Low Solar Concentration Research Articles

Highly efficient molybdenum nanostructures for solar thermophotovoltaic systems: One-step fabrication of absorber and design of selective emitter

Efficient solar energy harvesting materials or structures have become a topic of great interest in response to the ever-increasing energy demand. Refractory transition metals have been found to exhibit attractive properties in broadband absorptance, making them highly effective in solar thermophotovoltaic (STPV) systems that aim to surpass the Shockley-Queisser limit. Typically, broadband absorbers are made from hybrid materials or complex geometrical structures, which require multi-step fabrication technologies. In this study, we successfully developed a highly efficient broadband absorber based on molybdenum (Mo) nanosheets using a one-step physical vapor deposition process. The nanosheets result in the high absorber efficiencies of 96.1 % for blackbody radiation (BBR) at 5778 K and of 96.4 % for air mass (AM) 1.5 spectrum, and are insensitive to polarization and incident angle. After annealing at 1650 K for 24 h, the Mo nanosheets absorber demonstrates outstanding thermal stability without compromising the efficiency of the STPV system. To complete the STPV system, a selective narrowband Mo-based gratings emitter was further designed with a high emissivity of 96 % at a wavelength of 2.17 μm, resulting in a high PV cell efficiency of 41.8 % around 1723 K. Through a series of theoretical calculations, the proposed STPV system can achieve an overall conversion efficiency of 40.2 % by a trade-off between the structure temperature and the sun concentration. Furthermore, an efficiency of 35.5 % can be achieved at a low temperature of 1073 K with a low solar concentration of 500 suns.The results provide tremendous opportunities for widespread applications of high-performance STPV systems.

Experimental study on a concentrating solar photovoltaic/thermal system using different fluid spectral beam filters

Spectral splitting photovoltaic/thermal technology is the leading field in the area of extremely efficient utilization of solar energy. Due to its complexity, experimental research on spectral splitting photovoltaic/thermal devices is relatively limited, mainly focusing on the laboratory scale energy efficiency test or calculation process verification. These studies do not provide intuitionistic guidance for the actual operation of engineering systems. In this paper, a fluid filter-based compound parabolic concentrator solar photovoltaic/thermal system with engineering application dimensions is designed and experimentally studied under actual outdoor environments. Different fluid filters are prepared and used to regulate the spectral splitting characteristics of the proposed system, including the deionized water, silver/water nanofluids and silver-cobalt sulfate/water nanofluids. When the environmental parameters are relatively stable, the method of adjusting the composition of the fluid filter is used to obtain the dynamic operating characteristics of the system. The results show that silver nanoparticles and cobalt sulfate can improve the spectral absorptivity of the deionized water. Compared with the deionized water, when the silver/water nanofluids is used as the fluid filter, the thermal efficiency of the system increases, the electric efficiency and overall exergy efficiency of the system both decrease, and the three efficiencies are 56.0%, 8.5% and 19.6%, while for the silver-cobalt sulfate/water nanofluids, the efficiencies are 58.0% 8.0% and 20.0%. Moreover, the results reveal that for the photovoltaic/thermal system with a low solar concentration ratio, by enhancing the absorptivity of the fluid filter, the fill factor of the photovoltaic module can increase, but the output electric power will decrease. When the silver-cobalt sulfate/water nanofluids is used, the fill factor and maximum output power of the proposed system are 0.633 and 69.2 W. For a unit consisting of four sets of the proposed photovoltaic/thermal systems, the emission reductions of CO2, NOx, SO2 and dust for the whole lifetime are approximately 7794.19 kg, 181.48 kg, 191.95 kg and 111.68 kg, respectively.

Direct solar thermochemical CO2 splitting based on Ca- and Al- doped SmMnO3 perovskites: Ultrahigh CO yield within small temperature swing

Solar thermochemical CO2 splitting is a promising route to reduce environment pollution and mitigate energy crisis. However, traditional redox materials are limited by low CO production, slow reaction kinetics, and poor cycling stability, which results in low solar-to-fuel efficiency. Here, Ca- and Al- doped SmMnO3 perovskites are proposed for high-performance solar thermochemical CO2 splitting. The average CO yield of Sm0.6Ca0.4Mn0.8Al0.2O3 reaches a record-high value of 595.56 μmol g−1 when swinging between 1350 °C and 1100 °C. The CO yield is about 4.22 times as high as that of undoped SmMnO3 (140.97 μmol g−1) under the same conditions, and remains stable over multiple cycling. The ultrahigh CO yield can be attributed to the transformation of reaction mechanism from solid-state surface reaction (F2) to bulk diffusion model (D2). Besides, Ca- and Al- doped SmMnO3 perovskites possess high solar absorptance over 86.5% (in stark contrast to 12.1% of CeO2), so that only solar concentration ratio of 898 (Sm0.6Ca0.4Mn0.8Al0.2O3) is required to drive thermochemical reactions while benchmark CeO2 needs 1936. This work provides novel approaches for high-performance solar thermochemical CO2 splitting with high CO yield, good cycle stability, small temperature swing, and low solar concentration required.

