WO2011110946A2

WO2011110946A2 - Heat-exchange circuit

Info

Publication number: WO2011110946A2
Application number: PCT/IB2011/000616
Authority: WO
Original assignee: Schweyher, Holger
Priority date: 2010-03-11
Filing date: 2011-03-09
Publication date: 2011-09-15
Also published as: WO2011110946A3

Abstract

A water-filled heat-exchange circuit is provided. The circuit comprises a pressurising unit for pressurising liquid in the circuit and a heater for heating the liquid, e.g. a solar field heater. In accordance with best building practice, a pressure-release valve is located at a high point in the circuit. This valve is configured to open to release pressure in the circuit if a pressure threshold is exceeded. The circuit additionally comprises a second pressure-release valve that is located at a position in the pressurised circuit where, in normal use, the temperature is lower than at the location of the first pressure-release valve, e.g. because the second pressure-release valve is connected to the remainder of the circuit via a cold buffer, such as an un-insulated interception tank. The second pressure-release valve is furthermore configured to open at a pressure threshold that is lower than that of the first pressure-release valve. Thus in an over-pressure situation the second pressure-release valve will open to relieve pressure before the first pressure release valve. Because the second pressure- release valve is at a relatively cooler point in the circuit than would be the case for a conventional pressure-release valve, there is reduced risk of damage caused by uncontrolled steam generation as pressure is released in an over-pressure situation. At the same time, the provision of the first pressure release valve helps in ensuring compliance with building codes.

Description

TITLE OF THE INVENTION

Heat-exchange Circuit BACKGROUND OF THE INVENTION

The disclosed technology relates to heat-exchange circuits of the kind used in heating and cooling systems for buildings. In particular the disclosed technology relates to pressure- release safety valve arrangements in such circuits.

Figure 1 schematically shows a conventional heat-exchange circuit 2 for providing cooling in a building. The heat-exchange circuit 2 is water based and comprises various elements interconnected by pipe work 4 as shown in the figure. In particular, the circuit comprises a (heat) storage tank 6, an absorption chiller 8, a solar field water heater 10 and a sealed diaphragm expansion tank 1 provided to accommodate thermal expansion of the water as it is heated.

A pressurising pump 12 is provided to pressurise the circuit so the water can be heated to temperatures above 100 °C without boiling. An overflow valve 24 is provided in a branch of the circuit parallel to the pressurising pump 12 and connecting the pressurised side of the pressurising pump 12 to the sealed diaphragm expansion tank 14 on the low pressure side of the pressurising pump 12. The pressurising pump 12 and overflow valve 24 are operated to maintain a desired pressure in the circuit 2 in the normal way. These elements responsible for maintaining the desired pressure may collectively be referred to as a pressurising unit. In normal use for the example shown in Figure 1 the pressurising unit is operated to maintain a water pressure in the circuit of around 15 bar (1.5 Pa). The pressure at the expansion tank 14 (i.e. at the input side to the pressurising pump 12) in this example is below 1.7 bar (0.17 MPa). A heating-loop pump 16 and a cooling-loop pump 18 are provided to respectively circulate water from the storage tank 6 through the solar field heater 10 and absorption chiller 8. The section of the circuit 2 containing the heating-loop pump 16 and solar field heater 10 (with associated connections to the storage tank 6) may be referred to as the circuit's heating loop. Similarly, the part of the circuit containing the cooling-loop pump 8 and absorption chiller 8 may be referred to as the circuit's cooling loop. A pressure-release safety valve 20 is coupled to the storage tank 6 at a high point in the circuit 2. The safety valve 20 is configured to open and vent the circuit to a blow-down vessel 26 (open to atmospheric pressure) if the internal pressure in the circuit exceeds the valves' release pressure. This is a safety feature aimed at avoiding damage to the circuit, and the associated risk of explosion, in an over-pressure situation.

To heat water in the circuit the heating-loop pump 16 is operated to pump water from the bottom of the storage tank 6, through the solar field heater 10, and back to the top of the storage tank 6. In a steady state condition the water temperature in the circuit might typically range from around 160 °C to 180 °C. For example, the temperature of the water drawn from the bottom of the storage tank 6 for heating might be around 160 °C and the temperature of the water returned to the top of the water tank after heating in the solar field heater might be around 180 °C. This +20 °C temperature differential across the solar field heater 10 provides the energy input to the circuit.

