Flate plate collector

A brief description about flat plate collectors is presented.

1. WELCOME

2. FLAT-PLATE COLLECTORS PRESENTED BY: KIRTIKA SHARMA Department of Renewable Energy Engineering, CTAE, MPUAT, Udaipur (2017-18)

3. Solar collector – A device designed to absorb incident solar radiation and to transfer the energy to a fluid passing in contact with it , usually liquid or air. They can be classified in three groups : - Flat-plate collectors, - Evacuated-tube collectors - Focusing collectors. INTRODUCTION

4. FLATE – PLATE COLLECTOR A flat plate collector is basically a stationary black surface that is place d at a convenient path of the sun . Simple and effective means of collecting solar energy for applications that require heat at temperatures below 100 c. Figure1. Flat-plate collectors

5. However to reach higher temparatures evacuated-tube collectors and focusing collectors are used. In evacuated-tube collectors they use vacu m to reduce heat lost and to protect the absorber coating from deter eora tion.By this way they can reach temperatures up to 140 ° C and they can collect both direct and diffuse solar radiation . And focusing collectors, they are not stable and they follow the sun to get direct radiation; they can not utilize diffuse radiation. And they are also capable of producing high temperatures.

6. Classification: ( based on heat transfer fluid used ): Liquid heating collectors - used for heating water and non- freezing aqueous solutions 2) Air or gas heating collectors - used for heating, drying or curing of agricultural products, space heating

7. Components of a typical flat plate collector: A transparent cover – one or more sheets of glass or radiation transmitting plastic film or sheet. 2) Tubes, fins , passages or channels- integral with or connected to the absorber plate & conduct the working fluid through the collector. 3) The absorber plate- normally metallic or with a black surface 4) Insulation – provided at the back & sides to minimise heat losses 5) The casing or enclosure - encloses the other components & protects them from the weather.

8. Fig 2. Cross s ection of a b asic liquid heating f lat - p late s olar c ollector

9. A TYPICAL LIQUID FLATE PLATE COLLECTOR: Absorber plate: Is basically a flat metal plate, usually made of highly conductive and corrosion resistant copper or steel or aluminium or with a black surface with high absorptivity for solar radiation. Is made from metal sheet 1 to 2 mm in thickness. Generally corrugated galvanised sheet material is widely used. As copper is expensive, steel is widely used. The surface of the absorber plate determines how much of the incident solar radiation is absorbed and how much is emitted at a given temperature. Flat black paint which is widely used as a coating has an a b sor p tance of about 95 percent for incident shortwave solar radiation.

10. The tubings made of copper of diameter 1 to 1.5 cm are soldered in line or integral with the absorber plate with the pitch ranging from 5 to 15 cm. Fig 3. Cross s ection of a absorber plate & flow passages of a f lat p late c ollector For a copper plate 0.05 cm thick with 1.25-cm tubes spaced 15 cm apart in good thermal contact with the copper, the fin efficiency is better than 97 percent. Flow passages:

11. One or two sheets of glass of thickness 3 to 4 mm or radiation transmitting plastic film or sheet that is transparent to incoming solar radiation and opaque to the infrared re-radiation from the absorber. S hould have a high transmittance for solar radiation and should not detoriate with time . It r e duces convective and radiative heat losses from the absorber . Transparent Cover plate: Fig 4. Cross s ection of a cover p art of a f lat - p late c ollector

12. Thermal insulation of 5 to 10 cm thickness. Material is generally mineral wool or glass wool or a heat resistant fiber glass. Placed behind absorber plate to prevent heat losses from the rear surface. The collector enclosure is usually made from galvanized steel or aliminium . Slagwood , polyurethane foam, hay in polythene bags are other suitable insulation materials. Insulation & Enclosure: Fig 5. Cross Section of an Insulation Part of a Flat-Plate Collec tor

13. FLATE –PLATE AIR HEATING COLLECTORS (SOLAR AIR HEATERS) A conventional air heater is typically a flat passage between two parallel plates. One of the plates is blackened to absorb incident solar radiation. One or more transparent covers are located above the absorbing surface. The air is made to pass through the passage so that it gets heated. Insulation around the sides and base of the unit is necessary to keep heat losses to a minimum.

14. Fig 6. Cross s ection of a b asic air-heating f lat - p late s olar c ollector

15. PROPER OR I ENTAT I ON and ANGLE of SOLAR COLLECTOR Flat plate collectorts are divided in three main groups according to how they are oriented : Flat-plate collectors facing south at fixed tilt One-axis tracking flat-plate collectors with axis oriented north-south Two-axis tracking flat-plate collectors Most favourable orientation of a collector for heating only- collector facing due south at an inclination angle to the horizontal equal to the latitude plus 15 (s= ø + 15 ) for winter and is (s= ø - 15 ) for summer.

16. SELECTION OF MATERIALS FOR FLATE PLATE COLLECTORS: Absorber plate : High absorbtivity High thermal conductivity Adequate tensile & compressive strength Good corrosion resistance Less specific heat Easily workable Easy to handle Low cost Eg . Copper , Aluminium , Steel.

17. Material specification Material Density kg/m 3 Specific heat Kg/KJ Thermal conductivity W/m º c Aluminum 2707 0.896 204 Iron 7897 0.452 73 steel 7833 0.465 54 copper 8954 0.385 386

18. ii) Cover plate : Good strength, durability non-degradability efficient solar energy transmission Rigidity resistant to thermal shock Minimize convection loss Minimize radiation loss Eg . Tempered glass.

