GB2534608A

GB2534608A - Domestic hot water system

Info

Publication number: GB2534608A
Application number: GB1501518.3A
Authority: GB
Inventors: John Crawford Robert; Bertinat Charles; Henry Green Robert
Original assignee: EA Technology Ltd; C Tech Innovation Ltd
Current assignee: EA Technology Ltd; C Tech Innovation Ltd
Priority date: 2015-01-29
Filing date: 2015-01-29
Publication date: 2016-08-03
Also published as: GB201501518D0

Abstract

The system for providing domestic hot water from a heat pump 1 includes a hot water storage tank 15. The tank has a hot water input 16 and a hot water output 17 near the top and a cold water input 19 and a cold water output 18 near the bottom. The domestic hot water supply 27 is separated from water supplied from the heat pump. The system includes a second heat exchanger 21 connected between the top and bottom of the tank which exchanges heat between water heated by the heat pump and the domestic hot water supply. The water flow of heated water from the heat pump through the second heat exchanger is matched to the flow of domestic hot water. The tank maybe stratified with a pair of funnels (Figure 4).

Description

DOMESTIC HOT WATER SYSTEM

[0001] This invention relates to heat pump derived heating and hot-water systems.

[0002] Conventional central heating hot water systems employ a boiler supplying hot water for heating and domestic hot water purposes. In recent years increasing use has been made of ground or air source heat pumps to generate a primary source of hot water. A difficulty with heat pumps, and especially with air-source heat pumps, is that they operate most efficiently when the output water temperature is modest, and lower than the water temperature typically used in a central heating system heated by a gas boiler. Heat pumps operate in one of two modes: in one mode heating the domestic hot water (DHW) supply to a relatively higher temperature but less efficiently, and in another mode providing space heating at a relatively lower temperature. In practice these different objectives are difficult to balance effectively in one system and the result is relatively low coefficients of performance (COP) values, especially for air-source heat pumps in the UK. It is the objective of this invention to improve the COP of an air-source heat pump. The invention is also applicable to water-source and ground-source heat pumped systems [0003] In such a system for supplying domestic hot water, water passing through the heat pump is normally chemically treated and should be separated from domestic hot water.

[0004] According to the present invention, a system to provide domestic hot water has a heat pump, a hot water storage tank, the tank having a hot water input and a hot water output near the top of the tank and a cold water input and a cold water output near the bottom of the tank and in which the domestic hot water supply is separated from water supplied from the heat pump and in which the system comprises a second heat exchanger connected between the top of the tank and the bottom of the tank, the second heat exchanger exchanging heat between water heated by the heat pump and the domestic hot water supply and further in which the water flow of heated water from the heat pump through the second heat exchanger is matched to the flow of domestic hot water.

[0005] In such a system for supplying domestic hot water includes control of the rate of flow of water through the heat pump to achieve the desired water temperature at the outlet when the system is supplying domestic hot water.

[0006] In one arrangement in such a system, the heat pump having a cold water input and a heated water output and comprises a closed loop refrigerant cycle including a heat exchanger, expansion valve, evaporator, compressor, in which the refrigerant cycle additionally comprises a sub-cooler through which cold water entering the heat pump through the input passes before it is heated on passing through the condenser, the sub-cooler containing liquid refrigerant.

[0007] For economy the condenser and sub-cooler are combined as one unit. [0008] Ideally the level of liquid refrigerant is maintained at a constant level. [0009] In such a system for supplying domestic hot water includes control of the rate of flow of water through the heat pump to achieve the desired water temperature at the outlet when the system is supplying domestic hot water.

[0010] Preferably the storage tank is stratified to keep the temperature gradient as steep as possible so that mixing of hot and cold water in the storage tank is minimised. This has the desirable result of keeping the return feed to the heat pump from the bottom of the tank at as low a temperature as possible which in turn maximises the efficiency of the heat pump. In this context a stratified tank is one which has internal physical provision to slow intermixing of water layers in the tank.

[0011] In order that the invention may be more fully understood, examples of the invention will be described with reference to the accompanying drawings, in which: [0012] Figure 1 is a schematic illustration of an improved primary heat exchanger incorporating a sub-cooler in accordance with one embodiment of the invention.

[0013] Figure 2 is a schematic diagram of a first embodiment of the invention; [0014] Figure 3 is a second embodiment of the invention; and [0015] Figure 4 shows a stratified tank for use with the invention. It has been found possible further to improve the efficiency of heat pump heating and hot water systems by modifying the primary heat exchanger to include a sub-cooling function between the condenser and the conventional evaporator. In the arrangement shown in figure 1, the heat pump 1 has a sub-cooler; in the illustrated example, the sub-cooler function has between combined into the condenser by having a fixed level of liquid refrigerant 55 in the bottom of the condenser 51 of the heat pump, water to be heated in the condenser first passing through the liquid refrigerant. The condenser 51 has an outlet 2 for hot-water and an inlet 3 for cold water coming from, for example, the embodiments of figures 2 and 3. In figure 1 the water passing through the sub-cooler and condenser 51 is shown schematically as passing through a coil heat exchanger but in practical implementations a plate heat exchanger is often sed. Liquid refrigerant flows to expansion valve 52 to reduce its pressure and then onto evaporator 53 where it is evaporated by heat taken from the air or from a ground source. It is then compressed by compressor 54, and in so doing the refrigerant vapour is heated before being returned to the condenser 51 where it gives up its heat to water flowing between inlet and outlet 3 and 2, with the heated water leaving the heat pump though outlet 2. In giving up its heat, the vapour condenses.

