Virtually every week there are articles about new and innovative methods for harvesting wind energy. And every week more megawatts of capacity from three-blade horizontal-axis wind turbines (HAWT) becomes operational, despite all of the contenders. Why aren't these innovative new products knocking the iconic HAWT off its perch? Is it possible to tell which are likely to be viable? These eight points are a useful way to assess which technology has potential, and which are likely just hot air.
In 1919, Albert Betz calculated that the maximum energy that could be extracted from the wind regardless of the type of device used was 59.3 percent. This number, Betz' limit, has stood the test of time. No one in independent testing has exceeded it, yet many continue to claim that they do.
Most recently, a much hyped technology and company, Saphon, has made this claim, but there appears to be no evidence that this is actually the case.
If you see a new-fangled wind turbine claimed to exceed Betz' Limit, think of it as a red flag which could indicate that something is amiss. On a swept-area for swept-area basis, Betz' Limit still holds as the comparison has to be the total surface area of the wind capture device presented to the wind, whether we're dealing with funnels, sails or blades, and whether they're square or round.
Savonius wind turbines are basic torque engines which have a maximum rotational speed of the velocity of the wind. This makes them excellent for higher torque applications such as pumping water, but it makes them poorer generators of electricity. Darrieus wind turbines have aerodynamic blades, but the blades are only flying in clean air at the optimum angle for power generation 15–30 percent of the time. Between this and other challenges common to vertical axis wind turbines, their generation is better than Savonius turbines but has never come close to three-blade HAWTs.
One product took the Savonius, applied a lot of lipstick: golf-ball like dimples on the leading edges, a hollow interior claimed to double the surfaces for the wind to work on, stackable modules and magnetic-levitation bearings. All of these "innovations" merely made it a more expensive ineffective generator of electricity, based on my assessment of the details of the proposed investment and collateral. If a Savonius turbine is appropriate for a niche, the question is how cheaply and simply it can be constructed, not how to make it more productive.
Ducted and venturi-effect wind generators also resurface with disarming regularity. In these, some sort of shroud or funnel focuses the airflow on a smaller wind turbine, increasing the velocity of a given volume of air using well-known principles.
Most recently the Invelox has been capturing attention with its device. Turbines of this type have been tested extensively in the past and have never overcome the inefficiencies introduced by vortices created by the funnels, despite hypothetical improvements which ignore real world fluid dynamics. They are all less efficient at generating electricity from a given volume of air than a three-blade HAWT of similar surface area to the funnels. And they introduce a much heavier, bulkier and more susceptible to gusts shroud to the equation, as can be seen from the FloDesign cowled turbine:
Many people have ideas. Some of these people have access to reasonable quality graphics tools. They create fascinating-looking devices, often accompanied by some pseudo-scientific statement about which technical effect they are expecting to harness. If there isn't a working, tested prototype, red flags should be popping up that this is not a likely technology.
The Strawscraper concept is typical. It's solely an architectural rendering with added bafflegab about the piezo-electric effect.
There are several independent, reputable testing companies around which can do well formulated, proven and credible testing of wind generation devices. The best known of these is Sandia Labs, which has been testing and researching wind generation devices for decades. If a company doesn't have independent tests and is making extravagant claims, be skeptical.
Apparently Sheerwind, the company behind the Invelox system, has so far not allowed anyone outside the company to test the system.
As a positive example, consider the STAR (Sweep Twist Adaptive Rotor) blade, currently being incorporated in a portable wind generator by the start-up Uprise Energy in California.
On its website, Uprise Energy publishes a link to Sandia Labs' independent testing of its adaptive rotor blade documenting the 12-percent improvement in performance for this small generator technology. The principal of the company has a solid background of entrepreneurialism in related fields and they are targeting specific niches for portable small generation. This company is a good bet, unlike many of its competitors.
If patents are claimed to be pending or accepted that reflect the technology and its advances, it's worth having a look at them. Saphon's patent, for example, is for a device very different than the one that they show pictures of. This doesn't stop them from using the patent in their marketing hype. Here's what the patent, WO/2012/039688, says:
"The invention consists of a system for converting wind energy (SCEE) into mechanical and then electrical energy. This system (SCEE) is not subject to the theoretical Betz limit (59 percent). The system (SCEE) has a wheel (F) equipped with a series of blades arranged all around it."
A series of blades does not describe the disc-and-sail gizmo that Saphon shows.
