Coupling solar-air source heat pumps with sand-based thermal storage floor

Scientists in China have analyzed the performance of a system linking a solar-air source heat pump heating system to sand-based thermal storage floor and have found it can maitain an average indoor temperature of 18.8 C, even when outdoor temperatures range from −18.4 C to 12.3 C.

Researchers from China have proposed to combine solar-air source heat pumps (SASHP) with sand-based thermal floor storage in rural clean heating renovation projects.

“In rural areas where economic constraints and maintenance conditions are critical, there is still a lack of research on integrating cost-effective heat storage technologies with SASHP systems,” the team explained. “Moreover, the performance and cost-effectiveness of such integrated systems have not been thoroughly investigated. To address the identified research gaps, this paper introduces a novel thermal storage floor system that utilizes cost-effective, stable sand-filled coils, integrated with a SASHP for rural heating.”

The academics ran a series of tests in a real house in rural Inner Mongolia, China. The building in which the system was installed was a single-family rural residence in Chifeng, Inner Mongolia. The area has abundant solar energy in winter, with an average outdoor temperature of -3.4 C and the lowest temperature reaching -23.3 C. The heating season lasts six months, from mid-October to mid-April.

The installed system included a solar-thermal collector (STC), thermal storage tank (TST), ASHP, sand-based thermal storage floor (STSF) and water pumps. The STSF incorporates pipes with a 200 mm layer of sand around the coil layer. “When solar radiation is sufficient, the sand absorbs a substantial amount of heat and releases it into the room. During period of insufficient solar radiation or at night, the stored heat is gradually released into the room, thereby extending the duration of solar heating,” they explained.

The system had four operational modes. Mode 1 was for sunny days, enabling the system to operate solely with the STC to heat the TST. On an overcast and rainy day, the system used mode 2, relying entirely on ASHP to heat the TST. Two modes were used for complementary heating: in mode 3 the STC operates during the day and ASHP runs at night; while in mode 4 the STC operates during the day, and the ASHP is activated during the day when solar radiation is insufficient, but the ASHP is not operated at night.

The experimental system was measured 9 days from November 27 to December 6, 2023. “The STSF effectively maintains stable indoor temperatures between 17.4 C at night and 20.2 C during the day, even with outdoor temperatures ranging from – 18.4 C to 12.3 C (average -5.5 C). With an average coefficient of performance (COP) of 2.6 and solar friction (SF) of 50.9%, the system proves to be a feasible solution for heating in severe cold rural areas,” the scientists said.

After collecting the results, the academics have modeled the system in TRNSYS, and validation metrics showed it to be “both reasonable and reliable.” The simulation runs for the entire six months of the heating season, and mode 4 outperformed mode 3. Mode 4 had COP of 4.6 and SF of 77.9%, which increased by 15.9% and 20.3% compared to mode 3. Also, mode 4 consumed 405.2 kWh less energy than mode 3.

“Operating costs are significantly influenced by local electricity pricing policies. Without peak-valley pricing, mode 4 reduces operating costs by 28% compared to mode 3, while the operating costs in mode 4 are only 2.6 % higher than in mode 3 with peak-valley pricing,” the analysis showed. “This suggests that optimizing the operational mode according to local electricity pricing policies can minimize the operating costs. For rural households, mode 3 is the more economical option under peak-valley pricing, while mode 4 offers a more sustainable solution for energy conservation and emission reduction from the national government perspective.”

The results were presented in “Performance analysis of solar-air source heat pump heating system coupled with sand-based thermal storage floor in rural inner Mongolia, China,” published in Case Studies in Thermal Engineering. The study was conducted by researchers from China’s Zhongyuan University of Technology and Dalian University of Technology.