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Opinion: Are "school of fish" turbine arrays a red herring?

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July 23, 2013

John Dabiri has hypothesized and tested counter-rotating arrays of vertical axis wind turb...

John Dabiri has hypothesized and tested counter-rotating arrays of vertical axis wind turbines based on fish schooling vortices

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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.

Graphic from Dabiri's 2010 paper showing the insights from vortices from fish fins in scho...
Graphic from Dabiri's 2010 paper showing the insights from vortices from fish fins in schools to VAWT array placement

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.

Dabiri's test array of modified Windspires in Antelope Valley, northern Los Angeles County
Dabiri's test array of modified Windspires in Antelope Valley, northern Los Angeles County

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.

Table from Dabiri's 2010 paper showing source of calculations for energy density are wind ...
Table from Dabiri's 2010 paper showing source of calculations for energy density are wind farms world wide covering a variety of terrain and intermingled uses

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.

This is a more typical energy density picture for wind farms with other forms of agricultu...
This is a more typical energy density picture for wind farms with other forms of agriculture, in this case corn, and land use intermingled with wind turbines (Photo: Zepfanman

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.

About the Author
Mike Barnard Mike Barnard is Senior Fellow - Wind, with the Energy and Policy Institute. He has been a deeply interested observer of energy systems for three decades. His work as a business and technical architect on large initiatives in a variety of domains gives him the systems thinking perspective and stakeholder analysis skills to engage effectively with an area as complex as the grid. He’s regularly asked to peer-review academic and non-academic publications related to wind energy by journals, organizations and individuals.   All articles by Mike Barnard
18 Comments

The problem is that the wind shifts slightly and instead of the wake turbulence enhancing the airflow over the next turbine it is disrupting it.

Slowburn
23rd July, 2013 @ 08:41 pm PDT

The main issue with egg beater technology is not aerodynamics. The main issue is keeping the sand out of the massive thrust bearing that holds these things up. That has always been the main issue.

dchall8
23rd July, 2013 @ 08:56 pm PDT

Thanks for the review, Mike. Overall I found it quite reasonable, though we obviously reach different conclusions about the merits. Four points of correction:

1. Sandia's experimental program studied individual VAWTs, not arrays.

2. You state "He found that, unsurprisingly, differences in wind directions diminished the performance advantages for downwind VAWTs." In fact, we found the opposite: pairs of VAWT are largely insensitive to wind direction. See, e.g., figure 4a in the 2011 paper.

3. The statements "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" and "This solution comes at the expense of higher engineering costs, and greater visual, radar and environmental impacts" come from the same 2011 paper. In fact, they come from the same paragraph in that paper. The article gives the reader the incorrect impression that there has been a shift in our motivation for VAWT arrays, which is a bit disingenuous given the aforementioned placement of those statements. In any case, I think we probably agree that the most important metric is the levelized cost of the electricity.

4. You refer to "the level laminar flow necessary for his arrays to be most effective." In fact, the VAWT arrays we study in the field are in higher levels of turbulence than typical HAWT farms. Turbulence tends to decrease with altitude, so being closer to the ground (e.g. at 10 m vs 100 m) means turbulence is unavoidable.

These points aside, thanks again for providing a critical eye. As you know, wind energy has lots of snake oil salesmen, so I think your skepticism is healthy for the field. It's also why we are careful to limit our claims to what we can demonstrate in the field and publish in the peer-reviewed literature. I'll keep you posted on our current field test (now up to 24 VAWTs), which continues to support our hypothesis. More to come...

JOD

jodabiri
23rd July, 2013 @ 09:07 pm PDT

The problem with VAWTs is that they can only harness the wind's energy at certain points of their rotation, and all other angles are at less than optimal efficiency. HAWT blades are harnessing at maximum efficiency no matter their rotation, so long as they're pointed into the wind. Hence, you have to compensate with more of them, which increases the ground footprint of the installation, the cost, and the noise.

