Critique of Carbon Trust Report – Offshore Wind Power


 of the

Carbon Trust Report


Offshore Wind Power: big challenge, big opportunity

Maximising the environmental, economic and security benefit 

Contents of this Critique


1.0 Carbon Trust Report Section 3.  Grid and Planning                 

               1.1  Capacity Credit                                                                   

               1.2  Capacity Factor                                                                  

               1.3  Curtailment                                                                                             

               1.4  Grid Connections                                                                

2.0 Carbon Trust Report Section 7.  Cost/Benefit                                     

3.0 Title of the Report                                                                              

4.0 Concluding Remarks                                                                      

Appendices A, B, C                                                                                   


 This critique was prepared out of a belief that the Carbon Trust Report over estimates the amount of power that 40GW, (29GW offshore and 11GW of onshore wind), can contribute to annual UK electricity demand and under estimates the additional cost to consumers relative to the present day  prices of electricity.


By assuming wind provides a capacity credit, (i.e., a reduced need to invest in gas and coal generating plant) a high capacity factor and that manufacturing and construction costs will reduce due to learning, the Carbon Trust Report estimates the additional cost to consumers in 2020 of all electricity generated will be between 1% and 8% of present day costs.  That is £4.50 to £36.0 for an average present day household bill of £450 per year.


For the 8% scenario, the Carbon Trust has assumed the total construction cost of 40GW wind power (29GW offshore and 11 GW onshore) to be £79.00 billion, (£65.00 bn offshore and £14.00 bn onshore).  Average present day costs are higher.  A more realistic present day sum is £87 to £100 billion. 


This critique examines the statements made in the Sections 3 and 7 and the Title of the Carbon Trust Report.  It concludes that the probable increase in present day costs to consumers of all electricity from the construction of 40GW wind power will be significantly greater than forecast by the Carbon Trust and be in excess of 28%.  That is over £125 per year for an average present day household bill of £450.




1.0 Carbon Trust Report – Section 3.  Grid and Planning


1.1 Capacity Credit


The Carbon Trust Report states that the capacity credit for 40GW of wind power is 6GW and would allow 6GW of conventional capacity to be decommissioned. 


Capacity credit is defined in the Government’s Renewable Energy Strategy Consultation document  as:


“an indicator as to how much of the capacity can be statistically relied on to be available to meet peak demand”  


There is no doubt that on a normal day wind does have the ability to displace conventional generating capacity.  But, does this also apply on a windless or near windless day?


The Renewable Energy Foundation reports that recent Met Office data-based modeling, conducted by the Oswald Consultancy Ltd, demonstrates very large power swings dropping down to a minimum average output of 3.7% of installed wind power capacity, with minimum wind output tending to occur on colder days.


A recent study was undertaken, by myself, on a small number of wind speed records collected over the summer of 2008 and the spring of 2006.  See Appendix A.  The study found that there are several periods when the output of a large array of wind turbines spread evenly all around the UK and the seas surrounding the UK would be low and almost non existent.  For example on the 2nd March 2006 at 19.00 hours the demand on the National Grid was 57.85 MW and the theoretical output from 180GW of wind turbines distributed evenly between 73 observation points all around the UK was between 2.32 and 9.26 GW, 1.3 and 5.14% respectively.  The equivalent output from 40GW would have been between 0.5 and 2 GW.


A further calculation made for 17.00 hours on the 7th January 2009, using onshore wind data from the Met Office Observations and offshore wind data from found that the average output of 40 GW, 11 GW onshore and 29GW offshore, would have had been 2.84GW, 7.1%.  At this time, was recording an output of 140MW for an installed capacity of 1288MW. provides data on wind turbines located in Scotland.  The equivalent output from my calculations was 134MW, quite a close correlation.  The demand on the National Grid was 58.91GW, within 4.5% of the maximum simultaneous demand in 2007.  A large part of the wind output would have been used to serve the onboard systems of the majority of the wind turbines that did not record any output and would have been lost in transmission.  The contribution of 40 GW of wind power to the national grid would have been virtually zero.


