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.

53,760 hours.

 53,670 hours is the number of hours in just over 6 years, and this week I decided after this length of time it was time for the bearings to be changed; this blog illustrates what everything looked like when I took the bearing-housing apart having never disturbed it in all that time.

I installed the bearings on 5th July 2017 and they were SKF E2 Energy Efficient bearings; they have run continuously ever since bar one period of 48 hours in October 2018 when there was insufficient water; the only other times when the turbine shaft has not been turning is when a nozzle is changed, or the stator is changed, and such stoppages are typically for only a matter of minutes.

Of particular note is that the bearings have not been greased at all; neither was grease preloaded into the bearing housing when they were first installed; they have operated only on the grease put in by SKF at manufacture.

I made the decision to change them only because 6 years seemed a long enough interval; there were no warning signs of impending failure that prompted the change; literature from SKF suggests that the bearings can be expected to be serviceable for up to 9 years; after this length of time the grease will have come to the end of its ability to lubricate and 10% of a batch of apparently identical bearings will fail; this metric for predicting the likelihood of a bearing failing is called the T10 life expectancy; it is very much dependent on the conditions under which the bearing is operating, especially the load it is carrying, the temperature it is operating at, and most especially whether water and other contaminants can get to the rolling parts.

6 years is thus within the T10 life expectancy for the bearings - except the big unknown is the conditions under which they are actually operating.

So here is a pictorial account with captions of what the seals and bearings looked like: -

on my turbine, I have a specially made cover to help prevent water ingress; its purpose is to give a metallic face for the V-lip seal to rub on; it fits over the plastic 'Top-hat' and is held in place only by being a tight fit. 

a puller was needed to get it off.

another modification I have made on my turbine is this deflector to discourage water from entering the drainage hole of the 'Top-hat'

the first glimpse of the condition of the shaft indicated that little moisture was getting to it

for comparison, this is a picture from the previous bearing replacement in 2017, which was done after just 14,448 hours of operation and before steps were taken to prevent water ingress; it shows limescale encrusting the shaft, indicating that quite a bit of water was getting in.

detail of, and explanation of, the marks on the shaft; the brown colouring was of silt-like consistency and rubbed off very easily with wire wool; the surface of the shaft was not scored where it had been polished by the seal.

I was surprised by how much the stainless steel of the cap had been worn by the rubbing of the V-lip seal; it was almost as bad as the wear on the plastic of the Top-hat in the next picture, except that the plastic wore to be like this in a matter of months.

when I had removed the dust shields of the two bearings, the grease around the balls still looked pretty good

a close-up of the balls shows they were still well lubricated

SKF's E2 bearings have a very different looking ball cage; this is what the reverse side looks like.

the radial shaft seal on the inner end of the Top hat was a bit mucky; it has two lips and the second picture is a close up of the space between the two lips showing it to be full of the silt like material present on the shaft.

In summary, the bearings looked good enough to do another few years, but the seals were in need of being changed. The feel of the shaft rotating in the housing was of very free rotation as if the bearings were well 'run-in'. By comparison, when I had put in new bearings, of the same sort, the rotation felt rather stiff and not so free.

Unsurprising then that when I powered up the turbine with its new seals and bearings, power output was 16 watts down on what it had been, - that's a 4% loss of efficiency, - down from 44% to 40%, taking efficiency here to mean whole system efficiency, ie water-to-wire.

Ah well, - better a small drop in generation now than having the bearings fail later, - possibly in winter and having to do the job of changing them on a cold, wet day, as an urgent rather than an elective undertaking !

To change or not to change ...

In July, my Powerspout will have run for 4 years on the same set of bearings; in that time I've not greased them once; they've run only on their factory fill of grease and, having run continuously, the total hours will be just over 35,000.

EcoInnovation's recommendation is to grease regularly and to change the bearings every year (8,760 hours); I've not followed that advice, not because it's bad advice, but because I wanted to see how long I could get bearings to last.

