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.
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.
