Linear wind-power meter compensates for temperature
W Stephen Woodward, Chapel Hill, NC; Edited by Martin Rowe and Fran Granville - November 13, 2008
The rise of interest in renewable energy created by soaring fossil-fuel costs and global-warming fears has created a matching interest in associated support and demonstration instrumentation. This Design Idea hops on that bandwagon with the ability to directly and conveniently measure an important renewable-energy source: wind power. Handy for quick and easy preliminary evaluation of potential wind-turbine sites, it includes a wind-speed transducer, comprising an optically sensed vane anemometer, and a temperature sensor, comprising a diode-connected transistor (Figure 1). These components interface with a hybrid digital/analog-computation circuit. In combination, they provide a real-time, linear, temperature-compensated readout of wind-power density.
The power-generation potential of wind is ½×air density (kg/m3)×air speed (m/sec)3. To compute it, therefore, requires estimating air density, which is inversely proportional to absolute temperature; measuring air speed; and calculating a cube.
Here’s how the wind-power meter does it. Diode-connected Q1 has a bias of 550 µA for a 25°C (298K) base-to-emitter voltage of approximately 600 mV and a temperature coefficient of –2 mV/°C. Thus, Q1 is a voltage reference that tracks the approximate ideal-gas-law temperature dependence of air density: –0.3%/°C. Meanwhile, optical sensor O1 works with a free-spinning anemometer impeller to produce wind-speed-proportional frequency: FW=10 Hz/m/sec. Conversion of VQ1 and FW into a 1-mV=1W/m2 output signal is then the function of the third-order X×Y×Z-multiplying behavior of three cascaded CMOS-switch FVC (frequency-to-voltage-converter) charge pumps: S1, S2, and S3.
FVC S1/IC1A generates a negative voltage of –0.17×VQ1×FW; FVC S2/IC1B generates V2=–V×FW=0.17×VQ1×FW2; and FVC S3/IC1D generates –V3=–0.17×VQ1×FW3. Finally, differential inverter IC1C shifts and scales –V3 to output VOUT=0.42×VQ1×FW3=1V/kW/m2.
You can conveniently calibrate the wind-power meter in an automobile being driven on a windless day at a constant speed of 18.6m/sec=41.5 mph=66.8 kph. With the anemometer exposed to the external slip-stream, adjust the calibration trimming potentiometer for an output voltage of 4V or, for better accuracy, to the voltage that the following formula that accommodates true air density yields: VOUT=1.14V×air-pressure millibar/(273+ambient temperature Celsius).