Is Wind Power Reliable? Capacity Credit of Wind Energy

Is Wind Power Reliable

The following is a commentary on David Milborrow’s article in “New Power UK/Issue 1/February 2009”.

As David says, you would not design a thermal power generating system which did not have built in reserve. He has answered his comment about those letter writers being unconcerned (or unaware) that there was a shortage of nuclear output during the cold snap in early January.   “Why should they have been concerned/aware?”   The system is designed for this.

David quotes research to back up his assertion that wind has a capacity credit and the reserve margin of conventional plant may be reduced.   He indicates that based on statistical analysis this reduction might be 7 GW if 40 GW of wind power was to be installed.  More than the reserve available to the National Grid during the cold spell in the first week of January 2009.  He overlooks that two of the researchers, Grubb and the Oxford Economic Research Associates (OERA), quote periods bereft of wind.  For example:-    

• Grubb says there are on average 6 hours per month during the winter and twice this amount during summer when the wind is low or non existent.
   
• OERA say that there will be 23 one hour periods in the year when the output from wind turbines will be less than 10% of the rated output when demand is between 90% and 100% of peak demand. 

If, when preparing his Table 2, David had studied the wind speeds over the whole of the UK he might have drawn a very different conclusion.   His table quotes the recorded output of 1288 MW of wind turbines all of which are located in Scotland.   I do not have the record for the 6^th of January 2009 but do have the record for the 7^th January.   On this date, at 17.00 hours, www.bmreports.com, recorded a wind energy output of 140 MW.   It was a cold day, like it was on the 6^th of January, and the remainder of the UK was experiencing calm low wind speed conditions.  The electricity demand at 17.00 hrs was 58.9 GW, within 3% of peak simultaneous demand    If a calculation based on all the onshore Met Office wind speed observations and on the www.xcweather.co.uk offshore wind speed observations for 17.00 hours had been carried out, it would have found that if 40GW of wind power had been evenly distributed around the UK and the seas surrounding the UK the average output would have been 2.46 GW, 6.15% of the installed capacity.  The installed capacity for this calculation in Scotland was assumed to be 9.0 GW.

When broken down into regions and the installed capacities are redistributed as shown in the following table the average is similar.   Assuming an installed capacity of 1.29 GW (1288 MW) the output for Scotland would have theoretically been 0.13 GW (134 MW), very close to the output recorded by bmreports.com.    It is also noticeable that the theoretical output from a large array of wind turbines located in the North Sea off the coast Lincolnshire and East Anglia would have been zero. 

If you then consider that, 15% of the 8000 to 10000 turbines required to generate 40 GW might be out of action for repair and >75% of the remainder would be net consumers of electricity, keeping their onboard systems alive, and the losses in transmission might be in the order of 25%, then the output to meet demand would be considerably less than the value estimated and be almost negligible as far as the National Grid was concerned.

Notes to the Table: –

* Installed capacity monitored by bmreports.com

[1] The capacity assumed is 11 GW less that recorded by bmreports.com

[2] The totals are 29 GW.

[3] The total of 40GW is the total for onshore and offshore wind discussed in the Carbon Trust Report – Offshore Wind.  They assumed 11 GW onshore and 29 GW offshore.

[4] To allow for wind shear with height the Met office onshore wind speeds were increased by 35% and the xcweather offshore wind speeds were increased by 25%.   The detailed calculations can be obtained from denis.stephens@btinternet.com.                              

My studies have shown that a high proportion of recent simultaneous maximum demands on the National Grid have coincided with periods of low wind speeds, not windy conditions as reported by Palutikof et al. and some of these periods have lasted for more than 24 continuous hours.

David then says that no backup for wind is required because that backup already exists in the form of conventional power generation.    Obviously this is the case.   If there was no conventional backup and the UK relied solely on wind power large parts of the country would be without power for considerable parts of the year and the UK would have to build a conventional power generation system with the same margin of reserve as exists today.

David then states that in normal circumstances 26 GW of wind power, a 20% wind energy penetration, only displaces 5 GW of thermal plant.  That is, 20% wind energy penetration only reduces the requirement for conventional generation by 11%!

Conclusion

If any myths are to be debunked they are not those of the wind sceptics.

Post Script

Investment in wind energy is a luxury.  It is like investing in a second parallel electricity generating system.   The first is a conventional system where the reserve margin over demand is 25%.   The second is a wind powered system where the reserve margin is 125% and is wholly provided by the first system.   The main benefit of the second system is that it saves fossil fuels/carbon emissions but not significantly.      

David‘s article does not consider the cost of wind energy compared to conventional power generation.   He refers the reader to the Carbon Trust Report – Offshore Wind Power. This publication makes the assumption that the capacity credit of wind allows 6 GW of conventional power to be retired.   As a consequence, their calculations significantly under estimate the real additional cost of electricity when 40 GW of wind power is added to the UK generating capacity [1].

References

[1]   Critique of the Carbon Trust Report Offshore Wind Power: big challenge, big opportunity   by D Stephens posted on the Claverton Website

Posted by D S Stephens

Please note there is an extended learned discussion continuing on the topic of this article on Claverton Email Service –

See – https://claverton-energy.com/pipermail/claverton-group_claverton-energy.com/2009-May/000893.html