HOW TO SPECIFY PHOTOVOLTAICS
Solar panels have been available for decades, but they have mostly been clunky structures that could be tacked onto existing buildings. Even at their most elegant, these add-on solar panels have a hard time integrating into the rest of the building.
In the past decade, government subsidies around the world have fueled innovations in solar systems that mesh more neatly with the rest of the building. This building-integrated photovoltaics (BIPV) either replace conventional building parts or add another, almost invisible, layer onto conventional products.
The most famous of these products right now is the Tesla Solar Roof tile, but there aremany more products either on the market already or coming on shortly. While traditional building-applied photovoltaics are widely available from a range of manufacturers, BIPVare still fairly new. If you are interested in using BIPV, be sure to talk with your supplier and contractor to make sure that everyone is familiar with how they work.
BIPV are available either as integrated products or as thin, flexible ribbons that can be applied onto conventional surfaces. BIPV are relatively easy to clean and maintain. Natural rainfall is enough in most areas to keep them clear enough to work at optimum efficiency. A major advantage of using products with integrated photovoltaics is that they can replace traditional products rather than just layering over them. Rather than building a conventional roof and putting extra panels on top of it, you could just build one roof that can do it all. This keeps initial and lifecycle costs to a minimum.
When specifying photovoltaics, you have to select not only the solar panels themselves, but also the supportive hardware, like transformers and batteries. Consult with your manufacturer about how to get the right hardware to suit your needs. According to the National Renewable Energy Laboratory:
“Electrical issues primarily involve the performance and reliability of the inverters … A BIPV system designed so that multiple inverters work together ensures greater system reliability. If one inverter malfunctions or requires maintenance, it can be disconnected from the array and the BIPV system can still operate.”
Photovoltaics on the market fall into two broad categories: they can either be thick crystal products made of crystalline silicon, or they can be thin-film products made from a variety of metals. Generally, thick crystal products are bulkier and more expensive but can generate more power (10-12 watts per square foot of PV array under full sun). Most residential solar panels are crystalline silicon. Thin-film products are cheaper and more versatile, but because they are less efficient (generating 4-5 watts per square foot of PV array area under full sun), more of them will be required to reach the same power level.
Monocrystalline Silicon Solar Cells
Solar cells made of monocrystalline silicon (mono-Si), also called single-crystalline silicon (single-crystal-Si) are easy to recognize because of their dark, uniform appearance. They also come in distinctively shaped cells that are squares with chamfered edges, a result of their manufacturing process.
Monocrystalline solar panels are the most efficient kind of panel but are also the most expensive. They can convert 15%-20% of solar radiation into usable energy, or up to four times as much as thin-film products. Because these panels are the most efficient, they take up the least amount of space. They also have the longest life spans and perform better in low-light conditions than polycrystalline models.
Polycrystalline Silicon Solar Cells
Polycrystalline cells, also known as polysilicon (p-Si) and multi-crystalline silicon (mc-Si), are available in perfect squares, as opposed to their chamfered monocrystalline cousins. There are slightly less efficient than monocrystalline cells and have a speckled blue color, as opposed to monocrystalline’s deep, uniform blue. On the plus side, they are cheaper.
Thin-Film Solar Cells (TFSC)
Thin-film solar cells, or thin film photovoltaic cells (TFPV), are made by depositing layers of photovoltaic material on a flexible substrate. Efficiencies hover around 7%-10%. Although they are less efficient than crystalline cells, they are cheaper and because they are flexible, can be used in many ways their rigid counterparts cannot. They are also available in a wider range of reddish colors and can be placed on tinted substrates. Relatively new to the market, thin-film products are becoming increasingly common and popular.
Four different types of photovoltaic material are used:
- Amorphous silicon (a-Si): These are the kinds of cells that are used in small-scale applications like your pocket calculator. Because they are relatively inefficient, they haven’t been widely used in larger applications, but this is changing.
- Cadmium telluride (CdTe): These cells have passed crystalline silicon panels in energy efficiency and are increasingly widely used.
- Copper indium gallium selenide (CIS/CIGS): These cells have the highest potential for energy efficiency and are less toxic than other cells.
- Organic photovoltaic cells (OPC): Organic cells are still in development, but have the potential to be much cheaper and lighter than conventional cells.
Photovoltaics can generate energy upon any exterior surface that the sunlight strikes. These can be split into two broad categories:
BIPV façades often use photovoltaic glass panels in lieu of traditional window or spandrel glass. These products can be opaque, transparent, solid or patterned. Photovoltaic-integrated glass is available, although obviously, it comes at a price. Products from Onyx Solar use a tin oxide coating on the inner surface of the glass to conduct electricity out of titanium oxide cells. Others use a laminated construction where a PV cell is sandwiched between two panes of glass.
BIPV Roofing Systems
Roofing systems include BIPV shingles, metal roofing, and exterior insulation roof systems. These BIPV products can displace traditional construction materials. Flexible thin-film amorphous silicon BIPV shingles can replace asphalt shingles. This BIPV product is nailed to the roof deck, very much the way that traditional asphalt shingles are attached to a roof.
There are two primary factors to consider when it comes to orientation of photovoltaics: solar access and system orientation.
Solar access is the amount of energy from the sun, or insolation, that reaches the surface of a photovoltaic. It is a function of the building’s geographical location and is measured as kilowatt-hours per square meter per day (kWh/m2/day).
System Orientation and Tilt
To maximize solar access and power output, the physical orientation of the BIPV system and the tilt angle of the array should be considered relative to the geographical location of the building site. The National Renewable Energy Laboratory advises:
“As a general rule of thumb, BIPV installations north of the equator perform optimally when oriented south and tilted at an angle 15 degrees higher than the site latitude. Conversely, BIPV installations south of the equator perform best when oriented north and tilted at an angle 15 degrees lower than the site latitude.”
This could vary depending on seasonal needs. If the building will need more power for air conditioning in the summer, then panels should be oriented for maximum summer exposure.