Solar PV

Technology

Solar panels, also known as photovoltaics (PV) are made of at least two layers of semi-conductor materials. One layer has a positive charge, and the other has a negative charge. When light hits the top semi-conductor layer, a portion of the energy is absorbed, freeing electrons from the negative layer to flow through an external circuit and back in to the positive layer. This flow of electrons creates electric current. Individual solar panels can be connected to increase the power output, creating a solar array.

A more detailed explanation of how solar panels work can be viewed at UnderstandSolar.com

Sizing

The size of a solar panel or array of solar panels is usually given in kilowatts. The number of kilowatts is the maximum generating capacity of the panel or array. For example, the 7.2 kilowatt array on the roof of Morley Science Center can generate 7.2 kilowatts of electricity under ideal conditions.

Usage of electricity is often describing in kilowatt hours. The output of a solar panel in kilowatt hours is the output at any given time period multiplied by the amount of time. If a 7.2 kilowatt array were producing at peak for eight hours, it would have generated 57.6 kilowatt hours of electricity. However, it would be highly unusual for a solar panel to produce at maximum capacity for eight hours. The most output the Williams array has given in a 24 hour period has been around 50 kilowatt hours.

How conditions affect output

The more sunlight a solar panel is exposed to, the more electricity it will generate, up to its maximum capacity. Any shading on a solar panel from nearby trees or buildings will decrease the amount of electricity generated, as will snow cover in the winter. Solar panels also work most efficiently when light from the sun hits them at 90°, which is why solar panels are sometimes tilted to the south (in the northern hemisphere).  The degree of the tilt determines what part of the year the solar panels are optimized for: the greater the slant, the more the panels are optimized for production during the fall, spring, and winter when the sun is low in the sky.  To work well, solar electric systems need unobstructed light from the sun during most daylight hours for most of the year. A good site has no shading where the solar panels will be installed, either from vegetation, nearby buildings, or other parts of the building on which they are installed.

You can see the affects of weather conditions on the electricity output of the panels in a series of timelapse movies of the Morley PV array

Advantages and Disadvantages

Solar photovoltaic systems are generally reliable, well tested and low maintenance.  They’re quiet and less visually intrusive than many other sources of renewable energy, and can generally be designed to meet a wide variety of electrical requirements. In locations that don’t already have a power supply, an off-grid solar electric system may be more cost effective than running power lines.

PV systems continue to have a high initial cost, though ongoing research in to the materials and production methods involved in solar panel manufacturing may lead to future decreases in price.  Like several other sources of renewable energy, solar power is a variable energy source.  Solar panels can’t produce electricity at night or during periods of dense cloud cover, so a solar electric system must either have batteries for storing electricity or a backup source (such as a connection to the grid).