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Energies 2021, 14, 4103 12 of 28 excessive DHW demands will inevitably result in aggressive charging to meet demands. The control logic for the aggressive charging strategy is shown in Figure 4. Similar to the leveled charging strategy illustrated in Figure 3, six separate zones, Z, are applied. Also, Lmin and Lmax are defined as 50 kW and 110 kW, respectively. The starting point of the algorithm for the aggressive charging strategy is when the energy in the storage, Ei, reaches its minimum, Emin. The charging load, Li, is set to zero when the energy in the storage reaches its maximum, Emax. Calculate Ei and Zi DHW CONTROLLER INPUT T , T , ..., T .1 2 n Ei = Emin? YES QDHW,usage . NO QDHW,supply NO Li = Lmin Li = Lmax YES NO Li < Lmin? NO Ei = Emax? YES YES NO Zi < Zi-1? NO Zi > Zi-1? YES Li = Li-1 L = L + L i i s 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 Li = Lmax? Figure 4. Simplified decision tree control logic to determine DHW charging load with the aggressive charging strategy. 3.3. Operating Range The integrated CO2 heat pumps have been designed with the aim of achieving high performance through system flexibility. An ambient temperature span from −15 to 35 ◦C has been selected to demonstrate different operating modes in a typical Scandinavian climate. The assumed SH and AC cooling loads supplied by the heat pump at different am- bient temperatures are listed in Table 3. The loads have been established based on thermal demands of a medium-sized Norwegian hotel [29]. The setpoint curve for the SH water temperature is dependent on ambient conditions and is shown in Figure 5. The dimension- ing supply and return temperature of chilled water are 7 and 12 ◦C, respectively. The values have been defined as per the industry rule-of-thumb for AC cooling in Scandinavia [45]. Table 3. Ambient temperature dependent space heating and AC cooling loads supplied by the integrated system. Ambient temperature [◦ C] −15 −10 −5 0 5 10 15 20 25 30 35 Space heating load [kW] 180 180 140 100 80 60 40 0 0 0 0 ACcoolingload [kW] 0 0 0 0 0 0 0 40 80 150 220 Li = 0 Figure 5. Set point curve for hydronic circuit for space heating depending on ambient temperature.PDF Image | Evaluation of Integrated Concepts with CO2 for Heating
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