South Korean researchers have demonstrated a small-scale solar seasonal thermal storage system integrated with heat pumps for greenhouse heating over three years. The system achieved average thermal storage efficiency of 55.1%, while the dual-source heat pump recorded an average COP of 4.1.
A research team from South Korea’s Korea Institute of Energy Research (KIER) has built and demonstrated a small-scale solar seasonal thermal storage system integrated with heat pumps for greenhouse space heating. The researchers analyzed the system over three years at a two-section greenhouse used to grow Dekopon oranges on 2,090 m² and Setoka oranges on 1,760 m².
“This paper presents the results of a three-year performance analysis of a greenhouse heating system designed to reduce heating costs by using solar energy and a heat pump,” corresponding author Kyoung-Ho Lee told pv magazine. “In contrast to conventional large-scale seasonal thermal storage tanks, this study applied a small-scale storage tank suitable for greenhouse applications and installed it inside the greenhouse machine room to reduce thermal losses.”
Lee said that part of the solar heat stored during the summer was used as a heat source for the heat pump to improve its performance. The researchers also deployed a shallow-ground heat exchanger to provide a stable heat source.
To accommodate the equipment, the researchers demolished 114 m² of the existing greenhouse and built a compact mechanical room. They installed 96 m² of evacuated-tube solar collectors and 108 m² of flat-plate collectors on its roof. The system also included a 350 m³ seasonal storage tank, a 100 m³ buffer tank, a 45 RT dual-source heat pump, two 45 RT air-source heat pumps, piping, circulation pumps, and control equipment installed inside or beneath the structure.
During the heating season, the system supplied solar heat directly to the buffer tank. During periods without heating demand, it stored the heat in the seasonal storage tank. High-temperature water from the seasonal tank could be transferred to the buffer tank for direct heating, while water at temperatures ranging from 10 C to 35 C could serve as a heat source for the dual-source heat pump.
The system could also direct surplus heat to a shallow-ground thermal storage system comprising U-shaped pipes installed vertically to a depth of 5 meters and three tiers of horizontal pipes installed at a depth of 2 meters. The buffer tank received heat from the solar collectors, seasonal storage tank, and heat pumps before distributing it to the greenhouses through 25 fan-coil units rated at 10,000 kcal/h and 30 units rated at 20,000 kcal/h.
The researchers monitored the system from April 2022 to May 2025, covering three annual cycles. Annual solar irradiation totaled 1,958.3 kWh/m² in the first year, 1,744.1 kWh/m² in the second, and 1,817.2 kWh/m² in the third.
The researchers recorded monthly average minimum and maximum temperatures of 1.7 C and 26.7 C, respectively, in December of the first year. The corresponding values were 3.1 C and 27.5 C in November of the second year and 2 C and 29 C in January of the third year.
“The solar collector system demonstrated an annual average thermal production efficiency of 31.8%. Although the small-capacity seasonal thermal storage tank exhibited drawbacks of high unit cost per volume and lower efficiency compared with large-capacity tanks, the optimized valve configuration and enhanced insulation design achieved an annual average thermal storage efficiency of 55.1%,” the researchers said.
“The heat production efficiency of the multi-source heat pump utilizing the seasonal heat source of the storage tank offered an improved coefficient of performance (COP) as the water source temperature increased, with an annual average COP of 4.1,” they added. “When the multi-source heat pump operation was unavailable, an air source heat pump was used, which offered an annual average COP of 3.9. Shallow-ground thermal storage exhibited low long-term storage efficiency owing to rapid heat loss from poor ground conditions. However, the storage unit demonstrated the potential to stabilize the heat pump source temperature, contributing to the maintenance of stable performance even during peak heating periods.”
The researchers presented their findings in “Long-term thermal performance of a heat pump system with a small-scale solar seasonal hot water storage tank for greenhouse heating,” published in Applied Thermal Engineering.
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