat sink, then the heat is transferred directly to the only heat sink, being second thermal reservoir 104. Alternatively when the fluid in the third thermal reservoir 1 12 is sufficiently cool so that it can function as a heat sink, then the heat is transferred firstly to the third thermal reservoir 1 12 and, if any heat remains, then also to the second thermal reservoir 104.

It will be appreciated by the skilled addressee that the circuit 100 can be easily rearranged into a counter configuration without requiring any structural changes apart from altering the operation of valve 1 10. In the following description of such a counter configuration the respective parts are indicated by the same reference numerals but the heat sources and heat sinks are interchanged. In the counter configuration the circuit 100 is arranged to transfer heat between the first thermal reservoir 102, the second thermal reservoir 104 and the third thermal reservoir 1 12 to by pumping fluid through the circuit 100 so that it circulates from the first thermal reservoir 102 to the second thermal reservoir 104.

Accordingly the first thermal reservoir 102 functions as a heat sink and the second thermal reservoir 104 functions as a heat source. In this embodiment the third thermal reservoir 1 12 intermittently functions as an additional heat source.

The fluid is similarly pumped by pump 106. In the counter configuration the third thermal reservoir 1 12 is arranged to function as an additional heat source for reducing the heating burden on the second thermal reservoir 104. Also in this counter configuration, the third thermal reservoir 1 12 contains fluid that is either warmer or cooler than the fluid flowing from the first thermal reservoir 102, whereby valve 1 10 is configured to compare the temperature of the fluid flowing into the first inlet 1 10.1 with fluid flowing into the second inlet 1 10.2, but in this embodiment the valve 1 10 is arranged to promote the flow of the warmer fluid.

When the temperature of the fluid flowing into the first inlet 1 10.1 is cooler than the fluid flowing into the second inlet 1 10.2 the valve 1 10 is actuated to substantially or completely close off the first inlet 1 10.1 so that the fluid flowing from the first thermal reservoir 102 is substantially diverted through T-junction 108 to flow through and traverse the third thermal reservoir 1 12. In this manner the third thermal reservoir 1 12 functions as an intermediate heat source that raises the temperature of the fluid from the first thermal reservoir 102 before it enters the second thermal reservoir 104.

Conversely, when the temperature of the fluid flowing into the first inlet 1 10.1 is warmer than the fluid flowing into the second inlet 1 10.2 the valve 1 10 is actuated to substantially or completely close off the second inlet 1 10.2 so that the fluid flowing from the first thermal reservoir 102 substantially or completely bypasses the third thermal reservoir 1 12 and flows directly to the second thermal reservoir 104. In this manner unwanted cooling of the fluid by the third thermal reservoir 1 12 is substantially or completely avoided.

Therefore it will be appreciated that in this counter configuration the circuit 100 is arranged to transfer heat to a heat sink, namely first thermal reservoir, from either one or two heat sources. When the fluid in third thermal reservoir 1 12 is too cool so that it cannot function as a heat source, then the heat is transferred directly from only one heat source, namely second thermal reservoir 104. Alternatively, when the fluid in the third thermal reservoir 1 12 is sufficiently warm so that it can function as a heat source, then the heat is transferred firstly from the third thermal reservoir 1 12 and, if possible, then secondly also from the second thermal reservoir 104.

Valve 1 10 comprises a thermal actuator, being further described below with reference to Figures 4 and 5, which functions to compare the temperature of the respective fluids flowing into the first inlet 1 10.1 and the second inlet 1 10.2. In order for the thermal actuator to compare the respective temperatures of these fluids, it will be understood that the thermal actuator is arranged so that the valve member does not fully seal either of the first inlet 1 10.1 or the second inlet 1 10.2 all the time when these are closed off, but allows a small residual stream to flow in certain embodiments as will be further described below. The flow rate of the residual stream is partially dependent on the temperature difference between the fluids. In one embodiment the residual stream flow rate is <10% and in another embodiment the residual stream flow rate is <1 % of the full bore flow rate, but this can be selectively reduced to <0.1 % if the temperature difference is >2°.

In other embodiments it is possible for the valve 1 10 to close off one of the inlets 1 10.1 or 1 10.2 completely by attaching a biasing weight to the thermal actuator, whereby there will be no residual stream. The flow rate of the residual stream can be controlled to remain above 1 % of the full bore flow rate, for example by providing an obstruction to prevent the valve member from closing so that it remains open to a set degree. Those skilled in the art will appreciate that a higher residual flow rate is desirable if there is a large distance between the valve 1 10 and the third thermal reservoir 1 12, because that would enable a quicker response time in actuation of the valve member.

Although the above description has been made referring to the thermal reservoirs 102, 104, 1 12 it will be appreciated that these can be physical tanks but can also represent many other forms of heat sources and/or heat sinks, for example a solar collector, a heat exchanger or even piping subject to thermal losses or heating, such as by convection or radiation to or from the ambient environment. It will also be appreciated that some thermal reservoirs can function interchangeably as heat sources as well as heat sinks at different times, for example a solar collector is typically a heat source when it is able to absorb sunlight, but the solar collector is typically a heat sink when it is not able to absorb sunlight because of heat losses to the environment, e.g. at night. Thus a single solar collector can represent both the first thermal reservoir 102 as well as the second thermal reservoir 104 in the circuit 100.

Although the above description of the heat transfer circuit 100 makes use of the pump 106, it will be appreciated that in some systems the fluid flow can be driven by natural convection, i.e. caused by density differences of the fluid at different temperatures, and in such systems the pump 106 would not be required.

