I’ll follow the sun – even on cloudy days

NIWA is fitting new solar trackers to sites in the National Climate Network.

These trackers will strengthen the data quality and significantly improve the efficiency of solar monitoring that runs from Scott Base in the Antarctic to Kaitaia in the far north of New Zealand.

The sun is the Earth’s only significant source of new energy. Nearly all other energy sources, such as fossil fuels, store energy that has come from the sun previously. Changes in the amount of incoming solar radiation and how this energy is absorbed and reradiated into the atmosphere have a large effect on our climate.

NIWA has a network of long-term climate monitoring stations which continuously record solar radiation. There are five primary solar radiation stations with four spread across NZ: Kaitaia in the far north of the North Island, Paraparaumu near Wellington in the middle, Lauder in Central Otago and Invercargill near the bottom of the South Island. Prior to these new deployments, Lauder was the only site with a high-quality solar tracker. The fifth is located at Scott Base in the Antarctic.

What do these stations record?

These monitoring stations record three different aspects of solar radiation: its direct, diffuse and global components. While most sites in NIWA’s climate network record global radiation only, the five primary sites record direct and diffuse radiation.

Not all of the solar radiation headed toward the Earth’s surface reaches us. Some gets absorbed, or scattered in different directions, as it passes through our atmosphere.

Direct radiation is the raw electromagnetic energy that comes from the sun in direct, unshaded ‘line-of-sight’.

Diffuse radiation is indirect. It comes from all other angles because it’s been scattered in all directions. To measure it, we shade the diffuse radiation sensor (a pyranometer) from the direct sunlight to cut out the direct radiation, so we’re left with just the indirect diffuse radiation that we want to measure.

Global radiation is the radiation that comes from every angle; the sum of direct and indirect diffuse radiation.

Why do we need to know this?

We need to know how much energy is getting through the atmosphere and how much is being redistributed.

Solar energy can arrive directly or indirectly. Even indirect radiation has a large effect on animals, plants, and climate. Diffuse radiation can burn your skin when you aren’t in direct sunlight, causing implications for human health. Shaded plants can photosynthesize when diffuse radiation interacts with the chlorophyll in their leaves, causing implications for agriculture and irrigation.

Solar radiation drives climate. If we can understand and model the relationships between solar radiation and climate, then we can move closer to predicting how a given change might affect the environment.

New solar trackers for primary sites

The first of our four new fully automatic EKO solar trackers was installed at Scott Base in the Antarctic, where continuous climate observations have been made since 1957. Each of the primary sites has a long record of consistent, high-quality data from well-calibrated instruments.

The trackers have global, diffuse and direct solar radiation sensors located on a single fully automated platform. Previously, technicians had to climb the tower daily to manually unwind the cable, but the new tracker unwinds itself once a day. The new tracker is more reliable, more accurate and self-corrects if it gets out of alignment.

How does it track the sun on a cloudy day?

The tracker has two modes: ‘sun sensor’ and ‘calculation’ mode.
On a sunny day the tracker works in its preferred ‘sun sensor’ mode. But when the sun is obscured by clouds, the tracker goes into its fallback ‘calculation’ mode where its computer takes over the tracking until there’s sufficient sunlight to allow the tracker to revert to ‘sun sensor’ operation.
At midnight, the tracker automatically moves back to its starting position, ready to start the new day.

Where to now?

Two more solar trackers are to be installed, at Invercargill and Paraparaumu.

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Contact

Jeremy Bulleid
jeremy.bulleid@niwa.co.nz

Instruments on the new Kaitaia solar tracker follow the sun as it moves across the sky. The black sphere shades the diffuse sensor (pyranometer) from the sun’s direct radiation whereas the long tubular sensor (pyrheliometer) ‘looks’ directly at the sun. An older-style “Campbell-Stokes” sunshine recorder is in the left foreground Photo: Bob Newland
Our most southern solar radiation tracker - located at Scott Base. The sensors have fan heaters to prevent ice and snow from affecting the readings. (Thomas Matern)
Our most southern solar radiation tracker - located at Scott Base. The sensors have fan heaters to prevent ice and snow from affecting the readings. (Thomas Matern)
Research subject: Instrumentation