Gas is a lower-carbon fuel than coal and, unlike nuclear power, is relatively risk-free, says a German Wuppertal Institute for Climate, Environment and Energy report. The report, commissioned by environmental organisation Greenpeace, draws on work by the Fraunhofer Institute for Wind Energy and Energy Systems Technology regarding integrating renewable energies into the national energy system.
The Fraunhofer experts are exploring use of power-to-gas systems, where surplus wind power can drive electrolysis systems to produce hydrogen from water, then convert the hydrogen to methane, the main constituent of natural gas. The existing natural gas pipeline, storage and gas-fired power stations could provide the infrastructure for renewables-produced methane to secure electricity supply when the wind is not blowing.
Existing gas stores in Germany have a capacity of 200TWh of thermal energy, which could theoretically be used to generate about 100TWh of electricity or two months' worth of national electricity consumption, says the Fraunhofer team. The stores would have to continue their other functions, including holding reserves in the summer months for gas heating in the winter, but their potential for electricity generation could still be substantial.
In comparison, hydro-pumped storage systems and battery systems have a storage capacity of just 0.07TWh, which would cover electricity consumption for only a few hours. Even the battery storage that would be made available if around 45 million cars in Germany were replaced with electric vehicles would only amount to about 0.45TWh, enough to cover demand for about six hours.
While the Fraunhofer experts admit that work is at an early stage and that renewables power-to-gas storage systems would only be about 36% efficient, the report points out that, if longer-term storage facilities are not built, curtailment of output to prevent excess renewables electricity on the grid would also result in a loss. But the power-to-gas system is nearly three times more expensive than the current average price of natural gas imported into Germany. Electrolysis and methanisation units would require investment of about EUR1,000-2,000/kW. Surplus renewables electricity purchased for EUR0.02-0.04/kWh could be transformed into methane at an estimated cost of about EUR0.08/kWh heat content, while imports of natural gas cost around EUR0.03/kWh.
The Wuppertal Institute stresses that it is too soon for a reliable estimate of advantages and disadvantages of the power-to-gas concept.