Instrument Characteristics

Sensors, transducers, and signal conditioners measure and transform signals for recording. Resolution is the smallest unit of a variable that is detectable by a sensor. Recorders may limit the resolution. Reliability is a measure of an instrument's ability to produce useful data over a period of time. The best indicator of reliability is the past performance of similar instruments.

Accuracy and precision are two separate measures of system performance that are often treated ambiguously. Accuracy refers to the mean difference between the output of a sensor and the true value of the measured variable. Precision refers to the dispersion about the mean. For example, an instrument may produce the same measured value every time but produce a value that is off by 50%. That system has a high precision but low accuracy.

The accuracy, however, may be a function of time, or dependent on maintenance. Anemometers are calibrated in wind tunnels, where the airflow is steady. Another calibration of performance, scale and offset, of anemometers for wind resources assessment uses the controlled velocity method (boom mounted on truck). Generally, calibrated anemometers produce a signal that is accurate to within 0.5 to 2% of the true wind speed. Under normal use in the atmosphere, good anemometers should be accurate to around 2 to 4%.

The distance constant is the length of fluid flow past a sensor required to cause it to respond to 63.2% of a step change in speed. A step change is change from one value to another value, similar in shape to stair step. The larger and heavier cup anemometers usually have distance constants of 3 to 5 m. For light-weight and smaller cup anemometers, such as those used for turbulence, the distance constant is typically about 1 m. The time constant is the period that is required for the sensor to respond to 63.2% of a step change in input signal.

The damping ratio is a constant that describes the performance of a wind vane in response to a step change in wind direction. The damping ratio is dimensionless and is generally between 0.3 and 0.7.

The sample rate is the frequency (Hz) at which the signal is sampled. This may include the time for recording the data. Since a large amount of data requires large storage, wind speeds are averaged over a longer time period, and these are the values stored, along with standard deviations. Typical values for wind power analysis are sample rates of 1 Hz and averaging time of 10 min. Previously, 1 h averaging times were used for many resource assessment projects.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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