One of the best illustrations of solar power’s increasing capabilities can be found in the Coast Guard’s recent embrace of this power source for its offshore landfall lights. In the First Coast Guard district, five major lights have been converted to solar power in the past year and more are on the way.
For several years, solar cells have been used to charge batteries on lesser aids to navigation: small fixed lights and lighted buoys. But now the Coast Guard is relying on solar power to run its category 1 offshore lights. These installationstoo remote for commercial power cablespreviously relied on diesel generators to run lights and sound signals.
For intermittent use, a diesel generator makes sense, but for the constant operation required at an offshore lighthouse, diesel power is maintenance-intensive, failure-prone, and has ecological problems. While solar power seems like an attractive alternative, there have been technical drawbacks hindering its use at long-range offshore lights. In 1991, however, two factorsa major storm and a technical leap forwardconspired to pave the way for solar power.
One door was opened, literally, by the heavy storm that lashed the New England coast in October 1991. The light and fog horn installations at Isle of Shoals off Portsmouth, New Hampshire; Boon Island off York, Maine; and Halfway Rock in Casco Bay, were all substantially damaged by 80-knot gusts and 30-foot seas. At Boon Island, for example, heavy seas threw large boulders against the steel door to the generator house, eventually battering it down. Both primary and secondary generators were quickly destroyed by the steady cannonade of boulders.
With three major lights out of commission, the Aids to Navigation (aton) Branch at First District Headquarters had aton field teams place low-power temporary lights at each location. Meanwhile, the replacement cost for reinstalling generators and their associated equipment (fuel tanks, controllers, etc.) was put at roughly $500,000 per site. Given this cost and the incessant maintenance efforts required for operating diesel generators, the Coast Guard decided to take another look at solar power for these lights. “We decided to make lemonade out of lemons,” said Lt. Keith Bills, chief of the lighthouse and technical section in the First District aids to navigation branch. “Too many things go wrong with diesel generators. We decided to take the next step to solar power.”
Lamp developments
It was at this point that the second factor entered the picture. A new type of DC lamp was developed that required far less power for a given luminance. The lamp was jointly designed by the Xenell Corporation (now part of a company called Chicago Miniature Lamp), the Coast Guard, and Xenell’s Japanese subcontractors. Xenell, located in Canton, Mass., devised a 110-watt tungsten/halogen lamp that has a luminous output nearly equal to the 1,000-watt AC lamps used in major lighthouses. (For lights and sound signals that can’t be operated by commercial power cables, diesel generators run continuously, producing AC to power the equipment. As for range, two 1,000-watt lamps used together and will have a range of 23 to 26 miles. The 110-watt DC lamp has a maximum range of 20 miles.)
Xenell used a proprietary method for constructing this DC bulb and was able to produce a lamp that burns brightly at low power. “We used a combination of filament length, shape, and filament position to produce a brighter lamp,” said Randy Jay of Xenell.
In a tungsten-filament bulb, hot tungsten particles “boil off” the filament, weakening it and eventually causing it to break when current is applied. Filling the bulb with halogen gas causes this “free” tungsten to be redeposited on the filament, greatly increasing filament life. By coupling an improved filament with halogen, Xenell came up with a bright, long-lasting lampand removed a major drawback to a solar-powered DC power system for offshore lights.
In 1992, Coast Guard engineers designed just such a solar system around solar panels that charge lead acid cells and power a Xenell lamp assembly, a fog signal, a regulator, and a monitoring system, which includes a UHF radio link for remote monitoring.
This solar approach was approved in 1992, and the first installation began in 1993 on White Island at Isles of Shoals. Later solar packages were put in it at Boon Island and Mt. Desert Rock. Similar equipment is currently being put in place at Halfway Rock and Matinicus Rock. The cost for these installations is roughly $100,000 per site.
In addition to substantial savings in initial cost, the solid-state solar panels have little of the real and potential ecological impact of diesel power. The diesel units add to air pollution, produce some sound pollution, require the disposal of 40 gallons of used oil quarterly, and require the efforts of a 180-foot buoy tender to refuel the twin fuel tanks. Added to this is the potential for an oil spill should either fuel tank fail or there be a malfunction or mistake during fueling.
When lights are located on or close to the mainland, commercial power cables can be used. And that approach was tried at Boon Island, for example. However, each 6.5-mile submarine cable needed to be replaced about once a yearthey were severed due to chafe against underwater rocks.
Powerful array
Operating a landfall light, plus a fog signal, for hours on end requires a powerful solar array. The solar assemblies that drive these systems typically mount up to 30 panels in the main array, each with a power output of 40 watts, for a total of 1,200 watts. Each panel is composed of 29 individual single-crystal silicon cells. A separate 45-watt panel is wired directly to a NiCad battery to provide backup power should the lead acid battery fail.
