Operational characteristics of ground source heat pump coupled with seasonal solar thermal energy storage for greenhouse heating

As the world progresses towards achieving carbon neutrality, there is an urgent need to explore renewable energy sources, such as solar and geothermal energy, for greenhouse heating. Integrating these energy sources not only reduces greenhouse gas emissions but also ensures reliable and efficient heating, which is essential for maintaining optimal growing conditions in greenhouses. This study specifically focuses on the challenges associated with ground source heat pump (GSHP) systems, which have gained widespread attention for their stability when operating under low ambient temperatures. However, a well-documented issue with GSHP systems is the thermal imbalance in the soil, where prolonged use causes the ground temperature to drop, thereby reducing the system's efficiency over time. To address this issue, this study explores the operational characteristics of coupling seasonal solar thermal energy storage (SSTES) and solar thermal energy direct heating (STEDH) with the GSHP system. These methods aim to mitigate thermal imbalance and enhance the overall performance of the heating system. An experimental platform was constructed in northern China to facilitate this investigation, consisting of a 112 m2 Venlo-style glass greenhouse heated by a GSHP system integrated with solar energy. Over two heating seasons, the study conducted a series of experiments to compare the performance of the GSHP system with and without the integration of SSTES and STEDH. During the first heating season, which spanned from January 1st to March 15th, the GSHP system operated independently without SSTES. The results showed significant diurnal fluctuations in the greenhouse heat load, leading to intermittent operation of the GSHP system. This caused the soil temperature in the boreholes of the GSHP working wells to exhibit daily cyclical fluctuations, with a local temperature drop of up to 2.5 ℃ during heat extraction. The rate of soil temperature recovery was approximately 1.8 times faster than the rate of temperature decline, indicating a good daily thermal balance self-recovery capability. By the end of the heating season, a total of 16 934 kWh of heat had been extracted, with an annual heating coefficient of performance of 2.79. The soil temperature in the working wells dropped by 2.40 to 2.97 ℃ compared to the initial soil temperature, while the soil temperature in the monitoring wells within the well field decreased by 0.60 to 1.00 ℃. This temperature drop indicates that the soil thermal imbalance persisted, and even by early June, the soil had not recovered to its initial temperature, highlighting the severity of the thermal imbalance issue. In the subsequent non-heating season, from June 10th to November 7th, SSTES experiment was conducted. A total of 10 173 kWh of heat was stored in the soil, accounting for 60.1% of the total heat released during the previous heating season. The soil temperature at the monitoring points increased by approximately 0.2 ℃, demonstrating the effectiveness of SSTES in mitigating thermal imbalance. In the second heating season, from November 8th to March 15th of the following year, STEDH was coupled with the GSHP system. The results showed a 14.3% increase in the annual heating coefficient of performance, raising it to 3.19. At the same time, the rate of soil temperature decrease was slowed, indicating a reduction in thermal imbalance. In conclusion, coupling SSTES and STEDH with GSHP systems effectively addresses the issue of thermal imbalance and significantly enhances heating efficiency. Additionally, to further improve the overall efficiency of the system, it was recommended that the load-side return water temperature for the heat pump unit be maintained at 30 ℃, with the upper and lower temperature limits for SSTES set at 40 ℃ and 30 ℃, respectively. These findings provide valuable data and case studies for optimizing the design, operation, and control of greenhouse heating systems in engineering applications. This study contributes to the broader goal of sustainable energy use and carbon neutrality in the agricultural sector. © 2024 Chinese Society of Agricultural Engineering. All rights reserved.