Folks concerned about the environment gravitate towards using renewable energy. The sun provides peak power of about 1,000 watts per square meter (93W/sq ft) and a solar panel transforms this power into roughly 130W per square meter (12W/sq ft). This energy harvest corresponds to a clear day with the solar panel facing the sun. Surface dust on the solar panels and high heat reduce the overall efficiency.
Generating electricity by sunlight goes back to 1839 when Edmond Becquerel (1820–1891) first discovered the photovoltaic effect. It took another century before researchers understood the process on an atomic level, which works similar to a solid-state device with n-type and p-type silicon bonded together.
Commercial photovoltaic (PV) systems are 10 to 20 percent efficient. Of these, the flexible panels are only in the 10 percent range and the solid panels are about 20 percent efficient. Multi-junction cell technologies are being tested that achieve efficiencies of 40 percent and higher.
Global warming will affect solar panels negatively. A study from the Massachusetts Institute of Technology (MIT) reveals that a one degree Celsius increase in temperature reduces the photovoltaic power output by 0.45%. Like a battery, heat also reduces the lifespan of solar cells.
At 25°C (77°F), a high quality monocrystalline silicon solar panel produces about 0.60V open circuit (OCV). Like batteries, solar cells can be connected in series and parallel to get higher voltages and currents.The surface temperature in full sunlight will likely rise to 45°C (113°F) and higher, reducing the open circuit voltage to 0.55 V per cell due to lower efficiency. Solar cells become more efficient at low temperatures, but caution is necessary when charging batteries below freezing temperatures.The internal resistance of a solar cell is relatively high: with a commercial cell, the series resistance is typically one ohm per square centimeter (1Ωcm2).
A solar charging system is not complete without a charge controller. The charge controller takes the energy from the solar panels or wind turbine and converts the voltage so it’s suitable for battery charging. The supply voltage for a 12V battery bank is about 16V. This allows charging lead acid to 14.40V (6 x 2.40V/cell) and Li-ion to 12.60 (3 x 4.20V/cell). Note that 2.40V/cell for lead acid and 4.20V/cell for lithium-ion are the full-charge voltage thresholds.
Charge controllers are also available for lithium-ion to charge 10.8V packs (3 cells in series). When acquiring a charge controller, observe the voltage requirements. The standard Li-ion family has a nominal voltage of 3.6V/cell; lithium iron phosphate is 3.20V/cell. Only connect the correct batteries for which the charge controller is designed. Do not connect a lead acid battery to a charge controller designed for Li-ion and vice-versa. This could compromise the safety and longevity of the batteries as the charge algorithms and voltage settings are different.
A lower-cost charge controller only produces an output voltage when sufficient light is available. With a diminishing light source, the charge controller simply turns off and resumes when sufficient levels of light are restored. Most of these devices cannot utilize fringe power present at dawn and dusk and this limits them to applications with ideal lighting conditions.