Tuesday 5 March 2013

When comparing wind farms with conventional power stations we need to be clear about what is being compared, power or energy.  A large wind farm could have a capacity (maximum power rating) of 100 MW and this would be equivalent to a 100 MW conventional power station when both are running at full output.  (100MW would be a small conventional unit but units of this size do exist.)  The maximum energy output of the wind farm over a year, however, is driven by the variable wind speeds and is typically between 20 and 30 per cent of the maximum theoretical amount (maybe more for off-shore wind farms where the wind speed is more constant).


In practice a 100 MW wind farm with a capacity factor of 30 per cent will produce 30% x 100 x 24 x 7 x 365 MWh a year (1,839,600 MWh). A conventional power station, however, could produce 100 x 24 x 7 x 365 x 90 per cent i.e. 6,132,000 MWh, assuming the maintenance can keep it going for 90 per cent of the time and the plant is kept running.  So a 100 MW wind farm can only ever produce about 1/3rd of the energy of a 100MW conventional power plant.  This difference is just a result of the physics of wind turbines.


Now there is another interesting piece of analysis on wind turbine performance.  In 2012 the Renewable Energy Foundation published a report that showed that the output of individual turbines drops off with age.  This could be to increased wear and tear on the mechanical components and factors such as erosion of the turbine blades, or due more frequent breakdowns and operators taking more time to bring the turbines back into service.


This has implications for wind investors, particularly if they have not factored any performance degradation into their financial models.


Of course conventional power stations are also subject to degradation of output and many plants are down rated for their nameplate rating.  The difference is that operators and investors understand this but the REF report seems to be the first time the phenomenon has been studied in wind power.  The interesting question now is, does the same occur to solar installations?

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Dr Steven Fawkes

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