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Next GenerAtion MateriAls and Solid State DevicEs for Ultra High Temperature Energy Storage and Conversion

Periodic Reporting for period 2 - AMADEUS (Next GenerAtion MateriAls and Solid State DevicEs for Ultra High Temperature Energy Storage and Conversion)

Berichtszeitraum: 2019-01-01 bis 2019-12-31

Thermal energy storage (TES) for on-demand electric power generation is one of the most deployed energy storage options. Mostly used in concentrated solar power (CSP) plants, current TES systems are limited to temperatures of ~ 600 ºC due to high temperature thermal instability of currently available materials and devices. Ultra-high temperature systems that operate in the range of 1000 - 2000 ºC would offer up to 4.6 MJ/kg (~ 2.6 MWh/m3) energy density capacity, which is more than 20 times higher than that of today’s TES systems based on molten salts. This energy density is only comparable to liquid hydrogen storage, and would enable a drastic reduction of the overall size and cost of the system. This would broaden the number of applications of TES, not only in the field of CSP, but also for the storage of other variable renewable energy sources, such as solar PV or wind, or the cogeneration of heat and electricity in both industrial and residential sectors.

The targeted breakthrough of AMADEUS project is to develop novel materials and devices that enable energy storage and conversion at ultra-high temperatures, well beyond 1000 ºC. For this, AMADEUS is investigating phase change materials (PCM) based on silicon-boron binary (Si-B) and ternary (Si-B-X) alloys, with potential to surpass 2 MJ/kg of energy density. The most relevant technological challenges concerning the use of these materials are also being investigated, such as the refractory linings of the container, advanced thermal insulation casing, and hybrid thermionic-photovoltaic (TIPV) converters able to produce electricity from heat at those ultra-high temperatures.
During the first two years of the project, the research has progressed mostly in the development of the two main components of the system: the heat storage and energy conversion modules. Concerning the former one, a number of Si-B-X alloys has been produced and characterized. Among them, the binary Si-3.25B shows the highest latent heat in J/g, with a record measured value of 1.85 MJ/kg, which is higher than that of pure silicon (1.82 MJ/kg). Ternary alloys such as Fe-26.38Si-9.35B and Cr-43Si-5B have resulted in the highest latent heats per unit of volume (> 4 kJ/cm3 or > 1 MWh/m3), also higher than that of pure silicon. Besides, the investigated ternary alloys have shown volumetric contraction upon solidification, in contrast to pure silicon and Si-3.25B that increase ~ 10 % their volume during solidification. This is a key advantage because it minimizes the risks of crucible damage under thermal cycling. The high temperature reactivity of different liquid PCMs with selected solid refractory materials has been investigated through wettability and thermal cyclability tests. Only h-BN has been found non-wettable and barely reactive with all the tested PCMs up to 1750 ºC. On the contrary, graphite tends to react with Si and B to produce SiC and B4C. However, these reactions might create a protective layer in the crucible that prevents further infiltrations. Simulation work has been also conducted to design the optimal shape of the crucible that minimizes the mechanical stress and maximize the conversion efficiency, as well as the optimal thermal insulation layers that minimize the heat losses at the minimum cost. This extensive analysis has resulted in an optimum system design that is currently under construction and will be finalized in March 2019.

Concerning the development of a TIPV energy converter, the different constitutive elements have been individually developed and optimized. First, an emitter able to effectively radiate electrons has been accomplished by depositing thin layers of lanthanum hexaboride (LaB6) on tantalum substrates, resulting in work function of 2.6 eV and thermionic current density of 1.5 A/cm2 at 1650 °C. Ultra-thin (~ 1 nm) BaF2 layers on semiconductor substrates have resulted in very low work function of 2.1 eV. These layers will be eventually deposited on the PV cell to enable the collection of electrons. The PV cell and the emitter will be separated by micron-gap distances by means of ZrO2 micro-spacers, which have already demonstrated both thermal and electrical insulation. PV cells based on GaAs and InGaAs semiconductors have been manufactured, demonstrating open-circuit voltages beyond 1 V (for GaAs) and 0.5 V (for InGaAs), as well as photogenerated current densities as high as 60 A/cm2 (for InGaAs). Both two-terminal and three-terminal PV devices have been fabricated, which will eventually lead to two different proof of concept experiments. To characterize the converters at high temperatures, new vacuum systems have been developed. Preliminary experiments in one of these systems have been able to demonstrate a voltage boost of TIPV with respect to a reference device using a non-PV p-type GaAs anode. This PV enhancement represents the first proof of concept of a TIPV converter. Current activities are directed towards the integration of all these optimized elements in a final device. Very preliminary results indicate that the use of micro-spacers and the incorporation of BaF2 coatings are able to effectively produce an increment of the operation voltage, as expected.
During the first two years of development, the project has clearly progressed beyond the state of the art. Some of the key results obtained so far are: i) new PCMs with record latent heats beyond 1 MWh/m3 at temperature of 1200 ºC, ii) new characterization techniques for liquid metals at ultra-high temperatures up to 1750 ºC, iii) better understanding of the chemical reactivity between refractories and liquid Si-B alloys at ultra-high temperatures, iv) novel numerical methods to analyze phase change processes, v) optimized processes to fabricate and characterize low work function coatings for thermionic emission, vi) optimized micro-spacers that enable thermally and electrically isolate micron-gap separation between highly non-isothermal surfaces, vii) new characterization methods for ultra-high temperature solid-state energy conversion devices, and viii) the first experimental proof of concept of a new hybrid thermionic-photovoltaic converter. At the end of the project we expect to demonstrate a new energy storage concept by the integration of all these elements into a first prototype of this kind.

Owing to the novelty of the concepts involved and the challenges addressed, this project aims at initiating research on a radically novel technology for energy storage, establishing its proof-of-principle. Thus, it is the aim of this project positioning this technology in the future roadmaps of energy storage, together with other well-known solutions such as batteries, super-capacitors, fly-wheels, etc. This technology has the potential of providing one of the highest energy density storage solutions. Hence, it is aimed at causing an impact on several sectors of the economy, such as energy generation (new generation of solar thermal power plants) and transmission (enlarging the share capacity of other renewable energies and reducing the transmission and distribution costs by facilitating more efficient grid management).
AMADEUS partners during a meeting in Krakov
The AMADEUS concept
Schematics of the AMADEUS system
Latent heat of the new Phase Change Materials
Map showing the location of AMADEUS partners