It is exciting to see the helicopter we’ve been working with for the last 10+ years get some exposure, and that it seems like people are opening up to the idea of using unmanned aircraft for these purposes.
However, some of the claims, particularly those saying how much more drastically efficient the Invelox device is, seem a little too fantastical. I was going to address each of the claims using a back-of-the-envelope approach and a little bit of common sense. However, life in general and making progress on my thesis proposal took priority. I still want to make some general comments about the claims, though.
First, some background: This structure works on the principle that slow moving wind can be accelerated by capturing it in a duct that reduces cross section with length. The accelerated air is passed through a turbine to generate electricity. Additionally, the duct has openings arranged outward in an octagon, meaning that the structure doesn’t need to be rotated to face the wind, and the duct bends to be horizontal at ground level, meaning it can be as tall or short as necessary while maintaining accessibility to the moving parts.
Here is a diagram from Sheerwind:
Operates at wind speeds as low as 2 miles per hour and in low wind regimes (class 1 and 2), resulting in more candidate installation locations. I’ll buy this; the cut-in speed on a turbine is based on the wind speed at the turbine. So this could be used in places that have slower, but still consistent, wind.
Reducing installation capital cost to less that $750 per KW. I can’t say anything about the economics, but not having to install ginormous rotor blades, or mount a several thousand pound generator atop a tall tower has got to carry some serious savings.
No radar interference. Ok, I’ll buy this–not having giant blades whirling around, the INVELOX just looks like a building to a radar.
Reduced land use: INVELOX technology requires a much smaller footprint (90% less than traditional turbine generators). This is the first of the claims using percentages: “90% smaller footprint!” “Record high 72% production capacity!” “16% to 38% savings!” This is all bunk as far as I’m concerned. 90% smaller than what? If the comparison isn’t normalized to the power output of the device, then they’re comparing apples and oranges. Most commonly used wind turbines have disc area significantly larger than the Invelox intake–so they take up a lot more land, but they also produce way more power.
Less noise: By eliminating the need for massive tower-mounted rotors, INVELOX significantly reduces the high energy and low frequency noise nuisance created by wind farms and eliminates the shadow flicker issue.I’ll accept that you don’t get the ‘whoosh-whoosh-whoosh’ sound and definitely don’t have any shadow flicker. I don’t accept that these things will be noise-free. They’ll probably just make more constant and higher-pitched sounds.
No moving parts: INVELOX has little to no impact on wildlife, including bird migration.Yes, less likely a bird will get whacked by a blade swinging around. However, the company hasn’t addressed the likelihood of birds nesting or roosting in the structure, particularly on a downwind side, then getting sucked down when the wind starts blowing. I suspect this claim is more a reaction to the current controversy surrounding the wind power exception to endangered species laws.
Superior aesthetics: INVELOX uses a lower profile and a smart design, offering pleasing aesthetics for surrounding communities.Beauty is in the eye of the beholder. I personally like the way a spinning rotor looks. I doubt homeowners will think the Invelox is any better looking than a windmill.
Lower Costs, Higher Output. INVELOX promises superior power output and reduced generation costs, offering savings of 16% to 38% per megawatt-hour (MWh) produced. An INVELOX installation will cost nearly 7% to 11% less to construct than a traditional wind power setup, and 40% to 45% less for operation and maintenance. See above about percentages.
Weather resistance: Because turbines are installed inside the INVELOX unit, they are not subject to harsh temperature variations or icing. The wind that spins the turbine still comes from the outside air. If the air is cold and damp–and by the way, intentionally accelerated to high speed at the turbine disc–then the low static pressure at the turbine blades have the potential for icing. Again, since the turbine is on the ground, it could be serviced by heaters or easily replaced so that might mitigate the effects. But another thought–what happens to all the rain that will collect in the ducts?
Another fact that is ignored is that Invelox is much shorter than most common wind turbines. Being shorter means that it sees significantly slower winds, and consequently much less power available. On the pro side, big wind turbines see faster winds at the top of their disc than at the bottom, leading to cyclic loading that causes fatigue stresses much more quickly; the Invelox turbine wouldn’t have to be built for fatigue loading.
Produces 600% more electrical energy (kWh).First of all, 600% of WHAT? I was shocked to hear this claim because it sounds too good to be true. But, they did indeed make the claim. Let’s think about this from a fluid mechanics point of view:
What this says is that the power available in the wind, if we could extract ALL of it, is limited by windspeed and intake area. The analysis presented in Dr. Gordon Leishman’s textbook states that traditional wind turbines typically operate at about 66-83% of this maximum aerodynamic efficiency (page 731). So even if the Invelox operated without any losses, the most improvement they could get in electrical output is about 50% (66%*150% ~ 100%), which is nowhere close to the 600% claimed.
Once again, this seems like a good, and obvious, idea. However, I don’t think their cause is well-served by making dubious claims and by publishing, in my opinion, poorly explained research results.
Let’s also consider the costs; I don’t know enough to spitball numbers, but I do know that fuel, depreciation, insurance, and pilot fees are all costs that are significant for a manned mission and basically nil for the unmanned. Add to that the reduced risk of catastrophic failure, and it seems like a no brainer that this mission is better suited for UAVs, even if the manned mission could have been accomplished in three days instead of ten.