The promise of solar panels is sometimes disappointing and occasionally an expensive dead loss. Disappointment generally stems from over optimistic expectations; loses, in the main, from inferior panels that cannot
withstand the marine environment. Despite this, solar panels do have a definite, invaluable
role to play on a cruising boat. Let's take a look at where they fit into the power equation,
what to look for in a solar panel, and how to install a panel for a long and trouble-free life.
Solar Panel Rating.
Solar panels are rated in watts. You can convert this to amps by dividing by the system voltage. P = VI. The rating gives the panel's output in a certain set of standard test conditions, the two most important for the boat owner being the assumption of direct overhead sunlight and a panel temperature of 25 C.
On land, solar panels are set up on angled mounts designed to intercept as many of the sun's rays as possible. But if this is done on a boat, every time the boat turns the panel will lose the sun. As a result, a solar panel on a boat is almost always mounted more or less horizontally in a fixed location.
Even in the tropics, solar noon, when the sun is directly overhead, lasts only for a brief period. For the rest of the day the sun's rays intersect the panel at ever shallower angles and with decreasing intensity. Figure 1 indicates the decline in panel output on either side of solar noon. Should a cloud obscure the sun or a shadow be cast over the panel, perhaps from rigging, sails or a boom, there will be a further dramatic fall in panel output.
Thus even in sunny climates you cannot expect realistically to get more out of a panel than the equivalent of five hours a day at its rated output. A six-watt panel in a 12-volt system can only be expected to produce 2.5 amp hours per day; a 30-watt panel, 12.5 amp-hours per day. In many instances the panel produces less, and you will be sorely disappointed if you count on it for more than this.
Problems With Voltage.
In order to put even this limited amperage into a battery, a solar panel must have a higher voltage than the battery's voltage. The greater the differential between the panel and the battery, the more the current (amps) that will flow, up to the maximum output of the panel. When a battery is well-discharged its voltage falls and it readily accepts a charge. But as it comes up to charge, its voltage rises. If a solar panel is to continue to deliver its rated output, the panel must be able to maintain a healthy voltage differential over the battery's rising voltage. This requires a panel voltage of from 14.0 to 14.4 volts.
All panels have a grid connecting a series of silicon cells. Each cell produces around 0.5 volts in full sunlight. The voltage of a panel is raised to effective battery charging levels by connecting cells in series. On a 12-volt system, there will be anywhere from 30 to 36 cells in series, giving a nominal voltage of from 15.0 to 18.0 volts. This would seem to be more than adequate for battery charging, but in fact this is not always the case.
As solar noon approaches, the black silicon in a solar panel heats up. In the tropics the panel temperature is certain to exceed the 25° C temperature used for rating purposes, producing a decline in the panel voltage of approximately 1.0 volt for every 15° C temperature rise. At 50° C, which is not uncommon in the tropics, the nominal voltage of a 30-cell panel will be reduced to 13.3 volts; that of a 33-cell panel to 14.8 volts; and that of a 36-cell panel to 16.3 volts. The 30-cell panel is below effective battery-charging levels and will suffer a steady decline in its output as a battery comes up to charge.
While a solar panel "puts out" in sunlight, it "takes back" after dark, although the reverse current flow is much less. At the time of installation, a blocking diode is frequently added to the wiring to the battery. Such diodes are installed in addition to the bypass diodes found in some panel junction boxes and should not be confused with them.
A blocking diode has the effect of allowing the charging current to pass to the battery, but blocks any reverse current flow. However, such diodes cause a voltage drop of around 0.6 volts, which means that for a panel to remain an effective battery charging device, its output must now be rated at 14.6 to 15.0 volts. A 30-cell panel with a blocking diode, partictularly in a hot climate, will be almost compIetely ineffective; even a 33-cell panel will start to suffer a decline in its output as a battery comes up to charge.
Although blocking diodes are routinely added to solar panel wiring, in many instances they would be better left out. The voltage drop through a diode will often reduce the output of a panel by more than the nightime drain back into the panel. The key factor here once again, is the number of cells in series in a panel. A 36-cell panel generally has a high enough voltage to be able to handle a diode in all circumstances with no appreciable loss in performance. A 33-cell panel will suffer some loss of performance, especially in high temperature applications. A 30-cell panel will suffer a serious loss of performance in almost all applications.
The one situation in which a blocking diode is more or less mandatory is when covering the panel for any length of time (for example, when not using the boat) while leaving it connected to the battery, because it will steadily drain the battery. In this unlikely situation, put a switch in the circuit, rather than a diode, so that you can isolate the panel.
|Fig. 1. Solar panel output as a function|
of time of day.
|Fig. 2. Solar panel is tied into a charging system with diodes on the alternator side.|
|Fig. 4. The panels are tied in downstream of the isolation diodes, and each panel has its own diode.||Fig. 3. Shunt type voltage regulator with inbuilt diodes to avoid overcharging.|
Construction is of either amorphous or crystal cells. The former are composed of a silicon dust; the latter are cut from silicon crystals. Amorphous silicon produces a lower output for a given panel size. In addition, the output of many amorphous silicon panels declines by up to 10 percent in the first year or so of operation, and then stabilizes.
All flexible panels are made with amorphous silicon; rigid panels may be either amorphous or crystal. Some of the flexible panels have proven to be very short-lived in the marine environment, with the plastic cases delaminating or developing pinholes. In either case, water enters the panel and shorts it out. Other problems can develop with plastic-faced, metal-backed, rigid panels. The differential expansion and contraction of the various materials eventually breaks down the bonds, leading to delamination.
