Chasing small gains in efficiency

Operating one's Powerspout at best possible efficiency is self evidently sensible: you want to get as much power from it as you can.

Any opportunities to find efficiency improvements in an installation once the installation is completed are few.  Most factors making for poor efficiency are built into the installation from the outset and are not readily changed.  They often come from sub-optimal design and trying to save on expenditure, for example by opting for too small a penstock. This is always a temptation because the penstock is a significant cost burden but putting in too small a pipe leads to a loss of head pressure that will haunt the installation for its entire working life.  

Soon after I first got my installation going, I measured the 'water to grid watt' efficiency* to be 50% and in an early diary entry I speculated that this figure came about because of component efficiencies** as follows:

• penstock efficiency @ 3 lps                      0.95***
• manifold efficiency                                    0.98
• nozzle efficiency                                       0.98
• Pelton turbine efficiency                           0.77
• turbine / alternator drive efficiency           1.00
• SmartDrive PMA alternator efficiency      0.80
• transmission line efficiency                      0.98
• SMA 1200 inverter efficiency                   0.90

From this list, the only two which gave opportunity for obtaining any improvement seemed to be the nozzle and the inverter. 

Over the past 6 months I have been collecting data to investigate these two, the first by running with either one nozzle or two over the full flow range for my site, and the second by using a WindyBoy inverter in turbine mode and comparing with last year when a SunnyBoy in mppt mode was in use.

What I have found is that running on one nozzle gives a 2% efficiency improvement pretty well over the entire flow / power range:

I should add that when running a single nozzle it was always in the bottom position and when there were two nozzles, the top one only ever contributed 0.3 l/s, the bulk of the flow always being delivered via the bottom one.

As regards any benefit from changing inverters, - there really was none: the peak efficiency with the WindyBoy in two nozzle operation can be seen from the graph above to be a little over 49% and this is virtually identical (49.9%) to what I had observed the efficiency to be last year when using a SunnyBoy, also with two nozzles.

The small gain from using one nozzle rather than two is not unexpected: 

• there is an energy loss which takes place at a nozzle and by having one rather than two nozzles, there is only one loss
• one nozzle in the bottom position leads to less 'windage' i.e. the loss from spray inside the casing
• there is a marginal gain in head from using the bottom nozzle position.

As a result of this study, I'm going to continue with one jet operation.  I'll use the small top nozzle intermittently simply for the convenience of reducing the number of times I need to change the bottom nozzle to keep track of the ever changing seasonal flow.

This study doesn't provide a big step forward perhaps but nevertheless I thought the persuasiveness of the result interesting.

* The formula for calculating 'whole system' or 'water to watt' efficiency is:
Efficiency (%) = Power to grid (watts) x 100 / [ 9.81 x static head (m) x flow (l/s) ]

** Whole system efficiency is the product of multiplying together the efficiencies of all the components in the system; with the figures given this would make the whole system efficiency 0.496 (49.6%).

***This figure was 'guesstimated' before I had obtained precise measurements of the gross head and the head loss at full flow, which turned out to be just 0.6 m head of water. So the penstock efficiency is actually 98.9%.  Such a good figure is explained by the first 69 m of penstock being 110mm o/d MDPE pipe rather than the 90 mm o/d pipe from which the rest is made.

The UK "Energy Performance Certificate".

There might not seem to be any relevance on a blog devoted to a Powerspout for me to be writing about the UK Energy Performance Certificate (EPC).  Yet for readers in the UK, and this entry is really for UK readers alone, there is great relevance. Not only is the matter relevant to owners of Powerspout turbines but to owners of all 'domestic scale' hydros in the UK.

To rehearse the background: an EPC is required by law whenever a property is built, sold or rented.  It contains info about a property's energy use and typical energy costs, and it gives recommendations about how to reduce these. The certificate gives a property an energy efficiency rating on a scale from A (most efficient) to G (least efficient), is valid for 10 years and is issued following an energy assessment undertaken by a qualified assessor.

The origin of EPC legislation lies in the UK government's efforts to reduce carbon emissions and influence global warming: the energy we use for heating, lighting and power in our homes produces over a quarter of the UK's CO2 emissions. The certificate makes explicit this link to CO2 by including another scale which quantifies how good or bad the property is in terms of CO2 emissions for each energy efficiency rating A to G.

