Making liquid power

Mar 29, 2011 by Katie Gatto weblog

(PhysOrg.com) -- Power is a game of supply and demand. Unfortunately, it is also a very time sensitive game. You not only have to make the right amount of power, but you have to make it at the right time. Power is largely a perishable commodity. It can be stored in a battery, for a limited amount of time, but eventually it will wear out. This can be an issue with human generated power, but when you deal with greener methods of power, like solar and wind, we don't always have control over how much power is generated and when it is generated.

Since this creates a variety of issues with , and potential waste, researchers have been interested in ways to store the in the longer term. So, how do you keep your power in storage? You make it into a liquid.

Scientists at Britain's Highview Power Storage have figured out how to convert its excess energy into liquid air. The system that does this, which has been dubbed the CryoEnergy System, is like a very big power cold storage unit. The power uses the excess energy to power large refrigeration units. These units cool the powered air down to a temperature of -196 C, which causes it to liquefy. This air liquid, which is also known as cryogen, can then be stored in an insulated tank, that has an ambient pressure of about 1 bar.

When the power is needed, at high demand times, the air can be released into a sealed space. When the liquid reaches boiling temp, above -196C, the liquid will expand more than 700% and being to spin that staple of power generation, a .

The energy return on the process is about 50%. The CryoEnergy System has been in use at Scotland's Slough Heat & Power plant for the past nine months.

Explore further: Britain's first poo-powered bus takes to the road

More information: www.highview-power.com/wordpress/?page_id=5

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Parsec
not rated yet Mar 29, 2011
Th bp of O2 is about 90K, while the bp of N2 is 77K. I would be concerned about this scheme because of accidental fractional distillation of the air, because it could lead to a significant enrichment of the O2 content of the escaping air. While O2 doesn't burn, most other things burn quite nicely when the fraction of O2 in the air is increased much.
ereneon
5 / 5 (1) Mar 29, 2011
Is this really more efficient than just pressurizing the air normally? Cooling systems are notoriously inefficient.
Eikka
4.3 / 5 (6) Mar 29, 2011
Another similiar idea is pumping heat between two piles of gravel using a compressor and argon gas in a loop. One pile heats up, the other one cools down. Then when you want energy out, you reverse the pump and drive cold argon from the cold pile through the hot pile where it expands, drives a turbine, and returns to the cold pile again.

The efficiency can be up to 85%, but gets less as you draw more energy out, so the system has to be relatively large to be of benefit. At least liquid air always boils at -196 C so the temperature difference to ambient is constant, and so is the efficiency.

If they'd add a heat reservoir where they could dump the heat made by the liquefaction pump at a high temperature, and then use that instead of ambient air to boil and expand the air, then they'd get a higher return efficiency.
Eikka
1 / 5 (1) Mar 29, 2011
Is this really more efficient than just pressurizing the air normally? Cooling systems are notoriously inefficient.


The process is almost the same. You squeeze air, it heats up, you let it cool down and store it in a tank. Then you add ambient heat to expand it back to its original volume through a turbine.

Air heats up when compressed and cools down then expanded. This process simply stores the air in the densest possible way, which ultimately gives you the greatest pressure differential across the turbine.
GSwift7
3 / 5 (2) Mar 29, 2011
Stirring the liquid tanks prior to release should prevent O2 buildup.

Blairstown NJ has a storage facility where they pump water to a reservoir on top of a small mountain, then let it flow back down when needed. I believe this method is something like 75-80% efficient and has a huge capacity. I wonder what the load/duration of this compressed air system is? As far as I know, the most efficient system currrently under developement is the liquid salt thermal system (maybe 99%), but final cost/kWh is still up in the air pending more in-the-field experience.

Using liquid refrigerated air is different in one key way from using compressed air, and that is the problem of having to heat up compressed air before you can decompress it. Condensation has to be taken into account in the processes too. In the liquid air system, you should be able to us a sealed dry air source to reduce condensation problems.
GSwift7
1 / 5 (1) Mar 29, 2011
I wonder if the system above is a base load storage method or just a smoothing facility to absorb bumps in the load.

