HEATING SYSTEM

FIELD OF THE INVENTION

The present invention relates to a heating system and method, and in particular, but not exclusively, to a water heating system utilising a heat pump in combination with a refrigeration system such as an air conditioning refrigeration system.

BACKGROUND TO THE INVENTION The principal of transferring energy from a heat source to a heat sink by means of a heat pump has been recognised for a number of years and is utilised in numerous commercial and domestic applications, such as space heating/cooling and the like. A typical form of heat pump is the vapour compression heat pump which generally comprises a compressor, an expander such as a throttle valve, a condenser heat exchanger, and an evaporator heat exchanger. In use, a working fluid is delivered through the evaporator heat exchanger, positioned adjacent a heat source, such that said working fluid is heated by the heat source and thus evaporated. The evaporated working fluid is then compressed by the compressor, which requires an external energy source, to an elevated pressure and temperature, and is subsequently delivered to the condenser heat exchanger, wherein the working fluid is condensed to give up its latent heat to a heat sink. The condensed working fluid is then expanded and once again fed to the evaporator to repeat the cycle.

Known working fluids for use in heat pumps include CFCs (Chloro-Fluoro- Carbons), HCFCs (Hydrogen-Chloro-Fluoro-Carbon) and HFCs (Hydrogen-Fluoro- Carbons). However, these substances are environmentally hazardous; for example,

CFCs and HCFCs strongly contribute to the depletion of the ozone layer, and HFCs are strong greenhouse gases. Accordingly, there has been considerable pressure within the field of Refrigeration and Air Conditioning to eliminate use of such known hazardous working fluids in favour of more environmentally friendly alternatives. Additionally, it is to be noted that conventional heat pumps are known to suffer from the disadvantage that their efficiency is inversely proportional to the difference in temperature between the heat source and heat sink. This therefore limits the usefulness of conventional heat pumps in water heating, where relatively large temperature increases are required. One proposed alternative to conventional heat pumps to address such problems is a transcritical heat pump which utilises carbon dioxide as the working fluid. That is, refrigerant carbon dioxide (R744) is considered to be environmentally benign, and due to the requisite operation in a transcritical cycle, such heat pumps are thus capable of heating water through a large temperature range in counter flow with the working fluid. However, it is known that such transcritical heat pumps are generally low in efficiency.

In most domestic and commercial environments, there is a general requirement, and indeed growing environmental pressure, to minimise the expenditure of energy. Accordingly, inefficient processes are generally dismissed in favour of more efficient alternatives. However, even with such alternative, more efficient processes, considerable energy wastage may still be observed.

It is understood in the art that heat pumps typically utilise waste heat from a further system, such as the heat ejected from the condenser of a refrigeration system. This arrangement permits heat which would otherwise be wasted to be utilised in a useful manner. Additionally, it has been observed that utilising heat from a condenser

of a refrigeration system as a heat source in a heat pump produces a small reduction in power by lowering the condensing pressure. However, such power saving benefits are minimal and restricted only to the refrigeration cycle of the refrigeration system.

It is among the objects of the present invention to obviate or at least mitigate these and other problems in the art.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a heating system comprising a heat pump having a heat inlet heat exchanger and a heat outlet heat exchanger, said heat pump utilising a working fluid in a transcritical cycle, wherein said heat inlet heat exchanger is adapted to be positioned adjacent a heat source comprising liquid chilled by a separate refrigeration system.

Advantageously, in use, the outlet heat exchanger may be adapted to be positioned adjacent an appropriate heat sink comprising a medium to be heated by the heating system. Accordingly, the present invention may, in use, absorb heat energy from a liquid chilled by a separate refrigeration system, thus further chilling said liquid, and utilise the absorbed heat energy to appropriately heat a further medium. Such an arrangement provides the required heating while at the same time reduces the temperature of the chilled liquid heat source, providing corresponding energy efficiency benefits to the separate refrigeration system. That is, permitting the chilled liquid to be further chilled by the heating system of the present invention reduces the refrigeration effect required by the existing refrigeration system, while at the same time providing heat energy to heat a further medium.

This arrangement provides significant advantages over prior art arrangements in which waste heat from a separate refrigeration apparatus, for example from a

condenser, is generally utilised while offering very little gain in the energy efficiency of the apparatus, whereas the present invention provides a direct refrigeration effect producing a comparatively large reduction in power requirements.

