Sandia National Laboratories’ innovative design for offshore wind: no heavy tower, instead wind blades pulled taut with guy wires. (Image: Brent Haglund)

Dr. Brandon Ennis, Sandia National Laboratories’ offshore wind technical lead, had a radically new idea for offshore wind turbines: instead of a tall, unwieldy tower with blades at the top, he imagined a towerless turbine with blades pulled taut like a bow.

Tech Briefs: What got you started on this project?

Dr. Brandon Ennis: I work in the Wind Energy Department at Sandia. A lot of my research is focused on design of wind energy systems, from the design of actual materials used all the way up to the full system design and optimization. We have a lot of expertise and research on land-based wind turbines, but obviously offshore wind energy is gaining a lot of attention and traction in the U.S. — it’s becoming an industry here. The industry is currently mostly targeted toward fixed-bottom offshore sites, but the next step, and really one of the biggest remaining hurdles for wind energy, is floating offshore wind sites in deeper waters.

Dr. Brandon Ennis

The challenges for floating offshore wind are unique and the systems have different cost relationships than land-based wind energy. This distinction is interesting for us as researchers. So, we started thinking about this big problem — trying to understand what technologies might help to bring the cost down to a level competitive with fixed-bottom offshore and potentially with some land-based installation sites in the U.S.

One of the promising technologies is vertical-axis wind turbines. Having the drivetrain much closer to the water level, as well as this novel towerless design, presents opportunities to reduce the total mass of the turbine system — which can greatly reduce the system costs.

So, this project started with understanding the unique challenges for floating offshore wind and realizing that the solutions that have worked really well on land, and even for fixed-bottoms offshore, are likely not the same solutions that will make floating offshore wind successful.

Tech Briefs: I guess with a floating system you can get further out where there's more wind — is that it?

Ennis: There are several reasons to go to floating sites. One is exactly that, if you can be farther offshore then you can access better wind resources. The other reason is that on the West Coast of the United States, the continental shelf drops off very rapidly, so there's no fixed-bottom offshore option. That would require water depths that are less than 60 meters.

Tech Briefs: What's the end goal of your project?

Ennis: Our current project will conclude with the identification of an optimal system design that reduces the levelized cost of energy. We'll be designing the turbine, the platform, and the controls concurrently. We'll be identifying the system specifications and quantifying its levelized cost of energy to determine the value of this type of system relative to alternative systems. Additional work that will be important as a follow-on would then be actually demonstrating these technologies.

Tech Briefs: What level of specificity would it be? Would it be enough for someone to start building from your design or would they have to take the general principles you come up with and then decide how to make that happen.
A historical photo of Sandia National Laboratories’ experimental 34-meter-diameter, vertical-axis wind turbine built in Texas in the 1980s. (Image: Sandia National Laboratories)

Ennis: Great question. This will not result in a manufacturing drawing, but it is a 90 percent solution for both the platform and turbine. However, there will be some final details that would be required to actually take it to production. But this will be the level that gets you the bulk system design of the major components. Then there'll be some smaller components or slightly more detailed structural analysis required to produce what would be a final commercial system.

Tech Briefs: So, the co-design is between the turbine, the platform, and the controls?

Ennis: Yes, one of the unique things about this project — and it's a function of the program that that we're funded through — is that it's funded to perform the design in a different way than what's generally done by the industry.

The commercial design process that is being followed for offshore wind is divided by industry sectors. The turbine OEM designs their turbine independently of whatever floating platform it will ultimately be deployed on. They design their turbine and then that's a fixed design. The platform manufacturer takes that fixed turbine design, and they design a platform so that it can withstand the load passed from the turbine to the platform.

So, the traditional design approach is very much sequential and divided by the various industry sectors. The challenge with that approach is that you have very complicated and significant cost-performance relationships between turbine and platform and between system and the ultimate installation costs, and the operation and maintenance costs. If you design a system sequentially, based on components, then you really can't exploit those complicated cost-performance tradeoffs.

The alternative to that is what we're doing within this study, designing all of these systems concurrently. It's an approach for control co-design optimization where you're designing the physical systems of the turbine and platform structures simultaneously with their control, which affects the loads that are imparted on those systems.

