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[en] Highlights: • Steam generation is due to boiling/vaporization in localized solar absorption area. • Hypothesized nanobubble is unlikely to occur under normal solar concentrations. • A photothermal efficiency of 80.3% was achieved for 12.75 ppm GNP dispersion. • A specific absorption rate of ~50 kW/g was achieved for 1.02 ppm GNP dispersion. Steam production is essential for a wide range of applications, and currently there is still strong debate if steam could be generated on top of heated nanoparticles in a solar receiver. We performed steam generation experiments for different concentrations of gold nanoparticles dispersions in a cylindrical receiver under focused natural sunlight of 220 Suns. Combined with mathematical modelling, it is found that the initial stage of steam generation is mainly caused by localized boiling and vaporization in the superheated region due to highly non-uniform temperature and radiation energy distribution, albeit the bulk fluid is still subcooled. Such a phenomenon can be well explained by the classical heat transfer theory, and the hypothesized ‘nanobubble’, i.e., steam produced around the heated nanoparticles, is unlikely to occur under normal solar concentrations. For future solar receiver design, attention should be paid to focus and trap more solar energy at the superheated region while minimizing the temperature rise of the bulk fluid.
[en] Highlights: • Physics of black material for light-to-heat conversion. • The absorbers using various black materials are identified. • The state-of-the-art design of the photothermal sheets. • The devices with their steam releasing property are highlighted. Solar energy-to-heat conversion for steam generation is an essential metrology for power generation, water purification and desalination. Harvesting light energy and converting it to heat as terminal energy by black photothermal sheets is a novel strategy to attain this goal. This technology rely on use of black nanomaterials as light absorber to increase the absorption and conversion efficiency of solar energy. Fundamental understanding of their structure-property has to be fully exploited for further developing efficient solar-to-heat systems. This report summarizes physical understanding and experimental advances in development of black photothermal sheets for solar water evaporation. We examine the popular photothermal systems with remarkable vapor generation performance to identify the state-of-the-art of the device design. Three groups of the photothermal sheet are discussed in terms of different light-harvesting materials, such as carbon-based sheets, plasmonic sheets as well as semiconducting sheets. The physical difference of these novel devices with their steam releasing property are also highlighted.
[en] Highlights: • A subtle flag-like TENG with high adaptability is demonstrated for wind energy harvesting in diverse harsh environment. • A kite-shaped TENG can fly free in the sky and convert wind energy into electricity to drive some electronics. • A hybrid device based on the TENG can be used to construct a smart self-powered network node for multifunctional sensing. Establishing a smart sensing system is desirable but challenging as the conventional devices require an external battery or power grid which cruelly limits its applications, especially in remote areas or at the condition of power supply with difficulty. Here, we report a smart self-powered sensing network based on a hybrid nanogenerator (NG) that can harvest wind energy and solar energy simultaneously or individually to serve as a sustainable power source. A transparent flag-like triboelectric nanogenerator (TENG) plays a core role with both the output voltage and current positively proportional to the wind speed and the number of integrated units. By subtly designing, the bottom solar cell can absorb sunshine insusceptibly. The hybrid NG can not only be used as an active wind speed detection, but also be employed to construct a self-powered wireless temperature sensing system, further achieving multifunctional sensing. This work promotes the development of TENG-based renewable energy harvesting and puts forward the smart sensing network node that realizes multifunctional monitoring by scavenging environmental energy.
[en] Highlights: • A novel integration of carbon counter electrode based perovskite solar cell and thermoelectric generator was reported. • The integration broadened the use of solar spectra since the excellent light absorption property of carbon. • Thermal endurance of the integrated devices was studied. • The champion integrated device with efficiencies of 22.2% (in ice bath) and 12.6% (in ambient air) exhibited efficient solar energy utilization. We demonstrate a novel integration of carbon counter electrodes based perovskite solar cells (PSCs) and thermoelectric generators (TEs), which exhibits excellent thermal endurance and photo-electric conversion by use of good light-harvesting capabilities over the wide sunlight spectra. The carbon counter electrode based PSC owns a good prospect in development and commercialization, whereas photo-thermal effects induced thermal degradation will be more crucial due to notable photo-thermal conversion of the carbon. The photovoltaic performance of the PSCs will decline when the temperature increases and recover when decreasing. After integration with TEs, the carbon electrodes can act as the infrared light absorption layer for the TEs, which improve the photo-thermal ability and triple the output voltage of the thermoelectric devices, substantially compensating the thermal degradation of the PSCs. Electrical measurements reveal that the integrated devices exhibit much higher and more stable energy output. When tested in ambient air, the hybrid device exhibits obvious enhancement: the overall conversion efficiency increases from 9.88% to 12.6% after integration. When the cold side of the TE part is cooled by ice bath, the hybridization obtains a maximum VOC of 1.87 V under AM 1.5 G illumination with a temperature gradient of 15 °C, boosts a more than 124% increase of the photo-electric conversion efficiency (PCE), from 9.88% (photovoltaic) to 22.2% (photovoltaic-thermoelectric), and gets a higher maximum power output of 22.2 mW cm−2. By further optimization, larger improvements in PCE of the integration can be achieved. Our work opens up new avenues for the realization of high-performance, wide sunlight-harvesting photovoltaic-thermoelectric hybrid devices.
