Grid failure

This past week has seen snow falling where I live.  However well Western Power, the electricity distribution company for our area, has done its job of cutting back overhanging trees from the power lines, snow always brings problems.  In this past week we have had repeated power outages and I thought it worth writing a Diary entry on how a grid connected Powerspout pelton behaves in such circumstances.  

When the grid goes down, the turbine will continue to receive water and the energy of that water has to have somewhere to go,- where it goes depends on whether it is an older or newer type Powerspout and on what type of inverter is handling the interface between the turbine and the grid.

... the original "grid enabled" Powerspout, the GE 400, which is the one I have, manages the situation by diverting power which cannot pass to the grid whilst the grid is down by sending it to a small heater load which is splash cooled by the spray within the wet side of the turbine casing: 

 Diversion is controlled by an electronic control board, housed in the dry side of the casing, and this regulates the power fed to the heater element so that system voltage is kept at 380 volts DC.

The necessity of keeping voltage at this level is dictated by the inverter I have which is an early one marketed in 2011. It is unable to accept voltages higher than 400v DC.  The electronic control board locks system voltage at 380 v, just below the inverter's limit and thus keeps the inverter from being damaged by over-voltage.

... later, inverters came on the market capable of seeing an in-coming voltage considerably in excess of 400 v: - the 2 kW Enasolar inverter, for example, is quite happy with 600 v.  This development in inverter technology opened the door to managing grid outage situations in a completely different way, a way which was much simpler and did away with the need for a control board.  

Instead of diverting electrical power to a load, electrical power ceases to be created at all by keeping the system in open circuit: with no load connected what happens is that system voltage rises as the pelton runner goes to its run-away speed but, crucially, no current flows.  And so long as the system voltage rises to no more than 600 v the inverter remains safe.  What becomes important with this way of managing grid outages is that a stator must be selected which is wound in such a way that even at the highest run-away speed possible for the site (which is determined by the net head), the open circuit voltage will never exceed the inverter's limit.

You may well ask "but what happens to all the hydraulic energy in the system if none of it now finds an outlet by being turned into electrical energy?" The answer is that a lot of it never gets as far as being translated into shaft rotational energy. At the run-away speed of the pelton much of the water passes through the pelton runner, which is moving just about as fast as the water jet, without ever hitting the pelton cups.  The water ends up hitting the casing opposite the nozzle and its energy is dissipated as heat and sound. Some extra energy is also lost as heat in the bearings and shaft seal - a greater amount at the higher speed at which the shaft is revolving at run-away speed than is lost at normal operating rpm.

To illustrate some of this, here is a picture of a pelton at run-away speed showing how the water from both nozzles fails to be deflected in the normal way onto the front glazing because the velocities of both runner and jets are little different:

This pelton was coupled to an Enasolar 2 kw inverter which, at the time, was not connected to the grid; the run-away speed of the turbine which resulted was measured at 1440 rpm:

and because the stator of the SmartDrive had been carefully selected to be one which delivered just 0.266v /rpm in open circuit (it was a 100-14S-1P S stator), the open circuit voltage at runaway was measured at 383 v: well below the 600 v limit of the inverter.

So there have been two ways of managing a grid outage with a Powerspout pelton and both are good.  This past week with its numerous grid outages has reminded me just how "bomb proof" my system is: when the grid goes down, the turbine continues happily feeding power to its dump load and after the grid comes back on, seamlessly the turbine re-connects itself to the grid.  It's all clever stuff and it gives me great pleasure to see it in operation.

End of year results.

Good data about small hydros is hard to come by.  By good I mean energy data which is trustworthy, granular enough to show daily output and historical enough to allow comparison of one year with another. Measures such as 'Capacity factor' and 'Availability factor' are also nice parameters to see reported.

But such data is scarce so it's always a high point for me to come to the end of my 'accounting year' and be able to present my figures.  

The 'accounting year' I use runs from October 1st to September 30th so today is the day I have been able to wrap up the figures for the past 12 months.  Presented graphically as a cumulative plot of kWh's generated the results look like this:

...and the same data presented as a daily plot showing generation on each day of the year, the results look like this:

 Both plots show data for this year (purple) and for the previous three years. 

