Two alternatives to standard windows are clerestory windows – those high-up windows that sit between two levels of external roofing – and skylights. They both have their uses in passive solar design, though you have to be careful with their effects on light and heating.
As we mention elsewhere on our site, skylights are very popular in many homes as a means of bringing extra light into a space, especially an attic... but they come at the price of potential overheating in the warmer seasons.
The big problem with skylights placed for solar gain rather than daylighting (so those on a south-facing roof) is that they don't capture much of the winter sun's energy unless the roof is very steep. On a shallow or flat roof, skylights will generate intense solar gain in summer, when it is least needed, and do little except provide a bit of extra light in winter.
Even on a steep roof, skylights are an uncomfortable choice: for example, they are extremely hard to waterproof, insulate, and shade. With heat escaping through the skylight via air filtration and the possibility of rain leaking in, they can be an expensive option to maintain. And the harder it is to operate a shade (in summer, for instance), the less likely you are to operate the mechanism, so you end up with overheated rooms.
If you really want skylights, make sure you choose the best-quality model you can afford, and go for the best glazing, too. Make sure they are professionally installed and well-insulated. If you can, choose a shading system that is easy and convenient to operate – and insulated, if possible!
If you are thinking of installing skylights primarily for the extra daylighting, but want to keep most of the solar gain, you should consider tube skylights. These are special tubes which extend from the roof down into the ceiling. They have a light-gathering lens on the relatively small exterior end and are surfaced with reflective material which directs a massive amount of light into the home below.
The light is diffuse but brings most of its heat energy with it, without exposing the home so badly to unwanted solar gain in summer. The interior end of a solar tube looks like a ceiling lamp.