What’s it all about…? Energy storage

With energy saving policy dominating the headlines, significant savings could be achieved through the storage of energy, argues Tim Hennessy

Businesses are taking measures to minimise our usage of electricity and the output of emissions. But suppose we could extend that concept and truly ‘save’ the energy we produce – from large generation units – so that it can be stored and used as and when we need it? This would prevent any energy going to waste and maximise the energy being produced.

Well, we can. Energy storage systems, in the form of large flow batteries, are now up and running in a number of wind-farms and other applications around the world. They act as the missing link in renewable energy planning and are transforming the variable-output resource of wind into one of firm, reliable capacity.

Energy generated by wind-turbines is stored in the battery and released on demand, smoothing out the flow and reducing the volatility of even the largest wind farms. The energy comes in as DC and can be converted to AC for grid connection or DC for off-grid applications at extremely high rates of efficiency.

The system removes any need for back-up power from other sources – a common criticism of wind-power – and acts as its own form of back-up or spinning reserve. It means that all the energy generated, from gentle winds to raging gales, can be captured, stored and used when needed. Almost nothing needs to be wasted. In addition, by being able to store the energy generated at off-peak periods – at night for example, when electricity prices are lower – and then selling this to the grid at times of peak demand (e.g. during the day) wind farm owners and operators can achieve maximum returns from their business.

In many ways this is not a new concept. After all, re-chargeable batteries have been around for years.

However, charging them on this scale is something quite new. Historically, lead-acid batteries have been used for large-scale energy storage but these have proved very expensive, inefficient and incompatible with rapidly varying loads. They also pose a number of health and environmental problems in terms of disposal.

Alternative storage technologies have included pumped hydro and compressed air. Pumped hydro uses two water reservoirs, one higher up than the other. During off peak hours water is pumped from the lower reservoir to the upper reservoir. When required, the water is dropped back to the lower reservoir, generating electricity.

In compressed air energy storage, off-peak power is taken from the grid and is used to pump air into a sealed underground cavern to a high pressure. When needed, this high pressure can drive turbines as the air in the cavern is slowly heated and released.

A drawback with both of these is that large geological structures need to be implemented in the form of dams or deep mines, limiting the areas where they can be utilised.

Now, new technologies have emerged. Our own uses vanadium – a naturally occurring element often used to produce alloys. This is reliable, has a long-life, low-cost and negligible environmental impact.

Electrical energy coming into the system is stored chemically with the vanadium in a dilute sulphuric acid electrolyte. The electrolyte is pumped from separate plastic storage tanks into flow cells across a proton exchange membrane where one form of electrolyte is electrochemically oxidised and the other is electrochemically reduced. This creates a current that is collected by electrodes and made available to an external circuit.

The reaction is reversible allowing the battery to be charged, discharged and recharged over again at high rates in a fraction of the time needed for other batteries.

A major benefit of electro-chemical energy storage systems is that they can be scaled to suit any location.

There are several units in operation already on wind-farms in Japan and Tasmania and as part of a remote power supply in Canada.

As the price of all forms of traditional energy increases day-by-day, storage and firm supply solutions can make renewables an increasingly attractive and cost-effective option for both producer and consumer.

Tim Hennessy is CEO of VRB Power Systems

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