Contact | Imprint | Source | FAQ | Deutsch | Search

Special Reports


Made in Kassel: 30 Years of Wind Energy Monitoring - A Retrospective

 In 2018, employees and guests from business, science and politics celebrated the 30th anniversary of research and development for the use of renewables in Kassel. It all began in 1988 with the founding of the Institute for Solar Energy Supply Technology Iset as an independent affiliated institute of the University of Kassel. The aim was to advance energy system technology, i.e. the engineering understanding of the complexity of these new supply systems and structures. One of the highlights of 30 years of research and development is the monitoring of wind energy, which began at the end of the 1980s with the measurement and evaluation programme commissioned by the Federal Government for the »250 MW Wind« funding measure at Iset in Kassel. The tasks of wind energy monitoring have been continued to this day in several follow-up projects and their content has been broadened and deepened. Our authors Michael Durstewitz and Berthold Hahn have been involved in these monitoring projects from the very beginning and have contributed to their success over many years. In their review »30 Years of Monitoring« they review these times - to be read in the Wind Energy Report Germany 2018 (page 104). 


 Non scholae sed vitae: Study for the wind industry


The successful implementation of energy system transformation in Germany and the expansion of renewable energies worldwide, especially wind energy, need not only suitable locations, techniques and processes, but above all people with appropriate professional qualifications. As in many other areas, there is also a deficit between demand and supply in this special market or technology segment. Year after year, thousands of graduates leave colleges and universities, but very few of them have a sound knowledge of wind energy technology. One way of overcoming this shortcoming is to offer a comprehensive, in-service training program.
Various universities in Germany and internationally offer additional qualifications in wind energy technology. André Bisevic, Detlef Kuhl and Julia Merger present the course »Online M. Sc. Wind Energy Systems« in the Wind Energy Report Germany 2018 (page 96), which has been offered by the University of Kassel in cooperation with Fraunhofer IEE since the winter semester 2015 / 16.


Building and using knowledge together


After the commissioning of a WT, its economic success can essentially only be influenced by the operation and maintenance strategy. The goal must be a high availability and efficiency of the WT while at the same time keeping costs as low as possible. The introduction of the tendering procedure has further increased the pressure to succeed in the operating phase. In order to evaluate your own operation and maintenance strategy and, if necessary, optimize it, you need a benchmark and a broad database.
In the wind energy information data pool (WInDPool), Fraunhofer IEE is building up a corresponding knowledge and experience database as an industry initiative together with currently 15 industrial partners. In the wind pool, operating and maintenance data from over 3000 wind turbines are collected and evaluated across companies. In the special report »Windenergy-Information-Data-Pool« of the Wind Energy Report Germany 2018 (page 84), Philip Görg and Sebastian Pfaffel present results on site conditions, availability, operating conditions and events from the wind-pool.


PPAs - an alternative wind power market


PPAs - short for Power Purchase Agreements - are direct electricity supply contracts between producers and customers. In the USA and Scandinavia, PPAs are often used to market wind power. Google is a frequent example in the media: Google sometimes uses very long-term (20 years) PPAs to supply its energy-hungry data centers with wind and solar power. In Germany, PPAs are not common due to the framework conditions of EEG funding. With the abolition of this fixed remuneration for many old plants
In 2021, PPAs will also become interesting in Germany. With the help of PPAs, wind farm owners can calculate a fixed electricity price for continued operation and use the green electricity property for marketing purposes. In the special report »Wind Power PPAs: Potential, Opportunities and Risks« of the Wind Energy Report Germany 2018 (page 90) Fabian Huneke and Michael Claußner from Energy Brainpool present the concept of PPAs and their possible application in Germany.



Blowing in the wind? - Froenix knows the answer


The Fino measuring stations enable, among other things, high-resolution measurements of wind direction and wind speed far out in the sea. Over the years, they have contributed to the accurate quantitative and qualitative recording of offshore wind conditions in the North Sea and Baltic Sea and have thus provided essential information for the economic development of these marine areas for offshore wind energy use. One shortcoming of mast measurements is the only selective recording of wind conditions. In addition, there are the influences and impairments on the
Measurements by offshore wind farms that have meanwhile been erected in the vicinity. The Fraunhofer-Institute for Wind Energy Systems IWES offers assistance for operators of offshore wind farms in the German Bight to better assess the yields of their turbines in relation to the supply of wind energy with the product »Offshore Wind Energy Index«. Read more in our Special Report of the Wind Energy Report Germany 2018 (page 70).



