Solar Concentration Ratio Research Articles

Performance Analysis and Optimization of Concentrating Solar Thermoelectric Generator

A thermodynamic model for a concentrating solar thermoelectric generator considering the Thomson effect combined with Fourier heat conduction, Peltier, and Joule heating has been developed and optimized in MATLAB environment. The temperatures at the hot and cold junctions of the thermoelectric generator were evaluated by solving the energy balance equations at both junctions. The effects of the solar concentration ratio, input electrical current, number of thermocouples, and electrical load resistance ratio on the power output and energy and exergy efficiencies of the system were studied. Optimization studies were carried out for the STEG system, and the optimum number of thermocouples, concentration ratio, and resistance ratio determined. The results showed that the optimum values of these parameters are different for conditions of maximum power output and maximum energy and exergy efficiency. The optimum values of the concentration ratio and load resistance ratio for maximum energy efficiency of 5.85% and maximum exergy efficiency of 6.29% were found to be 180 and 1.3, respectively, with corresponding power output of 4.213 W. Furthermore, at higher concentration ratio (C = 600), the optimum number of thermocouples was found to be 101 for maximum power output of 13.75 W, maximum energy efficiency of 5.73%, and maximum exergy efficiency of 6.16%. Moreover, the optimum number of thermocouple was the same for conditions of maximum power output and energy and exergy efficiency. The results of this study may provide insight for design of actual concentrated solar thermoelectric generator systems.

A hybrid TEG/wind system using concentrated solar energy and chimney effect

This paper investigates a novel hybrid system combining thermal electrical generators (TEGs) and a wind turbine. The mathematical model of the system is derived and solved to investigate the performance of the proposed system. In the proposed system, solar energy is converted to heat by an absorber plate. Some portion of this heat is converted to electricity using TEG, while another portion of the heat is used to heat up air flowing in an inclined duct placed underneath the absorber plate. Heating the air inside the system enhances the current speed because of the effect of buoyancy. A wind turbine is placed inside the duct parallel to air flow before it exits to the atmosphere. The wind current is accelerated before passing through the turbine by using venture effect. The TEGs are exposed to the concentrated solar radiation. This can be obtained using a compound parabolic concentrator. The proposed configuration has multiadvantages: (1) the wind is used to drive a wind turbine; (2) air cools the rear surfaces of TEGs to increase the temperature difference between the opposite surfaces, thus generates more electrical power; and (3) it uses buoyancy effect to increase the wind stream speed, thus enhancing the power generated from turbine. It is found that the solar concentration ratio is the most important factor contributing to enhancing the TEG efficiency. The buoyancy effect leads to turbine power boost at high wind speeds more than at low wind speeds.

A detailed mathematical model for thermal performance analysis of a cylindrical cavity receiver in a solar parabolic dish collector system

Abstract In this study, a comprehensive mathematical model was developed and proposed for a cylindrical cavity receiver, placed at focal point of a parabolic dish collector (PDC) system. The main approach in the model is based on non-isothermal internal walls of receiver. The model was validated with experimental data and the statistical parameters of R2, RMSE and MBE show that there is a good agreement between the model results and experimental data. The effect of some operational parameters such as, HTF mass flow rate and global solar irradiation intensity, and some geometrical parameters of solar receiver such as, receiver aperture diameter and its length on the thermal performance of the system were investigated. For the parabolic dish collector and its receiver, a new definition called as solar irradiation concentration ratio (CRirr), was proposed. For a receiver with geometrical concentration ratio (CRgeo) greater than solar irradiation concentration ratio, by increasing in receiver aperture area up to 0.2 m the Heat Transfer Fluid (HTF) outlet temperature and receiver thermal efficiency are increased. For CRgeo less than CRirr, the mentioned parameters decrease due to increasing in heat losses. The proposed model can be applied in Design for Manufacturing (DFM) of novel high performance solar receivers.

