Science Questions

How do we store solar and wind energy?

Tue, 23rd Feb 2016

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Question

Signe Klange asked:

How do we store solar and wind energy?

Answer

Kat Arney asked Peter for a solution to this question...Wind turbine

Peter - Yes, well I assume the question means how do you store them, not in itís current form, so the concept of storing photonsÖ

Kat - Yes, the sun: thatís a good storage device.

Peter - Of course, there are all different kinds of ways of doing that.   There are three main ones which are: chemical - so you charge a battery and discharge a battery - so youíre storing the energy from solar or wind and you can use it again at other times,  particularly that moves the energy production from say daytime to nightime.  Kinetic - so fly wheels.  Things where youíre changing the energy into somethings thatís kinetic which can then be run down againÖ

Kat - So like winding up a spring effectively and then like "poing"Ö

Peter - Yes, thereís some work done here in Cambridge on superconducting high temperature magnets for for flywheels for storing energy in Saudi Arabia, specifically.  Or, of course, potential energy and thatís been done for many years in North Wales actually.  Thereís a power station called Dinorwig  (I hope Iíve pronounced that correctly), which is about 2 gw, which is done by pumping water up when youíve got spare energy capacity in the grid and letting it come back down again and generating power when the needís high because weíre getting close to to our peak production.

Kat - And where are the innovations coming from because I know people like Tesla in the States, they're very excited about trying to get better batteries.  But it does feel that, although thereís been a lot of emphasis on more efficient solar panels and stuff, it doesnít really seem to be a lot of emphasis on making better storage techniques or better batteries.

Peter - Well actually the batteryís a different issue altogether.  Thatís to do with the chemistry of the battery and thatís not satisfying us.  Our mobile phones donít last long enough, etc., or our smart watches which weíll talk about in a few minutes no doubt.  Itís not that, itís how this energy is being stored.  Now, if the amount of energy that can be produced by everything say in the U.K. is limited of course - in any country it is.  But if you can move the usage away from the peak times to other times by storing energy, then you donít need to build new power stations.  And thatís being done either at the edges of the grid with big containers full of batteries or actually being done in the home which is where the Tesla thing comes from.  So the Tesla Box which is quite expensive, but there are UK versions of it, where youíre actually storing the energy in the home from solar or from cheap energy overnight and then distributing it out, possibly as DC because more and more devices, your USB devices, you LED lighting is DC, so yes, there is lots going on.

Ginny - Are there issues not just with storing the energy but also then connecting everybody up. Because say if we all put solar panels on our house, we wouldnít necessarily only be using the amount of power made from them.  So how do you connect up all the different power stations?

Peter - Well thatís been done for several years now because you can sell your solar energy back into the grid.  So there is something called the feed-in tariff which allows you make money out of the solar panels so actually, with electricity, itís pretty well connected in the U.K. anyway.  If you were to connect other things like water - is not nearly as well connected.

Stuart - Itís really interesting is the other side of that as well which is the usage.  So thereís a lot of research into smart grids which is the idea of can you do clever things with all these connected electrical devices.  So thereís a company in London thatís looking at you know, can they shut down the air conditioning in a particular hotel for a few hours that would reduce demand, learn to moderate the flow of energy coming out of the system to try and meet some of those demands and actually do things on the other side, on the consumption side as well.

Chris - Yes, sort of knowing what are priority things. Like you wouldnít want to turn off someoneís life support machine, for example, in order to meet the demands of the grid,  but my fridge really wonít care if itís interrupted for five minutes when everyoneís turning their kettle on because their favourite tele programme has gone to adverts.  And so you can dynamically shift power around the grid to make it more efficient so we donít have to keep loads of power stations sort of running on standby to mop up the shortfall.

Peter - That is going to be happening.  The smart meters and all our devices, in time, so we wonít actually have control over when the fridge is or the freezer is on or off.  Of course, if we want lights on weíre allowed to switch those on.

