How much electricity do small wind turbines generate? The Small Wind Turbine Yield Estimator - A simple tool for yield estimation

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1 How much electricity do small wind turbines generate? The Small Wind Turbine Yield Estimator - A simple tool for yield estimation Paul Kühn Institut für Solare Energieversorgungstechnik, Germany Abstract A good estimate of the output is essential for the planning and realization of a small wind turbine project. But yield estimation of a small wind turbine is not a straightforward matter. This paper discusses the challenges of predicting the performance of small wind turbines and presents the Small Wind Turbine Yield Estimator a free, easy-to-use spreadsheet tool for estimating the yields of small wind turbines. Keywords: small wind turbine, SWT, yield estimation Introduction Playing with the idea of installing a small wind turbine (SWT) raises a number of questions: What is the wind speed at my site? What effects have turbine size and tower height on the kilowatt-hours to be generated per month or year? Shall I put one big turbine on a high tower or two smaller machines on lower towers? Add to this that planning aspects like building regulations, acceptance, safety, and noise emissions can pose restrictions to a SWT project, affecting the amount of available wind energy to be harvested. In other words, planning a SWT project and predicting its performance is a complex and challenging job. This is illustrated in Table 1. It shows typical tasks when planning a SWT and for reasons of comparison, when planning a photovoltaic system. A solar resource, for example, is generally easier to assess than a local wind regime. Furthermore, the developer of a SWT project is confronted with a complex SWT market offering a diversity of technical concepts of varying quality. Table 1 indicates that it is generally more complex to plan, install and operate a SWT than a PV-system. However, the technical and economical success of both system types, based on wind or solar energy, depends on how well the aspects in Table 1 are understood and investigated. In the next section a spreadsheet tool is presented that can be used to simplify some of the tasks listed in Table 1. Tel: Fax: pkuehn@iset.uni-kassel.de 1/5

2 Table 1: Key tasks and relevant aspects of planning photovoltaic and SWT projects Task Photovoltaic System Small Wind Turbine Resource Assessment Siting solar radiation module orientation and inclination angle, shadowing effects are visible average wind speed, main wind direction, turbulence, wind shear positioning of tower, effects (wind shadow, turbulence) of obstacles and terrain type are not visible Sizing collector area, peak power swept rotor area, rated power, tower height Choosing Technology Evaluating Operational Aspects module type, inverter no moving parts great variety of technical concepts due to moving parts potential safety risks, emission of noise and vibrations Small Wind Turbine Yield Estimator The spreadsheet tool presented here allows performing yield estimations of SWTs. Depending on the input data, i. e. site data and turbine specifications, the Small Wind Turbine Yield Estimator can be used for various purposes: beginners to wind energy can try different values of input parameters like average wind speed, terrain type, turbine size, and tower height so as to get a feeling on how these factors affect the performance of SWTs, power curves of different SWT models can be added, checked for plausibility and compared to one another, actual yield estimations can be performed, if reliable information on site-, windand turbine characteristics are available. Site data to be used with the Small Wind Turbine Yield Estimator The accuracy of the yield estimation highly depends on the available data of the local wind regime. Because this data is often unknown, the site characteristics e. g. the wind speed must itself be estimated, allowing only very rough estimations of the yield. The average wind speed is the most important parameter for the characterisation of the wind resource and ideally measured at the future site and at the hub height of the planned SWT. Additionally or alternatively to the average wind speed, Weibull parameters can be used with the Small Wind Turbine Yield Estimator. The spreadsheet tool contains a description of different terrain types and corresponding values for the roughness length as well as for the equivalent power law exponent. By selecting from these terrain types the surface roughness and obstacles are considered for, and hence allowing to estimate the wind shear. Tel: Fax: pkuehn@iset.uni-kassel.de 2/5

