Molten Salt CSP

- a promising link between Power Production and Energy Storage

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24 Nov 2019    By Poul Madsen           

The achilles heel for Renewables has for decades been the question of how to store energy. Wind turbines and solar panels have long proven their worth when it comes to power production, however none of these have managed to provide the final solution of how to deliver electricity when the wind is calm and the sun is gone for the night. In this blog we take a look at Molten Salt in combination with Concentrating Solar Panels and what appears like a promising solution...

CSP and steam turbines

Concentrating solar panels emerged as a new technology in the 1980s. Instead of generating power or warm water for hot baths and kitchen, by concentrating the sun rays to a small area, higher temperatures could be achieved. CSP panels could produce steam turbines to generate power. The plant at Ivanpah in California's Mojave Desert were built in the 1980s and is a very large plant generating some 392 MW. Other projects are also described in this article by sciencedirect.com [1].

Trough or Tower?

In Trough Concentrating Solar Panels, the parabols concentrate the rays from the sun to heat up a pipe with a fluid flowing inside. If the fluid is hot enough it can be utilized to create steam which in turn can drive a steam turbine.

The issue with trough CSP solar panels is that the panels only create heat at limited temperatures. To run steam turbines, you need high pressure steam at templeratures of nearly 300 degress centigrade - preferrably higher.

This weakness is overcome by using tower CSP. Here, large amounts of mirrors (called heliostats) focus the rays from the sun into a single point - the top of the tower, which then gets heated to very high temperatures. This way you can create heat of 600 degress centigrades and even more. The limit is how high temperatures the piping and turbines can handle. This goes for the liquid that runs through the trough pipes as well. The drawback of increased temperatures is the risk of corrosion and leaks.

Molten Salt

Until now, another major weakness of using solar panels has been the lack of options for storing power for when the sun is gone for the night. Here's where the molten salt comes into play. Molten salt keeps the heat for long, thus enabling high temperature steam generating at times when no other renewable sources are available; no sun, no wind (and no waves).

One of the benefits of molten salt is that it retains heat for very long. In an article from 2008 on renewableenergyworld.com Julie Way gives an overview of molten salt for thermal energy storage on [2].

Other benefits include the options for district heating. Having travelled thropugh the steam turbine, the steam has cooled off but is still hot. Using heat exchangers at this stage, steam can heat up water to be used for district heating. This, of course, requires the presence of residential areas within reasonable distance. Industries that need steam like aluminum production and laundries is another option.

Susan Kraemer in an article on solarpaces.org gives an overview of the Malta project using CSP molten salt [3]. Among benefits listed includes very long calendar life. Megan Geuss in an article on mentions that among investors for Malta are Bloomberg, Jeff Bezos with Bill Gates as the chariman [4].

There are some issues to keep in mind with molten salt. You wish to keep the salt flowing through the pipes, however, if the salt cools off, it solidifies. Different salt types has different charactistics [5]. You want a salt with a reasonably low melting point.

Sample Plants

Since molten salt in combination with CSP is a reasonably new technology, only a few plants have been built. According to www.theguardian.com, the world's first CSP molten salt plant was the Archimede 5 MW in Priolo Gargallo in Sicily [6].

The GemaSolar plant in Spain started generating power in May 2011 [7]. This tower plant using molten salt for energy storage has a reported output of 20 MW. The plant is located north east of Seville in southern Spain and is visible on satelite images on Google.

The Spanish power company Abengoa has built a number of CSP plants west of Seville. The northern most plants are the PS10 and PS20 plants at 11 and 20 MW respectively. They were commenced in 2009. They have towers though do not use molten salt technology. Just south of PS10 and PS20 you find the two 50 MW plants at Ecija. The latter are trough CSP technology. The plants can be viewed on Google satelite images.

Among the first large scale Molten Salt CSP plants is the Crescent Dunes northwest of Tonopah in Nevada and can generate 110 MW power [8]. The plant is built by SolarReserve.

By October 2019 the plant had run into issues and was not able to produce sufficient power for its customer, NV Energy. According to pv-magazine-usa.com SolarReserve has filed a lawsuit against the company who owns Crescent Dunes [9].

There are several molten salt CSP projects underway. newenergyupdate.com lists a number of projects and existing plants [10].

Financial Aspects

Sources estimate that towers are more economical than troughs. There are a number of reasons, but two reasons are that

towers can reach higher temperatures

they need less piping

Compared to other technologies like coal, it has been estimated that molten salt CSP proves itself much more competitive. In 2018 Insideclimatenews.org published an article which listed existing plants and included a comparison between power prices [11].

According to solarthermalworld.org, using molten salt for energy storage can be up to 33 percent cheaper than using batteries [12].