Severn Trent Wind Turbine Application  

Wind Turbine Planning Application 12 Month Wind Data/Generation Estimates Final Analysis   

Executive Summary

This exercise, which is entirely independent and self funded, was prompted by Severn Trent Water making a planning application to erect a wind turbine at Newthorpe Sewage Works, Halls Lane, Newthorpe, Nottinghamshire, the application stated that the turbine capacity would be 3.3MW and the objective has been to collect monthly 20 minute wind data from a Skylink weather station 19km away at East Midlands  Airport. The UK NOABL Windspeed Database software was used to estimate the annual average wind speed at East Midlands Airport and at the proposed location of the wind turbine. Following a comment from the consultants working for STW who suggested the Skylink data was probably collected by sensors only 10m above ground level and the proposed rotor hub height was about 80m agl the two locations (see table 1) were compared and a monthly height adjustment 5.3/6.6=20% was applied, the accuracy of the estimates being of paramount importance. Table 1 also highlights that the ENGX site has the higher average wind speed. In the final analysis it became evident that the simple height adjustment was insufficient and therefore table 3 and chart 4 below, relating to monthly height adjusted data have been added. This assumes the consultants were correct about the height of the sensors at Skylink station ENGX and must be seen as the maximum estimated generation.

STW have not specified the turbine they propose to erect and the nearest performance curve data available is for a Siemens 3.6MW-107m Unit. The monthly average wind data from ENGX in 2011 has been computed against the performance curve of the above turbine using The Windpower Program software. With 12 months of data in the completed table 2, mean average power output per month is 553kW (4800MWh/annum) with a capacity factor of 15.36%. Table 3 with 12 months of height adjusted wind speed data, the average mean power output increases to 898kW with estimated annual energy of 7847MWh at 24.93% capacity factor. The final capacity factor was boosted by the relatively high average wind speed of 8.72m/s in December 2011 otherwise the annual capacity factor would have been slightly more than 23%. If the proposed 3.3MW wind turbine has a similar performance curve it is reasonable to estimate its annual yield as 92% of the maximum estimate above (7219MWh).

The summary table below illustrates the relationship between the variable wind speed and potential output during 2011. The percentage capacity factors used were selected because they have an important relationship with the power performance curve (see chart 1). Zero % is based upon wind speed up to 3m/s (cut in point), less than 8% refers to wind speed below 5m/s and therefore beneath the “upper” variable wind mean power curve, when rotor efficiency is in a positive gradient. Less than 20% capacity factor is at wind speed below 5.7m/s and greater than 20% is above this threshold, approaching peak rotor efficiency and within the 20% to 30% operating band that is likely to be achieved with average UK wind speed. The fact that the hours in the year when the turbine would operate at less than 20% capacity factor is close to 50% and within this 22% of the year would see less than 8% CF, with the turbine not delivering for 10% of the year, highlights the need for conventional generation plant being available to cover the obvious shortfall of wind power. The table also identifies that the turbine would only operate at 100% CF for 151 hours per year. This makes the government guidelines that council planning committees may only consider the rated output of the wind turbine, stated by the applicant and not the potential delivery based on intermittency of wind, a rather worrying dictate.    

Annual Hours of Operation Summary Table    

 Although this analysis is an estimation of generated output from one future wind turbine based on wind speed data during 2011, compared with UK NOABL Windspeed database and Windpower performance software, an interesting comparison may be drawn with the conclusions in a report written by Stuart Young Consulting and supported by the John Muir Trust. His report was based on the delivery from all wind turbines metered by the National Grid between November 2008 and December 2010 http://www.jmt.org/assets/pdf/wind-report.pdf. He collected data originating from the National Grid relating to Historic Generation by Fuel Type and his conclusions most relevant to this analysis were…

 1) Between November 2008 and December 2010 the average capacity factor from metered wind turbines connected to the National Grid was 24.13%. The results of this analysis following high average wind speed of 8.72m/s recorded at station ENGX in December 2011 indicate an estimated annual capacity factor of 24.93%. The findings of both analyses are below the annual capacity factor claimed by those promoting wind turbines.

2) Also, his report concludes that during this period the total UK turbine output was below 20% capacity more than half the time, below 10% capacity over one third of the time, below 2.5% capacity for the equivalent of one day in twelve and below 1.2% capacity for the equivalent of just under one day a month. The similarities with the estimations in the Annual Hours Summary Table above suggest both reports, although approached from a different angle, arrive at similar conclusions!  

