The set up

The set up
5.36mm jet delivering 0.63 l/s to the pelton which is rotating at 870 rpm and generating 127 watts into the grid.

Sunday, 6 January 2019

Stopping water entry

I've been experimenting recently with trying to stop wetness from the pelton side of the turbine creating dampness on the alternator side. There's a seal which should prevent water in any quantity getting across but an investigation I've done using bags of silica gel indicates that in spite of the seal about 300mls per month still gets across.
These photos tell the story of the sequence of steps I've taken:

1. Limescale deposits on the shaft indicated that a considerable amount of water enters the top-hat labyrinth chamber

2. A V-ring seal (purchased here) was mounted on the shaft; the seal turns with the shaft and its lip seals against the plastic face of the top-hat, with the idea of preventing water tracking alongside the shaft

3. Inspection after 3 weeks running showed the seal had badly scored the plastic face of the top-hat, presumably from softening of the plastic by the heat of frictional contact, - I must have applied it too tight to the face.

4. A friend who is skilled on his metal lathe kindly turned a stainless steel cap to fit over the plastic top-hat so the seal rubbed on metal; the cap is held on only by being a tight fit.

5. Suspecting that water might also enter the top-hat via its drain hole, a deflector was devised to shield the hole from the upward direction of water leaving the pelton from the lower jet.

6. So the complete arrangement as it is at the moment looks like this:

Only time will tell if it makes any difference.  The early signs are that the silica gel bags do seem to be taking up less water but I'm yet to be convinced this is a genuine observation.

Whilst I was working through these stages of development, EcoInnovation have come up with a slightly different approach:

Theirs is a neater solution but care will be needed not to apply the V ring seal too tightly against the face of the top-hat.  The seal only needs to just touch. After observing the scoring illustrated above, a new top hat with the seal just touching ran for 3 weeks with not even a mark being caused.  A smear of grease is also a good idea.

Sunday, 9 December 2018

The benefit of offset

In the UK, people who generate electricity can be paid for what they generate. The scheme is called the Feed-in-Tariff scheme (FiTs), - and next year it comes to an end.  The end is on March 31st 2019.

It comes to an end only for new schemes. Schemes already accredited, like mine, will continue to receive payments for the duration of the original contract, - 20 years.

Feed-in-Tariff payment is made up of two parts: a payment for the total energy generated and a payment for the energy exported to the grid.  Payment for both components ends.

This change in government policy forces a re-think for those considering a renewable energy scheme: does it still make financial sense? The thrust of this blog post is to suggest that if the renewable scheme is very small hydro then there is still a strong case for going ahead.

The case rests on the idea of offset, by which is meant the expenditure saved when electricity is not purchased because you are generating it yourself. There are two aspects to the proposition:

First, in years to come, the cost of electricity purchased from the grid will go up. If you generate it yourself, so the value of what you save will increase over time. Since the design life of a hydro is upward of 40 years, these savings will have many years to play out.

Second, there is a benefit unique to very small hydros operating 24/7. Putting out power at the relatively low level of 500 watts (+/- 300), the turbine's output closely matches the base load demand of a property.  Base load is made up of that multitude of appliances which are 'on' all the time - from battery re-chargers to fridges, freezers, central heating pumps, computers and so on. Totted up their power requirement can typically be 400 watts. That translates over a day to an energy consumption of 10 kWh.

Purchased from the grid, day after day, week after week, 10 kWh per day amounts to a significant expense, but supplied from your own turbine what would have been an expense becomes a saving, - a saving which can be thought of as justifying the initial cost of the turbine.

To illustrate how great the saving is, consider some real life figures below showing energy taken from the grid (kWh) in the first quarter of each year for the 10 years before I installed my turbine and for the 5 years after:

... there is an unmistakeable step-down in kWh taken after the turbine was installed, - and this represents the saving made by installing it. (Incidentally, the increase seen in 2018 resulted from acquiring a Nissan Leaf in late 2017).

The value of the saving in this one quarter is £350.  For a whole year it is £546.

So, whilst Feed-in-Tariffs may be ending, a small hydro can still pay for itself through the savings of offset. Certainly, without FiT's payments the time taken to recoup the cost of installation will be longer.  With FiTs payments for me it has been 5 years.  Without FiTs and keeping to 2018 electricity prices it would have been 16 years. 

So 16 years is about the period of time someone starting out in very small hydro after 31st March 2019 should be thinking of as the time it might take to recoup the installation cost.  It seems a long time, but it could be shorter, perhaps much shorter if electricity prices really go through the roof.

Saturday, 24 November 2018


There are just two measures of how useful any renewable energy scheme is:

- how many days in a year it runs
- how much it generates when it is running

In renewables parlance, the 'days run' are spoken of as being the availability factor and how much is generated over a year when it runs as the capacity factor. 

More completely described, the capacity factor is how much energy (kWh) is generated in a year as a fraction of how much could have been generated if the installation had run at its rated capacity for the whole year.  Thus defined, it can be seen that capacity factor incorporates a scheme's availability factor since if a turbine isn't running for some days (↓ availability) necessarily the year's output will be less (↓ capacity factor).

I read recently that in the UK small wind turbines typically deliver a capacity factor of just 15%, whilst PV panels manage 17%.  These figures reflect the obvious: the wind doesn't blow all the time (↓ availability) and when it does blow it doesn't always blow sufficiently to drive a wind turbine to its full rated output (↓ capacity factor), - and for PV, the sun only shines in the day not the night (↓ availability) and even then is not always bright enough due to clouds or inclination in the sky to give maximal output from the rated capacity of the array (↓ capacity).

Comparing these figures with those of my turbine over the past year sheds a rosy glow on how useful the turbine has been: having run for 365 days its availability factor was 100% and the capacity factor 58%. (see plot below)

Figures as good as these are unusual for a very small hydro and it's worth touching on some of the reasons:

  • the source is a spring whose flow comes from groundwater; a spring depends in a loose way on rainfall and is more constant in its output than a source using a watercourse, where the flow will closely reflect whether it has rained or not
  • my abstraction licence doesn't specify a 'hands off flow' - meaning I'm free to take all the flow available, even in the dry times of a year
  • by having an orderly range of nozzles of different sizes and maintaining a regime of changing between them to suit what flow is available, nearly maximal use of flow is achieved 
  • by using washers to make the rotor stand off from the stator with the aim of keeping the speed of the turbine up at low flow times, output is always maximal for the flow available
  • by swapping from a 42 pole stator to an 18 pole in the dry months, the turbine can be kept in operation at low flows; also its speed can be kept up by using an 18 pole stator beyond what is achievable by packing off the rotor with washers; the issue is caused by too strong magnetic attraction, for the torque available at low flows, between the rotor and the iron cores of the many poles when a 42 pole stator is in place 
  • by using a Type 2+ (high power rotor) at high flow times, turbine speed is kept down when otherwise it would go too fast for optimal output
  • a full range of spares kept on-site means days are not lost to generation by waiting for spares to arrive
  • a plan of preventive maintenance and a sturdy initial installation has so far eliminated prolonged down-time from unexpected problems

Capacity factor = (3816 * 100) / (0.75 * 24 * 365) = 58%

Availability factor = 100% (turbine ran 365 days: the one day showing no output is Feb 29th, included for spreadsheet reasons, but 2018 was not a Leap Year: hence no output shown that day).