Since the 2015 Paris Climate Agreement, an increasing number of governments have declared a climate emergency and set out strategies to achieving a carbon-free future. In order to achieve these goals by 2050, almost 90% of the electricity produced globally will need to come from renewable sources, with the International Energy Agency estimating that 3,000 coal plants will also need to be shut down.
But as the world switches to higher renewable penetrations, new vulnerabilities are emerging in our energy systems. One of the most complicated challenges of a 100 percent renewable power grid is how to replace the inertial stability provided by traditional synchronous generators.
What is inertia in the power grid?
Inertia is the effect provided by spinning metal machinery in power stations resisting changes in the frequency of the electricity grid due to their momentum. This slows down the rate of change of frequency (RoCoF), a key variable in balancing a grid to provide resilient and reliable power thus avoiding operating conditions which could trigger a cascading failure leading to a power blackout.
Why does grid inertia matter?
Inertia has traditionally been provided by large, centralised fossil fuel generators connected to the National Grid transmission system. The kinetic energy of these rotating machines works like a shock absorber to keep grid frequency from dropping too fast when demand exceeds supply or rising too fast when supply exceeds demand. Without this stabilising force, the grid could face a greater risk of frequency excursions that could force generators offline or cause outages like the 2019 blackout that affected about 1 million consumers across Southern England. While a variety of factors contributed to the blackout, the loss of inertia was a critical element in the system failure.
As increasing penetrations of inverter-based resources like wind, solar and battery storage that do not inherently provide inertia are added to the mix, and more traditional generators are switched off, the level of inertia available to grid operators is decreasing. So how do we overcome this to ensure the grid remains stable in case of an emergency?
Today, multiple solutions are being considered to cover the upcoming problem of grid stability due to reduced inertia. Synchronous condensers, rotating machines linked to the grid to provide voltage stability via rotating masses have been proposed to provide inertia but they can be a costly add-on.
Battery storage can provide ‘synthetic inertia’ to replace the real inertia being lost by the closure of power stations and have also been proposed to tackle grid stability. However, the synthetic inertia offered by battery storage is an ultra-fast response time; any response time at all is a spike on the mains otherwise known as a RoCoF event. It was this reliance on synthetic inertia rather than real inertia that caused the 2019 blackout in Southern England. While synthetic inertia is excellent for recovering from faults, it cannot prevent them occurring in the first place.
Mechanical energy storage
Mechanical energy storage systems like flywheels and CAES solutions are naturally inertial and are an efficient and affordable solution to cope with the short-term challenges to grid stability. As a thermo-mechanical system, CEL’s eTanker solution has real inertia which provides very high-quality power output and fault tolerance relative to synthetic inverter-based alternatives such as lithium-ion batteries. The real inertia properties of eTanker’s spinning metal shafts deliver the same type of frequency stability benefits that we currently get from fossil-fired plants. The power train of an eTanker system consists of 3 stages of compression, which are embodied in two physical machines: a low pressure (LP) compressor and another unit that comprises the intermediate-pressure (IP) and high-pressure (HP) stages. Both compressors have a synchronous electric motor and an independently supported flywheel coupled to their shafts.
An increasing number of mechanical energy storage solutions have been deployed in countries like Australia and Ireland and here in the UK, National Grid ESO has launched pathfinder projects designed to roll out mechanical energy storage solutions to solve the inertia problem. With these energy storage solutions in place, the reduction in inertia would no longer be a barrier to the uptake of renewable energy generation, which is a win for the industry, customers and of course, the environment.
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