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SOLAR ENERGY COLLECTORS Non Conventional Energy Sources References : G. D RAI J K Nayak
SOLAR COLLECTORS: v Collector is a device which absorbs the incoming solar radiation, converts it into heat, and transfers this heat to a fluid (usually air, water, or oil) flowing through the collector.
COLLECTORS Collecting Flat (or) Non concentric Concentric (or) Focussing Mounting Stationary Adjustable Fluids Liquid Gas (or) Air
Flat or Non concentric type collector Flat-plate collectors, developed by Hottel and Whillier in the 1950 s, are the most common type of solar collector which are widely used for domestic household hot-water heating , air heating Temperature can be upto 900 In the Non-concentrating type, the collector area is the same as the absorber area.
Flat (or) Non concentric Fluid Type Air Type FLUID TYPE : Heat Transfer medium is water or any other liquid AIR TYPE : Heat Transfer medium is air or gas.
A typical diagrams of “ Flat plate solar thermal collectors”
Components of Flat plate collector GLAZING ABSORBER HEADERS INSULATION CONTAINER ** These can absorb both direct and diffused beam radiations
GLAZING: o Made of one or more covers of transparent material like glass or plastic. . o It is used to reduce convective and radiative heat losses from the absorber. o It is opaque to the infrared radiations. ABSORBER : o It is usually made of copper or steel metal. o The surface is covered with a thin coating of primer and then flat black paint of high absorptance huge paint may increase the resistance to heat tranfer o Coating is applied on the surface to maximize the absorptance of solar energy and minimizes the radiation emitted by plate. TUBES OR HEADERS : o A tube that is attached to absorber or sometimes it is a part of absorber plate. o If the working fluid is air , the flow passage should be below the absorber plate to minimize heat losses.
Insulation: v These are some materials such as fiberglass and they are placed at the back and sides of the collector to reduce heat losses. eg. wool, fiber etc. . . Container : v Which encloses the other components together and protects from the weather.
Flat plate collectors
Flat plate collector Fluid Type
Cross sectional arrangements through the collector plates Tube in strip type A Glazing Riser Absorbing plate B Glazing Riser Absorbing plate Insulation
Cross sectional arrangements through the collector plates Tubes brazed to absorber C Glazing Riser Absorbing plate Insulation D Glazing Riser Absorbing plate Insulation
Working Principle Sunlight passes through the glazing and strikes the absorber plate, which heats up, and solar energy is converted into heat energy. This the heat is transferred to the fluid passing through pipes attached to the absorber plate by means of convective heat transfer.
Glazing design Due to two glass sheets are used losses due to air convection are further reduced , this is mainly used and important in windy areas. Radiation losses in infrared spectrum are reduced to 25% from 50% due to use of two glass sheets. It is not economical to use more than two plates as if the plates increases it reflects the solar radiation by 15 % and efficiency decreases. The air space between the cover and the absorber prevents heat loss by convection However if one watches the electromagnetic spectrum
As cover is made of glass It allows the solar radiations of wavelength < 2µm and is opaque for wavelength > 2µm It is seen from electromagnetic spectrum that infrared radiations having wavelength > 2µm As a result infrared are opaque and does not pass Soo heat is trapped inside the space between the cover and absorber plate similar to green house effect and keeps it hot. However some of heat occurs due to conduction , convection , and radiation thereby effecting the overall efficiency
Glazing design considerations This can be met by following steps : -- It can be reduced by using "water white glass" with low iron content Reflection which is order of 15% can be met by gentlly itching the glass surface hydro flouric acid.
Absorber design Coating done on the absorber surface is called "selective coatings " "Black chrome" is extensively used coating which is of 2µm thickness Heat transport sytem Heat generated in the absorber is continuosly taken by the flow of medium through tubes.
Air Heaters Non porus Absorber Porus Absorber Non porus absorber : Air flows above and beneath the absorber. Porus Absorber : Air flows through the absorber.
Non Porus Air heater
Air Type Flat plate collectors Used commonly for space heating and agricultural heating applications. A Typical Flat plate collector Solar air heater
Collectors with porus absorbers
Design of Air plate collector Heavily insulated material like mineral wool is coated beneath the surface of absorber The most favourable orientation of the panel is facing due south with an inclination of 15 o latitude with the horizontal. Air as medium of heat transfer eliminates the corrossion and freezing problems which are more when water is used. Air is passed through the baffles arranged in zig zag pattern between the absorber and insulation where fins provides the more contact area thereby more amount of heat to transfer.
Applications Air collectors Heating Buildings Drying Agricultural Products Heating Green houses. Heat source for heat engines. Advantages Flat Plate collectors Both Beam and Diffuse radiations orientation need not be towards sun little maintaince simplier than concentric plate type
Thermal Analysis of Flat Plate Collectors
Thermal Analysis of Flat Plate Collectors If I is the intensity of solar radiation, in W/m 2 , incident on the aperture plane of the solar collector having a collector surface area of A, in m 2 Then the amount of solar radiation received by the collector is : However, as it is shown Figure 2, a part of this radiation is reflected back to the sky, another component is absorbed by the glazing and the rest is transmitted through the glazing and reaches the absorber plate as short wave radiation. Therefore the conversion factor indicates the percentage of the solar rays penetrating the transparent cover of the collector (transmission) and the percentage being absorbed. Basically, it is the product of the rate of transmission of the cover and the absorption rate of the absorber and is given by
Thermal Analysis of Flat Plate Collectors As the collector absorbs heat its temperature is getting higher than that of the surrounding and heat is lost to the atmosphere by convection and radiation. The rate of heat loss (Qo) depends on the collector overall heat loss coefficient (UL) and the collector temperature.
Thermal Analysis of Flat Plate Collectors Under steady state conditions The rate of useful energy extracted by the collector (Qu), expressed as It is also known that the rate of extraction of heat from the collector may be measured by means of the amount of heat carried away in the fluid passed through it, that is:
Thermal Analysis of Flat Plate Collectors It is convenient to define a quantity that relates the actual useful energy gain of a collector to the useful gain if the whole collector surface were at the fluid inlet temperature. This quantity is known as “the collector heat removal factor (FR)” and is expressed as:
Concentric type collector :
simple concentric collector
Types of concentric collectors a) Flat plate collector with adjustable mirrors b) Compounded parabolic collector c) Cylindrical parabolic collector d) Mirror strip reflector (or) Moving reflector with fixed concentrator e) Fresnel lens collector f) Parabolic trough collector
Cylindrical parabolic trough collector Beam radiation is reflected from the number of individual mirrors called heliostats to the central receiver at the top. Upto 20000 C
Fresnel Lens Collector It is flat on one side and has fine longitudinal grooves on the other The angles of these grooves are such that the radiation is brought to the focus Lens are made of plastic sheets. Used upto 4000 C
Parabolic trough collector solar radiation coming in a particular direction is collected and is collected on the focus of the parabola
Thermal Analysis of concentrate collectors Under the steady state conditions we have : Equation is written assuming that the contribution of diffuse beam radiations are negligible
And Q 1 the overall loss coefficient can be given as :
CONCLUSION Long lasting Cheaper than other water heating systems. However, they requires large areas Also solar energy is free If we do not include the initial cost for installation and the maintenance. Finally; besides these we should remember by using solar energy we can protect nature