Incident Sunlight Research Articles

Compact Flyeye concentrator with improved irradiance uniformity on solar cell

A Flyeye concentrator with improved irradiance distribution on the solar cell in a concentrator photovoltaic system is proposed. This Flyeye concentrator is composed of four surfaces: a refractive surface, mirror surface, freeform surface, and transmissive surface. Based on the principles of geometrical optics, the contours of the proposed Flyeye concentrator are calculated according to Fermat's principle, the edge-ray principle, and the ray reversibility principle without solving partial differential equations or using an optimization algorithm, therefore a slope angle control method is used to construct the freeform surface. The solid model is established by applying a symmetry of revolution around the opti- cal axis. Additionally, the optical performance for the Flyeye concentrator is simulated and analyzed by Monte-Carlo method. Results show that the Flyeye concentrator optical efficiency of >96.2% is achievable with 1333× concentration ratio and � 1.3 deg acceptance angle, and 1.3 low aspect ratio (average thickness to entry aperture diameter ratio). Moreover, com- paring the Flyeye concentrator specification to that of the Kohler concen- trator and the traditional Fresnel-type concentrator, results indicate that this concentrator has the advantages of improved uniformity, reduced thickness, and increased tolerance to the incident sunlight. © 2013 Society

Layer-by-Layer Assemblies of Semiconductor Quantum Dots for Nanostructured Photovoltaic Devices

A multilayer of quantum dots (QDs) is preferred for QD-sensitized solar cells over a monolayer counterpart to fully utilize the sunlight incident into a relatively thin-film-based photovoltaic device. A controlled assembly of QD multilayers such as layer-by-layer (LbL) assemblies can provide a model system to study the interactions between the QD layers and can offer an optimal device configuration for efficient solar power conversion. Recently, we have proposed a LbL QD assembly using electrostatic interactions of the surface charges and have successfully prepared a controlled multilayer of QD on the surface of mesoporous metal oxide films. The as-prepared tailor-made QD multilayers not only guaranteed the sufficient absorption of incident solar light but also provided a toolbox for the study and optimization of electron/energy transfers between QD layers.

Extraordinary Sunlight Absorption and One Nanometer Thick Photovoltaics Using Two-Dimensional Monolayer Materials

Graphene and monolayer transition metal dichalcogenides (TMDs) are promising materials for next-generation ultrathin optoelectronic devices. Although visually transparent, graphene is an excellent sunlight absorber, achieving 2.3% visible light absorbance in just 3.3 Å thickness. TMD monolayers also hold potential as sunlight absorbers, and may enable ultrathin photovoltaic (PV) devices due to their semiconducting character. In this work, we show that the three TMD monolayers MoS2, MoSe2, and WS2 can absorb up to 5-10% incident sunlight in a thickness of less than 1 nm, thus achieving 1 order of magnitude higher sunlight absorption than GaAs and Si. We further study PV devices based on just two stacked monolayers: (1) a Schottky barrier solar cell between MoS2 and graphene and (2) an excitonic solar cell based on a MoS2/WS2 bilayer. We demonstrate that such 1 nm thick active layers can attain power conversion efficiencies of up to ~1%, corresponding to approximately 1-3 orders of magnitude higher power densities than the best existing ultrathin solar cells. Our work shows that two-dimensional monolayer materials hold yet untapped potential for solar energy absorption and conversion at the nanoscale.

Empirical Regression Model for Biochemical Oxygen Demand Removal in Solar Enhanced Waste Stabilization Ponds

