The set up

The set up
5.46mm jet delivering 0.68 l/s to the pelton which is rotating at 900 rpm and generating 135 watts into the grid.

Thursday, 4 June 2015

On trial - ceramic bearings.

This week I have started a long term investigation for EcoInnovation into the use of ceramic bearings in a Powerspout.

These are ceramic versions of the standard issue 6205 and 6005 SKF deep groove ball bearings, but both the balls and the races are made of silicon nitride, Si3N4 (sometimes zirconium oxide, ZrO2) instead of steel.

According to Wikipedia: "since silicon nitride ball bearings are harder than metal, this reduces contact with the bearing track. This results in 80% less friction, 3 to 10 times longer lifetime, 80% higher speed, 60% less weight, the ability to operate with lubrication starvation, higher corrosion resistance and higher operation temperature, as compared to traditional metal bearings".

The desirable attributes out of this list which apply particularly to their use in a Powerspout are their resistance to corrosion, their reduced friction and their ability to operate with 'lubrication starvation'.

Their lubrication amounts only to a small amount of thin oil applied at the time of installation and being free therefore of the grease which surrounds the shaft when metal bearings are used, we had the expectation, having previously demonstrated that grease causes drag on the shaft, of seeing a significant improvement in power output.  

And indeed that is exactly what we saw.  The new bearing block was installed following a protracted run at constant flow with standard bearings, during which power output was measured to be 403 watts.  As I write, a 48 hour spell of running under identical conditions with the new bearings is just being completed, and the power level is now 412 watts.  So a gain of 9 watts.

OK, 9 watts isn't a huge amount but the thing to remember is that the power lost to frictional losses through shaft bearings and seals will be a fixed loss whatever level of power is being generated.  At the power level I am getting at the moment which is 412 watts, a 9 watt gain represents an improvement of just 2.2% over 403 watts.  But when flow diminishes and power yield falls to, say 175 watts, gaining 9 watts will be an improvement of 5%.

With the other improvements mentioned in earlier posts, all designed to boost output at low flow / low power times of year:

  • operating on one jet rather that two
  • changing to a reduced core stator
  • operating with a de-finned rotor
... there is the hope of greatly improving system efficiency in these drier months.

Already, the plot of flow vs efficiency suggests this might be so: the latest data points, which are indicated on the plot below, both lie well above the 'best fit line' for the other data points, all of which did not incorporate these improvements. 

If the left hand side of the above plot can just be flattened out a bit, which is to say if efficiency at low flow can be kept above the rather dismal levels it falls to below 1.5 litre per second, then this would amount to a real benefit.

Let's hope ! 

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