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Storing the sun’s heat for more efficient production

​Thermodynamic solar power plants:

Published on 24 November 2015

Thermodynamic solar concentrator plants serve two crucial functions. The first is to concentrate the warmth of the sun’s rays to produce electricity or heat; the second is to store heat so that plants can continue to produce energy outside of sunlight hours. Photovoltaic solar power plants offer heat-production yields of around 15%, while thermodynamic solar concentrator plant yields are between 50% and 60%. Of course, these plants require substantially higher CapEx than a traditional electrical installation. But because they rely on a totally free primary resource—the sun—their long-term ROI is guaranteed. And the world is paying close attention to thermodynamic solar technology as a way to bring the per-kWh cost of solar energy down. For the past seven years at Liten we have been working on a thermodynamic solar concentrator technology that leverages Fresnel mirrors. We are also looking at how thermal energy storage can be used to adjust electricity production to demand.

A Fresnel mirror is an interferometric system made up of two planar reflectors, one of which is positioned at a slight incline with regard to the other. Liten’s solution entails placing many of these reflectors side by side on the same planar surface and orienting them to follow the sun’s course and redirect the sun’s rays—concentrating them as effectively as possible—to a pipe-like heat collector installed horizontally above the reflectors. The technology’s main benefit is high yields at a cost below that of traditional cylindrical parabolic concentrators. We have signed a joint development agreement with a manufacturer to bring the technology to maturity and are helping them identify energy-sector partners for large-scale plant construction projects. Finally, we are also involved in the EU’s Solar Facilities for the European Research Area (SFERA) project alongside Europe’s major solar-industry stakeholders.
Thermodynamic solar power plants are of great interest due to the fact that they can offset some of PV solar’s shortcomings—intermittence and storage—and help align production capacity with demand, even for industrial energy customers. Thermodynamic plants currently produce less than 1% of all solar electricity, but they are rapidly gaining traction.

BENEFITS

Improved solar-energy storage for uninterrupted service

-    Thermodynamic solar technology harnesses a totally free resource—the sun—, guaranteeing long-term ROI.

-    Planar mirrors are much less costly than the parabolic mirrors used in other types of solar power plants.

-    Heat-production yields are 50% to 60%—much, much higher than the yields achieved by PV plants.

-    OpEx can be limited by using water instead of oil as the heat-transfer fluid.

-    The heat captured can produce electricity, of course, but it can also be used for absorption refrigeration systems, of interest to a vast array of industrial energy customers.

-    Heat storage can help make thermodynamic solar plants more responsive to demand, giving industrial energy customers access to solar power, even hours after the sun has set.

PROJECTS

- The Alsolen thermodynamic solar power plant demonstrator, developed for energy company Alcen, was completed in 2012. Located at the CEA’s Cadarache campus in southern France, the plant boasts 1,000 sq. m of mirrors and a capacity of 50 kWh during peak sun-hours. Oil is used as a heat-transfer fluid and heat can be stored for several hours. The plant’s waste heat is also recycled for cooling.

- Our researchers are looking at how to use water as a heat-transfer fluid, then generate steam directly in the heat collector. We hope to overcome two obstacles to the further development of this technology. The first concerns temperature: water must be heated to 450°C (as compared to 300°C for oil) to produce steam. The second issue is how to transform running water into steam in the same collector. A second demonstrator plant, Alsolen Sup—with 1,500 sq. m of mirrors—, is under construction at the CEA’s Cadarache campus. This new demonstrator plant will be used to validate the maturity of the technology, which will ultimately be used in plants of 50 MW minimum.

- We are also investigating three heat storage alternatives for thermodynamic solar power plants:

. So-called “sensitive” thermocline heat storage entails using tanks filled with solid materials that are then gradually heated. An experimental 3 cu. m storage tank prototype was built in Grenoble in 2010. The Alsolen demonstrator plant has another, larger tank prototype (35 cu. m), which provides heat to the Prohytech facility, demonstrating that thermal solar energy can be used for desalination, cooling, and electrolysis-based hydrogen production.
. Phase-change materials that go from a solid to a liquid state when heated can also be used to store heat. At Liten we have developed several, increasingly large, test facilities since 2009 (ranging from 1 liter to 5 tons). A tank measuring nearly 6 cu. m is currently being built at the Alsolen Sup demonstrator plant on the CEA’s Cadarache campus. This tank will contain sodium nitrate, compatible with water’s evaporation temperature of 300°C.
. Thermoreversible chemical reactions eliminate heat loss during storage, making the technology an attractive alternative for solar plants and domestic interseasonal heat storage. A test home has been equipped with this type of system at Ines (the French national solar-energy institute) in Chambéry and its performance is being assessed.
- In 2014 Liten and energy company Idex won a Bpifrance (France’s national investment bank) award for a heat storage system using phase-change materials destined for heating networks. We also benefit from the support of French energy agency ADEME when responding to calls of expressions of interest and help set evaluation standards.

FACTS AND FIGURES

-    Around 50 researchers
-    34 patents on concentrated solar energy
-    19 patents on thermal storage
-    Publications:


Girard R, Delord C, Disdier A, Raccurt O. May 2015. Critical constraints responsible to solar glass mirror degradation. Energy Procedia 69. International Conference on Concentrating Solar Power and Chemical Energy Systems, SolarPACES 2014: 1519–1528.
Rodat S, Bruch A, Dupassieux N, El Mourchid N. May 2015. Unique Fresnel demonstrator including ORC and thermocline direct thermal storage: operating experience. Critical constraints responsible to solar glass mirror degradation. Energy Procedia 69. International Conference on Concentrating Solar Power and Chemical Energy Systems, SolarPACES 2014: 1667–1675.
Garcia P, Olcese M, Rougé S. May 2015. Experimental and numerical investigation of a pilot scale latent heat thermal energy storage for CSP power plant. Critical constraints responsible to solar glass mirror degradation. Energy Procedia 69. International Conference on Concentrating Solar Power and Chemical Energy Systems, SolarPACES 2014: 842–849.
Bruch A, Fourmigue JF, Couturier R. July 2014. Experimental and numerical investigation of a pilot-scale thermal oil packed bed thermal storage system for CSP power plant. Solar Energy 105: 116–125.



FACTS & FIGURES