Consequent on the findings that a Solar Enhanced Waste Stabilization Pond (SEWSP) will increase treatment efficiency thereby reduce the large land area requirement; hence, this study aims at developing an empirical regression model for the prediction of the efficiency of Biochemical Oxygen Demand removal of the SEWSPs for sewage treatment. SEWSPs were constructed of varying sizes made of metallic tank with inlet and outlet valves, and a solar reflector to increase the incident sunlight intensity. Physio-chemical and biological characteristics of the wastewater samples were collected from different points (inlet and outlets) of the SEWSPs were examined for a period of two months. The examined parameters were: Efficiency of BOD removal in %, Efficiency of E Coli removal in %, Dissolve Oxygen in mg/l, Efficiency of COD removal in %, Efficiency of Suspended Solid removal in %, Temperature in0C, Detention Time in days and coliform. Discussions were made revealing the relationship between the depth of the SEWSP and treatment efficiency.An empirical correlation model predicting the efficiency of BOD removal for the SEWSP was developed thus y = - 0.292X1 – 0.1011X2 + 0.876X3 + 0.148X4 – 0.087X5 +0.012X6 + 22.939 together with a MATLAB solver for easy computation.

Investigating why recycling gravity harvested algae increases harvestability and productivity in high rate algal ponds

It has previously been shown that recycling gravity harvested algae promotes Pediastrum boryanum dominance and improves harvestability and biomass production in pilot-scale High Rate Algal Ponds (HRAPs) treating domestic wastewater. In order to confirm the reproducibility of these findings and investigate the mechanisms responsible, this study utilized twelve 20 L outdoor HRAP mesocosms operated with and without algal recycling. It then compared the recycling of separated solid and liquid components of the harvested biomass against un-separated biomass. The work confirmed that algal recycling promoted P. boryanum dominance, improved 1 h-settleability by >20% and increased biomass productivity by >25% compared with controls that had no recycling. With regard to the improved harvestability, of particular interest was that recycling the liquid fraction alone caused a similar improvement in settleability as recycling the solid fraction. This may be due to the presence of extracellular polymeric substances in the liquid fraction. While there are many possible mechanisms that could account for the increased productivity with algal recycling, all but two were systematically eliminated: (i) the mean cell residence time was extended thereby increasing the algal concentration and more fully utilizing the incident sunlight and, (ii) the relative proportions of algal growth stages (which have different specific growth rates) was changed, resulting in a net increase in the overall growth rate of the culture.

Towards Universal Wavelength-Specific Photodegradation Rate Constants for Methyl Mercury in Humic Waters, Exemplified by a Boreal Lake-Wetland Gradient

We report experimentally determined first-order rate constants of MeHg photolysis in three waters along a Boreal lake-wetland gradient covering a range of pH (3.8-6.6), concentrations of total organic carbon (TOC 17.5-81 mg L(-1)), total Fe (0.8-2.1 mg L(-1)), specific UV254 nm absorption (3.3-4.2 L mg(-1) m(-1)) and TOC/TON ratios (24-67 g g(-1)). Rate constants determined as a function of incident sunlight (measured as cumulative photon flux of photosynthetically active radiation, PAR) decreased in the order dystrophic lake > dystrophic lake/wetland > riparian wetland. After correction for light attenuation by dissolved natural organic matter (DOM), wavelength-specific (PAR: 400-700 nm, UVA: 320-400 nm and UVB: 280-320 nm) first-order photodegradation rate constants (kpd) determined at the three sites were indistinguishable, with average values (± SE) of 0.0023 ± 0.0002, 0.10 ± 0.024 and 7.2 ± 1.3 m(2) E(-1) for kpdPAR, kpdUVA, and kpdUVB, respectively. The relative ratio of kpdPAR, kpdUVA, and kpdUVB was 1:43:3100. Experiments conducted at varying MeHg/TOC ratios confirm previous suggestions that complex formation with organic thiol groups enhances the rate of MeHg photodegradation, as compared to when O and N functional groups are involved in the speciation of MeHg. We suggest that if the photon fluxes of PAR, UVA, and UVB radiation are separately determined and the wavelength-specific light attenuation is corrected for, the first-order rate constants kpdPAR, kpdUVA, and kpdUVB will be universal to waters in which DOM (possibly in concert with Fe) controls the formation of ROS, and the chemical speciation of MeHg is controlled by the complexation with DOM associated thiols.

