Energy Efficiency. Burning natural gas in an internal combustion engine (ICE) will reduce our dependence on imported oil, but is this the best use of natural gas resources?
We need to consider total system efficiency and also well-to-wheels greenhouse gas (GHG) pollution.
Even though converting natural gas to hydrogen is only 75% efficient, the much greater efficiency of a fuel cell electric vehicle (FCEV) (up to 3 times more efficient) compared to a conventional internal combustion engine (ICE) car means that a FCEV running on hydrogen made from natural gas can travel 1.99 times farther than using that natural gas in an internal combustion engine car. Alternatively, we could convert that natural gas to electricity to power a battery electric vehicle; in this case making hydrogen from that natural gas could propel the FCEV 2.3 times farther than using the natural gas to make electricity for a BEV.
Another option would be to convert natural gas to electricity for use in a battery electric vehicle (BEV). But converting natural gas to hydrogen is 1.3 to 1.9 times more energy efficient than converting it to electricity for a BEV.Thus converting natural gas to hydrogen for a FCEV will increase the vehicle miles traveled from a given quantity of natural gas by a factor between 2 and 2.3. Therefore our supply of natural gas for transportation will last 2 to 2.3 times longer.
Greenhouse Gas (GHG) Pollution. We also need to compare the GHGs resulting from using natural gas in an NGV compared to using hydrogen made from natural gas in a FCEV. In the near-term (before 2020), using hydrogen made from natural gas in a FCEV will cut GHGs approximately 2.1 times more than using that natural gas in an optimized NGV.
In the longer term, the advantage of FCEVs will grow dramatically, since hydrogen can be made from renewable sources, waste water treatment plants and from nuclear power with near-zero or zero carbon emissions. By the end of the 21st century, FCEVs will be able to reduce GHGs by 89% below 1990 levels, exceeding the climate change community’s goal of an 80% reduction. NGVs have no such practical option. BY 2100, optimized NGVs would increase GHGs by 41% over 1990 levels with the assumptions of increased vehicle miles traveled in our 100-year simulation model, natural gas hybrid electric vehicles (NG HEVs) would increase GHs by 29%. Natural gas plug-in hybrid electric vehicles (NG PHEVs) would reduce GHGs by 47% below 1990 levels, since the model assumes that the electrical grid also shifts to low- or zero-carbon sources by the end of the century.
Natural Gas Fueling Infrastructure Cost. While natural gas is available in most parts of the country, this gas must be pressurized and stored in high pressure tanks to rapidly fill the NGV car tanks. Thus expensive gas compressors, storage tanks, and high pressure dispensers will be required at fueling stations to service NGVs. These three pieces of equipment make up from 70 to 74% of the cost of a hydrogen fueling station. Thus the costs for a natural gas high pressure fueling infrastructure will not be much less than the costs of building a hydrogen infrastructure. So the question for society is whether to invest this money in a temporary, non-sustainable transportation option (the natural gas supply is finite, and NGVs cannot meet our GHG reduction targets and NGVs generate significant local air pollution.), or should we instead begin building the long-term sustainable solution: hydrogen powered FCEVs?
FOr more details comparing FCEVs and NGVs, see this paper