What Solar Panels Do?
Solar panels, or photovoltaic panels, produce electricity when exposed to the sun. This electricity is used to charge batteries, which can power DC loads, or panels can be connected to inverters to produce AC power. Two types of panels use either silicon crystals or thin chemical films to generate electricity. In crystalline panels, silicon, the primary material in quartz sand, is grown into crystals, refined, purified (by an expensive process), sliced into thin wafers and “doped” with the addition of chemicals. This process alters the wafer so that, when exposed to light photons, one side produces a surplus of electrons and the other side has a deficit. A voltage difference between the two sides of slightly less than 1/2 volt is created. Acting like an “electron pump,” solar cells create electrical current, the quantity determined by the size and efficiency of the cell and the amount of light. Solar modules are created by connecting cells in parallel to increase amperage and in series to increase voltage. Typical solar modules have 30 or 36 cells (generating between 14 and 18V DC).
Types of Solar Cells and Arrays
Types of panels most commonly used in boating applications contain either multicrystalline or amorphous thin-film cells. Single crystalline and multicrystalline (c-Si) panels are the oldest technology and also the most powerful. When sized correctly and matched to appropriate batteries, these are the panels to use for running large DC loads such as lights, a TV, radio or VCR.
Amorphous thin film Silicon (a-Si) panels are only about 50% as efficient as multicrystalline panels, but can be manufactured in flexible forms so they can roll or fold, or conform to the shape of a cabintop. They are more efficient in low or diffused light conditions and are less subject to voltage drops when they heat up. These are the panels most often used for low amperage charging and battery maintenance. They don’t generally have enough output for serious energy replenishment, but can be used to “float” or trickle charge a battery.
The most advanced thin-film panels are constructed from copper, indium, gallium and selenium (CIGS), a new blend of semiconductor materials. Unlike crystal silicon panels, so-called “Heterojunction” A-si and CIGS thin film panels feature multiple layers deposited on a metal surface, like a chemical layer-cake, with each layer responding to different light frequencies.
How Much Power Do Solar Cells Make?
Watts = Volts x Amps or Amps = Watts / Volts
So if we make a bunch of simplifying assumptions, the 12 volt, 90 amp hour battery requires 1,080 watts (or watt-hours, to be correct) of solar power to fully charge. An 85 watt solar panel could do the job in 12 hours, assuming optimal light and various other things that are never all true in the real world.But if you assume 3 hours of good light per day, the battery will be back to normal in 4 days. So even if we were to run it down pretty far over a 3 day weekend and then come back the next Friday night, all would be well.
Generally, we measure a solar array’s power output in watts or kilowatts of energy, and in watt-hours or kilowatt-hours per day (when estimating energy needs of a given application). Unless you are familiar with various formulas derived from Ohm’s Law (Watts = Volts x Amps) and can use this formula backward and forward, watts are not a useful method of determining power output. Since we use amps when discussing our boat electrical systems, we rate solar modules used onboard boats by their estimated electrical output, measured in Amp Hours per day.
We arrive at these values by averaging the number of hours the panel spends in full sun (defined as 1000 watts of energy per square meter). Full sun means enough light so you see fairly sharp-edged shadows, and most locations get no more than 80 to 85% of full sun. The amount of time a panel spends in full sun averages 4 or 5 hours per day.
Shadows covering even a tiny fraction of the panel have a dramatic impact on power output. All the individual cells are arrayed in series, and shaded cells show a large voltage drop that acts as a barrier to useful power production. Shading a single cell on a panel can cut its output by 20% or more. Shading two or more cells effectively turns off the panel until the shadow is removed. This defect is corrected somewhat by the use of bypass diodes across each cell, which allow the module to produce power even when partially shaded.
When Do Panels Require A Regulator?
As a general rule panels that produce less than 1.5% of a battery’s rated capacity in amp hours do not require regulation. This means that a 1.5 amp panel is the largest you should use without a regulator on a 100-amp-hour battery. Regulators should generally be used any time you have two or more large panels connected to your batteries. One caveat to the general rule applies to cruising boats that run an “energy deficit”. That is, when your boat removes more energy from the batteries than the panels replace. In this case, regulation might not be required. If you think you might fall into this category, it would be helpful to develop an “electrical budget” for your boat.