In order to meet the world’s energy demands in a sustainable manner, engineers need to deliver robust innovative technology. For 25 years, CD-adapco has enabled energy engineers to do just that in the ‘traditional’ energy sectors. Now we’re routinely applying our technology and expertise in the renewable energy sector as well.
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CFD can be used to simultaneously simulate the efficiency of a wind turbine while calculating the structural loading upon it.
A wind turbine is essentially a very large, inverse fan. Instead of using electricity to produce a small breeze, the wind pressure acting on the blades of the turbine turns a rotor which, via a gearing mechanism drives an electricity generator. The challenge for wind turbine designers is to extract the maximum amount of energy, in the widest range of conditions, at the least cost. Obviously no wind turbine is able to extract all the energy from the wind (operating at a theoretical 100% efficiency); if one did so the air behind the turbine would be perfectly still, even in the windiest conditions, preventing new air from passing through the blades of the turbine.
In order to maximize their efficiency, most current wind turbines were designed using extensive experimental model testing. Although experimental analysis provides considerable insight into the performance of a particular design, physical prototypes are expensive and time consuming to construct.
Unlike testing of physical prototypes, CFD simulations are typically carried out at full scale (the computer model has the same dimensions as the actual production wind turbine rather than those of a smaller experimental model). This has the considerable advantage that results can be interpreted directly and do not have to undergo scaling, a process that can introduce a significant uncertainty, especially for transient phenomena such as the impact of an extreme gust event.
Aerodynamics and Structural Loading
During the design of wind turbines, designers aim to simultaneously increase a turbine’s power output over the full range of operating conditions, while improving durability. In a recent project, wind turbine design company RingWing used STAR-CCM+ to improve the design of their innovative shrouded turbine. The simulations provided detailed insight into the aerodynamics of the turbine as well as the force loading on it, enabling engineers to quantify the effect of design changes.
Upfront design investigation can have a big impact on a project’s profitability. In another wind turbine design study, this time by the University di Udine, Italy, the business benefit of early understanding of designs, prior to prototyping, was recognized. Having compared various designs using STAR-CCM+, they gained a level of insight that had otherwise been unattainable. University di Udine’s Hans Grassmann states CFD’s impact on the bottom line.
“If the simulations had been carried out before the prototype stage, millions of dollars could have been saved with obvious benefits to the project profitability and overall success. But the success did not stop there. STAR-CCM+ was able to assist in finding a solution.”
Keeping turbines in a clean airflow at onshore wind farms significantly increases their power output and longevity. STAR-CCM+ has been used to simulate the airflow over a potential site and turbine configurations to enable wind farm developers to visualize complex wind patterns, identify areas of high wind speeds or turbulence and optimize turbine positioning.
A combination of atmospheric boundary-layer inflow conditions and coupling to meteorological codes ensures accurate representation of the local weather conditions.
As well as predicting how a given turbine design will perform under standard operating conditions, CD-adapco's CFD solutions are routinely used to understand how a turbine will react upon in extreme loading. This technology allows engineers to optimize the aerodynamic efficiency of the turbine and understand the range of conditions under which safe operation can be assured. The biggest advantage of CFD is that its rapid turn-around time helps to break the dependence of wind turbine design on pre-existing design codes. Although design wind conditions are a useful starting condition for analysis, CFD simulation allows designers to more easily pursue multiple “what if?” scenarios. Once a CFD model for a turbine is set up, it is relatively simple to repeat the calculation for multiple loading scenarios. A further advantage is that, instead of being restricted to retrieving data from a few experimental monitoring probes, data is available at every point on the turbine, at every discrete time interval for which the simulation is performed. The simulation results can be viewed from any angle, and the instantaneous forces acting on any part of the structure can be calculated. Data from CFD calculations can also be used to assist other types of analysis, for example, the forces acting on a turbine can be exported to a stress-analysis software package. In extreme cases, where fluid forces cause large deflections of turbine blades, the CFD simulation can be coupled directly with the stress analysis tool and both stress and fluid simulations can be performed simultaneously, each simulation feeding new boundary conditions to the other.
Offshore Wind Farms
Coping with higher wind-speeds, corrosion and wave impacts, the challenges of producing durable, cost-effective offshore wind farms are great, but far from insurmountable. One example where 3D flow simulation is having a significant impact is in modeling wave loading. The loading on the turbine-supporting structure can be determined for different wave heights, sea-states or wind speeds, at full-scale and without the expense of manufacturing prototypes or physical testing. Technology that’s been routinely used in the marine industry for years is now being applied to reduce the impact of the harsh offshore environment on wind turbines.