Userguide for V2.01c

8/13/2019 Userguide for V2.01c

1/27

1

User Guide for Version 2.01c March 2011(www.wind-power-program.com)

Copyright 2008 PelaFlow Consulting


2/27

2

CONTENTS

Page1.0 Introduction. 3

2.0 Entering data and calculating mean power outputs. 3

2.1 The Turbine Data and Power Curve Inputpane. 42.1.1 Turbine name. 42.1.2 The rotor diameter. 42.1.3 The cut-in speed. 42.1.4 The cut-out speed. 42.1.5 The power curve input table. 4

2.2 The Mean Power Output versus Mean Wind Speed Results pane. 62.3 The Wind Standard Deviation Input pane. 62.4 The comment box 72.5 Saving, loading and printing the mean power results. 7

2.5.1 Save Options. 72.5.1.1 Save turbine power data only. 7

2.5.1.2 Export power curve and mean power data as a csv file. 72.5.2 Load turbine data. 82.5.3 Print options. 8

3.0 Finance options. 83.1 Total returns to total cost ratio. 9

3.1.1 Print the returns ratio. 113.1.2 Export returns ratio and mean power as a csv file. 11

3.2 Calculating the payback time. 113.2.1 Print payback periods. 123.2.2 Export payback periods and mean power as a csv file. 13

3.3 Calculating the cost per kilowatt-hour. 133.3.1 Print prices. 143.2.1 Export prices and mean power as a csv files. 14

4.0 The wind turbine power profile. 144.1 Print. 165.0 Ratio of local energy used to the total energy produced. 166.0 Other features of the program. 177.0 Vertical axis wind turbines. ` 17

Appendix A. Two tutorial examples of wind turbine data. 18Appendix B. Obtaining a .powfile from PelaFlow Consulting. 19Appendix C. Calculating the total cost of a loan. 20Appendix D. Using the WindPowerProgram with 21

Vertical Axis Wind Turbines (VAWTs)Appendix E. UK feed-in tariffs for wind power 22Appendix F. Estimating the ratio of local energy used to total

energy production. 23Appendix G. Modification to power curves in the calculation of

local power usage. 26


3/27

3

The User Guide.1.0 Introduction.

This manual goes step-by-step through the use of the WindPowerprogram. In order tobecome familiar with using the program, appendix A contains two examples of wind turbinedata you need in order to use the program. One of the examples is a large commercial 2.5-megawatt turbine and the other a small domestic turbine with a rated out of 1.9 kilowatts. Inboth cases, the data comprises the rotor diameter, the cut-in and cut-out speeds, a table ofthe steady power output curve, an installation cost and an expected turbine lifetime. It isrecommended that you work through the manual using one or the other of these twoexamples as data sources. The WindPower program is not difficult to use and, after oneexample, further use should be straightforward.

As far as running the program is concerned, it should only be necessary to check that yourscreen size is a minimum of 1024 x 768 pixels with a screen font size of not more that 96 dpi.

2.0 Entering data and calculating mean power outputs.

The figure 1 below shows the opening form of the WindPowerprogram. It consists of threepanes, namely, (i) the Wind Turbine Data and Power Curve Inputpane where the basicturbine data and its power curve are entered, (ii) the output pane Mean Power or AnnualEnergy Output versus Mean Wind Speed Results where the final mean output power resultsare displayed and (iii) the Wind Standard Deviation Input pane where the magnitude of theunsteady component of the wind speed (i.e. its standard deviation) can be adjusted.

Figure 1. The opening form.


4/27

4

2.1 The Turbine Data and Power Curve Input pane.

This pane is where the basic turbine data are entered. The first data to be entered are (i) themanufacturers name for the wind turbine (ii) the rotor diameter in metres (iii) the cut-in windspeed in metres per second and (iv) the cut-out speed in metres per second. All of theseinputs will be found in the manufacturers description of a turbine.

2.1.1 Turbine name.The name of the turbine can be up to 50 characters long and, for identification purposes, it isgood practice to include in the name the rotor diameter and rated output of the turbine as wellas the manufacturers name. The practice of many manufactures is to give this information intheir turbine names so that, for example, the Vestas V90-3.0MW has a rotor diameter of 90mmetres and a rated output of 3 megawatts. Manufacturers of smaller turbines are often lessinformative in their names. For example, the Iskra AT5.1 does not obviously contain the basicinformation about the turbine and a name to enter might be Iskra AT5.1 5.4m 5kw showingthat the rotor diameter is 5.4 metres and the rated generator output is 5 kilowatts.

2.1.2 The rotor diameter.The next piece of information to be entered is the rotor diameter in metres. This is adjustedusing two scroll bars one for the whole number or integer part of the diameter and the otherfor the decimal part of the diameter. The default diameter at start-up is 20.0 metres and thiscan be scrolled over a range from the smallest turbine of 1.0 metre rotor diameter up to 150.9metres. No turbine of this size yet exists!

2.1.3 The cut-in speed.The cut-in speed in metres per second is next to be entered by a single scroll bar. The cut-inspeed is the lowest wind speed at which the turbine begins to rotate and the default value isset to 3.5 metres per second. However, this can be adjusted over a range from 1 metre persecond to 5 metres per second in steps of 0.5 metres per second.