Mind you, ...the bearings I'm using are a premium type from SKF, designated E2 Energy efficient and more expensive than their standard Explorer series; the claims for them are: "longer service life", "longer grease life", "reduced frictional loss" and "lower cost of ownership";

...and I've modified the sealing arrangement at the 'wet end' of the shaft to reduce the risk of water getting in and causing the grease there to be degraded; see here.

So the title of this post: "To change or not to change..." looks at whether after 4 years, the time is ripe for a new set.

According to the SKF leaflet, bearing service life is almost always limited by grease life; for their E2 bearings, SKF give the following chart to estimate how many hours the grease in E2 bearings should last; the estimate is based on an L10 grease life, and that is defined as the period of time at the end of which 90% of a sufficiently large group of seemingly identical bearings are still reliably lubricated.

The red arrow on the chart is placed to indicate the 'life curve' for an A value of 40,000 and that 'life curve' can be seen to intersect the logarithmic y axis at around 80,000 to 100,000 hours, the same as if the A value was 100,000. This means the L10 grease life should be at least 80,000 hours, - or 9 years of continuous running.

This being the case, I am not going to change the bearings this year; I'll hold out at least for another 12 months and think again when the next anniversary comes around in July 2022.

For the technically minded: 

The diagram shows 'grease life curves' for various values of A, under varying bearing operating temperatures, and for bearings working with a load value P = 0.05 * C; this value of P signifies the lowest loading and was assumed to be the value most likely to be applicable for the load experienced by the bearings operating in a Powerspout; the leaflet gives de-rating factors to lower the L10 hours for bearings subject to a higher load value.

the operating temperature of bearings in a Powerspout is almost certainly less than the lowest value given in the diagram (50 deg C); as the dashed lines of the life curves show, this will mean an arbitrary reduction in the value of L10 hours for any given A value.

the A value for the size of bearing used and the particular conditions under which the bearing is operating is calculated from:

A = n * dm

where: n   = rotational speed in revolutions / min (for my turbine 1000 r/m)

            dm = mean diameter, mm, of smallest bearing, 6005 size, ( [25 + 47] * 0.5 = 36 )

Thus: A = 1000 * 36 = 36,000 (which is near enough 40,000)

Post script added 11 September 2022.

One year on from when the above was written, the bearings are still sounding perfectly OK, so I have again decided not to change them and wait another 12 months.

Repairing and damp-proofing a V-Clamp board

A job, long delayed by uncertainty about how to proceed, has finally been taken off my "to do" list: how to restore Voltage clamp boards which have failed. Time to devote to it happened because of Covid, - a minor bonus in these otherwise difficult times.

Early Powerspout turbines had V-clamp boards to keep voltage within limits when the output from the turbine is to an open circuit; the function of the board is to divert power to a dump load and thereby hold the Powerspout's output to a voltage acceptable to whatever device the turbine is connected to: - either an MPPT controlled battery charger, or as in my case, a grid tied inverter.

When manufactured, the boards were covered with a conformal coating intended to keep the underlying electronics free of moisture, but the coating used was hot melt glue and at low temperatures this becomes brittle, separates from the underlying printed circuit board and develops cracks. Through these cracks, moisture could reach the live circuitry beneath and sooner or later this had terminally disastrous results.  

In the early months of my installation in 2013, two boards died in this way; but the years since have been completely trouble free through EcoInnovation having come up with a one-off special, - special in that the board was entirely encased in an epoxy resin block.

So the long delayed job has been to repair the two boards that died, and then to cast them in epoxy, as had been done for the special.  

Here, in pictures and captions, is the process:

An original board showing how the coating of hot melt glue cracks and separates from the underlying pcb, allowing moisture to enter.

