OPTIMIZATION of the GEOMETRY & MATERIAL of SOLAR WATER HEATERS.

Transcription

1 OPTIMIZATION of the GEOMETRY & MATERIAL of SOLAR WATER HEATERS. FLAT PLATE COLLECTORS ABSORBER PLATES OPTIMIZATION OF GEOMETRY SELECTIVE SURFACES METHODS OF TESTING TO DETERMINE THE THERMAL PERFORMANCE OF FLATE PLATE COLLECTORS BASIC PERFORMANCE EQUATIONS TESTING PROCEDURE 1

2 Flat Plate Collectors are the most common collector types for residential waterheating. The black absorber surface transfers the absorbed energy to the fluid. Cross Section of a Basic Flat Plate Solar Collector A typical flat-plate collector is an insulated metal box with a glass or plastic cover and a dark-colored absorber plate. The envelopes transparent to the solar radiation,reduce convection & radiation losses and back insulation to reduce conduction losses. 2

3 Optimization of the Geometry & Material of solar water heaters. Produce an economic & efficient flat plate solar collector. Instead of Cu, Al absorber plate, Fe absorber will be considered. Efficiency test and comparison with other collector types. Offer a new efficient &cheaper collector. *Fe is cheaper than the others *Galvanized iron is resistant to humidity & corrosion *Theoretically it is possible to produce cheaper collector absorber plates. 3

4 Four parameters will be changed by using a computer program to find an efficient and economic collector. Collector area will be kept the same as the others. Absorber plate & tubes join together by point welding. 4

5 GALVANIZATION PROCESS: To galvanize the Fe, the absorber plate will be dipped into the zinc pool and iron is covered with zinc. CHOICE OF THICKNESS AND PIPE SPACING Assuming that materials have been chosen for the pipe and fin, the material cost of the collector plate depends largely on the thickness of the fin and the spacing between pipes. 5

6 Material costs will obviously be reduced if fins are thinner and spacing between pipes is greater. However, this also leads to a reduction in the fin efficiency. Obviously a compromise is required which will minimize the overall cost of a system for a given energy output. To find the best combination price should be maximum Also collector should be efficient. Q u / 6

7 Selective Surface has high absorptance (α) & low emittance (ε). Examples for selective surfaces : Black Ni on polished Fe, Black Ni on Al, Black Cr on Cu, CuO on Al, For Fe absorber plate before application of selective surface, galvanization has to be done. Absorber surface & tubes will be galvanized together. By using suitable selective surface the efficiency can be increased. At least to the same efficiency with the others. 7

8 After these processes, the efficiencies of three collectors having Fe,Cu,Al absorbers will be compared. Methods of Testing to Determine the Thermal Performance of Flat Plate Collectors ASHRAE Standard This standard contains methods for conducting tests outdoors under natural solar irradiation and provides test methods and calculation procedures for determining steady state and quasi-steady state thermal performance, time constants of solar collectors. 8

9 Q u = F F = R A c tanh m W [ S U ( T T )] L [ m( W D) / 2] ( D) / 2 i a m = U L kδ Collector Thermal Efficiency: η = Q u / I T.A c η = actual useful energy collected solar energy incident upon or int ercepted by the collector 9

10 Experimental Determination of The Collector Time Constant: The first performance test to be conducted on the solar collector is the determination of its time constant. 10

11 Method : The inlet temperature of the transfer fluid, t f,i, is adjusted to within ±1 C of the ambient temperature while circulating the transfer fluid, water,through the collector at the flow rate specified, and maintaining steady state or quasysteady state conditions with an incident solar flux of greater than 790 W/m 2, the incident solar energy is then abruptly reduced to zero by shielding the collector from the sun. This may be accomplished most appropriately by shading with a white, opaque cover. The cover should be suspended of the surface of the collector so that ambient air is allowed to pass over the collector as prior to the beginning of the transient test. 11

12 The temperature of the transfer fluid at the inlet, t f,i and outlet, t f,e are continuously monitored as a function of time until the quantity t t f, e, T f, e, initial t f, i t f, i < 0.30 t f,e,t ; temperature of the transfer fluid leaving the collector at a specified time. t f,e,initial ;temperature of the transfer fluid leaving the collector area at the beginning of a specified time period. Experimental Determination of the Collector Thermal Efficiency: The testing of the solar collector to determine its thermal efficiency is conducted in such a way that a governing efficiency curve for near normal incidence is determined for the collector under test conditions. At least four different values of inlet fluid temperature are used to obtain the values of t/i t. 12

13 Experimental Determination of the Collector Thermal Efficiency: η Q u Instantaneous Efficiency = F R A Q FRU L ( Ti T ) a I u i = = FR ( τ α) n Ac IT c [ S U ( T T )] L i F R (τα) n and F R U L are two parameters that describe how the collector works. (α τ) :transmittance absorptance product of the absorber-plate couple. I T : incident radiation F R (τα) n : an indication of how energy is absorbed F R U L : an indication of how energy is lost Ti : inlet temperature Ta :ambient temperature a T 13

14 The Long term performance of collector can be characterized by the intercept & slope Slope : - F R U L The intercept :F R (τα τα) n Experimental Collector Efficiency Data Measured for a Liquid Heating Flat- Selective Absorber The efficiency curves will be established by data over a time period equal to the 4 times the time constant. The integrated value of energy obtained from the collector will be divided into the integrated value of incident solar energy to obtain the efficiency value for that test period. 14

15 At least four data points are taken for each value of t f,i ; two during the time period preceeding solar noon and two in the period following solar noon, the specific periods being chosen so that the data points represents times symmetrical to solar noon. This latter requirement is made so that any transient effects that may be present will not bias the test results when they are used for design purposes. η Q FRU L ( Ti Ta F ( ) ) I u i = = R τ α n Ac IT T If the difference between inlet temperature and ambient temperature is big then the efficiency will be small. If the difference between inlet and ambient temperature is small then the efficiency will be high. 15

16 RESULT : In this study; Al, Cu and galvanized iron were compared and it was observed that thickness of the absorber plate, spacing between pipes, diameters of pipes are the determining factors for the efficiency of the collectors. We will find a good alternative of the flat plate collectors for collecting solar energy. It was observed that since the optimized efficiency values of galvanized iron is very near to currently used materials,cupper and aluminum.that shows the best alternative among our elements is galvanized iron when its price is considered. 16

17 In this way people may use solar energy in their houses by paying less. 17