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Wind Energy

The power of the wind has been harnessed for generations, and improvements in technology has made captured wind energy increasingly more efficient and versatile.

Renewable Before It Was Cool

Wind power technology has been around for hundreds of years, but its recent resurgence has made it the fastest growing source of renewable energy in the world today. Wind power can provide the perfect complement to solar PV, and these hybrid systems have transformed and modernized off-grid living. Wind turbines combined with appropriately scaled storage can also power excellent stand-alone systems, on and off grid.

Hybrid_system_chart.jpg

The potential of wind power, like most renewable energy sources, is very site specific.  Alaska has many locations that are blessed with reliable and steady breezes. Even though Fairbanks is known for not having much wind, there are many micro-climate areas that are excellent sources. Ridge tops, funnel valleys, lakeshores and river banks are good places to check. It does not take hurricane wind, but only dependable breezes to make wind power viable. On frigid winter days, when Fairbanks is buried in stagnant ice fog, it is typical to have gentle breezes over the hills. This provides welcomed electrical generation when solar PV is sound asleep.

The National Renewable Energy Laboratory (NREL) produces maps of the broad-scale average annual wind speed across each state. These maps provide a basic idea of the gross wind potential in general areas; however, they do not address the micro-climates mentioned above. In areas with less population (most of Alaska) the data collection points are spaced farther apart and therefore produce less specific location data. However, the Alaska map does indicate noteworthy wind potential across much of the state. NREL’s website also recommends that “wind resource at a micro level can vary significantly; therefore, you should get a professional evaluation of your specific area of interest.” Arctic Sun can assist you with this.

Simple Design

Modern wind turbines are not complex devices, but subtle technological advancements and improvements in designs and materials have improved their efficiency and reliability. Most small-scale turbines are comprised simply of the blades, an alternator and a tail assembly. The differential pressures created by wind passing over the blades cause the blades to rotate. The turning blades spin magnets around a stationary coil of wire which induces an alternating current in the wires. The tail assembly's primary role is to keep the unit facing directly into the wind to maximize electrical production.  In many units, the tail is also used to prevent overspeeding the turbine in extremely high winds by furling or pivoting the unit out of the direct flow of air (see photo at left).

The power (P) available from a turbine is related to the velocity of the wind (V), the density of the air (d), and the size of the circular area the blades create as they spin (swept area)(A). The mathematical formula is:

 P = 1/2d x A x V3

Note that wind speed is by far the most important factor, as it is the cube of the velocity that matters. For Alaska, it is interesting to remember, the colder the temperature the denser the air. So the dense winter air improves the efficiency of wind turbines. Also, winter is typically the season with more consistent wind, so it is generally the best time for electrical production from turbines. These factors help make wind turbines an ideal complement to solar PV panels whose preferred season is obviously summer.

Mechanical Considerations

Turbines can be mounted on a variety of different types of towers. The type of tower is not as important as the height of the tower. The quality of air flow increases significantly with elevation. The velocity of the air increases, and the turbulence or swirling nature of the air currents decreases. Ground level obstructions such as buildings and trees cause disruptions in the flow of air that can reach to twice the height of the obstruction and for some distance downstream. Turbulent air flow is significantly less effective at producing power, and it is harder on the equipment. As we mentioned above, the velocity of the wind is by far the number one factor for power production, so spending a little more on the tower can be a wise investment. The normal guideline is the bottom of the turbine blade’s swept area should be at least 30 feet above the top of any obstacle within 300 feet of the tower. Taller and more horizontal clearance is always better. Also, don’t forget that trees grow, and a turbine tower is a long-term investment.

Initially, it may seem like mounting a tower on a rooftop would be a good idea to help gain height; however, this is rarely a good solution. Turbines produce significant vibrations that tend to travel very effectively down towers. The vibrating base of the tower can set up a resonance with the roof structure that can be quite loud and migrate throughout the structure. Being inside the building on a breezy day might sound like being inside a drum.

Some folks claim wind turbines are noisy. They certainly are not as quiet as a silent solar PV panel, but I think “noisy” implies a louder sound than I attribute to my 3 kW turbine. At slow operating speeds there is a swishing or swooshing sound that generally blends with other typical background noises. As wind speeds increase, so does the sound production of a turbine; although, other ambient sounds typically increase also. This generally keeps the sound from a turbine at similar levels to the surrounding noise, in the range of 50 to 60 decibels. At very high winds, a furling turbine gets a bit louder and changes tone, but this is normally not a frequent or extended-time occurrence. The bottom line is noise is in the ear of the listener, and a neighbor who does not like your turbine may have very sensitive ears. If you think this might be an issue, you should try and listen to a few existing turbines in various operating conditions before you buy. Not all turbines create the same amount of sound. Slower spinning units tend to run a little quieter, and they also tend to require less maintenance over time.

Every Case Is Unique

In choosing the proper turbine for your application, a common misconception is concern over the cut-in speed or the minimum velocity of wind required to start producing electricity. Most units begin production in breezes between 6 and 9 mile-per-hour. However, production at those wind speeds is very limited. If a unit sacrifices production at prime wind speeds to collect a couple of extra watts at slow speeds, the trade-off is a net loss in overall production. Most buyers’ guides don’t even evaluate a turbine’s energy output below 8 mph. The primary focus should be on the turbine’s total annual production potential matching the specifics of your site. 

 

Two excellent books for a comprehensive introduction to wind power are:
 
Wind Power Basics
by Dan Chiras

Wind energy is the fastest growing source of energy in the world, and by the year 2020 it is projected to supply at least 12 percent of global electrical demand. Wind Power Basics provides a clear understanding of wind and wind energy systems, including turbines, towers, inverters and batteries, site assessment, installation, and maintenance requirements. Whether you’re considering your own small-scale wind energy system or just want a straightforward, detailed introduction to the benefits and c…

 
Wind Energy Basics, Second Edition
by Paul Gipe

The availability of clean, renewable power is without question going to be the defining challenge and goal of the 21st century, and wind will lead the way.Internationally acclaimed wind energy expert Paul Gipe is as soberly critical of past energy mistakes as he is convincingly optimistic about the future. The overwhelming challenge of transforming our world from one of fossil carbon to one of clean power seems daunting at best—and paralyzingly impractical at worst. Wind Energy Basics offers a s…