2.1.4 The cut-out speed.The final basic piece of information is the cut-out speed in metres per second. The cut-outspeed is the high wind speed point at which the turbine is effectively shut down to avoiddamage. The default value is set to 25 metres per second and can be adjusted via a scroll barto cover the range from 10 metres per second up to 30 metres per second.

In the case of small turbines, there may not be a well-defined cut-out speed. In these cases,set the cut-out speed to the speed step above the last data point for which an output power isavailable.

2.1.5 The power curve input table.After the above data are entered, the green Initialise data input tablebutton can be clicked toset up the power curve table for data entry. It is important to double-check the accuracy ofthe above data before doing this because, apart from the turbine name, these inputscannot then be adjusted without resetting the whole data input process.After clickingthe Initialise data input tablebutton, the data input scroll bars will be disabled along with thebutton itself - which will now become grey.

At start-up, all the data cells in the power curve input table are grey. However, on clicking theInitialise data input tablebutton, the cells into which power data can be entered are turned

white. The power data cells below the cut-in wind speed and above the cut-out wind speedhave zero entries automatically put into them and these cells remain grey and disabled fromentry. The steady wind power values can now be entered into the white cells for each of thesteady wind speeds. When these have all been entered, the green Update power coefficientand mean power calculations button can be clicked.

1

1The Update power coefficient and mean power calculations button can be clicked at any

time so that, if you wish, you can check each data point as it is entered. Of course, the outputgraphs and output results wont be correct until all the data points are entered.


5/27


6/27

6

2.2 The Mean Power Output versus Mean Wind Speed Results pane.

When the Update power coefficient and mean power calculations button is clicked, the meanpower results are computed for the particular wind speed standard deviation shown in theWind Standard Deviation Input pane. The results are displayed both in tabular and graphicalform. The results are calculated for a range of mean wind speeds from 5 metres per second

to 10 metres per second in 0.2 metre per second increments. This range will cover the vastmajority of sites for which wind power might be considered. If a site has a mean wind speedof less than 5 metres per second, it is not really a viable site for generating wind power.In the graphical presentation, the mean power results are shown along with the turbine datafrom the power curve table in a steady wind. In general, the comparison shows that the meanpower output is greater than the steady wind output at the lower mean wind speeds but lessat the higher wind speeds (i.e. above about 8 metres per second).

If your local wind conditions have a different standard deviation from the reference case, usethe scroll bars in the Wind Standard Deviation Input pane to adjust the value. As the value ischanged, the mean power results will alter to reflect the change in standard deviation seenext section.

The mean power results are the most significant data to be produced by the program becausethey set the benchmark for assessing both the operational and economic benefit of aninstallation. In this context, it should be noted that the annual energy produced by the turbineis simply the mean power in kilowatts multiplied by the number of hours in a year 8,760.Thus, a turbine producing a mean power of 2 kilowatt will produce 17,520 kilowatt-hours ofenergy per year. For convenience, there is a check box that when selected or deselected willtoggle the results between annual energy output and mean power output.

2.3 The Wind Standard Deviation Input pane.

The mean power from a wind turbine is affected by the extent of the unsteady component of

the wind. The quantitative measure of this unsteadiness is the standard deviation, , which isa measure of the size of these fluctuations. For a natural wind, the histogram of the windfluctuations is not symmetrical but is skewed and the graph in the Wind Standard DeviationInput pane shows the shape of this distribution based on the Weibull equation.

From UK meteorological data, the value of the standard deviation of wind speed variationsrelative to the mean speed seems to be about 0.62 (corresponding to a Weibull shape factor,k, of 1.67) but it seems to be industry practice to calculate mean powers and energy outputson the basis of a value of k = 2. This is called the Rayleigh distribution and corresponds to astandard deviation that is 0.52 of the mean wind speed. This has been set as the defaultvalue but it can easily be altered using the standard deviation scroll bars in the Wind StandardDeviation Input pane. The allowed range is from 0.2 to 1.0, which comfortably exceeds anypractical values that will be encountered.

For those sites in open but undulating terrain, there will rarely be any need to change muchfrom the default value but, for small-scale turbines in an urban environment, the wind speedstandard deviation might be as high as 80%-90% corresponding to a Weibull shape factor inthe range from about 1.25 to 1.1. At the other end of the scale, there are sites in desert areasor island sites in the trade winds like Cape Verde where the wind variations are smaller at

around 40% - 45% corresponding to shape factors from around 2.7 to 2.4.

In order to gain some idea about the sensitivity of the results to standard deviation variations,it is interesting to scroll over a range of values to see the effect this has on the mean poweroutput. As the standard deviation is reduced (i.e. the wind becomes steadier), the meanpower output will approach nearer to the steady wind power output. As the unsteadinessincreases, the overall differences will increase. Figure 3 below shows the effect on the meanpower output of the Evance R9000 turbine of changing the standard deviation from 52% to80% of the mean wind speed. This would correspond to a turbine in a very turbulent urbanenvironment where the mean wind speed would, in any case, be quite low probably less


7/27

7

than 5 metres/second. As discussed on our website, small turbines in an urban environmentare almost never a worthwhile proposition.

Figure 3. The effect on the mean power of changing the wind standard deviation.

For those interested, the Weibull shape parameter, k, and the Gamma function for (1+1/k) areshown in the Wind Standard Deviation Inputpane.

