In a context where the world population is constantly growing and the economy of many developing countries is rising rapidly, global energy demand is not likely to decrease soon. Conventional sources of energy such as fossil fuels are finite and are the cause of many serious environmental threats. Developing new sources of clean and renewable energy is therefore a key issue in a sustainable development perspective. Energy production through the harvesting of marine currents is an innovative sector which is rapidly growing nowadays. Part of the reason stems from the fact that its enormous worldwide energy potential is yet almost unexploited. However, renewable energy sources such as marine-current and wind energy are diffuse compared to the energy coming from fossil fuels [1]. Consequently, it is critical to densify the energy production of hydrokinetic and wind turbines by suitably placing them in farms for this energy sector to become economically competitive. In this context, the choice of a particular technology over another does not depend solely on the efficiency of a single machine but the interactions between the turbines must also be taken into account [2]. Indeed, the close proximity of the turbines can create important blocking effects and the wake of an apparatus can greatly influence the performances of turbines positioned downstream. Moreover, it is found that the actual, fundamental source of energy is different depending on the size of the turbine farms. Indeed, the level of energy extracted by a farm depends on the upstream kinetic energy flux (0.5ρU∞^3) for small turbine farms (few turbine rows) while it depends more on the planform kinetic energy flux (-ρU∞ avg(u'w')) for increasingly large turbine farms (several turbine rows) [3,4,5,6]. As a result, the desired turbines and wakes characteristics one would hope for are not the same depending on the size of the turbine farms. It is therefore important to characterize different types of turbines in regard to the structure of their wakes, the turbulence they generate as well as the dissipation rate of the velocity deficit in their wakes.
A detailed wake analysis of three different hydrokinetic turbine concepts is conducted to gain a thorough understanding of the energy recovery processes at play (e.g., Figs. 1 and 2). The turbine technologies considered are the axial-flow turbine (AFT), the rotating cross-flow turbine (RCFT), also known as the Darrieus turbine, and the oscillating-foils turbine (OFT). The analysis is performed on single turbines placed in a uniform upstream flow and operating near their optimal efficiency conditions at Re =1×10^7. Three-dimensional DDES simulations are carried out using the Star-CCM+ 9.04.011 software from CD-adapco. An original methodology is proposed and used to assess the various contributions affecting the mean energy recovery in the turbines’ wakes. It is found that the most efficient turbine farm is not necessarily composed of the most efficient individual turbines and that the unsteadiness of the turbines operation, which is specific to RCFT and OFT turbines, plays an important and significant role in the mean energy recovery.
REFERENCES
1. Lago, L. I., Ponta, F. L., and Chen, L. Advances and trends in hydrokinetic turbine systems. Energy for Sustainable Development, 14(4) : 287–296, 2010.
2. Dabiri, J. O. Potential order-of-magnitude enhancement of wind farm power density via counter-rotating vertical-axis wind turbine arrays. Journal of Renewable and Sustainable Energy 3, 043104, 2011.
3. Abkar, M. and Porté-Agel, F. Mean and turbulent kinetic energy budgets inside and above very large wind farms under conventionally-neutral condition. Renewable Energy, 70 : 145–152, 2014.
4. Calaf, M., Meneveau, C. and Meyers, J. Large eddy simulation study of fully developed wind-turbine array boundary layers. Physics of Fluids. 22(1) : 015110, 2010.
5. Chamorro, L. Arndt R. E. A. and Sotiropoulos F. Turbulent Flow Properties Around a Staggered Wind Farm. Boundary-Layer Meteorology, 141(3) : 349–367, 2011.
6. Kinzel, M., Mulligan, Q., and Dabiri, J. O. Energy exchange in an array of vertical-axis wind turbines. Journal of Turbulence, 13 : 1–13, 2012.
Topics: Unsteady aerodynamics, vortical flows, aircraft wakevortex dynamics including DES, , Topics: Non-aeronautical aerodynamics, wind effects on structures, surface vehicle aerodyn , Topics: Computational Fluid Dynamics as applied to any of the above, including surface mod