An example of a practical application for the heat transfer circuit 100 is shown in Figure 2, which is illustrative of a solar heating system 200 for generating hot water. The system 200 comprises a hot water storage tank 202 being arranged to receive cold water from a cold water network 204 and to provide hot water to a sanitary hot water supply network 206. Storage tank 202 is a thermal reservoir and can also function as a heat accumulator. The storage tank 202 comprises a heat exchanger 208, through which it is in fluid flow communication with a heat transfer circuit 210.

Heat transfer circuit 210 comprises a pump 212, a solar thermal collector 214, and a valve 216 associated with T-junction 218. Valve 216 has a first inlet 216.1 , a second inlet 216.2 and an outlet 216.3. In this circuit 210 the valve 216 is arranged to cause fluid that is cooler than water contained in the storage tank 202 to bypass the heat

exchanger 208 while fluid that is warmer than the water contained in the storage tank 202 is directed through the heat exchanger 208.

Pump 212 circulates fluid through the heat transfer circuit 210, directing the fluid first through conduit 210.1 to the collector 214, and then through conduit 210.2 to the T- junction 218, after which the fluid can either flow into the valve 216 through the first inlet

216.1 , or it can flow through the heat exchanger 208 and into the second inlet 216.2. In both cases the fluid flow exits valve 216 through the outlet 216.3 for return to the pump 212. As described in relation to Figure 1 , in the solar heating system 200 the ability of the collector 214 to provide a heat source capacity (e.g. while absorbing sunlight) represents the first thermal reservoir 102, while the ability of the same collector 214 to provide a heat sink capacity (e.g. during a lack of sufficient sunlight) represents the second thermal reservoir 104. The conduits 210.1 , 210.2 also form heat sinks that are co-represented by the second thermal reservoir 104. The third thermal reservoir 1 12 is represented by the heat exchanger 208 of the storage tank 202. It will be appreciated that in some embodiments, at least parts of the conduits 210.1 , 210.2 could also function as a heat source, and that in such case these conduits would also represent the first thermal reservoir of Figure 1 , but in this example we have assumed that the conduits only function as a heat sink due to heat losses to the ambient.

As mentioned above, the valve 216 is arranged to promote fluid flow through the inlet where the fluid is the coolest, so if the temperature of the fluid flowing from the

conduit 210.2 into the first inlet 216.1 is higher than the temperature of the fluid flowing from the heat exchanger 208 into the second inlet 216.2, then the valve 216 directs the fluid to flow via the heat exchanger 208 by closing off the first inlet 216.1 . Contrary, if the temperature of the fluid flowing from the conduit 210.2 into the first inlet 216.1 is lower than the temperature of the fluid flowing from the heat exchanger 208 into the second inlet

216.2, then the valve 216 directs the fluid to bypass the heat exchanger 208 by closing off the second inlet 216.2. In this manner heat is not withdrawn from the storage tank 202 by being absorbed into cooler fluid flowing through the heat exchanger 208. In the embodiment shown in Figure 2 it is preferable that the second inlet 216.2 of valve 216 is completely closed off when the temperature of the fluid flowing from the conduit 210.2 into the first inlet 216.1 is lower than the temperature of the fluid flowing from the heat exchanger 208 into the second inlet 216.2, because the heat exchanger 208 should only function as a heat sink but never as a heat source. The valve 216 is arranged to completely close the second inlet 216.2 by fitting a biasing weight to the actuator of the valve 216, which causes the actuator to pivot to close the second inlet 216.2. The second inlet 216.2 is only opened when the temperature of the fluid flowing from the conduit 210.2 into the first inlet 216.1 is sufficiently higher than the temperature of the fluid from the heat exchanger 208 at the second inlet 216.2 so that the effect of the biasing weight can be overcome. The biasing weight is further described below with reference to details of how to arrange the valve 216 for this will be explained with reference to Figures 5a to 5d.

It will be appreciated that valve 216 operates to permit only one-way heat flow by only allowing heat to flow from heat transfer circuit 210 to the heat exchanger 208 while preventing heat from flowing from the heat exchanger 208 to the heat transfer circuit 210. This allows the solar heating system 200 to be operated in a manner that is different from the current state of the art for creating a high-efficiency solar heating system for hot water preparation, suitable for cooler climates such as in central and northern Europe, that makes use of a temperature differential controller to switch the pump on or off based on the difference in temperature between the solar collector and their hot water storage tanks. Contrary thereto, the provision of the valve 216 in the solar heating system 200 obviates the use a temperature differential controller by making use of a simpler control strategy.

In the solar heating system 200, a photovoltaic solar panel 220 is used to generate electricity for the pump 212. The electricity is transmitted via a cable 222 having a first section 222.1 leading from the photovoltaic solar panel 220 to a bimetal switch 224 or some other type of thermal switch. A second section 222.2 of the cable 222 connects the bimetal switch 224 to the pump 212. The bimetal switch 224 is normally on so that the pump 212 operates whenever there is sufficient sunlight to generate at least a threshold amount of electricity. The bimetal switch 224 is further exposed to the heat transfer circuit 210 and is arranged to interrupt the electricity supply when the temperature of the fluid in the heat transfer circuit 210 becomes too high or exceeds a prescribed limit, thereby stopping the pump 212 and preventing damage to the solar heating system 200. As will be understood by those experienced in the art, the heat transfer circuit 210 does not always generate heat when sunlight shines onto the collector 214 because heat losses in the heat transfer circuit 210, including within in the collector 214, can

occasionally be higher than the heat absorbed from the sunlight. This would typically be encountered when the ambient temperature is low, the temperature of the water in the storage tank 202 is high, or the sunlight intensity is low. In these circumstances the valve 216 prevents heat being lost form the storage tank 202, resulting in a high efficiency of the solar heating system 200.

It will be appreciated that the operation of the pump 212 can be achieved in many ways, whereby other examples are using an optical switch and an external power supply to switch the pump on, utilising stand-alone photovoltaic solar panels used for general power generation to power the pump 212, and using a timer and an external power supply to switch the pump on during normal daylight hours.

It will also be understood that the use of the bimetal switch 224 for over-temperature protection is not necessary if the stagnation temperature of the collector 214 is not high enough to cause any damage to the system. Also, there are many possible locations where the bimetal switch 224 could be provided, for example in, or close to, the storage tank 202, and multiple bimetal switches can be provided, for example both in, or close to, the collector, as well as in, or close to, the storage tank. The solar heating system 200 can be a closed pressurised system or a non-pressurised drain-back type systems. In the case of a drain-back type system, an external drain-back vessel will ideally be attached to the conduit 210.2.

It will be understood that different types of solar collectors can be used for the collector 214 in the solar heating system 200, for example different types of flat plate collectors can be used, glazed or unglazed, and also evacuated tube collectors can be used, directly circulated or using heat pipes.A further example of a practical application for the heat transfer circuit 100 is shown in Figure 3, which is illustrative of a combination solar heating system 300. The system 300 comprises a thermal storage tank 302 being arranged to provide heat to a sanitary hot water supply network through heat exchanger 304, e.g. in a domestic household. Storage tank 302 is a thermal reservoir and can also function as a heat accumulator. The storage tank 302 is in fluid flow communication with a direct heat transfer circuit 306 and an indirect heat transfer circuit 308.

The direct heat transfer circuit 306 is largely equivalent to the first heat transfer circuit 100 and comprises a thermal reservoir in the form of heat source 310, a first pump 312, a second pump 314, a radiator 316 (such as a room space heater) and a valve 318 associated with T-junction 320. Valve 318 has a first inlet 318.1 , a second inlet 318.2 and an outlet 318.3. In this circuit 306 the valve 318 is arranged to promote the flow of warmer water and to retard flow of cooler water.

The heat source 310 is used to supply hot water to the storage tank 302 when the first pump 312 is operated. Thus hot water is pumped from heat source 310 to enter the storage tank 302 through upper port 302.1 and then circulates through middle port 302.2 via valve 318 for return to the heat source 310.

When second pump 314 is operated, the water from heat source 310 first flows through radiator 316 to dissipate heat into a room causing the water to cool down to a certain degree. The water then enters T-junction 320 and valve 318 wherein the temperature of the water entering through first inlet 318.1 is compared to the temperature of water in the storage tank 302, i.e. exiting at a level of middle port 302.2, and entering the valve 318 through second inlet 318.2. As stated above, valve 318 promotes flow of the warmer water and thus if the water in the storage tank 302 is warmer, then the thermal actuator in valve 318 causes the valve member to close off the first inlet 318.1 and the cool water from the radiator 316 is directed to enter the storage tank 302 through lower port 302.3. Conversely, if the water in storage tank 302 is cooler, then the thermal actuator in valve 318 causes the valve member to close off the second inlet 318.2 and the warmer water from the radiator 316 is directed to circulate back to the heat source 310. Accordingly, it will be appreciated that the water from the radiator 316 only flows through the storage tank 302 when heat can be withdrawn from water in the storage tank 302 (i.e. when the storage tank 302 can function as an intermediate heat source), but the water from the radiator 316 bypasses the storage tank 302 when no heat can be withdrawn (i.e. when the storage tank 302 would function as a heat sink). The indirect heat transfer circuit 308 has a series layout and is used to transfer heat from thermal reservoirs such as first and second solar thermal collectors 322 and 324 to another thermal reservoir such as the storage tank 302. The circuit 308 comprises a pump 326 for circulating a heat transfer fluid in the circuit 308 so that it selectively flows through the collectors 322, 324 and/or through a first heat exchanger 328 and a second heat exchanger 330, which are all arranged in series with each other.

The heat transfer fluid flowing in circuit 308 is isolated from the water contained in the storage tank 302. In one embodiment the heat transfer fluid is water. In another embodiment the heat transfer fluid can be a combination of water provide with additives, such as anti-freeze or corrosion inhibiting additives. In yet a further embodiment the heat transfer fluid does not contain water.

In Figure 3 the first heat exchanger 328 is configured as an upper heat exchanger that is located between the upper port 302.1 and the middle port 302.2, whereas the second heat exchanger 330 is configured as a lower heat exchanger that is located between the middle port 302.2 and the lower port 302.3. In addition, as is often done in practice, the collectors 322, 324 have different orientations to each other so that they are configured to be facing towards the sun at different times of the day for absorbing solar energy. For example, the first collector 322 can be orientated in an easterly direction so that it mostly faces the sun in the mornings, whereas the second collector 324 can be orientated in a westerly direction so that it mostly faces the sun in the afternoons. It will thus be appreciated that the heat transfer fluid within the collectors 322, 324 may experience a net heat gain in one of the collectors 322, 324 while in the other collector 322, 324 it experiences a neat heat loss in the morning and the afternoons, but may experience a net heat gain in both collectors around midday. First collector 322 is associated with T-junction 332 and valve 334. Second collector 324 is associated with T-junction 336 and valve 338. Both valves 334 and 338 are arranged to promote the flow of warmer fluid and retard flow of cooler fluid.

