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A closed CO2 cycle between the fuel’s manufacture and its combustion
Audi e-gas project: CO2-neutral power generation
With a solution for surplus wind or solar energy storage

E-gas project: energy conversion and CO2 compensation.

20 May 2011 : Under the "e-gas project" title, Audi is presenting a clear plan to reach what it calls "balanced mobility", a completely CO2-neutral mobility, in an initiative from which both the energy-supply industry and society should benefit.

Audi says that the e-gas project can even help to solve the problem of how to store surplus wind or solar energy, as such a surplus energy can be fed into the natural-gas network  (see: methanation below) and from there, it can even flow back from the gas network back to the power grid at any time.

First, the Audi e-gas project consists of two main components: wind turbines and a new plant.

The wind turbines will generate clean power, part of which Audi will use in the future to build its e-tron vehicles (it will also play a key role in powering these vehicles).

The new plant will use the remaining green power to produce hydrogen by means of electrolysis. This source of energy, generated in a climate-friendly manner, can be used to power fuel-cell vehicles in the medium term.

Audi will also combine hydrogen with CO2 in an additional step to manufacture methane. Although this methane is also known as synthetic natural gas, the company refers to it as Audi e-gas. It can power combustion engines designed for use with natural gas; as of 2013, Audi will begin series production of such models, designated TCNG.

Methanation is particularly advantageous in that the reaction occurs with the aid of CO2, which consequently is not discharged into the atmosphere. This results in a completely closed CO2 cycle, which in turn facilitates cleaner long-distance mobility.

The German luxury car brand says that its e-gas project adds another advantage from which Germany’s energy-supply industry can benefit: in the form of methane, electricity generated via renewable energy can be fed into the natural-gas network – which can help to solve the problem of how to store surplus wind or solar energy. If necessary, this energy can flow from the gas network back to the power grid at any time.

Having completed the research phase of the e-gas project, Audi will soon take the second step (mid-2011): the construction of an industrial facility. Audi will thus kick off this large-scale energy project together with its project partners: SolarFuel GmbH from Stuttgart; the Centre for Solar Energy and Hydrogen Research (ZSW), also based in Stuttgart; the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) in Kassel, Germany; and EWE Energie AG.

 Offshore wind turbines

Audi E-gas project: neutral CO2 balance across the entire mobility chain.

A neutral CO2 balance across the entire mobility chain. Press on the e-gas project image to enlarge it in a different window.

Wind turbines constitute the first significant component of the e-gas project. During the project’s first phase, four large power plants at an offshore wind park in the North Sea are being financed by Audi and a regional power-supply company. Rated at 3.6 MW each, these four turbines are to supply some 53 GWh of electricity annually (equivalent to the requirements of a medium-sized city).

Offshore wind-power stations currently play a minor role in Germany. But when located far from the coastline, they harness wind averaging 30 km/h (19 mph) to produce about 40 percent more energy than onshore stations.

 E-gas plant

The second large component of the project is the e-gas plant, which will produce hydrogen and methane on an industrial scale. Ground is scheduled to be broken in Werlte, Germany in July 2011. The e-gas plant is connected to a waste-biogas plant, which supplies the concentrated CO2 necessary for methanation and which would otherwise pollute the atmosphere. The plant will annually produce some 1,000 metric tons of e-gas while consuming 2,800 metric tons of CO2.

The electrolyzer runs on electricity generated via renewable energy.

The electrolyzer runs on electricity generated via renewable energy. Press on the electrolyzer image to enlarge it.

The plant will comprise two main components: an electrolyzer and a methanation unit. There is also piping technology, tanks, open-loop and closed-loop control electronics, and compressors for feeding e-gas into the natural-gas network. In January 2011, a lab facility with an output of 25 kW was set up for testing purposes; it was possible straightaway to produce gas which meets feeding-in quality requirements.


The electrolyzer (picture above) runs on electricity generated via renewable energy. Aided by polymer electrolyte membranes, the electrolyzer splits water (H2O) into its components: hydrogen (H2) and oxygen (O2).


In the future, hydrogen will be able to power fuel-cell vehicles such as the Audi Q5 HFC. Since such vehicles have not yet reached product maturity, hydrogen will not be used directly during the project’s first phase; instead, after being separated and dried, it is placed into a storage tank and then the methanation unit.

Methanation: hydrogen is combined with carbon dioxide to create methane.

Methanation: hydrogen is combined with carbon dioxide to create methane. Press on the methanation image to enlarge it.

Here, the hydrogen is combined with carbon dioxide (CO2) to create methane (CH4) as per the Sabatier reaction; water (H2O) forms as a by-product. Methane, a synthetic natural-gas substitute, is thus produced; it is subsequently conveyed to Germany’s natural-gas network as well as the network of CNG stations. A long-proven technology in the chemical industry, it can in principle function wherever there is water, electricity and a CO2 source. Although CO2 can also be obtained from the surrounding air, doing so requires more resources.

Even during this initial phase of the e-gas project, the electricity generated by wind power and the methane produced at the plant will suffice for 2,500 motor vehicles in total. Some of the wind-generated electricity would be enough to manufacture 1,000 units of the A1 e-tron and propel them 10,000 km (6,200 miles) per year. An additional share will be fed into the grid; surpluses within the power grid would thus benefit the e-gas plant, too.

By means of the e-gas generated via renewable energy, 1,500 units of the A3 TCNG could each be driven 15,000 km (9,300 miles) annually. And there would still be 150 metric tons of e-gas for the public gas network. As needed, this gas could also flow back. All in all, that represents a big boost to the power grid and equates to far more than 30,000,000 climate-neutral kilometers (18,700,000 miles) driven every year.


With regard to environmental impact, if one considers the well-to-wheel analysis in lieu of exhaust emissions, then a compact natural-gas car powered by e-gas emits fewer than 30 grams of CO2 per kilometer (48.28 g/mile). And that includes all emissions created during construction of the wind turbines and the e-gas plant. Only electric vehicles which are directly supplied with wind-generated electricity perform even better: they emit under 4 g/km (6.44 g/mile). However, they exhibit a drawback in the overall energy picture regarding vehicle production: a lot of energy is needed to manufacture their batteries.

 Green power storage

The Audi e-gas project is capable of solving several problems faced by the sustainable energy-supply industry all at once. In the process chain, clean power, hydrogen and methane are produced: three key sources of energy for future mobility. In the medium term, this technology has the potential to establish a highly flexible power-supply infrastructure for electricity, heating and motor vehicles which is based entirely on renewable energies; in addition, the respective percentages of the three sources of energy can be adjusted as required.

Audi A1 e-tron: wind-turbine-to-wheel energy chain.

The Audi A1 e-tron is the concept of a purely electric vehicle.

The future of Germany’s power supply belongs to renewable sources of energy. Last year, their share of overall consumption of electricity, heating and fuel exceeded 10 percent for the first time. Renewable energies already account for 17 percent of electricity generated, whereby wind energy constitutes the largest share and figures vary considerably among German states. Renewable sources of energy are forecast to make up 77 percent of Germany’s overall electricity consumption by the year 2050.

Wind power has great potential. The Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) was commissioned to conduct a study by WindEnergie, a German association. According to the study, wind power could be harnessed to realistically generate some 390 terawatt hours (TWh) of energy – this would have satisfied 64.7 percent of Germany’s overall electricity consumption in 2010 (603 TWh). Overall output in the computational model amounts to 198 gigawatts (GW).

Audi A1 e-tron electric car.

The Audi A1 e-tron's electric motor supplies a continuous output of 45 kW (61 hp) and a peak output of 75 kW (102 hp).

The production of electricity via wind and sun, however, is subject to natural fluctuations and the necessary storage capacity is currently very low. Pumped-storage power plants are capable only of short-term storage: during an emergency in Germany, they could supply power for all of an hour. All other solutions, such as compressed-air energy storage plants, are similarly very limited regarding capacity and period of storage.