So you would logically conclude from all this that a property which is lucky enough to have a  hydro, which is to say a zero carbon energy source working 24/7, that such a property ought to score pretty well, at least in respect of that part of the assessment which deals with electrical energy consumption.

Moreover, you could not possibly expect the benefits from a hydro to be excluded when the certificate recommends one way of improving a property's EPC is to instal a wind turbine, - a technology which produces a far inferior energy flow compared to a hydro.

But you would be wrong if you concluded and expected these things: the reality is no electric generation from a hydro is permitted to count in the domestic energy assessor's survey of the property. The contribution from a hydro contributes not a jot to the calculation of the energy performance rating of the property.

The back story behind this completely daft anomaly is convoluted. It does not bear telling in full here. I have been arguing it this past 6 months all the way to the Minister of State for Energy, the Rt Hon Andrea Leadsom MP.  Her explanation in one sentence, though in the way of politicians she took several, is that assessing the contribution from a hydro is too complicated, therefore to train the energy assessors would be too expensive, and there are too few hydros to make it all worthwhile. End of matter.

So good bye fairness, logical thought, coherent reasoning, plausible argument and sensible decision taking in the legislative process !  Those of us with hydros simply have to take it on the nose that the point on the scale where the EPC places us will be lower than it ought to be, - in the process placing a lower value on the property, and in my case, dictating that I receive a lower Feed in Tariff payment for my solar generation.

As the French might say: "tant pis pour nous" (hard luck for us).  Which is indeed the only way to accept it, - but accepting it does nothing to suppress the irritation which still wells up because of such idiotic legislation.

Harvesting energy

Harvesting is a seasonal affair.  Whether the crop be potatoes, apples or kWh's, there is a season when the bulk of the crop is gathered in.  For my Powerspout the season for harvesting most kWh's is now showing signs of tailing off.

In the past seven days, I have seen the energy generated fall from its maximum of 18.86 to 13.82 kWh's per day, these figures representing the energy yield when instantaneous power is 786 and 576 watts respectively.

To track the diminishing water as tightly as possible requires having a series of nozzles of different sizes: - the available flow is steadily diminishing in a linear fashion but reducing the flow to the turbine necessarily has to follow a step-wise pattern as successively smaller nozzles are employed. 

The way I operate my Powerspout is to have a small nozzle at the top position and a large one at the bottom.  The top one delivers just 0.3 l/s and I never change this one, only turning it on or off.  The bottom nozzle delivers most of the flow and is always on. It has to be changed when in-flow to the header tank falls to be less than the nozzle's delivery rate.  

The difference in size of the orifices is really quite small, - just a difference in diameter amounting to fractions of a millimetre as the picture below shows.  

The nozzle I'm using at the moment is the one missing from the line up above: nozzle X; it's in the bottom position with the top, small nozzle turned off. Here it is in operation this morning, delivering 2.13 l/s and putting 576 watts into the grid:

As I go down through the nozzles, I'm measuring the speed of the turbine at each flow to see how far off the 'sweet spot' speed ( i.e. the optimum speed) the pelton is operating at:

The theoretical optimum speed* for my installation is 1200 rpm near enough, so with the rpm being 924, the operating rpm is 23% below optimum speed.  As discussed in an earlier post, whilst this is not ideal, neither is it as bad as it might seem.  The efficiency of the pelton in converting pressure energy into rotational energy is probably only diminished by 5% by operating at this slower than optimum speed, and this 5% loss of efficiency translates into a loss of about 30 watts, or 0.72 kWh in a day.  I think I can live with that though it would be nice to think of a work around to improve things.

Whilst the harvest of kWh's from hydro generation seems to be ending its season, the good news is that the harvest from solar panels is just starting.  I look forward to seeing how well the two blend their respective outputs and will report the outcome at the end of the year.

* to calculate theoretical optimum speed: 
1. calculate jet velocity (m/s): Vjet = 0.96√(2g x Hnet)
2. calculate optimum runner velocity at pcd (m/s): Vpcd = 0.46 x Vjet
3. optimum speed (rpm) = (Vpcd / 0.69**) x 60

So for my installation: 
Vjet = 0.96 x √(2 x 9.81 x 53) = 30.96 m/s
Vpcd = 0.46 x 30.96 = 14.24 m/s
optimum speed = (14.24 / 0.69) x 60 = 1238 rpm

** the pcd (pitch circle diameter) of a Powerspout pelton is 220mm; 0.69 is the circumference, in metres, of the circle having a diameter of 0.22m.