Here at Cape Cod Tech we have two different arrays of Photo Voltaic (PV) solar panels. The first array consists of six 300 Watt panels, or a 2 kilowatt (kW) array, placed right outside of our science labs. The second array is located outside of our Roots and Roses greenhouse and consists of four 300 Watt panels, or a 1.2 kilowatt (kW) array. On a sunny day, our Roots and Roses' greenhouse along with our science labs can be 100% self sufficient, and provide enough energy for most of its needs and sometimes more.
Our PV arrays are grid connected. This means that any energy not used in the science labs or in the greenhouse is distributed back throughout the school. Any energy not used by our school can be sold back to the electrical company.
Photovoltaic as the name implies (photo = light/voltaic = energy) converts the sun’s rays into energy we can use in a day to day basis. When the light strikes the photovoltaic cell some of that light energy is captured by a semiconductor material which the panel is made out of. In other words, light energy is absorbed by the semiconductor and electrons are knocked loose. Magnetic fields inside the panels make the electrons that were freed by the semiconductor to flow a certain way. This flow of electrons defines an electrical current. Metal contacts are placed at the top and bottom of the PV panel in order to use that energy externally.
In our school, we have two different types of solar panels. We have the evacuated tube collectors and we also have a flat plate collector. Heating the water to the HVAC shop and the plumbing shop are two evacuated tube collectors placed on top of trailers outside of the shops. At peak performance the evacuated tubes can easily supply enough water for a family of four.
Each of the collectors is rated at 30,000 BTUs at ideal conditions. The additional flat plate collector will be placed on the roof of the Renewable Energy Center building and provide hot water to heat the building through “radiated floor heating”. The evacuated tubes can get water up to and beyond boiling, even in the cold weather. The solar thermal collectors would normally augment to your existing conventional boiler, but can also replace it.
The evacuated tube is made up of several parts. The tube itself, usually multiple tubes, which runs up and down, gathers heat from the sun and heats up a metal component inside. This metal component transfers heat up to a reservoir on top of the tubes which contain a anti-freeze solution. This anti-freeze solution is run down to your solar thermal water boiler, and transfers its heat to the water. The tube in the evacuated tube collectors acts somewhat like a thermos. Like a thermos, there is a layer between the outside and inside of the tube made up of a vacuum. In other words, air molecules help transfer heat from your cup to the outside air. Once you put your liquid in a thermos, the reason it stays cold for longer is because most of the air molecules that help transfer that heat are removed. Since they are removed, much less heat energy will be transferred between your cool or hot liquid, and the air.
The same concept applies in evacuated tubes. Since the tubes in the system are made like a thermos, the temperatures inside of one evacuated tube can get very much warmer than temperatures outside, which then heat the anti-freeze solution and your home water much more. This same concept doesn’t apply to the flat plate collector.
The flat plate panel, instead of tubes, has a feel of a PV panel and looks like one. One might even confuse one as the other, but in reality they are totally different concepts. The flat plate panel is made up of copper or some form of piping inside. In these pipes, the anti-freeze flows back and forth through the flat plate collector and the solar thermal water boiler.
The part of the panel that faces the sun is painted black to absorb heat, and “glazed” in order to prevent heat from escaping. Since there is no vacuum, like the evacuated tube collector, some of the heat that is in the plate collector is lost through heat conduction. The evacuated tube collector works best in lower temperatures and is more efficient, but on the other hand it is a more expensive alternative. The second form, the flat plate collector, might not be as efficient as the evacuated tube solution, but it is a less expensive alternative in solar thermal heating.