Quantification of active-passive coupled system in Chinese solar greenhouse: thermal environment assessment

Frequent extreme weather in cold regions of northern China severely compromises the thermal stability of traditional Chinese solar greenhouses (CSGs). To enhance the resilience of CSGs against extreme cold and optimize the operating conditions of active heating systems, the synergistic mechanism between the passive internal insulation (II) and active earth-air heat exchanger (EAHE) is investigated. A full-scale unsteady CFD model was developed and validated, yielding high predictive accuracy (RMSE=2.49°C, MAE=1.69°C, and R2=0.865). Simulations revealed that while the standalone EAHE increased the average indoor nighttime temperature by 0.64°C, it occurred at the cost of excessive soil heat reservoir depletion. To mitigate this, the integration of II effectively reduced nighttime heat loss through the front roof. Consequently, this passive improvement delayed the start time of EAHE heating and further increased the minimum indoor temperature by 2.91°C. The synergistic effect significantly alleviated the operational burden on the active system. The average hourly nighttime heat extraction demand from the EAHE decreased from 1.65 MJ to 0.93 MJ, successfully preventing over-extraction from the soil. The II-EAHE coupling enhanced indoor thermal stability, achieving a damping factor (DF) of 1.58 and reducing the thermal load leveling (TLL) from 0.55 to 0.41, alongside a COP of 5.35. Over a complete winter production cycle under this coupled operation, the system demonstrated a daily energy profit of 10.9 kWh with total carbon emissions constrained to approximately 0.24 t. This study confirms the potential of the II-EAHE for CSGs in cold regions.