Money plays a role: Offshore electricity generation costs in international comparison


The level of the electricity production costs of electrical energy systems, in addition to environmental compatibility and social costs, for example, are the keys to whether and how a technology can position itself and hold its own on the market. In the case of offshore wind energy, these depend on the one hand on external boundary conditions, e.g. water depth, distance from the coast, wind conditions, etc., and on the other hand on regulatory, financial and fiscal conditions. These can vary considerably from country to country. The special report »Electricity production costs offshore in comparison with other countries« of the Wind Energy Report Germany 2018 (page 76) analyses country-specific framework conditions from seven countries.



Prospects for wind energy


Which technical developments are particularly important today to ensure that wind energy will provide the majority of the energy required in 2050? In which sub-areas of wind energy is there a great deal of potential? These questions are addressed in the Special Report "Wind Energy - Where do we stand, what’s next" in the Wind Energy Report Germany 2017.


In order to evaluate the status of wind energy, objective criteria are defined on which the development and maturity of the technology can be determined. It is thus possible to show the significant influence on energy saving, the contribution to climate targets and the cost efficiency of wind turbines in comparison to other technologies.


For the further expansion of onshore wind energy, the main challenges are to identify and optimally exploit suitable sites and to provide systems that are designed specifically for local/regional conditions and that are largely technically optimised. Examples include the LIDAR measurement technology to forecast wind conditions and detect approaching turbulence fields, or the development of "smart blades" using CFD-supported simulations that enable adaptive flow control and further increase the performance of the turbine. In addition, the operation management and maintenance processes must be designed in a cost-optimal manner and all environmental influences on wind energy use must be reduced as far as possible or the unavoidable effects must be accepted.



Improving supply through a new kind of forecast


One of the most important tasks for transmission system operators is to keep the grid frequency stable. This task requires a forward-looking overview of the entire power system, the feed-in points and their weather-related dependencies.


In the Special Report "Energy Forecasts 2.0" in the Wind Energy Report Germany 2017, Malte Siefert, Jan Dobschinski, and Andreas Röpnack show how this overview can be gained through a new kind of forecast. Through a joint effort between the transmission system operators, Fraunhofer IEE and the Deutscher Wetterdienst, goals were formulated to overcome known problems.


The targets were defined as the forecasting of the feed-in within a period of 15 minutes to one week and at the level of network areas, postal codes but also at certain transformer stations. Further goals were combining information about wind and solar power with weather data. Problems were to be found, among other things, in the weather models. Until now, the change between night and day and the associated dissolution of the stable boundary layer led to a deviation in the forecast. It was thus possible to significantly reduce the mean error at night by using the new type of forecast.



Bigger, higher, smarter?


Large rotors at low rated capacities on high towers. These turbines are known in Germany in particular as "weak wind turbines". The relationship between rated capacity and rotor area is also called specific output. There has been a trend towards smaller specific outputs for several years. This leads to a higher number of full-load hours and low electricity generation costs. The wind yields can also be increased via the hub height.


In the Special Report “Bigger, higher, smarter?" in the Wind Energy Report Germany 2017, Alberto Dalla Riva, Janos Hethèy and Elisabeth Buchmann examine how the use of systems with low specific outputs will affect the entire power system. Using a comprehensive model of the German and Danish markets, they show how the feed-in profile of wind turbines is expanding, how the market value of the wind power generated is increasing for systems with different designs and how system costs such as the construction of additional power plant or transmission capacity can be avoided. For the northwest of Germany, the study calculates an increase in the market value of €4.6/MWh, which can be achieved by choosing large hub heights and a low specific rated capacity. After deducting the estimated costs, an advantage of 3.3 ct/kWh remains.


The calculations also show savings in the overall system due to the broader feed-in profile in the form of lower fuel costs and, to a lesser extent, due to avoided investment in additional power plant capacity.



Lidar as a versatile tool for wind measurement


In the wind industry, detailed knowledge of wind conditions is essential for implementing successful projects. This starts with wind atlases to pre-select possible project locations, continues with detailed site-specific wind measurements to select turbine types and ends with specific project calculations. For scientific questions, e.g. new control strategies, the incoming wind field as well as the wake of existing wind turbines are intensively measured.


