GB2144762A - A process of pumping heat - Google Patents

Abstract

Heat is pumped from a relatively low temperature to a higher temperature by the use of an absorption heat pump wherein the absorbate comprises water and the absorbent comprises a mixture derived from reacting a water-soluble Group IIa metal compound (e.g. calcium chloride) with a Group Ia metal oxide or hydroxide (e.g. sodium hydroxide).

Description

SPECIFICATION A process of pumping heat This invention relates to a process of pumping heat using an absorption heat pump.

The partial pressure of water vapour above concentrated aqueous solutions of sodium hydroxide is very low. For example, a solution of 140 g NaOH/100 g H20 is in equilibrium with water vapour at a pressure of 3 mm of Hg when held at 600 C, but a solution containing 160 g of NaOH/100 g H20 at about 140 C is in equilibrium with water vapour at a pressure of 149.5 mm Hg, the vapour pressure of water at 600 C. It follows that in principle the NaOH/water pair can pump heat from below 0 C to above 60"C.

Unfortunately, very concentrated solutions of sodium hydroxide in water tend to freeze at around 60"C and the use of such solutions would pose some operational problems. In an absorption heat pump the concentrated absorbent solution should remain fluid at room temperature. Surprisingly we have found that the addition of a minor proportion of a water-soluble Group Ila metal compound, for example calcium chloride, achieves this end.

According to this invention heat is pumped from a relatively low temperature to a higher temperature by the use of an absorption heat pump wherein the absorbate comprises water and the absorbent comprises a mixture derived from reacting a water-soluble Group Ila metal compound with a Group la metal oxide or hydroxide.

A typical absorption heat pump comprises a generator connected to a condenser which is connected through a flow restrictor to an evaporator which in turn is connected to an absorber. The absorber is connected to the generator by a conduit having a flow restrictor therein and by another conduit provided with a circulation pump.

In operation the evaporator accepts heat from a relatively low temperature source and uses it to evaporate absorbate from the working fluid comprising absorbate and absorbent and the evaporated absorbate passes to the absorber. Here the absorbate is absorbed by absorbent containing solution and it delivers its heat of absorption to the load, usually via heat exchange, in the process.

The heat of absorption is greater than the latent heat of evaporation by a factor which depends on the strength of the absorbent solution. The stronger the solution the larger is the heat of absorption and the higher the temperature at which absorption can occur at the desired rate.

The effect of absorption is, however, to dilute the absorbent solution and in order to restore the solution strength the weak solution is pumped by a pump through a conduit to the generator. High temperature heat is supplied to the generator where absorbate is boiled off and passes through a conduit to the condenser where it is condensed delivering its latent heat of condensation, usually via heat exchange, to the load in the process.

The load temperature determines the pressure at which condensation occurs and this in turn is a factor in determining the temperature at which heat must be supplied to the generator.

In order to complete the circuit the stripped absorbent solution is returned from the generator to the absorber through a flow restrictor in a conduit connecting the generator with the absorber. The condensate is also returned to the evaporator via a flow restrictor in a conduit connecting the condenser with the evaporator.

If the absorption system is considered as a whole heat enters via the evaporator and the generator and leaves via the absorber and condenser and with the system in equilibrium these heat flows must balance. Approximately therefore, the latent heat of evaporation is delivered to the external load via the condenser whilst the heat of desorption is delivered to the load via the absorber. If the latent heat of evaporation was equal to the heat of absorption then each unit of heat put into the generator would result in two units being delivered to the load, one from the high temperature source and the other from the low temperature source. In practice, however, the highest coefficient of performance (C.0.P) which can be achieved is about 1.7.The countercurrent flow of absorbent solution from the absorber to the generator and back results in an unwanted transfer of heat from the generator directly to the external load. Heat transferred directly in this way does not pump any heat from the low temperature source and reduces the C.0.P.. In order to minimise this effect the streams passing in the conduits between the generator and absorber can be heat exchanged in a countercurrent heat exchanger usually known as the solution heat exchanger. As a further improvement the vapour leaving the evaporator may be pre-heated by heat exchange with the liquid draining from the condenser to the evaporator in a heat exchanger usually known as the pre-cooler. This increases the capacity of the liquid in the evaporator to receive heat from the low temperature heat source and again increases the C.0.P..

In accordance with this invention, the absorbent comprises a mixture derived from reacting a Group Ila metal compound soluble in water, e.g. a halide with a Group la metal oxide or hydroxide. One suitable alternative water-soluble compound is a nitrate, e.g. calcium nitrate.

Of the Group Ila metals the preferred are calcium and strontium, especially the former and chlorides are preferred to bromides, iodides or fluorides. The most preferred Group Ila metal halide is calcium chloride.

Group la metal hydroxides are preferred to Group la metal oxides and the most preferred are sodium hydroxide or potassium hydroxide.

