IN DEPTH: The upside-down turbine

It's a wind turbine, but unlike any you have seen before.

It still has a rotor, drivetrain, generator and so on, but it is shaped like a tornado — and early field tests suggest it can churn out up to 600% of the production of a like-rated horizontal-axis turbine.

The Invelox system, being developed by US start-up SheerWind, works by capturing wind as slow as 3.2km/h, feeding it into an array of intake vents at the top of its funnel-topped tower and channelling it down to power a turbine at ground level.

Trials of a 50kW unit last year in Minnesota exceeded expectations, opening the door to construction of a first 200kW prototype for a local factory. SheerWind — spun out of US renewables systems R&D outfit QRDc — sees this as a technology ranging from 50kW models up to a 7MW design.

The potential to scale up the technology is a strong suit. To take the concept from a 10-watt table-top unit to a 50kW machine called for a scale-up factor of 5,000; but to move the 50kW prototype to a 1MW turbine needs a scale-up of only 20 times.

To get to a machine of the size of the mightiest offshore turbines today would require a scale-up of seven or eight. For the largest units on SheerWind’s drawing board, electricity production works out at a per-kW price as low as $750; mass-produced medium-size conventional onshore turbines come in at $2,500-plus.

Set alongside similarly rated land-based machines, the numbers for a 1.8MW Invelox stack up well. The SheerWind design is less than 30 metres tall, compared with a conventional 130 metres to blade tip, with a much smaller rotor — less than ten metres wide instead of 85 metres. And because the Invelox machine does not create wind wake, each tower needs only 4.5 hectares rather than 32 hectares, meaning more units can be erected on a site.

According to computer simulations run by the company, this could translate into utility-scale power at a cost 30-50% lower than conventional wind farms today.

SheerWind calculates that an on-paper INV2.0 matches a 2MW Vestas V80’s annual energy production (AEP). But, with an ultra-high 67% capacity factor, it generates an actual AEP more than two times greater — around 12GWh versus 5.5GWh.

O&M costs have been screwed down by design in the architecture of the Invelox, which has no moving parts in the upper elements of the machine and positions the generator for easy, ground-level access, erasing crane costs. Repowering is simple, as new, larger transmission systems can be swapped for old without show-stopping problems of top-head mass.

One of the key innovations is that the concept puts distance between the intake vents and the turbine. Other “shrouded” turbine concepts place the two close together, limiting the ramp-up in speed of the incoming wind. The Invelox can be tailored for different wind speeds by lengthening the tunnel structure.

So far SheerWind has been bolting together off-the-shelf parts to build its machines, in recognition that using experimental components would demand capital-consuming, time-intensive trials. It plans to optimise elements of the technology, but has decided to leapfrog future testing in favour of moving straight into “build to order” mode.

The 200kW demonstrator should make the commercial-scale case for the technology on the grounds of more power per MW from a smaller footprint. SheerWind says it was “inundated” with enquiries after going public with test results from its 50kW prototype, and is discussing the development of larger Invelox pilot towers in the US, as well as having development licences awaiting sign-off in Italy, China, South Korea, the Czech Republic, New Zealand and the Middle East.

The company is also working on a suite of smaller-scale retrofit designs for everything from disused grain silos to conventional wind turbine towers. There are even sketch-ups for a hybrid wind/PV tree that looks like something out of The Day of the Triffids, and multi-hundred-kilowatt building-integrated concepts for office towers.

Unlike conventional wind farms, a complex of Invelox towers doesn’t need wide open spaces, as SheerWind founder Daryoush Allaei, notes: “We could be anywhere, like a water tower in any town — or even built into the structure of an existing water tower. As water towers have been accepted because they have an important purpose in communities, Invelox will serve a clear purpose in communities as well.

“We don’t need to be in the middle of nowhere [like conventional wind farms]. We only need the lightest wind — and for the industry to begin to get past its historical conservatism.”

How it works

Capture, accelerate, concentrate” is SheerWind’s mantra.

Wind enters intake vents arranged radially at the top of the Invelox tower. In the small-scale prototypes, this funnel-shaped structure was made of either steel or aluminium/steel architectural fabric, but it could be fashioned from other materials that can stand up to a site’s wind loads and weather.

As the wind travels down the tapering “runway”, it picks up speed due to the Venturi effect (a phenomenon in which the airflow is accelerated by its own changing pressure), boosting its kinetic energy as it approaches the ground-level turbine and generator set.

Channelling the wind from the top of the tower down to ground level allows for high-end power generation from small-diameter rotors: a 28-metre-tall Invelox with eight-metre blades should be able to go toe to toe with a traditional 1.8MW machine with a hub height of 90 metres and 85-metre-long blades.

SheerWind’s 50kW prototype in Minnesota ran at a capacity factor of 72% while flowing an average 314% more power than a like-rated horizontal-axis turbine.