A similar study is illustrated by Michael Laughton in the book ‘Renewable Electricity and the Grid – The Challenge of Variability’ edited by Boyle.  The study provided the wind speeds from 68 observation stations all around the UK on the 28th December 2005.  On this day, the maximum demand on the National Grid was 53.825GW.  Quite close to the maximum simultaneous demand in 2005.  The weather conditions were cold with high pressure. Wind speeds across the whole of the UK were low.  Wind speeds in Scotland were lower than in England.  This is not unusual. 


A calculation made by adjusting Laughton’s recorded wind speeds for a hub height of 150 metres using the calculator on the Danish Wind Energy Association web site shows that the theoretical output of an installed capacity of 40GW spread evenly across the 68 observation stations would have been about 1.5GW, 3.77%.  This combined output would have barley supported the onboard systems of the 75% of the wind turbines whose output would have been zero.


Laughton discounted his study on the basis that many of the measurements were recorded on airfields and do not reflect hilltop conditions.  However, not all new onshore capacity can be located on hilltops.  Many of the proposed large onshore arrays will be spread over wide areas some of which will be quite low lying.  For example, sites in Lincolnshire and the wind farm on Romney Marsh. 


Laughton has also stated that based on a Loss of Load Probability of 9% the capacity credit of wind power appears to be about the square root of the wind capacity installed measured in GW.  For 40GW, this is approximately 6GW.  This is the figure asserted by the Carbon Trust. A measure using the square root of the installed capacity does not seem to have any meaning.  For example, the square root of 1 is 1.  That is the capacity credit of 1GW of installed capacity is 100% whilst the capacity credit of 9GW is 3GW, 33% and 100GW is 10GW, 10%.  However, Laughton does state that the surprising outcome of his studies is that the total conventional plant capacity required in the National Grid will never be less than the peak load irrespective of the amount of wind added. 


The implication of Laughton’s statement is that the capacity credit will only apply to the margin above the peak load required to ensure that the peak load is met.  But, what is the maximum simultaneous load likely to be in the future.  Will it be higher or lower than the 2002 peak of 61.72GW?  Or, will it increase to 67.3GW by 2014/15 as forecast by the National Grid?


If the UK is to move to a carbon free emissions scenario then, irrespective of savings due to energy efficiency measures, population increase and the powering of transport by electricity and the conversion of household heating to electricity will increase electricity demand on the National Grid and peak demand will exceed present day peak demand.  


The fact is that high pressure over Europe extending over the whole of the UK and the seas surrounding the UK is a frequent event and that when it occurs over the winter period it is often accompanied by low temperatures, low wind speeds and high demand on the National Grid.  Consequently, there is a high degree of certainty that windless events will occur on days of high simultaneous demand, during which the output from a large array of wind turbines on the National Grid will be insignificant.  


To assign a capacity credit for wind other than zero is a gamble that should not be taken.


This is an important conclusion as it affects the cost of wind power calculated in Chart 7a of the Report.


1.2 Capacity Factor


On page 27, the Carbon Trust Report states that 40GW of wind power will generate 31% of the UK’s electricity over a year.  The footnote to Chart 7a says that total electricity generation is 405 TWh.  This figure was reported in the National Energy Statistics, 2008, for 2006 and includes imports and pumped storage.  Thirty one percent is 125.55 TWh.


The annual output of 40GW of installed wind power operating at maximum capacity is 350.4 TWh. 


The expected capacity factor is therefore 35.8%.


A capacity factor of 35% was used for the year 2020 in the Sustainable Development Commission’s (SDC) Report ‘Wind Power in the UK’ dated May 2005.  It is possible that individual wind farm installations will record such a factor and an even higher factor.  But the combined factor of all wind farms in the National Grid has, to date, not been so high.


Historically capacity factors for onshore wind farms in the UK have been less than 30%.  For 2007, National Statistics recorded a capacity factor of 27.5%.


Offshore records are not so extensive.  Since 2004, the recorded capacity factor has varied between 24.2 % and 28.7%.  For 2007, the recorded capacity factor was 25.6%.


