GB2138121A - Sorption heat pumps - Google Patents

Abstract

An apparatus for pumping heat comprises two sorption heat pumps (1a, 1b), means for delivering heat from a low temperature heat source to the evaporator (5a) of the first heat pump (1a) and means (14a, 15a) for delivering to the load heat of sorption produced by the sorption of the evaporated sorbate by the sorbent (10a) in the sorber (3a) of the first heat pump (1a), means (12b, 13b) for delivering heat from and stopping the delivery of heat from a high temperature heat source to the sorber (3b) of the second heat pump (1b), means (19b, 20b) for delivering to the load heat of condensation evolved in the condenser (5b) of the second heat pump (1b) and means (21,22) for allowing sorbate to flow from the sorber (3b) of the second heat pump (1b) to the sorber (3a) of the first heat pump (1a). <IMAGE>

Description

SPECIFICATION Sorption heat pumps This invention relates to sorption heat pumps and the operation of a pair of them coupled together.

Since a sorption heat pump delivers heat to the load in two distinct steps it is advantageous if a pair of them is connected together 180 out of phase so that not only is the delivery of heatto the load at an intermediate temperature virtually continuous butthe supply of lowtemperature heat and of high tempera ture heat is virtually continuous. We have found that the C.O.P. of a pair of sorption heat pumps coupled together 180 out of phase can be improved by introducing a step wherein sorbate vapour is allowed to flowfrom the sorber of pump which has been desorbing to the sorber of the pump which has been sorbing.

Asorption heatpump process involving the use of two sorption heat pumps according to this invention is one wherein in the first step whilst lowtemperature heat is supplied to evaporate sorbate from the evaporator of the first pump and heat is delivered to the load from the sorberofthefirst pumpdueto sorption at the evaporation vapour pressure of evaporated sorbate by the sorbent, high temperature heat is being supplied to the sorber of the second pump for desorption of sorbate from the sorbent and heat of condensation caused by the condensation of desorbed sorbate in the condenser ofthe second pump is delivered to the load.In the second step after desorption of sorbate from the sorbent in the sorber of the second pump has been completed attheconde- nservapour pressure, the supply of high temperature heatto the sorbent in the sorber of the second pump is stopped and sorbate vapour is allowed to flow at a pressure intermediate between said evaporation vapour pressure and said condenservapour pressure from the sorber of the second pump to the sorber of the first pump whilst heat of sorption is delivered to the load.

In this specification the terms sorber, sorption, sorbent and sorbate includethetermsadsorber, adsorption, adsorbent and adsorbate and include the equivalent univarient heat pump where the adsorbate is not merelyadsorbed bytheadsorbatebutcombines chemically.

A suitable sorbent/sorbate system for use in the sorption heat pump is a zeolite molecular sieve/water.

Alternatively, calcium chloride/ammonia or an aluminium silicate zeolite molecular sieve/water can be used.

The preferred adsorbent/adsorbate system is Yzeolite molecular sieve/water. Alternatively 4A-zeolite molecular sieve/water, 3A-zeolite molecular sieve/ watercan be used.

A suitable heattransferfluid 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 the first step ofthe process ofthe invention heat is delivered to the evaporator ofthefirst heat pump from a low temperature heat source. This lowtemperature source will be at the temperaturefrom which it is desired to pump heat to a highertemperature. This source can be for example water at ambienttempera- ture circulating through a heatexchangeriocated in the evaporator/condenser. This heat supplied to the evaporator results in vaporisation ofthe sorbate which is then sorbed atthe evaporation vapour pressure by the sorbent in the sorber of the first heat pump. The heatofsorption thereby produced at least in part is delivered to the load.

Whilst th is is going on heat from a high temperature heat source is supplied to the sorbent in the sorber of the second heat pump. Such a heat source may for example be an electrical resistive heater. Alternatively the heat may be supplied bythe circulation through a heat exchanger of heattransferfluid heated by a boiler. This supply of heat causes desorption of sorbateatthecondenservapourpressure.Thethus vaporised sorbate condenses and the heat of condensation thereby caused at least in part is delivered to the load.

The coefficient of performance (C.O.P.) (heating) of a basic sorption heat pump using a divarient pair such as water/zeolite is very much dependent on the temperature liftwhich is required. As the temperature lift increases so does the residual water content ofthe zeolite after regeneration to a fixed peak temperature.

The C.O.P. is dependent upon the swing in water content ofthe zeolite over the cycle of sorption and desorption and upon the sensible heat stored in the desorbed zeolite at the end of the desorption step. As the load temperature risesthe maximum water content ofthe zeolite falls and the minimum water content rises and consequently the C.O.P. is reduced.

In order to increase the C.O.P. the water content of the desorbing unit should be decreased at the conclusion ofthe desorption step and the water content ofthe sorbing unit increased. This is achieved by the second step ofthe process of the invention wherein after desorption of sorbate from the sorbent inthesorberofthesecond pump has been completed the supply of high temperature heatto the sorbent in the sorber of the second pump is stopped and sorbate vapour is allowed to flowfrom the sorber of the second pump to the sorber of the first pump.This causes the water content of the sorbent in the sorber ofthe second pump to decrease and the water content ofthe sorbent in the sorber of the first pump to increase by substantially the same amount as the decrease ofthe water content ofthe sorbent in the sorber ofthe second pump. Also the temperature of the sorbent inthesorberofthe second pump decreases. Howeversince heatofsorption (duetothe sorption of sorbate by the sorbent in the sorber ofthe first pump) is also delivered to the load the temperature of the sorbent in the sorber of the first pump remains substantially constant.

The flow of sorbate vapourfrom the sorber ofthe second pump to that ofthefirst pump may be achieved simply be interconnecting the two sorbers bya valved conduit. When the valve is opened the sorbate vapour will flow from the sorber ofthe second pump which is at a higher pressure than that of the first pump. Flow will continue until the two pressures equalise at an intermediate pressure and when this stage is reached there will be no more desorption of sorbatefrom the sorber ofthe second pump and no more sorption of sorbate bythe sorbent in the sorber ofthefirst pump.

After this second step of the process has been completed the whole process (i.e. step 1 followed by step 2) can be repeated indefinitely and the heat flows will be more or less continuous. However it will be appreciated that the roles of the heat pumps will be reversed; i.e. the pump that is sorbing in the first step will now be desorbing and vice-versa.

In the process ofthis invention exceptforthe transfer of sorbatevapourfrom thesorberofthe second pump to that ofthe first pump in the second step, all the heattransferand deliveries are usually by indirect heat exchange.

The various steps of the process can be initiated by microprocessors respondent to temperature and/or pressure signals. Thus, in the first step when the desorption has ceased the temperature ofthe sorber will be atthe designed maximum and the microprocessorwill send a signal causing the supply of external high temperature heat to be switched off and forflow of sorbatevapourfromthe sorber ofthe second pump to that of the first pump to commence, e.g. by causing a valve in a connecting conduitto open.This is allowed to carry on until the pressures in the two sorbers equalise.When this happens a pressure sensitive microprocessorcan signal a cessation offlowofsorbatefrom one sorberto the other, e.g. by closing the valve, and the first step ofthe process can be initiated, i.e. the commencement of the supply of lowtemperature heatto the evaporator of the second pump and the commencement of the supply of high temperature heat to the sorber ofthe first pump.

Apparatusforcarrying outthe process ofthe invention comprisestwo sorption heat pumps, means for delivering heatfrom a lowtemperature heat source to the evaporator of the heat pump, means for delivering to the load heat of sorption produced bythe sorption of the evaporated sorbate by the sorbent in the sorber ofthefirst heat pump, means for delivering heatfrom and stopping the delivery of heat from a high temperature heat source to the sorbet ofthe second heat pump and means for delivering to the load heat of condensation evolved in the condenser of the second heat pump.There are also means for allowing sorbate to flowfrom the sorber ofthe second heat pump to the sorber ofthefirst heat pump.

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/ condenserand means for evaporating condensed adsorbate in the evaporator/condenser so that it can be adsorbed bythe adsorbent housed in the adsorber.

The adsorption heat pump can be a series of trays one abovethe other housing adsorbentbelowwhich is the condenser/evaporator. Such a form of adsorption heat pump is described in otircopending patent application, GB.8222333. In this form of adsorption heat pump adsorbate when desorbedfrom the adsorbent is allowed to condense in the condenser evaporator. Atanotherstage in the process of using this adsorption heat pump, when heat is supplied to the condenser/evaporatorthe 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 meansfortransferring heat in the apparatus of the invention are preferably heat exchangers. There are heat exchangers associated with the two units of the sorption heat pumps. Preferably these heat exchangers are located inside each unit. Thus, for example a heat exchanger is located within the condenser ofthe sorption heat pump ratherthan outside this condenser. Conduits and where necessary, valves and pumps are used to interconnect the various heat exchangers. A valved conduit may be used to interconnectthe sorbers ofthe two heat pumps.

Preferably there is a header tank connected to the conduits interconnecting the heat exchangers. This tank contains heattransferfluid and this will enable any lost heattransfer fluid in the system to be replenished and also to allowforexpansion and contraction offluid in use due to temperature changes.

Sincewatercannot usually be conveniently used as the heattransferfluid in the apparatus of the invention, it is preferable that the heat provided by the system fordeliveryto the load is supplied to a heat exchanger and thatthis heat exchanger delivers heat two a load heat exchangerthroughwhich water can flowand which can deliver heattothe domestic heating system, etc.

An apparatus involving two adsorption heat pumps coupled together in accordance with the invention is described with reference to the drawings.

Thetwoadsorptionheatpumpslaand leach comprises four main sections. There are headertanks 2a and 2b, adsorbers 3a and 3b, sprinklers 4a and 4b and evaporator/condensers 5a and 5b.

Interconnecting each condenser and each header tank are respectively conduits 6a and 6b, each provided with a circulation pump 7a and 7b. There are also conduits 8a and 8b provided with valves 9a and 9b respectively for interconnecting the header tanks 2a and 2b with the respective sprinklers 4a and 4b.

The adsorbent used in this particular case is 4A-zeolite molecular sieve and this is provided intrays as described in copending application GB 8222333. In the drawing it is shown at 1 0a and 1 0b,thatrays being omitted to simplify the drawing. The adsorbate is water.

Each adsorber houses a heat exchanger (11 a and 11 b) which can be used both to supply high temperature heatto the adsorbent and to deliver heatto the load L. There are valved conduits 1 2a, 13a, 1 2b and 13bforconnectingthese heat exchangers 11a and 11b to the source of high temperature heat and valved conduits 14a, 1 Sa, 1 4b and 15b connecting these heat exchangers to the load L.

Each evaporator/condenser houses a heatexchan- ger (16a and 1 6b) which can be used both to supply lowtemperature heat to the evaporator and to supply heat from the condenser to the load L. There are valved conduits 17a, 18a, 17b and 1 8b for connecting the heat exchangers 1 6a and 1 6b to the source of low temperature heat and valved conduits 1 9a, 20a, 1 9b and 20b for connecting these heat exchangers 1 6a and 16b to the load L.

There is a conduit 21 with a valve 22 which connects the adsorber 3a with the adsorber 3b.

Heattransferfluid Dow Syltherm 800 flows through conduits 12a,13a, 14a, 15a, 12b, 13b, and 15b.

These conduits are connected to a headertank (not shown to simplifythe drawing) which contains a reserve supply of Dow Syltherm 800.

The apparatus operates as follows.

In the first step of the process valve 22 is closed as are the valves in conduits 1 9a, 20a, 1 2a, 1 3a, 17b, 18b, 14b and 1 Sb. Valve 9b is also closed and the valves in conduits 14a, 1 Sa, 17a, i8a, 1 2b, 13b, 19b and 20b are open.Valve 9a is opened and this allows waterto flow from the headertank2athrough conduit8a and into the evaporator 5a via the sprinkler 4a. Low tempera- tureheatobtained by the circulation of an aqueous ethylene glycol solution or other non-corrosive anti freeze solution atambienttemperaturethroughthe heat exchanger 16a and the connecting conduits 1 7a and 18a causes evaporation ofthewater in the evaporator5a and this rises and is adsorbed bythe 4A-zeolite molecular sieve 1 0a in the trays in the adsorber 3a.This causes heat of adsorption which by heat exchange through heat exchanger 11 a causes heat to flow to the load Via the heattransferfluid in conduits 14a and 15a.

Whilstthis is going on in the heat pump 1a, high temperature heat by circulation of Dow Syltherm 800 at300 Cthrough conduits 12b and 13b issuppliedto the heat exchanger 11 b. (The heattransferfluid, Dow Syltherm 800, may be heated by an external boiler).

The host fluid circulating through the heat exchanger 11 b causes waterto desorb from the 4A-zeolite molecular sieve lOb in the trays in the adsorber3b and the watercondenses and drains into the conde nser5b.Thewaterdraining into the condenser 56 is recirculated by pump 7b through conduit6bto the headertank2b.

When the water condenses in the condenser 5b it gives up its latent heat of condensation to an aqueous ethylene glycol solution flowing through heat exchanger 1 6b and conduits 19b and 20b and hence heat is delivered to the load L.

In this first step therefore heat is being delivered to the load from the adsorber 1 0a via heat exchanger 1 7a and conduits 14a and 15a andfromthecondenser Sb via heat exchanger 16b and conduits 19b and 20b.

When the desorption of water from the 4A-zeolite molecular sieve in the trays of the adsorber 3b has been completed, the valves in conduits 1 2b and 1 3b are closed so that no more high temperature heat is delivered to the adsorbent 1 Ob in adsorber3b. Also thevalves in conduits 19b,20b,17a and 18a andvalve 9a are closed. Atthe sametimevalve 22 in conduit 21 is opened and watervapourflowsfrom adsorber3b toadsorber3a,the pressure being higher in the former than the latter. Water vapour continues to flow until the pressures in the two adsorbers equalise and when this stage is reached there will be no more desorption ofwaterfrom adsorber3b and no more sorption of water in adsorber 3a.

The water adsorbed by the 4A-zeolite molecular sieve 1 0a in the adsorber 3a causes heat of adsorption which is delivered to the load L by means of heat transfer fluid flowing through conduits 1 4a and 1 5a and heat exchanger 1 lea.

When this stage is reached the first step can be repeated exceptthatthe roles ofthe two heat pumps are reversed because at the end ofthe second step, the adsorbent 10a in adsorber 3a is fully charged with adsorbate (water) and the headertank2b is charged with adsorbate (water). At this stage therefore the valves in conduits 17b, 18b, 12a, 13a, 19a and 20a, 14b and 15 are opened. Valve 22 is closed as arethevalves in conduits 14a and 15a. Valve 9b is opened so that water can flowfrom the headertank 2b through conduit 8b into the evaporator 5b via the sprinkler4b.

High temperature heat is applied via conduits 1 2a and 13a to the adsorber 3a and lowtemperature heat supplied via conduits 17b and 18bto the evaporator 5b and the first step repeated exactly except that the roles of the two heat pumps are reversed.

Atthe completion of this step the valves in conduits 1 2a and 13a are closed as are the valves in conduits 19a, 20a, 17b and 18b. Also valve 9b is closed and at the same valve 22 in conduit 21 is opened. Water vapour in this case flows from adsorber 3a to adsorber 3b but otherwise the procedure for this second step is the same as previously described exceptforthe reversal of the roles of heat pumps 1 a and 2a.

Thus it can be seen that the first and second steps can be repeated indefinitely with the two heat pumps alternating their roles and a virtually continuous supply of heat being delivered to the load.

The effect of the process of this invention on the C.O.P. ofthe heat pump can be demonstrated as follows.

With a commercial 4A-zeolite molecular sieve and pumping heat from 5"C to 90 C and when the desorptiontemperature is limited to 315 Cthen the maximum water content of the zeolite is 15% whilst the minimum water content is 6% by weight On average the heat of adsorption will be 1.4 times the heat ofvaporisation of water.

In normal operation with a swing of 15 to 6 wt.% in water content the heat flows per pound of water evaporated are as follows: wt zeolite/lb water = 1 = 11.1 Ibs 0.09 Heat content of zeolite plus 6% wtwater above 90 C after regeneration, (0.22 being the specific heat of zeolite and 1.8 being the conversion factor"C-"F).

= 91.1 x 0.22 x (31590) x 1.8 + .06 x 11.1 x (315-90) x 1.8 = 989 + 270 Btu/lb water evaporated.

Assuming the weight of the copper structure is about 20% by weightthat ofthe zeolite with the specific heat of copper being 0.9 the sensible heat =11.1 x0.2x.09(315-90)x 1.8 = 81 Btu/lb Average temperature of desorption = 250"C Sensible heat supplied to desorbed water = (250-90) x 1.8 = 288 Btu/lb water evaporated Latent heat of evaporation at 90 C =981.6 Btu/lb From tables the heat of desorption is 1.4 x latent heat of evaporation, so Heatofdesorption = 1374Btu/lb Total heatsuppliedfordesorption = 989 + 270 + 288 + 1374 = 3002 Btu/lb Latent heat of evaporation &commat;; 5 C = 1064 Bti/lb Heat delivered/lb water = total heat of desorption + heat of evaporation C.O.P. = 3002 + 10644 1.35 (saving 26% of fuel) 3002 (The saving of fuel is 1- 1 C.O.P.

Atan intermediate pressure of 55 torrthe water contentofthesorbing zeolite risesto 17.5% by weight whilstthat ofthe desorbing zeolite falls to 3.5% by weight at 240"C.

The gain in sorption capacity at the evaporating pressure (6-3.5 wt.%) is 2.5% by weight and the weight ofzeolite/lb of water = 0.115 = 8.7 Ib/lb Sensible heat available for 2nd stage desorption = 8.7 x .22 x (315 - 240) x 1.8 + .06 x 8.7 (315 - 240) x 1.8 =258+70.5+21 = 350 Btu Sensible heat required xT x 1035 1.4 approx =315Btu The system is approximately in thermal balance at the end ofthe first desorption step and sensible heat stored above 90"C.

= 8.7 x 0.22 (315-90) x 1.8 + .06 x 8.7 x (315-90) x 1.8 + 0.2 x 8.7 x .09 x (315-90) x 1.8 = 1050 Btu/lb water Average desorption temperature at stage 1 = 250"C.

Sensible heat supplied to desorbed water.

9 (11.5% water now = (250-90) x 1.8 x 1.b desorbed instead of 9% water) = 225 Btu/lb Latent heat of evaporation supplied at stage 1 9 = 1374 x 11.5 = 1075 Btu/lb Total heat input/lb water = 1050 + 225 + 1075 = 2350 Btu Heat pumped (latent heat of evaporation of water at 5"C) = 1064 Btu 2350 + 1064 C.O.P. = 2:ibU = 1.45 (31 % saving in fuel) 18% increase in fuel saved.

Claims (6)

1. Asorption heat pump process involving the use oftwo sorption heat pumps wherein in the first step whilst lowtemperature heat is supplied to evaporate sorbate from the evaporator ofthe first pump and heat is delivered to the load from the sorber ofthefirst pump due to sorption at the evaporation vapour pressure of evaporated sorbate by the sorbent, high temperature heat is supplied to thesorberofthesecond pumpfordesorption of sorbate from the sorbent and heat of condensation caused by the condensation of desorbed sorbate in the condenser ofthe second pump is delivered to the load, and wherein after desorption of sorbate from the sorbent in the sorber of the second pump has been completed atthe condenser vapour pressure in the second step, the supply of high temperature heat to the sorbent in the sorber ofthe second pump is stopped and sorbate vapour is allowed to flow at a pressure between said evaporation vapour pressure and said condenservapour pressurefromthe sorber ofthesecond pump to the sorber ofthefirst pump whilstheatofsorption is delivered to the load.

2. A process according to claim 1 wherein the adsorbent is 4A-zeolite molecularsieve and the adsorbate is water.

3. A process according to either of claims 1 and 2 wherein exceptforthe transfer of sorbate vapour from the sorber of the second pump to that ofthe first pump in the second step, the heattransfer and deliveries are by indirect heat exchange.

4. An apparatus for pumping heatwhich compris estwosorption heat pumps, meansfordelivering heatfrom a low temperature heat source to the evaporator of the firstheat pump, means for delivering to the load heat of sorption produced by the sorption ofthe evaporated sorbate by the sorbent in the sorber ofthe first heat pump, means for delivering heat from and stopping the delivery of heatfrom a high temperature heat source to the sorber of the second heat pump, means for delivering to the load heat of condensation evolved in the condenser of the second heat pump and means for allowing sorbate to flowfrom the sorber of the second heat pump to the sorberofthefirst heat pump.

5. An apparatus according to claim 4wherein each heat pump is an adsorption heat pump comprising a series of trays one above the other housing adsorbent, belowwhich is the condenser/evaporator.

6. An apparatus according to claim 4substantially as hereinbefore described with reference to the drawings.