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

This research investigates the thermal response and performance of a newly developed Hybrid Gravity Heat Pipe (HGHP), intended to provide soil warming during winter months for grapevine root systems in cold agricultural regions. Utilizing shallow geothermal energy, HGHP technology aims to address the need for an efficient yet economically feasible solution to sustain root temperatures and promote plant growth during colder seasons. The HGHP combines copper in the evaporative and condensative sections with Polypropylene Random Copolymer (PPR) in the adiabatic section, presenting a cost-efficient alternative to traditional all-copper heat pipes. The study was conducted in a vineyard region in Ningxia, China, where harsh winter temperatures have a significant impact on grapevine productivity and survival. The primary objective of this study was to evaluate the HGHP’s efficacy in increasing soil temperatures at grapevine root zones and to compare its thermal efficiency and economic benefits against the conventional Copper Gravity Heat Pipe (CGHP). This comparative analysis investigates the feasibility of HGHP in agricultural applications, focusing on its performance under varying environmental conditions and its ability to maintain a stable thermal environment conducive to grapevine growth. A field experiment was established at a vineyard in the Helan Mountain area of Ningxia, with both HGHP and CGHP installed under similar conditions to ensure an accurate comparison. The design and configuration of both systems were identical except for the materials used in the adiabatic sections. Each pipe’s evaporator was positioned in groundwater to absorb heat, while the adiabatic sections were embedded in soil with insulative covering to minimize external thermal influence. Temperature sensors were placed along each heat pipe’s evaporative, adiabatic, and condensative sections, as well as in surrounding soil, to capture temperature fluctuations. Observations were conducted across 121 days, covering distinct meteorological conditions including cloudy, snowy, extremely low-temperature, typical winter, and high-temperature scenarios to analyze 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 increased soil temperatures compared to the control group without heat pipes, contributing to an enhanced thermal environment for the grapevine root systems. The HGHP raised the soil temperature by an average of 3.08 ℃, whereas the CGHP achieved a more substantial increase of 4.94 ℃. Thermal profiles demonstrated that HGHP retained satisfactory isothermal performance, despite being constructed with PPR in the adiabatic section. Notably, CGHP outperformed HGHP in terms of heat transfer capacity due to copper’s superior thermal conductivity, with the CGHP condenser section showing an average temperature increase of 1.21 ℃ over that of the HGHP. However, in high-temperature conditions, the HGHP’s performance declined, with the condenser section reaching equilibrium with the evaporator, indicating cessation of heat transfer. Economically, the HGHP presented a substantial advantage, reducing costs by 48.39% compared to the CGHP. This cost reduction is attributed to the replacement of copper with PPR in the adiabatic section, which significantly lowered material and manufacturing expenses without severely compromising thermal efficiency. This study confirms that HGHP offers a cost-effective and operationally viable alternative to CGHP for agricultural soil warming applications, particularly in low-temperature settings where high thermal efficiency may not be as critical as affordability. The HGHP effectively leverages PPR’s cost benefits while maintaining functional heat transfer characteristics, making it suitable for geothermal applications where budgetary considerations are paramount. Although HGHP’s thermal performance was marginally lower than that of CGHP, the significant cost savings suggest its potential as a scalable solution for sustainable agriculture. Future research could explore improvements in HGHP’s design, especially in enhancing its adiabatic section to improve thermal stability across a broader range of weather conditions. Additional studies may also investigate HGHP’s application in other fields of geothermal energy, extending its utility beyond agriculture and into areas where soil temperature management is crucial.