The advantage of this approach is that you can now exploit those cost-performance tradeoffs to design the turbine differently so that the platform costs are minimized and ultimately to achieve the lowest levelized cost of energy.

Tech Briefs: It said in your press release that you adjust the tension on the support cables to correct for different conditions. Could you explain that?

Ennis: One of the innovations with this project is not only are we using a vertical-axis wind turbine, which of course is different than the industry standard of horizontal-axis wind turbines, but we've also designed a different type of vertical-axis wind turbine — we have a patent that was just granted for that design. Basically, it's similar to a traditional form of vertical-axis turbine called a Darrieus rotor. But the distinction is that the former design approach had a rigid tower in the center of the rotor and those towers were very massive. Although they do provide some structural support in certain directions, their massiveness is a drawback.

So, as we were designing this project and thinking through ideas, we came up with this towerless Darrieus rotor, which is an iteration beyond some prior work where we studied traditional Darrieus rotors. Part of the design philosophy is, of course, to minimize mass. So, we're doing that in two ways. One is by removing the tower and using blades that are pre-stressed into their position. We’re adjusting the loading in operation to shift the stress in the blades toward tension and away from compression by using the centrifugal forces to cancel out some of the pre-stress forces.

This design enables efficient material usage for the composites in the blades. And the second thing is we can control the load through actuation of the center support. By changing the tension or length of the center support, we can modulate the swept area of the of the turbine. And by doing that, we can reduce the forces and bring them closer to the water level. That reduces the overturning moment caused by the aerodynamic forces. We're seeing that this type of control is really meaningful for controlling the forces applied to the platform in high-wind-speed operational cases.

Tech Briefs: Is the platform held in a fixed position?

Ennis: In our project, we're looking at a floating platform — it's not rigidly connected to the seabed, but the structure is moored to it. These floating platforms are massive structures, traditionally constructed with steel, that trap large volumes of air which provides the buoyancy. For our project we’re studying a specific platform architecture, called a tension-leg platform, where the platform is connected to the seabed through vertical tendons that are tensioned. For this platform type, these tendons are what resist motion and overturning of the platform when loads are applied.

Tech Briefs: Is it almost like a little boat?

Ennis: Sort of — it's very similar to an oil rig. A lot of the technologies for floating platforms today are derived from the oil and gas industry. They have various fundamental architectures that have been proven in use.

Tech Briefs: Did you say the blades were a composite, so that they're somewhat flexible?

Ennis: Yes, that's right. The Department of Energy funded vertical-axis wind turbines in the early days of wind energy research in the U.S. The Sandia Wind Energy Department actually got its start studying vertical-axis wind turbines in the 1970s. At the time, composite materials were not really in production — there was very little body of knowledge on them. The blades from those early research studies were all made from extruded aluminum in the '70s through the '90s. One of the challenges with aluminum, and specifically extruded aluminum, was that they had bolted connections to join different blade sections together. There were fatigue issues with the early vertical-axis wind turbines in large part due to the material choice. Now jumping ahead 30 years, there's a large body of knowledge on composite materials, specifically for wind turbines — and these materials have exponentially better fatigue performance than aluminum.

So, there's a lot of opportunity for floating offshore vertical-axis wind turbines, bringing together the technology from the past studies with the new design approaches and materials.

Tech Briefs: So, if the wind speed changes, you want to change the tension on the support cables guy lines. Do you sense the speed electronically and have that activate some kind of winch to pull on this?

Ennis: Yes, exactly. We haven't designed the physical system for that control element, but very likely a winch controlled by either a motor or potentially by hydraulics. And that will pull the inner cables to reduce their length.

It's very common on wind turbines to have a sensor that's measuring the wind speed. There are control elements that are actuated based on the absolute wind speed, and others based on the rotor rotational speed. This rotor area control would be performed in a manner where, as the average wind speed changes the control system would respond accordingly.

Tech Briefs: What happens with a sudden gust?

Ennis: That's a design case that we simulate. There's a specific design case where there's an extreme gust with a wind-direction change. What's commonly done, even for horizontal-axis wind turbines, is that you would shut down the turbine so that it stops rotating.

In our case, depending on the rate of change of the gust, it will either continue operating but start adjusting the height of the rotor or it will go into a shutdown state where it stops the rotation. Once you've stopped operating, that reduces the load.

