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.

Tuesday, 20 December 2016

Knowing the flow.

Putting a numerical value on the flow being delivered to your turbine isn't important to most Powerspout owners; it's sufficient to open a valve, one or two of them for a pelton, or up to four for a turgo, and let the turbine spin; the business of knowing what flow is actually being delivered in litres per second, or gallons per minute, doesn't really matter; all that matters is that the flow delivered is not greater than what is available.

In the last diary entry I suggested I know what flow each of my nozzles delivers; that the figure for each can be calculated from a formula; that the accuracy of the result given by that formula depends rather too much on the value assigned to two inputs: the nozzle discharge coefficient (Cd) and the net Head (Hn).

Today, I've been doing an experiment to check the accuracy of the figure for Cd I've been assuming these past three years, - and I've found it's way out; the true flow from each nozzle is significantly less than I've been supposing; for me, and perhaps for some other Powerspout operators, this has important implications as I'll go on to show, but first the experiment:

It involved measuring flow rate by stopping the inflow to the header tank and measuring the time it took for the level to drop a measured amount; with knowledge of the dimensions of the tank, the depth dropped and the time it took, calculating the flow rate was easy. Small errors enter because the flow will change as the water level drops (but change only to a tiny extent), and because the internal dimensions of the tank are complicated by internal flanges (a 'best-guess' correction was incorporated).

To get a precise measurement of the drop in water level, two 3 mm knitting needles were mounted in a bit of wood, rigidly fixed to the tank; the tips of the needles were at different levels, a difference which could be measured to 0.02 mm with a vernier caliper; timing started when the falling water level broke the 'grab' on the tip of the higher needle, - the 'grab' being the attachment to the tip of the needle caused by surface tension, - and ended when it was broken on the lower placed needle:

Three runs were made and all provided near identical results: for the nozzle I was using it took 5 mins and 46.7 secs for the water level to drop from one needle tip to the other; calculation gave the flow rate as 1.25 litres per second whereas before I had thought it was 1.29 l/s.

Now you might think this difference to be small but its effects are big; using the figure for flow rate measured today and using it in the formula to do a 'back calculation' of Cd, instead of Cd being 0.91 as I had previously been assuming, it comes out at under 0.88.

Re-calculating the flow for each of my nozzles using a Cd of 0.88 makes each have a flow less than I had been reckoning on; what does this mean in the real world? - it means two things:

  • when I thought I was delivering the maximum flow my abstraction licence allows (3 l/s), in fact I was delivering less than this; for the future, delivering a true 3 l/s will mean more power when operating at the top limit of flow.
  • 'whole system efficiency' improves with this new information and this makes to be less the factor used in the annual calculation of the total volume of water abstracted, the so-called Hydro Abstraction Factor (HAF), - used to calculate, from the kWh's produced, the volume of water abstracted;  my HAF drops from 13  / kWh to 12  and the effect of this is to reduce the likelihood of my going over the volume I'm allowed to abstract in a 12 month period.
As carried out today, the test to verify Cd was performed using just one nozzle.  The question remains as to whether the value of Cd changes with nozzles having different orifice sizes; so I'll repeat the experiment as opportunity arises to use nozzles with bigger, or smaller, orifices.

Just at the moment though, it's so dry here that an opportunity to use bigger might be a while in coming.

Wednesday, 23 November 2016

Bringing order to nozzle changing

The heavens have opened at last !  From the middle of August until last week it has been very dry here with the water available for my Powerspout being less than 0.6 litre/sec; but a deep depression coming in from the Atlantic last week brought copious rain and has abruptly increased flow fourfold.

Although I have been looking forward to the winter rain coming, its suddenness has caught me on the back foot: in order to generate at all on a flow of 0.6 l/s, the SmartDrive had been set up with 18 pole stator and the rotor packed off maximally.  All this had to be changed and changed in the middle of a downpour: the 18 pole stator replaced with a 42 pole, the packing reduced and nozzles upped to match the greater flow available.

Despite my best efforts to have an orderly "take off" into each new water year when the rain comes, I have to admit I've failed this year.  I like to increase the nozzles in a steadily incremental way which matches as closely as possible the increasing flow. I don't like having to 'back track' to a smaller nozzle because I've put in too big a one. But a big element of getting things right is predicting how quickly the flow is going to increase and this year I've got it wrong; I've been beguiled by the torrent of water in the stream into which the turbine discharges (see pic below), and twice have put in too big a nozzle only to have to replace it with a smaller one within 24 hours.  I've been slow to follow my own adage that a source coming from a spring increases its output only gradually as ground water builds up; that I must not be misled by the early 'flashiness' of the run-off which appears in the stream.

In an attempt to bring a system to nozzle changing, I try to bring in a bit of measurement so the process becomes less of a guessing game.

For each nozzle I like to think I know what discharge the orifice will give; but in reality I know the accuracy of the figure will probably not be very good.  The lack of accuracy arises because the way of calculating the discharge involves a formula and that formula has rather too many inputs which are not known with great precision; the formula is:

 = CD Anoz √ (2g Hn)
Q is the flow from the nozzle (m³/s)
CD is the discharge coefficient for the nozzle (dimensionless; I take its value to be 0.91)
Anoz is the cross sectional area of the orifice (m²)
Hn is the net head (m)
g is 9.81m/s2

The term which is easiest to tie down in this formula is Anoz. By using the taper gauge that EcoInnovation supply in their tool kit, or a 'small hole gauge' and a vernier calliper (see here), the diameter of an orifice can be fairly accurately measured, and from that the area calculated. 

The difficulty comes with the other two terms: CD and Hn; as the orifice size changes so will the values of these two terms; but it is too complicated to try to measure them for every size of orifice; it is easier to assume constant values and recognise this will introduce an inaccuracy to the value calculated for Q.

