Optical Concentration Systems Research Articles

Full Spectrum Solar Thermal Energy Harvesting and Storage by a Molecular and Phase-Change Hybrid Material

Summary Efficient solar thermal energy harvesting and storage are critical steps toward utilizing the abundant solar irradiation that reaches the surface of the earth. Current solar thermal approaches rely on costly high optical concentration systems, leading to high heat losses by hot bulk materials and surfaces. At the same time, the energy stored in the form of thermal energy has inherently large temporal losses. Here, we combine the physics of molecular energy and latent heat storage to introduce an integrated, simultaneous harvesting and storage hybrid paradigm for potential 24/7 energy delivery. The hybrid paradigm utilizes heat localization during the day to provide a harvesting efficiency of 73% at small scale and ∼90% at large scale. Remarkably, at night, the stored energy by the hybrid system is recovered with an efficiency of 80% and at a higher temperature than that of the day, in contrast to all of the state-of-the-art systems.

Novel Receiver-Enhanced Solar Vapor Generation: Review and Perspectives

Efficient solar vapor/steam generation is important for various applications ranging from power generation, cooling, desalination systems to compact and portable devices like drinking water purification and sterilization units. However, conventional solar steam generation techniques usually rely on costly and cumbersome optical concentration systems and have relatively low efficiency due to bulk heating of the entire liquid volume. Recently, by incorporating novel light harvesting receivers, a new class of solar steam generation systems has emerged with high vapor generation efficiency. They are categorized in two research streams: volumetric and floating solar receivers. In this paper, we review the basic principles of these solar receivers, the mechanism involving from light absorption to the vapor generation, and the associated challenges. We also highlight the two routes to produce high temperature steam using optical and thermal concentration. Finally, we propose a scalable approach to efficiently harvest solar energy using a semi-spectrally selective absorber with near-perfect visible light absorption and low thermal emittance. Our proposed approach represents a new development in thermally concentrated solar distillation systems, which is also cost-effective and easy to fabricate for rapid industrial deployment.

Analytical and numerical analysis of a solar thermoelectric system cooled by an active system

ABSTRACTA solar thermoelectric generator (STEG) system composed of an optical concentrator system (OPS), a Bismuth Telluride thermoelectric module (TEG), and a cold plate-based cooling system (CPCS) was numerically simulated, to measure the efficiency of electric generation of a commercial thermoelectric module under controlled temperatures. The OPS is composed by a Fresnel lens that allows a temperature of around 200 °C, the OPS works with a solar irradiance of 1000 W/m2 (AM 1.5 Reference) and an optical concentration of 60. The OPS is coupled to the hot side of the TEG that consists of a commercially available thermoelectric module. The CPCS maintains a temperature of around 50 °C on the cold side of the TEG. To evaluate the configuration, a computational fluid dynamic (CFD) analysis was carried out to evaluated the thermal performance of the CPCS and the temperature achieved on the upper surface of the cooling device. Based on the numerical results generated by the CFD analysis, an analytical TEG efficiency of around 5% was achieved when a temperature difference, between the hot and cold sides of the commercial TE module, of 150 °C was maintained. We perform an analysis using the Hogan and Shih model that uses the thermoelectric material properties exposed by Chen et al.

Extremely Cost-Effective and Efficient Solar Vapor Generation under Nonconcentrated Illumination Using Thermally Isolated Black Paper.

Passive solar vapor generation represents a promising and environmentally benign method of water purification/desalination. However, conventional solar steam generation techniques usually rely on costly and cumbersome optical concentration systems and have relatively low efficiency due to bulk heating of the entire liquid volume. Here, an efficient strategy using extremely low‐cost materials, i.e., carbon black (powder), hydrophilic porous paper, and expanded polystyrene foam is reported. Due to the excellent thermal insulation between the surface liquid and the bulk volume of the water and the suppressed radiative and convective losses from the absorber surface to the adjacent heated vapor, a record thermal efficiency of ≈88% is obtained under 1 sun without concentration, corresponding to the evaporation rate of 1.28 kg (m2 h)−1. When scaled up to a 100 cm2 array in a portable solar water still system and placed in an outdoor environment, the freshwater generation rate is 2.4 times of that of a leading commercial product. By simultaneously addressing both the need for high‐efficiency operation as well as production cost limitations, this system can provide an approach for individuals to purify water for personal needs, which is particularly suitable for undeveloped regions with limited/no access to electricity.

Refraction-Assisted Solar Thermoelectric Generator based on Phase-Change Lens.

