WINDIE™ Computational Fluid Dynamics in Megajoule’s Wind Assessment Studies
a report by Carlos Silva Santos1 and Miguel Ferreira2 1. Managing Partner, Megajoule Inovação, Lda; 2. Founder and Executive Board Member, Megajoule
Uncertainty and Non-linear Models
The importance of wind resource assessment on the feasibility analysis for a wind farm project is obvious, as the wind characteristics determine the energy production and therefore the annual revenue. Indisputable as this statement might be, the reasons that lay behind it are often not fully perceived.
Considering that the wind power density varies with the cubic power of the wind speed, even a small variation in the annual average wind speed – even tenths of m/s – is sometimes sufficient to induce an important variation in the annual energy production.
Furthermore, the physical complexity of the wind flow leads to a significant spatial variation of the wind characteristics, mainly in orographically complex or densely forested regions where considerable differences can be identified across a wind farm area, sometimes even over a few hundreds of metres. A wind farm located in a very complex site, at one of the major mountains of Portugal, comprising 20 wind turbines on a ridge line with less than 2 km, is known for having energy production differences of around 40 %. Being so, wind measurement must be complemented with the use of simulation models, in order to map the wind characteristics over
Carlos Silva Santos is Managing Partner at Megajoule Inovação, Lda. He is also a Lecturer at the Porto Superior Institute of Engineering. Carlos’ research interests include atmospheric flows, computational fluid dynamics (CFD) for wind engineering and turbulence modelling. He received a PhD in Computational Fluid Dynamics, from the University of Bath and worked as a Senior Research Fellow at the Faculty of Engineering of Oporto at the Centre for Wind Energy and Atmospheric Flows. In 2009,
Carlos left academia to set-up the R&D company Megajoule Inovação, in collaboration with colleagues and the Megajoule Group. Its goal is to become one of the main players in the development of innovative solutions for the renewable energy market. Megajoule and International Student Exchange Programs (ISEP) have developed WINDIE™ CFD, which has already been applied to over 1 GW of installed power worldwide.
E:
carlos.santos@
megajoule.pt
Miguel Ferreira is Founder and Executive Board Member of Megajoule, as well as a university teacher in Renewable Energy. His core research interests include wind resource assessment and uncertainty in renewable energy assessment. Miguel also serves as Director of EnergyIn (Portuguese Association for the Competitiveness and Technology in the energy sector) and is a Member of European Wind Energy Technology Platform and the Programme Committee of the European Wind Energy
Association for its annual conference. He has an MBA in Management and Mechanical Engineering from the University of Porto and serves as a wind energy consultant, specialising in wind resource assessment. In 2004, he founded Megajoule, the Portuguese leader in wind assessment consultancy.
the area available for the wind farm and specifically calculating it for the wind turbines’ installation sites.
Also, due to the physical complexity of the wind flow, its numerical simulation is not straightforward. Historically, the wind energy industry has been using linear models, which make use of a set of simplifications in order to make it commercially usable. These models, like the industry-standard Wind Atlas Analysis and Application Programme
(WAsP) developed by the RISØ National Laboratory of Denmark, have been able to achieve both the necessary speed of calculation and user-friendliness to become usable at commercial level.
Simulation models, however, carry a significant uncertainty. Naturally, the simplifications, while permitting to reach results and perform dozens of calculations testing several different solutions, include a considerable amount of uncertainty. It has been a common assumption among the industry that, even making use of state-of-the-art wind measurements and simulation tools, a minimum level of uncertainty of 10 % should be considered, the value increasing for less well-described projects, and the wind flow simulation accounting for the major part of the uncertainty value.
The uncertainty of the wind assessment is a major issue when analysing project feasibility, not only from the investor point of view but also from the financing institutions’ perspective. In a time when credit access is becoming more and more difficult, the reduction of the uncertainty levels has become a major issue for the industry. In fact, the European Wind Energy Technology Platform (
www.windplatform.eu) has defined in its vision the goal of being able to decrease the uncertainty associated with wind assessment to a level of 3 % until 2030.
Computational Fluid Dynamics Modelling in Wind Energy To be able to achieve such an uncertainty target, a considerable evolution in the simulation models is necessary, mainly by bringing the use of computational fluid dynamics (CFD) models to industry standards. These models have historically been considered capable of achieving more accurate results but, on the other hand, deemed as requiring a much larger calculation load, leading to calculation times incompatible with the industry needs, and too complex to use, with only very experienced scientists being capable to extract all its potential. Furthermore, the danger of reaching completely wrong results when leaving it to inexperienced users has also been a source of risk when using CFD. Requirements that are often mentioned when discussing the spread of CFD simulations to the whole wind industry use are threefold:
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first and foremost, they must provide more accurate results than existing linear tools;
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Wind
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