Solid-state Energy Conversion Research Articles

Development of a high-temperature and high-uniformity micro planar combustor for thermophotovoltaics application

The micro-thermophotovoltaic (TPV) power generator is an important solid-state energy conversion system which utilizes high-temperature photons to generate electricity by the photovoltaics effect. Micro-combustor is an important component in this system. In order to obtain high system efficiency, a micro-combustor with high-temperature, high-uniformity and high-efficiency is strongly desired. Here we report a micro planar combustor with the desired features. The planar combustor is fabricated with two numerically optimized baffles which form a heat recirculation chamber. The thermal energy of the hot reacting gas could be utilized to preheat the fresh gas. The performances of the combustor under various operating conditions are investigated experimentally. Numerical simulation is performed to identify the flow field and mass fraction distribution in the planar combustor. A peak combustor wall temperature as high as 1354K is achieved with superior uniformity. By incorporating the designed micro planar combustor into the micro-TPV system, a system efficiency of 4.1% could be obtained. This system efficiency is twice higher than the system with only conventional planar combustor. As a result, the development of the planar combustor with baffles could pave a way to the development of high-performance of the micro-TPV system.

The effect of mechanical twisting on oxygen ionictransport in solid-state energy conversionmembranes.

Understanding 'electro-chemo-mechanics' in oxygen ion conducting membranes represents a foundational step towards new energy devices such as micro fuel cells and oxygen or fuel separation membranes. For ionic transport in macro crystalline electrolytes, doping is conventionally used to affect oxygen ionic association/migration energies. Recently, tuning ionic transport in films through lattice strain conveyed by substrates or heterostructures has generated much interest. However, reliable manipulation of strain states to twist the ionic conduction in real micro energy devices remains intractable. Here, we demonstrate that the oxygen ionic conductivity clearly correlates with the compressive strain energy acting on the near order of the electrolyte lattices by comparing thin-film ceria-based membrane devices against substrate-supported flat structures. It is possible to capitalize on this phenomenon with a smart choice of strain patterns achieved through microelectrode design. We highlight the importance of electro-chemo-mechanics in the electrolyte material for the next generation of solid-state energy conversion microdevices.

Accomplishments, Status, and Road Map for the US Department of Energy's Fossil Energy SOFC Program

The U.S. Department of Energy Office of Fossil Energy is charged with developing ultraclean (near-zero emissions), low-cost energy from fossil fuels to fuel economic prosperity, strengthen energy independence, and enhance environmental quality. A component of that effort, implemented by the National Energy Technology Laboratory, is the Solid State Energy Conversion Alliance, which is developing low-cost, highly efficient solid oxide fuel cell power systems that produce electric power from natural gas or coal with intrinsic carbon capture capabilities. In coordinated work involving industry, academia, and national laboratories, progress is being made in areas critical to technology commercialization, including seals, cells power density and stability, and low-cost interconnects. Cell tests have yielded acceptable performance and degradation rates. The program is now focusing on improving stack and system performance, reliability, and durability through progressively larger stack tests and integrated systems tests. Accomplishments, current program status, and the roadmap for continuing work are reviewed.

Towards a near-field concentrated solar thermophotovoltaic microsystem: Part I – Modeling

Modeling of a near-field concentrated solar thermophotovoltaic (STPV) microsystem is carried out to investigate the use of STPV-based solid-state energy conversion as high power density MEMS power generator. Near-field radiation can be realized between two closely separated surfaces (order of radiation characteristic wavelength), resulting in the enhancement of the heat radiation flux orders of magnitudes higher than the blackbody limit, consequently increasing cell output power density. The near-field STPV model incorporates a photonic crystal absorber which transfers absorbed concentrated solar radiation to a tungsten emitter. Thermal radiation from the emitter illuminates an In0.18Ga0.82Sb photovoltaic (PV) cell generating electrical power; waste heat is rejected from the backside of the PV cell via a microcooler. Based on the model, the near-field STPV performance is estimated for different emitter-to-PV cell separation distances dc, emitter temperatures Te, and emitter/absorber area ratios AR. Results from the numerical study showed significant enhancement of the heat fluxes due to tunneling of the near-field radiation, resulting in power densities as high as 60W/cm2 which is 30 times higher than the equivalent far-field power density for dc=20nm,Te=2000K and solar concentration of ×4350. For a emitter/absorber area ratio of AR=1, the emitter/absorber thermal efficiency and the overall solar to electrical conversion efficiency were 73% and 15.5%, respectively. Higher power densities are achievable (up to 50 times that of far-field values) however cooling requirements and solar concentration could be a concern.

Reduced Model and Application of Inflating Circular Diaphragm Dielectric Elastomer Generators for Wave Energy Harvesting

Dielectric elastomers (DE) are incompressible rubberlike solids whose electrical and structural responses are highly nonlinear and strongly coupled. Thanks to their coupled electromechanical response, intrinsic lightness, easy manufacturability, and low-cost, DEs are perfectly suited for the development of novel solid-state polymeric energy conversion units with capacitive nature and high-voltage operation, which are more resilient, lightweight, integrated, economic, and disposable than traditional generators based on conventional electromagnetic technology. Inflated circular diaphragm dielectric elastomer generators (ICD-DEG) are a special embodiment of polymeric transducer that can be used to convert pneumatic energy into usable electricity. Potential application of ICD-DEG is as power take-off system for wave energy converters (WEC) based on the oscillating water column (OWC) principle. This paper presents a reduced, yet accurate, dynamic model for ICD-DEG that features one kinematic degree of freedom and which accounts for DE visco-elasticity. The model is computationally simple and can be easily integrated into existing wave-to-wire models of OWCs to be used for fast analysis and real-time applications. For demonstration purposes, integration of the considered ICD-DEG model with a lumped-parameter hydrodynamic model of a realistic OWC is also presented along with a simulation case study.

Room-temperature sub-band gap optoelectronic response of hyperdoped silicon

Room-temperature infrared sub-band gap photoresponse in silicon is of interest for telecommunications, imaging and solid-state energy conversion. Attempts to induce infrared response in silicon largely centred on combining the modification of its electronic structure via controlled defect formation (for example, vacancies and dislocations) with waveguide coupling, or integration with foreign materials. Impurity-mediated sub-band gap photoresponse in silicon is an alternative to these methods but it has only been studied at low temperature. Here we demonstrate impurity-mediated room-temperature sub-band gap photoresponse in single-crystal silicon-based planar photodiodes. A rapid and repeatable laser-based hyperdoping method incorporates supersaturated gold dopant concentrations on the order of 10(20) cm(-3) into a single-crystal surface layer ~150 nm thin. We demonstrate room-temperature silicon spectral response extending to wavelengths as long as 2,200 nm, with response increasing monotonically with supersaturated gold dopant concentration. This hyperdoping approach offers a possible path to tunable, broadband infrared imaging using silicon at room temperature.

The prospect of high temperature solid state energy conversion to reduce the cost of concentrated solar power

At extreme temperatures, solid state energy conversion devices may be able to survive, because they do not involve moving parts.

SECA Program Overview and Status

The overall objective of the Solid State Energy Conversion Alliance (SECA), a U.S. Department of Energy program coordinated and managed by the National Energy Technology Laboratory, is the development of advanced power generation system technology. Specifically, the systems being developed, for central-station (>100 MWe) application, and based on solid oxide fuel cell (SOFC) technology, are to be cost-effective and operate with high electric efficiency. Further, their designs must provide for effective carbon (CO2) capture, and they must restrict the emission of other pollutants (eg, Hg, NOx, SOx) and conserve water. Historically, the fuel basis for this development has been coal, our major domestic energy source. This continues to be the focus. However, the program is cognizant that the SOFC technology developed to meet design objectives could be adapted for implementation in advanced power generation systems fueled with natural gas. Thus, there could be strong synergy between efforts to develop advanced coal-fueled power generation, as in the SECA program, and any parallel effort by the program participants to develop natural gas-fueled distributed-generation SOFC power systems. A natural gas-fueled distributed power generation system could be first to the marketplace, which would provide early manufacturing and operational experience on large commercial scale that would benefit SOFC power system developments with both fuels. SECA program status is reviewed.

Manufacturing Challenges in Wide Band Gap (WBG) Power Electronics

Wide bandgap (WBG) semiconductors, Silicon Carbide (SiC) and Gallium Nitride (GaN), have enormous potential for transformative impact on electric utility and transportation infrastructures. However, commercialization of WBG power electronics switching devices has been slow primarily due to high cost and unproven field-reliability. This paper reviews the manufacturing challenges facing the WBG power electronics industry in order to “unlock” the true potential of this “game changing” solid-state energy conversion technology.

Amphoteric oxide semiconductors for energy conversion devices: a tutorial review

In this tutorial review, we discuss the defect chemistry of selected amphoteric oxide semiconductors in conjunction with their significant impact on the development of renewable and sustainable solid state energy conversion devices. The effect of electronic defect disorders in semiconductors appears to control the overall performance of several solid-state ionic devices that include oxide ion conducting solid oxide fuel cells (O-SOFCs), proton conducting solid oxide fuel cells (H-SOFCs), batteries, solar cells, and chemical (gas) sensors. Thus, the present study aims to assess the advances made in typical n- and p-type metal oxide semiconductors with respect to their use in ionic devices. The present paper briefly outlines the key challenges in the development of n- and p-type materials for various applications and also tries to present the state-of-the-art of defect disorders in technologically related semiconductors such as TiO(2), and perovskite-like and fluorite-type structure metal oxides.

Near-Field Radiative Transfer Between Heavily Doped SiGe at Elevated Temperatures

SiGe alloys represent an important type of high-temperature semiconductor material for solid-state energy conversion. In the present study, the near-field radiative heat transfer between heavily doped SiGe plates is investigated. A dielectric function model is formulated based on the previously reported room-temperature mobility and temperature-dependent electric resistivity of several silicon-rich alloys with different doping type and concentration. Fluctuational electrodynamics is used to evaluate the near-field noncontact heat transfer coefficient. The variation of the heat transfer coefficient with doping concentration and temperature is explained according to the change in the optical constants and in the spectral distribution of the near-field heat flux.

Solid Oxide Fuel Cell Stack Module Development for Fuel Flexible Power Generation

FuelCell Energy is developing solid oxide fuel cell (SOFC) systems and technology for very efficient, economically viable, coal-to-electricity power plants utilizing synthesis gas (syngas) from a coal gasifier for the Department of Energy Solid State Energy Conversion Alliance (SECA) program. SOFC technology development includes cell performance enhancement, stack building block scale-up, stack tower design and testing, and development of a 250 kW module accommodating multiple stack towers.

Overcoming the black body limit in plasmonic and graphene near-field thermophotovoltaic systems

Near-field thermophotovoltaic (TPV) systems with carefully tailored emitter-PV properties show large promise for a new temperature range (600 – 1200K) solid state energy conversion, where conventional thermoelectric (TE) devices cannot operate due to high temperatures and far-field TPV schemes suffer from low efficiency and power density. We present a detailed theoretical study of several different implementations of thermal emitters using plasmonic materials and graphene. We find that optimal improvements over the black body limit are achieved for low bandgap semiconductors and properly matched plasmonic frequencies. For a pure plasmonic emitter, theoretically predicted generated power density of 14 W/cm2 and efficiency of 36% can be achieved at 600K (hot-side), for 0.17eV bandgap (InSb). Developing insightful approximations, we argue that large plasmonic losses can, contrary to intuition, be helpful in enhancing the overall near-field transfer. We discuss and quantify the properties of an optimal near-field photovoltaic (PV) diode. In addition, we study plasmons in graphene and show that doping can be used to tune the plasmonic dispersion relation to match the PV cell bangap. In case of graphene, theoretically predicted generated power density of 6(120) W/cm2 and efficiency of 35(40)% can be achieved at 600(1200)K, for 0.17eV bandgap. With the ability to operate in intermediate temperature range, as well as high efficiency and power density, near-field TPV systems have the potential to complement conventional TE and TPV solid state heat-to-electricity conversion devices.

Enabling high-temperature nanophotonics for energy applications

The nascent field of high-temperature nanophotonics could potentially enable many important solid-state energy conversion applications, such as thermophotovoltaic energy generation, selective solar absorption, and selective emission of light. However, special challenges arise when trying to design nanophotonic materials with precisely tailored optical properties that can operate at high-temperatures (> 1,100 K). These include proper material selection and purity to prevent melting, evaporation, or chemical reactions; severe minimization of any material interfaces to prevent thermomechanical problems such as delamination; robust performance in the presence of surface diffusion; and long-range geometric precision over large areas with severe minimization of very small feature sizes to maintain structural stability. Here we report an approach for high-temperature nanophotonics that surmounts all of these difficulties. It consists of an analytical and computationally guided design involving high-purity tungsten in a precisely fabricated photonic crystal slab geometry (specifically chosen to eliminate interfaces arising from layer-by-layer fabrication) optimized for high performance and robustness in the presence of roughness, fabrication errors, and surface diffusion. It offers near-ultimate short-wavelength emittance and low, ultra-broadband long-wavelength emittance, along with a sharp cutoff offering 41 emittance contrast over 10% wavelength separation. This is achieved via Q-matching, whereby the absorptive and radiative rates of the photonic crystal's cavity resonances are matched. Strong angular emission selectivity is also observed, with short-wavelength emission suppressed by 50% at 75° compared to normal incidence. Finally, a precise high-temperature measurement technique is developed to confirm that emission at 1,225 K can be primarily confined to wavelengths shorter than the cutoff wavelength.

