Cost and return

When it comes to renewable energy schemes, I find people are usually coy about the costs and rosy about the returns.  So in this post I have tried to set out as accurately as I can the bare facts relating to my set up.

Of course, this turns out to be not as simple as it seems: having the time and inclination to do the drawings for the planning application saved on paying someone else to do it; having a tracked excavator saved hiring one for digging the pit below the turbine; materials were sourced at auctions at less than true cost where possible, ... and so on.  So even with the firm intention of disclosing everything, circumstances particular to me will mean my costs will not necessarily be indicative for someone else.

Stating what the financial returns have been is a simpler matter than dealing with the costs, so we'll start with those: - overall it has worked out that instead of me paying Swalec, our electric utility company, £1,022,  which was the all inclusive bill for 2013 (daily standing charge and tax included), in 2014, with about the same total energy consumption and the same tariff charges, I paid them £592.  So that was a saving of £430.

 On top of that saving, they paid me £996 in Feed in Tariff and Export tariff for the energy I generated.  

So, to give two different ways of looking at these figures: 

• I had all my electricity for 2014 free and additionally I'm £404 in credit.  
• the worth of the turbine in 2014 was the sum of the saving from grid energy not purchased (£430) plus the revenue from tariffs (£996), ie £1,426.

But 2014 was a wet year and therefore may prove to have been more productive than the norm. Only time will tell.

Turning now to costs: below is a record of the expenditure incurred as the project was implemented during 2012 and 2013.  From the outset, I had wanted to see exactly how much the whole project ended up costing and with that intention, I kept a pathologically meticulous record of everything spent.

A word about VAT:  I had hoped that VAT would be chargeable at 5% and part of the reason for keeping such a meticulous record was to aid reclaiming the difference from the point-of-sale rate of 20%.  However, after a tedious correspondence, HMRC¹ eventually ruled that: ...since micro-hydros no longer came under the Micro Certification Scheme (MCS), under which they had qualified for a 5% rate, and were moved from that scheme to the ROOFIT² scheme in Dec 2012, - a move which opened up the way for people (like myself) who were not MCS accredited to do the installation, ... the 5% rate could only apply if an accredited installer did the installation.  Does that make sense ? - not to me, but what can one do ?

The costs given below therefore include VAT at 20%, where it was payable, and they also include the cost of carriage to bring everything on site.  In the case of the Powerspout turbine and spares, the cost also includes UK import duty.

Two grand totals are given at the bottom:

• 'All in' cost for installation as completed, with spares:          £10,387.96
• 'All in' cost excluding:  
• non-essentials (tank water level sensor, and spares) 
• re-saleable penstock construction equipment:              £7,125.97

¹ HMRC Her Majesties Revenue and Customs

² ROOFIT scheme : Renewables Obligation Order Feed in Tariff scheme

It's a J shaped curve.

The aim of this post is to say a bit more about the efficiency of my installation: in particular how the efficiency changes depending upon the flow delivered to the pelton.

In two earlier posts, mention has already been made that overall efficiency turned out to be 50% when water delivered to the pelton was 2.7 lps, and the efficiency dropped from this figure when flow was less than 2.7 lps, mostly due to inefficiencies attributable to the inverter.

Here, I want to show the relationship between efficiency and flow over the full range of flows I have used on my turbine, from under 1lps to the design flow of 3 lps.

Instead of using a numerical value for efficiency, eg 0.5 or 50%, as is usual, I have used instead a unit which is a direct indicator of overall efficiency: the volume of water required to generate one kWh.  This has the unit m³/kWh and is a measurement which I record anyway for the paper return I have to make yearly to Natural Resources Wales (NRW) to tell them how much water I've abstracted.  It's what they call the 'hydro abstraction factor' (HAF) and permits calculation of the volume abstracted simply by multiplying it by the number of kWh's generated.

It is a factor which is easily measured with a fair degree of accuracy: 

• flow rate from the sizes of the nozzles and the net head
• kWh's from the Elster generation meter (reads to 0.1 kWh)
• time from a digital, radio controlled clock with readings taken over a minimum of 24 hrs.

So here is the graph showing the relationship between flow and m³/kWh:

As can be seen, it turns out to be a J shaped curve with optimum efficiency occurring when flow delivered is just under 2.5 lps.

Why should the curve be shaped like this ?  The explanation for the fall in efficiency at flows below 2.5 lps, to the left side of the curve, has already been alluded to in earlier posts: factors intrinsic to the inverter and, at the lowest flows where system voltage rises towards 380 v dc, the onset of power dumping to the heater element within the turbine, which is triggered at that voltage.

As for the fall in efficiency on the right side of the curve with flows higher than the optimal 2.5 lps, the most likely explanation is the 'drag' imposed on the pelton runner from the spray created in the turbine casing by the extra water, - something the text books call 'windage'.

Does any of this have any practical application ? - well yes, it does:  NRW instruct that in working out one's abstraction return, the 'hydro abstraction factor' at Pmax must be used, ie at maximum power output.  For my installation, as can be seen from the plot, this would be 14.5 m³/kWh.  But the optimum point, which is the point on the graph where the installation will be operating for most of each 'water year', the HAF is nearer to 13.5 m³/kWh.

... and that is the figure I'll be using, rounded down to 13 m³/kWh, so as to be sure of keeping my abstraction return for the year as low as possible and within the limit imposed by my licence (49,982 m³).

49,982 m³  ! - it does make you wonder if the people who issue these licences stating such ridiculously specific volumes understand the limitations of the means which will be used to measure said volume.

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 !