Windage is defined in the dictionary as "the air resistance of a moving object ... such as a rotating machine part". For a pelton therefore it is the resistance encountered by the cups of the runner as they turn through the air in the casing. What is in the casing though is not just air but the more dense medium of water droplets suspended in air. There is a veritable fog of spray created by the water jets striking the cups.
Because windage is a resistance to rotation, it causes power to be lost from the power available for generating electricity. The loss is unavoidable, though it can be mitigated by designing the pelton casing so that spent water and spray are kept away from the rotating runner. In small turbines like the Powerspout this is almost impossible to do.
Normally, power lost to windage is very difficult to measure and can only be estimated mathematically. But in a 'Eureka moment' recently I realised that two features of my installation provide a unique opportunity to actually measure windage: the turbine is situated outside so removal of the transparent glazing wouldn't matter, and secondly I have a record of power output captured continuously and visible on-line*.
So this afternoon the experiment was done: at a precisely noted time, the glazing was removed whilst the turbine was operating; the turbine was left to run for just under 15 mins with the glazing off and then, without stopping the turbine, the glazing was put back on.
Here are some pictures and below them, the resultant power trace captured from the on-line record:
The power trace unequivocally shows a jump in power output coinciding precisely with when the glazing was removed and replaced. There can be no doubt about its authenticity. The actual gain in power amounts to 20 watts (~705 to ~725 watts).
One can speculate that the total power lost to windage will actually be double this because, ...on the assumption a jet strikes the splitter ridges dead centre and is split exactly in half, ...there will still be the windage loss arising from the half bouncing back from the bulkhead. So it's fair to say that windage loss probably totals 40 watts, which is just under 6% of the power output at the flow used for the experiment.
The flow used was 2.78 l/s. So in the 15 minutes of the experiment 2.5 tons of water sprayed out (2,500 litres): not an experiment for those whose Powerspouts are housed in smart enclosures !
It was fun to do and pleasing to get the result I sort of expected. My thanks to Paul Jones, my neighbour, for helping with it and being the one who got wet.
*You can view the actual record here but scrolling back to 4th April 2016 will be tedious for people who read this even a few days after the day of the experiment.