Recent developments in high-temperature photonic crystals for energy conversion

After decades of intense studies focused on cryogenic and room temperature nanophotonics, scientific interest is also growing in high-temperature nanophotonics aimed at solid-state energy conversion. These latest extensive research efforts are spurred by a renewed interest in high temperature thermal-toelectrical energy conversion schemes including thermophotovoltaics (TPV), solar‐ thermophotovoltaics, solar‐thermal, and solar‐thermochemical energy conversion systems. This field is profiting tremendously from the outstanding degree of control over the thermal emission properties that can be achieved with nanoscale photonic materials. The key to obtaining high efficiency in this class of high temperature energy conversion is the spectral and angular matching of the radiation properties of an emitter to those of an absorber. Together with the achievements in the field of highperformance narrow bandgap photovoltaic cells, the ability to tailor the radiation properties of thermal emitters and absorbers using nanophotonics facilitates a route to achieving the impressive efficiencies predicted by theoretical studies. In this review, we will discuss the possibilities of emission tailoring by nanophotonics in the light of high temperature thermal-to-electrical energy conversion applications, and give a brief introduction to the field of TPV. We will show how a class of large area 2D metallic photonic crystals can be designed and employed to efficiently control and tailor the spectral and angular emission properties, paving the way towards new and highly efficient thermophotovoltaic systems and enabling other energy conversion schemes based on high-performance high-temperature nanoscale photonic materials.

Nano-scale vacuum spaced thermo-tunnel devices for energy harvesting applications

ABSTRACTThe output power-density and the efficiency of thermo-tunnel devices are examinedas a function of inter-electrode separation, electrode work-function, and temperature. We find that these physical parameters dramatically influence the device characteristics, and under optimal conditions a thermo-tunnel device is capable of delivering a very high output power-density of ∼ 103Wcm−2. In addition, at higher temperatures, the heat-conversion efficiency of the thermo-tunnel device approaches ∼ 10%, comparable to that of a thermoelectric generator. We therefore propose that thermo-tunnel devices are promising for solid-state thermal energy conversion.

Role of solid oxidefuel cells in a balanced energy strategy

The 2012 Department of Energy (DOE) budget request significantly reduces fuel cell RD&D funding and would cease to support the Solid State Energy Conversion Alliance (SECA), DOE's solid oxide fuel cell (SOFC) program. This would be a grave mistake considering the US's historic dominance in fuel cell RD&D, exciting recent technological advancements in fuel cells, and clear positive market signals around the globe. In this paper we discuss DOE's energy RD&D policy, how SOFCs address every key DOE strategy, and why recent advances should make SOFCs an integral part of our energy RD&D policy. Moreover, we compare the prospects of low temperature SOFCs with the more common proton exchange membrane fuel cell (PEMFC) in the absence of a hydrogen infrastructure.

Development of Solid Oxide Fuel Cells at Versa Power Systems

Versa Power Systems (VPS) is a developer of solid oxide fuel cells (SOFCs) for clean power generation. VPS has been working with its development partner FuelCell Energy within the U.S. Department of Energy (DOE) Office of Fossil Energy's Solid State Energy Conversion Alliance (SECA) program to apply SOFC technology in large-scale power plant systems. As a result, initiatives such as scale-up of the cell and stack are being actively pursued for these applications.

Recent Developments in the SECA Program

The development of an electric power generation technology that effectively uses coal, our major domestic energy source, and meets environmental requirements and water conservation needs, is of crucial importance to the United States. Work to achieve that objective is being done by the U.S. Department of Energy Office of Fossil Energy. Embodied in the Solid State Energy Conversion Alliance (SECA) program, the development of the required power generation technology is centered on the solid oxide fuel cell as the key enabling technology, and is being done at the National Energy Technology Laboratory. The overall SECA program goal is the development of power generation technology for cost-effective, high-efficiency, central-station (>100 MWe) power systems that are equipped for effective carbon capture, and which restrict to specified levels the emission of other pollutants such as mercury, NOx, and SOx. SECA program status and accomplishments are reviewed.

Nonadiabatic Chemical-to-Electrical Energy Conversion in Catalytic Schottky Junction Nanostructures

ABSTRACTNonadiabatic energy dissipation by electron subsystem of nanostructured solids unveil interesting opportunities for the solid-state energy conversion and sensor applications. We found that planar Pd/n-SiC, Pt/n-GaP and Pd/n-GaP Schottky structures with nanometer thickness metallization demonstrates a nonadiabatic channel for the conversion into electricity the energy of a catalytic hydrogen-to-water oxidation process on the metal layer surface. The observed abovethermal current greatly complements the usual thermionic emission current, and its magnitude is linearly proportional to the rate of formation and desorption of product water molecules from the nanostructure surface. The possibilities and advantages of utilizing the nonadiabatic functionality in a novel class of chemical-to-electrical energy conversion devices are discussed. The technology has a potential for a very high volumetric energy density due to the intrinsically planar device architecture.