Performance parameters for residential heat pumps linked to PV, storage

German researchers measured a PV-powered heat pump with battery storage in a single-family home in Freiburg, Germany, for a period of a year. It features smart grid-ready tech that adjusts operations based on the grid.

Image: Ppntori, Wikimedia Commons

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Researchers led by the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) in Germany have studied a residential heat pump (HP) installation coupled with PV, battery storage, and a smart grid-ready system.

“In-depth research is missing in terms of the impact of smart control on the dynamic performance efficiency of the heat pump,” the researchers said. “Extensive work needs to be done to analyze the effect of PV and battery units on the energy consumed by the heat pump by redefining the system boundaries of the PV-HP systems.”

Their analysis is based on a year of data from a single-family home in Freiburg, Germany. It uses a ground-source heat pump with a nominal capacity of 13.9 kW. The PV unit has a module area of 60 square meters and a nominal power rating of 12.3 kW. The battery has a capacity of 11.7 kWh. The system had to account for heating a living space of 256 square meters, as well as a domestic hot water (DHW) tank.

The heat pump of the setup has an SG-Ready label, which means it can communicate with the grid and adjust its own operations. In the context of this research, however, it was used to maximize the PV self-consumption by adjusting the heat pump operation based on available PV electricity.

“In the analyzed system, the SG-Ready is configured to trigger the heat pump in boosted operation, whereby the space heating and DHW supply temperatures are increased,” the group explained. “The SG-Ready mode is activated when the battery is fully charged or is charging at its maximum power, and there is still a PV surplus available.”

When the instantaneous PV power stays below the total building demand for at least 10 minutes, the trigger-off condition is met, leading the heat pump to return to normal operation as defined by the system parameters, said the researchers. The thermal and electrical data of the systems used in the research was collected throughout the entirety of 2022 and analyzed at a one-minute resolution.

The researchers said the combined system achieved a self-consumption rate of 43% per year. During the winter, with reduced PV generation, the rate reached 94% to 100%. In the high-PV summer months, self-consumption dropped below 50%, reaching its lowest point in July at 25%.

The scientists also looked into solar fraction (SF), which refers to the ratio of PV electricity supplied directly to the heat pump load, or through battery discharge, to the total heat pump electricity consumption.

“During the evaluation period, the heat pump consumed a total of 5,064 kWh of which a major 63.8% was contributed by grid supply,” they said. “The PV array supplied 899 kWh (17.8%), and the battery unit supplied 934 kWh (18.4%), resulting in an annual solar fraction of 0.36.”

The researchers calculated the seasonal performance factor (SPF) for the combined system, yielding a rate of 6.7, a 59.5% improvement compared to a system solely reliant on the grid to feed the heat pump. Additionally, the smart grid-ready system increased supply temperatures for domestic hot water by 4.1 K and space heating by 1.8 K. However, only 5% of thermal energy in space heating mode and 28% in domestic hot water mode were provided at elevated temperatures.

“The temperature increase reduces the efficiency of 3.5 by 0.2 points and thus by 5.7%,” they said. “In space heating mode, the efficiency is reduced from 5.0 to 4.8 and thus by only 4.0%.”

They presented their findings in “Analysis of the Performance and Operation of a Photovoltaic-Battery Heat Pump System Based on Field Measurement Data,” which was recently published in Solar Energy Advances. The team included scientists from the Offenburg University of Applied Sciences.

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