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; 

End of year results 2019-2020

30th September marks the end of the 12 month period I use as my 'accounting year'; it is when I bring together data for how my Powerspout has done.  

The graphs below show how it has performed:

• in each graph the bold black line represents the data for the year just ended; 
• data is available because it is automatically captured by the installation's inverter (SMA WindyBoy) and transmitted by Bluetooth to a desk top display (SMA SunnyBeam), from which it is downloaded at the end of each month to a desktop pc.

Graph 1 shows power each day measured in Watts (left hand axis) and energy measured in kWh (right hand axis). 

• the power data is derived from the energy data by dividing the kWh figure by 24; 
• the value given for power is therefore the mean power generated in each 24 hours; 
• the figure will be reduced by any event during the 24 hours which interrupted generation, and such interruptions are seen as a downward spike in the trace;
• interruptions include events such as grid outages and turbine stoppages for maintenance; 
• it can be seen there is not a single day in the year when the turbine failed to generate anything at all; 
• sustained upward or downward steps in the trace are caused by a change of nozzle, either to a bigger or a smaller one, to suit seasonal changes in flow; 
• I had to change nozzles 22 times during the year; 

Graph 2 shows the cumulative energy, measured in kWh, generated over the 12 month period. 

• the output this year has been unprecedented, 5133 kWh, far exceeding the totals in the previous 6 years; 
• the cause of this bounty was a wet winter which started unusually early in October and continued through to March, with exceptionally heavy rainfall in February; 
• such wetness gave the turbine a stretch of generation at maximum output (920 W) lasting just under 160 days (26th Oct to 30th March).

Graph 3 shows how many days in the year a given level of power was achieved. 

• this way of displaying the turbine's output gives an idea of how much time the turbine spends generating at different power levels; 
• for example 200W was generated for the number of days between 260 and 315, i.e. 55 days; 
• but these 55 days would not have been in one stretch; this type of graph aggregates all the days in the year which saw generation at this level;
• Graph 1 shows most of the 55 days were in June / July with a few being added at the end of September; 

Graph 1

Graph 2

Graph 3

The Doble notch

Although the pelton wheel is named after Lester Pelton, - the man who saw the benefit of the splitter ridge in each cup, - another man, William A. Doble, invented a modification which was possibly of greater significance.

In 1907, Doble obtained a US patent for the notch which is seen at the outer edge of the cups of all modern pelton wheels. He realised two things: first, that as the cups entered the jet, they disrupted it, causing what was meant to be a clean, solid shaft of water to break up and fail to transfer maximum energy to the wheel; second, that the first bit of water entering each cup was deflected upward, where it hit the underside of the cup next to arrive. By hitting the underside of that cup, the force of the water was acting to make the wheel turn the opposite way, - and that was clearly a bad thing. 

By designing a notch which allowed the jet to pass through one cup and continue onward to the cup which preceded it, his invention ensured the next cup entered the jet when the wheel had rotated a bit further round. This meant the jet struck the cup more perpendicularly and the direction of the deflected exhaust water was no longer upward toward the following cup, but outward to the sides.

His design also described the shape and inclination of the splitter ridge so it was the sharp tip, the extreme tip, of the splitter ridge which was first to enter the jet. The effect of this was to make it as if a knife was cutting cleanly into the jet, disturbing the integrity of the solid column of water as little as possible. 

The sequence of events happening when a jet hits cups which have a Doble notch is seen in this photo from my last blog post:

• the jet can be seen passing through the notch of cup 1...
• ... and striking the splitter ridge of cup 2 almost perpendicularly, sending water to the sides and not back toward cup 1
• cup 3 is cut off from the jet by cup 2, but water which was entering it before cup 2 got in the way, is seen to be still passing across the floor of the cup and up its side wall where it is just beginning to exit 
• within a fraction of a second, the wheel will rotate a tiny amount clockwise bringing the tip of the splitter ridge of cup 1 cleanly into the path of the jet

Doble's patent can be viewed as he originally wrote it here.  It is worth reading for its clarity and conciseness.

These are two extracts from it:

 

William Doble became chief engineer in Lester Pelton's company from 1912, but history has ensured Pelton's name rather than Doble's has been tied to the impulse turbine they both worked to develop.   

It just goes to show that having a brilliant idea doesn't ensure history will remember you !

Frozen action.

Last night, after dark, I went down to my turbine with a camera, which was loaned to me, to take pictures I've been longing to take for ages.

