Two subjects to report back on in this diary entry: variable speed running and moisture.
1. Variable speed running
Since the last entry in which I floated the concept of variable head / variable speed operation, there hasn't been much opportunity to test out the idea. A spell of rainy weather has meant that instead of decreasing flow, I have needed to put in bigger nozzles to harvest as much as possible from the unseasonably generous flows available. I'm still aiming for that elusive total for the year of 4 MWh.
What has interested me is that I have come to read that variable head / variable speed operation isn't new and is by no means confined to the small scale of a Powerspout: pumped storage installations of several megawatts in size in Switzerland, France, India and elsewhere are being built to work this way, or in some cases converted to this way of operating when older, conventional synchronous pump-turbine units are refurbished. Apparently, the new way of operating confers efficiency gains on both the pumped and generating cycles. More can be read about it in Hydroworld magazine here and here.
The silica gel bags I placed in the SmartDrive enclosure 6 months ago have lately been seeming to come to the limit of their ability to take up moisture: relative humidity in the enclosure has risen to 51%. This is still below ambient RH which is usually about 70-90% but the figure is significantly up from the usual 10-20% which was evident when the bags were fresh.
Weighing the bags before and after recharging them in the oven (120℃ for 2 hours) showed they had taken up 245 mls of water during the six months. Certainly the bulkhead has always been completely dry on the few occasions when I have looked inside and I conclude that the use of silica gel has been a good thing. On the basis that moisture and electricity are never good bed-fellows, I plan to continue using silica gel and re-charge the bags every 6 months.
At the time of removing the silica gel bags, I took the opportunity of experimenting with packing out the rotor to see what effect it had on operating voltage and shaft rpm, - hence the tachometer and rubber O-ring visible in the picture above. The experiment needs to be repeated at different flow/power conditions before a conclusion can be drawn and if there's anything useful to feed back, it'll be the subject of another update.