June 2007 Issue
Choosing a Wind Generator
Practical Sailor begins its two-part report on wind generators for cruising sailboats begins with a look at features including blade size, number of blades, output considerations and installation.
When cruising sailors think of renewable energy, their thoughts immediately turn to the wind. When selecting a marine wind generator for your boat, several factors must be taken into account, and separating fact from fiction is hard. Claimed output data for specific units can vary greatly from real-world performance. In part one of our series we introduce the six units we test, the Air Breeze from Southwest Wind Power, the Superwind SW250, the KISS High Output, the Ampair 100, and the Rutland 913. The selection presents a good cross-section of micro wind turbines available today and allows us to make some conclusions regarding the best wind generator for particular marine applications.
Practical Sailortested six wind generators side-by-side over the course of four days. The last time we attempted a similar feat, it was a bust. The turbines spun feebly in a marina with little wind. Prior to that attempt, we long-term tested five models individually on a hilltop in Rhode Island ("Wind Generators, Part 1: Ten Years of Experience," Oct. 1, 1995, and "Fourwinds II Quietest Large Diameter Wind Generator," Nov. 15, 1995). Although that round of testing didn’t compare units under the same conditions, we took enough output readings at various speeds to create output curves and came to the dismaying conclusion that over the long haul, an average 50-watt solar panel would outperform the units we tested. (None of the units exceeded an average output of 10 amp hours per day.)
This time, we had plenty of wind.The test site was at the water’s edge, and five of the six wind generators spun simultaneously: the KISS High Output Wind Generator (made in Trinidad), the Rutland 913 (England), the Superwind SW350 (Germany), a prototype Air Breeze from Air-X makers Southwest Windpower (Arizona), and the Ampair 100 (England).
Conspicuously missing from our test were a pair of two-bladed units: one from Hamilton Ferris (reviewed in our Feb. 15, 2003 issue) and one model from Fourwinds. Both companies said they could not meet our timetable, despite our long lead time for delivering a unit. We’ve been assured that as soon as these units become available,
Practical Sailor will be able to test them. Another unit that looked very promising on paper was the Ampair 300. (There’s also an Ampair 600 for 24-volt systems.) This three-blade, large-diameter unit had a problem with the motor shaft on the first day of testing, and we returned it for repair. We expect to test the refurbished unit soon.
The topic of wind generators is not easily digested over a long lunch. Performance alone may not be the deciding factor, and several other details come into play&emdash;not the least of which is the possible mounting location for a set of blades whose tips slice the air at speeds as high as 200 mph. So before we dive into the results of our test in next month’s issue, we will focus here on key decision points in purchasing a wind generator and some general conclusions regarding wind generator selection based on our testing.
Wind Generator 101
Wind turbines convert the kinetic energy of the wind into mechanical power, and ultimately electricity. This electricity can be used immediately to power equipment, but is typically stored in batteries for future use. Larger turbines may generate enough power to carry or "float" larger loads (such as a small fridge during an overnight stay aboard), while smaller units produce enough electricity to power smaller loads for a few minutes (bilge pumps, etc.) or perhaps top off your battery banks after a weekend outing.
All generators share a few basic components: a rotor&emdash;they don’t propel, so they’re not propellers&emdash;with aerodynamic blades, an electrical generator, some form of rotor over-speed control, and a mounting system (pole, arch, etc.). Most also will have rotating electrical contacts, which enable the unit to operate in a continuous 360 degrees of rotation. All but one of the units in our test, the KISS, had this feature. The KISS generator has an internal spring (inside the mount) and a rope lanyard tied to the tail of the unit and mounting pole&emdash;the lanyard is a specific length to prevent the unit from rotating more than three or so times, after which the spring is supposed to return it to its original position once the wind dies down.
Wind turbines either produce direct current (DC) or alternating current (AC) power, which is then converted to DC via a rectifier. Of the models we tested, the KISS and both Ampair units utilize a rectifier to convert AC to DC, while the Rutland 913, Air Breeze, and Superwind 350 produce DC. Each approach has its pros and cons: AC can be transmitted over longer wire runs with less power loss (due to overall resistance of system wiring), even when smaller gauge wire is utilized. DC systems, on the other hand, don’t require the use of a rectifier, which reduces expense, cuts down on the number of parts that might fail, and eliminates a few installation steps. As for cons, DC motors have brushes and commutators, both of which require periodic maintenance to prevent generation of electromagnetic interference (EMI), which can disrupt onboard electronics. The rectifying diodes in AC-producing units can also be damaged if exposed to reverse-polarity voltages during installation or maintenance.
In terms of design evolution, no great technological breakthroughs have emerged since our last test. According to Betz’ Law (see "Estimating Wind Power") a wind turbine can theoretically use about 60 percent of the energy in any wind. Even the small turbines meant for use on land are still far from that ideal.
