GB2114724A - Heat pumps - Google Patents

Abstract

There is disclosed a heat pump system in which, in operation, a fluid is expanded after compression, in which system, in normal operation, fluid is accumulated and expanded substantially in batches in an expansion chamber located between the condenser and evaporator. A portion of each batch is removed by the compressor of the system to improve the efficiency of the system. <IMAGE>

Description

SPECIFICATION Heat pumps The present invention relates to heat pumps.

According to the present invention, there is provided a heat pump system in which, in operation, a fluid is expanded after compression, in which system, in normal operation, fluid is accumulated and expanded substantially in batches with significant recovery of work of expansion through a portion of each batch being removed by compressor means of the system and performing work as it enters the compressor means.

The present invention will now be described by way of example.

Many physical processes produce cooling. The examples of the invention which are to be described each foms part of a heat pump using a refrigeration cycle (whether used for refrigeration and/or heat supply) operated by means of mechanical power input and utilising a working fluid (of suitable chemical composition) which is subjected to a thermodynamic cycle which includes the following essential features (see Figure 1).

1. Relatively cold fluid absorbs heat from a source.

2. The fluid is subsequently compressed in a compressor.

3. After this, the fluid, at a higher temperature, gives up heat to an output heat sink.

4. The fluid is returned to substantially its initial state.

In practice, the first and third stages are usually achieved by vapourisation and condensation of the fluid respectively.

The energy efficiency of the complete system is given by the coefficient of performance, namely: Heat Output Power Input This cannot exceed the (reversed) Carnot efficiency, namely: Output Temperature Temperature Rise for a homogenous source and sink.

To achieve the theoretical maximum, the necessary and sufficient condition is that all processes in the system are thermodynamically reversible. In particular, heat flow must be due to infinitesimal temperature differences and fluid flow to infinitesimal pressure differences. There must also be degradation of energy into heat owing to friction, etc.

Of the four essential parts to the cycle, the first and third may be made essentially constant temperature processes by using change of phase, thus greatly easing design of components to approach the thermodynamic ideal. There is therefore no essential irreversibility. Actual irreversibilities are usually dominated by temperature drops external to the fluid, as required for the heat transfer.

In the compressor, there are irreversibilities due to engineering limitations. The compression process causes both temperature and pressure to rise. Ideally, they would so track one another that the compressor output would be a saturated vapour, ready for condensation (see Figure 2), where phase change is used (normal practice). In fact, either condensation or super-heating is normal, the latter being more common. In this case, there is a reduced risk of washing away lubricant but the heat of superheat must be removed before condensation can happen. In practice, this is achieved in the same unit as condensation in an essentially irreversible manner. The latent heat of condensation is generally much greater than this sensible heat so the degradation of performance due to superheat in the compressor output is usually negligible.

These three stages may therefore be implemented with very little inherent irreversibilities leading to efficiencies limited by engineering rather than the fluid thermodynamic cycle on which the system is based.

This, however, is not so for the last stage, the return of relatively warm fluid, typically a condensed liquid, to a cooler lower pressure state, typically with partial vapourisation. Any combination of the following techniques are currently used.

1. A heat exchanger to remove heat until the temperature is correct and a pressure reducing valve to return the pressure after cooling. The heat has to go somewhere and the simplest arrangement put its into the cold vapour stream by means of a heat exchanger thereby inducing considerable superheat even before compression.

(See Figure 3).

2. The fluid expands through an essentially adibatic pressure reducer. (See Figure 4). Unless external work is done, this expansion is at constant enthalpy apart from heat exchanged externally. There is therefore a degradation of energy-the work of expansion-and an increase in entropy which is predictable from the fluid properties regardiess of the construction of the pressure reducer. It is here that selection of fluid is most important in usual practice where efficiency is required.

3. In Figure 5, a mechanical means is used to effect the expansion. However, it is not usually practicable to effect the expansion via a mechanical means which recovers the work of expansion as mechanical power. It is were, the selection of fluid would be less critical.

In practice, expansion valves are usually used.

It is not necessary that the flow be continuous from the above limitations to apply.

