Opinion: Are "school of fish" turbine arrays a red herring?
Since we first looked at John Dabiri's hypothesis that vertical axis wind turbines should be arrayed like a school of fish to reduce the land area required for wind farm installations, the MacArthur Genius Grant recipient has continued to work on the idea. Following the latest round of coverage, Gizmag takes a deeper look at his concept, and wonders whether the idea of packing turbines into as tight a space as possible might overlook some wind energy fundamentals.
Dabiri's interest in wind energy became public knowledge with his 2010 paper [PDF] in the peer-reviewed journal Biomimicry and Bioinspiration. In it, he hypothesized that close spacing of vertical axis wind turbines (VAWTs) in an array would allow greater energy densities due to vortices enhancing the actions of downwind turbines in a manner similar to the fins of schools of fish. He found and reviewed the prior art from both a 2004 patent on paired VAWTs and 1990 assessments done by researchers associated with Sandia National Laboratories, world leaders in wind research and innovation. In general, any new innovation in wind energy that you read about will have already been investigated in some depth by Sandia at some point in the past four decades. While Dabiri has been transparent about prior attempts in the same space, press reports have typically not pointed out previous attempts at arrays of vertical axis wind turbines that have gone nowhere.
Dabiri's enhancement to the previously studied concept of an array of VAWTs was to include counter-rotating turbines in the mix based on an understanding of the way in which schools of fish took advantage of leading fishes vortices for greater efficiency in swimming. This is a well known concept at a superficial level that is visible in the V formation of flying geese or even of a peleton of bicycle racers drafting one another in the Tour de France. Dabiri's application of the deep math from fish schooling studies to the modelling of this indicated stronger potential.
Dabiri followed up this work with an experimental array of six VAWTs in a field in Antelope Valley in Northern Los Angeles County between June and September of 2010, publishing his results [PDF] in the Journal of Renewable and Sustainable Energy in July 2011. He used modified commercial VAWTs from Windspire, with support from that company. Windspire's VAWTs individually perform similarly to all vertical axis turbines, at levels of efficiency below that of horizontal-axis wind turbines. They have struggled in the relatively crowded small wind marketplace as a result.
He found that, unsurprisingly, differences in wind directions diminished the performance advantages for downwind VAWTs. The conceptual breakthrough of counter-rotation, after all, requires the counter-rotating blades to be in line with the right vortices from upstream blades. Schools of fish achieve this by following one another when direction changes. When the wind shifts by as little as 10 degrees, the necessary alignment diminishes. The results still show promise for small arrays of VAWTs generating power usefully when close together, as the overall performance was higher than individual VAWTs standing by themselves.
However, this is where a shaky premise starts to undermine his findings. In his own words in his 2011 paper, "existing renewable energy technologies require 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."
As those who follow energy systems know, there has been a long-running argument about whether highly centralized energy generation or distributed generation is more effective. Amory Lovins of the Rocky Mountain Institute is a primary proponent of distributed generation, and has been for decades. Opposing him intellectually have been the voices of traditional centralized generation, especially theorists associated with nuclear energy. Modern wind energy is arguably a pragmatic mix of the two ideologies with broader distribution of capital intensive, utility-scale wind farms instead of the more ubiquitous, smaller generation that Lovins envisioned. Naysayers of modern wind energy use an argument of energy density, the land required to generate a unit of energy, to assert that, for example, nuclear energy is better than wind energy. James Lovelock, in his 2007 book The Revenge of Gaia, pushes this point, arguing that it would take "1,000 square miles of countryside to provide enough land for a 1 gigawatt wind-energy source."
But as the National Renewable Energy Laboratory (NREL) points out in its land use guidelines, modern wind farms take up less than 1 percent to a maximum of 2 percent of the land that they are spread over. The base of a large modern wind turbine takes up about 10 percent of a hectare or 25 percent of an acre. Access roads take up more space, and are usually gravel. Wind farms in flatter landscapes, as opposed to ridge line wind farms, typically lease this small amount of land from land owners in return for anywhere from US$6,000 to $16,000 per year depending on the country. The rest of the land around the wind turbines typically continues to be used for whatever purpose it previously had, whether that is cultivation of crops, grazing of livestock or providing unused green space.
Dabiri's calculations of energy density assess the total land wind farms sit upon. In his 2010 paper, he averages the land area of eight wind farms from around the world, including ridge line wind farms, to arrive at an order of magnitude better energy density for his closely spaced arrays than for traditional wind farms. However, wind farms don't consume all of the land they spread over. At 1-2 percent land use, traditional wind farms currently approach an order of magnitude better effectiveness than Dabiri's still theoretical VAWT arrays, and as they increase in size and output, power goes up exponentially while spacing goes up in a straight line, increasing the gap between the HAWT approach and the still hypothetical array approach.
After all, his closely spaced VAWTs preclude almost every other potential use for the land. As inevitable realities intrude, Dabiri's envisioned VAWT arrays will likely become a very useful niche generation technology for specific areas where horizontal axis turbines are precluded. His more recent paper seems to acknowledge this, as he has started including other perceived problems with modern wind farms. "This solution comes at the expense of higher engineering costs, and greater visual, radar and environmental impacts," he writes. Most of these concerns have underpinnings at least as nuanced as his land density argument.
As for the limitation on placement, the interaction model Dabiri posits will likely be significantly challenged in a primary placement location for modern wind farms, ridge lines and sea shores, where the rise of the wind due to ridge line compression disrupts the level laminar flow necessary for his arrays to be most effective. As such, his approach is likely limited to the areas where wind farms have the greatest mixed use potential, limiting advantages further. This in turn likely diminishes his purported advantages, as the model is competing with wind farms that consume less than 1 percent of the land upon which they sit due to pre-existing farm roads and cultivated fields.
The 33 year old Dabiri is a compelling and brilliant character. He's richly deserving of the rare honor of a MacArthur Genius award. His 55 peer-reviewed papers (some still undergoing review) and 70 invited lectures to date show his deep understanding of fluid dynamics. Though his ideas in relation to wind energy are probably not going to pose a threat to currently established technology, his work in this space still bears watching.