The words "solar" and "cooling" might be an oxymoron, but the sun can
be a source of cooling in a number of ways. The simplest evaporative air conditioning system,
a pond of water on a flat roof, uses the sun to help evaporate the water. Evaporation of
the water cools the roof and the space below.
Any solar heating system, active or passive, can become a cooling system. Shading the collector
during the daytime and exposing it to cool night time temperatures allows it to lose energy.
As the outdoor temperature rises during the day, the cooler storage medium will absorb the
heat in the home. At night the cycle is repeated, and the stored heat is given up to the
cool night air.
Designing a passive system for cooling requires some form of daytime shading. A retractable
awning, or an overhang extension of the roof to shade the glazing from the high summer sun
but expose the glazing to the low winter sun will suffice. Moveable insulated panels work
better but add to the expense and mechanical complexity. Doors or glazing panels that open
will allow the cool night air in to remove stored heat.
Active systems can benefit from shading, but work almost as well without it. Operating
the collection system at night reverses the energy transfer. Heat is removed from the storage
facility and radiated out the collector. The storage facility can collect the heat of the
home during the daytime and expel it at night. Areas that do not experience dramatic temperature
swings from day to night, or that have warm nights need a heat pump solar system to capitalize
on the reverse flow. Areas that experience extreme temperature swings, such as the desert,
can construct heat pump systems efficient enough to produce refrigeration.
Solar cells that produce electricity put a new perspective on cooling. The electricity
generated can power conventional air conditioning equipment day or night. If sufficient
power is not available from the storage batteries, current from the local utilities can
run the system.