JET is one of a very few customers connected to the UK national ‘super grid’ at 400 kilovolts. Plasma operation is always timed to ensure that power is only taken at times when the grid is able to supply it without inconvenience to the grid operators. Evening operations are particularly sensitive to the TV schedules, which lead to quite predictable variations in the country’s power requirements as commercial breaks lead to higher power demands.
In common with most of the other large tokamaks around the world, JET has a mechanism to spread its power demand over a longer period helping to reduce the peak power requirement. To do this we need a way to store energy on site for a few minutes leading up to a JET pulse. JET achieves this using two large flywheels, each weighing 775 tonnes, and each capable of spinning at up to 225 revolutions per minute.
For a few minutes before the pulse, we draw about 10 megawatts from the grid to accelerate each flywheel to the speed required to store enough energy for the planned pulse. Then during the pulse, we draw energy out of the flywheels. One of them helps to drive the current required to produce the toroidal field (supplemented by power direct from the grid). The other drives current for the poloidal field coils.
The flywheels are carefully monitored in operation, and for such large and complex electro-mechanical devices they are highly reliable and essential components of the JET plant. This reliability is only maintained because of the good maintenance regime, and this year’s shutdown is a good opportunity to examine components that are normally inaccessible.