The report by Stuart Young Consulting, an engineer who has collected data from the internet and published a true record of this information in his report, has received criticism from wind industry representatives telling BBC News Scotland on 6^th April 2011, they had “no confidence” in the data and Jenny Owen for Scottish Renewables said no form of electricity worked 100%, 100% of the time. In the Ecologist on 4^th May 2011, Robert Norris of Renewable UK said “the report is based only on the 50 per cent of UK wind energy that passes through the metered stations and so is “visible” to the National Grid – most of it located in Scotland. None of these statements is backed up by facts to contradict the report and one would expect the other 50% of wind turbines to have the same wind intermittency limitations.

Professor Peter Tavner of Durham University stated that “fossil fired plants operate at roughly 25% of their capability, a similar statement to that by Jenny Owen above. These statements are very misleading, it has always been accepted in the electricity supply industry that coal fired power stations are about 33% efficient. This refers to the ratio of fuel burnt to electricity generated and not to the capacity factor of the turbines/generators. There is evidence on the internet that efficiencies for coal may be as high as 42%, 52% for natural gas and 45% for oil fired generation. For instance, a 500MW generator can deliver its rated output when required but a wind turbine is unpredictably variable according to wind velocity.

The Stuart Young Consulting report has received support from Dr David McHale at the following link.

http://www.tw312.org.uk/?page_id=1630

Everyone working in energy, whether they are engineers, suppliers, academics or politicians should be committed to transparency of accurate data and the periodic dated comments (below) on this webpage have been retained to illustrate how ones perception of a subject may change over a period of time due to ongoing conditions and increasing knowledge. Therefore, additional and necessary data, identified in the final analysis, has been added in order to arrive at accurate conclusions within this summary.

Ideally, wind farm owners should meter each turbine and monitor local wind speed, making the data accessible to anyone wishing to analyse the energy yield! 

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 15th February 2011 

 Severn Trent’s planning application for a wind turbine at the Newthorpe Sewage Works,Halls Lane, Newthorpe states that the turbine would generate 3.3 MegaWatts of electricity-equivalent to the annual electricity needs of 1857 homes-and would be used to power the sewage treatment works. It is assumed Severn Trent are committed to such applications because they are signed up to the CRC Energy Efficiency Scheme and although they emphasise they will not receive any subsidy for the installation, they omit to mention that they will benefit from the money they receive for selling the electricity generated (circa £45/MWh) plus the Renewable Energy Obligation Certificate Subsidy (£48/MWh). Also, there may be a further benefit in the form of a Climate Change Levy (CCL) exemption certificate (possible £3/MWh).  

Having looked at the application and the proposed site, the findings are purely objective and an opportunity to analyse wind/output ratio using live weather data and wind turbine database information for a unit similar to the one being proposed.

If the proposed wind turbine operated at rated capacity 24/7 it would deliver (8760x3300kWh) =28,908,000kWh (29millionkWh). This would, in fact, be enough to serve 6150 homes. Unfortunately, the biggest disadvantage of wind generation is the intermittent nature of the wind!

Simple calculation of wind turbine output (Aerodynamic rotor efficiency x  Swept area of blades (Πr²) x power density (0.6125/m³) x generator efficiency shows that the power output of a wind generator at 15m/s is 8 times greater than the power generated at 7.5m/s. Or, half the wind speed reduces the output by 87.5%! However, the Siemens performance curve chart below highlights that it is slightly more complex than the basic calculation suggests. 

Weather data downloaded from a local weather source demonstrates that between the 7^th January and the 3^rd February 2011, based on 20 minute integration periods there was one occurrence of wind speed exceeding 15m/s and 20 occurrences of wind gusts between 15m/s and 20m/s. The average wind speed during this period was 5.3m/s and gusts averaged 5.37m/s.

