Solar Updates
Technology advances, from panel construction to voltage regulation, are making onboard solar better than ever.
Sixteen years ago, I installed solar panels on my boat. At the time, the peak efficiency at converting sunlight to electricity was around 16%. Today’s panel technologies enable substantially more energy to be harvested from a given surface area, boosting efficiency as high as 24%—a 50% increase. Taken with other advances, notably how panel output is managed, we have a qualitative improvement in the benefits solar can bring to a boat’s energy systems.
Traditional Panel Technologies
One thing that hasn’t changed is the use of one of three silicon-based technologies: monocrystalline, polycrystalline, and thin-film.
Monocrystalline cells are created by placing a silicon crystal “seed” in a vat of molten silicon and then slowly withdrawing the seed. The molten silicon forms a solid single crystal cylinder around the seed. The cylinder is squared off and sliced into wafers—the basic building block of a cell. A fair amount of the silicon cylinder becomes waste, driving up the cost.
Polycrystalline (multicrystalline) cells also start as a seed in a vat of molten silicon. Once cooled and hardened, the resulting square-sided multicrystal solid is sliced into wafers. Compared with cylindrical, monocrystalline ingots, the square-sided ingots reduce waste and are significantly cheaper.
In theory, a monocrystalline cell is more efficient at converting sunlight into electrical energy than a polycrystalline cell. In practice, enough factors come into play that a high-grade polycrystalline cell in a well-built panel will be more efficient, and sustain that efficiency longer, than a low-grade monocrystalline cell in a poorly constructed panel.
Thin-film panels—less efficient than poly or monocrystalline-cell panels and susceptible to moisture intrusion—are rarely seen on boats these days.
The wafer of silicon in a solar cell is modified to create electricity when exposed to sunlight. One pole of the electrical circuit typically consists of three to five silver busbars embedded on the surface of the cell with a mass of barely visible fingers feeding into the busbars. The fingers collect the current generated by the cell, feeding it to the busbars. The electrical circuit’s other pole is on the underside of the cell, consisting of a conductive surface, often a thin layer of aluminum.
Each cell produces around 0.6v to 0.7v in sunlight. Cells are connected in series to boost the voltage (for a nominal 12v panel, there will be anywhere from 32 to 40 cells in series). The series connections are made through a strip of copper soldered to the busbars on top of one cell and connected to another strip of copper soldered to the back surface of the adjacent cell. Cells and wiring are laminated between sheets of plastic (flexible or semi-flexible panels) or between glass and plastic (rigid panels).
The ampere output of a panel is a function of cell size (surface area) and cell quality. A panel with 32 cells in series will produce the same ampere output as one with 100 similar cells in series, but the latter will have a much higher voltage. The rated power is the amperage multiplied by the voltage.
Innovating Cell Technologies
The Maxeon monocrystalline cell from Sunpower, sometimes referred to as Interdigitated Back Contact (IBC) technology, is different. The wafer’s positive and negative electrodes are both on the backside of the cell, eliminating the grid on the top. On the back of the cell, the typical thin aluminum layer is replaced with a more robust copper layer, improving electrical performance and eliminating the corrosion associated with aluminum.
Because the positive and negative electrodes are on the back of a Maxeon cell, the thin and vulnerable top-of-one-cellto-the-bottom-of-the-next-cell connection on conventional series-wired cells is replaced by a more rugged edge-to-edge connection (it looks like, and is called, a dogbone). The thin silver fingers printed on the front side of all other cells and the associated busbars are eliminated, along with numerous soldered connections.
Sunpower claims the various modifications in its cells and panels eliminate 85% of the failures in conventional designs, which, it contends, are due primarily to corrosion and electrical breaks. In the laboratory, Sunpower cells have achieved efficiencies above 24%. Sunpower manufactures its own panels from these cells and supplies cells to premium marine solar panel manufacturers such as Solbian (in Italy) and Solara (in Germany).
Another unusual approach to cell construction is the HIT cell from Panasonic. HIT stands for Heterojunction with Intrinsic Thin layer. HIT cells combine monocrystalline technology with ultrathin amorphous silicon layers to improve overall efficiency. In the laboratory, Panasonic has achieved efficiencies above 25%. Along with Sunpower IBC cells, Panasonic HIT cells are recognized as the highest-power commercial silicon cells available.
HIT cells use a conventional grid structure with its inherent weaknesses, but these can be substantially mitigated with the Merlin Advanced Metallization Technology (MTAT) grid. It can be used on all cell types except for Sunpower IBC cells.
The conventional silver fingers are first screen-printed onto cells. In place of the busbars, the MTAT grid is added. The primary conductors are tapered from one end to the other, gaining in cross-sectional area from one side of a cell to the other as they collect more and more current from
the cell. The conductors snake slightly back and forth, creating a spring effect that absorbs differential expansion and contraction with changes in temperature. This effect is particularly pronounced at the otherwise vulnerable cell-to-cell series connections, reducing the risk of fracture.
The MTAT copper grid structure adds stability to the brittle silicon cell wafers. It minimizes the failures associated with traditional soldered conductors. In the event of microscopic cell fractures that can severely impact cell performance, the multiple conductors and connection points (the front of each cell has around 2,000) minimize the loss of panel output. An MTAT grid on a HIT cell is an outstanding combination of efficiency and ruggedness.
Check the Warranty
Purchasing marine solar panels is a case of buyer beware. After manufacture, cells are tested and sorted based on their performance. Cells that look identical vary considerably in performance, and there may well be minute cracks and other flaws in lower-grade cells that accelerate performance degradation over time. These lower-grade cells will likely cost a fraction of high-grade cells, letting panel manufacturers produce much cheaper panels but with reduced performance.
Solar panels in home power applications and tied to the electricity grid must meet various standards, but the same is not
The solar panels on the author’s boat are partially shaded by the end of the boom, opposite page. This panel on a sailboat uses 36 half cells to achieve charging, left. The Merlin MTAT system helps add stability to the silicon cell wafers and minimize failures associated with traditional soldered conductors, right.
true for off-grid and marine applications, where third-party testing to verify claims isn’t required. Panel manufacturers have been known to quote the cell efficiency of high-grade cells from a manufacturer such as Sunpower, when in fact they are using significantly less efficient, lower-grade cells from the same manufacturer.
No matter how efficient the cells, if a panel is assembled from unsuitable materials, is poorly constructed, is damaged in shipping, handling, and installation, has
inadequately sealed wiring connections, or is connected to the boat with undersized wiring, it will perform poorly and likely fail prematurely. It is not unusual to see cheap solar panels fail in as little as two years.
A good indication of quality is the warranty—how many years it is valid, and what it covers in those years (e.g., full replacement for failures and loss of output, or prorated replacement). Note that most rigid household panels carry no warranty when used in marine and mobile installations.
Installation Considerations
With Bruce Schwab of
Ocean Planet
Energy, I conducted shading tests. We found that a hard shadow has the effect of knocking out a percentage of the panel’s output equal to the percentage shading of a single cell: shading one quarter of a single cell knocks out one quarter of the panel’s output. Soft shadows (for example, from rigging at some distance from the panel) have nowhere near the same effect.
When cells are shaded, they consume power from nonshaded cells in a series string. In the case of hard shadows, shaded cells can become hot enough to melt plastic cases and even start fires.
To prevent hot spots, bypass circuits and bypass diodes are often installed at strategic points. If a cell becomes shaded, back-feeding of the shaded cell is limited to that part of the string between the nearest bypass diodes.
Bypass diodes should be installed with any series string of 50-60 watts or greater. This will limit the maximum available back-feeding energy to levels that will not cause excessive heating or cell burning. The diodes can be built into a panel or mounted externally.
Although diodes in operation create a voltage loss of up to 0.7v, in normal panel operation bypass diodes are not part of the conducting circuit and do not consume energy or diminish panel output. However, if a cell is shaded and the diodes become part of the circuit, the combined effect of the shading and diodes will dramatically reduce the output voltage.
If panels are wired in parallel to a common voltage regulator and one is partially shaded, the higher-performing panel will back-feed the lower-performing panel. To prevent this, blocking diodes are required at panel outputs. These permanently wired components will create a permanent loss of voltage and will also absorb a very small amount of energy. To avoid this permanent inefficiency, add individual solar regulators to each panel to optimize each panel’s output.
Advances in Voltage Regulation
To charge a battery, we must raise the voltage above the battery’s at-rest voltage. This is why a 12v solar panel has anywhere from 32 to 40 cells in series, with each producing between 0.6v and 0.7v. In sunlight and disconnected from a battery, a voltmeter across the output terminals of a 12v solar panel will typically read anything from 16v to more than 20v.
