Power Management
Designing for green energy
Modelling and design simulation software are becoming increasingly important in the development and production of renewable energy systems, as Bruce Klimpke and Dr. Peter Van Duijnsen explain
G
reen/renewable/sustainable energy, carbon neutral - these, and many more, are the buzzwords bandied
around by politicians, engineers and the public. Promoting, creating and developing green energy is no longer the preserve of the eco-warrior, it is at the forefront of all up-and-coming software designs from fridges to large-scale industrial power plants. The 20th century was the era of establishing awe inspiring new technology; cars, the space shuttle, computers the list goes on. The beginning of the 21st century will be known as the epoch of redesign for a greener, more environmentally friendly world.
While using modelling and design simulation software is a well-established practice, it is becoming ever more important in the design and production of renewable energy systems. Not only for, say, solar cells and wind turbines but also in the redesign of existing power systems and consumer products. The engineering effort required to imagine new ideas and then realise them is large but with the help of simulation software the two biggest obstacles, reduction of cost and improving system efficiency, can be comprehensively overcome.
One of the most popular, and always
improving, types of sustainable energy is that of wind energy – in the form of a
wind turbine. The concept of wind energy has been around for a long time now and so, from its hypothetical beginnings through to its conception and common use, the technology behind wind turbines has always been cutting edge. One of the common, yet false, arguments against wind power is that it is inefficient. The Betz law limit which took into account the fundamental laws of conversion of mass and energy showed that no more that 60% of the kinetic energy of the wind was able to be captured. When taking into account other factors (some of which we will cover later on) the actual figure is more likely to be no more than 45%. While this conversion level may seem to be poor, when compared to that of coal and oil powered stations that are between 30-40% efficient, wind power seems a much more attractive solution.
When it comes to improving the
efficiency of wind turbines much of the effort is put on improving the Tip Speed Ratio (TSR) because power is proportional to velocity, therefore twice the velocity = eight times the power. However one area of wind turbine design that can be significantly improved by the use of design simulation software is that of the generator.
Figure 1: Finite Element simulation of the induction motor. The larger plot is the magnetic field magnitude; the insert is the induced current density.
32 October 2011
Generation Wind turbines generate electricity through asynchronous machines (induction motors) that are directly linked with the electricity grid and power is supplied to the motors by means of electromagnetic induction. One of the crucial areas to be explored through simulation software is the armature, which interacts with a field magnet enabling it to transfer current crossing the field and generate an electromotive force (EMF). As the motor in the turbine is acting as a generator, the EMF drives the armature current, and shaft mechanical power is converted to electrical power and transferred to the load. The role of simulation software is to make these motors work as efficiently as possible through analysing all the component parts and the area surrounding
Components in Electronics
them. As with all electrical products and systems, energy is not conserved and a proportion of it is converted into heat caused by eddy currents. This creation of heat through ohmic and or dielectric losses is highly important to the design as it will ultimately determine its performance. Even a 1% increase in efficiency can lead to a loss of 20kW.
Using modelling techniques such as Boundary Element Method (BEM) for electrical field calculation and Finite Element Method (FEM) for thermal analysis makes simulation software solutions for energy conservation simpler and more time efficient. Specialised FEM software that can model the generator in detail with manufacturer data for magnetic and ferromagnetic material is used here. Hybrid FEM/BEM solvers are also in favour, since they do not require meshing of the airgap and therefore are more accurate than only FEM solvers. From the hybrid simulation, look-up tables are generated that can be used in the system level or circuit level simulations.
Keeping the overall system in a state of stability is perhaps the key goal of design simulation. Having a turbine that can deal with and work efficiently throughout different wind speed conditions is vital and by using FEM/BEM, optimisation and parametrics the different components will work in harmony creating a much more efficient turbine.
Solar power is one of the most diversely used forms of renewable energy. The power of the sun can be harnessed on a large scale such as the 354 MW SEGS CSP solar power plant in the Mojave Desert of California, but also on a much smaller scale from solar roofing panels to minute solar cells used to power calculators. The conversion of sunlight into electricity is done either directly through photovoltaics (PV) or indirectly using concentrated solar power (CSP).
When dealing with solar modules the
first procedure is to get a complete picture of the power distribution in the system as well as looking at each individual system component and the load it takes. For example, the control system at its early
conception can be designed and tested in simulation. The maximum power point (MPP) in this instance, which tests the production of the cells and applies a resistance (load) to attain maximum power for any given environmental condition, can also be tested together with the solar module, DC converter and grid connection. The second step is to look at each component in more detail on the second, the circuit level. One technique is to replace the idealised system models with more detailed models. This increases overall simulation time, but gives greater detail in the simulation results. For example, a model that includes semiconductor switches (IGBT, Mosfet and diodes) can replace the ideal continuous model of a grid-connected inverter. The simulation results would now also include harmonics as well as typical modulation influences. The third step would be to look at each component more in detail on the third, the component level. Instead of using more comprehensive lumped circuit models with limited parameter sets from the second step, the models could be based on more detailed engineering software. For example, a non-linear model replaces the three-phase generator circuit model, where the input-output relations are pre calculated in Finite/Boundary Element Method (FEM/BEM) software.
This article has focussed on some of the
key areas that are vital for improving efficiency in the designs of green energy sources and devices. Design simulation software will help not only with improving efficiency and cutting costs with current up and coming technology but also provide a platform for the technology of the future. Enabling companies to greatly improve their costs when producing environmentally friendly products means that green energy will be at the forefront of new design possibilities.
Integrated Software |
www.integratedsoft.com Bruce Klimpke is Technical Director at Integrated Engineering Software and Dr. Peter Van Duijnsen leads Simulation Research
www.cieonline.co.uk
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