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Study suggests arrays of closely-spaced counter-rotating vertical-axis wind turbines could enhance power density of wind farms by up to an order of magnitude

18 July 2011

Dabiri
Dabiri suggests using optimized arrays of smaller vertical-axis wind turbines to increase the power density and reduce the cost, and visual, acoustic, radar and environmental impacts, of wind farms using giant horizontal-axis turbines. Source: Dabiri 2010. Click to enlarge.

Research by Prof. John Dabiri at Caltech suggests that using counter-rotating vertical-axis wind turbines (VAWTs) arrayed in layouts that enable them to extract energy from adjacent wakes and from above the wind farm could potentially achieve power densities (watts of power per square meter of land area) an order of magnitude greater than those of wind farms consisting of horizontal-axis wind turbines (HAWTs).

Moreover, Dabiri notes in a paper in press in the Journal of Renewable and Sustainable Energy, this improved performance does not require higher individual wind turbine efficiency, only closer wind turbine spacing and a sufficient vertical flux of turbulence kinetic energy from the atmospheric surface layer.

Modern wind farms comprised of horizontal-axis wind turbines (HAWTs) require significant land resources to separate each wind turbine from the adjacent turbine wakes. This aerodynamic constraint limits the amount of power that can be extracted from a given wind farm footprint. The resulting inefficiency of HAWT farms is currently compensated by using taller wind turbines to access greater wind resources at high altitudes, but this solution comes at the expense of higher engineering costs and greater visual, acoustic, radar and environmental impacts.

...To maintain 90 percent of the performance of isolated HAWTs, 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 power density of such wind farms, defined as the power extracted per unit land area, is between 2 and 3 W m-2.

Wind turbines whose airfoil blades rotate around a vertical axis (i.e. vertical-axis wind turbines; henceforth, VAWTs) have the potential to achieve higher power densities than HAWTs. This possibility arises in part because the swept area of a VAWT rotor (i.e. the cross-sectional area that interacts with the wind) need not be equally apportioned between its breadth—which determines the size of its footprint—and its height. By contrast, the circular sweep of HAWT blades dictates that the breadth and height of the rotor swept area are identical. Therefore, whereas increasing HAWT rotor swept area necessarily increases the turbine footprint, it is possible to increase the swept area of a VAWT independent of its footprint, by increasing the rotor blade height...The power density of the VAWT design is more than three times that of the HAWTs, suggesting that VAWTs may be a more effective starting point than HAWTs for the design of wind farms with high power density.

—Dabiri 2011

Dabiri hypothesized that counter-rotating arrangements of VAWTs can benefit from constructive aerodynamic interactions between adjacent turbines, thereby mitigating reductions in the performance of the turbines when in close proximity. By accommodating a larger number of VAWTs within a given wind farm footprint, the power density of the wind farm is increased.

Dabiri and his team have been conducting a field study at an experimental two-acre wind farm in the Antelope Valley of northern Los Angeles County, California. Dabiri’s experimental farm—the Field Laboratory for Optimized Wind Energy (FLOWE)—houses 24 10-meter-tall, 1.2-meter-wide VAWTs. Half a dozen turbines were used in a 2010 field test.

The turbines were a modified version of a commercially available model (Windspire Energy Inc.) with 4.1-m span airfoil blades and a 1200 W generator connected to the base of the turbine shaft. Three of the turbines rotated around their central shaft in a clockwise direction (e.g. from a top view) in winds above 3.8 m s-1 ; the other three rotated in a counter-clockwise direction when the wind speed exceeded the same threshold (which Dabiri calls the cut-in wind speed).

Averaged over the 48.6-m2 footprint of the six-turbine VAWT array, the daily mean power density produced by the array varied from 21 to 47 W m-2 at wind speeds above cut-in and 6 to 30 W m-2 overall. This performance significantly exceeded the 2 to 3 W m-2 power density of modern HAWT farms, despite the relatively low mean wind speed during this set of field tests (5.7 m s-1), Dabiri noted.

To extrapolate the present measurements to larger VAWT farms, we considered the present VAWT diameter (1.2 m) and inter-turbine spacing (4 diameters), and we made conservative estimates for both the total aerodynamic loss in the array (10 percent) and the capacity factor (i.e. the ratio of actual power output to the maximum generator power output; 30 percent). The calculated power density for a VAWT farm with these parameters is approximately 18 W m-2. This performance is 6 to 9 times the power density of modern wind farms that utilize HAWTs.

Furthermore, it is straightforward to compute combinations of VAWT rated power output and turbine spacing that can achieve 30 W m-2 (i.e. 10 times modern HAWT farms) by using 1.2-m diameter VAWTs like those studied here. 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 W generator on each turbine.

—Dabiri 2011

Having every turbine turn in the opposite direction of its neighbors, the researchers found, also increases their efficiency, perhaps because the opposing spins decrease the drag on each turbine, allowing it to spin faster (Dabiri got the idea for using this type of constructive interference from his studies of schooling fish).