First heat exchanger 328 is associated with T-junction 340 and valve 342. Second heat exchanger 330 is associated with T-junction 344 and valve 346. Both valves 342 and 346 are arranged to promote the flow of cooler fluid and retard flow of warmer fluid. During use and considering the pump 326 as a starting position, operation of pump 326 causes the heat transfer fluid to flow in conduit 308.1 via T-junction 332 into first inlet 334.1 of valve 334 so that its temperature can be compared to the temperature of heat transfer fluid located in and flowing from first collector 322 into second inlet 334.2. As described above, valve 334 promotes flow of the warmer fluid and thus if the fluid in the first collector 322 is warmer, then the thermal actuator in valve 334 causes the valve member to close off the first inlet 334.1 and the cooler fluid from the pump 326 is directed to enter the first collector 322 before circulating back to the valve 334 through second inlet 334.2 and exiting through outlet 334.3 into conduit 308.2. This will generally be the situation when the sun is shining on first collector 322 so that solar energy heats the fluid therein. Conversely, if the fluid in first collector 322 is cooler, then the thermal actuator in valve 334 causes the valve member to close off the second inlet 334.2 and the warmer fluid in conduit 308.1 is directed to flow directly through outlet 334.3 into conduit 308.2. This will generally be the situation when the sun is not shining on first collector 322. In a similar manner, the heat transfer fluid flows via T-junction 336 into first inlet 338.1 of valve 338 so that its temperature can be compared to the temperature of heat transfer fluid located in and flowing from second collector 324 into second inlet 338.2. As described above, also valve 338 promotes flow of the warmer fluid and thus if the fluid in the second collector 324 is warmer, then the thermal actuator in valve 338 causes the valve member to close off the first inlet 338.1 and the cooler fluid in conduit 308.2 is directed to enter the second collector 324. Again this will generally be the situation when the sun is shining on second collector 324 so that solar energy heats the fluid therein. Conversely, if the fluid in second collector 324 is cooler, then the thermal actuator in valve 338 causes the valve member to close off the second inlet 338.2 and the warmer fluid in conduit 308.2 is directed to flow directly through outlet 338.3 into conduit 308.3. This will generally be the situation when the sun is not shining on second collector 324.

Accordingly, it is apparent from the above that the heat transfer fluid only flows through the first and second collectors 322, 324 when they are able to heat the fluid, but that they will be bypassed if either of the collectors would function as a heat sink. The heat transfer fluid in conduit 308.3 subsequently flows towards T-junction 340 into first inlet 342.1 of valve 342 so that its temperature can be compared to the temperature of heat transfer fluid located in and flowing from first heat exchanger 328 into second inlet 342.2. First heat exchanger 328 is arranged to heat water in an upper part of the storage tank 302. As described above, valve 342 promotes flow of the cooler fluid and thus if the fluid in conduit 308.3 is warmer, then the thermal actuator in valve 342 causes the valve member to close off the first inlet 342.1 and the warmer fluid in conduit 308.3 is directed to enter the first heat exchanger 328 before circulating back to the valve 342 through second inlet 342.2 and exiting through outlet 342.3 into conduit 308.4. This will be the situation when the heat transfer fluid is able to heat the water in the upper part of the storage tank 302. Conversely, if the fluid in conduit 308.3 is cooler, then the thermal actuator in valve 342 causes the valve member to close off the second inlet 342.2 and the cooler fluid in conduit 308.3 is directed to flow directly through outlet 342.3 into conduit 308.4. This will be the situation when the heat transfer fluid in first heat exchanger 328 would function as a heat sink cooling water in the upper part of the storage tank 302.

In a similar manner, the heat transfer fluid in conduit 308.4 subsequently flows towards T- junction 344 into first inlet 346.1 of valve 346 so that its temperature can be compared to the temperature of heat transfer fluid located in and flowing from second heat

exchanger 330 into second inlet 346.2. Second heat exchanger 330 is arranged to heat water in a lower part of the storage tank 302. As described above, also valve 346 promotes flow of the cooler fluid and thus if the fluid in conduit 308.4 is warmer, then the thermal actuator in valve 346 causes the valve member to close off the first inlet 346.1 and the warmer fluid in conduit 308.4 is directed to enter the second heat exchanger 330 before circulating back to the valve 346 through second inlet 346.2 and exiting through outlet 346.3 for return to conduit 308.1 . This will be the situation when the heat transfer fluid is able to heat the water in the storage tank 302. Conversely, if the fluid in conduit 308.4 is cooler, then the thermal actuator in valve 346 causes the valve member to close off the second inlet 346.2 and the cooler fluid in conduit 308.4 is directed to flow directly through outlet 346.3 into conduit 308.1 . This will be the situation when the heat transfer fluid in second heat exchanger 330 would function as a heat sink cooling water in the storage tank 302. Accordingly, it is apparent from the above that the heat transfer fluid only flows through the first and second heat exchangers 328, 330 when they are able to heat the water in the storage tank 302, but that they will be bypassed if either of the heat exchangers would function as a heat sink. The configuration of the first and second heat exchangers 328, 330 is further arranged to promote thermal stratification of the water in the storage tank 302 whereby the hottest water is located towards a top of the storage tank 302 and the coldest water is located towards a bottom of the storage tank 302.

While the pump 326 is non-operational, due to the conduits 308.n having a smaller volume and larger surface area than the storage tank 302, it is likely that the heat transfer fluid in the conduits 308. n will radiate heat to the ambient environment and cool down much quicker than the water in the storage tank 302. The configuration of the

valves 342, 346 thus prevents heat being withdrawn from the storage tank during start-up of the indirect heat transfer circuit 308 and also if the circuit 308 is operated during times of low solar intensity, e.g. at dawn or dusk, when heat loss through the

collectors 322, 324, in the case when they are not equipped with bypass valves 334 and 338 as described above, may exceed heat absorption from solar irradiation.

The pump 326 may also be arranged to run in periods when the collectors 322, 324 are not generating any heat, but are acting as a heat sink because they lose more heat through outward radiation and conduction than they absorb from incoming radiation, an example of this is that the pump 326 may be powered by electricity from a photovoltaic solar panel, without the use of a temperature-differential controller to switch the pump on and off, so that the pump always runs when the sun shines, and therefore may also run when the sun doesn't shine brightly enough to generate net heat in the collectors 322, 324. In this case the valves 342, 346 prevent heat being drawn from the storage tank 302, so that the system can work effectively without using a temperature-differential controller. In the case of a hybrid solar collector, which provides both heat and photovoltaic electricity, the photovoltaic electricity to power the pump can be generated by the hybrid collector. To prevent the storage tank 302 from overheating, in such a system that doesn't use a controller to switch the pump on and off, the photovoltaic power supply to the pump can be interrupted by a bimetal thermostatic switch, installed in the storage tank 302, which interrupts the power supply once the storage tank reaches a predetermined temperature.