Typical tell-tale evidence of underlying pcb damage with blackening visible through the conformal coating and molten glue running out. The white stuff is heat conducting paste applied to the heatsinks on the board where they attached to the bulk head of the turbine; the bulk head, being water cooled on the pelton side, creates a good means of dissipating heat from the power electronics on the board.

Initial view of the damage once glue had been removed; since hot melt glue becomes brittle at low temperature, cooling the pcb in the freezer to -18 deg C allowed the coating to be chipped off, but it is a very time consuming process and care is needed not to accidentally remove any surface mounted electronic components. (addendum 9 Feb 2021: see below for an alternative method.)

Later view of damaged area after further cleaning, showing the glass fibre substrate of the pcb has carbonised and copper tracks have been lost; carbon being a conductor, this all had to be removed using a precision rotary tool (Dremel) and the resulting cavity filled with non-conducting epoxy.

Top-side view of the same area of damage; from some components such as the surge protection thermistor in the centre, the hot melt glue could not be removed, so this device was de-soldered and replaced with new; flakes of hot melt glue are still visible around surface mounted components.

Damaged area after repair; the next stage is a final clean with isopropyl alcohol then casting in epoxy

Topside view after repair with a new surge protection thermistor and adjacent capacitor

This is the 'special' made for me by Andrew Smithies, who designed the board; it was with his advice I arrived at my method of casting; one key piece of advice concerned 'thermal runaway'; this is the intense heat generated by two part epoxy compounds as they cure; the heat was so much when he did it that it melted the mould and let some of the epoxy escape; the resulting void had to be filled later with more epoxy; but epoxy added later to already hard-cured epoxy does not unify into one block, so I was keen to avoid this happening.

Before proceeding to casting the repaired pcb I wanted to check it was fully functional; at this stage it was vulnerable to moisture with the bare soldered components having no protection, so testing had to be done on a dry day with low ambient humidity; the test shows volt meters on the two outputs when the turbine was operating without power being fed to the inverter: - the digital voltmeter reads 381v dc, which is the correct voltage for the 400v version of the V-Clamp board, and the AVO analogue meter is reading the voltage being fed to the dump load. The turbine was then allowed to connect to the grid via its inverter and a test was performed to simulate a 'loss of grid' event to check the circuit board instantaneously diverted power to the dump load; all tests were satisfactory and I felt confident the board could be epoxied.

The secret to constructing a mould which can be used more than once is to use material for the mould to which epoxy does NOT bond, and that means polypropylene; a convenient source is a kitchen cutting board and such is what I used, price £7; for the curved bottom, on my first effort, I used a strip of flat rubber sprayed with silicon lubricant to act as a releasing agent; but the epoxy bonded to it and when the mould was disassembled, the rubber had to be carved off the cured epoxy block; on my second effort I used a non-stick sheet, PTFE coated, marketed as oven shelf  liner; it worked fine but needed to be supported by a backing for which lead sheet, as used in roofing, proved OK. Where pp or PTFE is used, no release agent is necessary, - the cured epoxy separates from the mould very easily.

The epoxy I chose to use was "Water clear" Transparent Epoxy Potting Compound from MG Chemicals; I bought it through RadioSpares; for the size of mould I had made each board needed 1.725 litres of epoxy; conveniently for two boards this meant buying one kit of 2.7 litres (RS no: 181-0370) and two kits, each of 375 mls (RS no: 181-0369); each kit comprises resin and hardener in a ratio of 2:1. It is not wise to attempt a mix of 1.125 litres to fill the mould in one go because this will lead to unmanageable thermal runaway; it could get hot enough to damage components on the board; I mixed 300 mls at a time (200 resin + 100 hardener) at half hourly intervals to stagger the curing time and reduce the temperature rise; it therefore took about 2 hrs to fill the mould; even doing it this way, the temperature still reached about 80 deg C and the 300 ml mixes added last cured more quickly than the earlier mixes because the mould had by then begun to get hot.

