In April 2023, the New Energy and Industrial Technology Development Organization (NEDO) will develop the first "Offshore Wind Observation Guidebook” has been published.
This guidebook provides a detailed summary of the necessary information about wind condition surveys, which are essential for business planning and wind turbine design for offshore wind power plants. To help you make the most of these surveys, Relatec will explain the key points from the perspective of a wind condition consultant.
The "Offshore Wind Observation Guidebook" was compiled incorporating the latest knowledge from Japan and overseas. Members of Reratec are involved in the aforementioned NEDO project as implementing companies, and are also cooperating in the creation of the guidebook.
In the first part, we introduced the overview of this guidebook and points ① to ③ that you should pay attention to. In the second part, we will continue with points ④ and ⑤, and talk about things to keep in mind when using the guidebook.
5 noteworthy points of the guidebook - ④⑤
④Specify the method for prior accuracy verification
Before conducting wind observations, it is necessary to verify that the accuracy of the observation equipment meets the standards.
In Europe, it is common to conduct pre-validation including LIDAR equipment to reduce the uncertainty of observations and increase their reliability. In particular, pre-validation is an important step for Dual Scanning LIDAR (DSL) observations, since the accuracy is expected to vary depending on the user's settings, etc.
Appendix B (p.47) of the guidebook describes how to verify the accuracy of DSL and Single Scanning Lidar (SSL) using wind mast observation data.
Figure 1. DSL accuracy verification examples and thresholds to refer to (Reference 2)
However, this section does not indicate at what site (observation location) the pre-accuracy verification should actually be performed.
Preparing one's own verification site not only incurs costs for setting up the site, but also requires demonstrating the validity of the site itself, which can be a high hurdle for pre-verification of accuracy.
Currently, ReraTech is working with Kobe University and the Japan Weather Association as part of a NEDO project to develop a pre-verification site at Mutsu Ogawara Port in Aomori Prefecture (References 2,3, XNUMX).
The site is open to the public and can be used by anyone, so please make use of it. (Website will be released soon.)
Figure 2. Mutsu Ogawara site in Aomori Prefecture, which is open as a test site
5) Valid data rate and method of filling missing data periods
When conducting annual observations, a certain amount of valid data must be obtained in order to perform a proper evaluation. The threshold for the amount of valid data that must be obtained is shown in the Guidebook (p. 4) after referring to existing guidelines (e.g., Reference 34).
For example, in observations using Doppler LIDAR, which measures wind speed by reflecting a laser off dust particles in the air, there can be periods when observation values are missing due to weather conditions. When the air is very clear, the laser passes through without being reflected, making it impossible to measure the wind, and when there is rain, snow, or fog, the laser is diffused and data cannot be obtained.
In addition, it is expected that there will be periods when data is unavailable due to maintenance, etc., for the Floating Lidar System (FLS).
When there are periods of missing data or abnormal values due to various factors like these, there are methods to supplement the data using observational data from wind masts (MM) and other sources.
The guidebook lists specific methods such as the Measure-Correlate-Predict (MCP) method and the method using double bias correction (p.35), and provides references. It also lists the acceptable criteria for using these imputation methods (such as the coefficient of determination of the regression equation).
Figure 3. Image of observation value complementation using the MCP method
One thing to keep in mind is that these standards in the guidebook are "values to be used as reference" and are not necessarily "equivalent" to the standards required for wind farm certification, etc.
Four issues not covered in guidebooks
This guidebook covers everything from obtaining the necessary permits for FLS to how to install the steam locomotive, making it easy to understand for those who are just starting to conduct offshore wind surveys. However, there are some topics that it does not cover in its entirety.
With the hope of future updates, we have picked out some issues that are of concern and not currently covered.
① Offshore wind survey method
Although the guidebook recommends DSL as the only lidar observation method for turbulence intensity, in reality DSL is not suitable for offshore wind surveys where floating offshore wind power is expected to be used.
The longest laser irradiation range of SL is 10 km, and from the viewpoint of data acquisition rate, the range where stable observation can be performed is thought to be about 4 to 5 km. In other words, the range that can be observed by DSL from the coastline can only cover shallow water areas (fixed-bottom offshore wind turbines).
As mentioned in Explanation ③, there are currently no sufficient technical results for measuring turbulence intensity using FLS, so it is not included in this guidebook. However, research results on measuring turbulence intensity using FLS have already been reported (for example, References 5 to 9).
We hope that further research into turbulence intensity measurements using FLS observations will provide additional information on offshore wind survey methods.
Figure 4. Image of offshore observation
②Further verification through DSL observations
The guidebook recommended DSL observation as a method for observing offshore wind conditions, including turbulence intensity. However, it is important to understand that DSL observation is not a perfect tool for measuring turbulence intensity, and there are still some technical issues that need to be resolved before it can be treated on an equal footing with wind condition masts (cup anemometers, vane wind vanes).
For example, as the guidebook states, "The length of the range gate may result in an underestimation of the wind speed standard deviation," it is necessary to understand how the settings on the device affect measurement accuracy.
In addition, thresholds and standards listed as recommendations, such as the accuracy of the standard deviation of wind speed (5% or less; Guidebook, p. 47) and the elevation angle of the line of sight (less than 5 degrees; Guidebook, p. 18), are also required to be updated based on accuracy verification that provides technical evidence.
In Europe, technology development is being carried out through the Joint Industry Project (JIP) method, and efforts are being made on turbulence intensity measurements using LIDAR (e.g., Ref. 10). In this case, the accuracy standards for turbulence intensity measured by LIDAR equipment are being discussed, taking into account wind turbine loads.
In Japan, measurements of turbulence intensity using DSL observations have already been adopted, so we believe that it is necessary to tackle the above-mentioned issues through speedy research and development, using the JIP method as an example.
Figure 5. Analysis results of turbulence intensity measurements using lidar and other instruments (Reference 10)
The vertical axis shows the relative error for each measurement method compared to the standard (cup anemometer).
3) Observation method for evaluating power generation amount
In a wind condition survey for the construction of a wind power plant, a "wind condition evaluation (wind turbine design evaluation)" and a "power generation evaluation" are carried out.
"Wind condition assessment" derives wind parameters necessary for wind turbine design, and requires a method that satisfies the guidelines for obtaining certification. The guidebook mainly introduces content aimed at obtaining this certification.
On the other hand, it does not describe important points to consider in wind observations or the concept of uncertainty assessment in order to carry out a "power generation assessment."
We hope that as more wind observations are conducted using this guidebook, the guidebook will be updated to incorporate the opinions of people involved in wind power in the field.
④Verification and review of standards
As described in Explanation ②, the standards set out in the guidebook may need to be verified and revised from the perspectives of both the actual conditions at the observation site and ensuring accuracy.
In fact, we hear voices from the field that "the installation standards for DSL and other networks are too strict and do not reflect the actual situation," so we believe that there is room for the standards themselves to be updated through future research and development.
For example, there seems to be room for debate about the "representative radius (the range represented by the observation point, set to include all planned wind turbines)" mentioned in the guidebook (p.11). The guidebook states that "the representative radius over the ocean is 10km, the same as the standard for flat land terrain," but if this is used as the standard, a large number of observation points will be required when planning a long stretch of ocean in the direction of the coastline or a vast offshore farm, which would require very strict wind observations in reality.
Given that there are no guidelines or standards overseas regarding the representative radius at sea, further consideration is required.
Figure 6. Wind turbine arrangement and representative radius at observation point (flat terrain, offshore)
Ueda et al. (2022) (Reference 10) provides an introduction to the guidebook from the perspective of its creators, which we hope you will find useful.
The latest publication is a technical guidebook summarizing recommended methods for offshore wind observation. As with guidelines, following the instructions does not guarantee that a wind farm will be certified. Furthermore, it is important to note that the farm will not necessarily be considered bankable in terms of power generation evaluation.
