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).