HEAT PUMP SYSTEMS

Abstract: The present invention relates to an improved heat pump system of a heating system which also comprises a utilization circuit. The heat pump system comprises a generator (2), an absorber (1), a condenser (5) and an evaporator (7), and the utilization circuit for circulation of a heat carrier medium comprises heat exchanger means (1 a , 5 a , 15, 17) for removing thermal energy from the heat pump system. To supplement the thermal output of the heat pump system, when the utilization circuit requires a greater thermal output than can efficiently be achieved by the heat pump system alone, a supplementary condenser (25) is proved in the generator, the condenser comprising a heat exchanger (25 a ) in heat exchange with vapour in the generator which can be connected in the utilization circuit by operation of a valve (WV) when additional thermal energy is required. Opening of the valve (WV) occurs simultaneously with an increase in the energy provided to the generator.

Description: The present invention relates to a circulating-fluid heating system according to the first part of Claim 1 in and relating to absorption heat pump heating systems, for example for providing the heat source of a domestic hot water and/or central heating system. Absorption heat pump systems conventionally comprise an absorber in which the refrigerant, for example ammonia, is absorbed in an absorbent, for example water, a generator to which the absorbent with absorbed refrigerant is pumped at an increased pressure and in which the refrigerant is driven out of the absorbent, the absorbent being returned to the absorber via pressure reducing means, a condenser to which the refrigerant from the generator is supplied and in which it cools and condenses, and an evaporator to which the condensed refrigerant is supplied via an expansion valve. Heat is supplied to the generator and evaporator, and the condenser and absorber are both associated with heat exchange means connected in a utilization circuit, for example the hot water and/or central heating system. It is found that, with such absorption heat pump systems, in maximising the efficiency of the system in relation to the cost of the installation, the maximum power and output temperature achievable are relatively low, for example of the order 8 kW at 55°C. While these levels are acceptable for the central heating and hot water systems in modern so-called low energy housing, they are not sufficient for the central heating and hot water systems in older or larger housing and particularly in colder weather conditions. This problem is addressed in European publication EP-A-0046196 by the incorporation of an additional bypass system. Thus EP-A-0046196 discloses a circulating-fluid heating system comprising an absorption heat pump system, a fluid heater and a utilisation circuit for transporting a fluid heat-carrier therebetween. The absorption heat pump system includes a refrigerant vapour generator and a refrigerant vapour rectifier. The utilisation circuit has heat transfer means for transferring thermal energy from the heater which includes a heat exchanger arranged to receive heat from the fluid heater having a switchable portion fitted with a valve which is operable by a controller to functionally couple the heat exchanger into the fluid utilisation circuit when the thermal output requirement of the system is above a predetermined level and to functionally disconnect the heat exchanger from the said circuit when the thermal output requirement of the system is below the predetermined level. In that system the switchable portion of the said heat exchanger comprises utilising-fluid coils which are directly exposed to the combustion gases of the heater. Inefficiency due to heat losses from the absorption heat pump system, which would occur when the system ceases to function as a heat pump due to cold weather conditions, is reduced by switching the utilising fluid from the portion of the circuit which includes the absorber, the condenser and the evaporator to a bypass loop comprising part of the additional bypass system. The present invention does not have the additional bypass system. Thus according to the present invention there is provided a circulating-fluid heating system comprising an absorption heat pump system, a fluid heater and a utilisation circuit for transporting a fluid heat-carrier therebetween, the absorption heat pump system including a refrigerant vapour generator, and a refrigerant vapour rectifier. The utilisation circuit has heat transfer means for transferring thermal energy from the heater, including at least one heat exchanger arranged to receive heat from the fluid heater and having a switchable portion fitted with valve means which is operable by control means to functionally couple the heat exchanger into the fluid utilisation circuit when the thermal output requirement of the system is above a predetermined level and to functionally disconnect the heat exchanger from the said circuit when the thermal output requirement of the system is below the predetermined level. The present invention is characterised in that the vapour generator and the vapour rectifier are contained by a closed vessel within the fluid heater, which vessel defines a liquid space and a vapour space, and the said switchable portion of the at least one heat exchanger