The skilled addressee will appreciate that although the solar heating system 300 of Figure 3 has multiple valves for bypassing thermal reservoirs, the system 300 can have more or fewer valves and thermal reservoirs. For example a basic solar heating system may have only a single solar collector and a single heat exchanger in a domestic hot water tank, whereby the heat exchanger would typically be located near a bottom of the hot water tank to prevent heat loss from the hot water tank at start-up.

Referring now to Figures 4a through 4d, there is shown a valve 400 configured to receive two fluid streams and is arranged to promote through flow of a warmer of the fluid streams. Accordingly the valve 400 is generally suitable for use in the solar heating system 300 as the valves 318, 334 and 338 but further integrally provides the respective T-junctions.

Valve 400 comprises a valve housing 402 defining a valve chamber 404. The valve housing 402 comprises a feed passage 406, a reservoir outlet 408, a reservoir return 410 and an outlet 412. Figure 4a is a top view so that valve 400 is shown looking axially along the outlet 412, whereas the various sectional views in Figures 4b to 4d are side views looking at the outlet 412 side on perpendicular to its axis.

Valve housing 402 further comprises a T-junction 414, so that a fluid flow entering through feed passage 406 can either flow through the reservoir outlet 408 or into the valve chamber 404. When the fluid flow exits through the reservoir outlet 408 the fluid will pass through a thermal reservoir and subsequently return to the valve 400 through reservoir return 410 and then enter the valve chamber 404. The fluid in the valve chamber 404 exits the valve 400 through the outlet 412.

Valve housing 402 further comprises a first inlet 413 leading from feed passage 406 into valve chamber 404 for receiving a first fluid stream and a second inlet 416 leading from reservoir return 410 into valve chamber 404 for receiving a second fluid stream.

A thermal actuator 418 is located in the valve chamber 404 and is pivotally mounted to the valve housing 402 at pivot joints 420 so that the actuator 418 can pivot within the valve chamber 404. The actuator 418 is joined to a valve member 422 that carries first plug 422.1 arranged to close first inlet 413 and second plug 422.2 arranged to close second inlet 416.

Actuator 418 is more clearly visible in Figures 4b and 4c. Actuator 418 comprises a hollow body 424 that is separated by a transverse wall 426 into two compartments 428 and 430. A fluid transfer conduit 432 extends through wall 426 to interlink the compartments 428, 430. A sealable filling tube 434 is provided for filling the hollow body 424 with a working fluid (not shown in the Figures) so that it contains both a liquid component and a gaseous component. The working fluid is arranged to flow through the conduit 432 between the compartments 428, 430 in response to changes in temperatures within the respective compartments. The operation of the actuator 418 is equivalent to an actuator as described in detail in EP 2781812. Briefly, a comparative increase in the temperature in compartment 428 over compartment 430 raises the vapour pressure therein so that the working fluid is transferred through the conduit 432 into compartment 430. It will be appreciated that the conduit 432 is located near an operative bottom of the wall 426 so that the liquid component flows through conduit 432 before the gaseous component is able to do so. This results in a change in the weight distribution between the compartments 428, 430 which causes the actuator 418 to pivot under gravity to lower compartment 430. Pivoting of the actuator 418 in an opposite direction is achieved by comparatively increasing the temperature of compartment 430 so that the fluid is transferred back to compartment 428.

As is shown in Figures 4c and 4d, when the actuator 418 is pivoted to lower

compartment 430, the joined valve member 422 is moved so that second plug 422.2 largely closes off second inlet 416. At the same time the first plug 422.1 is spaced away from the first inlet 413 so that it is opened to permit flow of the first fluid stream. The first fluid stream is thus able to flow into the valve chamber 404 and substantially surround compartment 428 before exiting through the outlet 412. In this manner, the

compartment 428 will be at substantially the same temperature as that of the first fluid stream.

In use, the second plug 422.2 does not fully close off second inlet 416 for substantial periods of time when in equilibrium because while completely closed the temperature difference between fluid in the valve chamber 404 and the working fluid in the hollow body 424 will lessen, thereby decreasing the closing force exerted by the actuator 418. As a result, a small volume of the second fluid stream will be able to flow into valve chamber 404 to surround compartment 430 so that the temperature within compartment 430 is influenced by the temperature of the second fluid stream. In the exemplary embodiment with the actuator 418 is the position shown in Figure 4c, the compartment 430 will be cooler than compartment 428. However, when the temperature of the second stream increases to exceed that of the first stream, the actuator 418 will pivot to open second inlet 416 and largely close off the first inlet 413.

T-junction 414 is integrally formed within the valve housing 402. One advantage of such integral formation is that it reduces the distance between the feed passage 406 and the first inlet 413, which improves the response time of the valve 400 to changes in the temperature of the heat transfer fluid entering feed passage 406.

In some embodiments, the valve 400 can be adapted to be more suitable for use with thermal reservoirs that cause a significant pressure head loss in the fluid flow, for example when the thermal reservoir is a heat exchanger with a narrow fluid passage. One adaptation is by reducing the diameter of the first inlet 413 relative to the second inlet 416. Alternatively, in a second adaptation the first inlet 413 may be positioned closer to the pivot joints 420 than second inlet 416. Either of these adaptations will make it easier to close the first inlet 413. Yet further a biasing weight may also be attached to the actuator to achieve this effect - the provision of such a biasing weight will be described in greater detail referring to Figures 5a to 5d.

Referring now to Figures 5a to 5d there is shown a valve 500 being largely similar to the valve 400. However, in contrast to the valve 400, the valve 500 is configured to receive two fluid streams and is arranged to promote through flow of a cooler of the fluid streams. Accordingly the valve 500 is generally suitable for use in the solar heating system 300 as the valves 342 and 346 and also integrally provides their respective T-junctions. The valve 500 is particularly suitable for use as the valve 216 previously described in the solar heating system 200.

Valve 500 comprises a valve housing 502 defining a valve chamber 504. The valve housing 502 comprises a feed passage 506, a reservoir outlet 508, a reservoir return 510 and an outlet 412. Figure 5a is a top view so that valve 500 is shown looking axially along outlet 512, whereas the various sectional views in Figures 5b to 5d are side views looking at the outlet 512 side on perpendicular to its axis.

Valve housing 502 further comprises a T-junction 514, so that a fluid flow entering through feed passage 506 can either flow through the reservoir outlet 508 or into the valve chamber 504. When the fluid flow exits through the reservoir outlet 508 the fluid will pass through a thermal reservoir and subsequently return to the valve 500 through reservoir return 510 and then enter the valve chamber 504. The fluid in the valve chamber 504 exits the valve 500 through the outlet 512.

Valve housing 502 comprises a first inlet 513 leading from feed passage 506 into valve chamber 504 for receiving a first fluid stream and a second inlet 516 leading from reservoir return 510 into valve chamber 504 for receiving a second fluid stream.

A thermal actuator 518 is located in valve chamber 504 and is pivotally mounted to the valve housing 502 at pivot joints 520 so that the actuator 518 can pivot within the valve chamber 504. In comparison to valve 400, it can be seen that in valve 500 the actuator 518 and pivot joints 520 are inverted so that the pivoting movement is in an opposite direction to that of actuator 418. However the remaining parts and working of the valve 500 is similar to that of valve 400. Thus the actuator 518 is joined to a valve member 522 that carries first plug 522.1 arranged to close first inlet 513 and second plug 522.2 arranged to close second inlet 516. Actuator 518 is more clearly visible in Figures 5b and 5c. Actuator 518 comprises a hollow body 524 that is separated by a transverse wall 526 into two compartments 528 and 530. A fluid transfer conduit 532 extends through wall 526 to interlink the

compartments 528, 530. A sealable filling tube 534 is provided for partially filling the hollow body 524 with a working fluid (not shown in the Figures) so that it contains both a liquid component and a gaseous component, whereby the working fluid is arranged to flow through the conduit 532 between the compartments 528, 530 in response to changes in temperatures within the respective compartments.

As is shown in Figures 5c and 5d, when the actuator 518 is pivoted to lower

compartment 528, the joined valve member 522 is moved so that first plug 522.1 is spaced away from the first inlet 513 so that it is opened to permit flow of the first fluid stream. At the same time the second plug 522.2 closes off second inlet 516. The first fluid stream is thus able to flow into the valve chamber 504 and substantially surround compartment 528 before exiting through the outlet 512. In this manner, the

compartment 528 will be at substantially the same temperature as that of the first fluid stream. In the exemplary embodiment with the actuator 518 is the position shown in Figure 5c, the compartment 528 will be cooler than compartment 530. However, when the temperature of the first stream increases to exceed that of the second stream, the actuator 518 will pivot to open second inlet 516 and largely close off the first inlet 513. As can be seen in Figures 5b and 5c, a biasing weight 536 is attached to the actuator 518 to influence the tilting behaviour of the actuator 518 and bias the actuator into a preferred position. In the exemplary embodiment, the biasing weight 536 is attached to the actuator adjacent to compartment 528. The additional weight results in compartment 528 being lowered (under gravity) also when there is no temperature difference between

compartment 528 and 530 so that the second plug 522.2 closes off the second fluid stream. Since the preferred position is maintained also when there is no temperature difference, the second fluid stream can be completely closed off in a steady state equilibrium condition.

While the actuator 518 is in the preferred position, and the second inlet 516 is completely closed off, the temperatures of compartments 528 and 530 will equilibrate due to heat conduction within the actuator 518. Subsequently, when the temperature of the first fluid stream increases, compartment 528 will become warmer than compartment 530, which will cause the actuator 518 to tilt to open second inlet 516 and allow the second fluid stream to flow into the valve chamber 504. The second fluid stream will flow around compartment 530, and the actuator 518 will be able to compare the temperatures of the first and second fluid streams. Accordingly it will be appreciated that by using the biasing weight 536, the valve 500 closes off the second inlet 516 completely in steady state conditions, but samples the temperature of the second fluid stream as soon as the temperature of the first fluid stream starts to increase. The biasing weight also introduces an offset in the temperature difference at which the actuator 518 will start to tilt, which can be used to ensure the actuator only allows the second fluid stream to flow into the valve, when the temperature of the second fluid stream is lower than that of the first fluid stream. In steady state conditions and while the temperature of the second fluid stream is higher than the temperature of the first fluid stream, the second fluid stream is completely blocked off. The equilibrium switching temperature difference (i.e. the temperature of the first compartment 528 minus the temperature of the second compartment 530) of the valve 500 can be changed by increasing or decreasing the weight of the biasing weight, or by moving the location of the biasing weight 536, e.g. typically to cater for temperature differences of to between 0 - 2 Kelvin.

Now again also referring to Figure 2, the solar heating system 200 is an example where valve 500 can be used as the valve 216, whereby the reservoir return 510 represents the second inlet 216.2. Thus when the reservoir return 510 (i.e. second inlet 516) is closed off, heat transfer fluid flow through the heat exchanger 208 is closed off. For this reason, when the temperature in the heat transfer circuit 210 is too low so as to be unable to add heat to the hot water storage tank 202, then the heat exchanger 208 is bypassed completely. When the temperature in the heat transfer circuit 210 starts to increase, for example due to increased sunlight, the valve 216 will sample the temperature of the fluid in the heat exchanger to determine whether the temperature in the heat transfer circuit 210 is higher than the temperature of the heat exchanger fluid. If it is not, then the second inlet 216.2 will remain largely closed until the temperature of the heat transfer circuit 210 does become higher than the second inlet 216.2, at which time it will be opened and the first inlet 216.1 will become largely closed off.

In the preferred position the valve 216 closes off the flow through the heat exchanger 208. When the temperature in heat exchanger 208 subsequently decreases, for example due to hot water being exhausted from the hot water storage tank 202 through the hot water supply network 206, this temperature decrease will not immediately register on the valve 216. However, the rate of detection of this change in temperature can be improved by installing the valve 216 close to the heat exchanger 208 and near or below the base thereof so that cold fluid in the heat exchanger will enter the reservoir return 510 leading to the second inlet 516 (due to natural flow effect within the piping caused by the temperature difference). This will result in heat conduction within the valve housing 502 cooling the second compartment 530, to the extent that it slightly opens second inlet 516 and starts to be impacted by the temperature of the second fluid stream, after which the change in temperature will be more quickly determined to cause the actuator 518 to fully pivot. The effectiveness of the valve 500 can be improved by locating the outlet 512 on the same side of the valve chamber 504 as the compartment 528. This results in the first fluid stream entering the valve 500 through the first inlet 513 not significantly flowing around the second compartment 530, but it mainly flows around compartment 528. Accordingly the heat conduction should be sufficient to cool the second compartment 530 even while the first fluid stream is flowing through the valve 500. With such a configuration the reaction time before pivoting of the actuator, in response to a decrease in the temperature of the hot water storage tank 202 while the valve 500 is closing the second inlet 516, may be reduced to less than 5 minutes, which is sufficient for an application as exemplified in Figure 2.

The effectiveness in closing the first inlet 513, while the temperature of the fluid flow into the second inlet is lower than the temperature flowing into the first inlet, is negatively influenced by the biasing weight. The degree of closure of the first inlet is normally less than 100%, though it is highly dependent on the temperature difference between the fluid flowing into the first and second inlets. The degree of closure is typically in the range of 95-99.5% when the temperature of the fluid flow into the first inlet 513 is 10 Kelvin higher than the temperature of the fluid flow into the second inlet 516, which is sufficient for the application illustrated in Figure 2. This reduced degree of closure rate improves the response time to any changes in the temperature of the fluid flow into the first inlet 513 whilst being closed off, typically to less than 30 seconds, which is positive for the use in the application illustrated in Figure 2.

Referring to Figure 6 of the drawings, there is shown a schematic flow diagram of a second heat transfer circuit 600, which is arranged to transfer heat between a first thermal reservoir 602, a second thermal reservoir 604 and a third thermal reservoir 606, by pumping fluid through the circuit 600. The first thermal reservoir 602 functions as a heat source, while the second and third thermal reservoirs 604, 606 intermittently function as heat sinks. The fluid is pumped by pump 608.

Intermediate the first thermal reservoir 602 and the second and third thermal reservoirs 604, 606, the circuit 600 comprises a first T-junction 610. Thereby the second and third thermal reservoirs 604, 606 are in parallel with each other.

Between the first T-junction 610 and the second thermal reservoir 604 the circuit comprises a second T-junction 612 associated with a first valve 614 that is arranged to selectively divert the fluid flowing from the first thermal reservoir 602 through the second thermal reservoir 604 before the fluid is returned to the first valve 614. Thus first valve 614 has a first inlet 614.1 and a second inlet 614.2, wherein the first inlet 614.1 is arranged to receive fluid flowing from the first thermal reservoir 602 and the second inlet 614.2 is arranged to receive fluid flowing from the second thermal reservoir 604.

Between the first T-junction 610 and the third thermal reservoir 606 the circuit comprises a third T-junction 616 associated with a second valve 618 that is arranged to selectively divert the fluid flowing from the first thermal reservoir 602 through the third thermal reservoir 606 before the fluid is returned to the second valve 618. Thus second valve 618 has a first inlet 618.1 and a second inlet 618.2, wherein the first inlet 618.1 is arranged to receive fluid flowing from the first thermal reservoir 602 and the second inlet 618.2 is arranged to receive fluid flowing from the third thermal reservoir 606.

First valve 614 has an outlet 614.3, and second valve 618 has an outlet 618.3, both of which are arranged to direct fluid to flow to a fourth T-junction 620, from where the combined fluid returns to the first thermal reservoir 602. The second and third thermal reservoirs 604, 606 contain fluid that is either warmer or cooler than the fluid flowing from the first thermal reservoir 602, whereby the first and second valves 614, 618 are configured to compare the temperature of the fluid flowing into the first inlets 614.1 , 618.1 with fluid flowing into the second inlets 614.2, 618.2 and to promote the flow of the cooler fluid.

As stated above, the second and third thermal reservoirs 604, 606 are arranged in parallel in the heat transfer circuit 600, with the purpose that the fluid flow from the heat source formed by first thermal reservoir 602 flows through the second and third thermal reservoirs 604, 606 when the latter can act as heat sinks, i.e. when their temperature is lower than the temperature of the fluid flow coming from the first thermal reservoir 602. However, the fluid flow from the first thermal reservoirs 602 can bypass either of or both the second and third thermal reservoirs 604, 606 when they cannot act as a heat sink, i.e. when their temperature is higher than the temperature of the fluid flow coming from the first thermal reservoirs 602. The effectiveness of the heat transfer circuit 600 can be improved by arranging the first inlets 614.1 , 618.1 of the valves 614, 618 to cause an obstruction to the fluid flow. For example, if the second thermal reservoir 604 acts as a heat sink with the first valve 614 directing the fluid flow through its second inlet 614.2, while the third thermal reservoir 606 does not act as a heat sink with the second valve 618 directing the fluid flow through its first inlet 618.1 , then the fluid flow from the first thermal reservoir 602 will be

predominantly directed towards the first valve 614 from the first T-junction 610, and only a residual flow is directed to the second valve 618 from T-junction 610. The arrangement of the obstruction will be explained further referring to Figure 7.

It will be understood to those in the art that a heat transfer circuit can comprise a large number of parallel thermal reservoirs, each of which can be bypassed by a valve as described above, to form a large heat transfer network. The heat sinks in such network can be domestic hot water tanks in an multi-family home, such as an apartment building, or a district heating network. The heat source can be any heat source, examples are solar collectors, heat pumps, geothermal heat, gas boilers and biomass boilers.

The heat transfer circuit 600 can also comprise other heat sinks arranged in parallel, which will not be bypassed using a valve as described above. For example, in some embodiments the heat transfer circuit 600 can be arranged to provide heat in a composite heating network comprising both space/room heating circuits and domestic hot water storage tanks. Is such case the space heating circuits will typically be controlled using a thermostat controller that switches a pump or a valve to activate the space heating circuit. Accordingly, only the domestic hot water storage tanks will make use of a valve for bypassing tanks in the manner described above referring to Figure 6. Such a heating network has the benefit that the heat source, or heat sources, can be operated at variable temperatures for optimum efficiency of the heat source, i.e. operating at lower

temperatures when only space heating is required but periodically operating at higher temperatures to heat the domestic hot water storage tanks. It will be understood by those skilled in the art that the heat transfer circuit 600 can also be used to combine space cooling with heating of domestic hot water tanks in a heating and cooling network, and that this can be used to create a network that provides space heating and heat for domestic hot water preparation in cold seasons, and space cooling and heat for domestic hot water preparation in warm seasons.

Other useful components to assist the valves in their bypass functionality may be added to the heat transfer circuit 600, for example a pressure relief valve arranged in parallel with the heat sinks in the circuit, that limits the pressure difference across the heat sinks. This can be particularly useful in systems where most or all the parallel heat sinks are bypassed in some circumstances, for example a heat transfer circuit with a solar collector heat source. Referring now to Figures 7a and 7b, there is shown a valve 700, which is largely identical to the valve 500 from Figures 5a to 5d. Accordingly the overall functioning of the valve 700 is the same as described with reference to Figures 5a to 5d and valve 500. The valve 700 is suitable for use heat transfer circuit 600, to cause an obstruction as required therein. The valve 700 differs from the valve 500 in that its first inlet 713 has a very small diameter of cross-sectional area in comparison to its second inlet 716. Preferably the cross- sectional area of the first inlet 713 is less than 20% and ideally less than 5% of the cross- sectional area of the second inlet 716. This causes the valve 700, in the exemplary position having the first inlet 713 open, to exhibit a much higher pressure loss than when the second inlet 716 is opened. This high pressure loss acts as an obstruction and effectively redirects the fluid to rather flow in a parallel circuit of thermal reservoirs.

The dimensions of the first inlet 713 should be selected taking into account that relatively little fluid should flow through first inlet 713, but the volume and flow rate should be sufficient so that the valve 700 can respond sufficiently quickly to any changes in temperature of the fluid flow received from the heat source, e.g. first thermal reservoir

602. When selecting the dimensions of the first inlet 713 it is necessary to account for the volume fluid retained in the piping between the first T-junction 610 and the valves 614, 618. In other words, a higher volume of fluid retained in the piping will require a higher flow rate through the first inlets 614.1 , 618.1 to achieve a suitable response time. As shown in Figures 7a and 7b, outlet 712 is located closer to first inlet 713 and first compartment 728, while being located further away from second inlet 716 and second compartment 730. This location of outlet 712 on the same side of the valve 700 as the first compartment 728 influences the operation of the valve 700 because fluid flowing through the first inlet 713 to the outlet 712 predominantly only surrounds the first compartment 728 whereas fluid flowing through the second inlet 716 to the outlet 712 surrounds not only the second compartment 730, but also the first compartment 728. For this reason the fluid flowing through the second inlet 716 is able to significantly influence the temperature of the first compartment 728 when the second inlet 716 is opened. This influence is increased, relative to the valve 500, because the fluid flow through the first inlet 713 is relatively low for the valve 700 compared to the valve 500. It should be understood that the influence is able to create a degree of proportionality in the valve, which can be beneficial for the operation of the heat transfer circuit 600, for example if both parallel thermal reservoirs 604, 606 are acting as heat sinks, the heat sink that has a lower temperature will receive a higher flow rate of fluid in the heat transfer circuit 600, so the heat can be distributed preferentially, based on heat demand. Moving the location of the outlet 712 closer to the second inlet 716, i.e. in the view shown in Figure 7b moving outlet 712 towards the right, reduces the influence that the fluid flowing through second inlet 716 will have on the first compartment 728 because the fluid will surround the actuator 718 to a lesser extent. Accordingly, such moving of the outlet 712 reduces the proportionality of the switching action of the actuator 718, but also reduces the amount of fluid flow required through the first inlet 713 for effective operation of the valve 700. It will therefore be appreciated that the effective operation of the valve 700 can be adjusted both by altering the diameter of the first inlet 713 and by relocating the position of the outlet 712. Depending on the specific requirements of the heat transfer circuit, the position of the outlet 712 and the size of the first inlet 713 can be selected for optimum performance.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

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