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July 30, 2008

Professor Plum, with the wrench, in the greenhouse

Any passive solar design would use the greenhouse effect. 

To understand the greenhouse effect, let me first explain that warming water using the black hose is not the greenhouse effect. The hose material is simply absorbing solar radiation and gaining heat, and then passing that heat through conduction to the water within it. (Black absorbs the most radiation of any color because it’s absorbing all the radiation. A green hose will absorb less because it’s reflecting the green part of the spectrum, which is why it appears green.)

The toasty car on a January afternoon (or, six months later, the sweltering car of a July afternoon) is an example of the greenhouse effect — as is, of course, an actual greenhouse. The solar radiation has enough energy (is of a short enough wavelength, or of a high enough frequency) to pass through the glass. The heated objects inside then re-emit a lower energy radiation (longer wavelength, lower frequency) that cannot pass through the glass. The heat builds up in the enclosed space.

A house in Louisville, properly oriented with a bank of south-facing glass walls, can be heated easily on cold, sunny winter days using only the greenhouse effect. Any passive design at this latitude will also utilize dense interior materials that will hold heat, giving it back throughout the night. Such a design might also incorporate solar panels that use a recirculating fluid to build up heat in a reservoir (not photovoltaic solar panels, but basically the black hose in a glass-topped box). And any design will use lots of insulation. 

Carbon dioxide in earth’s atmosphere provides the same effect as the glass in the greenhouse, an effect that made life on planet earth possible, and an effect that has supported that life since. The exaggeration of that effect, almost certainly a byproduct from the activities of one of those life forms, has led to an increase in planetary surface temperature, which is being called global warming. —F.B.