AUTOMATED WIND FARM SIMULATION (AWFS)

Transcription

1 AUTOMATED WIND FARM SIMULATION (AWFS) Dipl.-Ing. Markus Weimbs, Prof. Dr. Wolfgang Schellong, Omar Herrera Sánchez (Eng.) Cologne University of Applied Sciences Betzdorfer Str. 2, D50679 Cologne, Germany ABSTRACT Regarding a possible climate change and the increasing prices concerning fossil fuels (e.g. mineral oil, natural gas, coal, etc.) the usage of renewable energies is getting more and more important. The past decade outlined that the establishment of a sustainable energy supply is admittedly a difficult task, but that this branch of business is full of innovations. And as a matter of fact there is still enough space for new ideas. The present software concept, called Automated Wind Farm Simulation (acronym: AWFS), is designed to handle three planning challenges regarding the site assessment of wind energy projects: The development of new potential sites, the so-called Repowering of existing wind farms and as there is an increased demand for non-commercial or rather private systems the particular site assessment regarding small wind turbines (SWT). The innovative thought regarding the AWFS is up to the term Automated based on a comprehensive investigation concerning the current state of the art of computer-aided wind farm simulation, which results in the cognition that there is a high innovative potential regarding the decisionmaking referring to the selection of wind turbine positions and the general handling (user-friendliness, system requirements, computing-time, etc.). KEY WORDS Wind Energy, Repowering, Small Wind Turbines, Wind Farm Simulation. 1. Introduction As a consequence of the upcoming discernment that there is a necessity for a reversal point regarding the utilization of fossil fuels (e.g. mineral oil, natural gas, coal, etc.), because of an emerging climate change [1] and finite resources, the importance of using renewable energy sources [2] is getting more important or rather indispensable. Regarding the market for renewable energies it is obviously that, even during the financial crisis, there is a strong rate of growth concerning both: the overall amount of employees and the degree of innovation. But as a matter of fact there is still enough space for new ideas. The AWFS project, which represents the basis for this paper, is concerned with the currently most promising area of the renewable energy family: the wind energy. In terms of this energy source there are three important challenges regarding the site assessment: AWFS New Site Repowering SWT Figure 1: General Structure of the AWFS Software On the one hand there is the fact that the amount of suitable sites to establish a wind energy project is finite (even regarding offshore projects). As a consequence of this circumstance the given sites have to be utilized to the maximum point, which is processed by the AWFS module New Site. Until today many scientists faced up to the topic Site Assessment of Wind Energy Projects [3]. But at least in all cases the user himself is at 100% responsible concerning the later position of a wind turbine (even when using state of the art simulation software). This might or rather will lead to the fact that the selected site will not be exploited to its maximum point, which dissipates important resources. The main unique characteristic of the AWFS is to automate the lions s share of all decisions and thus facilitate the work for the user, which leads to several advantages. First there will be a considerable saving of time regarding both, the training and the actual planning period. Additional to this the amount of input errors and wrong decisions will be reduced to a supportable value. But finally the major advantage is represented by the optimum utilization of a specific site or rather the substantial increase regarding the energy yield of a wind farm. Of course all the listed advantages will have a positive effect on the planning costs and the amount of money gained from the electricity feedin. The second challenge is represented by the topic Repowering, which gained more and more importance regarding the last years [4]. To repower a wind farms means that the existing wind turbines will be replaced by new ones offering all state of the art technologies. This will lead to an increase of the wind farm s energy yield, but in full compliance with the restrictions by law. The AWFS module Repowering offers an automated process, supported by a special database, to check which new wind turbine has to be selected in order to reach both: an increased energy yield and the mentioned adherence of the requirements of the local authorities.

