Accessibility Links

Skip
to content

Skip to search
IOPscience

Skip to Journals
list

Accessibility
help

IOPscience

Login

Please login to access our web services, or create
an account if you don't yet have one.

You must have cookies
enabled in your web browser to be able to login.

Username

Password

Forgotten your password? Get
a new one here.

Remember me

Create
account

Athens/Institutional
login

IOPscience

Environmental Research
Letters

All
Fields Title/Abstract
Author Affiliation Fulltext PACS/MSC
Codes Last Week Last Month This Year Last Year
3 Years 5
Years 10 Years All Dates All journals This
journal only

Home

Search

Collections

Journals

About

Contact us

My IOPscience

Authors

Referees

Librarians

Environmental Research
Letters

Volume 4 number 4

Abstract

References

Related articles
beta

Supplementary
info

Environ. Res.
Lett. 4 (October-December 2009) 044011
doi:10.1088/1748-9326/4/4/044011

Economic and environmental evaluation of compressed-air
cars

Felix Creutzig1,2,
Andrew Papson3, Lee Schipper4,5 and Daniel
M Kammen1,2,6

1 Berkeley Institute of the Environment, University of
California, Berkeley, USA
2 Renewable and Appropriate Energy
Laboratory, University of California, Berkeley, USA
3 ICF
International, 620 Folsom Ave, Suite 200, San Francisco, CA 94107,
USA
4 Precourt Energy Efficiency Center, Stanford University,
USA
5 Global Metropolitan Studies, University of California,
Berkeley, USA
6 Energy and Resources Group, University of
California, Berkeley, USA

E-mail: [email-masked]

Received 29 June 2009
Accepted 3 November
2009
Published 17 November 2009

Abstract. Climate change and
energy security require a reduction in travel demand, a modal shift, and
technological innovation in the transport sector. Through a series of
press releases and demonstrations, a car using energy stored in compressed
air produced by a compressor has been suggested as an environmentally
friendly vehicle of the future. We analyze the thermodynamic efficiency of
a compressed-air car powered by a pneumatic engine and consider the merits
of compressed air versus chemical storage of potential energy. 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, a pneumatic–combustion hybrid is technologically feasible,
inexpensive and could eventually compete with hybrid electric vehicles.

Keywords:  compressed-air car, life-cycle
analysis, greenhouse gas emissions, innovation, electric
mobility

Contents

1. Introduction

2. Thermodynamic
efficiency

2.1. Compression

2.2. Expansion

3. Model
considerations

3.1. Total
energy requirement

4. Performance

4.1. Fuel
storage weight and volume

4.2. Primary
energy required

4.3. Greenhouse
gas emissions

4.4. Consumer
savings, break-even costs

4.5. Capital
costs

4.6. Summary
of performance comparison

5. Discussion
of assumptions and costs

6. Hybrid
solutions

6.1. The
pneumatic-combustion hybrid

6.2. Air
engine hybrid

7. Conclusion

Acknowledgments

References

1. Introduction

There is an urgent need for mobility technologies and infrastructures that
are based on a technology other than oil and that have acceptable costs.
Consumers are affected by high and fluctuating oil prices, and in 2008 total
vehicle miles decreased for the first time in decades in the United States.
Manufacturers must address plummeting car sales. The US government spends
billions on national oil security while worldwide greenhouse gas emissions are
still increasing at a rapid pace.

The compressed-air car has been promoted by companies such as Motor
Development International, France, and Energine, Korea as an
environmentally friendly car of the future. As a form of storage, compressed air
is nothing new. Indeed, compressed-air engines were utilized in power mining
locomotives in the United States and Europe at the beginning of the 20th
century [1].
It is also reported that compressed-air engines have powered tramways in Bern,
Switzerland and Nantes, France [2].
However, a tramcar kilometer needed more than 7 kg coal because the loading
stations were run by steam engines. This corresponds to
13 kg CO2 km–1. To get a feeling for the
magnitude: a fully occupied tram with 31 passengers has a worse carbon footprint
per person km (410 g CO2 pkm–1) than a
Porsche Cayenne (358 g CO2 pkm–1). Thus, a
compressed-air tram had ten times more emissions than a modern electric tram per
passenger km. In fact, tramways and locomotives were soon powered by more
efficient electricity. This time, compressed air is proposed as a propellant for
automobiles. Do high oil prices offer a second chance for the compressed-air
engine?

