Seeking a Ceramic Solution for Solar Application
Can ceramics provide the key to the perfect Trombe wall?
Trombe walls, which are named for their French inventor, are the simplest of solar heating systems. A Trombe wall is typically a solid, south-facing wall (generally 8-14 in. thick) that is constructed of adobe, concrete or filled concrete blocks, and covered with glass (see Figure 1). As the sun shines through the glass onto the darkened surface of the wall, the energy in the sunlight warms the mass of the wall. A large part of the energy in the sunlight is in the visible part of the spectrum and passes through the glass to be absorbed by the dark surface of the wall.
As the wall warms up, it conducts heat into the building. Some of the heat is lost back out through the glass—but not much. The heat loss is in the form of conduction, which is interrupted by the layers of glass (typically two), and infrared radiation, which doesn’t readily pass through glass. This is called the greenhouse effect: visible light is easily absorbed, and infrared is not so easily lost.
Selective surfaces are a useful enhancement in many solar technologies. They capture heat in a way that is analogous to the role of glass in producing the greenhouse effect. Most surfaces are equally good at absorbing visible light and radiating infrared. A pale paint is poor at both, and a dark paint is good at both. Some surfaces, however, are much better at the absorbing part than the radiating part. Such surfaces are termed selective.
It is possible to create such surfaces using various methods, and they are used in a variety of places. Thirty years ago, the first generation of selective solar collectors had a black chrome surface. The copper substrate was plated with shiny chrome and covered with a suitably thin layer of chromium oxide particles of a particular size. (Chromium oxide particles are black, and the size of the particles determines the behavior of the surface relative to the light of corresponding wavelength.)
As a result, the surface looked flat black and absorbed visible light just the way you would think it should based on the appearance of the surface. At infrared, which has a longer wavelength, it radiated much like the shiny surface under the black coating. Such a surface has a coefficient of absorption of .98, which is to say 98% of a perfect absorber, and an emissivity of .12, or 12% of a perfect emitter. Some selective surfaces are naturally occurring, such as oxidized zinc, which has an absorptivity of 65% and an emissivity of 30%. Weathered galvanized roofs have been used as solar heat sources, but most good selective surfaces are carefully designed.
Building a Better Wall
Trombe walls could be much improved by the addition of a selective surface. Various attempts have been less than satisfactory for a few reasons. The surface of the mass wall is not metal, so the sort of plating processes available for solar collectors cannot be used. In addition, selective paints also require a shiny metal substrate in order to be selective.
In order to function properly, the mass wall itself must have no thermal breaks in it. Consequently, the outside of the wall must be stuccoed or painted, and the inside must be plastered, producing a clean path for the heat to flow from the outside surface to the inside, with no gaps that present obstacles to conduction.
In the past, products have been available that were high-quality selective surfaces provided on a thin copper or nickel foil, with an adhesive on the back side. Although these have been used effectively on Trombe walls, the foils were not stretchy enough to adhere smoothly to the whole outside surface of the mass wall, so they looked bubbly and unprofessional. However, the selective surface performed so well that the air gap behind it did not degrade the performance very much. Unfortunately, the foils are no longer available.
The ideal Trombe wall would be: A dense, conductive ceramic block or brick of some kind (perhaps 6 x 18 x12 in.), with one of the 6 x 18-in. surfaces already coated with a durable selective surface. The blocks could be dimpled to interlock and stack nicely into a 12-in.-thick wall, with minimal mortar to bind them together. The selective surface would likely be in the form of a multilayer glaze that has been fired onto the surface of the block. Many chemistries are possible for producing selective surfaces, so those that are appropriate for a ceramic block need not be the same as those used with metal substrates. Even a shiny, low-emissivity surface that readily accepts selective paint would be a big improvement.
So far, no one has created this product, but the world could benefit from some excellent, easy Trombe walls. Who will make this possible?
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