Hydrogen
A common method to produce hydrogen is steam methane reforming, in which hydrogen (H2) is split from natural gas (that is mainly methane, CH4) according to the overall reaction CH4 +2H2O ↔ 4H2 +CO2. Larger quantities of H2 are produced this way for NH3 production (see Sec. 23.10). This process generates CO2 and thus is not carbon neutral. If the natural gas is used in a combined cycle power plant with thermal efficiency of 60% (Sec. 13.6), there will be less CO2 produced per kWh of electricity than if H2 from steam methane reforming is used in a fuel cell.
By means of electrolyzers, hydrogen can be produced from any primary source of electrical power, including carbon neutral power sources like nuclear, solar, wind, or tidal. As opposed to electricity, hydrogen can be stored and distributed in pipelines and tanks, and therefore offers a means to store power produced from intermittent sources (solar, wind, tidal).
The stored hydrogen can be reconverted into electricity either in traditional combustion systems (e.g. combined cycle power plants, Atkinson cycle), or in fuel cells. The latter offer an elegant and efficient means to use power produced by stationary non-carbon power plants (solar, wind, …) for transportation (cars, trucks, busses, trains, … ). The use of hydrogen produced by carbon neutral energy sources could play a role in the future energy system, which must aim to reduce the emission of greenhouse gases, including carbon dioxide.
Due to irreversibilities, only a portion of the energy fed into the electrolyzer is retrieved from the fuel cell. Moreover, one needs to account for the efficiency of processes required to store and distribute the hydrogen. Indeed, at normal conditions hydrogen is a gas which, due to its low molar mass, assumes a relatively large specific volume. In particular for use as transportation fuel, the hydrogen needs to be compacted, either by compression of the gas, or by liquefaction. The (irreversible) processes involved can be described by a storage and distribution efficiency measure ηII .
With the second law efficiencies for electrolyzer, storage and distribution, and fuel cell, the ratio between the power provided by the fuel cell, W˙ FC , and the power W˙ E which was consumed to produce the hydrogen in the electrolyzer, is
Typical values for these efficiencies are ηII so that only 27% of the energy provided at the source is finally recovered.
Doubtless, new materials and better designs will lead to efficiency improvements in the future. Meanwhile, other storage concepts might have higher efficiencies, e.g., batteries or pumped hydro.