Analysis of heat transfer characteristics during thawing of frozen soil in solar greenhouses incorporating cylindrical heat sources in severe cold regions

As the primary facility for winter agricultural production in severely cold regions, solar greenhouses often suffer from insufficient soil temperatures to meet crop growth requirements. This limitation is mainly attributed to the effects of seasonal frozen soil. To address this issue, a solar concentrating and soil heating (SCSH) system with heat exchange pipes in soil is proposed to mitigate the impact of seasonal frozen soil on winter cultivation in solar greenhouses. In addition, a two-dimensional unsteady heat transfer model for cylindrical heat sources in soil is developed, incorporating freeze–thaw phase change. The coupled thermal response of the SCSH system is then investigated. The effects of surface solar radiation and counter-flow heat transfer from cylindrical heat sources on soil temperature distribution and energy transport characteristics are further analyzed. In-situ monitoring results indicate that after five consecutive days of continuous operation under sunny conditions, soil temperatures of the measurement points 1–9 above the heat exchange pipes increased by 2.89–4.72 °C. The vertical distance at which the soil temperature rise is actually measured from the cylindrical heat source is approximately 0.5–0.6 m, forming a high-temperature zone within the 0.6–0.8 m soil layer that supplies heat upward. Throughout the test period, the heat flux at the top surface of the deep unfrozen zone peaked at 53.3 W/m 2 and averaged 38.52 W/m 2 , respectively. Its contribution to heat transfer consistently remained above 50%, indicating a significant capacity for continuous heat supply. In contrast, the peak heat flux from the surface frozen zone into the soil was −44.07 W/m 2 , but the duration of inward heat flux was brief with the average heat flux being only −3.81 W/m 2 . Overall, the proposed system effectively promotes frost thawing and improves soil temperature conditions. It offers a viable solution for enhancing the soil thermal environment and supporting winter crop production in severely cold regions.