April 22, 2008
Letters, Scotland

Wind and diesel

James Mackenzie raised a couple of questions about emissions from the diesel engines used to generate electrical power. (‘Emissions will just keep rising’, Shetland News, 16 April 2008)

1. “What happens when the engines are ‘ramped up’ (accelerated) to provide power when the wind drops?”

The engines actually run at approximately constant speed, pretty much regardless of load. This is necessary because frequency of the ac mains is required to be 50 Hz (i.e. 50 cycles per second) and this is determined by the speed of rotation of the alternator which is driven directly from the engine crankshaft.

Each of the alternators (of the engines which are generating) are connected together so that the engines run in synchronism. When there is an increase in load, all the engines slow down and the engine controllers detect this drop in speed and increase the amount of fuel into the engines to increase the engine power and thus restore the mains frequency to 50Hz.

The inverse happens when there is a decrease in load. A drop in wind speed would have the effect of increasing the load on the engines and thus the engine controllers would increase the fuel into the engines to compensate.

2. ” What if they are idling?”

It would not be normal for engines to idle except when being started or stopped. Usually, all the engines that are generating share the load. Sufficient generating capacity must be provided so that the engines can run below their maximum output and thus ensure that there is always some “spinning reserve” that can be called on to supply short term increases in load. Daily trends in load and wind speed forecast would influence how many engines and of what maximum output would be required.

These comments are fairly general. There are lots of diesel power stations around the world, often supplying small and island communities. I don’t think the arrangement in Shetland is unusual.

The situation for large connected systems like the UK grid with its variety of generating power stations is more complicated. Coal, oil and nuclear power stations take hours to reach maximum efficiency from start-up and are best used for long term generation i.e. days or weeks or longer. Short term variation in load can be addressed with gas turbine and hydro generation and pump storage schemes which respond more quickly.

Wind speed can be forecast with a level of accuracy that is sufficiently useful to ensure that enough alternative generating capacity is made available in a reasonably efficient manner.

Tony Erwood,
Lunna,
Shetland.