It is understandable that as technology improves less down time will be required for repair and maintenance and over the initial years of operation the capacity factor should improve. However, to date not many wind farms have had long lives.  It is probable that with age down time for repairs will increase and the capacity factor will reduce. 


The capacity factor will also vary from year to year depending on the frequency and strength of the winds arising over the year and the difficulty of offshore access during spells of inclement weather.


It is therefore optimistic speculation that, on average, the capacity factor will rise to 35.8%, a 30 to 40% improvement on today. 


For purposes of investment, it is prudent to err on the safe side and assume a capacity factor of 27.5%.  This would result in a reduced output of 96.36 TWh from 40GW of wind, 23.8% of the UK’s electricity demand, over the year compared to 31% assumed in the Carbon Trust Report.


This is an important conclusion as it directly affects the cost of wind power calculated in Chart 7a of the Carbon Trust Report


1.3 Curtailment


Chart 3c of the Carbon Trust Report is meaningless.  Wind curtailment will depend on the base load of other renewables and nuclear power in the National Grid and when available carbon capture and storage plant.


For example, if it is assumed that by the time 40GW of wind is available the installed capacity of nuclear will be less than 2GW then wind curtailment will be low.  However, if the existing nuclear base load was to be renewed and the output of other renewables was to increase then, once 40GW of wind is installed, curtailment of the wind output is a very real possibility.  If further nuclear and or CCCS plant was to be added to the grid curtailment will be a regular occurrence.  See Appendix C. 


This is a further indication that the capacity factor of 35.8% assumed by the Carbon Trust is overstated.


1.4 Grid Connections


The grid connection price quoted for 29GW of offshore wind power is £8.16 bn.  It represents a cost of £0.281 bn per installed GW. 


There will, in addition, be onshore costs.


The price quoted is less than the estimate of £13 bn stated in Box 3.4, Page 84, of the Government’s UK Renewable Energy Consultation for the onshore and offshore costs.  That is a cost of £0.325 bn per GW.


The source for the Carbon Trust’s costs appears to be same as the Government’s costs, namely, SKM.  


These sums are significant.  They affect the O&M costs.


2.0 Carbon Trust Report – Section 7.  Cost/Benefit


Chart 7a of the Carbon Trust Report states that the net cost of incorporating 40GW of wind power into the UK’s electricity system, assuming 9% learning, is £8.60 per MWh the equivalent of adding 8% to retail prices.


Chart 7a states:-


  • Wind is 31 % of annual electricity supply.  I.e. 125.55 TWh out of a total supply of 405 TWh.
  • The capacity factor is then (125.55 TWh *1000)/(40GW*8760 hrs/yr) =  0.358. 
  • The additional capex of offshore wind is £17.5 per MWh spread over all supply of 405 TWh.
  • This makes the extra cost of offshore wind to be £17.5 *405/125.55 = £56.45 per MWh. 
  • With a base cost of £45.0, the total cost of offshore wind is therefore £101.45 per MWh.
  • On the same basis, the total cost of onshore wind is £56.31 per MWh
  • 40GW of wind would due to its capacity credit replace 6GW of gas generation. 

By reference to Page 108, Chart A1, Appendix 1 of the Carbon Trust Report, the following can also be noted:-


  1. An offshore cost of £101.45 is equivalent, with a capacity factor of 35%, to a 2008 capex of £2.27 bn/GW of installed offshore wind power .     
  2. By proportion, the equivalent capex of onshore wind is £1.26 bn/GW.    

 The modified Carbon Trust Report Chart, shown below, reflects the comments made in Section 1.0 and the calculations in Appendix B of this critique.  It is based on the following:-. 



  • A historical capacity factor of 0.275
  • Consequently, 40 GW of wind capacity provides 23.80 % of annual electricity supply.  I.e. 96.4 TWh out of a total supply of 405 TWh.
  • The 2008 capex of offshore wind is £2.50 bn/GW.  Thus, 29GW of offshore wind power has a capex of £72.50 bn.  Due to the falling value of the UK Pound against the Euro the present day capex is probably higher and approaching £3.0 bn/GW.
  • The capex of onshore wind is accepted at £1.26 bn/GW.  Thus, 11GW of onshore power has a Capex of £13.86 bn.

