Renewable Energy Global Innovations features: Predicting the performance of a floating wind energy converter in a realistic sea

Significance Statement

This research paper focuses on the methods for predicting the working of a floating wind energy converter, for instance, a wind turbine set up on a floating platform moored to the sea bed. Practically, the performance analysis of the floating wind energy converter is normally performed by solving the converter motion equation with a complex convolution integral term that represents the hydrodynamic effects.

The computation of the convolution integral term is complicated, time consuming and demands a large amount of memory on the selected computing machine. Above all, most research works have investigated the performances of the wind energy converters in an ideal unidirectional random sea. The researchers have applied the wave spectra in these works as uni-directional spectra, meaning that the wave energy travels in one direction. However, wind-generated energy propagates in various directions.

Researchers led by Professor Yingguang Wang from Shanghai Jiao Tong University, China, investigated rigorously the performance of a floating wind energy converter set up in a realistic, multi-directional random sea. They considered in the numerical simulation process, all the wind loads acting on the energy converter. Above all, the authors adopted the new state space model, the FDI-SS model, to solve the motion equation of the floating wind energy converter. This was in a bid to enhance the efficiency of the simulation. Their work is published in Renewable Energy.

The authors adopted a 5MW wind turbine with a 3-blade turbine with 126m rotor diameter. The hub diameter was 3m and was 90m high above the still water surface. In order to analyze the performance of the proposed wind energy converter, they numerically integrated the vector-form time domain motion equations with a convolution integral term.

The researchers selected specific load cases for the operation condition. The 10-min average wind speed was 11.2m/s at the top of the tower and 0.15 turbulence wind intensity. They applied a Kaimal power spectrum to characterize the turbulence wind random field over the turbine’s rotor plane. The turbulence wind information was applied to calculate the wind loads prior to the numerical integration. The JONSWAP wave spectrum was adopted in the simulation process of the unidirectional random waves.

In the study, the wave height for a selected sea state was 5m, spectral peak period was 12.4s and peakedness factor was 2.0. The JONSWAP wave spectrum was unidirectional in the sense that the wave energy was travelling in one direction. However, in a realistic sea, wind generated wave energy propagates and spreads in various directions. Therefore, the authors multiplied the uni-directional wave spectrum by a spreading function in a bid to obtain a directional spectrum.

The results of their paper demonstrated a great need for using a realistic, multidirectional sea state when determining electrical generation as well as dynamic responses of the floating wind energy converter. Above all, with an aim of improving the simulation efficiency, the authors used a new state space model to estimate the convolution integral term when solving the motion equation of the floating converter.

Yingguang Wang and Lifu Wang systematically analyzed and compared the simulation results and confirmed the precision and efficiency of the proposed model. They found that the new FDI-SS model could be a helpful tool for the design of floating wind turbines, therefore, helping in the exploitation of renewable wind energy.

Figure caption: Simulated sea surfaces on a square of 128 [m] by 128 [m] based on a directional JONSWAP wave spectrum with Hs=7m, Tp=11s and a cos-2s type spreading function (s=15)).

About The Author

Dr. Yingguang Wang earned a B. S. degree and a M. S. degree from Shanghai Jiao Tong University and the University of Washington at Seattle, respectively. He received his Ph. D degree from Shanghai Jiao Tong University in 2008.

Presently he is an Associate Professor in the Department of Naval Architecture and Ocean Engineering at Shanghai Jiao Tong University (SJTU). He has been teaching in the Department of Naval Architecture and Ocean Engineering of SJTU since February, 2003. The SJTU course “Principles of Naval Architecture” he teaches was honored in 2007 by the Chinese Ministry of Education as a national level excellent course. The textbook “Marine Structural Analysis and Design” sole authored by him is honored as a “China’s 12th Five-Year Plan national key book”.

Dr. Yingguang Wang’s research efforts have focused on the development of techniques for predicting the dynamics and stability of ships and floating offshore systems subject to random environmental loads. Systems exhibiting nonlinear behavior and/or exposed to risk inducing conditions receive his particular attention. In addition, his international academic renown also has much to do with his superb work on the prediction of extreme waves critical to the design of marine structures, including oil and gas facilities as well as ocean renewable energy systems. During the past ten years, his research work in the aforementioned areas has resulted in 32 outstanding journal papers as the sole author or first author in peer-reviewed journals, many of which are leading academic journals in the world. One of his first-authored papers was honored in 2015 as one of the “Top articles in outstanding scientific and technical journals of China” by the Chinese Ministry of Science and Technology. On November 2010, he won a prestigious second-class Science and Technology Advancement Award bestowed by Shanghai municipal people’s government.