For any method to be viable, the final coat per unit over the life of the system needs to be cheaper than installing a natural gas turbine system.
sender
not rated yet Mar 29, 2011
With ideal gases and induction plasma the power conversion efficiency should be near 100%

Surprising.
GSwift7
1 / 5 (1) Mar 29, 2011
With ideal gases and induction plasma the power conversion efficiency should be near 100%


Too costly. Keep in mind that power generation is a for-profit business.
Eikka
1 / 5 (1) Mar 29, 2011

Using liquid refrigerated air is different in one key way from using compressed air, and that is the problem of having to heat up compressed air before you can decompress it.


Not really. When you put liquid air into a bucket, it boils away until the bucket and the surroundings have cooled down to -196 C and then it stops boiling. It no longer expands into gas.

It has to be heated just the same, which is done by blowing warm ambient air through a heat exchanger, or by waste heat from a nearby power station.
Eikka
1 / 5 (1) Mar 29, 2011

Blairstown NJ has a storage facility where they pump water to a reservoir on top of a small mountain, then let it flow back down when needed. I believe this method is something like 75-80% efficient and has a huge capacity.


Pumped hydro storage usually has a poor energy density. If you can pump a ton of water a kilometer up the side of a mountain, you get about 2 kWh of electricity back assuming 75% efficiency.

For one average household for one day, you would need about 40 tons of water lifted up the mountain. That would be about a small swimming pool's worth.

Now multiply that by 10 000 and you get the point.
GSwift7
3.7 / 5 (3) Mar 29, 2011
Yes, the lake at the top (and bottom) of the mountain is very large and the pipe leading down the mountian is about 15 feet in diameter, so the volume of water is huge. According to the above article, it is way more efficient than the liquid air method. The 75-80% number includes evaporation and water that leaks out through the ground as well as gains from rainfall. They use it for main load generation at peak times, and they have been using that facility for something like 25-30 years I think. The plant generates so much money that people in that town don't pay any property tax. I lived there for a few years. The area around the top reservoir is a nature preserve, with miles of walking paths. You have to watch out for rattlesnakes though, and of course the lake itself is fenced off and is unsafe for swimming. On a side note, that's the town where the original Friday the 13th movie was filmed. The later films were filmed somewhere else.
zevkirsh
not rated yet Mar 30, 2011
the reason this beats compressed air is because compressing air into a container requires too much volume per kilowatt of energy you put into the system to be industrially useful.

the storage tank needs to be too big and it needs to handle very high pressures, and is thus impractical and unsafe and less cost effective.

eric96
1 / 5 (1) Mar 30, 2011
Lol, this technology is nothing impressive. For instance, electrolysis can be used to store hydrogen...and then use a fuel cell to convert back into electricity again with a loss of 50%. Of course, the mentioned technology would presumably be safer assuming nothing explosive is in the vicinity otherwise it would amplify it at least 10 fold if not more. Storing energy really isn't as much of a problem as people make it out to be. With hydrogen or liquefied oxygen...the problem is the cost of hardware needed to make it work...probably similar or a little cheaper but much more dangerous to the cost of equivalent batteries. I suppose Best case scenario, would be to use a non-reactive gas that liquefies at a much higher temperature than -196C which would solve the pressure danger, explosion danger, and the need for lots of energy to cool gas to liquid state. However, their process is feasible for industrial and civil use...not so much for home haha.
eric96
1 / 5 (1) Mar 30, 2011
Correction:

Nothing new. You can use electrolysis (near 100% efficient) to store hydrogen and then use a fuel cell (50% efficient) to convert back into electricity, total loss of 50%. Liquefied oxygen firstly would be energy costly for refrigeration...and very explosive should anything explosive be in the vicinity. So really not safer and less efficient than electrolysis method. Cost of hardware is similar to equivalent needed in batteries and inverters etc. The problem
is that 50% loss, whereas batteries although they lose charge overtime are more or less 100% efficient. The problem with batteries is energy density. Best bet is in large capacitors.
unknownorgin
1 / 5 (1) Mar 30, 2011
The diagram is poor in that it fails to show the compressor is mechanicaly linked to the expander so the heat generated by compressing air gets converted to mechanical energy by the expantion turbine to help power the compressor. This is the same as using a stirling cycle engine to produce low tempertures by putting mechanical power into it. When generating electricity if they could use solar generated heat to boil the liquid air it would result in better energy return.

