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temperature, when the defrosting mode stops. After the defrosting process, the ice built up on the evaporator will melt completely. One of the advantages of a CO2 heat pump is the temperature glide in the supercritical region, which fits the water heating process better than cycles with conventional refrigerants (se figure 1). This enables CO2 heat pumps to achieve a high theoretical COP for these specific conditions when producing hot water only. Therefore, if the CO2 heat pump is used mainly for space heating instead of hot tap water heating, the advantage may not be obvious and such applications will not make CO2 heat pumps appear as a future alternative. Thanks to the temperature glide of supercritical CO2, it is possible to arrange tap water production at different temperature levels by better gas cooler layout designs to meet the demand of both warm radiator water and hot tap water. However, to make the CO2 heat pumps more competitive than conventional heat pumps, the ratio between hot tap water demand and warm radiator water demand has become a critical factor. In principle, the higher the ratio between tap water and radiator heating, the better the CO2 heat pump will be. This ratio is at the same time closely linked to the building standards. In old houses, with a less effective climate shell, this ratio will always be much smaller than for modern well insulated houses. Therefore the CO2 heat pump should fit well in modern low energy houses, with a high ratio between hot tap water demand and space heating demand. Any future development should aim in this direction also realizing the fact that the heat pump size (capacity) needs to be much smaller. The gas cooler outlet temperature has a critical influence on the COP of CO2 heat pump. A low gas cooler outlet temperature can greatly reduce the CO2 heat pump’s throttling losses and increase its COP. Therefore, surrounding system must be designed so that the temperature of the CO2 at the gas cooler outlet temperature is as low as possible. This makes the water tank design critical in order to achieve a good stratification in the tank and ensure low temperature water being delivered to the CO2 heat pump. It can be noticed from the test results that the current CO2 heat pump achieved lower COP than typical traditional heat pumps due to the fact that the water being delivered to the heat pump from water tank always has a relative high temperature It has been noticed that when the heat pump is running in a mode without the electric heater, the tank stratification is destroyed every time the heat pump start to deliver hot water to the tank. It is observed that the highest temperature in the tank appears below the partition plate. Based on the test results, several improvements are therefore suggested: • The inefficient temperature stratification in the water tank is mainly due to the fact that the connection tube (dark blue color in Figure 31) is not exactly connecting two tank parts at the partition plate’s (green color) level, but below it. Consequently, the space between the tube inlet and the partition plate will reserve some of the hot water produced by the heat pump and these results in high water temperatures below the plate. • Based on the findings mentioned above, it is suggested that the return loop (red line in Figure 31) from the heat pump should be inserted slightly above the partition plate, instead of below it. After the modification, the hot water produced by the heat pump will 39PDF Image | CO2 heat pumps for the Swedish market
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CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
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