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3 – Theoretical Background and System Analysis Table 3.3 presents the required charging periods at typical DHW demands when the total heating capacity of the DHW gas cooler units ranges from 1 to 3 kW. For the calculations it has been assumed a city water tempera- ture of 10oC and a DHW temperature of 70oC. The water consumption data are from Appendix F, Characteristic Properties of DHW Systems. Table 3.3 Tapping Sites Washbasin x 3 Shower x 2 Bath-tub x 1 Aggregated demand 1) DHW flow rate (40oC) [l/s] 0.1 x 3 0.2 x 2 0.3 - [min] [litres] 1.1 20 5.0 120 7.8 140 at 1 kW at 2 kW at 3 kW 0.7 0.4 0.2 4.2 2.1 1.4 4.9 2.4 1.6 9.8 4.9 3.2 The required charging period τ [hours] at different DHW demands and gas cooler heating capacities. m 1) Time V 2) τ [h] τ [h] τ [h] T T123 - 280 2) Total DHW demand (40oC) The duration of the charging period at the actual operating conditions is about 10 to 50 times longer than the duration of the tapping period. 3.3.2.3 The Reheating Mode In periods with little or no DHW demand in the house, the water tempera- ture in the tank may drop below the set-point due to heat loss from the tank shell, and the water needs to be reheated. Owing to the relatively high temperature level in the tank, only the DHW reheating gas cooler unit (C) can be used during the reheating mode. The DHW preheating gas cooler unit (A) can be by-passed either on the water side or the CO2 side. When by-passing on the water side, two solenoid valves and a temperature sensor are required. Figure 3.23 shows the principle of the by-pass arrangement. The extra tubing and components will ensure proper operation of the tripartite gas cooler, but will increase the cost and complexity of the system. An alternative solution for the reheating mode is to use the electric immersion element located near the bottom of the tank for reheating of the DHW. When the heat pump unit is operated in the combined heating mode, the by-pass arrangement will be needed if the inlet water temperature from the DHW tank exceeds the CO2 outlet temperature from the space heating gas cooler (B). 54PDF Image | Residential CO2 Heat Pump System for Combined
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