VAWTs with HAWTs
The energy available in the wind is the cube of the wind speed. This physical law means creating small increases in wind speed will result in significant increases in a wind turbine’s energy output.
This fundamental physical law of wind energy drives developers to invest in high wind resource sites and to place their turbines where they can harvest the highest velocity wind. It also drives the importance of researching how Vertical Axis Wind Turbines (VAWTs) can be positioned to increase the wind speeds realized by Horizontal Axis Wind Turbines (HAWTs).
A key maintenance and repair difference between VAWTs and HAWTs is that HAWT blades send the vibrations caused by turbulence in the wind to the drive train, causing harmful wear and tear to the bearings and gearbox. Near-ground wind is turbulent, a fundamental reason HAWTs are installed high above the ground. This does, however, leave wind farms with good near-ground wind resources, such as those with hills, ridgelines or mesas, or those set in passes, with an untapped, valuable resource that no HAWT can harvest.
If VAWTs can be installed below and around HAWTs without causing any harmful wake or turbulence to HAWT blades, then the understories of wind farms can be developed. If VAWTs also are able to increase the wind speeds that reach the HAWT rotors above them, the economics and incentives to build out these unharvested resources improve.
Capacity Factor Enhancement
A 1-MW project operating at full power all 8760 hours in a year would produce 8760 MWhs of energy and achieve a 100% Capacity Factor (CF). Most wind farms operate with a CF of 25-35% and produce only 2200-3000 MWhs annually per installed MW. (1)
Including VAWTs among the HAWTs without triggering the need for additional substation capacity is called Capacity Factor Enhancement (CFE). The number of VAWTs that could profitably be installed in such a project depends on:
- The wind speeds at the site and the power performance curves of the HAWTs and VAWTs.
- The wear and tear on the HAWTs and the economic benefits of resting them during high wind events.
- The Power Purchase Agreement for the energy produced, including time of day production bonuses.
- The estimated annual VAWT production that would be lost during high wind speed events when (if) all HAWTs are operating at full capacity.
- The VAWT project savings that come from not having to invest in new land, roads, and infrastructure.
- The potential increase in wind speeds and Capacity Factors that could be realized by HAWTs when VAWTs are installed strategically nearby.
Research and pilot studies will be important to accurately size CFE projects and determine the best way to increase the Capacity Factors of existing wind farms with new VAWTs. WHI has submitted a Research and Development proposal in response to the CEC EPIC Program’s grant opportunity GFO-16-310 - Improving Performance and Cost Effectiveness of Wind Energy Technologies. The R&D project is titled “Researching and developing the potential of VAWTs to double capacities of California’s wind resource regions while preventing harm to birds – Phase I”. This would be a foundational study upon which more research on more site-specific proposed CFE projects can be built.
We are interested in hearing from wind farm owners and others interested in how VAWTs could be used in CFE projects. Let us know your thoughts at firstname.lastname@example.org.
Porous Wind Fence Effect
Wind fences (aka windrows or windbreaks) “slow the wind in one place by deflecting it to another” and thus reduce the wind speed immediately downwind. Generations of land owners have used windbreaks and wind fences to protect their crops and prevent erosion, but it is only recently that science has been brought to bear on the details of the wind flow patterns they create.
Different porosities of a windbreak will affect the wind speed over its top and downwind (see Figure 1). Note that the highest speed-up zone is above and slightly downwind of the “fence.” A row of VAWTs is similar to a row of trees, except the porosity of the VAWTs is significantly higher. With a highly porous “fence” of VAWTs, Dr. Marius Paraschiviou predicts that the speed-up effect will occur nearly directly above a 2-4m zone of high turbulence located immediately above the rotating blade tips.
Though Dr. Paraschiviou predicts a speed-up effect above the row of VAWTs, its location and how far it will extend requires field data to validate and model. WHI proposes to use Doppler LiDAR, calibrated in the field with sonic anemometers, to collect high quality wind-speed data immediately above the row of VAWTs.
This field research will provide the data needed to validate a Stanford-created CFD Large Eddy Simulation model that will be able to predict how VAWT arrays of differing solidity and in different configurations could be optimized to create increases in the wind speeds that reach HAWT blades and thus increase their energy output.
The fact that a “fence” of VAWTs increases the wind speed over their tops creates another opportunity of benefit to HAWTs—that increased wind speed creates a zone of lower air pressure. When that zone reaches up into the space directly downwind of the lower sweep of HAWT blades, the differential between the front and back of the HAWT increases and so should the wind speed through its rotor. A setup of two rows of VAWTs around the base of a HAWT, as shown in the figure to the right, may not be possible in some sites where it would interfere with HAWT crane access for repairs. It is an example of how VAWTs could be placed both geographically and vertically to maximize the often narrow space available on ridges, where the near-ground wind shear can disappear, and winds at 10m above ground level (agl) can achieve speeds at at 30- 80m agl.
All objects in the landscape, including VAWTs, create obstacles that block the wind, forcing it to change direction, speed up and swirl around them. When VAWT rotors are in motion, they create three types of downwind wake and turbulence: blade-shed vortices, blade tip-shed vortices, and shear-layer turbulence. This complex flow can increase the wind speeds realized by HAWTs and allow the next row of VAWTs to be placed much closer downwind of another row of VAWTs.
