Warming effect of grape overwintering root zone using gravity heat pipe heating

The Eastern Helan Mountain can be the largest concentrated and contiguous producing areas for the wine grape in China. Specifically, the wine grape planting area reached 388.7 square kilometers in Ningxia by the end of 2022, accounting for nearly 1/3 of the nation, and the comprehensive output value exceeded 30 billion Yuan. However, the extremely low temperatures (down to -30.0℃) in winter in the eastern part of the Helan Mountains can lead to the occurrence of overwintering frost damage, which greatly limits the sustainable development of Ningxia's grape industry. In this study, a gravity heat pipe system was designed using shallow groundwater geothermal energy for the soil warming in the root zone of grapes. Numerical simulation was also carried out to analyze the influence range and distribution of the gravity heat pipe temperature field under different spacing. A series of field experiments were used to verify the working performance of the gravity heat pipe and the warming characteristics of the soil. The results show that the soil temperature field was evenly spread around the gravity heat pipe. The temperature field around the heat pipe was divided into four regions, according to the temperature level: the temperature range was 5.0-8.50℃ high-temperature affected region Ⅰ, the temperature range was 0-5.0℃ medium-temperature affected region Ⅱ, the temperature range was -2.57-0 ℃ low-temperature affected region Ⅲ, and the temperature field was not affected region Ⅳ. The highest temperature (8.50 ℃) appeared in the temperature field area Ⅰ condensation section pipe wall, whereas, the lowest temperature (-2.57 ℃) was in the temperature field unaffected area Ⅳ. In addition, the diameter of the medium temperature affected area of the heat pipe in the soil was 30.8 cm horizontally and 32.8 cm vertically, where the grape roots were not affected by frost. There were evenly distributed temperature fields around the heat pipes partially overlapped and the temperature fields under the conditions of 10 cm burial depth and 15 cm spacing between adjacent heat pipes. The soil temperature increased by 7.0 ℃ on average in the experimental group during the test period, compared with the control group, indicating the outstanding increase in temperature. The maximum temperature difference was 3.70 ℃ between the evaporation and condensation sections during the normal operation of the gravity heat pipe. The maximum temperature differences were 1.0 and 0.2 ℃ within the evaporation section, and within the adiabatic section, respectively, indicating the excellent isothermal distribution. The average start-up and operating temperature difference of the heat pipe were 5.4 and 2.9 ℃, respectively. The clean and renewable shallow geothermal energy was taken as a source of heating energy in the root zone of grapes, which was in line with the current green development concept. The finding can provide the theoretical basis and technical support to treat the frost damage in the root zone of grapes. In addition, a new way was proposed to realize the development and utilization of shallow geothermal energy using gravity heat pipes.