Stage-Specific Environmental Characterization and Decision-Support Methods for Intelligent Control in a Chinese Solar Greenhouse

To quantitatively evaluate the environmental performance of a Chinese solar greenhouse (CSG) in Northern China during the autumn–winter season and explore its matching relationship with the growth requirements of autumn–winter tomato production, continuous monitoring data collected throughout the tomato growing season were analyzed. The growth period was divided into four stages: Seedling, Flowering, Swelling, and Fruiting. An indicator framework was established to analyze air temperature, solar radiation, and humidity inside the CSG. Results showed that total indoor growing degree days (GDD) reached 1326.77 °C·d by harvest, substantially higher than outdoor conditions (476.30 °C·d). Stage-wise GDD exhibited a bimodal pattern, with the Seedling and Fruiting stages contributing 37% each, whereas the Swelling stage contributed only 10.38%. Notably, the relative GDD index (RGI) revealed a stage-specific thermal mismatch, peaking at 1.64 during the Seedling stage (indicating thermal surplus) and dropping to 0.67 during the Fruiting stage (reflecting heat insufficiency in deep winter). Across all growth stages, indoor daily mean accumulated radiation ranged from 7.05 to 8.12 MJ·m−2·d−1, and transmittance increased from 61% to 85%. Nevertheless, indoor radiation supply remained strongly constrained by the seasonal decline in outdoor radiation. Humidity regulation displayed pronounced diurnal asymmetry and stage-specific differences. Relative humidity (RH) suitability rates across the four growth stages were 57%, 16%, 12%, and 17%, respectively. Low humidity was prominent during the Seedling stage, with RH below 60% accounting for 32% of the time. High humidity dominated the Swelling and Fruiting stages, with RH above 80% accounting for 67% and 72% of the time, respectively, accompanied by frequent short-term low-humidity fluctuations. Overall, although CSGs in Northern China can effectively buffer external low temperatures during autumn–winter production, they still face challenges such as stage-specific imbalances in solar thermal energy storage and release, weak indoor light availability (especially during winter), and insufficient synergistic humidity regulation.