 The following chart indicates that the Carbon Trust Chart 7a under estimates the net cost of incorporating 40GW of wind power into the UK’s electricity system.  The additional cost will add at least £30.21 per MWh.  That is, the equivalent of adding 28% to present day retail prices.   If the capital cost was assumed to rise to £3.0 bn/GW the increase in present day retail prices would be 30%.


Modified Carbon Trust Chart


 The Chart will not import onto the Claverton site.   For a copy email


Legend                                                                                                                        £/MWh


Column 1 – Wind Capital Cost                                                                                      30.10 

Column 2 – Wind O&M, including grid reinforcement costs                                             8.00

Column 3 – Reduced Fuel Cost of displaced Gas Turbine Plant                                     – 9.14

Column 4 – Reduced Carbon cost of displaced Gas turbine Plant                                  – 2.45

Column 5 – Balancing Cost                                                                                             1.70

Column 6 – Load Factor Cost                                                                                         2.00

Column 7 – Impact of 40GW wind /MWh over all electricity generation                         30.21


3.0 Title of the Report


The subterfuge demonstrated in Sections 1 and 2 above starts with the title of the Carbon Trust Report.  For example:


Is the ‘big opportunity’ for, the consumer or the wind farm developers?  


The renewable obligation ensures/will ensure developers will make a return on their investment without much risk.  They have no responsibility for providing power when the wind does not blow. 


The consumer will pay through increased electricity bills and have to risk the possibility of power cuts.


Does wind provide ‘environmental’ benefits?  


Wind reduces the amount of carbon emitted to the atmosphere from the generation of electricity by displacing coal and gas fired generating plant.  It therefore has an environmental benefit. 


Onshore wind on the scale stated by the Carbon Trust will damage the landscape and cause a noise nuisance.  Both are environmentally detrimental.  Just type ‘Stirling Wind Farms’ into Google and open the first web site that comes up and view the destruction to the landscape that a large array of wind turbines can cause.


If carbon reduction is the only environmental benefit then other forms of power generation can achieve this with considerably less destruction and disfiguration of the natural environment. 


Is wind ‘economic’?


Contrary to the statements made by the Carbon Trust the resulting cost per kWh will add more than 28% to the present day cost of the annual household electricity bill of £450.00.  See section 2.0 above for the detailed calculation.


The increased cost will put more households into fuel poverty. 


Does wind provide ‘security’? 


Wind’s only value in security is that it displaces conventional gas and coal electricity generation thus saving on fuel imports and their associated politics.  It does not provide security against power cuts.  Nor does it replace the need for full conventional power generation as a backup.  


If the UK moves to a European/Scandinavian/North Africa/Middle East super grid and dispenses with conventional and nuclear power generation in favour of renewable power then, unless the political dimension can be solved, security of supply will be highly problematic. 


 4.0 Concluding Remarks


The Carbon Trust web site states that it is an independent company funded by the Government.  That is you and I.  Consequently, the advice it gives should be conservative and impartial.  However, their report stretches the boundaries and makes assumptions that cannot be verified until the proposed 29GW of offshore and 11GW of onshore wind power has been constructed and has been operational for a reasonable period. 


The costs presented by the Carbon Trust assume a reduction in manufacturing and installation costs due to learning and repetition in manufacture and installation of wind turbines.


The SDC’s report on Wind Power dated May 2005 also stated that costs should reduce with repetition and learning.  Since then costs have more than doubled! 


A simple change in the rate of exchange between the Pound and the Euro and the Pound and the Dollar, as has been experienced over the last few months, can nullify any savings due to learning and add considerably to the costs.


