Feasibility Analysis of Biogas Based Polygeneration for Rural Development in Bangladesh

Sammanfattning: Around three-quarters of Bangladeshis (total population 164 million) live in rural areas: only 25% of these households have access to grid electricity with non-reliable supply despite the country’s successful rural electrification program, kerosene is the predominant source for lighting, and woody biomass is virtually the only option available for cooking. Aside from this energy service challenges the rural population also struggles with unsafe drinking water in terms of widespread arsenic contamination of well water. Access to electricity, clean cooking gas, and safe drinking water services are genuine needs of the rural poor and are essential to improving welfare. These needs can be addressed individually or using an integrated approach. Anaerobic digesters are now a proven technology and remain economically promising in the rural setting, where connection to the public electric and gas grids are not available/either not cost effective or feasible, and where energy and water scarcity are severe. As the technologies continue to improve, and as energy and safe water becomes scarce and fossil fuel energy prices rise, renewable energy based services and technological integration becomes more viable techno-economically. In these circumstances, the integration of biogas digester with power generation and water purification unit is an innovative concept that could be applied in remote areas of Bangladesh. This work presents a new concept for integrated polygeneration and analyzes the techno-economic performance of the scheme for meeting the demand of electricity, cooking energy and safe drinking water of 30 households in a rural village of Bangladesh. This study considers a holistic approach towards tackling both of these issues via integrated renewable energy-based polygeneration employed at the community level. The polygeneration unit under consideration provides electricity via cow dung-fed digester, which in turn is coupled to a gas engine. Excess digester gas is employed for cooking, while waste heat from the process drives a membrane distillation unit for water purification. The specific technologies chosen for the key energy conversion steps are as follows: plug-flow digester; internal combustion engine; and air-gap membrane distillation. The technical features, energy consumption, and potential of renewable energy use in driving the main integrated processes are reviewed and analyzed in this thesis. This study also examines one approach by investigating the application of suitable membrane technologies, specifically air gap membrane distillation (AGMD), as a promising method for small-scale, low cost deployment.  Experimental results show that the tested AGMD prototype is capable of achieving high separation efficiency, as all product water samples showed arsenic levels below accepted limits. Mass flows and energy balance, life cycle cost (levelized cost) of producing electricity, cooking gas and safe drinking water as well as the payback period of such a polygeneration system were studied. The results indicate that this polygeneration system is much more competitive and promising than other available technologies when attempting to solve the energy and arsenic-related problems in Bangladesh. One of the main encouraging issues of this integrated system is the levelized cost of the three major services: cooking gas (0.015 USD/kWh), electricity (0.042 USD/kWh–an orders of magnitude lower than comparable photovoltaic or wind systems) and safe drinking water (0.003 USD/liter). Additionally, the payback period is between 2.6 to 4 years.