The trouble is that in the drier times of year when not much water is available, the efficiency of the system becomes very poor and the amount of power produced, already low because of the reduced flow, becomes even worse because of deteriorating system efficiency.
The reasons for this deteriorating efficiency are many, but the principle ones for my grid connected Powerspout GE 400 are:
- the fixed loss due to energy lost to shaft friction: seal and bearings
- the fixed loss due to conversion of dc to ac power in the inverter
I call these losses fixed because they are not proportional to the output of the turbine: whatever the output is, these will be subtracted from it. When the output is low anyway because of low flow, they assume an ever greater proportion of the total and therefore drag the efficiency down markedly.
From the time I installed my turbine, I have been collecting data on just how efficient my system is at different flows. From last year's data, it was evident that with the full core stator in the Smart Drive alternator (ie a 42 pole stator) plus standard bearings on the shaft plus using both jets, it was not possible to keep generating below a flow of about 1.5 lps, which was equivalent to an output to grid from the inverter of about 300 W.
This year, by using a reduced core stator (which has just 18 poles) plus reduced friction bearings (either ceramic or SKF E2 bearings) plus using only the bottom jet, it has been possible to keep generating down to a flow of 0.56 lps, which is giving 95 W into the grid.
Operating with the changes introduced for this year has significantly improved efficiency at low flow, and has permitted me to keep generating way beyond the date at which I had to stop last year. But still, the efficiency is terrible at the lowest flow levels, and this is because of the reasons explained above: reasons which cannot be avoided.
For those who like and can interpret graphs, here are what the figures look like: