Review Article

J. Sol. Energy Eng. 2016;138(4):040801-040801-8. doi:10.1115/1.4033596.

This review describes the methodology for the analysis of environmental damages and presents key results obtained by the external costs of energy (ExternE) projects of the European Commission as well as analogous work in the U.S. The classical air pollutants (PM, NOx, SO2, and O3) due to the combustion of fossil fuels cause significant damage costs. The costs of global warming from the emission of greenhouse gases are also large. We show results for the damage cost per kilogram of emitted pollutant for typical conditions in Europe; they are based on the last version of ExternE (published in 2008), but with a major upward adjustment of the monetary values. We also show results that have been published in the U.S. Combined with the emissions data per kilowatt hour, they yield the damage costs of electric power. For the choice between different power technologies, one should take into account not only the emissions from the power plant but also from the entire fuel chain, using life cycle assessment (LCA) inventories. The damage costs of fossil fuels are much higher than most renewable energy sources. The results provide crucial input for the formulation of rational environmental policies, for example, the appropriate level of pollution taxes and the promotion of cleaner technologies.

Commentary by Dr. Valentin Fuster

Research Papers

J. Sol. Energy Eng. 2016;138(4):041001-041001-10. doi:10.1115/1.4032943.

This paper presents assessments of the sensitivity of the performance of high flux solar simulators to the key variables of conical secondary concentrators for metal halide lamps, which offer complementary benefits compared with xenon arc lamps. The assessment is performed for both a single-lamp configuration and a seven-lamp array, each lamp close-coupled with its own elliptical reflector, and then aligned with a common conical secondary concentrator. The simulation of heat flux from both the single- and the seven-lamp systems was performed with the Monte Carlo ray-tracing code, which was validated with the experimental results from the single-lamp system. The calculated heat flux at the focal plane agrees with the measured peak flux to within 5% and to within 13% of the measured half width. Calculated results also show that the addition of the secondary concentrator to the single-lamp system can increase the peak flux by 294% and the average flux by up to 93% within a target of 100 mm in diameter, with a corresponding reduction in total power by 15%. The conical secondary concentrator is less effective for a seven-lamp system, increasing the peak and average fluxes by 87.3% and 100%, respectively, within 100 mm diameter focal plane, with a corresponding reduction in total power by 48%. The model was then used to assess the sensitivity of the geometry of the secondary concentrators for both the single- and seven-lamp systems. The results show that the average heat flux is sensitive to the surface reflectance of the secondary concentrator, with the average flux decreasing almost linearly with the surface reflectance. The presence of the secondary cone greatly reduces the sensitivity of the concentrated heat flux to misalignment of the tilting angle of the elliptical reflector relative to the arc.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):041002-041002-6. doi:10.1115/1.4032944.

A new symmetric nonimaging parametric trough collector (PmTC) for an evacuated circular receiver is proposed having an absorber diameter of 70 mm similar to the LS3/Eurotrough absorber one, and a circular secondary. The optical design method is explained and a sensitivity analysis is conducted with respect to the diameter of the secondary and to the acceptance angle. In the design process, transmission, absorption, and reflection losses are neglected. A secondary mirror radius of 145 mm and an acceptance angle of ±14 mrad were chosen as realistic values. For these values a concentrator geometry having 8.7 m gross aperture and 100% intercept factor for rays impinging on the primary within the referred acceptance angle is obtained producing a net concentration ratio relative to the thermodynamic ideal limit of 0.52 compared to 0.31 for traditional parabolic trough collectors. The new concentrator is composed of a primary discontinuous reflector with two symmetrical portions with parametric geometry, and a central parabolic portion located between the other two. The ensemble secondary receiver can be dressed up with the secondary concentrator manufactured by partially mirroring a diameter adapted glass tube—either internally or externally—or alternatively by means of a commercial evacuated receiver and an independent 145 mm radius arc of circumference external secondary reflector. Monte Carlo ray-tracing results show that only 15% of the rays undergo secondary reflection before arriving to the absorber and a sharp angle transmission curve. The new concentrator is proposed for application in solar thermal energy (STE) trough power plants.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):041003-041003-5. doi:10.1115/1.4032977.