The main driver for the upward pressure on costs is Government policy.  Political desire rules over cost.  Blind acceptance of the EU target for 15% of all energy to come from renewables by the deadline of 2020 is driving up costs.  It does not matter whether the RO is maintained, or exchanged with feed in tariffs, or what cost is put on carbon.  The effect will be the same.   For example; large prestigious multidiscipline construction projects with a deadline for completion have a history of industrial action resulting in the need for site agreements which have increased the labour and the overall costs of the projects.  40GW of wind power by 2020 is a major project involving the construction of 8000 to 10,000 wind turbines.  It is the equivalent to the expenditure required for the construction and operation of the 2012 Olympic Games every year for each of the next 10 years.  The consequence is that the subsidy on wind or penalties on coal and gas will have to be increased to ensure that wind power is competitive.


The subsidy is tantamount to providing a blank cheque to the suppliers to wind farm developers and the developers.  Recent press coverage indicates that this is already happening for projects like the London Array. 


The Carbon Trust Report implies that wind power is affordable.  Currently this is not the case.  Wind, especially offshore wind, is three to four times more expensive to construct per GW of output than nuclear.  If you take into account the reduced life of wind turbines compared to nuclear plant the ratio is higher.  The unit cost of offshore wind power per kWh is over three times that of nuclear plant.  


Electricity generation from wind is a diversion from other carbon free technologies such as nuclear and carbon capture and storage.  The Carbon Trust’s efforts should be directed to the means by which funds can be made available for carbon capture development.  The Carbon Trust should lobby the Government to put in place a banded Obligation in favour of electricity from all forms of low/zero carbon emission electricity generating plant not only renewable power.  If the recommendation of the Climate Change Committee to phase out coal plants in the early 2020’s is taken on board then an incentive mechanism for Carbon Capture and Storage coal fuelled plants is urgent.


Without incentives for coal, carbon capture and storage, plants the existing RO combined with the EU emissions trading scheme will dictate the market and may result in reduced investment in coal fuelled plant.  This may increase the risk that, at times of maximum demand on the National Grid, there will be power cuts for parts of the UK.


The Carbon Trust Report writes off nuclear power.  It states nuclear will not be delivered in time to meet the EU and the carbon emissions targets for 2020.  There does not seem to be any basis for this statement.  Previous nuclear construction programmes, notably in France, have average the commissioning of 3 to 4 nuclear plants per year.  Even with the Governments present extended programme for design assessment, site licensing and planning permission, it should be possible to commission three or more 1.6GW nuclear stations per year from the beginning of 2017.  This would provide 19GW of new nuclear capacity by 2020.  The construction cost would be 50% of that for 40GW of wind.  The annual electrical output available to meet demand and the savings in carbon emissions would be 50% greater than 40GW of wind and the present day unit cost per kWh of electricity met by consumers would not increase.  


Chart 7a of the Carbon Trust Report implies that wind replaces some conventional generating plant which is not the case.  If the same logic was applied to nuclear power the saving in the need to invest in coal and gas fired plant would be much greater and result in a considerable reduction in the unit cost of electricity.


In Conclusion


The promotion of the construction of 40GW of wind power on the scale recommended by the Carbon Trust Report does not appear to be in the interests of the nation.  Wind power:-


  • increases electricity demand when the wind does not blow.
  • does not have a capacity credit and consequently does not replace other forms of electricity generation.
  • does not reduce the need to invest in coal and gas fired power plants to replace ageing plant and to provide a margin over maximum demand.
  • is 3 to 4 times more expensive to construct per GW of output than nuclear power
  • costs, per kWh of output,  are 3 to 4 times greater than that of nuclear power and CCCS.  
  • has a life expectancy of 20 years after which replacement or repowering of turbines is required.  This is up to 20 years shorter nuclear or CCCS
  • is damaging to the visual environment and
  • will place more households into fuel poverty.


Notwithstanding this conclusion, the Author of this critique does believe that there is a future for wind power in a decentralised power system where wind turbines are embedded into local distribution networks.  Under such a system local communities, industrial estates and port areas would elect to erect wind turbines to provide local power which is supplemented, when the wind speed is low or does not blow, by centralised low/zero emission power generation.



