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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Hydrogen from Sunlight and Water: A Side-by-Side Comparison between Photoelectrochemical and Solar Thermochemical Water-Splitting

Photoelectrochemical (PEC) and solar thermochemical (STCH) water-splitting represent two promising pathways for direct solar hydrogen generation. PEC water-splitting integrates multiple functional materials and utilizes energetic electrons and holes generated from sunlight to produce hydrogen and oxygen in two half-reactions, while STCH water-splitting couples a series of consecutive chemical reactions and uses absorbed heat from sunlight to generate hydrogen and oxygen in two full reactions. In this Focus Review, the basic operating principles, sunlight utilization, device architecture, reactor design, instantaneous and annually averaged solar-to-hydrogen (STH) conversion efficiency, and the operating conditions and constraints of both pathways are compared. A side-by-side comparison addresses some common sources of confusion and misinterpretation, especially in the evaluation of STH conversion efficiencies, and reveals distinct features and challenges in both PEC and STCH technologies. Furthermore, this Focus Review also addresses materials and device challenges in PEC and STCH for cost-competitive hydrogen generation.

08 HYDROGEN↗
DOE OSTI2022

Protecting ice from melting under sunlight via radiative cooling

As ice plays a critical role in various aspects of life, from food preservation to ice sports and ecosystem, it is desirable to protect ice from melting, especially under sunlight. The fundamental reason for ice melt under sunlight is related to the imbalanced energy flows of the incoming sunlight and outgoing thermal radiation. Therefore, radiative cooling, which can balance the energy flows without energy consumption, offers a sustainable approach for ice protection. Here, we demonstrate that a hierarchically designed radiative cooling film based on abundant and eco-friendly cellulose acetate molecules versatilely provides effective and passive protection to various forms/scales of ice under sunlight. This work provides inspiration for developing an effective, scalable, and sustainable route for preserving ice and other critical elements of ecosystems.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2018

The role of iron-oxide aerosols and sunlight in the atmospheric reduction of Hg(II) species: A DFT+ U study

Experimental and field measurements have shown that, in the presence of both iron-containing aerosols and sunlight, oxidized mercury species such as HgCl 2 and HgBr 2 undergo reduction to elemental mercury (Hg°), which remains in the atmosphere longer than oxidized mercury species due to its higher volatility. We performed density functional theory (DFT, PW91+U) calculations to elucidate the reduction mechanism for atmospheric HgCl 2 and HgBr 2 to Hg° on several iron-oxide aerosol surfaces relevant in the troposphere. On the OH-Fe-R-terminated α-Fe 2 O 3 (0001) surface, predicted to be most prevalent under ambient conditions, we show that: (1) the first Hg-X bond is broken via either thermal or photolytic activation depending on the ambient temperature; (2) photons with an energy of 2.69 eV (461 nm) are required to break the second Hg-X bond; and (3) a photo-induced surface-to-adsorbate charge-transfer process can promote Hg° desorption with an excitation energy of 2.59 eV (479 nm). All the calculated excitation energies are below the threshold value of 3.9 eV (320 nm) for photons in the troposphere, suggesting that sunlight can facilitate mercury reduction on iron-oxide aerosol surfaces. In contrast, the gas-phase reduction of HgCl 2 (HgBr 2 ) involves photoexcitation requiring an energy of 4.98 (4.45) eV (249 (279) nm); therefore, the energy range of sunlight is not suitable for gas-phase reduction. Our computational results provide the first evidence on the detailed mechanism for the combined role of aerosols and photons in the reduction of HgCl 2 and HgBr 2 . In conclusion, our methodology can be adapted to study other photochemical heterogeneous processes in the atmosphere.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Hybrid Solar Cells with a Sunlight Concentrator System

Hybrid solar cells based on InGaP/Ga(In)As/Ge multijunction structures integrated into crystalline Si heat-removal base and provided with sunlight concentrator system based on linear Fresnel lenses and carboplastic mount structure have been developed and investigated. The hybrid solar cells with sunlight concentrators in the photovoltaic module provide a specific electric power of 390 W/m{sup 2} (AM0, 1367 W/m{sup 2}) at a photoconverter unit specific weight reduced to 1.0 kg/m{sup 2}. Improved photovoltaic characteristics and high radiation resistance allow using the proposed hybrid solar modules with sunlight concentrators in space solar batteries and autonomous power supply facilities.

