Recent Researches in Environment, Energy Planning and Pollution

Roughness Parameter of the Land Verification of Values Based on Measurements Obtained Through the Use of Anemometers Poles

IOAN ION, VIOREL CAMPIAN ADRIAN CUZMOS, COSMIN DUMBRAVA Center for Research in Hydraulics, Automation and Thermal Processes – CCHAPT “Eftimie Murgu” University of Resita Traian Vuia Square, No. 14, 320085 Resita [email protected] http://www.cchapt.ro

Abstract: The investment management of a fleet of airelectrical generators, the most important activity that takes place in the opening phase is to assess the wind potential of the area which will have wind turbines. If the evaluation is only based on general information from weather stations, wind speed at heights above calculation is made taking into account the "roughness of the zone characterized by specific parameters recommended by the literature. Based on experimental data, their values can be verified.

Key-words: Wind, wind speed, wind potential, specific energy / power of the wind, graphic variation, roughness parameter

1 Introduction. General Specifications which will be the cost price (production) of Every project involves risk. In large projects, such electricity when connected to the download as the one seeking to achieve a park of wind network? generators, the risk should be minimized. In fact, the which is the average lifespan of the turbine itself project of a wind turbine park, involves both certain and related facilities, thereby being able to factors and elements that are characterized as safe, estimate the rate for the investments recovery but also factors which by definition are and its economic profit? unpredictable and can lead to positive results, but The safety of the response to the first question is also failures. in inverse correlation with the risk assumed under Most technical products have a behavior that can the project to build a wind generator park. be anticipated with high probability. Since their Evaluation of wind potential, in a point or an area design phase (even experimental models and can only be based on the measurement of prototypes) their technical characteristics can be parameters (factors) of those atmospheric spots. estimated with great precision because, in general, The values of atmospheric parameters can be the operation does not depend only to a limited obtained from weather stations (if they exist in the extent by climatic conditions. area) an indication by the websites which provide In the case of the generators in question, they information on movements of air masses and/or have wellestablished technical characteristics, but using the facilities and specialized equipment for the electricity they produce is entirely dependent on monitoring the atmosphere and which are mounted atmospheric stability and wind speed in particular. on special poles with different heights. We can say with certainty that the risk management, Background difference between data recorded at in this case involves a thorough assessment of weather stations and those obtained from potential wind measurements consist of the following aspects: The intention of investment in wind energy at the from weather stations single values are initial stage of implementation, must find the answer obtained for size and wind direction. In to some questions: addition, generally anemometers from weather which is the wind energy potential in the future stations are placed at low heights, of several location of wind turbines? meters, which make wind power to be impacted how much electricity will be produced in a time (reduced) significantly from the roughness of by a single plant (elected) whose technical and site. Until the use of modern equipment, wind structural characteristics are known? direction and intensity values are read from a certain period by time, mediation was made somehow subjective, there are no recorded data

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base for fixed time intervals, allowing a relative air humidity in [%]; calculation and hence an objective assessment . air temperature [o K] × 10; Obs. For a general estimation, to conduct tests for Atmospheric pressure [hPa]; determining the place of the arrangement of Storing data in columns was made with a Data anemometric columns maps with weather Logger 32 and was used for transmitting mobile information on the intensity of the wind are more telephone network. than welcome and necessary. Downloading data was done daily for all the anemometric pillars (specialized) may have columns, followed by validation and processing several anemometers at heights preestablished, values. with which the wind force can get the Each data file with file extension. row (for each measurements averaged over the desired time day and location) was transformed into an Excel file intervals (preestablished); to validate both the performance and subsequent disposal sites in the field, the anemometric poles calculations. can be chosen depending on the land It is noted that by setting the equipment was configuration, by access roads, electricity intended and achieved an average of values for a transmission networks and distribution of period of 10 minutes per day resulting in 144 lines existing and especially curves (based on the of eight fields containing values of parameters existing primary information’s) for the existing tracked. potential in the area . In Figures 1 and 2 were presented the measured The county of CarasSeverin, integrating the values (in m/s) of wind speeds (at h=50m) for all 10 objectives and priorities of the National locations (July 2009). Development Plan, namely the Joint Programming 12 Document Romania, Serbia, has realized the project viteza măsurată [m/s] "Assessment of wind potential in the south of la h=50m 10 Banat from the perspective of sustainable Bolvasnita development." The project provided specialized Gradinari technical support (as plants and transmission 8 Moldova Noua characteristics of weather values, and pillars) for the Resita Semenic following ten locations: Anina BolvaniŃa, Brebu 6 Nou Buchin, Gradianri, Moldova Noua, Pojejena,