Here we report on the thermodynamic limits, the overall efficiency, the
environmental impact, the propellant volume and the cost-benefit balance of the
compressed-air car (CAC) and compare it with a battery electric vehicle (BEV).
We discuss some of the advantages and disadvantages of this method of energy
storage and propulsion mechanism while pointing out the range of uncertainty on
real performance. We conclude that the main drawback is the thermodynamic
efficiency loss and uncertainties about required technologies. Hence, in
contrast to the BEV, the CAC is not likely to successfully compete against
gasoline cars. The compressed-air technology, however, might be feasible in a
hybrid configuration.

All calculations, graphs and consideration are also specified in a
spreadsheet model. The model, compressed-air car analysis meta-model (CACAMM)
can be downloaded at stacks.iop.org/ERL/4/044011/mmedia where
users can verify the influence of different assumptions and variable
values.

2. Thermodynamic
efficiency

We first investigate the thermodynamic efficiency of compressed-air storage.
The different stages of efficiency loss are depicted in figure 1.
We focus on air compression and air expansion, two stages that are specific to
the compressed-air car. Tank leakage loss is negligible compared to the loss of
air compression and air expansion. A similar analysis was conducted by
Bossel [3].
Our investigation differs from that study in assuming cooling at constant
pressure [4,
5].
Furthermore, we specify that compression is polytropic and expansion adiabatic.

2.1. Compression

Our reference scenario corresponds to data published by Zero Pollution
Motors, a subsidiary of MDI [6].
A volume V1 = 100 000 l is compressed from normal
pressure p1 = 1.01325 bar to p2 =
310 bar (=  4500 psi), resulting in a final volume
V2 = 327 l. The technical work required for filling the
tank under isothermal conditions is

Isothermal conditions are not achievable in practice. Real processes deviate
from the isothermal optimum. A lower bound is given by an adiabatic process,
i.e. when there is no heat exchange and no rapid compression. The adiabatic
coefficient is n = 1.4. The process is then characterized by

Then the overall efficiency of the process is the energy in the tank divided
by the work done,

There are two practical measures to increase efficiency: (A) multi-stage
processing with inter-cooling at constant pressure and (B) slow compressing
allowing for concurrent heat exchange [4,
5].
Compressors working in 4 stages are commercially available. MDI indicated that
they use 2-stage compression. In this case, two subsequent compression processes
have volume ratio  ~ 17.5 instead of one compression with volume ratio
~ 310. Furthermore, using a 5.5 kWh compressor charging takes
4–5 h allowing for heat exchange, by this increasing energy efficiency.
Taking this diabatic process into account, we assume n = 1.2. Hence, the
overall work is calculated by repeatedly applying and updating equation (2),
resulting in Wcom ~ 61.9 MJ and
Ecom ~ 93.6%.

Figure 1. The different stages
of efficiency loss for transportation with the compressed-air car. The two
stages that are specific for compressed-air storage are marked in
gray.

2.2. Expansion

The expansion work can be retrieved similar to compression work by
equation (2).
In this case, we conservatively assume the process to be adiabatic, n =
1.4, as the expansion is instantaneous. If there is heat exchange, expansion
would be more efficient. The pressure in the storage chamber can be kept
constant by an adjusting valve. It is assumed that all expansion work is used to
drive the car—with constant torque a normal driving cycle would be much more
inefficient (see section 5).
A single-stage expansion results in expansion work
Wexp ~ 22.8 MJ, i.e.,

As for compression, a multistep expansion can increase efficiency, in this
case Eexp = 49% for 2 steps as used by MDI. The overall
efficiency of the compressed-air storage only is Eair =
EcomEexp ~ 45.7%.

3. Model considerations

How does the compressed car perform compared to the conventional internal
combustion engine and battery electric vehicle? In particular, we want to
measure the overall energetic performance and the total fuel volume required to
drive a reasonable distance. Furthermore, there are two major cost drivers for a
switch in mobility technology: environmental externalities in terms of
greenhouse gas emissions and increasing gasoline prices and price fluctuations.
Hence, we compare different car technologies with respect to (A) required
energy per km (B) greenhouse gas emissions per km (C) fuel volume and
(D) cost per km. The real energy performance of CAC has not been verified
in independent tests, and, like virtually all other vehicles, will depend on
both technologies and behaviors, such as speeds and driving cycles.

