Compressed-air vehicle
A compressed-air vehicle is powered by an air engine, using compressed air,
which is stored in a tank. Instead of mixing fuel with air and burning it in
the engine to drive pistons with hot expanding gases, compressed air
vehicles (CAV) use the expansion of compressed air to drivCompressed air
propulsion may also be incorporated in hybrid systems, e.g., battery
electric propulsion and fuel tanks to recharge the batteries. This kind of
system is called a hybrid-pneumatic electric propulsion. Additionally,
regenerative
Engine
One can buy the vehicle with the engine or buy an engine to be installed in the vehicle.
Typical air engines use one or more expander pistons. In some applications it is
advantageous to heat the air, or the engine, to increase the range or power.braking can
also be use
The storage tank may be made of:
steel,
aluminium,
carbon fiber,
KevlarThe fiber materials are considerably lighter than metals
but generally more expensive. Metal tanks can withstand a large
number of pressure cycles, but must be checked for
Compressed air has a low energy density. In 300 bar containers,
about 0.1 MJ/L and 0.1 MJ/kg is achievable, comparable to the
values of electrochemical lead-acid batteries. While batteries
can somewhat maintain their voltage throughout their discharge
and chemical fuel tanks provide the same power densities from
the first to the last litre, the pressure of compressed air tanks
falls as air is drawn off. A consumer-automobile of conventional
size and shape typically consumes 0.3-0.5 kWh (1.1-1.8 MJ) at
the drive shaft[4] per mile of use, though unconventional sizes
may perform with significantly less.corrosion Advantages
Compressed-air vehicles are comparable in many ways to electric vehicles, but use
compressed air to store the energy instead of batteries. Their potential advantages over
other vehicles include:
Much like electrical vehicles, air powered vehicles would ultimately be powered
through the electrical grid. Which makes it easier to focus on reducing pollution
from one source, as opposed to the millions of vehicles on the road.
Transportation of the fuel would not be required due to drawing power off the
electrical grid. This presents significant cost benefits. Pollution created during
fuel transportation would be eliminated.
Compressed air technology reduces the cost of vehicle production by about 20%,
because there is no need to build a cooling system, fuel tank, Ignition Systems or
silencers.[6]
Air, on its own, is non-flammable.
High torque for minimum volume.
The mechanical design of the engine is simple and robust.
Low manufacture and maintenance costs as well as easy maintenance.
Compressed-air tanks can be disposed of or recycled with less pollution than
batteries.
Compressed-air vehicles are unconstrained by the degradation problems
associated with current battery systems.[3]
The tank may be able to be refilled more often and in less time than batteries can
be recharged, with re-fueling rates comparable to liquid fuels.
Lighter vehicles would mean less abuse on roads. Resulting in longer lasting
roads.
The price of fueling air powered vehicles will be significantly cheaper than current fuels.
PeriodicallyCompressed air energy storage can be done adiabatically, diabatically, or
isothermally:
With adiabatic storage, the heat that appears during compression is also stored,
then returned to the air when the air is expanded. This is a subject of ongoing
study, but no utility scale plants of this type have been built. The theoretical
efficiency of adiabatic energy storage 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%[2]. 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 a molten-salt
(600 °C).
With diabatic storage, the extra heat is removed from the air with inter coolers
following compression (thus approaching isothermal compression), and is
dissipated into the atmosphere as waste. Upon removal from storage, the air must
be re-heated (usually in a natural gas fired burner for utility grade storage or with
a heated metal mass for large Uninterruptible Power Supplies) prior to expansion
in the turbine to power a generator. The heat discarded in the intercoolers
degrades efficiency, but the system is simpler than the adiabatic one, and thus far
is the only system which has been implemented commercially. The McIntosh
CAES plant requires 0.69 kW·h (2,355 btu) of electricity and 4,100 btu (LHV) of
gas for each 1.0 kW·h of electrical output [3]. A GE 7FA 2x1 combined cycle
plant, one of the most efficient non-CAES natural gas plants in operation, uses
6,293 btu (LHV) of gas per kW·h generated[4], a 54% thermal efficiency
comparable to the McIntosh 6,455 btu, a 53% thermal efficiency.
Isothermal compression and expansion approaches (which attempt to maintain
operating temperature by constant heat exchange to the environment) are only
practical for rather low power levels, unless very effective heat exchangers can be
incorporated. 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 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 (or seawater as in early compressed air torpedo [5]designs)
over an air-to-air (or air-to-seawater) heat exchanger between each expansion stage. This
warms the exhaust of the preceding stage and admits this preheated air to the following
stage. This was widely practiced 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, expansion with
turbo 'expanders' [8] or air engines driving electrical generators to produce electricity.
Air is stored in mass quantity in underground in a cavern created by solution mining (salt
is dissolved away) [9]or an abandoned mine. Plants are designed to operate on a daily
cycle, charging at night and discharging during the day.