Beneficially, the heat sink may form part of the present invention. In one embodiment, the heat sink may comprise a supply of liquid. Preferably, the supply of liquid is arranged to flow adjacent the heat outlet heat exchanger, preferably in counter flow relative to the working fluid. This arrangement assists to maximise the heat transferred to the liquid, and also more closely matches the temperature gradient of the liquid with that of the working fluid within the heat pump when operated in the transcritical cycle. The supply of liquid may comprise water, such that said heating system may be operated as a water heating system, wherein the outlet heat exchanger beneficially functions as a water heater. Heated water or other liquid may be utilised directly, or alternatively may be utilised in a space heating system or the like. Alternatively further, heated water or other liquid may be stored, and may optionally be recirculated to be reheated, in accordance with system output requirements.

Alternatively, the supply of liquid may comprise oil or the like.

In an alternative embodiment, the heat sink may comprise a gas, such as air or the like. Alternatively further, the heat sink may comprise a solid body, such as heat- retaining bricks or the like. Preferably, the heat pump comprises a compressor, such as a scroll compressor, piston compressor or the like. It should be understood, however, that any suitable compressor for use in a transcritical cycle may be utilised.

Preferably also, the heat pump comprises an expansion valve, such as a constant upstream pressure type valve or the like, or any other suitable type which would be selected by a person of skill in the art.

Advantageously, in use, the working fluid may be compressed by the compressor to a pressure greater than the critical pressure for the particular working fluid, wherein the compressed working fluid is delivered to the heat outlet heat exchanger positioned adjacent the heat sink comprising the medium to be heated. The supercritical working fluid is then cooled in the heat outlet heat exchanger while heating the medium. It should be noted that as the working fluid is in a supercritical condition, the heat transferred from the working fluid is sensible heat, maximising energy transfer and causing the working fluid to be cooled. Accordingly, the heat outlet heat exchanger may be described as a gas cooler. This transfer of sensible heat advantageously permits efficient use of a counter flow arrangement between the working fluid and the medium to be heated, as the heating and cooling temperature gradients may be closely matched to maximise heat transfer efficiency.

Upon exiting the heat outlet heat exchanger, or gas cooler, the working fluid is expanded through the expansion valve and subsequently passed through the heat inlet heat exchanger. At this stage, heat energy from an adjacent chilled liquid source is absorbed and utilised to evaporate the working fluid, while simultaneously reducing the temperature of the chilled liquid. The heat inlet heat exchanger may therefore be described as an evaporator. The evaporated working fluid may then be returned to the compressor to repeat the cycle. In a preferred embodiment of the present invention, the working fluid comprises carbon dioxide. Thus, in use, the carbon dioxide is compressed by the compressor to pressures above the critical pressure of 73.748 BarA, and may be

capable of heating water, for example, to temperatures of up to 8O ⁰ C depending on

the water flow rate.

Preferably, the separate refrigeration system comprises a liquid circuit and a refrigeration unit adapted to reduce the temperature of the liquid within the liquid circuit to provide chilled liquid. Advantageously, the separate refrigeration system may further comprise pump means adapted to circulate liquid within the liquid circuit. In one embodiment of the present invention, the separate refrigeration system may form part of an air conditioning or cooling system, wherein chilled liquid within the liquid circuit is utilised to cool air within a room or the like, for example via a liquid/air heat exchanger.

Advantageously, the heat inlet heat exchanger of the present invention is adapted to be positioned adjacent at least a portion of the liquid circuit, such that heat energy within the chilled liquid flowing therethrough may be transferred to the working fluid within the heat inlet heat exchanger. Accordingly, the heat transfer effect at the heat inlet heat exchanger advantageously reduces the temperature of the liquid within the liquid circuit. In a preferred embodiment, the heat inlet heat exchanger is positioned adjacent the liquid circuit at a location upstream of the refrigeration unit of the separate refrigeration system, such that liquid may be pre- chilled prior to being directed towards said refrigeration unit. Accordingly, the necessary refrigeration effect and thus energy required by the refrigeration unit may be decreased. Beneficially, the heat source comprising the chilled liquid may be at a

temperature in the range of, for example, 5°C to 25 ⁰ C, preferably in the range of 5 ⁰ C

to 15 ⁰ C.

Advantageously, the heating system may be adapted to heat a medium to a

temperature in the range of, for example, 20°C to 100 ⁰ C and preferably in the range

of 40 ^o C to 80°C.