2.4 The comment box.In the centre of the opening form is a scrollable text box into which details about a turbine andits associated data can be entered. Such comments might include the manufactures websiteand important mechanical features of the turbine. Comments on the origin of the power curveand its likely reliability can also be useful. Any comments are saved with the Save command

2.5 Saving, loading and printing the mean power results.

2.5.1 Save Options.

If you click on the menu item Save Options, you will be offered the two choices describedbelow

2.5.1.1 Save turbine power data only.

Having manually entered the wind turbine power curve data, this can then be saved for lateruse. From the Save Options, select Save turbine power data only. A standard dialog boxopens and you can select a file name and location to store the data. The data is stored as asimple text file (see Appendix B) with the extension .powso that it can be recognised as adata file for the WindPowerprogram. The mean power data is not stored in this file.

2.5.1.2 Export power curve and mean power data as a csv file.

A user of WindPower might wish to carry out further analysis of the data or to plot the data indifferent ways using spreadsheets like Microsoft Excel. In order to do this, the data (both thesteady and mean power data can be saved as a comma-separated-variable file (a csv file).Such a file can be opened directly in Excel where its format will be obvious although thespreadsheet column widths will have to be adjusted to see all the data clearly. However,having done this, the data can be saved in Excel format and all the facilities of Excel are thenavailable for further analysis or data presentation.


8/27

8

2.5.2 Load turbine data.

Having saved a power curve file, it can then be reloaded with the Load turbine data option onthe menu bar. A standard dialog box opens and files with the .powextension listed in thedirectory where the original files were saved. Additionally, the WindPowerwebsite contains adatabase of turbine power curves and these are downloadable from the site.

2

2.5.3 Print options.

For hard copies of the wind turbine data, two options are available from the Print options itemon the menu bar. The first is a printout of the turbine power curve with its power coefficientdata and the second is a hardcopy of the mean power results.

3.0 Finance options.

The main menu contains a Finance option with three sub-options, namely(i) calculation of the ratio of the total returns from electricity produced by a wind

turbine to the total costs of the turbine.(ii) calculation of the payback period(iii) cost per kilowatt-hour of the electricity produced by the turbine

The first two of these are the more important from the point-of-view of an investor wishing tomake a decision about the financial viability of an installation. In both cases, they require aninput of the price received and/or money saved from the production of electrical energy.Because of the wide variety of incentive schemes in operation in different countries andstates, it is impossible to include all of them in a single program. Instead, the effective price ofthe electricity produced by a turbine is represented by a single figure. For large turbines, thisreference price would be simply the amount that the generator received from the powercompany for electricity exported to the grid plus any incentive payments received from thegovernment or state. For the smaller turbine, the incentive schemes might be morecomplicated. Most schemes give a payment for each kilowatt-hour of electricity generated but,beyond this, there might be additional payments for electricity exported to the grid. On theother hand, if the electricity is used locally, no additional payment is received but, of course,the generator saves the price of the electricity that would otherwise be imported from theutility company. Thus, the reference price could be the price paid for each unit generated pluseither the export tariff or the utility supply tariff. By using both values, the results would giveupper and lower limits on the financial viability of a scheme. Appendix F describes a simplebasis on which the ratio of locally used energy to total energy produced can be estimated.

The other pieces of information needed are the cost of the turbine including installation costsand annual maintenance costs. The website www.wind-power-program.comgives generalguidance about turbine costs but for a more accurate assessment of the financial viability of ascheme, it would be necessary to get up-to-date estimates from a turbine supplier.

2If a wind turbine is not in the database, any purchaser of the WindPowerprogram can have

a pow file put together for them as part of the purchase arrangement. Details of how to goabout this are given in appendix B.


9/27

9

3.1 Total returns to total cost ratio.

The total return on the investment option is selected from the menu bar select Financeandthen Total return/Total cost.The form shown in figure 4 opens.

Figure 4 The returns to cost ratio form (blank)

The name of the turbine and the standard deviation of the wind speed are automaticallybrought forward from the basic data entry page along with the mean power results.

Turbine and installation costs are entered into the appropriate text boxes. Any grants can bededucted from the turbine costs if so desired. Annual recurrent costs such as maintenance

charges can also be entered. For a small turbine, the maintenance costs are usually smallcompared with the fixed costs but the option to include them is there. It is one of the featuresof wind power that the main costs are committed at the start and, unlike fossil fuel or nuclearpower stations, subsequent running costs are relatively small. It is important to exercisecare in entering the installation and other costs because these numbers will be large(in the thousands for smaller turbines and millions for the largest ones) and it is veryeasy to enter a digit too many or too few! It should be noted that only numericcharacters can be entered. Anything else will cause a warning message box to bedisplayed.

After entering the turbine costs, the Calculate (Total Return)/(Total cost) button should beclicked to calculate the returns ratio.

The total return on the investment divided by the total cost is given by

m m

365 24 Lifetime(years) P (U ) TTotal return=

Total cost Turbine and other fixed costs+(Annual recurrent costs Lifetime)ref

where Pm(Um) is the mean power in kilowatts at the mean velocity U mand Trefis the referenceprice per kilowatt-hour in the units of the installation and maintenance costs. This ratio isdisplayed both in tabular and graphical form as a function of the mean wind speed.

