Solar flat plate collector is basically a heat exchanger transferring the radiant energy of the incident sunlight to the sensible heat of the working liquid. The solar collector has a metal sheet with channels for liquid flow and painted black. When exposed to solar radiation the metal sheet heats up and increases in temperature until a value at which the rate of input of solar energy equals the rate of heat energy output from the system.
The rear side of the plate is insulated to minimize conduction loss. Leaving an air gap above the metal sheet, it is covered by a glass cover on the top to minimize convective heat loss. Flat plate collectors are normally fixed in one position and do not track the sun. Hence the value of optimum tilt is an important parameter to collect maximum solar radiation throughout the year. The optimum tilt for the collector installation could be calculated by the method of Kern and Harris1. In general, the optimum tilt could be, Latitude ± 10o. The collector should be facing south.
In order to maximise the useful energy from the system, the collector absorber is painted with special coatings known as Solar selective coatings which are characterized with high solar absorptance and low thermal emittance. In some installations, the collector can be an integral part of the south facing roof of a building also.
Schematic representation of a solar flat plate collector is shown to the right
a – solar flat plate collector. b- hot water storage tank , Arrows indicate the direction of flow of water based on thermo siphoning.
For installation of a solar flat plate water heater system, the following components are required.
- Solar flat plate collector
- Storage tank
- Water tubes with no return valve.
In order to calculate the collector area required for a domestic installation, the following equation can be used.
Where, m=mass of water (L/day) , s=specific heat capacity of water (4.187 Joule.kg-1. K-1), DT=the temperature difference between inlet and outlet water, I = intensity of solar radiation (W.m-2) and F= Collection efficiency.
About 10% of the solar radiation is lost even before reaching the collector by transmission and reflectance losses. Hence while designing the system, an area 10% more than that theoretically calculated could be used.
Factors for change in efficiency of the collector system, and maintenance:
Dust deposit on the cover system reduces the performance by decreasing the transmittance of the cover glass and hence the glass cover should be periodically cleaned. Freezing of the heat transfer fluid (water) is one of the problems of flat plate collector at locations with low atmospheric temperature. In such cases, antifreeze solutions can be used in the collector and the heat transfer fluid is contained in a closed loop, transferring heat to a secondary heat transfer fluid. The heat transfer fluid is not used directly.
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