Learning Objectives - After this course, you should be able to:
- Understand the basics of how PV systems work.
- Discuss several options for designing with PV.
- Explain different financing options for PV.
- Tap into resources for local incentive programs.
Photovoltaics, the conversion of solar radiation into electricity, reached two important milestones in 2010. First, new installations of photovoltaic (PV) modules were expected to surpass 14 gigawatts worldwide by year's end-at least doubling the 2009 installations. That's the equivalent of about 14 coal-fired plants. Also, for the first time anywhere, PVs crossed the "nuclear threshold": energy from photovoltaics is now more cost effective for North Carolina consumers than nuclear energy, according to a Duke University study.
Photons into Electrons
A basic PV module consists of two semiconductor layers. As light energy strikes the first layer, electrons are "excited" and are captured in the second layer. The amount of energy captured is limited to a small bandwidth of the sun's available energy. Current commercial solar cells range from 7 to 24 percent in efficiency, but given the quantity of solar radiation available, this percentage can amount to significant power.
For optimal performance a solar panel should be roughly perpendicular to the sun's rays. Since the position of the sun changes seasonally and throughout the day, arrays are often fixed to the best average south-facing position according to latitude. This position can be tweaked for certain demands. For example, if cooling loads are higher in the afternoon, the array can be shifted slightly toward the west to harvest more afternoon sun.
Illustration by La Boca
The two most common types of PV systems are crystalline silicon and thin-film modules. Crystalline cells are used in typical flat solar panel systems. The most common and efficient crystalline cells are monocrystalline, which are made from a single continuous silicon crystal. Polycrystalline cells are made from multiple crystals and have a more varied texture. They are easier to produce, making them less expensive than monocrystalline cells. All PV panels should be well ventilated since excessive heat build-up can lower the efficiency of the units.
Thin-film modules utilize very fine layers of semiconductors deposited on an electrically conductive surface. While they are cheaper than crystalline cells, they are less efficient so they require more area to generate an equivalent amount of energy. Some thin-film modules have the advantage of being light and flexible, making them a good option for building retrofits where roof loads are a concern.
PVs produce direct current (DC) so most systems require an inverter to transform the power into alternating current (AC) for use with all standard fixtures and equipment. Typical inverter losses drop efficiency by about 5 percent. Since solar panels are wired in a series, when one portion of the array gets shaded then the efficiency of all the panels in the series drops. If it's unavoidable that one or more panels get occasional shade, then a bypass diode can be installed to keep the efficiency of the other panels stable. Another option is to use micro-inverters on each panel or a portion of an array.