Thermo-Economic Optimization of a Solar-Biogas Hybrid Heating System for Controlled Environment Greenhouses in Temperate Climates

The research hypothesis proposed that a hybrid renewable energy system integrating evacuated tube solar collectors (ETSC), phase change material (PCM) thermal storage, and a biogas combined heat and power (CHP) unit could supply more than 75% of winter heating demand for a greenhouse in a temperate climate (latitude 52.5°N) at a levelized cost of heat (LCOH) competitive with natural gas ($0.022–0.025/kWh), while also providing CO₂ enrichment benefits for crop growth. The data was gathered through experimental measurements from a pilot facility over 90 days (December 2023–February 2024) and numerical simulations using TRNSYS 18 software. The pilot facility was a 1000 m² Venlo-type glass greenhouse in Berlin, Germany equipped with 120 m² ETSC, a 15,000 L PCM storage tank (sodium acetate trihydrate, melting point 58°C, latent heat 265 kJ/kg), and a 40 kWₜₕ / 20 kWₑₗ biogas CHP. Twelve Type-T thermocouples (±0.3°C), a pyranometer, humidity sensors, and a CO₂ sensor recorded data at 1-minute intervals via a datalogger. For crop yield analysis, 300 tomato plants were divided into three treatments: control (420 ppm CO₂), medium enrichment (800 ppm), and high enrichment (950 ppm), with weekly harvest measurements over 12 weeks. The data shows that the optimized hybrid system achieved a seasonal renewable fraction of 78.3%, reducing LNG consumption by 92% (from 1,850 m³ to 148 m³). The parametric optimization revealed a minimum LCOH of $0.024/kWh at 120 m² solar area and 40 kWₜₕ CHP capacity, corresponding to a solar-to-biogas thermal capacity ratio of 2.5:1. The CO₂ enrichment experiment demonstrated a statistically significant effect on tomato yield (F(2, 297) = 68.4, p < 0.001), with the 950 ppm treatment producing 18.2% higher yield (5.46 ± 0.44 kg/plant vs 4.62 ± 0.38 kg/plant for control). Including CO₂ enrichment revenue of $4,200 per year made the hybrid system generate a net annual profit of $1,760, with simple payback of 6.8 years with 30% subsidy. The TRNSYS model validation achieved MAPE of 4.8% for temperature and 6.2% for storage state of charge. These findings indicate that solar-biogas hybrid systems with PCM storage are technically and economically viable for greenhouse heating at latitudes up to 52.5°N. The optimal design rule (120 m² solar, 40 kWₜₕ CHP, 15,000 L PCM per 1000 m² greenhouse) provides a practical guideline for engineers. Other researchers can use the validated TRNSYS-MATLAB framework to test different climate conditions or greenhouse sizes. Limitations include the single location study and single-season crop trial requiring multi-year replication.