The performance curve for a 3.6MW wind turbine (see chart 1 below) shows that it has a cut in speed of 3m/s and cuts out at 25m/s. It generates to its rated capacity between 15m/s and 25m/s. However, the data confirms that at wind speed of 5m/s it will generate only 296kW (9% of rated capacity) but, with peak Cp efficiency between 5m/s and 10m/s and compensated for variable wind speed, its output will be approximately 575kW . If this were to be the approximate average annual wind speed, it would only supply (575x24x365=2592960) 2.6millionkWh at steady wind speed but 5.04millionkWh in a variable wind! Enough to serve the annual needs of approximately 553 or 1072 homes on the higher curve therefore, it seems the Severn Trent calculations are based on the unit having an average output of 996kW (8.73millionkWh) at wind speeds of approximately 6.6m/s (30% of capacity). The NAOBL UK Windspeed database estimates annual average windspeed at OS SK4745  to be 6.6m/s(8.5millionkWh) at 80metre hub height.

If the authorised supply capacity of the sewage works is say approximately 250kVA with a load factor in excess of 68%, annual electricity usage at the site would be approximately 1.5millionkWh at unity pf.

Finally, Severn Trent stated at the meeting that they need to use the local high voltage network as a “buffer”! The reason for this is that the generator output will be at high voltage and they will have to export (sell) all its energy into the local distribution company’s network. Network  distribution losses of circa 7% have not been factored into the estimates.

It is anticipated that considerable work will be required on the local 11kV network in order to accommodate the potential exported energy based on full output capacity.

 Chart 1:

Chart 2 below forJanuary 2011 has integrated wind data with performance curve data to illustrate the potential delivery when steady wind speed averaged 5.3m/s and there was only one occurrance, on the 15th January 2011, when a wind speed of 15m/s was recorded and rated output would have been achieved. It is noticeable that the low wind speeds largely restricted estimated generator output to less than 500kW. 

Chart 2:

Table1 represents UK Wind Database NOABL average annual wind speed for the site of the proposed STW wind turbine and also the location of the historic wind speed data illustrating monthly local conditions. It is worth noting that the predicted annual wind speed for Newthorpe is 6.6m/s and the site from where the historic data is downloaded is 7.0m/s.  Also, Severn Trent Water have not specified the exact wind turbine they propose to use and the Siemens turbine, selected from the available database, is the best alternative. Also, the height of the source wind speed sensor is not known.   

However, the turbine output predictions should fall within acceptable limits.

Table 1 

Table 2 and Chart 3 will be populated by monthly average wind speed data from OS4525 and matched with data from the Wind Power program relating to the Siemens performance curve. Estimated monthly and annual turbine kWh will be shown, with total kWh to date for monthly estimates and average kWh to date in the yearly estimates columns.

Table 2:

Chart 3:

Table 3 and chart 4 illustrate the height adjusted data mentioned in the executive summary and represent the estimated maximum output that may have been generated by a Siemens SWT-3.6Mw-107m(MG) wind turbine given the monitored windspeeds during 2011. 

Table 3

Chart 4

 January 2011:

 During January 2011 there was only one occurrence, at 13:55 on 15/01/11, of wind speed in excess of 15m/s where the turbine rated output of 3600kW would have been achieved. Turbine capacity factor estimated at 16%.

 February 2011:

Winds speeds between 15m/s and 20m/s were recorded during 52 integration periods between the 4^th and the 7^th February 2011. The maximum recorded wind speed was 19.44m/s and estimated output was higher than January. Turbine capacity factor estimated at 22%.  

14^th March 2011:

Following an enquiry to Nottinghamshire County Council about their objections to the proposed Severn Trent Water wind turbine, Councillor Richard Butler emailed the report supporting their concerns (see attached file).    

Wind turbine Newthorpe Report March.pdf

March 2011:

Wind speeds between 15m/s and 20m/s were recorded between 09:55 and 13:55 on Thursday 10th March 2011 and between 12:15 and 13:55 on Thursday 31st March 2011; during these times peak output of 3.6MW would have occurred. However, the average monthly wind speed of 4.01m/s was the lowest this year. With three monthly average wind speed of 5.1m/s and an estimated average annual output of 4.3million kWh. After the sewage works electricity usage is factored, if this trend continues, the turbine is unlikely to supply the annual needs of more than 595 homes.  

April 2011: 

Detailed analysis of the STW Application to Broxtowe Borough Council has highlighted some inconsistencies in their documentation.