In addition to measuring masts, Doppler lidar (light detection and ranging), which can measure wind conditions remotely by means of laser light using the Doppler effect, are increasingly becoming state-of-the-art technology for wind measurements. Constant development and research work has resulted in lidar now being used in all of the above-mentioned applications and thus it not only provides valuable information, but also saves costs compared to conventional measuring masts. Due to the widely differing requirements depending on the application, lidars are used in various configurations. As a measuring buoy for offshore wind energy or as a spinner lidar directly in the hub of wind turbines. Lidar will continue to be the subject of research and development in order to reduce the uncertainty of measurement and to cater to other fields of application.


The special report "Multitalent Lidar" by Florian Jäger and Doron Callies in the Wind Energy Report Germany 2017 offers an overview of how lidar works, the possible fields of application and current research questions.



Evaluation of the tendering procedure for onshore wind energy


In 2017, the Bundesnetzagentur conducted the first three tendering rounds for onshore wind energy. A total of 198 contracts for 2820 MW of new wind power were awarded. The bid values are between 2.2 ct/kWh and 5.71 ct/kWh, the volume-weighted average contract value is 4.53 ct/kWh.


At first glance, the tendering rounds were a success and led to significantly lower remuneration compared to the fixed remuneration rates under the EEG. On closer inspection, however, the result is less clear. If the tender results are compared with the remuneration under the previous system, other factors must be taken into account. For example, the reference yield model for adjusting the remuneration to the location has once again changed as part of the tendering process. The fact that the remuneration for a wind turbine under the previous EEG depends on the date on which the wind turbine is commissioned is even more important. In contrast, an implementation period of up to 2.5 years or 4.5 years applies during the tendering procedure for citizens’ energy companies, which have received 93 percent of the contracts. The comparison must therefore always take into account the future degression of the remuneration during the implementation period.


Whether onshore wind energy has actually become cheaper under these conditions as a result of the tendering procedure is discussed in the Special Report "Really cheaper through auctions?" in the Wind Energy Report Germany 2017. In this, Fraunhofer IEE and the Institute for Future Energy and Material Flow Systems (IZES) at Saarland University in Saarbrücken present the joint study to evaluate the first tendering rounds.



Storing wind power - but how?


Regulation, shutdown, negative electricity prices - all these measures and effects could be cushioned, at least in part, by power stores in the current inflexible power plant system. The "superfluous" wind power is temporarily stored in these power stores if it is not required whenever there is a high proportion of wind power but low demand. A storage concept was developed and tested in a joint project between Hochtief Engineering Consult IKS and Fraunhofer IEE. The storage concept, which functions in a similar manner to a pumped storage power plant, is based on one or more spherical concrete cavities. These are anchored to the seabed. Depending on the situation, the store can be loaded or unloaded. A functional model was successfully tested in Lake Constance. The exact functioning of this concept is described in the Special Report "Submarine Energy Store – Concrete Ball in Lake Constance" in the Wind Energy Report Germany 2017. The authors of the article are Stephan Fromknecht, Julian Meyer and Matthias Puchta.



More electricity in the heating and transport sector


The European Union and the German government have both set climate targets that will see the reduction of emissions by 80 to 95 per cent of the 1990 levels by 2050.


The road to replacing fossil fuels means not only considering the electricity market but also then transport, industry and heating sectors.


In their special report in the Wind Energy Report Germany 2016, Norman Gerhardt and Philipp Haertel from Fraunhofer IWES show how the coupling of sectors and the use of electricity in heating and transport can contribute to meeting these climate protection targets.


They show how the use of electric vehicles and heat pumps can lead to immediate reduction of emissions, even with the existing grey electricity mix. Electrode boilers and power-to-gas applications on the other hand will first come into their own later, where additional flexibility options are necessary for when 100 per cent renewable power is used for longer periods of time.


Key new areas of application include a variety of uses for electric cars, overhead cable trucks and heat pumps, which will in conjunction with electrode boilers and power-to-gas applications double our conventional electricity needs by 2050, and in the case of particularly ambitious climate goals almost triple the requirement. It will thus be possible to replace great quantities of fossil fuels in the heating and transport sectors with electricity generated from renewable sources.



Wind measurement using aerial drones


The expansion of land-based wind power means there is an increasing frequency of more complex sites in regions with low mountain ranges or forests. Detailed knowledge of the wind conditions in such locations is therefore especially important.


Met masts and lidar devices are currently the most commonly used methods of measuring wind conditions. It is possible to determine the wind speed very accurately using a met mast, but these can only be employed at one specific site. Lidar devices can be moved much more easily, and the combination of several devices has already been used to measure wind fields in a number of research projects. Where there is complex terrain there is however a need for subsequent CFD simulation to correct the lidar measurements.


Unmanned aerial drones (UAVs) can also be employed to measure the actual wind field or flow conditions at a complex site, since they can take measurements at any position in a particular location. The data thus obtained can also be used to validate computational models.


The corresponding research work and the results obtained by the universities of Stuttgart and Tubingen universities have been presented in the special report on Aerial Measurement Systems in the Wind Energy Report Germany 2016.



Survey of experts on the future cost development of wind energy


Will the costs of wind energy continue to decrease? If yes, how quickly and by how much? Which factors have the greatest influence and what measures can industry, government and science take to realise such cost reductions as quickly as possible?


The participants of the IEA Wind Task 26 Cost of Wind Energy put these questions to over 160 experts worldwide in the biggest wind sector survey to date. The study examines onshore, offshore fixed-bottom and floating technologies. The experts see cost reduction potential in all these areas. Cost reduction in the onshore sector will primarily be driven by the increased yields expected, while offshore the costs will be reduced primarily through further upscaling of the turbines.


Dr. Ryan Wiser from the Lawrence Berkeley National Laboratories in California presents the detailed results of the survey in the special report on Future Cost Development of Wind Energy in the Wind Energy Report Germany 2016.



Floating offshore wind turbines


The overwhelming number of offshore wind turbines built to date get their stability from pile foundations that have to be sunk deep into the seabed. Different types of foundations are employed depending on the depth of water and the condition of the seabed, such as monopiles, tripods or jacket foundations. These types of foundation can however only be technically and economically employed in relatively shallow regions of the sea and only up to a certain water depth.


In many parts of the world the seabed falls away relatively quickly and steeply, meaning that these foundation structures can no longer be used. For such regions a solution is floating foundations for offshore wind turbines. With this kind of foundation, the wind turbine is mounted on a flotation unit that can be anchored to the seabed with chains. One concept is the tension leg principle, where the float in kept in position by means of prestressed cables connected to foundation elements on the seabed. Such a test system is due for trials in the relatively shallow Baltic Sea off Mecklenburg.


Dr. Frank Adam and his colleagues present this project in their special report in the Wind Energy Report Germany 2016.



...and now the weather


Wind and weather and the accompanying external influences such as storms, wave conditions and currents also play a key role in the installation and servicing of offshore wind turbines. Adverse weather conditions can soon become a cost driver when such conditions mean it is only possible to carry out work offshore to a limited extent, if at all.


In their special report on the impact of weather on the installation of offshore wind farms, Marcel Wiggert and Maxim Hartung from Fraunhofer IWES introduce several logistic concepts for commissioning offshore wind farms. The report analyses and compares weather risks, resources and costs based on a generic wind farm.


The colleagues from Fraunhofer IWES in Bremerhaven present their work in a special report on Analysis, Optimisation and Comparison of Logistic Concepts for the Commissioning of Offshore Wind Farms in the Wind Energy Report Germany 2016.



Wind expansion

  • Critical raw materials in wind power expansion
    • The expansion of renewable energies is increasingly being discussed in terms of resource consumption as well as in terms of cost, security of supply, acceptance issues and effects on land utilization and landscape appearance. It is beyond dispute in the discussion that overall resource utilization of an energy system is generally considerably lower the more it is based upon renewable energy (and not aligned primarily towards biomass). This does not necessarily mean however that renewable energy can be seen as unproblematic with regards to deployment of resources.

      In particular, little investigation has been conducted into the consumption and long-term availability of mineral raw materials generally required to manufacture energy converters and infrastructure. A current study (Wuppertal Institute 2014) contributes towards closing this analysis gap, providing information on whether and how energy transition with a high level of renewable energy expansion can be structured more resource efficiently.
  • Balancing power from wind turbines
    • During the course of the energy transition in Germany, more than 25% of electricity consumed is now provided from renew-able energies. This energy comes primarily from wind turbine and photovoltaic installations. It is therefore becoming increasingly necessary that renewable energies are also involved in the provision of system services. The reform of the REA (Renewable Energy Act) has meant since the start of 2012 that renewables are able to participate in the market. This explicitly includes participation in the markets for the provision of system services. Within this context, balancing power is already being provided successfully by biogas and hydroelectric power plants (exceeding 1 GW in total). Wind turbines have yet to contribute any balancing power however. This is mainly because the formalities for the balancing service market are not rendering participation possible.
  • Regional market value factors for wind power
    • The share of fluctuating renewables as a percentage of total power generation means a greater call for better market integration of renewable produced electricity. Within this context, the question arises as to the value of the electricity generated from renewables dependent on supply. One possible parameter is represented by market value factors - providing information on the value level of the electricity generated as compared to the average exchange electricity price. Different time-related generation characteristics mean this value can vary from region to region. Given the mandatory direct selling of electricity generated by wind turbines, introduced in the 2014 Renewable Energy Act, the regional market value factors for wind energy are of special interest. Also, the level of financial funding for electricity
      from renewables is to be determined by means of a tender system no later than 2017. The analysis of regional market
      value factors for wind energy is the subject of a study carried out by Fraunhofer IWES on behalf of BNetzA (the Federal Network Agency).

