SENSORS

The basis for monitoring wind resources is collecting data on wind speed and direction, and air temperature and humidity. These data are basic for acquiring key information needed to assess the reasonableness of establishing a wind farm.

1. Wind speed

Wind speed data are the most important indicator of a site’s wind energy resources. Multiple measurement heights are encouraged for determining a site’s wind shear characteristics and conducting turbine performance simulations at several turbine hub heights.
A cup anemometer is the most commonly used type for the measurement of near-horizontal wind speed. The instrument consists of a cup assembly (three cups) centrally connected to a vertical shaft for rotation; at least one cup always faces the oncoming wind. The aerodynamic shape of the cups converts wind-pressure force into rotational torque. The cup rotation is nearly linearly proportional to the wind speed over a specified range. A transducer in the anemometer converts this rotational movement into an electrical signal, which is sent through a wire to a data logger. The data logger then uses known multiplier and offset constants to calculate the actual wind speed.

Wind direction information is important for identifying preferred terrain shapes and orientations, and for optimizing the layout of wind turbines within a wind farm.
A wind vane is used to measure wind direction. The vane constantly seeks a position of force equilibrium by aligning itself into the wind. It uses a potentiometer-type transducer that outputs an electrical signal relative to the position of the vane. This electrical signal is transmitted via wire to a data logger and relates the vane’s position to a known reference point (to the true north). Therefore, the alignment (or orientation) of the wind vane to a specified reference point is important.
The data logger provides a known voltage across the entire potentiometer element and measures the voltage where the wiper arm contacts a conductive element. The ratio between these two voltages determines the position of the wind vane. This signal is interpreted by the data logger system, which uses the ratio and the offset to calculate the actual wind direction.

Air temperature is an important descriptor of wind farm’s operating environment, and it is normally measured either near ground level (2 to 3 m), or near hub height. In most locations, the average near ground level air temperature will be within 1°C of the average at hub height. It is also used to calculate air density, a variable required to estimate the wind power density and a wind turbine’s power output.
A typical ambient air temperature sensor is composed of three parts: a transducer, an interface device and a radiation shield. The transducer contains a material element with a relationship between its resistance and temperature. The resistance value is measured by the data logger, which uses an established and accepted equation to calculate the actual air temperature. The transducer is housed within a radiation shield to protect it from direct solar radiation.
Relative humidity may be defined as the ratio of water vapour density (mass per unit volume) to the saturation water vapour density.

Remote communication with the use of an internal GSM/GPRS modem is used for data transfer. The advantage of this method is that data can be sent at any desired time interval (depending on the settings), several times a day. This enables simultaneous data checks and fast interventions in cases of possible operation failure or incorrect measurement data. The only condition that must be fulfilled is a sufficiently strong GSM signal. If this is not present on the area, such a mode of data transfer is not possible. On the areas with poorer signal strength, additional external antennas that strengthen the GSM signal can be used.

The measurement device is powered exclusively by an internal battery. The measurements are recorded on an internal flash memory. Minimal energy consumption is achieved in the low-consumption operation mode. In practice, this means that the peripheral units are turned off when they are not needed. For example, the modem is turned on only when data needs to be sent to the data server; analogous adjustments at the input of the measurement part are activated only during measurement, and the CPU is in sleep mode during measurement.
The internal battery is used as the only energy source. A connected solar cell acts as the battery charger in the part of the day when the sunlight is at the level where the solar cell can transmit the energy needed for charging. The battery is able to maintain a charge for several “dark” days, when the solar cell energy is not sufficient for charging; this occurs in winter. Because of this, the current level of the battery (voltage) is sent with every communication of the measurement device as a precautionary measure. We thus know the state of the battery at all times, and we can take the necessary steps (replace it) before its failure.