Small and simple gas turbine power plants to produce electricity for communities isolated from a grid or a community wishing to remain so can be a reliable means of power supply at reasonable rates. Such power plants need to be simple in order to reduce maintenance needs. Research engineers love to play with thermodynamics and will go to any lengths to increase thermal efficiency even if it means that the final constructed plant is complex, expensive and results in frequent maintenance shut downs. For a gas turbine plant this implies using multistage turbines with intermediate heating and several heat exchangers. While the additional complexity may be justified for large gas turbine plants where thermal efficiency has an important bearing on running costs, it is not advisable for small independently managed gas turbine power plants that range from a few kilowatts to a Megawatt capacity.
A gas turbine plant involves a compressor stage coupled to a turbine for expansion. Heating of the working fluid is carried out after compression. The fluid that leaves the turbine after expansion carries a considerable amount of heat and thermodynamic principles suggest its utilization to partially heat the incoming working fluid before compression in a heat exchanger. It is suggested here that atmospheric air be the working fluid for simple gas turbine plants and it be heated in a furnace after compression without the use of any heat exchangers. The surplus heat carried in the exhaust air can be utilized by using it as the air required for combustion in the furnace. A single stage turbine may be utilized to further eliminate the need of reheat and additional heat exchanger and stages.
The heat of combustion can then be generated by burning various materials such as wood, waste, coal if available etc. Bio-gas if available in the community is best reserved for cooking and heating needs. The power so generated may be augmented by partial use of wind and solar energy. The plant furnace can be enclosed in a hot chamber that can be utilized for various purposes such as a bread oven or brick making plant.
The proposed gas turbine here is an external combustion type with only pure air running to the turbine and compressor. In this manner the maintenance of the turbine set is minimized and most maintenance is confined to the furnace area that is easier to maintain.
UPDATE 2013
Falling costs of solar panels is becoming an encouraging option for small communities wishing to generate their own energy. however, battery costs are still high and a combined system of solar, wind and the small turbine system running on waste etc. is a very practical option that can minimize or even eliminate the use of storage batteries. With such a system the turbine can be shut down during hours when sunshine is available.
In parts of the developed world such as India an electric supply does not reach every home, especially in rural areas. The shortage is particularly acute in eastern provinces such as Bihar. A news report in the media described how some young enterprising engineers have set up small power plants in several such villages using waste rice husk as the source of energy. The precise method of energy generation they are using is not known to this author but it appears that the cost of the electricity produced is competitive. Perhaps these engineers should read this blog post too to check if they can use some of the ideas here, or perhaps they have already it in the past and that is why they installed such systems.
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