GSwift7
1 / 5 (1) Mar 30, 2011
The problem with batteries is energy density.


The problem with batteries is cost. They are very expensive per unit of energy stored and they wear out after a relatively few number of charge cycles.

They are mainly used to fill the time between when a main load supply goes down and a backup natural gas or deisel generator can come online.
RealScience
5 / 5 (1) Mar 30, 2011
eric96 - even the best capacitors today have an energy density much lower than that of batteries.
GSwift7
1 / 5 (1) Mar 30, 2011
Energy density isn't a concern for grid power storage anyway. The three factors are 1)lifetime cost per kWh, 2)efficiency, and 3)lifetime cost per kWh.
Sepp
4 / 5 (1) Mar 31, 2011
Simple compressed air in large underground storage caverns could store energy from both windmills and wave power systems in an admirable way. With gradual (stepped) decompression, condensation should not be a big issue. A 'quasiturbine' could convert the stored pressure into motion in a very efficient way, and could even be used inbetween storage sections that have not-too-great pressure differentials, where too high a pressure differential would need to be bridged.

This would permit to smooth out energy production, both from variable wind speeds and different wave activity, and guarantee constant generator rpm. It would be a rather simple matter to associate such caverns with windmills. Heavy equipment (such as turbines and generators) would no longer have to sit high up in the air as they do today. They could be located at ground level or underground...

No need to go into cryogenic storage. Space is not, generally, a big problem.
Eikka
1.5 / 5 (2) Apr 03, 2011
Lol, this technology is nothing impressive. For instance, electrolysis can be used to store hydrogen...and then use a fuel cell to convert back into electricity again with a loss of 50%.


The total round-trip efficiency with water filtering, electrolysis, leakage, pressurization, storage and pumping is about 20% at best, so hydrogen really isn't a viable method for storing energy.

You really have to be hell-bent to seriously use it in any way.
Quantum_Conundrum
3 / 5 (2) Apr 03, 2011
I don't think physical or thermal forms of energy storage make much sense. To me, chemical storage via electrolysis or other endothermic chemical reactions is much more energy dense and also less complex. Complexity is a bad thing, because it increases the likelihood of something breaking, and therefore increases maintenance costs.

The other advantage of electrolysis of water is that you already have access to water at the power plant anyway, even a large solar plant. So what you can do is use some of the energy to electrolysis water, and then after sunset and after stored steam has been used up, you then burn the hydrogen in a generator with a double carnot cycle, the same way natural gas power plants work, or in a fuel cell, to re-capture this power when it's needed.
Quantum_Conundrum
3 / 5 (2) Apr 03, 2011
The total round-trip efficiency with water filtering, electrolysis, leakage, pressurization, storage and pumping is about 20% at best, so hydrogen really isn't a viable method for storing energy.

You really have to be hell-bent to seriously use it in any way.


The total round-trip efficiency of a cryo-liquid isn't going to be anywhere near the ridiculous 50% listed here either. The carnot limit is 66%, and the best real-world generators are around 41%. You can look this up on the internet. Which means that even a double cycle would only get you to about 65% energy return, even if all the other steps in the process were 100% efficient.

Now that doesn't consider losses due to less-than-ideal insulation, it doesn't consider the cost of refrigerating the air, etc.

So their 50% number in this article is scientifically and fundamentally impossible. They are ignoring and/or under-reporting certain losses and costs in the cycle.

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