However, I feel that the establishment of clear standards for all observation items to enable highly accurate observations is a major step forward for the domestic wind power industry.
In the future, as more knowledge and information is collected and updated, a more practical guidebook will be compiled. We hope that the publication of this guidebook will lead to even more excitement for offshore wind power generation.
References
- NEDO, Offshore Wind Observation Guidebook, 2023,
https://www.nedo.go.jp/library/fuukyou_kansoku_guidebook.html
Kobe University Mutsu Ogawara Offshore Wind Observation Test Site,
https://www.lab.kobe-u.ac.jp/gmsc-airsea/mutsu/ - Mizuki Konagaya, Teruo Ohsawa, Susumu Shimada, Shogo Uchiyama, Kazuhiro Kawamoto, The necessity of pre-validation in lidar observations and consideration of establishing an offshore research platform at the Mutsu Ogawara site, Proceedings of the 44th Wind Energy Utilization Symposium, pp.124-127, 2022.
- Carbon Trust, OWA Roadmap for the Commercial Acceptance of Floating LiDAR Technology, 2018.
- S. Uchiyama, T. Ohsawa, Y. Aso, M. Konagaya, T. Misaki, R. Araki, and K. Hamada, Understanding the accuracy characteristics of floating lidar at the Mutsu Ogawara site, Proceedings of the 44th Wind Energy Utilization Symposium, pp.120-123, 2022.
- S. Asakura, T. Osawa, Y. Aso, A Land Motion Experiment for Performance Evaluation of Floating Lidar (Part 44), Proceedings of the 116th Wind Energy Utilization Symposium, pp.119-2022, XNUMX.
- Fuuma Fujimoto, Teruo Ohsawa, Mizuki Konagaya, Takeyuki Misaki, Kohei Hamada, Observation characteristics of vertical lidar at a coastal site, Proceedings of the 44th Wind Energy Utilization Symposium, pp.132-135, 2022.
- Moreno MA, Bellanco MJ, Aghabi, R., EOLOS FLS200 Test Case: accuracy and sensitivity analysis of the three-second gust and turbulence intensity when compared to an offshore met mast. Wind EUROPE OFFSHORE 2019, 2019, .
- Kelberlau, F., Neshaug, V., Lønseth, L., Bracchi, T., Mann, J. Taking the Motion out of Floating Lidar: Turbulence Intensity Estimates with a Continuous-Wave Wind Lidar. Remote Sens. 2020, 12, 898.
https://doi.org/10.3390/rs12050898 - St. Pé, Alexandra, Weyer, Ellie, Campbell, Iain, Arntsen, Alexandra E., Kondabala, Nikhil, Mibus, Marcel, Coulombe-Pontbriand, Philippe, Black, Andrew H., Parker, Zach, Swytink-Binnema, Nigel, Jolin, Nicolas, Goudeau, Barrett T., Meklenborg Miltersen Slot, René, Svenningsen, Lasse, Lee, Joseph CY, Debnath, Mithu, Wylie, Scott, Apgar, Dale, Fric, Thomas, … Matthew Meyers. (2021). CFARS Site Suitability Initiative: An Open Source Approach to Evaluate the Performance of Remote Sensing Device (RSD) Turbulence Intensity Measurements & Accelerate Industry Adoption of RSDs for Turbine Suitability Assessment. Zenodo. https://doi.org/10.5281/zenodo.5529750
- Yuko Ueda, Tomoya Iwashita, Fuminori Heki, Hiroshi Imamura, Introduction to the NEDO Offshore Wind Observation Guidebook, Proceedings of the 44th Wind Energy Utilization Symposium, pp.136-139, 2022.
As wind condition consultants, Rera Tech Inc. will conduct optimal wind condition surveys that combine "observation" and "estimation" for wind power generation. Please feel free to contact us if you have any inquiries regarding wind conditions.