by Jonathan Knott, Associate Research Fellow at the Institute for Superconducting and Electronic Materials, University of Wollongong

Energy storage has been the subject of intense interest from consumers, industry and government recently – and was even part of the focal point for the 2018 South Australian election. While the bulk of the world’s energy storage capacity will remain in the form of water in dams for use in hydroelectric generators, there has been an explosion in the development and deployment of battery-based energy storage systems as their price continues to plummet. While lithium-based systems are currently king, the huge demand for energy storage is driving the development of alternative technologies – particularly for residential and utility-scale energy storage applications.

The demand for energy storage has been increasing rapidly, and the market is anticipated to go through a similar expansion to that of solar PV in the early 2000’s.

This demand is occurring across many scales and applications, including portable electronics, electric vehicles, residential energy storage (coupled with solar PV systems), and utility-scale solutions.

Residential and utility-scale energy storage products are being developed and deployed in response to a number of factors including an overall increase in energy demand, renewable energy generation methods becoming an increasingly large portion of the generation capacity (both in Australia and worldwide), and because consumers are starting to take an active interest in managing their energy generation and consumption.

This is especially true in Australia, where there are approximately 1.79 million solar photovoltaic installations, from small home rooftop systems to megawatt solar farms.

Many owners of these systems are looking to add energy storage, with 6750 battery energy storage systems installed in 2016 (up from approximately 500 in 2015), 20,000 installations in 2017, and predictions the numbers will continue to rise into the future.

Ways of storing energy

There are many ways energy can be stored, and methods that are applicable to residential to utility-scale applications include mechanical, electrochemical and thermal energy storage.

Mechanical energy storage: Energy can be stored as kinetic energy – such as in a spinning flywheel or as gravitational potential energy – such in a mass of water in a dam. Flywheels have found use in a wide range of applications, from the International Space Station to remote Western Australian communities. Pumped hydro energy storage systems are prevalent worldwide and form the backbone of many grid-scale energy storage solutions.

Electrochemical energy storage: Rechargeable batteries, hydrogen fuel cells and supercapacitors are all technologies that use the principles of electrochemistry to store and provide energy on demand. In essence, energy is stored and released via the reduction (adding electrons) and oxidation (removing electrons) of particles. This form of energy storage is arguably the most flexible as it can be efficiently (and as of late, relatively cheaply) applied across a huge range of energy storage scales, from milliwatt-hour watch batteries to megawatt-hour utility-scale solutions.

Thermal energy storage: Latent heat is a physical property of materials that has found use in a wide array of applications, from keeping drinks cold in an esky on a hot day to providing a mechanism to store energy as heat for use in industrial processes and generating electrical energy. Latent heat is defined as the energy required to transition a material from one phase to another (i.e. solid-to-liquid or liquid-to-vapour). This energy can be extracted from the material later as heat, leading to a transition of the material back to its initial state.

Current technologies

Pumped hydro storage accounts for almost 95 per cent of currently installed energy storage worldwide, however there are almost three times as many utility-scale electrochemical storage installations. This is unsurprising as pumped hydro systems typically incorporate large reservoirs with significant capacities, however electrochemical storage systems do not require specific geographic features or a large capital outlay to construct.

Lithium-ion battery energy storage systems are currently the most widespread solution available for residential energy storage solutions. Similarly, lithium-ion batteries dominate utility-scale battery energy storage solutions, with the famous “world’s biggest battery” in South Australia based on Tesla’s lithium-ion batteries.

There are, however, significant issues that come along with manufacturing and using lithium-ion batteries. Although the relative scarcity and difficult process for extracting lithium has been widely reported, the current issues facing lithium-ion batteries are around the cobalt and nickel required to provide the highest levels of performance.

These materials are costly to extract – in dollar, human, health and environmental terms – and mining of cobalt in the Democratic Republic of Congo has been criticised for its use of child labour to meet demand.

The storage evolution

There are many new and innovative technologies being developed for residential to utility-scale energy storage applications, with many demonstrations planned, announced or currently underway.

Flow batteries

Flow batteries are a hybrid of fuel cell and traditional battery cell technologies – the anode and cathode materials are liquids (rather than solids as in standard rechargeable batteries). Flowing the electrolyte liquids between metal electrodes and a membrane that allows ions to flow and energy to be extracted via the electrodes.

Unlike fuel cells, however, the used liquids can be flowed back past the membrane to be recharged, allowing the battery to cycle back-and-forth to release and store energy.