Tech Briefs: A recent study looked into the problem of matching the speed of the rotor to that of the generator — the rotor tends to go at low speed, high torque, and the generator likes high speed, low torque. They proposed a system for power transmission using a hydraulic system rather than gears to deal with the problem. Have you thought about that?

Ennis: Yes, but not specifically for this project. There are certainly different approaches for drivetrains and that's absolutely an area of need for research and innovation for offshore wind. The other consideration is the reliability. Land-based wind systems almost exclusively use geared systems. They have a gearbox to handle the difference in the desired speeds for the generator and the rotor. With the gearbox and using a certain type of generator, the challenge for offshore applications is its reliability. Those are components that can fail. Even though it's a small failure rate, when you go to offshore operation, having any sort of additional maintenance is very costly because you have to access the turbines. If the weather conditions aren't good, you have to wait before you can maintain the system so you will accrue additional losses of energy production and revenue.

For offshore systems, to my knowledge, they've transitioned away from geared systems to direct drive systems using a permanent magnet generator. However, those systems are expensive and massive, so you get increased reliability, but the downside is your generator is more massive and more expensive. So far, that's viewed as favorable relative to the drivetrain systems used for land-based service. But ideally there would be a system that you could implement offshore that's both low mass and high reliability.

Tech Briefs: Is there another project coming to look into that?

Ennis: Yes, there are certainly researchers looking into various approaches, hydraulic drivetrains being one possible approach.

Tech Briefs: What kind of size are you talking about, in megawatts?

Ennis: Currently we're designing a 20 MW turbine.

Tech Briefs: Would your design be scalable?

Ennis: It would certainly be scalable. We are currently designing that size because one of the challenges for a vertical-axis wind turbine is that you have long blades. Our blade length sweeps the perimeter of the swept area, whereas for a horizontal-axis turbine, the blade length is the radius of the swept area. So, our blades are on the order of three times that of an equivalent horizontal-axis turbine — that's one of the challenges for Darrieus rotors.

For our initial studies, we've looked at different sizing designs — different power ratings and capacities — and we found that there are numerous cost components that favor larger systems because that helps to reduce some of the fixed costs like installation, operations, and maintenance. With these larger systems you can increase the energy capture offshore, although one of the challenges is how to manage the mechanical loads for those very large systems, in addition to the logistical challenges.

Tech Briefs: How long are the blades?

Ennis: The blades for our system are 330 meters long — they're massive — by far the longest composite structures in existence. We have an advisory board made up of different industry members: turbine OEMs, blade manufacturers, offshore developers. We have performed a logistics study on this specific blade length, to understand whether it can be manufactured, whether it can be transported, whether it can be installed; and everything looks favorable, like it’s very possible within existing facilities and supply-chain logistics. But it's certainly an area where we'll focus future work as we move toward advancing this technology to a higher readiness level.

Tech Briefs: Would this be put together on land and then moved as a completed device out to sea or would it be assembled at the site, or both.

Ennis: Great question. You can take either approach — both are being explored by the industry. For fixed-bottom offshore sites, your only option is to install the turbine offshore because it's not a floating platform. Floating offshore sites give you the opportunity to be able to install everything at a port and then tow it out. However, in some cases, different platform manufacturers have decided to install the turbine offshore, similar to what's done for fixed-bottom sites.

We think that a lot of the value of floating offshore systems, from a transportation and installation logistics perspective, is you can transport an integrated system with the turbine already fully assembled and installed, connected to the platform.

As a result of this benefit, for our project we have required that the design be able to satisfy the constraints for a quayside integration of the turbine with the platform, just connect it all at the port and then tow it out — the goal is to minimize operations that are performed offshore.

Tech Briefs: What about the variability of wind throughout the day?
An image of an analysis of Sandia National Laboratories’ innovative floating vertical-axis wind turbine by the team’s new design tool. (Image: Kevin Moore)

Ennis: It’s very site-specific — even for land-based sites in the U.S., different areas have a different distribution of wind speeds throughout the day. In some places the wind speeds are highest toward the end of the day, although in other locations, or at other times of year, it can be highest during daytime hours. Offshore winds are typically quite consistent, and one of the other advantages of offshore installations is that you have a higher average wind speed than most sites on land. So, as a result, you have a more consistent energy production profile. Typically, you have higher capacity factors offshore so that you're producing closer to the maximum power output more regularly throughout the day and throughout the year.

