OK, so I'll have a punt at this. You can challenge all of my assumptions as you like.

OK, so 1 litre of water falls on each square cm per year (I think that's your statement). That gives us a pulsating 2.5x10^15 litres of rainfall in the UK per year. (2,500,000,000,000,000 litres).

Nicely enough, that's the same number of kilos of rainfall. OK, so if we with 100% efficiency extract 10m of potential energy from this water (PE = mass x g x height) we get 2.45x10^17 J of energy (per year).

There are 31.5 million seconds in a year, so the power output is 2.45^17 divided by 31.5^6. Which is about 8000MW, or the output of 2-3 coal fired power stations.

Clearly we won't a) get our hands on all the water or b) get 100% efficiency from our mythical PE recovery engine.

I think the key point is that you need the big drop to get the power output through a tubine that would even approach this sort of efficiency.

Unfortunately they tend to break A LOT quicker when exposed to salt and moisture. At Toyota we had numerous forklifts on long-term lease in seafood factories, canneries, salt distributors etc. Unlike other customers whose trucks we almost never saw returned to the depot for repairs within the five year extended warranty these often lasted no more than a year before manifesting major faults. And when they came back they looked not years but [idecades[/i old. Wheel nuts, load chains, king pins, tie rods etc. were seized solid and usually required the help of burning gear to remove. The lads hated working on them because even simple tasks ended up taking hours to accomplish. Salt is hideous stuff and it wreaks havoc with not just metals but electronic circuitry, too. It was very common to find a truck inoperable because of a dead short within a corroded wiring loom.

Of course, there are materials resilient to salt corrosion. But they tend to be expensive and/or unsuitable for engineering tasks. This is the reason wave energy didn't take off years ago.

I know.


It's one of the reasons. However, engineers have been coping with salt water issues for centuries and none of these problems are insurmountable.

They also require a 3rd component - Air or more specifically oxygen (hence the term oxidation). Otherwise we'd still be picking at salt with hand tools and getting it to the surface in wooden barrows. Steel and iron ships that were sunk over the decades are still pretty much whole. So yes, your fork-lifts working in seafood environments corroded out quicker than in other environments but that was because the perfect storm of all three components required for oxidisation were present.

As for tidal power, there are a number of sites that at first glance appear to be perfect, the Bristol Channel being a prime one. The strength of tide ripping threough the channel, coupled with the 2nd greatest tidal range in the world also has to be tempered by the amount of silt and other debris (including trees, dead animals and humans) that is also carried by the tide. Tidal barrages also divert the natural water course and could pose other environmental problems, up or downstream, such as unexpected erosion or silt deposits. About five years ago, when the Bristol Channel Tidal Barrage was being considered, another company came up with the idea of anchoring sub-surface turbines to the sea bed, thereby leaving the channel navigable and causing negligible water diversion. As with most things though, no money was thrown at the idea.

There has also been a hell of a lot of environmental opposition to the Bristol Channel barrage as large areas of tidal mud flats may be affected by any alteration in the natural tides - the St Malo barrage in France seems to have worked for 40 or more years though.

There used to be a great ferk-off, working model of the River Humber in an enormous hangar on King George Dock, that was used for prctical modelling of tide flows and effects. Unfortunately once BTDB was disbanded and the docks privatised, no one saw a need for it anymore.

Now that would beat anybody's model railway layout