The scroll bars can be used to alter both the turbine lifetime and the reference price of akilowatt-hour of electricity. The default value of the turbine lifetime is 20 years but it can be


10/27

10

changed from anything between 5 and 50 years. The default value for the reference price ofthe generated electricity is 10 monetary units per kilowatt-hour but this can be varied between1.0 and 99.9. This should cover incentive schemes in the common currencies of US/eurocents or UK pence and many other currencies as well. It is important to note that themonetary units of the price per kilowatt-hour are 1/100

thof the units used for the

turbine, installation and annual recurrent costs i.e. if turbine costs are in dollars, thereference price per kilowatt-hour for generated electricity will be in US cents.

If the returns ratio is less than one then the investment would result in an overall loss.

Below in figure 5 is an example of the display for the Evance R9000 metre turbine. Theturbine cost is taken as 25,000, installation costs as 2,000 and annual maintenance costsas 250. These figures should be treated as approximate. The turbine lifetime has been left atits default value of 20 years but the reference price has been adjusted to 33.3p per kilowatt-hour. This is based on the feed-in tariff in the UK that pays of 26.7p per kilowatt-hour for everykilowatt-hour generated for turbines between 1.5 and 15 kilowatts. In addition, a further 3p perkilowatt-hour is paid for all electricity exported to the grid but, if the electricity is used locally,this results in an additional saving of about 13p per kilowatt-hour for displacing utility companyelectricity by that generated by the turbine. The balance between electricity exported and thatused locally is discussed in appendix A where an example is given in which 26% of theelectricity is used locally and it is shown that this is equivalent to an overall tariff of 33.3p perkilowatt-hour.

Figure 5 The returns to cost ratio form (completed)

Apart from maintenance, the investment in a wind turbine is essentially an initial up-frontinvestment. It can be compared with making the investment in a straightforward compoundinterest savings account for which the returns to cost ratio is simply

(1 )Np+ wherep is the interest rate and Nis the number of years that the investment is held. Theequivalent percentage interest rates corresponding to the returns to cost ratios are alsoshown in the table. This gives a good indication of the financial merit of the investment.

As with the other finance options, the returns ratio is a basic calculation that takes no inbuiltaccount of interest payments that would be incurred if the turbine installation were financedwith, say, a bank loan. In such cases, it would necessary to carry out a more careful cash flowanalysis than is possible in a generalised program like WindPower. Nonetheless, if the


11/27

11

equivalent interest rates in the table were somewhat greater than the bank interest rate, itwould indicate that a scheme was financially viable whereas, if they were less, then it is mostlikely that the scheme was not a financially viable one.

3

The form can be cleared for new data by clicking the Reset button.

3.1.1 Print the returns ratio.

If a hard copy of the returns ratio results is required then this can be printed by clicking on thePrint (Total Returns)/(Total Cost) item on the menu bar.

3.1.2 Export returns ratios and mean power as a csv file.

The returns ratio data and mean power data can be exported as a csv file for use inspreadsheet programs like Excel. The spreadsheet column widths will have to be adjusted inorder to see the table contents. The format of the data can be adjusted and the file can thenbe saved as an Excel file.

3.2 Calculating the payback time.

The WindPowerprogram also provides an option to estimate how long it will take to recoverthe cost of an investment in a wind turbine. The expression used for payback period is

m m ref

m m ref

Turbine and other fixed costsPayback period (years)=

Annual recurrent costs365 24 P (U ) T 1

365 24 P (U ) T

where, as before, Pm(Um) is the mean power in kilowatts at the mean velocity U mand Trefisthe reference price per kilowatt-hour in the units of the installation and maintenance costs. Itis important to stress that the installation and other fixed costs are treated as upfront costswhereas, for example, if the installation costs were funded through a bank loan, therepayments would be spread over a period and a proper cash flow forecast would have to beproduced to determine the economics of an installation. However, by treating all theinstallation costs as upfront costs gives the most pessimistic estimate of payback period.

By clicking on the menu item Payback period, a new form opens as shown in figure 6 below.

Figure 6. The payback period form (blank).

3The turbine and installation costs could be replaced by their basic cost plus interest

payments. However, treating the interest payments as an upfront cost will give a pessimisticimpression of the financial viability of a scheme.


12/27

12

As with the return on investment form, the name of the turbine and the standard deviation ofthe wind speed are automatically brought forward from the basic data entry page along withthe mean power results.

Once again, turbine and installation costs are entered into the appropriate text boxes along

with annual recurrent costs. From the above expression, it should be noted that if an annualrecurrent cost is entered that is greater than the cost of the electricity produced in a year, thepayback period becomes infinite! No commercial turbine is going to have recurrent costs ofthis size but, if an incorrect figure is entered by mistake that leads to this error, the programwill label the payback period as Never! and colour the label pink. If the maintenance costsare 70% of the production costs, the payback labels will be coloured pink to warn the userthat there is probably an error in the recurrent cost. Finally, if the payback period is greaterthan 50 years, a label will be shown >50 years. An upper limit of 50 years is set both on theplotting and printing displays. Generally speaking with realistic data, none of these limits orconditions should be invoked. A turbine that had a payback period for 50 years wouldcertainly not be a viable proposition.

