The next stage in the water wheel construction is nearing completion. Because the ‘engine’ of the system is a permanent magnet generator from a wind turbine, and a water wheel turns much more slowly than a wind turbine, the wheel needs to be linked to the PMG via a gearing system that steps up the number of revolutions. To work out the optimum gear ratio for this installation, we needed to measure the actual rpm the wheel produces at present water volumes. For that to be done, we needed to construct a chute to deliver the water to the wheel.
After debating the relative merits of plastic piping versus wood, we settled on wood. Wayne measured up, I ordered from the woodyard. The wood should have been delivered to us a fortnight ago, but hey, this is Portugal. After two visits to the yard from me and a fair bit of hassling to no avail, Wayne and I turned up at the yard on Wednesday, helped mill the timber on the spot, and took it away with us on the roof of the van. British Health & Safety would have been apoplectic at the mere thought! And this being Portugal, getting out my purse prompted the woodyard owner to suggest he should wait as long for my payment as I had to for the wood … I love this country!
Today Wayne put the chute together on a temporary framework and we measured the rpm. 65-66. Better than expected, and developed with a very small volume of water. My original research on Pelton-wheeled impulse type turbines indicated I could generate 150-200W with a flow of 3-5 litres per second, using all 15m of potential head available down the steepest section of the barroco, but this wheel could possibly generate as much sited only a third of the way down that section. Add to this the absence of any problem with debris coming down the barroco (it’s simply thrown straight off the top of the wheel), or any need for civil works in the form of dam construction to feed a penstock, and a system like this becomes a complete no-brainer. I love it!
It makes the power calculations almost impossible though. Generally, ‘head’ is taken to be the vertical height that the water falls in its descent from a collecting point (often a small dam) to the generator, via a relatively frictionless delivery system like a plastic pipe. This makes for a mostly closed system, so you measure the vertical height between point A and point B, make adjustments for the length of the pipe run and the number and angle of bends in it (since there will be losses due to friction and the braking effect of bends), measure your volume, do your calculations and you end up with a pretty good indication of the power you can extract from the water.
But mountain streams are open systems. There is no clearly definable point A because the wheel is installed in the middle of a running stream with the chute’s main purpose being to deliver the entire flow of the stream to the top of the wheel before it carries on its way down the valley. So what is my head? The height difference between the wheel site and the stream source (190m)? Clearly not. The course of the stream is straight enough over the half kilometre or so it takes to drop from that height, but it’s not comparable to a closed pipe run. There’s much more friction from the stream bed, many changes of gradient, falls and pools, trapped debris, overhanging vegetation, etc, on the way down, all of which affect the power of the water. The volume top to bottom is not consistent either. But yet the impact on the water’s power of its 174m drop before it reaches our quinta boundary and gets channeled into the barroco can’t be wholly ignored either.
At the other end, the vertical height of the chute is only 2-3m, but there’s a steep fall down smooth rock of another 3m immediately above its entry point. The water doesn’t slow down much in the transition. And while the gradient above that fall is much less, the water is still travelling at some speed as it’s being channeled down a straight barroco. You see my difficulty? It’s almost a case of pick a number, any number, between 5 and … well … Let’s say 5 and 25. That equates to potential power of between 100 and 500W with a flow of 2 litres per second (the lower end of our winter flow range) before subtracting efficiency losses. 100W is marginal, 500W comfortable (but probably unachievable).
At the end of the day, we’re having to go as much by noses and seats of pants as physics. Seat of the pants says around 200-250W ought to be achievable with winter volumes we’ve seen to date, but we could be talking out of our arses. Only time will tell.
The gear wheel and chain can now be ordered. Should be roughly 2 weeks.