This paper describes and evaluates accelerated aging of a titanium dioxide (TiO2) filled polyamide (PA) based backsheet film for photovoltaic (PV) modules. Damp heat exposure (85%RH, 85 °C) was carried out up to 2000 hrs. The backsheet was characterized using microscopic, spectroscopic, thermoanalytic, chromatographic, and mechanical methods. While Raman microscopy, infrared spectroscopy in attenuated total reflection mode (IR-ATR), scanning calorimetry (DSC), and thermal gravimetric analysis did not reveal aging-induced changes, significant yellowing was detected by ultraviolet visible near infrared (UV/VIS/NIR) spectroscopy. Depending on the stabilizer type (UV-absorbers, hindered amine light stabilizers (HALSs), and antioxidants), rather different consumption rates were ascertained by high-performance liquid chromatography (HPLC) and gas chromatography (GC). Although the ultimate mechanical properties decreased significantly, no full embrittlement was obtained after damp heat exposure of up to 2000 hrs. The observed physical and chemical aging mechanisms were classified as within the induction period without premature failure.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):041004-041004-9. doi:10.1115/1.4033068.

The problem of particle deposition on the glass window of a solar receiver has restricted its continuous operation by reducing solar radiation transmission. A rigorous attempt has been made in this analysis by exploring the understanding of particle deposition mechanisms and their mitigating strategies. A simplified form of a fluidized bed solar receiver (FBSR) having the same flow phenomena of FBSR is chosen for the numerical analysis. In the numerical analysis, the turbulent flow in the receiver is investigated by renormalized group (RNG) theory based k–ε models. The validation of the numerical model is carried out by measuring the turbulent flow properties using a turbulent flow instrumentation (TFI) Cobra probe. The results of this analysis revealed that mass flow into the secondary concentrator of the receiver was reduced significantly when the ratio between the outlets and inlet areas was 0.5, and the ratio between the aperture and receiver diameter was 0.41. When using window shielding jets, only 5% of the inlet mass as a window jet was sufficient to prevent any particle deposition on the glass window, however, the number of jets was found to be an important factor. At a constant mass flow rate, increasing the number of window shielding jets reduced the suction pressure from the core to the aperture, which helped to restrict the inlet flow in the secondary concentrator.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):041005-041005-8. doi:10.1115/1.4033402.

Thermal performance of solar air heater does not take into account the energy loss due to friction for propelling air through the duct. In this work, an experimental investigation has been carried out to study the effect of heat transfer and friction characteristics of turbulent flow of air passing through rectangular duct which is roughened by circular arcs having gaps of 2 mm in between arranged in angular fashion, and the roughened wall is uniformly heated. The thermal and friction characteristics are governed by duct aspect ratio (W/H), hydraulic diameter (D), and relative roughness pitch (P/e), angle of attack of angular arc (α), and Reynolds number (Re). Experiments encompassed the Reynolds number ranges from 3600 to 15,100, P/e ranges from 6 to 20, and angle of attack of angular arc of flow over the protrusions ranges from 15 deg to 75 deg. The results have also been compared to W-shaped roughness inclined at 45 deg. The maximum enhancement in heat transfer and friction factor is 3.15 and 3.93 times as compared with smooth duct. Arc with gaps have also been observed to be better than their W-shaped counterparts. These experimental results have been used to study their influence on Nusselt number and friction factor, and empirical relations have been derived using regression analysis.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):041006-041006-10. doi:10.1115/1.4033403.

Volume-filling ratio of the working fluid has a predominant effect on the system performance of a closed two-phase solar water-heating (SWH) system. To study this effect, a prototype of a loop thermosyphon SWH system, which uses remolded flat-plate solar collector as the evaporator and the coil pipe in the water tank as the condenser, was set up. A set of long-term outdoor experiments under 10%, 20%, 30%, 50%, and 70% volume-filling ratios were conducted in this paper. R600a was used as working fluid. Loop thermosyphon solar collector thermal performance and system thermal performance under different volume-filling ratios, including the temperature distribution of loop thermosyphon evaporator, were presented. It is shown that the loop thermosyphon solar collector and the system had a better thermal performance than the conventional ones under 30% and 50% volume-filling ratio, and the loop thermosyphon evaporator had an even temperature distribution when the volume-filling ratio was higher than 30%. The optimum volume filing ratio lies in between 30% and 50% of the whole system volume.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):041007-041007-11. doi:10.1115/1.4033333.

