By Francesca Boothby

The announcement of the 19.9MW Gemasolar Power Plant, Spain, has been widely regarded as a breakthrough in the CSP industry. It has been called ‘a milestone’ of technical and engineering achievement. It will produce energy for 25,000 homes and can generate electricity in the absence of sunlight for seven or eight hours longer than parabolic trough systems.

Currently, the main advantage that parabolic troughs have over towers is maturity. They are a more proven technology and this translates into a more attractive option for the risk averse investment community.

This however may be about to change. The Gemasol plant has now proven that tower technology combined with molten salt energy storage (MMES) can outperform competition in terms of storage, efficiency and intermittency issues.

Tom Georgis, Vice President of Development at Solar Reserve, commented “the future of renewable energy is storage and this technology is at the forefront of that”.

Molten salt has inherent technical advantages over oil. It uses much less piping for example and can reach higher temperatures of up to 1000 degrees Farenheit compared to oil at 700 degrees. It also retains up to 90% of captured heat with 15 hours of storage capacity.

From an investment and engineering point of view Tom Georgis spoke of the “validation of a superior technology” he felt had been achieved through Gemasol. He predicts the market will now “fully recognize the value of storage and fully dispatchable energy” and a system that “promotes stability and reliability”.

Parabolic trough systems

This is not to say however that the industry will now dramatically turn away from parabolic trough systems. Craig Turchi who works on the CSP program at the National Renewable Energy Laboratory, recently co authored a paper comparing costs and merits. It states that both systems are highly vulnerable to risk.

The fact is that tower and molten salt technology is still in the early stages. In the US there is only one operational 5MW power tower. For their part, trough systems need to increase in efficiency, and this may depend on their ability to incorporate molten salt, something the report says could occur by 2015. In that case trough systems would rival tower systems in terms of storage and cost projections.

Turchi also points out the risks of using molten salt. Although the replacement from oil is “conceptually a relatively small change”, the risk of freezing means a threat to kilometers of piping. This is where “all the research is focused now”. It hasn’t happened yet but if something were to go wrong there could be serious consequences.

Other factors to consider are land and water use. If both technologies use dry cooling systems there is not an enormous disparity, towers may have a slight advantage. In terms of land use, this depends on the way the technology is deployed. ESolar, for example, has a modular system where multiple smaller towers feed into a single central tower using smaller heliostats.

Craig Turchi says the comparison to trough systems is “not great enough to be a major factor”. Solar Reserve pointed out that in terms of siting, tower systems may face height issues. At 200 metres tall they need to be away from flight paths and gain environmental and cultural permits.

The announcement of the Gemasol plant has sent encouraging messages to supporters of tower and salt technology in the CSP industry. Despite these exciting developments parabolic trough systems should not be underestimated. In this extremely expensive industry, proven technologies are vital and if engineering can produce a system that incorporates molten salt adequately the gap between the two systems may become a lot more subtle.

To respond to this article, please write to: Francesca Boothby

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Rikki Stancich: rstancich@csptoday.com