Joel Detrow
23rd July, 2013 @ 09:44 pm PDT

How about wind farms built on man made lakes such that fish farming can take place? One could even open the bases of the windmills such that fish could use the submerged portion of the base as habitat. Servicing the windmills could be done by boat. Awnings with solar cells could shade much of the lake so that we can grow good food, gather wind and solar power all in the same space.

Victory is as easy as we allow it to be!

Jim Sadler
24th July, 2013 @ 10:26 am PDT

Why VAWT's are less sensitive is the sad fact they can't capture much of the wind's energy. Thus one downwind would have of a VAWT would have wind with more energy than downwind of a HAWT.

The facts are VAWT's take 3-10x's the material with far more parts, failure points to get 1/3 of the energy the same area of a good 3blade HAWT would that costs only 10-30% of a similar output HAWT.

I've done VAWT's and found they are great as linear ones on sailboat/the sails. But for generating electrical power from the wind you need torque and speed to get power. VAWT's lack speed.

The sad thing is all these VAWT people know this too. An even worse it costs 75% less to do it right in a HAWT!!

Nor is this rocket science as they perfected small WT's in the 30's before subsidized utility power killed them off. Many of them are still running today and are prized power producers. Why can't they do that now cost effectively? They are really simple machines.

jerryd
24th July, 2013 @ 11:46 am PDT

I have followed Amory Lovins since 1983. In the field of energy he is the top scientist/engineer. So if he advocates decentralized (distributed), I will bet on it. However, this is a very complicated subject. As technology changes, the best solution changes.

We can be sure of two things remaining constant: 1. Govt. interference never helps, and sometimes makes progress impossible. 2. No "energy problem" would exist in the first place if energy suppliers operated on a level playing field, i.e., in a free market.

voluntaryist
24th July, 2013 @ 02:09 pm PDT

I can't help but wonder if gobbling up wind energy will at some point begin to have climatic effects. I wonder the same about current/tidal turbines in bodies of water. Both will certainly have local effects. Just sayin'.

Fritz Menzel
24th July, 2013 @ 07:42 pm PDT

@Fritz, I agree with your sentiment. Even by tapping Solar power you are stopping a portion of the environment of absorbing the normal amount of heat. - It is just one of life's little general rules: there is no such thing as a free lunch.

When alternative energy gets going in a BIG way, then it will become more visible.

In whatever way we generate our energy currently we will be having some side-effect, but the burning of fossil fuels is probably the worst offender. Our challenge is to spread our generation of power such that it is having a minimal impact on world and its environment.

Riaanh
25th July, 2013 @ 05:06 am PDT

Why is Mike so eager to discredit new technology? Does he work for a wind turbine manufacturer? In the interests of full disclosure I would like to know. John is a dedicated researcher in search of better methods of energy extraction from the wind. His published results should have everyone in the wind industry buzzing. Imagine having the potential to extract 10 times more energy from a given area of land using his practices.

Mike says 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. This may be of importance to the farmers that own the land, but it is of little concern to a wind farm developer who has to build roads and transmission lines to turbines spaced far apart. Subsequently, other wind farm developers must

settle for secondary locations with lower wind, which only increases the Levelized Cost of Energy.

I think John is doing great work and I am anxiously awaiting the results from his current projects.

Lux Wind
25th July, 2013 @ 11:55 am PDT

Mike, I agree with all your points. There are some areas where Dabiri's biomimicry ideas hold promise and others where they fall flat. His work on VAWTs is the latter. Plus, from an avian/bat perspective, his tight array is a step in the wrong direction. Imagine the poor critter stumbling into that tight array at night.

CliffG
30th July, 2013 @ 06:53 pm PDT

.."I don't mind critter carcasses", says the coyote,"it feeds the pups".

Don't higher rates of avian/bat mortality correlate with conventional HAWTS on taller towers and higher rpm/tsr (blade tip speed specifically) instead of the slower tsr, lower altitude VAWT?