Design and optimization of nanoparticle-pigmented solar selective absorber coatings for high-temperature concentrating solar thermal systems

We present a systematic approach for the design and optimization of nanoparticle-pigmented solar selective absorbers for operation at 750 °C. Using the scattering and absorption cross-sections calculated by Lorenz-Mie scattering theory as input, we employ a four-flux radiative transfer method to investigate the solar selectivity mechanism and optimize the optical-to-thermal conversion efficiency (ηtherm) as a function of the metallic nanoparticle material, the nanoparticle diameter, the volume fraction, and the coating thickness. Among the nanoparticle material candidates in this study, C54-TiSi2 is the best option with an optimized ηtherm = 87.0% for a solar concentration ratio of C = 100 and ηtherm = 94.4% for C = 1000 at 750 °C. NiSi is also a promising candidate comparable to TiSi2 in thermal efficiency. Experimentally, an un-optimized 200 nm-diameter TiSi2 nanoparticle-silicone solar selective coating has already achieved ηtherm = 89.8% for C = 1000 at 750 °C. This performance is consistent with the theoretical model and close to the thermal efficiency of the commercial Pyromark 2500 coatings (90.1%). We also demonstrate that Ni/NiSi core-shell structures embedded in the SiO1.5 matrix is thermally stable at 750 °C for 1000 h in air. These results indicate that silicide cermet coatings are promising to achieve high optical performance and high temperature thermal stability simultaneously.

Enhancing the performance of a solar driven hybrid solar still/humidification-dehumidification desalination system integrated with solar concentrator and photovoltaic panels

A new solar driven desalination system is developed using hybrid solar still/two effects humidification-dehumidification desalination system combined with solar concentrator and two thermally cooled photovoltaic panels. The system performance is investigated under different operating conditions including varying the basin water height, circulating air mass flow rate, and solar concentration ratio. A transient mathematical model based on the conservation of mass and energy equations for the system components is developed and the predicted results are validated using the available experimental and numerical data. The results indicated that the system productivity decreases with the increase of the basin height and the circulating air mass flow rate. Integrating the photovoltaic panels along with solar concentrator leads to a significant increase in the fresh water yield at high concentration ratio. The maximum temperature of photovoltaic panels, electrical output power and efficiency of both panels are presented. Accordingly, selection of the optimal operating conditions is developed within the allowable maximum basin water temperature (to avoid a potential scale formation) and the maximum photovoltaic panels' temperature.

Modelling the efficiency of a nanofluid-based direct absorption parabolic trough solar collector

In this paper we propose an approximate analytic solution to the steady state, three-dimensional model of the efficiency of a nanofluid-based direct absorption parabolic trough solar collector under a turbulent flow regime. The model consists of a system of equations: a partial differential equation describing the conservation of energy, and a radiative transport equation describing the propagation of radiation through the nanofluid. Writing the model in non-dimensional form leads to four controlling non-dimensional numbers, specifically one describing the relative importance of conduction and advection and three representing the heat loss to the surroundings. We use realistic parameter values to reduce the model further and show that two of the non-dimensional groups have a much lesser impact on the performance of the solar collector. Our reduced model suggests that the nanofluid’s temperature rise is linear as it flows through the receiver. The resulting solution is used to investigate the efficiency of the collector and permits optimisation of design parameters such as particle loading, particle type, solar absorption characteristics of the fluid, receiver dimensions, the inlet temperature, and solar concentration ratio. Further analysis of the collector efficiency reveals an inequality that determines whether or not it is reasonable to incorporate a heat-mirror into the solar collector’s design.

Design optimization of ultra-high concentrator photovoltaic system using two-stage non-imaging solar concentrator

This paper presents a systematic approach for optimizing the design of ultra-high concentrator photovoltaic (UHCPV) system comprised of non-imaging dish concentrator (primary optical element) and crossed compound parabolic concentrator (secondary optical element). The optimization process includes the design of primary and secondary optics by considering the focal distance, spillage losses and rim angle of the dish concentrator. The imperfection factors, i.e. mirror reflectivity of 93%, lens’ optical efficiency of 85%, circumsolar ratio of 0.2 and mirror surface slope error of 2 mrad, were considered in the simulation to avoid the overestimation of output power. The proposed UHCPV system is capable of attaining effective ultra-high solar concentration ratio of 1475 suns and DC system efficiency of 31.8%.

