Testing the system.

I've always wanted to know what would happen to the turbine and its electrical output if the header tank were to drain down to empty. A few days ago I had the opportunity to find out.

After the wetness of mid November, things have turned dry again, so I was in the situation of still having the largest nozzles in the turbine, delivering the design flow of 3 lps, with a supply of water which was slowly diminishing.

Inevitably, a time came when water inflow to the tank fell below outflow down the penstock to the turbine, and instead of overflowing, as is the normal operating condition, the tank gradually began to empty.

The dimensions of the tank are 2.74 x 1.52 x 1.37 metres but since the bottom 83mm cannot be accessed, the useable volume is 5.3 m³.  With this generous volume of reserve, and only a small difference between inflow and outflow rates, it actually takes several hours, more than 12 on this occasion, for the tank to drain down. Somewhat inconveniently this meant all the action was set to happen after dark.

Here is an aerial picture (actually taken from up a tree) of the header tank and the arrangement for gathering water into it:

And here is the record of power output from the inverter into the grid over the period when the tank was emptying.  The record was captured using Wattson power monitoring and the data downloaded to my computer using the Holmes software that Energeno provide free for this purpose:

What did I learn from it all ?  The most useful lesson was that the system is resilient to such an event: nothing occurred which could foreseeably damage the key components of the system, namely: turbine, pma alternator and inverter.

If anything, the system 'fails safely' because the dc voltage falls to a lower operating level than normal (in this event it was 265 v dc at its lowest) because with the lower 'head' created in the penstock, turbine rpm will have decreased marginally.  Against this effect, there will have been a counter tendency to raise voltage created by the inverter lowering the load it places on the pma.  That the overall result was a drop was interesting.  It was not obvious by observation and listening that the turbine rpm was very different from usual.

So, all in all, an exercise which was worth doing even if it did give me a sleepless night !

More paper work !

At the end of each month I need to remember to keep my records up-to-date.  

This November has been a bonanza month for generation, especially considering it was only from Nov 5th that there was sufficient flow to generate anything at all.  By the end of the month, 393.7 kWh will have been clocked up, and the capacity factor for the month will be a very presentable 0.73.

I need to remember because a return stating the volume of water I've abstracted has to be sent to Natural Resources Wales (NRW).  NRW like the year to be broken down into individual months, and you have to give the volume abstracted in each month. The return has to be sent in within 28 days after 31st March but fortunately they post the form to you as a reminder.

For my scheme, to generate each kWh requires 13 m³ of water;  this is a figure the NRW call the Hydro Abstraction Factor and they tell you how to calculate it for your scheme.  The inputs to the calculation, which will vary from site to site, are the 'water-to-wire' efficiency and the head of the scheme.

Here is the spreadsheet I keep of monthly abstraction so everything is to hand at the end of March to complete the annual return:

So that was one bit of paperwork I had to remember to do.  The other was applying for payment for the kWh's generated in the past 3 months.  Since the turbine doesn't operate for most of that quarter, for lack of water, a November claim is the lowest of the year: just £71 on this occasion.

Claiming feed-in-tariff is a much more straightforward business. I opted for a contract with Scottish and Southern Electricity (SSE) simply because they're the company we purchase power from and it seemed to make sense to stick with them.  Submitting a generation-meter reading on-line is simplicity itself at the SSE webpage. You immediately get email confirmation of your claim being processed and payment comes through direct to your bank account, - somewhat unusually on this occasion, within 48 hrs.

Co-incidently, I had a letter from SSE today telling me they're in the process of updating their FIT payment system.  When completed, I'll be assigned a new date in each quarter on which to submit my FIT claim and more detail will be provided about how it is calculated.  Additionally, their man who comes to read the meter will, in future, read both consumption and generation meters at the same visit.

So the take-home message is: in the UK at least, there is a fair amount of bureaucracy involved in operating even a small hydro installation, but like a tax return, so long as you keep on top of all the figures and the deadlines, it isn't burdensome. 

TIC, DNC, Abstraction and all that stuff.

For the first time, I now have the Powerspout operating at 'design flow', - at least it's the first time I've had any certainty the flow being delivered is the design flow: 3 lps. So for the first time too, I have reliable figures for those two parameters so loved by OFGEM: Total Installed Capacity (TIC) and Declared Net Capacity (DNC). OFGEM is the UK body giving accreditation for an installation to receive payment for the electricity it generates.

Just to set the record straight: DNC is TIC "less any electricity consumed by the generator".  So for a Powerspout the power taken by the inverter (see last post) has to be deducted from TIC to get DNC. Both of these figures need to be specified at the "highest capacity at which the generating station could be operated for a sustained period ... without causing damage to the generating station". I took that to mean at 'design flow'.

I may have taken it to be at design flow but in fact a Smart Drive alternator can put out in excess of 1kW for a sustained period and without being damaged, so there is nothing actually to stop me from delivering a flow in excess of 3 lps, when it's available, and getting more power.  That would've meant giving OFGEM a higher TIC and higher DNC.  So where do you draw a line in the sand ?

I opted to give values for TIC and DNC based on 3 lps because my abstraction licence limits me to that flow.  Which begs the question "why didn't you ask for a higher abstraction volume?" And the answer to that is another line in the sand: it seemed from the flow duration plot for my site to be the optimum peak flow when the pattern of flow over a full year is taken into consideration.

I relate all this to make the point that filling in all the forms to make an installation 'legal' is a headache.  Moreover what you put on one form, the Abstraction application, has a bearing on what you put on your OFGEM form to get accredited for Feed in Tariff payments.

To get back to real figures, below is the actual return I submitted to OFGEM last year for my FIT accreditation...

... and as you can see, I put 0.8 for TIC and 0.75 for DNC. These were, at the time, the best estimates I could come up with.  They were supported, as the extract above requires, by independent confirmation from Michael Lawley, the manufacturer of Powerspouts, who used the calculator tool on the Powerspout website to confirm my expected output.

But the thing is: it's very difficult to get the accuracy of the inputs to that calculator tool to be spot on; now I have real data available, I find the true TIC and DNC aren't what I submitted.  The DNC turns out to be 0.92kW and the TIC 1.1 kW.*

Do these small differences matter ?: no ! - no one at OFGEM is going to be any the wiser that my submission figures were 5% out.  But the saga does draw attention to the fact that in completing these forms, what you enter on one, the abstraction application, has a bearing on what you should enter on the other: the FIT application. And deciding where 'the lines in the sand are to be drawn', I found to be a hard call.

Finally, if you're wondering what OFGEM's definition of DNC is in full, as hyperlinked in the above extract, this is it:

Confused ? - so was I, - and to a large extent, I still am.  The forms are, truly, a headache.

* figures amended April 2020 with knowledge gained over the 6 years since original post was written.