Numerical evaluation of one-dimensional transparent photonic crystal heat mirror coatings for parabolic dish concentrator receivers

High temperature solar receivers can convert concentrated solar power to heat to drive chemical production or mechanical cycles to produce electricity. Transparent windows with selective coatings that reflect thermal radiation can be used to cover receivers to improve their efficiency. However, in parabolic dish concentrators (PDC) these windows can reflect or absorb incoming solar radiation, and may actually reduce the efficiency and power output from PDC receivers. In this work numerical analysis shows that one-dimensional transparent photonic crystal heat mirrors (TPCHMs), which have the form of a modified dielectric mirror, can be designed to be highly transmissive to solar radiation but highly reflective towards thermal radiation. Results show that TPCHMs can be designed to provide significant enhancements to PDC receiver efficiencies operating at lower solar concentration ratios. Specifically, numerical analysis shows efficiency improvements of at least 62%, and 193% compared to either open-face receivers or the case when the receiver window is coated with a transparent conducting oxide film at operating temperatures of 1000 K and 1500 K, respectively. These results suggest that TPCHM covers have potential for enhancing the performance of smaller PDC systems operating at relatively low solar concentration ratios.

One of the most efficient methods to utilize full-spectrum solar energy: A photovoltaic-thermoradiative coupled system

Solar photovoltaic cells are widely used in many aspects, but a mass of solar energy cannot be utilized, resulting in much waste heat being dissipated into the environment, which limits the performance of photovoltaic cells. To improve the utilization of solar energy, a novel model of the concentrated solar spectrum photovoltaic-thermoradiative coupled system is established, where main irreversible losses in the system are considered. Expressions for the power outputs and efficiencies of two subsystems and coupled system are derived. The effects of the voltage outputs and bandgap energies of two cells, area ratios of different units, and the concentration factor on the performance of the coupled system are investigated in detail. The maximum efficiency and corresponding power output density of the coupled system are numerically calculated. The optimal selection criteria of the main parameters affecting the systemic performance are provided. Two subsystems are optimally matched. The performance characteristics of the coupled system are revealed. Compared with the photovoltaic cell and thermoradiative cell alone, the proposed system has a large efficiency enhancement. Compared with other photovoltaic-based coupled systems, the proposed system can be operated at low solar concentration and the maximum efficiency can attain 44.91%, which has a significant efficiency improvement.

High-temperature, spectrally-selective, scalable, and flexible thin-film Si absorber and emitter

Solar thermal technologies have great potential to provide low-cost storage for solar energy. However, their efficiencies are limited by a lack of scalable, mechanically flexible, durable, yet highly-efficient spectrally-selective solar absorbers suitable for high temperatures at low solar concentrations. Here, we overcome these challenges by fabricating a scalable free-standing spectrally-selective thin-film Si absorber and emitter (SSTFS) composite. Its high-temperature emittance shows strong spectral selectivity, even at 595 °C. Thermal stability is proven by measuring optical properties before and after thermal cycling equivalent to one day of concentrated sunlight. Despite the use of crystalline Si, the fabricated SSTFS composite exhibits exceptional mechanical flexibility to cover most surface geometries. The SSTFS composite demonstrates the potential of high-temperature, efficient and flexible solar absorbers and thermal emitters to advance renewable solar energy with storage.

III‐V//Si multijunction solar cells with 30% efficiency using smart stack technology with Pd nanoparticle array

AbstractMultijunction (MJ) solar cells achieve very high efficiencies by effectively utilizing the entire solar spectrum. Previously, we constructed a III‐V//Si MJ solar cell using the smart stack technology, a unique mechanical stacking technology with Pd nanoparticle array. In this study, we fabricated an InGaP/AlGaAs//Si three‐junction solar cell with an efficiency of 30.8% under AM 1.5G solar spectrum illumination. This efficiency is considerably higher than our previous result (25.1%). The superior performance was achieved by optimizing the structure of the upper GaAs‐based cell and employing a tunnel oxide passivated contact Si cell. Furthermore, we examined the low solar concentration performance of the device and obtained a maximum efficiency of 32.6% at 5.5 suns. This performance is sufficient for realistic low concentration photovoltaic applications (below 10 suns). In addition, we characterize the reliability of the InGaP/AlGaAs//Si three‐junction solar cell with a damp heat test (85 °C and 85% humidity for 1000 h). It was confirmed that our solar cells have high long‐term stability under severe conditions. The results demonstrate the potential of GaAs//Si MJ solar cells as next‐generation photovoltaic cells and the effectiveness of smart stack technology in fabricating multijunction cells.