To provide cooling, the cooling-loop pump 18 is operated to pump the hot water from the top of the storage tank 6, through the absorption chiller 8, and back to the bottom of the storage tank 6. In accordance with the typical steady state conditions noted above, the temperature of water drawn from the top of the storage tank 6 is around 180 °C. The water is cooled as its thermal energy is used to drive the cooling process in the absorption chiller 8 and is returned to the storage tank at a temperature of around 160 °C. This -20 °C temperature differential across the absorption chiller 8 represents the energy extracted from the circuit which is used to drive the cooling. The absorption chiller 8 is thermally coupled to an air conditioning system of a building (not shown) to provide cooling for the building.

Thus the circuit 2 of Figure 1 provides a solar-powered cooling system using heated water as a working liquid to provide energy to drive a cooling system.

A potential issue with sealed pressurised hot water circuits of the kind shown in Figure 1 is a risk of over-pressure, e.g. in a fault situation. For example, if the electric motor of the heating- loop pump 16 of Figure 1 were to fail, the stationary water in the solar field heater 10 would continue to be heated. Eventually this water may become so hot it begins to boil and cause a rapid build-up of pressure in the circuit. In another possible failure mode the overflow valve 24 may become stuck in a closed state so pressure from the pressuring pump 12 cannot be released. The pressure-release safety valve 20 is provided to reduce the risk of an explosion in these types of situation. For the example shown in Figure 1 , the activation pressure for the pressure-release safety valve 20 is 18 bar (1.8 MPa). This is above the normal working pressure of the circuit maintained by the pressurisation unit and so the safety valve 20 is normally closed. However, if the pressure in the circuit exceed 18 bar (1.8 MPa) because of an over-pressure situation, the safety valve 20 opens to vent the circuit with a view to preventing damage.

It can be a legal requirement for safety valves to be used in circuits of the kind shown in Figure 1. In particular, a country's building code (boiler code) may require pressure-release valves to be provided at high points in the circuit, e.g. above the storage tank 6 as shown in Figure 1. Such valves might also be required at high points in any separately isolatable parts of a pressurised circuit that contain a heater. For example, if the heating loop of Figure 1 had valves to allow the solar field heater 10 to be isolated from the storage tank 6, e.g. for maintenance, the isolatable part of the loop containing the heater 10 may also require its own safety release valve.

The inventor has identified a drawback in the conventional safety valve arrangement in heat- exchange circuits of the type shown in Figure 1. In particular, problems can be caused by the rapid generation of steam when the pressure-release safety valve 20 opens. In normal operation the elevated pressure in the circuit prevents the water from boiling, even at a temperature of around 180 °C. However, the sudden drop in pressure as the pressure- release safety valve 20 opens can lead to uncontrolled steam generation through sudden boiling of the liquid. Not only does this present a significant hazard as superheated steam is expelled from the safety valve at high speeds, the process can also damage the circuit. For example, sudden large pressure gradients in the circuit created as the safety valve opens can cause steam and water to be accelerated to high velocities in the circuit which can cause severe water hammer effects that damage the circuit. Furthermore, the uncontrolled generation of steam in the safety valve itself can stimulate resonant oscillations which can potentially drive the valve to destruction.

There is therefore a need for heat-exchange circuits having safety valve arrangements that help to avoid the above-identified problems with conventional circuits while maintaining compliance with best building practice for these types of circuit. SUMMARY OF THE INVENTION

According to a first aspect of the disclosed technology there is provided a heat-exchange circuit comprising: a pressurising unit for pressurising liquid in the circuit; a heater for heating the liquid; a first pressure-release valve located at a first position in the circuit and configured to open to release pressure in the circuit if a first pressure threshold is exceeded; and a second pressure-release valve located at a second position in the circuit and configured to open to release pressure in the circuit if a second pressure threshold is exceeded, wherein the second pressure threshold is lower than the first pressure threshold, and wherein the first and second positions are arranged so that in normal use the temperature of the liquid at the second position is lower than the temperature of the liquid at the first position.

