Electricity generation – why do we want ‘flexibility’?
31st January 2021
This short article explores what ‘flexibility’ is, and why we seem to want more of it, and quite quickly, in power systems. It is inspired by a Sunday Times article describing the UK’s difficulties in attracting new investment for nuclear plant (31st Jan 2021). Hitachi is the latest potential investor to back out of new nuclear prospects having found it impossible to develop as a project.
The graphic below describes the evolution of the UK power grid over the last decade. The ‘nuclear’ point is that apparently slow nuclear capacity/generation decline will shortly accelerate as we will have to retire ageing plant and there may be few, if any, replacements. But look at the rest of the chart. Coal has vanished, and that’s the big #1 disruption over the decade, now factored fully into the system. That was about ‘fuel switching’, from high emissions coal to a mixture of natural gas and renewables generation. The big capacity and generation volume winner can be seen to be renewables, mainly wind and then photo-voltaic sources.
Next, lets consider what this plant is like:
-Nuclear. Base load large scale units, Built to start and then not to stop again until inspection or maintenance require it to. Its ability to change load is extremely limited. Its ability to be ‘scheduled on’ ie made to run is high.
-Coal – no longer, practically, exists in the mix. Dramatic to be able to simply ‘move on’ from this one.
-Oil – very small in the mix and exists as back up, emergency start, and system support. A legacy fuel, these days, but it is still there and it is able to change load fast and be run when its wanted. Fuel cost is high and emissions are high, which excludes this from frequent operation.
-Gas – in Combined Cycle Gas Turbines and Open Cycle Gas Turbines. In the 1980’s the CCGTs were built as base load units, now they are built as multiple-despatch and flexible load units. The OCGTs easily change load and are quickly despatched – often called ‘peakers’ these days. By re-engineering the older CCGT’s they have been repurposed as much more flexible plant – not without additional capex and opex costs.
-Renewables – now the quickly increasing block of capacity and generation, shown powerfully on the graphic above. Very low flexibility (i.e., you can’t really make them increase or reduce generated power in the moment) and very low despatch ‘ability’. They generate when the wind blows or the sun shines.
‘Flexibility’ – the ability to, at will, increase or reduce generation, has been valued ever since we had a grid. During the rapid growth of CCGT plant build in the 1980’s we talked of flexibility and haven’t stopped talking since. Flexibility also now includes the concept of varying fuel mix widely as fuel sources, such as LNG and Hydrogen, are added to the pipelines.
The point here, is that flexibility now matters a great deal more than it used to. That is due largely to:
-The renewables system share has grown from (generation volume) about 5% in 2014 to around 12-15% now. This volume is, basically, very low in flexibility rankings and despatch rankings and so does not contribute to our ability to manage the grid. In fact, arguably it makes matters significantly more complicated as the other system elements have to compensate.
-Nuclear is still here, but will decline. It is also very low in the flexibility and despatch rankings.
-Gas plant, CCGT and OCGT. This is now one of the most significant sources of grid flexibility and, as matters stand today, it is the place to look for short term grid support when the system operator needs it. The degree to which this element of the system needs to change load on demand is becoming much greater as the rest of the system proportionately, cannot.
Of course this is, and will be, addressed by new system developments including:
-Short term power storage and this is likely to include additional battery storage systems as costs reduce.
-Demand side management, which should be, and needs to be, quite powerful and which will involve changes at local distribution and consumer levels.
-Chemical energy storage, not least by using hydrogen created from electrolysis powered by excess renewable generation. This hydrogen can be, and is in pilot form increasing being, used to displace natural gas in CCGT’s and other combustion systems.
The reason that Oxsensis is interested in this, is that it is powering rapid gas turbine combustion development activity to transition the, already clean, gas turbine power systems into hydrogen-enabled units with a pathway to zero carbon emissions from these. This work will involve the retrofit of existing CCGT assets, which have service lives of four decades and the upgrade of plants to next generation combustion systems.
For more information on our work [email protected] www.oxsensis.com
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