Maximum efficiency and parametric optimum selection of a concentrated solar spectrum splitting photovoltaic cell-thermoelectric generator system

An updated model of the solar spectrum splitting photovoltaic cell-thermoelectric generator system is used to derive the analytical expression of the systemic efficiency. The cutoff energy of the system, cell voltage, and dimensionless current of the thermoelectric generator are optimized. Particularly, the influences of the area ratio of the collector to photovoltaic cell on the performance of the spectrum splitting system are discussed in detail. The maximum efficiencies of the spectrum splitting system are calculated for given solar concentration factors. The results obtained are compared with those of the single photovoltaic cell with the same solar concentration factor. It is found that the maximum efficiency of the system can be further improved when the area ratio is optimized. The expressions of the upper and lower bounds of the optimized area ratio are derived. The spectrum splitting system can more efficiently harvest the full solar spectrum than the single photovoltaic cell, especially at low solar concentration factors. For example, when solar concentration factors are, respectively, equal to 30 and 100, the maximum efficiencies of the spectrum splitting system attain 39.5% and 40.2% and increase approximately 2.67% and 2.19%, compared to those of the single photovoltaic cell.

Experimental and numerical investigation of hybrid concentrated photovoltaic – Thermoelectric module under low solar concentration

The quick progress in solar energy technology has made it one of the most promising alternatives to conventional energy systems in recent years. In this work, in order to make efficient use of the solar energy, a hybrid system composed of the concentrated photovoltaic cell and thermoelectric generator (CPV-TEG) is studied using both experimental and numerical approaches. The experimental study is carried out under concentrated radiation of a solar simulator, and the numerical simulation is accomplished using finite volume method. The results are presented for various solar concentration (SC) values ranging from 8 suns to 37 suns. The variation of the temperatures, open circuit voltage, and short circuit current are discussed. I-V-P curves for both CPV and TEG are obtained and evaluated experimentally and numerically. The results show that contribution of the TEG in the total electrical power produced by the hybrid system enhances with increasing the solar radiation. Furthermore, the experimental results indicate that the maximum and minimum efficiency of the CPV is reached to 35.33% and 23.02%, while these values for the TEG are 1.20% and 0.63%, respectively.

Charge transfer mechanics in transparent dye-sensitised solar cells under low concentration

In dye-sensitized solar cells (DSSCs), the incoming sunlight can influence many of the key electron transfer processes occurring at the TiO2/dye/electrolyte interface. So, the intensity of incident light is crucial in determining the overall efficiency of the devices. Here, transparent DSSC exhibiting an average light transmittance of 53% and high energy conversion efficiency (5.93%) is fabricated. A low solar concentrator with 3X optical concentration is coupled with the device to improve the photovoltaic performance without affecting its transparency. The internal resistances of the fabricated transparent DSSCs with and without the low solar concentrator are analysed using impedance spectroscopy. The results indicate that the charge transfer resistance at the TiO2/dye/electrolyte interface of concentrator coupled devices are lower than the bare devices. Moreover, the device active area is scaled up and charge transport properties are compared with a low concentrator coupled device. The overall results show that the concentrator coupled device performs better than the scaled-up device.

A bioinspired capillary-driven pump for solar vapor generation

Harvesting solar energy for vapor generation has attracted large amount of attention due to its promise for applications in water purification, desalination, power generation, and so on. Many structures based on the two-layered design or the plasmonic enhanced evaporation have been reported to promote the efficiency of solar vapor generation. Inspired by the transpiration phenomenon in plant, we report that a capillary-driven pump can be used for highly efficient solar vapor generation. The pump is mainly consisted of a porous hydrophilic modified NiO (M-NiO) disc and a one-dimension water supply channel. The M-NiO with a three-layered structured TiAlON-based nanocomposite absorber deposited on its surface can efficiently capture solar radiation (absorptance of 0.97). Driven by the capillary force in the porous M-NiO, the pump can continuously wick water via the one-dimension channel to the solar absorber for evaporation, achieving solar-to-vapor efficiency of 73% at 1 sun and 90% at 4 suns. The high conversion efficiency can be attributed to the high absorption ability of the M-NiO disc and the one-dimension water supply design to limit the thermal loss. This new design with advantage of easy scale-up provides an efficient approach to harvest sunlight for solar vapor generation in low solar concentrations.