Thus a circuit may be provided that has a first pressure-release valve positioned to comply with building (boiler) code requirements and a second pressure-release valve arranged in a relatively cool part of the circuit. In an over-pressure situation the second pressure-release valve will open first because of its lower pressure threshold. This allows the circuit to be vented to relieve pressure through the expulsion of cooler liquid than would be expelled if the first pressure-release valve were to be the first to open. The circuit may thus provide a generally safer pressure release mechanism while still complying with the relevant building / boiling codes.

The first pressure-release valve may be a full-lift valve, e.g. for building code compliance, while the second pressure-release valve may be a proportional-lift valve. This allows for improved control of the pressure release through the second pressure-release valve in an over-pressure situation which can help to reduce the risk of damage to the circuit still further.

The first position may be at a location in the circuit which is higher than the second position. Building codes may require a pressure-release valve to be positioned at a high point in a circuit, and the first pressure-release valve may be located to meet this requirement. By locating the second pressure-release valve at a lower position in the circuit, the process of gravity driven temperature stratification can naturally help to ensure the temperature at the second pressure-release valve is lower than at the first pressure-release valve.

The heat-exchange circuit may further comprise an interception tank arranged between the first and second positions in the circuit and having an upper connection port coupled to a part of the circuit including the first pressure-release valve and a lower connection port coupled to a part of the circuit including the second pressure-release valve.

Thus the interception tank may act as a thermal buffer between the hotter parts of the circuit containing the first pressure-release valve and the second pressure-release valve. This can help in some situations where a relatively large amount of liquid may need to be vented to relieve pressure (although in most situations only relatively small amounts of liquid will need to be released to sufficiently reduce pressure). The contents of the interception tank thereby provide a source of cooler water to vent through the second pressure-release valve.

The interception tank may, for example, be provided with less thermal insulation than other parts of the circuit, or no thermal insulation at all, such that in normal use the liquid in the interception tank more readily cools to a temperature that is lower than the temperature at the position of the first pressure-release safety valve.

The pressurising unit for pressurising liquid in the circuit may also be coupled to a lower connection port at the bottom of the interception tank. This can help isolate the pressurising unit from the hottest parts of the circuit. Furthermore, the pressurising unit may be coupled to an expansion tank to accommodate thermal expansion in the liquid, and the interception tank can thus also help protect the expansion tank from over heating. This can be particularly useful during start-up, for example, when there is likely to be a relatively large transfer of heated liquid to the expansion tank as it undergoes its initial expansion. During steady state operation there is likely to be relatively little transfer between the expansion tank and the rest of the circuit.

The first and second positions may be arranged so that in normal use the temperature of the liquid at the first position is above a temperature at which the liquid boils at atmospheric pressure and the temperature of the liquid at the second position is below a temperature at which the liquid boils at atmospheric pressure. Thus the liquid expelled from the second pressure-release valve in an over-pressure situation does not rapidly boil as the valve opens to atmospheric pressure. This helps to reduce the risk of damage to the circuit during pressure release still further since there is reduced risk of spontaneous steam generation.

The heat-exchange circuit may further comprise a main storage tank arranged to store heated liquid received from the heater. This may provide thermal store, e.g. to smooth out variations in the thermal input from the heater. For example, in one implementation the heater may be a solar field heater and the circuit may further comprise an absorption chiller arranged to receive liquid heated by the heater to drive a cooling process, e.g. to cool a building. The storage tank may thus act as a thermal store for the heat from the solar field array so that cooling may continue to be provided through the thermal chiller during periods of diminished sunlight.

The first pressure-release safety valve may be located in a branch of the circuit that is connected to an upper port near the top of the main storage tank while the second pressure- release safety valve may be located in a branch of the circuit connected to a lower port near the bottom of the main storage tank. This can help provide the desired difference in temperature between the first and second pressure-release safety valves and also help in complying with good building / boiler practice as regards the position of the first pressure- release safety valve. According to a second aspect of the disclosed technology there is provided a method for releasing pressure in a heat-exchange circuit, the heat-exchange circuit comprising: a pressurising unit for pressurising liquid in the circuit; a heater for heating the liquid; and a first pressure-release valve located at a first position in the circuit and configured to open to release pressure in the circuit if a first pressure threshold is exceeded; and wherein said method comprises: providing a second pressure-release valve located at a second position in the circuit where the temperature of the liquid is lower than the temperature of the liquid at the first position, and opening the second pressure-release valve to release pressure in the circuit when a second pressure threshold is exceeded, wherein the second pressure threshold is lower than the first pressure threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed technology is now described by way of example only with reference to the following drawings in which:

Figure 1 schematically shows a conventional solar-powered heat-exchange circuit for providing cooling;

Figure 2 schematically shows a solar-powered heat-exchange circuit for providing cooling according to an embodiment of the disclosed technology;

Figure 3 schematically shows in more detail a solar-powered heat-exchange circuit similar to that shown in Figure 2 according to another embodiment of the disclosed technology; Figure 4A shows a legend for various ones of the elements of Figure 3; and

Figure 4B shows a template layout for the two-by-two tables of thermodynamic data shown for various locations in Figure 3.

DETAILED DESCRIPTION

Figure 2 schematically shows a heat-exchange circuit 102 according to an embodiment of the disclosed technology. The circuit may be used, for example, to providing cooling for a building (not shown). Several elements of the circuit 102 are similar to and will be understood from corresponding elements of the conventional heat-exchange circuit 2 of Figure 1.

The heat-exchange circuit 102 is liquid based and comprises various elements interconnected by pipe work 104 as shown in the figure. In this example the working liquid is primarily water (e.g. water plus corrosion inhibitors), but in other situations other liquids could be used. The choice of liquid in a given example may be based, for example, on the same considerations as for a conventional heat-exchange circuit of the kind shown in Figure 1.

The circuit comprises a main storage tank 106, an absorption chiller 108, a solar field water heater 110 and a sealed diaphragm expansion tank 114 for accommodating thermal expansion of the water as it is heated. (In principle an open (i.e. non-sealed) expansion tank could be used. In this case it may be appropriate to provide for continuous water treatment, e.g. on-going chemical treatment / degassing.) The circuit further comprises an interception tank 130 and a secondary pressure-release safety valve 132 which are discussed further below.

A pressurising pump 112 is provided to pressurise the circuit so the water can be heated to temperatures above 100 °C without boiling. An overflow valve 124 is provided in a branch of the circuit parallel to the pressurising pump 112 (i.e. connecting the pressurised side of the pressurising pump 112 to the sealed diaphragm expansion tank 114 on the low pressure side of the pressurising pump 112). The pressurising pump 1 12 and overflow valve 124 are operated to maintain a desired pressure in the circuit 102 in accordance with known techniques. These elements responsible for managing the desired pressure in the circuit may collectively be referred to as a pressurising unit 136. The operation of the pressuring unit 136 may be wholly conventional.

A heating-loop pump 116 and a cooling-loop pump 118 are provided to respectively circulate water from the storage tank 106 through the solar field heater 110 and absorption chiller 108. The section of the circuit 102 containing the heating-loop pump 116 and solar field heater 110 (with associated connections to the storage tank 106) may be referred to as the circuit's heating loop. Similarly, the part of the circuit containing the cooling-loop pump 118 and absorption chiller 108 may be referred to as the circuit's cooling loop.

A primary pressure-release safety valve 120 is coupled to the main storage tank 106 at a high point in the circuit 102. This is similar to the safety valve 20 shown in Figure 1 and discussed above. The safety valve 120 is configured to open and vent the circuit 102 to a 1 m³ blow-down vessel 126 open to atmospheric pressure if the valves' release pressure is exceeded. The primary safety valve 120 in this example is a full-lift valve (i.e. designed to "snap" fully open) and is configured to release pressure in the circuit if the circuit pressure exceeds 18 bar (1.8 MPa). The primary safety valve 120 is a safety feature provided to comply with building code requirements as discussed above.

The interception tank 130 is provided in a branch of the circuit 102 between the main storage tank 106 and the pressurising unit 136. The circuit connection between the main storage tank 106 and the interception tank 130 is from the bottom of the main storage tank 106 to an upper port near the top of the interception tank 130. The circuit connection from the interception tank 106 to the pressurising unit 136 is from a lower port near the bottom of the interception tank 130 to the high pressure side of the pressurising unit 136. The secondary safety valve 132 is coupled to the circuit between the interception tank 130 and pressurising unit 136. The secondary safety valve 132 is a proportional release valve (i.e. opens progressively with increasing pressure) and in this example is configured to start to open if the circuit pressure exceeds 17.1 bar (1.71 MPa). The operation of the circuit 102 of Figure 2 will now be described. For the sake of providing a concrete example, particular example operating parameters for the circuit will be given, e.g. in terms of temperatures and pressures (such as the example valve release pressures given above). However, it will be appreciated these are simply specific example values for one implementation of an embodiment of the disclosed technology. In other example embodiments other parameters will be used as appropriate.

