By Kevin Wolf / President / Wind Harvest International

Wind farms in California and other regions of the world exist only in relatively small geographic regions.1 Most of these resource areas have reached their physical or political2 limits in their ability to install additional propeller-type, horizontal axis wind turbines (HAWTs).3 Nonetheless, many have topographies that create excellent near-ground wind speeds.

To profit from the energetic wind below their HAWTs, wind farm owners need cost-effective vertical axis wind turbines (VAWTs) that operate efficiently in high turbulence and that do so without wake4 from the added rotors negatively impacting their existing turbines. They also need turbines that are wildlife friendly.

Near-ground turbulence

The good-to-excellent average annual wind speeds (6 to 9 m/s, 14 to 20 mph) found at 10 to 25-m above ground level in wind farms in California5 and other regions are well known to wind industry meteorologists.6 Passes and ridgelines accelerate near-ground wind and cause wind shears to decrease, often significantly. Meteorological data also document that thermal and obstacle-induced turbulence in the high-energy, near-ground wind is found in many wind farms, including in four of California’s five Wind Resource Areas.

One reason near-ground wind resources haven’t been developed is that HAWTs have increased failure rates when their blades pass through turbulence.7 As a result, rows of HAWTs are hundreds of meters downwind of each other, and the bottom tips of their blades range between 20m and 50m above ground level.

HAWT turbulence-loading problems arise primarily from their long blades connecting to the drive shaft at only one end and their large rotor having to operate in changing wind speed and direction. The blades and bearings used in modern HAWTs would have to be substantially strengthened to withstand the high peak and cyclic loads from the near-ground layer of extreme turbulence.8

Why VAWTs now

VAWTs are intrinsically less sensitive to turbulence than HAWTs because their blades are attached to the rotating shaft at two or more locations. Another beneficial outcome of their geometry is that VAWTs don’t have to yaw and turn into the changing wind direction.

At least one such wind turbine (i.e., Wind Harvest International’s (WHI) Harvester VAWTs[ix]) is ready for certification and operation underneath HAWT10 Other turbines could also soon be capable of achieving a 20+ year service life in high turbulence and be ready for industry-scale sales(e.g., Stanford/Dabiri’s VAWTs), once they can comply with the IEC 61400 certification process and become UL listed.

Historically, VAWTs have had trouble with mechanical design and durability because they lacked field-validated, aeroelastic modeling that HAWT engineers use. That has been resolved by building on VAWT modeling developed by Sandia National Labs and advanced at Delft and Danish Technical University. The engineers of the WHI Harvester used a suite of a prototype-validated finite element, frequency response, and fatigue analysis models that together function as an aeroelastic model.11

Aerodynamic modeling funded by a 2010 California Energy Commission (CEC) EISG grant [xii] to WHI proved that modern VAWTs, when placed close together would also create the “coupled vortex effect”. The one-meter close spacing and counter rotations let them produce 20 to 30% more energy per pair than from two VAWTs operating separately. This offsets the problem VAWTs face that HAWTs don’t: Their blades create drag as they return into the wind. Historically, this increase in drag prevented them from realizing more than a 45% efficiency,[xiii] whereas HAWTs can achieve 50%. With the coupled vortex effect, VAWTs in arrays can theoretically realize the efficiencies of HAWTs.





Another problem hindering VAWT development is that smaller VAWTs like WHI’s Harvesters use more steel and material per rotor-swept area and MW of installed capacity than do large HAWTs. However, with large-scale use possible in wind farms:

The mass manufacture of the smaller VAWTs offers significant savings. 20

Their shorter towers use less material 21 and smaller and easier to build foundations.

and smaller and easier to build foundations. They make dual use of valuable land and infrastructure 22 when installed in existing wind farms.

An additional benefit modern inverter-based VAWTs have for repowering wind farms is that they can help solve the grid harmonics and reactive power problems that are caused by older HAWTs using “induction generators”. A megawatt of VAWTs like the WHI Harvester 70 with inverters similar to the ones in Northern Power Systems’ 100kW HAWTs can, independently of wind speed instantaneously source or sink 450 KVARs 23 of the problematic reactive power produced by the older HAWTs. Solving the reactive-power problems of older wind farms can increase their power quality and real output.

VAWT impacts on HAWTs

Aerodynamics predict that the wake from VAWTs won’t harm HAWTs, and may, in fact, help them. The wake and vortices shed from an array of tightly spaced VAWTs should stay in the same wind layer that passes through their vertically spinning rotors. Modeling shows that downwind by five rotor heights24 or ~eight rotor diameters 25 , the wake of VAWTs is gone, their vortices have disintegrated, and the wind speed has recharged, in part due to the vertical mixing that their spun-off vortices create.