Appendix A

Calculation of Wind Turbine Output


The calculations may be obtained by email from  Alternatively please see Claverton 2008 Conference updates.


Appendix B


Calculations with respect to the Carbon Trust Report Chart 7a


Chart 7a. – General


Chart 7a states:-


  • Wind is 31 % of electricity supply.  I.e. 125.55 TWh out of a total supply of 405 TWh.
  • The capacity factor is then (125.55 TWh *1000)/(40GW*8760 hrs/yr) =  0.358. 
  • The additional capex of offshore wind is £17.5 per MWh spread over all supply of 405 TWh.
  • Consequently the extra cost of offshore wind is £17.5 *405/125.55 = £56.45 per MWh. 
  • And with a base cost of £45.0, the total cost of offshore wind is £101.45 per MWh.
  • On the same basis, the total cost of onshore wind is £56.31 per MWh.

 By reference to Page 108, Chart A1, Appendix 1 of the Carbon Trust Report, the following can be noted:-


  1. A cost of £101.45 approximates  to a 2008 capex of £2.27 bn/GW of installed offshore wind power.
  2. By proportion, the equivalent capex of onshore wind is £1.26 bn/GW.  Based on present day costs the capex is low but for purposes of the following is accepted.

 Chart 7a, Col. 1 – Wind Capital Cost


As discussed under the headings ‘Capacity Factor’ and ‘Curtailment’ a capacity factor of 0.358 appears to be high.  A factor nearer 0.275 providing 96.36 TWh per year, 23.8% of all electricity, would appear to be more realistic. 


The increased onshore wind Capital Cost is then (((£56.31*0.358/0.275)-£45))*96.36/405)- £3.50 = £3.24 per MWh.


The offshore capex is low.  At the time of the Carbon Trust Report, present day capital expenditure was in excess of £2.5 bn/GW.  It is now approaching £3.0 bn/GW.  By reference to Chart A1 the resulting cost per MWh is greater than £110 and approaching £118 *.


The increase in offshore Wind Capital Cost Column 1 is => (((£110*0.358/0.275)-£45))*96.36/405)- £17.5 = £5.86 (£8.34)


Chart 7a, Col. 2-Wind O&M


Table 5 of the SDC Report ‘Wind Power in the UK’ suggested costs in the region of £35 to £42 per kW.  This equates to £3.50 to £4.14 per MWh over all electricity supply.  These figures apply to 2004/2005.  They are superseded by inflation. 


The Carbon Trust O&M figure of £3.50 appears to be on the low side.  It includes the connection costs between the wind farms and the National Grid.  Consequently, these costs are not shown separately in Chart 7a.  They are quite significant.  For example, assuming a return on capital invested of 8% and a repayment period of 20 years, £8.0 bn capex for the transmission assets equates to a cost of £2.0 per MWh over all electricity before any mark up by the transmission asset operator. 


*Note the figures above, in orange, are for an offshore capex of £3.0 bn/GW.



If the capex of the transmission assets are £13 bn, as forecast in the Government’s Renewable Energy Strategy Consultation document, then the cost rises to £3.25 per MWh, before any mark up by the transmission asset operator.


The O&M costs should also include legacy costs for the removal of the wind turbines and their foundations once they have past their useful life.  This could amount to 5% to 10% of the Wind capital costs.  That is, £0.6 to £1.2 per MWh over all electricity generation.


A more realistic, present day, figure for Column 2 O&M costs would be in excess of £8.00 per MWh.


Chart 7a, Col. 3 – Avoided Capital Cost of Displaced Plant


As discussed under ‘Capacity Credit’ wind does not have a capacity credit at times of peak demand on the National Grid. 


