Windsmithing – Not Recommended on a Windy Day
The Nordic Folkecenter (NFC) is full of wind turbines, some big, some small and even more that don’t work. There is a Test Field with platforms and foundations for testing of electricity producing windmills of 1 KW up to 30 KW as well as mechanical wind pumps. The Test Field is equipped with data loggers, wind measurement masts, towers for installation of wind turbines and water wells where the performance of small windmills for electricity and water pumping can be measured.
Over the years they’ve tested numerous wind turbine designs and last week Michelle, Jesse (the resident Californian) and I were tasked with fixing two of these delinquent turbines. We were all really happy with this task as it is one of the best ways to learn about how turbines work. Tupac (the lead engineer) also claims that fixing poorly designed windmills has the added bonus of teaching us how not to design them!
In order to fix a turbine you need two things: (i) slow wind speeds ( >~8m/s) and (ii) a man lift. Tupac and I drove off to Hurup to pick up a sketchy man lift from the local tool rental shop. We hooked up to the lift and towed it back to NFC so that we could get to work. The first turbine called the ?Uni? had two small problems which took us most of the day to fix. The 7.5 kW Uni is a Folkecenter designed turbine which is built like a brick sh_t house. Most turbines of this size are not built to last but this one is basically a scaled down version of a Vestas 1 MW machine. It uses a uniframe design which allows it to be compact, robust and very reliable. The turbine has a hydraulic disk break on a fast shaft for stopping at slower speeds and centrifugal air breaks on the blade tips for over speed protection. On the larger machines you can not use a disk break to stop that much momentum without a catastrophic failure so you use a type of aileron which is activated when the rotational speed is too high.
The first order of business with the Uni was to refasten the yaw pinion which allows the machine into the wind. Replacing the pinion was no problem, it was getting to it that was hard. Unfortunately the design of Nacel (wind turbine body) was poor and so accessing various components while 15 m in the air is not all that easy. After we had fixed the yaw pinion Jesse and I were asked to reinforce the Nacel on the Uni as it shakes like Nitin Parimi on the dance floor when the winds are high. While we were fixing the Uni the winds were quite low and the sketchy man lift worked alright. The next day however was a totally different situation.
The second turbine that we had to repair shall remain unnamed on this blog as it is a P-o-S. The manufacture of the turbine appears to happen somewhere in China and everything that could go wrong did go wrong.
The turbine is a simple 1 kW machine with a tail furling mechanism. It employees a slip ring under the turbine to allow the turbine to orientate itself to the wind without being restricted by a fixed cable going to the ground. The turbine, installed just over one year ago, was not properly designed for the monster Danish winds prevalent here. After one year of pummelling from the wind (more than 20 m/sec in the winter time) the turbine had to be removed, dismantled and rebuilt.
After ordering and installing a new insert fitting from China, we ascended 15 meters to the top of the tower to discover that the new insert had a larger diameter than the original and therefore would have to be ground down. After grinding the insert down, we returned the next day to ascend in the man basket to the top of the tower again. The wind speeds were high and we hesitated, but decided that we did not want to keep the man lift rented for another day. Once all the way up, Tupac and I immediately regretted it.
First off, we found out that the new insert bolt pattern was different and therefore we could not install the machine. Second, the wind had picked up enough that the man basket started to sway significantly from side to side. Out of fear that the lift would tip over we actually tied the man basket to the tower (thank-god we had brought up some rope!). We initially thought to leave the man basket in the air and climb down the tower but decided to use a makeshift pulley system with the rope and attempt to kept tension on the basket to minimize swaying as we slowly lowered ourselves. It took us about 45 minutes to get safely back to the ground. I was very glad to plant my feet onto a solid surface again.
I guess you could say we learnt two valuable lessons- how not to design a wind turbine and how not to underestimate the wind load. You bet I’ll be checking the wind speeds before climbing into that basket again!
As an aside, this last weekend the wind speeds toped out at 20 m/s and Denmark was producing around 80% of it’s energy from wind mills. The wind turbines were making so much power they actually surpassed the capacity of the central plants. Some critics of wind power claim that the surges in wind power that we experienced this weekend are the reason that wind power cannot exceed 20% of the total grid capacity. For instance, recently announced in Alberta by the Utilities Board was a wind power production upper limit of 900MW (~12%). As demonstrated here in Denmark and at the Folkecenter it is possible to have much higher (some even argue 100%) Renewable Energy and in one of our future posts we will talk about how the Folkecenter is dealing with wind variability. In fact, one of my projects deals directly with the management and control of excess wind power.