• Energy Research

Hybrid thermionic-photovoltaic (TIPV) converters are efficient and clean solutions for the direct conversion of thermal energy to electricity, taking advantage of both the photovoltaic and thermionic phenomena. An important hurdle for their efficient operation is the overheating of the PV cell integrated within the TIPV anode, due to partial conversion of the emitted electron and photon fluxes to thermal heat. This obstacle needs to be overcome with an efficient, yet practical, cooler. In this work, a copper plate heat spreader is experimentally tested for TIPV cathode temperatures up to 1450 degrees C, whilst its performance is also assessed using a validated CFD model for temperatures up to similar to 2000 degrees C. A multi-parametric analysis is conducted testing two coolants: i) a water/ethylene glycol mixture at various temperatures (-5-40 degrees C) and mass flow rates (0.05-0.4 kg.s(-1)), and, ii) cryogenic liquid nitrogen at a temperature of -196 degrees C and mass flow rate of 0.074 kg.s(-1). Numerical results reveal that with water/ethylene mixture the PV can withstand heat fluxes up to 360 W.cm(-2), without its temperature exceeding 100 degrees C. For higher thermal fluxes (360-600 W.cm(-2)), cryogenic liquid nitrogen is found to prevent the PV overheating and, therefore, is an attractive coolant; however, it poses safety concerns due to its possible boiling. Finally, two additional cooling system designs are proposed, a heat sink with straight fins and another with copper pipes, which offer higher heat transfer areas, but are more difficult to manufacture, than the copper plate heat spreader.

AMADEUS es un proyecto europeo que investiga materiales y dispositivos de estado sólido para almacenar energía a muy alta temperatura. Usando aleados de silicio como materiales de cambio de fase se alcanzan calores latentes superiores a 1000 kWh/m3, propiciando la obtención de altísimas densidades energéticas. Dichos aleados suponen temperaturas de almacenamiento por encima de los 1000 ºC, muy por encima de las de los sistemas actuales de acumulación térmica. El artículo describe las actividades del proyecto y sus primeros resultados, explicando los principales retos de este nuevo sistema que combina la acumulación de energía en forma de calor en silicio fundido con dispositivos de estado sólido termiónicos y termofotovoltaicos para la posterior conversión en electricidad. AMADEUS is a H2020 project that researches on materials and solid-state devices for very high temperature energy storage and conversion. By exploring silicon-based alloys as new phase change materials (PCMs), latent heat higher than 1000 kWh/m3 is achievable, which implies a very high energy density. In addition, silicon-based PCMs lead to storage temperatures well beyond 1000 ºC, well beyond that of current state-of-the-art thermal energy storage (TES). This paper describes the project R&D activities and first results, and comments on challenges towards a new kind of systems combining latent heat energy storage in molten silicon with thermionic and thermophotovoltaic solid state heat-to-power conversion.

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Starting in January 2017, AMADEUS (www.amadeus-project.eu) is the first project funded by the European Commission to research on a new generation of materials and solid-state devices for ultra-high temperature energy storage and conversion. By exploring storage temperatures well beyond 1000 ºC, one of the main objectives of the project is to create new PCMs (phase change materials) with latent heat in the range of 1000-2000 kWh/m, an order of magnitude greater than that of typical salt-based PCMs used in concentrated solar power (CSP), along with developing advanced thermal insulation, PCM casing designs, and novel solid-state thermal-to-electric energy conversion devices able to operate at temperatures in the range of 1000-2000 ºC. In particular, the project is investigating silicon-boron based alloys as PCMs and hybrid thermionic-photovoltaic (TIPV) devices for energy conversion. This paper describes the main project R&D activities and the results that have been attained during the first two years of the project. This includes the thermophysical characterization of Si-B alloys, the wettability and solubility analysis of said alloys with solid refractory materials, the numerical simulation of phase-change and heat losses through thermal insulation cover, as well as the realization of the two main proof-of-concept experiments: the TIPV converter, and the full latent heat energy storage system.

Dielectric microspacers (DMS) are important components in thermal energy converters. Engineered DMS are fabricated and characterized on different substrates by depositing patterned ceramic thin films of alumina (Al2O3) and zirconia (ZrO2) with a thickness ranging from 0.3 to 3 μm. Both Al2O3 and ZrO2 films are electrically and thermally optimized, finding zirconia more suitable as a thermal and electrical insulating material at high temperature, whereas the developed DMS are morphologically analyzed by scanning electron microscopy. The analysis of thermal simulations carried out with COMSOL Multiphysics allows identifying the best geometrical constraints for each single structure, whereas simulations carried out by the Fluent software allow identifying the best arrangement for DMS, leading to a solution with optimized pattern in terms of amount and spatial distribution so to achieve the required electrical and thermal insulation for practical applications. DMS are integrated within thermionic‐photovoltaic devices to be validated experimentally, and enhanced electron emission measurements are successfully performed at a cathode temperature up to 1350 °C to verify the operational feasibility and potential of this technology.

AMADEUS es un proyecto europeo que investiga materiales y dispositivos de estado sólido para almacenar energía a muy alta temperatura. Usando aleados de silicio como materiales de cambio de fase se alcanzan calores latentes superiores a 1000 kWh/m3, propiciando la obtención de altísimas densidades energéticas. Dichos aleados suponen temperaturas de almacenamiento por encima de los 1000 ºC, muy por encima de las de los sistemas actuales de acumulación térmica. El artículo describe las actividades del proyecto y sus primeros resultados, explicando los principales retos de este nuevo sistema que combina la acumulación de energía en forma de calor en silicio fundido con dispositivos de estado sólido termiónicos y termofotovoltaicos para la posterior conversión en electricidad.