2 The third module of the AWFS software is called SWT and handles the site assessment of small wind turbines. The market for those small wind turbine systems grows in a continuous way, although there is no computer-aided tool to plan the installation. Recent investigations show that an intense site assessment concerning small wind turbines is even more important than regarding their big brothers, because of the enormous dependence on the local wind conditions (e.g. air turbulence). The AWFS module SWT will provide a solution to calculate the optimum position for single small wind turbine systems in order to e.g. replace a diesel generator regarding a project site without a grid connection. The next sections of this paper consist of information about the AWFS software regarding its concept, structure and handling. 2. New Site This section contains information about the structure of the AWFS module New Site, including its single simulation steps, which will be processed in a successive way. The principle is to start with the entirety of all potential positions (elements) within a predefined area (matrix) and end up with only the optimum positions remaining. Among these two states the mentioned simulation steps will eliminate positions, which are not suitable to host a wind turbine because of a specific reason. Site specific Conditions Data Input Infrastructure Mathematical Model Simulation Steps Infrastructure Figure 2: AWFS Procedure - New Site Restrictions by Law Like in every simulation there has to be a collection of information, which can be (in this case) divided into three groups: The site s specific conditions (wind conditions, terrain complexity, local surface roughness, etc.), the local infrastructure (e.g. roads or power lines) and the restrictions by law. With the help of this data input the module New Site is able to feed a mathematical model to calculate the optimum wind turbine positions and thus the wind farm s optimum layout. 2.1 Basic Framework With reference to the last published status [5] of the project (June 2009) many innovations have been added, which resulted in interesting changes regarding the basic framework of the AWFS software. The idea of using different simulation steps is quite the same, whereas the amount and the set-up of the steps is changed. AWFS Simulation Steps (New Site) Economical Considerations (ECon) General Availability (GAv) Wind Resource Assessment (WRA) Restrictions by Law (RbL) Grind Connection (GC) Result Utilization Figure 3: AWFS Simulation Steps - New Site 2.2 Step 0 - Economical Considerations (ECon) This step is no simulation step in the proper meaning of the word; it rather represents the possibility for the user to adjust options before starting the actual simulation. These options are designed to lead the later simulation decisions in a certain way regarding the local electricity demand (e.g. How many and which wind turbines have to be installed? ) and financial aspects (e.g. What is the budget for the planned wind energy project? ). A significant default option for example is the later usage of the wind energy project regarding the grid connection. Will the project feed electrical energy into the public electrical grid (commercial activity) or is it a stand-alone system (island system) to supply a local independent consumer? During this preliminary stage it is also possible to adjust options concerning requirements from e.g. local authorities, such as a maximum hub height because of visual reasons in relation to the landscape (perhaps when the site of the planned wind energy project is located near to a recreational area for example. To alleviate the user s investigation regarding the local requirements, there will be a data base with restrictions depending on the specific country. 69

3 2.3 Step 1 - General Availability (GAv) This first real simulation step consists of the basic consideration or rather decision if it is generally possible to place a wind turbine at a specific position within a predefined site. The intention of this simulation step is the reduction of elements within a matrix by filtering out the unsuitable positions, which will result in a decrease regarding the simulation s overall computing time. Strategy: A matrix system, which consists of single elements representing a defined area with a specific resolution, is placed on top of a satellite photo (e.g. using Google Earth ). These elements are connected to a SQL data base to save the appropriate properties. A catalog containing possible reasons, which speak against the construction of a wind turbine at a corresponding point, is established. Examples for these reasons are the existence of buildings, roads, forests, water surfaces (e.g. lakes or rivers), etc. At the moment there are two basic ideas to handle the property determination. On the one hand there is the manual solution, which holds the advantage of being quite simple in its implementation concerning the programming. Whereas due to the fact that the user has to select or rather mark the particular elements by hand the processing time is depending on the user, the map size and the complexity of the area. Additionally to this the probability for input errors is increased. Figure 4 shows the actual state of the graphical user interface (GUI) of