The Jellyfish: Doing the Math

This Worldchanging piece about the Jellyfish caught my eye, but it sounded too good to be true. I emailed my neighbor Tony about it—he's all about science—and he responded with the following:
It is a toy but it has some attractive features.

First of all, the claim of 40 kilowatt-hours per month says nothing about under what conditions one might expect to generate this much energy.  If you simply do the math, and assume that the thing is going all the time, this means that it is generating 55 Watts constantly, 24/7.  We also see that it does nothing below 5 mph and it feathers out at 30 mph.  As I sit here typing this, I'm guessing that it's blowing about 7 mph or about 3 meters per second. 
So now we will estimate the power output of the thing.  It is essentially a Savonius Darrius (actually a variant of a Gorlov) rotor, with a power coefficient of maybe 35% at best. The thing is about a meter tall and a third of a meter wide, so it has a rotor area of 0.3 square meters.  The density of air is about a kilogram per cubic meter and the power output is one half the product of rotor area, air density, power coefficient and the cube of velocity. 
Do the math: I get about 1.4 watts. If I plug in all the numbers and crank the windspeed up to about 23 mph, I get close to 55 watts. So you want 40 kilowatt-hours a month?  It's gotta blow 23 miles per hour 24 hours per day, 30 days a month. What's it gonna generate today?  Let's be optimistic and say it blows like this all day long.  .0014 kilowatts x 24 hours = .03 kilowatt-hours.  It comes with it's own grid-tie inverter (an attractive feature) that probably runs at 50% efficiency so you get .015 kilowatt-hours back to the grid.

So now lets compare it to a single 150 watt Sharp solar panel that you can buy at Frys for about 750 bucks.  If positioned properly, it's out there right now generating perhaps 100 watts and averaged out over 24 hours, it will generate maybe .6 kilowatt-hours.  Again, throw half away for the inverter and you're at .3 kilowatt hours.  A small grid-tie inverter will cost you maybe as much as the solar panel.

So now we compare:

  • Jellyfish operating all day today: .015 kilowatt-hours for 400 bucks.
  • Sharp solar panel and inverter operating all day today:  .3 kilowatt-hours for 1500 bucks.
Of course, perhaps I've picked a day that favors solar power and not wind power.  But to get this thing up to competing with the solar panel, it's gotta blow a lot. 
Now that said, there are a variety of competing systems out there that are a step up from this system that have better rotors (higher power coefficient), more rotor area, require a little better tower, a little fancier inverter, and cost quite a bit more.  But they are a better investment, i.e. 2 real wind turbines is more cost-effective than 4 or 5 jellyfish. 
And around here, solar beats wind. And if I lived in Nebraska, I wouldn't fuck with this tinkertoy, I'd put up a real wind turbine. 
All of their assumptions about time-to-payback are based on how much the wind blows.  Additionally, they are touting this thing as a "plug in the wall socket" toy while at the same time, talking about sticking them on top of power poles. 
The rugged-izing needed to stick it on a power pole in a utility environment is an order of magnitude greater than that required for a "gee-whiz" home installation. How much did your simple breaker box cost that you stuck in your wall?  And finally, cost efficiency goes up as the size of the wind turbine goes up. That's why industrial strength wind turbines are the size of a house, not the size of a suitcase. So they are being loosey-goosey with their numbers, i.e. they are comparing apples with oranges and frankly, being a little disingenuous. If you want to look at what a real system of roof-mounted residential turbines should look like, check out the AeroVironment wind turbine systems. 

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