How much sunshine makes a ‘sunburnt country’?

A few years ago, cartoonist Cathy Wilcox drew a panel poking fun at Australia’s uptake of solar energy. In it, two Australians wearing broad hats and sunglasses are talking to each other. ‘The Germans plan to generate a quarter of their power from solar energy by 2020,’ says the Hawaiian-shirted man. ‘If only we had access to German sunshine!’ replies the woman.

In this cartoon, Wilcox’s implication is: we get much more sun than other countries, so why don’t we use it?  It’s true we’re self-described as a ‘sunburnt country’, but exactly how does our sunshine compare to other parts of the world? I thought it’d be interesting to show you some of the data here.

There are two images below, adapted from satellite data available from NASA’s Surface Meteorology and Solar Energy site. The first one shows total sunlight on a flat surface averaged over a certain period of time (in this case 25 years), which we call Global Horizontal Irradiation or GHI. The second image shows how much of the direct sunlight component (i.e. that part of sunshine which casts crisp shadows) falls on a surface tilted to face the sun, which is known as Direct Normal Irradiation or DNI. This latter type is what we need for concentrating solar thermal systems like solar towers. Redder areas on the maps mean more sunlight of this type at ground level, on average. These images are powerful because they show at a glance which parts of the world have the best solar resource.

On a worldwide scale, you can see from both images that solar conditions don’t change uniformly with latitude. There are also complicating factors like local geography, nearby industries and population, which affect annual levels of sunlight-blocking cloud cover, water vapour and smog.

At first glance at the two maps you might be surprised to see that the DNI values in the bottom plot are higher than the GHI values in the upper plot. Doesn’t DNI measure just a part of the total or GHI levels? Yes, but remember that DNI gives the energy falling on a ‘normal’ plane, i.e. one held facing the sun (like a sun-tracking panel), while GHI gives energy on horizontal ground (like a flat, stationary panel). Making measurements on a plane tilted towards the sun effectively removes some of the ‘penalty’ that locations at higher latitudes (i.e. closer to the poles) get from having the sun lower in the sky on average, and it also results in higher readings for energy per square metre in early morning and late afternoon.

Going back to the original question, what about Australia in particular? Here’s a close-up comparing the direct sunshine falling on Australia to that of two regions with rapid solar thermal energy development, Europe and the USA:


So you can see that Wilcox’s ironic point is correct: as we expected, we clearly get a lot more sunshine in Australia than in Germany. It’s also interesting to note that we also compare favourably to Spain, where there’s been a real solar thermal construction boom over the last few years.

Of course, there are a lot more factors than sunlight that influence this. Just one of these is how close the sunniest regions are to the major population centres and transmission lines. In this respect, California, Nevada and New Mexico are extremely well-equipped for solar power production, and you can see why there is more than a gigawatt of new concentrating solar thermal power capacity being installed there right now.

It’s also easy to understand from the maps why there’s so much interest from European countries in generating some of their electricity from solar thermal plants in the sun-drenched areas of North Africa. Those countries have one of the most solar-rich resources in the world – and, as you can see, Australia is right at the top of the list with them.


External links last accessed 14/11/11

Perfect day for solar power

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.

These little gizmos are probably the most important solar instruments we have on site. Without them we’d have no idea how much energy was available for use in our solar facilities.


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