Lisa Kivirist and John Ivanko stand next to a solar thermal system that heats a greenhouse at their bed-and-breakfast.

AP Photo/Andy Manis

Solar Thermal Greenhouses

The idea of using thermal mass materials -- materials that have the capacity to store heat -- to store solar energy is applicable to more than just large-scale solar thermal power plants and storage facilities. The idea can work in something as commonplace as a greenhouse.

All greenhouses trap solar energy during the day, usually with the benefit of south-facing placement and a sloping roof to maximize sun exposure. But once the sun goes down, what's a grower to do? Solar thermal greenhouses are able to retain that thermal heat and use it to warm the greenhouse at night.

Stones, cement and water or water-filled barrels can all be used as simple, passive thermal mass materials (heat sinks), capturing the sun's heat during the day and radiating it back at night.

Bigger aspirations? Apply the same ideas used in solar thermal power plants (although on a much smaller level) and you're on your way to year-round growing. Solar thermal greenhouses, also called active solar greenhouses, require the same basics as any other solar thermal system: a solar collector, a water storage tank, tubing or piping (buried in the floor), a pump to move the heat-transfer medium (air or water) in the solar collector to storage and electricity (or another power source) to power the pump.

In one scenario, air that collects in the peak of the greenhouse roof is drawn down through pipes and under the floor. During the day, this air is hot and warms the ground. At night, cool air is drawn down into the pipes. The warm ground heats the cool air, which in turn heats the greenhouse. Alternatively, water is sometimes used as the heat-transfer medium. Water is collected and solar heated in an external storage tank and then pumped through the pipes to warm the greenhouse.