3 Turbine specifications The diversity of SWTs available and the heterogeneous data provided by manufacturers make a comparison of different SWT models and different sizes difficult. For example, there is no standard definition for rated power, i. e. at which wind speed a SWT is to be rated. In the diagrams of Figure 1 data of about 100 SWTs, having a rated power of up to 20 kw, are presented. The data used in Figure 1 was taken from manufacturers web sites, catalogues and SWT data sheets. Figure 1 (a) depicts the rated wind speeds of the different SWT models. It shows a wide scattering of points for the rated wind speeds, with values mainly between 9 m/s and 14 m/s. Moreover, it shows the trend for vertical axis turbines to be rated at higher wind speeds than horizontal axis turbines. Because of the broad range of rated wind speeds, it is difficult to compare the size of SWTs by their rated power only. The rotor size, measured by the rotor swept area or rotor diameter, is the better choice for comparing the size of SWTs. However, even the comparison of the power performance of SWTs of similar size can be quite difficult. Figure 1 (b) depicts SWT power coefficients at 9 m/s wind speed over the rotor diameter. The scattering of points can be explained by a number of reasons including different technical concepts, turbine designs having different efficiencies of rotor, generator, and gearbox, insufficient test data as well as inconsistent and incorrect methods to measure the power curve. Besides the large range of power coefficients, some turbine data, particularly data of smaller machines with less than 5 m rotor diameter, show inconsistent values, with a few power coefficients even exceeding the Betz limit 1. Rated wind speed m/s (a) horizontal vertical Rated power kw Power coefficient at 9 m/s 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 B e t z l i m i t 0, Rotor diameter m (b) Figure 1: Specifications of about 100 SWTs: (a) Rated power and corresponding rated wind speeds (b) power coefficients at a wind speed of 9 m/s Turbine data to be used with the Small Wind Turbine Yield Estimator The spreadsheet tool uses the rotor diameter to describe the size of a SWT. In case of a vertical axis turbine the equivalent diameter of a horizontal axis turbine, i. e. a circular rotor shape having the same rotor swept area, is to be used. Besides all required site data and the planned tower height, the spreadsheet requires no SWT data other than the turbine size, i. e. the rotor diameter, to perform yield 1 The theoretical maximum power coefficient is called Betz limit. It is the theoretical maximum fraction of the power in the wind to be utilized by a wind turbine and has a value of 16/27 (0,593). Tel: Fax: pkuehn@iset.uni-kassel.de 3/5

4 estimation. For this, synthetic power curves for turbines with rotor diameters between 1 m and 15 m were created of which the user can choose from. The calculations for these synthetic power curves result in an overall conversion rate of about 20 % (at 5 m/s average wind speed and Rayleigh distribution). This means that 20 % of the total available wind energy would be harvested. The synthetic power curves provided with the Small Wind Turbine Yield Estimator have similar performance characteristics, i. e. the same power coefficient at corresponding wind speeds. At 9 m/s the power coefficient has a value of 0,22, compare to Figure 1 (b). Alternatively to the provided synthetic power curves, new performance data, i. e. power curves, may be entered, saved and used with Small Wind Turbine Yield Estimator. However, SWT power curves must be used carefully as the information provided by manufacturers, distributors or other sources might not always be reliable. Output data of the Small Wind Turbine Yield Estimator The yield estimation spreadsheet presents a number of results with special focus on the annual yield of the selected SWT. In addition to the yield at the user-defined hub height, estimations of the average wind speeds and annual yields are calculated for alternative hub heights. An output table and several graphs show the estimations for up to seven heights between 10 m and 40 m, allowing to compare estimations for different possible tower heights. In the following, some example output data are presented for a SWT with a rotor diameter of 3 m and a planned tower height of 18 m at a site with 5 m/s average wind speed measured at 10 m/s above ground 2. Figure 2 depicts the power curve of the SWT. Furthermore it gives the annual number of hours in each wind speed bin based on the estimated wind speed distribution. This allows assessing how many hours in a year the turbine would generate above or below a certain power level, e. g W. Power output W h 417 h 316 h h 597 h h h 816 h h h 956 h h h h h h h 894 h 907 h 584 h 706 h 785 h 386 h 522 h 644 h 236 h 363 h 504 h 134 h 238 h 375 h 71 h 147 h 267 h 35 h 86 h 182 h 16 h 48 h 119 h 7 h 25 h 74 h 3 h 12 h 45 h 1 h 6 h 26 h 3 h 14 h 1 h 8 h 4 h Wind speed m/s Annual operating hours at: 10 m 18 m (user-defined) 40 m Figure 2: Power curve and annual hours in each wind speed bin at three different heights 2 2 input data: average wind speed = 5 m/s measured at 10 m above ground, roughness length = 0,1 m, Rayleigh distribution, air density (standard atmosphere) = 1,225 kg/m 3, hub height = 18 m, rotor diameter = 3 m, SWT power performance characterised by synthetic power curve Tel: Fax: pkuehn@iset.uni-kassel.de 4/5

5 Moreover the hours in special operating modes can be calculated, e. g. standstill due to low winds below 4 m/s or furling at wind speeds above 15 m/s (see Table 2). Table 2: Estimated annual output and annual hours of different operating modes for three different hub heights 2 Hub height Annual output Annual number of hours the SWT is: generating idling furling 1 kw or more 10 m 1800 kwh m 2500 kwh m 3600 kwh Summary This article highlighted challenges when installing a SWT with focus on yield estimation. An easy-to-use spreadsheet tool is presented allowing to perform yield estimations for SWTs. The Small Wind Turbine Yield Estimator is available at a free of charge database for research and education in renewable energies. Use the keywords SWT yield estimation in the Search field for a free download of the Small Wind Turbine Yield Estimator. Tel: Fax: pkuehn@iset.uni-kassel.de 5/5