On holiday but in touch.

I've just been away on holiday, - and owners of Powerspout turbines have decisions to make when they go away.

Those lucky enough to have a water supply which is constant can decide without thinking: - the turbine can be left running. But for those, like me, who have to change nozzles, planning is needed.

One option would be to shut down the turbine, but that would mean missing out on useful generation; an empty house still uses electricity and it's nice to know the hydro is meeting that demand.  Besides, I've set myself the target of generating all through each year without any breaks, and that means I don't come close to considering shutting down.

The alternative then is to keep going, and this is where it gets tricky; my crystal ball has to be dug out so I can gaze into the future to make a prediction about how much water is going to be available while I'm away.

The reason for this is that Powerspouts have nozzles with an orifice size which is fixed, and they are not automated in any way; many small hydros are automated; they have a nozzle orifice which is variable in size and can be adjusted dependent on how much flow is available, the flow being sensed by the level of water in a tank at the top of the penstock.

People can, and do, automate their Powerspouts along this line but keeping things simple is the line I prefer to take.

So the exercise with the crystal ball is to discern what size of fixed nozzle to leave the turbine with, which will still, I hope, be the right size for whatever is the flow at the end of my time away.

The things to look for in the crystal ball are recent rainfall, time of year, anticipated rainfall, temperature, and vegetation cover in the catchment area of the water source.

It doesn't much matter if you compute everything and get it wrong; if the flow dries up more than you thought it would, it's not a disaster.

What happens in my set-up is the head tank empties, water coming into it immediately goes out again down the penstock, setting up a new operating head somewhere down the pipe. This lower level of head ensures flow through the nozzle is less and the end result is a new equilibrium is reached where water arriving equals water leaving.

The operating efficiency of the system is rubbish when the turbine runs like this, - it labours at a much too slow rpm and power output is greatly reduced, - but the system does manage to keep generating and no damage is done.

Because of the technological wonders of the age we live in, there is no need to be in ignorance of how your turbine is doing while you're away. The photo shows that at the top of Carn Llidi in West Wales, I could check to see if the crystal ball had spoken truthfully.

In fact, when I got home I discovered it had not; far from drying up, 5 inches of rain had fallen in the ten days I was away; I could have been generating twice as much as the 220 W I set the turbine up for.

I need to get a new crystal ball !