This article is to introduce a new concept of a tethered wind energy generation after a short overview of airborne wind energy generation to utilize strong and steady wind on high altitude of sky. The tethered wind energy generation system consists of a generator placed on the ground, a windmill on high sky, and tether connecting both of them. The weight of the windmill is designed to be light and the heavy generator is placed on the ground. Energy generated by the windmill in high sky is transferred to the generator on the ground through sophisticated tether technology. Roadmap and overview of the present project is introduced at the end of the article.

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Because a significant portion of the topography in Japan is characterized by steep, complex terrain, which results in a complex spatial distribution of wind speed, great care is necessary for selecting a site for the construction of Wind Turbine Generators (WTGs). We have developed a Computational Fluid Dynamics (CFD) model for unsteady flow called Research Institute for Applied Mechanics, Kyushu University, COMputational Prediction of Airflow over Complex Terrain (RIAM-COMPACTR). The RIAM-COMPACTR CFD model is based on Large-Eddy Simulation (LES) technique. In this paper, the numerical wind simulation over the Shiratakiyama wind farm was executed using the high resolution elevation data. In order to reproduce terrain-induced turbulence numerically, it is shown that both of the horizontal grid resolution and the time increment are extremely important. As a result, the numerical results also showed that it is possible to reproduce energy cascade of actual terrain-induced turbulence well in the frequency-wavenumber domain.

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At the Atsumi Wind Farm in Aichi Prefecture, Japan, damage to wind turbines occurred frequently due to terrain-induced turbulence. In the present study, numerical wind diagnoses (analyses of terrain-induced turbulence) were conducted by reproducing the topography in the vicinity of the wind turbine sites in high resolution and using RIAM-COMPACTR natural terrain version, which is based on large eddy simulation (LES). The results of the diagnoses indicated that, in the case of east southeast wind, terrain-induced turbulence is generated at a small terrain feature located upstream of Wind Turbine No.2, which serves as the origin of the turbulence. The generated turbulence strongly affects the wind turbine, causing the wind speed and direction at the wind turbine site to change significantly with time. At the Atsumi Wind Farm, a combination of the series of wind diagnoses and on-site operation experience led to a decision to adopt an "automatic shutdown program" for WTs No.1 and No.2. Here, "automatic shutdown program" refers to the automatic suspension of wind turbine operation upon the wind speed and direction meeting the conditions associated with significant effects of terrain-induced turbulence at a wind turbine site. The adoption of the "automatic shutdown program" has successfully resulted in a large reduction in the number of occurrences of wind turbine damage, thus, creating major positive economic effects. Specifically, the following economic effects have been realized based on converting the performance of the two wind turbines combined from the two years preceding and the two years following the adoption of the "automatic shutdown program" into the performance of a single wind turbine per year: 1) a reduction in the repair costs by 9.322 million yen per year per wind turbine, 2) an increase in the availability by 8.05 %, and 3) an increase in the capacity factor by 1.70 %.

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