[ExI] Are mini nuclear power stations the way forward?
eugen at leitl.org
Sun Mar 27 11:38:06 UTC 2011
On Sat, Mar 26, 2011 at 06:58:45PM -0600, Kelly Anderson wrote:
> On Tue, Mar 22, 2011 at 3:21 AM, Eugen Leitl <eugen at leitl.org> wrote:
> >> I think the future in offline storage MAY lie in compressed air. Large
> > No, only for very large scale. The thermodynamics of it doesn't allow
> > small scale.
> Tell it to Tata motors.
The principal disadvantage is the indirect use of energy. Energy is used to compress air, which - in turn - provides the energy to run the motor. Any conversion of energy between forms results in loss. For conventional combustion motor cars, the energy is lost when chemical energy in fossil fuels is converted to heat energy, most of which goes to waste. For compressed-air cars, energy is lost when chemical energy is converted to electrical energy, and then when electrical energy is converted to compressed air.
When air expands in the engine it cools dramatically (Charles's law) and must be heated to ambient temperature using a heat exchanger. The heating is necessary in order to obtain a significant fraction of the theoretical energy output. The heat exchanger can be problematic: while it performs a similar task to an intercooler for an internal combustion engine, the temperature difference between the incoming air and the working gas is smaller. In heating the stored air, the device gets very cold and may ice up in cool, moist climates.
Conversely, when air is compressed to fill the tank it heats up: as the stored air cools, its pressure decreases and available energy decreases. It is difficult to cool the tank efficiently while charging and thus it would either take a long time to fill the tank, or less energy is stored.
Refueling the compressed air container using a home or low-end conventional air compressor may take as long as 4 hours, though specialized equipment at service stations may fill the tanks in only 3 minutes. To store 14.3 kWh @300 bar in 300 l (90 m3 @ 1 bar) reservoirs, you need at least 93 kWh on the compressor side (with an optimum single stage compressor working on the ideal adiabatic limit), or rather less with a multistage unit. That means, a compressor power of over 1 Megawatt (1000 kW) is needed to fill the reservoirs in 5 minutes from a single stage unit, or several hundred horsepower for a multistage one.
The overall efficiency of a vehicle using compressed air energy storage, using the above refueling figures, cannot exceed 14%, even with a 100% efficient engine—and practical engines are closer to 10-20%. For comparison, well to wheel efficiency using a modern internal-combustion drivetrain is about 20%, Therefore, if powered by air compressed using a compressor driven by an engine using fossil fuels technology, a compressed air car would have a larger carbon footprint than a car powered directly by an engine using fossil fuels technology.
Early tests have demonstrated the limited storage capacity of the tanks; the only published test of a vehicle running on compressed air alone was limited to a range of 7.22 km.
A 2005 study demonstrated that cars running on lithium-ion batteries out-perform both compressed air and fuel cell vehicles more than threefold at the same speeds. MDI has recently claimed that an air car will be able to travel 140 km in urban driving, and have a range of 80 km with a top speed of 110 km/h (68 mph) on highways, when operating on compressed air alone, but in as late as mid 2009, MDI has still not produced any proof to that effect.
A 2009 University of Berkeley Research Letter found that "Even under highly optimistic assumptions the compressed-air car is significantly less efficient than a battery electric vehicle and produces more greenhouse gas emissions than a conventional gas-powered car with a coal intensive power mix." however they also suggested, "a pneumatic–combustion hybrid is technologically feasible, inexpensive and could eventually compete with hybrid electric vehicles."
> If I could score some parts from a Tata Nano, I think I could make a
> nice storage system... I don't understand what you mean by the
> thermodynamics of the situation in detail, I do understand that
Just ideal gas law.
Compression of air generates a lot of heat. The air is warmer after compression. Decompression requires heat. If no extra heat is added, the air will be much colder after decompression. If the heat generated during compression can be stored and used again during decomression, the efficiency of the storage improves considerably.
There are three ways in which a CAES system can deal with the heat. Air storage can be adiabatic, diabatic, or isothermic:
Adiabatic storage retains the heat produced by compression and returns it to the air when the air is expanded to generate power. This is a subject of ongoing study, with no utility scale plants as of 2010. Its theoretical efficiency approaches 100% for large and/or rapidly cycled devices and/or perfect thermal insulation, but in practice round trip efficiency is expected to be 70%. Heat can be stored in a solid such as concrete or stone, or more likely in a fluid such as hot oil (up to 300 °C) or molten salt solutions (600 °C).
