Beyond lithium-ion: Energy storage technologies

Storage of renewable energy requires low-cost technologies that have long cycle lives — where they can be charged and discharged many times — are safe and can store enough energy cost effectively to match demand.

● Vanadium redox-flow batteries use two tanks containing positive and negatively charged liquid vanadium electrolytes that are pumped past a membrane in a cell. The batteries have less degradation than lithium-ion and a longer cycle life.

● Liquid air is cooled to minus 196C, after which it is stored in tanks. It is then heated, which drives a turbine to generate power. An alternative uses heated compressed air to store energy in purpose-built caverns.

● Gravity storage involves lifting heavy blocks up and down abandoned mine shafts as a way to store and generate energy.

● Thermal energy storage Malta, a company backed by Bill Gates’ Breakthrough Energy Ventures, stores energy as heat in the form of molten salts. The company says the technology can last longer than 20 years and is suitable for six-plus hours of storage.

● Liquid metal batteries use metals that naturally separate when heated to form a cathode and anode separated by a salt electrolyte. Once initially heated, the battery maintains its high operating temperature by generating heat on discharge and charge.

● Low-cost batteries using cheap raw materials such as iron, sulphur and zinc offer alternatives to lithium-ion battery technology. Zinc-based battery developer EOS, for instance, says its battery has capacity to discharge energy over three to 12 hours. Form Energy, a start-up backed by Bill Gates, says its battery can store energy cost effectively for up to 150 hours.

● Hydrogen Using electricity to produce hydrogen is a way of storing energy, but there is a substantial loss of energy in the process, making it less efficient than batteries.