As far as the reference cost per kilowatt-hour is concerned, this can be adjusted with thescroll bars. The default value is 10.0 monetary units but, as already mentioned, it can bescrolled over a range from 1.0 to 99.9 monetary units. This should cover all reasonable costsper kilowatt-hour in the common currencies of US/euro cents or UK pence.

Figure 7 shows an example of a completed payback form after clicking on the Calculatepayback period (years) button. In addition to the payback period, the table also lists theannual income plus savings generated by the turbine. This is simply the reference price perkilowatt-hour times the annual energy production in kilowatt-hours.

Figure 7. The payback period form (completed).

3.2.1 Print payback periods.

If a hard copy of the payback period results is required then this can be printed by clicking onthe Print payback periods item on the menu bar.


13/27

13

3.2.2 Export payback periods and mean power as a csv file.

As with the other forms, the payback periods and mean power data can be exported as a csvfile for use in spreadsheet programs like Excel.

3.3 Calculating the cost per kilowatt-hour.

The returns ratio and payback period are the important financial calculations for anyoneinterested in investing in wind energy. However, some may also be interested in the intrinsiccost per kilowatt-hour of wind turbine generated electricity. Accountants have variousmethods for making such calculations but the estimate in the present case is obtained simplyfrom the total cost of installing and running the turbine divided by the number of kilowatt-hoursgenerated over the turbine lifetime, namely

m m

Turbine and other fixed costs costs+(Annual recurrent costs Lifetime)Cost per kilowatt-hour =

365 24 Lifetime(years) P (U )

where Pm(Um) is the mean power in kilowatts produced at a mean wind Um. The factor(365x24) just converts the lifetime in years into the lifetime in hours. Once again, if interest

charges are ignored, this will lead to a low estimate of the cost per kilowatt-hour and, in mostanalyses of costs from various power sources, it is usual to include interest payments in someway. A simple approach is to assume that a loan is taken out for the turbine and its installationand then calculate the overall cost of repaying this sum with interest payments over thelifetime of the turbine. Appendix C describes how this calculation can be done.

By clicking on the menu item Cost per kilowatt-hour, a new form opens as in figure 8 shownbelow.

Figure 8. The cost per kilowatt-hour form (blank)

The name of the turbine and the standard deviation of the wind speed are automaticallybrought forward from the basic data entry page along with the mean power results.

After turbine, installation and recurrent costs are entered, the Calculate costs per kilowatt-hour button is clicked. Figure 9 below shows the costs per kilowatt-hour based on these basic


14/27

14

costs. Of course, if other costs like interest payments for, say, financing the purchase of theturbine were included, the costs per kilowatt-hour would be higher still. Appendix C containsformulae and a graph of the interest paid on a reducing loan and, if the turbine and installationcosts were financed by a bank loan with an annual interest charge of 5%, the total amountpaid over twenty years would be 40,000 for an initial loan of 25,000. Although this is onlyan example, it demonstrates why small turbines only become a worthwhile investment if thereare fairly generous feed-in tariffs to support them. Very larger turbines of the sort used in

commercial wind farms produce electricity at a far more competitive rate.

Figure 9. The cost per kilowatt-hour form (without interest payments).

3.3.1 Print prices.

If a hard copy of the costs per kilowatt-hour form is required then this can be printed byclicking on the Print prices item on the menu bar.

3.3.2 Export prices and mean power as a csv file.

Once again, the cost per kilowatt-hour and mean power data can be exported as a csv filefor use in spreadsheet programs like Excel. As already mentioned, the spreadsheet columnwidths will have to be adjusted in order to see the table contents. The format of the data canbe adjusted and the file can then be saved as an Excel file.

4.0 The wind turbine power profile.

Unlike conventional power stations, wind turbines have a power output that is not under thecontrol of the operator. It is therefore of some interest to know what proportion of time thewind turbine produces different levels of power. This information can be obtained by clickingon the menu item Power-output profile when the following form will appear.


15/27

15

Figure 10. The opening power profile form.

This form displays the percentage of time that the wind speed lies between two selectablevalues. This is a function only of the two speeds and the standard deviation of the unsteadycomponent of the wind. However, by displaying the steady power curve for the particularturbine, it is possible to select wind speed ranges that correspond to a specific range of powervalues.

In the opening default form, the speed range is set from zero to the cut-in speed of theparticular turbine. The graph on the right displays the percentage time for which the turbineproduces no power at all as a function of mean wind speed. As can be seen from the examplein figure 10, this Evance turbine would produce no power at all for about 25% of the time if the

mean wind speed were 5 metres per second. However, at 8 metres per second, this hasfallen to around 10% of the time.

By contrast, it might be of interest also to know what proportion of time the turbine producessome higher output level say 4 kilowatts in this case. In this case, the upper scroll bar in theright hand pane is adjusted to the power output of 4 kilowatts. The second scroll bar controlsthe extent of the speed range and is adjusted so that the upper speed limit is equal to the cut-out speed or, as in this case when there is no cut-out speed, 30 metres/second. Figure 11shows the results of these adjustments. It should be noted that the scroll bars change theirspeed values in steps of 0.1 metre/second.

In this example, the right hand graph shows that at a mean wind speed of 5 metres persecond, the percentage of time that the turbine produces 4kW to 5kW is only about 4% of thetime whereas at 8 metres per second, this percentage has risen to around 30% of the time.