The reason for it being after dark was so the speed of the flash, rather than the speed of the shutter, froze the action.

Inevitably, a lot of water sprayed onto the camera lens and that has taken away some of the quality I was hoping for, but the pictures do show nicely how: 

• the cups cut into the jet and chop it up 
• the jet is carved in two by the splitter ridge
• the notch in each cup ensures the splitter ridge is the first part to enter the jet 
• water then passes down and around the floor of the cup 
• water is thrown up and away by the side walls
• pulses of water are created exiting to the side

The photos were made possible only by choosing carefully how to capture them. I use an SMA Windy Boy inverter to connect to the grid, and it takes 4 minutes from the time it first receives power before connecting to the grid. During this time, the pelton overspeeds and causes exhaust water to exit in a different way to when it's at its operational speed. A line of approach relatively free of spray is created which gives a good view of the jet.

The rotational speed in the photos was 1270 rpm, giving a linear velocity for the runner at the pcd (pitch circle diameter) of about 15 m/s. 

At this speed, the time taken for a cup to move to the position of the cup ahead of it is about one 400th of a second.

The nozzle orifice was 6.48 mm diameter, the flow 0.91 l/s and the jet velocity about 31 m/s.

The camera was a Sony Cybershot DSG TX10.  It is water proof, - and it needed to be !

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)

Changing a nozzle - in 28 seconds !

Covid 19 lockdown is making me experiment with new ways (for me) of using digital media.
Here, filmed in time lapse, is the bottom nozzle being changed for a smaller one because water supply is drying up.
The smaller nozzle also required two spacer washers to be added under the Smart Drive rotor - but filming of that didn't work out; maybe I'll try again and add it later .
Enjoy !

...next day:
it turned out that two washers generated fewer watts than one washer so today I removed one; here is the video in real time:

you'll see that: 

• the rpm before removing the washer, when the turbine was connected to the grid, was 1152
• the rpm after removal was 1218, but this was before the inverter had established connection to the grid
• when the inverter established grid connection, rpm came down to 1058

The effect of this exercise is seen in the record below of output to grid: power output rose from 443 W to 468 W.
And this means that overall efficiency is better at 1058 rpm and one washer (53.6%) than at 1152 rpm and two washers (50.9%).
 A nice demo of how fine tuning can gain a few watts.

More graphs.

All things, both good and bad, come to an end.  This week, as the world still battles the flu pandemic, my Powerspout has ended its unprecedented good run of generating at full power: 156 days continuously at between 900 and 928 watts. That's 3.4 mega watt hours of energy, - more than it usually generates in a whole year.

The reason for this bonanza was the exceptional winter rainfall, which as well as bringing generating power also brought us floods. Starting early in October and peaking in February, the rainfall on the hillside where I live comfortably exceeded the long term average until March, as the graph below shows.

I have written before about wanting to understand the relationship between the amount of rain and the amount of energy the turbine can generate. The matter was what prompted me to start measuring rainfall 3 years ago. The relationship ought to have a quantitative element: more rain leads to more energy, - and ought also to have a temporal element: a relationship in time between rain falling or not falling and generation changing.

I now have enough rain and energy data to show these relationships in a graph: (note added 10 Jan 2021: the graph has been updated with additional data)

What it seems to show is the quantitative relationship is confirmed: more rain leads to more energy; but with the proviso that since turbine output is limited to 600 - 700 kWh/month, if there is rain in excess of what will generate that number of kWh's, the relationship no longer holds true; rainfall of 125 mm per month seems to be enough to generate maximally.

The temporal relationship is also evident: in autumn several months of rain needs to fall before turbine output picks up: generation can be said to lag behind rainfall; after a dry summer (e.g. June 2018) this can especially be seen and is doubtless due to ground water being much depleted and needing topping up before the spring, on which my turbine depends, begins to have a good flow.

There is a corresponding mismatch at the start of each year but here generation exceeds what might be expected from rainfall, i.e. turbine output persist for longer than expected; this is understandable as being because the spring continues to flow strongly from ground water accumulated over preceding winter months. Such 'bonus' generation tends to get terminated rather abruptly in April when trees start coming into leaf, so making their own demands on groundwater for transpiration.

Graphs are a useful way of understanding data and we are all being given an education in the use of graphs by the present flu pandemic. When I write again, life will probably be returning to normal from the point of view of flu, though it will be later than that when turbine output returns to the good levels of the past 156 days.