High temperature ceramics are used in a variety of harsh environments due to their mechanical strength and thermal stability. However, applications requiring high emissivity and/or absorptivity typically rely on “dark” ceramics, such as carbides, which are often expensive and complex to manufacture. In this work, a nanosecond pulsed laser is used to texture the surface of alumina, drastically increasing its emissivity in the near- and mid-infrared regions to levels comparable to those of silicon carbide. Emissivity was determined from reflectivity and transmissivity measurements at room temperature, and these measurements were used to estimate a hemispherical emissivity of up to 0.9 at 1000 °C, comparable to that of intrinsically “dark” ceramics such as graphite and silicon carbide. This technique offers a rapid and cost-effective method for enhancing the emissivity of alumina, thereby expanding its potential for applications requiring increased thermal radiation or absorption, such as emitters for thermophotovoltaic systems or absorbers in concentrated solar technologies.

This study assesses the role of Power-to-Heat-to-Power Storage (PHPS) systems, also known as Carnot batteries, in a stylized national energy system of a wind-dominated (Denmark) and a solar-dominated region (Spain). Using the open-source PyPSA framework, we model sector-coupled electricity and heating systems and evaluate individual (decentralized) and district (centralized) configurations for heat provision at the residential level, with and without PHPS waste heat recovery. The results show that PHPS contribution is more pronounced in wind-dominated systems with decentralized heating, where PHPS emerges as the dominant storage option. PHPS also plays a significant role in other configurations, including wind-dominated regions with centralized heating and solar-dominated regions with decentralized heating. Across all scenarios, waste heat recovery proves essential to enhancing PHPS competitiveness, in some cases enabling it to partially displace lithium-ion batteries. Sensitivity analysis highlights that PHPS viability depends primarily on achieving low energy capacity costs (l < 10 €/kWh) and maintaining high heat-to-power conversion efficiencies (>30%). Overall, while PHPS complements existing storage technologies by providing dispatchable electricity and heat, its potential adoption is highly context-dependent, influenced by climate, heating system configuration, and competing storage costs.

Concentrated solar power coupled with thermal energy storage enables dispatchable solar electricity production, a key step towards the penetration of solar energy in the renewable energy mix. However, the high cost and moderate operating temperatures (similar to 565 degrees C) of current systems limit their competitiveness compared to photovoltaic-battery solutions. To overcome these limitations, this study introduces a conceptual solar-powered latent heat thermophotovoltaic battery coupled with beam-down concentrated solar optics for ultra-high temperature (similar to 1157 degrees C) energy storage and conversion. Designed with a modular architecture, the system allows for scalability and flexibility, enabling configurations that meet diverse energy demands. Operating in four distinct modes - charging, discharging, simultaneous charging/discharging, and storage - the system addresses critical challenges associated with modular solar thermal power systems. Minimizing lateral heat and solar-aperture re-emission losses and selecting thermophotovoltaic cells with band gaps tailored to the emitter temperature are key elements for an optimized system efficiency. Furthermore, the system's dimensions significantly influence the optical, thermal and conversion performance, affecting radiation distribution, thermal insulation losses, and overall efficiency. A multi-physics model coupling optical, thermal and electric analyses reveals that, by minimizing the lateral thermal losses (<10 %) and maintaining a solar absorber efficiency over 65-70 %, the system can achieve round-trip efficiencies above 20 % for configurations with crucible heights between 0.48-0.72 m and thermophotovoltaic cell band gaps of 0.74 eV. These results demonstrate that the proposed system offers a compact, high-temperature, and dispatchable alternative to molten-salts, paving the way for next-generation solar energy storage and conversion.

Latent-heat thermophotovoltaic (TPV) batteries offer a pathway for long-duration energy storage, but their scalability is limited by the low thermal conductivity of high-temperature phase change materials (PCMs), which restricts heat extraction during discharge as a solid crust forms. This work presents an idealized theoretical analysis of a TPV battery architecture employing optically transparent PCMs to enable combined conductive and radiative heat transfer through the storage medium. A quasi-one-dimensional model is used to compare opaque PCMs with two idealized transparency scenarios: fully transparent and phase-dependent (transparent solid, opaque liquid). Under the simplifying assumptions of perfect transparency, blackbody emitters, and adiabatic boundaries, the model predicts that transparency can sustain higher emitter temperatures during discharge and increase the average power density by mitigating the thermal resistance of the solid crust. A parametric sensitivity analysis further examines the influence of key PCM properties—including thermal conductivity, refractive index, and effective radiative attenuation—showing that the qualitative performance advantages of transparency persist over a relatively broad parameter range. These conditions relax the conventional trade-off between PCM thickness and power output, establishing an upper bound on the performance improvements that transparency could provide. Since no currently known high-temperature PCM satisfies these ideal transparency assumptions, the results should be interpreted as theoretical limits intended to guide future searches for materials with partial or wavelength-dependent transparency under operating conditions.