Yet another common source of problems is moisture ingress into the panel terminals, leading to corrosion, which in turn generates electrical resistance. Given the low output of a solar panel, any unwanted resistance can cripple its performance. All connections should be made inside watertight junction boxes.
Selecting A Panel.
You need to be realistic about what you can expect from a panel: Use the five-hour rule at rated output as a general guide to maxirnurn output. You will immediately realize that on a modern electrically loaded cruising boat, a solar panel can be no more than an adjunct to other charging devices. (The one exception to this is where a solar panel is used to " float charge" a battery - that is, maintain the battery in a state of full charge when the boat is not in use.)
Next, realize that at elevated temperatures, and with a diode in the system, many panels will not even reach this limited projected level of output. In the tropics, any 30-cell panel, with or without a diode, is likely to prove a marginal battery-charging source. A 33-cell panel will develop effective battery charging voltages, but with little margin for other losses, such as voltage drop in transmission lines, resistive connections or poor sunlight. A 36 cell panel will develop effective battery-charging voltages in just about any application.
Does this mean you should always buy a 36 cell panel? Not at all. Being larger than the others, these panels are more expensive. Many times you will be wasting the additional money. What it does mean is that you must review carefully your installation to see if a diode really is necessary, and then consider the climate in which you will be operating the panel.
A 30 cell panel with a diode in any climate in any circumstances, is not recommended, and should only be considered in temperate climates. A better choice in temperate climates would be a 33 cell panel which, even with a diode, will develop adequate battery-charging voltages on all but the hottest days. In the tropics, with or without a diode, a 36 cell panel is recommended.
Lastly, there is no question that the crystal panels in general are better constructed and have a longer life expectancy than many amorphous panels. If a manufacturer cannot offer you at least a three to five-year warranty on a panel, one should question the panel's ability to withstand the marine environment.
If you have selected a panel that will maintain an effective battery-charging voltage, you may find that when your boat is not in use the panel has enough capacity slowly to overcharge your battery.The critical point comes if the panel's rated capacity at 14.0 volts is above 0.5 percent of the amp-hour rating of the battery to which it is connected (for example, above one amp when connected to a 200 amp-hour battery bank). If the panel's capacity exceeds this level, either turn it off when leaving the boat, or else fit a shunt-type voltage regulator. This monitors a battery's rising voltage, diverting the solar panel's output to a fixed resistance as the battery comes up to full charge. Shunt-type regulators are available anywhere that solar panels are sold.
As shunt-type regulators come with blocking diodes, no other blocking diode should be placed in the system. Because the combined effect of the regulator and diode will be to curtail output as a battery continues to charge, and because the only time the regulator is likely to be needed to protect the battery is when the boat is not in use (unless you have a very large array of solar panels), you should fit a bypass line with a switch around the regulator so that you can remove the regulator from the circuit when using your boat.
When left idle, a battery will slowly self-discharge. In time this leads to sulfates forming on the battery plates, which can permanently lower the battery's capacity. To combat this, at dockside many boat owners leave their batteries hooked to a battery charger, but this can lead to equally damaging levels of persistent overcharging. What is more, the shore power connection brings with it the risk of stray currents that will consume sacrificial zines and then attack underwater hardware.
The answer is a small solar panel booked to each battery bank on the boat. All that is needed is a rated panel capacity at 14.0 volts of around 0.25 percent of the amp-hour capacity of the battery bank to be floated (that is, a 0.5 amp, six watt panel on a 200-amp-hour battery bank). So long as there is no drain on the battery bank, and the batteries are in good condition, this will indefinitely maintain the batteries in a state of full charge with only minimal water consumption. If you have good-quality deep-cycle batteries (as you should have), the panels will pay for themselves in extended battery life alone.
Any kind of a resistance in solar panel circuits can play havoc with the output. You must use marine-grade wiring and terminals when making the installation. The terminals at the panel are particularly vulnerable to corrosion and need to be completely sealed. There should be no other connections above deck. Run a continuous length of cable through a deck seal, and make any connections inside the boat.
A good rule of thumb for determining the cable size is to take the maximum short-circuit current of the panel (it will be given somewhere in the panel specifications), multiply this figure by 1.25 and treat this figure as the required current-carrying capacity of the panel wiring. A larger cable than that recommended can only be beneficial. An undersized cable will cause unneccessary voltage drop.
If the panel is to be hooked directly to a battery, which it will have to be for float charging (see circuit diagrams), there must be a fuse in the wiring as close to the battery as possible. Without such a fuse, any short in the wiring will cause a dead short across the battery and likely start a fire.
Solar Panels In Perspective.
Watt for watt, solar panels are one of the most expensive methods of generating DC power on board a boat. For perhaps $1,600 you can buy panels capable of providing around 60 amp hours a day at 12 volts (that is, if you can find the space to mount the panels). For the same price you can buy a wind generator that will average at least twice this output, or a high-output alternator and "smart regulator," together with sophisticated DC monitoring devices that will give you the same output in 30 minutes!
Nevertheless, the steady trickle from a solar panel can come in handy. There are times when you need to run no more than the odd fan and a little electronic equipment. There may be no wind to run the wind generator, and you have no inclination to fire up an engine. A solar panel will keep up with such demands silently, effortlessly and without appreciable wear and tear. But even more important is the fact that the solar panel can correctly float your boat's deep-cycle batteries when you are not using the boat. In this role a solar panel will frequently repay more than its purchase price in extended battery life alone, while your batteries will be fully charged when needed and you can eliminate your dependence on shore power.
When you think about it, there are not many pieces of equipment on board that actually pay for themselves, so in a sense this becomes virtual free energy from the sun.
Sourced from Cruising World magazine.
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