Compressed air energy storage can also be used to describe
technology on a smaller scale such as exploited by air cars or
wind farms in steel or carbon-fiber tanks. Physics of isothermal
compressed air storage
One type of reversible air compression and expansion is described by the isothermal
process, where the temperature remains constant. Compressing air heats it up and the heat
must therefore be able to flow to the environment during compression for the temperature
to remain constant. In practice this is often not the case, because to properly intercool a
compressor requires a compact internal heat exchanger that is optimized for high heat
transfer and low pressure drop. Without an internal heat exchanger, isothermal
compression can be approached at low flow rates, particularly for small systems. Small
compressors have higher inherent heat exchange, due to a higher ratio of surface area to
volume. Nevertheless it is useful to describe the limiting case of ideal isothermal
compression of an ideal gas:
The ideal gas law, for an isothermal process is:
PV = nRT = constant
By the definition of work, where A and B are the initial and final states of the system:
where, PAVA = PBVB , and so,
is the absolute pressure,
is the volume of the vessel,
is the amount of substance of gas,
is the ideal gas constant,
is the absolute temperature,
is the energy stored or released.
This amounts to about 2.271 ln(PA/PB) kJ at 0 degrees Celsius (273.15 kelvins) or 2.478
ln(PA/PB) kJ at 25 °C (298 K), per mole, or simply 100 ln(PA/PB) kJ/m³ of gas (at 0.1
MPa = approx. atmospheric pressure).
An isothermal process is thermodynamically reversible, so to the extent the processes are
isothermal, the efficiency of compressed air storage will approach 100%. The equation
above represents the maximum energy that can be stored. In practice, the process will not
be perfectly isothermal and the compressors and motors will have heat-related energy
losses.
When gas is compressed adiabatically, some of the compression work goes
into heating the gas. If this heat is then lost to the surroundings, and
assuming the same quantity of heat is not added back to the gas upon
expansion, the energy storage efficiency will be reduced. Energy storage
systems often use large natural underground caverns. This is the preferred
system design, due to the very large gas volume, and thus the large
quantity of energy that can be stored with only a small change in pressure.
The cavern space can be compressed adiabatically and the resulting
tempSafety
As with most technologies, compressed air has safety concerns, mainly the catastrophic
rupture of the tank. Highly conservative safety codes make this a rare occurrence at the
tradeoff of higher weight. Codes may limit the legal working pressure to less than 40% of
the rupture pressure for steel bottles (safety factor of 2.5), and less than 20% for fiber-
wound bottles (safety factor of 5). Design rules are according to the ISO 11439 standard.
[14]
High pressure bottles are fairly strong so that they generally do not rupture in crashes.
erature change and heat losses are smEngine
Main article: Compressed air engine
A compressed air engine uses the expansion of compressed air to drive the pistons of an
engine, an axle, or to drive a turbine.
Sometimes efficiency is increased by the following methods:
A turbine with continuous expansion at high efficiency
several stages of expansion
use of waste heat, notably in a hybrid heat engine design
use of environmental heat
A highly efficient arrangement 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
heat exchanger between each expansion stage. This warms the exhaust of the preceding
stage and admits this preheated air to the following stage.[7].
The only exhaust gas from each stage is cold air which can be as cold as −15 °C; this may
also be used for air conditioning in a car.
Additional heat can be supplied by burning fuel as in 1904 for Whitehead's torpedoes[15].
This improves the range and speed available for a given tank volume at the cost of the
additional fuel.
As an alternative to pistons or turbines, the Quasiturbine is also capable of running on
compressed air, and is thus also a compressed air engine.
[edit] Hybrid systems
The system can be a hybrid power generation system, with the stored compressed air
mixed with a fuel suitable for an internal combustion engine. For example, natural gas or
biogas can be added, then combusted to heat the compressed air, and then expanded in a
conventional gas turbine engine (or the rear portion of a jet engine), using the Brayton
cycle.
In addition, Compressed air engines can be used in conjunction with an electric battery.
The compressed air engine, drawing its energy from compressed air tanks, recharge the
electric battery. This system (called a Pne-PHEV or Pneumatic Plug-in Hybrid Electric
Vehicle-system)[citation needed] and was being promoted by the apparently defunct
Energine[16]
Filling
Filling the tanks with compressed air takes 3–4 minutes;
therefore, the cars will also be able to be used for longer
journeys. In addition to the compressed air refuelling option, the
car has a built-in air compressor that can plug into any standard
electric outlet and refill the tanks in 4 hours[edit] How it works
It utilizes air expansion as an energy source by releasing compressed air out of tanks with
extreme cold internal temperature and high pressure, about 300 Bar. The resulting air
expansion is used to move a piston or turbine attached to a transmission
Compressed air car specification
Acceleration:- 0-80 km/h
Maximum speed:- 130km/h
Fuel economy:- 450km/filling of tank
Body & dimensions
Front brake:- disc
Rear brake:- drum
Front track:- 1525mm
Rear track:- 1424mm
Ground clearance:- 150mm
Front suspension:- mcpherson strut
Rear suspension:- independent coil spring
Wheel:- 18-inch