Computational Fluid Dynamics

Transcription

1 Computational Fluid Dynamics A better understanding of wind conditions across the whole turbine rotor

2 INTRODUCTION If you are involved in onshore wind you have probably come across the term CFD before but may not fully understand its application and benefits. This guide describes how, if used to its full potential, CFD aids understanding of turbine performance in complex wind regimes leading to higher P90s. Why CFD matters Photo 1 - Simple terrain Photo 2 - Complex forested terrain These photos show sites with very different terrain and ground cover. These aspects, along with atmospheric stability (relative buoyancy of air, often due to temperature variation) are what primarily influence wind flow. On simple flat sites, as in photo 1, wind characteristics tend not to vary from one location to the next, so wind measurements combined with traditional flow models (eg. WAsP) do a good job of representing the wind at each turbine. Not so in photo 2, where trees and slopes cause large variations in turbulence, rendering traditional models for predicting wind speed and shear ineffective. Given that it s not economically viable to blanket a site with masts to get measurements at every turbine location, CFD allows accurate prediction of wind characteristics at these complex sites.

3 HOW CFD HELPS Reducing development equity A better P90/P50 ratio means less capital investment by the owner. One of the best ways to do this is to reduce uncertainty in the energy yield prediction. CFD has the double benefit of reducing both horizontal extrapolation i.e. wind flow modelling, and turbine performance uncertainties. Even if we consider a small wind farm, the reduction in equity can be up to 20 times the original cost of CFD modelling. For more detail on this, see our upcoming guide on how to reduce energy yield prediction uncertainty. Picking the best place to put turbines Imagine the kind of performance you would get driving a car down a narrow lane with two wheels on the tarmac and two in the ditch. Placing a turbine where a portion of the rotor is subject to high turbulence and shear has a similar effect with consequent impact on O&M costs and performance. CFD allows these areas to be identified for hilly and forested sites so they can be avoided. Real world turbine performance Turbine power curves are measured in simple wind conditions, as depicted in photo 1 on the previous page. If this power curve is then applied to the complex wind conditions in the other photo, as is common practice, the resulting energy yield prediction is likely to be optimistic. Many consultants use CFD to predict wind speed at hub height only, but at Prevailing we are unique in using CFD output to understand wind flow across the whole rotor. This brings realism to turbine performance predictions in complex wind conditions. CFD is advantageous for the following reasons: Higher certainty wind speed, shear and turbulence predictions; Optimal turbine positioning; Accurate turbine performance predictions; Improved project finances due to higher overall P90.

4 Examples 200 MW project, UK - Maximising the energy yield and P90 and reducing lifetime O&M costs. CFD was employed to highlight areas of highly volatile flow. These areas were then avoided during turbine micro-siting, maximising turbine performance and lowering lifetime operations and maintenance costs. CFD results also reduced the energy yield uncertainty, giving the developer a double benefit. 75 MW project, UK Minimising energy yield loss from trees The client needed to determine which of 3 possible felling strategies would lead to the lowest energy yield loss. The influence of each of the felling strategies on wind flow was modelled using CFD and the one with lowest energy yield loss selected. 150 MW project, Chile Improved energy yield estimate for large complex terrain site The client was developing a 35km² complex site in Chile. There were masts on site but some turbine locations were situated 11km away in locations where the wind flow differed markedly due to upwind terrain features. The use of CFD results gave the client far higher confidence in the energy yield estimate because they explained the influence of the complex terrain lying upwind of the site. Additionally, the wind conditions predicted by CFD modelling were used to decide where best to position a LiDAR in order to gather additional wind measurements.

5 WHAT IS CFD? You ve now grasped that CFD is essential to accurately model wind flow across complex wind farm sites, but how... Meshing Imagine a hilly, forested wind farm site in 3D. We first define the region to be considered by the model. This region is called the domain. The domain must be significantly larger than the site to allow the simulated flow to develop. Next, we divide the domain into millions of individual cubes or cells. This is referred to as a mesh. Information about topography, atmospheric physics, tree heights, dimensions of buildings or other solid obstacles and surface roughness is provided to the mesh. The 3D mesh fills the domain which covers the wind farm site and surrounding region Simulation Wind from each direction is then pushed through the domain. Fundamental equations describing fluid flows are solved iteratively within every cell of the mesh to determine how the wind will behave. The result of this calculation is a HUGE amount of data (around 100 GB for an average wind farm). The complex physics that can be included within a CFD model allows the effect of complex terrain features and atmospheric variations to be determined more accurately than ever before. Results processing The trick is to process the data produced by the simulation to obtain wind parameters such as wind speed, turbulence, shear, veer and inflow angle at all locations within the 3D mesh across the whole wind farm, then interpret these parameters in a way that provides most benefit to the wind farm development. Powerful stuff!

6 FAQs Do the benefits of CFD offset the cost? CFD requires some additional investment above more typical analysis methods because running the model requires a cluster of computers and interpreting the data requires a high degree of specialist knowledge. If CFD is applied at feasibility stage for an average northern European project, the capex and opex benefits from increased yield, better capex deployment, reduced wind farm O&M costs and lower energy yield estimate uncertainty far outweigh the cost. Even if CFD modelling is only used in the final financial grade study, the financial gains from the increase in P90 are considerable. Will CFD significantly increase the analysis timescale? Prevailing can typically deliver an energy yield assessment including CFD results within a 3-4 week timescale, and regularly do so to support investors in making more informed acquisition decisions. We re deliberately set up to deliver results quickly, enabling our clients to bid with confidence. Which sites benefit most from the application of CFD? Sites with steep slopes, forests or buildings are where CFD really excels. CFD applies more complex turbulence modelling, enabling flow in these areas to be more accurately predicted than by using WAsP or other simplified methods. However any site where there is a considerable distance between turbine locations and the on-site measurement location is likely to benefit from the application of CFD. At which point in the project life cycle should I apply CFD? The results from a CFD simulation remain valid throughout the project development life cycle. Therefore to get most value from the investment, CFD should be carried out as early as possible in the development of the project. Applying CFD early in the development process minimises the number of iterations required to design an optimal wind farm and provides considerable cost savings. As more on-site measurements become available the simulation results can be re-applied to subsequent energy analyses with minimal further investment. Lots of consultancies offer CFD but their services and costs vary considerably. Why the difference? For CFD to provide maximum value a high quality simulation must be carried out. It is also essential that the results of the simulation are properly interpreted to benefit the energy yield prediction. Prevailing is unique in considering flow conditions across the whole rotor and the resultant impact on turbine performance and lifetime O&M costs. We also specialise in communicating results clearly and concisely to the client.

7 CONTACT HEADQUARTERS UK - ENGLAND Prevailing Ltd 10th Floor Tower House Fairfax Street Bristol BS1 3BN GERMANY Prevailing Europe c/o Combinat 56 Adams-Lehmann-Strasse Munich UK - SCOTLAND Prevailing Ltd 2 Woodside Place Glasgow G3 7QF USA Prevailing North America 707 SW Washington Street, Suite 1100 Portland, OR tel. +44 (0) enquiries@prevailinganalysis.com Correct at time of print 01/2017