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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.
Uganda’s total population is currently 32,369,558 with an annual growth rate of 2.8%. The urban population is 12%, with the remaining 88% living in the rural areas with a density of less than 100 persons per km². More than 37% of the population lives below the poverty line with an average annual per capita income of USD293.
The entire country consumes approximately 5 million TOE/year of which 94% is derived from biomass (wood, charcoal and agricultural residues). The national electrical grid reaches only 200,000 users with an annual increase between 5.5% and 7.5%. The remaining majority of the population, who are not connected to national grid, utilizes various local solutions to generate electricity (diesel generators, photovoltaic panels, batteries). The electricity provided by the grid is utilized as follows: 55% for residential/domestic use, 44% for production activities/services and 1.1% for public illumination.
The problem of access to electricity and programs of rural electrification are foremost among national objectives in the fight against poverty, for example: PEAP (Poverty Eradication Action Plan), RESP (Rural Electrification Strategy and Plan).
[Source: Investing in Uganda’s Energy Sector, Uganda Energy Assessment, UIA – Uganda Investment Authority, 2002].

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.
Besides the fulfilling of the energy requirements, the technology must sustain the environment and be easily transferable. Without any doubt, the Stirling engine meets these requirements. It even possess the capacity to run on solar energy, and as already mentioned, due to its better combustion, it enjoys higher fuel economy compared to an internal combustion engine at the same power output.     

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. 
Neither regenerator nor auxiliary heat exchanger are foreseen which in this case could negatively affect the engine (creation of dead volumes and pressure loss)

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 interest of the Ugandan government in the development of energy sources in the country, the abundance of natural resources and of renewable energy sources: solar, water, and biomass  represent favourable conditions to adopt various different solutions/technologies to resolving the problem of electricity generation. Certainly, the Stirling engine can be considered as an option to consider, offering various interesting scenarios.

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.

For information

Prof. Andrea Micangeli
CIRPS - Self-Reliance and Environment Technologies
Sapienza University of Rome
E-mail: andrea.micangeli@gmail.com

Antonello Binni (Ph.D., Eng)
via Terme di Tito 92 - 00184 Rome (Italy)
E-mail: antonello.binni@uniroma1.it

Author: Jeffrey Pepe (Eng.)
E-mail: jeffrey.pepe@fastwebnet.it


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