This post is part of an ongoing series of energy storage posts by intern Brett Szmajda.
When I say ‘solar power’, most people conjure up images of the thin, iridescent blue panels that make a patchwork quilt out of the roofs of suburban houses. But photovoltaic solar power — converting the sun’s rays directly to electricity — is a youngster in the field of solar energy. Its great, great grandfather is solar thermal power; and with the looming threat of climate change, heat from the sun could be a significant part of Australia’s renewable energy transformation.
The principle behind solar thermal power should be familiar to anyone who has ignited dry leaves with a magnifying glass. Solar thermal power utilises the heat from the sun’s rays to do useful work. This object from the collection, invented by Lawrence Hargrave, illustrates the Australian inventor’s early attempts to heat water using the sun’s heat. Sunlight is focused by the conical dish onto the central pipe, which is closed at one end so it can hold a small volume of water. As best as we can tell, this was a hobby or proof-of-concept by Hargrave, who was also making small steam engines. However, around the same time as Hargrave was toying with solar, inventors on the other side of the world were patenting larger solar water heaters that could heat water for a household.
Utility companies have taken these basic small-scale ideas and supercharged them, creating solar thermal power stations to yoke the sun’s heat and turn it into electricity. (There are many alternative designs; most involve a lot of mirrors). For example, ‘power tower’ solar thermal power plants use several hundred mirrors to concentrate the sun’s rays on a central tower containing a column of water; this causes the water to boil, producing steam that drives a turbo-generator.
The big problem for solar thermal power generation is that sunlight isn’t constant — a solar thermal plant must contend with clouds, inclement weather, and of course, nightfall. The Solar Tres power plant (a ‘power tower’ design) in Andalusia, Spain has overcome this using a novel form of energy storage: molten salt. Instead of heating water directly, sunlight is concentrated onto a column containing a mix of 60% sodium nitrate and 40% potassium nitrate. The heat from the molten salt boils water and turns a turbine, as usual. The advantage of this additional step is that the molten salt can store the accumulated heat (for the electronics junkies in the audience, it’s almost like a ‘heat capacitor’). So when the sun goes behind the clouds or night falls, the heat from the molten salt continues to boil water, turning the turbine and keeping the power flowing. The simple addition of molten salt to the system allows 15 hours of heat storage, meaning that Solar Tres can run around the clock.
Solar thermal plants have been rolled out in a number of locations world-wide, but the uptake in Australia has been limited to two small plants: a 1.5 MW demonstration solar thermal plant has been added to the coal-fired Liddell Power station, and CSIRO has a 0.5 MW solar thermal power station in Mayfield. The biggest recent development was in June 2011, when a 250 MW solar thermal/gas hybrid plant (Solar Dawn) was given 464 million dollars of government funding as part of the Australian Government’s Solar Flagships program. Solar power is a natural fit to the Australian climate, so I’d expect some considerable growth in this sector. Until then, we’re left to wonder why Germany has invested more in solar infrastructure than Australia, when the majority of Australia has more sunshine hours per day than the German average.