Why Solar Power?
What can you do with Solar Power?
How are Solar Panels Constructed?
What are the Mounting Considerations for Solar Panels?
What are my Choices for Panels?
What are my Choices for Charge Controllers?
What are the Installation Considerations for Charge Controllers?
And how do I wire it all up?
Why Solar Power?
Solar power is a safe, reliable, maintenance-free, and non-obtrusive way to generate power aboard. Solar power avoids the noise and physical danger of wind turbines, the noise, smell, and maintenance of engines, as well as the need to carry extra fuel. And for you racers, advanced solar power systems compare favorably even with fuel cells.
What can you do with Solar Power?
Solar power installations may be scaled to meet almost any need. A small panel may be all that's required to keep the keep the batteries topped up for day sails, and run the bilge pump while the boat is moored. Larger installations may reduce the need for running the engine or generator on longer sails, while more substantial installations may provide all the power needed for liveaboard cruising! Several friends of the author have modest sized installations which run refrigeration systems. The draft of this article was written on a laptop aboard an engineless sailboat during an Atlantic crossing, where the only source of electricity was the sun!
How are Solar Panels Constructed?
The basic building blocks of solar panels are solar cells. Each cell is a very thin piece of silicon, doped with special impurities which allow it turn light into electricity. Since silicon is not a very good conductor, aluminum is deposited on the front and back of the cell to collect the electricity efficiently. These aluminum fingers form the familiar grid pattern seen on solar cells.
Each cell will only produce about half a volt in full sunlight, so generally 32-36 cells are connected in series to form a 12V nominal panel. The aluminum contacts are difficult to make good connections to, so the resulting connections can be fragile. To hold the cells together, and protect them from physical damage and the elements, the cells are then sandwiched between a transparent material and a back panel with a refined version of hot glue, such as ethyl vinyl acetate (EVA) to hold everything together. Front and rear materials include glass and plastic, while the rear panel may also be stainless steel. These materials are laminated in large ovens that apply a vacuum remove air bubbles from the EVA, in much the same way as a prepreg composite structure might be cured.
After encapsulation, wires or terminals are added, and an aluminum frame is added if the panels is glass, or a plastic edge if the panel is intended to be flexible. Recently, with all the investment in solar energy, other processes have emerged. One process directly deposits the silicon cells onto a glass front panels, eliminating the need for both cell interconnections and encapsulation.
What are the Mounting Considerations for Solar Panels?
Before you rush to the store, you must make sure that your solar panel and intended mounting location are in harmony. Ask yourself the following questions:
How protected will the panel be from physical damage? Manufacturers claim that flexible panels, if well-supported, can actually be walked on without immediate damage. Your author's experience, it should be noted, gives him a strong bias against flexible panels. He has seen several instances of failed cell connections in flexible panels that were treated very kindly, and has serious doubts about the long-term reliability of flexible panels placed underfoot. Glass panels, on the other hand, are completely immune to bumps and scrapes, but are prone to shatter with large impacts. Glass panels generally have a higher profile than flexible panels, and triangular trim strips may be advisable if there is a danger that lines may get caught on them.
Will the panel be shaded? Of course you know that solar panels won't work well in the shade, but did you know that even a small patch of shade on your panel can reduce your output to a small fraction of its full output? Since a solar panel is made up of cells in series, the output current of the panel is generally limited to the output of the weakest cell. If the only locations available to you will be shaded often by lines, sails, spars, etc., all is not lost. There are special shade-resistant panels that work in one of two ways. The first type uses very long and narrow cells stretching the length of the panel, reducing the probability that a patch of shade will cover much of any one cell. The second type adds a diode in parallel with each cell, which shunts the current from stronger cells around a weak cell.
Will the panel remain cool? Temperature is an enemy of silicon solar cells. Every degree Celsius of temperature rise reduces the voltage, and hence power available by 0.3-0.5%. So, if your cells are sitting in the sun, glued right to your insulating foam-core deck, and get to be 40 degrees above ambient, you would be throwing away up to 20% of your available power. With flexible cells, not too much can be done other than making sure they are not sheltered from the wind. Glass cells may be mounted slightly off the deck, or on a railing to allow air circulation to the rear of the panel. It's important to note that your choice of charge controller will determine how much your system is affected by temperature. Conventional controllers throw away 10-30% of the available power from a solar panel. In a sense, high temperatures cut into this discarded power first, so the overall drop in power is small. MPPT controllers, on the other hand, always operate the panel at peak efficiency, and will be directly hit by the reduced power.
Will the panel be able to track the sun? While not much of an option for deck-mounted panels, keeping the panel pointed at the sun can deliver roughly 40-100% more power over the course of a day. This can be accomplished with panels mounted on a railing or small tripod. Options for aiming the panel include an adjustable gimbal, or a universal joint (such as the base of a windsurfer mast) with small cords with rolling hitches leading to the corners of the panel for adjustment. Aim needn't be perfect, since power falls off slowly with the first few degrees of misalignment. A panel off by 10 degrees will only lose about 1.5%. Taking 30 seconds a few times a day to aim your panel will deliver much larger gains than anything else you can do for your system.
Keep in mind that the best mounting for your panel may be no mounting at all! Lightweight panels may be stored inside, under a seat cushion perhaps, and brought outside when needed and laid on deck, tied down at the corners if necessary.
What are my Choices for Panels?
In the marketplace, the main decision you need to make for your solar power system is whether to use flexible or rigid (glass) panels. The choice here is mostly determined by your mounting location. Flexible panels will bend to accommodate decks curved in one dimension. Glass panels with aluminum rails are simple to mount to a railing or any sort of a bracket. For flat decks, it's a toss-up. Glass panels are heavier, but will probably last longer . Flexible panels may initially be more tolerant to abuse.
Most flexible panels are made of the same silicon cells as are inside glass panels. The are flexible in the same way that a thin pane of glass would be considered flexible. That is to say, they can bend to accommodate gentle, even curves. Most panels should have a maximum curvature specified, so you can double-check your mounting location before committing.
More recently, truly flexible panels have come on the market that can even be rolled. These may seem attractive, but their efficiencies are very low at present, about half that of glass and semi-flexible panels. Watt for watt, you would have to determine for yourself whether having a truly flexible panel is worth carrying a panel that's twice as large as it would otherwise need to be.
And of course, if it's extreme performance you seek, Genasun is here to help.
What are my Choices for Charge Controllers?
(see also, About MPPT.)
Solar charge controllers regulate the flow of electricity from the panel into the battery, and for units with MPPT, run the panel at its highest efficiency to deliver extra power. There are three basic choices: none, a conventional controller, or a controller with Maximum Power Point Tracking.
If your goal is to keep your batteries topped off, and maybe run your bilge pumps, your solar panel power will probably be small in proportion to the capacity of your batteries. In this case, the danger of overcharging is small, and a controller might not be necessary. If your panel power is relatively larger, and you'd like to avoid using a controller, it would be best to use a "self-regulating" panel. These panels have a lower output voltage which reduces the risk of overcharging, at the price of reduced system efficiency. Without a controller, you will need to make sure your system has a blocking diode, which is a small and inexpensive electrical component that prevents electricity from flowing from your batteries back into your panel at night. Blocking diodes also prevent possible panel damage if your batteries are charged from another source such as an alternator or a shore-power charger.
Most systems today use a conventional solar charge controller. In their basic form, these are dead-simple pieces of electronics. If the battery voltage is lower than a threshold, the controller connects the panel to the battery. Once the battery charges to a somewhat higher threshold, the controller disconnects the panel. Nearly all charge controllers perform the function of blocking diodes (check the manual), and including a diode unnecessarily will only add cost and reduce system efficiency.
There are a large variety of conventional controllers available with all sorts of bells and whistles. Features you might be interested in include: LED status indicators, adjustable float voltages for sealed or flooded batteries, built-in meters, optional remote meters, low-voltage load disconnects to prevent your batteries from going flat, load bypass outputs, which supply power to secondary loads once the batteries are fully charged, manual or automatic battery equalization, temperature compensated battery charge voltage, remote temperature sensors, panel mount or surface mount enclosures, sophisticated 3-state charging, conformal coating, etc., etc., etc.
There is, however, a problem with conventional controllers. Solar panels will only deliver their rated power when running at a particular voltage. A typical "12V" 50W panel will typically put out 17V and about 3A. Operation at any other voltage will result in reduced power. If you connected that 50W panel to a conventional controller, it would deliver only slightly more current than 3A, but would be doing so at battery voltage of 12-13V, resulting in a power output of 40W, not the 50W you paid for. If your batteries were down around 11V, then you would be doing even worse, at around 35W. The metric for performance in these situations is called "tracking efficiency." Tracking efficiency is the ratio of delivered power to available power, and is a measure of how good a job a charge controller does at extracting power from a solar panel. For the examples above, the tracking efficiency would be 80%, and 70%, respectively. The conventional controllers that boast 99.5% electrical efficiency generally keep quiet about their poor tracking efficiency.
