Year end results for 2023-24 water year.

The year to September 30th has seen the most generation in the eleven years my Powerspout has been running. The total energy generated was 5254 kWh.

Understanding that this figure is really quite good for a small turbine is best captured by considering what is called capacity factor.

Capacity factor is the energy generated in a year divided by the energy that could have been generated if the turbine had run everyday for 365 days at the rating specified in its technical specification.

The rating specified for my turbine is 750 watts, which means the maximum power it is meant to be able to produce is 750 W. And using this figure, the maximum energy that could be produced in a year is 6,570 kWh (calc: 0.75 x 24 x 365).

So the capacity factor for the 2023-24 water year turns out to be near enough 80% (calc: 5254 / 6570 = 0.799).

But there is a slight deception in this figure. And this is because the maximum power my Powerspout can actually produce is more than 750 W, and this is evident in the first of the plots below.

There is a reason why there is this deception: the figure of 750 W had to be specified before the turbine had even been commissioned and was a figure derived purely from theoretical calculation. This theoretical calculation had to make assumptions about a lot of things such as friction loss in the pipeline, the efficiency of the 3 phase alternator, the efficiency of the dc/ac grid-tied inverter, ...and many more factors, - all of which were little more than educated guesses.

When the turbine actually got to run, many of the guesses turned out to be on the pessimistic side and the turbine actually was found to produce nearer 800 W at full flow, and even 900 W if slightly more water than the specified maximum flow of 3 l/s was used.

So the 80% capacity factor figure is inflated, and if the true maximum power of 900 W is used to calculate it, the figure drops to 66%.  But since officially my turbine is rated at 750 W, that's the figure I'm going to continue to use for my capacity factor calculations. But I will point out that in the interests of 'honesty and transparency' you'll notice that I do say in the Scheme Details section in this blog where each year's capacity factor is given, that the figures are "taking as datum DNC 750 W" (DNC = Declared Net Capacity).

For those who like to appreciate how a small scale hydro works out in the real world, here are the figures for this past year's generation. The year 2023-24 is the black plot line, and previous years are coloured lines:

Usefulness

There are just two measures of how useful any renewable energy scheme is:

- how many days in a year it runs
- how much it generates when it is running

In renewables parlance, the 'days run' are spoken of as being the availability factor and how much is generated over a year when it runs as the capacity factor. 

More completely described, the capacity factor is how much energy (kWh) is generated in a year as a fraction of how much could have been generated if the installation had run at its rated capacity for the whole year.  Thus defined, it can be seen that capacity factor incorporates a scheme's availability factor since if a turbine isn't running for some days (↓ availability) necessarily the year's output will be less (↓ capacity factor).

I read recently that in the UK small wind turbines typically deliver a capacity factor of just 15%, whilst PV panels manage 17%.  These figures reflect the obvious: the wind doesn't blow all the time (↓ availability) and when it does blow it doesn't always blow sufficiently to drive a wind turbine to its full rated output (↓ capacity factor), - and for PV, the sun only shines in the day not the night (↓ availability) and even then is not always bright enough due to clouds or inclination in the sky to give maximal output from the rated capacity of the array (↓ capacity).

Comparing these figures with those of my turbine over the past year sheds a rosy glow on how useful the turbine has been: having run for 365 days its availability factor was 100% and the capacity factor 58%. (see plot below)

Figures as good as these are unusual for a very small hydro and it's worth touching on some of the reasons:

• the source is a spring whose flow comes from groundwater; a spring depends in a loose way on rainfall and is more constant in its output than a source using a watercourse, where the flow will closely reflect whether it has rained or not
• my abstraction licence doesn't specify a 'hands off flow' - meaning I'm free to take all the flow available, even in the dry times of a year
• by having an orderly range of nozzles of different sizes and maintaining a regime of changing between them to suit what flow is available, nearly maximal use of flow is achieved 
• by using washers to make the rotor stand off from the stator with the aim of keeping the speed of the turbine up at low flow times, output is always maximal for the flow available
• by swapping from a 42 pole stator to an 18 pole in the dry months, the turbine can be kept in operation at low flows; also its speed can be kept up by using an 18 pole stator beyond what is achievable by packing off the rotor with washers; the issue is caused by too strong magnetic attraction, for the torque available at low flows, between the rotor and the iron cores of the many poles when a 42 pole stator is in place 
• by using a Type 2+ (high power rotor) at high flow times, turbine speed is kept down when otherwise it would go too fast for optimal output
• a full range of spares kept on-site means days are not lost to generation by waiting for spares to arrive
• a plan of preventive maintenance and a sturdy initial installation has so far eliminated prolonged down-time from unexpected problems

Capacity factor = (3816 * 100) / (0.75 * 24 * 365) = 58%

Availability factor = 100% (turbine ran 365 days: the one day showing no output is Feb 29th, included for spreadsheet reasons, but 2018 was not a Leap Year: hence no output shown that day).

Happiness is...

... the start of a new generating year !  11 am yesterday saw all systems go, and so far everything is performing exactly as it should, right down to the in-house monitoring and on-line visibility of live-power (see link to right).

Now, all that remains is to get the capacity factor for the 2014 / 15 "water year" above the 59% recorded for last year.

"Capacity factor" and "Water Year"

Capacity factor and water year are two useful concepts.

The capacity factor for a scheme is a measure of its productivity over time.  It is calculated from the actual energy generated over a year as a proportion of how much energy could have been generated if the scheme had run at full capacity "24/365".  Like overall efficiency, it is one of those things which would be nice to know for a scheme before it is implemented, - but cannot be with certain accuracy.

No scheme can run 24/365. Most 'run-of-river' schemes have to reduce generation or shut down in the drier months for lack of flow. Then there is down-time for maintenance, breakdowns and grid-out spells.

For a Powerspout installation there is the added factor that nozzles have to be changed manually. As the flow changes with the seasons, so the size of the nozzle has to change.  It is somewhat of a "hands-on" business doing this, with quite a learning curve for reading the signs as to when to make a change, but the assiduousness of the operator in doing it  will improve the capacity factor. 

For my scheme in 2013/14, the total generation was 3,672 kWh.  The peak power output possible in that year was 0.713 kW which makes for a maximum possible energy generation of 6,245.9 kWh. So the capacity factor was 0.59.  My hope is to better that in this next year.

Which leads on to what period of 12 months is best taken as the period of time over which to base such a study: a calendar year ?  a financial year ? an Environment Agency year (which runs 1 April to 31 March) ? - or a "Water year" ?

Since the operation of any hydro site falls into a natural rhythm governed by the seasons and rainfall, it makes sense to use such a natural pattern and call it a "Water year".  It should begin and end in the middle of the driest season.  Here, that runs from August to October, so my water year starts 1st October and ends 30th September.

One advantage of doing things this way is that if there is a particularly severe dry season, and they happen every few years, its effects on the output record will be spread over two water years, rather than giving one apparently very bad year.  It only makes the figures appear to be better !