Proposal and assessment of a solar thermoelectric generation system characterized by Fresnel lens, cavity receiver and heat pipe

A solar thermoelectric generation system characterized by Fresnel lens, cavity receiver and heat pipe, i.e., FCH-STGS, was proposed. To analyze the performance of FCH-STGS accurately, a comprehensive theoretical model was built, where a finite element method was employed to consider the Thomson effect and temperature-dependence of thermoelectric materials. With this model, the influences of emissivity of ceramic plate and environment factors such as solar irradiation, optical concentration ratio, wind speed, wind incident angle, system tilt angle and ambient temperature were discussed. Results show that the FCH-STGS has the potential to reach the output power and efficiency of 45.73 W and 3.289%. Both output power and efficiency of FCH-STGS can be improved by decreasing the emissivity of ceramic plate or ambient temperature, or increasing the total solar irradiation (product of solar irradiation and optical concentration ratio). However, the efficiency reduces slightly when the total solar irradiation exceeds a certain value. As the wind speed increases, both output power and efficiency rise firstly and then fall. Due to the high thermal efficiency of cavity (exceeding 85%), the effects of wind incident angle and system tilt angle are approximately negligible. Compared with the previous STGSs, the FCH-STGS is excellent in the perspectives of performance, economy and practicability.

Explore the performance limit of a solar PV – thermochemical power generation system

Performance limit of a solar hybrid power generation system integrating efficient photovoltaic (PV) cells and methanol thermal (T) decomposition is explored from a thermodynamic perspective within the capability of state-of-the-art technologies. This type of PVT system features potentially high “net solar-to-electric efficiency” in general, primarily resulting from a key difference in the design of the thermal part compared with conventional PVT systems, i.e. replacing heat engines by a thermochemical power generation module for thermal energy utilization. Key design parameters of the system, including PV cell type, emissivity, solar concentration ratio and solar concentrator type, are individually studied. A system combining all such optimized aspects is projected to achieve net solar-to-electric efficiencies up to 51.5%, after taking all major (e.g. optical, radiative) losses into consideration. This study reveals important insights and enriches understanding on design principles of efficient PVT systems aimed at comprehensive and effective utilization of solar energy.

Analysis of a temperature dependent optical window for nanofluid-based spectral splitting in PV/T power generation applications

As an emerging technology, nanofluids can be used to split the solar spectrum for PV/T applications At present, most of the nanofluids developed for solar applications have been at low temperature, but it may be possible to develop fluids for high temperature systems. This study takes an in-depth look at the allowable temperature of the nanofluid and at how this directly affects the performance of hybrid system. In order to analyze this issue, the present work develops and experimentally validates a model to predict the performance of high temperature nanofluid-based spectral splitting PV/T systems for a wide range of optical properties, flow rates, and solar concentration ratios. The efficiency of system, the allowable temperature of fluid, and heat to electricity ratio are the main metrics of comparisons for this parametric analysis. It was found that the optimum ‘cut-on’ wavelength of nanofluid transparency should be set at 620–680nm for silicon solar cells, for concentration ratios of 10–50, when the allowable temperature of nanofluid is 400°C. This value will then move towards longer wavelengths as the allowable temperature of the nanofluid increases. The ‘cut-off’ wavelength for nanofluid transparency should be the bang-gap of the cell, or 1100nm in the case of silicon cells. When an optically ideal nanofluid is adopted, it was found that the power generated in the proposed hybrid system will come mainly from the thermodynamic conversion process. Therefore, we can conclude from a technical perspective that nanofluid-based PV/T systems should focus on developing fluids with as high as possible allowable temperature ratings.