The amount of water-soluble compound compared with the hydroxide or oxide is preferably a minor proportion and more preferably 0.5 to 1.5 moles, e.g., about I mole of halide per 10 moles of hydroxide or per 5 moles of oxide are suitable.

The amount of hydroxide or oxide per mole of absorbate (water) is preferably 0.5 to 1.25 moles per mole of water, e.g. about 0.75 moles of hydroxide or oxide per mole of water. A typical for mulation which gives satisfactory results and which is liquid at temperatures above 20"C is 40 parts by weight sodium hydroxide, 10 parts by weight of calcium chloride and 22 parts by weight of water, i.e., 0.09 moles of calcium chloride and 1.22 moles of water per mole of sodium hydroxide.

It will be appreciated that the concentrations of components will vary in different parts of the heat pump and at different temperatures of operation.

In practice the working fluid, i.e., absorbate plus absorbent is prepared by mixing the appropriate quantities of hydroxide or oxide and halide. Thereafter the appropriate quantity of water is added when it is usually found that the temperature rises rapidly and a white creamy suspension or slurry is formed. Alternatively one may dissolve the watersoluble compound, e.g. a halide in water and then add the hydroxide or oxide to this solution. Unlike concentrated solutions of Group la metal hydroxide in water, the creamy suspension thus formed still remains fluid when the temperature drops to ambient temperature.

The invention is now described with reference to the drawing which shows an absorption heat pump.

The generator 1 housing a heat exchanger 2 is connected by a conduit 3 and expansion valve 5 to an absorber 7. It is also connected to the absorber 7 by a conduit 4 having a circulation pump 6. The two conduits 3 and 4 are arranged to heat exchange with one another, thus forming a solution heat exchanger 8. The absorber 7 houses a heat exchanger 9.

The absorber 7 is connected by conduit 10 to an evaporator 11 housing a heat exchanger 12. This evaporator 11 is connected by conduit 13 and expansion valve 14 to a condenser 17. The conduits 10 and 13 are arranged to heat exchange with one another, thus forming a pre-cooler 15.

The condenser 16, housing a heat exchanger 17,is connected by conduit 18 to the generator 1.

The working fluid in the absorption heat pump is the slurry formed from calcium chloride, sodium hydroxide and water in a respective weight ratio of 40 parts to 10 parts to 22 parts. The slurry was formed by first mixing the calcium chloride with the sodium hydroxide and then adding the water.

Low temperature heat at about 0 C from atmospheric air is supplied to the evaporator 11 via the heat exchanger 12. The water which evaporates, i.e. steam, transfers to the absorber 7 via the conduit 10. The heat of absorption of water vapour produced in the absorber 7 is delivered to the load at a temperature of about 60"C via the heat exchanger 9.

The thus diluted working fluid is pumped by means of pump 6 through the conduit 4 to the generator 1 where high temperature heat at about 1500C is delivered via the heat exchanger 2. This delivery of heat causes water to be boiled off from the working fluid in the generator and the steam passes to the condenser 16 via conduit 18.

Meanwhile stripped and hence strong working fluid is transferred from the generator 1 to the absorber 7 via the conduit 3 and expansion valve 5, exchanging heat in the solution heat exchanger 8 with the weak working fluid passing through the conduit 4 when transferring from the absorber 7 to the generator 1.

The water vapour passing through conduit 18 condenses in the condenser 16, giving up its heat of condensation and delivering heat to the load at about 60"C via heat exchanger 17.

The condensate is returned to the evaporator 11 via the conduit 13 and expansion valve 14 having delivered heat in the pre-cooler 15 to the water vapour evaporated from the evaporator 11 and passing through conduit 10.

In this manner low temperature heat is delivered to the evaporator 11 (via heat exchanger 12) and heat is delivered to the load at the absorber 7 (via heat exchanger 9) and at the condenser 16 (via heat exchanger 17).

Claims (7)

1. A process of pumping heat from a relatively low temperature to a higher temperature by the use of an absorption heat pump wherein the absorbate comprises water and the absorbent comprises a mixture derived from reacting a watersoluble Group Ila metal compound with a Group la metal oxide or hydroxide.

2. A process according to claim 1 wherein the water-soluble compound is a Group Ia metal halide.

3. A process according to claim 2 wherein the Group Ila metal halide is calcium or strontium chloride.

4. A process according to either of claims 1 and 2 wherein the Group la metal hydroxide is sodium or potassium hydroxide.

5. A process according to any one of the preceding claims wherein 0.5 to 1.5 moles of halide per 10 moles of hydroxide or per 5 moles of oxide are reacted.

6. A process according to any one of the preceding claims wherein the amount of hydroxide or oxide is 0.5 to 1.25 moles per mole of absorbate.

7. A process according to claim 1 substantially as hereinbefore described with reference to the drawing.