Posted by Bryce Alsten on June 2016
University of Florida and WindAid volunteer
Research and design; it’s the meat and potatoes of any engineering project. It is also an ongoing process, because no matter how good you think your product or system is, there’s always room for improvement.
The collective understanding that our wind turbine systems need continuous development is what makes working at WindAid a truly immersive learning experience. Volunteers from all over the world with a myriad of skills and experiences come together to learn from each other and contribute their own unique knowledge towards the ultimate goal of improving the way we empower developing communities by providing them with electricity.
There are many factors and constraints that go into the successful design of small wind turbines, but above all others, the most important factor is safety. Currently, one of our main focuses is on ensuring safety over the lifespan of our 500W WindAid 1.7 model turbine by analyzing and improving the structural integrity of the tower assembly.
During our most recent installation in one of our communities, Playa Blanca (on the northern desert coast, where there is little rain, strong sun and very sandy soil), we took note of some concerning levels of degradation in the towers and foundations of some previously installed turbines. The most concerning signs were structural cracks in the concrete foundations (pictured below).
There are two types of cracks in concrete: structural cracks and non-structural cracks. Non-structural cracks are normal and to be expected. They are unavoidable and are often caused by variations in moisture and temperature. A common adage is that there are two guarantees with concrete: one, it will get hard and two, it will crack. Structural cracks on the other hand, are larger (generally wider than 1/8”, or the thickness of a credit card) and must be addressed before they become larger and propagate, leading to eventual structural failure. Structural cracks can be caused by many factors including improper mixing and pouring of concrete, as well as excessive forces acting on the concrete.
It is our opinion that these structural cracks are being caused by a combination of the two previously mentioned factors. Because of the concern over the long-term structural integrity and safety of the turbines that these cracks raise, we have begun the process of completely redesigning the way be build and install the towers. Wind turbines experience a variety of different forces (pictured below), and in general the stresses caused by these forces build up throughout the turbine’s tower, especially at the base of the tower where it is connected to the foundation.
Our current tower design is simple – like the example depicted above minus the tension cables (guy wires). The tower shaft is a circular post that is approximately 8m long and 13cm in diameter made of eucalyptus wood. We use eucalyptus because it is low cost, relatively strong, and readily available in Playa Blanca. The actual turbine assembly is bolted onto the top of this post. We then place the bottom of the tower into a hole that is approximately 1.5m deep and 0.75m in diameter and backfill the hole with concrete.
Based on the evident structural cracking in the foundations, it is clear that we must design a new tower structure that provides more support than our current design does. There are three common types of tower structures used to support wind turbines: lattice, monopole, and guyed mast (all pictured below). Our current design is a monopole. After extensive research, we have decided that the best option to properly support our turbines is to use a guyed mast structure, the most commonly used type of support structure in smaller scale turbines.
However, using guy wires in our redesign brings a new problem – a significant increase in the footprint area of the tower. This is a problem is because in Playa Blanca, the tower foundations for all the existing turbines are located within 0.5m from the walls of the owners’ houses. This means that a traditional guyed mast tower setup would require anchoring one or more of the guy wires inside of the owner’s houses, which is simply not a realistic option. This issue has forced us to get creative. We want to use guy wires but we don’t want to increase the size of the tower’s footprint.
Our preferred solution is to add horizontal crossbeams towards the top of the tower that allow us to redirect the guy wires back towards the tower’s base while still maintaining proper tension. Think of the new tower design as a lower case letter t. The guy wires are attached at the top of the t, then at the ends of the cross of the t, and then again at the bottom. The only difference between our theoretical design and this simplified letter t example is that there will be three guy wires spaced 120 degrees apart so there will also be three beams supporting them as opposed the two that you would visualize on the t –shape.
Decisions now must be made on several factors including but not limited to: how high up the guy wires should be installed, how long the cross beams should be, and how much pretension should be installed in the guy wires. We will be able to answer these questions when we finish analyzing the forces and moments experienced by our turbines in Playa Blanca. When we are confident with the accuracy of our numbers, we can begin optimizing the equations and running finite element analysis (FEA) on the new structure design in programs like SolidWorks so that we can be confident that our design will remain structurally safe and sound for the entire lifespan of the turbine.
If you have any comments or suggestions on this article, we would be interested to hear from you at [email protected]