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.

Friday, 17 October 2014

Accurate to two decimal places.

What I like about operating my Powerspout is both the unpredictable element, namely the dependance on the vagaries of rainfall, yet the very precise predictability of how much power will be generated for a given flow.

So I have taken to operating my installation rather like a laboratory experiment.  I measure, record and calculate various inputs and outputs which enable me to better understand the quite complicated engineering which underlies such an apparently simple machine.
Most especially what this admittedly 'anorak-like' approach shows is how losses, or inefficiencies, in the system drastically reduce the electrical output.  I want to draw attention and explain this further in the next posts on this blog.

To make any kind of scientific investigation, I have had to decide just how accurate I intend to be, and an accuracy of two decimal places is the tolerance I aim for in most readings.  This should mean the conclusions drawn are reasonably valid.

To measure available flow, I divert water from going into the header tank and measure with a stop-watch the time it takes to fill a coal scuttle (11.36 litres).

Today, it was 20 seconds, the same as yesterday, making the flow 0.57 lps.  Still not enough to run the turbine continuously without draining down the header tank.  The premise on which my scheme works is that outflow from the tank cannot exceed inflow if the turbine is to operate continuously. The corollary of this is that the tank must always be at least slightly overflowing.

The out flow from the tank to the turbine is determined by the size of the two nozzles jetting water onto the pelton wheel.  The out flow can't readily be measured but it can be calculated. The formula is: 
Q = CD × Anoz × √[2g × Hn
where Q is flow, CD the discharge coefficient for the nozzle, Anoz the cross sectional area of the nozzle hole, g is acceleration due to gravity and Hn  is the net head of the scheme.  

So three unknowns: Hn, Anoz and Chave to be discovered.  Here's how I have measured each:

  1. Net Head

2. Nozzle area

3. Discharge coefficient
Powerspouts are provided with well designed, 40° tapered nozzles having a discharge coefficient (CD) of 0.89-0.91.  I established this value one afternoon by using the same equation above, but this time in-putting the flow as a known. I had somewhat laboriously measured it by measuring the turbine discharge flow. (note added 6 Feb 2018: the Cd was later measured to be 0.85 not the figure given here; see this diary entry).

Solving the equation for the two nozzles fitted in the turbine at the moment, I know that the outflow will be the sum of 0.83 and 0.30 lps, ie 1.13 lps.
Since this is twice what the inflow is, it is clear continuous operation of the turbine is still not possible.
Patience is needed, but the good news is that the weather forecast is for a warm, wet week ahead.


TommyTheCat said...

Hi Bill,

Really interesting site, I'm just in the process of looking into a powerspout.

Just out of interest (as I've not been able to find a figure for it anywhere else), what sort of charge do you incur from the Environment Agency for your yearly abstraction?



Bill said...

Hi Paul - glad there's something of interest to you in my musings !
There's no annual charge for abstraction for the purpose of generating electricity. However, there is a hefty charge for the process of obtaining an abstraction licence.
I think this charge has risen to £1500 recently, both in England and Wales, though when I got my licence in 2013, it was just £135.
Obviously £1500 is a disproportionate fee for an installation of Powerspout size. The charge is the same whether you might be generating 1 kW or 1 MW. This is totally unfair but the Environment Agency WILL not listen to reason on the issue.
Sadly, it will kill off many schemes which otherwise might have gone ahead.

TommyTheCat said...

Thanks Bill,

As I'm looking at this as a long term investment as I'm totally off grid, so this is still preferable to running my generator all the time to charge my batteries.

Fortunately I live between two burns (15 & 50m either side of the house) and they both have good flow all year round so even at £1500 it will probably still make sense in the long run.

I'll be following your posts for more inspiration!



Bill said...

Sounds like you're in Scotland, - SEPA might have different charges to what I gave you.

Best of luck with your project, it'll need graft but will be worth it if the scheme is a good one, and Powerspouts are excellent in my opinion.


TommyTheCat said...

Cumbria, not Scotland, but no shortage of water that's for sure.