The biggest player in what makes a micro-hydro productive is the rainfall in its catchment area, and so it's useful if you can relate the amount of rainfall to how much generation will result.
One reason it's useful is that data for rainfall go back many years whilst data for generation, especially for a recently installed turbine, go back no time at all. If a relation between rainfall and generation can be established it makes possible a feeling of right perspective when an exceptional year comes along, - you are able to say to yourself "Ah yes, from past rainfall records this is something I know happens once in 'X' number of years".
To give an example: of late, I have been noting with consternation, - consternation bordering on pain, - how generation at the moment is lagging well behind previous years. By the end of February, kWh's generated were more than 700 less than last year and less also than the two years prior to that:
I know, of course, that rainfall varies from year to year but the question in my mind is "What value does 'X' have in this situation? - how often should I expect to have a bad year such as this?". With only 4 years of my Powerspout being operational, I don't have enough years of generation to feel I can give an answer.
As good fortune would have it, I happen to have a guide who can help. A lady called Ena who used to live on our hillside collected rainfall data for the UK Meteorological Office. She lived at the top of the hill that forms the catchment for the spring which is my turbine's source and so her records are ideal for my purposes. Unfortunately she moved away in 2012, and 2012 was, crucially as I'll explain below, just before I started generating. Before she went, she let me have her data for the previous 23 years, - data which I am sure are as accurate and reliable as is possible.
Re-arranging her 'calendar-year' data into time periods of 'water-years' (Oct 1st to Sep 30th) to make them coincide with the same 12 months over which I measure the generation from my Powerspout, reveals that in those 23 years there were just 2 years when the amount of rainfall fell below the distinct cluster of values which account for 17 of the 23 years:
...so this would suggest that dry years only happen once in ~10 years, with very dry happening once in ~20.
But what I would really like to do is establish a more definite relationship between annual rainfall and annual energy generation, - establish a conversion factor, albeit an approximate one, - so I can convert Ena's 23 years of rainfall data, measured in mm/year, into its electrical equivalent measured in kWh/year. I can't make that correlation immediately because Ena moved away before I had my Powerspout up and running, and to establish the correlation I need at least one year, preferably two, for which there is both rainfall data and generation data.
The direction this train of thought is pushing me is to embark on measuring rainfall; not to measure it for ever, only for two years. The bonus from doing it would be that I would be able to compute surrogate generation data going back to 1988 from Ena's rainfall measurements; the data would become the earliest entries in a reference list of kWh's generated, - a list which would be added to each year with actual data; no need to wait for years of actual data to accumulate, - from the moment it has been compiled the list would go far enough back in time for a reasonable perspective to be gained.
It must have been a labour for Ena to measure rainfall for 23 years. It would be nice if all that labour found a purpose by being applied in this very specific way.
Below are two pictures, a before 🙁 and an after 😊; what do you think it was that made the difference between them ? - and before you jump to thinking the water flow or head were somehow the cause, let me assure you the two pics were taken under identical hydraulic conditions.
The answer is that the second picture was taken with a Type 2+ rotor installed in the permanent magnet alternator (pma), - a Type 2+ rather than a Type 2.
So what is a Type 2+ rotor and why should it increase the power whilst reducing the rpm? The standard rotor for a 42 pole Smart Drive is the Type 2 and it's capable of generating 0.75 W / rpm. The Type 2+ is magnetised to a higher flux strength and is capable of making 1 W / rpm.
For most purposes, which is to say for most operating conditions of head and flow that characterise an installation, the Type 2 will output as much electrical power as the pelton is capable of extracting from the flow available, and it will do this whilst keeping the speed of the pelton at, or near enough at, optimum speed.
But at the very top end of an installation's design flow, when the power put out by the water side of the turbine is greatest, the Type 2 may not be capable of converting all that 'hydraulic-cum-shaft' power to electrical power without rpm's increasing significantly. Not that there's any problem with this: electrical power WILL increase with more water flow because the turbine will simply spin faster - but it won't increase linearly and in the extreme situation power output can actually DECREASE with more flow.
All this is best understood by reference to the following diagram which relates the power output of a pelton to its speed:
From the shape of the curve it can be seen that a pelton has an optimum speed (speed /optimum speed = 1); that at optimum speed, the power output is greatest (power/max power =1); that away from optimum speed, both below and above it, power as a proportion of max power diminishes.
In the situation illustrated by the before and after pictures above, the difference between them is represented by a shift of the point on the curve at which the Powerspout is operating from A to B: speed was too HIGH in the before picture, which is to say the turbine was operating to the RIGHT of the optimum speed point, and by REDUCING it (point moves leftward toward Speed/Optimum speed = 1), power output as a proportion of max power increases. In effect, there is an efficiency improvement being gained which produces more watts output.
For those who read my earlier diary entries about Rotor packing, the above diagram also helps in giving understanding there: in that situation, power output was lower than it could have been because speed was too LOW: - the Powerspout was operating at a point on the LEFT side of the curve; by changing the magnetic environment of the pma, (which is achieved by reducing the flux linkage by packing the rotor away from the stator coils), the speed is allowed to INCREASE, and with that increase comes an increase in watts output as the operating point moves rightward up the slope of the hump. On the diagram this is a move from C to D.
For the owner / operator of a Powerspout who really wants to maximise output under all flow conditions by playing around with the strength of the magnetic field, the challenge is knowing what the optimum speed for their particular installation is. For me, and as a result of this experiment using a Type 2+ rotor, I'm settling on a figure somewhere between 950 and 1000 rpm. But take note, this is for my site only; it won't hold true for somewhere else.
