Vertical Wind Turbine - Some Basics (and a word of caution)

A vertical wind turbine or more accurately, a vertical axis wind turbine (aka. VAWT) is a specialized turbine design gaining popularity for deployment in the suburban residential, rural and some commercial wind power markets (see recommended guides, plans and kits below). They tend to be more easily installed than their horizontal cousins (HAWTs) and can have a much smaller footprint generally. They are so named because the main rotor shaft and axis is arranged vertically, as opposed to a horizontal propeller type rotor that faces into the wind to extract energy. A vertical wind turbine can be of two distinct types, distinguished by how it extracts energy from the wind (lift vs. drag). Just the same, all VAWTs are driven by wind striking them from the side (perpendicular to their rotational axis). This gives them the ability to harvest wind energy from various rapidly changing directions with no directional (yaw) adjustment of the mechanism (a key advantage and drawback). A relatively small footprint allows a vertical wind turbine to be installed on rooftops, in backyards, on or off towers (usually towers aren't used). Worldwide, commercial HAWT installations far out number VAWTs because HAWTs are generally considerably more efficient. Just the same, a closer look at the vertical wind turbine, its operation and components, will aid in understanding its appeal. If you are seriously considering a VAWT installation (especially at a suburban residential site) please be sure to read our word of caution at the bottom of this page.

Classic VAWT Designs

There are several basic design archetypes for the vertical wind turbine category. They are forerunners of today's modern vertical wind turbine designs. Differentiated by their rotor configurations (Darrieus, Giromil, Savonius) each fall into a category of deriving propulsion from aerodynamic "lift" or aerodynamic "drag". Each has its advantages and disadvantages. Note that these are not brand names. They are subclasses that various current day designs fall into. For each type, see the link for an image of a current day design and/or brand.

Darrieus "eggbeater" Design - Named for its creator Georges Jeans Mary Darrieus, this turbine essentially has two large curved airfoils (referred to as a troposkien shape) that resemble an "eggbeater". The Darrieus design is fairly efficient and can produce significant amounts of torque (if geared), rivaling that of a HAWT. In its basic dual bladed rotor design however, it develops torque ripple (or pulsations) corresponding to maximum and minimum instants of wind interaction, as well as rocking stresses, on the main lower bearing, that correspond to wind gusting pressures. These forces if not countered will greatly accelerate wear, even for small machines. Improved variations of this design use multiple blades to smooth the pulsations and an external guyed super structure to eliminate (or at least greatly minimize) structural oscillations. This design also requires a start-up motor because the pitch angle of the blade/foils are fixed at 0 degrees, providing very low start-up torque. Note that the Darrieus design uses aerodynamic lift for propulsion. The rotor can move at speeds greatly in excess of actual wind speed. Therefore, its relatively high velocity and rpm are ideal for power generation. See a larger image of this commercial scale "eggbeater" vertical wind turbine design (courtesy Sandia National Labs) here. This design is not commercially adopted on a large scale due to its inherent weaknesses (blade fatigue, high bearing stress and wear, etc.).

Giromil (gyro-mill) - or H-bar rotor type, classically has two vertical airfoil/blades symmetrically arranged about the turbines central axis. Torque ripple can be a problem for dual blade designs, but multiple blades are an effective counter-measure. This design also uses aerodynamic lift for propulsion with the added advantage that its straight foils can be pitched to overcome startup inertia and set for maximum lift. For a larger image of the classic Giromil click here. For an image of a modern day small scale equivalent being installed (manufactured by Mariah Power: model Windspire) click here.

Variable pitch foils are used on cyclo-turbines, a Giromil variant, (click here for image) that can automatically adjust their blade angle of attack (using a directional vane or other means) for optimum and consistent torque output. They are also able to self start by adjusting the blade to "drag" the wind, thus gathering initial start-up momentum.

Another variant is the helical rotor turbine. This is essentially a Giromil with blades (typically three) canted at a 60 degree spiral about the center axis. The spiral eliminates torque pulsations and will also extract power from vertical updrafts. Due to their relatively low mass, these turbines are capable of very high rotor velocities. Unfortunately, this also accelerates mechnanical wear for all but the toughest bearing materials.

Savonius Turbine - Invented by the Finnish engineer Sigurd J. Savonius in 1922, the classic design uses scoops (or for more efficiency, cups), symmetrically arranged about a central axis, to drag the wind capturing its kinetic energy directly. With a typical efficiency of about 15% , it is much less efficient than a Darrieus design. Still, Savonius designs develop excellent torque and are well suited to applications such as water pumping and grain grinding. Note, this vertical wind turbine type can never develop rotor speeds above that of the driving wind velocity. To see a helical Savonius turbine design (manufactured by Helix Wind) click here.

In the past few years, a number of new manufacturers have come out with variations on the turbine designs above intended to make vertical wind turbine installation simpler and viable for tightly packed suburban and urban sites. Most are mounted near ground level to take advantage of (slower) swirling wind turbulence and facilitate maintenance. In addition, many modern designs are quieter, and have smaller overall footprints (with or without a tower).

Vertical Wind Turbine Guides, Plans and Kits

Build A Wind Generator - There are several guides that offer information for generating residential power through wind turbines (such as Earth4Energy or GreenDIYenergy). However, these guides provide wind power technology information as a secondary offering or after-thought, focusing primarily on solar enegy production. We feel that "Living Off The Grid - Build A Wind Generator" offers first rate practical, usable, field tested information that concentrates primarily on wind power generation, with solar power as a supplementary method. Their main objective is to help the reader go off-grid with wind power, should they so desire. They even share resources for acquiring free equipment and provide free updates to their material. It's an excellent value.

Now, this is not to say that the other guides offer less than useful information. Nothing could be further from the truth. Earth4Energy and GreenDIYenergy are both very usable solar guides that provide wind power information as a supplement. If your primary interest is focused on solar energy systems you really can't go wrong with either of the two guides (although we give the edge to GreenDIYenergy for it's breadth and completeness)

Other Resources:

We would be remiss if we didn't point out that VAWT kits and product lines are currently, at best, a developing technology (even some well known products fail to complete independent performance tests, as can be seen here). The best VAWT product can in no way compete with the energy output efficiency of a similar HAWT (at least not yet!). It's a simple fact of physics due to differences in how the two devices harvest wind energy. Therefore:

* * * A * W O R D * O F * C A U T I O N * * *

When considering a VAWT we recommend extreme caution about any power production claims. Many VAWTs can produce power. That isn't the issue. The problem is many suppliers make performance claims that are misleading or that are simply not true. In addition, reliability, noise pollution and durability performance has been poor for this device class.

We strongly recommend that you protect yourself with knowledge. One reference work that provides excellent information is Paul Gipe's, Wind Power: Renewable Energy for Home, Farm, and Business, 2004 Revised Edition or his "Wind Energy Basics, 2nd Edition May 2009" (See the "Other Resources" book carosel above). Each contains very detailed information about vertical and horizontal turbines and when to use them. But don't take our word for it, click on the links above and see the customer reviews (try the book preview sample tool, it's useful). We feel that from an economic standpoint, it's just prudent and good sense to consider spending $31.50 or $19.77 to protect tens of thousands of dollars that could be tied-up in a VAWT investment.

At the very least, before completing a purchase, investigate as to whether or not the unit has been independently tested by one of the industry research groups (NREL, AWEA, SWCC, etc...). If it hasn't, again, caution is warranted. Is the amount of money you're about to spend worth demanding independent verification of the vendors product performance claims? We say, "Absolutely!!". These wallet safeguarding steps could save you a huge chunk o' change (and your sanity). Good luck.