According to a second aspect of the present invention, there is provided a method of heating a medium, said method comprising the steps of: providing a heat pump having an inlet heat exchanger and an outlet heat exchanger, said heat pump utilising a working fluid in a transcritical cycle; locating the heat inlet heat exchanger adjacent a source of chilled liquid from a separate refrigeration system; locating the heat outlet heat exchanger adjacent the medium to be heated; and operating the heat pump to transfer heat energy from the chilled liquid to the medium to heat said medium. Advantageously, the method assists to further chill the chilled liquid, reducing the refrigeration requirement of the separate refrigeration system, while providing heat energy to heat the medium with minimal additional energy input.

In a preferred embodiment, the separate refrigeration system forms part of an air conditioning system. Advantageously, the medium to be cooled is arranged to flow in an opposite direction to the working fluid within the heat pump.

Preferably, the medium to be heated comprises water.

Preferably also, the working fluid within the heat pump comprises carbon dioxide, which may advantageously be utilised to heat water, for example, to

temperatures up to 80°C depending water flow rate.

According to a third aspect of the present invention, there is provided a water heating system comprising: a heat pump having a heat inlet heat exchanger and a heat outlet heat exchanger, said heat pump utilising carbon dioxide in a transcritical cycle; a heat source positioned adjacent the heat inlet heat exchanger; and

a supply of water arranged to flow adjacent the heat outlet heat exchanger; wherein the heat source comprises liquid chilled by a separate refrigeration system.

Preferably, the refrigerant utilised in the heat pump is carbon dioxide. Advantageously, the separate refrigeration system forms part of an air conditioning system, wherein the chilled liquid is for use in achieving cooling of air in the air conditioning system.

According to a fourth aspect of the present invention, there is provided a combined cooling and heating system comprising: a heat pump having a heat inlet heat exchanger and a heat outlet heat exchanger, said heat pump utilising a working fluid in a transcritical cycle; a medium to be cooled positioned adjacent the heat inlet heat exchanger; and a medium to be heated positioned adjacent the heat outlet heat exchanger; wherein said medium to be cooled comprises liquid chilled by a separate refrigeration system.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a water heating system in accordance with an embodiment of an aspect of the present invention; and

Figure 2 is a plot of pressure against enthalpy representing the operation of the water heating system of Figure 1 in one specific example.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 is a diagrammatic representation of a water heating system, generally identified by reference numeral 10, in accordance with an embodiment of an aspect of the present invention. The system 10 includes a heat pump 12 which utilises carbon dioxide as the working fluid in a transcritical cycle, wherein the heat pump 12 comprises a heat inlet heat exchanger or evaporator 14, a compressor 16, a heat outlet heat exchanger or gas cooler 18, and an expansion valve 20.

The evaporator 14 is positioned adjacent a portion of a chilled liquid circuit 22 of an air conditioning system, generally identified by reference numeral 24, wherein chilled liquid circulating within the circuit 22 is utilised as a heat source to operate the heat pump 12, whereas the gas cooler 18 is positioned adjacent a water circuit 26 containing water to be heated. The evaporator 14 and the adjacent portion of the chilled liquid circuit 22 together form a first heat exchanger arrangement 28, and the gas cooler 18 and adjacent water circuit 26 together form a second heat exchanger arrangement 30. In use, the carbon dioxide working fluid is compressed by the compressor 16 to a pressure above its critical pressure (73.748 BarA), wherein the supercritical gas is then delivered to the gas cooler and flowed therethrough in the direction of arrow 32 to thus heat the water within circuit 26 via the second heat exchanger arrangement 30, wherein the water flows in the direction of arrow 34, i.e., counter flow to the carbon dioxide within the heat pump 12. It should be noted that as the carbon dioxide is in a supercritical condition, the heat transferred therefrom is sensible heat, maximising energy transfer and causing the carbon dioxide to be cooled. This transfer of sensible heat advantageously permits efficient use of the counter flow arrangement between the carbon dioxide and the water being heated, as the heating and cooling temperature gradients may be closely matched to maximise heat transfer efficiency.

Upon exiting the gas cooler 18, the carbon dioxide is expanded through the expansion valve and subsequently passed through the evaporator 14. At this stage, heat energy from the adjacent chilled liquid circuit 22 is absorbed and utilised to evaporate the carbon dioxide, while simultaneously reducing the temperature of the liquid within the chilled liquid circuit 22, which will be discussed in more detail below. The evaporated carbon dioxide is then returned to the compressor 16 to repeat the cycle.

The water to be heated may be provided from a fluid source 36 and after heating may be removed via fluid outlet conduit 38 for immediate use. Alternatively, or additionally, heated water may be stored within a storage tank 40 for subsequent use by removing via fluid outlet 42. In this storage arrangement, stored water may be recirculated via fluid conduit 44, shown in broken outline, to be reheated.