We’re on the right track, but this is by no means “mission accomplished”. The next steps are to scale up the field demonstration and to improve upon the off-the-shelf wind-turbine designs used for the pilot study.

—John Dabiri

This summer, Dabiri and colleagues are studying a larger array of 18 VAWTs to follow up last year’s field study.

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July 18, 2011 in Power Generation, Wind | Permalink | Comments (32) | TrackBack (0)

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Potentially a game changer - much smaller turbines giving much higher power in the same amount of space (3-10X). Great upgrade for existing terrestrial farms, and new ones where the visual pollution of the tall towers was prohibitive.

VAWT's have been hyped a lot over the years, over and over again. But in real life they were huge disappointments. That's why HAWT's reign supreme.

So forgive me for being sceptical. I'll postpone my enthusiasm for when it is independently confirmed with a real, commercial windfarm.

By the way, the footprint of a windfarm is a question of definition. The space occupied by the turbines is very small, most of the land is available for agriculture and therefore not wasted. The closer spacing of the turbines will simply squeeze the same footprint in a smaller area. The main advantage is a reduced visual impact on the landscape.

Oh, and another thing. This test windfarm is small. I wonder if it scales to hundreds of turbines.

There is a limit to how much energy you can extract from the wind over a given area. This limit is the average kinetic energy in the air flowing past your windfarm.

Those wind turbines are very small. Vertical 1,2 kW wind turbine vs 5 MW commercially viable horizontal. 4000 times smaller!!! It looks like a baby toy vs Ferari.
Somehow was published many developments of vertical commercial wind turbines but in fact no one in operation since.

Wind velocity and reliability increases markedly at heights above 50 metres, twice the average wind velocity gives 8 times the average power, so tall HAWTs will always have that advatage over low-lying smaller VAWTs.

The biggest problem is not the footprint it take but what to do with the electricity obtained. Most wind turbines of today are almost useless because they don't harness efficiently the electrical output and cannot match it to the grid, the impedance don't match the constant impedance of the grid.

Only audi with their e-gas and hydrogen project have a correct and efficient method of harnessing the wind energy. They also add mass(water and hydrogen) to obtain high output and efficiencies contrary to actual wind farm that obtain quasi negative outputs trying to match the grid with a faulty method.

Anne, the reason VAWTs were huge disappointments was that they were seen as an alternative to HAWTs and put into places a HAWT should go. The strengths and weaknesses of the two are different and we should put the right turbine in the right place.

A.D.

What are you trying to say with that 'impedance' thingy? Do you know what you are talking about? All wind turbines adhere to the electrical standards from the grid operators.

Your claim that most wind turbines today are useless is not based in fact. Most countries (like Denmark, Spain and Germany) produce data about wind power production. It is easy to deduce from these numbers that your assertion is incorrect.

The need for storage is much overrated. The penetration of variable resources is still relatively low and existing CCGT and hydro can easily compensate. At least for the next 1 or 2 decades. I always point to the nice live charts that the Spanish grid operator publishes and where you can see in real time how they combine variable resources, variable demand, and dispatchable resoures to balance production and consumption. https://demanda.ree.es/generacion_acumulada.html

ai_vin,

As far as I know, that is not true. They were mostly placed on rooftops, because they were supposed to better deal with the turbulent wind around buildings. Still, they did not live up to their promise (= marketing blabla). I still think the weaknesses of the VAWT (low yield, expensive) vastly outweigh the strengths (less susceptibilty to turbulence).

Ignore A.D. If he's human (e.g. not some troll running a Markov chain generator), he's incompetent and not well connected to reality.

Some of Dabiri's claims may be technically correct (he admits in the paper that his major claims are theoretical, not proven by test results) but in practice are meaningless. The figure of merit isn't watts per square meter, but watts per resources invested (dollars, euros). Also, a forest of VAWTs spinning at high RPM represents a much greater danger to birds and bats than a much smaller number of very large HAWTs with blades passing by at much longer intervals.

Dabiri's biggest claim, that VAWT's are more efficient per unit of area than HAWT's, is based on his apples-to-oranges comparison of the HAWT's full rotor area with the VAWT's ground footprint (Table 1). An apples-to-apples comparison of rotor area facing the oncoming wind stream reduces the VAWT's power density to 127 W/m^2. The VAWT is not a contender.

I have seen several vertical designs in the Altamont Pass
area. Vertical designs do seem to be more prevalent and probably produce more power.

I think ai_vin has a valid point. Some locations may not be suited to larger vertical wind turbines. This idea could have some merit and should not be totally dismissed.

I meant to say horizontal designd produce more power per unit. It would depend on turbine cost and land cost. If aesthetics were important on expensive land, then this idea could have merit.

Good points, ai_vin and SJC. It seems that Prof. John Dabiri at Caltech is on to something, but not as a direct competition to the MegaWatt-size HAWT. These humongous HAWT structures are very efficient and very cost-effectively bringing down the price of wind electricity on par with coal-fired electricity.

