Energy storage: the way of the future

Organic battery under development at the University of Southern California

By Paul Grad, engineering writer

The emergence of low-cost energy storage technologies has become a crucial factor in the development of the “smart grid” – the management and effective integration of renewable energy into the grid. Renewable energy sources such as wind and solar are intermittent and therefore effective energy storage is essential to integrate them into the grid.

The introduction and increasing use of renewable energy technologies has become a mixed blessing to the utilities, which call them “disruptive technologies”. The utilities were at first supportive of renewable energy systems, but some of the utilities are now weary of them due to the management complexities they have created. And, with low-cost energy storage technologies, electricity users could become independent of the grid altogether.

It is not yet clear what the most cost-effective energy storage system will turn out to be, but batteries, particularly lithium-ion batteries, are among the strongest candidates.

Lithium-ion batteries are the most popular energy storage option due to the advantages they offer over other rechargeable batteries. They are lighter than other rechargeable batteries for a given capacity, they deliver a high open-circuit voltage, they have a low self-discharge rate (about 1.5 per cent per month), and they do not suffer from battery memory effect. They can store 100 to 265Wh/kg.

They also entail some disadvantages, however, such as poor cycle life, rising internal resistance with cycling and age, and safety issues if overheated or overcharged.

The materials used for the anode and cathode can dramatically affect battery performance, including capacity. Graphite has traditionally been the anode of choice for commercial use.

Other materials have been investigated, with silicon offering the highest capacity (mAh/g – milli ampere hours per gram). Nexeon, of Abingdon, Oxfordshire, UK, is offering a lithium-ion battery with silicon anode with greater energy density properties than those of lithium-ion batteries with graphite anodes.

Several lithium compounds are used as positive electrodes (cathodes) in lithium-ion batteries. Handheld electronics use mostly lithium cobalt oxide (LiCoO2). Lithium iron phosphate (LiFePO4) or LFP (“Lithium ferrophosphate”), lithium manganese oxide (LiMn2O4) or LMO, and lithium nickel manganese cobalt oxide (LiNixMnyCozO2) or NMC are widely used for electric tools. NMC is a leading contender for automotive applications.

According to the Clean Energy Council, energy storage technologies are becoming increasingly viable in Australia, though. Currently storage systems are economical in off-grid markets and those on the fringes of the grid, where energy is often unreliable.

For example, in the towns of Marble Bar and Nullagine in Western Australia, Horizon Power has installed a flywheel 2x500kW storage system to support solar power stations.

Ergon Energy has installed modular 5kW batteries to support solar energy on Magnetic Island, Queensland. Hydro Tasmania has installed a vanadium-redox flow battery to support wind power on King Island. Ausgrid has installed 5kW batteries at volunteer properties in Newcastle, Scone and Newington, NSW.

Solar360, of Melbourne, has a division called 360Storage, which has partnered with several manufacturers to offer on-grid storage systems. The company’s products include an integrated system of an inverter, solar MPPT (maximum power point tracking – an electronic DC to DC converter that optimises the match between a solar array of photovoltaic panels and a battery pack or utility grid), LiFePO4 batteries and BMS (battery management system) in portable enclosure.

Redflow Limited, of Brisbane, is offering a 3kW continuous/8kWh zinc-bromide battery module designed to be integrated into electricity storage systems. The company says the batteries are well suited for storage of intermittent renewable energy, managing peak load on the grid as well as for supporting off-grid power systems.

Palladium Energy, of Woodridge, Illinois, is offering lithium-based battery packs with chemistries such as lithium iron phosphate and lithium thionyl chloride (Li-SOCl2), which feature high energy density (3.6V/19Ah), long life of up to 20 years, and low self-discharge – less than 1 per cent per year.

Although modern lithium-ion batteries hold more than twice as much energy by weight as the first commercial versions sold by Sony in 1991, and are 10 times cheaper, they are still not energetic enough to match the performance of petrol in motorcars. To stimulate development beyond Li-ion technology, the US Department of Energy granted the US Joint Center for Energy Storage Research (JCESR) based at the Argonne National Laboratory, in Chicago, US$120 million in funding. The goal was to achieve cells that would be five times more energy dense than the existing standard and five times cheaper, when scaled up to the commercial battery packs used in electric cars. The JCESR set itself a target of 400Wh/kg by 2017, which some researchers thought impossible to achieve.

In answer to the challenge, the centre developed lithium-sulfur (Li-S) technology, which uses cheap materials and can theoretically pack five times more energy by weight than Li-ion. One of the advantages of Li-S is that it eliminates the weight of a Li-ion battery. Inside a Li-ion battery, space is taken up by a layered graphite electrode that acts only as a host to lithium ions. In a Li-S battery, the graphite is replaced by a sliver of pure lithium metal that doubles as the electrode and the supplier of lithium ions. Also, the metal oxide is replaced by cheaper and lighter sulfur.

Lithium-sulfur batteries could eventually replace lithium-ion batteries. For applications in energy storage for the electricity grid, however, size does not matter. Instead of a small, light battery with a high energy density, you need a battery that stores energy cheaply and with little maintenance. The JCESR aims to produce such batteries lasting 7000 cycles – about 20 years.

However, not all people are as happy with lithium-based batteries. Kurtis Kelley, of Firefly International Energy Co, of Peoria, Illinois, has written a fairly angry paper titled Lead-acid vs lithium for the smart grid: balancing the Illinois battery research facility efforts, where he expresses concern about the potential for one-sided efforts at the new Illinois Battery Research Center. Secondly, he tries to dispel some myths and show that advanced lead-acid chemistry is much better than usually believed. Kelley said the potential for lead-acid in creating energy-efficient, sustainable technology to support the need for high-performance battery storage is undervalued. The specific energy of a lead-acid battery is 160.9Wh/kg.

But apart from lithium-ion and lead-acid, many battery systems are possible. To provide a low-cost, flexible, reliable and long-lifespan grid-scale electricity storage system, scientists at the Massachusetts Institute of Technology (MIT), led by Prof Donald Sadoway, have developed a system with two layers of molten metal as electrodes, separated by their different densities and by a layer of molten salt electrolyte. The metal layers swell or shrink as ions pass between them, storing or releasing energy. Called the Liquid Metal Battery, it is now manufactured and sold by Ambri, Inc, of Cambridge, Massachusetts.

Other groups are developing flow batteries, where the fuel consists of two liquids that pass ions to each other through a membrane. The liquids can be kept in tanks outside the battery and pumped through when needed. Thus large amounts of energy can be stored by using larger tanks. Commercial flow batteries use vanadium ions on both sides of the barrier. However, vanadium and the membranes are expensive, and pumps and valves require considerable maintenance.

To avoid the cost of metals, researchers at the University of Southern California, in Los Angeles, led by Prof Sri Narayan, are developing an all-organic, water-based battery, redox flow battery for “clean storage”. Narayan said when batteries are scaled up for use as energy storage systems on the power grid, they will have to be of fairly large sizes, and then using metals and toxic substances will be a problem. The organic battery uses quinones for their electroactive part. Quinones are energy-storing compounds found in plants and animals, used in photosynthesis in plants and in cellular respiration. The battery uses an adapted version of benzoquinone.

Whatever the best option will turn out to be, it will help integrate renewable resources, creating a cleaner electricity infrastructure. It will offset the need to build additional transmission, generation and distribution assets, which will lower electricity costs. And, it will improve reliability in the face of an aging grid.