Venezuela, a nation where 82% of its electricity comes from renewable energy...

[BUFF]

One possible use (assuming a suitable power grid is set up) would be to use the electricity to power pumps which pump water to higher altitude reservoirs from lower altitude ones. When more power is needed this can be generated by allowing the water to flow back into the lower altitude reservoirs through a hydroelectric dam. Such reservoir setups exist in scandinavia.

It's called pump storage - they also exist at least in Scotland & Wales, probably in England as well I would imagine but you are limited to sites that are suitable.

[mediaboy]

I thought pump storage was used in most Hydro-electric powerstations?

What they teach us anyway :S

[BUFF]

it is which is why nvening said "through a hydroelectric dam" ...

the electricity for pumping back up comes off the grid from other power stations (oil/gas/nuclear etc.) surplus, usually during offpeak. No reason that it couldn't be provided by wind/wave power generators connected to the grid.

[Whiternoise]

Pump storage is the most common way - and the most efficient currently - of storing most forms of electricity.

However, there is a difference between hydroelectric power and pump storage.

Typically there'll be a power station nearby (or it can use the grid) which provides cheap electricity to fill up the reservoir (off peak). Then, when there's demand, they release the gates, electricity is generated and they can sell it off to make a profit. A bit like buying stocks when they're low and selling later. Trouble is, pump storage is actually an energy sink rather than a source. It's up to 80% efficient if well designed, and as thermodynamics prohibits free energy, it can't give out more energy than it took pumping it up there.

Hydroelectric power generators are electricity sources and work in more or less the same way, however the water going into the reservoir arrives by river or stream or otherwise. The hoover dam, for instance, provides a electricity by damming a river in a canyon. They took advantage of what was once a huge waterfall, popped a wall with a hole in way and thus use the gravitational potential energy of the water to power the station. Critically, it's self-replenishing. The river naturally stocks the dam of water to run it, and as it's got a much smaller exit hole than a waterfall, there's not much chance of it drying up!

So yeah, key difference:

Pump Storage: Electricity storage - but overall takes in more than it gives back and has negative net electricity output
Hydroelectric Station: Electricity production - requires a self replenishing water suppy, but gives positive net electricity output


Now, it's a great and very simple way of storing leccy. However, what happens when you have 30 odd gigawatts to store? That's a lot of water to pump up and a massive reservoir needed. This is where the trouble of storage comes along. A reservoir large enough to take that kind of power would be massive and expensive and there isn't really anywhere to put it - it would need to be artificial and that generally means damming and flooding a valley in Scotland (which funnily enough conservationists don't like).

The Hoover Dam provides 2GW. The government wants to 33GW to be in operation by wind power by 2020. Now, even if we wanted to store a third of that (off peak) we'd need something in the region of 5 hoover dam's worth of reservoir to cope! There simply isn't that sort of space. It depends how the government feel like doing it. If they start replacing old stations with equivalent wind farms then presumably they'll just link them up to the current reservoirs and build a couple more when they find a good spot to do it - i.e. replacing rather than increasing, and with current figures they'd be looking to directly replace 20GW of power station with wind/renewables by the 2020 deadline.

Sorry it's a bit long, but physicists y'know...

[jim]

We should really be pressing ahead with plans like the Severn barrage in my opinion.

Hydroelectric is a proven technology, so we should be trying to apply its fundamentals to our natural environment - we might not have the huge valleys, but we do have some huge estuaries.

On another topic, would hydrogen not have some potential as a form of energy storage, in cycles of electrolysis and combustion? I guess the problem would be in scaling it up - and how efficient it would be, but I'm not seeing any major problems at the moment. Bear in mind that I've not read anything about it, don't know the first thing about electrolysis of water, and it only just occurred to me, so I don't doubt it has major flaws .

[Whiternoise]

You're hitting on hydrogen fuel cells there. And yes, they are effectively electrolysis batteries. It's one of the most basic "redox" reactions you learn about in any basic chemistry course and even at GCSE these days (Oxidation Is Loss, Reduction Is Gain - OIL RIG). You have two plates, an anode (negative) and a cathode (positive). Hydrogen is split into positive ions and electrons. As you should know, positive attracts negative - and the membrane between the two plates only allows ions through. The electrons thus travel down a separate channel - and that is by definition a current (where the electricity comes from). The ions go through the membrane and combine with the oxygen from the oxidant to make water - hence the clean byproduct. The electrons having flowed through the channel rejoin at the cathode and are also a component in the water making.



The only major flaw is availability.

It's described as a chicken/egg scenario. Why should you buy a car with a fuel cell when there are no hydrogen pumps (there are around 60 in california) and why should Shell/BP/etc build an H pump when there are no cars to fuel up. Once it gets going, if it gets going, the cost will decrease - arguably to less than petrol - and in theory things will become cleaner.

Don't forget that hydrogen still has to be extracted somehow, but there are plans to develop fridge sized hydrogen "factories" that you can put in your house to charge up the cells. Whilst it'll still likely rely on fossil fuels for the electricity, it's possible to use renewables to extract it. Oh, and hydrogen is about as abundant as it gets, so definitely "renewable".

Incidentally it's a well proven technology - the shuttle uses fuel cells to power their avionics i believe, and the by product is clean drinking water

PS: With the government a bit strapped for cash, plans to green up the country have sagged a bit. The barrage will likely be a very expensive project - circa £23b.
http://www.guardian.co.uk/environmen...-barrage-tidal

[Marcos]

EDIT: @ Marcos, warmer climate makes very little difference to anything except solar power. Hydroelectrics work by pumping water into a reservoir and letting it drain back down again - this works regardless of the temperature (we've got a fair few here).

what i meant by warmer climate is that a lot less electricity would be spent on heating buildings through the winter. Which lets face it, takes a big chunk of our usage

[jim]

You're hitting on hydrogen fuel cells there. And yes, they are effectively electrolysis batteries.

Well, quite. I appreciate its potential as a form of transportable energy, but obviously in its current form it is completely and utterly useless... its sole benefit would be to focus the pollution somewhere else... and I'm not sure that really counts as a benefit.

Used as a form of energy storage, mind, where the power itself comes from is really not the issue.

And yeah, regarding the Severn, doesn't it always come down to cost at the end of it

[Whiternoise]

I wouldn't call it completely useless. With petrol there are two polluting stages. You've got to extract and refine the crude - which is pretty bad for the environment. So that takes some of the "bad" electricity, as well as polluting. Secondly when you combust it you make plenty of horrible stuff that has to be filtered with the catalytic converter - as well as the carbon emissions.

With hydrogen cells, the only stage is the extraction of "raw" hydrogen. This can be done cleanly - perhaps with a solar panel/wind turbine on your house, or using dirty electricity from the mains. That is the only polluting stage. When you convert it into electricity, the key point is that the only byproduct when straight oxygen is used is water.

As i said, it's not completely and utterly useless - it's a tried and tested technology with NASA.

It's not a particularly good way of storing energy from another source, it's fiddly and around 80% is the absolute maximum efficiency that people are claiming for electrochemical storage. Reservoirs are reasonably cheap and can be naturally topped up (via rivers, rain, etc) to gain back some of the energy used in the pumping (i.e. by extra work such as heat, friction). However, on small scales, it's vastly more efficient than petrol and diesel (diesel is 45-50% efficient - tons of energy is wasted as heat). Hydrogen cells can also be recharged using their own fuel - remember the output is water, so you just make a closed loop and re-extract the H and O2 from the waste (using say solar panels on the roof of the car).