A low-cost high-efficiency crystalline silicon solar cell based on one-dimensional photonic crystal front surface textures

In this paper we analyze the influence of active layer thickness on the light absorption processes of c-Si solar cells by the finite-difference frequency-domain (FDFD) method and show that the ideal c-Si active layer thickness range is from 10 to 20 μm. By evaluating the absorption enhancement factor, we find that c-Si solar cells with thicknesses within this range can give full play to the synergistic effects of back reflectors and antireflection coatings. By using a simple light trapping scheme, which is composed of a Ag back reflector, a bilayer antireflection coating and a one-dimensional photonic crystal front surface texture structure with rectangular profile, a c-Si solar cell with an active layer thickness of 12 μm can obtain a higher photocurrent density of 32.6 mA cm−2 for normally incident sunlight and a broader acceptance angle range of −60°–60° for obliquely incident sunlight.

Environmental Quality Predicts Optimal Egg Size in the Wild

Parents can maximize their reproductive success by balancing the trade-off between investment per offspring and fecundity. According to theory, environmental quality influences the relationship between investment per offspring and offspring fitness, such that well-provisioned offspring fare better when environmental quality is lower. A major prediction of classic theory, then, is that optimal investment per offspring will increase as environmental quality decreases. To test this prediction, we release over 30,000 juvenile Atlantic salmon (Salmo salar) into eight wild stream environments, and we monitor subsequent growth and survival of juveniles. We estimate the shape of the relationship between investment per offspring (egg size) and offspring fitness in each stream. We find that optimal egg size is greater when the quality of the stream environment is lower (as estimated by a composite index of habitat quality). Across streams, the mean size of stream gravel and the mean amount of incident sunlight are the most important individual predictors of optimal egg size. Within streams, juveniles recaptured in stream subsections that featured larger gravels and greater levels of sunlight also grew relatively quickly, an association that complements our cross-stream analyses. This study provides the first empirical verification that environmental quality alters the relationship between investment per offspring and offspring fitness, such that optimal investment per offspring increases as environmental quality decreases.

Trends and Opportunities in Direct-Absorption Solar Thermal Collectors

Efficient conversion of sunlight into useful heat or work is of increasing global interest. Solar-to-thermal energy conversion, as opposed to solar-to-electricity, is enabled by solar thermal collectors that convert sunlight into heat at some useful temperature. We review here recent developments in solar thermal energy conversion. Our emphasis is on “direct-absorption” solar thermal collectors, in which incident sunlight is absorbed directly by a working fluid. This contrasts with conventional solar thermal collectors where the sunlight strikes and is absorbed by a solid receiver, which then transfers heat to the working fluid. Both liquid-based and gas-based direct-absorption collectors are described, although liquid-based systems are emphasized. We propose that if “direct-absorption” technologies could be developed further, it would open up a number of emerging opportunities, including applications exploiting thermochemical and photocatalytic reactions and direct absorption of a binary fluid for absorption refrigeration.

High performance isothermal photo-thermionic solar converters

A new class of solar energy converters is described, based on photon-enhanced thermionic emission of electrons into a vacuum region between two electrodes. In contrast to conventional thermionic converters, the two electrodes are at the same temperature, and the cathode is side-illuminated rather than front-illuminated. This configuration leads to several advantages: close coupling of multiple units in a series connection high-voltage configuration; higher area for electron emission; and recovery of waste heat at a higher temperature compared to heat recovery from the anode of a conventional thermionic converter. The conversion efficiency from solar radiation to electricity of the new photo-thermionic device may be in the range of 30–40% at moderate operation temperatures and moderate concentration of the incident sunlight. Recovery of the waste heat and further conversion from heat to electricity may raise the overall conversion efficiency to the range of 40–50%.

