Screw Hydro!

2009-09-03

The Archimedean screw. A venerable machine for lifting water. You can run it in reverse to generate power. How much?

New Mills, where the Sett meets the Goyt, has a community owned Archimedean screw. From their blog the energy generated for each month is:

September: 11108 kWh
October: 25356 kWh
November: 24232 kWh
December: 29513 kWh
January: 19512 kWh
February: 9185 kWh
March: 20330 kWh
April: 3091 kWh
May: 4436 kWh
June: 1389 kWh

Somewhat arbitrarily, but giving them some benefit of the doubt, I’ll replace September’s figure with October’s (perhaps the low September output was mostly teething troubles), and for the missing July and August figures I’ll use May’s.

So the total is: 25356 + 25356 + 24232 + 29513 + 19512 + 9185 + 20330 + 3091 + 4436 + 1389 + 4436 + 4436 = 171272 kWh per year.

or 19.6 kW. This is considerably lower than the 31 kW quoted by one of their investors.

Nice rule of thumb I discovered whilst writing the post: 1 kWh per year is 0.1 W.

The people who built it give it a plate rating of 63 kW (it’s capacity, or maximum power output). So it’s load factor is a little less than 1/3 at 0.31. They also quote a flow rate of 2860 l/s with a drop of 3m. Neglecting the water’s kinetic contribution (which I’m not sure is reasonable), the water has a power of about 86 kW (2860 litres of water is about 28600 Newtons, dropping 3m every second). So the extractive efficiency is about 73%. Quite impressive. I wonder if it can really be that high? Perhaps at high flow rates the kinetic energy is a more useful contribution.

The seasonal nature of the power is clear from the graph:

(the empty bars are missing data, not zero generation)

Basically, you only get power in winter, when it rains. The rest of the load factor gets eaten away by maintenance (oiling, fishing, that sort of thing), high water flow (!) and HSE requests (which I take to mean noise complaints).

David MacKay, in his book “Sustainable Energy – without the hot air” has a cute chapter about hydro. He analyses the total energy of the rain falling on our land and concludes that we can only ever produce about 1.5 kWh per person per day from hydro. After that there’s not much to say, and the chapter is correspondingly short. His figures for actual UK production (page 56) suggest a load factor of 0.29 for large scale hydro, and 0.16 for small scale hydro. So Torrs Hydro is doing atypically well (or I’ve been overly generous in filling the data).

The thing that surprises me is that the Archimedean screw produces a solution that is comparable, in load factor and efficiency, to large scale hydro.

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4 Responses to “Screw Hydro!”

  1. Gareth Rees Says:

    1.5 kWh/day from hydro

    You need to add “per person” here.

  2. Nick Barnes Says:

    The other useful rule of thumb is that 1 W = 1 GBP/year.

  3. Ted Lemon Says:

    This no doubt explains why New Zealand has such a terrible power shortage…


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