Installers particularly of these smaller domestic turbines might find it helpful to explain to apotential customer these proportions of time so that the client fully understands theintermittent character of the power generated and is not therefore surprised or disappointedby, say, the proportion of time that the turbine is stationary.


16/27

16

Figure 11. The power profile form set for the rated power speed range.

4.1 Print.

A hard copy of any of the percentage time data can be obtained by clicking on the menu itemPrint.

5.0 Ratio of local energy used to total energy produced.

When the Local energy usage menu item is selected, the following form is displayed figure12.

Figure 12. The power cumulative probability and the level of local energy usage.


17/27

17

The graph on the left is the cumulative power probability distribution and shows the proportionof time that the wind turbine spends delivering power below some reference power level, P,as a function of the mean wind speed. As discussed in appendix F, it is possible from thiscumulative probability distribution to calculate the ratio of the energy used locally to the totalenergy produced as a function of the local power usage and the mean wind speed. The graphon the right shows this relationship.

As a simple example, if the turbine was connected to, say, a house using one kilowatt ofpower and where the mean wind speed was 6 metres/second, the graph shows that the ratioof energy used locally to the total energy produced is 0.41. With this ratio, the reference priceof the electricity to be used in the financial calculations would be

Generation tariff + 0.41 x Utility company tariff + (1- 0.41) x Export tariff

Using a generation tariff of 26.7p per kw-hr, a utility company tariff of 13p per kw-hr and anexport tariff of 3p per kw-hr gives a reference price for the turbine power of 33.8p per kw-hrcompared to the case of 29.7p if all the electricity was exported to the grid. This can have asignificant effect on the financial viability of a scheme and so it is important to obtain someestimate of local usage.

Of course, a house or business does not use power at a constant rate and, in appendix F, it isshown how to calculate the local usage rate when the local power consumption is varying.However, in most cases, a good estimate of the local usage can be obtained by using themean local power usage.

6.0 Other features of the program.

The wind speed units throughout the program are in metres per second. However, by clickingon the menu item Units converter, a simple conversion chart is displayed that givesconversions from metres per second into knots, mph or positions on the Beaufort scale.

To ensure commonality of the data used in the forms, if any controls or menu items areclicked on the main form when another form is open, this latter form will be automaticallyunloaded. By the same token, the program only allows one of the supplementary forms to beopened at anytime.

To close any form, just click on the standard Windows close button on the top right handside of each form.

When you opt to close the main form, you will be prompted with a reminder to save any datathat you might want to use again before the form finally unloads.

7.0 Vertical axis wind turbines.

The WindPower program is aimed essentially at horizontal axis turbines (HAWTs) and so onlya horizontal axis rotor diameter is available as an input characteristic of the turbine and theBetz limit -whose derivation assumes a horizontal axis turbine - is used as a check on theturbine power coefficients. However, vertical axis wind turbine (VAWT) power curves can beinputted into the WindPowerprogram but some equivalent rotor diameter will have to becalculated. Appendix D outlines some ways of doing this but it is worth noting that the powerclaims of many small VAWTs are clearly exaggerated and so need to be treated with caution.The introduction of certification schemes like the UKs Microgeneration Certificate Schemewill do much to prevent outlandish claims for turbine performances being made.


18/27

18

APPENDIX A.Examples of wind turbine data needed to use the WindPowerprogram.

(All data is for illustration only)

A domestic wind turbine Skystream 3.7m 1.9kw.Rotor diameter = 3.7 metres.

Cut-in speed = 3 metres per second : Cut-out speed = 25 metres per second.

Estimated lifetime = 20 years.Estimated turbine cost (no interest payments or grants) = 7,000Estimated installation costs (no interest payments or grants) = 1,500

Estimated annual maintenance costs = 200Reference price of electricity = (Generation) + (Export) = 26.7 + 3 = 29.7p

Skystream 3.7m 1.9kw Power Data

Steadywind

speed(m/s)

Poweroutput

(kilowatts)

Steadywind

speed(m/s)

Poweroutput

(kilowatts)

1 0 16 2.400

2 0 17 2.300

3 0 18 2.265

4 0.084 19 2.203

5 0.203 20 2.2036 0.391 21 2.203

7 0.643 22 2.2038 0.968 23 2.203

9 1.333 24 2.203

10 1.748 25 2.203

11 2.106 26 0

12 2.301 27 0

13 2.403 28 0

14 2.425 29 0

15 2.414 30 0

A large commercial wind turbine General Electric 100m 2500kw.Rotor diameter = 100 metres.

Cut-in speed = 3.5 metres per second : Cut-out speed = 25 metres per second.Estimated lifetime = 20 to 25 years.

Estimated turbine cost (no interest payments or grants) = 1,500,000Estimated installation costs (no interest payments or grants) = 500,000

Estimated annual maintenance costs = 25,000Reference price = 4p(Utility tariff) + 4p (ROC) = 9p

General Electric 100m 2500kw power data

Steadywind

speed(m/s)

Poweroutput

(kilowatts)

Steadywind

speed(m/s)

Poweroutput

(kilowatts)

1 0 16 25002 0 17 2500

3 0 18 2500

4 49 19 2500

5 184 20 2500

6 390 21 2500

7 652 22 25008 972 23 2500

9 1368 24 2500

10 1865 25 2500

11 2241 26 0

12 2432 27 013 2500 28 0

14 2500 29 0

15 2500 30 0


19/27

19

APPENDIX B.Obtaining a .powfile from PelaFlow Consulting.