Prediction of a Double-antireflection Coating Made Solely with SiNx in a Single, Directional Deposition Step

Silicon solar cell modules, where the EVA layer is replaced by an air gap, are able to produce the same electric power as standard modules with EVA only if their anti-reflective properties are enhanced. We propose a method to do this by exploiting the fact that, on Si surfaces textured with random pyramids, light incident from near normal angle always hits at least two pyramidal faces before being reflected back toward the sun. If these two faces are covered with an anti-reflective coating (ARC) made of one and the same material but with two different thicknesses, the coating acts as a double ARC. Such a coating can be produced by depositing the SiNx layer from an oblique angle,optimally from 14.7°. Our detailed raytracing analysis predicts that Jsc can then be improved by 0.2mA/cm2 for normal incident sunlight and AM1.5g standard illumination, and is improved for all angles within a cone with an apex angle of approximately 64°. Furthermore, the coating can be optimized for modules in vertical mounting, where a Jsc gain of 0.1mA/cm2 is predicted for an angle of incidence of 40°.

EFFECT OF MOLE FRACTION OF HF AND H202 ONMORPHOLOGY OF POROUS SILICON FORMEDBY Ag ASSISTED CHEMICAL ETCHING

The research of silicon solar cells mainly focuses on reducing cost and improving con- version efficiency, and one of the effective methods of improving photoelectric conversion efficiency of solar cells is to decrease the reflection of incident sunlight onto the light-receiving surface. The porous Si layer can work for the antireflection of Si surfaces, which can be prepared by noble metal assisted chemical etching. In this work, the effects of HF, H2O2 and their volume ratio on morphology and growth of pores on single-crystalline Si surface by using Ag (with a small cluster) metal assisted etching were investigated in order to produce a highly efficient antireflecting structure. The metal particles were deposited onto Si wafer by electroless deposition from a metal salt solution including HF. The surface and cross-sectional morphologies of the porous Si surfaces were observed with field-emission scanning electron microscope. The reflectivity of the etched Si surface was measured with a UV-Vis spectrophotometer equipped with an integrating sphere accessory. The experimental results show that the growth rate and morphology of the pores formed on the Ag metallized Si surfaces are strongly de- pendent on the volume ratio of HF and H2O2. It is not beneficial for the pore growth when mole fraction �=(HF)/((HF)+(H2O2)) is too low or too high, and the etching goes well only whenis between 60%— 80%; in this case, the pore growth rate is up to 1050—1260 nm/min, and the pores grow straightly and

Effect of Light Intensity during Stenting Propagation on Rooting and Subsequent Growth of Two Rose Cultivars

This study was conducted to investigate the effect of light intensity during a winter season on rooting and subsequent growth of stenting-propagated domestic cut rose (Rosa hybrida Hort.) ‘Pink Aurora’ and ‘Yellow King’ in an effort to develop an efficient stenting propagation method for rose cultivars. To facilitate graft joining, both base of scion and top of rootstock, removed leaves, were cut together at a 45˚ angle. Single node scions, each with a five-leaflet leaf, were grafted on Rosa indica ‘Major’ as the rootstock. A scion-rootstock union was stuck in a rockwool cube (5 cm × 5 cm× 5 cm, Grodan, Denmark) and was placed in a graft-take chamber for five days before being moved to a misted greenhouse bench. Plants were grown under 650 (0%), 228 (35%), or 125 (55%) μmol·m -2 ·s -1 of the incident sunlight. Rooting and growth were affected by the light intensity and cultivar. In both cultivars, root growth was accelerated and percent rooting increased under higher light intensities. No shading generally showed the highest percent rooting, shoot length, shoot weight, length of longest root, and root weight. Chlorophyll content was not significantly affected by the light intensity. The greatest rooting and subsequent growth of stenting-propagated plants were found in the no shading. The results suggested that shading was not necessary for the stenting propagation of domestic cut rose cultivars during a winter season.

