Investigation of the effect of storage operation in Single-Family Homes with Separate Tanks for Space Heating and DHW
While heat pump systems in existing residential buildings already achieve significant CO2-savings compared to fossil heating systems, field measurements still show optimization potential regarding their performance. One key factor influencing the operation of heat pump systems is the integration and operation of the thermal energy storage, as it couples heat generation and heat demand side. This study investigates a typical heat pump system hydraulic, comprising a parallel buffer storage for space heating (SH) and a domestic hot water (DHW) tank. The analysis is based on monitoring data of single-family houses in Germany. It involves a total of seven heat pump systems from five different manufactures, consisting of four air source heat pumps and three ground source heat pumps, all with a nominal thermal capacity of less than 12 kW and inverter-controlled compressors. The measurements and evaluations were conducted throughout the year 2023. The measurement concept was designed to monitor the efficiency and operational behavior of heat pump systems. To achieve this, all necessary energy flows are recorded with minute-by-minute precision. The cooling mode available in part of the installations was not evaluated. The results reveal a tendency for the heat pumps to produce heat at a rate exceeding the actual heating load of the houses, suggesting that the systems may be overdimensioned and/or that modulation control is suboptimal. Additionally, the analysis uncovered unbalanced flow rates between tank charging and discharging. These imbalances can cause unfavorable thermal conditions within the storage, including mixing effects and poor thermal stratification, ultimately compromising the overall performance of the heating system and leading to unfavorable operation with frequent on/off cycling of the heat pump. To further understand this behavior, simulations were conducted to evaluate smoother and favorable operational.