Wednesday, February 11, 2009

More windfarm power per unit area

Executive summary:
The windfarm with the highest load factor in the British Isles has a power per unit area of 6.5 W per square metre.

Commentors on my previous article on
the power per unit area of windfarms
queried whether any cherry-picking might have happened
in the selection of Blood Hill windfarm; it was also suggested that we should work out the numbers for Burradale, the famous windfarm in Shetland with the highest load factor in Britain. The answer to the first query is "no, not at all" - Blood Hill was selected at random, and as I said, I would encourage anyone who can be bothered to look up the data for other windfarms to do so and add the results to the "withouthotair" wiki.
And now, to satisfy the request for cherry-picked facts about wind in the UK, I am happy to present...

The data: Burradale has five wind turbines: three in "phase 1" and two in "phase 2". Their capacities are 660 kW and 850 kW respectively, and their average outputs over the last few years have been 357 kW and 446 kW respectively. (That corresponds to load factors of 52% and 54%.) I judged the "area occupied" by the five turbines to be 0.3 square kilometres. The average power per unit area of this windfarm is 6.5 W per square metre.

This number can be compared with my assumed figure of 2 W per square metre for typical onshore windfarms in the UK.

So, a cracking good place for wind, Shetland! What does it need? Obviously what this place really needs is a campaign group opposed to expanding wind farms in Shetland. And The Good Lord hath provided "Sustainable Shetland".


Chris said...

Do you know what percentage of that 6.5W is lost getting to the grid where it can be of some use? My guess is that it's not even connected to the grid and if it was only about half of it would make it down to the heavily populated parts of Scotland.

David MacKay said...

The population of Shetland is about 23000.
The average output of this windfarm (2000 kW) is therefore easily consumed by the local population (the wind output works out to 2 kWh per day per person; average electricity consumption in Britain is 17 kWh per day per person. So there's no need for a big connection to the mainland. Yet. Of course if they expand their windfarm, they will need a new interconnector. And that's one of the initiatives that "Sustainable Shetland" are opposed to.

David MacKay said...

By the way, though, it's a myth that "half of electricity is lost in the long-distance grid". For the UK as a whole, the total losses of energy in the transmission network are only 8% of the generated electricity, and almost all of that loss is in the local network. Only about 1.5% of the generated electricity is lost in the long-distance network. I don't know what the loss in a Shetland-to-Edinburgh link would be, but it definitely would not be 50%. I would guess 10% loss, tops.

Chris said...

Thanks for the info.

IIRC, I think the 50% losses estimate come from including the cost of transforming to and from HT.

David MacKay said...

As I said, the losses in the national grid, INCLUDING TRANSFORMING TO AND FROM HIGH TENSION, are only 8%. Please banish this myth about the grid being terribly inefficient. It is not true. A typical transformer has a loss of roughly 0.5%.

Paul said...

Hi David,

Thanks for posting the follow-up with truly cherry-picked data - maths is not my strong point, so I wouldn't have been able to work it out :-)

That's an impressive site up there... what I am wondering now - is why the turbines are so feeble? With such a high load factor - shouldn't there be a bunch of 2GW devices up there? Would that increase or decrease the 6.5 W per sq metre figure?

Sustainable Shetland? Bah! About as 'appropriately' named as the Renewable Energy Foundation ;-)

Paul said...

And yes - I meant 2MW not 2GW ;-)

matty said...


The capacity of the turbine isn't particularly important to power density, except that newer turbines (which tend to be bigger) may have higher average efficiency.

That said, increasing the hub height increases the available power (due to decreased wind shear) and hence power density.

The average wind speeds on this site are already much higher than what modern turbines are optimized for, so increasing the height may may not be optimal.

This particular wind farm would operate at a much lower average efficiency that most wind sites as the power coefficient (which you can think of as a measure of efficiency) tends to be highest around say 7 to 11 m/s or so. The majority of this turbine's power is produced at much higher wind speeds where the power coefficient is lower (and the extractable power limited by the turbine capacity).

So to answer your question, you could design a turbine with this standard of site in mind and likely get a much higher power density. However, perhaps counter intuitively, this turbine would have a much lower capacity factor.

Indeed, if turbine manufactures were concerned about power density they may build very different turbines. However, most manufacturers would consider capacity factor more important than power density as such. Due to the nature of the wind resource, if turbines extract more energy from the wind the capacity factor tends to go down, not up.

e.g. in this case you could replace only the generator, increase the rated speed of the turbine, and thus you would extract more power at a lower capacity factor.

matty said...

I should add that a turbine manufacturer is going to be most concerned about maximizing profit above anything else!

(which seems to be with a design capacity factor of 20 to 40 %)