Hydronic root zone temperature control system for a Chinese solar greenhouse

The mono-slope Chinese solar greenhouses (CSGs) are the main facilities for year-round vegetable production. A hydronic root zone temperature control system (HRZTCS) was tested to improve the greenhouse environment and increase production in a CSG. This system utilizes a chiller as the source of cooling, a flat-plate solar collector to collect solar heat as the heat source, and water as the heat exchange medium, which is pumped into the plant root zone using polyethylene piping with thermal insulation. In the summer experiment, the test areas T S and T D (T S represents a single cooling pipeline, T D represents double cooling pipelines) and a control area (CK) (without cooling) were established. In the winter experiment, trial areas T S and T N (where N and S represent north and south, respectively, with double heating pipelines) and a control area (CK) were established. The HRZTCS achieved a cooling heat-transfer efficiency of 74 % in summer and a heating heat-transfer efficiency of 60.7 % in winter. The coefficient of system performance (COP) of the refrigeration system was 6.2–7.3, effectively reducing the root zone temperature by 5.9–7.5 °C. The temperature was controlled at 24.1–25.5 °C. In winter, the average heat collection efficiency was 66.1 %, resulting in a 5.5 °C increase in the root zone temperature. HRZTCS had a positive impact on the root environment of tomatoes, promoting tomato yield and improving survival, thereby significantly extending the plant growth season of CSGs.