[0016] To maintain the static level of liquid refrigerant, the pressure of the vapour 56 in the upper part of the condenser is measured as is the pressure of the liquid refrigerant 55, the difference between these two pressures is monitored by a computer 57, any change in the difference indicates a change in the height of liquid refrigerant and the pressure difference is used to adjust the flow of refrigerant through the expansion valve 52 to maintain the liquid refrigerant at the desired value.

[0017] As an alternative to measuring the pressure difference, the same control can be achieved by measuring the temperature of the incoming water at input 3 and of the refrigerant leaving the sub-cooler, these should be similar, any changes are identified by computer 58 and used to adjust the flow of liquid refrigerant liquid through the expansion valve 52.

[0018] The heat pump 1 including a sub-cooler shown in figure 1 as part of a heating and hot water system is illustrated in figures 2 and 3.

[0019] In figure 2, heated water leaves the heat pump 1 through outlet 2. The flow can then split, with one pipe leading to the heating system 9 the other to the domestic hot water system described in more detail below. Access to the domestic hot water system is controlled by a demand controlled valve 5. When valve 5 is open water flows from the heat pump 1 through valve 5, at the same time valve 4 is close preventing flow to the heating system 9. When there is no demand for supply to the hot water system valve 5 closes and valve 4 opens supplying the heating system.

[0020] Generally the flow of water through the heating system is much higher when the valve 4 is open than when supplying heat to the domestic hot water system through valve 5. Generally the heating circuit 9 is supplied with water at a lower temperature than the hot water system (typically 35°C as opposed to up to 65°C for the domestic hot water system) [0021] In figure 2, which illustrates a thermal store arrangement in which hot water supplied from the heat pump to the hot water system is stored, the hot water supply goes through an optional thermostatic divert valve 13 to the hot water input 16 of a hot water storage tank comprising an open vented stratified storage cylinder 15. Cold water from the cold water output 18 near the bottom of cylinder 15 is returned to the heat pump 1through its inlet 3 by the action of a variable speed pump 11. The variable speed pump 11 controls the flow rate to deliver water from the fixed pump to cylinder 15 at a fixed temperature; alternatively the flow rate from the heat pump 1 to cylinder 15 can be controlled by a regulating control valve. Water from the heating system 9 pumped by pump 7 also enters the heat pump 1 through inlet 3. Pumps 7 and 11 do not pump when valves 4 or 5 are closed respectively. At the start of hot water production, the pipe from heat pump outlet 2 and connections to the cylinder would be full of water that is cooler than the top of the cylinder 15. To avoid this cooler water entering the cylinder 15, optional divert valve 13 diverts the water away from the cylinder until water reaching the divert valve 13 is hot enough to be allowed through to the cylinder -the divert valve can be omitted, as its impact on efficiency is quite small.

[0022] A hot water output 17 near the top of cylinder 15 is connected to second heat exchanger 21; water is pumped from the top of cylinder 15, through the auxiliary heat exchanger 21 by pump 23 located between the auxiliary heat exchanger and a return input 19 to the cylinder situated near its base.

[0023] A cold mains water supply 25 is connected through a flow measuring device 24 to second heat exchanger 21 where it is heated from water passing from the cylinder 15, the heated fresh water then leaves the second heat exchanger to the outlet 27 for domestic use and on to taps etc..

[0024] It is important in the operation of the embodiment shown in figure 2 that the water flows from the cylinder 15 and supply 25 through the second exchanger 21 are matched. To achieve this variable speed pump 23 adjusts the flow rate from the cylinder 15 to match that measured by flow measuring device 24, to provide substantially the same flow rates through them. Alternatively the flow rate can be adjusted by variable speed pump 23 to achieve a similar temperature change in each fluid (i.e. the temperature difference between hot water outlet 17 and return input 19 is similar to that between outlet 27 and supply 25) [0025] Another embodiment is shown in figure 3. In the embodiment domestic hot water is stored. The arrangement of the heating system 9, supply conduit 2 from and return conduit 3 to the heat pump, valves 4 and 5 are exactly the same as described in figure 2. In this embodiment, however, valve 5 controls entry of hot water from the heat pump 1 to second heat exchanger 21 from where it is pumped by variable speed pump 23 and returned to the cold side of the heat pump 1 as in figure 2. In the embodiment of figure 3, heat is exchanged in second heat exchanger 21 between water flowing from the heat pump 1 and the domestic hot water system.

[0026] The hot water system comprises a stratified cylinder 15 having a hot inlet 16 from second heat exchanger 21 and cold outlet 18 connected through a variable speed pump 29 to the second heat exchanger 21. A thermostatic divert valve 13 is provided to divert hot water away from the cylinder 15 if it is at a temperature that is below the required value for storing in the cylinder 15. A hot water output 17 from the top of cylinder 15 is connected to outlet 27 for domestic use and on to taps etc. A cold water input 19 at the bottom of cylinder 15 is fed from mains water supply 25. As before an optional thermostatic valve 13 can divert water though a by-pass conduit until it has reached a desired temperature.

[0027] It is important in the operation of the embodiment of figure 3 that the flows through second heat exchanger are matched. To achieve this variable speed pumps 23 and 29 are controlled to achieve substantially the same flow rates on both sides of second heat exchanger 21. Additionally in this embodiment the flow rate to input 16 of cylinder 15 is controlled by variable pump 23 or, alternatively by a regulating control valve.

[0028] In figure 4 one example of a stratified cylinder as used in the embodiments of figures 2 and 3 is shown. A funnel 31 having mouth 33 and stem 32 is mounted within cylinder 15 near the top of the cylinder, with the mouth 33 facing towards the top of the tank and stem downwards. The hot water input 16 and hot water output 17 are connected to the stem 32, but the bottom of the stem is not open to the cylinder 15. When hot water is demanded through the output 17, it is taken down through mouth 33 into stem 32 and thus through output 17. Replenishing hot water enters through input 16 through stem 32.

[0029] A second inverted funnel 35 having mouth 37 and stem 36 is mounted within cylinder 15 near the bottom of the cylinder, with the mouth 37 facing downwards towards the bottom of the cylinder 15 and stem 36 upwards. The cold water input 19 and cold output 18 are connected to the stem 36. When hot water is demanded through the hot water output 17 and cold water returned through cold water input 19, cold water descends through mouth 37, where is displaced upwards by any colder water in the cylinder, the coldest water in the cylinder is thus driven upwards through the mouth 37 into stem 36 and out though cold water outlet 18.

[0030] An alternative to the design of figure 4, porous plates have been mounted immediately below the hot water input 16 and hot water outlet 17 and above the cold water input 19 and cold water outlet 18.

[0031] Although the specific examples of the invention in figures 2 and 3 are in relation to a novel heat pump having a sub-cooler as shown in figure 1, the invention can be used with conventional heat pumps, either air sourced or ground sourced.

Claims

Claims 1. A system providing domestic hot water from a heat pump having a hot water storage tank, the tank having a hot water input and a hot water output near the top of the tank and a cold water input and a cold water output near the bottom of the tank and in which the domestic hot water supply is separated from water supplied from the heat pump and in which the system comprises a second heat exchanger connected between the top of the tank and the bottom of the tank, the second heat exchanger exchanging heat between water heated by the heat pump and the domestic hot water supply and further in which the water flow of heated water from the heat pump through the second heat exchanger is matched to the flow of domestic hot water.
2. A system according to claim 1 in which the second heat exchanger is connected between the hot output and cold input of the tank.
3. A system according to claim 1 in which the second heat exchanger is connected between the hot input and cold output of the tank.
4. A system according to any preceding claim including control of the rate of flow of water through the heat pump to achieve the desired water temperature at the outlet when the system is supplying domestic hot water.
5. A system according to any one of claims 1 to 4 in which the storage tank is stratified.
6. A system according to claim 5 in which the stratification comprises a pair of funnels, each funnel having a mouth and stem, one funnel near the top of the tank with its mouth upper-most and the other funnel near the base of the tank with its stem upper-most, in which the hot water input and hot-water output are connected to the stem of the upper funnel and the cold water input and cold water output is connected to the stem of the lower funnel..
7. A system according to claim 6 in which the tank comprises at least two porous plates one near the top of the tank below the hot water input and hot water output of the tank, the other near the bottom of the tank but above the cold water input and cold water output of the tank.
8. A system according to any one of claims 1 to 7 in which the heat pump has a cold water input and a heated water output and comprising a closed loop refrigerant cycle including the heat exchanger, expansion valve, evaporator, compressor, characterised in that the refrigerant cycle additionally comprises a sub-cooler through which cold water entering the heat pump through the input passes before it is heated on passing through the condenser, the sub-cooler containing liquid refrigerant.
9. A system according to claim 8 in which the condenser and sub-cooler are combined as one unit.
10. A system according to claim 8 or 9 in which the level of liquid refrigerant is maintained at a constant level.
11. A system according to any one of claims 8 to 10 in which the level of liquid refrigerant is maintained by measuring difference between the pressure of refrigerant vapour above the level of the liquid refrigerant and the pressure of the liquid refrigerant below the surface of the liquid refrigerant maintaining that difference a constant value.
12. A system according to any one of claims 8 to 11 in which the level of liquid refrigerant is maintained by measuring the temperature of water entering the sub-cooler and the temperature of the liquid refrigerant leaving the sub-cooler and adjusting the flow of liquid refrigerant through the evaporator to maintain the two temperatures close together.
13. A heating and domestic hot water system substantially as herein before described with reference to the accompanying drawings.