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After the pretesting, the main tests for heat pump COPs at varying water inlet temperatures to the gas cooler were done at the following outdoor temperatures: -10, -5, 0 and 7 °C. During the main tests, the heat pump heating COP were recorded during significantly longer period of constant water inlet temperature than for the pre-tests. As mentioned previously the performance of CO2 heat pumps is strongly influenced by the water inlet temperature to its gas cooler due to the high throttling losses for cycles operating near the critical point. For applications in real buildings, operating under varying conditions, the heating demand should be divided into two main categories: space heating (radiator water heating) and tap water heating. The space heating can be further roughly divided into three sub-categories: high temperature space heating, medium temperature space heating and low temperature space heating (I.e. floor heating). Depending upon the building design and type of heat distribution system (e.g. old poor insulated houses, new well insulated houses), different temperature levels for space heating (the heating system) will be required. Accordingly, the ratio between the energy demand for space heating and tap water heating will be different. All these differences are important for selecting the right heat pump systems and will decide whether the performance of a CO2 heat pump can compete with conventional heat pump systems or not. The European test standard (EN14511) specifies a heat pump testing standard with a constant water inlet temperature of 40°C and an outlet temperature 45°C at an outdoor temperature of 7 °C (with variations for different temperature levels). This specified condition however does not reflect all possible “real house conditions” found on the market. Furthermore, it also does not show the difference between modern houses, where the heating demand for tap water heating is much higher than space heating and the old houses, where the heating demand is relatively large. This means that tests done at a 40°C water inlet temperature not necessarily reflect conditions when the CO2 heat pump could (or should) be used (i.e. a space heating system with low return water temperature or. modern houses with floor heating systems). Test results based on the standard EN14511 therefore cannot give a good comparison between CO2 and traditional heat pumps. For the reasons stated above, the heat pump unit is here tested based on testing condition given in the heat pump manual and the testing results will thus be compared with the results given by the manual. During the testing, data is recorded and calculated when the heat pump working conditions are stable for about 30 minutes. An example of recorded results at 7 °C outdoor temperature is shown in figure 12. With this manner, the heat pump’s COP is tested against different water inlet temperatures at different outdoor temperatures (i.e.-10, -5, 0 and 7 °C respectively). Figure 13 shows all the test results. As shown in figure (Figure 13), the heat pumps heating COP decreases with increasing water inlet temperature as expected and it decreases with decreasing outdoor temperature (as usual). One may notice that the points in Figure 13 are not all monitored at the same water inlet temperature. This is due to the reason that the water pump, which controls the water flow rate from the water tank to the heat pump, was controlled by the built-in control algorithm. This algorithm changes the flow rate dependant upon the tank conditions, which influences the water inlet temperature and so on. 21PDF Image | CO2 heat pumps for the Swedish market
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