Thought that compressed air energy storage was ridiculously inefficient? Well, it looks like they’ve managed to best it with a new concept in energy storage: liquid air.
Yes, liquid air, as in cryonic liquid air. In other words, this is a combination of liquid nitrogen, liquid oxygen and little bit of liquid argon.
Via Discovery News:
Frozen Air ‘Battery’ Stores Wind Turbine Energy
Liquid air, which can be frozen, stored and warmed later, could work better than batteries or fuel cells to store energy from wind turbines or other renewables.
The technology was originally developed by Peter Dearman, a garage inventor in Hertfordshire, U.K., to power vehicles. For the past several years, U.K. tech firm Highview Power Storage has been working to transfer Dearman’s innovation to a system that can store energy for power grids.
Dearman’s idea works like this: electricity generated by wind farms at night is used to chill air to -310 Farenheit — its cryogenic state — turning it into a liquid. The liquid air is then stored in a giant vacuum flask until it time to be used again. This is done at night when demand for electricity is low and the energy from wind would otherwise go wasted
When demand increases during the day, the air can be warmed to ambient temperature. As it vaporizes, it drives a turbine to produce electricity, according to the BBC’s Roger Harrabin.
In July, Highview Power Storage signed a commercial agreement with a German firm to develop “frozen air” plants in Sub-Saharan and South Africa. And it now has a pilot facility near a traditional gas-powered plant outside London. That way it takes advantage of the plant’s waste heat to warm the liquid air, making the entire process more efficient and less costly. Company officials say their energy-storage system is best designed to help smooth out the peaks and valleys of energy production that often occur with wind, solar and other renewable energy project.
And the video…
Liquifying air is a common industrial process. It is most often used as the first step in certain types of air separation techniques. Partial liquification of air can be used as the first step for the separation of nitrogen and oxygen, followed by additional liquification to separate out argon, or it can be used to produce liquid air which is then boiled in a series of distillation columns. Occasionally, air is liquified and used in its mixed state as an ultra-low temperature refrigerant.
The process is very simple. Air is compressed to extreme pressure, which causes it to heat up. The heat is removed by passing the air through heat exchangers, which may be actively refrigerated to aid the process. Once the heat is removed from the highly-compressed air, it is reexpanded back to ambient pressure. Some of the air boils off in the process, taking additional heat with it and resulting in a super-cooled liquid. In practice, of course, it’s a little bit more complex than this. There are additional heat exchangers to reclaim some of the heat from the process stream and the process may be done in stages. After the air is liquified, it is stored in an insulated tank, often a Dewar container.
Although the principle is simple enough, the process is incredibly energy intensive. Even when the most modern and efficient equipment is used, the amount of energy required to liquify air enormous. So much so that plants that liquify atmospheric gasses are normally located near cheap sources of electricity or even have their own generation capability on site.
Most of the energy is lost in the process. The final product does contain some recoverable energy, by virtue of the fact that it has a much different temperature than the ambient environment and can be expanded when warmed. However, the best of the best gas refrigeration systems only manage to achieve a Carnot efficiency of about 25-50%. That also does not include the loss that occurs as a result of storing the liquid for any length of time, during which, as a result of imperfect insulation, some inevitably evaporates away. Thus, for every two joules of energy that goes into producing liquid air, only one joule is actually retained.
In fact, existing large plants are much less than 50% efficient. The figure comes from a hypothetical proposal of a purpose-built energy storage plant where the cold temperatures of the expanding air is re-captured into some kind of intermediate storage mass and then used to aid in the pre-cooling of air that is being compressed. Of course, this would vastly complicate the procedure and as yet, it has not been validated as workable. These ideas are common in various proposals for compressed air or liquid air storage. Unfortunately, as a consequence of thermodynamics, you cannot keep re-capturing and reusing the same heat (or cold) without losing most of it.
Unfortunately, it gets even worse from here, because getting that energy back from the liquid air means even greater loss. In order to convert the energy back into mechanical and ultimately electrical energy, the liquid air must be heated so that it expands back into a gas. In principle, this could be done by just exchanging the heat with the atmosphere to return it to the ambient temperature, but doing so is more difficult than it might seem. For one thing, frost is quick to build up on any radiators used, reducing their ability to exchange heat. Of course, it could be heated by using a gas flame or some other heat source, but that would also mean a significant amount of the energy would be coming from burning fuel. You may as well just burn the fuel to begin with and dispense with the ridiculously cumbersome and lossy process of pre-cooling the air to a liquid.
