Storing up the sun
If green electricity is to power us in future, it needs two things: larger power grids and new storage systems. The latter must be capable of providing calculability as we make the transition to a supply system where the amount of electricity being fed into the grid can fluctuate. “From today’s perspective, there is no doubt that new storage solutions are crucial to developing a stable system,” says Eicke Weber, head of the Fraunhofer Institute for Solar Energy Systems in Freiburg. But as yet it is unclear exactly when and to what extent we will need pumped storage plants, batteries, and electrolysers for producing hydrogen. A lot depends on the German government. Will it stick to its 2010 Energy Concept goal, which foresees a relatively modest 50-percent share for renewables in the overall electricity supply by 2030? Or will it focus on the current network development plan, which aims to have renewables supplying around two-thirds of the country’s electricity by as early as 2033. If the government takes that route, then we will see a rapid increase in the need for technologies that can balance out fluctuations between generation and consumption.
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As things stand, however, work on developing high-performance batteries and power-to-gas technology (which uses green electricity to produce storable hydrogen and methane) is still very much in its infancy. Researchers still have a long way to go before the solutions make economic sense. Uwe Leprich, an economist and scientific director of the Institut für Zukunftsenergiesysteme (IZES) in Saarbrücken, doesn’t see this as a problem. He says that a lack of storage systems won’t be the downfall of the switch to renewables, and that the energy needed to cover shortfalls can initially come from flexible gas-fired power plants or combined heat and power plants (see interview on page XX). He adds that hourly and daily reserves can be handled by pumped storage plants. These systems pump water from a low reservoir to a higher one. When the water is released, it flows down drainpipes and drives turbines, which in turn produce electricity. Intelligent load management could reduce the amount of balancing power needed. Electricity rates that vary depending on the time of day encourage consumers to link their consumption to the supply/demand ratio. Electricity is cheap when solar power plants and wind turbines are working to full capacity. It gets more expensive during periods of peak demand. Load management could become even more effective if, in the future, owners of electric cars agree to charge and discharge their vehicles at specific times of the day.
Leprich says that long-term storage solutions like power-to-gas technology will only be needed once renewables make up 80 percent of the mix – a scenario that is still a long way off. “The storage capacity of the natural gas network makes this technology a fascinating prospect. But it is only financially viable if very large quantities of surplus electricity are available,” he says. Norman Gerhardt, head of the Energy Economy and Systems Analysis group at the Fraunhofer Institute for Wind Energy and Energy System Technology in Kassel, also sees no immediate need for batteries and the like: “We’ve still got time for that.” He says that although expanding photovoltaics would create a need for storage solutions that can carry energy over into the evenings (when demand spikes) and that can supply operating reserves to stabilise grid frequencies, these tasks could be handled in the short term by advanced pumped storage plants. According to Gerhardt, we will probably start to need additional compressed air storage technology from 2030. This solution uses excess green electricity to compress air into caverns. When released, the air powers turbines, which in turn generate energy. He says that the technology would be particularly suitable for northern Germany, since it could store offshore wind energy in the region’s many underground salt domes.