According to the following examples of the invention, I propose a heat pump with an expansion system, which in principle, effects an insentropic expansion and does not require the use of a heat exchanger. The cycle efficiency will then closely approach the theoretical maximum and the performance limits in practice will be due to engineering. I anticipate that such a system does not significantly alter the capacity of the whole system and can be implemented at a modest cost. I estimate a saving in power input of heat pumps for domestic heating of 40%.

Each of the proposed systems effects the expansion of fluid to cooler, lower pressure conditions in an essentially reversible, adiabatic manner, recovering the work of expansion by means of the compressor. The specific loading (i.e. per mass flow) is reduced as the inlet pressure is raised, the expansion system exploiting this.

Each of the expansion systems operates on batches of fluid. A batch is allowed to accumulate until of suitable size before expansion is started.

At the beginning of expansion, the properties of the batch are (substantially) those of the fluid accumulating. Suction is then applied to the batch. The fluid expands progressively, the portion being drawn off doing work as it enters the compressor. As the remaining fluid drops in pressure, the specific loading on the compressor rises (assuming other operating conditions to remain constant). In a simple system, the initial specific loading is virtually nil, rising until the fluid is expanded sufficiently.

Where the system uses the usual change of phase, the compressor draws of principally vapour, leaving behind liquid for use in the evaporator(s).

After expansion, the compressor is free to be used, if desired, in its basic use to compress fluid which has received external heat before giving it out again.

The system may be implemented quite simply.

Where the operation of devices controlling the flow are indicated, these may be self-acting or controlled.

Examples 1) In Figure 6, the evaporator, compressor and condenser are conventional devices. V1 is a selfacting one-way valve. V2 and V3 operate as follows: a) The initial conditions are: 1. The expansion chamber is full of vapour.

2. V1 passes vapour from the evaporator to the compressor with negligible pressure drop.

3. Condensed liquid accumulates in the reservoir.

4. V2 is closed.

5. V3 is closed.

b) At a suitable point, V2 opens. Fluid passes through it into the expansion chamber, raising the pressure, temperature and liquid level there. A negligible expansion is incurred in this simple arrangement. The condition of the fluid batch in the expansion chamber is then close to that while it was accumulating.

c) V2 closes, isolating the fluid from the reservoir.

d) Vapour passes to the suction line of the compressor, shutting off Vi. Initially then, the specific loading on the compressor is exceedingly small.

e) The vapour being pumped continues to be condensed in the usual way, giving out its latent heat in the condenser.

f) Just before the pressure in the expansion chamber reaches that in the evaporator, V3 opens to allow the fluid to pass into the evaporator. V3 closes before V2 opens again.

g) The compressor now handles vapour from the evaporator, the expansion cycle being complete. During the expansion, heat output is maintained, though subject to fluctuations. Heat input is suspended owing to lack of suction on the evaporator. However, this is compensated by the increased heat removal during normal operation, arising from the fact that the pumped vapour all derives from input heat, there being no vapour to pump uselessly, from the expansion process. The corresponding vapour is that removed during the reversible expansion.

2) In Figure 7, V1 operates periodically in such a way that the conditions in the evaporator swing from close to those of the condensed liquid to much colder, lower pressure conditions in order to effect heat withdrawal. The evaporator is shown with its own reservoir.

a) The initial conditions are: 1. The evaporator is full of liquid in the usual way.

2. Condesned liquid accumulates in the reservoir.

3. V1 is closed.

b) When the evaporator liquid level, is at a suitable minimum, V1 opens. Fluid passes into the evaporator and, provided the evaporator capacity is considerably greater than its operating minimum and thermal capacities are not excessive, the temperature and pressure rise to approach those of the condensed liquid.

c) V1 closes, enabling normal refrigeration to resume.

d) The specific loading on the compressor is now relatively low. As vapour is withdrawn, pressure and temperature fall.

e) This continues until the next time V1 is opened. This example results in temperature swings in the evaporator. It is desirable that the evaporator be thermally isolated while its temperature is in excess of the source. This may be effected by turning off any pumping or fan system used to facilitate heat transfer to the evaporator.

In this example, in contrast to the operation of the expansion valve in a standard system, the flow through the valve is sufficiently modulated to result in large temperature changes in the evaporator by itself during normal operation.

The temperature swings have the advantage of providing a defrost action.

