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Research Papers

J. Sol. Energy Eng. 2016;138(5):051001-051001-9. doi:10.1115/1.4033592.

A simple unsteady blade element analysis is used to account for the effect of the trailing wake on the induced velocity of a wind turbine rotor undergoing fast changes in pitch angle. At sufficiently high tip speed ratio, the equation describing the thrust of the element reduces to a first order, nonlinear Riccti's equation which is solved in a closed form for a ramp change in pitch followed by a constant pitch. Finite tip speed ratio results in a first order, nonlinear Abel's equation. The unsteady aerodynamic forces on the NREL VI wind turbine are analyzed at different pitch rates and tip speed ratio, and it is found that the overshoot in the forces increases as the tip speed ratio and/or the pitch angle increase. The analytical solution of the Riccati's equation and numerical solution of Abel's equation gave very similar results at high tip speed ratio but the solutions differ as the tip speed ratio reduces, partly because the Abel's equation was found to magnify the error of assuming linear lift at low tip speed ratio. The unsteady tangential induction factor is expressed in the form of first order differential equation with the time constant estimated using Jowkowsky's vortex model and it was found that it is negligible for large tip speed ratio operation.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(5):051002-051002-14. doi:10.1115/1.4033711.

This study investigates the efficiency of application of phase change materials (PCMs) in a solar cooling system. The proposed system consists of an adsorption chiller and a latent heat storage unit (LHSU) containing PCMs. The PCM stores solar energy during daytime and at nighttime, the thermal energy stored in the PCM is utilized to drive the adsorption chiller. An auxiliary heater is also used to provide the required energy in addition to the LHSU. To verify the accuracy of the obtained results, the modeling of the solar adsorption system and the PCM unit are validated separately. Moreover, the whole system performance is verified by evaluation of the conservation of energy in the system. The performance of the system is compared with a similar solar adsorption chiller lacking LHSU. Also, the parameters which affect the performance of the LHSU are studied. It is found that application of LHSU decreases auxiliary energy consumption and increases solar fraction. Solar fraction goes up more if larger amount of PCM is used. However, there exists a maximum mass of PCM which can be charged during the sunshine hours. The maximum chargeable mass of PCM goes up by increasing the solar collector area, which leads to decreasing auxiliary energy consumption and increasing solar fraction. The results also show that enlargement of the hot water storage tank reduces auxiliary energy consumption and enhances solar fraction, but decreases thermal storage efficiency. In order to achieve higher thermal storage efficiency and also less auxiliary energy consumption, it is suggested to use average-sized hot water storage tanks.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(5):051003-051003-7. doi:10.1115/1.4033574.

Buildings account for a significant portion of the total energy consumption in the U.S., especially the energy-inefficient commercial building sector. As part of the future path toward realizing net zero energy buildings, innovative energy-efficient technologies must be developed. In this study, the potential of phase-change material (PCM)-enhanced constructions to lower heating, ventilating, and air conditioning (HVAC) energy consumption in a commercial restaurant building was investigated. A commercially available fatty acid based PCM product was selected due to their promising thermal and chemical properties. Differential scanning calorimetry (DSC) was used in isothermal step mode to accurately measure the latent heat energy storage of the PCM. A U.S. Department of Energy (DOE) commercial reference building model with a PCM-enhanced ceiling was simulated using a finite-difference conduction heat transfer algorithm in EnergyPlus to determine the effects of the PCM on the building energy performance. It was found that, although the PCM-enhanced ceiling had a beneficial stabilizing effect on the interior surface temperature of the ceiling, the zone mean air temperatures were not significantly altered. As such, minimal HVAC energy savings were seen. Future work should focus on active PCM systems, which utilize heat exchanging fluids to discharge the PCM to remove the stored thermal energy of the PCM during the night in summer, overcoming the fundamental issue of the passive PCM system returning stored thermal energy back into the building.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(5):051004-051004-7. doi:10.1115/1.4034027.

In this research, the thermal performances of flat, finned, and v-corrugated plate solar air heaters were investigated experimentally. A solar air heater with single glass cover, single pass was designed and tested under prevailing weather conditions of Tanta city (30°43′ N, 31° E), Egypt. The solar air heater was designed to be easy to replace the absorber plate from one to another one. Comparisons between the temperature difference of air across the heater and thermal efficiencies of the flat, finned, and v-corrugated plate solar air heaters were presented. The effect of change in the mass flow rate of air on the outlet air temperature and the thermal efficiency of the heater were also studied when the mass flow rates were 0.062, 0.028, and 0.009 kg/s. The experimental results showed that the maximum value of outlet temperature of the v-corrugated plate solar air heater was 5 and 3.5 °C more than that of flat and finned plates when the mass flow rate was 0.062 kg/s, respectively. And, it increased to be 8 and 5.5 C when the mass flow rate was 0.009 kg/s. It is also indicated that the thermal efficiency of the v-corrugated solar air heater is 8–14.5% and 6–10.5% higher than that of the flat and finned plate heaters, respectively, when the mass flow rate was 0.062 kg/s under the considered configurations and operating conditions. The experimental results also indicated that the convective heat transfer coefficient of the v-corrugated heater reached up to 1.64 and 1.36 times than that of the flat and finned heaters, respectively, when the flow rate was 0.062 kg/s.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(5):051005-051005-11. doi:10.1115/1.4034072.

This research is to assess effects of a partition on thermal comfort, indoor air quality (IAQ), energy consumption, and perception in an air-conditioned space via computational fluid dynamics (CFD) analysis. The variables of indoor air are numerically determined before/after installation/removal of a partition. Accordingly, predicted mean vote (PMV) of thermal comfort, carbon dioxide concentration, rate of energy consumption in making up air, and an overall perception index are proposed to quantify effects in a partitioned space. For a case study, a partition is used to tightly separate a study area from a rest area in a library during peak time. The CFD analysis is performed so that the mean differences between the measured and simulated variables at 14 locations are less than 5%. After partitioning in the CFD analysis, it is found that the average PMV value decreases to −1.4 in the rest area, and it remains at −0.7 in the study area where occupants perceive a slightly cool sensation. In the study area, the carbon dioxide concentration increases to 450–500 ppm, while the rate of energy consumption increases by 8.3%. From the overall perception index of 0.9, the occupants feel spacious in the partitioned areas. Therefore, installing the partition is encouraged with the recommendation that cooling supply can be reduced for energy savings. It is apparent that the proposed methodology yields quantitative indicators for decision making of installation/removal of partitions. The interior investigation of partitions in buildings can be performed before making real physical changes.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(5):051006-051006-6. doi:10.1115/1.4034028.

In this work, we have chosen the low cost natural dye extracted from Saraca asoca flowers to act as a sensitizer dye for TiO2-based dye-sensitized solar cell (DSSC). UV–visible spectroscopic studies of ethanolic extract of dyes have been done in order to understand light absorption behavior of dye. The natural dye extract covers appreciable spectrum of solar radiation, 400–500 nm with an absorption maximum at 450 nm that makes it suitable for use as a photosensitizer in DSSC application. The dye adsorbed onto the semiconductor facilitates electron transfer across the dye/semiconductor interface. FTIR spectra of extract revealed the presence of anchoring groups and coloring constituents. DSSC fabricated with TiO2 and natural dye extract obtained from Saraca asoca flowers as sensitizer has shown open-circuit voltage (Voc) 516 mV, short-circuit current density (Jsc) 0.29 mA/cm2, fill factor (FF) 0.65, incident photon-to-current conversion efficiency (IPCE) 43%, and conversion efficiency of 0.09%. This work briefly discusses the simple extraction technique of natural dye and its performance in DSSC.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(5):051007-051007-8. doi:10.1115/1.4034070.

