Jet propulsion, - for the mathematically (and musically) minded.

When the turbine was off its plinth last summer for its annual overhaul, I took the opportunity to photograph one of the jets in operation, - something which wouldn't be possible in most Powerspout installations and something you don't normally get to see.  Here's what it looked like:

The distance the jet travelled was 15 metres.

Quite early after emerging from the orifice the jet can be seen to begin breaking up, probably due to a tiny irregularity in the plastic of the nozzle at the orifice.  Better transfer of power to the pelton runner would be achieved if the jet remained more compact. Tidy orifices are important.

The net head recorded on the test gauge is 53.3 metres, and with the orifice diameter being 7.46 mm, the calculated flow should be 1.28 litres per second. 

The formula for calculating this is: Q (m³/s) = CD x Anoz(m²) x √ (2g x Hn),  where CD is 0.91

Also by calculation, the velocity of the water at the orifice is 31.6 metres per second, which is 70.6 miles per hour !  The formula is vjet = Cv x √(2g x Hn), where Cv is 0.98

To get optimum energy transfer to the pelton wheel, the cups of the wheel should be moving away from the jet at a velocity just a little bit less than half (0.46 to be exact) of the velocity of the jet.  This wheel velocity needs to be measured at the circumference of an imaginary circle defined by where the jets strike the runner, - a circle which has a diameter, in the case of a Powerspout pelton, of 220 mm, - what is often referred to as the pcd or "pitch circle diameter".  Putting all this together, it can be calculated that the optimum speed at which my runner should turn is 1260 rpm, or 21 revs per second.

This speed of revolution is relevant because it defines the frequency of the sound emanating from a turbine, at least from the hydraulic side but not the SmartDrive side, which tends to have a higher pitched sound. 

The frequency of the sound can be calculated knowing there are 20 cups on a Powerspout runner: each jet will therefore 'see' 20 x 21 cups passing it each second. The resulting sound should have a frequency of 20 x 21 ie 420 Hz, - not far off Standard Concert pitch A, 440 Hz, the note used for tuning musical instruments to in an orchestra.  See what you think from the video clip below; I think it's turning more slowly than 1260 rpm; you can 'tune in' your ear here.

I have found it really interesting getting to grips with the maths which underpins the operation of a micro hydro.  Jeremy Thake's book on the subject is easy to read. I recommend it.

Upkeep

How demanding is it having a Powerspout and keeping it operational to maximum advantage ? - that's what I'm setting out to answer in this post. Every site is different though, and others may have very different levels of upkeep: I'd be interested to hear.

1. Trash clearing is seasonal: every day when leaves are falling, but once per day is always enough.  In times of strong winds, clearance is also needed daily because of the large fall of twigs and branches that rain down onto my abstraction site. For the rest of the time, weekly clearing is enough, although I often find myself up there for other reasons and brush off the screen anyway:

As my scheme has a good head but low flow, the nozzles have small orifices: diameters of between 3.64 and 8.90 mm.  Being so small, it would be easy for them to get blocked so having a screen taking out all the trash before the water drops into the header tank is crucial. The screen is of perforated stainless steel with 3mm holes at 5mm centres, and its value has been proved in that I have only ever had one nozzle blockage from trash.

Whilst only having had one blockage from trash, I used to have a more frequent problem with blockage from another cause: toads finding their way into the tank and thence down the penstock. The old galvanised sheets now covering the tank have put an end to that, which is probably of equal relief to both me and the toads.

2. Greasing is every ~500 hours, which is about every three weeks.  I have opted to retain the manual method using a grease gun, rather than going for the self-dispensing GreaseMax canisters recommended by EcoInnovation because I fear too much grease can be as bad as too little.  The canisters contain 125 mls grease which is dispensed over 12 months.  My turbine only runs for 9 months each year and the greasing schedule I work to only delivers 20 mls in that time.

3. Nozzle changes have been necessary 16 times between Nov 5th 2014, when generation started for this year, and today, 7th Feb 2015. They are necessary in order to keep the flow delivered to the turbine matched to the flow available from the spring source.
  
Each change takes about 10 mins and rather than being a chore, is something I enjoy.  In this second "water year", the shortest time between changes has been 1 day and the longest 23 days.  Last year I had a spell at full flow which lasted 75 days which is the record so far.  It all depends on what the rainfall is.  Designing your installation for good access does make this job very much easier if it's something you're likely to have to do as often as I do.

4. General oversight is something which goes on all the time: a water turbine is NOT a generating source like pv panels which can be said to be "fit and forget".  Having said that,  there has not been one instance of malfunction from my turbine apart from some initial teething problems after commissioning over a year ago: it just keeps on going.  In the 3+ months which have been completed in this second 'water year' of its operation,  it has not ceased generating once, other than for nozzle changes.

Occasionally I notice from the continuous record of power output to the grid that the inverter has disconnected from the grid on account of a transient grid failure or voltage fluctuation. When this happens, the Powerspout copes with it seamlessly by diverting output to its inboard dump element until the inverter has satisfied itself that the grid parameters are back to being OK again.

Oversight is helped in my situation by the turbine being sited close to the track to our house.  Every time we come and go, it is easy to glance at it to check that all is well.  If the season is a time when water availability is changing, whilst passing a check can also be made on the fullness of the header tank using the display housed in the pillar by the turbine. More about that in a future post.

If we go away for a few days, I now have enough confidence in everything to leave it running.  Sometimes this will mean putting in smaller nozzles before going away to be sure the system will not drain down while I'm away.  The 'on-line visibility' of its power output also means I can give myself peace of mind by checking on its wellbeing when I'm away.

In summary, for someone who needs to be around their property most of the time, a Powerspout is very little extra work.  In truth, I keep a closer eye on mine than is needful: I walk with the dog down to the turbine last thing every night, - mostly because it is so nice to hear it humming away to itself !

Small fish: big pond.

I read something interesting today:  

"The UK has topped 3GW of renewable capacity under the feed-in tariffs scheme, according to Ofgem. The scheme started in 2010 and there have to date been 568,612 accredited projects, almost double the amount forecast by DECC*. The figure breaks down as 561,185 for solar PV, 6,256 for wind, 546 for hydro, 490 for microchip and 135 for anaerobic digestion."

So, I'm one of the 546 FIT accredited hydro sites ... and I bet I'm the smallest.  

With January ended, I've clocked up the most productive month ever: 524 kWh.  This represents a capacity factor of 94%, meaning the very most I could have generated would have been 558 kWh.

A tiny drop in the ocean of renewables generation to be sure, but still worthwhile.

* Department of Energy and Climate Change