Prototype sodium-ion battery cells. Source: UOW/Paul Jones

A significant advantage of this technology is that the power and energy capacity in the system are largely decoupled – the power is a function of the size of the membrane, however the energy capacity is dependent only on the amount of electrolyte liquid.

As such, additional storage capacity can be added to a system by installing larger electrolyte tanks, allowing greater flexibility both during and after deployment.

This technology is gaining traction in off-grid and large-scale energy storage applications, with Australian companies Redflow and VSUN Energy targeting these markets.

This is timely, as a recent survey of 500 energy professionals saw 46 per cent of respondents predict flow battery technology will become the dominant utility-scale battery energy storage method in the future.

Sodium-ion batteries

Sodium-ion batteries are similar to lithium-ion batteries in terms of operation, energy density and construction, however sodium is used as the primary element in the oxidation/reduction cycle. Sodium-ion batteries do not require cobalt, nickel or lithium, which allows many of the issues surrounding sourcing materials for lithium-ion batteries to be sidestepped.

An additional benefit of this technology is that the manufacturing processes and plant used to produce lithium-ion batteries can be used for sodium-ion batteries, allowing sodium-ion batteries to piggyback on 25 years of manufacturing optimisation and supply chain development.

The University of Wollongong (UOW) is currently leading a $10.5 million ARENA-funded project to develop and demonstrate sodium-ion batteries in a number of applications.

We’re collaborating with a consortium of national and international partners, including several Chinese manufacturing companies, to scale-up manufacturing of materials developed at UOW.

We have also engaged with Sydney Water to demonstrate the sodium-ion battery technology at their Bondi Sewage Pumping Station. There are also several startup companies that have formed around commercialising sodium-ion batteries, and their innovative approaches will further cement this technology as a viable large-scale energy storage solution in the future.

Molten silicon

Molten silicon is an emerging thermal energy storage method that is seeing interest from industrial-scale energy consumers.

Silicon has a melting temperature of 1414 degrees Celsius, and a large latent heat (almost 2000 kilojoules per kilogram), meaning very high energy densities are possible with this technology.

Another key advantage of this technology is that stored energy can be extracted as electricity or heat, which can be useful for industrial applications that require process heat (such as
in furnaces).

The Australian company 1414 Degrees has been working on commercialising molten silicon energy storage for over ten years and have recently announced a project to install one of their systems at the Glenelg Wastewater Treatment Plant in South Australia.

Concentrated solar power

Concentrated solar power (CSP) with thermal storage is a method of storing energy that combines renewable generation and storage in one process. Light from the sun is focused onto a working fluid – typically a molten salt – which is either used immediately to drive a heat engine or stored in insulated tanks for later use. Many CSP plants use vast arrays of sun-tracking
mirrors to focus sunlight on a central tower, in which the working fluid can be heated up to 1000 degrees Celsius.

The key advantage of this technology is that it is essentially a dispatchable generator, allowing solar energy to be time-shifted to when it is needed or even to allow continuous generation with appropriately-sized thermal storage tanks.

CSP with thermal storage has undergone a renaissance in recent years, with many innovative technologies and large-scale deployments being announced. Vast Solar, an Australian company, has developed a modular and efficient ‘CSP plus storage’ system that is currently being demonstrated in regional NSW.

At the other end of the scale, Solar Reserve is currently building the world’s largest CSP with thermal storage plant in South Australia, a 150 MW/1100 MWh facility that is anticipated to be
operational by late 2020.

Future integration

The utility of energy storage for a multitude of applications – including black start services, frequency regulation services, peaker plant replacement, renewables generation smoothing along with many others – is opening doors for the wide-scale deployment of energy storage technologies.

Further, a recent CSIRO report stated there is no technical reason the Australian NEM could not move to 100 per cent renewables generation. Such a high renewables penetration would require substantial energy storage capacity to be installed to maintain stability and provide resilience.

While the NEM generation mix will not change overnight – and it may not ever reach 100 per cent renewables generation – it is clear energy storage has a key role to play in increasing resilience, efficiency and cost effectiveness of the grid.

As new and innovative energy storage technologies continue to be developed, demonstrated and optimised, even more applications and value propositions will become viable.

About the author: Jonathan Knott is an Associate Research Fellow at the Institute for Superconducting and Electronic Materials at the University of Wollongong. He is currently the Project Manager for the ARENA-funded Smart Sodium Storage Solution (S4) Project. The S4 Project consortium includes the University of Wollongong, Liao Ning Hong Cheng Electric Power Co., Hebei ANZ New Energy Co., McNair Technology Co., and Sydney Water Corporation.


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