The Potential of Sky Radiation for Humidity Control

The potential of sky radiation (SR) to serve the latent space cooling loads was evaluated. Using ASHRAE standard 55 comfort limits (room temperature 22 °C, relative humidity 60%, and dew-point temperature 13.9 °C), condensation was the chosen mechanism for humidity reduction. Typical meteorological year (TMY3) weather data were used for eleven ASHRAE climate zones. Three values of load-to-radiator ratio (LRR) (infiltration/ventilation volume flow rate times the ratio of building floor area to radiator area) were evaluated: 0.35, 3.5, and 35 m/h. Three thermal storage cases were considered: 1. Annual cooling potential, 2. Diurnal storage, and 3. Minimum storage capacity to serve the entire annual load. Six SR temperatures T_rad = 13.9 to −26.1 °C were tested. Even in the most challenging climates, annual SR potential exceeded the total sensible and latent cooling load, at least for the lowest LRR and the highest T_rad. For diurnal storage, SR served less than 20% of the load in the hot and humid southeast, but the entire load in the mountain west. The minimum storage capacity to meet the entire annual load decreased with decreasing LRR and decreasing T_rad. For the southeast, large capacity was required, but for Louisville, for instance, sufficient capacity was provided by 0.05 m³ of water per m² of floor area for LRR = 0.35 m/h. These results demonstrate that for much of the U.S., sky radiation has the potential to serve the entire annual sensible and latent cooling load.