Storage is one of the highest technological barriers to the spread of renewable energy. When the sun is shining, the tide turning or the wind howling, how do we collect that energy and keep it to use when generation is down?
There are many different types of energy storage technology available or under development. But each technology has some inherent limitations or disadvantages that make it practical or economical for only a limited range of applications.
Some technologies are mechanical (flywheels, pumped hydro, compressed air). These have low energy efficiencies and slow response times. Pumped hydro and compressed air storage systems are also restricted by special geological and geographical requirements, high investment costs and long construction times.
Electrochemical energy storage systems - batteries - offer many benefits and advantages compared with other forms of energy storage.
Amongst the different types of battery technologies currently available, the ones receiving the most attention for large-scale energy storage applications are: - lead-acid - lithium-ion - sodium sulphur - flow batteries.
Lead-acid batteries are low cost. But their application for large-scale energy storage is limited by their short cycle life and limited rechargability.
CSIRO has recently developed the UltraBattery. This hybrid energy storage device integrates a supercapacitor with a lead acid battery. The UltraBattery can be charged much faster than conventional lead-acid batteries.
This type of battery is well suited to hybrid vehicles and could be used to smooth out short-term power fluctuations in wind turbines (though this ability has not yet been demonstrated).
Lithium-ion (Li-ion) batteries
The main advantages of Li-ion batteries, compared to other advanced batteries, are:
- high energy density - they can store a large amount of energy in a smaller physical space
- high efficiency
- relatively long cycle life, but still not adequate for many application.
Lithium-ion batteries perform well and are widespread in portable devices, such as mobile phones.
But a number of challenges need to be overcome if they’re to be used in large-scale grid-connected applications.
The main hurdle is the high cost (above $600/kWh). Li-ion batteries need special packaging and internal overcharge protection circuits to overcome safety issues that can lead to fires and potential explosions. Some of these safety issues have been addressed with the use of new electrode materials that operate at much lower voltages, but this reduces their energy density (and thus the main advantage of Li-ion).
Several companies are also working to reduce the manufacturing cost of Li-ion batteries. The electric vehicle industry is pushing this development.
Sodium-sulphur (Na/S) batteries
The sodium-sulphur or Na/S battery has a liquid (molten) sulphur positive electrode and liquid sodium negative electrode separated by a solid beta alumina ceramic electrolyte. Na/S battery cells are about 89% energy efficient.
The battery has to be kept at above 300°C to prevent electrolyte freezing and irreversible damage to the cells. These batteries also have safety issues.
The main difficulty with the Na/S technology is producing the solid beta alumina tubes that act as both separator and solid electrolyte. These are difficult to mass produce at an affordable cost.
Despite their high cost however, the Na/S technology has been extensively implemented in a large number of energy storage field trials and demonstrations around the world.
Flow batteries are a cross between a conventional battery and a fuel cell. They have up to 80% energy efficiency. In contrast to conventional batteries, the power generation (kW) and energy storage capacity (kWh) can be independently varied to suit the job the battery is doing.
Flow batteries are the cheapest energy storage technology for applications requiring storage of more than four hours, such as large-scale renewable energy storage.
Of the different types of flow battery chemistries that have been explored, only two types - zinc bromine (Zn/Br) and all-vanadium redox (VRB) - have reached commercial fruition.
Three companies are developing the Zn/Br battery commercially. RedFlow, an Australian company, says they will release an improved battery this year. This is a 120 kVA, 240 kWH grid-connected energy storage system designed to store off-peak electricity for time-shifting and network stabilisation.
VRB was pioneered at the University of New South Wales, Australia. The largest VRB installation to date is a 4 MW/6 MWh demonstration system built by Sumitomo Electric Industries to store energy at a windfarm on the island of Hokkaido in Japan. Up to 200,000 charge-discharge cycles were demonstrated over the three-year life of the project, as well as very high energy efficiencies and fast response times that enabled output power stabilisation for the wind turbines.
The cost per kWh of generated energy of a VRB system can be less than half that of an equivalent lead-acid battery system for storage capacities in excess of four hours. This makes the VRB one of the cheapest energy storage technologies for large-scale renewable energy storage.
Several companies are now commercially manufacturing VRB systems, and VRB is likely to be of the leading battery technologies in the expanding global energy storage market being fuelled by the push towards renewables and the smart grid.