With or without PV, air-source heat pumps are among cheapest residential heating sources

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Researchers at the Technical University of Munich in Germany have evaluated the environmental, economic, and eco-efficiency performance of 13 different heating systems for a typical German two-story dwelling and have found that air-source heat pumps and gas systems are the most cost-effective solutions.

The scientists specified that identifying the cheapest residential heating source remains challenging, as the final results depend on the assumptions made and energy prices. But they also pointed out that differences in costs between all the analyzed systems are small, which they said makes it challenging to establish a clear ranking based solely on economic evaluation.

For this reason, their methodology includes both environmental and economic aspects into a single indicator. The life cycle assessment (LCA) method was applied to evaluate the environmental impact of the systems and the net present value (NPV) approach was utilized to assess their economic viability. The analysis also included a best-case and a worst-case scenario with different initial investment costs.

For the two-story dwelling, the researchers assumed it operates under climate conditions in Munich with a standard heating load of 5 kW and an annual heating requirement of 8,066 kWh. They also considered that all the systems have a thermal storage tank that heats the hot water continuously and a lifecycle of 20 years.

The 13 systems taken into account by the analysis were: Natural gas condensing boiler; natural gas condensing boiler with solar thermal system; pellet boiler; pellet boiler with solar thermal system; wood gasification boiler; wood gasification boiler with solar thermal system; air-source heat pump; air-source heat pump with PV system; ground-source heat pump with geothermal probe; ground-source heat pump with geothermal probe and PV system; water-source heat pump; water-source heat pump with PV system; and ground-source heat pump with ice storage and solar collectors.

“A target flow temperature of 35 C is specified for the considered standard outdoor temperature of -12.9 C,” the research team explained. “The calculation of the hot water demand is based on a daily hot water demand of 40 l per person at 50 C, using a typical consumption profile with peak loads in the morning, at noon, and in the evening.”

The academics also considered that only 6% of the electricity generated by the PV system is consumed directly by the heat pump, with the remaining percentage being injected into the grid or used by household appliances. “It should be noted that the systems were not optimized for PV self-consumption,” the scientists highlighted.

The analysis showed that air-water heat pumps with and without a PV system, as well as the gas heating systems are the most cost-effective solutions among the 13 systems considered. In terms of “eco-efficiency,” the scientists found that the air source heat pump with a PV system and the log wood gasification boiler perform better than all other systems.

“Three other heating systems — water and ground source heat pumps with PV and air-source heat pumps — also exhibit higher eco-efficiency than the gas heating system,” they added. “In contrast, the ice-storage heat pump and the pellet heating with solar thermal support show the lowest eco-efficiency.”

Their findings can be found in the study “Environmental, economic, and eco-efficiency assessment of residential heating systems for low-rise buildings,” which was recently published in the Journal of Building Engineering.

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