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Technological innovations in heat pump systems 277 Figure 19. Percentage of total effective heat pump operating hours used for defrosting (mean values for the heat pumps measured; HG = hot-gas defrosting, PR = defrosting using cycle reversal; for PR: bottom part is for actual defrosting process, and upper part is for heating up the heating system again). The required energy to operate defrosting is not negligible. Fig. 18 reports the percentage of energy required for defrosting for different working conditions for the outside air and the heated water temperature [7]. The bars refer respectively to defrosting by hot gas injection or cycle reversing. About 10% of the energy required by the heat pump is due to defrosting with worse performance of the hot gas injec- tion mode that is, however, less penalising for thermal comfort. The analysis also shows the required time to provide defrosting (Fig. 19). It is a function of outside air conditions and it sometimes exceeds 10% of the heat pump total working period. In the case of cycle reversing mode the upper bar gives the time fraction required for heating up the heating system again. A recent proposal to mitigate the energy request and thermal discomfort due to defrosting was produced by Oak Ridge National Laboratory [8]. The starting point is the survey that the frost formation is not significant if the surface of the outside battery is kept over −2.8oC when the outside temperature is between 0oC and 5oC. A small thermal power is provided to the liquid accumulator before the compressor. For a heat pump whose capacity is between 10–15 kW the required thermal power is of about 1 kW. This heat (that returns as room heating effect) allows increased suction pressure and temperature with a delay in frost formation. Laboratory tests revealed that for the same working conditions frost develops after 100 minutes in the traditional system, whereas the proposed system requires defrosting after about International Journal of Low Carbon Technologies 2/3 Downloaded from https://academic.oup.com/ijlct/article/2/3/262/758006 by guest on 21 June 2022PDF Image | Technological innovations in heat pump systems
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