PRACTICAL ASSESSMENT OF CUT IN WINDSPEED

15. We return to the practical assessment of the cut-in windspeed of a WEC-load combination as mentioned in (8) above. One way of doing this is by sampling data for the WEC at one relatively high windspeed and 2 or more moderate windspeed; while doing this try to exclude inertia effects caused by the WEC speeding up and slowing down - i.e. try to get steady wind and a constant rotor speed - 'steady-state' conditions.

Then with your 3 or more data points, apply the formula:

(v/vci)^2 = a - (b * vblade/v)

where v = windspeed, vci is cut-in windspeed, vblade is speed of blade (tip or mean speed may be used - it is immaterial for this purpose - or you can even use revolutions per minute, revs per second, or radians per second angular velocity), and a and b are constants.

You have 3 unknowns - vci, a and b - of which only vci is of interest; if you have 3 equations you can solve for vci, by algebra or by computer iteration; if you have more that 3 data points use any 3 to establish the values of vci, a and b and then run through the other data points to check accuracy.

To do this type of measurement well, I strongly recommend using a video camera and the following technique:

Paint or otherwise mark one of the WEC sails or blades; use a digital watch or stopwatch watch which counts ideally to 0.01 seconds; use some kind of windspeed measuring device - ideally an expensive hand-held anemometer with low or zero starting torque and analog dial or digital speed readout. Film all three simultaneously with the watch and anemometer in the near foreground and the WEC in the background; probably use manual focus and keep focus on the 2 instruments - the WEC should still be visible. Play back on slow motion and/or freeze frame/still and write down or computer enter the relevant data for analysis. As an alternative to the expensive hand-held anemometer use a home made device such as a standard table tennis ball on a fixed length of thread or standard-size plastic straw - watch out of course for wind direction shifts... Such a device has of course to be calibrated against such an expensive hand- held anemometer, against a cca, or in a windtunnel...

16a. Alternatives.

An anemometer can be fitted with some inexpensive electronics so as to feed signals to the com ports of a standard PC - thus with some fairly basic database software with telecomms built in then a complete log of instantaneous windspeed and direction for 1 or several years can be collected; to do this probably 2 comms ports would be used - one for speed and one for direction, but a more sophisticated box could feed one comms port only - the signal comes once per anemometer revolution, and the direction determines the character (e.g. ascii 40 through 127 or 40 to 240 ) transmitted; the PC clock is used to determine the exact timing of each revolution (to 0.01 second); additionally, if it is desired to monitor a windpump, then another comms port can receive a signal every time the pump shaft reaches top or bottom of stroke (through another black box and a proximity or electro-mechanical switch). Note that a fast PC could operate this software under Windows and still be used at the same time for accounting, word processing, spreadsheet, other database etc applications without serious performance degradation - i.e. a dedicated PC would NOT be required.... Most PC's can take up to 4 comms port cards without problems.

16b. One suggestion - since cca's appear to give a good indication of windpower availability, why not remount them at 6 or 10 metres above ground level to give readings less prone to interference by long grass or buildings - they can be read using binoculars, and readings can be corrected for the height difference; the catch of course to this is that remounting at 6 or 10 metres modifies the cut-in speed - a large reason why there are good correlations between cca data and real or simulated impulse type WEC's is that a cca is itself an impulse type WEC with a cut-in windspeed not much below the band in which we are interested; mounting cca's at a higher height would take their operating conditions further from this practical band range......