14 SOLAR ENERGY↗
DOE OSTI2019

Sunlight collection system with lightguide having solid and fluid components

The system captures and concentrates sunlight for transmission to interior spaces or to a PV system. A solar collector uses arrayed refractive lenses and opposing concave focusing mirrors and a movable coupling sheet forming part of a lightguide. The transparent sheet contains small angled mirrors, where each angled mirror corresponds to a particular set of the lenses/focusing mirrors and is in the focal plane. The lightguide also includes a fluid surrounding the transparent sheet, and lower index cladding layers sandwich the fluid. The sheet is translated within the fluid by an actuator to position the angled mirrors at the focal points of the sunlight for maximum deflection of the sunlight to an edge of the lightguide for extraction to a light transmission system or to a PV system. A position sensor on the sheet provides feedback regarding the position of the angled mirrors relative to the focal points.

Kozodoy, Peter↗
DOE OSTI2019

Sunlight collection system with asymmetric catadioptric optics

The system captures and concentrates sunlight for transmission to interior spaces or to a PV system. A solar collector uses arrayed refractive lenses, opposing concave focusing mirrors, and a movable coupling sheet forming part of a lightguide. The lenses and mirrors have an asymmetric shape, such as having aspect ratios of 3:4 or 1:2, so as to have an asymmetric aperture to better receive light at the different ranges of angles of the sun's rays over the course of a year. The long axis of the apertures is generally oriented in an East-West. The movable sheet contains small angled mirrors, and the sheet is translated to position the angled mirrors at the focal points of the sunlight for maximum deflection of the sunlight to an output of the collection system. A position sensor provides feedback regarding the position of the angled mirrors relative to the focal points.

Kozodoy, Peter↗
DOE OSTI2017

Photocatalytic Degradation of the Nanocomposite Film Comprising Polyvinyl Chloride (PVC) and Sonochemically Synthesized Iron-Doped Zinc Oxide: A Comparative Study of Performances Between Sunlight and UV Radiation

Photocatalytic degradation of a composite film made of PVC and Fe-doped ZnO nanoparticles under both UV and sunlight have been accomplished and compared. The metal-doped semiconductor nanoparticles were previously synthesized using ultrasound. The nanoparticles had average crystal size of 33.67 nm and a bandgap of 2.69 eV. Catalysts loading were varied. Composite film with 0.033 g Fe-ZnO per g PVC was found to be the optimum with respect to degradation and mechanical properties. Degradation was monitored by loss in weight. Under sunlight 12% degradation could be achieved with the composite film in 210 min whereas under UV light the degradation was 10%. For pure PVC film, the corresponding degradations were 4.2 and 4.84% respectively. A plausible mechanism has been suggested and validated with experimental results which fitted the proposed model satisfactorily.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Synthesis and Characterization of NiFe{sub 2}O{sub 4} Nanoparticles for the Enhancement of Direct Sunlight Photocatalytic Degradation of Methyl Orange

The current investigation shows the simple and direct sunlight-mediated photocatalytic degradation of methyl orange dye by quasi globular NiFe{sub 2}O{sub 4} nanocrystals synthesized from the high-temperature chemical co-precipitation method. The experiment was carried out under direct sunlight which shows significant degradation results lead to the practical possibility of heterogeneous photocatalysis towards environmental remediation. The as-synthesized quasi globular NiFe{sub 2}O{sub 4} nanocrystals also were characterized by well-known analytical measurements of their structural, morphological, bonding, surface area, band gap and magnetic properties prior to the photocatalytic experiments. The presence of active free radicals formed during the photocatalytic reaction was confirmed from the EPR signals recorded for the solution containing the photocatalyst and dye solution, and accordingly, the photocatalytic degradation mechanism was discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Si Microwire-Array Photocathodes Decorated with Cu Allow CO 2 Reduction with Minimal Parasitic Absorption of Sunlight

High loadings of Cu were integrated on the light-facing side of Si microwire arrays used under simulated sunlight for the photoelectrochemical reduction of CO 2 (aq) to hydrocarbons in 0.10 M KHCO 3 (aq). Radial-junction n + p-Si microwire arrays decorated with Cu exhibited absolute photocurrent densities comparable to an uncovered Si surface. Moreover, with respect to a Cu foil electrode, the positive shift in the onset potential for hydrocarbon formation at n + p-Si/Cu microwire arrays was equal to or greater than the photovoltage of the semiconductor alone. Selective electrodeposition of Cu on the tips and sidewalls of Si microwires was responsible for the minimal parasitic reflection and absorption exhibited by the catalyst, such that light-limited, absolute current densities >25 mA·cm –2 were sustained for 48 h under simulated sunlight. Photoelectrodes prepared from semiconductors with low diode quality factors and electrocatalysts with large Tafel slopes are shown to benefit from increased microstructured surface area. Si microwire arrays are thus suitable for photoelectrochemical reactions requiring high loadings of metallic and reflective electrocatalysts.