comprises a utilisation circuit coil situated in the said vapour space so as to be subject to vapour condensing conditions. The switchable utilisation circuit coil accordingly experiences a constant temperature environment, notwithstanding an increasing heat demand as the cold weather intensifies. The heat transfer means of the utilisation circuit may comprise first and second heat exchangers associated with the absorber and condenser for withdrawing thermal energy therefrom and the heat exchanger having said switchable portion may be connectable in series with the first and second heat exchangers. The said heat exchanger having a switchable portion may be connected in parallel with a respective bypass loop and the control means may include a valve for controlling flow of the heat carrier medium through the said heat exchanger or through the respective bypass loop. The control means may include means controlling the supply of energy to the generator to increase the supply of energy thereto with operation of the valve to permit heat carrier medium to flow through the said heat exchanger. The absorption heat pump system may include a pump for pumping liquid from the absorber to the generator, the pump being driven by fluid from the generator which is then passed to the absorber. The fluid may comprise liquid and/or vapour from the generator. Embodiments according to the invention will now be described, with reference to the accompanying drawings. In the drawings: Figure 1 illustrates diagrammatically an embodiment of an absorption heat pump system and part of a utilization circuit of a heating system according to the present invention; Figure 2 illustrates diagrammatically part of the utilization circuit of the heating system of Figure 1; Figure 3 illustrates diagrammatically another embodiment of a heating system according to the present invention; Figure 4 is a section through a generator of the heating system of Figure 3; and Figure 5 is a section on the line V-V of Figure 4. The heating system shown in Figures 1 and 2 comprises a heat pump system shown in Figure 1 with a utilization circuit, parts of which are shown in Figure 1 and which are also shown in Figure 2. The heat pump system shown in Figure 1 comprises an absorber 1 in which a refrigerant, e.g. ammonia, is absorbed in an absorbent, e.g. water, and which includes a heat exchanger 1 a , a boiler 2 containing a vapour generator 2 a to which the absorbent with absorbed refrigerant (called the strong solution) is supplied via a line 3 by a solution pump at an increased pressure, and in which the strong solution is heated to drive off the refrigerant, the absorbent depleted of refrigerant (called the weak solution) from the vapour generator being returned via a line 4 and pressure reducing means to the absorber 1, a condenser 5 to which the refrigerant vapour is supplied at high pressure via a line 6 and in which it condenses on heat exchanger 5 a , and an evaporator 7 to which the condensed refrigerant is supplied via a line 8 provided with an expansion valve 9, in which the refrigerant evaporates and from which the evaporated refrigerant is supplied via a line 10 to the absorber. The absorber 1 may be a conventional absorber or, as shown, it may be as described in U.K. Patent 2,169,069. Such an absorber comprises a generally upwardly extending passage 100 through which liquid flows upwardly and a return path 101 including a reservoir 102. Weak liquid from the vapour generator 2 a is supplied to the lower end of passage 100 together with refrigerant from the evaporator. The refrigerant is absorbed in the weak solution as it flows upwardly through passage 100 and at the same time causes circulation of the liquid, independently of the flow caused by the solution pump, upwardly through passage 100. Strong solution is withdrawn from the reservoir 102 along line 3 to the vapour generator. Passage 100 is surrounded by a jacket 103 providing heat exchanger 1 a and through which the heat carrier medium of the utilization circuit flows counter-current to flow of liquid in passage 100 and to cool the liquid as absorption proceeds. The pump and pressure reducing means in lines 3 and 4 between the absorber 1 and generator 2 a for the strong and weak solutions may be conventional or may, as shown, be provided by a pump 11 as described in U.K. Patent No. 2,086,026. The pump 11 is, as described, generally operated to pump excess of the weak solution which is then accommodated in a lock vessel 12 and returned to the generator 2 by gravity flow. The strong solution from the absorber 1 may, as shown, be utilised in the rectification of vapour in the refrigerant vapour rectifier 2 b and may thus be pre-heated before entering the vapour generator 2 a by heat exchange in dephlegmator or rectifying heat exchanger 13 with the refrigerant vapour as it leaves the vapour rectifier 2 b and then by heat exchange in heat exchanger 14 with the weak solution leaving the vapour generator 2 a . The weak solution may be additionally cooled in a further heat exchanger 15 which is in heat exchange relation with the heat carrier medium of the utilization circuit. The heat input for the boiler 2 is derived from any suitable means such as oil burners or electrical heaters but the boiler is preferably, as shown, gas powered. When gas powered the boiler may operate as a fully condensing boiler and to this end is provided with a vessel comprising the vapour generator 2 a having a liquid space 207 and a vapour space 208 and with a flue 16 having two heat exchangers 17, 18. The first heat exchanger 17 is connected in the utilization circuit as shown in Figure 2 and the second heat exchanger 18 is connected as part of the heat input to the evaporator 7. Evaporator 7 may also, as is conventional, have an external low grade energy source, heat carrier medium, which may be air or water or other suitable fluid, being pumped through a heat exchanger at the low grade energy source and heat exchanger 18 to a heat exchanger in the evaporator 7. Heat exchangers 17 and 18 are advantageously of sufficient size to be capable of recovering energy efficiently in the flue 16. The utilization circuit of the above described heat pump system comprises the heat exchangers 1 a , 5 a , 15 and 17 which may for example be connected as shown in Figure 2 with valves 19 and 20 provided to control the relative flows of the heat carrier medium, for example water, through the heat exchangers 1 a , 15 and 5 a both during start up of the heating system and during normal operation. During normal operation, valves 19 and 20 are operated so that the heat carrier medium flows in parallel through heat exchangers 15 and 5 a of the condenser and heat exchanger 1 a of the absorber, the relative flows being adjusted by valve 19, and all the fluid then flows through heat exchanger 17. For a domestic hot water and/or central heating system, the heat exchangers are connected to a heat exchanger in a hot water tank and/or the radiators of the central heating system. The heat pump system as described above may, for example, be designed to have a maximum output of 8 kW at 55°C with the power supplied to the generator being approximately 6 kW. To increase the maximum output power and temperature, the vapour generator 2 a is modified by the association with it of a supplementary condenser 25 which, as shown in Figure 1, is arranged in a region of the vapour space 208 thereof and is therefore subjectable to the refrigerant vapour, but which does not form part of the heat pump system. As shown the condenser 25 comprises a heat exchanger 25 a which is connectable in the utilization circuit and is located in the said vapour space 208. The heat exchanger 25 a may, as shown in Figure 1, be provided in a housing 26 projecting from the top of the vapour generator 2 a , which would enable the condenser 25 to be isolated from the vapour space 208 when the heat exchanger 25 a is not in use, for example by a retractable flap closing the lower end of the housing 26, or it may be permanently exposed to the vapour in the vapour space. As shown in Figure 2, the heat exchanger 25 is connected in the utilization circuit in parallel with a by-pass line 27 and valve WV so that by operation of valve WV the heat exchanger 25 a can be isolated from the utilization circuit or connected in the utilization circuit. It will be appreciated that, when the heat exchanger 25 a is isolated from the utilization circuit, the condenser 25 will cease to operate as such and will not withdraw thermal energy from the boiler. In use of the above described heating system, when the requirements of the utilization circuit are at or below the maximum of the heat pump system, operating as a heat pump system, heat exchanger 25 a is isolated from the utilization circuit. When more heat is required, valve WV will be opened, the condenser 25 will operate and simultaneously the power supplied to the generator is progressively increased, depending on the requirements of the utilization circuit. Initially the heat pump system will continue to operate as a heat pump system, with the output of the heat pump system being supplemented by the output of the heat exchanger 25 a . However, as the temperature of the heat carrier medium in the utilization circuit rises and specifically as the temperature of the medium flowing through the heat exchangers in the absorber and condenser rises, there will come a point, which depends on the operating conditions of the heat pump system, when the evaporator and condenser will cease to function and the efficiency of the system will then become equivalent to that of a conventional boiler system. Until such time as heat exchanger 25 a is brought into the utilization circuit, the coefficient of performance of the system will be that of the heat pump system in the absence of the supplementary condenser 25. When the heat exchanger 25 a is brought into the utilization circuit, the coefficient of performance of the entire system will be reduced. The heating system shown in Figure 3 is similar to that of Figures 1 and 2 but has different connections of the various heat exchangers in the utilization circuit a different form of boiler 2 and a different form of pump 111 replacing pump 11 in the embodiment of Figure 1. The same reference numbers are used in the embodiment of Figure 3 for the same parts used in the embodiments of Figures 1 and 2. The pump 110 used in the embodiment of Figure 3 comprises a pumping chamber 111 to which strong liquid from the absorber is supplied via one-way valve 112 and from which it is pumped via one-way valve 113 to the generator via line 3, the dephlegmator 13 and heat exchanger 14 as in the previous embodiment. Chamber 111 is defined by a piston 114 providing a first driving chamber 115 and a second chamber 116. The driving fluid for the pump is mainly weak solution flowing at high pressure from the generator to the absorber along line 4 which is supplied to chambers 115 and 116 via a solenoid operated valve SV10 and flows therefrom to the absorber via a solenoid operated valve SV12, valves SV10 and SV12 being operated alternately, valve SV11 being, under these circumstances permanently open and valve SV13 permanently closed. The return force on the piston 114 is, as shown, provided by a spring 117 and a bias provided by connecting the chamber housing the spring permanently to line 4 downstream of the pump so that it receives low pressure liquid. The process of absorption increases the specific volume of the strong solution compared to that of the weak solution by an amount which depends on the respective concentrations of refrigerant in the respective solutions. Thus the average volume flow rate of solution from the absorber to the generator will be greater than that from the generator to the absorber. The effective areas of the chambers in the pump 110 are arranged so that there is an imbalance in the volume of liquid flowing to and from the generator, less liquid being pumped to the generator than flows back to the absorber. Thus in time the level of liquid in the absorber will rise above a predetermined level, sensed by a liquid level sensor in the reservoir 102 of the absorber. When this occurs supply of liquid to chamber 116 of the pump is replaced by supply of vapour from the boiler 2 and for this purpose the vapour space of the vapour rectifier 2 b is connected by a line 119 and the solenoid operated valve SV13 to the chamber 116. When the level of liquid in reservoir 102 rises above the preset level, valve SV13 is operated with valve SV10 to open and close with valve SV10 and valve SV11 is operated with valve SV12 to open and close with that valve so that during the pumping phase of the pump, liquid from the vapour generator 2 a is supplied to chamber 115 and vapour is supplied to chamber 116 and during the return phase both the liquid and vapour are supplied together to the absorber. When the level of liquid in the absorber falls below a preset level, valve SV13 is again permanently closed and valve SV13 is permanently open to resume operating in the mode when both chambers 115 and 116 are supplied with liquid from the vapour generator 2 a . As in the preceding embodiment, the evaporator 7 is provided with heat from heat exchanger 18 in the generator flue and a heat exchanger 180 subject to a low grade energy source, for example an external source, heat from heat exchangers 18 and 180 being supplied to a heat exchanger in the evaporator though a circuit 181 through which a heat carrier medium is pumped by pump PU2. To control relative operation of the evaporator and the condenser over a range of performance, an additional expansion valve 9 a together with a solenoid operated valve SV6 is connected in parallel with expansion valve 9, valve SV6 being operated in dependence on the level of liquid in the condenser which is sensed by level sensor 12. In this embodiment, the heat exchangers 1 a , 15 and 5 a are connected in series in the utilization circuit and, as in the previous embodiment, in series with the heat exchanger 25 which is arranged in parallel with a by-pass line 27 and under the control of a solenoid operated valve WV. It will be appreciated that the interconnection between heat exchangers 1 a , 15 and 5 a depends on the requirements of the utilization circuit and the connections shown in this embodiment and in the embodiment of Figures 1 and 2 are shown by way of example only. As shown, the utilization circuit includes a pump PU1 for circulating the heat carrier medium at a constant rate, together with a solenoid valve V for controlling supply of the heat carrying medium to a heat exchanger in a hot water tank 306 alone or to both the tank and to radiators shown diagrammatically at 307 of a central heating system or to the radiators 307 alone. Control means 400 for the heating system is provided which includes controls for the heat pump system for controlling solenoid operated valves SV10, SV11, SV12, SV13 and SV6 in dependence on the level sensed by level sensors L1 and L2 and for controlling the thermal output of the system by control of the air and gas supplied to the boiler in dependence on the temperatures in the hot water tank, in the ambient air in the region of the central heating radiators, in the heat carrier medium flowing in the utilization circuit, in the boiler and in the boiler flue, which are sensed by temperature sensors STW, STA, T0, T2, T1 and T8, temperature sensors STW and STA for sensing the temperatures in the hot water tank and ambient air being thermostatic. It will be appreciated that the controls described above for the embodiment of Figure 3 can similarly be applied to the embodiment of Figures 1 and 2. When the thermal output required of the heating system exceeds that which can be provided by the heat pump system operating as a heat pump system, control 400 causes opening of valve WV to bring the heat exchanger 25 a into the utilization circuit together with the progressive increase in energy supplied to the boiler, i.e. the progressive opening of gas valve GV and increase in speed of air supply fan F, until the required thermal output is achieved. As with the system of Figures 1 and 2, if the thermal output required is sufficiently high, the temperature of the heat carrier medium of the utilization circuit flowing through the heat exchangers in the absorber and condenser will rise to a temperature at which the absorber and condenser cease to operate as such. The heat pump system then converts to a conventional boiler system. For a heat pump system which is designed to operate with a high pressure in the boiler of the order of 350 psi and an evaporator temperature of the order of 0°C, the temperature of the heat carrier medium leaving the heat exchanger 25 a , i.e. that sensed by temperature sensor T2, at which this will occur is about 60°C. The boiler of the heating system of Figure 3 is shown more fully in Figures 4 and 5. The boiler comprises a main body 200 provided in its lower region with a high temperature lining 201 and surrounded by thermal insulation 202. The body 200 provides a lower chamber 203 housing gas burners 204 to which gas is supplied via valve GV and air by fan F. Gases from the burners 204 rise into a heat exchange region 205 of the vapour generator 2 a , circulate in that region and exit through flue 16. In the heat exchange region 205, an array of tubes 206 is provided, the tubes being closed at their lower ends and open at their upper ends which communicate with the liquid space 207 in the body 200 to which strong liquid is supplied by duct 3. The weak liquid exits from the vapour generator along duct 4 which opens into the liquid space 207 above the level of the tubes 205. The lower region of the body 200 is generally cylindrical and the upper region providing a vapour rectifier 2 b is of reducing and reduced section. Vapour is withdrawn from the boiler along duct 6, the vapour having passed through the rectifying portion which, as is conventional, comprises rectifier trays 209. The rectifying heat exchanger 13 is provided at the upper end of the vapour space of the vapour rectifier and the supplementary condenser 25 with its heat exchanger 25 a is provided in the vapour space 208 above the level of the liquid in the vapour generator 2 a and below the rectifier trays 209. As shown in Figure 5, the heat exchanger 25 a is in the form of a pipe having a length which is determined by the maximum thermal output required of the supplementary condenser 25. As shown, to minimise the vertical extent of the vapour generator 2 a , the heat exchanger 25 a is in the form of a sinuous planar coil but it may alternatively have a plurality of coils extending upwardly within the vapour space 208 of the vapour generator. In the above described embodiments, the heat pump system is intended to operate using ammonia as the refrigerant and water as the absorbent but it will be appreciated that the invention is equally applicable to heat pump systems using other refrigerants and absorbents. There is thus provided a heating system comprising a heat pump system which can be arranged to operate as a heat pump system for the majority of the time at maximum efficiency but, in which, when the requirements on the thermal output of the system exceed a preset level, the thermal output of the heat pump system can be supplemented and it is only at the upper end of the range of operation of the system that the heat pump system may cease to operate as such and convert to a conventional boiler system.

Claims: A circulating-fluid heating system comprising an absorption heat pump system, a fluid heater (2) and a utilisation circuit for transporting a fluid heat-carrier therebetween, the absorption heat pump system including a refrigerant vapour generator (2a), and a refrigerant vapour rectifier (2b), the utilisation circuit having heat transfer means for transferring thermal energy from the heater (2) which includes at least one heat exchanger (25a) arranged to receive heat from the fluid heater (2) and having a switchable portion fitted with valve means (WV) which is operable by control means (400) to functionally couple the heat exchanger into the fluid utilisation circuit when the thermal output requirement of the system is above a predetermined level and to functionally disconnect the heat exchanger from the said circuit when the thermal output requirement of the system is below the predetermined level characterised in that the vapour generator (2a) and the vapour rectifier (2b) are contained by a closed vessel within the fluid heater (2), the said vessel defining a liquid space (207) and a vapour space (208), and the said switchable portion of the at least one heat exchanger (25a) comprises an utilisation circuit coil situated in the said vapour space (208) so as to be subject to vapour condensing conditions. A heating system as claimed in Claim 1, characterised in that the utilisation circuit comprises first and second heat exchangers (1a, 5a), associated with the absorber (1) and condenser (5) for withdrawing thermal energy therefrom and the switchable utilisation circuit coil (25a) is placed in series connection with the first and second heat exchangers (1a, 5a), by operation of a bypass valve (WV) in a bypass loop (27).

Inventors: PAGE, JOHN KENNETH RURIK
Applicants: BRITISH GAS PLC
Application year: 1986
Main classification: F24D3
IPC codes: F25B29, F24H4, F25B15, F25B33