Linear wind-power meter compensates for temperature

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: F_W=10 Hz/m/sec. Conversion of V_Q1 and F_W 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/IC_1A generates a negative voltage of –0.17×V_Q1×F_W; FVC S₂/IC_1B generates V₂=–V×F_W=0.17×V_Q1×F_W²; and FVC S₃/IC_1D generates –V₃=–0.17×V_Q1×F_W³. Finally, differential inverter IC_1C shifts and scales –V₃ to output V_OUT=0.42×V_Q1×F_W³=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: V_OUT=1.14V×air-pressure millibar/(273+ambient temperature Celsius).