Recently, accurate modeling of the differences between the current and voltage (I–V) characteristics of solar cells has been the main focus of many research studies. Mostly the results were obtained only for single diode or double diode solar cells, not for both or even for photovoltaic (PV) modules. Moreover, the effect of different shading conditions and different temperatures should be considered; otherwise, the obtained results would be reliable for specific weather conditions and unreliable for all real conditions. In this study, a novel nature-inspired optimization method known as the lightning search algorithm (LSA) was developed to extract the parameters of single diode and double diode solar cells as well as for a PV module. LSA is formulated based on lightning, which originates from thunderstorms. Experimental data from multicrystalline KC200GT solar panels were used to test the single diode and double diode solar panel models, and experimental data from the monocrystalline SQ150-PC solar panels were used to test the PV module model. The experimental data are first collected at the same temperature at five different irradiance levels. In the second stage, variations in temperature are considered at the same irradiance level. The extraction results in the LSA I–V curves accurately fit the entire range of the experimental data, while many fluctuations were seen in the particle swarm optimization (PSO) and bee colony optimization (BCO) I–V curves. The convergence characteristics of LSA were also evaluated in terms of accuracy and speed. For all cases, when LSA was used, the accuracies matched well with the entire range of experimental data. In addition, the value of the objective function using LSA was lower, and that method converged much faster than PSO and BCO.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):041008-041008-9. doi:10.1115/1.4033500.

The tower represents a significant portion of the materials and cost of the small wind turbine system. Optimization techniques typically maximize the tower loading capability while reducing material use and cost. Still, tower design focuses mainly on structural integrity and durability. Moreover, tower motion that intensifies drivetrain and structural loads is only rarely considered. The environmental impact of the wind turbine must also be considered since wind energy promotes sustainability. Trade-offs between the structural performance, cost, and environmental impact are examined to guide the designer toward a sustainable alternative. Ultimately, an optimal design technique can be implemented and used to automate tower design. In this study, nine tower designs with different materials and geometries are analyzed using finite element analysis (FEA). The optimal tower design is selected using a multilevel-decision-making procedure. The analysis suggests that steel towers of minimal wall thickness are preferred. This study is a continuation of the previous work that optimized energy production and component life of small wind systems (Hall et al., 2015, “An Integrated Control and Design Framework for Optimizing Energy Capture and Component Life for a Wind Turbine Variable Ratio Gearbox,” ASME J. Sol. Energy Eng., 137(2), p. 021022). The long-term goal is to develop a tool that performs optimization and automated design of small wind systems. In our future work, the tower and drivetrain designs will be merged and studied using higher fidelity models.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):041009-041009-10. doi:10.1115/1.4033501.

The objective of the present work is to conduct a worthwhile experimental study of the performance of a parabolic solar concentrator for solar cooking. The literature survey briefly highlights the standard performance tests of solar cookers and gives the experimental studies obtained by some authors. Our experimental device, made from simple means using local materials, consists of a parabolic concentrator having a 0.80 m diameter and 0.08 m depth as well as a cylindrical absorber with a 0.10 m diameter and is 0.20 m long. The testing period started on April 24th, 2014 and continued till July 10th of the same year, in Rabat (33°53′ N, 6°59′ W), Morocco. The average ambient temperature is 24 °C. The results show that using synthetic oil as the heat transfer medium has achieved a maximum temperature of 153 °C against 97 °C with water. The overall heat loss coefficient is estimated to be 17.6 W m−2  °C−1. The energy and exergy efficiencies are, respectively, 29.0–2.4% and 0.1–0.5%. Adding a glass cover on the front face of the absorber improved the maximum temperature by 15 °C. Automatic two-axis sun tracking system also increased the maximum temperature by 13 °C compared to manual tracking system.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):041010-041010-7. doi:10.1115/1.4033498.

The design, construction, and characterization of a solar simulator are reported. The solar simulator consists of an optical system, a power source system, an air cooling system, a control system, and a calibration system. Seven xenon short-arc lamps were used, each consuming 10 kW electricity. The lamps were aligned at the reflector ellipsoidal axis. The stochastic Monte Carlo method analyzed the interactions between light rays and reflector surfaces as well as participating media. The seven lamps have a common focal plane. The focal plane diameters can be changed in the range of 60–120 mm with the lamp module traveling the distance in a range of 0–300 mm. The calibration process established a linear relationship between irradiant fluxes and grayscale values. The measures to reduce irradiant flux error and fluctuations were described. The irradiant flux distribution can be changed by varying the power capacities and/or moving the focal plane locations. The peak fluxes are 1.92, 3.16, and 3.91 MW/m2 for 25%, 50%, and 75% of the full power capacity. The peak flux and temperature exceed 4 MW/m2 and 2300 K, respectively, for the full power capacity. A 8 cm thick refractory brick can be melt in 2 min with the melting temperature of about 2300 K when the solar simulator is operating at 70% of the maximum power capacity.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):041011-041011-8. doi:10.1115/1.4033595.