  •  Effects of the bidding process
    • REA 2014 contains for the first time the political agreement to introduce generally by 2017 at the latest bidding processes
      for the financial funding of electricity from renewable energy (§ 2 Section 5 REA). To gain experience, the REA provides for
      pilot bids for ground mounted PV installations (§ 55 REA). The associated power to issue statutory instruments contains a
      number of specifications for this bid process (§ 88 REA). Included here is the requirement to retain the diversity of players
      in electricity generation with renewable energy installations.

      During the course of the legislative procedure, different studies have been published which deal critically with the planned tender process. Accordingly, experience abroad with tender systems is mixed. The positive effects expected with auctions were rarely realized as implementation was difficult. There is also concern that all forms of tender process do not maintain the diversity of players and systematically disadvantage public energy cooperatives and citizen participation in particular.

  • German offshore expansion scenarios
    • Since the initial deliberations on expanding wind energy at sea, different scenarios have existed which represent a potential development of installed offshore capacity for Germany.

      This article provides an overview of the different early and current scenarios, and enables classification of the scenarios and estimation of the future development of offshore wind energy in Germany on the basis of the current expansion status and the existing project pipeline.


Wind in the renewable energy mix

  • Business model for renewable energy
    • The debate about renewable energy is currently dominated by the costs. Dr. Carsten Pape, Fabian Sandaua, and Norman Gerhardt of Fraunhofer IWES are of the belief, however, that the debate here is focusing on too short a timescale. In their Special Report they show that that the required investment can be turned into profit.

      They compare the costs of fossil fuels for conventional power stations to the investment costs for renewable energies. They show that after about 20 years the switchover to renewables saves one money.

      From an economic point of view, it is important that transport and heating also switchover at an early stage from being powered by fossil fuels to being powered by electricity. This is because although the amount of primary energy required for power generation is a similar order of magnitude to the amounts of primary energy required for heating and transport, the purchase costs are relatively low. In contrast, oil and gas are expensive and are difficult to substitute. Energy efficiency measures are also vital.

      The report makes it clear that the investment opportunities and business models for renewable energies are more salient topics than the costs.
  • Models for funding renewable energy generation
    • The expansion of renewable energies cannot be funded in its entirety via the existing wholesale markets. The current and expected prices in these markets are too low for this, and indeed are not even sufficient to fund conventional power stations. The control energy markets are also not the answer, as the market volume is almost negligible compared to the required level of funding. As such, this therefore concerns not a “subsidization“ or “sponsoring“ of renewable energies, but rather their funding.

      Prof. Dr. Uwe Leprich and Dr. Uwe Klann of the Institut für ZukunftsEnergieSysteme (Institute for Future Energy Systems) discuss this issue further in their Special Report.

      The difference between the funding models is whether they take into account differences between the individual technologies, whether they are technology-neutral or not, and whether they bring electricity into the system via a physical transfer or marketing approach. A distinction is made between marketing models with a market premium and a capacity premium, with the former being a variable or (ex-ante) fixed premium.

      Leprich and Klann recommend using different funding models depending on the renewable technology. For offshore wind energy they recommend an auction procedure with compulsory direct marketing. For onshore wind energy, PV, and hydroelectric energy, an options model with feed-in remuneration for small, risk-averse investors and a capacity premium with direct marketing for professional investors will entice a wide range of different investors.
  • The Energiewende in North Hesse
    • A switch to electricity generation entirely from renewables is a topic being discussed by an increasing number of cities and communities. Using the example of the region of North Hesse, Dr. Thorsten Ebert of Städtische Werke Kassel and Katharina Henke of Fraunhofer IWES show in their special report how transformation of the electricity supply system to decentralized generation from renewables is possible.