As a reference vehicle with internal combustion engine (ICE), we rely on an
established mini-car, the Smart fortwo [7,
8].
This car drives 5.2 l/100 km (45 mpg) in the European drive
cycle, weighs (including driver) ca 900 kg and has a maximum speed of
145 km h–1. This choice is appropriate as the same model
will be available as an battery electric vehicle, the Smart fortwo ed. We
compare the conventional internal combustion engine powered by gasoline with the
compressed-air car (CAC) and the battery electric car (BEV), i.e. the Smart
fortwo ed which drives 13.7 kWh/100 km (as measured from the grid),
weighs 100 kg more than the Smart fortwo, has maximum speed
100 km h–1, and a range of 115 km. Hence we limit our
analysis to urban transportation. BEVs and CACs are commonly classified as
electric cars as both types of car obtain their energy from the grid.

To achieve ambitious climate change mitigation goals, not only must fuel
economy must be improved but also total energy requirements (and travel demand)
must be reduced. To elucidate the weight factor, we compare the Smart fortwo and
its electric cousins with a hypothetical super-light car, or golf cart, that
weighs 300 kg and is assumed to drive 1.7 l/100 km. The improved
fuel economy is achieved mainly by weight reduction, as well as other feasible
technological improvements, and reduced motor power. Such a vehicle is suited
only for urban transport but not for highways and has a maximum velocity of
around 60 km h–1. As such it is more suitable for
relatively dense European or Asian cities that have high accessibility and short
distances.

3.1. Total energy
requirement

First, the efficiencies of the three different storage technologies is
calculated. The grid-to-wheel efficiency of BEVs is 77.5% [9–11].
Comparing energy content of fuels, an approximate fuel-to-wheel efficiency of
21.2% for gasoline can be inferred. Furthermore, one sixth of all carbon
emissions of gasoline are upstream in the supply chain, resulting in a
well-to-wheel efficiency of ca  17.7%. For CAC, we conservatively assume
that additional to 45.7% storage efficiency, 10% are lost due to mechanical and
flow losses. Furthermore, another 29% are lost due to shaft energy
requirements [10,
12].
For the electric modes, an average grid transmission loss of 9.5% must be
included [13].
If electricity is generated by thermal power plants, plant efficiency loss must
be included, e.g.  0.4 for a relatively efficient coal power plant. The
resulting efficiency values are summarized in table 1.

Table 1. Efficiencies for
propellants. For the electric modes, specific power plant efficiencies
must be added.

Smart

CAC

Smart ed

Coal/well-to-wheel (%)

17.7

11.7

28.3

Wind/well-to-wheel (%)

17.7

29.2

70.8

Grid/pump-to-wheel (%)

21.2

26.7

77.5

Propellant-to-wheel (κ) (%)

21.2

34.6

90.0

4. Performance

4.1. Fuel storage weight and
volume

The propellant weight is not significant as a fraction of total weight for
conventional cars, but the weight of batteries can be considerable for BEVs.
Here, we also compute the fuel weight for compressed-air cars. The weight of
propellant needed is a function of the range required, the efficiency of
conversion of stored energy to work at the wheels and the average storage
capacity in MJ. The cruising range from here on is r = 115 km, as has
been specified for the electric Smart. In a CAC or gasoline vehicle but not in a
BEV, the weight of the fuel changes over time. To take this into account,
average filling is a = 0.6 for gasoline and CAC, and a = 1 for
BEV. The relationship between fuel weight and vehicle properties can be stated
as follows.

where wf is the propellant weight, ef the
energy per weight fuel (gasoline: 45 MJ kg–1, CAC:
1.94 MJ kg–1, BEV: 0.40 MJ kg–1) and κ
the fuel-to-wheel efficiency. Defining the range-specific energy required per
weight as ,
the required propellant weight is

The fuel weight for different storage technologies and car sizes is
summarized in table 2.
Compressed-air weighs ten times more than gasoline with similar energy content,
but three times less than batteries with similar energy content.

Table 2. Fuel
weight.

