Adoption of Solar Thermal Energy for Optimal Production of Biogas in Lesotho

Purpose: The purpose of this study is to investigate the technical and economic viability of using solar thermal collectors to provide the necessary heat to keep the temperature of the biogas digester above sub-optimal conditions. Design/Methodology/Approach: The case study consists of a 5 m3 biogas digester, located at Maseru in Lesotho, whose contents are to be heated by a solar thermal system comprising solar collectors, which have to be selected and sized, and a solar hot water tank, also to be sized. A Microsoft Excel spreadsheet program was developed, which can aid the selection of the type of solar thermal collector that is most economic under Lesotho environmental conditions and to determine the economically optimal size of the collector for use with a specified size of biogas digester. Maximizing the thermal energy output per dollar expended was used as the objective function for selecting the best collector type, and the collector size resulting in the maximum Net Present Value (NPV) was specified for the system design. Meanwhile, the hourly temperature of the bio-digester contents and the related biogas output is related to the solar system temperature using heat balance equations and a relationship of volumetric biogas output with digester temperature obtained from the literature. Findings: The results select an evacuated tube branded Sun Power, whose operating characteristics are rated by the Solar Ratings and Certificate Corporation (SRCC), as the suitable collector type based on the highest energy per dollar (25.3 kWh/$) amongst its competitors. This system required an optimum collector area of 16 m2 and solar storage tank size of 800 L. The solar system was economic compared to energizing with utility electricity, with an NPV of $1854, an Internal Rate of Return of 10.36%, a Payback period of 9 years, and a Benefit–Cost Ratio of 2.01. Comparing the biogas digester system with-solar and without-solar showed that the solar-assisted biogas digester is 11.5% more productive (produces an extra 393 m3/annum). Practical Implications: Solar collector type to be deployed should be selected using the energy per dollar criterion and the optimum size of collector should be specified using techno-economic considerations, as done in this study. Social Implications: The socio-economic and environmental benefits of using biogas are many. They include reduced firewood and kerosene consumption required for cooking, lower emissions of greenhouse gases and indoor air pollutants and the possibilities to use biogas to treat human and animal wastes. Originality and Value: This study contributed to the pool of knowledge on sizing and construction of a solar assisted bio digester. It offered reliable and efficient supply of energy using modern technology with more economic benefits.