Photovoltaics and geothermal heat pumps for domestic hot water heating

Researchers from the Lappeenranta University of Technology (LUT) have outlined a series of control methods for domestic hot water (DHW) heating produced by a ground source heat pump (GSHP), in combination with a rooftop PV system, in Nordic climates.

GSHPs, which are also known as geothermal heat pumps, utilize shallow-ground energy to achieve space heating and cooling and are able to transfer heat to or from the ground.

The four methods were tested on two system configurations located in Finland. One system is based on an F1255-12 R EM heat pump with a capacity of 12 kW provided by U.K.-based provider Nibe Energy Systems Limited, and an east-south-west-oriented 21 kW rooftop PV system. The second system relies on the same kind of heat pump and a 5 kW south-oriented solar array.

Both systems include a hot water storage tank with a capacity of 500 liters and, for both configurations, solar power is first used to meet the requirements of the baseload demand of the household and then is made available for DHW heating. Space heating is not provided by the proposed solution and the heat pump is used exclusively to heat the hot water tank storage.

“Energy consumption of DHW is calculated by assuming that the house is a typical four-person household with a DHW consumption of 200l per day,” the researchers specified. “The cold inlet water temperature is assumed to be 10 degrees Celsius, while the hot outlet water temperature is 60 degrees Celsius.” Based on these data, the electricity demand for DHW was found to be 11.64 kWh per day. Heating is performed between 8.00 a.m. and 8.00 p.m.

With the first of the four methods, which the Finnish group defined as a “base case,” the water tank is heated from 45 to 55 degrees Celsius twice a day and the highest consumption peaks occur in the morning and in the evening. As for the second method, which the scientists called “clock control,” the tank is heated once a day from 45 to 65 degrees Celsius, and heating is carried out mostly at noon hours when PV power generation is higher.

The third method, which was labeled as “energy-optimal,” is aimed at optimizing the coefficient of performance (COP) value of the heat pump. The COP defines the ratio between the useful heat transfer for heating or cooling and the required drive energy. This approach is said to reduce electricity consumption in water heating by maintaining the storage tank temperature at 45 degrees Celsius. “The low chosen-temperature limit can be justified by the fact that it is just enough to guarantee the comforts provided by hot water,” the academics explained.

The fourth method, dubbed as “cost-optimal,” is intended at minimizing heating costs and using cheap grid electricity together with solar power. Under this configuration, water is heated once a day, from 45 to 64 degrees Celsius, and heating duration is established based on the COP curve, DHW demand, PV generation forecasts, and expected spot market prices.

“The results in the period between June and September 2020 show that the developed cost-optimal control reduces DHW heating costs compared with the other three control cases, even with the use of an actual PV output forecast,” the research team stated. “Considering the share of PV production in DHW heating, the cost-optimal control with the 5 kWp PV system shows a greater PV production share than the other three control methods, even though the imperfect PV forecast is used.”

With the 21 kW array, the share of solar power is higher in the clock control than in the cost-optimal approach and, if PV forecasts are accurate, DHW heating costs are lower. “The simulation results also indicate that as the size of the PV system increases, the accuracy of the PV production forecast loses its significance,” it further explained. “In the case of the 21.1 kWp PV system, the heating cost increased by 9% when the actual PV forecast was applied, instead of the perfect forecast while, in the 5 kWp case, the heating cost increased by 11%.”

The four approaches are described in the paper Ground source heat pump control methods for solar photovoltaic-assisted domestic hot water heating, published in Renewable Energy.