If a turbine manufacturer or supplier provides power curve details in a tabular form then it isstraightforward to enter the details into the WindPowerprogram. In many instances, thepower curve is given only in a graphical form and it can be a little time-consuming to extract

reasonably accurate numerical values of the power curve from a graph. Pelaflow Consultinguses a graphics software package to convert graphical data into a numerical file and anypurchaser of the WindPowerprogram is entitled to receive a single conversion from a graphto a .pow file for a commercial turbine. In order to do this, details of the turbine need to besubmitted to us including website details if available. It should be noted that manufacturers ofsmall turbines do not always have a power curve for their own product. Nevertheless,provided some broad details are available, it is possible to construct a power curve that will beaccurate enough to give good estimates of the mean power produced by the turbine.

As many details of the turbine and the purchase order number should be submitted by e-mailto

[email protected]

It is, of course, impossible for Pelaflow Consulting to provide a power curve data file if certainbasic data cannot be obtained.

Any data file produced will be added to the general database that can then be accessed byother users.


20/27

20

APPENDIX C.Calculating the total cost of a loan.

If the installation of a turbine is financed through a loan then the total cost of the installationshould include the interest payments in the installation costs. For a straightforward repaymentloan extending over N years and with a fractional annual interest rate of p, the annualrepayment rate is given by

( )( )

+=+

N

N

0

p 1 pXY 1 p 1

where Y0 is the initial loan.

The total cost of the loan is simply NX and the figure below shows the ratio of the total cost ofthe loan to the initial loan as a function of annual interest rate and for a range of repaymentperiods from 10 to 25 years.

If you want to know the loan outstanding after n years, this is given by

( )

= + +

n

n 0

X XY 1 p Y

p p


21/27

21

APPENDIX D.Using the WindPowerProgram with

Vertical Axis Wind Turbines (VAWTs)

One of the aims of the WindPowerprogram was to provide wind turbine installers, users andconsultants with a means of checking on the plausibility of performance claims made byturbine manufacturers. The program was essentially aimed at horizontal axis wind turbines

(HAWTs) and one of the important checks on performance figures is the efficiency (oftenreferred to as the power coefficient cp) of the turbine.

p 23

Power output(watts)C

1 DU

2 4

=

where is the air density in kg/m3, U is the wind speed in m/sec and D is the turbine diameterin metres.

In the case of HAWTs, there is a well-known upper limit on this efficiency (known as the Betzlimit) of 16/27 or 59%. In practice, very large turbines can achieve peak efficiencies ofbetween 40 45% but smaller turbines are likely to have much lower peak efficiencies ofbetween 20 35%. In the WindPowerprogram, any power that a user tries to input into theprogram that leads to an efficiency of greater than 59% is blocked and a warning message

box is displayed showing the maximum power that is possible if the Betz limit is not to beexceeded.

In the case of VAWTs, it is less obvious how to check for the plausibility of the manufacturerspower claims because it is not clear how to apply the Betz limit to them. This is because thereis as yet no clear way of determining the equivalent radius of a VAWT from its cross-sectionalarea. The most obvious starting point is simply to calculate the diameter of the circle whosearea is the same as the VAWT cross-section area. Thus,

VAWTequivalent

4 AD

=

whereVAWT

A is the cross-sectional area of the vertical axis wind turbine.

If usingequivalentD as the wind turbine diameter leads to efficiencies that are still greater than

59% then it would be wise to treat the power data with some suspicion as being far toooptimistic. However, if a user still wanted to enter power values into the program withoutbeing blocked by the Betz limit criterion, it would be necessary to increase the equivalentdiameter further, say, to twice the VAWT cross-sectional area i.e.

VAWTequivalent

8 AD

=

If, using this diameter, the Betz limit was still exceeded then there is little doubt that the powercurve is wrong and any calculations about mean power, annual energy output and financialcalculations like the payback time would be inaccurate and misleading.


22/27

22

APPENDIX E.UK Feed-In Tariffs for Wind Power.

In order to encourage investment in renewable energy, feed-in tariffs are paid to users of windturbines. There are two tariffs, namely, (i) a generation tariff which is a sum paid to the windpower producer for every kilowatt-hour of energy that is generated (ii) an export tariff which isa sum paid to the producer for every kilowatt-hour of energy that is exported to the grid. The

tariffs are only guaranteed at the moment up to 31

st

March 2013. However, anyone installinga wind turbine, say, in 2010/2011 will receive the tariff for that year for the next twenty years.

UK Generation Feed-In Tariffs (Pence per kilowatt-hour)

Power rangeFrom

1/4/2010 to31/3/2011

From1/4/2011 to31/3/2012

From1/4/2012 to31/3/2013

Tarifflifetine(years)

Less than or equal to 1.5 kW 34.5 34.5 32.6 20

>1.5 kW to 15 kW 26.7 26.7 25.5 20

>15 kW to 100 kW 24.1 24.1 23.0 20

>100kW to 500 kW 18.8 18.8 18.8 20

>500 kW to 1500 kW 9.4 9.4 9.4 20

>1500 kW to 5000 kW 4.5 4.5 4.5 20

UK Export Feed-In tariff = 3 Pence per kilowatt-hour


23/27

23

APPENDIX F.Estimating the ratio of local energy used to total energy production.