During his years of working on wind farms, Bob Thomas observed the vertical mixing that HAWTs were creating in the San Gorgonio Pass and hypothesized that the addition of VAWTs in the “bush-tree” formation would increase that mixing and bring faster-moving wind into HAWT rotors.
In recent years, a growing body of field data and research, led in large part by Dr. John O Dabiri, has demonstrated how counter rotating VAWTs, when placed upwind of HAWTs throughout a wind farm, would bring higher, faster-moving wind down into the rotors of HAWTs. 
"For the vertical axis wind turbines, what you get, especially as you place them in close transverse proximity to each other, is that they can actually interact positively," Dr. Ann Craig said. "Although it is still an active area of research, we think that the VAWTs can have blockage effects causing speedup around the turbines that helps downstream turbines. They can also have vertical wind mixing in the turbine's wake region, which assists in the wind velocity recovery."
Arrays of closely spaced VAWTs enhance vertical mixing mainly due to the vortices of swirling wind produced by their blades that are moving much faster than the wind, as well as by shear-layer turbulence induced by wind flowing over the closely spaced VAWTs in an array. The large, vertically oriented vortices that spin off at the back of the rotating blades produce a low-pressure zone at their center that sucks in higher pressure air from above or below. This, along with the shear-layer turbulence, creates vertical mixing of the layers of slower- and faster-moving wind at different altitudes.
Though the basic physics of vertical mixing are relatively well understood, research will be needed to confirm that VAWT arrays won’t cause harmful turbulence to reach the HAWT blades and damage their bearings and drive trains. Wind farm owners and managers will want solid science based "proof" that VAWTs won't cause the repair costs of HAWTs to increase. This can be proven using LiDAR and sonic anemometers to validate CFD modeling. For more information on WHI's research proposal to accomplish this, see Phase I research, below.
Phase 1 Research
Researching and developing the potential of VAWTs to double the capacities of California’s wind resource regions while preventing harm to birds
A proposal presented by WHI to the California Energy Commission EPIC Program on June 20, 2017
WHI is seeking grant funding to conduct research on the effects of integrating VAWTs into existing and new wind farms. Before such VAWT developments would be acceptable, verification is needed to ensure that the new, vertically spinning turbines do not create wake and turbulence that damages the HAWTs above and downwind of them. If these wind farms include habitat of rare and endangered birds, then third-party collected data will be needed to demonstrate either that the VAWTs won’t harm these species or that they can be operated safely when these birds and bats are in their vicinity.
In April 2017, the CEC’s EPIC Program released a grant opportunity entitled “Improving Performance and Cost Effectiveness of Wind Energy Technologies.” In response, WHI is presenting its application that will make innovative use of Doppler LiDAR, sonic anemometers and computational fluid dynamics Large Eddy Simulations (CFD LES) to map out and model the wake and turbulence that result from closely spaced pairs of WHI’s G168 VAWTs.
At the same time, the grant will fund the evaluation of the DTBird detection and video recording technology that can rapidly and inexpensively increase the data on what happens when birds are in the vicinity of VAWTs. This first round of basic research conducted with a field "mortality study" should lead to a solid understanding of how well the camera technology can work in detecting birds before they can fly into harm's way.
Schematic of LiDAR vertical scanning geometry for near wake measurements. Not drawn to scale.
The proposed project will be conducted primarily on a ranch north of Hwy. 12, adjacent to the Solano Wind Resource Area (WRA) in Northern California. First, a pair of G168 VAWTs, followed by another pair, would be installed in 2018 to create a 280kW four-VAWT array.
The area north of Hwy 12 to Dixon has a height restriction, preventing the installation of turbines with blade tips exceeding 100 feet above ground level because of aviation concerns of nearby Travis Air Force Base. Nearly 100 square miles of some of the windiest land in California could be opened up to many rows of VAWTs that can easily keep the tops of their blades below 100 feet above ground level.
A smaller part of the project will take place at UL's Advanced Wind Turbine Testing Facility to evaluate the effectiveness of the DTBird motion detection system's ability to video record bird activities around the VAWTs. The recorded video will also provide an additional year of field data on how non-endangered hawks and birds of prey that are plentiful in this region of Texas react to the VAWTs.
The objectives of the proposed R&D project include:
- Doppler LiDAR and 3-D sonic anemometers will produce the data needed for both an empirical analysis and CFD Large Scale Eddy modeling on how wind speed and turbulence changes after moving around, over, through and downwind of counter-rotating pairs of 70 kW-sized VAWTs.
- Modeling and analysis to help assure HAWT owners that a VAWT array can be safely placed a distance upwind (to be determined by Phase I research) or a short distance downwind of their turbines. A Phase II project could then take place among the HAWTS in the 1000 MW Solano or 700 MW San Gorgonio Pass WRAs to begin analyzing HAWT-VAWT wake interactions.
- Data on the "porous wind fence" speed-up effect that will help model the extent above a VAWT array that the effect lasts and where it is the strongest.
- Data and analysis on how birds interact with VAWTs and whether the DTBird motion detection and recording technology can be relied upon to accurately record whether the turbines harm wildlife.
WHI is pleased to have assembled an excellent team of scientists to collaborate on this proposal.
Drs. Craig Clements and Neil Lareau, San Jose State University, LiDAR
Joe Drennan and Eric Jepsen, Garcia and Associates, Bird research
Dr. Sanjiva Lele, Stanford University, CFD Large Eddy Simulation Modeling
Kevin Wolf, Chief Operating Officer, WHI, and Project Manager