Resita, Semenic and Vrani. Installation of 4 equipment was conducted in summer 2009 and then started collecting the measurements. 2 The lack of equipment with which the monitoring of atmospheric parameters is made, zilele lunii iulie 2009 0 there can be used approximate methods of work 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 which can be determined based on average wind Fig. 1. Speeds measured in 10 locations h = 50m speed chart for the location of interest. Material object is to make a check (and possibly 6 viteza medie măsurată [m/s] a correction) of values by roughness of parameters, 5.167 la h=50m which are presented in the literature, based on actual 5 measurements obtained by using columns 4.024 anemometric. 4 3.882 3.892

3.252 3.12 3.027 3 2.744 2.761 2 The Measuring of Weather 2.376 Characteristics with Anemometric 2 Pillars. Parameters and Units 1 The equipment, which was disposed, the monitoring was done by measuring the weather characteristics 0 as follows: Anina Bolvasnita Brebu Nou Buchin Gradinari Moldova Pojejena Resita Semenic Vrani Noua wind speed at three heights (50, 25 and 15 m) in Fig. 2. Averaged speeds measured in 10 locations [dm / s]; h=50m wind direction at two heights (50 and 25 m) in sexagesimale degrees;

ISBN: 978-1-61804-012-1 217 Recent Researches in Environment, Energy Planning and Pollution

3 Calculations for Wind Potential 3.2.2 Math Computing Assessment The above assumptions can be analytically written as follow:

3.1 Commercial Software ⋅vm 2 E = (1) There are commercial software that can estimate the c 2 wind potential for higher heights then column height (which is properly placed and anemometer). m Vp ⋅⋅=⋅ TR (2) Depending on the results we can estimate the M power/energy and also specific wind generator can be chosen optimally. α v  h    =   (3) v  hrr  3.2 Own Software Based on the reasoning of the underlying where: Ec is air mass kinetically energy; phenomenon which transforms kinetic energy (air m – air mass that spin the turbine runner; masses) into electrical energy, taking into account v – air speed at height h corresponding to the measurements of anemometric columns was turbine shaft; prepared a program that can determine the wind p – measured atmospheric pressure; potential at the desired height (usually the upper V – air volume corresponding to air mass pole height ). that spin the turbine runner; M – air molar mass; 3.2.1 Working Assumptions R – ideal gas constant; The following assumptions were made: T – measured air temperature; the wind exposed area, for which the kinetic vr – reference air speed; energy are computed, is perpendicular by wind hr – reference height; direction; h – turbine shaft height; is considered that the air mass which spin the α boundary layer exponent. generators propeller, have the same velocity (on The coefficient that characterized the exponential entire propeller surface) and is equal with the profile of the boundary layer for a height h > hr = 50 measured or computed (based on extrapolation) m can be computed using the determined for the height at which the rotor shaft are coefficients for height interval that the wind speed installed; was measured (h1 = 15 m, h2 = 25 m i hr = 50 m). the pressure and temperature variation of free The specific power (measured in W/m2) will be air are neglected (between disposal point of computed with the following formula: measuring instruments – approx. 5 ÷ 6 m and height h of the turbine axis); P E P == c (4) humidity influence against air density is sp A ⋅ At neglected (his influence on temperatures lower than 30°C is insignificant); After the replacements and taking in account the is considered that the measured value of M and R values, the specific power math formula velocity and brought into account is the will be obtained, where the measured values for velocity’s vector horizontal component; pressure (hPa) and air temperature (°K) will be the generator impeller has the capacity to take replaced: and send the kinetic energy of the air mass that it strikes. The transmission of the kinetic energy −3 h 1 3 1 10483925,3 ⋅⋅ p 3 and obviously electricity production are made sp ρ vP ⋅=⋅= ⋅ vh (5) 2 2 T with an efficiency. from the thermodynamic point of view, the air is For the wind speed the computed value like a perfect gas; corresponding to rotor shaft height will be accepted. in conformity with international standards the The calculus of this speed will be made using, for boundary layer has an exponentially profile. boundary layer coefficient, the average values of the coefficients resulted (computed based on measurements) from the following two intervals:

ISBN: 978-1-61804-012-1 218 Recent Researches in Environment, Energy Planning and Pollution

