The set up

The set up
5.46mm jet delivering 0.68 l/s to the pelton which is rotating at 900 rpm and generating 135 watts into the grid.

Tuesday, 13 January 2015

Rain, glorious rain.

 Yesterday, it rained steadily all day.  

As a general rule, one third of the rain hitting the ground runs off immediately into surface water courses, one third soaks down to top up ground water stores, and one third returns to the atmosphere as evaporation, which includes, in summer, transpiration from vegetation.  

Since it is winter, there won't be much evaporation or transpiration, and because the soil on our hillside is made up of geological deposits of sandstone and mudstone which have been weathered down to leave a fairly absorbent soil on the surface, a good proportion of our rainfall seeps down to reach ground water.

This is good news for me as the source for my turbine is a spring. The springs, and there are many of them on the hill, all occur on particular contours along the hillside because of thin beds of limestone sandwiched between the sandstone/mudstone deposits. These 'limestone developments' create the springs by allowing the groundwater back onto the surface. What is interesting is the time it takes between the rain falling and the springs gushing.

Yesterday evening, after the day's steady rain, the yield of my spring had not picked up very much at all.  But by this morning it was beginning to show an increased flow, and, because the ground acts as a reservoir giving up its rain-replenished water slowly, this increased flow should continue to gush for some time to come.

In anticipation of this imminent increase in flow, I changed the Powerspout's nozzles this morning to a combination which will give just short of maximum power output: lower nozzle delivering 1.6 lps, the upper 1.18 lps, together giving 718 watts out of the inverter to the grid.

It's always a bit of a guessing game deciding how much 'spare' flow there is, and just which nozzles can be put in.  It's even more of a gamble when you're trying to guess in advance of the spring actually giving up its water: choose too big and the header tank drains down; choose too small and you chide yourself that you're not maximising generating capacity.

This morning I seem to have got it right.  After putting in my chosen nozzle combination, there was still water in excess overflowing the tank and running down to join the stream. That stream will take it to the River Usk and thus it will return to the sea whence it came in the first place.

Conveniently, I can visualise how much 'spare' water I have available at any time: in its drop to join the stream, water from the spring, which has not been diverted to the turbine, falls over the rock face of a quarry at the bottom of the hill.  By the trajectory of the spout from the pipe, I have educated myself as to what volume of flow there is.

Here are before and after photos of that flow, the difference between them being 0.22 lps, the extra volume diverted to the turbine by putting in a bigger jet this morning:


OK, there isn't much difference  - but to the educated eye, there is !  I had hoped for a more depleted flow in the 'after' photo to make the point, but it wasn't to be.

So today, 13th January, I have almost reached peak power output.  Last year this point was reached on 7th January and it then continued un-interruptedly for 75 days until 23 March, contributing 1.28 MWh of last year's total energy output of 3.67 MWh.

I wonder how this year will turn out in comparison.

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