Diabatic storage dissipates the extra heat with intercoolers (thus approaching isothermal compression) into the atmosphere as waste. Upon removal from storage, the air must be re-heated prior to expansion in the turbine to power a generator which can be accomplished with a natural gas fired burner for utility grade storage or with a heated metal mass. The lost heat degrades efficiency, but this approach is simpler and is thus far the only system which has been implemented commercially. The McIntosh, Alabama CAES plant requires 2.5 MJ of electricity and 1.2 MJ lower heating value (LHV) of gas for each megajoule of energy output. A General Electric 7FA 2x1 combined cycle plant, one of the most efficient natural gas plants in operation, uses 6.6 MJ (LHV) of gas per kW–h generated, a 54% thermal efficiency comparable to the McIntosh 6.8 MJ, at 53% thermal efficiency.
Isothermal compression and expansion approaches attempt to maintain operating temperature by constant heat exchange to the environment. They are only practical for low power levels, without very effective heat exchangers. The theoretical efficiency of isothermal energy storage approaches 100% for small and/or slowly cycled devices and/or perfect heat transfer to the environment. In practice neither of these perfect thermodynamic cycles are obtainable, as some heat losses are unavoidable.
A different, highly efficient arrangement, which fits neatly into none of the above categories, uses high, medium and low pressure pistons in series, with each stage followed by an airblast venturi that draws ambient air over an air-to-air (or air-to-seawater) heat exchanger between each expansion stage. Early compressed air torpedo designs used a similar approach, substituting seawater for air. The venturi warms the exhaust of the preceding stage and admits this preheated air to the following stage. This approach was widely adopted in various compressed air vehicles such as H. K. Porter, Inc's mining locomotives and trams. Here the heat of compression is effectively stored in the atmosphere (or sea) and returned later on.
Compression can be done with electrically powered turbo-compressors and expansion with turbo 'expanders' or air engines driving electrical generators to produce electricity.
The storage vessel is often an underground cavern created by solution mining (salt is dissolved in water for extraction) or by utilizing an abandoned mine. Plants operate on a daily cycle, charging at night and discharging during the day.
Compressed air energy storage can also be employed on a smaller scale such as exploited by air cars and air-driven locomotives, and also by the use of high-strength carbon-fiber air storage tanks.
> compressed air can get very hot and needs to be cooled... but it would
> SEEM that Tata has resolved these issues to some extent.
Tata can't magically route around thermodynamics.
> >> building sized batteries also have some interesting potential. An
> > The car industry will bring you pretty powerful batteries within
> > the next 10 years.
> I hope so. Battery power stored per kilogram follows a Law of
> Accelerating Returns curve, does it not?
Not at all, progress is linear, and will be sublinear as
it asymptotically approaches the ceiling of the storage
> > Yes, but one of the most inefficient things you can do with PV
> > panels you rely on to sit under snow. Climbing up the roof to
> > clean them off is not a particular sane way of dealing with the
> > situation.
> I think it is the only sane way to deal with it... putting in a
> heating system is just not practical. If I had it to do over again, I
It is not that difficult to add adhesive resistive heating pads to the
back of the panels even after the fact. (More adventurous natures
could attempt to bypass the panel diodes, and use the panel
itself for heating, e.g. this is a problem with monocrystalline
cells parts of which are shaded off, but I wouldn't do that).
> would not have put the PV on the roof.
> > If I knew I had to do that, I'd have a roof which is trivial
> > to access and safe to be on, or built electric heating, starting
> > with small segments below so that the PV panels assist with self-dethaw,
> > or install combination solar thermal/photovoltaics (I presume you
> > have Si panels, these would profit from liquid cooling) and dethaw
> > them by running a warm liquid until snow slides off.
> How would you keep the tubes of warm liquid from freezing? Run it all the time?
Just use ethylene glycol or another antifreeze mix, picking a mix that
will survive your worst case without freezing.
> >> likely not pay back for a week or more, by which time it would have
> >> snowed again.
> > Again, you can do things very hard for you, if you want to.
> The question is whether it can be profitably done cheaply.
> >> I have no alternative. If I could easily hook up a little coal power
> >> plant, you bet I would... :-)
> > You already have gasoline generators, why are you not running these to
> > defrost the panels? If you have gasoline generator backup, why do you
> > have batteries? I don't know the details of your installation, of course.
> The reason for batteries is to run off of the solar at night, when the
> sun has shone all day. Ideally, the generator would only come on once
> a week or so, say on a night after a cloudy day.
What is your battery capacity, in Wh? What exactly are you running
at night? Is your diesel on-demand or has to be switched on manually?
Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
ICBM: 48.07100, 11.36820 http://www.ativel.com http://postbiota.org
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