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I've always advocated using spare electricity to electrolyse water. The existing gas grid can store and distribute hydrogen either by itself or mixed with methane (old fashioned town gas was 50% hydrogen) and already has sufficient capacity and distribution reach to supply about half of all industrial and domestic power requirements, with existing storage capacity for  several weeks' supply. Why invent anything else?  alancalverd, Tue, 23rd Feb 2016




My favorite has always been to use gravitational potential as a storage system. 
This is more of a local storage system and is not necessarily connected to and distributed by the grid, although there is no reason why it couldn't be. 
And it is not a new invention. 

The system consists of two water tanks separated by enough height to allow an on demand hydro generator enough head to supply all the requirement that a household can demand. Obviously the water capacity of the system has to be enough to maintain that output for a minimum of 24 hours. 
It uses solar energy and any other energy input you want to couple to it, to pump water from the bottom to the top.
It then uses this gravity battery to run the hydro generator. 
It can be a stand alone Off-Grid system, or hooked into the Grid to sell power back once the top storage tank is at capacity. 
In Off-Grid mode such a situation would just overflow the top tank back in to the bottom tank. 
In a Grid-connect system this overflow is run through the hydro unit into the Grid.  $$$$$ 

The normal problem with local storage of excess power is the batteries. They are expensive and need regular replacing. 
Done properly this Gravitational potential battery, has minimum maintenance and no replacement costs.

arthur.manousakis, Wed, 24th Feb 2016

The problem with gravitational storage is its very low energy density.

The energy stored in 0.5 ton of water at 4 meters height (that's about the mass of your coldwater tank in the loft) is mgh = 500 x 9.81 x 4 = 19,620 joules. Enough to heat your 1 liter kettle by about 4.5 degrees.

Your 100 Ah car battery stores Vit =  12 x 100 x 3600 = 4,320,000 joules in a lot less space and weight - say 10 kg

1kg of methane or hydrogen will release about 45,000,000 joules on combustion.

Go for gas! alancalverd, Wed, 24th Feb 2016

But what about the energy cost in storage of the gas; high pressure needed, embodied energy in infrastructure etc...? chris, Wed, 24th Feb 2016

We already have a gas grid - it feeds factories, hospitals, your cooker and boiler, and about 60% of the UK electricity generating industry!

It used to transmit town gas, now methane. Town gas was stored at low pressure, methane is now imported and stored as liquid.

The entire infrastructure already exists and is in everyday use. alancalverd, Wed, 24th Feb 2016

I agree with alan regarding the storage of energy in the form of hydrogen gas. Not only can it be mixed with methane for use in home and industrial heating purposes, it can be used in the hydrogen cell and become an efficient form of electrical energy for use in transportation as well. Ethos_, Fri, 26th Feb 2016




My favorite has always been to use gravitational potential as a storage system. 
This is more of a local storage system and is not necessarily connected to and distributed by the grid, although there is no reason why it couldn't be. 
And it is not a new invention. 

The system consists of two water tanks separated by enough height to allow an on demand hydro generator enough head to supply all the requirement that a household can demand. Obviously the water capacity of the system has to be enough to maintain that output for a minimum of 24 hours. 
It uses solar energy and any other energy input you want to couple to it, to pump water from the bottom to the top.
It then uses this gravity battery to run the hydro generator. 
It can be a stand alone Off-Grid system, or hooked into the Grid to sell power back once the top storage tank is at capacity. 
In Off-Grid mode such a situation would just overflow the top tank back in to the bottom tank. 
In a Grid-connect system this overflow is run through the hydro unit into the Grid.  $$$$$ 

The normal problem with local storage of excess power is the batteries. They are expensive and need regular replacing. 
Done properly this Gravitational potential battery, has minimum maintenance and no replacement costs.


The average american household draws about 27 kwh per day. If you wanted to store that gravitationally, it would require pumping about 100000 liters (100 m3) of water to a height of 100 meters.

Even accounting for 50% round-trip efficiency of electrolysis, converting 15 liters of water into hydrogen would store the equivalent amount of useful energy.

