FACTBOX - Types of electricity storage

BERLN (Reuters) - Electricity storage will become essential for mass adoption of wind, solar and other forms of renewable energy, because their output varies drastically by the hour or the day.
Below are the five main types of storage technology, which are of limited maturity and/or under development, as identified by a Boston Consulting Group study:
1. Mechanical storage * Pumped hydroelectric storage+++ * Compressed air energy storage (CAES)++++ * Flywheel storage
2. Thermal storage * Hot-water storage * Molten-salt storage * Phase-change material storage
3. Electrical storage through supercapacitors and superconducting magnets
4. Electrochemical storage through batteries, either static or in electric cars
5. Chemical or hydrogen storage
+++ Pumped storage is the most established method, used in conjunction with hydroelectric plants. When electricity is cheap and plentiful, it is used to pump water to high basins, from where the water can be released to power turbines when additional power is needed and prices are high. The biggest planned German project is at Atdorf in the Black Forest, which could supply 1,400 MW of capacity from 2020, but there is local opposition to new infrastructure. The technology suits mountainous countries such as Austria and Switzerland, which already use it. But in the centre of Europe, there are geographical limits and opposition from the population to major interference with nature and with residential areas.
++++ In a poll by Trend Research of Bremen last October about the potential for storage technologies, the most significant role was assigned to hydrogen/methane (power-to-gas), followed by electric cars, batteries and compressed air, in that order.
(Reporting by Vera Eckert, editing by Jane Baird)