The invasion Orson Wells described so vividly on the radio October 30, 1938, was only sensational because of its time and place in history. What did Wells know about future events that we don’t know? Maybe he foresaw last year’s supernova or exploding star (it was best seen between the big and little dippers September 7-9, 2011), or the growth of solar power generation (photovoltaics = PV) or the implosion of Solyndra. Maybe he foresaw the green movement (little “green” men).
Now that I’ve got your attention, what does this have to do with today’s topic? Other than to introduce a discussion on the power of the sun, not much. Over the next two posts I will discuss photovoltaics (PV), a.k.a. solar power, its history, and the types available.
The most common type of solar module utilized today, crystalline silicon panels, are encapsulated in glass. They make up about 95% of all PV systems installed. Monocrystalline cells invented by Bell Labs in 1954 were cut as wafers from specially grown cylindrical silicon crystals. They are still among the most efficient PV systems, but they have poor tolerance for low light, are fragile and, very expensive, and require very heavy frames for rooftop mounting.
Polycrystalline cells are made from multiple sources and are not as dependent on perfect crystal growth. They are less expensive than monocrystallines, extremely fragile, and less efficient at converting sunlight to electricity.
While some crystalline manufacturers claim higher levels, typical silicone-based PVs have power production between 12 and 18 watts per square foot and operate with 14-20% efficiency. High temperature and shade reduce their output.
Thin-film PV systems don’t use crystalline silicon, but very thin layers of materials such as amorphous silicon, a mixture of copper-indium-gallium-diselenide (CIGS), or cadmium telluride. They can be flexible or rigid and can be adhered to a roof covering or rigid material.
First generation thin-films are mounted on a glass substrate and are relatively inexpensive to produce, but they are about 50% less efficient than monocrystallines. A heavy support frame is required and there have been issues with longevity and durability.
Second generation thin-films are mounted on a flexible substrate. They also do not require crystalline silicon and are easier to manufacture than first generation thin-films at the same cost. There is no requirement for special framing or support structures because they are much lighter than other PV systems. These thin-films are rugged and can often be integrated with modern roofing membranes after they are installed.
Because thin-films are typically surface-mounted, heat gain is an issue and these systems can compromise the benefits of reflective roof systems. Thin-film systems have power production of 5 to 10 watts per square foot and operate with 6-12% efficiency. Compared to crystalline silicon systems, thin-films are more effective in low light situations and are less affected by high temperatures.
In the concluding post I will introduce another type of rooftop power generation that produces electricity from the sun: Concentrated Solar Power, or CSP.