Solar thermoelectric generators (STEGs), which are used for various applications, (particularly small size electronic devices), have optical concentration systems for high energy conversion efficiency. In this study, a refraction-assisted STEG (R-STEG) is designed based on phase-change materials. As the phase-change material (PCM) changes phase from solid to liquid, its refractive index and transmittance also change, resulting in changes in the refraction of the sunlight transmitted through it, and concentration of solar energy in the phase-change lens. This innovative design facilitates double focusing the solar energy through the optical lens and a phase-change lens. This mechanism resulted in the peak energy conversion efficiencies of the R-STEG being 60% and 86% higher than those of the typical STEG at solar intensities of 1 kW m−2 and 1.5 kW m−2, respectively. In addition, the energy stored in PCM can help to generate steady electrical energy when the solar energy was removed. This work presents significant progress regarding the optical characteristic of PCM and optical concentration systems of STEGs.

Toward a standard testing methodology for solar thermal collectors with variable-geometry: The direct radiation incidence angle modifier issue

Solar thermal energy is able to provide a considerable fraction of the medium-temperature energy demand and should be more widely used in specific industrial sectors in 100–250°C range. For energy collection within this temperature range, solar collector designs usually have optical concentration systems, and therefore, the optical behavior of these devices, especially Incidence Angle Modifier (IAM) term in the standard equations, is of paramount importance to the energy equation balance. In the last few decades various new designs have been developed specifically for industrial applications. At the end of 2013, the new revised version of International Standard ISO 9806 for solar thermal collector test methods was published for worldwide certification. This new Standard unifies all the former American, European and International Standards. However, some solar collector types still do not fit to the Standard testing methodology, in particular solar collectors with variable-geometry. This paper describes the points in International Standard ISO 9806 defined specifically for testing solar collectors with solar tracking and concentration, and present a theoretical model to specify the range of the incidence angles which should be tested on variable-geometry collectors to validate the IAM terms in the energy equation balance. Finally some issues are proposed to extend the scope of the current standards for testing collector designs with variable-geometry, in particular for medium-temperature applications.

Comparative Evaluation of Reflective and Refractive Optical Concentration Systems in Tropical Climate

Convergence of global economic inequalities and greenhouse emissions makes it imperative that fossil- fuel dependence be replaced by renewable energy revolution. Sunlight is the only truly free and abundant global energy resource capable of replacing fossil fuels. Historically, thermal and electrical forms of energy have been generated through Concentrated Solar Power (CSP) systems. A major disadvantage of existing CSP systems lies in their long lead times, large start-up costs and integration with an advanced electricity transmission grid. Research work reported here is focused on evaluation of reflective and refractive optical concentration systems in tropical climate with the aim of developing small scale distributed electricity generation systems linked to micro grids. Evaluation of seven optical concentrators in reflection and refraction modes in tropical Malaysian climate has been carried out. The experimental methodology was based on measurement of temperature at the focal point of the optical systems as a function of time and solar irradiance. Highest temperatures achieved with reflective systems were in 200-300°C range, in contrast, Fresnel-lens based refractive systems approached temperatures in excess of ~1300°C. For the Fresnel lenses investigated, an approximate logarithmic temperature dependence on lens diameter was determined. For the Malaysian climate, sunlight to thermal energy conversion of refractive systems was determined to be significantly superior to reflective systems.

Numerical modeling of loss mechanisms resulting from non‐uniform illumination in multijunction concentrator solar cells

AbstractQuasi‐3D distributed emitter models utilizing a unit cell‐based methodology have been successfully applied to the analysis of lateral current flow in solar cell emitter layers. However, the analysis of the specific loss mechanisms resulting from this flow has not been given adequate attention. In this work, a quasi‐3D model for the simulation of effects related to the lateral flow of current in a solar cell emitter layer, particularly under non‐uniform illumination, is developed. The model is applied to a specific case in which a GaInP/GaAs‐like two‐terminal solar cell is illuminated with a Lorentzian irradiance pattern, which is the expected pattern for parabolic trough concentrator. It is shown that bias‐point loss, which results from the variation in the local operating condition of the cell, is significant under highly non‐uniform illumination at low–moderate optical concentration. Understanding this loss mechanism is useful in considering trade‐offs related to the design of the optical concentrator system, as well as, the design of a grid electrode pattern for maximum power output. Copyright © 2013 John Wiley & Sons, Ltd.

Solar hydrogen production by water splitting with a conversion efficiency of 18%

An integrated system combining highly efficient III–V solar cells in an optical concentrator system with a polymer electrolyte membrane electrolyser is presented. Hydrogen and oxygen are produced by solar water splitting. The integrated design and the high concentration factor of 500 allow material savings and high system efficiencies. The thermoneutral solar to hydrogen conversion efficiency of a prototype system with an area of 96 cm 2 was measured outdoor to be 18%.