Monday, September 01, 2008

Energy in your Attic

As those who read this column regularly know, I have a bit of an obsession with attics, and with the energy available in our attics. So it will be no surprise that, while here at the farm in central Texas, I’ve been doing some investigation above the ceiling.

Previously, of course, I build a solar heat collector in the conventional way-an insulated box with clear polycarbonate sheeting on top. The collector did a credible job as a solar heat collector, generating temperatures inside the box over 80 degrees F above ambient. Though I wasn’t able to generate the desiccant or absorbent cooling I was hoping for, and didn’t get around to quantifying the amount of energy gathered, it is clear that given enough area and storage volume and no worries about aesthetics, it was practical to supply essentially all the heat required by a typical house very cost effectively with this type device. And I’m still convinced it could generate most of the cooling required as well.

But, what if you could generate these benefits in your attic? Without the clear sheeting? Without the additional box/framing? Without the aesthetic issues? Obviously, without the clear sheeting you would collect less of the energy hitting the surface and collect heat at lower temperatures, but might you be able to improve economics and aesthetics by reducing cost and making the collector invisible from outside the attic?

The first step to finding out was to take some temperature measurements. Obviously, the best attics are built with good ventilation. This preserves the integrity of the insulation and minimizes heat transfer into the house, reducing your utility bills. But, it also reduces the temperature in the attic. So, to effectively collect heat from the attic, especially in winter, you would need to insulate below the roof decking and collect heat from the space in between.

To get a feel for whether the temperature would be high enough to make collection worthwhile, I stuck a small patch of fiberglass insulation under the roof decking and inserted a thermometer in the space between. On clear days, the temperature measured was about 150-160 degrees F, or about 50-60 degrees above ambient. Even on partly to mostly cloudy days, the temperatures were about 40-50 degrees above ambient.

This means that it is quite possible to obtain all the heat needed for hot water heating during the summer. And, south of the Mason-Dixon, where high temperatures in the winter average around 60 degrees, it is possible to collect the heat needed for space heat, and much of what is needed for water heating.

Of course, possible is far different from practical. It remains to be seen how much this type collector would cost, and the amount of heat which would be generated from each square foot of collection area. The answers to these questions will have to wait, as I’m moving on to new adventures, but with collector costs a fraction of those for conventional collectors, it seems a good bet that this could be practical as well.