[en] Highlights: • A high-performance near-field TPV is reexamined for waste heat recovery. • With a 900 K source, the system yields 11 W/cm2 power and 40% efficiency. • A thin metal layer further enhances the power density to 31 W/cm2. The US industries reject nearly 20–50% of the consumed energy into the environment as waste heat. Harvesting this huge amount of heat can substantially improve the energy usage efficiency. For waste heat in the medium temperature range (~ 500–900 K), traditional solid-state waste heat recovery techniques like thermoelectric generators and thermophotovoltaics (TPVs) are still suffering from relatively low efficiency or power density. In this work, we analyze a near-field TPV system consisting of a plasmonic emitter (indium tin oxide) and a narrow-bandgap photovoltaic cell (InAs) that are brought to deep sub-wavelength distances for high-efficiency and high-power-density waste heat recovery. We show that despite the inclusion of realistic nonradiative recombination rates and sub-bandgap heat transfer, such a near-field TPV system can convert heat to electricity with up to nearly 40% efficiency and 11 W/cm2 power density at a 900 K emitter temperature, because of the spectral reshaping and enhancement by the thermally excited surface plasmons and waveguide modes. Thus, we show that for waste heat recovery, near-field TPV systems can have performances that significantly exceed typical thermoelectric systems. We propose a modified system to further enhance the power density by using a thin metal film on the cell, achieving a counterintuitively “blocking-assisted” heat transfer and power generation in the near-field regime.
[en] Highlights: • A multifunctional hybrid power unit for harvesting blue energy has been proposed. • The complementary output of TENG and EMG can harvest blue energy in a broad frequency range. • The attractive force of magnet pairs enables the fully enclosed packaging of the TENG part. • The power unit can harvest energy as a practical power source under all weather conditions. • The power unit can be easily driven owing to ingenious design and multifunctional outputs. The complementary output of triboelectric nanogenerator (TENG) and electromagnetic generator (EMG) can be hybridized and maximized for harvesting blue energy in a broad frequency range. How to optimally design and construct the hybrid structure still remains a challenge. In this work, we proposed a multifunctional hybrid power unit for harvesting blue energy, which consists of contact-separate mode triboelectric nanogenerators (CS-TENGs), freestanding sliding mode electromagnetic generators (FS-EMGs) and commercial water-proof silicon based solar cells (WS-SCs). When harvesting ocean wave kinetic energy, the bottom magnet in FS-EMG moves forth and back driven by the wave motion and makes the top magnet shake upward or downward, thus the two triboelectric layers of CS-TENG contact and separate constantly. The magnet pairs produce the noncontact attractive force that enables the fully enclosed packaging of the TENG part, protecting it from ambient environment. The TENGs effectively harvest low-frequency (< 0.5 Hz) motion and the EMGs enable to produce larger output at relatively high frequency, achieving the purpose of harvesting blue energy in a broad frequency range. In addition, considering the adequate illumination on the sea, solar cells are easily integrated to collect solar energy simultaneously. The hybrid power unit has been demonstrated to harvest energy as a practical power source to drive LEDs directly or charge commercial supercapacitors under all weather conditions. The hybrid unit can be easily driven owing to ingenious design and multifunctional outputs.
[en] Highlights: • Carbon nanoparticles (CNPs) were prepared from biomass. • CNPs work as a robust anode material with high discharge capacities for NIBs and LIBs. • CNPs exhibited superior rate and cycling performance for both LIBs and NIBs. In this paper, we report a flame deposition method to prepare carbon nanoparticles (CNPs) from coconut oil. The CNPs were further modified with a piranha solution to obtain surface-carboxylated carbon nanoparticles (c-CNPs). When used as an anode for sodium-ion batteries, the CNPs and c-CNPs respectively delivered discharge capacities of 277 and 278 mA h g−1 in the second cycle at a current density of 100 mA g−1. At the 20th cycle, the capacities of CNP and c-CNPs were 217 and 206 mA h g−1 respectively. The results suggest that modification of the CNPs with the piranha solution improved neither the charge storage capacity nor the stability against cycling in a sodium-ion battery. When the CNP and c-CNP were used an anode in a lithium-ion battery, 2nd-cycle discharge capacities of 741 and 742 mA h g−1 respectively at a current density of 100 mA g−1 were obtained. After 20 cycles the capacities of CNP and c-CNP became 464 and 577 mA h g−1 respectively, showing the cycling stability of the CNPs was improved after modification. The excellent cycling performance, high capacity and good rate capability make the present material as highly promising anodes for both sodium-ion and lithium-ion batteries.
