CUT IN WINDSPEEDS

5. The optimal cut in speed for (4a) above is also well correlated to the average windspeed as obtained by a Casella Mark II cca, by the following formula:

vciopt = 2.2 + (0.78 * vcca)

where vciopt is optimum cut-in windspeed at 2 metres above ground level over short grass.

and vcca is indicated average cca wind velocity at 2 metres etc etc..

6a. For cases where the optimum vci was departed from, then the following relationships were found to apply:

- if the vci were lowered so that the extractable energy fell to 80% of the maximum possible (as per (4) above, then the vci was approx 0.67 times its

optimum value (as per (5) above.

i.e. if vci = 0.67 * vciopt then E = 0.80 * E max.

- similarly if lowered to produce an extractable energy of 90% of maximum, vci was found to be 0.78 times optimum

i.e. if vci = 0.78 * vciopt then E = 0.90 * E max.

- similarly if RAISED to produce 90% energy level, then the vci raised to 1.26 times its optimum value.

i.e. if vci = 1.26 * vciopt then E = 0.90 * E max.

- finally if raised to produce 80% of maximum energy level then vci became 1.38 times vciopt.

i.e. if vci = 1.38 * vciopt then E = 0.80 * E max.

Thus relatively high extraction efficiencies are still obtainable over a wide range of cut-in speeds; this is fortunate, since most sites have significant seasonal variations and also since most WEC's do not have easily variable cut-in speeds - thus it is important to tune the WEC to the load so as to optimise energy yields for the site in question.

6b. The WEC indicated windspeeds were also computed by the same iteration techniques for the 13 Beaufort Data Sites. The findings were well correlated to mean indicated cca windspeed, by the formulae:

vwecind = 0.50 + (0.60 * vcca) for vci = 0.67 * vciopt

vwecind = 0.45 + (0.53 * vcca) for vci = 0.78 * vciopt

vwecind = 0.30 + (0.35 * vcca) for vci = 1.00 * vciopt

vwecind = 0.15 + (0.214 * vcca) for vci = 1.26 * vciopt

vwecind = 0.10 + (0.16 * vcca) for vci = 1.38 * vciopt

Note that the above potentially enables assessment of existing windpower sites if the revolutions or pump strokes are recorded for weekly, monthly and yearly periods, AND if the blade tip speed ratio is known (i.e. revolutions per minute at a high known windspeed, when for example v/vci > 5); for such conditions, and where cca data is available or can be guesstimated, then the existing set cut-in windspeed can be estimated without relatively complicated testing as in (15) below.... Then if it is found to be too high or too low to produce a best energy yield pattern over the year, it can be adjusted. Restating the above in mathematical terms, if vwecind and vcca are both known or can be estimated, then vci/vciopt can be calculated; getting a good ratio of vci/vciopt is important to optimise annual and seasonal energy yields.

6c. The WEC (operational hours/total period hours) were also computed by the same iteration techniques for the 13 Beaufort Data Sites. The findings were very variable and could not be well correlated to mean indicated cca windspeed; BUT in general terms one could say that a WEC which operates 40% of the total time during a period is probably operating at the vciopt (optimum cut-in windspeed for that period); similarly operating 60% of the total time indicates that it is set with a vci such that the lower 80% energy extraction level is attained (i.e. at 0.67 * vciopt); and operating at only 20% of the total time during a period indicates that it is probably operating at the upper 80% energy extraction level (i.e. at 1.38 * vciopt). These measures, although very variable for the 13 locations and 12 months, may be a good rough-and-ready measure to evaluate the cut-in windspeed matching of existing WEC's (but who is going to stay up all night and watch the thing going round or not?; computer monitoring of a WEC can of course easily produce such utilisation factors.....).

7a. The significance of (6a) above is to do with seasonal windspeed and windpower variations - often for practical reasons it is impossible or undesirable to alter the cut-in speed of the WEC-load combination (e.g. by varying the length of the crank or the cylinder diameter on a windpump). Thus with seasonal variations obviously it is not possible to operate at maximum extraction efficiency during every month. Thus it is desirable to choose the vci so as to best meet energy demands throughout the year. Different strategies may be chosen, depending on the application, for example it may be decided to maximise energy output during the low season, even at the expense of total energy yield over the 12 months; or it may be decided to maximise annual energy yield, despite operating less efficiently during the low season....