19. Specification of transparent cover Material Thickness (mm) Solar transmisivity (%) Thermal transmission (%) Glass 3-4 91-95 3-5 Pvc 0-3 85 32

20. Properties of insulating materials Material Density Kg/m 3 Thermal conductivity W/m º c Timber 720 0.1442 Thermocol 22 0.0314 Saw dust 188 0.0511 Glass wool 65 0.0418 Fiber glass 32 0.0372

21. A. COLLECTOR PERFORMANCE The thermal performance of a collector can be calculated from a first-law energy balance. A ccording to the first law of thermodynamics, for a simple flat-plate collector an instantaneous steady-state energy balance is : Useful energy = energy absorbed – heat loss to the gain (Qu) by the collector surroundings

22. Q U = A C H R ( τ . α ) - A C U L (T p -T a ) Equation 1 . Useful energy gain equation where Q U = useful energy delivered by collector, W (kcal/ hr ) A C = Collector area, m 2 H R = solar energy received on the upper surface of the sloping collector structure, W/m 2 ( kcal/ hr m 2 ) H = rate of incident beam or diffuse radiation on a unit area of surface of any orientation F = Factor of convert beam or diffuse radiation to that on the plane of collector

23. τ = fraction of incoming solar radiation that reaches the absorbing surface, transmissivity ( dimensionless ) α = fraction of solar energy reaching the surface that is absorbed , absorptivity ( dimensionless ) U L = overall heat loss coefficient, W/m 2 c T p = average temp. of the upper surface of the absorber plate, c T a = atmospheric temperature, c τ . α = a transmittance-absorptance product that is weighted according to the proportions of beam, diffuse, and ground reflected radiation on the collector .

24. A C H R ( τ . α ) = Absorbed energy A C U L (T p -T a ) = Effective heat loss T o improve the performance of solar collector it is necessary either to reduce the overall energy loss coefficient or reduce area from which energy is lost. That is; the maximum possible useful energy gain (heat transfer ) in a solar collector occurs when the whole collector is at the inlet fluid temperature; heat losses to the surroundings are then at a minimum.

25. B. Collector heat removal factor (F R ): A quantity that relates the actual useful energy gain of a collector to the useful gain if the whole collector surfaces were at the fluid inlet temperatur e. The quantitiy FR is equvialent to the effectiveness of a conventional heat exchange, which is defined as the ratio of the actual heat transfer to the maximum possible heat transfer. Equation .2 . the collector heat removal factor F R Where ; m’ = Fluid mass flow rate, kg/s Cp = Fluid specific heat, J/kg ° C

26. C. Overall heat loss coefficient (UL): In equation 1 . UL is the collector overall loss coefficient and it is equal to the sum of the top, bottom,and edge loss coefficients : Equation 3. Overall loss coefficient U L U L = U top +U bottom +U edge , W/m ² K

27. 92 % of the total sunshine reaches to the copper absorber . 8 % of the total sunshine is reflected from glass . 5 % of the sunshine is emitted from the panel . 12 % is lost through convection and conduction. Fig.6. E nergy diagram of typical flat flate collector

28. D . COLLECTOR EFFICIENCY It is the measure of collector performance and is defined as the ratio of the useful gain over any time period to the incident solar energy over the same time period. The basic method of measuring collector performance is to expose the operating collector to solar radiation and measure the fluid inlet and outlet temperatures and the fluid flow rate. ƞ =   Equation.7. Collector efficiency equation

29. I NSTANTANEOUS SOLAR COLLECTOR EFFICIENCY can be defined as the ratio of the actual solar energy collected to the solar energy incident on or intercepted by the collector. Equation 8. Instantaneous solar collector efficiency formula

30. METHODS TO IMPROVE EFFICIENCY OF FLATE-PLATE COLLECTOR: By increasing the transmission of energy through the collector to the working fluid. By decreasing the thermal losses from the collector to the ambient by reducing conductive, convective and radiative losses. Conductive losses can be reduced by using a sufficiently thick layer of thermal insulation & also by increasing the thickness of the air gaps. Convective losses can be eliminated by evacuating the space between the absorber and the cover plate. Radiative losses can be reduced by a spectrally selective coating on the absorber plate.

31. APPLICATIONS Domestic applications Domestic hot water Air conditioning Cooking B) Commercial applications laundromats car washes military laundry facilitie s Space heating Power generation Water pumping

32. Figure 7. Flat plate collectors used for heating buildings

33. Figure 8. Flat-plate collectors used for heating swimming pools

34. COMPARISON OF LIQUID AND AIR HEATING FLAT PLATE COLLECTORS S.NO. PARAMETRS LIQUID HEATING TYPE AIR HEATING TYPE 1. Volume of storage required 1/3 rd of vol. of rocks necessary to store equal quantities of heat for air systems Roughly 3 times as much vol. as for water heat-storage (due to low density of air as working fluid) 2. Noise level Less noisy Higher noise level 3. Energy requirements for pumping working fluid Much less Much more(require blowers) 4. Energy supply to absorption air-conditioners Easily adapted Has difficulty 5. Fluid circulation costs Low high

35. CONCLUSION Flat-plate collectors which are used for water heating, are long lasting, and also in long term they are cheaper than other water heating systems.However,they requires large areas if high energy output is a requirement. Than 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.

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