Linear wind-power meter compensates for temperature
Texas Instruments » CD74HC4053, LM4040, LMC6484
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 = 1 W/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
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
You can conveniently calibrate the wind-power meter in an automobile being driven on a windless day at a constant speed of 18.6 m/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 4 V or, for better accuracy, to the voltage that the following formula that accommodates true air density yields:
VOUT = 1.14 V × air-pressure millibar/(273 + ambient temperature °C).
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