Design, construction and test of a passive solar tracking device

Thesis Overview

<p> INTRODUCTION<br>1.1 SOLAR ENERGY<br>Solar energy is an important clean, cheap and abundantly available renewable<br>energy. The sun radiates heat and light. The heat and light received from the<br>sun support the environment on the earth through the following well-known<br>natural effects.<br>– Temperature balance on the earth<br>– Photosynthesis by biological plants, production of oxygen and organic<br>materials. Production of organic chemicals and biomass<br>– Wind due to unequal heating of water, land surfaces.<br>– Water cycle: Evaporation – Clouds – Rain – water – evaporation –<br>clouds – …<br>– Heating of ocean water: Ocean thermal energy (OTEC)<br>– Waves in Ocean: Ocean wave energy<br>– Tides in Ocean: Ocean tidal energy (due to gravitational forces._<br>The sun produces enormous amount of heat and light through sustained<br>nuclear fusion reactions. The solar energy received on earth and can be used for<br>heating and producing electrical energy.<br>The first person to use the sun’s energy on a large scale was Archimedes<br>who reportedly set fire to an attacking Roman fleet at Syracuse in 212BC. He<br>accomplished this by means of a burning glass composed of small square<br>mirrors moving everywhere upon hinges so as to reduce the Roman fleet to<br>ashes at a distance of a bowshot. Serious studies of the sun and its potentials<br>began in the 17th century when Galileo and Lavoiser utilized the sun in their<br>researches. By 1700 diamonds had been melted and by the early 1800s heat<br>20<br>engines were operating with energy supplied by the sun. In the early twentieth<br>century, solar energy was used to power water distillation plants in Chile and<br>irrigation pumps in Egypt. (Kreider and Kreith, 1981).<br>About 170 trillion KW of solar energy are intercepted by the earth, an<br>amount which is 5000 times greater than the sum of all other energy inputs,<br>{Dickinson and Cheremisinoff, 1980). 30% of this amount is reflected back into<br>space as short wave radiation, 45% is absorbed by the atmosphere, the land<br>surface and oceans and it is converted to heat as the ambient surface<br>temperature of the planet. The remaining 25% powers the evaporation,<br>convection and precipitation cycles of the biosphere.<br>1.2 SOLAR TRACKING<br>Although it is abundant, solar energy impinging on the earth’s<br>atmosphere is relatively small due to attenuation, local weather phenomena and<br>air pollution. Also solar energy is received in cyclic, intermittent form with very<br>low power density from 0 to 1kW/m2. The direction of solar rays changes<br>during the day and with season. Solar energy received on ground level is<br>affected by atmospheric clarity, degree of latitude etc.<br>There is therefore a need to harness the cyclic and intermittent though<br>abundant solar energy. One way of doing this is using solar collectors.<br>Collectors are devices which absorbs and transfers the solar energy to heat<br>transport fluids. Solar Collectors could be flat plate, paraboloic trough,<br>Heliostat – reflector or Fresnel.<br>These collectors are usually operated in a fixed position oriented in a<br>north-south direction in the northern hemisphere titled at an angle<br>approximately equal to the latitude angle of that location. The collector<br>performance and efficiency are usually expressed in terms of solar energy<br>21<br>incident on the collector. A method that has been receiving attention in recent<br>times for increasing the output of the collectors is that of making the collector<br>track the sun.<br>For the rays to be normal to the collector surface requires that the<br>collector follows the sun both in the east- west and in the north- south directions<br>continuously. This type of collector is known as the fully tracking collector. A<br>restricted form of tracking, called semi-tracking may however be obtained by<br>restricting the collector to rotate about a north – south axis, with the collector<br>inclined to the horizontal at an appropriate slope. Two cases may be<br>distinguished: one when the normal to the collector rotates at a uniform speed<br>of 150 per hour and the other when the normal to the collector is always in the<br>azimuthal plane. The later case, though better, requires that the collector be<br>rotated at rotational speeds which vary over the day. The last type of semi<br>tracking is by constraining the collector to rotate about a east – west axis, which<br>itself is inclined to the horizontal at an appropriate slope usually the latitude<br>angle of the location. This is usually done on a monthly basis.