Custom-Built Passive Solar Appliances

The Greenhouse Effect:

All of our hand-built solar contraptions operate using the "greenhouse effect." We like to bring this up for two reasons: 1) most people have heard about the greenhouse effect but don't fully understand it, and 2) once you understand how it works, you can use it to design and build your own contraptions.

Most people know that the sun emits a broad spectrum of radiation including ultraviolet, visible light, and infrared heat. The part most people don't know is that visible light radiation travels through glass (and clear plastic) while infrared heat radiation does not. Glass is opaque to heat but transparent to light.

You may have noticed that if you have the windows rolled up in your car on a sunny day, hot or cold, the inside of your car always get hotter than the outside ambient temperature. Why? The greenhouse effect! Yes, so, this is how it works. Visible sunlight passes through the transparent windows. When the light hits a surface in your car, some of that light is absorbed and some is reflected. The reflected light may pass back through the windows or hit another surface and get absorbed or reflected again. Anyway, all the light that gets absorbed is converted from light energy to infrared heat energy. And, as we pointed out above, glass is opaque to infrared heat radiation, so that heat energy is trapped inside and it warms up interior of the car. This also is how most solar heating devices - greenhouses, solar ovens, solar water heaters, etc. - work.

We also know that dark colored objects are dark because they absorb more visible light than they reflect, black being a color that absorbs all the frequencies of visible light. So, most solar contraptions have an absorber, usually painted black or a dark color. And the second important element is the glazing, usually glass or clear plastic that enables the greenhouse effect to happen.

Can you identify the absorber in our solar heating contraption examples below?

Cold Frames

Solar Oven

In the case of the cold frames, the windows of course are the glazing and the soil and plants are the absorber. Our particular solar oven is all reflective, so the pot, pan or casserole dish should be a dark color to act as the absorber. In the case of the solar dehydrator, the dark colored metal underneath the glass acts as the absorber. It gets hot and air flows on both sides of the absorber up into the cabinet. Yes, this is one other point to mention in design of solar contraptions - heat rises - and we can often use that to construct a passive device (no fans or pumps required) that operates quite efficiently using natural heat convection.

Okay, thanks, but will you build it for me?

Well, if we didn't convince you how easy it is to design and built your own contraptions, you could persuade us to build a unit for you. We prefer to use recycled materials as much as possible, so give us some time to gather the necessary materials once you let us know what you'd like.

Asher Gelbart

575.574.7119

agelbart@gmail.com