Waste heat may be able to help, but it can only do so much and would limit this to a very secondary method of energy storage, making the claims that it is somehow going to have a major impact impossible. It does, however, make this a great way of adding a “green” addition to a thermal power plant, which always seems to makes people feel good.
Once the air is heated enough to cause expansion back to a gas it would be used to drive a turbine or some other engine. This is actually a thermal engine, although the method of energy storage is the reverse of how thermal engines are typically thought of, since in this case, it’s the environment that is hot. As such, it is possible to use basic formulas to calculate the Carnot limit of an engine that runs on a liquified gas. An engine running on liquid nitrogen and at an ambient temperature and pressure of STP would be expected to have a total Carnot efficiency of about 74%. Nitrogen composes the majority of air, but since oxygen boils at a higher temperature, a liquid air engine would have a Carnot efficiency of a bit less.
Of course, Carnot efficiency is the theoretical limit of an engine’s efficiency but no engine ever reaches it. It would presume that the liquid air were warmed all the way to ambient temperatures without loss (which it wouldn’t be) and that there was no fluid friction involved (which there would be) and that all other aspects of the engine were otherwise perfect and free of any resistance. This never happens. So, really, the engine would not achieve anywhere near 74% or even 70%. The best turbines out there can get about 75% of their Carnot limit. That would mean that realistically, a liquid air engine might be able to achieve about 50% efficiency.
I should note that if we compare this to the actual historical performance of liquid gas engines then these numbers turn out to be exceptionally generous, because such engines have been built and they tend to be very very inefficient. However, there will be those who claim that they can somehow stop frost from becoming a problem and push everything to its limit to get a 50% efficiency rating.
Note that the efficiency will go down significantly during cold weather.
Therefore, we can approximate how much energy you can get out of a liquid air storage system:
Based on current air liquification technologies and the current standard for small to medium thermal engines:
25% * 33% =8.25%
Best case, if the turbine preforms as well as the best large turbines do and the refrigeration is 50% efficient (Which is highly suspect):
50% * 50% = 25%
Of course, none of this actually considers the other losses, such as the fact that electric motors and generators are only 98-99% efficient and that some of the liquid air will evaporate. These may seem like small losses, but they compound!
By comparison, the current standard for grid energy storage is pumped hydro. Pumped hydroelectric storage systems can achieve efficiencies of between 66 and 75%, which blows away the idea of liquid air energy storage. Utility scale batteries are currently expensive, but a good battery and inverter plant can return more than 90% the energy put in.
And NO, this is not a new idea!
Based on the amount of press out there and the number of pundits jumping up and down and saying this will be the end of batteries and that it’s a revolutionary way of storing energy, you might think that this was actually new and that someone had only recently come to realize that liquified air can be used to store energy. Unfortunately, it’s an all too familiar pattern. Journalists and politicians seem to think every tired old idea was just invented and is brilliant and the perfect place to throw some money.
In reality, this idea has been around for a very long time. In the 1970’s, 1980’s and 1990’s, the term “liquid nitrogen economy” was being thrown around right along side all the other “economies” (methanol, ethanol, sugar, hydrogen etc) that were being proposed. In the late 1800’s, liquified air was tried for a variety of vehicles from cars to flameless locomotives. They were even less successful than their compressed air cousins and the concept was dropped entirely as soon as electricity and gasoline engines became available. One company claimed their car could drive 40 miles at 12 miles per hour using a 18 gallon tank of liquified air. Even by standards of the day, that was not very impressive.
In the late 1800’s, an inventor by the name of Charles Triplet began developing a means for liquification of atmospheric gas on an industrial level. Among other uses, he promoted liquified air as a possible source of energy. The systems he developed did prove commercially viable for the purposes such as deep refrigeration and gas storage, but they failed to ever succeed for energy storage.
The image to the left is of a liquid-nitrogen powered vehicle (basically the same concept as liquid air) that was constructed in 1997 at the University of Washington. Another similar vehicle was constructed at the University of North Texas that same year. Many many others have been built, both in the form of vehicles and static engines, powered by liquid nitrogen or liquid air. Every time one of these vehicles or motors is built, the press seems to gather around it and assume it must be something new and amazing, despite the fact that this has been done for more than a century.
These engines are interesting as scientific curiosities and they certainly do a good job at demonstrating just how diverse thermal engines can be and the energy that can be produced through phase changes. That’s about their only practical use, however. Perhaps a winning science fair project but definitely not a practical way of storing energy.
This entry was posted on Monday, January 7th, 2013 at 9:12 pm and is filed under Bad Science, Enviornment, History, Just LAME, Misc. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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