In normal use for the embodiment shown in Figure 2 the pressurising unit is operated to maintain a water pressure in the circuit of around 15 bar (1.5 MPa). The normal operating pressure at the expansion tank 114 (i.e. at the input side to the pressurising unit 136) in this embodiment is typically around 1.7 bar (0.17 MPa). The main storage tank 106 is thermally insulated and has a capacity of around 16 m³. The pipe work comprising the heating and cooling loops and the connection from the interception tank 130 to the main storage tank 106 is also thermally insulated. The interception tank 130, however, is not thermally insulated and in this example has a capacity around 2.5 m³.

To heat water in the circuit 102 the heating-loop pump 116 is operated to pump water from the bottom of the storage tank 106, through the solar field heater 110, and back to the top of the storage tank 106. A typical flow rate provided by the heating-loop pump 1 16 might be around 31 gallons(US)-per-minute (gpm) (117 litres per minute) with a differential pump pressure equivalent to a hydrostatic head of around 3 m. The solar field heater 110 may be any conventional solar heater, for example, based on fluid through a heat exchange element at the focus of a solar reflector. In a steady state condition the water temperature in this example circuit 102 typically ranges from around 160 °C to 180 °C. For example, the temperature of the water drawn into the heating loop from the bottom of the main storage tank 106 might be around 160 °C, and the temperature of the water returned to the top of the water tank after heating in the solar field heater 10 might be around 180 °C.

To provide cooling, the cooling-loop pump 118 is operated to pump the hot water from the top of the storage tank 106, through the absorption chiller 108, and back to the bottom of the storage tank 106. A typical flow rate provided by the cooling-loop pump 118 might be around 34 gpm (129 litres per minute) with a differential pump pressure equivalent to a hydrostatic head of around 12 m. The absorption chiller 110 may be wholly conventional. In accordance with the typical steady state conditions noted above, the temperature of water drawn from the top of the storage tank 106 is around 180 °C. The water is cooled as its thermal energy is used to drive the cooling process in the absorption chiller 108, and is returned to the storage tank at a temperature of around 160 °C. This -20 °C temperature differential across the absorption chiller 108 represents the energy extracted from the circuit which is used to drive the cooling. The absorption chiller 108 is thermally coupled to an air conditioning system of a building (not shown) to provide cooling for the building.

Thus in normal operation the circuit 102 of Figure 2 provides a solar-powered cooling system using heated water as a working liquid to provide energy to drive a cooling system in a manner that is broadly conventional and similar to that shown in Figure 1. However, the circuit of Figure 2 behaves differently should an over-pressure situation arise, for example because of a failure / blockage in the over-flow valve 124 in the pressuring unit 136 preventing controlled pressure release back to the expansion chamber 1 1 . In particular, the circuit of Figure 2 behaves differently from a conventional circuit in an over-pressure situation because of the different safety valve arrangement.

As noted above, the circuit 102 of Figure 2 comprises a conventionally-located pressure- release safety valve 120 at a high point in the system. This valve is located where the water in the circuit is typically close to its hottest. However, as also noted above, the circuit 102 of Figure 2 further comprises the secondary pressure-release safety valve 132 configured to open at a lower pressure (in this example 17.1 bar (1.71 MPa) as opposed to 18 bar (1.8 MPa)). Significantly, the secondary pressure-release safety valve 132 is located at a point in the heat exchange circuit 102 where the water temperature is lower than at the location of the primary pressure-release safety valve 120. Typically this can be achieved by locating the secondary pressure-release safety valve 132 at a position that is physically lower in the circuit as compared to the location of the primary pressure-release safety valve 120. Lower locations are generally cooler than higher locations in such circuits in view of gravity-driven temperature stratification effects typically seen in these types of circuit. Alternatively, or in addition, the secondary pressure-release safety valve 132 may be located at a position where the temperature is lower because of differences in thermal insulation.

