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Solar thermal: the search for cheaper storage solutions

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A new Australian research program is looking to develop new materials that could reduce the volume of storage systems for solar thermal energy by a factor of 10, and deliver a significant reduction in costs.

Most of the storage systems being rolled out for solar thermal systems in Spain and elsewhere have used some form of molten salt, but a project being led by the University of South Australia is looking to exploit so-called “phase change” materials that can operate at higher temperatures, and either store more heat within the same volume, or require significantly less space.

The project – which also includes the Barbara Hardy Institute, and the University of Lleida in Spain, along with the Solar Oasis consortium which is developing a 44MW “big dish” solar power project in Whyalla, South Australia, and AORA Solar in Israel – is backed by grant funding from the Australian Solar Institute.

Project leader Professor Wasim Saman, from the department of sustainable energy engineering at the University of South Australia, said his team was likely to look at a number of different materials, including inorganic salts.

Professor Saman said his team is looking at a number of potential materials that can freeze and melt at 400°C to 800°C. In order to test these materials, a specialised test facility will be built which will be able to accommodate storage systems operating at temperatures up to 900°C.

He described the molten salt storage systems being used in Spain – where the salt is melted at high temperatures and stores in huge tanks – as similar to storing water in a giant thermos.

“So what we are looking to do, similar to what happens when you melt and freeze ice, is to store a lot of heat through the change of phase from solid to liquid and vice versa – and hopefully improve the economics of the storage system,” he told RenewEconomy.

“Hopefully we can reduce the size of those tanks by factor of five to 10. That would reduce costs, but we need the technology to enable a quick enough heat transfer between material and heat source.”

Saman says his team has been working with the Solar Oasis project for some time, and the university has a campus at Whyalla. The team intends to create a small-scale storage system to use with the big-dish technology. The partnership with AORA Solar in Israel will focus on smaller concentrating solar power systems. “In both cases, the differentiation is the potential to be dispatchable. That is something that solar thermal can do.”

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  • http://www.sunoba.com.au Sunoba

    Phase-change materials certainly offer the prospect of storing a lot of energy in a small volume, so I commend Prof Saman’s research effort.

    I think there is merit in another option, namely air-blown thermal storage in a loosely packed bed of rocks. Such a storage material is cheap, durable, widely available, non-toxic and stable over a wide range of temperatures. Some details of recent work on this topic are given in posts on 16 March 2012 and 21 May 2012 at http://www.sunoba.blogspot.com

  • John Newton

    Solar Thermal is one of, if not the most promising technologies for beating our fossil fuel addiction. Is this why it gets absolutely no press?

  • http://yes2renewables.org Ben Courtice

    The higher the temperature, the greater the logistical challenges in storing and transferring that heat. For example, any solar concentrators that move to track the sun (big dish, trough etc) need to have moving joints in the piping that transfers the fluid.

    At high temperatures that is increasingly difficult and/or expensive: flexible materials like PTFE are ruled out over a few hundred degrees.

    I’m not aware of the specific materials or methods used at high temperatures, (I’m more familiar with high pressures in hydraulics); but I suspect there might be a “sweet spot” where the fluid is hot enough to give some efficiency benefits (eg running a supercritical steam turbine); yet not hot enough to be prohibitively expensive in materials and engineering/maintenance. Does anyone else know more than me on this?

  • Derek B

    Current molten salt systems carefully avoid letting the salt freeze. It has to flow through the system to collect heat then use it to make steam. Indeed, they have to have emergency systems for keeping the salt hot should the collector fail for a period of weeks.
    Presumably phase change storage would keep the material in one place and use another fluid passing through heat exchangers to move the heat around. The phase change itself is likely to generate large stresses on the vessels and pipes. So I imagine it will be some years before this becomes commercial.

  • Alastair Leith

    @Ben
    Power towers do not have a moving receiver, the mirrors do all the moving and have no contact with the heat transfer systems. Some trough systems have a fixed collector and a moving trough, again the piping for heated liquid does’t require flexible joints for automated sun tracking movement. The salts seem to get more expensive as their maximum heat capacity increases, for superheated steam it’s a mixture of salts. There’s a fair introductory discussion of CST technologies in the Beyond Zero Emissions Stationary Power Report.

    Goto beyondzeroemissions.org and download the ≈100pp PDF.