      Several municipal utility companies have formed an alliance in order to plan and coordinate measures to switch over to renewables. The study shows that there are good opportunities for decentralization and regionalization of energy provision. In North Hesse there is an interesting mix of urban and industrial areas and large areas with a low concentration of buildings. The rural regions make about 1.2% of their area available for wind energy utilization. This meets 60% of the electricity demand. In urban areas the focus is on utilizing the surfaces of roofs for photovoltaic installations. For flexibility, electricity is also generated from biomass. A total of 80% of the electricity demand in the region is generated decentrally from renewables.

      The switchover to electricity generation from renewables also increases local value creation in the energy sector. Henke and Ebert estimate that 90% of current payment streams, namely hundreds of millions of euros, would remain in the region.
  • Direct marketing of wind energy 
    • The new version of the REA in 2012 extended the regulations for direct marketing of electricity generated from renewable sources. According to §33a of the REA, grid operators can directly market the electricity and receive a so-called market premium for this. Participation in direct marketing requires the operators to provide feed-in forecasts.

      Direct marketing also stimulates operators to retrofit their wind turbines with remote control devices to shut down the wind turbines when prices are negative. Furthermore, it is expected that the quality of feed-in forecasts will improve and that more wind turbines will join the control power market. In the medium term this may decrease the costs for the REA surcharge.

      The market premium is receiving lots of interest amongst wind turbine operators. At the end of 2012, 80% of electricity generated from wind was directly marketed. Developments to date and further details are described by Christoph Richts of Fraunhofer IWES in his Special Report.
  • Wind energy utilization onshore
    • Finding suitable locations is vital in order to expand wind energy utilization onshore. New wind turbines are increasingly being installed inland away from the shore and also in the low mountain regions. At existing locations older wind turbines are being repowered, namely replaced by fewer but more powerful wind turbines.

      In the Special Report Dr. Bofinger presents the results of a study commissioned by the BWE. The study appraises the land area of Germany for its suitability for wind energy utilization. The conclusion is that 8% of the total area is available for classical wind energy utilization. If forested areas and conservation areas are also included, then the available land area increases to 12.3% and 22.4% respectively. For a scenario in which solely 2% of the land was utilized for wind energy utilization, a total nominal power of 198 GW could be installed. For about 2000 hours operating at full load per year, ca. 65% of the total electricity requirement of Germany in 2010 could be generated.


Grid integration

  • Technical grid assessment 
    • Grid expansion in Germany reached an important milestone in 2013. The grid development plan was assessed and approved by the Federal Network Agency. The German government passed the Federal Requirements Plan Act as the basis for specific planning procedures. What grid expansion measures are, however, necessary to keep pace with the expansion of renewable energy generation?

      The Federal Network Agency carries out a technical grid assessment to determine whether the proposed measures are effective and necessary. Besides assuring electricity provision, it also ensures that the grid expansion measures are proportional, economically viable, and robust.

      Based on the assumptions in the scenarios in the grid development plan, the assessment involves determining the hourly electricity inflow and outflow to the grid and performing load flow calculations. Based on these models, the effect of the grid expansion measure and potential utilization of the new line are determined.

      How the Federal Network Agency actually undertakes the technical grid assessment and approaches for improving the assessment procedures are described by Dr. Swantje Heers, Thomas Dederichs, and Achim Zerres of the Federal Network Agency in the Special Report.
  • Provision of system services by using wind power plants
    • The need for stable grid operation is becoming ever more pressing with the large increase in energy generation from renewables in the German and European grids. Thermal power stations, with their large rotating masses, will be increasingly removed from the mix of power generators. The frequency and voltage stability they provide must be taken over by the new power generators.


      Prof. Dr. Lutz Hofmann outlines in the Special Report how wind farms and wind farm clusters can provide system services in the future. Using weather and feed-in forecasts, the feed-in can be actively controlled over time by a wind farm cluster management system (WCMS). In addition, wind farms can make available positive and negative control energy and can generate reactive power for voltage maintenance. Furthermore, wind farm clusters can be controlled in such a way using a predictive grid management system that grid bottlenecks and voltage errors are avoided.


      The Special Report describes the services that can be provided by wind energy by integrating forecasts, wind turbine data, and grid information into a wind farm cluster management system.



  • Recycling of wind turbines
    • Wind turbines are designed to generate clean electricity for a period of 20 years. Once this period lapses, wind turbines are either decommissioned or repowered (namely replaced by a new wind turbine). In both situations, the end-of-life wind turbine must be disassembled and, where possible, recycled.