Though the figures for hydro generation are interesting enough in themselves, many very small hydro installations will be coupled on the same premises with a small solar installation, and it's nice to be able to see how the one compliments the other.  Here then are the corresponding graphs for my 3.3 kWp PV which has been operational for only two years:

It doesn't need a magnifying glass to spot some of the comparisons to be made between the hydro and solar yields:

• solar is hugely variable on a day-to-day basis but the cumulative plot in one year follows almost identically that of the previous year;  hydro by contrast has almost no day-to-day variation (a step change occurs only when a nozzle change is made) but the cumulative plot lines are widely divergent from one year to the next.
• the yield from hydro does well from November to May, whilst solar does well from March to September; late October is the time when total 'domestic generation'(i.e. the daily sum of the two) is at its lowest.
• on the evidence so far available, the total per year for my domestic generation will lie between 5800 and 7200 kWh.

This last bit of information, i.e. generation being between 5800 and 7200 kWh/year, has been a key driver in my exploring 'battery-on-the-wall-storage' at home.  Our domestic consumption of electricity is about 6000 kWh/year but because cooking (our heftiest use) always takes more power than is being generated, we end up having an annual take from the grid of about 1700 kWh.  I have been exploring whether it might be feasible to reduce this by storing domestic generation into a battery when there is a surplus, - a few hundred watts at any given moment but over several hours, - so that it can be used for cooking when the need is for 3-4 kW but for a short time only. 

And a 'battery-on-the-wall' offers two other possibilities: the possibility of purchasing and storing grid energy when the tariff is cheap then using it at a time when the grid tariff is high, and the possibility of having an 'uninterruptible power supply' for the whole property for times when grid outages occur.

So data collection has its uses. It can inform how best to move with the times as new developments like 'home battery storage' come on the market. The man is coming to see me about battery storage next month.  I'll report back.

Peltons past and present.

I've recently acquired a new pelton wheel, - only it isn't new, - it was made in 1905 by an engineer who, at that time of his life, lived and worked from a small town about 40 miles from here. His name was Percy Pitman and here is an article he wrote for the October edition of 'The Engineer' in 1905. I guess it's probably him in the picture:

It's worth remembering that 1905 was only 25 years after Lester Pelton, living in Ohio, USA, filed his patent for an impulse turbine having cups with a splitter ridge, so the cups effectively became two cups side by side. Prior to this, cups were more like a single bucket and the water entering them had to bounce back rather than be streamed outwards to each side.  Pelton's design made the wheel significantly more efficient.

The wheel which has come to me was for sale in a farm auction.  Where I live, when a farmer retires, it is usual for him to have a sale of his old equipment, - everything from tractors to scrap wood and steel. The farmer who was retiring had bought this pelton at a similar farm auction some years before, but had never done anything with it. By the look of it, the lack of wear on the buckets indicated it had NEVER done any real work:

Back home, shot-blasted, painted and mounted on a frame which I picked up from another farm auction, the wheel is beginning to look presentable.  I'm intending to put it close to my Powerspout so people passing can understand what a pelton wheel is.  I'm amazed at how many people have never heard of the word !  Here's how it looks now with a Powerspout pelton next to it to give an idea of size:

Life sometimes throws up odd coincidences !  A few weeks after getting my pelton, a friend drew my attention to an advertisement for the sale of ANOTHER pelton wheel, almost certainly by Percy Pitman because of having identical construction and being located in the vicinity of Bosbury where he lived, but this time a more complete example than mine:

To have TWO historic pelton wheels become available within weeks of each other is a truly unusual event.  And to learn that one hundred years ago, in this part of Wales, there was a man making his name by the manufacture of such wheels gives a nice historical dimension to my Powerspout operating today.  

There's one curiosity that remains with me though: - my pelton is NOT well made; the cups are steel castings bolted to a circular steel plate, but they have not been attached around the circumference in a way that makes them equi-distant from each other, - at one side they are crowded together and at the opposite side spaced more apart.  The effect is that the wheel is badly un-balanced and a rather crude effort was made to balance it by bolting on a strip of steel. I have removed it because it was rusting badly and the rust was eating into the circular steel plate, but where it was attached can just be seen by the presence of two bolt holes at the wheel's 12 o'clock position.

Perhaps Percy Pitman was not the meticulous engineer he advertised himself to be ! Certainly he quickly left behind the manufacture of 'industrial' pelton wheels to pursue the manufacture of "hydraulic equipment for educational establishments".  But judging by the progressively more prosperous houses he occupied in London he must have died a well-to-do gentleman and a far cry from his humble beginnings in Bosbury, Herefordshire where he started out.

Postscript added Nov 2019:
More about Percy Pitman can be found here.

How the finished wheel looks: