GB2114730A - Absorption heat pump - Google Patents

Abstract

An absorption heat pump comprises a generator 22, a condenser 9, an expansion valve 33, an evaporator 34, an absorber 19, a sorber 1 containing sorbent, a second evaporator 4, means 2 for supplying heat intermittently to sorber 1 to vaporise sorbate from the sorbent, means 7,37 for effecting heat exchange between the sorbate and generator 22 and for passing condensed sorbate to evaporator 4, means 11 for passing heat to evaporator 4 from condenser 9 or absorber 19 to vaporise sorbate which is then sorbed in the sorbent in sorber 1, and means 3, 7 for transferring the heat of sorption to the generator 22. <IMAGE>

Description

SPECIFICATION Heat pump This invention concerns a heat pump which can in fact be a converted chiller and a method of pumping heat.

There are a number of ways wherein heat operated heat pumps may be constructed which, by the use of staged operation, will pump heat from --50C to 600C with coefficients of performance (C.O.P's) greater than 2.0. It is however especially advantageous if existing designs of water chiller can be converted to staged operation without any major modifications. This can be done by plumbing in an adsorption or sorption heat pump into the heat input heat exchanger.

According to this invention a heat pump comprises an absorption system having a condenser, an evaporator, an absorber and a generator. There is a sorber and means for supplying heat intermittently to the sorber so as to vaporise sorbate from sorbent housed in the sorber. There is a conduit connecting the sorber with a heat exchanger capable of exchanging heat with liquid housed in the generator. There is a conduit connecting said heat exchanger with a secondary evaporator and means for supplying heat to the secondary evaporator either from the heat of condensation produced in the condenser or from the heat of absorption produced in the absorber. There is a conduit connecting the secondary evaporator with the sorber means for transferring heat of sorption produced in the sorber to the liquid in the generator.There are also means for delivering heat to the load from the heat of absorption produced in the absorber and from the heat of condensation produced in the condenser.

The process of pumping heat involving an absorption system is one wherein intermittently heat is supplied to a sorber to vaporise sorbate from sorbent housed therein and vaporised sorbate passes to and condenses in a heat exchanger capable of exchanging heat with liquid housed in the generator. Thereafter condensed sorbate is transferred to a secondary evaporator housing liquid sorbate, the level of which is thereby raised. Heat is obtained for delivery to the load from the condenser and the absorber. After this, the supply of heat to the sorber is stopped and the secondary evaporator is heated either by transfer of heat of condensation from the condenser or by transfer of heat of absorption from the absorber.In this manner sorbate is evaporated from the secondary evaporator, the evaporated sorbate is sorbed by the sorbent in the sorber and the resulting heat of sorption is used to heat the liquid in the generator. Heat is obtained for delivery to the load either from the absorber when the condenser is used to heat the secondary evaporator or from the condenser when the absorber is usedx6 heat the secondaryevaporator.

The cycle wherein external heat is used to heat the sorber and then the sorber is charged with vaporised sorbate can be repeated indefinitely.

The terms sorbate, sorbent and sorber are meant to indicate respectively a substance which is absorbed or adsorbed, a substance which absorbs absorbate or adsorbs adsorbate and a container for housing absorbent or adsorbent.

Whilst evaporated sorbate vapour is sorbed in the sorbent housed in the sorber, heat of sorption is transferred to the generator, preferably through heat exchange. This will cause working fluid to be stripped from the generator and this stripped working fluid should be condensed in the condenser, heat of condensation being used to heat the secondary evaporator. The condensed working fluid is then returned after being expanded to the evaporator.

In the absorber and generator the working fluid can be various substances, for example (1) a solution of lithium bromide in methanol, (2) a solution of lithium bromide in a water/n-propanol azeotrope, (3) a solution of lithium bromide in aqueous ammonia or (4) an ammonia solution of sodium thiocyanate. In the evaporator and condenser the respective working fluids will be (1) methanol, (2) the azeotrope, (3) ammonia and (4) ammonia.

In the case where the working fluid comprises methanol, the methanol will be the absorbate, where the working fluid comprises the water/propanol azeotrope this azeotrope will be the absorbate and where the working fluid comprises ammonia, ammonia will be the absorbate.

Usually but not necessarily, there will be a different working fluid for the secondary evaporator and sorber. Since the temperature in the secondary evaporator will usually never be below OOC, the most convenient working fluid to use will be water. However other fluids, e.g.

methanol, butene-1 or cyclohexane could be used, for example in conjunction with activated charcoal.

The sorbent which is used in the sorber can either be a solid, e.g. a zeolite molecular sieve such as the A or Y-type, or a viscous liquid supported on a suitable porous substrate, e.g.

aqueous lithium bromide supported on coke breeze. Other suitable sorbents include activated carbon or silica gel.

The absorption system includes a condenser communicating e.g. by conduits with both the generator and the evaporator.

The evaporator is designed to house the absorbate solution and to receive heat from a low temperature source, e.g. ambient air, either directly or indirectly.

There is an absorber in communication, e.g. by conduit, with the evaporator. The transfer of heat generated in the absorber to the load is preferably by means of a heat exchanger, preferably located inside the absorber and less desirably outside the absorber.

Connected to the absorber in the usual manner for an absorption system is a generator. There are two separate conduits connecting together the absorber and the generator, one conduit being provided with an expansion valve. Weak working fluid is conveyed from the absorber to the generator through the non-valved conduit by a solution pump, whilst strong working fluid from the generator is conveyed to the absorber through the conduit having the expansion valve.

The apparatus of this invention also includes a secondary evaporator, heat to which is supplied from the absorber or from the condenser or both.

This transfer of heat is preferably by means of a heat exchanger one section of which is preferably located inside the condenser and the other section of which is peferably located inside the secondary evaporator. Less desirably the sections of this heat exchanger are located outside both the secondary evaporator and the condenser. The sections are usually connected together through a conduit having a circulating pump or through a valved heat pipe. In the alternative embodiment there can be two linked heat exchangers, one located in the absorber and one located in the secondary evaporator.

The secondary evaporator communicates preferably by means of a valved conduit with the sorber, i.e. the vessel where absorption or adsorption occurs. There are means capable of supplying heat to the sorbent housed in the sorber. These means can be an electrical resistive heater and the system can be designed so that this heat is only operated during cheap off-peak periods. Preferably however a simple heat exchanger is used this being located in the sorber and the heat being supplied by hot liquid usua!ly supplied by a boiler heated by fossil fuel and passing through the coils of the heat exchanger.

Sorbate from the sorber is preferably transferred to the heat exchanger capable of supplying heat to liquid housed in the generator by means of a conduit connected to the upper part of the sorber and connected to the coil of the heat exchanger located preferably inside the generator or less desirably outside the generator. Preferably this conduit has a valve. Here sorbate vapour at least partially condenses and latent heat of condensation is transferred to the generator by these means.

The condensed sorbate or partially condensed sorbate should be transferred to the secondary evaporator via a valve which controls the pressure of condensation and hence the condensing temperature. This is preferably achieved by a conduit connected at one end to the downstream end of the above-mentioned heat exchanger capable of supplying heat to liquid housed in the generator, the other end of the conduit being connected to the lower region of the secondary.

evaporator.

The means for transferring heat of sorption produced in the sorber to the generator is preferably by means of a heat exchanger associated with the sorber which is connected to the heat exchanger capable of exchanging heat with liquid housed in the generator. The connection between the two heat exchangers is preferably by means of a valved conduit. The coils of the heat exchanger associated with the sorber are preferably housed within the sorber.

It is preferred that there be a conduit connecting the heat exchanger associated with the sorber with the secondary evaporator. This conduit should be provided with a pump. In this manner liquid can be pumped from the secondary evaporator to the two connected heat exchangers and back to the secondary evaporator. If the sorber is located physically at a higher level than the secondary evaporator when the pump is not working liquid can drain from the sorber to the secondary evaporator.

It can be seen therefore that an existing water/lithium bromide water chiller can be relatively easily converted into the heat pump of this invention.

The apparatus of the invention operates as follows: In the desorption step of the cycle of operations the absorption system operates in the normal manner. That is strong working fluid is transferred from the generator to the absorber and relatively weak working fluid is transferred from the absorber to the generator. Vaporised working fluid is evolved from the generator passing to the condenser where it condenses. Condensed working fluid then passes to theevaporator after expansion through an expansion valve. The input of heat to the evaporator causes absorbate to be evaporated from the evaporator and to pass to the absorber where it is absorbed in the working fluid, thereby weakening the solution.

In the first i.e. desorption stage, external heat isapplied;to the sorber and sorbed sorbate is desorbed, passing to the heat exchanger capable of supplying heat to liquid in the generator, where it condenses. The latent heat of condensation is transferred to the working fluid in the generator and the condensed sorbate is transferred to the secondary evaporator.

In the second stage external heat is no longer delivered to the sorber but instead heat of sorption is used to-drive the system. The absorption system works in the same way as in the first stage except that either the heat of condensation in thecondenser of the heat of absorption in the absorber is used by heat exchange to evaporate sorbate from the secondary evaporator instead of being delivered to the load. Sorbate evaporated from the secondary evaporator passes to the sorber where it is sorbed. The.heat of sorption is transferred by heat exchange in the generator.

Condensed sorbate is returned to the secondary evaporator where the sensible heat of the condensed sorbate is delivered to the sorbate in the secondary evaporator. When no more sorbate can be sorbed in the sorber the cycle of operations can be repeated. In the first stage heat is delivered to the load by heat exchange from the absorber and from the condenser. In the second stage heat is delivered to the load from either the absorber or the condenser.

The invention is now described with reference to a preferred heat pump as shown in the drawing.

The absorption system consists of a condenser 9 connected by conduit 23 to a generator 22 which in turn is connected by conduit 14 having a valve 15 and also by conduit 16 having pump 17 to an absorber 19 housing a heat exchanger 18.

The absorber is connected by conduit 20 to an evaporator 34 housing a heat exchanger 35 and this evaporator 34 is connected by conduit 32 having an expansion valve 33 to the condenser 9 which also houses a heat exchanger 26. In the evaporator 34 and condenser 9 the working fluid is an azeotrope of n-propanol and water. In the absorber 1 9 and generator 22 the working fluid is a solution of LiBr in the azeotrope.

There is a secondary evaporator 4 which is connected by a conduit 25 having a valve 12 to a sorber 1. This sorber 1 is packed with Y-type zeolite molecular sieve as the sorbent. The sorber 1 is provided with heat exchangers 2 and 3. The latter is connected via a conduit 36 having a valve 6 to a conduit 8 connecting the top of the sorber 1 with the heat exchanger 7. This conduit 8 also has a valve 24.

The other end of heat exchanger 3 is provided with a conduit 21 having a pump 5 passing into the secondary evaporator 4, the lower end of conduit 21 dipping below the surface of the liquid sorbate in evaporator 4 which in this case is water. The-lower end of heat exchanger 22 is also 'connected by a conduit 37 having a pressure control valve 13 to the secondary evaporator 4.

Heat is supplied to water in the secondary evaporator 4 by means of a heat exchanger 11 which via Conduits 30 and 31, valve 29 and pump 28 is connected to the heat exchanger 26 located in the condenser 9. A valve 27 controls the supply of heat from the condenser 9 to outside the system.

In operation heat is supplied to the sorber 1 at about 300"C by passage of hot liquid, i.e. a proprietary heat transfer fluid, through the coils of the heat exchanger 2. Desorption occurs and -desorbed fluid (water vapour) passes through conduit 8 and valve 24 and condenses in heat exchanger 7 where it delivers its heat of condensation to the working fluid (n-propanol azeotrope solution of lithium bromide) in the generator 22.

The condensed water passes through conduit 37 and valve 13 to the secondary evaporator 4 where it delivers its sensible heat, say above 600C to the sorbate (water) in the evaporator 4, the water level thereby being raised.

During this step of the cycle and indeed the - other step of the cycle the relatively strong working fluid is transferred from the generator 22 to the absorber 1 9 via conduit 1 6 and pump 1 7 and relatively weak working fluid is transferred from the absorber 1 9 to the generator 22 via conduit 14 and valve 1 5. The heat of absorption is delivered to the external load heat exchanger 1 8.

Azeotrope stripped in the generator 22 by this heat input passes as vapour through conduit 23 to the condenser 9 where it condenses. In this step of the cycle the heat of condensation is delivered to the external load via valve 27 (valve 29 being closed and pump 28 inoperative) by the heat exchanger 26. After condensation the azeotrope passes to the evaporator 34 by way of the conduit 32 and expansion through expansion valve 33.

Condensed azeotrope then passes to the absorber 19 via conduit 20 where it is absorbed in the solution of LiBr in azeotrope causing heat of absorption to be evolved.

When all the sorbate is desorbed from the sorber 1, the supply of heat to the heat exchanger 2 is stopped.

Heat is provided in this embodiment of the invention from the condenser 9. With valve 27 closed and valve 29 open hot liquid, again a heat transfer fluid such as water which has been heated in the heat exchanger 26 in the condenser 9, is transferred through conduit 31 to heat exchanger 11 in the secondary evaporator where it delivers its heat to the sorbate (water) housed therein and returns via conduit 30.

Valve 12 is open and water vapour is driven off passing through conduit 25 and valve 12 and is sorbed in sorber 1 which houses the Y zeolite molecular sieve as sorbent. The heat of sorption is transferred to the heat exchanger 7 in the generator 22 by evaporating sorbate (water) in the heat exchanger 3. This vaporised water is consequently condensed in the heat exchanger 7 and liquid water passes through conduit 37 and pressure control valve 1 3 to the secondary evaporator 4. The pressure of condensation and hence the temperature of condensation is controlled by this pressure control valve 1 3.

Whilst the sorption of water vapour by the sorbent takes place the water is circulated from the secondary evaporator 4 through heat exchangers 3 and 7 and back to the evaporator 4 by the circulation pump 5.

