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

Description

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.

Steps to reproduce

The data was generated through two parallel workflows: experimental measurements from a pilot-scale greenhouse facility and numerical simulations using a validated TRNSYS-MATLAB co-simulation framework. The pilot facility was a 1000 m² Venlo-type glass greenhouse located in Berlin, Germany (52.5°N, 13.4°E). The greenhouse was equipped with 120 m² of evacuated tube solar collectors (SunRain SR-898) installed at a 45° slope facing south. Thermal storage consisted of a 15,000 L cylindrical steel tank filled with water and 1,500 encapsulated capsules containing sodium acetate trihydrate PCM (melting point 58°C, latent heat 265 kJ/kg). The biogas CHP unit was a Capstone C30 micro-turbine modified for biogas operation (40 kWₜₕ / 20 kWₑₗ) fueled by on-site anaerobic digestion of crop residue and dairy manure (62% CH₄, 34% CO₂). Twelve Type-T thermocouples (±0.3°C accuracy) were positioned at six indoor locations (2 m height), storage tank top and bottom, and collector inlet and outlet. A Kipp & Zonen CMP3 pyranometer measured global horizontal irradiance. Three Vaisala HMP155 sensors measured relative humidity. A Vaisala GMP343 infrared sensor measured CO₂ concentration at canopy height. All sensors were connected to a Campbell Scientific CR1000 datalogger recording at 1-minute intervals, later averaged to 5-minute means. Data was collected for 90 days from December 1, 2023 to February 29, 2024. For the CO₂ enrichment experiment, 300 tomato plants (Solanum lycopersicum cv. 'Merlice') were divided into three bays (100 plants each): control (420 ppm), medium enrichment (800 ppm via 3:1 dilution with fresh air), and high enrichment (950 ppm via undiluted biogas exhaust). Weekly harvest measurements of marketable fruit weight per plant were taken over 12 weeks. The simulation workflow used TRNSYS 18 with components: Type 71 for ETSC, Type 60 (modified) for PCM storage, Type 907 for biogas CHP, and Type 56 for greenhouse thermal load. Co-simulation with MATLAB (R2023b) implemented predictive control using 24-hour weather forecasts from the DWD API. The model was validated against experimental data using MAPE, RMSE, and R² metrics. All simulation files and experimental data have been deposited in a public repository with a DOI for reproducibility.

Institutions

• Royal Agricultural University

  England, Cirencester
• Queensland University of Technology

  Queensland, Brisbane
• Tashkent State Technical University named after Islam Karimov

  Tashkent, Tashkent

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