potential and considerations for control
8 Application to other types of heat pump system
This chapter aims to generalise the previous results to heat pump systems other than hydronic ground-source systems.
8.1 Outdoor air source heat pumps
Outdoor air is a very common heat source for heat pumps, also in cold climates.
As already discussed in chapter 7 the defrost function is an extra consideration beside the aspects of variable-speed control of compressors and circulators as discussed before. The energy saving potential for outdoor air-source heat pumps should be larger than for ground-source systems as the evaporation temperature, with standard design of the evaporator, increases more for air-source heat pumps when the compressor capacity is reduced than it does for ground-source systems.
The greater temperature change is due to the normally smaller heat transfer capacity of standard designs of finned-tube coils compared to standard designs of plate heat exchangers. Also the frosting will be less and thus there will be less need for defrosts and the efficiency will increase further because of this.
Considering the optimisation of pump, fan and compressor speeds the same principles apply as described in chapters 5 and 6. The power use of the evaporator fan generally is about the same as the power use of the ground-source circulator.
Hence, it should be more important to control and optimise the evaporator fan than the condenser fan or pump.
8.2 Exhaust air heat pumps
Exhaust air heat pumps recover heat from the ventilation exhaust air flow and rejects it back to the building, normally via a hydronic heating system, and/or to sanitary hot water, see Figure 8.1. Due to the limited heat source, these heat pumps normally cover a smaller part of the total heating demand than do ground-source systems. In a test from 2001, reported by Lagergren[51], the four heat pumps evaluated covered 67 – 82 % of the annual heating demand for a building with a total demand for space and domestic hot water heating of 15 MWh. The capacity of the heat pump can be increased by increasing the ventilation air flow or by increasing the capacity of the compressor. However, increasing the air flow will also increase the heating load, as described by Eq. 8.1, where transmission and ventilation losses are separated.
(
in out)
a bldg
bldg C t t t
Q R ⎟⎟⋅ −Δ −
⎠
⎞
⎜⎜
⎝
⎛ +
= 1 & 0
& Eq. 8.1
The design compressor capacity will be limited by the evaporation temperature and the penalty paid by reduced efficiency when the heat pump operates under frosting conditions. As the indoor air is the heat source for the exhaust air heat
pump, the source temperature is almost constant throughout the year. Thus, if the heat pump is designed such that frosting occurs on the evaporator during the coldest days (when the heat demand is high) frosting may occur throughout the year. Using a variable-speed compressor in the heat pump makes it possible to increase the capacity of the heat pump without the draw-back of frosting during the major part of the year. As in ground-source systems the efficiency of the heat pump system will increase when operating at reduced capacity when the heat pump does not operate intermittently.
Considering defrosting, there is no need for a special function as the high temperature heat source can be used for melting the frost. It is sufficient to just stop the operation of the compressor and let the warm exhaust air melt the frost.
Figure 8.1 Schematic diagram of exhaust air heat pump rejecting heat to a hydronic heating system. Normally these systems also reject heat to a sanitary hot water tank.
As the drive power of the compressor in an exhaust air heat pump for domestic use is rather small (approximately 500 W) the efficiency and operation of fans and pumps is even more important than it is for ground-source heat pumps. Thus, the potential for applying efficient variable-speed pumps and fans and applying an on-line optimisation routine should have a greater potential than for ground-source heat pump systems.
The optimisation of fan, pump and compressor speeds will have more boundary conditions to consider than is the case for outdoor air and ground-source systems.
To summarise, there seem to be a clear potential for improved energy efficiency of exhaust-air heat pumps by applying variable-speed compressors, pumps and fans. The potential lies in both increased energy coverage and improved performance at part load.
8.3 Other heat sinks
The methods and principles presented in this thesis should apply also if the heat pump is connected to warm-air distributed systems or larger hydronic heating systems in for example office buildings. For larger systems there is the possibility to apply sequencing of several compressors instead of applying variable-speed capacity control. One option could be to have a number of single-speed
compressors and use only one variable-speed compressor for adapting the capacity to the load. A steady-state on-line optimisation method should also be possible to apply, if having the distribution pumps controlled by their inherent control algorithms, for example proportional pressure difference. For fan-coil systems the control of the fan speed should also be included in the optimisation process as they otherwise will reduce system efficiency at reduced capacities in the same way as previously discussed for the circulators.
Sanitary hot water is another common heat sink for hydronic heat pump heating systems. The most common solution is that the heat pump is used both for space heating and sanitary hot water heating. Normally, the hot water is stored in a double-walled storage tank, where the high-temperature heat transfer medium coming from the condenser is heating the hot water inside the tank, see Figure 8.2.
The heat transferred from the high-temperature heat transfer medium to the hot water is limited by the heat transfer capacity of the double-wall. Thus, a number of on and off cycles are normally needed for heating up the water. Using a variable-speed capacity controlled compressor instead, it will be possible to heat the tank to the set temperature in fewer cycles and also get a higher temperature of the sanitary hot water. This is possible because the capacity of the heat pump can be adapted to the heat transfer capacity of the heat exchanger (double wall). As the temperature of the water in the double-wall increases the capacity of the heat pump can be reduced, thus transferring heat at a lower capacity. This allows more heat to transfer into the storage tank before the temperature of the water in the double-wall gets too high and the operation is shut off. It is, of course, possible to heat sanitary hot water with a low capacity heat pump and in this way achieve the high temperature level, but then it will take a long time to heat up the water in the storage. Using a variable-speed capacity controlled compressor makes it possible to combine both high capacity, thus fast reheat, and low capacity and high temperature. There are other possibilities of heating the sanitary hot water but generally the better the heat transfer capacity of the heat exchange, the less important should be the possibility of adapting capacity.
Figure 8.2 The figure shows the parts and flows of a typical ground-source heat pump. The high-temperature heat transfer medium can either be used for space heating or for sanitary hot water heating. The use is
controlled by the three-way valve. (Source: Nibe)