Car weight (kg)

Gasoline (kg)

CAC (kg)

BEV (kg)

900

4.8

53.0

140.3

300

1.6

17.7

46.8

One of the main objections against electric-only vehicles is their limited
range. Above, we required that all vehicles have a 115 km range. This
requires a significant amount of compressed air, adding more than 50 kg.
However, the real issue is the low energy density: a large storage volume is
required. For the 900 kg CAC, 780 l storage is required—more than
double the trunk volume of the Smart—and hence posing a serious challenge for
car design. In contrast, for gasoline less than 4 l is required. Li-ion
batteries have values of 400 whr l–1 and
150 whr kg–1 [14].
This implies 0.375 kg l–1 density. In table 2
we specify 140.3 kg battery pack for the Smart, and 46.8 kg battery
pack for the super-light car. Using the density above, this corresponds to
374 l for the normal car, a little above the trunk volume of the Smart, and
125 l for the small ca. Note that battery densities are defined in two
different ways: either just the active material, or for the active
material plus packaging. The numbers above refer to packaging, which is
appropriate for this use since were looking at the volume of battery packs.
Figure 2
summarizes the volume requirement for the different energy storage technologies.
Whereas battery volume is already at the upper limits of what can be deployed
for small cars, the volume of compressed-air cars poses a very serious hurdle
for vehicles with suitable range.

Figure 2. Compressed-air stores
energy only at low density, and a combination of large tank volume, high
pressure and low vehicle weight is required to provide an acceptable
range. From here onwards normal car refers to the 900 kg model
whereas light car refers to the 300 kg city
car.

4.2. Primary energy
required

From the efficiency table 1,
the primary energy per distance can be calculated, given the car plus fuel
weight. The results are summarized in figure 3.
The primary energy requirement is crucially dependent on the power plant. If
electric cars are powered by renewable energies, less primary energy is required
than for the gasoline car. If electric cars are powered by coal power plants,
more primary energy is required for CACs but still less for BEVs. From the
overall efficiency perspective battery cars are much more efficient.

Figure 3. Primary energy
required. When powered by renewable energies, the compressed air needs
less energy per km than the gasoline car but significantly more than the
battery car. The CAC needs 75% more primary energy than the gasoline car
when the CAC is powered by a conventional coal plant.

4.3. Greenhouse gas
emissions

Environmental performance as zero local emissions is one of the
primary arguments for the compressed-air car. Indeed, noxious matter is not
emitted locally when driving, thereby one of the main contributors to urban air
pollution is eliminated. This benefit is shared by battery carsNote7. However, compressed-air tanks can be disposed
of or recycled with less toxic waste pollution than batteries, depending on the
precise recycling requirements. One important environmental concern relating to
car use is the impact of cars on climate change. Greenhouse gas emissions
themselves depend critically on the source of electricity used for charging
batteries or running the CAC compressor. Whereas a compressed-air car or the BEV
do not emit greenhouse gases (GHG) when operated, emissions are shifted to power
plants. Emission levels then depend on the power plant characteristics.

In general there is a great deal of uncertainty associated with modeling the
effect of electro-mobility on the grid. It is agreed that even huge penetration
rate of electric cars can be serviced using the present power plant capacity if
cars are mostly plugged-in at off-peak hours [15].
When advanced scenario-based simulations of overall grid growth, electricity
dispatch and geographic generation distribution are performed, results are
region specific. Usually emission reductions are larger than when assuming the
current power mix [15].

Here, we calculate the emissions produced by electricity purchase from two
different German utilities, RWE and EWS Schönau. Providers can be chosen in the
German electricity market. Note that RWE produces the charging stations for the
Smart ed in the Berlin trial. The power plant mix of RWE is dominated by coal
plants, producing 887 g CO2 kWh–1 [16],
whereas EWS Schönau relies nearly exclusively on renewables and hydro-energy,
producing 17.3 g kWh–1 [17].
The future power plant mix of RWE crucially depends on policy decision, such as
permissions for new coal plants.

Results are summarized in figure 4.
The choice of power plant mix has significant impact on greenhouse gas
emissions. The compressed-air car indirectly emits more than twice the
greenhouse gases than its conventional counterpart when powered by RWE. The poor
environmental performance of the CAC is due to its thermodynamic inefficiencies,
see section 4.2.
Even the BEV performs only similar to the gasoline car. In contrast, the CAC and
the BEV mitigate greenhouse gas emissions considerably when a renewable energy
provider is chosen. PG&E, a major Californian energy provider, has medium
GHG emission (238 g CO2 kWh–1). As a
result, the deployment of both CAC and BEV produces some environmental benefits
in California.

Figure 4. Greenhouse gas
emissions per km of the compressed-air car are even higher than those of
the conventional gasoline car. Total emissions are crucially dependent on
the choice of provider in Germany. The BEV has lower emissions for typical
Californian conditions.