Tech Briefs: How far offshore will you be going?

Ennis: For our study, we are looking at a specific site in the Gulf of Maine. The distance from shore is specified by our funding program as 30 kilometers and we're looking at a 100-meter water depth. The University of Maine has a long-standing research program where they've developed a specific floating platform and they're moving that design through various levels of commercialization. So, we’re using metocean data from their test site.

Tech Briefs: Is there anything you’d like to add?

Ennis: Yes, with this project we've partnered with a company from the oil and gas industry with a lot of expertise designing floating platforms: FPS Engineering & Technology. And our other project partner is the American Bureau of Shipping, which is a classification agency that helps to certify platforms for oil and gas, for maritime purposes, as well as for offshore wind energy.

These two partners have brought a lot of commercial, real-world experience into our project team to increase our confidence in the final project results.

Also, one of the conclusions of our project, one of the goals for us, is to quantify the levelized cost of energy of this co-designed, optimized system of turbine, platform, and controls. We will be comparing that to reference costs for horizontal-axis wind turbines.

The big aim of this project is to identify the opportunity space for a floating vertical-axis wind industry and to identify whether this is a truly ideal solution from a cost perspective.

Based on our current findings regarding costs and the required mass of the various systems, it seems promising that this system will provide a meaningful reduction in the cost for floating offshore wind energy relative to the current projections — and we believe relative to the likely future costs as well.

Tech Briefs: So, you're thinking there's not just the difference in installation costs, but the actual operation would be more efficient with the vertical systems?

Ennis: Yes, it's both a challenge and a benefit for vertical-axis turbines, that you have fewer active sub-systems, which helps you with reliability. The other benefit is that all of the control systems are at the platform level. Whereas, for horizontal-axis wind turbines they would be within the turbine nacelle at the top of the tower, 150 meters above the top of the platform.

That distinction affects your maintenance weather windows. It's very likely that maintenance on a vertical-axis turbine closer to the water level will not have as strict weather window requirements as maintenance on a horizontal-axis turbine where the components you're maintaining are 150 meters above the water level. So that would be a second mechanism for reducing the operations and maintenance costs for this vertical-axis system.

Reducing capital costs is the other big opportunity for improvement, both in terms of reducing the platform costs and then, based on our current level of analysis and findings, there will be opportunity to reduce even the cost of the turbine itself.

Tech Briefs: And I imagine it would be much easier to stabilize your system versus something on a tall mast.

Ennis: Yes, having a lower center of gravity is really meaningful from a performance viewpoint on the platform. Certainly, I would say the greatest opportunity for vertical-axis wind turbines is because you have that much lower center of gravity.

Tech Briefs: By the way, you said something about levelized energy costs — what does that mean?

Ennis: That's the metric used to make comparisons across energy systems. This metric, to a large degree, has typically controlled which energy systems have been deployed throughout time — whichever system has the lower levelized cost of energy.

Specifically, that term is the annual cost divided by the annual revenue. The annual cost is the capital cost multiplied by the finance rate and the annual operating expenses — that's the numerator. The denominator is the revenue, which is the annual energy production.

Just to give you a little reference, wind energy has been incredibly successful in the U.S. because of its low levelized cost of energy. There are analyses that track these costs over time and there have been substantial reductions for land-based wind systems in the U.S, to the point now, where for new energy generation, wind energy is among the top two in terms of lowest levelized cost of energy when installing on land. So, it’s between wind energy and solar, depending on the site and its resource. Combined cycle natural gas would also be within that range, depending on the cost of fuel.

Although wind energy has been very successful on land in the U.S., the challenge is that when you go offshore, specifically floating offshore, it's not a mature technology. In addition, the majority of the costs are for the non-turbine components like the floating platform and the installation costs. On land, the non-turbine costs account for about 35% of the levelized cost of energy. When you move to floating offshore sites the non-turbine costs rise to about 80 percent of the levelized cost of energy. The goal of the turbine design now has to be reducing the 80 percent system costs to achieve competitive values with alternative generation sources.