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Hybrid Renewable Energy Converter for continuous and flexible power production

Final Report Summary - HRC POWER (Hybrid Renewable Energy Converter for continuous and flexible power production)

Executive Summary:
Many countries develop renewable energy sources to satisfy future demand for electricity as well as to increase the contribution of renewable energies in their energy mix. A strong limitation results from the fact that many renewable energies are intermittent and there is a clear need for new technological solutions to overcome this issue. A power source must ideally be able to provide reliable and continuous base-load power as well as peak-load power when it is needed in order to match the supply demand. It should have a high Annual Capacity Factor (ACF), which is the ratio of its realized output over one year to its potential output if it had operated at full capacity over the same time period. Providing peak-load power means that it should be flexible enough to adjust its output during expected or unexpected peak demands.
Renewable energies generally have quite low ACF compared to conventional power plants, especially nuclear. For renewable energies, a high level of unused capacity comes from the high variability of the source over time. Their intermittent nature has a large impact: low capacity factors, low flexibility, and higher amortization costs. It also limits their penetration into the energy mix, as the current grids are not ready to compensate their large variability.
The HRC Power project proposes a radically new approach combining novel advanced materials and an innovative hybridization technology to make breakthroughs at materials and concept levels: very high temperature operation up to the 1000-1300°C with high Carnot efficiency, round-the-clock operation for 95% ACF, high flexibility, low water consumption with no thermodynamic cycle. Novel materials consist of advanced absorber metamaterials based on self-organized structure and advanced infrared selective emitter refractory crystals. Novel technology / concept consist of a specific micro-combustor operating at very high temperature. This concept is a radically new path for renewable energy hybridization in a solid state device able to provide high quality thermal energy from solar and H2 or Biogas sources to thermal / electrical solid state converters. The HRC converter unit will be heated by high solar radiation concentration or by an internal combustion. This heat could be then converted into electricity by thermoelectric or thermo-photovoltaic devices.

Project Context and Objectives:
The main objectives of the HRC Power project address both novel materials and a novel technology concept for energy:
➢ Design the innovative converter concept to hybridize solar and combustion for continuous and flexible operation
➢ Develop 3 advanced building blocks for solar, combustion, and dual-modes:
o New refractory advanced absorber metamaterials for high temperature operation and high spectral selectivity in order to unlock new concepts for high efficiency solar thermal energy harvesting
o New micro-combustor technology compatible with solar energy input, highly thermally conductive, with enlarged flame stability envelop
o New infrared emitter materials consisting of advanced metallic films with controlled emissivity spectrum and directionality for thermal energy transfers
➢ Develop technologies for the integration of the building blocks into a solid state converter
➢ Provide the proof of concept of this new technology with a performance assessment.

Project Results:
WP1 had been dedicated to the simulation and design of the novel HRC converters. Thermal and combustion simulations were conducted. The main result was the definition of two generations of converters able to achieve temperatures corresponding to the domains targeted by the project. GEN1 was defined to achieve temperatures between 500 and 900°C and used parallel channels in a steel body. For GEN2, two main options were investigated and defined. The first one uses parallel channels defined for catalytic combustion and laminar gas motion in a silicon carbide ceramic block. It is well adapted for hydrogen / air combustion in a 3x3cm2 ceramic block. The second one is based on the swirling motion of gases in parallel chambers located as well in a ceramic block. The chambers can be connected or not. The minimum diameter to length aspect ratio necessary to maintain combustion was defined for various combustion scenarios. This design was optimized to operate with hydrogen but also with methane, which combustion is much more difficult to stabilize. The architectures of HRC converts were defined to be fabricated thanks to the building blocks developed in the other work packages.
The development of solar absorbers was done in WP2 for the efficient capture of solar radiation in the solar mode operation. They are based on refractory materials. The first achievement was the development of very efficient absorbers made by nanostructuration of refractory molybdenum films. Absorptivity performances of more than 95% were achieved thanks to a double structuration process using reactive plasma etching of molybdenum films through a single layer of self-arranged particles. The process was further developed to be simplified by removing the particle layer step. An excellent absorptivity performance of 99% was achieved. The effort to develop innovative materials for solar absorption able to operate in air at high temperature conducted CEA to investigate structuration of silicon carbide materials. New metamatarials able to absorb more than 95% of the solar radiation and to sustain operation in air were developed and implemented on the HRC converters in the last period of the project. These new absorbers are able to withstand extremely high temperatures in air, up to 1000°C. Their development contributes to increase the performance of the technology by improving solar capture. It contributes also to increase the durability of the converters and reduce the cost of the technology.
In parallel, WP4 was focussed on developing materials to control the infrared emitted spectrum to make the technology compatible with a good conversion efficiency on thermophotovoltaic cells. The simulation of the IR spectra emitted by various structurations and thin film solutions resulted in the choice of a tungsten / alumina stack which was optimized, fabricated and characterized. Simulation and experimental work provided solutions to adapt the cut-off wavelength of the stack to a TPV cell. Nevertheless, the material is still quite sensible to high temperature, especially when operated in air. This remains a challenging point for the technology.
WP5 was key in the project with a lot of work on technologies suitable for the integration of the different parts and materials to build the full hybrid converter. Following all the work done on materials, design, assembling options, two main routes were defined. The first one is based on the brazing of the body, thermal insulators and inlet/outlet tubes with optimized geometries able to reduce stress on the brazing surfaces. The second one is based on the introduction of platinum tubes in the converter body and uses only brazing for the assembly of platinum tubes with steel tubes. This second option was successfully integrated in a fully functionalized converter including high temperature silicon carbide structured solar absorber and interferential selective emitter.
WP6 was dedicated to the performance assessment of the converters. The main expected final result of the HRC Power project was the performance measurement of the fully integrated HRC converter in both combustion and solar modes in terms of operating temperatures. The two temperature ranges targeted in the project were achieved. The first one (500-900°C) was achieved with a full steel combustor without additional functionalization of surfaces (GEN1). The highest temperature range of 900-1300°C was achieved with the integration of ceramic silicon carbide converters with lateral thermal insulation. Highest temperatures were achieved by adding high performances solar absorbers as well as selective emitters.

Potential Impact:
The project has various impacts because of its multidisciplinar nature. The first one is on material science with advances on ceramic materials, integration of multi-materials for high temperature operation, new metamaterials for solar absorption including in air operation and infrared emission. The second one is related to the energy field with the demonstration of this new concept of very high temperature hybrid converters able to operate with solar concentration as well as internal combustion, at least up to 1100°C in both modes. It is very flexible as a large range of temperatures can be achieved by varying the chemical input with good flame stability in case of hydrogen / air combustion as well as by varying the solar input with facetted mirrors. The high temperature operation is the condition to benefit from interesting thermal to electrical conversion efficiencies with the available thermoelectrical or thermophotovoltaic technologies. In the energy sector this FET project allowed to do the demonstration of this new hybridization concept. The next steps are the coupling with thermal to electrical conversions as well as optimizations needed to improve the performance / cost ratio for which it has a strong advantage based of the full time operation allowed by the hybrid operation.
More generally, it is a contribution to the societal objective of reducing human impact on global warming by proposing a new hybridization pathway to efficiently use renewable energies.

List of Websites:
The address of the project web site is