The air conditioning system 24 in the example shown in Figure 1 is of the space-cooling type, wherein the chilled liquid within the chilled liquid circuit 22 is utilised to cool a space 46, such as a room or the like, via an air cooler heat exchanger

48. The air conditioning system 24 comprises a separate refrigeration unit, generally identified by reference numeral 50, which is utilised to chill the liquid within circuit 22 to the required temperature via heat exchanger arrangement 52. The refrigeration unit 50 may be of a conventional design. The air conditioning system 24 also comprises a water pump for pumping chilled liquid around circuit 22 in the direction of arrow 56. Accordingly, it should be noted that the first heat exchanger arrangement 28 formed between the heat pump 12 and chilled water circuit 22 is located upstream of the refrigeration unit 50. Thus, in use, the heat pump 12 may be utilised to heat water within circuit 26 while at the same time reducing the temperature of the liquid within the chilled liquid circuit 22 prior to

being fed to the refrigeration unit 50 for further cooling. Consequently, pre-cooling the chilled liquid permits the refrigeration unit 50 to be operated to provide a reduced refrigeration effect, while at the same time providing heated water with minimal additional energy input, as will be demonstrated in the Example below, and with additional reference to Figure 2 of the drawings.

EXAMPLE

Figure 2 is a pressure-enthalpy (Mollier) diagram representing the operation of the heat pump 12 shown in Figure 1, utilised to supply heat to raise the temperature of

water within the water circuit 22 from 15 ⁰ C to 50°C. In the following example, the following parameters are assumed:

Evaporating temperature of carbon dioxide within the evaporator 14 is 0°C; Heat rejection at the gas cooler 18 is achieved at 80 BarA;

Supercritical carbon dioxide is cooled to 20°C within the gas cooler 18; Isentropic efficiency of the compressor 16 is 0.6;

Coefficient of Performance (CoP) of the existing refrigeration unit 50 is 3.0.

When the heat pump 12 is in use, the carbon dioxide is compressed by compressor 16 from 35 BarA to 80 BarA, which is above the critical pressure (73.748

BarA) of carbon dioxide. This compression is accompanied by a temperature increase, and thus enthalpy increase from 735 kJ/kg (hi) to 795 kJ/kg (h2). This gas compression is represented by line 60 in Figure 2. The supercritical gas is then cooled within the gas cooler 18 while heating water within the water circuit 26, which is represented by line 62 in Figure 2. As noted above, the supercritical gas is cooled to

20 ⁰ C and an enthalpy of 560 kJ/kg (h3). The carbon dioxide is then expanded

through valve 20, which is represented by line 64 in Figure 2, such the pressure is

reduced to 35 BarA while at a constant enthalpy of 560 kJ/kg (h4=h3). The expanded carbon dioxide is then evaporated in the evaporator 14, which is represented by line 66 in Figure 2, such that the enthalpy is again returned to 735 kJ/kg (hi).

Calculation:

The work input (i.e. to drive compressor 16) is: h2 - hi = 795 - 735 = 60 kJ/kg The heating effect is: h2 - h3 = 795 - 560 = 235 kJ/kg The refrigerating effect (heat pump 12) is: hi - h4 = 735 - 560 = 175 kJ/kg

Thus, the work required to produce the equivalent refrigerating effect from the refrigerating unit 50 of the air conditioning system is: refrigerating effect (heat pump 12)/CoP(unit 50) = 175/3 = 58 kJ/kg Therefore, the net work required to produce hot water is: refrigerating effect (heat pump 12) - equivalent refrigerating effect (unit 50) = 60 - 58 = 2 kJ/kg.

Thus, the electrical power cost of producing hot water is considered to be negligible when there is a simultaneous requirement to chill liquid, as is the case in the present example.

It should be understood that the embodiment described is merely exemplary of the present invention and that modifications may be made thereto without departing from the scope of the invention. For example, the heat pump may be located adjacent any region of the chilled liquid circuit. Additionally, the heat pump may be used in conjunction with any other system which incorporates a chilled liquid circuit, and is

not limited to air conditioning systems. Furthermore, the heat pump may be utilised to heat any other suitable medium, such as a gas or a solid or the like. For example, the heat pump may be utilised to directly heat air, such as in a space heating system. Additionally, the heat pump may be utilised to heat a solid material such as heat- retaining bricks or the like, to be used in a space heating system, such as a storage heating system or the like.