@JOD- great work and research! any experimental data/results to release this year on the VAWT array? is the study you are conducting equipment and geometry/planform sensitive to array performance? I ask because the equipment maker referenced in the opinion piece listed, Windspire, is now defunct. Would array performance vary or is the array design resilient to designs: Savonius vs Darriues or vs Helical. And one more question-did the pilot trial require an avian take permit/monitoring or is that a non-issue?

Definitely need more diversity (Big/Small) in RE equipment and I hope your research opens new frontiers.

Big Wind, while the standard, cannot alone move the needle on the Keeling Curve. It is glaringly obvious. Only a single turbine commissioned in the US (first 2 quarters 2013-source AWEA) at 1.6MW.

@Mike-can you answer Lux Wind's question to clear the air, no?

AeroFox-Full disclosure: I work the trails in Appalachia country (a few states). Farmer J.Rock. and his ranch hands put a big moat around the farm. So had to make some wings to get those chickens.!

AeroFox
31st July, 2013 @ 05:36 pm PDT

oops and I forgot this one. I won't call it gospel, since I am not religious, but if I were, this would be the words to work by..

http://www.motherjones.com/politics/2008/05/power-qa-amory-lovins

pretty good stuff.

AeroFox
31st July, 2013 @ 07:30 pm PDT

@AeroFox: We're finishing up a 2-year study of an 18-VAWT array. Since we're testing at full-scale in the field, we need several thousand hours of data in order to achieve converged statistics for the array performance at different wind speeds and directions. I can say preliminarily that the results are consistent with what we saw in the published study of the 6-VAWT array. But I'll wait for the paper to come out in the peer-reviewed literature so you don't have to take my word for it :) Given the typical length of the review process, the paper probably won't be out till next year.

Our research is not constrained to a particular VAWT design (which is important, because the Windspire has serious reliability issues), though it does appear that a TSR in the 2-3 range is preferable to lower values in order to achieve beneficial wake dynamics/interactions. So, I don't think a Savonius rotor would be as effective. But we haven't tested any yet.

At 30 feet and with the TSR/blade speeds of the VAWTs, the risk to birds/bats is significantly reduced. We've had no animal strikes in our array since we began field testing in 2010, but to be fair our array is relatively small. We're in early stage planning for a formal study next year, funding permitting.

jodabiri
3rd August, 2013 @ 10:46 am PDT

A combination of triangle arrayed VAWT and vertical axis triple tier sails with opposing aerlion control placed in the center of each triangle may work better.

GRich
3rd August, 2013 @ 05:00 pm PDT

@JOD. Thanks for taking the time to reply. No peer review is necessary for me to understand it, but always good to have. Your work speaks for itself as it does with other researchers who spend an inordinate amount of time to compress thought into code and then diffuse code into a physical product or high performance array.

Impressive stuff!

AeroFox
6th August, 2013 @ 02:45 pm PDT

I am wondering if same number and same power rate of small horizontal wind turbines are used to replace the vertical wind turbines., will produce more Kwh or less?

Can someone give me some idea?

Thanks a lot!

WayneLee
11th August, 2013 @ 07:25 am PDT

@JOD: I would like to specifically ask you a question. First, let me say that the primary detractor for this type of array is its niche market. Currently, an extensive VAWT array requires much more ground space than the equivalent HAWT wind farm. Therefore, I have a design question for you:

Is it possible that, in R&D, there is an air-based design. What I mean by this is a design that uses a triangular support system anchored by 3 footpoints, the same or similar in spacing than the equivalent HAWT wind farm, but that supports a VAWT network array up above between those three points. I'm not sure that's entirely clear, but I can't exactly draw pictures here. Picture a VAWT array in a triangular form, floating above the ground. Then connect all the VAWTs together and anchor the array to the ground at 3 points. Hopefully that will be clearer. Any ideas?

-Jason

Jason Shepard
19th October, 2013 @ 02:16 pm PDT
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