For turbines connected to a house or business, a critical factor in their financial viability is theproportion of the electricity generated that is used locally compared to that which is exportedto the grid. In the UK, for electricity used locally, the value of each kilowatt-hour of electricityproduced is the generation feed-in tariff (26.7p for turbines under 15kW) plus the amount

saved by not using electricity supplied by a utility company (about 13p per kilowatt-hour in2011 and likely to rise). By contrast, for electricity exported to the grid, the user receives thegeneration tariff plus a further 3p for each unit exported. Thus, the effective value of electricityused locally is about 39.7p per kw-hr as against 29.7p per kw-hr for exported energy. It istherefore of some consequence to be able to estimate how much electricity is used locallyand how much is exported. This appendix provides a simple means of doing this.

Using the Local energy usage menu option in the WindPowerprogram, it is possible to plotthe fraction of time that the wind turbine spends producing between zero and some other levelof power, P. As an example, the figure A1 below shows the results for the Evance R9000turbine for five values of the mean wind speed.

Figure F1 Cumulative power probability function for an Evance R9000

This plot is, in fact, the cumulative probability function for the power output of the turbine sothat the mean power produced by the turbine at different mean wind speeds is simply thearea under these curves.

Let us consider a curve for a single mean wind speed of 6 metres/sec. Suppose now that anoperator is using locally some level of power PL= 1 kilowatt as shown in the next figure F2.The ratio of local to total energy usage is simple the area shown in grey compared to the areaunder the whole curve - the grey area plus the pink area. The pink area is the exportedenergy proportion.


24/27

24

Fig F2 Local power usage at 1 kilowatt and 6 metres/second

The details of the calculation of the areas under these curves is not relevant to the use of theinformation so only the results will be shown. The figure F3 below shows the ratio of localenergy usage to total energy production for a range of mean wind speeds and as a function ofthe local power usage level. For a local power usage of 1 kilowatt and a mean wind speed of6 metres/second, the ratio would be 0.41. As already illustrated in section 5, this can be usedto adjust the reference price per kilowatt-hour of the electricity produced by the turbine.

Fig F4 The ratio of local energy usage to total energy

In reality, the variation of local power usage may fluctuate significantly with time and so it isappropriate to see what effect this has on the overall ratio of local usage to total usage.


25/27

25

As an illustration, the figure F5 below show an imaginary breakdown against time of localpower usage in a home over a twenty-four hour period. It consists of 8 hours when there is nolocal usage say, during the night. There is then 8 hours of 1 kilowatt usage, 4 hours of 2kilowatts and 2 hours of 4 kilowatts. The average power usage is still 1 kilowatt.

Fig F5 Pattern of local power usage against time

The pattern of power consumption is listed below along with the ratios of local to total powerusage obtained from figure F4 and a calculation of the average value of the ratio of localenergy usage to total energy production.

Time in a day, Thours

Local powerusage

(kilowatts)

Ratio of local tototal usage, R

R x Thours

8 0 0 0

8 1 0.41 3.28

6 2 0.67 4.02

2 4 0.93 1.88

Sum of R x T = 9.18

Average ratio of local to total energy usage = 9.18/24 = 0.382

The calculation of the average ratio of local to total usage turns out to be 0.382 comparedwith 0.41 obtained by just using the average local power usage. This demonstrates that usingthe average local power usage will generally be of sufficient accuracy for correcting thereference price of the generated electricity and that there is little to be gained by doing moreelaborate calculations using a detailed pattern of local power usage. However, as has beendemonstrated, the more elaborate estimate is not difficult to obtain either.


26/27

26

APPENDIX G.Modifications to power curves in the calculation of local power usage.

Wind turbine power curves for medium to large turbines are almost always like the one shownbelow in figure G1. They are characterised by a power control mechanism that results in aconstant power output beyond the so-called rated wind speed.

Figure G1 Schematic of a wind turbine power curve.

For power curves of this sort, it is straightforward to compute the cumulative powerprobability distribution and, from this, to compute the ratio of the local power usage to the totalenergy produced as outlined in appendix F.

For small turbines, the power control mechanisms are less sophisticated. A number of smallturbines use a tail fin mechanism to turn the turbine away from the wind direction at high windspeeds and, as a result, their power curves are more irregular. An example of the effect of a

tail fin furling mechanism on a power curve for a small Bergey turbine can be seen in thefigure G2.

Figure G2 The actual and modified power curves for a Bergey Excel-S

For reasons connected with the numerical integration, it is computationally messy to carry outthe numerical integration in these cases and so the simple expedient has been adopted ofreplacing the actual power curves with power curves that have the same maximum power as


27/27

the actual one but thereafter have a constant power level up to some cut-out speed which isfixed to produce the same mean power as the original power curve at 10 metres/second. Thisequivalent power curve is shown in figure G2. The effect this has on the calculation of thelocal energy usage is negligible for all practical purposes but, where this has been done, anote is added to the left hand graph in Local Energy Usemenu option.

Documents

Userguide for V2.01c