4.4. Consumer savings, break-even
costs

One of the dominant concerns for consumers is high fuel prices. What fuel
costs do consumers save when driving the compressed-air car? Two gasoline price
scenarios and two regions are considered. The regions are California and
Germany. California has a low gasoline price scenario with 2$/gal and high gas
price scenario with 4$/gal. The price for electricity is taken as
0.128$ kWh–1, the Californian average retail price in
2007 [18].
Road maintenance costs are usually recovered via fuel taxes, e.g.
$0.37/gal in California, corresponding to roughlyNote8 to 0.0065$ km–1 which must be
added to the marginal cost of using cars that are run by electricity. Assuming
that this charge is levied as tax on electricity for cars, the price would
increase by another $0.046 kWh–1 for the average 900 kg
electric vehicle and $0.017 kWh–1 for the more inefficient
compressed-air car (taxes are proportional to km kWh–1). Results
as marginal costs per km driven are presented in table 3.

Table 3. California: marginal price
(c km–1) with $4/gal ($2/gal).

Gasoline

CAC

BEV

Normal fuel

5.5 (2.7)

—

—

Light fuel

1.8 (0.9)

—

—

Normal grid

—

5.2

2.3

Light grid

—

1.7

0.8

Break-even normal $

300 (–2700)

3500 (400)

Break-even light $

100 (–900)

1200 (100)

For Germany, the low gasoline price scenario is
($6.4/gal), the high price scenario
($8.4/gal). The higher gasoline prices favor electric cars. However, both
electricity prices and fuel taxes are significantly higher in Germany than in
California, and

respectively [19].
This translates into very high electricity charges for electric cars, with road
taxation amounting to more of 50% of the total price of
0.57$ kWh–1. Results are displayed in table 4.

Table 4. Germany: marginal price
(c km–1) with $8.4/gal ($6.4/gal).

Gasoline

CAC

BEV

Normal fuel

11.5 (8.8)

—

—

Light fuel

3.8 (2.9)

—

—

Normal grid

—

13.6

7.6

Light grid

—

4.5

2.5

Break-even normal $

–2400 (–5400)

4300 (1300)

Break-even light $

–800 (–1800)

1400 (400)

In the following, we focus on the high fuel costs scenarios which are more
optimistic for the compressed-air cars. Consumer have—for a variety of
reasons [20]—a
high discount rate of ca 16% [21].
Taking the respective gasoline car (Smart fortwo) as a benchmark, in California
the CAC has total usage savings of only $300 whereas the Smart ed accumulates
savings of $3500 over its lifetime. In this calculation, a user fee or road
charge for electric vehicles substitutes for fuel taxes. If there are no road
charges, and electric vehicles are subsidized, the CAC saves $1000, and the BEV
$4200. In Germany, the CAC produces additional usage costs of $2400 whereas the
Smart ed accumulates savings of $4300 over its lifetime. This is due to
comparatively high fuel taxes (or equivalently road charges) in Germany. If
there are no road charges, the CAC saves $2000, and the BEV $8700.

All results are summarized in figure 5.
Figure 5(a)
displays the annual usage costs for the 2 different storage technologies in
California and Germany with and without user fees and compares them with varying
gasoline prices. In figure 5(b),
the total savings for each scenario are displayed as a function of the gasoline
price. Only the usage of battery electric vehicles allows significant savings in
each region. Furthermore, the political choices of how and when to introduce
user fees for electric vehicles has a significant influence on the costs and
benefits of electric cars.

These numbers can be interpreted as break-even costs, i.e. the total
costs that the storage technology can costs, including for the BEV battery
costs, battery maintenance, battery substitution, and for the CAC compressed-air
tanks plus compressor. Only if the storage technology produces fewer additional
costs than total usage savings, can the respective technology becomes
economically reasonable.

Hence, if costs for batteries fall below $290 kWh–1 in
California or below $360 in Germany, it becomes economical to drive the BEV,
assuming all other costs remain the same. The CAC produces lifetime savings only
in California. It becomes only economical to drive the CAC, if the engine
construction saves costs compared to the combustion engine.

Figure 5. Costs and savings of
different propellants as a function of the gas price. (a) Annual
costs of each propellant for driving 17.700 km. (b) Total
savings for CAC and BEV. Total savings are considerably higher for BEV
drivers. The CAC does not provide saving for gas prices below $4/gal. Note
that we included a road charge in the electricity price, equivalent to
today's gasoline tax. Without the road charge, both modes are more
profitable.

