Natural gas vehicles are seen by some as a bridge technology on the way to completely CO2-free transportation. But they are good as an environmentally friendly transitional solution until electric cars are suitable for everyday use? Senior researcher moritz mottschall answers the question and comes into contact with his personal history in the process.
Before living at the eco-institute, i was a cab driver in berlin. the company i drove for had a very special cab: on the side was a green sticker that said "TUT – a thousand environmental cabs for berlin". This "eco-taxi" had a modern bi-fuel drive with two tanks, one for compressed natural gas and one for gasoline.
These insights into reality gave me the decisive plus points in my interview: for example, i knew the (open) secret that many eco-taxis from a certain manufacturer ran on super gasoline instead of natural gas for a long time due to problems. The gasoline was even paid for by the manufacturer, to the delight of the contractors. The question of the environmental benefits of natural gas vehicles could not be answered clearly at the time. But how does it look today?
Today, natural gas vehicles are sometimes referred to as a possible bridge technology on the way to completely CO2-free transport, for example with electric cars with batteries or hydrogen fuel cells. The idea in the background: natural gas causes less direct CO2 emissions than gasoline. This is because natural gas has a better so-called C/H ratio. This means that for every carbon atom (C), there are a particularly large number of hydrogen atoms (H). That’s why natural gas causes fewer direct CO2 emissions than gasoline.
Gas as a source of electricity
a major argument for natural gas is that it could be replaced by biogas in the future, making the car almost CO2 neutral. Despite this, many millions of euros are currently being invested or planned in fossil natural gas infrastructure. For example, the northstream 2 pipeline or terminals for liquefied natural gas in northern germany.
In the long term, it is clear: even the cleanest gas can only be burned once! So we asked ourselves: aren’t there more sensible ways of using natural gas or biogas than burning it in a vehicle engine with very low efficiency and blowing more than two-thirds of the energy into the environment in the form of waste heat?? One possibility would be to use the gas in highly efficient gas and steam power plants to generate electricity. This electricity could be used where the bridge is supposed to go: in an electric car.
A comparison of e-cars and natural gas cars
So we did the math. A VW e-golf and a natural gas-powered VW golf TGI have been launched. For both, many users have entered real consumption data in the spritmonitor database.De entered, which we have taken into account. However, this is not a completely fair comparison. the two vehicles differ significantly in terms of performance: the e-golf’s output of 100 kilowatts is 23 percent higher than the natural gas vehicle’s 81 kilowatts. We therefore include a generic vehicle from the compact class in the race, which we have taken from our technology data base, the TEMPS model (transport emissions and policy scenarios) of the oko-Institut.
Final energy consumption of the electric and natural gas cars considered, source: oko-institut
In addition to the electricity consumed in the vehicle, part of the electrical energy is lost on the way to the battery: during electricity transport or during the charging process. Total losses of 16 percent are estimated here.
In our comparison, the burner uses the gas directly. The electric car uses electricity generated from natural gas in the power plant. In the european average (73.2 percent combined electricity and heat generation, 26.8 percent exclusively electricity generation), this results in 475 grams of CO2 equivalents per kilowatt hour (reuter, benjamin; hengstler, jasmin; whitehouse, simon; zeitzen, lena (2017): greenhouse gas intensity of natural gas. Final report. Hg. V. NGVA europe, last checked on 08.04.2019.). In a particularly efficient gas-fired power plant in germany, it is even only 423 grams of CO2 equivalent per kilowatt hour (ecoinvent v.3.5; electricity production, natural gas, combined cycle power plant).
When used as a fuel, emissions depend on the origin and composition of the natural gas, as the content of methane, ethane and propane in the natural gas can differ, for example. But the upstream emissions caused by the supply of natural gas, which are also referred to as emissions in the upstream chain, also differ. upstream emissions are caused by leakage and energy consumption in the extraction, processing and transport of the gas to germany.
These direct and upstream emissions can be related to the energy contained in the natural gas. In the calculation, we assume around 68 grams of CO2 equivalents per megajoule. Of this, about 18 percent is accounted for by upstream process steps and 82 percent by direct emissions in the vehicle.
For operation, the picture is clear: both for a particularly efficient natural gas power plant and the european mix of natural gas power plants, the electric car performs better than the natural gas burner. the fact that such a large difference arises even when taking into account the electricity losses during transmission and charging is due to the different efficiencies: modern gas-fired power plants have an electrical efficiency of well over 50 percent, in some cases up to 60 percent. Gasoline-powered internal combustion engines, on the other hand, can generally convert 20 to 30 percent of the energy into motion. Even if a particularly inefficient gas-fired power plant were assumed, operation of the electric car with the burner would result in comparable or lower emissions.
Specific greenhouse gas emissions from vehicle operation for natural gas-powered passenger cars and natural gas-powered electric cars for different types of power plants (including emissions from the upstream chain), source: oko-institut
However, a fair comparison must also take into account the costs of vehicle production. This is important because the production of electric cars leads to higher emissions, especially from batteries. Estimation of greenhouse gas emissions from battery production is currently subject to great uncertainty. Often outdated emission factors and wide ranges are used. Our estimate is based on the calculations of the ifeu institute, which they prepared for agora-verkehrswende.
Accordingly, an internal combustion engine in the compact class – gas-powered cars were not explicitly considered – has an emissions advantage of almost six metric tons. The electric car produces more than 13.2 metric tons of CO2 equivalents through production and disposal, while the internal combustion engine produces only around 7.4 metric tons.
Greenhouse gas emissions from operation, vehicle production and disposal of a compact class vehicle as a function of lifetime mileage, source: oko-institut
Depending on the emission factor used, the electric vehicle, taking into account production and disposal, has a lifetime mileage of about 82 kilometers.000 (efficient natural gas power plant in germany) or 93.000 kilometers (mix of natural gas power plants in the EU) a climate advantage over the gas car. With a typical lifetime mileage of 150.000 the greenhouse gas emissions of an electric car powered by natural gas are between three and five tons lower than those of a natural gas burner. This corresponds to a greenhouse gas reduction of between 12 and 17 percent.
In fact, this is just a game of arithmetic. Because in a balance sheet, it is best to also use the electricity mix in the respective network. And gas electricity tariffs are at least not known to me.
However, the great advantage of electromobility becomes apparent: the high energy efficiency at the vehicle level. Impressively, the double conversion of natural gas into electricity and then into kinetic energy is more efficient than the direct conversion of natural gas into kinetic energy in a gasoline engine.
Besides, the reality is more complicated: gas-fired power plants, for example, perform certain functions in the power grid to stabilize the grid. Due to their high flexibility, they can be operated in such a way as to compensate for fluctuations in power demand on the electricity grid.
On the other hand, greenhouse gas emissions from the transport sector, which are currently not traded, would be shifted to the energy sector, where emissions are already traded and capped.
Nevertheless, my comparative view can serve as a suggestion to use the available energy, be it natural gas, biogas or synthetic methane, where it can be used most efficiently. Natural gas cars are not. That’s why we shouldn’t focus on a supposed bridge, but rather tackle the goal of zero-emission driving head on.
Moritz mottschall is a senior researcher in the field of resources& mobility in berlin. One focus of our work is energy efficiency and alternative drive concepts in transportation.