In the example shown in Figure 2, the temperature at the location of the secondary pressure- release safety valve 132 is low compared to the temperature at the location of the primary pressure-release safety valve 120. This is because of the un-insulated interception tank providing a source of heat loss between the hottest parts of the circuit and the secondary pressure-release safety valve 132. For the example operating conditions described above, the temperature at the secondary pressure-release valve in the embodiment of Figure 2 might be around 70 ^DC, for example.

Thus in an over-pressure situation in the circuit 102 of Figure 2, the secondary pressure- release safety valve 132 will open to reduce pressure in the circuit 102 when the pressure exceeds 17.1 bar (1.71 MPa). At this stage the primary pressure-release safety valve 120 remains closed because its higher release pressure (18 bar (1.8 MPa) in this example) has not been reached. The opening of the secondary pressure-release safety valve 132 releases pressure in the system as water is expelled from the circuit through the open valve. Significantly, the expelled water at the secondary pressure-release safety valve 132 in this example is at a temperature of only around 70 °C, and so there is no sudden boiling as it is exposed to ambient atmospheric pressure. Furthermore, because the secondary pressure- release safety valve 132 is a proportional valve (as opposed to a full-lift valve as normally required by building codes for a main pressure-release safety valve), the pressure release may be less sudden, further reducing the chances of damage to the circuit cause by the relief of the over-pressure situation. In the event the secondary pressure-release safety valve 132 fails, or is unable to fully relieve the over-pressure situation, the primary pressure-release safety valve 120 provided in accordance with conventional building codes acts as a back-up and opens if the pressure in the circuit exceeds 18 bar (1.8 MPa). Thus in accordance with embodiments of the disclosed technology, a heat-exchange circuit is provided that includes a first pressure-release valve located at a first position in the circuit, e.g. so as to comply with building code practice, and in addition comprises a second pressure-release valve located at a second position in the circuit which does not in itself satisfy building code requirements. The second pressure-release valve is configured to open and release pressure in the circuit if a pressure threshold is exceeded that is lower that the opening-pressure associated with the first pressure-release valve. What is more, the locations of the first and second pressure-release valves are such that in normal use the temperature of the liquid at the first location is higher than the temperature of the liquid at the second location. Thus together the pressure-release valves provide an arrangement that complies with best building / boiler practice (by virtue of the first pressure-release valve), while at the same time allowing for a more controlled release of pressure in an over-pressure situation (by virtue of the second pressure-release valve).

It will be appreciated this principle may be embodied in many different pressurised water (liquid) circuits and is not limited to the specific configuration of Figure 2. For example, whereas Figure 2 schematically shows a solar-powered cooling system, embodiments of the disclosed technology may equally be applied in a solar-powered heating system, for example with the absorption chiller 108 of Figure 2 replaced with one or more heat radiators internal to a building. What is more, the heat exchange circuit need not rely on a solar field heater. For example, a circuit similar to that seen in Figure 2 may be based on an alternative heat source provided to replace, or supplement, the solar field heater 110.

What is more, different configurations and arrangements for locating a secondary pressure- release safety valve and interception tank may be provided. For example, in Figure 2, the secondary pressure-release safety valve and the interception tank are located in-line with the pressurising unit 136. This provides an added advantage of providing a temperature buffer (cold buffer) to protect the pressurising unit 136 and expansion tank 114 from high temperature water. For example, in typical operating conditions for the circuit of Figure 2 the water temperature at the bottom of the interception tank 130 to which the pressurising unit 136 is connected might be only around 70 °C, whereas the pressurising unit in the conventional circuit of Figure 1 will typically be exposed to higher temperatures. Nonetheless, in some other embodiments the secondary pressure-release safety valve and the interception tank may be provided in a spur to the circuit that is separate from the pressuring unit. Furthermore still, in some examples there may not be a structurally-distinct interception tank. Instead the secondary pressure-release safety valve may be located towards the end of an extended un-insulated length of pipe work such that the temperature of the water at secondary pressure-release safety valve is cooler than elsewhere. In general some form of interception tank may be preferred as it can provide a larger volume of cool water to be expelled through the secondary pressure-release safety valve. This allows pressure to be relieved through continued venting of relatively cool water even in situations where a relatively large amount of water needs to be expelled to relieve the pressure (although in practice only small amounts of water will typically need to be released to relieve sufficient pressure).