Multi-parameter optimization of double-loop fluidized bed solar reactor for thermochemical fuel production

In this paper, the design of a double-loop fluidized bed solar reactor, involving CeO2 nanoparticles and two gas streams of N2 and CO2, for efficient thermochemical fuel production, has been optimized in a six-dimensional parameter space by means of a multi-parameter optimization algorithm. The system under investigation is capable to develop a thermochemical two-step cycle, producing CO by means of the overall reaction CO2→CO+1/2O2. The use of nanoparticles as catalyst allows maximizing the performance of the reactor; actually, nanoparticles increase surface area of reaction, with respect to common catalysts and, at the same time, allow realizing the reactor as double-loop fluidized bed, which can operate without alternating flows of CO2 and inert sweep gas. A genetic algorithm coupled with a quasi-random Sobol design population has been used, to find the optimal configuration of the double-loop fluidized bed solar reactor.The results highlighted the very important role of several factors, as radius of fluidized beds, mean residence time of reactor, mass of nanoparticles within reactor, solar concentration ratio, etc., on the performance of the system under investigation and allowed to find the best configuration of the system, reaching the mean global efficiency over a period of time of 1 year equal to 29.96%, with a maximum of 59.46%.

Solar electricity via an Air Brayton cycle with an integrated two-step thermochemical cycle for heat storage based on Co3O4/CoO redox reactions III: Solar thermochemical reactor design and modeling

A two-step solar thermochemical cycle based on Co3O4/CoO redox reactions integrated into an Air Brayton cycle is considered for thermochemical heat storage. The two-step cycle encompasses (1) the thermal reduction of Co3O4 to CoO and O2 driven by concentrated solar irradiation and (2) the re-oxidation of CoO with O2 to Co3O4, releasing heat and completing the cycle. An evacuated horizontal solar thermochemical reactor is proposed with an inclined slope and quartz window for promoting direct irradiation of dense, granular Co3O4/CoO flows. Mechanical analysis of flat and spherical quartz window designs for a 5kWth scale prototype was performed to ensure window stability. Detailed mass and heat transfer analysis for a 5kWth scale prototype was performed coupling Monte Carlo ray tracing for radiative heat exchange to the energy balances for the bed and the reactor. A parametric study of the reactor design was performed with varying cavity depth, particle inlet temperature, and solar concentration ratio. The optimal solar reactor design maximized conversion of Co3O4 to CoO and particle outlet temperature while preventing particle overheating and achieved a Co3O4 to CoO conversion of 0.91, particle outlet temperature of 1385K, maximum flow temperature of 1572K, and absorption efficiency of 0.76.

Glass melting using concentrated solar thermal energy

Glass melting using concentrated solar thermal radiation is demonstrated on the kilogramme scale using a high flux solar simulator (HFSS). The melting process involved a novel furnace design utilising a downward orientated concentrated solar beam coupled with back-up integrated electrical heating elements, which provided secondary heating to maintain a melt when the HFSS beam was unavailable (i.e. equivalent to cloudy conditions or at night). With 5·26 kW radiative power from the HFSS input to the furnace through a 6 cm diameter aperture, pelleted soda–lime–silica batches were melted. Repeated additions of ∼300 g of batch pellets were made to the melt with each ∼300 g addition requiring ∼15 min for the reactions to complete and the melt temperature to recover. This is equivalent to a glass batch melting thermal efficiency of 16% at a solar concentration ratio of 1857 suns. The areas of land required for the heliostat field and, for the electrical backup elements, photovoltaic field are shown to be significant for even moderate daily glass production tonnages.

Rectifying structural deflection effect of large solar concentrator via correction of sun-tracking angle in the concentrator photovoltaic system

The influence of self-weight structural deflection of large solar concentrator has caused significant impact to the quality of concentrated solar flux. The resulted negative outcomes from structural deflection, including image distortion and pointing error, can deteriorate the electrical power generated by dense-array concentrator photovoltaic (CPV) module located at the target of large solar concentrator. In this article, a novel corrective measure for sun-tracking angle is proposed to rectify the structural deflection effect on the large solar concentrator and hence to optimize the electrical output power of dense-array CPV module. The methodology of the proposed corrective measure has been formulated and discussed in details using numerical simulation via ray-tracing technique. The simulated result has shown that the proposed method is proficient to recover the deviated and distorted solar flux distribution in the receiver without affecting the maximum solar concentration ratio and hence to reduce the electrical power loss. For the overall gain, the maximum output power has been increased from 3562W to 4030W and the electrical power loss has been reduced from 12.4% to 0.8% at elevation angle of 60° in the dense-array CPV system.

