Perfect day for solar powerPosted: Wednesday, 18th May, 2011
You know you’re a solar scientist when you’re outside on a Saturday, looking at the beautiful blue skies and perfect weather, and you find yourself thinking, “If only today was a work day.”
It may sound crazy, but this has been know to happen — usually when experiments have been delayed for a few days because of cloud only to have it clear up on the weekend. Luckily we’ve been having stunning solar conditions every day recently, and Thursday in particular had flawlessly clear skies. We know exactly how good the conditions were, because we have instruments here on site that record the amount of sunshine hitting the ground at all times. It’s obviously important to have this data when you’re testing solar technology.
The graph above shows the intensity of sunlight at our site on Thursday. The red curve is the data we’re interested in for concentrated solar power, because it’s the ‘direct irradiance’ — the intensity of direct (shadow-casting) sunlight on a surface that’s tilted to face the sun. Because there was no cloud it’s almost a perfectly smooth curve, starting at sunrise (just before 7 am) and dropping off at 5 pm. The little dip just after 7 am is where the shadow of Solar Tower 2 passes over the measuring instrument.
The green ‘total horizontal’ irradiance curve, on the other hand, is what you’d be interested in if you wanted to measure all the light falling on a surface lying flat on the ground — say, a book you’re reading. It includes the direct sunlight as well as the indirect rays—the ones that arrive on the page after having been scattered off clouds or other objects—and also takes into account the fact that the angle between the sun and the book changes throughout the day. The blue ‘diffuse horizontal’ curve shows just the indirect rays. This is how much light you’d still have available to read by if someone cast a shadow over your book.
If you’re following along, you may be wondering why the curves don’t add up – specifically, why the ‘direct’ plus the ‘diffuse’ curves don’t equal the ‘total irradiance’ curve. The answer is simply because here the ‘direct’ curve refers to shadow-casting light on a surface tilted to face the sun. The other curves refer to a surface lying flat on the ground. That’s all.
Below are the three instruments we use to take these measurements. The two pyranometers measure the sunlight falling onto a flat surface. One of them is kept shaded by a little black disc that tracks the sun, so it can measure the diffuse (non-direct) rays only. The pyrheliometer measures the direct radiation only, and a tracking mechanism makes sure that it always faces the sun directly.