Session F01: Focus Session: Turbulence in Wind and Solar Energy

Is turbulence wind and solar energy's best friend?

Fluid mechanics plays a critical role in renewable energy systems. The role of turbulence in particular has been of growing interest in the community. The APS-DFD wind energy continue to increase in number and recently the role of fluid mechanics in solar energy has also received attention. Turbulence plays a key role in the atmospheric and energy harvesting processes and can lead to both, improvement in energy production and turbine and solar panel failures. The purpose of this session is to: 1.) Have a unified theme for wind energy and solar power and 2.) Provide a platform for those interested in wind energy, solar energy and fluid mechanics to share commonalities between problems observed in both fields. This talk will provide an introduction to the session and expose relevant topics/problems.   

Big wind power: open questions for turbulence research

The growing deployment scale of wind power is pushing the technology into areas where a number of science uncertainties still exist, including fluid dynamics aspects. In particular,  turbulence plays a vital role in interactions between wind turbine blades, wind turbine wakes, wind farm wakes and the atmospheric boundary layer. In this presentation, we briefly review relevant aspects of various research questions that have been articulated recently (J. Turbulence vol. 20, 2-20 (2019)). These questions touch upon the need for better analytical, synthetic and reduced order models of turbulence, better model coupling methods to e.g. couple Large Eddy Simulations to meso- and global scale models, and basic understanding of flow phenomena governing kinetic energy entrainment and limiting power density. We will focus discussion on the latter question, specifically reviewing the budget of mean flow kinetic energy, i.e. the budget of the resource of interest, and the often opposing trends associated with turbulence. These opposing trends are reflected in increased turbulent mixing which tends to mitigate the wake strength downstream and thus increase wind energy production while also increasing turbulent dissipation of mean kinetic energy, which is associated with increased overall losses. Progress along the articulated research directions are needed for robust continued improvements in wind energy and widespread implementation.    

Microscale Winds in Complex Terrain: Insights from the Perdigão Mega Project

More than 70% of the Earth’s land surface is in complex terrain, and thus mountain meteorology has attracted the atten­tion of a wide range of constituencies, including meteorologists, fluid dynamists, and wind engineers. Most past research on winds in complex terrain has been on mesoscales (~1–100 km; hours to days), spurred by air pollution, aviation, warfare, and energy applications. Comprehensive field projects with mesoscale emphasis abound, some examples being VTMX, T-REX, MATERHORN and WFIP-2. More recently, however, microscale (1-500 m; seconds to hours) flows in complex terrain have received increasing interest owing to growing applications in wind engineering, urban modeling and firefighting. To this end, after a decade of preparation, the Perdigão mega-project came to fruition in 2017, participated by EU and US groups that included some 70+ research and technical personnel.  The terrain, instrumentation, and collaborative arrangements of the Perdigão project were all unique, for more information see: https://doi.org/10.1175/BAMS-D-17-0227.1.  This presentation describes the evolution of the project, its conduct, as well as selected results of flow through small topographic features and over heterogeneous surfaces relevant to wind energy prospecting.   

Wind farm flow control: Demonstrating potential at utility-scale

The magnitude of wake interactions between individual wind turbines depends on the atmospheric conditions. Wake interactions are suppressed by mixing in highly turbulent atmospheric flow. For inflow atmospheric conditions with lower ambient turbulence, flow control has demonstrated potential in idealized numerical and wind tunnel experiments to increase wind farm power production. We investigate flow control methodologies to increase utility-scale wind farm energy production in field experiments of turbines of rotor diameter greater than 100 meters. We first characterize the power production of the individual wind turbines and the wind farm depending on the ambient conditions and the turbine control strategy. We then leverage a novel hybrid physics- and data-driven wake modeling framework to optimize the yaw misalignment angles for wake steering control to maximize wind farm power. We impose the yaw misalignment angles at a utility-scale wind farm and measure their impact on the power and energy production of the farm over a period of months. Wake steering control results in a statistically significant increase in wind farm power and energy production.   

Numerical simulations of wind−wave interactions for applications in offshore wind energy

As a renewable energy resource, offshore wind exhibits several advantages than inland wind, such as faster wind speeds, larger available spaces for building wind farms, and relatively less impact caused by the visual and acoustic pollutions of wind turbines. However, offshore wind also possesses some complex flow physics due to its dynamic interactions with the sea-surface waves, making it a challenging task to accurately predict the fluid dynamics and wind power production for designing offshore wind farms. In recent years, considerable efforts have been made for developing computational fluid dynamics (CFD) tools that can capture the complex flow physics in offshore wind farms. This talk will focus on one specific CFD model, in which the large-eddy simulation of wind turbulence is dynamically coupled with the high-order spectral modeling of sea-surface waves and the accurate disk model of wind turbines to simulate the wind–wave–turbine interactions in offshore wind farms. The simulations results provide some useful insights on how the sea-surface waves (e.g., broadband wind-waves and long-crested swell waves) affect offshore wind energy harvesting through wind–wave interactions.   

Enhancement of solar PV systems efficiency through changes in array configuration: The role of fluid dynamics in solar energy.

Efficiency of solar PV cells is linearly dependent on its temperature. Per each degree Celsius above standard test conditions (STC, traditionally 25ºC), efficiency of traditional solar PV cells decreases by 0.5%. Depending on the installation site, operational temperature can be more than 50ºC above standard temperature, with a corresponding reduction in efficiency of more than 25%. Therefore, there is a strong interest not only to develop new solar PV cells made of materials with lower thermal sensitivity (i.e. minimization of the thermal sensitivity), but also to find strategies that can help mitigate the thermal losses.

In this work, we have focused our efforts to enhance convective cooling of solar PV systems with the design of innovative solar module arrangement strategies. The objective is to find new approaches to the installation of solar modules that boost energy harvesting efficiency through the reduction of thermal losses by simple and costless changes of arrangement. For this purpose, we have developed an array of field experiments, scaled wind tunnel measurements, and controlled numerical simulations. With these combined strategies we have quantified the effect on the convective cooling of changes in module row spacing, module height, module orientation, and the use of flow deflectors and vortex generators. The rich dataset has enabled us to develop a new scaling relation between the Nusselt, Prandtl, and Reynolds numbers that takes into consideration not only the flow momentum and thermal characteristics, but also the spatial arrangements of the system. To encapsulate all the variables defining the geometry of the PV system, we have developed a new length-scale based on the metric of lacunarity.

Results from this work illustrates the relevant role the fluid dynamics community can play in developing another prominent renewable energy system.