solar hot air collector

If you mean double-glazing, yes, it will definitely be more effective at trapping heat energy.

This is what I though ,that a thermopane window (double glass) would be better then a single pane.Because once I put one on a wooden table outside in the sun ,and when I went to lift the window ,I almost got burnted from the heat between the window and the wood table.

The collector will be of 1/2 inch cement board painted flat black.This will have the mass to collect and store some heat when the fan turns on at a certain temp.

tks

Johnny

johnny; be sure you use a paint that won't gas off fumes that could harm you. perry

Using black anodized aluminum plate would eliminate outgassing toxic effects of volatile organic compounds.

Just consider the double-glazed window. It is a lot better option than a single pane one however, a vacuum between the glasses makes it a good insulator. Even in severe sub-zero temperatures it can function well and a gap >0.5cm is really good.

Isti80,

The space between the glazing layers is not a vacuum, but is filled with a selected gas. (Often air or nitrogen, but frequently argon, and occasionally at a high cost krypton; listed in order of decreasing thermal conductivity.) Further efficiency will be achieved by using a low-e coating on the inside of the inner glazing layer, because this will reflect the longer-wavelength radiation from the collector surface back to the collector while transmitting the shorter-wavelength solar radiation quite well.

--John M.

Do you have to use gases? Why can't use vacuum?

Isti80,

If you have a means of keeping the two glazing layers apart, other than by air, then use it. Two parallel flat panes of glass, with a vacuum between them, would be forced together by the atmospheric pressure on the outside of this assembly.
For a collector cover 36" by 84", this force is more than 40,000 pounds.

A very good vacuum has no thermal conductivity or convection, just radiative heat transfer, so heat loss through a glazing from the collector would be minimal. Any way of keping the panes apart, in the presence of the vacuum, adds complexity and cost. Spacers have their own thermal conductivity problems and can adversely harm the transmission of radiative energy. A gas fill at a pressure close to the average atmospheric pressure is a good balance of cost and heat loss. (The minor changes in atmospheric pressure will cause the two panes to bow slightly apart or together until the pressures on the two sides of the glass are nearly equal.)

Depending on the thickness of the spacing and the temperature difference between the two sides, convective heat loss can be kept minimal. Then, one can choose which gas to use, because some have greater or lesser thermal conductivity than others.

The "noble" gases are mono-molecular and have less conductivity than all other gases. The choice among them is a balance between materials cost/availability, conductivity, and stability of the gas fill. Over time, each of the gases will diffuse out of the space into the surrounding atmosphere (and be replaced with in-diffusing nitrogen or oxygen). The higher the molecular weight of the gas, the slower this diffusing process is. Therefore, although fairly abundant, helium is a poor choice (its small molecular size allows rapid diffusion through the best of seals and even through solid materials). Argon is the normal choice because its slower diffusion and relatively low cost are a good compromise. Krypton, though slower to diffuse, has a much higher cost, and therefore is only used in the most expensive windows.

Regards--JMM

GA

Thank you all ,for all the comments and help.

Johnny