Comparison of Solar Thermal Collectors

Solar thermal means using the energy from the sun to make hot water. There are two types of solar thermal collectors used in residential applications, flat plate collectors and evacuated tube collectors. I have an hour long presentation that I do on this topic, but I'll keep it brief here.

Flat plate collectors are the same type of collectors commonly installed in the 70's and 80's. They haven't reinvented the wheel. They are basically the same as they were then, but with better insulation and more efficient absorbers. They consist of a 4 ft. by 10 ft. heat collection box (smaller sizes are available) with a series of pipes encased within it beneath a glass cover. These collectors can resemble skylights. As fluid (usually a glycol mix or plain water) flows through the collector it is heated by the sun. The heat of the fluid is transferred to the hot water storage tank. Flat plate collectors are very much affected by the ambient air temperature. They will perform much better in warm conditions than in cold conditions.

Evacuated tube collectors have become increasingly more popular over the last 4-5 years. These collectors use absorber plates on heat tubes to collect solar energy. There can be anywhere from 16 to 30 evacuated tubes in a collector. There is a small amount of water within the tube that boils. The steam rises up the tube and transfers it's energy to a glycol solution in a header assembly which then transfers the heat to the hot water tank. When the steam cools and condenses, it drips back down the tube to repeat the process. The heat tube is encased in a glass vacuum tube. The insulating characteristics of the vacuum shields the heat tube from ambient conditions. Because of this, evacuated tube collectors perform nearly the same in warm conditions as in cold conditions.

The question I hear most is, "Well, which one is better?" It really depends on where you are and what you want to heat. Here, where I am subjected to the chill of the Mid Atlantic winters, I have two evacuated tube collectors, a 24 tube and a 30 tube, heating my hot water as well as the radiant heat and growing table heat for my wife's greenhouse/potting shed. If I was heating a pool in the warm summer months, I'd be using flat plate collectors.