While calculating the efficiency of evacuated tube collector Solar air heater, how to calculate the Intensity of solar radiation? Is it measured directly using a solar radiation measuring instrument or can also be calculated using equations ?
2. Also what is the aperture area of the collector to be used in the same formula for calculation of efficiency. ?
The formula i am using for efficiency calculation is -
Q = m * c * (Tout - Tin ) / ( A * S )
Where
S = Solar intensity
The easiest way is to measure solar intensity using radiation instrument instruments. However, there are models that can be used to estimate solar intensity such as the clear sky model.
The area A in collectors is the area where radiation is converted in to heat. This is the absorber plate that contributes heat to the gas/liquid as it circulates.
Even though commercial instruments are available in the market, I would still suggest for taking down the reading using both methodologies: Instrument and numerical model; for validating the data.
The area to be used for calculating efficiency will depend on the end purpose. If the efficiency is used finally to calculate the total area required for meeting the heating requirement, then you should use the total area covered by the ETC collector and NOT the area which absorbs the solar radiation.
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