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101 renewable - weather station used in solar data acquisition

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Last Updated
4th of October, 2018

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Irradiations the way it works

Critical to understanding about weather stations is that they do not measure some of the parameters a solar system it is direct influenced or in no case the solar productivity for a given site.

The weather station provides the weather data necessary for computation of system efficiency and is offering general prognoses which in turn can be considered to estimate the near future energy production for that solar site . Not to forget that weather aberration can occur at any time and the weather charts are a general guidance during the time of assessment and design only and they do not have to be considered as an absolute.

The four weather variables used in this type of estimation are:
>_ Solar Radiation;
>_ Wind Speed;
>_ Temperature;
>_ Humidity;

It is important for owners of weather stations to understand that estimates are only as good as the weather data it will prove as valuable to determine the site energy production. Poor quality weather estimation will produce erroneous values and will make use of information very difficult.

There are two main causes of poor quality weather data:
>_ Improper sitting of the weather station;
>_ Poor station maintenance;

Sitting the weather station:~

Proper sitting is essential if the weather station is to provide the data necessary to estimate solar or wind energy production in a consistent and reliable manner. Solar radiation, for example, is an extremely important variable in the event of such a procedure. A weather station must, therefore, be placed in a location where no shading can occur. It is important to remember that shade patterns vary considerably with the season due to changes in earth-sun geometry. Thus, it is best to place the station well away from large obstacles if at all possible.

An open location is also necessary to measure wind speed. A station hidden from view behind an outbuilding or a solid wall will not accurately measure the wind speed over more open areas such as fairways or sports fields. As a general rule, weather stations should be isolated from large obstacles such as fences, trees or buildings by a distance equal to 7-10 times the height of the obstacle. Using this rule, one should place a station 70-100' away from a 10' high building to ensure proper wind flow at the site.

Temperature dependent PV panels

Maintenance of the weather station elements:~

Perhaps the most important maintenance activity involves keeping the solar radiation sensor clean and level. The small white sensing surface or the glass dome (thermopile pyranometers) must be kept free of dirt, dust, debris, and bird droppings. Weekly exam cleaning of the sensor should be adequate for most locations. Simply remove any debris from the top of the sensor and wipe off the white circle with a damp cloth.

Actual maintenance of the temperature and humidity sensors should be left to skilled technicians. Local personnel can extend the longevity of these sensors by regularly removing accumulated dirt and debris from the radiation shield and by adjusting sprinkler heads to minimize direct water contact with the shield and sensors.

The main problem causing poor anemometer performance is dirt in the sensor bearings. This problem is best observed when winds are light — usually early in the morning. An anemometer that emits a grinding sound does not turn smooth at low wind speeds, or halts abruptly with a lull in the wind likely has bad bearings. Faulty bearings will generate erroneously low wind speeds and should be replaced as soon as possible.

Rain gauge maintenance involves keeping the gauge level and collection funnel clean. To check the gauge level, simply place a carpenter's level across the opening of the collection funnel. Turf clippings and dirt can occasionally plug the catchments funnel and prevent rainwater from reaching the recording bucket mechanism.

The power supply of the weather station may also require some routine maintenance. Most stations operate off a 120 volt A.C. supply with a rechargeable battery serving as a backup when the power fails. Loss of data following a power failure is a good indicator of problems with the battery and/or charging circuit. As an alternative, a solar panel may provide power to weather stations located away from a reliable source of A.C. power. The solar panel provides power to operate the station during the day and also charges a battery that supplies power for night time operation.

Weather station common sensors:~

Weather station is equipped with a set of sensors all of them facilitating the proper data acquisition for the microclimate that particular weather station it is installed. In general, a weather station typically monitors five meteorological parameters solar radiation, wind speed, temperature, humidity, and precipitation.

The silicon cell pyranometer is commonly used to measure solar radiation (Figure 2).The sensor is a small (1" diameter) black cylinder that is situated on an adjustable base plate containing a bull’s eye level. The actual sensing portion of this instrument is the small circular white “eye” that rests on top of the sensor. Some stations may employ a thermopile pyranometer to measure solar radiation. The sensing element of a thermopile pyranometer is typically located underneath a glass dome.

Temperature and humidity:~

Are commonly measured with a combination sensor which is located in a naturally ventilated radiation shield that shades the sensor from direct sunlight. Various types of shields are used to protect the sensor, but the most common shields look like an upside-down stack of white plastic plates (Figure 3). Temperature is measured with thermostats or platinum resistance thermometers while humidity is measured with a resistance or capacitance chip. A combination temperature and relative humidity sensor are housed within this radiation shield to protect the sensor from direct exposure to sunlight.

Anemometer (above) and wind vane (below) commonly used on automated weather stations. Wind speed and wind direction are measured with a cup anemometer and wind vane, respectively . The anemometer has three or four cups attached to spokes which in turn connect to a hub. The cup assembly connects to a sensing element that generates an electrical signal (e.g., switch closure or voltage) proportional to the rate of cup rotation. Cup rotation is in turn proportional to wind flow. The wind vane is designed to rotate freely and point into the wind. A variable resistor (potentiometer) attached to the vane hub provides an electronic signal proportional to wind direction.

Precipitation:~

Is measured using a tipping bucket rain gauge which typically is a large (6-8" diameter) cylindrically shaped instrument (Figure 5). A funnel on the top of the gauge captures the precipitation and directs the water to a tipping bucket mechanism that rotates on a pivot each time a known quantity of water is collected (usually 0.01).


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