"What they are getting is a piece of that 60 percent Betz limit," said Jim Johnson, a mechanical engineer for In the Wind at the National Resources lab. "The better units will produce about 40 percent of that limit."
Ongoing research at National Wind Technology Center (www.nrel.gov/wind/nwtc.html)&emdash;including the development of more efficient, quieter blades&emdash;will eventually trickle down to micro-turbines (as the boat-sized units are called). However, the limited marine market, price-point competition, and design limitations imposed by marine applications likely will slow this process. Advances generally have been baby-step improvements in rotor noise, more efficient blades, reduced shaft friction, and smarter regulators. If the last 12 years are an indication of what’s to come, what we buy this year probably won’t be much different than what will be available five years down the road, when our turbine will likely need an overhaul or replacement. Like any moving part on a boat, these things do break down.
Wind-turbine makers often bear a "label rating" according to potential output under ideal conditions. For instance, the Ampair 100 will produce 100 watts (volts x amps = watts) in a 28-knot breeze. Aside from the fact that no one purposely chooses to anchor for long in a 28-knot breeze, these numbers can be deceiving.
Turbine manufacturers will typically provide speed-output curves that graph output at all wind speeds within their unit’s range of operation. Others will simply indicate projected output at a sampling of fixed, steady wind speeds. Either approach can yield a distorted picture of real-world output. Some makers base their steady-wind output projections on absolutely fixed wind speeds (impossible, except in a wind tunnel). Other makers reach their output numbers by using a standard wind distribution model known as the Rayleigh distribution, a statistical method used by wind power experts to translate average annual wind speed data into potential wind power estimates (see chart below).
"You should take any output figures published by the manufacturers with three very large grains of salt," say Paul Gipe, whose website (www.wind-works.org) and book ("Wind Power: Renewable Energy for Home, Farm and Business"), covers the topic of wind power for land applications in great detail.
This, of course, is one of the reasons we are looking at these units in a real-world application.
Wind Turbine Types
Wind turbines can usually be classified as either small rotor units (blade diameters less than 48 inches) or large rotor units, with typical blade diameters of around 60 inches. All things being equal, the highest potential output will increase with the diameter of the rotor. A rough rule of thumb is that larger units typically generate around 4 amps in 10- to-15 knots of steady wind, while smaller units average about 1.3 amps.
The main challenge confronting any wind generator is the fickle nature of wind itself. Wind generators present a Catch 22 scenario. While they are most effective when exposed to steady winds with the vessel at anchor, the best anchorages tend to be sheltered from the wind. As such, the cut-in speed of a wind generator (the point where it actually starts producing electricity) and its output in lower winds (10 to 15 mph or less) can be more important than maximum rated output.
Smaller, multi-blade units (typically six blades) have an advantage in this respect. These blades have less inertia, so they require less wind to start turning, allowing them to reach their cut-in speed and start producing power sooner in light winds. So, if your cruising anchorages are characterized by light breezes, a small blade is the way to go ... or is it?
A key factor in potential power output is the cube rule: Available wind power varies as a cube of wind speed. So if wind speed doubles, energy content (measured in kilowatts per square meter) increases eight times. A 10-mph wind has one-eighth the power of a 20-mph wind (103=1,000 versus 203=8,000), and a seemingly insignificant increase in wind speed from 10 to 12 mph can increase available wind power by 73 percent.
What this means from a practical standpoint is that if you choose a quiet anchorage that experiences occasional higher-than-normal gusts (squalls or katabatic winds, for example), a wind turbine could potentially yield more energy than it would if you were anchored in a steady, moderate breeze during the same time period.
Wind generators with fewer larger blades have higher maximum outputs and can produce more power in higher winds. (A one-bladed rotor, odd as it may seem, has greater potential for output than one with multiple blades.) This means that while a large-blade turbine might not match the output of a small-blade wind generator in light winds, its higher output in gusts can compensate for its higher cut-in speed and poor performance in lighter winds. A key factor is whether the occurrence of higher gusts is high enough to keep up with power demands.
Another consideration output-wise is that while sailing downwind, you have to subtract the boat’s speed from the wind speed to get the apparent effective wind speed at the generator. If the true wind speed is 14 knots and boat speed is 7 knots, your generator is actually "seeing" only 7 knots, meaning output will be greatly reduced.
Rotor Speed Control
While wind generators obviously require wind to operate, at some point (typically around 35 knots of sustained wind), you’re approaching the too-much-of-a-good-thing level, and some form of blade speed control mechanism is required to prevent physical damage to the unit and, in some cases, the boat’s batteries. Braking, or blade speed control, can be accomplished in a number of ways. Some units have "self-braking" blades that stall at certain speeds, while others are designed to gradually turn away from the wind as higher than acceptable speeds are reached. Friction or air-brake systems are also used, as well as electrical stop switches. Finally, some turbines require you to physically tie or secure the blades, often an unattractive prospect in a rocking boat, considering the speed at which the blades can rotate. For extreme weather conditions, even the makers of units with stop switches recommend that you physically secure the blades and rotate the units to reduce windage, or remove the unit altogether.
