Everyone is familiarised with how a battery works: it provides electricity as long it has charge, and modern units can be recharged and reused when depleted. Depending on the materials used to manufacture a battery, its service life can range from a few hundred cycles to many thousands. In the context of wind turbines and solar photovoltaic systems, Batteries are viewed as a powerful addition that ensures a continuous supply of electricity when there is no sunlight or wind, eliminating the main limitation of modern renewable sources.
Conventional lead-acid batteries are a mature technology with a well established supply chain, but there is a limit to how fast you can charge and discharge them: their efficiency is reduced drastically as current increases. In addition, lead-acid batteries suffer a very rapid deterioration if they are not fully recharged between discharging periods.
- Lead-acid batteries have been successful in remote applications, as a complement for solar or wind power. For example, you can have a house in the countryside running on solar panels during the day and on batteries at night.
- On the other hand, these batteries are unsuitable for the fast pace of modern power grids, where supply and demand can change drastically within seconds. A power plant fault can take several megawatts of generation away in an instant, and a sudden breeze can increase the output of wind turbines without warning.
Modern power grids not only face the challenge of storing energy from variable renewable sources. With more distributed resources and variability in both supply and demand, networks have also become more difficult to balance, and utility-scale batteries can be of great help.
Power Grid Stabilisation with Large-Scale Batteries
Operating a power grid is an complex balancing act: for decades, grid operators have adjusted generation according to consumption. However, renewable power sources such as solar arrays and wind turbines have introduced variations in supply as well. Sudden imbalances between generation and consumption can destabilise a power grid, causing voltage and frequency to deviate from their standard values.
- Oversupply of electricity tends to raise voltage and frequency.
- Excessive demand tends to lower voltage and frequency.
Most conventional generation systems cannot respond fast enough to compensate for the rapid changes in supply and demand in modern power grids. The generation systems offering the best response for grid stabilisation are hydroelectric turbines and natural gas turbines, but both have limitations: hydropower is only viable with specific terrain conditions, and gas-fired power plants are susceptible to volatility in the fossil fuel market, not to mention that they produce emissions.
Large-scale batteries have emerged as a third option, and one that responds even faster than hydroelectricity and natural gas. In addition, batteries are free from the limitations affecting the alternatives: they can be installed almost anywhere and are independent from a fuel input.
The Hornsdale Power Reserve in South Australia
The Hornsdale Power Reserve, better know by the media as the “Tesla big battery”, started operating in late 2017. However, it has demonstrated the capabilities of utility-scale batteries in just a few months, by participating in frequency control and ancillary services (FCAS). The results have been impressive for such a short timeframe:
- The cost of FCAS in South Australia has been slashed down by more than 90%. Grid stabilisation with natural gas turbines had costs above $10,000 per MW in some days, and the Tesla battery has reduced them to under $300 per MW. FCAS costs above $7 million per day had been reported before the Hornsdale Power Reserve started operating.
- The 100-MW battery only accounts for 2% of the installed capacity in South Australia. However, thanks to its extremely fast response, it has managed to rake in over 55% of the grid stabilisation revenue in the state.
In its first four months of operation the Hornsdale Power Reserve achieved around $35 million in savings for electricity consumers in South Australia. The financial performance is exceptional, considering that the project cost was around $50 million.
Power networks in Australia are becoming very expensive to operate, and this is the main reason why power bills have become so high. South Australia is the only state that has experienced a reduction of grid regulation costs, in great part thanks to the 100-MW battery.
In a few words, utility-scale batteries are both faster and more accurate than the conventional hydroelectric and gas turbines used in power grid stabilisation. Originally, the project faced some criticism because its storage capacity of 129 MWh is small for the demand of South Australia. However, the value of utility batteries comes from fast and accurate response, not sustained energy output.
How Distributed Batteries Can Have the Same Performance
Although the Hornsdale Power Reserve has all its capacity in a single location, the concept is viable with distributed batteries as well, as long as they are linked to a common platform. South Australia has started development of a “virtual power plant” project with Tesla, which will encompass 50,000 households. Each will be equipped with:
- A 5-kW solar photovoltaic system
- A 5-kW, 13.5-kWh battery
Adding up the capacity across all participating households, the total is 250 MW and 650 MWh, resulting in a much larger system than the Hornsdale Power Reserve. With a smart network connecting all 50,000 units, the virtual power plant can provide the same grid stabilisation services as a centralised system.
Australia is becoming a global leader in energy storage, since there are many other projects under development in addition to the 100MW Hornsdale battery and the 250MW virtual power plant. For example, OneSteel has announced a 120 MW and 140 MWh battery for the Whyalla steelworks, and more utility batteries are be.
Distributed battery systems have a higher cost per kW and kWh, and the control system required for coordinated operation is more complex. However, they bring the benefit of power grid de-congestion, since electricity generation and storage are closer to the point of use.
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