the manual input procedure. Figure 4: Actual State of Step 1 - General Availability On the other hand there is the idea for an automated solution where it is only necessary to predefine some conditions before starting the marking procedure (e.g. the grade of selectivity). The major disadvantage of this solution is represented by the extremely complex parametric procedure to teach the software how to select and mark a single element within the matrix system. But of course the effort by the user would be minimized. The assumed percentage of eliminated elements should amount about 65-75%. But as a matter of fact this first step only identifies the elements within the matrix, which are in general not available to host a wind turbine. In order to calculate the optimum positions within a predefined site the following simulation steps are designed. Starting with the most important one: The Wind Resource Assessment. 2.4 Step 2 - Wind Resource Assessment (WRA) The term Wind Resource Assessment is based on the relation between the local terrain structure and the wind conditions of all elements within the predefined area. Actually this simulation step is placed second in the chronology because of the fact that it is way easier and especially faster to run General Availability before. A huge amount of elements within the matrix will be eliminated, so it is only necessary to accomplish this complex simulation step (WRA) for the remaining points. There was the development of a system to accomplish a general assessment of the single elements of the mentioned matrix regarding their specific local wind conditions. This data is saved in a connected database. The intention of this simulation step is to eliminate elements, which are in general not suitable to host a wind turbine in an economical way. After this assessment there is an additional problem that might or rather will occur when the n (e.g. 100) best elements are determined. There will be an accumulation of good elements quite near to each other, which leads to a decrease concerning the quality of the affected elements. The reason for this situation is represented by the so-called park-effect, which describes the negative interference of one wind turbine in wind direction on the rear one. When a wind turbine extracts energy from the wind a wake is created downstream, where the velocity of wind is decreased. As the wind flow proceeds downstream the wake spreads and recovers towards free stream conditions. To avoid the park-effect distance ellipses surrounding the potential wind turbine positions are used. Based on this idea the optimum elements without mutual interference are calculated, which will decimate the amount of elements in an additional way. In addition to the filtration of the best points there is an investigation regarding critical elements concerning extreme wind loads within the correspondent area. As the AWFS is meant to act in a global way several countries with a high probability of extreme wind phenomenons, such as hurricanes, are taken into account. These elements are marked within the results as dangerous positions. After this simulation step the elements with the best wind conditions to host a wind turbine are remaining. But this 70

4 is not the final result of the AWFS. There are several other things that have to be taken into account following in the next simulation steps. For example it might be possible that it is not allowed (concerning requirements of the local authorities) to build up a wind farm within a specific area because of different reasons. 2.5 Step 3 Restrictions by Law (RbL) This simulation step contains restrictions by law, which are determined in order to save human life, their wellbeing and of course the surrounding environment of a wind turbine. The following sub-sections describe the work on the two main aspects regarding the restrictions by law related to wind energy: The so-called shadow flicker and the noise immission perceived in inhabited areas near to wind turbines. As a matter of fact the most important restriction by law, the safe distance, is already included in simulation step 1 - General Availability Shadow Flicker / Shading Short definition: The so-called shadow flicker is an alternating shading effect, which occurs when different conditions suit (e.g. daytime, degree of cloudiness, distance between the inhabited area and the respective wind turbine, the wind turbine s hub height and rotor diameter, etc.). Currently there are two common methods to avoid or rather handle this effect. On the one hand it is possible to disable the wind turbine during the time frame the mentioned conditions suit. On the other hand there has to be an adjustment concerning the turbine s position (Secure Position - Method). Regarding the Secure Position - Method the AWFS will procure that the calculated (based on several parameters combined in intercept theorems) wind turbine positions are selected in order to avoid the exceeding of the respective shading values. In Germany for example the maximum shading time within an inhabited area is restricted to 30 minutes per day and 30 hours per year. Additional to this the software will show good positions within the predefined area although there will be an exceeding of the maximum shading time. The situation might occur that these positions are even better than the Secure Position - Method regarding its energy yield even when the wind turbine is disabled for a specific time frame Noise Immission The second important aspect regarding the restrictions by law is represented by the noise immission caused by a wind turbine within an inhabited area. Based on the local terrain complexity, the specific noise emission of a wind turbine (data base) and the possible wind conditions (in main case regarding a kind of worst case scenario) it is possible to calculate a minimum distance between a wind turbine and a specific area, which is taken into account regarding the determination of the best wind turbine positions. 