VAWT placements are theorized to increase the wind speeds entering the rotors of the HAWTs above them in two major ways.

Lowering the wind shear

A growing body of field data and research, led in large part by Dr. John O. Dabiri, has demonstrated how counter-rotating VAWTs lowers wind shears by bringing higher, faster-moving wind toward the ground and replenish the

wind speeds lost to the energy and turbulence the VAWTs produce.26 As a result, faster moving wind from above will drop down into HAWT rotors and increase their energy output.27

Stanford University doctoral candidate Anna Craig led a study that modeled various VAWT arrangements. Their results 28 indicate that VAWTs can interact positively when placed in close proximity to one another. Craig noted that “We think that the VAWTs can have blockage effects causing speedup around the turbines that help downstream turbines. They can also have vertical wind mixing in the turbine’s wake region, which assists in the wind velocity recovery.”

In Benefits of collocating vertical-axis and horizontal-axis wind turbines in large wind farms, the authors stated, “Because of the presence of the VAWT layer, the turbulence in the wind farm is increased, which enhances the wake recovery of the HAWT. The faster wake recovery more than compensates for the additional momentum loss in the wind because of increased effective surface roughness associated with the VAWTs.”

Porous wind fence effect

Dr. Marius Paraschivoiu’s modeling shows that there will be a few meters of high turbulence directly above an array of closely spaced VAWTs. Above that, there will be a zone where the wind speeds increase above ambient. This is caused by the blockage effect of the VAWTs.

A row of VAWTs could be placed upwind of a HAWT at just the right height so that the HAWT blade enters a zone of higher wind speed with no significant increase in turbulence. Arrays of VAWTs placed a short distance downwind of a HAWT can also create a speed-up effect for the upwind HAWT, but the physics are different. The wind speeding up over the VAWTs decreases the pressure there, which increases the pressure difference between the front and back of the HAWT rotor. This, in turn, would increase the wind speed through the HAWT rotor and thus its energy output.

Just how much this porous wind fence effect could benefit HAWTs was to be a significant focus of the LiDAR studies WHI proposed as part of its R&D proposal to the CEC EPIC Program.

VAWTs potential to increase wind farm energy output

HAWTs in wind farms are placed substantial distances apart. Below is a table comparing land used in some wind farms in California’s Wind Resource Areas to other means of estimating the amount of land a HAWT wind farm needs.

Modeling and field testing show that the relative distances between rows/arrays of VAWTs can be much shorter than with rows of HAWTs without the downwind row losing wind speed and energy.23

The table below shows the VAWT energy densities that can be developed with the following assumptions:

• One-third meter between 3kW VAWTs in a four-turbine array

• One meter between G168 VAWTs in a four-turbine array

• Two rotor diameters (6m and 24m) between arrays in a row

• 5 times rotor height between G168 rows (70m)

• 8 times the rotor diameter between rows of 3-kW VAWTs (24m)

The layouts in illustrations would lead to about 150 to 200 MWs of VAWTS on the same land on which 32 MWs of HAWTs now operate in one of the best near-ground wind resources30 in the San Gorgonio Pass. Dabiri’s research on VAWTs predicts a 5 to 10 times increase in energy density is possible from VAWTs compared to HAWTs. This seems to be eminently doable with two layers of VAWTs set among the same MWs of HAWTs.

In the Mountain View Power wind farm, the wind is unidirectional from the west. In the illustration below:

• A row of small VAWTs (3 kW) maximizes the porous wind fence effect increasing the wind speed into the taller G168 VAWTs a few meters downwind.

• The G168s are a short distance (10 to 25m) downwind of a HAWT such that the bottom blade tips of the HAWT pass at just the right height above the VAWTs to safely maximize pressure difference the downwind VAWTs create.

• More rows and arrays of small- and medium-sized VAWTs are placed upwind and downwind at roughly a 7-rotor diameter distance apart.

Next steps to adding VAWT layers to wind farms

In the last round of wind energy grants through the CEC’s EPIC Program, there were three applications to advance the development of VAWTs for installation in existing wind farms. All three were either disqualified or failed to score enough points. Some of the main objections from reviewers included the following misunderstandings:

• Near-ground wind resources won’t be developed because wind shear causes the wind speed to be too low to ever be competitive with simply adding new wind farms of HAWTs.31 (Note: High wind shear occurs in wind farms in the Great Plains but not in California or other areas with passes and ridgelines).

• VAWTs have such a terrible history of commercialization that grants should not be made for their development until after they have been certified.32 (Note: Grants should be available to help renewable energy technology get through the most difficult part of the commercialization process – certification.)

