October 29, 2010 | 13 Comments
John O. Dabiri at the California Institute of Technology’s Graduate Aeronautical Laboratories & Bioengineering makes the assertion that an order of magnitude increase in the energy harvest can be made with wind turbine density increases and wind turbine design. The full paper is available at arXiv.org > physics > arXiv:10103656. (Direct pdf download link here.)
Odds are, Dabiri is right. That would be a bit unsettling for those who can’t grasp wind as a viable alternative energy source. Its unsettling as the case against wind today is correct – its expensive, has an intermittancy and storage problems. Wind is good, but problematic. Increased density would solve part of the transmission line issue and allow closer less costly storage, however that matter comes to maturity. But without doubt – wind is here to stay, so any piece of the cheaper, better, more reliable issues solved is great news.
Wind harvesting requires substantial land resources in order to extract appreciable quantities of energy. This limitation of land use is especially acute in the case of wind energy, which currently faces an additional constraint in that the conventional propeller-style wind turbines seen in horizontal-axis wind turbines (HAWT) designs must be spaced far apart in order to avoid aerodynamic interference caused by interactions with the wakes of adjacent turbines. This requirement has forced wind energy systems away from high-energy demand population centers and toward remote locations including, more recently, offshore sites. There’s the transmission issue in a distance illustration.
To get to the 90% of the performance of an isolated HAWT, the turbines in a HAWT farm must be spaced 3 to 5 turbine diameters apart in the cross-wind direction and 6 to 10 diameters apart in the downwind direction. The overall performance of such wind farms, as quantified by the power extracted per unit of land area, is between 2 and 3 watts per square meter.
Dabiri and his colleagues suspected that counter-rotating arrangements of wind turbines whose airfoil blades rotate around a vertical axis (VAWT) can benefit from constructive aerodynamic interactions between adjacent turbines, thereby maintaining the performance of the turbines when in installed close up. By accommodating a larger number of VAWTs within a given wind farm area without adversely affecting the performance of the individual turbines, the power density of the wind farm is increased.
The field tests indicate that power densities approaching 100 watts per square meter can be achieved by arranging vertical-axis wind turbines in layouts that enable them to extract energy from adjacent wakes.
That number is now experimentally shown as factual. Take those results out to the world view calculation and the global wind resource available to small 10-m tall turbines based on the present experimental approach is approximately 225 trillion watts (TW), which significantly exceeds the global wind resource available to 80-m tall, propeller-style wind turbines, approximately 75 TW. This improvement is due to the closer spacing that can be achieved between the smaller, vertical-axis wind turbines. The results suggest an alternative approach to wind farming, in which many, smaller vertical-axis wind turbines are implemented instead of fewer, large propeller-style turbines.
All this from just an experiment.
Now this is only an experiment, that doesn’t address the practical limitation a direct evaluation of turbines surrounded on all sides by neighboring VAWTs, as would be the case for the majority of turbines in a wind farm. But a comparatively simple extrapolation can show using the experiment’s 1.2-meter diameter wind turbine set at 4 diameters apart with a conservative estimates for both the total aerodynamic loss in the array at a doubled 10 percent and the capacity factor (i.e. the ratio of actual power output to the maximum generator power output) set at a low 30 percent. The calculated power density for a VAWT farm with these parameters is still approximately 18 watts per square meter. This performance is 6 to 9 times the power density of modern wind farms that utilize the HAWT design. There might be something way wrong with plunging forward with those giant propeller wind turbines.
Even more concerning is it is straight forward to compute combinations of VAWT rated power output and turbine spacing that can achieve 30 watts per meter getting 10 times a modern HAWT farms output or even 200 watts per meter yielding 66 times the modern HAWT farm by using 1.2-m diameter VAWTs like those used in the study. Higher VAWT rated power outputs can be achieved by taller turbine rotors than the 4.1-m structures used in these experiments, and by connecting the turbine shaft to larger generators. Indeed, in initial field tests with 6.1-m tall rotors, the captured wind power exceeded the capacity of the 1200 Watt generator on each turbine.
The modern VAWT designs can really crank out the power.
Now this is just a seed, but the gauntlet is thrown between the horizontal shaft builders and the vertical shaft builders. Today the horizontal shaft builders are on a run. But investors, lenders, landowners and power buyers can’t remain the suckers for long. Something has to give, and a war of sorts is sure to come.
It will be interesting to see what imaginative words will support the two sides. Dabiri and his team have set out some very hard facts using elegantly simple experimentation and measurements. There are practical landscape issues and a wealth of inputs to consider, one being a vertical farm could install within a horizontal farm. Wind power is going to get far more interesting in new ways and fast. Lets go.