Monday, January 7, 2008

How much hydro does it take to "power Glasgow"?

Whenever a renewable power facility is described they always say how many 'homes' it will power. Today's news says The 100MW Glendoe Hydro Scheme will be able to power around 250,000 homes – equivalent to a city the size of Glasgow.

I think this 'homes' description is really misleading, because I bet people confuse 'powering all the homes in Glasgow' with 'powering all Glasgow's electricity' or even 'powering all Glasgow's energy'.

Let's do a simple calculation.

The average expected power from Glendoe is 180 GWh per year [source]. Now
if we take 180 GWh per year and share it between a Glasgow of people
(616,000 people), we get 0.8 kWh/d per person.

OK; what is the average electricity consumption per person (including all forms of electricity, not just domestic)? Answer: 16 kWh/d per person. So Glendoe actually provides 5% of the electricity consumption of Glasgow.

So if people get the impression from the press releases that Glendoe will power Glasgow, they have been misled by a factor of twenty!

This is a bigger factor than the normal factor by which people are usually misled. The statement
that Glendoe (180 GWh/y) would power 250,000 homes implies that each 'home' uses just 720 kWh per year. But the normal assumption in press releases about wind or tide is to assume the average home uses 4000 kWh/y or 4700 kWh/y. What's going on? The ratio between 720 kWh and 4000 kWh (18%) is suspiciously similar to the ratio between the average power production of Glendoe (180 GWh/y) and its capacity (100 MW is equivalent to 877 GWh/y). Methinks that someone at Scottish and Southern must have screwed up (or deliberately misled the public) by pretending that Glendoe will produce 100MW 100% of the the time, whereas in fact it will have an average load factor of 20%.

3 comments:

David MacKay said...

Whahey! The Scotsman published my letter

David MacKay said...

I see that (As of December 2008) Press releases are still perpetuating the twaddle that 'Glendoe could power Glasgow'.

Helen Highwater said...

I was happy to see that someone else noticed this vast discrepancy between what the media were persuaded to say and reality (the 5% of Glasgow figure is on Wikipedia too). I used to walk in that area pre-hydro and knew those small streams no way had the energy to power Glasgow. I obtained figures for the reservoir filling and emptying times and came up with it would need roughly 25 Glendoes to power Glasgow inc. non-domestic.
We had to wait years for actual results due to the tunnel collapse. The load factor achieved was only 16%. A very senior SSE employee inadvertently gave me their calculation method and you're right- they didn't factor in the fact that the water would soon run out and take much longer to refill the reservoir. In June they only ran it for 2 hours! The other cheat that they used was to use 3300kWh as the average domestic consumption. DECC give this figure as the average for STANDARD meters, and 4227kWh as the overall average, and 6600 kWh for houses having off peak meters. Instead of using the average, they cherry picked the lowest figure. At the time in 2006 the average was c. 4600kWh so 9 Glendoes would have been needed to provide the electric for the homes in a city the size of Glasgow. Conspiracy theorists would claim that renewable energy companies are deliberately misleading the politicians and public the generate support for ineffective schemes in the wrong places. Claims like "wind farm x provides the power for y,000 homes" should be ignored as totally inaccurate, irrelevant and meaningless.
And yes, the term "power" means all forms of energy; how many Glendoes would it need to replace all the gas central heating in Glasgow, which is what the govt. wants eventually. And what about the push for electric cars?