Efficiency at low flows

If you have a water turbine, it's nice to keep it operating for as much of the year as possible.

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:

Energy conversion.

I've had a bit of fun this morning trying to photograph the Powerspout with its front window off. Operating as I am now at pretty well the lowest flow possible (0.56 lps), it was the best opportunity to see, without causing a major flood, what the pattern of water discharge from the runner buckets looks like unobstructed.

The physics underlying the operation of a pelton is, on the surface, simple: the pressure energy of the water in the penstock is converted to kinetic energy in the water of the jet emerging from the nozzle, which is then converted into rotational energy in the turbine shaft by the transference of momentum from water to runner. 

This time last year I posted this picture of a jet without the turbine being in place, - the jet travelled for a distance of 15 metres:

The picture gives a good idea of the energy there is in the jet but today's photographs show how completely that energy is transferred to the shaft by means of the pelton cups:

The point to note is that the energy of the water leaving the runner buckets is now only sufficient to carry it a short distance, about 2 metres, and the direction it travels in is more or less at 90⁰ to the original line of the jet: all the forward momentum has been extracted and transferred to the runner.

I had hoped to get a close-up shot of the jet hitting the splitter ridges of the runner, but insufficient light and too much spray thwarted this.  Nevertheless, it was nice to see the pattern of discharge free from it being blocked by the perspex window.  Though a simple machine, an analysis of what goes on as the pelton cups pass through the jet leads to an appreciation of how complex the pattern of flow actually is.  Something of this complexity can be seen in the three dimensional 'fan' of water which emerges.

It's good to be reminded that things which appear simple at first glance sometimes turn out to be quite complex.

Little but worthwhile.

I'm happy !  

Last year I had to stop generating in the first week of July but this year I've been able to keep going all through the month.  It's not that there has been more water this year, in fact there has been less.  No, the reason is because of the changes made to the turbine which mean it can keep generating when flows are low.  The reduced core stator is the foremost of those changes but other changes have included using reduced friction bearings and learning that operating on the bottom jet alone gives better efficiency.

Here is what the output has been through July this year:

The step in the middle of this graph was when I changed nozzles: from a nozzle delivering 0.96 lps to one giving 0.83 lps, followed on the same day by another change to one delivering 0.68 lps. 

At 0.68 lps, the daily yield has been 3.02 kWh per day, - not very much admittedly but if one works out what the cash value of that daily level of production is over a full month, it comes out at £37.

£37 is about what it costs to fill our car with petrol, so put that way, if we get a free full tank each month on the back of what the Powerspout is generating, even this low level of output is worthwhile.