Powered!December 9th, 2010. Post by Quinta do Vale
It was almost time enough to have a baby in, and in many ways it’s felt a bit like a pregnancy, but finally we have hydro power!
Today the sprockets arrived for the water wheel’s gearing. They have been waiting for the last couple of weeks to have US threads machined into them to fit on the spindle of the new permanent magnet alternator from Presto Wind in the USA. So it was just a matter of fitting the M-24 plus framework to the existing framework housing the water wheel’s gear wheel, chain and chain tensioner, adjusting the chain to the correct length, connecting up the M-24 to the junction box and switching on the generator and its charge controller. It only took about half an hour.
Then all we had to do was direct the water back down the chute onto the wheel. I’ve become so accustomed to seeing the freewheeling wheel flying round at anything between 65-85rpm over the last couple of weeks that memories of its sluggish performance with the Miniwind permanent magnet generator we originally installed had been temporarily obliterated and the penny didn’t drop for at least a full minute.
The wheel was still flying round.
We didn’t count the rpm before diverting the water, but having done so so frequently over the last month, I’ve got pretty good at estimating the rpm from just looking at the water volume coming down the barroco. About 3 litres per second. So about 70rpm. After connecting it up with a 4:1 gear ratio, it was doing 56rpm equating to 14rpm of resistance against a loss of 35-40rpm for the Miniwind PMG. 56rpm for the wheel means 224rpm for the alternator which would, according to Presto Wind’s power curves, translate into around 80W for the M-24: with the 2 stators in the M-24 wired in parallel, that would be 20V at 4A, not far off the 24V it needs to be to start putting power into the batteries. We still have to wire in an ammeter and play around with different gear ratios.
Meaningless gibberish? It was to me to start with but I think I understand it a bit better now. With independent (ie. off-grid) renewable energy systems, any input over the system voltage – 24V in our case – charges the batteries, and it’s not really of benefit to have too high a voltage. Over a certain level, the batteries don’t absorb the majority of it so a lot of the power is wasted. This is what happens when you use a portable generator to charge solar batteries. It keeps them going but doesn’t really do much for the state of charge. Simply put, when you want to store electricity, you don’t want speed (= voltage) because speed can’t be stored. You want volume (= amperage). So out of the total wattage (volts x amps) you want to maximise the current (ie. the amps) rather than the voltage. 4A is a nice steady healthy input to have 24/7 – about the same amperage that a portable generator running at 240V and blasting in 1.0-1.4kW is producing – and is what we’re aiming to achieve once we get the system running optimally with the right gearing. At the moment it’s running just under the threshold to contribute.
After running-in the M-24’s bearings and rotors, it will take about 2 weeks of constant charging to get our 770Ah batteries up from their very low state of charge to anywhere approaching full, so the LPG generator hasn’t completed its tour of duty quite yet, but the long wait is finally over.
Big thanks to Wayne Sutton for his genius design and impeccable engineering, and to the guys at Presto Wind for what appears to be an excellent product.
Now we celebrate! Premature? Possibly, but it’s a good excuse for some champagne anyway …
Update, December 16th:
We’ve now upped the gearing on the wheel to a 5:1 ratio, and in the process discovered that part of the resistance was down to an overly tight gear chain. Slackening this off increased the wheel’s speed at 5:1 from 35 to 48rpm under about 2.5 litres per second flow (at 4:1 it had been doing 52rpm). This equates to 240rpm for the M-24 and around 100W open circuit once the M-24 is fully run-in at water levels more usual in June than December. At the moment it’s producing 11V from one stator and 16V from the other (open circuit) after 6 days’ operation. Ammeter still to be connected.
Interestingly, extrapolating the rpm increase resulting from loosening the chain from a 5:1 to a 4:1 ratio would give almost 70rpm. This is pretty much what would be expected from a free-wheeling wheel at these water volumes, suggesting that the M-24 has next to no resistance. With only another week or so to go before it’s run-in, we’re like a bunch of kids waiting for a big treat and can’t wait to see how it’s going to perform.