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.
