Simulation study and test on the solar photovoltaic-ground source heat pump cooling system for grain warehouse

High energy consumption and operating costs have affected the development of green and efficient granary warehouses. In order to promote energy saving and emission reduction in the reconstruction of granaries, this paper proposes a special solar photovoltaic-ground source heat pump cooling system for grain warehouses. The study investigates the feasibility and energy-saving effects of this combined system in grain warehouses, aiming to achieve "zero energy consumption" and "zero emissions" for green storage. Bungalow warehouses are typically single-story buildings widely used in grain reservoir, characterised by large roof area and spacious room between building rows. These features facilitate installing both underground heat exchangers and solar photovoltaic panels. Compared with residential buildings, adquate space can enable the solar photovoltaic-ground source heat pump system to integrate more well with warehouse buildings, and provide more cooling and power capacity. In summer, the electricity generated by photovoltaic panels is primarily consumed by the ground source heat pume for grain warehouse cooling. Then, the electricity is transmitted to living and office buildings in the grain reservoir, meeting the lighting requirements. Finally, excess power is sold to the State Grid. During other seasons when cooling is not required, the generated electricity first supplies the living and office buildings, and extra power is sold to the State Grid, too. Therefore, operating costs can be reduced through the surplus electricity sale. To maximize the combined advantages of solar and geothermal energy technologies in low-temperature grain storage, it is essential to conduct thorough research and design focused on the function and structure characteristics of warehouse buildings. This study aims to achieve deeper energy savings in storage application with current technologies. Simulation software was used to analyze the operation characteristics of the proposed system. Firstly, according to the actual size and working conditions of a grain warehouse, a simulation model for an air source heat pump system was established using TRNSYS software, which was verified with test data, and the the temperature response mechanism used in later simulation was obtained. Then, a TRNSYS simulation model for the solar photovoltaic-ground source heat pump cooling system was developed, considering the temperature and humidity requirements for low-temperature grain storage. System performance was simulated by this model over short and long-term periods, computing its operational characteristics. Finally, the cooling system was evaluated according to operational performance, such as photovoltaic adaptability, economic efficiency, energy savings, and environmental benefits. The results show that after one-year operation, the system maximum COP is 4.76 and the minimum value is 3.30; after ten-year running, the maximum COP is 3.96 and the minimum is 3.07. In one-year operation, the soil temperature around underground heat exchangers increased by 0.65 ℃, and after ten years, it increased by 5.4 ℃. Underground thermal balance is an important factor for long time working performance. To avoid continuous soil temperature rise, the system should include a cooling tower into the equipment or use the ground source heat pump for space heating in the office and living buildings during winter. The ratio of system annual power generation to consumption is 192% to 270% for the warehouse and the investment payback period is 7.8 years. With ground source heat pump system, the standard coal consumption for cooling grain is 1.2 kg/(t·a), compared to 2.93 kg/(t·a) for the air source heat pump system, resulting in an energy saving rate of 59%. Additionally, harmful emissions are reduced by 3 841 t/a annually. These findings indicate that the solar photovoltaic-ground source heat pump cooling system is more suitable for warehouses than traditional grain cooling methods. This research can be expected to provide theoretical support and design references for developing more efficient granary cooling systems.