There is no avoided capital cost of displaced plant.  Column 3 is zero


Chart 7a, Col. 4 Avoided O&M of displaced Plant


There is no avoided O&M cost of displaced plant.  Column 4 is zero


Chart 7a, Col. 5, – Reduced fuel Cost


With a reduced capacity factor of 0.275 the reduced fuel cost, Column 5, will reduce in proportion from £11.9 to £11.9*0.275/0.358 = £9.14


Chart 7a, Col. 6, – Reduced Carbon Cost


With a reduced capacity factor of 0.275 the reduced carbon cost, Column 6, will reduce in proportion from £3.2 to £3.2*0.275/0.358 = £2.45.


Chart 7a, Col. 7, – Impact of 40GW wind (ex balancing load factor costs)


The above result in an increase in the impact of 40GW from £4.90 to (£30.10 +£8.0 -£9.14- £2.45) =£26.51   (£28.99)  


Chart 7a, Cols. 8 and 9,   Balancing and Net Load factor Cost


The figures of £1.70 for balancing costs £2.0 for the net load factor costs are accepted. 


A simple calculation indicates that, based on the installed capacity in 2006, the Carbon Trust figure of £2.0 for the net load factor costs may be low and that the costs vary depending on the amount of other fixed capacity such as nuclear in the National Grid.


Chart 7a, Col 10, Impact of 40GW wind


The resulting impact of 40GW wind power is to increase the unit cost of all electricity supply by => £26.51+£1.70+£2.0 = >£ 30.21 per MWh. (£32.69)


That is a =>67% (73%) increase on wholesale electricity prices and a =>28% (30%) increase on retail prices.




The above figures when compared to the figures quoted in Chart 7a of the Carbon Trust Report illustrate that the assumptions made can provide a wide variation in the value of the increased costs due to the addition of 40GW wind power to the National Grid. 


The higher value ascribed above is still probably an underestimate.


*Note the figures above, in orange, are for an offshore capex of £3.0 bn/GW.



Appendix C


Curtailment Diagrams



The diagrams may be obtained by email from



Critique compiled by



D S Stephens



4 comments on “Critique of Carbon Trust Report – Offshore Wind Power

  1. There seems to be some basic disagrements here about what is meant by statistical reliabilty. No plant is 100% reliable. If you required 100% abilty to meet peak demand then every plant, nuclear coal, gas or whatever would have to have 100% back up (and also pressumably back up to that backup, just in case). Laughton based his square root rule for wind on a ‘Loss of Load Probability’ of 9%.

  2. Dave Elliot

    I agree, no machine is 100% reliable. The reasons are mechanical failure, lack of fuel etc. etc.

    In the case of a steam turbine powered by steam generated from a nuclear reaction, or the burning of coal, oil or gas, the fuel is under the control of the operatives. In the case of a wind turbine the fuel is controlled by nature.

    Coal, oil and gas fuel starvation is rare.

    Wind starvation is quite common. There are many days during the year when over the whole of the UK and the seas surrounding the UK the wind does not blow at a sufficiently high speed to enable any or hardly any electrical output from a large array of wind turbines. In winter, these periods usually coincide with cold spells and high electricity demand.

    Rather than talk about statistical reliability perhaps it would help if the problem was thought of in the following manner:

    In the UK the generating capacity of conventional plant, coal, gas and nuclear, is approximately 75GW some 14GW, 23%, more than maximum demand. Historically this margin of capacity over demand has proven to be sufficient to provide 100% reliability of electricity supply.

    When 40GW of wind capacity is added the excess capacity over demand rises to 54GW.

    But, if we look closely, we find that the average output of this amount of wind capacity is about 11GW which equates to a wind speed of 8m/sec. If we study the shape of the power output curve of a wind turbine we see that the area under the curve between zero wind speed and the average wind speed is small compared to the area under the curve between the average wind speed and the rated wind speed.

    This leads to the conclusion that a wind turbine operates, for perhaps 75% of the time, in the zone of the curve between zero and the average wind speed and for a large part of this time its output is zero or near zero.

    Consequently, the additional wind capacity does not always add to the margin over demand and therefore often does not provide any capacity credit.

    The corollary of this is that a large array of wind turbines requires 100% backup whilst a large number of conventional power generating plants operating together require about 20% to 25% backup.

    Denis Stephens

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