Browsing by Author "Kinyua, R."

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    Investigating a Prototype Heat Exchanger for Steam Storage
    (2014-07) Kawira, M.K.; Kinyua, R.; Kamau, J.N.
    A coiled tube single pass counter flow heat exchanger was designed and fabricated using locally available materials and its capacity to transmit heat to water to make steam was tested. The heat exchanger was part of components used in solar thermal power production using a parabolic trough solar concentrator. The design of the heat exchanger storage system was done using Auto CAD 2010 software. Higher temperatures of steam were realized when the heat exchanger was used as a steam storage system. The heat transfer fluids used were locally available and they were water, sunflower oil, Rina oil, used engine oil, unused engine oil, 2 M sodium chloride salt solution, 4 M sodium chloride salt solution and 6 M sodium chloride solution. For each of the heat transfer fluids, heat exchanger operating points were obtained and it was operated between 1 Nm-2 to 1.0 Nm-2 of pressure. In the study the number of heat transfer units of the heat exchanger obtained was 0.61 and the thermal efficiency was found as 0.91. The average rates of heat transfer were 68.4 J for 6M sodium chloride solution, 62.8 J for 4 M sodium chloride solution, 57.9 J for 2M sodium chloride solution, 55.4 J for water, 49.1 J for sunflower oil, 46.3 J for Rina vegetable oil, 59.7 J for unused engine oil and 54.1 J for used engine oil. Thermal conductivity of the salt solutions was better than for other heat transfer fluids although they were more corrosive on the system. Thermal efficiencies of the heat transfer fluids were 6 M sodium chloride solution; 0.89, for 4 M sodium chloride solution ; 0.84, for 2 M sodium chloride solution ; 0.80, water ; 0.78, sunflower; 0.69, Rina oil ; 0.66, unused engine oil ; 0.75 and used engine oil; 0.71. Storage of solar thermal energy will address the problem of low solar density and its variability. The heat exchanger storage system achieved a thermal capacity of 3.26 kJ at a maximum temperature of 249.4 oC and at a pressure of 7.2 Nm-2. The use of heat exchangers in solar thermal storage will increase the overall efficiency of total system
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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 PTSCsĀ“ 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.