Wind Turbine Design, Cube Laws, Efficiency and Cock Ups

Well, I have to 'fess up to having made an error regarding the output characteristics of modern day Industrial Wind Turbines.

A silly mistake at that.

But possibly a mistake that also reveals some interesting possibilities with wind turbines. Especially related to reducing their size, noise and increasing useful power output.

Crack Pottery? Possibly. But I've not been at the cider yet. (honest)

First, in order for this post to make sense, let me summarise some things that ARE true.

• The energy in the wind is a cube of the speed. In other words if you double the wind speed – wind contains 8 x energy. Halve wind speed- wind contains one eighth the energy.

• The theoretical maximum amount of this raw wind energy that can be harvested is 59.3% (Betz's Law)

• In reality the most efficient turbines manage about 45% (at a wind speed of around 7-8 m/s).

All of the above are correct. (Or I really am in trouble!)

My mistake in some earlier posts was to assume the efficiency of a wind turbine was roughly constant across the operational wind speed range (up to maximum output). 

Sadly this is nowhere near true.

In reality the efficiency (or how much energy the wind turbine can actually suck out of the wind) drops like a stone as the wind speed increases.

For most industrial wind turbines the highest efficiency (at around a wind speed of 7m/s) is about 45%. But as the wind speed increases, the efficiency falls to around 10% at a 90% loading.

The overall effect of this is to roughly linearise the power output to the wind speed. So instead of getting eight times the power out when you double the wind speed you only get double the power out. The rest is spilled.

So what does this matter if the thing is only as efficient as a 19^th century steam engine when confronted with a high wind?

It matters a lot.

Way back in 2002 at the Lee Ranch wind turbine research facility in New Mexico, it was discovered that 50% of the annual energy output of a wind turbine was delivered in 15% of the time. 

My own analysis done back in 2011 showed that for a three month period the whole UK wind turbine fleet delivered 50% of its energy in 25% of the time. But remember that was for the whole distributed fleet. 

It would be reasonable to assume that for a single facility, the Lee Ranch figures are roughly correct for the UK too. Also, there is no reason to think any design change to wind turbines since 2002 will have significantly affected these Lee Ranch findings.

In order to harvest the 50% of the energy that is smeared out over 85% of the year you have to compromise the turbines efficiency at higher wind speeds. The result today is an enormous unreliable monster.

So, for a moment, let us forget about the grindingly low 50% of energy generation that gets smeared over 85% of the year. Let us concentrate on the other 50% that arrives in 15% of the time (currently at an efficiency of a measly 10-15%).

For arguments sake, let us design a turbine that may not cut in until the wind speed is 12 or 14 m/s but then delivers an efficiency of 40%. It will (MWhr for MWhr) be very much smaller, simpler and more robust than a conventional turbine. 

OK it will only operate for 15% of the time and it is truly intermittent. But all wind is intermittent. Remember a conventional turbines output during 85% of the year is pretty derisory anyway. Often it is so low that it might as well not be there.

So, build smaller more efficient turbines. Crucially, in order to make these turbines more efficient, they only operate at higher wind speeds. We then rely on gas backup for the rest. More predictable, less environmental impact and more reliable (due to narrower operating domain). 

Tell me where I'm wrong. (Seriously - I may well be)

Of course this is still all window dressing. This (and the rest of RE) is just Care Bear fluff. Nuclear plant (with some gas) is the ONLY viable option to cut GHG and air pollution.  

But I hope that this is at least “interesting” fluff.