[ExI] Are mini nuclear power stations the way forward?

[Kelly Anderson]

On Sun, Mar 27, 2011 at 5:38 AM, Eugen Leitl <eugen at leitl.org> wrote:
> 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.
>> http://en.wikipedia.org/wiki/Compressed_air_car
>
> Disadvantages
>
> 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.

Absolutely! And we have the same problem with batteries and fuel cells
as well. Since we're talking about storing excess solar energy, it is
all the same kind of problem, although the specific numbers may differ
for particular systems.

>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.

Yup.

> 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.

I agree that the heating of air on compression, and the cooling of air
on decompression are the greatest problems to overcome in a compressed
air power storage system.

> 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.

That is a correct analysis.

> 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.[3] 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.[6]
>[citation needed]

Ok, correct me if I'm wrong on this one... but if you have compressed
air in a large storage tank at the gas station, and you merely
transport that compressed air into the car's tank, that doesn't
involve active compression, and thus for that particular exchange,
there is no heating or cooling issues. Getting the air compressed at
the gas station is admittedly a big issue, but fueling up is not, I
think, a big problem. Going down the road of course is another time
where the temperature issues come up.

> 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%.[7] For comparison, well to wheel efficiency using a modern
>internal-combustion drivetrain is about 20%,[8] 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.

Not if the compression were accomplished with solar or wind... right?

> 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.[9]

I had heard Tata had gone above 20 miles, but that may have been with
a hybrid system compressing air as you go...

> 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.[10]

Interesting.

>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,[11] when
>operating on compressed air alone, but in as late as mid 2009, MDI has still not produced any
>proof to that effect.

Yeah, got to see something actually working to totally believe it.

> 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."[12]
>
>
>> 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.
>
> http://en.wikipedia.org/wiki/Compressed_air_energy_storage
>
> Compression of air generates a lot of heat. The air is warmer after compression.

Understood.

>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.

Perhaps something can be accomplished with this more effectively in a
home electric storage system than in a car where weight is more of an
issue... just a thought.

> 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%.[3] 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).

fascinating.

> 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.[4] 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,[5] a 54% thermal
>efficiency comparable to the McIntosh 6.8 MJ, at 53% thermal efficiency.

So how much does the natural gas heating eat into the storage capacity
and 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.

Yes, conversion of energy type always results in some heat loss. That
is to be expected. Low power levels sounds like it might work in a
home storage environment. How much do you think the rules change when
dealing with a home electricity storage unit vs. running a car? i.e.
what can you gain by having the luxury of extra weight in the system?

> 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[6] and trams.[7] 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'[8] 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)[9] 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.

Yes, I understand that the carbon fiber tanks are a necessity for
weight and safety issues.

>> 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.

No, they can't. It would seem that they have backed off of some of
their initial claims and schedules as well :-(

>> >> 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
> technology:
>
> e.g. http://www.kk.org/thetechnium/archives/2009/07/was_moores_law.php
>
> http://www.kk.org/thetechnium/Battery%20Energy%20Density.jpg
>
> http://en.wikipedia.org/wiki/Energy_density
>
> http://upload.wikimedia.org/wikipedia/commons/c/c6/Energy_density.svg

Pity that.

>> > 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).
>
> Just use ethylene glycol or another antifreeze mix, picking a mix that
> will survive your worst case without freezing.

That makes sense.

> 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?

My generator is gasoline, not diesel. Currently, I have no batteries,
but previously I had 8 large L16P Trojan batteries.

http://www.trojanbatteryre.com/PDF/datasheets/L16PAC_Trojan_Data_Sheets.pdf

I can't get it to download right now, so I don't know the Wh off hand.

-Kelly