Solar energy trapping with modulated silicon nanowire photonic crystals

We demonstrate the efficacy of nanostructured thin film silicon solar cells to trap and absorb approximately 75% of all sunlight incident (400 nm–1200 nm) with an equivalent bulk thickness of only 1 micron of silicon. This is achieved by sculpting the collection zone into a three-dimensional, simple-cubic-symmetry, photonic crystal consisting of modulated silicon nanowires embedded in SiO2 and sitting on a quartz substrate with no metallic mirrors. A specific modulation of the radius of nanowires provides antireflection, strong light trapping, and back-reflection mechanisms in targeted spectral regions. This modulation is linear at the top of the nano-rods leading to nanocones at the solar cell to air boundary. These silicon nanocones are very good absorbers at short wavelengths and act as broadband coupler to a light-trapping region below at longer wavelengths. In the light trapping region the modulation is periodic to form a simple cubic photonic crystal exhibiting a broad spectrum of strong parallel interface refraction resonances. Here, light incident from most angles is deflected into slow group velocity modes with energy flow nearly parallel to the interface, long dwell times, and strong light intensity enhancement (up to 150 times the incident intensity) in specific regions. Finally, a stronger and chirped modulation of the nanowire underneath provides back-reflection by means of a one-dimensional depth-dependent photonic stop-gap. The possibility of absorbing light at energies below the electronic band gap of silicon is illustrated using a graded index SixGe1−x alloy in the bottom section of each nanowire. Each nanowire is amenable to a radial P-N junction for proximal charge carrier separation and efficient collection of photo-generated current.

Broadband optical absorption enhancement in silicon nanofunnel arrays for photovoltaic applications

Silicon photonic nanostructures have received considerable attention for light trapping in recent years. In this work, we demonstrate silicon nanofunnel (SiNF) array as a broadband light absorber for silicon photovoltaic devices via simulation. Due to more excellent optical couple between SiNFs and incident sunlight as well as the gradual change of the effective refractive index, the SiNF arrays exhibit significantly better optical absorption over a broad wavelength range and higher ultimate efficiencies compared to silicon nanohole array counterparts. The proposed SiNF arrays are promising for low cost ultrathin silicon solar cells and other photoactive devices.

Cloud pattern and water relations in Picea rubens and Abies fraseri, southern Appalachian Mountains, USA

The spruce-fir [Picea rubens Sarg.-Abies fraseri (Pursh) Poir.] forests of the southern Appalachian Mountains are considered refugial, endangered communities that exist on only seven mountaintop areas in Virginia and North Carolina, USA. These relict forests continue to be threatened by stress factors such as logging, acid rain deposition, attacks from invasive insects, and climate change. It has been suggested that these communities have persisted because of frequent cloudiness and periods of cloud immersion (fog), although few studies have examined corresponding effects on microclimate and tree ecophysiology. Incident sunlight (PPFD), air temperature, vapor pressure deficit (VPD), and xylem water potentials were measured throughout the summer growing season in Mount Mitchell State Park, NC (35°45′53″N, 82°15′54″W), along with continuous camera recordings of the forest canopy and accompanying cloud conditions. Approximately 60% of all summer days had at least 2h of cloud immersion with the large majority (80.3%) of immersion events occurring during morning hours. Cloud-immersed days had the greatest reduction in cumulative daily PPFD compared to clear days (11.09molm−2day−1 vs. 38.03molm−2day−1, respectively), as well as substantially reduced mean VPD (0.98kPa vs. 1.81kPa) but only slightly lower mean air temperatures (14.5°C vs. 14.9°C, respectively). Moreover, xylem water potential (Ψ) increased significantly (∼0.2MPa; values) from morning to afternoon on cloud-immersed days. In contrast, clear days showed no afternoon recovery in Ψ, but a continued decrease during the afternoon. Juvenile Ψ was more responsive to daily cloud regime compared to adult Ψ and had a strong negative correlation with vapor pressure deficit. When all measurement days were sorted by the cloud pattern of the previous day, there was a strong response in juveniles trees, i.e. Ψ increased following previous afternoon cloud-immersion. Juveniles of both species also had greater seasonal decreases in Ψ than adults (P. rubens, adult: 0.05MPa, juvenile: 0.13MPa; A. fraseri, adult: 0.06MPa, juvenile: 0.20Mpa). For climate change models that predict a higher cloud base (resulting in less immersion) and dryer conditions, the water relations of Abies fraseri and Picea rubens could be substantially and negatively influenced.

Effects of Aspect Ratio on Solar Enhanced Waste Stabilization Ponds

The simulation of solar radiation in waste stabilization ponds is a developing tool worth studying in order to understand its effects on wastewater quality in waste stabilization ponds. Hence, the research is aimed at investigating the effect of length to width ratio (L/W) in solar enhanced waste stabilization ponds (SEWSPs) treatment efficiency. SEWSPs of varying sizes made of metallic tank with inlet and outlet valves, and a solar reflector was constructed to increase the incident sunlight intensity. Wastewater samples collected from the inlet and outlet of the SEWSPs were examined for physico-chemical and biological characteristics for a period of twelve (12) months. The parameters examined were Temperature, pH, detention time, total suspended solids, dissolved oxygen, biochemical oxygen demand (BOD5), chemical oxygen demand (COD), algae count, faecal coliform and E-Coli . The efficiencies of the SEWSPs with respect to these parameters fluctuated with width variation, with the smallest SEWSP in width giving the highest treatment efficiency. The research revealed that with the incorporation of solar radiation in WSPs, a length/ width ratio of 1:0.2 should be used in the design of SEWSPs for maximum treatment efficiency. Keywords: waste stabilization pond, efficiency, solar radiation, aspect ratio, wastewater.

Angular selective semi-transparent photovoltaics

Conventional semi-transparent photovoltaics suffer from an inherent tradeoff between the amount of visible light transmitted versus absorbed, reducing energy conversion efficiency when higher transparency is desired. As a solution to lift this tradeoff, we propose a wavelength and angular selective reflector and demonstrate a potential implementation utilizing high aspect ratio metal nanoparticles. Using the anisotropy in the localized surface plasmon resonance wavelength, the proposed device can selectively harness sunlight incident at an elevated angle, increasing the power conversion efficiency by a factor of 1.44, while maintaining 70 percent optical transparency at normal incidence.

Luminescent solar concentrators – A low cost photovoltaics alternative

The development and current status of luminescent solar concentrators is reviewed. These solar concentrators generally consist of transparent polymer sheets doped with luminescent species; presently mainly organic dye molecules are used as luminescent species, however semiconductor nanocrystals are gaining interest. Direct and diffuse incident sunlight is absorbed by the luminescent species and emitted at red- shifted wavelengths with high quantum efficiency. Optimum design ensures that a large fraction of emitted light is trapped in the sheet, which travels to the edges where it can be collected by one or more mono- or bifacial solar cells, with minimum losses due to absorption in the sheet and re-absorption by the luminescent species. Optimized luminescent solar concentrators are predicted to offer potentially lower cost per unit of power compared to conventional solar cells. Various design and material aspects will be discussed using thermodynamic and ray-trace modeling techniques and recent experimental results are presented.

Io: Charting thermal emission variability with the Galileo NIMS Io Thermal Emission Database (NITED): Loki Patera

We have calculated the ≈5‐μm radiant flux for every volcanic hot spot in every one of the 190 GalileoNear‐Infrared Mapping Spectrometer (NIMS) tube observations of Io obtained between 28 June 1996 and 16 October 2001 in order to determine the variability of thermal emission from Io's volcanoes at local, regional and global scales, and to identify individual eruption episodes where thermal emission waxes and wanes. The resulting NIMS Io Thermal Emission Database (NITED) allows the comparison of activity at individual volcanoes and different regions of Io. The database contains over 1000 measurements of radiant flux at approximately 5μm, corrected for emission angle, range to target and incident sunlight (where necessary). We examine the data for Loki Patera, Io's most powerful volcano. For data acquired in local darkness we use two‐temperature fits to nighttime spectra and prior knowledge of emitting area to determine total radiated thermal emission. For other data we use the constancy of the integrated thermal emission spectrum to determine total thermal emission from measurements of radiant flux at 5μm. As seen by NIMS, total thermal emission from Loki Patera varies between 7600 GW and 17000 GW. We revise upwards previous estimates of thermal emission from NIMS data. NIMS 3.5‐μm radiant fluxes (both measured and estimated) are consistent with measurements from ground‐based telescopes. This work highlights the value of NITED as a research tool.