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.This is achieved when incomingsolar radiation is transmitted through the transparent cover, travels in the inner space ofthe collector and finally reaches the surface of the converter.There radiation istransformed into heat and increases the temperature of the plate, at this moment theplate starts to emit radiation with a greater wavelength (less energy) that is nottransmitted to the environment through the transparent cover.Then, this radiation staysstacked between the cover and the converter, increasing the inner temperature of thecollector.Covers are made of transparent materials, mainly glass or plastic.This secondone has the inconvenient of not being stable under the action of ultraviolet radiationbesides of being so soft, what causes that it can be easily scratched, making necessaryits often replacement.Also some polymers allow the pass of UV radiation but don't stop______________________________________________________________________- 61 -Master Thesis Alvaro Escudero Muñoz______________________________________________________________________the exit of that infrared, so the greenhouse effect doesn't take place and good efficiencyis not reached.However, plastics have the good point of their low price and lightness,much better than glass.In the case of the glass, its main drawback is its fragility,making this material unable to face environmental agents like hard snow or hail, orvandalism acts.Nowadays, however, some special glasses are used able to absorbimpacts of medium-high intensity.Infrared radiation coming from absorber produces a increasing of temperature inthe transparent cover and it emits radiation to both the inner and the outer spaces.Onesolution for this problem is using a multilayer cover.This brings a better greenhouseeffect but also it diminishes the solar radiation that reaches the absorber and it makesthe installation more expensive.This solution is useful in locations with lower averagetemperatures and often strong winds but it is now recommended for warm locations likein Spain.Transmittance of glass, τ, depends on its quality and on the incident angle of thesolar radiation.For a perpendicular incidence, a normal glass in a window reflectsapproximately 8% of the solar radiation, and two layers with air between them, 15%.Impurities in the glass, mainly iron, increase the average capacity of absorption.Atypical glass 3mm thick can absorb radiation in the range of 1% - 5%, being 2% a goodaveraged value.Transmittance when solar radiation is not perpendicular to the surfaceof the collector is a little bit lower.Bearing in mind that solar rays change theirincidence angle with the glass along the day, there can be written the following valuesfor the coefficients:− Absorption - 2%− Reflection - 10%− Transmission – 88%Finally, we must pay special attention to the insulation of the collector where thecover is jointed to the box.Circulation of air between cover and absorber creates heatlosses for convection, and also a good insulation avoids the problem of dust andhumidity inside of the collector, improving the efficiency and increasing the lifetime ofthe collector.______________________________________________________________________- 62 -Master Thesis Alvaro Escudero Muñoz______________________________________________________________________C.Thermal fluid circuitThe flowing fluid must be in direct contact with the absorber to permit a goodheat transfer from the plate to the fluid.Heat is propagated following the temperaturegradient, from the plate of the converter to the zone where plate and pipe are joined.Theunion between plate and pipe usually is made by welting, in the whole longitude of thepipe or in several points.If there are zones where the union has not been done properly,like for example when there are cavities of air inside of the welding rope, then the heatis transferred not by conduction but by convection, creating a higher thermal resistanceand making more difficult the heat transfer from the plate to the pipe.D.Thermal insulationThe converter is a higher temperature than the surroundings, that's why,spontaneously, it transfers heat to the environment.The heat emission in the form oflong wavelength radiation in the face where solar radiation is received is diminished bythe greenhouse effect.In order to reduce the heat emission in the rear face it is installeda layer of thermal insulator 4 – 5cm thick between the back face and the box.The capacity to avoid the heat loss depends on the physical properties of theinsulator, the main ones are:− Low thermal conductivity− Low apparent density− High allowed working temperature− Good behavior in humid environment− Chemical stability− Low price− Etc.______________________________________________________________________- 63 -Master Thesis Alvaro Escudero Muñoz______________________________________________________________________A very often used material for insulation is the silicate wool, which has a verylow thermal conductivity and a working temperature of 150ºC.Actually the insulatingaction is performed by the air stacked in the pores of the silicate wool: the more air theinsulator is able to house, the better the insulating properties.Due to the much higherconductivity of water, humidity must be absolutely avoided.E.BoxThe box houses all the components that form the collector.Its most importantfunction is to assure the water tightness of the collector.If in the seals there are holeswhere air can flow, it enters into the box and when contacting the converter it getswarm.As finally the air will go out of the collector, it means that it will steal some heatand besides this heat loss is not easy to calculate.Not only is the air dangerous because of stealing heat.Also it is absolutely notrecommended because it brings contaminants and vapor of water.Contaminants mustbe avoided because they cause corrosion in the converter.When the collector loosestemperature the vapor of water condenses on the inner surface of the cover in the formof drops of water that finally fall to the plate of the converter.When, again, the collectorincreases its temperature this water is evaporated and it creates a sort of fog thatdiminishes the transmittance of the glass and reduces the flow of radiation, what have atriple effect:− Less absorption of radiation− Less heat transferred to the thermal fluid− Less efficiency of the collectorThe box bears the environmental conditions of the place where the collector islocated, more or less corrosive depending on the chemical composition of theatmosphere.As the solar installations must have a great lifetime to be profitable, theproperties of the box must remain in a proper range of usefulness as many years aspossible, what demands a high quality of the materials used in its construction.Plasticsand woods are not recommended because their shape or properties don't remain constant______________________________________________________________________- 64 -Master Thesis Alvaro Escudero Muñoz______________________________________________________________________with time [ Pobierz całość w formacie PDF ]