With this limitation in accuracy admitted, my orifices are so arranged as to have a difference in flow between one and the next of about 0.3 l/s.  The smallest orifice, Roman numeral # 1, is cut to deliver 0.3 l/s.  By keeping this nozzle permanently in the top nozzle position and bringing it into use only when there appears to be sufficient water to be able to use it, I can test whether the time has come to increase the bottom nozzle by another 0.3 l/s increment: - if I find that employing # 1 causes the header tank to stop overflowing, the time to up the bottom nozzle has not yet come; but if the header tank still overflows, I can up-size the bottom nozzle.

Intuition might say that employing nozzles of markedly different size in the top and bottom positions is a bad thing.  True, it will produce a vector thrust which has to be born by the bearing on which the shaft is carried; a vector thrust which is cancelled by an equal and opposite force if the two jets are equal in size.  But the size of this thrust when calculated at the bearing is small and is unlikely to be a factor in limiting its life. 

The more important consideration relates to the velocity of the water in the two jets; it may be counter-intuitive, but the velocity is actually the same.  This comes about because jet velocity relates only to net Head, not to orifice size.  So the two jets actually deliver water to the pelton cups at the same velocity even though one is delivering more mass of water than the other.  Having the same velocity, the relative velocity of each jet to the speed of the runner is the same for both jets and this is what is important for good pelton efficiency.

As I finish writing, it has not rained for over 24 hours; all today I've been running on nozzles # 1 and # VIII delivering respectively 0.3 and 1.6 l/s, and giving 448w into the grid; the header tank is still overflowing and the forecast is for no more rain for several days.  With that forecast, the likelihood is my next move will be to turn off # 1.

It's good to have a plan but better still would be to have more rain !

Wednesday, 2 November 2016

Is it worth it in 2016 ?

If you have a stream with promise, the decision to proceed or not with installing a small hydro usually boils down to money: - will it be worthwhile ? 

Working through to a solution of this question is not very straightforward; the factors which  determine if a scheme will be cost effective are constantly changing. So in this diary entry I thought it might be useful to touch on how things stand in the UK as of now, November 2016, whilst also casting an eye to the future to bring in issues which are already in the pipeline (sorry for the pun) and which will inevitably come to bear on the matter sooner or later.

Income from energy generated
  • Feed in Tariff (FIT) payments have dropped colossally; in 2013 when my scheme gained accreditation with OFGEM, the payment per kWh for small hydro was 21.65 p; for new schemes now it is 7.65 p; the rate is due to drop further in more leisurely stages to reach 7.52p by Jan 2019.
  • the export tariff has gone up;  this payment is additional to the FIT payment but is paid only on 75% of the total kWh's generated;  it was 4.64p /kWh; today it is 4.91p.
  • receiving export payments on 75% of generated units is called 'deeming' and deeming has worked very much to the advantage of Powerspout owners because the output of a Powerspout is so low that in reality most generated power gets used 'in-house'; little or none is actually exported; owners who are canny have been able to go further in ensuring no export happens by installing a diverting device to send excess power to a heat storage load such as an immersion or room storage heater.  But deeming is about to change.  A recent consultation paper made it clear that the government's intention is that homes having renewable generation will need to have a Smart meter recording energy flowing each way, - into and out of the premises; no more deeming; export payments in future will be based only on an actual reading of energy exported.
  • to flesh out what these changes mean with real figures: it used to be the case that a Powerspout owner would receive payment of £251 for every 1000 kWh's generated; now the figure is just £113 whilst deeming continues; £76 when it stops and if no power is exported.

The effects of sterling devaluation
  • at the rate today, the value of sterling has fallen 16% against the US dollar since the Brexit referendum; as the price of a Powerspout is denominated in US dollars, this is going to make Powerspouts and their spares more expensive; however, the cost of a Powerspout accounts for only about 1/4 of the total cost of an installation so this effect is not particularly off-putting.
  • by contrast, the fall in the value of the pound will have a more significant opposite effect which will make turbine installation attractive;  this will come about because of the effect on the cost of electricity; right now as I'm writing, the UK national grid is importing 4.3% of its requirement from France, 2.2% from Holland and is generating 50% of its load from gas; much of the gas is also imported now that North sea gas is dwindling; all this importation paid for with a weak pound will soon force supply companies to put up their prices; the effect will be to make it increasingly valuable to avoid buying energy by producing it yourself.  
  • the rate at which prices will rise is going to be steep; at the moment the tariff I pay is 18.3p for day units and 7.67p for night (VAT included); these rates have been fixed for over 2 years but are nevertheless nearly double what they were 10 years ago; there is no escaping that the pause of the last two years has been an aberration in the longer term upward trend and that soon that upward trend is going to reassert itself; as it does so, the case for a Powerspout is made to be ever more attractive.
  • to illustrate how much more attractive: in the past two years, for each 1000 kWh I've generated, my electricity bill has been reduced by £152 ( a calculation which assumes I've used everything generated and none was exported); if prices rise by 5% per year for the next 5 years, the saving at the end of the 5 years will have grown to £194 per 1000 kWh generated; at the end of 10 years, it will be £247.

To conclude, I offer no answer to the question "is it worth it in 2016 ?"; the answer is too specific to each scheme, - how great is the promise of the stream; how willing is the scheme owner to rise to the challenge of devising a way of making the scheme work, of tackling the bureaucracy involved, of doing the installation work themselves.  

In all this however, too much should not be made of the strength of the business case in reaching a decision; there is the feel good factor to consider as well; the feeling of becoming a generator connected to the national grid, contributing in a small way to the energy needs of the country in a sustainable way. 

It might not count for anything on a balance sheet but in the bigger picture, the feel good factor is a potent force for making a scheme seem worthwhile.