<br>1.3 SOLAR COLLECTORS<br>When sunlight strikes an object, a portion of the energy bounces off<br>(reflection). The remainder serves to increase its molecule activity causing a<br>corresponding rise in temperature.<br>The above discussion shows the relationship of sunlight to thermal<br>energy conversion process. The conversion takes place automatically on contact<br>between sunlight and a material. To be able to utilize the sunlight – to – thermal<br>phenomenon effectively requires a device capable of heating up efficiently, able<br>to transfer the collected heat to some kind of heat transfer fluid (either air or a<br>liquid) and be able to do this while exposed to outdoor weather conditions. The<br>22<br>most popular device at present capable of fulfilling this is called a “flat – plate<br>collector”. All flat plate collectors have five basic components in common<br>namely:<br>a. The absorber surface<br>b. The heat transfer interface/fluid passage<br>c. The Glazing<br>d. The insulation and<br>e. The protective casing<br>Flat plate collectors could either be Liquid cooled or air cooled Flat plate<br>collectors. Water or a water/antifreeze solution is used predominantly as the<br>heat transfer (heat removing) Fluid in liquid cooled flat plate collectors. Non<br>toxic anti freezes and silicon based heat transfer fluids are also being developed<br>specifically for use in solar energy collection systems. Air cooled Flat plate<br>collectors use air as the heat transfer medium.<br>1.4 SOLAR PHOTOVOLTAIC CELLS<br>Photovoltaic (PV) is the direct conversion of sunlight to electricity<br>through photoelectric effect. The smallest unit of a solar electric device is the<br>solar cell. Solar cells are made of semi conductor material of which the<br>commonest is silicon. The silicon cell may be from monocrystaline,<br>polycrystalline or amorphous silicon. Monocrystaline refer to cells cut from<br>single crystal of Silicon while polycrystalline refers to cells made from many<br>crystals. Amorphous type cells are made from silicon that is not in crystalline<br>form. Instead silicon is deposited on the back of a glass or surface in very thin<br>layers. The surface is then scored to divide it into a number of cells and<br>electrical connections are added.<br>23<br>Mono and poly crystalline silicon cells are silicon wafers sliced from<br>cylindrical silicon crystals using very precise saws. These wafers are then<br>chemically treated in furnaces to enhance their electrical properties, after which<br>an anti reflective coating is applied to the cell surface to help it absorb radiation<br>more effectively. Thin metal wires are soldered to the front of the cell. These<br>ribbons of metal on the cell act as the contact, whereas a solid layer of metal on<br>the back side of the cell acts as a negative contact.<br>1.5 SOLAR MODULE<br>Arrangement of solar cells wired in series sealed between glass and<br>plastic and supported inside a metal frame is called a solar cell module. A<br>standard module will produce 12 volt, 3.5 Amp at maximum power point.<br>While a solar array is a group of modules mounted together.<br>1.6 NEED FOR TRACKING<br>Tracking requires that the collectors receive sunshine early and late in the<br>day to be effective. At mid and high latitudes the summer gain is significantly<br>greater than the winter gain which typically is not more than 20%.(Paruleka and<br>Rao, 1999).<br>Solar tracking increases the amount of time solar collectors are oriented<br>towards the sun and therefore increase the solar collector output. Solar tracking<br>could to a lesser degree be used for control functions. In a situation where an<br>optical lens is to be used to track the sun’s movement, there would be a<br>reduction in the cost of the lens since the tracker allows the incident rays to<br>strike normal to the lens and therefore smaller lenses are required.<br>24<br>1.7 THE AIM OF THE PROJECT<br>The aim of this research work is to design, construct and test a solar<br>energy tracking device to which a solar module can be incorporated.<br>The aim is to be achieved by the design of a single axis variable speed<br>passive solar tracking device after which construction of the tracker using the<br>design criteria would be undertaken.<br>A solar tracking device will maximize the amount of time the solar<br>module is oriented towards the sun thereby maximizing the energy output from<br>the solar module. It is also hoped that after the construction and<br>experimentation on the performance of the device, some useful information<br>would be gained which could lead to improvement on the design.<br>25 <br></p>