Photovoltaic Modules

Skeptics and pessimist knew it all along: Solar photovoltaic modules could not possible be all that simple. Even the most elegant technology is never perfect, so here are a few things to watch out for.

Wattage ratings on photovoltaic modules are given under ideal laboratory conditions at room temperatures. There are two factors which directly affect module performance out in the real world: percentage of full sub, and temperature.
Full Sun

Most of us seldom see 100% of full sun conditions in our locations. If you are not getting full, bright, shadow-free sun, then your photovoltaic output will be reduced. If you are not getting bright enough sun to cast fairly sharp-edged shadows, then you do not have enough sun to harvest much useful electricity.

Temperature

The current from all modules fades somewhat at higher temperatures. This is not an important consideration until ambient temperatures climb above 80 deg.F, which is not uncommon in the sun. The backs of modules should be as well-ventilated as possible. In very hot installations where surplus water is available, sprinklers are sometimes employed to cool the modules. On the positive side of this same issue, all modules increase output at colder temperatures, as in the wintertime when we need all the help we can get. We have seen cases when the modules were producing 30 % to 40% over specs on a clear, cold winter morning with a fresh reflective snow cover and hungry batteries. What this all boils down to is this: Derate your module output by 15% as a general rule of thumb. If you are designing a panel-direct system (where the modules are connected directly to the pump or fan without charge controllers or batteries), derate by 20%, or by 30% for direct applications in a hot climate.

Shadows

Even a tiny amount of shading dramatically affects module output. Electron flow is like water flow. It flows from high voltage to low voltage. Normally the module is high and the battery or load is lower. A shaded portion of the module drops to very low voltage. Electrons from other portions of the module and even from other modules in the array will find it easier to flow into the low voltage shaded area than into the battery.

Maintenance For Photovoltaic modules

It’s almost laughable how easy maintenance is for photovoltaic modules. Having no moving parts makes them practically maintenance-free. Basically, you keep them clean. It if rains irregularly or if the birds visit often, you would be wise to wipe the modules down. Do not hose them of when they’re hot, since uneven thermal shock could theoretically break the glass. Wash them in the morning or evening.

Control Systems

Conventional controls for photovoltaic systems are usually simple. When the battery reaches a full-charge voltage, the charging current can be directed elsewhere, or the circuit opened. Without a load, module voltage rises 5 to 10 volts and stabilizes harmlessly. When the battery voltage drops to a certain set-point, the charging circuit is closed and the modules go back to charging. Most controllers offer a few other whistles and bells like nighttime disconnect, LED indicator lights, etc.

You may be interested to explore more on photovoltaic cells and how do they work.

Useful information Solar photovoltaic:
http://www.greenearth4energy.com/solar_photovoltaic.html

Useful information photovoltaic systems:
http://www.greenearth4energy.com/photovoltaic_systems.html

Useful information photovoltaic cells:
http://ezinearticles.com/?How-Crystalline-Solar-Photovoltaic-Cells-Work&id=2768475