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Energies 2021, 14, 4103 11 of 28 3.2. DHW Charging Strategy The DHW charging strategy is a key influencing factor to achieve a high overall system performance in CO2 heat pumps [29]. The thermal storage of 6 m3 provides a buffer that enables a high degree of flexibility with regard to operating strategy. Two different charging strategies, leveled and aggressive charging, are investigated to evaluate the influence of thermal storage operation in light of design and overall performance. 3.2.1. Leveled Charging Charging at reduced loads has the potential to limit return temperatures from the secondary systems and, by this, enhance system performance. A control scheme that aims to reduce DHW charging load and increase charging time has been formulated based on the storage volume and the temperature span across the storage. The simplified decision tree describing the outline of the leveled control strategy algorithm to determine the DHW charging load at time i, Li, is shown in Figure 3. Calculate Ei and Zi DHW CONTROLLER INPUT T , T , ..., T .12 n Zi = Zi-1? YES Li = Li-1 Li = Lmax Li = Li + Ls NO NO Li = 0 YES Ei = Emax? NO YES Li = Li - Ls NO Li = Lmin? Zi > Zi-1? YES NO Li = Lmax? YES NO QDHW,usage . QDHW,supply Lmin Minimum charging load [kW] Lmax Maximum charging load [kW] Ls Change in charging load [kW] Ei Energy in storage at time i [kWh] Zi Storage zone Z(Ei) at time i [-] Li Charging load at time i [kW] YES Ei = Emin? Figure 3. Simplified decision tree control logic to determine DHW charging load with the leveled charging strategy. The DHW storage is divided into six separate zones, Z, which are formulated based on the maximum available energy reservoir of the storage,Emax. For instance, zone 1 applies when the DHW storage has a low temperature and, thus, no useful energy reserve. Zone 6 is reached when all tanks attain a temperature of 70 ◦C. The energy in storage at time i,Ei, is calculated based on DHW controller input variables, which include the temperatures across the storage, T1 − Tn, the rate of energy entering, Q ̇ DHW,supply, and exiting the storage, Q ̇DHW,usage. These values are attained directly in the simulations but could easily be calculated in a real-life system as a function of measured mass flow rates and temperatures of water entering and exiting the DHW storage tanks. The zone at time i,Zi, is further calculated to determine Li. The minimum and maximum loads in which the heat pump can actively produce hot water, Lmin andLmax, are defined as 50 kW and 110 kW, based on the load profile and the size of the heat exchangers. The step value in which the charging load increases or decreases, Ls, is fixed to 10 kW. 3.2.2. Aggressive Charging The aggressive charging strategy represents a common practice in regards to the operation of DHW thermal storage’s [29,30]. The aim of the strategy is to charge the DHW storage tanks periodically from low to high temperature, which results in charging over several periods during the day, usually at high loads and intervals of 8 to 12 h. Typically, the aggressive charging strategy is applied when DHW is produced through heat recovery, e.g., from a supermarket, as the heat source is more available for particular hours of the day [44]. In addition, heat pump operations with under-dimensioned thermal storage orPDF Image | Evaluation of Integrated Concepts with CO2 for Heating
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