Penstock pipeline purging.

I hadn't been thinking that silt accumulation would have amounted to much in the penstock delivering water to my Powerspout. Yes, I did do 'flush throughs' to remove silt back in 2014 and 2018, but I haven't done one since.

But as the years have clocked by since 2018, I and others began to notice that power output for each nozzle was getting to be less in each successive year; this was most noticeable for the biggest nozzle - which used to generate over 900W in the winter months, but by 2024/5 was only producing 885 W.

There were a number of possible explanations for this fall-off in generation, - wear on the pelton runner, or changes I had made to the algorithm in the grid-tied inverter, - but when I'd excluded these, the only likely cause remaining was silt accumulation in the penstock.

So this week I set about flushing through the penstock thoroughly.

In previous 'flush-throughs' I had either removed the turbine completely from its plinth or only flushed from the top nozzle holder; this time I wanted a way of doing it which didn't disturb the turbine's seating on the plinth and which flushed the more important bottom nozzle, - which is the one in constant use. The pipe bend in the picture is 2" bore and it attaches to the 2" BSP parallel threading of the nozzle holder with a 2" union, - which has BSP taper threads.

The silt seems to have been cleared from the pipeline in the first few seconds but to be sure, we did two header-tank-fulls of flushing, with each full tank being 6 cubic metres, - it discharged in just 4m 30s.

Afterwards, it was gratifying to see that electrical generation had been improved.

Before flushing, the ac watts generated from the grid-tied inverter was 391.5 W, and afterwards it was 418.5 W; that's 27 W of improvement, representing an improvement in water-to-wire efficiency from 50.7 to 54.1%. 

Pre-flush:

Post-flush:

I'm going to make 'penstock pipeline purging' a yearly maintenance job from now on, alongside the need to de-silt the header tank each year.

After 12 years continuous operation, what does a Powerspout PLT runner look like.

 1. On the side where sunlight reaches it, despite the force of the water jetting around, there is considerable growth of algae and moss which is not present on the inner side.

Inside face:

Outside face:

2. The amount of wear on those places where the jets hit hardest is negligible. The splitter ridges of the pelton cups are as sharp as in a new runner. In my installation, the water is free of silt and free of lime and that is why the runner maintains such good condition.

Old runner:

New runner:

3. From the way the heads of the stainless steel retaining screws have been polished by water as it emerges from each pelton cup, it is evident that some of the water is directed up toward the shaft. This is important, - because it is this water which can enter alongside the shaft and reach the ‘wet side’ bearing. The V-lip seal on the shaft is placed there to prevent such water tracking along the shaft, and the twin lip seal at the inner end of the ‘Top hat’ assembly is the final barrier stopping water reaching the bearing. Both seals need to be kept in good condition.

4. My conclusion: I was pleasantly surprised by how little wear there was after 12 years of operating 365 days per year. I only replaced the runner so I could thoroughly inspect it, and having seen it's in such good condition, I will put it back for at least a further decade of use.

Year end results for 2023-24 water year.

The year to September 30th has seen the most generation in the eleven years my Powerspout has been running. The total energy generated was 5254 kWh.

Understanding that this figure is really quite good for a small turbine is best captured by considering what is called capacity factor.

Capacity factor is the energy generated in a year divided by the energy that could have been generated if the turbine had run everyday for 365 days at the rating specified in its technical specification.

The rating specified for my turbine is 750 watts, which means the maximum power it is meant to be able to produce is 750 W. And using this figure, the maximum energy that could be produced in a year is 6,570 kWh (calc: 0.75 x 24 x 365).

So the capacity factor for the 2023-24 water year turns out to be near enough 80% (calc: 5254 / 6570 = 0.799).

But there is a slight deception in this figure. And this is because the maximum power my Powerspout can actually produce is more than 750 W, and this is evident in the first of the plots below.

There is a reason why there is this deception: the figure of 750 W had to be specified before the turbine had even been commissioned and was a figure derived purely from theoretical calculation. This theoretical calculation had to make assumptions about a lot of things such as friction loss in the pipeline, the efficiency of the 3 phase alternator, the efficiency of the dc/ac grid-tied inverter, ...and many more factors, - all of which were little more than educated guesses.

When the turbine actually got to run, many of the guesses turned out to be on the pessimistic side and the turbine actually was found to produce nearer 800 W at full flow, and even 900 W if slightly more water than the specified maximum flow of 3 l/s was used.

So the 80% capacity factor figure is inflated, and if the true maximum power of 900 W is used to calculate it, the figure drops to 66%.  But since officially my turbine is rated at 750 W, that's the figure I'm going to continue to use for my capacity factor calculations. But I will point out that in the interests of 'honesty and transparency' you'll notice that I do say in the Scheme Details section in this blog where each year's capacity factor is given, that the figures are "taking as datum DNC 750 W" (DNC = Declared Net Capacity).

For those who like to appreciate how a small scale hydro works out in the real world, here are the figures for this past year's generation. The year 2023-24 is the black plot line, and previous years are coloured lines: