Photovoltaic (PV) Solar Electric Systems
In most installations, PV electric works just like power from the grid. No batteries are required to store power because you remained grid tied with a net-use meter. If connecting to the power grid is cost prohibitive or undesirable, a battery bank will be necessary to provide power throughout the day. With a net-use meter you are credited for any extra power you generate. At night you pull from these credits. In some states the electric company will buy your excess production, but not in Maine. Our electric companies use a 12 month window to evaluate your credits and debits.
Solar electricity is generated by a group of solar collectors called an array, which is normally installed on a roof or a ground-mounted rack. When the sun’s radiant energy hits the solar modules, DC electrical current is created. The DC electricity feeds into an inverter that changes it to standard AC electricity - the same kind your home already uses. This power is feed into the grid. As needed, you pull your power from the grid as before. Should the grid go down, you would lose power despite the fact your PV system is producing. This is a safety feature to prevent power from entering a disabled line and harming line workers. It is possible to install an automatic grid disconnect switch and small battery bank to switch to PV power in case of an outage.
PV panels like to face south and must be in the sun, especially from 10:00-2:00. It is possible to install tracking devices that rotate the PV panel to follow the sun’s angle throughout the day, and the season. This can increase the panel output, but also increases the cost and maintenance needs.
The price on PV panels has sharply dropped in 2011. In our area we are seeing prices in the $4 per watt installed. After the federal 30% tax credit and State incentives (visit www.DsireUSA.org) this put the pay-back time for an average family under 10 years. You can expect your system to be running long after the 25 year warranty. That is a lot of free electricity. Plus Maine offers PACE and PowerSaver low interest loans where families can borrow up to $40,000 for energy upgrades including solar.
The PV world keeps changing. Thin film is said to be the up and coming technology. It can be applied to any smooth surface and is flexible. Dow just released their solar shingle product and Dawn Solar makes a integrated metal roof, PV and solar hot water system that is nice for new construction. For large high rise buildings, there is clear PV window glass. Traditionally DC was brought from the panels to a large central inverter. On most residential installs we now use Enphase micro inverters. A small inverter is located at each panel. This make system expansion a breeze, the overall output is better, and nothing beats the Enphase panel by panel output reporting. You can monitor every panel from your wall mounted Envoy, the web, and of course there is an app for your smart phone.
A standard electrical meter tracks the power you pull from the grid. A net-use meter has the ability to track not only use, but also credit your account for the power your PV system feeds into the grid. With a net-use meter a battery system is not necessary since you will pull power from the grid to meet your needs. The use charges are offset by the power you feed into the grid. The power company uses a 12 month window to calculate your net use. Often there is excess production during the long days of summer and you draw from the grid during the shorter days of winter. The ideal is to “net to zero” as excess production is forfeited.