4.5. Capital costs

It is instructive to compare the break-even costs with known capital costs
for the storage technology. Note that the storage technology costs are not
necessarily equivalent to the additional full vehicle costs. The front-up cost
for the CAC are mostly (a) the on-board compressor and (b) the carbon-fiber
tank. A commercially available 4700 PSI compressor costs ca
$3000 [22].
Carbon-fiber tanks with ca 300 l total volume cost $3500 [23].
However, wholesale prices should already be much lower. Nonetheless, the low
savings achieved by driving the CAC (see table 4)
do not allow for costs above $300 for the storage technology; the CAC technology
is not competitive. The break-even costs for batteries above corresponds to a
battery price of ca  $290–360 kWh–1. Current prices are in
excess of $1000 kWh–1 [24],
and there are additional costs for ancillary cooling and electronics. That is,
total costs for batteries must come down by a factor of at least 4.

4.6. Summary of performance
comparison

From the analysis above, it is clear that the CAC is outperformed by the BEV
and even the gasoline car in most dimensions. The only exception is primary
energy required—the gasoline car has higher total energy needs. The results are
summarized in a radar plot (figure 6,
left).

5. Discussion of assumptions and
costs

There is reason to believe that currently projected CAC costs are optimistic
because current models have much lower real performance.

We specified two expansion steps. Commercially available air motors work with
one expansion step. There is no physical reason why both expansion steps cannot
be used, although this may be technologically very challenging. To circumvent
this challenge, one could use the second expansion step alone to power the
vehicle. For example, assuming zero technological innovation and using a
commercial air motor that is driven by input pressure of 9 bar for the
second expansion step only, would reduce expansion efficiency from 68% to
maximal 16% and total efficiency from 34% to not more than 8%. In this scenario,
we still assume storage at 300 bar and the first expansion step is only
used to bring down the pressure to the appropriate input pressure for the
commercial air motor. Of course, such low efficiency translates into poor
performance of the compressed-air car, e.g. more than four times higher GHG
emissions than a conventional gasoline-powered car. A comparison of this
modification in comparison to the default model is drawn in figure 6,
right panel.

As a limitation, we did not carry out a drive-cycle analysis here. Instead,
we assumed that the air motor can operate at equal efficiency across different
output power regimes. However, at least with conventional commercially available
air motors, such as piston air motors, this is not true. More specifically, the
air flow rate translates into rotational speed and torque of the air motor and
from their into output power. Hence, the optimal efficiency regime is a property
of the air motor [25].
A complete drive-cycle analysis would certainly lead to inferior results.

6. Hybrid solutions

Internal combustion engines are usually driven usually at low load and hence,
in a low efficiency regime. Also their thermodynamic cycle cannot be reversed
and in braking situations kinetic energy dissipates as heat. A hybrid powertrain
is one solution to these problems, requiring a battery as the storage system.
The downside of this solution is the additional weight and costs of the battery,
electric motor and generator. The obvious question is whether hybrid
pneumatic–combustion engines offer better performance.

Two different hybrid approaches have been proposed, one based on the
combustion engine, the other on the air engine. We explore both approaches
conceptually.

6.1. The pneumatic–combustion
hybrid

This approach takes the conventional internal combustion engine and adds an
additional valve that connects the combustion chamber to an air tank. The tank
is charged by the combustion engine when performing below peak efficiency and
adds power to the engine in supercharged mode, i.e. when additional power
is required. Additionally, the tank can be charged when the engine operation is
reversed upon regenerative braking. One theoretical study found that optimizing
a hybrid air tank to 16 kPa and 80 l with combined engine downsizing
can improve fuel efficiency by 31% [26].
Fuel economy improvements of 64% in the city and of 12% on the highway have been
reported in another model [27].
Experimental work demonstrated the feasibility of this concept, recovering up to
half of the energy content of the compressed air; the expansion efficiency is
> 48% [28].

In contrast to the battery hybrid, the pneumatic hybrid approach does not
require a second propulsion system nor does it increase the car's weight
considerably. The main costs of pneumatic hybrid arise from the variable valve
actuation system. As no batteries are required, costs are considered to be lower
than in the hybrid electric vehicles. A switch from a two-stroke to a
four-stroke pneumatic engine would further reduce costs for actuated
valves [29].

6.2. Air engine hybrid

The other hybrid concept is focused on the air engine. Here, a combustion
engine would be used to recharge the air tank. Here the internal combustion
engine can constantly work in the maximum efficiency regime. Energy is lost in
the compression and expansion stages, comparably to section 4.2.
However, waste heat of the combustion engine can be used to heat up the expanded
air and, hence, increase expansion efficiency. Modeling studies claim that such
an air engine hybrid can reach total vehicle efficiency
> 33% [30,
31],
compared to vehicle efficiency of 20% of the conventional car in our study.
These results have to be independently verified.

