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Solar Panels - The Silent Power PlantBoat owners have few choices when it comes to charging batteries while away from mains power - generators, engine driven alternators, wind or tow generators and solar panels. In this article we investigate the practical use, advantages, disadvantages, and cost of using solar panels.
Solar panels convert sunlight directly into electricity with an efficiency of about 13%. The earth's surface receives close to 1000 watts per square meter of energy from the sun. Solar panels can convert this 1000 watts into about 130 watts of electricity per square meter of solar panel surface. Since solar panels are usually mounted in a fixed position, the sun does not always strike it straight on and we can't count on getting the full 130 watts all day. Early in the morning and late at night solar panels have a much reduced output with maximum output near noon. A good rule is you can get the equivalent of about 42% of maximum output per 12 hours of sunlight - less on cloudy days. So a one square meter solar panel should provide 0.42 x 12 hours x 130 watts = 655 watt-hours per day. Mismatches between optimum solar panel output voltage and actual battery voltage results in losses that further reduce the practical output of the solar panel to about 500 watt-hours per day per square meter of solar panel surface.
500 watts represents an actual achievable charging current of about 36 amps for one hour into a 13.8 volt battery or an average of 3 amps over 12 hours - not bad considering the typical usage of energy on a boat. This amount of power is more than enough to run a small 12 volt refrigerator, a few lights and a vhf radio. It won't run your hair drier or air conditioning for very long so don't look to solar energy if these toys are in daily use aboard your boat. Panels sizes range from 11 watts to over 100 watts, with the larger units most popular and the smaller units being useful only to keep your batteries topped off when you're not using your boat.
Some advantages of solar power are:
a) Simple installation
b) Long life (30 years)
c) Low maintenance
f) Light weight
g) No moving parts
h) Environmentally friendly
Some disadvantages are:
a) Limited power
b) Up-front expense ($15.00 per peak watt)
c) Don't work well on cloudy days
d) Drastic power reduction if any part of panel is shaded
Practical solar panels have only been around for about 30 years and were initially so expensive that their use was pretty much restricted to satellites. The costs have steadily decreased from about $400 per peak watt to around $15.00 per peak watt today. New technologies are being investigated to further reduce the cost of manufacturing to allow solar power to be competitive with other means of power generation. If this is achieved, the prices of solar panels can further slide to $3.00 per peak watt.
Single crystal silicon cells is the premium technology. They are grown as large crystals from semi-conductor grade silicon and sliced into thin wafers with special saws. They are rigid and expensive to manufacture but provide the greatest efficiencies and have a stable output during their lifetime. Polycrystalline cells are cast, costing less, but have lower efficiencies. Thin film amorphous cells are manufactured by depositing photo-voltaic materials on a substrate. These are inexpensive to manufacture, can be made on flexible materials, but are not as stable as single crystal or polycrystalline cells. The output power of amorphous cells go down with time until a stable point is reached, this usually occurs during the first 1,000 hours of use. A bonus of this technology is that manufacturers state the output power after stabilization is reached - meaning you get a little extra output power at first. Single crystal and polycrystalline panels are available only in rigid panels. Amorphous panels can be had in rigid or flexible panels with the flexible panels costing a bit more.
In addition to the solar panel you will require some kind of voltage regulator to keep the panel from overcharging your battery if left connected for long periods of time. Even small solar panels can erode the battery plates and boil away electrolyte on lead-acid batteries requiring expensive battery replacement. Some solar panels are called "self-regulating" - which is not entirely true. A self regulating solar panel has fewer cells, giving a lower open circuit output voltage closer to 15 VDC, hence less likely to overcharge your battery. Charge controllers are high efficiency (low loss) voltage regulators that are wired between the solar panel and the battery bank being charged. Their function is to disconnect the solar panel when the battery has fully charged and automatically re-connect the panel when the battery voltage drops. If not disconnected, a solar panel will try to drive the battery voltage to as high as 18-19 VDC - a level most batteries take exception to.
Many charge controllers have set points that can be varied so both wet and gell batteries can be protected. Some charge controllers have indicator lights that tell you whether the solar panel is charging the battery or "floating" so you don't have to guess whether it's working. The simplest charge controller have a fixed voltage set point and no indicators of any kind to tell you what's happening. The most sophisticated charge controllers have digital LCD display of battery voltage, charging current, and load current and may offer "Load Disconnect" - the ability to remove any drain from the battery if the battery voltage drops too low.