Compressed air energy storage and use system   

From Appropedia
Jump to: navigation, search

A compressed air energy storage and use systems (CAES&US) is a system for the storing of energy generated at one time, so that the energy can be extracted again at another time. Such systems are useful both for stationary applications aswell as mobile (transport) applications.

Contents

[edit] The compressor

By far the most important part of a CAES&US is the compressor. This, as it generates the "fuel" for the system for later use. The compressor is often the most energy-inefficient part of the system, unless it's properly designed (using a heat exchanger, and cold source). A cold source is needed as air that is compressed heats up a lot, causing waste of energy, aswell as problems during the compression itself.

[edit] Types

[edit] Main types and their compression ratio's

  • a reciprocating compressor (piston) can reach a compression ratio of 10
  • A roots blower can reach a compression ratio of 2 [1]
  • a turbocompressor can reach a compression ratio of 2,5 [2]
  • a rotary vane compressor can reach a compression ration of 6[3]
  • a scroll compressor can reach a compression ratio of 3
  • a screw compressor can reach a compression ratio of 15-20 [4]

[edit] Oil-lubricated and non-oil lubricated compressors

A rotary vane compressor system

Most of the main types presented here above come in "oil-lubricated" and "non-oil lubricated" form. Oil lubrication sprays a mist of oil trough the whole of the air compression system (via the air inlet). This lubricates the part (making them more durable) and also plugs little holes between the moving parts where air can escape trough; hence making the system more efficient. A downside is that (besides requiring more cleaning), the compressor can then not be used for compressing breathing air for living organisms.

[edit] Stages & combining compressors

To attain any useful compression ratio, a compression system should have several stages. A stage is basically a seperate compressor. Stages are required to discard more heat from the compressor-part of the system, and teh stages also created in progressively smaller sizes so as to improve the efficiency. Although a stage is basically a seperate compressor, they often hang together (composing a single part).

[edit] Combination 1: the breathing air compressor (non-lubricated)

This compressor is made using 3 stages:

turbocompressor
rotary vane compressor
reciprocating piston compressor
3 X 6 X 10 = 180 bar

Why this combination; ie instead of using another reciprocating piston at stage 2 ? The answer is that using a rotary vane compressor at stage 2 instead of a piston compressor increases the energy efficiency (pistons are very energy inefficient). The reason why pistons were used at all (despite their inefficiency) is because they are most useful for dry (non-lubricated) applications. Also, they are one of the most durable compressors, and thus they are placed on a later (3rd) stage.[5] Note btw that all three compressors (or atleast the last 2) can still be cooled by oil by using an oil bath for the compressors. The oil however is NOT mistified inside the air inlet. Note finally, that the 180 bar is all directed into 1 compressed air tank (most diving cylinders allow 200-300 bar of pressure, so we're quite close to this). Finally note that a quasiturbine can also be used instead of a reciprocating piston compressor for stage 3 (quasiturbine also has 10:1 compression ratio); given the much higher efficiency this would be an even better combination; however it may be difficult to acquire this compressor.

[edit] Combination 2: the CAES&US 1 (lubricated; intented for stationary use)

This compressor is made using 3 stages:

rotary vane compressor
rotary screw compressor
rotary screw compressor
6 X 15 X 15 = 1350 bar

The 1350 bar is divided over 6 compressed air tanks; each with 225 bar

[edit] Combination 3: the CAES&US 2 (lubricated; intented for mobile use)

Most mobile applications lack the presence of a compressor alltogether. A small, lightweight compressor could however be added to deplete any remaining electricity in the lead-acid battery (which needs to be taken along anyhow for powering other electric machinery (ie valves, regulators, ...) In some instances, the battery btw can itself be recharged using pedaling power.

[edit] Variable speed compressors

Variable speed/variable displacement seems to be a must to increase efficiency, atleast when the full power isn't used (else it doesn't seem to matter). If the power ratio is thus perfectly chosen to the needs, we don't need it. (not sure btw how variable speed works; ie is the electric motor simply fed with less power or are gears present)

[edit] The motor

A pneumatic motor is a motor that uses compressed air as its fuel source. The pneumatic motor is a very important part of the system. It is often an energy-inefficient part of the system. Unless it's properly designed (using a heat exchanger, and heat source). A heat source is needed as air that is expanded cools off a lot, causing less efficient use of the motor aswell as problems during the operation itself.

[edit] Types

Several types exist, and they have varying efficiencies

[edit] Workings of a regular system

Stationary pneumatic system 2 (ie for energy storage/use)

According to MDI (http://www.mdi.lu/english/moteurs.php ): Ambient air is compressed in the vehicle’s tank (A). The air coming from the high pressure tank (A) crosses a pressure reducer (which allows a quasi isothermal transformation; B). It is then used in a system of expansion with work, consisting of an active chamber (C) and an expansion cylinder (D).

According to Wikipedia: 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.

As I understand it:

  • the pressure reducer (A) only allows a limited amount of air to pass hence reducing the pressure
  • the active chamber (C) hence improves the efficiency by heating the air
  • D is the piston that provides the useful work due to the compressed air. The piston seems designed with a flywheel so that the piston comes back up after it has been forced down. The secondary piston seems connected to the same axis ?, and plugs the exhaust/recompression lines intermittently to the first piston.
  • E is the exhaust when not recompressing the outgoing air
  • F is the outlet line when recompressing the outgoing air
  • G is the compressor

[edit] Energy density of compressed air compared to SLA batteries

I did some comparing to regular lead-acid batteries,

Compressor systems for stationary applications
Compressor systems for mobile applications

According to Wikipedia (http://en.wikipedia.org/wiki/Compressed_air_car ), the tanks used operate at 4500 psi (or 300 bar) and compressed air at 30 MPa (4,500 psi) contains about 50 Wh of energy per liter. Lead-acid batteries have instead 180W/kg (see http://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery ) So, if a 1 liter compressed air tank with air weighs less than 1/3 kg (or 0,333 kg), the efficiency would be comparable; I'm not sure of this however.

For my project, I stated I wanted a range of 100km, but if we could attain 50km with compressed air, that might also allready be suitable, given that refilling stations can refill quickly. Although your APUQ car didn't achieve this (20 mins at 70 km/h), I think it may be possible for the velomobile, if a suitably dimensioned engine is used (ie the APUQ car had a 600cc/800HP engine, where only 150HP is required). Given that my velomobile is no way comparable to a car (ie weight, efficiency, ...), it should be possible to use compressed air.

Regarding the efficiency, I'm wondering whether the air compression can't even be lowered a bit or done a bit slower to crank up efficiency. According to wikipedia: current compressors use 14.3 kWh @300 bar in 300 l (90 m3 @ 1 bar) reservoirs, needing at least 93 kWh on the compressor side (with an optimum single stage compressor), or rather less with a multistage unit. 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%.

It is clear that the type of air compressor used, aswell as the pressure on which the air is stored in the air tank are of great importance. Multistage compressors, using a rotary screw are probably best used; the Eole-Tract project appearantly uses a very efficient air compressor. The pressure at which the air is stored is 300 bar/4500psi with the MDI car. This might however be too high, resulting in too great temperature differences at the pipes/components; also the pressure may be too great to allow any reasonable efficiency using the compressor.
  • At 300 bar, energy density of the tank is 50W/kg --> assuming a weight of 0,333 kg for a 1l tank, this makes the system almost as efficient as lead-acid batteries, which have 180W/kg (50W x 3 = 150W/kg), overall efficiency however is 11% (14% x 80% engine efficiency = 11,2%)
  • At 180 bar, energy density of the tank is 30W/kg --> assuming a weight of 0,333 kg for a 1l tank, this makes the system not as efficient as lead-acid batteries, which have 180W/kg (30W x 3 = 90W/kg); overall system efficiency however is ?

Another idea I had is in regards to the air preheating (active chamber). Although MDI uses a fuel (or adjuvant) to heat the air, I think that the heating can also be done by means of an electrical heater, powered by a "magneto" (as found in IC engines) and this on the wheel. As the wheel turns, some power is used to preheat the air, and this (unlike MDI) without any emissions.

