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.

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 fitted out with 18 pole stator and 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 spring source 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 inaccuracy comes because the way of calculating it involves a formula which 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 (no unit; 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 micrometer (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 that 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.

Sunday, 16 October 2016

Sizing turbine to stream

In the planning stages of implementing a small hydro scheme, one of the biggest challenges is to decide what power the flow in the stream will support.  To put in a scheme which under-utilises the available flow is to save money. But it will create a lingering feeling of not making the most of the resource which is available; and having installed too small a scheme, up-sizing is not something easily done at a later time.  Conversely, to put in a scheme which is over-sized such that full power is realised only for a small part of each year, wastes money on the cost of a bigger machine and bigger penstock and bigger everything without seeing a commensurately bigger return.

So how does one hit upon the right balance?  

The first thing to know with as much accuracy as possible is the flow in the stream and how this varies through the seasons of a year. Even when you think you have this knowledge however, there is a caveat. By whatever means this knowledge is gained it will not be information that can be wholly relied upon: - year to year changes in wetness happen, and climate change may introduce longer term changes too. "Past performance is not a reliable guide to future returns" is an epithet often applied to financial investments; it applies in hydrology too.

In the previous diary entry, I mentioned that it is possible to purchase information about the flow in any river or stream in the UK, made available in the form of a flow duration curve (FDC); that the accuracy of such a computer generated FDC is questionable for the smallest streams of the kind where a Powerspout is likely to be used; and that purchase is expensive. But purchasing knowledge is one way of getting the knowledge you need about your stream's flow.

An alternative way which is cheap but time consuming is to measure flows oneself; this is what I did: every week for a year I measured the time it took to fill a coal scuttle thrust into the flow at a point where the entire flow fell freely over a rock face and I had arranged for it to discharge from a pipe: 

Not being able to hold a stop watch at the same time as holding the coal scuttle, I simply counted the seconds it took to fill its 10 litre capacity.  The method was probably not very accurate but it gave data which was useable to make the following plot:

Once gathered, this time-sequenced data can be manipulated; such manipulation is how an FDC is constructed. Instead of showing what flow was present in each week, an FDC shows the length of time (i.e.duration of time, expressed as a percent of the recording period) specified flows were equalled or exceeded. There is a lot of number crunching which goes into creating an FDC and it needs to be done using a spreadsheet programme such as Microsoft Excel. The easy to follow description of how to do it which I gave before is given again in this link.  

Once constructed, an FDC is useful because it begins to indicate what size of turbine might be suitable for the flows in the stream.  But it only begins to give an idea; many are the factors which massage the actual flow figure that can be used, foremost of which is the amount of flow the regulatory body in your country will allow you to take.

For the flows I measured in my stream, this is the flow distribution curve I constructed; on it I have marked a point which is called Qmean*. As can be seen it is at a flow of 1.86 l/s, a flow which is equalled or exceeded, on aggregate**, for 39% of the year. But, and it is an important but, - 1.86 l/s was the Qmean only in the year in which I took the flow measurements. This one year will not necessarily be predictive for future years and as we will see below, it wasn't.

Qmean is an important term to comprehend; the value of it for your stream is a good first-off guesstimate of the size of turbine that suits your site; a rough rule of thumb is that the Qmean value, or a flow close to it, will be the flow to use in your calculations of the maximum power the installation will be capable of producing. Although flows higher than the Qmean flow will be present during the course of a year, experience shows that sizing the installation to the Qmean flow gives a good compromise between being able to use the higher flows of winter and also the lower flows of the drier months.

The way to calculate Qmean depends on the way you do it.  If you have a series of flow measurements taken at equal intervals over a period of time***, then Qmean is straightforward; it is simply the arithmetic mean (sum of flows divided by number of measurements).  But if you have a flow duration curve without actual flow measurements, then Qmean is the point where the area under the curve to the left of the Qmean point equals the area under the curve to the right of it.  Determining this is not difficult in a spreadsheet programme; this link shows how it can be done. 

In terms of reliability, the weight that can be born by the figure for Qmean really depends on the way it is reached.  The figure given above for my stream,1.86 l/s, is not reliable; it was derived from measurements, not very precise measurements, taken only weekly, over only one year. To illustrate how unreliable it was, the plot below shows the FDC for that year, 2008/9, together with the FDC for the year just ended (2015/16); in the latter year, the flow mesasurents were calculated from power generated each day****; seen together, the two years give very different Qmean values: 2.43 l/s vs 1.86 l/s:

Only when the period upon which a flow duration curve is based is long enough to represent the long-term picture can the curve be considered reliable and begin to be used in a predictive way. Even this reliability will not be guaranteed if climate change is having an effect, for then even a historical long-term record will not hold true for the future.

To conclude and to emphasise the importance of Qmean, let me relate how abstraction is adjudicated here in Wales. Since the time, 3 years ago, when I applied for my abstraction licence, there has been a complete overhaul of the guidelines for the abstraction of water for micro-hydro. The new recommendations attach great importance to the value of the Qmean measurement; by relating it to the type of stream under consideration, a formula is employed which determines what amount of abstraction will be permitted.  The guidelines can be seen in full here (especially page 5) but in summary either Qmean or a small multiple of Qmean (a factor of 1.3) is set as the maximum flow which can be abstracted from a water course.

The change in the guidelines is a welcome simplification of what existed before; but the pivotal place given now to Qmean in fixing the size of the installation, and take note, fixing it so that future change can scarcely be considered, makes it a very important parameter to get right; the difficulty is that it's a parameter which doesn't lend itself easily to precision.

*Q is the symbol for flow; mean is the average; so Qmean is the point signifying average flow.

** by "on aggregate" it is meant that within the year the time where 1.86 l/s was equalled or exceeded in total came to 39% of the year; this 39% of the year will not be a continuous stretch of time during which 1.86 l/s was equalled or exceeded.

*** only complete years of flow measurements should be used and the records for partial years discarded; otherwise the flow data may be skewed by seasonal wet or dry periods.

****by using power data, the flow duration curve is 'capped' by the design flow of the turbine; the effect of this will be to under-estimate Qmean; in this year, Qmean would actually have been higher than 2.43 l/s.