      Recycling is becoming an ever more important issue given that a growing number of wind turbines are approaching the end of their service lives. Prof. Dr. Henning Albers and Saskia Greiner, authors of the Special Report, explain that this does not solely concern bulk materials such as concrete and steel. For these materials there are already established return and recycling systems. Rather, wind turbines also contain large amounts of glass fiber reinforced plastics from the rotor blades and small but valuable amounts of heavy metals and rare earth metals.

      Albers and Greiner outline the targets, tasks, and responsibilities in the process chain, quantify the mass flows, and summarize the available recycling technologies. They highlight that there are only a small number of options at present for recycling rotor blades and the recovery of heavy metals and rare earth metals is still unresolved. The wind energy industry must meet the challenge of developing material-efficient and environmentally-friendly recycling systems for end-of-life wind turbines.
  • Test-based development of wind turbine control systems
    • The development of control systems for wind turbines is increasingly being aided by automated test systems. Martin Shan and Dr. Boris Fischer of Fraunhofer IWES describe in the Special Report how automated testing of control systems can be used for developing wind turbines and the operation of wind farms.

      The use of hardware-in-the-loop (HiL) systems allows simulation of the behavior of the total system when testing individual components. It enables development and commissioning times to be significantly shortened and the testing of extreme or abnormal situations without risks to safety. The tests may in the future play an important role for the certification of wind turbine control systems.

      The benefits of HiL systems also apply for the development of wind farm control systems. With in-line wind turbines, for example, more power can be generated overall if the power output of the first wind turbine is reduced. The simulation of shadowing effects on the HiL test stand means that such optimization tasks, taking into account wind farm communication, can be undertaken in the laboratory in comfort.
  • Extended operation of wind turbines beyond their planned service lives
    • The matter of extended operation has only been discussed and evaluated in recent years. This topic is becoming ever more relevant as more and more wind turbines reach their 20 year design age over the coming years. Wind turbine operators are faced with the question as to whether the wind turbines can continue to be operated at their location. The alternative to extended operation is repowering, namely replacement by more powerful wind turbines. Repowering is stimulated by a repowering bonus of 0.005 €/kWh on the feed-in remuneration.


      In the Special Report Jürgen Holzmüller, a wind turbine expert, describes the technical, economic, and legal aspects of extended operation and suggest guidelines and procedures for making decisions about extended operation. Using case examples, he shows how it is possible to estimate the total utilization period for a wind turbine.


      In particular, there is the potential for extended operation in instances where the actual loads on wind turbines are less than the loads assumed by the design criteria. This may, for example, be a location where there are lower winds than the wind turbine was designed for or where there is less turbulence.


      Read this Special Report for further information about the conditions under which wind turbine operation can be extended and about matters that have to be taken into account here.

  • Developments in rotor design
    • The largest wind turbines currently have a rotor diameter of 126 m. Larger wind turbines for both onshore and offshore are being actively developed by all the turbine manufacturers. At present Fraunhofer IWES is testing rotor blades having lengths up to 83.5 m. So what is the reason for this continuous trend towards ever larger turbines and can it continue? What are the technical and economic limits of wind turbine size?


      Dr. Arno van Wingerde of Fraunhofer IWES addresses these issues in the Special Report and considers alternative approaches for utilizing wind energy.


      Whilst the rotor diameter increases by the square of the rotor blade length, the mass of the blade increases in three dimensions (length/width/height) and hence more than the rotor area. Manufacturers are having to come up with ever more sophisticated designs, use ever more advanced materials, and employ ever more efficient and reliable production processes to compensate for the loads and stresses caused by the higher weight of the wind turbine.


      The situation is the same for other components of wind turbines, with costs rising by more than the second power of the rotor diameter. According to Wingerde, larger wind turbines will in the future not automatically guarantee lower electricity generating costs.

  • Drive concepts and direct drive wind turbines
    • The key task of a wind turbine in converting wind power to an alternating current is to convert the rotary motion of the rotor into electrical energy. Although direct drive wind turbines dominate the German market, most other wind turbine manufacturers worldwide make wind turbines with gears. These gear systems convert the rotor speed to a higher generator speed, so allowing compact design of the generator and turbine housing.

      Direct drive wind turbines, due to their design, have a generator with a large diameter, meaning high material usage and a greater tower head mass. Wind turbines with permanent magnet generators (PMGs) have this

      disadvantage to a lesser extent but require the use of
      rare earth elements (neodymium and dysprosium). The existing mining capacity for these metals is mostly in China. Export is limited by the Chinese government.

