northO GOOD DEED, says an old saying, goes unpunished. That is certainly true of the introduction of green energy. The unreliability of solar and wind power compared to that generated by fossil fuels is well known, and with it the attendant need for storage facilities, such as large battery packs, to smooth things over.
But green energy brings another more subtle problem. Modern power grids run on alternating currents (AC), and these must be of a fixed and reliable frequency (usually 50Hzo 60Hz). The stability of this frequency is maintained by a phenomenon called lattice inertia, which results from the actual physical inertia (as described in Isaac Newton’s first law of motion) built into fossil fuel power-generating turbines (and also nuclear and hydroelectric). seasons
Taking a walk
These turbines act as massive flywheels that store inertia. As long as their outputs are in sync (and an important part of network management is to keep them that way), the resistance to change provided by their inertia stabilizes the entire network. The fewer these turbines (unlike wind turbines, which do not rotate in sync with the grid, and solar panels, which do not rotate at all), the less inertia a grid has. And in some particularly green countries, this is becoming a problem, as non-power-generating flywheels are added to the system to provide the missing inertia.
One such place is Britain, which generates about 30% of its electrical power from wind and sunlight. On March 17, for example, National Grid THAT— the company that, as its name suggests, operates the country’s electricity grid — cut the inaugural ribbon for a plant built near Keith, in northern Scotland, by Statkraft, a Norwegian renewable energy company. The inertia in this plant is stored by a pair of steel flywheels (see image of the road train required to deliver them). Each of these flywheels weighs 194 tons and spins at up to 500 revolutions per minute (rpm).
A second Statkraft plant is due to open in the fall, near Liverpool. Instead of the large masses turning relatively slowly, the smaller ones will turn rapidly (1500 rpm). Both approaches incorporate roughly the same amount of inertia, and in combination, the pair will store around 2% of the inertia currently required to support Britain’s grid. That is equivalent to the inertial contribution of a conventional coal power plant. Also, later in the year, National Grid THAT it plans to add two more systems, built by Siemens, to further increase its buffer storage potential.
However, there is an alternative to building new steering wheels, and that is to reuse the old ones; in other words, redesigning existing fossil fuel stations simply to store inertia, rather than generate electricity. National Network THAT it’s also testing that idea at an old gas station in North Wales. This has been open for business as a fly shop since 2021.
The firm hopes, in addition to all this, to develop ways to stabilize the network without spinning bits of metal by themselves. That will involve the use of what are known as network-forming inverters.
Both solar energy, which is a direct current (DC) when it leaves the generator panel, and the wind power, which is AC but it still needs to be adjusted before being fed into a grid, they are first processed by semiconductor-based devices called inverters. This is also true of the DC extracted from storage devices such as batteries, which are used to smooth out irregularities in solar and wind power.
Existing investors are described as “network followers”. This means that they monitor and adjust to the set frequency of the network they are feeding. That suits grid managers well enough when solar and wind power contribute only a small fraction of a grid’s total power, but is progressively less suitable as that contribution increases. However, inverters can be designed to “network,” meaning that the current they output mimics the external stabilizing effect of mechanical inertia. The use of grid-forming rather than grid-following inverters should allow much more wind and solar power to be easily integrated into a grid.
Until recently, network-forming inverters have only been tested on a small scale. In January, however, Britain’s energy regulator Ofgem signed off on a technical standard acceptable to both manufacturers and service providers. That will allow for large-scale deployment, and Julian Leslie, National Grid THATThe chief engineer of , says he expects the large investors that make up the network to provide inertia within two years.
Being an island, Britain has a more or less autonomous power grid. This makes it a good place to try such an experiment. Success would encourage other island networks, both real (Australia and Ireland, for example) and metaphorical (such as Texas, which has few ties to the rest of North America) to try. The largest networks in North America and Europe will no doubt be watching from the wings.
The search for grid inertia, then, is an example of the essential adjustments needed to accommodate the shift in energy production and use that is happening now. Other technologies, from electric cars to hydrogen gas supplies, may have higher profiles. But what is happening in the engine room of the green economy is just as important, if not more so. ■
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