It will be appreciated an implementation of a circuit such as shown in Figure 2 would typically include various additional features, e.g. isolating valves, temperature and pressure sensor, electric motors for the pumps, and so on in line with normal practice for constructing such types of circuits. These are not shown in Figure 2 because they are not directly relevant to the underlying principle of the circuit's operation as described above.

Nonetheless, and for completeness, Figure 3 shows an engineering schematic diagram of a heat exchange circuit 202 according to another embodiment of the disclosed technology. The circuit 202 of Figure 3 is similar to that of Figure 2 and corresponding elements are identified with the same reference numerals and are not described again in the interest of brevity. The circuit 202 of Figure 3 differs from that of Figure 2 in that it is shown to include various additional elements typically seen in a hot pressurised water circuit. Figure 4A shows the legend for various ones of the elements of Figure 3. These elements are all conventional and may be implemented and operated in the usual way except where described otherwise. The operation of these standard features is not described in detail for brevity. Also shown in Figure 3 are thermodynamic data at various locations in the circuit 202. The thermodynamic data are presented in two-by-two tables adjacent the relevant parts of the circuit following the template layout shown in Figure 4B. Thus the top two numbers respectively show the water temperature in °C and pressure in bar, and the bottom two numbers respectively show the flow rate in kg/h, and the specific enthalpy of the liquid (water) in kJ/kg.

Among other things, the circuit 202 of Figure 3 includes various valves that may be closed to isolate sections of the circuit. For example, the valves 240A and 240B may be used to isolate the heating-loop part of the circuit containing the solar field heater 1 10 from the storage tank 106, e.g. for maintenance. It is noted that because the heating-loop part of the circuit includes a heater and can be isolated from the pressure-release safety valve 120 it is provided with its own pressure-release safety valve 250. This is similar to the primary pressure-release safety valve 120 in that it is provided in accordance with best building practice and is configured to release at the same pressure (in this example 18 bar (1.8 Pa)). The pressure-release safety valve 250 of the heating-loop part of the circuit is also vented to the blow-down vessel 126. In operation the secondary pressure-release safety valve 132 on the cold side of the cold buffer provided by the interception tank 130 will open before either of the two pressure-release safety valves 120, 250 provided in the upper part of the circuit to release pressure with reduced risk of uncontrolled steam generation as described above in relation to Figure 2.

It may be noted the circuit 202 of Figure 3 also includes a pressure-release safety valve 260 on the low-pressure side of the pressurising unit 136. This valve 260 has a release threshold in this example of around 2.5 bar (0.25 MPa) and is provided to protect the low-pressure side of the circuit, e.g. the expansion tank 1 14, from an over-pressure situation in this part of the circuit. The pressure-release safety valve 260 thus performs a role which is different from the role of the secondary pressure-release safety valve 132 that provides a controlled release of pressure in an over-pressure situation in the high pressure side of the circuit (i.e. the secondary pressure-release safety valve 132 has a pressure-release threshold that is higher than the normal operating pressure provided by the pressurising unit to the pressurised part of the circuit).

Thus in accordance with embodiments of the disclosed technology a water-filled heat- exchange circuit is provided. The circuit comprises a pressurising unit for pressurising liquid in the circuit and a heater for heating the liquid, e.g. a solar field heater. In accordance with best building practice, a pressure-release valve is located at a high point in the circuit. This valve is configured to open to release pressure in the circuit if a pressure threshold is exceeded. However, the circuit additionally comprises a second pressure-release valve that is located at a position in the pressurised circuit where, in normal use, the temperature is lower than at the location of the first pressure-release valve, e.g. because the second pressure-release valve is connected to the remainder of the circuit via a cold buffer, such as an un-insulated interception tank. The second pressure-release valve is furthermore configured to open at a pressure threshold that is lower than that of the first pressure-release valve. Thus in an over-pressure situation the second pressure-release valve will open to relieve pressure before the first pressure release valve. Because the second pressure- release valve is at a relatively cooler point in the circuit than would be the case for a conventional pressure-release valve, there is reduced risk of damage caused by uncontrolled steam generation as pressure is released in an over-pressure situation. At the same time, the provision of the first pressure release valve helps in ensuring compliance with best building practice.