Anyone can claim greater efficiency, but what do they mean by it? The gold standard is Levelized Cost of Energy (LCOE), in which all of the costs associated with raw material, manufacturing, transportation, construction, operations and maintenance are factored into a cost per kWh based on expected output over the life of the device. ISO standards exist around the LCAs and the attendant calculations so that apples-to-apples comparisons can be done across different forms of generation. Anything less than that requires attention to exactly what is being claimed, what specific device the claims are about, and in fact whether they spell out what they mean at all.
Invelox, for example, makes claims of 81–660 percent efficiency gains. It appears that the company hasn't done a full LCOE, and it appears that it hasn't had independent testing done. The explanation is that it took the relatively high-speed optimized small wind turbine that it uses in its device, left it in the open, and compared that to the same wind turbine in its device. This has multiple failings including the lack of a comparison to a wind turbine appropriate to wind speeds at the same height and with the same surface area as its funnel openings. All that it appears to have proved is that it can make a small wind turbine generate more electricity by putting a great huge whopping funnel on top and exploiting the Venturi effect. This has been well understood for decades, and is understood to not justify the cost, complexity and lack of scalability of the device.
Many wind energy innovators speculate that they will be easily able to incorporate storage into their devices based on their unique characteristics as if this is an advantage. Once again, Saphon is a poster child for this claim, stating that its very lossy hydraulic system could include a storage reservoir. For context, wind energy is on track to exceed nuclear generating capacity in the next 3-4 years with virtually no storage, which only off-grid applications require, usually as a separate component and almost invariably in the form of an electric battery.
Organizations such as GE are an exception. It knows the market, it knows what is required, and its devices are producing a significant percentage of the total world wide wind energy every day. When it announces integrated storage, as it did recently with its Brilliant turbine, it's for an understood target market, engineered and scaled appropriately.
High-altitude wind energy is a constantly recurring meme in wind generation. As people continually point out, the wind is stronger and more consistent the further you get from the ground. That's why HAWTs keep getting taller and people are looking at finding ways past current height limitations.
Many different groups and individuals are looking at flying generators of one sort or another higher into the atmosphere attached to cables that are attached to winches on the ground.
The varieties include airfoil kites, blimp-style Savonius generators, flying cowled turbines where the cowl is a blimp, and small solid kitecraft with generating propellers on them (Skysails, Altaeros and Google's new acquisition Makani respectively). These are some of the more visible examples from recent years.
These devices are likely to remain a niche for the simple reason that putting lots of them up high in the atmosphere would require 1–4 km long, effectively invisible cables which stretch over a broad and shifting downwind range. This would require a significant area to be declared a no-fly zone for most forms of aviation, although passenger jets could still fly overhead. If the system failed, and the device fell from the sky, it would drape those kilometers of cables over everything downwind, including roads and buildings, requiring that a large area downwind be fairly free of any human structures. And for the solid flying wing, a very heavy object with rotating propellers would fall out of the sky somewhere between 1 and 10 km downwind in the event of a failure. That's why, after a period of assuming they these could make major onshore contribution, most of these devices are now aimed at servicing remote locations or offshore sites.
It doesn't help that the lighter-than-air variants require increasingly rare helium which is also required for other, arguably much more valuable, uses including as a coolant in medical imaging machines. There are significant scalability issues with such turbines, and, given the increasing height of HAWTs, they are dealing with diminishing returns in any event.
There are about 240,000 HAWTs worldwide in sizes ranging from a few kilowatts to 7 MW capacity, both onshore and offshore, in rural and urban areas. Four out of five of the top selling small wind turbines are horizontal axis two- and three-blade wind turbines (with only a single two-blade one, which is a reasonable choice at this scale.) They are generating all but a tiny fraction of a percentage of the electricity harvested from the wind in the world. They are undergoing constant incremental improvements in design including:
As examples of the types of innovations that are constantly appearing, yet aren’t particularly sexy, here are two recent stories. In the first, Magdy Attia and Marko Ivankovic of Embry-Riddle Aeronautical University realized that they had a design for a gearbox that would last longer than current gearboxes and are looking to put it into wind turbines. In the second, software-based predictive maintenance that has been used for years in other industries is being applied to wind farms to optimize maintenance schedules, purchases and hence costs. These may not be as eye-catching as a newfangled wind turbine, but there is enormous money in shaving a percent here or a percent there off of costs when the costs are in the billions.
The wind industry is disruptive because it is supplanting fossil fuel generation at a reasonable cost. That reasonable cost is due to decades of incremental improvement and major supply chain and business innovations, not radical technical innovations. The most effective technology was chosen a few decades ago, and it's been getting steadily better ever since.
If someone is selling on a "new" wind generation technology, be aware. The wind industry is unlikely to be disrupted by someone with an idea and a Powerpoint pitch.
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