The panels are mounted on a 20-foot high aluminum frame, and this is angled at 70° to provide the best overall light-gathering performance. The angle for each installation is calculated by combining the changing declination of the sun over the year with the latitude of the installation. The optimum angle is heavily weighted toward winter light-gathering since there is an inverse relationship between the need for the light and amount of daylight available to power it. “We use a computer program to determine the best angle for the array,” said Jon Grasson, project engineer in the ocean engineering branch at Coast Guard headquarters. “In winter, we have the least amount of sun and the most power needed for the light.” And since the sun’s altitude is lower in winter months, the array angle must be fairly steep.
Set on concrete footings and guyed with steel cable, the solar frame holds panels that are wired in groups of six to local terminal boxes. The outputs from two local boxes are combined into one circuit. There are two of these 12-panel circuits, and one six-panel circuit (2 x 12 =24 + 6 = 30 total). The result is three circuits connected to a master junction box where the circuits are fused for circuit overload protection. Lightning protection is provided by metal oxide varisters tied in ahead of the circuit fuses. The panels are paralleled, so their current output is additive.
From the master box, three cables carry power from the two 12-panel circuits and one 6-panel circuit into a charge controller. Each of the three inputs has its own breaker, and the two, 12-panel circuits have relays to open their circuits and disconnect them from the main battery when battery voltage reaches the chosen set points. (According to Jon Grasson, the solar systems are presently being reconfigured; when battery voltage reaches 15 volts, both circuit relays open and both sets of 12 panels are taken off line. The solar controller was originally set up to stagger the relay set points so that first one set of 12 panels dropped out and then the second set was open-circuited.) To provide a float charge to the battery, the six-panel circuit remains permanently connected to the system. Since solar panels will discharge a battery at night, blocking diodes are used.
One large battery
Interestingly enough, the 1,200-watt maximum output (84.5 amps at 14.2 volts) of the solar array is routed not to a bank of batteries, but to a single lead acid battery composed of six individual cells; each massive unit weighs roughly 330 pounds. Since putting all six cells into one case would make it prohibitively heavy, the 2-volt cells are mounted independently and wired in series. (All lead acid batteries have cells connected in series, but this is usually done inside the case.) Should one cell fail, it can easily be disconnected and replaced, rather than replacing one massive battery. A high-powered, deep-cycle marine battery might have a capacity of 250 amp hours. The monster Exide battery used in these solar-powered installations has a capacity of 2,915 amp hoursif one could convert this directly to work, it would equal roughly 50 horsepower. Since the primary lamp, the light motor, fog signal, and associated circuitry require about 15 amps (this maximum load will be at night, or about half the time), this battery could theoretically supply power (until it reached 50% discharge) for about eight days.
Should this powerful battery somehow fail (like any 12-volt storage battery, if one cell fails, the whole unit is out of business), there is a backup 400-amp-hour NiCad battery that is charged by a dedicated 45-watt panel. Monitoring circuits in a device called the solar distribution box determine system voltage. Should that fall below a set point, an alarm is engaged and a relay isolates the main light and sound signal and switches the auxiliary battery in to power an emergency light and sound signal.
Since the backup battery cannot power the emergency light indefinitely, any failure of the main battery must be corrected quickly. The first step in getting the main battery and light back on line is initiated by a monitoring and RF signaling system that operates on UHF frequencies. A malfunction in the system would result in an alarm broadcast to Coast Guard aton personnel in South Portland. “The alarm will tell us if a part of the system has malfunctioned,” said Petty Officer Tim Gass, a member of the South Portland aton team. “We can also do some basic checks on system status by querying the monitor with the UHF link.”
The First District won’t be the only area of the country that will benefit from these solarized lights. The Coast Guard has installed a solar package at Anacapa Island off Oxnard, Calif. Other locations that are being considered for solar power are Long Beach Light; Farallon Island off San Francisco; the light at Destruction Island off Taholah, Wash.; Cape Flattery on the Strait of Juan de Fuca; and Southwest Pass Light on the Gulf of Mexico.
“The decision on solarizing other lights will depend on user needs,” said Grasson at Coast Guard Headquarters. “There might be some reduction in range using the 110-watt lamps.” Even this problem may be solved by improved optics on the 110-watt lamps. “There are new optics around the corner that can increase the range,” said Grasson. “Or we may choose to maintain the same range but drop the power needed to run the light.”
Given the cost advantages of both less equipment and less projected maintenance, plus the public relations bonus of reduced environmental impact, solar power for offshore lights is undoubtedly an idea whose time has come.