While construction, size, weight, and ease of installation are all important considerations when choosing a wind turbine, noise is often a deciding factor. All models are noisy to some extent. However, some units are as loud as an engine or genset running at anchor, which defeats one of the reasons folks turn to renewable energy & emdash;peace and quiet.
Much of the noise from a wind generator is caused by air movement at the tips (tip vortices) and back edges of the blades, which is why there is constant refinement in blade design. Blades with fine, smooth trailing edges and smaller tips will generally be quieter. Although noise can be reduced by factors such as construction and blade design, as a general rule, units with smaller blades are quieter than those with larger blades. The number of blades is a factor as well&emdash;a six-bladed unit will always be quieter than a two- or three-bladed unit, provided the blade diameter and design is equal.
Some folks don’t mind the noise of a larger unit, equating it to the sound of "money" flowing into the proverbial energy bank. Others (often those anchored beside you) will find it annoying. A good way to compare noise levels of various units "in the wild" is to walk the docks of your local marina or dinghy around the mooring field and observe others’ wind gens&emdash;it also gives you the opportunity to ask how satisfied the owners are with each unit.
Wind-power study is rich with mathematical formulas, and there’s one to account for mounting height as well. According to the Wind Profile Power Law, wind speed rises proportionally to the seventh root of its height above sea level. By this formula, doubling the height of a turbine, then, increases the expected wind speeds by 10 percent and the potential power by 34 percent. However, at the slight altitude changes that are possible on a boat (say the 20 feet between a pole mount and a mizzen mount), this formula will likely have little bearing.
More important for our discussions of boat mounting is the "roughness" factor, which accounts for obstructions that impede windflow. The slight shift from pole mount to masthead will clearly alleviate roughness. How this will affect output will vary from boat to boat. Estimated increases in output range as little as 10 percent to more than 20 percent.
Mounting a wind generator is often a balancing act of aesthetics and performance, meaning your choice can look good but operate poorly or vice versa. The best spots are those that offer an unobstructed flow of wind while keeping whirling blades well clear of rigging, self-steering vanes, davits, or, most importantly, the outstretched arms of the tallest crew member.
Stern Poles and Arches
Stern poles and arches are popular mounting choices&emdash;both keep your wind generator in place where it can be tied down or serviced, but up and out of the way of outstretched arms. Stern poles are less expense, but proper bracing is crucial not only for strength, but to reduce movement of the pole (which, in turn, minimizes vibration and noise transmission belowdecks). Arches cost more, although the added expensive of having one fabricated can often be justified if it will serve multiple purposes (i.e., mounting for radomes and solar panels as well). The multiple attachment points on deck can also serve to dissipate vibration on the deck.
A boat with two masts has the option of mounting its wind generator about two-thirds up the mizzen or at the very top. Both choices offer more exposure to wind and provide a cleaner-looking deck, however, they do add weight aloft and the units will be more difficult to service. They’ll also require longer cable runs, meaning you may have to upgrade to larger wire sizes to address voltage drop concerns. Securing them will also be more of a challenge, particularly those that have to be physically tied off in high winds.
Rigging-suspended mounts, such as a fore-triangle hoist, are a good alternative when you just can’t seem to locate that perfect mounting spot. This option produces less vibration, and units that are designed to be deployed in this manner can easily be removed and stored to clear the decks when needed, however, they can’t be used while underway.
Based on our research (including the most recent data that we’ll report next month), a large-diameter, three-bladed unit is a good choice if maximum potential output is a chief concern. Small-diameter units can’t be written off, however. If low noise, small size, and a low cut-in speed (for low wind areas) are your first priorities, these units have much to offer.
Three of the units in our most recent test & emdash;the Superwind 350, the Air Breeze, and the Kiss High Output&emdash;had best days of 88-115 amp-hour production. Worst days were less than 10 amp hours. This is enough, or nearly enough, to meet the average amp-hour requirements aboard a modern cruising boat fitted with a watermaker and refrigeration.
Despite these persuasive numbers, our evaluations and experience in the field indicate that relying on a single wind turbine for one’s primary energy source is not the most sensible way to optimize for efficiency, particularly while under sail, when the rocking motion of the boat further inhibits performance. Solar panels have no moving parts, are durable, and in many ways are better suited for a lifestyle that tends to follow the sun. With the assistance of today’s Multi Point Power Tracking Technology (See "Boosting Solar Panel Output," Chandlery, August 2006), a single, 80-watt solar panel can replenish as much as 60-80 amp hours on an ideal summer day. Wind turbines, in our opinion, should be regarded as a viable option for a cruising sailboat with high energy needs to supplement its solar panels, genset, or high-output alternator&emdash;not as the ultimate solution to onboard energy production. Next month, we’ll take a close look at the performance and features of each of the units.