Simulation and validation of a model for heating underground biogas digesters by solar energy

• Simulation & validation of solar heat transfer were conducted on an AD system.
• AD system was placed in a coverall greenhouse with an inner greenhouse to heat inlet pipe.
• Results from validation and simulation were similar (≤ 0.34 °C difference).
• Digester design allowed the process temperature to be ≥ 9.8 °C for the whole year.
• Model could be used to determine effectiveness of the design in different locations.

The current study examines the impact of using solar energy, specifically the use of a solar greenhouse to surround the digester and a second inner greenhouse to heat the digester inlet (System A), by simulation and validation for warming biogas digester. The simulation study compares the heat balances of System A and System B, which had no heating resource. Both systems were assumed to be located in Xianyang, Shaanxi province, northwest China. The simulation and modeling were undertaken using EPBSS, OpenStudio, and SketchUp. The simulation had the following results: in System A, the monthly average temperature of the process during the winter was maintained high enough (≥9.8 °C) to support biogas production for the whole year, in contrary to System B, which had a temperature of <8 °C for 5 months of the year. The results also show that System A could only be viable in regions where the average temperature during the winter is above −5 °C. The validation study was conducted on an actual digester with the same model and location as System A, and the results were within the range of results from simulation results (≤0.34 °C observed difference). Consequently, this model could be used for predicting the geographic locations where System A’s design could be used. Economic analysis of System A showed that the installation of the two greenhouses could increase revenue from biogas by US$1,290 per year, which could cover the costs of the greenhouses within 14 months of operation.

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