2.6 Step 4 Grid Connection (GC) This last simulation step is designed to handle the connection of the planned wind turbines with the local electrical grid in order to save costs regarding new power lines. To put it simply this step will decide between various possible positions, which one is closest to the next power line. Another important aspect regarding this simulation step is the calculation concerning the capacity of the existing power line. 2.7 Reports When all simulation steps are accomplished the best potential positions within the predefined area are calculated. The amount of the mentioned positions depends on the adjustments made in the economical considerations and the quality (Wind conditions, terrain complexity, restrictions by law, etc.) of the site. The results are displayed in reports, which can be exported using.pdf documents. Additional to the calculated optimum positions for wind turbines within the regarded area there are reports concerning the restrictions by law, especially the shadow flicker and the noise immission, which are very important regarding the building license of the wind farm. 3. Repowering The term Repowering describes the replacement of old electricity plants by new, modern and in major case more powerful systems. Regarding the wind energy every wind farm operator has to decide if the existing wind turbines will be fully reconditioned or will they be replaced by new ones after the estimated lifetime is up. For this reason the replacement of old wind turbines by larger and newer ones represents a big challenge for the next future. As in Europe the first wind farms were installed in the early 90 s [6] there are a lot of turbines achieving the estimated life cycle of 20 years. On the other hand in some countries there is a first decrease of new installed wind power as shown in figure 5 for Germany as one of the leading wind power producers. An essential reason for this fact is the lack of appropriate areas for new wind farms. Therefore repowering is a good alternative to continue the increasing use of wind energy. As a rule of thumb the double of the wind power could be produced by the half number of turbines in the same area. 71

5 MW Installed Wind Power in Germany MW Installed Year Accumulated Power Figure 5: Installed wind power in Germany [6] But repowering includes not only the simple replacing of the old wind turbines by new ones in the same area. Additional influences and restrictions have to be proofed: The park effect (see section 2.4) depends on the size of the new turbines. The restrictions by law concerning the shadow flicker and the noise immission (see section 2.5) must be fulfilled. The existing height limits for the turbines may cause the search for new areas. New turbines with larger capacities obtain higher requirements for the integration to the grid. The number of turbines to be installed in the wind farm is influenced by the profitability of each turbine. Therefore a computer-aided system is necessary to simulate the energy yield of the new wind farm for different repowering scenarios based on a detailed wind turbine database. The AWFS module Repowering will offer an automated process including all steps described in sect. 2. In order to do so there are two ideas. On the one hand roofs with a small slope (such as flat roofs) are suitable to use a kind of indirect roof tower system, which means that a separate tower is mounted on the roof without a physical connection to the roof itself. The advantage of this method is to save material, because of the additive height of the building, whereas the disadvantage is represented by a high system complexity in relation to a standard pipe tower. On the other hand there is the option to avoid building up the small wind turbine system on top of a roof and using instead a stand-alone tower in a specific distance to the building. In combination with some roof conditions (type, slope, orientation, etc.) this method might involve the advantage of obtaining a higher energy yield by exploiting accelerated wind flows. Figure 6 shows an example of flow simulation results regarding a flat roof with a slope of about 5 to 7 degrees. This is a extremely simplified exposition and the later AWFS SWT module will simulate the predefined building including the close surrounding environment in more detail. In this case the basic building is a shape with the measures 10 m (width) by 10 m (height). The wind comes from the left side with an initial velocity of 5 m/s based on a logarithmical wind profile. The very dark areas (in front and in the first layer on top of the building) represent wind conditions with low velocities, in main case caused by turbulence and back-flows. 