Solar monitoring instruments

Solar monitoring site
Figure Solar-equipment. A typical layout of the latest version of the Bureau's solar monitoring equipment.
Solar monitoring tracker
Figure Solar-tracker. A closer view of the instruments mounted on the solar tracker.

Pyranometer

Global and diffuse solar irradiance are measured by ground-based pyranometers. The two basic commercial types employ a sensor comprising either a series of thermocouples or a photovoltaic (PV) sensor to measure the incident irradiance. Thermocouple devices have a broader spectral response, and are generally better suited for observing the full visible spectrum; hence, their use by the Bureau. Pyranometers using a PV sensor are sometimes recommended for monitoring performance and degradation of PV modules because there is generally a closer spectral response match between the module and the pyranometer.

The pyranometers currently used within the Bureau's network employ a black painted ceramic (Al2 O3) disc as the sensing element which absorbs radiant energy. One hundred thermocouples are imprinted on this disc. The one hundred hot junctions are located near the centre of the disc in a rotationally symmetric arrangement, while the cold junctions are in close contact with the pyranometer body, which acts as a heat sink. The rise of temperature is easily effected by wind, rain and thermal radiation losses to the environment ('cold' sky). Therefore, two glass domes shield the detector.

The glass domes allow isotropic transmission of the solar component from every position of the sun in the sky, however the spectral range of the pyranometer is limited by the transmission of the glass. The sensing element absorbs all wavelengths equally well, but the absorptance will vary with the angle of incidence. For most pyranometers the absorptance remains constant until the incident angle reaches about 70°. Beyond this point, the absorptance drops rapidly as the angle of incidence approaches 90°. Fortunately, at low solar elevations the energy contained in the solar beam is very small and a small percentage change in the measurement is non-critical, and reflections from the dome compensate for loss of absorptance.

Pyrheliometer

Schematic of solar pyrheliometer.
Figure Pyrheliometer-angles. Pyrheliometer slope and full opening view angles.

The pyrheliometer is used to measure direct beam irradiance. The one second samples of direct irradiance are also used to derive the number of sunshine seconds in each minute. It consists of a thermopile which senses the temperature difference between the exposed black sensor surface and the massive body of the instrument producing a voltage output proportional to the direct irradiance. The temperature of the sensor body is also monitored to enable quality control of instrument performance. A glass window at the end of the instrument protects the sensor from the weather. A dioptre is attached to the body to allow for checking of instrument alignment to the Sun.

The field of view of the sensor is defined by the full opening view angle, which for the current Bureau instruments is 2.5° (see figure Pyrheliometer-angles). Of greater importance when considering tracking accuracy and measurement of circumsolar irradiance, however, is the slope angle. For the pyrheliometers currently used by the Bureau the slope angle is 1°. As the angle subtended by the sun is approximately 0.53°, this will allow for a slight tracking error within the pyrheliometer guiding equipment. However, as a result the pyrheliometer will measure not just the irradiance from the area of the solar disc, but also circumsolar irradiance from the annulus around the solar disc caused by forward scattering of light through small angles by aerosols in the earth's atmosphere.

Pyrgeometer

The pyrgeometer is used to measure downward long-wave irradiance. It consists of a thermopile which senses the temperature difference between the exposed black sensor surface and the massive body of the instrument, producing a voltage output proportional to the incident long wave radiation. A white shield protects the body of the instrument from the effects of direct beam radiation. A silicone dome acts both as a filter to allow only long wave energy to reach the sensor element and to protect the sensor element from the weather. The temperature of the sensor body and that of the silicone dome are monitored as they are required to calculate long wave irradiance and to enable quality control of instrument performance.

Sun photometer

Sun photometer
Figure Sun-photometer. A close view of a sun photometer mounted on the solar tracker.

A sun photometer (sometimes referred to as a spectral radiometer) is a device used for taking spectral measurements of the direct beam irradiance. It is used to derive atmospheric transmission and in particular aerosol optical depth (the amount of extinction of solar energy by small particles in the atmosphere).

The instrument used by the Bureau is a 4 channel device with the ability to take four spectral measurements simultaneously. Appropriate bandpass filters are selected for the wavelengths of interest. One or two sun photometers may be installed at the station. At single photometer stations, the wavelengths are typically 412, 500, 610, 778 nm, while at dual photometer sites the second instrument will typically house 368, 500, 812 and 868 nm filters.

Shading arm

The tracker head is fitted with a shading arm fixed with two sun-occulting discs to provide for measurement of diffuse and long-wave radiation. The angle subtended by the shading disc to the instrument sensor is 5° (2 x 2.5°).

Tracker

The solar tracker is an instrument designed to accurately track the Sun's position under any sky conditions. The tracker provides mechanisms to align the pyrheliometer and sun photometer accurately and continuously at the Sun, as well as shade instruments from the Sun's direct beam (such as the diffuse pyranometer and the pyrgeometer). Initial installations within the Bureau's high quality surface and terrestrial radiation monitoring network used two trackers. More recently, an improved design has enabled all tracking to be accomplished with a single tracker. It achieves this by means of two computer-controlled stepper motors within the instrument, one for azimuth and one for elevation.

The tracker operates under one of four modes:

  1. Active mode: When the sun is visible the tracker actively seeks, then locks on to, the sun and follows the movement of the sun using feedback from the tracker eye.
  2. Active Reference Mode: When the solar position is well known with respect to the tracker coordinates, the Sun is tracked by counting the number of steps from this reference point rather than by feedback from the tracker eye.
  3. Passive Mode: When the sun is partially or wholly obscured (i.e. cloudy days) and the tracker position is determined using the sun position algorithm.
  4. Park Mode: This is the mode when the sun is below the horizon and the tracker is parked overnight.

Tracker eye

The tracker eye (or active sun sensor) contains four sensing segments; each produces a voltage output proportional to the magnitude of energy from the Sun's beam incident upon it. In clear sky conditions, the tracker is moved by the software so that these voltage outputs remain equal and maximized.

Routine maintenance

The high quality solar stations are located at sites which are staffed for the majority of the year, where the trained observers undertake a variety of maintenance operations and fault recovery procedures (should one occasionally occur). These include:

Additional instruments

Each of the Bureau's solar monitoring stations is co-located with an Automatic Weather Station (AWS), which provides a wide range of one minute weather data (derived from one second samples). Not only do these observations potentially add value to the majority of solar data applications (for example, relative humidity and wind gusts can affect the efficiency of a solar concentrating power system), they assist in the quality assurance process of the solar data; for example, helping with the detection of fog. Instruments associated with the AWS are listed in the station metadata file.

References