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Design of a prototype Stirling engine for didactic use and utilization in Uganda. By Jeffrey Pepe
Going into a third millennium globalized world, many people still do not have access to electricity. More than a quarter of the world’s population does not have access to this resource which is of fundamental importance for economic development and well being, especially in remote areas of the world.
Uganda, despite the richness of its land which could offer sufficient resources for the population, has a per capita consumption of energy among the lowest in the world. The annual per capita consumption of energy is 0.3 TOE.
Possible solutions for this reality can be sought in the research and development of appropriate technology which represents a concrete opportunity for sustainable development.
Proposals for supplying electricity to the rural and isolated zones of the country are arising from collaboration between the “Sapienza” University of Rome and the University of Makerere, Kampala. The idea is for a stand-alone generating plant using a Stirling engine that is fueled by biogas which is produced in the plant, itself, by a biomas digester.
Considering its characteristics, the Stirling engine would be an optimum choice to fulfil the required needs and within the feasible of a country like Uganda.
The Stirling engine is an external combustion engine with high performance capacity, and it requires little maintenance due to its simplicity. With correct adjustment to the burner or heat exchanger any heat source can be used to fuel the engine. Because the combustion is external, it allows for a better control of the flame and for a better more complete combustion which is cleaner. With this in mind, the future study and design of the burner will be of fundamental importance. The abundance of easily available biomass in the African rural areas, once appropriately treated and transformed could constitute the required fuel at an affordable price and everywhere.
Regarding the collaboration mentioned between the two Universities, the project idea was for a prototype 90°cylinder Gamma type Stirling engine. A demonstrative prototype was fabricated using PMMA (Plexiglas), for didactic use in the University of Makerere.
The idea behind this engine is to offer a small electricity generating unit (<1kWe). In fact, a space is allocated within the engine frame for mounting the alternator. The project was designed with the idea of the engine working under pressure, therefore sealing elements were foreseen such as O-rings for the joining of certain components.
In view of the careful attention that was given to insure good sealing, the possibility was also considered of using a working fluid other than air, for example helium. Although helium is more volatile, it is better suited chemically and physically for use in the engine.
In addition, a system to balance and to recuperate the pressure within the engine was designed by means of a small piston in solid connection with the guide of the kinematism which works in counter phase with the displacer. Therefore by using the motion of the pistons and with the assistance of an electrovalve, it is possible to regulate the internal working pressure..
The estimated running temperature of the engine is between 400°-800°. At this temperature, the austenitic stainless steel, the material designated for the fabrication of the hot cylinder, still resists well. The cooling is guaranteed by the water jacket which surrounds one end of the hot cylinder which houses the displacer.
The designs and the model of the prototype produced herewith constitute the preliminary phase. Ulterior study is required to specify the interventions and modifications necessary for the eventual realization of a working prototype. A detailed study of the burner is necessary considering the issues linked to the low caloric value of the biogas.
The designs in this work were carried out using as software CATIA. Below are two 3-D designs made with CATIA and a photo of the model in PMMA 1:1 scale.
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