The Weekly Geek: micro-hydro power

Posted by bex — 27 February 2008 at 8:23pm - Comments

It's Weekly Geek time, and this week we're looking at micro-hydro power: a truly reliable, highly efficient, and extremely clean (it has no direct carbon emissions) way of generating electricity.

It needs no fuel but offers a constant supply of electricity which often increases in winter, along with demand. It has a long life cycle (typically 25 years or more). It can have low implementation and maintenance costs. And, unlike some large scale hydroelectric power schemes, it has minimal environmental and visual impacts.

It's so brilliant, in fact, that the Ancient Greek Antipater of Thessalonica was prompted to write:

Hold back your hand from the mill, you rinding girls, even if the cock crow heralds the dawn, sleep on. For Demeter has imposed the labour of your hands on the nymphs, who leaping down upon the topmost part of the wheel, rotate the axle; with encircling cogs it turns the hollow weight of the Nisyrian millstones.

OK, he was talking about ancient water wheels, but the two technologies are based on pretty much identical principles (and I'll take any opportunity to shoe-horn a bit of poetry into this column…). Water wheels have been around for millennia; Ancient Indian texts seem to refer to a water wheel being used as far back as 350 BC, although the first time we know for sure one was used was in ancient Greece and Asia Minor, around 240 BC.

Essentially, waterwheels convert kinetic energy in flowing water into motive power, by re-directing flowing water from a stream and dropping it onto a water wheel to turn it. In the UK, water wheels were (and occasionally still are) used for all manner of agricultural and industrial processes - from milling grain and cutting timber to weaving cloth and shaping metal.

Around 150 years ago, some bright spark decided to borrow from the principles of the water wheel and started developing technologies to generate electricity from flowing water. Nowadays, micro-hydro is a hugely popular way to bring electricity to rural communities around the world (Vietnam has over 2,500 micro-hydro schemes supplying over 200,000 household), and it's deservedly enjoying a renaissance in many of our own rural communities.

(Micro-hydro, by the way, usually refers to plants generating up to 100KW, or 100 standard units of electricity in an hour. Large scale hydroelectric power plants generate a lot more, but have all sorts of damaging environmental and social impacts. Micro-hydro has none of these drawbacks.)

Micro-hydro plants are very site-specific and the kind of plant built depends on two factors: the amount of water flowing past and the "head", or distance of the drop onto the turbine.

How it works

First, water is diverted from a stream or river along a channel known as "leat", "flume" or "head race". Sluice gates control the passage of water into the channel (so it can be shut off during maintenance, and the plant can be somewhat protected during floods).

After passing through a screen to get rid of debris, water drops onto a turbine. The type of turbine chosen (there are all sorts out there) depends on the flow of the river and the head. (At a push, an original water wheel can also be used, usually the overshot style, although this is far from ideal.) For simplicity, we'll describe a cross-flow turbine.

The force of the water on the turbine's bladed surface rotates the turbine around its horizontal axle. Historically, this axle was used to drive a mill's machinery. With micro-hydro, it drives a generator which converts the mechanical power into electricity.

Water leaving the wheel is drained through another channel – also known as the "tail race" - back into the river.

As this animation from EfficienCity explains:

Where they're built

Micro-hydro plants can be built in old, redundant water mills scattered around our countryside; a number of old mills are being converted because re-using structures like the weir, the leat and the original stone millhouse (to house the machinery) mean using fewer resources, less time and less money to build the plant. And because they're pretty. Pedley Wood in Cheshire and Gants Mill in Somerset are both sited in converted mills.

Another option is to build micro-hydro plants in entirely new sites. The same considerations (flow rate and head) apply, but if a spring-fed stream has enough of a drop, new sites can be developed without the need for structures like weirs.

Either way, most micro-hydro schemes are "run-of-river", meaning they don't have a reservoir and only take water from the stream when there's water available.

As concerns about climate change and fuel security grow, hydro-power is getting a fair bit of attention as a small, clean electricity source that can fit perfectly into a decentralised energy system.

More information

We'll keep advocating a decentralised energy system to government and local councils, but if you can't wait that long and want to install your own micro-hydro plant, here are a few resources that may help:

  • You can calculate the hydropower available in a stream, work out the timescale and estimate the potential revenue at hydrogeneration.co.uk.
  • The Centre for Alternative Technology advises you to also discuss the details with the local planning officials, in case the work needs planning permission. Obviously, you also need permission from the landowner.

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