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3 – Theoretical Background and System Analysis The mean temperature drop in the tank during a 24 hour period is also influenced by the insulation standard, the tank volume and the ambient air temperature. Table 3.4 shows the estimated annual heat loss from a 200 litre DHW tank at different temperatures when using different insulation materials. The boundary conditions are as in Figure 3.24. Table 3.4 The annual heat loss from a 200 litre DHW tank at different DHW temperatures and insulation types. Annual Heat Loss [kWh/year] Insulation type 60°C 70°C Glass-wool 880 1110 XPS 700 880 80°C 90°C 1320 1550 1060 1230 By replacing the glass-wool with expanded polystyrene (EPS), the annual heat loss from the DHW tank will be reduced by about 20%. The annual exergy loss in J due to the heat loss through the tank wall is calculated as: ⎛T−T T−T⎞ ∆E=(Ein−Eout)=Q⋅⎜S 0− R 0⎟ (3.22) where Q is the annual heat loss from the tank, TS is the DHW storage temperature in the tank, TR is the room temperature and T0 is the reference temperature (e.g. the outdoor temperature). At 70oC storage temperature, 20oC ambient air temperature and 0oC reference temperature, the exergy loss constitutes about 15% of the heat loss from the tank. 3.3.3.2 Mixing of Hot and Cold Water During the tapping and charging (heating) periods, the inlet water flows will lead to inevitable mixing of some of the hot and cold water in the DHW tank. This will in turn increase the average inlet water temperature to the DHW preheating gas cooler unit during the charging period, and with that reduce the COP of the CO2 heat pump (ref. Figure 3.4 in Section 3.2.2.1). Figure 3.25 illustrates an idealized mixing process, where Tm and VM refer to the average temperature and the water volume of the mixing zone, respectively. ⎝TS TR⎠ 57PDF Image | Residential CO2 Heat Pump System for Combined
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