His VAWT idea is best deployed in urban areas in order to produce wind electricity right where the bulk of electricity is consumed. Thus, even if VAWT is more expensive per unit of output than HAWT, the cost of powerlines would be nil, cost of transportation to sites would be far less, and maintenance is far easier when deployed within vicinity of a city.

Someone will need to calculate the entire cost of wind turbines part and installation labor + powerline installation cost + maintenance cost = total cost/ kWh for each HAWT and VAWT concepts.

I envision someday that all roofs will have mounted PV panels supplemented by an array of VAWT producing DC voltages to be converted to AC via power inverters, with optional BEV-worn-out array of storage batteries. In winter, output for PV panels will plummet while output from wind turbines will pick up...these will really put big dents in the consumption of fossil fuel energy and will be our main hope for minimizing the looming AGW catastrophe. Imagine also the tens of millions of jobs will be created locally in installing and maintaining these things...Any politician should love that!

Many built-up areas lack air circulation in the amounts necessary to e.g. remove pollution and excess heat. Sucking out the energy of the local winds would not make these places better.

Good point, EP. The trouble is, summer time, the wind hardly blows anyway, when we have the most pollution in the form of ozone and heat from all the concrete surfaces. Transforming from petroleum-burning vehicles and coal burning power plants to BEV's, PHEV's, and renewable energy will greatly reduce if not eliminate local air pollution.

In the winter, ground-level ozone is almost non-existent, no heat problem, and the wind often blows hard. Perhaps city building code should take into account the issue of local air circulation before granting each VAWT permit.

There was a vertical design that was mounted horizontal on tall building roof tops. It looked like it could be used in several locations. The building roofs were at elevation, so no tall masts were needed and they were not seen from street level.

And the available wind energy at such altitudes close to roofs was, on most occasions, zero.

Here's an idea for urbin use of VAWTs I found; http://www.nlarchitects.nl/slideshow/71

It's a slideshow so just click through the small ovals beneath the picture.

Wind has a place in the portfolio - though it will find it increasingly difficult to compete with lower cost, more reliable sources of sustainable energy.

Thanks, ai_vin, for the referenced slice show of very aesthetically rendered versions of urban VAWT.
The specs look real good. For example, for the size of a large tree, a 12-unit tree-shaped VAWT array on a single tree-like trunk is rated at 48 kWh. Yearly production at 5m/s average wind speed is rated at 55,000 kWh, enough for 4 average-size houses. If the whole tree-like VAWT structure is painted green, it will blend right in with the landscape, and no visual aesthetic issue whatsoever. The ground-level noise level at 12m/s windspeed is barely a whisper at 48 db. The wind itself at 12m/s makes noise against the trees at that speed, so the 48 db will blend right in and hardly audible.

Overall, very promising rendering. Some birds will be hurt initially, but natural selection over time will select for those birds who can successfully avoid the whirling blades and will proliferate, so the death rate of birds will decline over due time. We have no choice but to save our human specie from the looming catastrophic of AGW first!

Hi Reel$$.
Could you please elaborate with peer-reviewed references and numerical data on "lower cost, more reliable sources of sustainable energy." as you mentioned.

Wind energy is supposed to offer electricity as low as one can get, on par with coal-fired electricity. With cut-in wind speed of 3.5 m/s and cut-out wind speed of 30 m/s, those VAWT design in the referenced slice show can produce power on the widest range of wind speeds typical of spring, fall and winter. Short-term energy storage can be in the form of used-up batteries from current HEV's or future BEV's and PHEV's. Long-term seasonal energy storage may be in the form of H2, electrolyzed from water and stored locally from excess wind and solar electricity. When these H2 will be used in home-based combined heat and power generators in the winter, the H2 utilization efficiency will be ~90% or more.

EP,

If you are going to make critical statements like that, at least show evidence for your statements.

http://www.metaefficient.com/renewable-power/rooftop-wind-turbines-ready-for-commercial-use.html

Sure Roger - "lower cost, more reliable sources of sustainable energy."

Hydro.

@Roger: When these H2 will be used in home-based combined heat and power generators in the winter, the H2 utilization efficiency will be ~90% or more.

Agree. CHP (SOFCs PEMs) utilizing green H2 (electrolyzed with renewable) will make a very efficient package for home heat and electricity. Especially in cool climates. Further development to shrink chiller equipment will make these systems effective in warmer climates.

I would expect this design to be most effective somewhere the wind is funnelled into a tight area (such as a ridge line like the Altamont Pass). In ideal sites, it makes sense to take several swipes at capturing the energy.

I think I expect more out of high altitude winds such as the tethered airborn designs of Makanipower.com or jobyEnergy.com but those are high-risk and high-reward designs...it's just that there's so much energy higher up.

Thanks HB, I'll add those links to my collection.

BTW;
http://www.magenn.com/
http://www.ampyxpower.com/
http://kitegen.com/
http://www.skywindpower.com/ww/index.htm
http://www.airborne-wind-turbine.com/airborne_wind_turbine_englisch/airborne-wind-turbine-start.htm
http://www.youtube.com/watch?v=Ddliyfspmr4

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