The heat balance of an unsheltered black bag.

A rectangular black rubber bag 1m×1m×01m with walls 5 mm thick is filled with 100 litres of water, supported on a thin, non-conductive, horizontal grid well above the ground, and exposed to a solar irradiance G = 750 Wm⁻² (Figure).

 The ambient temperature T_a is 20 ⁰C, and the wind speed is 5ms⁻¹. Calculate the resistance to heat losses from the bag. Hence estimate the maximum average temperature of the water, and also the time taken to reach that temperature.

Transcribed Image Text:

Sky G 750 Wm-2 Radiative losses - Ambient u= Sms- Wind: Convective losses 0, Im --Fluid • Thin supporting grid of non-conductive material -I m- P Losses RL Useful Trel Tsef Losses "u", Losses Radiation R - Plate Convection R.p Radiation Convection т. -Ambient Ambient- ---Sky Sky G 750 Wm-2 Radiative losses - Ambient u= Sms- Wind: Convective losses 0, Im --Fluid • Thin supporting grid of non-conductive material -I m- P Losses RL Useful Trel Tsef Losses "u", Losses Radiation R - Plate Convection R.p Radiation Convection т. -Ambient Ambient- ---Sky