• Funding should not be spent on how VAWTs can best be sited in a wind farm because R&D on HAWT placement is already commercialized.33 (Note: Little is known about how VAWT wakes will interact with HAWT wakes in different topographies and wind conditions.)

WHI’s proposal to the EPIC Program called for the use of its G168 VAWTs in a 140 to 280 kW project on ranch land in the Solano Wind Resource Area. There, San Jose State University LiDAR and its transportable meteorological mast with sonic anemometers would have been used to collect wake data from the VAWT array. Modeling would have been done with the help of Stanford University’s Large Eddy Simulation CFD model. WHI committed to placing the resulting data into the public domain, so other universities and companies could begin to validate their own modeling codes for VAWT wakes.

More field data on arrays of VAWTs is needed. LiDAR is the key to measuring the changes in wind speeds and turbulence created from different ways of linking together co-and counter-rotating VAWTs. Then data will be needed on how those VAWT wakes interact with HAWT wakes. Given the capabilities of modern wake and topography modeling, it shouldn’t take long to confirm basic “rules of thumb” for how vertical turbines can be safely and effectively installed along ridgelines and among the wind turbines in rich resource areas like the flat desert lands of the San Gorgonio Pass.

Wildlife

VAWTs will eventually enter the wind farm market. Before large numbers are installed, their potential impact on birds and bats needs to be evaluated and mitigated through the California Environmental Quality Act (CEQA) and other land use planning processes. Both Dabiri’s and WHI’s most recent EPIC proposals included new ways of documenting how birds and bats react to VAWTs. Biologists34 theorize that these animals evolved to fly around three-dimensional objects, such as trees and VAWTs, and will have an easier time avoiding their blades than they do those of two-dimensional HAWTs. Producing field data to try to disprove this hypothesis is a fundamental first step.

The fastest way to produce the data would be for all 100+kW VAWT projects like the one WHI is pursuing to use 24/7 motion detection and recording systems with binocular, high-definition cameras. Such tools can be field validated and then relied upon to capture far more animal-turbine interactions than traditional field observation methods. In-field, mortality studies should accompany the camera data analysis to compare the two methodologies until the mortality studies are no longer needed to accurately count fatal interactions.

Given the potential for VAWTs to be safely installed in valuable wind resource lands containing endangered species’ habitat, grant funding of VAWT wildlife research would be a wise investment.

Benefits to ratepayers and slowing climate change

Installing thousands of megawatts of new VAWT capacity in existing wind farms with good near-ground wind resources promises to be of significant help to ratepayers and local economies, especially if some of the VAWT components can be manufactured near the new installations.

Layering VAWTs among HAWTs in high-value wind resources should result in a lower Levelized Cost of Energy than any other renewable energy option. With 40% Capacity Factors and a 14% “learning curve” for price reductions in the technology, by 2025, the LCOE of VAWTs installed among HAWTs should drop to $.05 per kWh, which is less than the wind energy alternatives for places like California, where it is very difficult to permit new wind farms, and the alternatives are expensive offshore projects or installing new transmission lines to new projects in places like Wyoming.

According to Project Drawdown, the second-best way to meet carbon reduction goals is with on-shore wind development. Making double use of existing wind farm infrastructure to harvest the lower wind layers of some of the best wind resources in a region should be a priority on the world’s roadmap for achieving its carbon and pollution-reduction goals while keeping ratepayer costs low.

Key research questions

1. Rows of VAWTs will enhance recovery of the mean wind speed above, but the turbulence fluctuations in the

wind layer above will likely increase significantly35.

• How far above and downwind of a field of VAWTs must HAWTs be so that the higher turbulence doesn’t harm the lifetime of the HAWTs’ blades and drive trains?

• How problematic is the turbulence created by different types and placement patterns of VAWTs?

• Does the shape of the VAWT blade tips matter to the turbulence that might impact HAWTs?

2. VAWTs and HAWTs layered in the densities of Figures 1, 2 and 3 could increase the 800 MWs of HAWTs in the San Gorgonio Wind Resource Area to more than 6,000 MWs and rotor-swept area by a similar percentage. Could this more intense extraction of energy and lowering of the boundary layer have either positive or negative impacts for regional and even distant weather patterns and intensities?

3. Families of species have similar attributes, behaviors, and physiologies. How much field research needs to done before scientists can accurately predict whether a species that has never seen a VAWT will be able to avoid being hit by its blades? For example, if research proved that vultures were able to consistently avoid arrays of VAWTs with carrion placed underneath, would studies still be needed on whether condors in Chile were always able to avoid VAWT blades before VAWTs were allowed in condor habitat in the U.S. where the bird is an endangered species?

The complete 12-page article in PDF format is available on the windharvest.com home page



Footnotes and further reading