PowerSwitch

Here's a question for all you engineering types...

Every weekend I cross over a weir into Reading and notice the enormous force that the water has as it passes under it. Is there any practical reason why a turbine or two couldn't be built into weirs? Is it simply not worth the bother or too expensive?

It seems such a waste.

What's the level-drop (Head) like? It might be too low for any design to get a reasonable %-age of the flow energy out.

I also know the bureaucracy is a pig (planning permission, extraction licence from EnvAgency, finding out who owns the river in the first place, etc). Plus there's always some eejit who says it "doesn't look nice".

But apart from that no, no good reason.

Correct me if I'm wrong, but wasn't London Bridge damaged by ice which built up because the bridge created too many obstacles in the river?

The drop is about a metre I guess but the Thames is quite wide at that point. You can see by looking at the water at certain points that there is enormous force in it.

These guys are planning a new hydro station on the Wharfe:

http://www.derwent-hydro.co.uk/

we should see a lot more like this at old mill sites

Here's an outfit using Archimedes screw technology to harvest energy from weirs:

http://www.torrshydro.co.uk/

Thanks for that Andy, I think I'll splash out on it (I notice your avatar has changed a litte )



I thought the Royal Family were commissioning a hydro scheme at Thames lock?

Weirs are often used around locks on canals, so output could drop when the locks are busy. Also the volume of water (and hence current) varies tremendously according to rainfall. On the Thames, current warnings are sometimes issued, advising boats to stay put.



More here:

http://www.the-river-thames.co.uk/navigation.htm#five

On my MSc we looked at low-head hydro projects. I seem to recall they could operate as low as 2m head, and maybe even 1.5m. The trouble is that the machinery is large - with a small head, you need a large flow, and that means large pipes and turbines - lots of metal and concrete. Weirs however are a special case, because the civil works are already in place. I saw a design for a turbine in a kind of submersible pod, that you inserted into the weir. Completely out of sight. I think this is what's being used by the Royal scheme.

If you own a sailing boat, you can buy turbines which trail behind the boat and turn in the water. The power output isn't high because the flow is usually only 4 or 5 knots but I wonder if you could have the same sort of thing on weirs which is much greater than 5 knots at the weir itself.

As others point out, it is possible to harness the energy wasted at a weir, however due to the small head the results tend to disappoint.

A few years ago a study was carried out at a weir in Salisbury town center (near the aptly named bishops mill pub)

If I remember correctly, the expected output was about 10KW and this was not considered worth the capital expense.
The retail price of electricity was then about 5p, it is now about 12p which may make the scheme more viable, though of course capital costs have also increased.

10KW at present is hardly worth exploiting since oil, gas and mains electricicty are still cheap and readily available.
In a lower energy future 10 KW could be very valuable indeed, it would light 1,000 low energy lamps, enough for for a fair size town.
In daylight hours, 10KW used wisely would save a great deal of hand labour, for example milling grain, cutting wood, weaving cloth.

WolfattheDoor wrote:If you own a sailing boat, you can buy turbines which trail behind the boat and turn in the water. The power output isn't high because the flow is usually only 4 or 5 knots but I wonder if you could have the same sort of thing on weirs which is much greater than 5 knots at the weir itself.

It should work fine, though I have never heard of it being done.
Dont know how durable those marine turbines are, possibly only intended for short term use ?

AFAIK the output is less than 100 watts, not much by todays standards, but more than enough for lighting a home, and with battery storage would allow modest use of power tools etc, and possibly a high efficiency fridge.

RenewableCandy wrote:What's the level-drop (Head) like? It might be too low for any design to get a reasonable %-age of the flow energy out.

I also know the bureaucracy is a pig (planning permission, extraction licence from EnvAgency, finding out who owns the river in the first place, etc). Plus there's always some eejit who says it "doesn't look nice".

But apart from that no, no good reason.

There must be some minimum level below which bureaucracy doesn't apply. I remember reading something in Nick Rosen's "Off-Grid" about someone called Judy of the Woods who used hillside springs gathered in giant orange juice containers.

Anyway, the problem for both "tree-huggers" and "power-mad" has been harnessing the weird.

mikepepler wrote:On my MSc we looked at low-head hydro projects...

I saw a design for a turbine in a kind of submersible pod, that you inserted into the weir. Completely out of sight. I think this is what's being used by the Royal scheme.

Wow, that's just...out of sight!

A pod with an archimedes' screw-type turbine might work quite well on a weir. And in fact if you hung it there "temporarily" no-one need ever know

Cambridge City Council wasted several thousand pounds on a feasibility study into harnessing the water at Jesus Lock. This was after several learned people had told them the head was insufficient.

Just another example of Cambridge City Council doing what its best at. Wasting large sums of money

I'm hoping to insert a pipe in the river near my doomstead and pipe it to the farm. I think 2-300m will give me enough head for irrigation. For electricity there is a small waterfall and I hope to install a Banki turbine (without permission as I don't think anyone will notice. These turbines work on a low head. Thers information here

http://www.picohydro.org.uk/

about low head turbines

Andy