Solid particles to harness the Sun's energy

22nd December 2016
Enaie Azambuja

Scientists replaced the conventional molten salts in solar technologies with a suspension of solid particles that can capture and store heat at higher temperatures. Solar energy is an important alternative renewable resource that helps mitigate the negative environmental impact from carbon dioxide emissions. Concentrated solar power (CSP) uses mirrors to concentrate sunlight and capture it as heat, which then drives an engine or turbine.

One of the main advantages of CSP with respect to highly variable renewable electricity production technologies like photovoltaic or wind power is the capacity to supply electricity on demand thanks to thermal storage.

But to produce electricity on a competitive basis, the efficiency and cost of CSP plants have to be improved. One of the possible routes to improve CSP performances is increasing working temperature of the heat transfer fluid (HTF) and of the storage medium.

Within the EU-funded project CSP2 (Concentrated solar power in particles), scientists proposed an alternative HTF, based on dense suspension of solid particles. This can operate at up to 750°C with 70-90% efficiency, thus considerably exceeding state-of-the-art CSP plants using molten salt as HTF.

Consortium members have developed and operated successfully a pilot solar receiver with 100-150 kW thermal capacity that was placed on the focus of the 1 MW solar furnace of the National Center for Scientific Research (CNRS), located in France.

The solar loop was tested for 8 hours, with a flow rate of about 0.7 to 1.8 tonnes per hour. The solar receiver was composed of 16 one-metre-long vertical tubes in which the dense gas-particle suspension (DPS) flowed vertically upward.

The HTF flowing through the tubes is a DPS. The tubes set in a bundle constitute the solar absorber (receiver), placed at the top of a central receiver CSP system. The new HTF is totally safe and does not become solid at low temperatures. It behaves just like a liquid, although it allows working temperatures higher than 550ºC and up to 750°C.

The solid phase consists of any particulate mineral that withstands high temperatures. In addition, it can be used as an energy storage medium due its good thermal capacity. Large quantities of the particles can be easily produced at low cost without any chemical process development.

Project members also performed scale-up studies for industrial CSP facilities. Economic assessments allowed comparing this new DPS technology to the molten salt one.

CSP2's solution of using solid particles in a solar receiver is a radical alternative to the hitherto-used liquid or gas HTFs. Given that elevated temperatures require less heat transfer material, the DPS enables a power plant configuration that uses high efficiency thermodynamic cycles such as supercritical CO2 and combined cycles and that leads to cost reduction.

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