Browsing by Author "Kawira, M."

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    Design, Fabrication and Characterization of an appropriate Solar Thermal Electricity Generating System
    (Jomo Kenyatta University of Agriculture and Technology, 2015-07) Kawira, M.
    The sun provides an abundant and clean source of energy. However the supply of this energy is periodic following yearly and diurnal cycles, intermittent, unpredictable and it is diffused. Its density is low compared to the energy flux densities found in convectional fossil energy sources like coal or oil. There have been attempts to produce solar thermal power using parabolic trough technology as was demonstrated by Luz Company which built a solar electricity generating station with a power output of 354 MW in USA. Also the largest solar power plants in the world using parabolic trough technology are the Andasol 1 to 3 which are established in Spain. Therefore it was necessary to undertake design and fabrication of a solar thermal electricity generating system consisting of a collector, steam storage system, heat exchanger, turbine and generator as a local solution for power production. The design layouts were done using auto cad. The testing of the steam storage system and the heat exchanger were done using TEMA (Tubular Exchangers Manufacturers Association Standard and ASME (American Society of Mechanical Engineers). Locally produced heat transfer fluids (water, saline solutions, vegetable oils and engine oils) were tested for their suitability in solar power production using guidelines obtained from National Renewable Energy Laboratories. Some of the parameters investigated included mass flow rates, power output, efficiency, steam flow rate, heat absorbed, heat emitted, evaporation ratio, proportion of flash steam, number of heat transfer units among others. The efficiencies of the concentrator when using the heat transfer fluids were in the range of 48.8% to 60.1% for closed collector and in the range of 46.7% to 56.6% for the open collector. The length of complete discharge for the steam storage system ranged from 4.4 hrs to 6.9 hrs. The power output for the heat transfer fluids were in the range of 287.9 W to 467 W. The steam storage was found to have an efficiency of 93.5 % and a thermal capacity of 4.54 kJ. The rate of heat transfer was an average of 68.4 Js-1kg-1 to 46.3 Js-1kg-1. The thermal efficiency for the heat transfer fluids was in the range of 0.85 to 0.66. Available power from the impulse type turbine was 478.4 Watts, isentropic efficiency was 83.5 %, cycle power output was 497.8 W, turbine output was 468.2 W xxii and gear efficiency was 87.9 % and generator efficiency of 86.9 %. The overall efficiency of the system was 34.97%. Coupling of the steam storage system and the heat exchanger increased the capacity of steam storage to 4.15 KJ, at a maximum temperature of 249.5 °C and at a pressure of 7.2 Nm-2. Coupling of the steam storage system and the heat exchanger increased the capacity of steam storage to 4.15 kJ, at maximum temperatures of 249.5 °C and at a pressure of 7.2 Nm-2. A single stage impulse turbine was fabricated which had an average efficiency of 61.6% and a maximum power output of 498 W. The solar thermal characterization and collection was done in solar intensities of the average range 700 Wm-2 to 1100 Wm-2. In Coolidge irrigation facility, a thermocline storage tank has a capacity of 19.8 GJ and operates at a temperature of 288 °C. The thermal capacity of the storage system used in this study was 4.15 kJ and was operating at a temperature of 249.9 °C.
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    A prototype parabolic trough solar concentrators for steam production
    (Jomo Kenyatta University of Agriculture and Technology, 2012) Kawira, M.; Kinyua, R.; Kamau, J.N.
    In this work, the potential for a solar-thermal concentrator to produce steam has been studied. Three parabolic trough solar concentrators (PTSCs) of dimensions: - aperture width of 1.2 m, Collector length of 5.8 m and aperture area of 6.95 m2 were investigated. The absorber pipe was a copper tube which carried water as the heat transfer fluid, were designed, fabricated, characterized and their efficiencies compared when closed and when open. The PTSCs´ were made of appropriate materials and were manually tracked. They were designed with principal focus at 0.4 m so that the receiver heat loss is minimized by covering the collectors with glass which was 0.0025 m in thickness. The concentration ratio of the solar concentrators was 128. The concentrator testing was carried out for each of the concentrators. The maximum temperature of steam obtained was 248.3oC while average temperature of steam was produced was 150oC. When closed their efficiencies were: Aluminium sheet reflector PTSC; 55.52 %, Car solar reflector PTSC; 54.65 % and Aluminium foil reflector PTSC; 51.29 %. The open solar concentrator efficiencies were 32.38 %, 34.45 % and 27.74 % respectively. The efficiency of car solar reflector when open was higher than for aluminium sheet since it was less prone to thermal degradation when exposed to weather elements. The results obtained show that production of power using the sun flux is a viable undertaking. The concentrators can be used to provide power to remote areas which are far away from the power transmission gridlines. This will make power readily available to the marginalized rural people. Improvement of the tracking system and optical efficiency can improve the efficiencies of the fabricated concentrator systems.
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    A Prototype Steam Storage System for Power Production
    (2014-08-01) Kawira, M.; Kinyua, R.; Kamau, J.N.
    Use of solar energy on a large scale is mainly limited to the sun duration hours, weather conditions and adequate solar thermal storage technology. A means of addressing this problem using local materials is provided. A prototype pressure storage system was designed using auto cad 2010 and fabricated using locally available materials. The steam storage system was tested using ASME 2000b guidelines for boiler and pressure vessels at a small scale. The maximum continuous rating for the storage system was 60 kg maximum instantaneous demand 40 kg , storage capacity 50 kg and mean off peak load of 100 W was realized. The maximum test temperatures of operation obtained using seamless galvanized iron pipe was 264.7 o C and 140 o C using polypropylene pipes. The steam storage system operated between 1.0 Pa and 1.0 Pa. Its charging duration was twenty minutes and would discharge at the rate of 50 l continuously after full charge with solar irradiance between 800 W/ and 1020 W/ . The length of complete discharge for the heat transfer fluids was found to range between 4.4 hours. and 6.9 hours. The power output for the heat transfer fluids were in the range of 251.8 W and 486.9W. The steam storage system was found to have an efficiency of 93.5 % and a thermal capacity of 4.54 k J. This system presents a means of storing solar energy in form of steam during low energy demand and its conversion to power during high peak demand twenty four hours a day depending on the size of the storage and application and hence addressing the problem of variability and low density of solar energy.
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    Staff Profile - Dr. Millien Kawira
    (University Of Embu, 2015-07) Kawira, M.;
    Dr. Millien Kawira is the Deputy Registrar, Admission & Teaching Programmes (ATP)/Senior Lecturer. Her area of specialization is Energy Technology. She holds a Doctor of Philosophy in energy technology at JKUAT, and a Master of Science degree in Physics from the same University.