A flat plate solar collector of 3 m ? 5 m area has an absorber plate that is to operate at a temperature of 70°C. To reduce heat losses, a glass cover is placed 0.05 m from the absorber. Its operating temperature is estimated to be 35°C. Determine the rate of heat loss from the absorber if the 3 m edge is tilted at angles of inclination from the horizontal of 0°, 30°, and 60°.

Question

GIVEN

• A flat plate solar collector

• Area = 3 m × 5 m

• Absorber temperature (Ta) = 70°C

• Glass cover temperature (Tc) = 30°C

• Distance between absorber and cover (?) = 0.05 m

FIND

Heat loss by natural convection from the absorber of angles (?) of (a) 0°, (b) 30°, and (c) 60° from the horizontal

ASSUMPTIONS

• The space is air filled

SKETCH



PROPERTIES AND CONSTANTS

for dry air at the mean temperature of 52.5°C

Thermal expansion coefficient (?) = 0.00307 1/K

Thermal conductivity (k) = 0.0274 W/(m K)

Kinematic viscosity (?) = 18.7 × 10–6 m2/s

Prandtl number (Pr) = 0.71

Answer 1

The Grashof and Rayleigh numbers for this geometry are



The heat transfer coefficient is given by Equation (8.31), where the quantities enclosed by [ ] are to be set to zero if they are negative: At ? = 0°. Since the aspect ratio (L/?) = 3/0.05 = 60, the critical angle is 70°



The rate of natural convective heat transfer is given by



Performing a similar calculation for the other angles yields the following results



COMMENTS

Heat transfer by radiation will also be significant in this case.