A pressurized high-flux solar reactor for the efficient thermochemical gasification of carbonaceous feedstock

We report on the design and first-ever experimental demonstration of a 3kW pressurized solar reactor for thermochemically converting carbonaceous feedstocks into gaseous fuels. It uses a windowless SiC cavity to efficiently absorb and transfer concentrated solar radiation to an annular gas-particle vortex flow created by injecting tangentially a charcoal/water slurry at high pressures. Experiments were carried out in a high-flux solar simulator under a solar concentration ratio equivalent to 3718 suns. For slurry feeding rates in the range 0.42–1.26g/min, H2O:C molar ratios in the range 1.48–1.98, and absolute reactor pressures in the range 1–6bar, the nominal reactor temperature was between 1009 and 1273°C yielding high-quality syngas with a carbon conversion up to 94% within residence times of less than 5s. The peak solar-to-fuel energy conversion efficiency, defined as the ratio of the heating value of the syngas produced to the solar radiative energy input plus the heating value of the slurry converted, reached 20%. The calorific value of the feedstock was solar upgraded by 35%, thus outperforming autothermal gasification in addition to delivering higher syngas output per unit of feedstock and eliminating the need for upstream air separation. The performance of this robust windowless design also was compared with that of an equivalent windowed configuration in which the gas-particle vortex flow was directly exposed to high-flux solar irradiation.

Thermodynamic analysis of a combined-cycle solar thermal power plant with manganese oxide-based thermochemical energy storage

We explore the thermodynamic efficiency of a solar-driven combined cycle power system with manganese oxide-based thermochemical energy storage system. Manganese oxide particles are reduced during the day in an oxygen-lean atmosphere obtained with a fluidized-bed reactor at temperatures in the range of 750–1600°C using concentrated solar energy. Reduced hot particles are stored and re-oxidized during night-time to achieve continuous power plant operation. The steady-state mass and energy conservation equations are solved for all system components to calculate the thermodynamic properties and mass flow rates at all state points in the system, taking into account component irreversibilities. The net power block and overall solar-to-electric energy conversion efficiencies, and the required storage volumes for solids and gases in the storage system are predicted. Preliminary results for a system with 100 MW nominal solar power input at a solar concentration ratio of 3000, designed for constant round-the-clock operation with 8 hours of on-sun and 16 hours of off-sun operation and with manganese oxide particles cycled between 750 and 1600°C yield a net power block efficiency of 60.0% and an overall energy conversion efficiency of 41.3%. Required storage tank sizes for the solids are estimated to be approx. 5–6 times smaller than those of state-of-the-art molten salt systems.

Performance analysis of a hybrid solar thermoelectric generator

ABSTRACTIn this paper, a theoretical model is developed to investigate the performance of the hybrid solar thermoelectric generator (HSTEG) system, which is designed without (B-HSTEG) and with an evacuated glass tube (V-HSTEG). The heat loss, power output, thermal efficiency, and electrical efficiency of the B-HSTEG/V-HSTEG system are evaluated by analyzing the design parameters such as geometric solar concentration ratio, thermoelectric figure of merit, and cold-side inlet fluid temperature. The performance of the B-HSTEG is compared with the V-HSTEG system using two heat transfer fluids: water and Therminol VP-1. The maximum electrical efficiency of the B-HSTEG and V-HSTEG is estimated to be 12.2 and 15.6% (ZT = 3) with a corresponding thermal efficiency of about 61.9 and 60.3%, respectively. Overall, this paper provides a systematic performance analysis of HSTEG systems.

Comparison of selective transmitters for solar thermal applications.

