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♦ Double-shell DHW tanks are constructed from a cylindrical primary vessel for the DHW and a secondary vessel or water jacket. Typical water volumes for the primary/secondary vessels are 200/120 and 300/120 litres. The hot water from the heat pump condenser circu- lates through the secondary vessel and preheats the DHW. Reheating and back-up heating of the DHW are usually provided by an electric immersion element. 2.4.2.3 Examples of System Designs Utilization of Condenser Heat for Hot Water Heating The large majority of residential brine-to-water and water-to-water heat pumps preheat DHW by means of the condenser heat, and the higher the outlet water temperature from the condenser, the less reheating is required in order to meet the minimum DHW storage temperature of 55 to 60oC. The minimum storage temperature is set to prevent growth of the legio- nella bacteria that cause the fatal legionnaires’ disease (USHA, 2003). Figure 2.4 shows the principle of two different residential pump systems for low-temperature space heating and DHW heating. In alternative A, the system comprises an accumulator tank and a single- shell DHW tank with an integral tube-coil heat exchanger. Double-shell DHW tanks are also commonly used. The shuttle-valve directs the water flow either to the accumulator tank for the space heating system or the DHW tank, and heating of DHW is prioritized. When the heat pump unit supplies heat to the space heating system, the required supply temperature from the heat pump condenser is determined by the outdoor temperature and the return temperature in the space heating system, i.e. variable condensation temperature. During operation in the DWH mode, the water flow rate through the condenser will be lower and the return temperature will be higher than that of the space heating mode. Depending on the type of working fluid, the heat pump unit is able to preheat the DHW to a temperature of 45 to 60oC. In alternative B, the secondary vessel in the double-shell DHW tank acts as the thermal storage, and heat is transferred to the DHW through the bottom of the primary vessel. When the heat pump unit delivers heat to a low-temperature floor heating system, there will be an inevitable trade-off between the supply temperature from the condenser, which determines the COP of the heat pump unit and the degree of DHW preheating, and the need for supplementary heating in the DHW system. The lower the supply temperature, the more supplementary heating is required. 2 – Technological Status 15PDF Image | Residential CO2 Heat Pump System for Combined
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