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The compressor’s influence on the efficiency of a heat pump

17 May 2012 | Author: Danfoss
Heat pumps are continually gaining in popularity. Their operation is emission-free at the point of use and the power for their compressors, pumps and fans can be drawn from the ubiquitous national grid. Whether air to air, air to water or water (brine) to water, the compressor always constitutes the core element of the heat pump. And since the compressor is the largest individual consumer in a heat pump we have to ask ourselves what needs to be taken into account when selecting and optimising a compressor to make the system as a whole as efficient as possible.
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Minimizing the dead space
When using a reciprocating compressor for a heat pump it is necessary to pay particular attention to minimising the dead space. The dead space is the volume remaining in the cylinder when the piston is nearest to the compressor’s valve system. This is the state of the system when it has just expelled its compressed refrigerant through the pressure valve, just before more low-pressure low-temperature refrigerant is drawn in through the suction valve. The reason why minimizing the dead space is so important is that any compressed refrigerant remaining in the cylinder after the expulsion stage is going to re-expand, and this naturally reduces the volume of refrigerant that can be drawn in. For this reason it is of critical importance that the piston of a reciprocating heat pump compressor precisely fits its valve system. In series MTZ Danfoss compressors, the piston effectively travels right into the valve system, though naturally without any actual contact. This is possible only because, at peak compression, the outer rings of the central milled depression in the upper surface of the piston are level with the central portion of the valve system. Since MTZ compressors use the same valve systems and piston diameters for various performance ratings and one, two and four-cylinder compressors are also used, optimizing the dead space is particularly important at the lowest performance rating for a given number of cylinders. That means that this measure will be especially effective for the MTZ 18 (the smallest one-cylinder MTZ compressor), MTZ 44 (the smallest two-cylinder) and MTZ 100 (the smallest four-cylinder), because here the ratio of dead space to piston displacement is most disadvantageous. In larger configurations with the same number of cylinders the piston displacement increases with the length of the stroke while the dead space remains the same, so its effect is less critical.

Utilising compressor cooling
Another aspect that affects the efficiency of a heat pump is compressor cooling. Compressor cooling is of only minor significance for refrigeration applications, but in heat pumps waste heat from the compressor can be used to generate additional heating power. Compressors that are cooled only by suction gas, such as MTZ reciprocating compressors and HHP scroll compressors, are particularly suitable for heat pump operation because their waste heat, which would otherwise be lost to the environment, is given up to the refrigerant.  This enables the refrigerant to pass additional heat to the heating system via the condenser.  Compressors with 100% suction gas cooling can be integrated into heat pump systems with no additional technical effort and constitute a well-tried and reliable solution, although this type of compressor cooling is often not available for small heat pump systems. Compact single-cylinder compressors generally have to be able to dissipate their heat into the environment. They may be able to do this statically, and if not they may need a fan to provide forced ventilation, but even in such cases there is a solution that will improve the efficiency of the heat pump. If you use a compact heat pump compressor with oil cooling then the waste heat from the compressor can also be passed to the heating system. One example of such a compressor is the SC15GHHX for refrigerant R134a with heat pump optimisation. This calls for a two-part condenser, where the first part cools and at least partially liquefies the refrigerant. This partially or completely liquefied refrigerant is then passed to the lower part of the compressor via the oil cooler. This has the effect of cooling the compressor while the acquired heat begins to re-evaporate the refrigerant, which then enters the second part of the condenser where it does completely liquefy. In this way, even small-scale systems can use waste heat from the compressor to generate additional heat for heating purposes.

Scroll compressors with additional valve technology
Another way of having a positive effect on heat pump efficiency is to use a scroll compressor with additional valve technology. This is particularly useful where the operating conditions are such that it is not possible to maintain the installed pressure ratio – the ratio at which the compressor operates with optimum efficiency. Such conditions are bound to occur in some parts of the annual cycle, and – depending on the type of heat pump – more frequently than you might expect. With a standard scroll compressor such situations produce a marked over or under compression, both of which have a noticeable effect on the operator's electricity bill. This is why Danfoss HHP scroll compressors for heat pumps are equipped with an additional valve system. At low compression, a standard compressor with no additional valves will always carry out the entire compression cycle, placing the refrigerant under an unnecessarily high pressure only for it to immediately expand to the lower compression as soon as it is expelled. A valve scroll compressor does not need to do this. The refrigerant in a scroll compressor is compressed from the outside of the scroll set towards the centre, and the additional valves enable it to take a “short cut” as soon as it has been compressed as far as necessary. When a compression pocket passes the flap of one of the additional valve system valves, if the contrary pressure from the high pressure side is lower than the current pressure in the compression pocket the valve will open to the pressure side. There is thus no excessive compression and therefore no need to pay for extra drive power to enable the compressor to carry out the extra work. There is a similar effect in situations where a standard compressor would produce under-compression. At the optimum pressure ratio, while the condensing pressure remains too high for the compressed refrigerant to be expelled, the compression process will continue until it has built up sufficient pressure to expel the refrigerant via the additional central valve. This considerably mitigates the deleterious effects of under-compression in traditional scroll compressors. This clever trick makes it possible to always operate the compressor and hence the heat pump at high efficiency even under fluctuating operating conditions.

The compressor influences the efficiency of a heat pump in manifold ways, some larger than others, but their total contribution is considerable . The above section on “Utilising compressor cooling” also showed that the heat pump is not simply a refrigeration unit running in reverse, but that specific measures may be of quite different significance for refrigeration units and heat pumps. When we take all the various effects into account we see that the compressor has an enormous effect on the efficiency of a heat pump.

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