4. Small Wind Turbines (SWT) Small wind turbines represent a good combination, together with e.g. photovoltaics, for island power supplies used in interior areas, which can be found for example in Africa, Asia or Latin America [4]. Regarding small wind turbines there has to be a particular investigation concerning the surrounding environment. Every obstacle within the current wind direction will have an affect on the turbine s energy yield. So it is even more important to find the optimum installation position than regarding large scale wind turbines. First of all it is quite useful to mention that experts meanwhile advise operators against installing a small wind turbine on top of a roof in a direct (physically) way. The reason for this advice is based on the perception that vibrations, which are caused by the alternating impinging wind on the rotor blades, are transferred into the building using the direct tower installation as a kind of transmission medium. This might or rather will cause damages on the building, which must be prevented. Figure 6 : Particular Site Assessment SWT (CFD) The gray colored areas on top of the roof represent wind conditions where the incoming flow is accelerated in a strong way. These conditions have to be used to operate a small wind turbine system in an optimum way. Regarding the optimum installation site of a small wind turbine the AWFS module SWT contains the necessary tools to plan the best position and hub height for a predefined location. 72

6 5. Results In order to investigate the credibility regarding the calculated optimum wind turbine positions a validation of the simulation results has to be accomplished. Several scenarios were modeled and simulated to prepare a comparison of the AWFS results with models of common computer-aided wind farm simulation tools, such as WindPRO2 made by the Danish company EMD International A/S [7] or WindFarmer developed by the British company Garrad Hassan [8]. Further tests consist of aspects regarding the simulation speed (computing time), system requirements, user friendliness, etc. The first trials were accomplished by trying to theoretically repower an existing wind farm in the rural Eifel area in Germany. The original wind farm consists of 5 ENERCON E-40 wind turbines. The results after several scenarios showed that the best option in order to repower the wind farm is to use 4 instead of 5 turbines (To avoid the mentioned park-effect) and use the new model ENERCON E-44. and improvements. Such as to automate the lion s share of the user activities to a minimum to avoid wrong decisions. Regarding the finite amount of potential new sites the socalled Repowering gains more and more importance. A comprehensive inquiry results in the cognition that there is also no automated computer-aided simulation tool to handle this new challenge. Additional the planning and repowering of large scale wind energy projects the market for small wind turbines, for noncommercial or rather private use, is growing in a continuous way. As a consequence of this it is necessary to develop a computer-aided simulation tool to find the optimum position within a specific area to install such a system. In summary the described AWFS software offers three unique selling propositions: An automated Wind Farm Site Assessment (New Site), an automated Repowering - procedure (Repowering) and last but not least an automated Site Assessment regarding Small Wind Turbines (SWT). Scenario Alternation of wind turbine positions. Exchanging the old wind turbines by new models; The old positions are maintained. Combination of both scenarios.* Potential 10% 32% 55% References [1] World Energy Outlook 2009, International Energy Agency (IEA), Paris, 2009 [2] J. Randolph, G. M. Masters, Energy for sustainability: technology, planning, policy, Island Press, Washington, 2008 [3] P. Gipe, Wind Power,James & James, 2004 Table 1. Optimization Potential * One existing position was deleted in order to avoid the mentioned park-effect. The other 4 wind turbines (E-40, 600kW Nominal Power / Turbine) were exchanged by new ENERCON E-44 (900kW Nominal Power / Turbine) systems. By doing this the annual wind farm energy production was increased slightly, but as a matter of fact the aim was to optimize the wind farms efficiency, which was reached by using the AWFS Repower module. [4] Wind Energy International 2009/2010, World Wind Energy Association, Brussels, 2010 [5] W.Schellong, M.Weimbs, Site -Planning and -Optimizing of Wind Farms, Proc. Conf. On Enviromental Management and Engineering, Banff, Canada, 2009 [6] EMD International A/S, [7] Garrad Hassan & Partners Ltd, 6. Conclusion Regarding the mentioned facts of an emerging climate change and finite resources the usage of renewable energy sources, such as wind energy, is getting more and more important. In order to be able to develop new potential sites for wind energy projects a particular investigation concerning the site assessment is necessary. State of the art computer-aided wind farm simulation software establishes the basic conditions to plan a new wind energy project, but as a matter of fact there is still enough space for new ideas 73