Development of a New Biomass Stove Integrated with a Stirling Engine

Abstract: 1.2 billion people around the world, mainly in the developing countries, live without access to electricity. To increase the living standard for this people and make socio economic development possible, supplying rural areas with electricity is crucial. In rural areas around the world 2.7 billion citizens use biomass through simple stoves or open fire for heating, lighting and cooking in the households. This leads to indoor pollution which both is unhealthy and dangerous to inhale and ultimately leads to the prematurely death of 4 million people every year. Bolivia is no exception since it is one of Latin Americas poorest country. 1.2 million Bolivians still have no electricity access and fuel wood accounts for 47 % of the energy used for cooking in the country. One solution to minimize the premature deaths caused by pollutants is to improve the cook stoves to become cleaner and more efficient. A product that can contribute to both solve the problems with indoor pollution and no electricity access is an improved cook stove integrated with electricity generation. This thesis will present the first design approach in the development of a new biomass stove with an integrated Stirling engine, primarily adapted for rural Bolivians. The Stirling engine is an external combustion engine that can convert thermal energy to electricity. Through literature survey and field studies information to formulate a requirement specification was collected. A Quality Function Deployment (QFD) was formulated to transfer the requirements into technical measurable qualities. This constituted the guidelines for the development which led to five different concepts. With support from a Decision Matrix, where the different concepts where compared to each other, it was shown that the best concept was MAYA, a stationary stove with capacity for two pots, one combustion chamber, an ash trap and an attached chimney. Combustion and heat transfer processes appearing in a stove are complicated and depending on a number of different parameters. To evaluate a good stove design testing is required which is why the design approach was to look at successful improved stoves and to follow design principles and guidelines. The improved MAYA is a 70 cm high metal stove with a scaffold base constructed with double metal walls with insulation in between. The two pots can together fit 10 liters and are sunken down in the stove body. To create a good draft in the stove the combustion chamber, flue gas path and chimney are designed with a constant cross sectional area. Three different placements for a 95 We Stirling engine prototype are suggested but the final placement has to be further investigated since it depends on the temperatures in the stove. To increase the accessibility of the produced electricity a battery is advisable connected to the engine. A battery with a capacity of 25 000 mAh is shown sufficient to store the daily produced electricity. MAYA is a design draft based on design principles of wood burning cook stoves and can be further used in physical modeling and testing to create a final design.