Picking a Base Absorber Material for Your Flat Plate Solar Collector
A flat plate solar collector is the most common type of solar collector. They consist of a rectangular panel with a dark absorber inside and a transparent cover. Behind the absorber are parallel tubes. Inside these tubes is a fluid (antifreeze or water) that absorbs the heat and transfers it to a hot water tank or a heat exchanger for space heating.
If you build a flat plate collector you will have to decide which materials will work best for your application. A couple of those decisions are choosing the plate material and the coating that will be applied to it. These are important choices because what you choose will determine how much heat can be absorbed and how much heat will be lost.
Absorber Plate Material
Absorber plates are usually made of metal. Copper, aluminum and steel are used because they are good heat conductors. Copper is the most expensive but also the best conductor and corrodes less than aluminum.
Coatings that can absorb a high amount of visible (short-wave) light are used on the surface of the absorber. Their dull or “flat” appearance let them absorb solar radiation coming from all directions equally. Either paint is applied to the absorber or a coating is plated onto it. This results in a black surface that will absorb over 95 percent of the solar radiation that enters the collector.
Since black is normally used to boost the absorber’s thermal capacity flat black paint is option. It does absorb a great amount of heat but it unfortunately also loses much of the heat. Another problem is that it can produce a gas that will fog over the cover glass interfering with the amount of solar rays that can get through. This problem can be lessened by buying a “low gassing” solar paint.
Blackened copper is a much better than paint. Copper, which is an excellent heat conductor, can be blackened with Liver of Sulphur which is sold at arts and crafts stores. Be sure to use it in a well ventilated area.
Some options come already applied to the absorber or have to be applied professionally. These are selected for their superior ability to absorb solar radiation and retain that heat without emitting it back outside. These coatings include black chrome, black nickel, and aluminum oxide with nickel. These are applied galvanically (a hot-dip process that protects the metal from corrosion). A newer type of coating is notable because it is produced with less emission and energy. This is a coating made of titanium, nitride and oxide. It is applied with steam in a vacuum process.
Choosing a material for the absorber that is a good heat conductor and applying a coating that will both absorb solar radiation and retain the heat will make your solar collector much more efficient and provide you with the heat you need to have hot showers and maintain the comfort of your home.