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5.9.1. Operation principle

A desuperheater is a heat exchanger that uses extra heat available from the refrigerant vapour at the exit of a heat pump compressor to heat water. It comes as an option on most ground source heat pumps for domestic applications, and on some air source heat pump as well. Because it only recuperates extra heat when the heat pump is on, a desuperheater can only provide part of the annual DHW needs. Figures 5.17 and 5.18 show how the desuperheater works when the heat pump is respectively in cooling or heating mode.

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Figure 5.18 – Operation of a heat pump with a desuperheater in heating mode

When the heat pump is in cooling mode, it takes heat from the air (in this case, through the fan coil), compresses the warm refrigerant to increase its temperature and then rejects the heat to a heat sink, which is the outside air for an air source heat pump, and the ground loop fluid for a ground source heat pump. Because the heat needs to be rejected and because it is available at a high temperature at the exit of the compressor, the domestic hot water can then be heated efficiently and for free under these conditions.

When the heat pump is in heating mode, the compressed, high temperature and high pressure refrigerant is used to heat the air. Therefore, removing heat from the refrigerant is not free. However, this heat is provided to the water with the rather high COP of the heat pump, so it makes sense to run the heat pump slightly longer to provide DHW.

116 5.9.2. Modeling

The modeling of this system is based on a methodology previously described by Picard et al. (2007). Because the DHW heating capacity of the desuperheater is given as a function of the entering ground loop fluid temperature and the status of the heat pump (first or second stage, in heating or cooling mode), it can be decoupled from the whole house simulation. A result file, obtained from the house simulation, contains the status of the heat pump and the temperature of the fluid entering the heat pump for each time step. A model is then developed to simulate the whole system, including the desuperheater, the circulation pump, the electric DHW tank and the DHW usage profile, as can be seen in Figure 5.19.

Figure 5.19 – TRNSYS model for desuperheater and electric DHW tank

As for the the solar water heater system and the electrical DHW tank, Type9a reads the DHW usage profile and Type28 (DHW_Consump) calculates the energy consumption for the auxiliary heating of water in the DHW tank.

Type9a-2 reads the results file which includes the heat pump status and the entering fluid temperature for each time step. The heat pump status is represented as follow: 1 or 2 refer to the stage in which the heat pump is operating, and a negative value means that it is in cooling mode. When the heat pump is turned off, its status is set to 0.

117 Type581c is a multi-dimensional interpolation tool. In this case, it is used to interpolate one dependent variable (the DHW heating capacity) for two independent variables (heat pump status and entering group loop fluid temperature). The table giving DHW heating capacity for the desuperheater, as presented in the technical literature provided by the manufacturer, is available at Appendix C.

The Desuperheater equation block calculates the temperature of the hot water after it passes through the desuperheater, according to equation 5.6.

(Eq. 5.6)

where Td,in is the temperature of water at the inlet of the desuperheater in °C, DSHcap is the

desuperheater water heating capacity in W, is the mass flow rate of water through the desuperheater in kg/s and cpwater is the specific heat of water (4.19 kJ/kg·K)

Because heating the water through the desuperheater when the heat pump is in heating mode takes heat out of the liquid that would otherwise heat the air inside the house, the heat pump requires to run for a longer time. Therefore, the water is produced not with waste heat that does not affect the electricity consumption of the heat pump, but with the same COP as the heat air. Therefore, the extra electricity consumption to produce that hot water has to be calculated, using Equation 5.7.

(Eq. 5.7)

where qcompDHW is the extra electrical input needed by the compressor to heat the water in W, COPGSHP is the coefficient of performance of the ground-source heat pump, and gt(Stage,0) is a Boolean operator that returns 1 when Stage, the heat pump stage, is positive (therefore in

118 heating mode), and 0 when it is negative (in cooling mode). This value in integrated over the year to obtain an electrical consumption in kWh.

Type3b is used to model the energy consumption of the circulation pump. According to Picard et al. (2007), for a 2 ton heat pump, a 50 W circulation pump is required for the desuperheater. The pump is turned on (at full flow) when the heat pump is in function, and its power consumption is integrated by Type28 to obtain the annual electricity consumption.

However, it is to be noted that the results given for the desuperheater are slightly optimistic. The error comes from the entering water temperature, which is assumed to be 32 °C for the desuperheater capacity data given by the manufacturer. Because the desuperheater system presented here does not have a pre-heating tank, the entering water temperature is about 45°C, which means that the efficiency of the heat exchanger will be reduced because of the smaller temperature difference between the cold and the hot sides. As the results already show that, even in an ideal system setting (with a pre-heating tank), the system would not provide significant energy savings, the possibility of either improving the mathematical model or adding a pre-heating tank is excluded.