Reference

Yingguang Wang, Lifu Wang. Predicting the performance of a floating wind energy converter in a realistic sea.  Renewable Energy,  volume 101 (2017),  pages 637-646.

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Renewable Energy Global Innovations features: Potential threshold of anode materials for foldable lithium-ion batteries featuring carbon nanotube current collectors

Significance Statement

Energy storage devices present a noteworthy challenge in creating a robust deformable system because they must be flawlessly integrated with intelligent wearable devices such as stretchable circuits, epidermal health electronics and flexible displays. The lithium ion batteries are the preferred source of power for such deformable functional devices if they can be compressed, rolled, stretched, buckled and folded. However, selecting an active material for convectional lithium ion batteries poses a major challenge since it easily delaminates or fractures upon being deformed. Moreover, the high-density attribute of the metal restricts the energy density of the lithium ion batteries. Therefore, replacing the metallic foils with light weight, flexible and highly conductive current collector can impart flexibility to the devices and may also rise the energy density of lithium ion batteries.

In a recent paper published in Journal of Power Sources, researchers led by professor Zi Ping Wu at Jiangxi University of Science and Technology in China proposed a study on Potential threshold of anode materials for foldable lithium-ion batteries featuring carbon nanotube current collectors. They aimed at providing a principle for the selection of an active material based on flexible current collectors for foldable lithium ion batteries.

First the research team prepared flexible carbon nanotube macro-film composed of carbon nanotube bundles by chemical vapor deposition. They then fabricated the electrodes by using slurry-based preparation technique. Next, they assembled coin-type half-cells in an argon filled glove box with bare flexible carbon nanotube macro-film disks as working electrodes, lithium titanium oxide and GPE that use copper foil as current collector for loading, lithium cobalt oxide that uses aluminum foil as current collector and lithium metal as reference electrode. These cells were then assembled in a de-humidified room with moisture content less than 2%. Eventually, cyclic voltammetry measurements were carried out using standard electrochemical instrumentation.

From the set up described it was observed that the electrochemical behavior characteristic for the two electrodes, lithium cobalt oxide -aluminum and lithium cobalt oxide – Flexible carbon nanotube macro-film was similar except for the current of oxidation peak. No notable difference was observed in their CV curves and galvano-static discharge/charge profiles. It was noted that when the anode material had a potential higher than 0.9V, good performance of the flexible carbon nanotube macro-film based foldable lithium ion batteries is obtained since the lithium ion passed the potential threshold and the flexible carbon nanotube macro-film retained its electrochemical inactivity. However, if the potential anode is lower than 0.9V several free lithium ions will be constrained. In return the capacity of the flexible carbon nanotube macro-film based electrode will be lowered and the cycling performance will be poor.

The flexible carbon nanotube macro-film based foldable lithium ion batteries generally performed well experimentally. Therefore, it is expected that the mechanism of potential threshold of anode materials will provide new impetus to both industry and academia to explore the development of lithium ion batteries in flexible and foldable energy storage devices.

About The Author

Dr. Wu is an associate professor at Jiangxi University of Science and Technology. He currently specializes on carbon nanotube macro-films for flexible batteries and electromagnetic shielding applications. Under his guidance, a folding insensitive and high energy density lithium-ion battery has been reported in 2015.

Reference

Qing Hui Wang, Sheng Wen Zhong, Jing Wei Hu, Ting Liu, Xian Yan Zhu, Jing Chen, Yin Yan Hong, Zi Ping Wu. Potential threshold of anode materials for foldable lithium-ion batteries featuring carbon nanotube current collectors. Journal of Power Sources volume 310 (2016) pages 70-78.

Show Affiliations

1. School of Materials Science and Engineering, Jiangxi University of Science and Technology, 86 Hong Qi Road, Ganzhou 341000, PR China
2. School of Materials Science and Engineering, Guilin University of Technology, 12 Jiangan Road, Guilin 541004, PR China

 

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