Wednesday, October 22, 2008
Shooting the breeze
We've started on the road to finding out whether a wind turbine is feasible on the farm. The turbine we have in mind, a Proven 6kW, costs in excess of £20,000 fully installed so we want to be sure that this is money well spent. So we've started logging wind speed and we'll continue to do this for 6 months at least. To be properly viable, we need average speeds of over 5 metres per second (m/sec or about 11 mph) at the height of the turbine hub; that's about 15 metres above the ground. As I write, the wind is blowing at double this speed. But how did we get a setup working to do this? That's the point of this post because I could find very little help on the practicalities of how to do this. I hope others will find our experiences helpful.
Logging wind speed: The basic idea is to get an anemometer up in the air as high up as the turbine would be and then log its readings for as long as possible. Fifteen metres is quite a height so we settled for 11.5m which we achieved by buying 8 lengths of swaged (one end made smaller so that it fits inside the next pole piece) aluminum pole with the other bits and pieces needed to support it in strong winds. I found an excellent company called Moonraker which sells masts, spiders (clamps which fit on the mast at intervals and enable you to attach guys) and guys. I've already given details of the anemometer we're using. There's a slide show here so that you can see the stages of erecting the mast.
How to put up a mast: First, do a scale drawing. The scale I used was 1cm to 1 metre (1:100). You need a map view (as if seen from above) and a vertical section. These drawings allow you to work out exactly how long the guys will be. The angles between the guys and the ground should be 45 degrees. For an 11-metre mast, experience tells us that you need four spider clamps for attaching guys at 120 degrees (map view) to each other to give maximum possible stability. (We tried putting up the mast using just two spiders and quickly realised that they were not sufficient and so took the mast down again.)
Then you need to lay out the locations on the ground of the guy anchors and mast base. This is easily done with a 30m tape and compass. Extreme accuracy isn't needed. Mark the positions with sticks. Next, number the mast sections and work out from your drawing where to attach the spiders so that they are equidistant all the way to almost the top (thus leaving space for the actual anemometer). For guy anchors, I used pieces of reinforcing bar and other pieces of scrap steel I had lying around. Hammer these into the ground at 45 degrees away from the pole and attach the guys to them. I made fencing wire to make eyes for the nylon 3mm guys, top and bottom, so they don't chafe. It's all rather like putting up a tent.
Up it goes: Erecting the mast requires you to have some kind of platform at least half the height so that someone - in our case, Mark, our neighbour - can stand safely and guide the pole as you add pieces from the bottom. I have a scaffold tower and this was perfect for the job. You can hire these. When all is ready, you start by fixing the anemometer to the top piece and then attaching the first 3 guys to the spider directly below (see photo above and also the slide show). I held the mast and lifted it up as Val added the next pieces. The whole mast is no more than about 10 kilograms so this is easy enough. Each time a piece with a spider went on, Val would fetch the respective guy, pre-laid out and cut to the right length, and attach it to the spider with a secure knot. And so on... Mark was up on the platform guiding it and monitoring the spider angles so that the anemometer itself was aligned East-West and the spiders were all in line. Val would also tape the anemometer feed cable to the mast every metre so that it didn't flap about. Sounds complicated but it wasn't. The whole thing went up in about 10 minutes. The secret is in good preparation and treble checking that everything is where it should be and amply long enough in the case of the guys. So far, the mast has stood up to a gale without problems. Plenty more gales can be expected, for we're approaching winter and the jet stream steers plenty of heavy weather across Wales.
Sit back and watch the data! Not just yet. The final stages are to adjust all the guys so that they mast is vertical as seen from anywhere. It helps to have a long spirit level to do this. Guy adjustment is easy using standard tent guy adjusters, available from camping shops. We were tweaking the adjustments for the next day or so as the winds picked up, but don't overtighten. The anemometer feed cable plugs into a battery-powered radio transmitter which we taped near the base of the mast. This transmits the wind speed and direction data to the receiving base station. I find that the reception is fine on the kitchen windowledge, within line-of-sight and about 70 metres away from the mast. Once this is all set up, the data starts rolling in. The base station can collect data every 5 minutes for about 6 days and has a port which allows you to connect it to your computer and download all the stored data. There's a program which comes with the weather station which processes this data and presents it to you either as a list or a graph. And that's it. I've had it running for 5 days so far and it's fascinating to see the results (image below: green lines are gust speeds and red the general wind speed. Click this image for a larger version). Next, I have to find a program which will read tens of thousands of datapoints (1 every 5 minutes for months) and give me the figure we need for average wind speed. I can import the data into Excel but I don't know how to get the program to calculate an average. I'm sure I can find out!