For instance 

“Supporting Documents with Planning Application 06/01/2011” states that this proposal would help tackle the causes of climate change by: off-setting the generation of between approximately 1472 tonnes and 2971 tonnes CO2 annually and between 36,808 and 74,283 tonnes CO2 over the proposed lifetime of the development. Providing enough clean renewable energy to power approximately 1470 households annually (based on BWEA average 4700kWh/household/annum).

Previous calculations on this webpage have been based on 3500kWh/household/annum and will therefore be corrected.

This is followed by

“Environmental Report and Technical Appendices (Part 1)” which says “On this basis the CO2 reduction of the proposed Newthorpe Wind Turbine is estimated to be between approximately 1546 tonnes and 2065 annually; 37,100 and 65,491 tonnes CO2 over 24 years. Clearly differs with the previous document!

Peak wind speed of 11.67m/s was recorded on the morning of Tuesday 5^th April 2011 but the recorded monthly average wind speed was 4.58m/s. The capacity factor for April would have been 6.08% with average output of 219kW. If the yearly averages to date applied after 12 months the estimated annual generator output would be 3.71million kWh. If the sewage works consumed 1.5million kWh the surplus would supply the annual needs of approximately 470 homes. The average estimated capacity factor to date this year is 11.76%.

May 2011:

Average wind speed of 6.03m/s during May was the highest recorded this year with maximum wind speed of 15.83m/s recorded between 14:15 and 15:00 on Sunday 22nd May; this was the only period during May when rated output (>3MW) would be achieved! Gusts between 20m/s and 25m/s were recorded on the 22nd and 23rd May. Estimated average mean power output for for May 2011 is 798kW at 22.17% capacity factor. This shows an improved annual yeald to 4.4 million kWh and after SWT sewage works consumption, the surplus would serve approximately 610 homes, with an annual capacity factor of <14%. However, if circa 7% distribution losses are factored in, it would benefit approximately 567 homes annually.

June 2011:

Average windspeed was again below 5m/s in June  with an estimated capacity factor of 7.44%.

The comment below refers to the planning application approval by Broxtowe Borough Council and therefore monthly summaries will not continue. However, it is intended to continue logging wind data and updating the monthly data table and chart. When 12 months of data are illustrated a final summary and conclusion will be presented. Currently, the estimates relative to annual yield are calculated averages based on the first 6 months, at year end the results should provide a more accurate assessment of probable performance.

The local wind turbine application was approved by Broxtowe Borough Council on Wednesday 22^nd June 2011. The vote was split 5 all and approved by the Chairman’s casting vote. The research, published on this website, brought some criticism from BCC. Also, criticism came from the Severn Trent Consultants; obviously representing STW they are well paid for attempting to discredit alternative data! They both  pointed out (I already knew this) that the Government guidelines dictate that planning departments may only consider the rated capability of wind turbines and not the potential delivery based on intermittency of wind (efficiency).  However it was interesting to note that the original application stated that the output would be sufficient to serve 1850 homes annually, or 8700MWh (capacity factor 27.6%) ; in last nights local paper the quoted claim has become 1350 homes or 6000MWh (capacity factor 19%). My calculations based on monthly wind data to date this year and published on my website suggest 4029MWh (12.8% capacity factor). A 100% capacity factor, which is not possible, would deliver 31500MWh!

One of the criticisms from the consultants was that the data is not from the location of the proposed wind turbine and in probability the height of the wind speed sensor was likely to only be at 10 metres rather than the 80 metre proposed hub height. This has been mentioned previously (see table 1comment) stating the wind data base for the weather station was 7m/s rather than 6.6m/s at hub height at the proposed location. Their comment and a 20% upward adjustment based on the wind data base for both locations at 10 metres and 80 metres has been made, the revised estimates are illustrated in the bottom row of the data table and suggest to end of June this year, the wind turbine would generate circa 4800MWh (15% capacity factor). It’s interesting that STW have diluted their original claims and the height adjustment in response to their comments about height of wind data have finally brought estimates closer but, there is still a considerable differential.

Sadly, as I said to a councillor on the planning committee who contacted me with concerns about the approval, it’s like buying a Ferrari with a Mini engine. Please circulate to anyone who may be interested, because these applications will increase over time; I am always happy to discuss energy matters and would fully support reliable projects.

Finally, this does not allow for the fact that 100% of output will be exported to the local distribution network and incur further 7% distribution losses. The consultants claim they have considered this…?