Integrating solar energy into an urban small-scale anaerobic digester for improved performance

In this research, the feasibility of using an air-source heat pump as the main heating source for a 30 m3 mesophilic digester located on the roof of an urban building was investigated during the coldest month of the year. A heat loss model for an insulated digester tank and adjacent rooftop greenhouse were developed as well as a heat transfer model for an air source heatpump. A 3-D model of the rooftop and surrounding buildings was developed in order to determine the effect of shadowing on solar energy gains of the greenhouse. Simulations of tank temperature fluctuation and resulting theoretical biogas production were performed for cases in which the heat pump was active only during daylight hours for each day of the month, every other day of the month, and for one third of the month including a full week without any heating. Results of the simulations show a daily temperature fluctuation of 0.2 °C for the first case, a fluctuation of 0.5 °C for the second case, and a maximum of 2.1 °C for the third case.