Multi-axis turbines?

02.17.07 | 1 Comment

Traditional wind systems have had either a horizontal axis with a single blade (95+% of what is in use today) or vertical axis with a single blade (think large egg beater anchored to the ground.) A company called Mass Megawatts Wind Power Inc. is advancing the application of multi-axis vertical turbines, that is many blades mounted vertically in a box-like array around 20 meters off the ground.

The thinking behind this technology is that with more smaller blades, lower wind speeds will be able to output power than in traditional horizontal turbine systems. The design has two generator modes, one 7.5 kilowatt that cuts in at under 4.5 meter/second (~10mph) and another 22.5 kilowatt that cuts in at 6.25 meters/second (~14 mph.) Each unit has 32 generators attached to 32 shafts with blades. That means in a 6 m/s wind, 100% output would be around 864 kilowatts/hour and in a 8 m/s wind, 100% output would be 2,592 kilowatts hour. The image above is a smaller prototype system.

Wind systems rarely operate at 100% capacity, in fact, the average capacity factor is 30% across the installed wind farms. If we take this into account, the MAT system would produce somewhere between 2,267 and 6,811 megawatt hours per year depending upon which generator has the dominant share of time in action. At the prevaling wind rate of $45/MWh, that gives a revenue spread per unit per year of $101,015 on the low end to $306,495 on the high end. The acquisition and construction cost of each unit is estimated (by Mass Megawatts) to be $210,000 per unit. If the MAT unit performs as above and maintenance costs don’t kill it, it might be economically viable.

The science of harvesting kinetic energy from the wind is pretty straight forward, the density of the air moving through the swept area of the turbine at some speed dictates how much energy can be harvested. The formula is: Power = 0.5 x Swept Area x Air Density x Velocity3. Without accurate measurements of the blades on the MAT we can’t make the calculation, but at first blush, the swept area doesn’t look sufficient to produce the stated power levels. This is physics, the equation has to solve so if the area is smaller, then the velocity has to increase and/or air density has to increase.

The other claim made by the MAT folks is that the maintenance costs are lower. I’m high skeptical of this as basic physics dictates the more moving parts an object has, the more opportunities for things to break. In a traditional horizontal turbine there is one blade assembly, one gearbox, one turbine etc you get the picture. On the MAT, there are 32 turbines, 32 shafts, who knows how many blades; it’s at least 32x more likely to experience failure in a given time frame. Now, with the horizontal turbine, a failure tends to be complete. In a MAT, that’s not necessarily so, if 1 shaft/generator/blade combo goes, theorhetically, the other 31 are still in business.

I’m all for experimentation and open minded that breakthroughs can and do happen, but I don’t think we’re seeing one here. A part of me hopes that there is something to this, but the evidence seems to suggest that this design introduces more problems than it solves. We’ll see as Mass Megawatts installs their wind farm, the business plan calls for 100 units to be installed this calendar year. Proof will be in production and we’ll be able to see that as a result of their quarterly reports. We’ll be watching with interest.

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