Silence on the hook
As part of my hybrid-propulsion research I have been looking carefully at large-capacity Litihum-Ion batteries as a possible energy source on modern cruising sailboats. Lithium-ion batteries are common in cell phones and laptops, but they are rarely used in higher capacity applications. This may rapidly change, because the hybrid automotive and electric vehicle markets are in desperate need of better battery technology.
Regardless of whether we are sailing in a hot or a cold climate, we never take our bimini down. We always want protection from either the sun or from the weather, so for our latest boat—Nada IV—I had Malo Yachts in Sweden build me a hard top. Over time, the hard top should prove cheaper because it will never need replacing. It is also much stronger than a canvas bimini (I can stand on it), with supports that do not obstruct winches, as is normally the case. Best of all, it provides a platform for mounting lots of solar panels.
There are three basic types of solar panel: single crystal (monocrystalline), where each cell is cut from a single silicon crystal; polycrystalline, where individual cells are composed of multiple smaller crystals; and amorphous silicon, where vaporized silicon is deposited on some substrate (for recent developments, see below). Once panels have aged a little, their efficiency in terms of converting sunlight into electricity is around 16%, 10%, and 6-8% respectively. Prices pretty much correlate with efficiency.
I was looking for efficiency – i.e. monocrystalline panels. Beyond this, I wanted the greatest panel surface area I could fit on the hard top while still leaving space in the middle for a hatch so I can see the set of the mainsail. The best fit I could find was achieved with four Kyocera 85-watt panels. These panels are rated at 12 volts. My house systems run on 24 volts, so I wired two sets of two panels in series to get 24 volts for each set, then wired the sets in parallel to supply my house circuits.
Cabling and Installation
The maximum nominal output of my panels is 4 x 85 = 340 watts, which is 14 amps at 24 volts (340/24 = 14). The cable run from my panels is approximately 30 feet. With solar-panel circuits it is important to keep voltage drop to a minimum, which requires relatively large cables—in this case at least 10 AWG.
It is essential to provide appropriate overcurrent protection (a fuse or circuit breaker) at the point of connection of the positive cable to the boat’s electrical system. This prevents the wiring from melting down in the event of a short circuit. As such, the maximum allowable overcurrent rating is based on the cable size, rather than the solar panel output. A 10 AWG cable is rated from 40 amps on up (depending on the quality of the insulation). Although my maximum panel output is only 15 amps, I installed a 30-amp fuse to eliminate any potential for nuisance fuse blowing. This is well above the panel’s maximum output, but still below the cable rating.
The panels are wired such that when I shut down the house-battery isolation switch, the panels are still connected to my batteries. This way, they can keep the batteries charged when I am not on the boat.
Space Age Regulation
When the boat is idle the panels have more than enough capacity to overcharge my batteries, so a voltage regulator is needed.
If you look at the output curve for a solar panel, it shows the amperage produced at any given voltage, with the amperage declining as the voltage goes up. The wattage—the power produced—is determined by multiplying the amperage by the voltage at any given point on the curve. In any given set of conditions, the peak wattage, which is what is used to rate the panel, occurs at only one voltage. Typically, this is higher than the voltage of the batteries to which the solar panel is connected.
In the past, solar panels were wired directly to the batteries, holding the panel voltage at the battery voltage so that the panel almost never operated at peak efficiency. Voltage regulators simply disconnected the panel, or dissipated the output as heat, when the battery was fully charged. Modern regulators effectively disconnect the panel from its batteries and allow its voltage to climb to the point at which the panel is operating at peak efficiency in the specific operating conditions. A DC-to-DC converter then steps this voltage down to an appropriate voltage for charging the batteries.
This type of regulation is known as Maximum Power Point Tracking (MPPT). It increases solar panel output by around 15% (depending on the losses through the regulator —crude ones may only be 85% efficient, but the best are up to 98% efficient). A good MPPT regulator can be a little expensive, but it will more than pay for itself over its lifetime.
Because my panels are installed flat, and there is some shading from the boom, performance is far from optimal. In this kind of situation I conservatively estimate I will get the equivalent of four hours of full-rated output a day, i.e. 4 x 340 = 1,360 watts-hours, or 1.36 kilowatt-hours (kWh). The common industry rule of thumb is to assume full output for 4.8 hours a day.
The total cost of the panels, regulator, and installation was around $2,000. At this price it would take about 27 years for the panels to pay for themselves if I installed them on the roof of my house, assuming a cost of around 15 cents per kWh for electricity from the grid. This doesn’t look too good, which is why we need better solar panel technology to make this energy source appropriate on a mass scale.
On a boat, however, the equation changes dramatically. The normal mechanisms for generating electricity on board are an engine-driven alternator or a generator powering a battery charger. Both mechanisms are chronically inefficient. If you add the amortization cost of the engine driving the alternator or generator to the fuel bill and maintenance costs, the cost of a kWh is frequently well over $2 and can be as high as $5.
For a full-time cruiser, the payback period on solar panels is now somewhere between 1 and 3 years, which looks pretty good. For someone who only uses the boat on the occasional weekend, the panels will, in effect, be idle for most of their life, so the payback period is much longer.
Of course, it’s not just about payback. Through the use of fluorescent and LED lighting, and other conservation measures, we have reduced our at-anchor electrical load on Nada IV to the point where the solar panels can often meet the entire 24-hour load, including using my laptop computer for a good bit of the day. If there’s enough wind for the Air Breeze wind generator (www.windenergy.com) to kick in as well, we have surplus energy.
There is some satisfaction, on which you cannot put a price, in being able to sit on the hook day after day without cranking an engine.