It will be appreciated that although particular embodiments of the disclosed technology have been described, many modifications/additions and/or substitutions may be made within the spirit and scope of the present disclosed technology. It will further be recognized that the foregoing recital is not intended to list all of the features and advantages that may be associated, with embodiments of the disclosed technology Those skilled in the art will appreciate that they may readily use both the underlying ideas and the specific embodiments disclosed in this description as a basis for designing other arrangements for carrying out the same purposes of the present disclosed technology and that such equivalent constructions are within the spirit and scope of the disclosed technology in its broadest form. Moreover, it may be noted that different embodiments of the disclosed technology may provide various combinations of the recited features and advantages of the disclosed technology, and that less than all of the recited features and advantages may be provided by some embodiments. It will be further be appreciated that features described above in connection with aspects of the disclosed technology will often be equally applicable to, and may be combined with, other aspects of the disclosed technology. In particular, features of embodiments of the disclosed technology may be combined in any appropriate way and not just in the specific combinations recited in the attached claims.

Claims

1. A heat-exchange circuit comprising:

a pressurising unit for pressurising liquid in the circuit;

a heater for heating the liquid;

a first pressure-release valve located at a first position in the circuit and configured to open to release pressure in the circuit if a first pressure threshold is exceeded; and

a second pressure-release valve located at a second position in the circuit and configured to open to release pressure in the circuit if a second pressure threshold is exceeded, wherein the second pressure threshold is lower than the first pressure threshold, and wherein the first and second positions are arranged so that in normal use the temperature of the liquid at the second position is lower than the temperature of the liquid at the first position.

2. A heat-exchange circuit according to claim 1 , wherein the first pressure-release valve is a full-lift valve.

3. A heat-exchange circuit according to claim 1 or claim 2, wherein the second pressure-release valve is a proportional-lift valve.

4. A heat-exchange circuit according to any preceding claim, wherein the first position is at a location in the circuit which is higher than the second position.

5. A heat-exchange circuit according to any preceding claim, further comprising an interception tank arranged between the first and second positions in the circuit and having an upper connection port coupled to a part of the circuit including the first pressure-release valve and a lower connection port coupled to a part of the circuit including the second pressure-release valve.

6. A heat exchange circuit according to claim 5, wherein the interception tank has less thermal insulation than other parts of the circuit or no thermal insulation such that in normal use the liquid in the interception tank cools to a temperature that is lower than the temperature at the location of the first pressure-release safety valve.

7. A heat exchange circuit according to claim 5 or claim 6, wherein the pressurising unit for pressurising liquid in the circuit is also coupled to a lower connection port towards the bottom of the interception tank.

8. A heat-exchange circuit according to any preceding claim, wherein the first and second positions are arranged so that in normal use the temperature of the liquid at the first position is above a temperature at which the liquid boils at atmospheric pressure and the temperature of the liquid at the second position is below a temperature at which the liquid boils at atmospheric pressure.

9. A heat-exchange circuit according to any preceding claim, further comprising a main storage tank arranged to store heated liquid received from the heater.

10. A heat-exchange circuit according to claim 9, wherein the first pressure-release safety valve is in a branch of the circuit connected to an upper port towards the top of the main storage tank and the second pressure-release safety valve is in a branch of the circuit connected to a lower port towards the bottom of the main storage tank.

11. A heat-exchange circuit according to any preceding claim, wherein the heater is a solar field heater.

12. A heat-exchange circuit according to any preceding claim, further comprising an absorption chiller arranged to receive liquid heated by the heater in order to drive a cooling process.

13. A method for releasing pressure in a heat-exchange circuit, the heat-exchange circuit comprising:

a pressurising unit for pressurising liquid in the circuit;

a heater for heating the liquid; and

a first pressure-release valve located at a first position in the circuit and configured to open to release pressure in the circuit if a first pressure threshold is exceeded; and wherein said method comprises:

providing a second pressure-release valve located at a second position in the circuit where the temperature of the liquid is lower than the temperature of the liquid at the first position, and opening the second pressure-release valve to release pressure in the circuit when a second pressure threshold is exceeded, wherein the second pressure threshold i lower than the first pressure threshold.