Generating in a drought

The drought of 2026 in the UK is going to be remembered for a long time and in spite of no rain for 2 months my Powerspout is continuing to generate.

The flow available has dropped to half a litre per second (0.53 l/s) but that small amount at a head of 53 metres and an overall efficiency of 40% is enough to make 111 Watts (2.66 kWh per day).

0.53 l/s pictured today being strained of trash before dropping into the header tank.

The onset of the drought can be seen graphically from daily energy output declining from March onwards:

Rainfall had been unusually heavy during the five months October to February, totalling 944 mm which is not far short of the total rainfall expected for 12 months. Yet in spite of this generous topping up of groundwater in the hillside, the water issuing from the spring fell off steeply from mid-March onwards.

In many years, the month of June sees a good amount of rainfall, what I call the 'June Monsoon', but in 2026 it didn't happen and the decline in yield from the spring continued inexorably.

Happily though, the rate of fall in spring water yield seems to have flattened off after May and from experience I know it will be quite likely that even with no rain falling, the 0.53 l/s issuing at the moment will continue to be available for several weeks to come. 

The reason for this ongoing yield of water, with no rainfall to support it, is that the geology of the hillside above is a type of rock called Old Red Sandstone. It is a rock which can take up and hold prodigious volumes of water with the result that the yield of its springs is largely unaffected by droughts of just a year's duration.

The nozzle I am using to deliver a flow of 0.53 l/s is Roman numeral II in my numbering of nozzles. I have been curious to observe how good the efficiency is with this smallest of nozzles because efficiency always drops off at low flows. 

It turns out Nozzle II is giving a respectable efficiency of 40.1% and that is not bad. For reference, the best efficiency I get is 56.4% which happens when flow is between 2.30 and 2.74 l/s.

To know efficiency for your hydro scheme, you need to know accurately two parameters: output power and flow.

The new monitoring system I installed in January of this year (see previous post) allows me to know power very accurately indeed.  The meter measuring it is one accredited by the UK's Measuring Instruments Directive (MID), which means it is in a class of meters accurate enough to be used for billing purposes.

The power output trace from this meter for nozzle II today, over one hour, is shown in the picture below:

The meter is so accurate that it records the 'hunting' of the Powerspout's inverter as the inverter seeks its 'set point'. 

This is seen as the saw-tooth pattern in the power output trace. 

The 'set point' it is seeking is determined by the inverter measuring incoming dc voltage from the Powerspout's Smart drive alternator and then trying to deliver the output, in watts ac current, that its 'table', - stored in memory in the inverter, - tells it is the appropriate power output for the incoming dc voltage.

But like all control systems which change what they're controlling, it continuously overshoots and undershoots its set point and fails to give a straight line.

You can see that the time in takes to complete one full cycle of this unwanted oscillation, - what is called the 'period', - is between 5 and 10 minutes.

Before I installed this MID accredited meter, because of this oscillation I had difficulty measuring power output accurately, but now calculating efficiency correctly is easy using the MEAN power output, which can be read in the red oval. 

It is 111.8 W.

Another thing I've learned from this spell of using Nozzle II is that my previous obsession with keeping rpm (revs per minute) close to the optimum rpm for my pelton (1000 rpm) has been wrong.

I used to keep rpm high by packing off the alternator rotor. 

In fact I packed it off so much that rather little of the shaft's splines engaged in the plastic splines of the rotor, - and this led to rotors being damaged.

What I've learned now is that whatever gain in efficiency might result from having the pelton operate near its optimum speed, is counteracted at least as much by a loss of efficiency in the alternator because of impaired flux linkage between permanent magnets and stator cores.

So the efficiency I'm seeing now with Nozzle II, the respectable 40.1%, is actually obtained with little packing off and with a resulting rather slow rpm, - just 742 rpm .

This slow rpm can be seen in the concluding pic below, where the splash pattern of water exiting the pelton cups is way off to the right. At the optimum rpm of 1000 it is at 6 o'clock. Note that only the bottom jet is in use.

Turbine on 19 August 2026, on 0.53 l/s of water, at 742 rpm, generating 111.8 W to grid.

To round off this blog post, I have to admit that with such a memorable drought happening, it amuses me that I can still be using water to generate a bit of electricity. 

Long may it continue !