Electrical Storage

[andrewducker]

Anyone able to pluck some figures out of the air for the efficiency of converting electricity to heat and then back to electricity again?

(Based on the idea that molten salt at solar plants is apparently very efficient for concentrating and storing the energy that's being collected. Could it be used as a battery in general?)

Comments

[andrewducker]

Idle googling on molten salt batteries also found me this:
http://nextbigfuture.com/2011/03/low-temperature-molten-salt-battery-ten.html

Which looks to be pretty revolutionary, if it makes it to mass production.

[gonzo21.livejournal.com]

Yes, that would be fabulously elegant wouldn't it.

Would probably make desalinisation plants a lot more economical too?

[nancylebov.livejournal.com]

My back of the mind guess [1] is that the amount of salt wanted for batteries is small compared tot the amount produced by desalinization plants.

[1] Something even more casual than the back of the envelope.

[del-c.livejournal.com]

The brine that comes out of water desalination is usually pretty dirty, containing not just salt, but everything else that was in the water, so the plant would then have to get rid of the rest of the water, and then clean all the dirt out of the salt. Good clean fresh salt's not hard to get, cheaper than any desal plant could make it.

[steer.livejournal.com]

Electricity to heat, near 100% -- dead easy. Going the other way it's pretty hard to know what you mean by efficiency.

One way:
http://en.wikipedia.org/wiki/Thermoelectric_generator

The thing is that it really depends hugely on the situation. I imagine you want to know something like, if I created 100 Joules of heat then how "much" of that could be converted to electricity. The thing is that the answer really is "it depends" -- if you heat something slowly over a period of days you're going to be able to get nearly nothing useful out of it. On the other hand, if you make something incredibly hot, it'll be easier to get some electricity generation, but it will cool into the environment quickly too.

If your salt is NaCl then that's pretty damn hot. Drive a steam engine from it? (Don't laugh, essentially that's what a nuclear reactor does). Your efficiency is theoretically limited by the Carnot cycle here so "efficiency" is perhaps not the best measure. Wikipedia says "In practice, a steam engine exhausting the steam to atmosphere will typically have an efficiency (including the boiler) in the range of 1-10%, but with the addition of a condenser and multiple expansion, it may be greatly improved to 25% or better." But note that 100% is not achievable.

[andrewducker]

The insulators used in places like the solar plants are pretty darn good, apparently.

But yes - that doesn't sound terribly effective. Oh well.

[steer.livejournal.com]

I guess that depends what your expectations are. Lots of "state-of-the-art" power generation uses this component. Actually, I should have been fair and said steam turbine not steam engine.

When we're talking about generating energy from heat, it's better to throw away your normal definition of "efficiency" in any case.

[danieldwilliam.livejournal.com]

There are three problems in energy storage for static applications. They all relate to the length of time you need to store the energy. They are the in-day problem, the three day (weekend) and the six month problem. Basically, can I shift supply demand from 5am to 5pm, from Saturday lunchtime to Monday tea time or from winter to summer.


In order to be economically viable the storage mechanism needs only to be more efficient than the difference between the price of energy at peak demand and the price of energy at slack demand.


From memory, off peak prices are circa £25 per megawatthour and peak prices £100 per mwh. As long as the storage mechanism is 25% efficient (plus an allowance for capital costs and running costs) you’re in the money. Of course, adding significant amounts of storage will tend to flatten prices as off-peak prices are bid up and peak prices bid down by the effect of being able to shift supply around in time.

The question is how much usable and valuable energy that I put into this thing can I get out? If you are only converting electricity into heat and then into electricity you are limited to the volatility in the power market. Building one of these things is a bet on the volatility of the various energy markets.

One thing you can do to make a storage mechanism like molten salt more economically efficient is to plug the back end into a combined heat and power scheme. Probably some form of district heating scheme but not necessarily. Access to a heat grid make the ability of heat loads to buy or sell heat energy greater.

If you back the molten salt store onto a CHP scheme you can sell the value in storing the energy in-day or for three days for electricity and then sell the waste heat to people who want water at 40 degrees or there abouts.

One of the exciting things about the 21st energy market is the process of matching much more accurately and efficiently demand and supply of electricity, heat and motive power. They all have different periods of peak demand and efficient delivery.

[jarkman.livejournal.com]

It depends on the temperature your heat is at - the hotter the source (and the colder your heatsink), the more efficient the conversion can be.

The theoretical limit for conversion from heat to mechanical work is given by Carnot's theorem:
http://en.wikipedia.org/wiki/Carnot%27s_theorem_(thermodynamics)
which is 1 - cold/hot (with temperatures in Kelvin).

If your salt is at 500C, 773K, then you get something like 1-273/773, or about two thirds. And that's the theoretical limit, real turbines will give you rather less.

[steer.livejournal.com]

Heh... that's says concisely what I took a few wordy paragraphs to say. Thanks.

[ami-bender.livejournal.com]

I seem to recall that some of the new gas turbines (e,g, co2) have an improved efficiency of around 40 or 50% (if you believe the blurb). The question I have is how efficient does it have to be to become a viable alternative for a storage mechanism? I could see an inefficient mechanism still being useful for surpluses from renewables like solar/wind/tide.

Link
http://www.solarthermalmagazine.com/2011/03/06/supercritical-co2-brayton-cycle-turbines-promise-giant-leap-in-thermal-to-electric-conversion-efficiency/

[steer.livejournal.com]

Impressive -- though sort of worrying that there is so much of an efficiency gap left to gain.