With reducing energy demand and required installed mechanical system power of modern residences, alternate heat pump system configurations with a possible increased economic viability emerge. Against this background, this paper presents a numerically examined energy feasibility study of a solar driven heat pump system for a low energy residence in a moderate climate, where a covered flat plate solar collector served as the sole low temperature heat source. A parametric study on the ambient-to-solarfluid heat transfer coefficient was conducted to determine the required solar collector heat transfer characteristics in this system setup. Moreover, solar collector area and storage tank volume were varied to investigate their impact on the system performance. A new performance indicator “availability” was defined to assess the contribution of the solar collector as low temperature energy source of the heat pump. Results showed that the use of a solar collector as low temperature heat source was feasible if its heat transfer rate (UA-value) was 200 W/K or higher. Achieving this value with a realistic solar collector area (A-value) required an increase of the overall ambient-to-solarfluid heat transfer coefficient (U-value) with a factor 6–8 compared to the base case with heat exchange between covered solar collector and ambient.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):041012-041012-8. doi:10.1115/1.4033511.

To harvest more energy from wind, wind turbine size has rapidly increased, entailing the serious concern on the reliability of the wind turbine. Accordingly, the international standard requires turbine designers to estimate the extreme load that could be imposed on a turbine during normal operations. At the design stage, physics-based load simulations can be used for this purpose. However, simulating the extreme load associated with a small load exceedance probability is computationally prohibitive. In this study, we propose using importance sampling combined with order statistics to reduce the computational burden significantly while achieving much better estimation accuracy than existing methods.

Topics: Stress
Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):041013-041013-10. doi:10.1115/1.4033594.

A volumetric solar receiver for superheating evaporated sulfuric acid is developed as part of a 100 kW pilot plant for the hybrid sulfur (HyS) cycle. The receiver, which uses silicon carbide foam as a heat transfer medium, heats evaporated sulfuric acid using concentrated solar energy to temperatures of 1000 °C or greater, which are required for the downstream catalytic reaction to split sulfur trioxide into oxygen and sulfur dioxide. Multiple parallel approaches for modeling and analysis of the receiver are used to design the prototype. Focused numerical modeling and thermodynamic analysis are applied to answer individual design and performance questions. Numerical simulations focused on fluid flow are used to determine the best arrangement of inlets, while thermodynamic analysis is used to evaluate the optimal dimensions and operating parameters. Finally, a numerical fluid mechanics and heat transfer model is used to predict the temperature field within the receiver. Important lessons from the modeling efforts are given, and their impacts on the design of a prototype are discussed.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Sol. Energy Eng. 2016;138(4):044501-044501-5. doi:10.1115/1.4033404.

This article introduces an implementation of an insulated alternative energy system. The objective of the study is the analysis of energy generation in communities that are located in insulated areas. The study focuses on the development of a photovoltaic hybrid microgrid, with storage in stationary lead-acid batteries and energy from fuel cell. The photovoltaic hybrid microgrid must be sized to produce enough energy to supply to a community of houses. This study attempts to elaborate a standard computational simulation method using the Matlab® Platform, to size the system independent of region, but taking the impact of local global irradiance, air temperature, and the energetic demand into consideration. This simulation type allows an estimation of the photovoltaic power to be installed and the nominal battery capacity.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):044502-044502-5. doi:10.1115/1.4033593.

A low-temperature (<120 °C) solar organic Rankine cycle (ORC) power generation experimental facility is designed and built. The influence of light intensity on the system performance is investigated using the experimental facility. The results indicate that the system efficiency can reach 2.2%. The temperature of heat transfer fluid (HTF) decreases linearly with light intensity (I). However, both system efficiency and thermoelectric efficiency first decrease linearly and then drop sharply as I decreases at working fluid flow rates (Vwf) of 200 and 160 L/hr, while they only decrease slightly with I at Vwf of 120 L/hr. The light intensity of the turning point is 824 W/m2 at Vwf of 200 L/hr, which corresponds to an HTF temperature of 75 °C. In addition, it is found that the influence of light intensity on the performance of ORC becomes stronger for higher working fluid flow rate. Moreover, the light intensity and HTF temperature at the turning point increase with working fluid flow rate. The experimental results are of great significance for the design and operation of low-temperature solar ORC power generation system.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(4):044503-044503-6. doi:10.1115/1.4033646.

Numerical simulation results and modeling on the electrical features of concentrating photovoltaic-thermal (PVT) using the free circuit simulation package from linear technology corporation (LTSPICE) are presented. The effects of partial shading of cell strings and temperature are analyzed, showing very good agreement with the results obtained experimentally in lab, at Lisbon University, and under outdoor testing using similar receivers, at the SME Solarus Sunpower AB, a Swedish company whose mission is the development, production, and marketing of concentrated solar technology to the world market. The potential of the used methodology for the design of the solar cell configuration is emphasized as an important tool to optimize PV and PVT performances in the energy conversion process.

Commentary by Dr. Valentin Fuster

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