These days, anyone who wants to buy a new car faces a difficult decision. That's because there have long been more questions to consider than the model and financial affordability: The environmental aspects of both the manufacture and operation of a vehicle are becoming increasingly important – primarily with respect to the emission of climate-damaging gases such as CO2. This is particularly true in Switzerland, because here road travel is of great importance for climate protection: It causes around one-third of all greenhouse gas emissions – more than industry and also more than the residential sector. Politicians are creating the framework conditions for the most environmentally compatible production and operation of cars, for example through laws, emissions standards or taxes.
A PSI study can help both politicians and consumers. On behalf of the Swiss Office of Energy, former doctoral candidate Brian Cox and project leader Christian Bauer examined the environmental impact of passenger cars with different engines. For the sake of comparison, they have stated their results in CO2-equivalents. Since 2010, they have been collecting data, combing through literature and databases, interviewing manufacturers and asking for information from research experts, including colleagues at PSI. The result is a highly differentiated picture of how the various automotive power systems stand today in terms of the ecological balance sheet – and how they are expected to be in 20 years. The study dares for the first time to make a forecast of their likely environmental impact in 2040.
"With such analyses, it is important to consider the entire life cycle of the vehicle," says Brian Cox, who now works for the consulting firm Infras. In calculating the climate footprint, carbon dioxide emissions during operation are not the only thing that counts. There are also the emissions arising from the production of the car and its components as well as from its disposal. In addition, the emissions from road construction and from the production of the respective fuel must be included. For cars with petrol-burning engines, the latter are incurred through oil production and refining, for fuel cell vehicles through hydrogen production and with battery-powered electric vehicles through electricity production. All in all, a gigantic research effort. "Especially since some data is difficult to obtain," says Christian Bauer. "Just how such an electric car is made, nobody will tell you. The companies treat this as top secret information."
Nevertheless, the researchers ultimately were able to compile everything that was necessary and came to a clear result: In the area of passenger cars, we should rely on battery-powered electric motors to protect the climate. Summing up all factors, this has the lowest climate impact. This is already the case in Switzerland, since the electricity here is mainly produced by hydropower and nuclear power. As a result, the Swiss electricity mix only generates a little more than 100 grams of CO2 per kilowatt hour, even taking energy imports into account.
By the way, it is important to note that before the first kilometres are driven, the balance sheet for the electric car is worse than for other vehicles – though with low-CO2 electricity it compares better the more it is driven. This is because the manufacture of the battery causes a lot of emissions, among other things because it contains special metals that are mined far away and transported long distances. However, the electric car more than makes up for this shortcoming with its low emissions during operation.
The study indicates that the fuel cell is currently the second most climate-friendly automotive power source – under the right conditions. Here too, in the classic version, the wheels are driven by an electric motor. However, the electricity does not come from the electrical socket, but is produced by a fuel cell that converts hydrogen from an on-board high-pressure tank and oxygen from the air into water, releasing energy. The decisive factor is how the hydrogen was produced: If it comes from electrolysis – that is, the splitting of water into oxygen and hydrogen by means of electricity – and if this is carried out with solar electricity, this is very favourable for the climate. If it is produced with the Swiss electricity mix, it is a little less so. And the balance becomes really unfavourable if the hydrogen is obtained from a fossil fuel such as natural gas.
Cars that run on natural gas have a carbon footprint roughly comparable to diesel. But so-called synthetic natural gas (SNG) has a special role to play. This artificial natural gas substitute is produced by first obtaining hydrogen from water by electrolysis, as with the fuel cell. This is then added to carbon dioxide, which can for example be separated out in a biogas or waste incineration plant, in a coal-fired power plant or directly from the air. The result is a hydrocarbon that is so similar to natural gas (both consist mainly of methane) that it can be fed into the normal natural gas network. "In the overall balance, natural gas cars that run on SNG do consume five to six times as much electricity as electric cars," says Bauer, "because there is a lot of energy loss along the SNG production chain and the combustion engine is inefficient."
Why consider such an engine at all? The idea with SNG is that it is produced using surplus green energy. So if, in the summer, there is more energy available from solar installations and wind turbines than we can use, it has to be stored somehow. Then it would be available in winter, when the renewable energy sources are weak and the electricity demand is high. The chief attraction is that SNG is easy to store. Small surpluses are simply fed into the existing natural gas network, where it can be tapped at any time. And for large surpluses, there are inexpensive and largely loss-free tanks. "If we actually have a lot of green electricity in the future, SNG will definitely make sense to power passenger cars," says Bauer.
So the big question for the climate footprint of all automotive power systems is what the European electricity network of the future will look like. Do politicians and society have the courage to rapidly switch the system to renewables? Then the alternative motors could fully demonstrate their strengths.