Fully electrifying the building sector requires not only the widespread adoption of photovoltaic (PV) self-consumption and heat pumps, but also the integration of cost-effective energy storage solutions. Hybridizing lithium-ion (Li-ion) batteries with power to heat to power storage (PHPS) systems, thermal batteries capable of thermal-to-electric energy conversion, offers a promising and economically viable solution. PHPS systems dispatch combined heat and power by utilizing the low-temperature waste heat generated during the thermal to electric energy conversion process. This study investigates the technoeconomic impacts of waste heat use in PHPS systems integrated with Li-ion batteries and heat pumps to support the decarbonization of the building sector. Two distinct strategies are evaluated: direct use of waste heat to meet heating demands; and the use of waste heat to enhance the heat pump's coefficient of performance. Results show that supplying the waste heat at the demand setpoint temperature is the best solution to integrate PHPS cost-effectively, although enhancing the heat pump's COP with waste heat also yields notable economic gains. Additionally, leveraging waste heat significantly lowers the minimum thermal-to-electric conversion efficiency required for PHPS systems to achieve economic viability. Optimal PHPS designs enable large-scale energy storage and charging capacities, thereby enhancing PV self-consumption rates and reducing the levelized cost of energy. The analysis also reveals that hybridizing PHPS with Li-ion batteries may rise as the optimal solution for moderately priced PHPS systems, with the reduction in levelized cost being more pronounced in solar-dominated regions.

Thermionics and thermophotovoltaics are solid-state technologies that convert high-temperature heat into electricity by utilizing fundamental particles, electrons in thermionics and photons in thermophotovoltaics, as energy carriers. Both systems have the potential to achieve high efficiency and power density, contingent on the optimization of radiative/electronic energy fluxes. A critical factor in enhancing energy flux in these devices is the introduction of microscale (thermionics) or nanoscale (thermophotovoltaics) gaps between the hot thermal emitter and the cooler receiver. In thermionic converters, microscale gaps mitigate space charge effects that create energy barriers to electron flow. For thermophotovoltaic systems, nanoscale gaps facilitate photon tunneling, significantly boosting photon flux towards the thermophotovoltaic cell. Forming these small-scale gaps often necessitates intermediate materials or spacers between the emitter and receiver. Over the past few decades, various spacer designs have been proposed and studied, demonstrating their effectiveness in enhancing energy transfer and conversion. However, challenges remain regarding their reliability and scalability. This article provides a comprehensive overview of spacer technologies for thermionics and thermophotovoltaics and summarizes recent advancements, current capabilities, and persistent challenges.

A promising solution to fully decarbonize the energy consumption of buildings consists of hybridizing solar PV installation with lithium-ion (Li-ion) batteries and heat pumps. However, the high capital cost per unit of energy storage of Li-ion batteries often results in systems with relatively small storage capacities, leading to low selfconsumption ratios. Thermal batteries with power generation capacity, such as Power-to-heat-to-power storage (PHPS), leverage the significantly lower cost of thermal energy storage to increase the overall storage capacity of the system. In addition, PHPS systems generate heat as a by-product during the energy conversion, which can be used directly in the building to supply its heating demand. The goal of this study is to assess the profitability of integrating PHPS systems with heat pumps and Li-ion batteries. A techno-economic analysis, based on a fully-electrified building, demonstrates that the hybridization of PHPS and Li-ion batteries yields the lowest levelized cost of consumed energy, regardless of the coefficient of performance (COP) of the heat pump. This hybrid configuration takes advantage of the lower cost of the energy subsystem of PHPS, which is mostly used for baseload power generation (long duration discharge), and the higher efficiency and lower cost of power capacity of Li-ion batteries, which are optimized for peak power generation (short-duration discharge). Under the assumptions of this study, hybrid solution reduces the levelized cost of consumed energy by 7 % compared to a system relying solely on Li-ion batteries, while simultaneously increasing PV self-consumption by up to 20 %.

Ultra-high temperature thermal energy storage (UHTES) and conversion is an emerging field of technology that enables much higher energy densities (>1 MWth) and conversion efficiencies than conventional thermal energy storage technologies. Our research group of Solar Energy Institute is currently developing a novel latent heat thermophotovoltaic (LHTPV) battery that utilizes Si-based alloys to store either surplus renewable electricity or concentrated sunlight in the form of latent heat at temperatures close to 1200 ºC and convert it back to electricity on demand. Determining the State of Charge (SoC) of this ultra-high temperature thermal battery is imperative to regulate its real-time operation and optimize its performance. However, using of several sensors within the storage system –as mainly done in low temperature phase change materials (PCMs) to quantify their SoC - becomes costly and challenging for this range of operating conditions. This study presents a numerical method, which is used to get an understanding of the physical processes taking place during the LHTPV operation and capture comprehensive data of time varying flow variables that can be difficult to record during real-time operation. Our results indicate that we can describe the system’s SoC by measuring the time-varying temperature at its sidewalls and the input/output heat flux values, without the need of knowing beforehand the thermophysical properties of the used materials. Based on these variables we can define several indicators that can help us obtain a better understanding of the required physical signals to be measured in order to determine its SoC, during real-time operation.

Thermophotovoltaic (TPV) energy conversion has gained significant attention in recent years, leading to the funding of numerous research and business initiatives, particularly in Europe and the US. This growing interest stems for the remarkable efficiency of TPV devices, which can convert radiant heat directly into electricity with efficiencies exceeding 40 %. This positions TPV as the most efficient solid-state heat engine, with the potential to outperform traditional turbogenerators. This article explores the techno-economic parameters essential for the profitability of TPV systems and examines their primary applications and future research challenges. It begins by outlining the key factors influencing TPV viability, including electric power density, cost per unit area, and TPV cell efficiency. Using the levelized cost of electricity as a central framework, the study identifies suitable applications for TPV technology, emphasizing its potential in military and space applications, waste heat recovery, and thermal energy storage. Finally, the article addresses ongoing research challenges, detailing innovations in TPV technology that could significantly reduce costs and expand its applicability across a wider range of heat source temperatures. The study concludes that the development of TPV batteries-systems that store surplus electricity as ultra-high-temperature heat and reconvert it into electricity using TPV technology-represents a unique opportunity for TPV to move beyond niche applications and toward broader commercial viability.

Crystalline germanium (c-Ge) has historically been regarded as a cost-effective alternative to III-V semiconductors for thermophotovoltaic (TPV) device fabrication. However, Ge-based devices have not yet reported high efficiencies, partially due to the lack of an efficient back-surface reflector that turns back to the heat source out-band (sub-bandgap) thermal radiation. The difficulty of implementing back surface reflectors in Ge TPV cells is related to the simultaneous requirement of good back surface passivation, low electrical resistivity, and high out-band optical reflectivity. In this study, we demonstrate a highly reflective ohmic contact to p-type c -Ge (doping concentration of 2 x 1015 cm-3) made of an aSiCx(1 nm)/Al2O3 (50 nm)/aSiC (45 nm) stack that is laser processed using Nd:YVO4 laser emitting at 355 nm to create punctual p+ contacts (locally diffused Al regions). This stack is finally caped with a thick (1000 nm) Al layer that behaves as a metallic mirror and back electrode. As the laser processed area increases from 0.1 to 3 %, which is the typical range in the final devices, the surface recombination velocity increase from 10.5 to 60.0 cm/s, while the effective contact resistance reduces from 0.462 to 0.036 omega cm2. Moreover, a sub-bandgap reflectance of 90-98 % is achieved. Simulations assuming ideal device configuration indicate that implementing these back contacts could potentially enable TPV cell conversion efficiencies comparable to the reported high-efficiency c-Ge TPV cells operating at similar illumination temperature.

Thermophotovoltaic (TPV) energy conversion efficiency has recently surpassed 30%. The key behind such high efficiency is the inclusion of a highly efficient mirror in the rear of the TPV cell that turns back to the thermal emitter the outband energy photons. Efficiencies over 50% could be theoretically attainable by approaching a mirror reflectance of 100%. However, the very few percent of outband absorption significantly deteriorate the conversion efficiency, especially at low emitter temperatures. Thus, current research focuses on developing advance mirror designs able to reach an extreme high outband reflectance over 95%. In this article I propose a bifacial TPV cell that enables very efficient photon recycling without using mirrors and that is less sensitive to outband optical losses. The key to this design is that the cell is introduced in a thermal emitter enclosure where it is irradiated from both sides. Then, outband photons transmit through the cell and are reabsorbed in the emitter. Therefore, the optical losses linked to the mirror/cell interface are eliminated, potentially enabling higher photon recycling efficiencies. This article presents a detailed balance simulation of an edge-cooled bifacial TPV cell to demonstrate that bifacial configuration enables higher conversion efficiencies and twice much as power density than monofacial designs, the latter being an advantage for moderate temperature and low-cost TPV power generation. Therefore, bifacial TPV cells are appealing for developing practical high-efficient and low-cost TPV devices for power generation in an extended range of heat source temperatures.

A standardized method for measuring thermophotovoltaic (TPV) efficiency has not been yet established, which makes the reported results difficult to compare. Besides, most of the TPV efficiencies reported to date have been obtained using small view factors, i.e., large cell-to-emitter distances, so the impact of the series resistance is usually underestimated, and the optical cavity effects, i.e., the multiple reflections taking place between the emitter and the cell, are not accounted for experimentally. In this work, we present an experimental setup that is able to measure the TPV efficiency under high view factors (up to 0.98) by using small emitter-to-cell distances (<1 mm). This allows a more accurate direct measurement of the TPV efficiency at higher power densities than previous works. As a result, a TPV efficiency of 26.4 +/- 0.1% and a power density of 4.3 +/- 0.8 W/cm2 have been obtained for an InGaAs TPV cell with a back surface reflector irradiated by a graphite thermal emitter at 1592 degrees C.

Thermophotovoltaics (TPV) is the direct conversion of radiant heat into electricity through the photovoltaic effect. In space applications, TPV has been primarily investigated for the development of radioisotope power systems that could be used in missions where the solar resource is too weak or intermittent, such as deep-space or planetary settlements. Other possible applications include solar-powered thermal generators, small-scaled fission reactors, or hybridization with solar thermal propulsion systems. The main advantages of TPV are its high efficiency, the lack of moving parts, and the fact that it produces DC power. The main drawbacks are unproven reliability and the requirement of low rejection temperatures, which necessitate the use of large heat radiators. This chapter reviews the state of the art of TPV, along with other technologies for converting thermal to electrical energy that are being developed within the framework of space power research programs.

Latent heat thermophotovoltaic (LHTPV) batteries store electricity as latent heat at very high temperatures (1,000 degrees C) and convert this heat back to electricity on demand using thermophotovoltaics (TPVs). In this study, we discuss the techno-economics of LHTPV systems, focusing on parameters such as the round-trip efficiency, energy-to-power ratio, cost per energy and power capacities, and levelized cost of storage. The very low cost of the heat storage media (<4 euro/kWh) results in optimal designs with high energy-to-power ratios, fitting long-duration storage (LDS) applications. Shorter duration storage applications are also possible by increasing the overall round-trip conversion efficiency through cogeneration, that is, combined heat and power (CHP) generation. Results indicate that LHTPV systems can provide lower levelized cost of storage than Li-ion batteries in both LDS and CHP applications. Preliminary experimental results are provided to illustrate the real operation of a LHTPV system.

This chapter presents a general description of systems that store energy in the form of heat at temperatures above 1000 °C and then convert that thermal energy into electricity on demand. These extreme temperatures allow for very high thermal-to-electrical conversion efficiencies and storage energy densities, ultimately resulting in lower costs of energy and power capacities. High efficiency and energy density enable very compact system designs suitable for decentralized storage applications, and the low cost of energy capacity allows its use in long-duration storage applications characterized by high energy-to-power capacity ratios. Ultra High Temperature Thermal Energy Storage (UH-TES) systems can store solar energy, high temperature waste heat or electricity, and deliver both heat and electricity on demand. Therefore, they are also a versatile solution for combined heat and power (CHP) generation. This chapter provides an overview of the technical and economic aspects of emerging UH-TES systems.

Hybrid thermionic-photovoltaics (TIPV) are solid-state thermal-to-electric energy converters that rely on the non-isothermal transport of photons and electrons through a vacuum gap. In contrast to pure thermionic converters, the absorption of photons in a photovoltaic anode produces an electrochemical potential that can be delivered as electricity, ultimately boosting the power generation capacity of the device. In this work, the proof of concept of a three-terminal TIPV converter where thermionic and photogenerated currents are collected independently is reported. Thermionic electrons are injected in the conduction band of a semiconducting anode (n-type InP), from where they are directly extracted. Photogenerated electrons are also extracted from the conduction band of the anode, but they are then reinjected in the valence band through an independent hole-selective contact (p-type InGaAs). By using a low workfunction engineered anode (BaF x /InP) and cathode (Sc x O y /W) a maximum power generation capacity of 125.6 and 0.35 mW cm −2 for PV and thermionic sub-devices, respectively, is demonstrated, operating at 1400 °C. This proof of concept paves the way for the development of efficient hybrid thermionic and photovoltaic converters for the direct conversion of heat into electricity, and subsequently contributes to finding an efficient alternative to thermoelectric generators.