Fortunately, there is a solution to the problem of poor tracking efficiency. Advanced charge controllers use a technology called Maximum Power Point Tracking (MPPT) to operate solar panels at their most efficient voltage, converting the power down to battery voltage with very little loss. Until recently, these controllers had only been used large or high-performance applications like home power, solar generating stations, satellites, or solar race cars. Now, with the renewed interest in solar power and the plummeting costs of power electronics, these technologies are within the reach of everyone. The field is narrower for these controllers, with companies like Genasun, Blue Sky Energy, and BZ making controllers suitable for boats. MPPT controllers share the same features as conventional controllers and are generally slightly larger, but will deliver typically 10-30% more power out of a solar array. This translates into quicker battery charges and more amp-hours available for your safety or enjoyment.
The maximum power point of a solar panel installation varies with a number factors, including amount of light, shading, temperature, and the length and gauge of wire used to connect the panel and controller. Manufacturers and researchers have come up with a number of ways of finding the maximum power point, some better than others. Some controllers briefly disconnect the solar panel from the battery, measure its voltage, then estimate the maximum power point based on that. These controllers will certainly deliver better performance than their conventional counterparts, but will not deliver full power under all conditions. The better controllers, such as our own GV series, will continually measure the power output of the panel, and maximize it by adjusting the panel operating voltage, assuring top performance. MPPT controllers generally have electrical efficiencies (a measure of the power lost in the controller itself) of 95%, which is less than the best conventional controllers, but their tracking efficiencies of 95-99+% more than make up for it.
One interesting feature of some MPPT controllers is the ability to charge a 12V battery from 24V of solar panels. This allows two 12V panels to be connected in series and wired to the controller, permitting the use of lighter gauge, cheaper wire, for the same power loss.
At present, MPPT controllers are economical for installations down to the 50W range. If panel area or weight are the major concerns, then MPPT controllers make sense for even smaller installations. If your current solar installation falls short of your needs, replacing your conventional controller with an MPPT controller is a cost-effective and much simpler way of increasing your system capacity than adding more solar panels.
What are the Installation Considerations for Charge Controllers?
There are a few installation considerations that apply to any charge controller. The controller should be mounted as close to the batteries as possible. This ensures quickest charging by making sure the controller doesn't see an artificially high battery voltage because of voltage drop in the wiring. Proximity to the batteries is also important if the charger provides temperature compensation, but does not have an external sensor. For these reasons, controllers with built-in meters seem to be something of a waste. If metering at a convenient location is desired, it's best to choose a controller with a remote metering option, or use a separate volt, amp, or amp-hour meter.
Like any electronics, controllers should be kept away from sources of heat, particularly MPPT controllers, which tend to run hotter than conventional controllers. And, as a matter of common sense, charge controllers should be kept away from water, either direct splashes or drips of seawater, or condensation. The better controllers will have corrosion-resistant hardware and enclosures and a conformally coated circuit board to resist moisture, making them suitable for marine use, but to the author's knowledge, there are no waterproof controllers on the market.
Interestingly, solar panels will put out their highest peak power during cloudy weather. When the sun moves into a hole between clouds, the panel will see full direct sunlight, plus light reflected from the clouds. This effect may produce peaks of 50% or more over direct-sun output! Make sure your controller has some headroom over the rated panel output power to absorb these surges in output. Most controllers will have some headroom built in, but only a few controllers have any sort of internal current limit beyond a fuse, so if your array is pushing the ratings of your controller, you may be asking for headaches.
Make sure your controller has an appropriate float voltage for your batteries. 13.5V-14.4V float voltages are common for sealed lead-acid batteries, while a lower voltage, around 13.2V, is preferred for flooded lead-acids. Some controllers have a user-adjustable float voltage, some are available from the factory with different voltages, and others are intended to be one-size-fits-all.
Most larger installations will consist of several panels: how many controllers are necessary? With conventional controllers, all panels may be connected in parallel, up to the controller rating, with no loss of efficiency above that inherent in a conventional controller. Note that whenever solar panels are connected in parallel, each panel must have its own blocking diode connected in series. Otherwise, especially when the batteries are charged, current from several panels can flow through a single panel, causing thermal runaway, hot spots, and melting of the panel encapsulation. Blech.
With MPPT controllers, the question of multiple panels becomes more complicated. Panels with the same maximum power point may be connected in parallel freely. In practice, this means that panels of the same make and model, pointed in the same direction, and subject to the same shading (none!) may be connected together with no loss in efficiency. Connecting mismatched panels in parallel with an MPPT controller will cause no damage, but will reduce the power delivered, substantially in some cases. Therefore, for highest performance, use a separate MPPT controller for each panel or set of matched panels.
And how do I wire it all up?
Click here for a sample diagram of a small-boat electrical system. This is essentially how the mini S/V Genasun is wired up, minus the mains charger, of course! Note that the image is a big .gif; you may want to save it and open with an external viewer if your browser doesn't display it correctly.
