Components for heat pumps: compressors

Wednesday, February 1, 2012

The compressor is the core component of a heat pump. It is the primary factor that determines whether the heat pump is good or bad. What counts is not merely the energy efficiency of the compressor but also the limits placed on applications by the evaporating and condensing temperatures, highly durable mechanical parts and quiet operation. 

A compressor in a heat pump application operates in the same way as in a compression refrigeration plant. It conveys refrigerant in the form of vapor at low pressure and low temperature to a high pressure and high temperature stage. The motive force for this is generally an electric motor fed from the mains power grid and integrated into the compressor. “Electrically operated” is by no means a rarity in everyday life but it is not yet the standard for domestic heating. This sector offers a variety of diverse solutions, many of which burn fossil fuels such as oil and gas. Advantages other than efficiency are therefore important arguments, such as the fact that no oil tank is needed, and no gas connection (which will often be unobtainable, particularly in rural areas).

Compressor technologies

Of all the possible types, two compressor technologies have conquered the heat pump market for domestic and commercial use, namely fully hermetic reciprocating and scroll compressors. This fully hermetic construction, which dispenses entirely with the screw joints that would otherwise permit access to the interior of the compressor, is vital to avoid leaks and therefore an essential contribution to environmental protection. Reciprocating compressors are the classic solution and are particularly robust. Scroll compressors are a more modern variant that is increasingly being used. Where reciprocating compressors use one or more reciprocating pistons to draw in refrigerant via a suction valve, compress it and return it to the heat pump circuit via a pressure valve, in a scroll compressor this function is carried out by the scroll set. The two scroll spirals orbit one within the other to compress the refrigerant. Unlike reciprocating compressors, standard scroll compressors have no valve system, there is only a check valve on the pressurized gas outlet in the center of the scroll set. This check valve prevents the orbiting scroll half from permanently rotating backwards because of the pressure difference, thereby minimizing unpleasant noise and preventing the refrigerant from condensing inside the scroll set.

HHP heat pump scroll compressor with additional valve system

Selection

When selecting a heat pump compressor the first consideration is the refrigerant. The compressor must be approved for the desired refrigerant and must be usable for a wide range of applications corresponding to the current requirements. Any fluctuations in the temperature of the heat source over the course of a year will determine what evaporation temperatures will be required. For example, if the heat pump is to be used only during interseasonal periods – with some other form of heating, such as gas or oil, being used in winter – then the compressor will not need to be suitable for extremely low evaporation temperatures. Conversely, on the condenser side we then have to consider the necessary water supply temperature. A heat pump and an underfloor heating system thus constitute a perfect team. Underfloor heating systems do not need high inlet temperatures, hence only low condenser temperatures will be needed, which increases the efficiency of the heat pump. The worst possible case is when an old building’s heating system is brought up to date but its ancient cast-iron radiators are retained. These call for the highest input temperatures – by comparison, modern radiators are usually content with 10K lower. For cast-iron radiators in an old building the compressor needs to be geared for condenser temperatures of up to 65°C. Such systems do offer the additional assurance that, when (also) used to provide hot water they will not constitute a Legionella hazard. Temperatures higher than 60°C offer reliable protection from Legionella bacteria and the diseases they cause. Indeed, where large volumes of hot water are stored at temperatures lower than 60°C the water should be heated to 60°C at regular intervals. Manufacturers generally attempt to keep the compressor design as simple as possible, but where a very wide field of application is required it may be necessary to employ additional measures to ensure that the hot gas temperatures in the heat pump do not get too high.

Additional valves on the HHP

Extending the field of application

One good way of achieving this is liquid injection in the compressor. The Danfoss PSH series for heat pump manufacturers already incorporates such an electronic solution. The controller board is located on the compressor terminal board, the hot gas temperature sensor is directly on the discharge connector and the electronic injection system is attached to the compressor. Because high hot gas temperatures are not a bad thing – particularly for a heat pump – it is especially important that post-injection activates only when the temperature at the discharge connector is really excessive – higher than 135°C. For temperatures lower than this, the extra recuperative heat from the condenser simply makes a further contribution to the generated heat. However, if the temperature exceeds 135°C, liquid refrigerant is injected in the mean pressure area of the scroll set. Since scroll compressors draw in refrigerant in the outer part of the scroll set and convey it to the middle for expulsion, with the compression pockets becoming continually smaller, the mean pressure area containing the post-injection inlet is located about half way in. In the mean pressure area the refrigerant temperature is thus reduced by the injected liquid refrigerant . The pressure difference between the liquid presenting at the injection valve (high pressure) and the outlet (mean pressure) reduces the pressure of the refrigerant and hence the temperature in the mean pressure pocket. 

Post-injection function on the PSH

Improving the seasonal performance factor

Whatever refrigerant and whichever temperature range for evaporation and condensing are decided on, we of course also want a compressor that promises a good coefficient of performance (COP) value under the expected operating conditions. The COP of a heat pump compressor is the quotient of the delivered heating power and the consumed drive power, so we should always be aiming for a high COP. To get an even better prediction of compressor performance over the heating period of the year we should also consider its seasonal coefficient of performance (SCOP), which is the average or measured COP value over an entire year. Considering the SCOP ensures that the compressor functions efficiently under a variety of operating conditions and is optimized for a broad operating range. 

One measure that specifically addresses this point is to equip the scroll compressor with a supplementary valve system. This has been done for the Danfoss heat pump scroll compressors in series HHP. The design of a scroll compressor gives it a built-in pressure ratio. This may differ from device to device, but for a given scroll compressor it is always the same and does not change. It is the pressure ratio at which the compressor operates most efficiently and it would be optimal if this defined ratio were present in the compressor all year round, regardless of the momentary heating and water-heating situation. Unfortunately, for most of the year, this optimum pressure ratio is not maintained because of the varying evaporation and condensing temperatures. Even a slight deviation from this optimum value, in whatever direction, has a markedly deleterious effect on the efficiency of the compressor and hence that of the heat pump as a whole. These losses result from the fact that – even when the condensing pressure is low – a scroll compressor always begins by compressing to its built-in internal pressure ratio, and it does this even when this pressure is higher than the condensing pressure at the expulsion point of the scroll set: in this case the compressor is doing unnecessary work. In such a situation the supplementary valve system of HHP series scroll compressors gives the refrigerant the chance to exit from the scroll set before the end of the compression process. During the compression process, whenever a compression pocket passes the outside of a supplementary valve that can open because the pressure at the other side (the condensing pressure) is sufficiently low, the refrigerant can take this “short cut” to the compressor outlet. This avoids unnecessary pressure build-up and has a similar effect on the operator’s electricity bill. A similar effect is achieved by the supplementary valve located in the center of the scroll set when the condensing pressure is higher than the end pressure of the built-in pressure ratio. 

Log p h diagram - temperature value 4 lower due to post-injection

Quiet operation

The operating noise of a compressor belongs in the area "customer comfort", so the rule is "the quieter the better". But it must be said, particularly for modern fully hermetic scroll compressors, that they already achieve a noise level that leaves little to be desired. If the compressor is contained in a separate – often insulated – compressor compartment within the heat pump, its noise level will be barely detectable to the outside world. Where additional sound damping measures are necessary, compressors that are cooled only by suction gas can be fitted with an acoustic hood. With this approach you must also consider whether the acoustic hood is suitable for the expected temperatures: unlike fully hermetic reciprocating compressors, the temperatures in the upper part (the discharge side) of a scroll compressor can reach well over 100°C. Prospective operators of heat pumps are often particularly exasperated by the complex nature of sound-related information. If you need to discuss noise levels with customers it is important to first clarify the terminology. The sound power level is expressed in "dB(A)" and is always the highest value stated. This value is already weighted and therefore does not need to be qualified with a distance measurement. Sound pressure levels are also expressed in "dB(A)", but are always associated with a specific distance. They are also always numerically lower than the sound power level. The more remote the sound source the smaller will be the noise level. Unlike sound power levels, sound pressure levels are values that can be verified by measuring instruments. However, such measurements need to take into account additional reflections from walls or equipment housings, which will further increase the measured value. This needs to be allowed for particularly when dealing with noise readings that are qualified as "free-field measurements". All of these sound quantities are calculated from the individual frequency ranges relevant to human hearing ("dB(A)" is most usual, but "dB(B)" and "dB(C)" values, which are evaluated differently, are also used). 
However, such discussions with plant operators are increasingly becoming unnecessary thanks to modern compressor technology, in which low noise emissions are already written into the specification at design level. 

PSH 38 - 77 Application limits for R410A – with post-injection

Conclusion

A good heat pump compressor will offer an outstanding seasonal performance factor that is kind to both natural resources and the operator's wallet. It will also be as flexible as possible as regards evaporation and condensing temperatures and will emit no obtrusive noise. With good compressors there will be no stopping the triumphal progress of the heat pump.