Imagine every household having a 100 m high tower with a 10m3 capacity, plus the pumps and dynamos required for conversion: $$$$ indeed! chiralSPO, Fri, 26th Feb 2016


Perhaps it does sound like a bit of an ask when you put it that way and for the average house in America it would not be feasible. For the average house in America putting excess power into the existing grid is probably the best available option at the moment. 
I live in Australia and at the moment we pay an average of about 25 cents a kilowatt hour. The current offering for putting your own generated power into the grid is about 6 cents per kilowatt hour and there is talk of that coming down. I have a roof full of solar panels and the Only way to see any value out of them is to use the power they generate myself. Grid storage is a joke. 
Alan's idea sounds fine but it would require new technology. 
New metering and a pressure system to override the supply pressure so gas can be stored that way. There would also major safety concerns with such a generation and pumping system in a private home. Research, development, installation and maintenance costs are at this stage unknown as I have not heard of a feasibility study on such a thing. 

I live out of the city and don't have piped gas anyway. 
What I do have is over 100000 litres of water storage already and the cost of that storage is not as expensive as you might think. 
I also have 68 metres of fall. This is not an unusual situation in hilly country. I agree you would not consider such a system if you had to do it with towers. 
But for an initial set up cost that is not really outrageous you end up with a power storage system that done properly should last for several generations with minimal maintenance. And you can always have the option of further hibridising if you want to sell excess into the electric or gas grid.

So what is the best way to store solar and wind power? 
It really depends on country and more importantly situation.. arthur.manousakis, Fri, 26th Feb 2016


No need for a feasibility study. Nobody needs to generate gas at home, nor is it remotely sensible to do so when surplus electricity is produced in bulk from otherwise pointless windmills. The entire UK, and AFAIK most of Europe, and all US cities, have had a piped gas supply to homes and workplaces since around 1820. Investors are still building gas-fired power stations, and consumers complain if their new homes are not on the gas grid. The entire economy of Russia seems to depend on exporting gas to the rest of the world. I think 200 years' experience is enough to prove that it works.

If anyone wants to use small-scale storage from off-grid electricity, I'd advocate generating Brown's Gas by simply electrolysing water with alternating current, inside an ordinary steel gas bottle fitted with an ordinary gas regulator. This will give you all the delivery pressure you could possibly want, without pumping. The stoichiometric mix of hydrogen and oxygen can be burned in an unventilated space, unlike methane or pure hydrogen, which should make for a very efficient miniature central heating boiler. And you can condense the exhaust for drinking or recycling. alancalverd, Sat, 27th Feb 2016



I don't think storing Brown's gas would be sensible (certainly not pressurized), as it could explode. I have detonated small quantities of Brown's gas as part of a demo for school children (50 mL of atmospheric-pressure brown's gas gives quite a bang!!) Then, if you can't store it, and just convert it to heat as it's produced, why not just drive an electric heater with the excess energy and avoid conversions altogether?

I think rechargeable zinc-air batteries could do nicely for storing significant amounts of energy in a home. With an energy density of 38 MJ/L (40 khw/gal), as well as significant power density, and no risk of fire, explosion or toxicity, zinc makes an ideal stationary energy carrier (it's a bit heavy for use in a mobile setting, only storing about 5.3 MJ/kg).

Zinc air batteries are also a fairly mature technology, and the rechargeable versions are advancing quickly. chiralSPO, Sat, 27th Feb 2016


The problem, particularly for the UK, is that the hottest and coldest days are those with no wind, so we absolutely need to store energy from windmills if they are going to replace more than 10% of our electricity supply. alancalverd, Sun, 28th Feb 2016


The problem, particularly for the UK, is that the hottest and coldest days are those with no wind, so we absolutely need to store energy from windmills if they are going to replace more than 10% of our electricity supply.


Oh, I quite agree that storage is necessary. I was just pointing out that Brown's gas is not suitable for storage, and that if it is to be converted immediately to heat by combustion, why not just cut out the dangerous intermediate and go directly to heat? chiralSPO, Sun, 28th Feb 2016

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