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Friday, 2 December 2011

How much sunshine makes a ‘sunburnt country’?

Posted: Monday, 14th November, 2011 | Author: Tania | Filed under: Solar photovoltaics, Solar thermal | Tags: insolation, sse |1 Comment »
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:

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