Solar thermal collectors are radiative heat exchangers. Their efficacy is dictated predominantly by their absorption of short wavelength solar radiation and, importantly, by their emission of long wavelength thermal radiation. In conventional collector designs, the receiver is coated with a selectively absorbing surface (Black Chrome, TiNOx, etc.), which serves both of these aims. As the leading commercial absorber, TiNOx consists of several thin, vapor deposited layers (of metals and ceramics) on a metal substrate. In this technology, the solar absorption to thermal emission ratio can exceed 20. If a solar system requires an analogous transparent component-one which transmits the full AM1.5 solar spectrum, but reflects long wavelength thermal emission-the technology is much less developed. Bespoke "heat mirrors" are available from optics suppliers at high cost, but the closest mass-produced commercial technology is low-e glass. Low-e glasses are designed for visible light transmission and, as such, they reflect up to 50% of available solar energy. To address this technical gap, this study investigated selected combinations of thin films that could be deposited to serve as transparent, selective solar covers. A comparative numerical analysis of feasible materials and configurations was investigated using a nondimensional metric termed the efficiency factor for selectivity (EFS). This metric is dependent on the operation temperature and solar concentration ratio of the system, so our analysis covered the practical range for these parameters. It was found that thin films of indium tin oxide (ITO) and ZnS-Ag-ZnS provided the highest EFS. Of these, ITO represents the more commercially viable solution for large-scale development. Based on these optimized designs, proof-of-concept ITO depositions were fabricated and compared to commercial depositions. Overall, this study presents a systematic guide for creating a new class of selective, transparent optics for solar thermal collectors.

Performance enhancement of concentrated photovoltaic systems using a microchannel heat sink with nanofluids

A new cooling technique for low concentrated photovoltaic–thermal (LCPV/T) systems is developed using a microchannel heat sink with nanofluids. In this study, Aluminum Oxide (Al2O3)–water and Silicon Carbide (SiC)–water nanofluids with different volume fractions are used as cooling mediums. The influence of cooling mass flow rate and nanoparticles volume fractions on the performance of LCPV/T system is investigated at different values of concentration ratio. A comprehensive model is developed which includes a thermal model for the photovoltaic layers, coupled with thermo-fluid dynamics of two-phase flow model of the microchannel heat sink. The model is numerically simulated to estimate the performance parameters such as the solar cell temperature and the electrical and thermal efficiency. Results indicate that a significant reduction in solar cell temperature is attained particularly at the high concentration ratio by using nanofluids compared to using water. Using SiC–water nanofluid achieves a relatively higher reduction in cell temperature than Al2O3–water nanofluid. By increasing the volume fraction of nanoparticles, both SiC–water and Al2O3–water nanofluids accomplish a major reduction of cell temperature. As a result, the use of nanofluids achieves higher solar cell electrical efficiency, particularly at lower Reynolds number (Re) and higher concentration ratio, than the use of water. The influence of nanofluids on thermal efficiency varies according to the concentration ratio. Furthermore, friction power increases with the increase in both Reynolds number and nanoparticle volume fraction. By increasing the volume fraction of the nanoparticle, the net electrical power increases at high concentration ratio while the thermal power decreases. The results of this study indicate that the use of nanofluids is effective cooling technique, particularly at high solar concentration ratios where the solar cell temperature reduces to 38°C, and electrical efficiency improves up to 19%.

Copper-alloyed spinel black oxides and tandem-structured solar absorbing layers for high-temperature concentrating solar power systems

Although renewable solar power plants are rapidly proliferating, high cost and the intermittent availability of solar power are still significant barriers for its penetration into the energy grid system. Concentrating solar power (CSP) offers an attractive alternative due to its integration with cost-effective thermal energy storage systems. To further reduce the cost of CSP, it is imperative to operate the plants at higher temperatures for enhanced efficiency. One of the key components for next-generation high-temperature CSP is the solar absorbing coating materials. In this work, we have developed tandem-structured solar absorbing layers with CuFeMnO4 and CuCr2O4 black oxide nanoparticles (NPs). These tandem structures exhibited a remarkably high solar-to-thermal conversion efficiency, or figure of merit (FOM), of 0.903, under the condition of 750°C operating temperature and a solar concentration ratio of 1000. More importantly, the coating showed unprecedented durability, as demonstrated from long-term isothermal annealing at 750°C in air as well as rapid thermal cycling between room temperature and 750°C. Our results suggest that the tandem black oxide coating is suitable to meet the stringent demand of next-generation high-temperature CSP systems. The coating materials synthesis, structures, optical as well as thermal properties will be discussed.