Designing a Broadband Pump for High-Quality Micro-Lasers via Modified Net Radiation Method.

High-quality micro-lasers are key ingredients in non-linear optics, communication, sensing and low-threshold solar-pumped lasers. However, such micro-lasers exhibit negligible absorption of free-space broadband pump light. Recently, this limitation was lifted by cascade energy transfer, in which the absorption and quality factor are modulated with wavelength, enabling non-resonant pumping of high-quality micro-lasers and solar-pumped laser to operate at record low solar concentration. Here, we present a generic theoretical framework for modeling the absorption, emission and energy transfer of incoherent radiation between cascade sensitizer and laser gain media. Our model is based on linear equations of the modified net radiation method and is therefore robust, fast converging and has low complexity. We apply this formalism to compute the optimal parameters of low-threshold solar-pumped lasers. It is revealed that the interplay between the absorption and self-absorption of such lasers defines the optimal pump absorption below the maximal value, which is in contrast to conventional lasers for which full pump absorption is desired. Numerical results are compared to experimental data on a sensitized Nd3+:YAG cavity, and quantitative agreement with theoretical models is found. Our work modularizes the gain and sensitizing components and paves the way for the optimal design of broadband-pumped high-quality micro-lasers and efficient solar-pumped lasers.

Front contact optimization of industrial scale CIGS solar cells for low solar concentration using 2D physical modeling

Cu(In,Ga)Se2 (CIGS) technology is one of the best absorber materials with record efficiencies among photovoltaic thin-film technologies (22.3% at lab scale and 16% at large commercial module). Although research on this material was originally motivated by low-cost, glass-glass applications focusing to fixed photovoltaic structures, the high efficiency values make CIGS an interesting alternative for low concentration systems. In this paper a 2D model for Cu(In,Ga)Se2 (CIGS) solar cells under low solar concentration is described and contrasted with experimental data. Using simulation, the effect of front electric contact design parameters: finger width, finger separation, and number of buses are analyzed for solar concentrations from 1 up to 10 suns. Efficiency maps allowing front contact grid optimization are shown and analyzed for each concentration factor (Cx), assessing the viability of CIGS solar cells for low concentration applications, where commercial CIGS solar cells may exhibit 35% of electrical power increases with proper front grid optimization under low concentration respect to conventional grid design.

Mechanical design of a low cost parabolic solar dish concentrator

The objective of this research was to design a low cost parabolic solar dish concentrator with small-to moderate size for direct electricity generation. Such model can be installed in rural areas which are not connected to governmental grid. Three diameters of the dish; 5, 10 and 20m are investigated and the focal point to dish diameter ratio is set to be 0.3 in all studied cases. Special attention is given to the selection of the appropriate dimensions of the reflecting surfaces to be cut from the available sheets in the market aiming to reduce both cutting cost and sheets cost. The dimensions of the ribs and rings which support the reflecting surface are optimized in order to minimize the entire weight of the dish while providing the minimum possible total deflection and stresses in the beams. The study applies full stress analysis of the frame of the dish using Autodesk Inventor. The study recommends to use landscape orientation for the reflective facets and increase the ribs angle and the distance between the connecting rings. The methodology presented is robust and can be extended to larger dish diameters.

Numerical and Experimental Analysis of a CPV/T Receiver Suitable for Low Solar Concentration Factors

Solar energy conversion is one promising technology to provide the building required energy. Generally, the main used technologies are the PV and thermal flat panels, but this situation provides separately electricity and thermal energy. Electrical and Thermal power combined production is available for the concentrating solar but, usually, this technology is applied to devices working at high concentration factor (over 100), which are large and, therefore, are not suitable for roof installations. At lower concentrating factors (less of 50 suns) small linear, mono-axial, roof integrated devices can be designed and built. The solar receiver plays a key role in the performance of energy generation because it houses the solar cells and itis used to recover the thermal solar power: actually, this is the device where solar energy is converted in electrical and thermal power. The radiation flux distribution on the receiver affects the efficiency of the linear solar concentrator system, because in a mono-axial sunrays are not perpendicular to the receiver. This paper describes the numerical and experimental investigation useful to evaluate the performance of a linear low (20 suns) CPV device and to understand the thermal working condition of the solar receiver. The experimental study focuses to a quantitative analysis of the energy transfer from sun to the water. The numerical activity is a CFD conjugate analysis where the solid volume and the fluids are investigated together; the scope is to individuate how the energy flux cross the device.

