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John O | March 2018

New solar reactor uses thermal energy storage to run day or night

By Josh Perry, Editor


Scientists from the German Aerospace Center (DLR) and the Aerosol and Particle Technology Laboratory of CPERI/CERTH Greece have designed and fabricated a new solar reactor, named CONTISOL, that runs on air and uses concentrated solar power (CSP) and thermal energy storage to make any solar fuel, such as hydrogen, day or night.


CONTISOL was tested at Cologne, Germany using simulated ‘suns’, rather than an actual solar field, and the storage and heat exchanger was also simulated, because the reactor itself is the innovation being tested. (SolarPACES)


According to a report from SolarPACES, solar fuels offer the potential for cleaner energy, without carbon emissions and fossil fuels and scientists have turned to CSP to provide an efficient energy source rather than electricity, photovoltaic solar power or wind.


In order to create a reactor that could work around the clock, CONTISOL is essentially two reactors in one, combining storage capabilities with a direct thermochemical reactor to ensure there is enough thermal energy to perform the high-temperature chemical processing.


“CONTISOL uses an open-air receiver, based on the volumetric air receiver operated at its test solar tower at Julich by DLR (Deutsches Zentrum für Luft- und Raumfahrt), which can heat air to 1,100°C,” the report explained. “There an open-air receiver takes air from the atmosphere and pulls it through small channels in a monolithic material.”


Initially, researchers have used silicon carbide for the receiver, but they will also try Inconel as the material. The reactor needed to operate at 800-900°C to create solar fuels and during tests the new design operated at 850°C at 5 kW.


“CONTISOL was tested at Cologne, Germany using simulated ‘suns’, rather than an actual solar field, and the storage and heat exchanger was also simulated, as the reactor itself is the innovation,” the report added.


Air was chosen as the heat transfer medium in the reactor because it is abundant in the atmosphere and did not require additional pumps or components required of a liquid cooling loop. As researchers noted, most of the reactions that take place in the reactor will be at temperatures that few, if any, liquids would stay liquid anyway.


“Solar reactors don’t include the large power block of a CSP plant, which is a full thermal power station producing electricity (except with heat supplied by the sun),” the article continued. “Solar reactors don’t need the big turbine or generator for making electricity, but only consist of a tower, a solar field, a receiver and the reaction chamber. To this, CONTISOL adds a storage system, transferring the heat from the air into the heat exchanger.”


The research was recently published in Applied Thermal Engineering. The abstract stated:


“The CONTISOL concept is a new vision of an integrated solar receiver/reactor for a variety of thermochemical processes. The concept includes a single monolithic solar absorber with two inter-mixed, but nonintersecting sets of gas channels.


“One set of channels is always used for a chemical process. During daytime operation, the other set of channels is used to heat air which is sent to thermal storage. During nighttime operation, the air flow is reversed, transferring heat from thermal storage to the monolith through the same set of channels, thus providing energy to continue chemical processing continuously through day and night.


“In this paper we introduce the general operation of the system and discuss its benefits applied to solar methane reforming as an example process. Past solar reactors which influenced the development of CONTISOL are discussed. A 5 kW scale demonstration prototype has been constructed at DLR and thermal experiments have been conducted using the DLR high flux solar simulator.


“A statistical design-of-experiments procedure has been applied to evaluate the influence of absorber temperature, gas flow rates, and gas inlet temperatures on heat transfer rates to gas streams, and to construct a thermal performance map of the device.


“The target gas outlet temperatures of over 850°C were reached during these tests. Limitations on the initial design of the monolith are discussed including recommendations for future improvements.”

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