Figure 6. Performance summary
(Germany, electric cars powered by RWE power mix). Left: overview on the
performance of the CAC, BEV and gasoline car. GHG emissions, primary
energy requirement, storage volume and marginal costs are plotted against
each other. Performance is better for points further inwards. With the
exemption of primary energy requirement, the CAC performs worse than the
other vehicles. Right: comparison of two CAC scenarios. The scenario with
two expansion steps corresponds to the scenario used on the left side. The
1-step expansion scenario is most likely used in existing air
cars.

7. Conclusion

The compressed-air car should be regarded as a car similar to the common BEV,
powered by electricity from the grid but different in storage technology. In
principle, compressed-air cars could compete with BEVs in substituting for
gasoline cars. The life-cycle analysis of the compressed-air car, however,
showed that the CAC fared worse than the BEV in primary energy required, GHG
emissions, and life-cycle costs, even under our very optimistic assumptions
about performance. Compressed-air energy storage is a relatively inefficient
technology at the scale of individual cars and would add additional greenhouse
gas emissions with the current electricity mix. In fact, the BEV outperforms the
compressed-air car in every category. Uncertainty in technology specifications
is considerably higher for CACs than for BEVs, adding a risk premium. We provide
a transparent spreadsheet model that can be used to replicate results or
experiment with other values.

A hybrid concept, where the air tank is recharged with an internal combustion
engine, is more efficient but has yet not been experimentally verified. However,
a pneumatic–combustion hybrid is similar to the hybrid electric vehicle in
concept and efficiency gain, offers potential cost and weight advantages and is
closest to implementation.

Overall, the CAC does not appear to offer any advantage over purely
electrical means of storing energy on board a vehicle. Batteries are common and
improving almost daily, while the compressed-air cycle has no present role in
any popular automobile platform. Since there are great pressures on battery
performance from other applications such as cell phones, it is hard to imagine
that CAC will gain an advantage over BEV in the foreseeable future. Automobiles
must become lighter and more efficient if even the best batteries are to provide
longer autonomous ranges. At the same time, combustion technology itself is
evolving rapidly in the face of concerns about oil and climate change. As long
as there are no substantial innovations in compressed-air technology and its
deployment, the real progress in this sector may be the emphasis on light
materials and small car design, for which the competition between batteries and
fuel will just intensify.

Acknowledgments

We thank Emilie C Mathieu and four anonymous reviewers for helpful
comments on the manuscript. The work was supported by the European Recovery
Program and by a grant from Vulcan, Inc. We thank Jim Boyden for discussions and
assistance.

References

[1]

Gairns J F 1904 Industrial locomotives for mining,
factory, and allied uses. Part II. Compressed air and internal combustion
locomotives Cassier's Mag. 16 363–77

[2]

http://www.tramwayinfo.com/tramways/Articles/Compair2.htm
accessed 23 June 2009

[3]

Bossel U 2005 Thermodynamic Analysis of Compressed Air
Vehicle Propulsion European Fuel Cell Forum

[4]

Barber A 1997 Pneumatic
Handbook (Amsterdam: Elsevier)

[5]

Rajput R K 2006 Thermal Engineering (New
Delhi: Laxmi) ISBN: 8170088348

[6]

For information on the companies seehttp://zeropollutionmotors.us/ and
http://www.mdi.lu/, both last accessed 23
June 2009 For information on the companies seehttp://zeropollutionmotors.us/ and
http://www.mdi.lu/, both last accessed 23
June 2009

[7]

http://www.smart.com/ accessed 26
August 2009

[8]

Sueddeutsche Zeitung, August 20Smart fortwo electric drive—Zweiter Akt http://www.sueddeutsche.de/automobil/391/484824/text/ 2009 Sueddeutsche
Zeitung, August 20Smart fortwo electric drive—Zweiter Akt http://www.sueddeutsche.de/automobil/391/484824/text/

[9]

Wang M 2001 Development and Use of GREET 1.6 Fuel-Cycle
Model for Transportation Fuels and Vehicle Technologies www.transportation.anl.gov/pdfs/TA/153.pdf

[10]

Delucchi M A and Lipman T E 2001 An analysis
of the retail and lifecycle cost of battery-powered electric
vehicles Transp.
Res. D 6 371–404
CrossRef
link