The higher the voltage, the greater the ability to maintain charging voltages in the face of voltage losses from connections and wiring, and losses through diodes and regulators. But now, if a solar panel is
unregulated, the output of even a moderately sized panel is sufficient to destroy a battery over time through overcharging. We need a voltage regulator on all but the smallest panels. These regulators also incorporate a diode that prevents the battery from back-feeding a solar panel overnight or when the panel is seriously shaded.
Two types of regulators are commonly used: Pulse Width Modulated (PWM) and
Maximum Power Point Tracking (MPPT).
A PWM regulator feeds panel output directly through to a battery. The battery determines the solar panel’s output voltage. A significantly discharged battery will initially accept everything the solar panel can throw at it. The regulator will do nothing. As the battery becomes more fully charged, its voltage creeps up. At a pre-determined acceptance voltage, the regulator kicks in to hold the voltage at this level. It does this by disconnecting and reconnecting the solar panel at a high frequency, pulsing the battery with charging current. As the battery state of charge climbs and its ability to absorb charging current continues to drop, the regulator’s off periods get longer than the on periods; this is the pulse width modulation. When the battery is fully charged, the regulator trips to a lower voltage float setting.
In different light conditions, the output voltage at which the most energy can be extracted from a solar panel varies. Because the output voltage of a panel connected to a PWM regulator is controlled by the battery voltage, the regulator cannot modify the voltage to optimize panel output.
An MPPT regulator, on the other hand, effectively disconnects the solar panel from the battery, determines the optimum panel voltage in the given light conditions, and loads the panel in a manner that holds it at this voltage.
In less-than-ideal light conditions, an MPPT regulator can push the output of a panel up by as much as 30% compared to a PWM regulator. However, the electronics in the MPPT regulator absorb energy and negate some of this gain. The best are now over 95% efficient, with some claiming more than 99% peak efficiency.
We have a recent adaptation of MPPT technology. Traditionally, all regulators have required a panel voltage that is above battery voltage, hence the 32 to 40 cells in a 12-volt panel. In smaller panels, cells must be cut up to achieve the requisite number in series, reducing panel efficiency. We now have MPPT regulators that can boost voltage. We can put as many full-sized cells in a panel as will fit that particular size, and then boost the voltage as high as we want— for example, 10 cells with a nominal voltage of 6v to 7v boosted to levels that will charge a 48v battery bank.
The link between cell numbers and battery voltage has been broken, giving us another significant improvement in performance.
With PWM regulation, it is common to wire multiple panels in parallel to a single regulator, with blocking diodes to prevent panels back-feeding one another. Although this can also be done with MPPT regulators, it undermines the benefits of the MPPT technology, especially if the panels are in different light conditions. To fully optimize panel output, a separate MPPT regulator should be used on each panel.
Note that there is often a relatively long cable run between solar panels, regulators, and batteries. These cables must be sized to minimize voltage losses, and any cabling that attaches directly to the panels and is in the open air needs to be sunlight, UV, and ozone resistant.
Conclusions
The key to an effective, optimized, solar installation on a boat is to buy quality panels with an excellent marine warranty from a recognized marine vendor. Install them with care, providing adequate support in a location that limits shading. Ensure that the wiring is sized to minimize voltage losses and that the connections are watertight. Use panels with full-sized cells. Add an MPPT regulator to each panel with a boost capability if necessary. The result should be an installation that gives at least a decade of service with minimal degradation in output.
Increases in efficiency combined with MPPT controllers, and recently boost controllers, has transformed what we can expect in terms of output. On many boats, especially catamarans, it is possible to keep up with all house loads, up to and including modest air conditioning, frequently eliminating the need for a generator or long hours of battery charging at anchor.
The further offshore you intend to sail, the greater the lifestyle benefits from installing as much solar as possible.
SAIL
Nigel Calder
Boatowner’s Mechanical and Electrical Manual and Marine Diesel Engines. A longtime member of the American Boat and Yacht Council, his boathowto.com provides dependable technical information for sailors. He and his wife, Terrie, have sailed from the Faroe Islands to Portugal, the Bahamas, the U.S. East Coast, and Caribbean, and he’s written guides including Nigel Calder’s Cruising Handbook: A Compendium for Coastal and Offshore Sailors.