Analysis of temperature response and thermal performance of hybrid gravity heat pipes for winter soil warming of grapevine root systems

A hybrid gravity heat pipe (HGHP) can greatly contribute to the soil warming of the grapevine root systems in cold agricultural regions in winter. According to the shallow geothermal energy, HGHP technology can provide an efficient yet economically feasible solution to sustain the root temperatures for plant growth during cold seasons. Among them, copper is used in the evaporative and condensative sections, while the polypropylene random copolymer (PPR) is in the adiabatic section, indicating a cost-efficient alternative to traditional all-copper heat pipes. This research aims to investigate the thermal response and performance of the HGHP. A series of tests were conducted in a vineyard region in Ningxia in northwest China. Particularly, the harsh winter temperatures shared a significant impact on grapevine productivity and survival. The efficacy of the HGHP was also evaluated on the soil temperatures at grapevine root zones. A comparison was then made on the thermal efficiency and economic benefits against the conventional copper gravity heat pipe (CGHP). The feasibility of HGHP was verified to evaluate the performance under varying environmental conditions. A stable thermal environment was also maintained on the grapevine growth. A field experiment was conducted at a vineyard in the Helan Mountain area of Ningxia, particularly with both HGHP and CGHP installed under similar conditions. Both systems shared the identical configuration, except for the materials in the adiabatic sections. Each evaporator of pipe was positioned in groundwater to absorb the heat. While the adiabatic sections were embedded in soil with insulative covering to minimize the external thermal influence. Temperature sensors were placed along each evaporative of heat pipe, adiabatic, and condensative sections, as well as in surrounding soil, in order to capture the temperature fluctuations. Observations were conducted across 121 days, thus covering the distinct meteorological conditions, including cloudy, snowy, extremely low-temperature, typical winter, and high-temperature scenarios. A systematic analysis was implemented on the thermal response, heat transfer efficiency, and stability of both HGHP and CGHP under real field conditions. The results indicate that both HGHP and CGHP effectively enhanced the soil temperatures for the grapevine root systems, compared with the control group without heat pipes. The HGHP raised the soil temperature by an average of 3.08℃, whereas the CGHP was achieved in the more substantial increase of 4.94℃. Thermal profiles demonstrated that the HGHP retained isothermal performance in the adiabatic section. Notably, the heat transfer capacity of CGHP outperformed that of the HGHP, due mainly to the superior thermal conductivity of copper. The CGHP condenser section shared an average temperature increase of 1.21℃ over that of the HGHP. However, the performance of the HGHP declined at high temperatures. The condenser section reached the equilibrium with the evaporator, indicating the cessation of heat transfer. Economically, the HGHP reduced the costs by 48.39%, compared with the CGHP. This substantial cost advantage was attributed to the replacement of copper with PPR in the adiabatic section, which significantly lowered material and manufacturing expenses without severely compromising thermal efficiency. Therefore, the HGHP can offer a cost-effective and operationally viable alternative to CGHP for agricultural soil warming applications, particularly for low-temperature settings. The cost benefits of PPR greatly contributed to the functional heat transfer suitable for geothermal applications. Although the thermal performance of HGHP was marginally lower than that of CGHP, significant cost savings can be expected for the promising scalable solution for sustainable agriculture. Furthermore, the adiabatic section of the HGHP system can be expected to improve thermal stability across a broader range of weather conditions. The HGHP can also be extended in fields of geothermal energy beyond agriculture.