Do Balloon Mounted Wind Turbines Work?

2015-05-06

The idea of a balloon mounted wind turbine is a nice one.

There is always a need for power in remote places and where people face the exceptional circumstances that happen around the world each day, e.g. natural disasters and war. Sometimes there is no grid and getting a line to the nearest connection is impractical or impossible. But are they practical?

In situations where a grid connection is difficult, the standard plan is to deploy mobile generators, usually fuelled by diesel or petrol. This is often environmentally sub-optimal and, depending on the location, getting fuel to the site can be tricky.

A floating wind turbine is neat: it can be packed onto the back of a truck or into a container and dropped where the power is needed. Depending on automation, deployment might be easy.

The wind speed is higher the further up you go and, since a wind turbine's power output depends on the cube of the wind speed, a floating wind turbine has a big advantage over one stuck to an expensive tower.

We can assume a deployment height of 600 m, about four times higher than the tallest conventional wind turbine towers.

Some floating wind turbine designs have an additional apparent advantage: with a centrally mounted wind turbine, the dirigible balloon accelerates the airflow through the rotor, further increasing the wind speed at the rotor.

How much power might you reasonably expect from such a machine?

The change in wind speed with height at a site follows a power law, more or less, of the form,

V = V₀*(h/h₀)^a

Where V is the wind speed at the higher altitude h, V₀ is the wind speed at the reference, lower altitude h₀, and a is a number that depends on the how cluttered the landscape is. At a perfect site, i.e. with little clutter close to the surface, a might have a value of around 0.1. In a cluttered location, corresponding to an exponent of 0.25.

For an imagined system, (h/h₀)^a may have a value of around 1.51 at a good site, where a is 0.1, h₀ is 10 m and h is 600 m.

This means that if the wind speed at 10 m is 5 m/s , the wind speed at 600 m will be 7.5 m/s.

The ducted design of a balloon mounted machine may further increase the wind speed by a factor of about 1.15 (or a power increase of about 1.5 times an unducted machine), meaning that the wind speed at the turbine may be up to 8.7 m/s, at a good site.

At an average wind speed of 9 m/s, a 4 m wind turbine - which seems to be about the size used in such systems - will produce on average, over a year, 1 kW continuously.

In one hour you'd have energy to:

power a Samsung S3 for two months or so (~ 1.8 x 10^7 J per year)
power an Apple iPhone 5 for four months or so (~ 1.3 x 10^7 J per year)
run an average EU household for two hours.

This is more than a trivial amount of energy. Yet it's not a huge amount. My car, an old Audi A2, has a 1.4 litre 55 kW petrol engine. Running this will produce 55 times the energy of this imagined machine in a good wind site each hour.

To keep my car running, you'd need about four litres of petrol per hour. In 24 hours, you'd need 96 litres, and so on. Each litre is about a kilogramme, so in under two weeks, almost a tonne of fuel would be required. Trucking in a tonne of fuel every two weeks is not an insignificant matter!

This is the selling point of the balloon mounted turbine.

Is it a good idea? Yes, it's nice. Towers are awful. You need foundations and trucks ... as with batteries, nobody that designs wind turbines much likes them.

Is it practical? This is an open question. Paul Gipe has written a good critique - helium is a limited resource (it's mined and running out because of party balloons and medical applications) and there's a lot of regulation, understandably, around things flying at 600 m. It's also a lot of equipment for a modest output.

The principal technical challenge is the output and cost. You'd need a lot of these to make up for a diesel generator - 55 of the imagined machines to match my car engine, not accounting for variability. Solar is getting cheaper. Designing a safe and robust machine that conforms with the IEC 61400 series standards, with independent and credible power curve tests and in an open and engaged manner - this is the challenge.

As with regular wind turbines, safety, performance and cost will determine the future of balloon mounted systems. For sentimental reasons, I'd like such a thing to exist. Economic considerations might not allow it.

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