Regarding the efficiency

Higher the pressure, more heat will be lost at the compressor, and more heat will be needed at the expansion engine. This is because higher pressure distroy the reversibility of gas systems. Pin hole regulator are totaly dissipative pieces of equipment, and rotary expander must be used to manage air tank expansion. (compressors use 14.3 kWh @ 300 bar in 300 l (90 m3 @ 1 bar) reservoirs, needing 93 kWh on the single stage compressor side). The overall efficiency of a vehicle using compressed air energy storage, using the above refueling figures, cannot exceed 14% (for comparison, gasoline efficiency form well to wheel is about 10 %). Do not confuse low system efficiency with much higher engine efficiency, where single stage flow-pressure engine efficiency can reach 80 % in Quasiturbine.

[edit] Improving the efficiency

--> May be incorrect ! Appearantly, the optimal system efficiency depends to a large amount on the components used in the system and the system dynamic has to be a compromise. Hence, the higher the power ratio (in HP) of the compressor, the more efficient the system becomes (as with a higher power ratio, the compressor can work at a lower percentage of its capacity (ie 70%) and still attain a high air pressure). HP can be upto 1200 HP, but the most powerful compressors will be much more costly. Hence a compressor with a power ratio of ie 1000 HP will need to be used (cost-power ratio will need to be assessed on sight).

A multi-stage compressor seems to works similar to a compound engine. Basically, it uses several chambers, with the first one ("first stage") being the largest, and the next one being smaller, with a piston that is larger in surface area. Another advantage of the multi-stage compressor seems to be the better heat removal (as the casing's surface area too is larger).

A question that arises though is whether a single compressor can be hooked up to various chambers, or whether each chamber requires its own compressor (this would increase the cost heavily).

Update --> "capacity" may refer to amount of PSI attained at that moment, rather than the % of the used wattage of the electric motor); other questions at http://en.wikipedia.org/wiki/Talk:Gas_compressor#Optimal_efficiency
Update 2: bigger compressors may be more efficient, not because their size is different and/or size of inlet/outlet hole, but simply because they use several stages. A stage seems to be a seperate compressor, which is just a bit smaller (hence not a chamber an thus a lot more costly). One way to reduce the costs is to use less powerful compressor types in the first stage, and more powerful ones in second or third stage. See http://www.sullairinfo.com/industrial/tandem_200804/LIT_TS_TS01E.pdf

http://www.tpub.com/content/NAVFAC/mo206/mo2060037.htm

[edit] Advantages of CAES&US

Compared to an energy storage system using electrochemical batteries;

  • the system can be built a lot more environmentally (no use of sulpheric acid), ...
  • unlike batteries, tanks don't need to be replaced every 5 to 7 years.

[edit] Modelling

QT flow diagram and control

QT air expander has 2 equivalent flow circuits with distinct intake and exhaust. Simple operation let air flows in both intake simultaneously. Heating air by heating the QT engine block could be a convenient approach (unconceivable with piston engine). Air storage management could be improved by using only one QT circuit (higher rpm) at low power demand. Sophisticated integration may require a small QT of higher pressure, and a larger QT for low pressure.

schematics quasiturbine: http://quasiturbine.promci.qc.ca/FQTAffiches.html and http://peswiki.com/index.php/PowerPedia:Quasiturbine

Reciprocating piston compressor

Rotary piston compressor:

Turbocompressor http://www.youtube.com/watch?v=z_xmCKaXcY4&feature=related


[edit] See also

[edit] External links

  1. ↑ [http://www.everestblowers.com/technical-articles/understandin-blowers.pdf Compression ratio - roots blower
  2. ↑ http://www.gnttype.org/techarea/turbo/turboflow.html
  3. ↑ [http://www.compasscompression.com/vane.html Rotary vane compressor - compression ratio
  4. ↑ [http://www.scribd.com/doc/51962006/20/Max-Compression-Ratio-s Screw compressor compression ratio
  5. ↑ Durability of compressor type being more important as total pressure increases; not just pressure difference (ie compression ratio)
  6. ↑ [www.lineymachine.com/ Liney machine Halo 3/5 pneumatic rotary engine]
  7. ↑ Taiyo rotary pneumatic engine
  8. ↑ Tesla Turbine pneumatic motor
Retrieved from "http://www.appropedia.org/Compressed_air_energy_storage_and_use_system"
Personal tools