Expert Insight

The Claverton Energy Group investigates........

This week has seen the announcement by Gordon Brown of a “huge increase in off shore wind power” along with the planned nuclear expansion.  These two are fundamentally incompatible, despite the Government oft repeated mantra “of needing a balanced portfolio of nuclear and renewables.”

Whilst you can replace the annual power output of today 70 GW of power stations, with 100% wind, which would need 140 GW of wind, you can’t do a lot of wind and a lot of nuclear.

Dr Fred Starr believes, “At the moment with just 2GW of wind energy, it has a minor affect on the operation of power plants. In the 5-10GW range fossil fuel will be able to handle the intermittency but it will have an affect on maintenance costs and it will result in some increase in fuel.  Above 10 GW of wind fossil, given say 20GW of nuclear, plants will increasingly have to be shut down for days at a time, and maintenance costs and the need for start fuel will increase disproportionately.  Above 25GW of wind, plus 20 GW nuclear, some fossil plants will need to be off for weeks at a time and the cost of keep in these plants ready, but not producing power will become a major headache.  Above 40 GW wind plus 20GW nuclear. It becomes worth replacing some existing fossil plants with true cheap standby capacity.”

Another contributor  presents an excerpt from his consultancy report referring to
Thermal station dynamics, that supports Dr Starr’s insights.

Thermal generation scheduling consists of putting units into one of three states:
1)       Preparation. Preparing for generation – bringing the unit to a state where it can generate, this generally includes heating the unit such that temperatures are high enough and temperature gradients low enough for stable, reliable and efficient generation. Fuel supply, heat supply and emission control systems may also require preparation. Fuel is used to heat up the unit: the longer the unit has been shut down, the colder it is, and the more fuel is needed to get it to operating temperature.

2)       Generation. The unit is prepared and can generate an amount (MW) between a minimum and maximum available capacity. For most thermal plant the minimum stable output is greater than zero (for flame stability etc.). The reserve is the difference between current and maximum unit output, and plant output can be increased rapidly by this amount. The thermal efficiency is dependent on the power generated (load) by that unit.

3)       Shut down. The transitions between these states takes time, with the longest transition usually being heating the unit from cold (after it has been shut down for a long time) to operating temperature. This can take hours.

Electricity modelling includes detailed modelling of power stations running through these phases, and the use of DSM to reduce the variability in thermal station output. It requires sophisticated modelling to optimise (minimise cost) plant scheduling and DSM accounting for these generator parameters; The authors  program iss quite good but the real ones are huge.

Renewables and 'base load' stations
As soon as variable renewable output from wind/wave etc. exceeds some 30-40% of annual TWh demand, then there will be times when no 'back-up' of any sort will be needed. Obviously this depends on details of renewables, demand, storage etc. So a renewable fraction greater than 30-40% is incompatible with nuclear, and indeed nuclear would 'shut out' a higher renewable fraction than this for decades as surpluses couldn't be handled economically.

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