[en] In any solar thermal application, such as solar space heating, solar hot water for domestic or industrial use, concentrating solar power, or solar air conditioning, a solar receiver converts incident sunlight into heat. In order to be efficient, the receiver must ideally absorb the entire solar spectrum while losing relatively little heat. Currently, state-of-the-art receivers utilize a vacuum gap above an absorbing surface to minimize the convection losses, and selective surfaces to reduce radiative losses. Here we investigate a receiver design that utilizes aerogels to suppress radiation losses, boosting the efficiency of solar thermal conversion. We predict that receivers using aerogels could be more efficient than vacuum-gap receivers over a wide range of operating temperatures and optical concentrations. Aerogel-based receivers also make possible new geometries that cannot be achieved with vacuum-gap receivers.
[en] Developing means to reduce the cost of solar energy is vital to curb our carbon footprint over the upcoming decades. A luminescent solar concentrator (LSC) is a potential solution as it provides light concentration without any tracking device and can be readily integrated into the built environment. In this study we report on an advanced LSC design that employs quantum dots as absorption fluorophores and organic dye molecules as emission fluorophores. By linking the two types of fluorophores to each other, energy is transferred efficiently via Förster resonance energy transfer (FRET) from the quantum dot to the dye molecule. This novel method makes use of the quantum dot's spectrally wide absorption profile and the higher quantum yield of the dye. We show that our design can overcome the losses normally incurred due to a low quantum yield emitter by transferring the absorbed energy to a linked fluorophore with a higher quantum yield. Our experimental measurements show FRET can enhance the optical efficiency of a LSC by at least 24.7%. The maximum theoretical efficiency has been investigated by ray-tracing and has been found to be 75.1%; this represents a relative improvement of even 215.5% compared to a LSC doped with quantum dots only (23.8%), showing the great potential of our concept. Our design will initiate interest in fluorophores which have not been considered for LSC applications thus far because of their low quantum yield or small Stokes shift.
[en] Highlights: • 2D nanosheets of 3D microspheres TiO2 nanostructure was synthesized. • It provides high dye loading per unit area and superb light scattering property. • High porosity of 3D structure renders a facile mass transfer in cobalt-based DSSCs • 11.43%-efficient cobalt-based DSSC is thus obtained with TiO2 microspheres WE. • This gives 29.7% higher cell efficiency than that of cell with Ref-TiO2 WE. To cope with the mass transport problem of a Co(bpy)3]2+/3+ electrolyte as well as to maintain a high dye loading, mono-dispersed microspheres of anatase TiO2 with well-defined (001)-facets (10%, 36%, 58%, and 84%), high surface area (106.5, 102.9, 97.6, and 88.3 m2/g), and large mesopores (26.6, 30.6, 33.7, and 38.5 nm) were synthesized by a facile hydrothermal route. We demonstrate that, owing to the large voids among microspheres, an extremely high porous TiO2 film is formed, thus offering a facile freeway for electrolyte diffusion. Besides, with the increase of (001)-facets, the TiO2 film shows an increase in its dye coverage (dye molecules per unit internal surface area). The coverage of porphyrin dye on a highly exposed (001)-facets TiO2 has been discussed. The mirror-like, (001)-facet planes of the film enabled a superior reflectance of >60% at the wavelength region of 400 to 800 nm. It is a crucial property for YD2-o-C8 dye to compensate its weak absorbance in the spectral range of 480 to 620 nm. A superior electron diffusibility through the 2D nanosheets of 3D microspheres was observed. Moreover, a negative shift in the flat-band energy was observed in the case of high content of (001)-facets; this has enabled a higher open-circuit voltage for the dye-sensitized solar cell (DSSC). By optimizing the condition of post-treatment and TiO2 film thickness, a cell efficiency of 11.43% under 1 sun illuminasion was obtained. In addition to their promising use in cobalt-based DSSCs, we anticipate that TiO2 nanosheet assemblies will provide long-sought-after material solutions in photocatalysis, water splitting, and lithium ion battery.