The methanation of hydrogen using renewable energy helps solve this problem: the power grid is linked to the underground gas network, which can store surplus power supplies for months. The gas network has a potential capacity of 217 TWh, in contrast to the power grid’s storage capacity of just 0.04 TWh. The latter’s transport capacity, moreover, is just one tenth of that of the gas network.

Energy can be conveyed from the gas network – perhaps by means of gas-fired power plants or, in a decentralized manner, in block-type thermal power stations – back to the power grid at any time. New, decentralized cogeneration power plants can boost efficiency even more. In addition, methane is also suitable for the supplying of gas to private residences or providing high-temperature process heat.


The efficiency ratio of the e-gas pilot plant – from wind turbine to methane gas – is about 54 percent. If the dissipated heat is also used, this value is considerably higher still. The aim is to achieve an efficiency ratio above 60 percent in the future. The potential to store large quantities of energy – made possible by pairing electricity with gas on the one hand as well as wind energy and solar energy on the other – can invigorate the expansion of renewable sources of energy. The fact that the Audi e-gas project can easily be replicated in any country with an existing natural-gas network hints at the technical and economic significance of this project.

 E-gas project cars

Audi will supply three sources of energy in the scope of the e-gas project: electricity, hydrogen and methane gas. Respectively, each one is suitable for a very different type of drive concept: for electric cars, fuel-cell vehicles and CNG vehicles.

 A1 e-tron

The small compact Audi A1 e-tron is the concept of a purely electric vehicle. If necessary, a range extender can recharge its battery; the A1 e-tron is propelled exclusively by the power of its electric motor. The four-seater is a zero-emission vehicle during short city drives.

The Audi A1 e-tron's electric motor supplies a continuous output of 45 kW (61 hp) and a peak output of 75 kW (102 hp), transmitted to the front wheels via a single-stage transmission. The peak torque of 240 Nm (177.01 lb-ft) is available right from the off. The A1 e-tron dashes from zero to 100 km/h (zero to 62.14 mph) in 10.2 seconds and boasts a top speed above 130 km/h (80.78 mph).

It draws its energy from a package of lithium-ion batteries arranged in a T pattern beneath the center tunnel and rear bench seat. The liquid-cooled battery stores 12 kWh of energy, which suffices for more than 50 km (31.07 miles) of driving. High-voltage current will recharge the battery in less than an hour. As of longer distances, a range extender operates. A small rotary-piston engine underneath the luggage compartment recharges the battery by means of an alternator.


The Audi A3 TCNG can run on the e-gas which Audi produces in the methanation unit. Its four-cylinder TFSI engine and the exhaust system’s catalytic converter were designed with natural gas in mind. In Germany alone, natural gas is available at some 900 CNG stations and counting.

The Audi A3 TCNG can run on e-gas produced in the methanation unit.

A3 TCNG can run on e-gas produced in the methanation unit.

Via the “balanced cycle method” – similar to the purchasing of green power – A3 TCNG owners should be able to fuel their vehicles with wind energy starting in 2013. When a driver refuels with e-gas, the corresponding amount of renewable energy required to produce this e-gas is fed into the grid.

The volumetric density of e-gas is equal to that of fossil-based natural gas and is thus lower than premium unleaded. Similarly to natural gas, the combustion of e-gas also creates far less CO2 than premium unleaded does. Concerning the e-gas project, this means that CO2 emissions are very low not only in the overall picture (well-to-wheel), but also at the exhaust pipe (tank-to-wheel). Not one gram of CO2 is emitted via the exhaust pipe which would not have been consumed during the manufacture of e-gas, says Audi. In other words, there is a closed CO2 cycle between the fuel’s manufacture and its combustion.

The high octane rating of approx. 130 RON for natural gas, biomethane and also for e-gas facilitates a high compression ratio in the turbo engine – which ensures high efficiency. The A3 TCNG's gas tanks, which store the e-gas at a pressure of 200 bar, offer enough capacity for long drives. The Audi A3 TCNG also boasts a bivalent configuration: if the natural-gas tanks run empty and there is no CNG station nearby, the vehicle can run on conventional gasoline with no drop in performance.


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