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A step forward for renewable energy storage?

Energy storage will be critically important as we work towards sustainable living. Developing cost-effective ways to store large amounts of electricity from wind turbines and solar farms will be essential in turning from fossil fuels to renewables as our primary source of energy.

As these technologies develop and our reliance on them grows, there will be an increasing need for rechargeable energy storage capability. Although wind and solar generated electricity is becoming increasingly popular in many countries including Australia, these natural sources provide only intermittent energy; thus energy storage systems (like batteries) are required to store the energy until needed by the electrical grid.

A water-based sodium battery is an affordable and safe option to store power from renewable generation.

Batteries with high energy density (large storage capability) enabling back up for wind and solar power typically can’t store much energy. They have historically been based on lead-acid (Pb-acid) chemistry. As a positive, Pb-acid batteries are known to last up to a decade depending on the depth of discharge (if you discharge them regularly they last longer; irregular discharges shorten their life). However, they employ highly toxic electrodes and use highly corrosive sulphuric acid as the electrolyte, and only provide meagre energy density.

These issues highlight the critical need to replace the Pb-acid battery. But high cost and safety issues of alternative battery chemistries present significant challenges. The lithium-ion battery is one viable alternative, although the trade-offs between performance, safety and cost have significantly hampered its utility.

Lithium batteries can have an extended cycle life if they are used relatively gently (they can’t be fully discharged, for example). This means their energy density is inadequate for the price. The electrolytes in lithium batteries have some problems: they can be toxic, flammable and have other safety issues. They’re also expensive, meaning the manufacturing costs of lithium batteries are high.

For safety concerns, water based electrolytes are the natural choice in this field. Water is cheaper than organic solvents and has fewer disposal and safety issues. The ionic conductivity of aqueous electrolytes is two orders of magnitude greater than that of organic electrolytes. This allows higher discharge rates (you can run the battery flat more often) and lower voltage drops due to high conductivity of the water based electrolyte.

The aqueous battery is particularly interesting in land-based applications, such as electric grid stabilisation. In these situations, low weight and high capacity are not critically important.
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