ScienceWise - Nov/Dec 2008

Bottled sunlight

Article Illustration
Dr Keith Lovegrove with one of the mirror panels from a newer and larger Big Dish under development. You can see the frame of the original Big Dish in the reflection
Article Illustration
Schematic an ammonia based stored energy solar-thermal plant. Ammonia is dissociated into hydrogen and nitrogen gas at the extreme temperatures of the focus of a solar concentrator. The gasses are returned to a shared liquid/gas storage vessel where any un-converted ammonia return to the liquid supply. When power is required such as after dark, the hydrogen and nitrogen are pumped to a catalatic reactor where they recombine into ammonia releasing large quantities of heat in the process. This heat is then used to create steam for electricity generation.

Developing a practical system for storing solar energy

The ANU campus is home to the Big Dish, the largest parabolic dish solar concentrator in the world. The development of the Big Dish and its related technologies is undertaken by the ANU solar thermal group, headed by Dr Keith Lovegrove.
The dish's massive 400 square metre parabolic mirror concentrates the sun's rays to heat water and create superheated steam, which can be piped to a turbine to generate electricity. The same system can also be used to store energy in the form of high temperature reversible chemical reactions and it's this capability that gives it a special advantage in the world of solar power.

Conventional Photovoltaic solar cells are an excellent technology for direct generation of electricity from sunlight to power daylight needs such as industry and air-conditioning. However, one of the challenges facing the solar power industry is storage of solar energy for use either at night or in vehicles or locations very far from the solar plant. "This is the competitive advantage of solar thermal systems," Dr Lovegrove explains. "It's actually a lot easier to store heat, for example, in the form of hot oil or molten salt or using appropriate chemistry, than it is to store large amounts of electricity."

Part of the Solar Thermal Group's work is based on an ammonia based energy storage system. The basic idea is that ammonia is pumped into the focus of a large solar collector where temperatures reach several hundreds of degrees. At these temperature the ammonia dissociates into hydrogen and nitrogen.

"This is an attractive reaction because it's chemically simple. It has no side products and is fairly benign in that even in the event of an accident there would be no long lasting toxic legacy at the site."

In this system, the product gasses are returned to the ammonia storage vessel via a heat exchanger. This has the advantage of automatically returning any un-dissociated ammonia to the supply vessel. The hydrogen and nitrogen gas can then be tapped from the same chamber to fuel a recombination system liberating the chemically stored heat.
The use of ammonia in industrial processes is not new. In fact ammonia products such as fertilizer, explosives and household cleaners represent one of the world's largest chemical industries. "As a result the industrial storage, handling, creation and dissociation of ammonia are well established technologies." Dr lovegrove says, "Industry has been doing this kind of thing for over a century so we're confident that we can integrate our solar energy storage with current practices and off the shelf components."

Of course ammonia isn't the only way to store solar power. In a collaboration with the ANU School of Biology, the group are also investigating the use of algae as a fuel source. Under the right conditions algae grow prolifically given a rich supply of water, nutrients and sunlight. Of course what is happening is that these microorganisms are consuming carbon dioxide from the atmosphere and using sunlight to combine this with hydrogen and oxygen from water to create the complex molecules that form their unicellular bodies.

To extract a useful fuel, the filtered algae slurry is pumped into the focus of the solar thermal concentrator at pressures of around 25MPa and temperatures up to 700°C. This results in the dissociation of the slurry into carbon dioxide, carbon monoxide and hydrogen. "What I find exciting about this work is the potential to create methanol from the carbon monoxide and hydrogen in the reactor product." Dr Lovegrove explains. "Here we have a system capable of generating vast quantities of easily stored liquid fuel with a high energy value. Methanol generated in this way may well be the petrol of the future."

"It's an exciting opportunity to use green technology in an area with tremendous commercial potential. Far from undermining Australia's economy, a solar generated methanol fuel industry has the potential to vastly expand our economy. And do so in a way that's good for the environment."

 

 

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