Power from the Sun
In 1982, I mounted four ARCO Solar M-55 monocrystalline panels to a stainless steel frame above the dodger on my Ericson 41 sloop, Wind Shadow. On sunny days during the hours closest to noon, my amp meter indicated an 8 to 10 amp charge. Before and after the midday feast, the flow tapered to 2 to 6 amps. On good days I harvested a total of about 50 amp-hours (AH)—more than enough to offset our daily refrigeration load.
The fact that this output remains virtually unchanged nearly 30 years later is testimony to what a good investment solar panels can be. Even when I cut my 50AH/day calculation in half to compensate for things like clouds, I estimate I’ve harvested a staggering 3,449,250 watts of energy over the years. That’s nearly three decades of free electricity with no oil changes, no bearings to replace and no other maintenance beyond occasionally hosing down the panels.
As wonderful as this sounds, solar panels are not actually very efficient at what they do. In the upper atmosphere the energy density of sunlight is about 1.3kW per square meter. At sea level in the equatorial region, we see approximately 1kW per square meter of energy available. The maximum output from Wind Shadow’s 1.5 square meter array of 10 amps at about 13.8 volts, or 92 watts per square meter (138/1.5), is 11.5 percent of the about 800 watts of energy that is potentially available at mid-latitudes.
In truth, this 11.5 percent slice of the pie was about the best that could be expected from 1980s technology. Monocrystalline panels, where each cell in a rigid panel is composed of a single silicone crystal, are still the most efficient commercially available technology. A quick check of contemporary panel specs reveals that the efficiency of monocrystalline panels like those I mounted on Wind Shadow is now only 12 to 18 percent. Top-quality monocrystalline panels mounted under glass with well-sealed terminal boxes and alloy frames come with 20 to 25 year warranties and can last even longer than that. The downside is they become less productive if even a part of the panel is in shade.
The other two primary technologies are polycrystalline panels, where each cell in a rigid panel is composed of many smaller silicon crystals, and so-called “amorphous” panels, where a substrate material is coated with vaporized silicon. Polycrystalline panels are historically less efficient than monocrystalline panels (typically 10 percent), but are less affected by shade. Amorphous panels are the least expensive, but are also the least efficient (typically 6 percent). Even worse, they lose about 10 percent of their productive capability over time. They are, however, much less affected by shade than polycrystalline or monocrystalline panels and can be made flexible.
While the above efficiencies are the norm today, advances are now being made in both the efficiency and price-per-watt basis of all three types of panels. Kyocera, for example, has devised a coating system that increases the efficiency of its well-built polycrystalline panels to 16 percent. Meanwhile, Sharp and a growing number of other manufacturers have developed the ability to “stack” two layers of silicon in amorphous flexible panels. In this configuration, the translucent top layer stops only certain light frequencies, allowing the lower layer to react to the remainder of the spectrum. This results in an efficiency boost from 9 to about 14 percent.
Sunpower Corp. has recently put into production a Stanford University monocrystalline solar-cell design that reportedly has a 24 percent efficiency rating. The new technology puts all of the metal conductor strips on the backside of the cell, freeing up more surface area for photon contact and electron excitation. The space program is also driving innovation and has developed a new technology that substitutes gallium arsenide cells for silicon and delivers an astounding 42 percent efficiency rate. Time will tell whether such advances will come down enough in price to find their way to the waterfront.
In addition to price and panel efficiency, those pondering solar power need to consider where panels can be realistically mounted on boats. Most panels have sizable surface areas and need to be kept as perpendicular to the sun as possible. The incident angle, or the angle of the panel to the sun, is vital to efficiency—the closer to 90 degrees the better—which is why many solar farms ashore have automatic tracking systems that keep panels perpendicular to the sun at all times. About the best you can do on a sailboat is make sure that the angle is most favorable near midday, and that there are as few shadows from shrouds and spars as possible. Some boat mounts do allow angles to be altered; the added output from these is noticeable.
Another issue is vulnerability. Some years back we fell off a wave in the Tasman Sea that caused me to think twice about strapping solar panels, fuel jugs or fender boards to relatively fragile lifeline stanchions. Whenever an arch or other panel support structure is added to a vessel, the engineering of the structure should take into account susceptibility to damage from wind and waves.
One great advantage of flexible amorphous panels is that they can be strung up like an awning or made part of a bimini. They can then be rolled up and easily stowed away where they will be much less susceptible to damage. Windage, however, can be an issue. Watching an unrolled flexible solar panel take flight across an anchorage would be a less than welcome experience.
Turning Sunlight Into Ice Cubes
I teamed up with refrigeration guru Rob Warren of Coastal Climate Control to take a closer look at the variables involved in using photons to run a marine refrigeration system. His goal was to get some baseline data on what it takes to run a sunlight-powered refrigeration.
Rob set up a closed system comprised of a Frigo-boat fridge with a Danfoss 35 compressor coping with a fixed heat-removal demand. His reefer was powered by a battery bank fed by a Kyocera solar panel with a rated output of 85 watts and a measured peak output of 6.4 amps. The fridge was thermostatically controlled to keep the box temperature between 40 to 45 degrees F night and day. Computer-linked thermal probes also tracked ambient air temp, solar panel temperature and voltage, current and power readings. I was curious to see how the data from a closed-loop solar-fridge system compared with the system I had installed and run for 28 years.
In addition to being able to track the solar energy input, compressor current use and battery voltage, the temperature of the solar panel and the box temperature, Rob also could track the effect of overcast conditions. It was interesting to note that as midday sun angles increased, so did the charge output of the panel, but as heat built up in the dark panel, efficiency dropped.
At night, though, the ambient temperature decreased, but the full load of the compressor's power demand was carried by the battery bank. The discharge profile revealed that the smaller the battery bank, the greater the voltage drop each time the compressor cycled on. In the morning, both battery charging and the compressor's demand kept the solar charge at its maximum output. Theoretically, if the duty cycle of the compressor remains modest, by late afternoon the battery should be recharged and the solar panel's regulator will start to taper the output in order to not overcharge the batteries. Whenever the compressor restarts, the voltage will drop and the regulator will allow the panel's full output to again energize the charging circuit.
Coming up with the right blend of solar capacity, battery-bank size and refrigeration system efficiency is crucial if autonomous operation is the goal. In this bench trial there was not quite enough capacity for autonomous operation. Another panel and another battery would have allowed the system to cope with more overcast conditions. This is one reason why the larger 220-watt installation aboard Wind Shadow handled the job better.
In sum, what Rob saw was a bit of Refrigeration 101 logic. Also, insulate your box as much as possible, and keep it as small as possible. Don't make ice if you don't have to, and try to cool off provisions before loading them into the fridge. The bottom line is it's not yet time to chuck the alternator overboard and swear off diesel fuel.