2.7 Heat Pumps
2.7.1 Heat Pumps in General
The greatest trait of a heat pump, is that it manages to get more energy out of a heat source than originally seemed possible. This is done through a process that will be explained a bit further into this section, but it means that a heat pump can cover a demand much higher than the power it requires. This ability makes the use of heat pumps to replace heaters running purely on oil or electricity one of the main measures to ensure a more sustainable energy deliverance [60].
Heat pumps exploit natural temperature differences to slightly rise the tempera-ture of a circulating refrigerant, before compressing it to a higher temperatempera-ture level.
This means that no electricity goes directly to the production of heat, but instead is used for the work load of the compressor.
Boiler El. radiator Heat pump 40
60 80 100 120
140 130
100
Energydemand[kW] 35
Figure 2.20: Typical energy demands for different methods of 100kW heat production The use of heat pumps is an efficient way to produce energy for space heating, heating of DHW and space cooling. Figure 2.20 shows the principal of this. The energy delivered by the systems is 100 kW, and the graph shows how much energy the different systems need in order to cover this. Using equation 2.18, the efficiency, η
[−], of the boiler, electrical radiator and heat pump are 0.78, 1.0 and 2.9, respectively.
η= delivered energy
used energy (2.18)
A boiler system includes a lot of heat losses, which makes it consume more energy than it delivers. This is the reason why boiler systems are energy demanding. The electrical radiator has approximately no losses and is therefor 100 % efficient. The heat pump produces more energy than it consumes, and has thereby an efficiency of more than 100%.
This efficiency of a heat pump, and thus the energy used, is dependent on the refrigerant, which will be discussed in more detail in section 2.7.2. The energy savings, however, can be as large as 50 to 80 % compared to that of conventional heating systems [60].
Industrial heat pumps (IHP) are heat pumps that use excess heat from other industrial processes. Regular heat pumps use heat sources like geothermal wells or water basins. IHP are preferred due to their high COP as a result of low temperature lifts. They also usually have low installation costs, and the distance between the heat source and sink is generally short. [56]
The Process of a Heat Pump
The concept of a heat pump, is to transfer heat from one place with low tem-perature, to another place with higher temperature. According to the 2nd law of thermodynamics, this is not possible in a natural manner. However, with the help of compressors, valves and heat exchangers, heat pumps are able to do it none the less. [58]
The main components of a heat pump that need to be present in order for the process to work, are pipes, one compressor, one expansion valve, and two heat exchangers, also referred to as a condenser and an evaporator. Other components, like accumulation tanks and additional heat exchangers could also be present to get even higher efficiencies.
Figure 2.21: The simplified process of a heat pump
Further, a heat pump consists of three main circuits. These are the low temper-ature side, also referred to as the heat source, the high tempertemper-ature side, referred to as the heat sink, and a refrigerant circuit. Figure 2.21 shows the composition of a heat pump, where the refrigerant circulates between the condenser and evaporator.
As seen in the figure, the evaporator draws energy,Q˙E, from the heat source, and the condenser delivers energy,Q˙C, to the heat sink. In the condenser, the refrigerant cools down and condenses while delivering heat to the high temperature side liquid.
In the evaporator, the refrigerant warms up and evaporates while receiving heat from the low temperature side liquid.
To make this happen, the compressor and expansion valve regulate the pressure levels of the refrigerant. This to make sure the evaporation temperature in the evaporator is lower than the heat source temperature. Similarly for the condenser, they work together to make sure the condensing temperature is higher than the heat sink temperature. The supplied energy,W˙ , in the compressor is the only non-natural energy transfer happening in the heat pump.
Heat Pump in ph-Diagram
The process shown in figure 2.21, can also be represented by the ph-diagram shown in figure 2.22. This diagram follows a refrigerant cycle of ammonia, with the specific enthalpy, h [kJ/kg], plotted against the corresponding pressure, P [bar]. Specific enthalpy represents the energy present in a system as a result of the pressure and temperature of the system. The figure is plotted in the simulation tool DaVE, which will be explained more in depth in chapter 3. [58]
Figure 2.22: The heat pump process in a ph-diagram
This diagram represents the phases the refrigerant goes through in the heat pump. The dome in the middle represents the two-phase stage, being both liquid and gas. The red lines represent constant temperature levels, with a total difference of 20 K between each line. Where the black and red line meet to the left of the dome, everything is liquid. Moving along the red line to the other side of the dome,
more liquid turns into gas without changing the temperature, until everything is gas on the right side of the dome.
In the figure, the refrigerant warms and evaporates form point 1 to point 2, gaining energy from the heat source at constant pressure. From point 2 to point 3, the compressor compresses the refrigerant, causing T and P to increase rapidly.
Then, at point 3, the refrigerant enters the condenser, gas turns into liquid, transfers heat to the heat sink, and cools to a lower temperature at constant pressure from point 3 to point 4. Finally, the refrigerant enters the valve where the pressure is relieved, and T drops back to point 1. The enthalpy value, h, however, stays the same between point 4 and point 1.
In figure 2.22, it can also be seen that point 2 is slightly extended into the gas region. This is to ensure that only gas enters the compressor. Liquid is not com-pressible, and if liquid is to enter the compressor, the efficiency will drop. Therefore, it is often common to have a superheat, ∆TSH [K], of about 5 K, meaning having a temperature of the fluid at 5 K higher than the evaporation temperature for that pressure level.
Energy Evaluation
Figure 2.21 and 2.22 indicates that, for a heat pump delivering heat, the delivered energy at the high temperature side, corresponds to the supplied energy in the compressor and the drawn energy from the low temperature side combined. This is also illustrated by equation 2.19. [56]
Q˙C = ˙QE + ˙W [kW] (2.19) This equation shows that the heat transfer rate in the condenser,Q˙C [kW], equals the heat transfer rate in the evaporator, Q˙E [kW], and the work performed on the system, W˙ [kW], combined.
In real life, there will be losses in the compressor, causing it to deliver less energy than what it requires. Isentropic efficiency, ηis [−], is the ratio between the power input in the compressor and the power input from the compressor to the heat pump process.
Usually, for heat pumps used for buildings, it is true that a heat pump that can cover 50 % of the peak demand can also cover 90 % of the total demand.
Coefficient of Performance
To evaluate how well a heat pump operates, the efficiency of the system needs to be calculated. The efficiency of a heat pump is determined by the coefficient of performance (COP). This value states the ratio between useful heating or cooling provided, Q, and the amount of work needed, W. Thus, the COP for a heating process can be calculated by equation 2.20.
COPH = QC
W (2.20)
Equation 2.21 shows the COP for a cooling purposes. For both equations, the higher the COP value is, the less energy demand is needed in the compressor for
the system to deliver the necessary energy. However, to compensate for this, more energy is needed from the energy source.
COPC = QE
W (2.21)
10 20 30 40 50
10 20 30
Temperature rise [K]
COP[−]
Figure 2.23: Theoretical COP related to the temperature rise of the heat pump There is a close relationship between the temperature rise within the refrigerant and the COP value. This i shown in figure 2.23, where the graph indicates the lower the temperature difference between the evaporator and condenser, the higher COP values can be reached [58].