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 CD have 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.