GB2130713A - Staged heat pumps - Google Patents

Abstract

A method of operating a sorption heat pump provided with a heat store comprises alternate steps (a) and (b). Step (a) comprises delivering heat to the evaporator (3) of the heat pump (1) from a low temperature heat source (5) and delivering to the heat store (12) heat of sorption produced by the sorption of the evaporated sorbate, delivering heat from a high temperature heat source (4) to the sorber (2) of the heat pump (1) and delivering to the load (35) heat of condensation of the thus evaporated and subsequently condensed sorbate. Step (b) comprises delivering to the load (35) sensible heat from the sorber (2) and then delivering heat from the heat store (12) to the evaporator (3) of the heat pump (1) wherein sorbate evaporates and is sorbed in the sorber (2) and delivering to the load (35) heat of sorption produced in the sorber (2). <IMAGE>

Description

SPECIFICATION Staged heat pumps This invention concerns the operation of sorption heat pumps.

A factor limiting the utility of sorption heat pumps is the temperature lift between the evaporator and the sorber at useful sorption rates.

When calcium chloride and methanol are used as sorbent and sorbate this temperature is limited to about 40-450C. It is possible to deliver the heat of condensation of the sorbate at a higher temperature than the sorption temperature and the sensible heat stored in the sorber can usually be delivered at a higher temperature. However, the heat of sorption which can account for about half of the heat delivered to the load cannot normally be provided at temperatures suitable for the operation of a piped hot water central heating system in cold weather.

In accordance with this invention such difficulties are substantially overcome by the method of operating a sorption heat pump which is provided with a heat store which comprises (a) delivering heat to the evaporator of the heat pump from a low temperature source and delivering to the heat store heat of sorption produced by the sorption of the evaporated sorbate. Thereafter heat from a high temperature heat source is delivered to the sorber of the heat pump and heat of condensation of the thus evaporated and subsequently condensed sorbate is delivered to the load. Alternately to step (a) above in a step (b) alternately (i) sensible heat from the sorber is delivered to the load and (ii) heat from the heat store is delivered to the evaporator of the heat pump wherein sorbate evaporates and is sorbed in the sorber and heat of sorption produced in the sorber is delivered to the load.

In this specification the terms sorber, sorption, sorbent and sorbate include the terms adsorber, adsorption, adsorbent and adsorbate and include the equivalent univarient heat pump where the adsorbate is not merely adsorbed by the adsorbate but combines chemically.

The preferred sorbent/sorbate system for use in the sorption heat pump is calcium chloride/ methanol. Alternatively, calcium chloride/ ammonia or an aluminium phosphate zeolite molecular sieve/water can be used.

A suitable heat transfer fluid for use in the heat exchangers and in the connecting conduits is a low vapour pressure liquid, for example a silicon liquid, e.g. Dow Syltherm 800. Other suitable fluids are thermally stabilised aromatic hydrocarbons.

In step (a) of the process of the invention heat is delivered to the evaporator of the heat pump from a low temperature heat source. This low temperature source will be at the temperature from which it is desired to pump heat to a higher temperature. This source can be for example water at ambient temperature circulating through a heat exchanger located in the evaporator/condenser. This heat supplied to the evaporator results in vaporisation of the sorbate which is then sorbed by the sorbent in the sorber of the heat pump. The heat of sorption thereby produced at least in part is transferred to the heat store. This results in the melting of the substance housed in the flexible receptacles when the heat store is a latent heat of fusion heat store.

Thereafter in step (a) heat from a high temperature heat source is delivered to the sorber of the heat pump. Such a heat source may for example be an electrical resistive heater.

Alternatively the heat may be supplied by the circulation through a heat exchanger of heat transfer fluid heated by a boiler.

The thus vaporised sorbate condenses and the heat of condensation at least in part is delivered to the load.

In step (b) of the process which is carried out alternately to step (a) alternately (i) sensible heat from the sorber is delivered to the load and (ii) heat from the heat store is delivered to the evaporator of the heat pump wherein sorbate evaporates and is sorbed in the sorber and heat of sorption produced in the sorber is delivered to the load. After the last action of step (a) i.e. delivering heat to the sorber one is left with a hot sorber.

Hence in stage (i) this sensible heat at least in part is delivered to the load. After this sensible heat has been delivered in alternate stage (ii) the heat store is used to provide heat, heat having been supplied to the heat store in step (a), and this is delivered to the evaporator of the heat pump. As a result sorbate evaporates and is sorbed in the sorber and the heat of sorption thereby produced is at least in part delivered to the load.

In the process of this invenion all the heat transfers and deliveries are usually by indirect heat exchange.

Apparatus for carrying out the process of the invention comprises a sorption heat pump, a heat store, means for delivering heat from a low temperature heat source to the evaporator of the heat pump, means for delivering to the heat store heat of sorption produced by the sorption of the evaporated sorbate by the sorbent, means for delivering heat from a high temperature heat source to the sorber of the heat pump and means for delivering to the load heat of condensation of the thus evaporated and subsequently condensed sorbate. There are also means for alternately (i) delivering to the load sensible heat from the sorber and (ii) for delivering heat from the heat store to the evaporator of the heat pump and means for delivering to the load heat of sorption produced in the sorber.

An adsorption heat pump usually comprises an adsorber and an evaporator/condenser. There are means for desorbing adsorbate adsorbed by the adsorbent housed in the adsorber, there are means for condensing the desorbed adsorbate in the evaporator/condenser and means for evaporating condensed adsorbate in the evaporator/condenser so that it can be adsorbed by the adsorbent housed in the adsorber.

The adsorption heat pump can be a series of trays one above the other housing adsorbent below which is the condenser/evaporator. Such a form of adsorption heat pump is described in our copending patent application, GB.8222333. In this form of adsorption heat pump adsorbate when desorbed from the adsorbent is allowed to condense in the condenser/evaporator. At another stage in the process of using this adsorption heat pump, when heat is supplied to the condenser/ evaporator the condensed adsorbate is evaporated and this rises and is adsorbed by the adsorbent housed in the trays. The heat of adsorption can be transferred elsewhere by means of a heat exchanger associated with the trays.

The sorption heat pump used when the sorbate combines chemically with the sorbent is usually of the same construction as the adsorption heat pump.

The heat store which is used in conjunction with the heat pump is preferably a latent heat of fusion heat store. Such heat stores usually comprise a plurality of flexible receptacles housing a substance with a relatively low melting point and a relatively high heat of fusion, for example sodium sulphate decahydrate (mp 320C), calcium chloride hexahydrate (mp 290C), sodium carbonate decahydrate (mp 330C), sec sodium phosphate heptahydrate (Na2HP047H20) (mp 480C) sodium thiosulphate pentahydrate (mp 480C) or stearic acid (mp 680C). When heat is supplied to the heat store the substance with the relatively low melting point which initially is solid melts, the heat of fusion coming from the heat supplied externally.Heat is stored in this substance housed in the receptacles and when heat is abstracted from the heat store the substance freezes, evolving latent heat of fusion.

The means for transferring heat in the apparatus of the invention are preferably heat exchangers. There are heat exchangers associated with the two units of the sorption heat pump.

Preferably these heat exchangers are located inside each unit. Thus, for example a heat exchanger is located within the condenser of the sorption heat pump rather than outside this condenser. Conduits and where necessary, valves and pumps are used to interconnect the various heat exchangers.

Preferably there is a header tank connected to the conduits interconnecting the heat exchangers.

This tank contains heat transfer fluid and this will enable any lost heat transfer fluid in the system to be replenished and also to allow for expansion and contraction of fluid in use due to temperature changes.

Since water cannot usually be conveniently used as the heat transfer fluid in the apparatus of the invention, it is preferable that the heat provided by the system for delivery to the load is supplied to a heat exchanger and that this heat exchanger delivers heat to a load heat exchanger through which water can flow and which can deliver heat to the domestic heating system, etc.

Since heat is being delivered intermittently to the load it is preferable if the load itself is provided with a heat store so that the supply of heat to the ultimate load, e.g. domestic water heaters and radiators, can be made more uniform.

The method of using the sorption heat pump and apparatus of this invention should enable one to pump heat satisfactorily from --50C to +600C.

At the beginning and end of the domestic heating season when conditions are mild and only partload operation is required, the use of a low temperature heat store as a buffer between the heat pump and the heat collector (load) will ensure that the evaporator temperature is always maintained at a reasonable level, say 1000, When this is so a sorption temperature of say 550C can be quite useful and the balance of the heat can be delivered at a somewhat higher temperature, if required. In the depths of winter however when the ambient temperature falls below OOC a sorption temperature of only 400C will not be very useful.In these circumstances the intermediate temperature heat store, e.g. of the hydrated salt type, as provided by the process and apparatus of the invention will be used to store the heat of sorption during one cycle so that it may be fed to the evaporator during the next cycle.

Since only about 70% of the heat delivered to the evaporator during the staged sorption step will have come from the low temperature source and since two desorptions will be required in order to deliver it to the load, the coefficient of performance (C.O.P.), in this mode of operation will be reduced from 1.60 to 1.20. However, since the sorption rate increases as the temperature lift is reduced and since the desorption rate may be increased as required, a heat pump operating in this manner can be made smaller and cheaper than one designed to meet maximum load in normal operation at a lower heat delivery temperature. It is likely therefore that the seasonal C.O.P. can be held in the region of 1.4 in the U.K.

with a maximum heat delivery temperature in the region of 700 C.

An apparatus suitable for carrying out the process of the invention is shown schematically in the accompanying drawing.

A sorption heat pump is shown at 1 with a sorber section 2 and an evaporator/condenser section 3. Heat exchanger 4 is capable of supplying heat to the sorbent (calcium chloride) located in trays in the sorber 2 and the sorbate is methanol. Low temperature heat is supplied to the evaporator/condenser section 3 by means of a heat exchanger 5. Also located in the evaporator/ condenser section is a heat exchanger 6 capable of supplying heat to a heat exchanger 35 via conduits 7 and 8. Heat exchanger 35 exchanges heat with the load heat exchanger 36 through which water circulates, enabling heat to be transferred to the domestic hot water system, etc.

There is a heat exchanger 9 located in the sorber section 2 and this connects via conduits 10 and 11 to a latent heat of fusion heat store 1 2 containing flexible bags 1 3 containing sodium sulphate decahydrate, each bag being immersed in the heat transfer fluid 14 which circulates through the conduits of the apparatus and which in this case is Dow Syltherm 800.

Conduits 14 and 1 5 connect the heat store 12 with conduits 7 and 8 and conduits 1 6 and 1 7 connect respectively conduits 8 and 11 and 10 and 15.

There is a header tank 1 8 housing the heat transfer fluid Dow Syltherm 800 and this tank 1 8 is connected by conduit 1 9 to conduits 7 and 10.

There are valves 20 and 21 in conduits 10 and 11, valves 22 and 23 in conduits 8 and 7, valves 24 and 25 in conduits 14 and 15 and valves 26 and 27 in conduits 16 and 1 7.

There are pumps 28, 29,30 and 31 in conduits, 10, 1 5, 1 7 and 7 respectively.

The apparatus operates as follows: With valves 22, 23, 24,25, 26 and 27 closed and valves 20 and 21 open and pump 28 operating and pumps 29,30 and 31 inactive low temperature heat is supplied to evaporator 3 by means of the circulation of a calcium chloride solution at ambient temperature through heat exchanger 5. This causes evaporation of methanol in the evaporator 3 and the vaporised methanol is sorbed by the calcium chloride in the sorber section 2. The heat of sorption thereby produced is transferred by heat exchange to the heat store 12. This is achieved by the circulation by pump 28 of the heat transfer fluid Dow Syltherm 800 through conduits 10 and 11 connected to the heat exchanger 9 and heat store 12.

The supply of heat to the heat store 12 causes the sodium sulphate decahydrate to melt in the bags 13.

When the sorption is finished, valves 20 and 21 are closed, pump 28 inactivated and valves 22 and 23 opened at the same time as heat from the high temperature source is supplied to the sorber by the circulation of Dow Syltherm 800 at 3000C through the heat exchanger 4. Valves 24, 25, 26 and 27 remain closed and pumps 29 and 30 remain inactive. This causes the methanol to be desorbed in the sorber 2 and to condense in the condenser 3. The heat of condensation is transferred to the load by circulation of the heat transfer fluid Dow Syltherm 800 through conduits 7 and 8 by means of pump 31 which is now activated.

After the heat of condensation has been transferred to the load valves 22 and 23 are closed and valves 26 and 27 are opened. Valves 20,21,24 and 25 remain closed. Pump 30 is activated, pump 31 inactivated and pumps 28 and 29 remain inactivated. By this means sensible heat from the sorber section 2 is transferred to the heat exchanger 35 and hence to the load heat exchanger 36, via heat exchanger 9 and conduits 10,17,15,7,8, 16 and 11. When this transfer of heat has been completed valves 24, 25, 26 and 27 are opened and pumps 29 and 31 activated.

Valves 26 and 27 remain open, valves 20, 21,22 and 23 remain closed, pump 30 remains activated and pumps 28 and 31 remain inactivated. At this stage heat is being transferred from the heat store 12 to the heat exchanger 6. This causes the temperature of the heattransferfluid circulating in the conduits 14 and 1 5 to drop and the sodium sulphate decahydrate in the bags 1 3 will freeze, giving up its latent heat of fusion.

The heat delivered to the heat exchanger 6 causes the sorbate (methanol) to evaporate in the evaporator section 3 and this is sorbed by the calcium chloride sieve in the sorber 2. Heat of sorption is evolved and this is transferred via heat exchanger 9 and heat transfer fluid circulating through pumps 30 and 31 in conduits 10, 11, 16, 1 7, 7 and 8 to the heat exchanger 36. When this transfer of heat has been completed the cycle of operations can begin again and the cycle repeated indefinitely.

Automatic means, e.g. temperature sensors and microprocessor can be provided for opening and shutting the valves when required.

Heat can therefore be delivered from as low as about -50C to a temperature of about 500C.

Claims (10)

1. A method of operating a sorption heat pump being provided with a heat store which comprises alternately (a) delivering heat to the evaporator of the heat pump from a low temperature heat source and delivering to the heat store heat of sorption produced by the sorption of the evaporated sorbate, delivering heat from a high temperature heat source to the sorber of the heat pump and delivering to the load heat of condensation of the thus evaporated and subsequently condensed sorbate and (b) alternately delivering to the load sensible heat from the sorber and (ii) delivering heat from the heat store to the evaporator of the heat pump wherein sorbate evaporates and is sorbed in the sorber and delivering to the load heat of sorption produced in the sorber.

2. A process according to claim 1 wherein the sorbent is calcium chloride and the sorbate is methanol.

3. A process according to either of claims 1 and 2 wherein the high temperature heat source is an electrical resistive heater.

4. An apparatus suitable for carrying out the method according to any one of the preceding claims which comprises a sorption heat pump, a heat store, means for delivering heat from a low temperature heat source to the evaporator of the heat pump, means for delivering to the heat store heat of sorption produced by the sorption of the evaporated sorbate by the sorbent, means for delivering heat from a high temperature heat source to the sorber of the heat pump and means for delivering to the load heat of condensation of thus evaporated and subsequently condensed sorbate, means for alternately (i) delivering to the load sensible heat from the sorber and (ii) for delivering heat from the heat store to the evaporator of the heat pump and means for delivering to the load heat of sorption produced in the sorber.

5. An apparatus according to claim 4 wherein the sorption heat pump comprises a series of trays one above the other housing adsorbent, below which is the condenser/evaporator.

6. An apparatus according to either of claims 4 and 5 wherein the heat store is a latent heat of fusion heat store.

7. An apparatus according to claim 6 wherein the substance with a relatively low melting point and a relatively high heat of fusion is sodium sulphate decahydrate.

8. An apparatus according to any one of claims 4 to 7 which includes a header tank connected to conduits interconnecting heat exchangers, said heat exchangers and conduits being provided to transfer heat to and from the heat pump and the heat store and to the load.

9. An apparatus according to any one of claims 4 to 8 wherein the load includes a heat store.

10. An apparatus according to claim 4 substantially as hereinbefore described with reference to the drawing.