between h1 = 15 m and h2 = 25 m respectively INMH, at a height of 10 m) and the average wind between h2 = 25 m and hr = 50 m. velocity for a higher heights will be computed. The subject of this paper is to verify the value of α roughness parameter based on measurements  h    vh vr ⋅=   (6) obtained from anemometric poles.  hr  The input data in this case are wind average where: velocities for heights of 15 and 50 m (v15 and v50), the proposed value for roughness parameter (z0) and +αα α = − −50252515 (7) a required value v. For these locations is well 2 known geographical coordinates, appearance and shape of the relief area, appearance of buildings and vegetations. 3.3 Roughness Parameter The software will compute the wind velocity at It is considered that the data base based on the 50 m (based on the velocity measured at 15 m and weather stations network measurements is roughness coefficient) and will compare with unsatisfactory for air power evaluations. However measured wind velocity at 50 m. The software will for micro units and/or in emergency cases is be successive runs, with a range of value for possible to take in account this kind of date, but will roughness coefficient, until between those two be accepted only the average values from measured velocity values will not be a greater difference then heights. With this values, and taking in account the required value. conditions from analyzed location, can be With this method, the roughness parameter determined a average wind speed for desired height. values were verified starting from literature Obs. With average value, the insurance and wind recommended values. frequency curves can be obtained for an “average Thus, based on experimental measurements, one year” admitting that the phenomenon is quite can verify the roughness parameter values starting accurately reproduced by Weibull distribution. With from the values recommended in the literature. resulted values for average wind speed and time To illustrate, the location chosen was Vrani, intervals, graphically can be computed the specific situated on the crest of a hill, surrounded only with power/energy presumed for that location. crops whose height in the vegetation is relatively For this kind of situations (when special low (maize), and the rest (from November to May) measurements doesn’t exist) when is used the there are no actually vegetation. average wind speed computer based on INMH measurement, for the velocity computation at a superior height the following formula can be used: 3.4 The Results After computations, for each type of location a chart β 50  h  form can be draw: v(z0)=v(z0)v . It’s obviously = vv   (8) r  h  than the z0 value for which the chart v(z0) have a  r  minimum is the real value for area roughness parameter. where v, vr, h and hr have the same signification For designated area the program was used with described in 3.2.2 For β the following math will be the central value of roughness parameter z0 = 0,02 used: and the step of 0,002. Were obtained graphs like those listed below for July 2009, January 2010 and α=β − vlog55.01 0 ( r ) (9) October 2010.

2,0  z  and α =  0  (10) 0  h   r  where z0 is land roughness conventional parameter. In [4] are presented the numerical values for land roughness parameter. When roughness parameter is used for wind velocity approximation in land boundary layer, will be take in account the average wind velocity (from

ISBN: 978-1-61804-012-1 219 Recent Researches in Environment, Energy Planning and Pollution

July 2009 If used (for evaluation calculations based on 2.00 meteorological stations measurements) values of Delta V mediat 1.50 roughness parameter outside the real interval, is 1.00 reached, usually at an overstatement of wind 0.50 potential. 0.00 0 0.005 0.01 0.015 0.02 0.025 0.03 Can say with certainty that the roughness factor 0.50 is a function of following variables: relief, 1.00

1.50 buildings, vegetation, climate but also the wind

2.00 Zo velocity. 2.50 The method can be used, if are available values for wind speeds measured on at least two known January 2010 heights on the same pole, for checking the 3.50 roughness parameter of any type of land. Delta V mediat 3.00 The activity materialized in present paper

2.50 allowed researchers improved ability to guide

2.00 himself and to use easier the recommendations from

1.50 literature related to roughness parameter

1.00 characterizing terrestrial boundary layer by wind

0.50 velocity.

0.00 Zo 0 0.005 0.01 0.015 0.02 0.025 0.03 0.50 5 Aknowledgement October 2010 The work has been cofunded by the Sectoral 4.00 Operational Programme Human Resources Delta V mediat 3.50 Development 20072013 of the Romanian Ministry 3.00 of Labour, Family and Social Protection through the 2.50 Financial Agreement POSDRU/89/1.5/S/62557. 2.00 1.50 1.00 6 Bibliography 0.50 Zo 0.00 0 0.005 0.01 0.015 0.02 0.025 0.03 [1] Vlădea, I., Tratat de termodinamică tehnică i transmiterea căldurii, Editura Didactică i Pedagogică, Fig. 3. Graphs of variation v(z0) Bucureti, 1974. Basically, following the calculations made for [2] Nedelcu, D., Microsoft Excel – Concepte teoretice i aplicaŃii, Editura Orizonturi Universitare, Timoara, studied area, it can be stated that the average 2003. roughness parameter is z0 = 0,0151. [3] *, Manual Data Logger Meteo 32, EdiŃia 1.4.0, Ammonit, 2005. 4. Conclusions and Further Directions [4] Bej, A., Turbine de vânt, Editura Politehnica, Following the calculations made based on measured Timioara 2003. values of velocity, it was found that in fact the [5] Spera, D.A., Wind turbine technology, ASME Press, parameter of roughness of the terrain for an exact New York, USA, 1994. [6] *, SR 13465, 2007. location uses a range of values and is difficult to say [7] www.wasp.dk that it has a certain fixed value. At the same time it [8] www.windatlas.dk was found that the roughness parameter is [9] www.windenergy.com influenced by the wind speed. Using experimental data obtained from measurements of wind speed for a position over a period of time (at least one year), can be determined with sufficient accuracy for the parameter range of roughness values. The set of values obtained (for various periods of time minutes, hours, days, years) can be analyzed statistically, and thus attain the maximum value of probability.

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