The solar panel is the starting point of the whole solar electric generation process. This is the fun part of solar power, well, one of them at least. However, choosing solar panels for the home is hard to do unless you have a grasp of the technology and terminology involved.
Solar panels are made up of individual photovoltaic (photo=light, voltaic=electricity) cells connected in series and parallel to deliver electricity at a specific voltage and current. A collection of solar cells wired together are often refered to as a “module.”
Photo by pixor via flickr
Commercially produced modules consist of cells mounted on a substrate, covered with transparent glass, and framed in an aluminum enclosure. The panels can then again be connected in combinations of series and parallel to build large high voltage and/or high current arrays.
The reason I mention “commercially produced” modules is that there are a ton of eBooks and the like promoting the building of your own solar panels and the supposed savings of going this route. You can certainly DIY your own panels, but just have realistic expectations about what you’ll get in terms of quality and savings.
Modules come in different physical sizes for different applications. A small, 12″ x 18″ panel delivering 15 watts would be suited to powering smaller attic fans or garden fountain pumps. Larger panels delivering 80 to 120 watts are typically used in residential and commercial power arrays.
Photovoltaic (PV) cells can be made from any number of semi-conductor materials. Although, silicon has emerged as the most prevalent PV material. The cells are manufactured in a variety of configurations, each with its own cost/efficiency ratio.
Panel Efficiency
Cells cut from a single piece of silicon are called mono-crystalline. Pieces of silicon are fused together to make poly-crystalline cells. Thin-film PV material is created by deposition or by drawing out the silicon material from a molten pool. The efficiency of these different constructions are roughly as follows:
- Mono-crystalline 13.8%
- Poly-crystalline 12.3%
- Thin Film 7.5%
The efficiency determines the amount of light energy converted to electricity. For example, a poly-crystalline panel intercepting 100 watts of solar radiation will deliver about 12 watts of electrical energy.
Mounting Solar Panels
The roof is the most common place to mount solar panels. The roof usually affords the best view of the sky and panels are safely out of the way. On pitched roofs, panels can normally be mounted flush to the roof. Flat roof mounts usually have the panels at a fixed tilt to the site’s latitude. One inconvenience of roof mounted PV arrays is maintenance.
Roof mounted solar panels
Soot, pollen and other debris builds of on panels over time and they should be cleaned periodically for optimal performance and efficiency. This job is a lot easier if the panels are close to the ground.
Panels can be mounted literally on the ground with some support structure to prop them up. But the ground is a rough place. It’s much better to get your PV array off the ground a bit. A fixed mounting system may allow you to adjust your panel tilt seasonally for best performance or you may choose to put them on a pole mounted rack. Pole mounted systems can use passive or active trackers to keep the array pointed directly at the sun all day long.
Lacking a tracking system, the most effective tilt equals you latitude and the best direction, or azimuth, is due south in the northern hemisphere. The same tilt rule applies in the southern hemisphere, except here the best azimuth is north.
It’s not always possible to achieve ideal orientation and tilt. Sometimes you’ll just have to settle for getting direct sun for part of the day. Consider this when surveying your site for solar panel installation. If you have property you have your choice of location, other than having to go with roof mount and whatever orientation that provides.
Effects of Shading
A PV module produces best when in full sun. Similarly a full solar array produces best with no shading. A series array’s performance will be limited to that of the least productive panel when the array is in partial shade. There is emerging technology that reduces the detrimental effect of partial array shading. By locating micro-inverters on each panel and having independent returns to a combiner, the panels in full sun are not degraded when other parts of the array are in shade.
Portable Solar Panels
Rigid, aluminium framed panels are not the only game in town. Some innovative solar panel formats exist that integrate the PV material into the roof shingles, so the roof is the solar panel. Some people find rigid, mounted panels unsightly, so the roof integrated PV is an option worth the expense.
For portable recreational use, PV cells are affixed to a flexible backing and the whole panel can be rolled up when not in use. Such panels are typically used for lower power applications like radio, cell phone, or gps battery charging.