While solar energy provides a source for passive space heating across a variety of climates, other ambient energy sources may be more appropriate for passive space cooling. These ambient resources include ambient air at dry-bulb and wet-bulb temperatures, ground temperature at locations where the soil is cooler than the indoor comfort temperature, and night-sky radiant temperature, which is substantially lower than ambient air in most climates. The focus of this study was on comparing these sources to cooling loads across climates in the US. Using a degree-day approach, annual cooling potentials were calculated for over 800 TMY3 locations. Color-themed maps for each ambient source at several indoor comfort temperature ranges were constructed as visual references for design purposes. In addition, eight US cities (Denver, CO, Los Angeles, CA, Louisville, KY, Madison, WI, Miami, FL, New Orleans, LA, Phoenix, AZ and Washington DC) were selected to represent a range of climate characteristics, including seasonal ambient temperature, diurnal temperature swings, humidity and sky clearness. For each city, an ambient potential to cooling load ratio (ALR) was calculated, with the potential based on an indoor comfort temperature range of 68°F – 72°F and the load calculated with a base temperature of 65°F. ALR, which neglects phase lags between source and load and the associated need for thermal storage, exceeded one for dry-bulb air and for ground temperature for all locations except Miami, New Orleans and Phoenix. Wet-bulb ALR exceeded one for all locations except Miami, and sky ALR exceeded one for all locations. Finally, the effect of limited thermal storage was estimated by calculating daily ambient source fraction, fas, which is the daily ambient cooling potential divided by the daily cooling load. fas thus approximates the cooling potential of systems with one day’s worth of thermal storage, and has an upper limit of one. Fas, the annual sum of fas, equaled one for ground temperature for Los Angeles and Madison and for sky temperature for Denver and Los Angeles. Fas for ground temperature was above 0.9 for all locations except Miami, New Orleans and Phoenix. Fas for sky temperature exceeded 0.6 for all locations. By utilizing all possible combinations of ambient sources, half of the selected locations attained Fas equal to one and the minimum for all locations still exceeded 0.65.

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