May 14, 2008

Windows and solar hot-air collectors are about equal in collection capability. In fact, a window that is designed for collection should beat a dedicated collector, but a lot of windows will not do so well (see note below). I think the choice boils down to which works best for your situation. To me, the pros and cons would go something like this.

Window Pros:

*Collect heat and provide daylight

*Views

*Nice bright interior

*With good thermal mass they can carry some heat into the evening.

Window Cons:

* High heat loss at night and on cloudy days (can be somewhat overcome with thermal shades).

* Can be undesirable in some situations (glare, loss of privacy, etc.)

* Can lead to overheating — you need overhangs for summer, and a house with adequate thermal mass to absorb the heat.

* Your south wall has to have a good view of the south sky.

* You have to have a floor plan in which it makes sense to have windows in the south wall.

* Harder to distribute the heat into the parts of the house where you want it.

This is a fairly large list of cons, but that's what passive solar house design is all about. If you do it all right, the windows both collect heat and make the house a bright and pleasant place to be. In a retrofit situation, you just have to be lucky.

Collector Pros:

* More flexible in location — south roof, south wall, even detached from house — easier to find a location with good sun.

* Can get the heat from the collector to where you want it more easily.

* It is possible to store heat for later use (at the cost of more complexity).

* No night heat loss problem.

Collector Cons:

* You lose the daylight and the views.

* Often have controls and fans that must be maintained.

* Typically they don't look as good as windows.

It’s not really an either or — you can (and should) use both on the same house. You can even mix them on the same south wall. I did not put cost down as either a pro or con for either, since it can vary so much depending on how you go about it.

Note: All things being equal, you would expect windows to be more efficient collectors. The sun that penetrates the window gets absorbed deep in the room. Even if some of the sunlight is reflected off room surfaces, very little gets back out the window. Since there is no hot absorber just inside the glazing, the heat loss from the window is lower that it would be from a collector. So, in the ideal window case, you have efficient absorption coupled with low heat loss — a good combination.

The things that are often not "equal" are that glass in windows is not generally designed for high transmission, and if it is not it will absorb a lot of light before the light can get into the room. A good clear, non low-e, double glazed window might transmit 70 percent of the light incident on it, but this can be (and typically is) a lot less depending on type of glass and low-e coatings.