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Data loggers in the wind industry are electronic recorders that receive and process signals from wind sensors such as anemometers (wind speed measurement) and wind vanes (wind direction measurement).
Information from wind data loggers is used to select sites for development
and often for ongoing monitoring of operating projects. The wind data
from data loggers is used
to generate production estimates and determine the feasibility of development
at a site.
Modern wind data loggers are little computers with microprocessors that process
the signals from the sensors they monitor. To conserve power, they do not
continuously measure the inputs but rather sample the
signals.
Data samples are collected and processed according to the logger's configuration.
The configuration is the program that tells the logger what it is connected
to and what calculations are required. Examples of calculations or "functions" the logger may make are average, maximum, minimum, and standard
deviation.
The industry standard averaging interval is ten minutes, but some loggers
may use an hourly averaging interval, or offer configurable averaging periods.
No matter what function is being calculated, the time period is still called
an averaging interval.
Data loggers used for siting wind projects are designed to be rugged, so they can deal with sometimes-extreme climates. They are designed for low power consumption, because wind sites are remote and site visits add to the cost of prospecting. They need to be reliable, because no one is nearby to fix them. And they need to accommodate enough inputs to instrument a 60-meter mast with sensors at multiple heights, plus redundant anemometers. Wind
prospectors don't just measure wind speed and direction. They are also interested in temperature and pressure because these influence air density, and thus the amount of energy in the wind. The logger should also be able to monitor itself, to report its memory status and battery voltage level in case one should become full or the other empty.
Parts of a wind data logger include:
Sensor inputs: Wind data loggers always have several inputs designed for counter-type
anemometers. Sensors with counter or pulse
outputs, where the strength of the signal increases with the number of pulses per second, are not common outside the wind industry, which is why wind-specific data loggers are best for wind measurement. Meteorologists like to see wind speeds from anemometers placed every ten meters or so. Redundant anemometry--instrumenting the tower with two or more sensors at each height being measured--is often used because a failed sensor might mean having to extend the study and delay the project. The logger will also have several inputs for analog sensors, including wind vanes (which measure wind direction), temperature sensors, and pressure sensors.
Power supply: Wind data loggers are designed
for low power consumption so they can run unattended for months at a time.
Each subsystem of the logger reflects this need. The processor will "sleep"
in between the one-second readings to conserve power. Any power signals
needed for the sensors will only be activated as needed and not continuously.
Sensors chosen for remote data logging applications should also be matched
to the available power.
User Interface: Most data loggers have a control panel with a display for operator use. When setting up the logger, the navigation keys and display are used to interact with the logger to check settings, such as date and time, and to review sensor values.
Enclosure: Wind data loggers are sited in harsh environments and
need to be protected from the sun, wind and rain. Most wind prospectors either use a logger
with a sturdy steel enclosure like Nomad® 2 , or get a separate enclosure to protect the logger on site. The main
compartment of the logger needs to be watertight to protect the electronics
and avoid condensation. The National
Electrical Manufacturers Association (NEMA) classifies enclosures
by their resistance to environmental conditions, and this degree of watertightness
is referred to as NEMA 4. An external enclosure may have openings for
sensor cables to pass that are not sealed, but will protect the logger
from the rain. This level of protection is classified as NEMA 3R.
Processor Board: Logging wind data may seem like a simple
task, but complex programming is involved in running a wind data logger. The
logger not only samples the sensors, but also performs statistical calculations
every ten minutes. It handles memory
management, both of the samples and the data stored on removable
or fixed media. Modern wind data loggers run an operating system, just like
a PC, that sequences the tasks they perform and allocates processing
and memory resources to each task. Telecommunications requirements add
another layer of complexity. The logger must provide power as needed
to the modem, redial the server as needed, and exchange data upon request.
With the data exchange, the logger may receive instructions to change
its own programming, which requires that it keep the old and new instructions
straight and not get confused in between.
Data Storage: Even though the majority of wind prospectors are receiving their data via remote telemetry, data storage at the logger remains critical. Virtually all loggers now offer removable storage media.
Remote Communications: A remote communications system is comprised of a modem to communicate with the server or host, an antenna to boost the signal, and programming to activate the modem on schedule and transfer the data. Remote communications will significantly increase the power consumption of the logger and will require additional battery capacity. Solar charging systems are one popular way of doing this. A solar panel, typically 5 or 10 watts, is connected to a solar
charge regulator and a rechargeable battery. In locations where sun is not reliable, large batteries such as automotive batteries are used.
Many types of remote communications are available for wind data loggers. For sites that are near buildings, loggers can be equipped with either wireless Internet or wired telephone modem. Some loggers, like Second Wind's Nomad® 2 , can provide a continuous stream of real-time information. For sites with cell coverage, cellular modems are available for a variety of carriers. While GSM
service is standard in Europe and in much of the world, carriers in the US each offer their own method that is incompatible with the others. Various locations are better served by different providers, and each provider needs a different kind of modem to operate. Yagi
style directional antennas are helpful in improving cell reception as long as they are pointed toward the nearest cell tower and the antenna cable is not too long. Some wind data loggers use satellite service for remote communications. There are two main commercial satellite providers: Globalstar and Iridium. Both have a network of low earth orbiting satellites, with Globalstar's satellites at a somewhat higher altitude. The Globalstar system offers a higher data transfer rate, though, because it uses a signal processing technology called CDMA (the same as Verizon currently uses). Satellite data service is becoming more competitive in cost with cellular service, and offers the appeal of universal coverage. The challenge for both providers is gaps in coverage, which can mean no signal or dropped calls at times. Logger programming needs to be flexible to accommodate quirks in the network.
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