When no more sorption can take this step in the cycle is terminated, i.e. by shutting valves 12 and 19 and stopping pumps 5 and 28 and desorption is resumed by supplying heat to heat exchanger 2. Valve 6 is shut and since pump 5 is stopped sorbate (water) will drain back from heat exchanger 3 to secondary evaporator 4 through conduit 21.

In this manner the steps of the cycles can be repeated indefinitely.

In both steps of the cycle heat can be pumped from a temperature of around -50C to a temperature of about 600C delivered at the condenser and the absorber. External heat is supplied to the sorber usually at a temperature of about 3000C and the generator usually operates at about 1 500 C. The secondary evaporator usually operates at a temperature of about 600C. Either the condenser or the absorber is used in the second and sorption stage to deliver heat to the secondary evaporator. This means that the condenser or the absorber in this stage does not need to operate at a temperature close to 600C and the temperature can vary somewhat.

Water is the preferred sorbate and a suitable zeolite molecular sieve may be used as the sorbent, but it may be possible, by running heat exchanger 26 at a higher temperature than the load temperature at the outlet of heat exchanger 18, to use an active grade of silica gel for this purpose. With a heat input temperature of --50C and a load temperature of 600C heat will have to be supplied to the input heat exchanger 7 at about 1 500C and when zeolites are used the desorption temperature must be in excess of 3000 C. When silica gel is used the desorption temperature will be lowered to below 3000C because the heat of sorption is lower. The utility of this sorbent will however depend on its sorption capacity at 1 5000 and at the vapour pressure available in 4. An alternative sorbent could be a very strong LiBr solution being viscous enough to be supported in coke breeze. This could give a quite high stage efficiency, perhaps in the region of 1.7.

If for example the COP. of the absorption system and that of the sorption system are both 1.6. then the overall C.O.P. will be 1.96 approximately. The stage efficiencies will depend upon the temperature range over which the heat pump is operating and when the ambient temperature exceeds 5 C, which it commonly does in the U.K., then the overall C.O.P. may exceed 2.0.

Claims (9)

1. A heat pump comprising an absorption system having a condenser, an evaporator, an absorber and a generator, said pump including a sorber, means for supplying heat intermittently to the sorber so as to vaporise sorbate from sorbent housed in the sorber, a conduit connecting the sorber with a heat exchanger capable of exchanging heat with liquid housed in the generator, a conduit connecting said heat exchanger with a secondary evaporator, means for supplying heat to the secondary evaporator either from the heat of condensation produced in the condenser or from the heat of absorption produced in the absorber, a conduit connecting the secondary evaporator with the sorber, means for transferring heat of sorption produced in the sorber to liquid housed in the generator and means for delivering heat to the load from the heat of absorption produced in the absorber and from the heat of condensation produced in the condenser.

2. A heat pump according to claim 1 wherein the means for supplying heat intermittently to the sorber comprises a heat exchanger.

3. A heat pump according to either of claims 1 and 2 wherein the conduit connecting the heat exchanger capable of exchanging heat with liquid housed in the generator is provided with a valve which controls the pressure and temperature of condensation of sorbate in said heat exchanger.

4. A heat pump according to any one of the preceding claims wherein the means for transferring heat of sorption produced in the sorber to liquid housed in the generator comprises a heat exchanger housed in the sorber.

5. A heat pump according to claim 4 wherein the heat exchanger housed in the sorber is connected by a conduit having a pump to the secondary evaporator.

6. A heat pump according to claim 1 substantially as hereinbefore described with reference to the drawing.

7. A process of pumping heat involving an absorption system comprising a condenser, an evaporator, an absorber and a generator in which intermittently heat is supplied to a sorber to vaporise sorbate housed therein, vaporised sorbate is passed to and condenses in a heat exchanger capable of exchanging heat with liquid housed in the generator, condensed sorbate is thereafter transferred to a secondary evaporator housing liquid sorbate, heat is delivered to the load from the condenser and from the absorber and thereafter said supply of heat to the sorber is stopped, the secondary evaporator is heated by transfer of heat of condensation from the condenser or by transfer of heat of absorption from the absorber, sorbate is evaporated from the secondary evaporator and passes to the sorber where it is sorbed by sorbent, the resulting heat of sorption is used to heat the liquid in the generator and heat is delivered to the load either from the absorber or from the condenser.

8. A process according to claim 7 wherein the working fluid in the absorber and the generator is a solution of lithium bromide in an n-propanol/ water azeotrope and in the evaporator and condenser the n-propanol/water azeotrope.

9. A process according to either of claims 7 and 8 wherein the sorbate is water and the sorbent is Y-type zeolite molecular sieve.