Let us consider a solar powered water heater. The solar collector surface is coa

ID: 1863054 • Letter: L

Question

Let us consider a solar powered water heater. The solar collector surface is coated

with black chrome, whose emissivity can be approximated to be 0.9 for radiation at

wavelengths less than 5 m and 0.05 for radiation greater than 5 m. How much heat

does this surface deliver to the water circulating in the collector when solar radiation

is incident on the surface at a rate of 600 W/m2 and the absorber surface temperature

is 70C. The air and eective sky temperatures are 25C and 15C, respectively, and

the convection heat transfer coecient is 10 W/m2

-K. Assume that the temperature

of the sun is 5800 K.

the isolation given = 600 W/m2

but some energy loss in radiation the energy transmitted to water = 0.95*600 = 570 w/m2 (i.e. considering radiative heat loss)

because the energy in solar spectrum for more tha 5 m wave length is is very less and energy density is also less that so why emissivity is less

heat loss due to convection = 10*(70-25) = 450 W/m2

then net energy tranfer to the water = 540 -450 = 90 W/m2

if we observ result the heat loss due to convection is high that so why for proper utilization we need to insulate the system

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