You are helped to a good outcome in epoxy potting by being well prepared: graduated beakers for mixing resin and hardener, stirring sticks, cleanliness in the work area, a means of de-aerating the mix before pouring it into the mould, a kitchen timer or clock, warming the resin and hardener in the oven to 45 deg C before you start so their viscosity is as low as possible to facilitate mixing and de-aeration. Thorough mixing is particularly important because any resin not mixed with hardener will not cure; it will lead to a sticky patch in the finished block; I stirred for 5 mins timed on a timer; the type of epoxy I was using will remain workable for up to 1 hour at 20 deg C but for less time if it is warmer; each successive mix of 300 mls will bond to the previously poured batch as long as the cure has not reached the stage of being hard, as estimated by whether a finger nail can indent the surface.

Getting the bubbles out after mixing the resin and hardener is important; it can be done by waiting if the mix is not too stiff, or it can be done by placing the mix in a chamber and reducing the pressure; I chose the latter method and used a vacuum cleaner to suck out from a closed container; it seemed to work and I supposed it worked by making the air bubbles in the mix expand and so rise to the surface more quickly.

There are two stages to the curing process, soft cure and hard cure; you don't want to remove the block from its mould until hard cure has reliably been achieved throughout its mass; this can be ensured by 'post-curing' in an oven at 80 deg C for 4 hours, and this is what I did, waiting until the next day when it had cooled down to disassemble the mould.

The result was better than I had expected: the epoxy had filled all crevices and the mould left clean edges; the original V-Clamp board attached to the Powerspout bulkhead with self tapping screws driven in through the bulkhead from the pelton side but a modification in this epoxied board was to incorporate M5 machine screws into the epoxy, onto which Nyloc nuts fasten to draw the heatsinks tight against the bulkhead. I had expected to have to reduce the bulk of the finished block with a disc sander in order to get it to fit in the confined space it occupies above the Powerspout's bearing housing, but in the event it fitted perfectly by entering it from the bottom right side and then rotating it anticlockwise into its final position. 

Conclusion:
The board depicted was installed in the turbine on 7th October 2020 and has so far worked perfectly; it should, I hope, go on working for years to come.

Repairing and epoxying these two boards has not been without expense: the more severely damaged one was beyond my repair abilities and cost £320 to have done professionally. The other board only cost the price of replacing one MOSFET, - a few pence.

Clear epoxy of the type I used is not the cheapest available but I wanted to be able to inspect the encapsulated components should failure occur, - not that repair would be possible once potted.

For each board the epoxy cost worked out at £232.

I did have the option of ditching the present system in which a V-Clamp board is necessary, either by changing to using a Klampit device and continuing with the SMA inverter I use, or by discarding the SMA inverter and changing to one which can accept 600v dc rather than the 400v dc the SMA is designed for.

Either alternative would have meant disrupting a system which has proved bomb-proof for 7 years, and having several spare SMA inverters and V-Clamp boards, I decided to stick with that way of dealing with voltage capping.

Getting to the point of having these two boards repaired and ready for use has been a long journey; for several years I didn't think it was going to be possible; but having done them, and having learned a good deal in the process, I'm glad of having pursued it.

An alternative method to cooling to remove hot melt glue is to use heat; at 120 deg C the glue becomes liquid and drips off; the circuit board is hung in an oven; 

An engineering solution to cover fixing.

Owners and operators of Powerspout turbines, especially the pelton sort, will know that the front and back covers are held on by self-tapping screws which screw into the plastic of the turbine carcass.

The method has something of the wood-worker about it.

Repeated screwing and unscrewing of these fixtures inevitably causes them to de-thread eventually, whereupon one solution is to drill a new hole and continue to use a self tapping screw in the new hole.

Although the holes have not yet de-threaded for me, they're beginning to feel that way, and since that is after nearly 7 years of fairly frequent removal of both front and back covers, the self tapping screw method does have merit in lasting for quite a while.