25 ENERGY STORAGE↗
DOE OSTI2019

A high-performance oxygen evolution catalyst in neutral-pH for sunlight-driven CO 2 reduction

The efficiency of sunlight-driven reduction of carbon dioxide (CO 2 ), a process mimicking the photosynthesis in nature that integrates the light harvester and electrolysis cell to convert CO 2 into valuable chemicals, is greatly limited by the sluggish kinetics of oxygen evolution in pH-neutral conditions. Current non-noble metal oxide catalysts developed to drive oxygen evolution in alkaline solution have poor performance in neutral solutions. Here we report a highly active and stable oxygen evolution catalyst in neutral pH, Brownmillerite Sr 2 GaCoO 5 , with the specific activity about one order of magnitude higher than that of widely used iridium oxide catalyst. Using Sr 2 GaCoO 5 to catalyze oxygen evolution, the integrated CO 2 reduction achieves the average solar-to-CO efficiency of 13.9% with no appreciable performance degradation in 19h of operation. Our results not only set a record for the efficiency in sunlight-driven CO 2 reduction, but open new opportunities towards the realization of practical CO 2 reduction systems.

36 MATERIALS SCIENCE↗
DOE OSTI2020

THE CASE FOR MULTICRITERIA ANNUAL SUNLIGHT EXPOSURE GUIDELINES

This paper explored the effects of the current Annual Sunlight Exposure (ASE1000 lux, 250 hours) requirements on energy consumption and thermal comfort of an office space in five cities representing different U.S. climates. Specifically, the goal was to quantify the extent to which the current ASE metric requirements may be adjusted allowing for more/ less annual sunlight exposure to address climatic differences, energy, and thermal comfort criteria. A south-facing perimeter office space was simulated in the five climates to assess differences in energy and thermal comfort between the current ASE requirements and best-case scenarios. The results highlight the need to incorporate thermal comfort and energy as primary criteria informing target ASE requirements and guidelines. Lastly, the paper discusses research pathways to help develop multi-criteria ASE guidelines.

Abboushi, Belal K.↗
DOE OSTI2023

Thermally Responsive Hydrogels for Passive Temperature Regulation under Direct Sunlight

By spontaneously emitting midinfrared radiation to outer space through the atmospheric window and reflecting sunlight, daytime radiative coolers achieve notable passive cooling performance. However, existing daytime radiative cooling systems generally lack the ability to adaptively switch between heating and cooling states based on ambient conditions. Herein a passive thermal regulation system that features a temperature‐dependent switchable solar reflectance from 0.05 (low temperature) to 0.8 (high temperature) is presented. This, along with a ≈0.95 midinfrared emittance, it enables automatic switching between radiative cooling and solar heating. Switchablity is enabled using a poly( N ‐isopropylacrylamide) (PNIPAM) hydrogel which exhibits high solar scattering above its tunable lower critical solution temperature (LCST) and transparency below its LCST. The lower part of the hydrogel is loaded with graphite to absorb solar energy in the heating state. In testing under sunny and partly cloudy outside conditions, this system maintains a temperature close to the set LCST.

Xie, Dajie↗
DOE OSTI2021

Storing sunlight at low temperatures?

Here, artificial leaves that convert sunlight to fuels could provide a sustainable energy future. But several challenges must be overcome to improve their economic viability. Publishing in Energy & Environmental Science, Kölbach, Rehfeld, and May describe the potential of decentralized hydrogen production using thermally integrated architectures that operate under sub-freezing temperatures.

14 SOLAR ENERGY↗
DOE OSTI2021

Ruthenium Dye Excitations and Relaxations in Natural Sunlight

Solar harvesting devices using dyes convert the sun's energy to usable forms. The photophysics involved are generally investigated using time-resolved spectroscopic experiments with femtosecond to nanosecond resolution. We show that a kinetic framework constructed from transient and linear absorption measurements of metal-ligand charge transfer states for a set of ruthenium complexes in solution can be used to simulate the steady-state dynamics of dyes adsorbed on a substrate under diffuse solar radiation. Even though the intensity of sunlight is relatively low, double excitations to higher excited states can occur. The steady-state populations show that the dyes' triplet state is the main species present besides the ground state. While small, these persistent excited populations can influence reactivity over the extended periods of time that the systems operate. The results show that non-radiative and optical events (dye -1 s -1 ) within the singlet manifold and from the triplet state exhibit a dependence on ligand substituents.

14 SOLAR ENERGY↗