Latent heat storage integration into heat pump based heating systems for energy-efficient load shifting

Integrating latent heat thermal energy storage (LHTES) units into building heating systems has been increasingly investigated as a heat load management technology. A conventional LHTES integration method for heat pump based heating systems is to connect the heat pump's condenser for charging the LHTES unit. This integrating layout however usually leads to increased electricity input to the heating system. To underline this issue and provide solutions, this paper presents three new LHTES integrating layouts where the LHTES unit is connected with the de-superheater of the main heat pump (Case 2), the condenser of a cascaded booster heat pump cycle (Case 3), or a combination of using both the de-superheater and the booster cycle (Case 4). In the context of a multi-family house in Stockholm, a quasi-steady state heating system model was developed to evaluate the new integrating layouts, which were benchmarked against the baseline heating system without storage (Case 0) and the conventional integrating layout using the main heat pump condenser (Case 1). Hourly electric power input to the heating system was modelled for calculating the performance indicators including the heating performance factor, the operational expense and justifiable capital expense, and the indirect CO2 emissions. Two load shifting strategies were simulated for an evaluation period of Week 1, 2019: 1) charge during off-peak hours (8 pm to 6 am) and 2) charge during daytime hours (10 am to 7 pm). The simulation results of the off-peak charging strategy show that, in Cases 2-4, the heating performance factor is 22%-26% higher than Case 1 and the operational expense can be reduced by 2%-5% as compared with Case 0. The savings in the operational expense can justify the capital expense of 11 k-25 k Swedish Krona (SEK) for the LHTES systems in Cases 2-4 assuming a 15-year operation. Furthermore, the advantage of using the daytime charging strategy is principally the mitigation of CO2 emissions, which is up to 14% lower than the off-peak charging strategy. In summary, higher energy efficiency for heating is validated in the three new proposed integration layouts (Cases 2-4) against the condenser charging layout.