Posted 5.3.2016 in Articles
Understanding the Fundamentals of Photovoltaic Power Production
Having a powerplant in your house that can produce all the energy you need was once the stuff of science fiction. Today, the technology does exist that can create power without consuming any fossil fuel, is totally silent, has no moving parts, requires no maintenance (barring an occasional rain shower that washes of dust and pollen), and lasts more than 30 years. Even more intriguing, this residential powerplant technology produces its energy directly from the sun. By using the sun's energy, you can power all the electric modern conveniences from computers to entertainment devices to kitchen appliances to even your electric car.
Turning the sun's energy to power
In simplest terms, photovoltaic (PV) solar panels absorb photons, which are the energy packets in sunlight. When the photons strike the semiconductor material that is used to make the solar cell, electrons are knocked loose. Since the cell's semiconductor material is treated so it is positively charged on one side and negatively charged on the other, as the electrons break loose, they flow in one direction creating current that moves between the cells via electrical conductors attached to the positive and negative sides of the solar cell.
The current flows through conductors between the cells called busbars. This energy flow is called a direct current, or DC. Homes run off alternating current, or AC. So, solar power produced by photovoltaic panels must to go through one more process before it can start powering your home. The electricity must go through an inverter that converts the DC current to an AC current before it can be used by the appliances in your home.
Crystalline silicon solar panels
There are two main types of crystalline silicon solar panel technology used in residential settings: Monocrystalline and Polycrystalline.
Monocrystalline and polycrystalline silicon panels are the type most associated with solar panels. These are the panels that range in color from blue to black and have a metal frame and some sort of racking system attaching them to the roof.
Monocrystalline solar cells are made by creating large cylindrical ingots that are sliced into wafers. This type of cell has the highest efficiency rate since it is made out of the highest-grade silicon. The efficiency rates of monocrystalline solar panels are typically 15-20%. These cells also tend to perform better than similarly rated polycrystalline solar panels at low-light conditions.
Companies like SolarWorld and Trina Solar have invested capital and R&D into creating processes to manufacture the monocrystalline cells they use in their solar panels. They control every step of production from the raw material to final assembly in an effort to offer customers highly-efficient monocrystalline panels at the best price-point possible.
There are some trade offs to monocrystalline panels though. If the monocrystalline solar panel is partially covered with shade, dirt or snow, the entire panel will produce less electricity. To manufacture the monocrystalline silicon, the Czochralski process is used, which produces the large cylindrical ingots. Four sides are cut from the ingots to make the square silicon wafers more effectively fill the entire surface of the solar module. Because they are made of higher grade silicon and are more expensive to manufacture, these panels cost more. You can tell which solar modules are monocrystalline because the panels are comprised of a set of round or square cells resembling tiles.
Polycrystalline cells, also sometimes called multi-crystalline, are made by melting raw silicon and casting the material into square ingots. The process used to make polycrystalline silicon is simpler and costs less. The amount of waste silicon is also less compared to monocrystalline, which means these panels cost less. But, depending on the panel manufacturer, the efficiency of polycrystalline-based solar panels is typically lower, on the order of 13-16%. As the technology to manufacture polycrystalline panels develops, companies are pushing efficiencies to higher levels. Panasonic has developed poly cells for the residential market that meet and exceed the efficiency of some monocrystalline panels.
Thin film solar panels
Thin film solar panels are made by depositing a thin film photovoltaic material onto backing material. These panels are generally less efficient than crystalline panels. Depending on the technology, thin-film modules have reached efficiencies between 6–14%. Thin film panels roll off the assembly line as if it were coming off a printing press. This ability to mass manufacture these solar power is what could potentially drive the cost lower than fossil fuels. These panels are flexible and can be laid out in an almost endless array. So, even though the panels are less efficient, they are less expensive to manufacture and more can be used to achieve the same level of output.
There are three basic types of thin film solar panels with a range of efficiencies: Amorphous silicon have an efficiency of around 6%; Cadmium Telluride has reached efficiencies of approximately 11%; and Copper indium gallium (di) selenide (CIGS) achieve efficiencies of 12-14%.
Showcasing how thin film panels allow companies to rethink the placement of solar panels, Dow Chemical has incorporated CIGS film into roofing shingles that can be used in place of traditional shingles.
Besides the rated power output of a PV panel, the next most talked about specification is the panel efficiency. Solar panel efficiency refers to the ratio of output power to input power. In other words: How much electricity can a solar panel produce from a certain amount of sunlight is the panel's efficiency. So, if a solar panel creates 100 watts of power from 1,000 watts of sunlight, it has a solar power efficiency of 10%.
Various companies claim to have the most efficient panel, but they can make their claims based on internal test parameters. That can make judging panels tricky. But all companies should provide an efficiency rating under standard test conditions (STC). During STC testing, solar panels are exposed to a flash of artificial sunlight with an intensity of 1000 watts per square meter. The temperature is 25°C (77 °F) and the atmospheric density is 1.5. These conditions correspond to a clear day with sunlight striking a sun-facing 37-degree tilted surface with the sun at an angle of 41.81° above the horizon. This represents solar noon near the spring and autumn equinoxes in the continental United States with the surface of the cell aimed directly at the sun. Think of this rating of a solar panel as being similar to an EPA mileage rating for a vehicle.
In general, the more efficient a panel is, the more it usually costs. Whether you choose monocrystalline, polycrystalline or thin film panels usually depends on the space available for installation, budget and esthetics.
Manufacturers are constantly working on ways to increase efficiency. New approaches include cells that mix crystalline with thin film, new busbar and conductors that transfer electricity with less resistance and loss, non-reflective coatings that absorb more energy, half-cut cells resulting in lower resistance, and residential tracking systems that move panels so they are receiving optimum amounts of sun for extended periods.
There are a multitude of decisions that will need to be discussed and will influence the cost of your solar installation. Those decisions will be made with input from the homeowner. Through thorough discussion with you, almost all of this complexity will be resolved by your system designer, generally a member of the team that installs your system. Some decisions, like the size of your system, will be made in consultation with you, who ultimately determine the budget and make the final call on esthetics.