3) In Figure 8, fluid accumulates in the expansion chamber itself. To allow continuous operation, the system is duplicated as shown. V4 and V7 are introduced to allow fluid to accumulate while the compressor is applying suction.

4) In Figure 9, the layout and operation are identical to Figure 6 in what may be called normal operation of the expansion system. If, however, V3 opens well before the normal pressure reduction is complete, possibly with V2 open as well, fluid passes into the evaporator in sufficient quantity to raise its temperature. In this case, the system operates in the same way as Figure 7 (which also results in defrost action).

The foregoing are just examples of systems according to the invention and there is a number of generalisations which may be made as regards the specific description.

1. Valves may be self-acting or controlled. They may be replaced by pumps or other flow control devices (or possibly omitted with some loss of efficiency).

2. Evaporation and condensation are not essential. Where a system is implemented that does not use them, they are replaced by mere heat transfer, the terminology of the heat exchangers being altered from evaporator and condenser.

3. Even where change of phase is used, it need not be complete. The terms vapour and liquid refer in general to a proportion of the fluid, the particular terms being chosen for clarity in the context of a simple system.

4. All devices such as compressors may be composite.

5. The valve systems which lead into a common suction line may be replaced by separate inlets to the compressore. A dedicated compressor may be used for expansion purposes.

6. Any combination of the basic systems is possible. Actual complete systems may be more elaborate.

7. There may be insignificant departures from the ideal operations described. Batching may be replaced by merely substantial changes in flow rate. By-pass flow have been ignored.

8. The described phases may not be precisely defined in practice, with overlaps and separations.

This is especially true for the systems using a large capacity evaporator as in Figure 7 in which expansion runs into normal evaporation.

9. There is a number of conventional systems which have features whereby they may be modified to operate in accordance with the present invention. For example: 1. Fluctuations in flow through a conventional expansion valve may be quite large, but normally fail to swing the downstream conditions enough to bring significant thermodynamic benefits.

2. In some refrigeration systems, batches of fluid are held back purely for defrost purposes, normal operation being by an expansion valve.

In some refrigeration systems, in the 'pumpdown' cycle, the supply of fluid to the evaporator is cut off before the compressor is cycled off.

Again, normal operation uses an expansion valve.

There is therefore the possibility of modifying the design operation of such conventional systems so as to operate in accordance with the present invention and therefore increase the efficiency without major changes to the fluid system.

Claims (7)

Claims

1. A heat pump system in which, in operation, a fluid is expanded after compression, in which system, in normal operation, fluid is accumulated and expanded substantially in batches with significant recovery of work of expansion through a portion of each batch being removed by compressor means of the system and performing work as it enters the compressor means.

2. A system according to claim 1, which comprises first heat exchange means, having an input connected to an output of the compressor means; a reservoir having an input connected to an output of the first heat exchange means; first flow control means, between an output of the reservoir and an input of an expansion chamber; a flow path between an output of the expansion chamber and an input of the compressor means; second flow control means, between another output of the expansion chamber and an input of second heat exchange means; and a non-return valve between an output of the second heat exchange means and the input of the compressor means.

3. A system according to claim 1, which comprises first heat exchange means, having an input connected to an output of the compressor means; a reservoir having an input connected to an output of the first heat exchange means; and flow control means connected between an output of the first heat exchange means an an input of second heat exchange means, an output of the second heat exchange means being connected to an input of the compressor means.

4. A system according to claim 1, which comprises first heat exchange means, having an input connected to an output of the compressor means; first and second expansion chambers each having an input connected to an output of the first heat exchange means via a respective one of first and second flow control means; and second heat exchange means having an input.

connected to a first output of each of the expansion chambers via a respective one in each case of third and fourth flow control means, an output of the second heat exchange means being connected with an input of the compressor means via a non-return valve and a second output of each of the expansion chambers being connected to the input of the compressor means, in each case via a respective one of fifth and sixth flow control means.

5. A system according to any of claims 2, 3 and 4, wherein the or each flow control means is a controlled valve.

6. A system according to any one of claims 2 to 5, wherein the first heat exchange means comprises condenser means and the second heat exchange means comprises evaporator means.

7. A heat pump system according to claim 1, substantially as herein described with reference to Figures 6 to 9 of the accompanying drawings.