The Cooling Potential of Sky Radiation With Variations in System Parameters

This study evaluated the building cooling capacity of sky radiation, which was previously identified to have the greatest cooling potential among common ambient sources for climates across the U.S. A heat pipe augmented sky radiator system was simulated by a thermal network with nine nodes, including a thin polyethylene cover with and without condensation, white (zinc oxide) painted radiator plate, condenser and evaporator ends of the heat pipe, thermal storage fluid (water), tank wall, room, sky and ambient air. Heat transfer between nodes included solar flux and sky radiation to cover and plate, wind convection and radiation from cover to ambient, radiation from plate to ambient, natural convection and radiation from plate to cover, conduction from plate to condenser, two-phase heat transfer from evaporator to condenser, natural convection from evaporator to water and from water to tank wall, natural convection and radiation from tank wall to room, and overall heat loss from room to ambient. A thin layer of water was applied to simulate condensation on the cover. Nodal temperatures were simultaneously solved as functions of time using typical meteorological year (TMY3) weather data. Auxiliary cooling was added as needed to limit room temperature to a maximum of 23.9 °C. For this initial investigation, a moderate climate (Louisville, KY) was used to evaluate the effects of radiator orientation, thermal storage capacity, and cooling load to radiator area ratio (LRR). Results were compared to a Louisville baseline with LRR = 10 W/m² K, horizontal radiator and one cover, which provided an annual sky fraction (fraction of cooling load provided by sky radiation) of 0.855. A decrease to 0.852 was found for an increase in radiator slope to 20 deg, and a drop to 0.832 for 53 deg slope (latitude + 15 deg, a typical slope for solar heating). These drops were associated with increases in average radiator temperature by 0.73 °C for 20 deg and 1.99 °C for 53 deg. A 30% decrease in storage capacity caused a decrease in sky fraction to 0.843. Sky fractions were 0.720 and 0.959 for LRR of 20 and 5, respectively. LRR and thermal storage capacity had strong effects on performance. Radiator slope had a surprisingly small impact, considering that the view factor to the sky at 53 deg tilt is less than 0.5.