But I am not enthusiastic about drilling new holes and continuing with self tappers so for some time I have been looking around for a better solution, - some way of fixing that comes more from the toolbox of an engineer than a wood-worker.

During this past week of continuing Covid lockdown, I have received the parts I'd decided on and have put them in: this post tells the story.

I've opted to use M5 stainless steel flanged screws and have these screwing into brass inserts fixed into the holes previously used by the self tapping screws.

Links to where I purchased the bits are at the bottom and are correct at the time of writing.

Brass inserts with M5 internal thread to accept M5 threaded stainless steel, flanged, screw.

Close up of brass insert to show coarse outer thread with longitudinal tracks to aid it to lock into the plastic and not unscrew.

Method used for placing insert by mounting it on an M5 socket headed screw with two lock nuts so it can be screwed into the plastic; a socket headed screw was used for placement because quite a bit of axial force is needed to get the coarse outer threads to bite.

Screwing the insert into place taking care to keep perpendicular to the face.

The insert in place with its surface flush with the plastic; no prior drilling out of the hole was done; it was found that the size of the hole after it had been used by self-tapping screws was about right to get a really tight fit of the brass insert into the plastic; a round file used for sharpening a chain saw (7/32", 5.5 mm size) was used to tidy up the hole beforehand but the hole should not be enlarged too much by the use of the file.

To finish off, a new neoprene self adhesive strip was placed; to make holes in it for the screws, a hot nail poked through does the job neatly.

The completed job, - no leaks ! The turbine here is running on the bottom jet only, rotating at 940 rpm, generating 322 W into the grid at a water-to-grid efficiency of exactly 50%.

And to finish off, a neat tool for dealing with the 8 mm hex-headed screws.

- M5, flanged, A2 stainless steel screws, 16 mm long were purchased here (£2.90 for 20, Free postage to UK) 
- Threaded brass, double ended, self tapped, screw fit inserts, M5 internal thread were purchased here (£7.30 for 25, Free postage to UK)
- Black Neoprene self-adhesive sponge, 6 mm thick x 15 mm wide x 5 m long purchased here (£5.50 Free postage in UK)
- Britool 8 mm nut-spinner, available while stocks last,here (£3.85 Free postage in UK)

Stopping water entry

I've been experimenting recently with trying to stop wetness from the pelton side of the turbine creating dampness on the alternator side. There's a seal around the shaft which should prevent water in any quantity getting across but an investigation I've done using bags of silica gel indicates that in spite of the seal about 300mls per month still gets across.

These photos tell the story of the sequence of steps I've taken:

1. Limescale deposits on the shaft indicated that a considerable amount of water enters the top-hat labyrinth chamber

2. A V-ring seal (purchased here) was mounted on the shaft; the seal turns with the shaft and its lip seals against the plastic face of the top-hat, with the idea of preventing water tracking alongside the shaft

3. Inspection after 3 weeks running showed the seal had badly scored the plastic face of the top-hat, presumably from softening of the plastic by the heat of frictional contact, - I must have applied it too tight to the face.

4. A friend who is skilled on his metal lathe kindly turned a stainless steel cap to fit over the plastic top-hat so the seal rubbed on metal; the cap is held on only by being a tight fit.

5. Suspecting that water might also enter the top-hat via its drain hole, a deflector was devised to shield the hole from the upward direction of water leaving the pelton from the lower jet.

6. So the complete arrangement as it is at the moment looks like this:

Only time will tell if it makes any difference.  The early signs are that the silica gel bags do seem to be taking up less water but I'm yet to be convinced this is a genuine observation.

Whilst I was working through these stages of development, EcoInnovation have come up with a slightly different approach:

Theirs is a neater solution but care will be needed not to apply the V ring seal too tightly against the face of the top-hat.  The seal only needs to just touch. After observing the scoring illustrated above, a new top hat with the seal just touching ran for 3 weeks with not even a mark being caused.  A smear of grease is also a good idea.