Advanced thin-film chip concepts for efficient InGaN solar cells

Two chip concepts based on thin-film technology are presented and compared regarding their suitability for concentrating photovoltaic applications. The main difference is given by the n-contact design: the first concept relies on a metal grid (MG) while the second one is based on vias (V) connected from the bottom of the solar cell. Although the MG-chip demonstrated a better performance at low solar concentrations, the V-chip was found to be more efficient at high concentrations being the relevant operation range for InGaN-based solar cell applications. Detailed investigations of J–V characteristics applying varied light concentrations revealed a significantly lower series resistance as the main advantage of the V-concept leading to considerably better fill factors at high solar concentrations.

Upconverter Silicon Solar Cell Devices for Efficient Utilization of Sub-Band-Gap Photons Under Concentrated Solar Radiation

Upconversion (UC) of sub-band-gap photons has the potential to increase the efficiency of solar cells significantly. We realized an upconverter solar cell device, by attaching an upconverter layer of β-NaYF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> doped with 25% Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> embedded in the polymer perfluorocyclobutyl to the rear side of a bifacial silicon solar cell. We determined the external quantum efficiency of such upconverter solar cell devices under broad-band sub-band-gap excitation. Under consideration of spectral mismatch, we calculated the expected increase of the short-circuit current density due to UC under the air mass 1.5 global illumination. We determined an enhancement of 2.2 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for a spectral excitation band ranging from 1450 to 1600 nm and a comparatively low solar concentration of 78 suns. Subsequently, a system of concentrator lens and upconverter solar cell device was characterized with a solar simulator. We determined an increase of the short-circuit current density due to UC of sub-band-gap photons of 13.1 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> under a concentration of 210 suns. This corresponds to a potential relative increase of the solar cell efficiency of 0.19%.

Absolute upconversion quantum yield of β-NaYF4 doped with Er3+ and external quantum efficiency of upconverter solar cell devices under broad-band excitation considering spectral mismatch corrections

For applications to harvest solar energy, it is essential to characterize upconverter materials under broad-band excitation at reasonable irradiance levels achievable by concentration of solar radiation. We present a method to determine the absolute upconversion quantum yield (UCQY) under broad-band excitation by photoluminescence measurements. We introduce a spectral mismatch correction that allows calculating the UCQY that can be expected under illumination with the solar spectrum with a certain solar concentration. Applying these methods to β-NaYF4 doped with 25%Er3+, we determine an external UCQY of 2.0% in the spectral range from 1450nm to 1600nm under a comparatively low solar concentration of only 50suns. This value corresponds to a potential increase of the short-circuit current density of 3.89mA/cm2. Subsequently, we measure the external quantum efficiency due to upconversion of sub-band-gap photons for a bifacial silicon solar cell with upconverter attached to its rear side under the same broad-band excitation. We determine an additional short-circuit current density due to upconversion of 4.03mA/cm2 for a solar concentration of only 77suns. This value agrees very well, with the expected additional short-circuit current density due to upconversion, calculated from the external UCQY values as determined by the photoluminescence measurements, by considering the transmittance of the solar cell for sub-band-gap photons and the external quantum efficiency of the solar cell for photons emitted by the upconverter. Finally, a comparison with different literature values based on a set of figures of merit shows that our results currently constitute the largest enhancement of the short-circuit current density due to upconversion.

Solar absorption heating in horizontal channel: Influence of absorbing plate location on thermal performance

Thermal analysis of solar absorption heating system is considered and the influence of absorbing plate location on the performance characteristics is examined. The study is extended to include the effects of solar concentration and the Reynolds number on the performance characteristics. A numerical scheme incorporating the full radiation heating model is used in the simulations. It is found that the performance parameter increases with increasing Reynolds number. The performance parameter attains the highest value for the absorbing plate location at the mid-height of the channel for low solar concentration (C=1). In the case of high solar concentration (C=5), the performance parameter becomes the highest for the absorbing plate location at the top of the channel. The pump power loss parameter remains high for the absorbing plate location at the mid-heights of the channel.

The Problem of Automation of Solar Concentrator Assembly and Adjustment

Recently we have proposed a low cost solar concentrator based on a large number of small flat mirrors that approximates parabolic surface needed for solar light concentration. In this work we describe an improvement of the concentrator support frame assembly. The improvement is connected with the design of the gauge/guages for support frame adjustment. The gauge has the parabolic edge and rotates in the central tube. Special nuts are moved up to the contact with parabolic edge of the gauge