[11]

Kammen D M, Lemoine D M, Arons S M
and Hummel H 2008 Evaluating the cost-effectiveness of
greenhouse gas emission reductions from deploying plug-in hybrid electric
vehicles Brookings-Google Plug-in Hybrid Summit

[12]

Mazza P and Hammerschlag R 2005 Wind-to-Wheel
Energy Assessment European Fuel Cell Forum www.efcf.com/reports/E18.pdf

[13]

United States Department of Energy Office of Electricity Delivery and
Energy Reliabilityhttp://sites.energetics.com/gridworks/grid.html
last accessed 28 June 2009 2009 United States Department of Energy
Office of Electricity Delivery and Energy Reliabilityhttp://sites.energetics.com/gridworks/grid.html
last accessed 28 June 2009

[14]

Linden D 2001 Handbook of Batteries 3rd
edn (New York: McGraw-Hill)

[15]

Bradley T H and Frank A A 2007 Design,
demonstrations and sustainability impact assessments for plug-in hybrid
electric vehicles Renew. Sustain. Energy
Rev. 13 115–28
CrossRef
link

[16]

RWE. 2007 Corporate Responsibility. Report.

[17]

EWS Schoenauhttp://www.ews-schoenau.de/ accessed 26
August 2009 2009 EWS Schoenauhttp://www.ews-schoenau.de/ accessed 26
August 2009

[18]

Energy Information Administration 2008 Form eia-861, `Annual
Electric Power Industry Report'

[19]

Statistisches Bundesamt. 2008 Finanzen und Steuern,
Energiesteuer Fachserie 14 Reihe 9.3

[20]

Delucchi M A 2007 Cost-benefit analysis of
fuel-economy improvements Discussion Paper

[21]

Greene D L
and DeCicco J 2000 Engineering-economic analyses of
automotive fuel economy potential in the united states Annu. Rev.
Energy Environ. 25 477–535
CrossRef
link

[22]

http://www.northshorecompressor.com/

[23]

Personal communication with Bauerairhttp://www.bauerair.com/ 2009 Personal
communication with Bauerairhttp://www.bauerair.com/

[24]

Lemoine D M, Kammen D M
and Farrell A E 2008 An innovation and policy agenda
for commercially competitive plug-in hybrid electric vehicles Environ.
Res. Lett. 3 014003
IOP
ArticleCrossRef
link

[25]

Ingersoll RandIndustrial Air Motors, IND-0804-036 2004 Ingersoll
RandIndustrial Air Motors, IND-0804-036

[26]

Higelin P, Vasile I, Charlet A
and Chamaillard Y 2004 Parametric opimization of a new
hybrid pneumatic–combustion engine concept Int. J. Engine
Res. 5 205–217
CrossRef
link

[27]

Tai C, Tsao T-C, Levin M B, Barta G
and Schechter M M 2003 Using camless valvetrain for
air hybrid optimization SAE
Trans. 112 196–210

[28]

Trajkovic S, Tunestal P
and Johansson B 2008 Investigation of different valve
geometries and valve timing strategies and their effect on regenerative
efficiency for a pneumatic hybrid with variable valve actuation SAE Paper
2008-01-1715

[29]

Dönitz C, Vasile I, Onder C H
and Guzzella L 2009 Modelling and optimizing two-and
four-stroke hybrid pneumatic engines Proc.
IMechE 223 255–80
CrossRef
link

[30]

Huang K D and Tseng K-T 2005 Development of
a hybrid pneumatic power vehicle Appl.
Energy 80 47–59
CrossRef
link

[31]

Huang K D, Quang K V
and Tseng K-T 2009 Study of recycling exhaust gas energy
of hybrid pneumatic power system with cfd Energy Convers.
Manag. 50 1271–8
CrossRef
link

Notes

Note7  Modern superlow emission vehicles,
comprising standard models such as the Toyota Prius, also emit substantially
lower levels of hydrocarbons, carbon monoxide, nitrous oxides and particulate
matter than conventional vehicles; hence, for addressing air pollution concerns
neither a BEV or CAC is required.

Note8  For the Californian value, the US
Environmental Protection Agency rating of 36 mpg is assumed for the Smart
fortwo.

Your
last 10 viewed

Economic and
environmental evaluation of compressed-air cars

Felix Creutzig et al 2009 Environ. Res. Lett. 4
044011

© IOP Publishing
2010

help

site map

privacy
policy

terms &
conditions

disclaimer