      In the Special Report Dr. Jan Wenske presents the various drive concepts and outlines their advantages and disadvantages. With regard to new concepts for offshore wind turbines, new emphasis is being put on servicing and maintenance aspects and availability.
  • Wind measurement technology
    • The exploitation of inland locations using wind turbines with high towers makes detailed knowledge of wind conditions ever more important. The topography inland is more complex and this is especially so in forested areas. Detailed knowledge of the wind conditions at specific locations is hence vital for optimizing the location of wind turbines and the wind turbine design. Wind measurement technology is being advanced in order to procure this information. Measurements using LiDAR, which uses the Doppler effect on reflection of optical signals, have already been successfully used for locations of simple orography.


      In the Special Report Tobias Klaas presents how LiDAR technology is being advanced for new application fields using comparative values from an additional 200 m high measuring mast.



  • Floating Lidar System
    • The recording of wind measurements for evaluating potential locations for offshore wind farms provides a challenge for wind project development. Floating LIDAR systems are a useful alternative to installing wind masts at high sea and they considerably reduce the cost of the development and planning phase for an offshore wind farm. As part of IEA Wind Task 32, recommendations were made for using this technology in the wind industry.

      Within the framework of the “Offshore measuring buoy“ project funded by the Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety (BMUB), Fraunhofer IWES developed a floating LIDAR system based on an adapted light buoy. An island system comprising three small wind turbines, PV modules, and three battery banks provides the power. An offshore test from August to October 2013 in the direct vicinity of the FINO1 measuring mast allowed considerable operational experience to be gained and allowed the measurements to be compared with the FINO1 data. In addition, correction algorithms were developed and verified. These algorithms allow errors in the wind speed and turbulence measurements due to buoy movements to be compensated.

      The tests showed that the LIDAR buoy meets the key requirements of high data accuracy and good system reliability. A detailed account of the technical properties of the system, the correlation of the measured wind speeds with reference measurements, and the effect of buoy movements on the measurements is given by Julia Gottschall in the Special Report.
  • Acceptance of offshore wind energy utilization
    • Offshore wind energy is being increasingly accepted, by both those who live near the coast and tourists. Acceptance is, however, higher if the wind turbines are far out to sea and are not a hazard to ships.

      These are the findings of an interdisciplinary project entitled „Acceptance of offshore wind energy utilization“ funded by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety. This project has been ongoing since 2009 and is being undertaken by environmental psychologists from the University of Halle-Wittenberg, specialists in rural planning from the Technical University of Berlin (Dr. Elke Bruns) and Technical University of Munich (Prof. Dr. Sören Schöbel-Rutschmann), and an expert in maritime tourism (Prof. Dr. Michael Vogel, Bremerhaven University of Applied Sciences).

      Two surveys were carried out, in the summer of 2009 and 2011, in which more than 300 people living at the coast and more than 700 tourists were asked for their experience and opinion of offshore wind energy utilization. The Special Report entitled „Acceptance of offshore wind energy utilization“ by PD Dr. Gundula Hübner and Dr. Johannes Pohl provides insight into selected results of the surveys.
  • ORECCA - marine energy
    • The move to alternative energy sources not only involves the transformation of energy generation on land but also the utilization of energy sources in the sea. The potential of marine energy is being researched in the ORECCA research project which is being funded under the EU 7th Framework Programme for Research, Technological Development, and Demonstration Activities (FP7).

      The first results indicate that offshore wind is the biggest offshore power source (over 90%). This requires usage of areas of the sea having water depths of greater than 50 m. Wave and tidal power plants can also make useful contributions. The combined usage of wave and wind energy on the Atlantic coasts of Ireland, Great Britain, and France is also being discussed.
  • Offshore support structures
    • One of the special challenges for offshore wind energy utilization is the development of safe, reliable, environmentally compatible, and economically viable foundation structures for wind turbines at sea.


      Offshore locations are being exploited in ever deeper waters and at ever greater distances from the shore. In an international comparison, German offshore wind farms are playing a pioneering role and many are in water depths of over 20 meters. Gravity and monopile foundations predominate near the shore, but more complex support structures such as tripods and tripiles are necessary at greater water depths.


      In order to aid the development and testing of these foundation structures, a test stand is being constructed at the Fraunhofer IWES in Hannover which will enable dynamic tests to be carried out.