Rotating shadowband irradiometers (RSIs) are a common type of radiation sensors for measurement of direct normal irradiance (DNI) at remote sites where daily maintenance of the instruments is not feasible or practicable. Their primordial lower accuracy due to systematic deviations of the photodiode response can be improved significantly with a thorough calibration of each RSI against high precision sensors and application of suitable corrections on the raw data. With different available correction functions for the systematic errors, RSI data coincide with first class reference sensors within 2–3% root mean square deviation (RMSD) for 10 min averages of DNI and meet the annual irradiation sum within 1.5%. Such comparisons of RSI data to reference irradiances have only been published for a small number of sites. To endorse the credibility of RSI measurements, it has to be shown that these accuracies derived for certain locations are also valid at other sites with differing atmospheric conditions. Therefore, a parallel measurement campaign with six RSIs and a reference station with first class and secondary standard instrumentation has been performed in the in the extreme climate of the United Arab Emirates (UAE). The results of this comparison are presented in this paper. The stated empiric accuracy could be validated and confirmed for the UAE.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(5):051008-051008-9. doi:10.1115/1.4033573.

This paper evaluates cost and performance tradeoffs of alternative supercritical carbon dioxide (s-CO2) closed-loop Brayton cycle configurations with a concentrated solar heat source. Alternative s-CO2 power cycle configurations include simple, recompression, cascaded, and partial cooling cycles. Results show that the simple closed-loop Brayton cycle yielded the lowest power-block component costs while allowing variable temperature differentials across the s-CO2 heating source, depending on the level of recuperation. Lower temperature differentials led to higher sensible storage costs, but cycle configurations with lower temperature differentials (higher recuperation) yielded higher cycle efficiencies and lower solar collector and receiver costs. The cycles with higher efficiencies (simple recuperated, recompression, and partial cooling) yielded the lowest overall solar and power-block component costs for a prescribed power output.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(5):051009-051009-18. doi:10.1115/1.4034241.

A modified version of the Beddoes–Leishman (B-L) dynamic stall model is presented. A novel approach was applied for deriving the effective flow separation points using two-dimensional (2D) static wind tunnel test data in conjunction with Kirchhoff's model. The results were then fitted in a least-squares sense using a new nonlinear model that gives a better fit for the effective flow separation point under a wide range of operating conditions with fewer curve fitting coefficients. Another model, based on random noise generation, was also integrated within the B-L model to simulate the effects of vortex shedding more realistically. The modified B-L model was validated using 2D experimental data for the S809 and NACA 4415 aerofoils under both steady and unsteady (oscillating) conditions. The model was later embedded in a free-wake vortex model to estimate the unsteady aerodynamic loads on the NREL Phase VI rotor blades consisting of S809 aerofoils when operating under yawed rotor conditions. The results in this study confirm the effectiveness of the proposed modifications to the B-L method under both 2D and three-dimensional (3D) (rotating) conditions.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(5):051010-051010-8. doi:10.1115/1.4034357.

This work presents a simple parameter estimation approach for a photovoltaic (PV) module using a single-diode five-parameter electrical model. The proposed approach only uses the information from manufacturer datasheet without requiring a specific experimental procedure or a curve extractor. The number of parameters to be determined is first reduced from five to two by gaining insight into electrical equations of the model at the standard test conditions (STCs). A nonlinear least square (NLS) objective function is then constructed and minimized by a complete scan for all possible values of the two parameters within some specific ranges based on their physical meaning. Consequently, the single-diode five-parameter electrical model at the STC is determined based on two optimal parameter values. Besides, a PV full characteristic model with consideration of both the irradiance and temperature dependencies is also constructed by using the data at the nominal operating cell temperature (NOCT) test conditions. The proposed approach is easy to implement and free of the convergence problem. The evaluations on several PV modules show that the proposed approach is capable of extracting accurate estimates of the model parameters.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(5):051011-051011-9. doi:10.1115/1.4034228.

Solar combi-storages are used in many countries for storing solar heat for space heating and domestic hot water (DHW) in one device. When a combi-storage is used in combination with a heat pump, the temperature stratification efficiency of the storage is a decisive factor for the overall efficiency and thus, for the consumed end-energy of the system. In particular, fluid that is entering the storage with a high velocity may cause considerable mixing, thus, destroying stratification and leading to poor system performance. This work presents computational fluid dynamics (CFD) simulations of direct horizontal inlets at midheight of a typical solar combi-storage of about 800 L volume. Different inlet diffusor designs were simulated, and laboratory measurements were used to validate CFD experiments. For the given tank geometry, mass flow rates, and inlet position, it is found for a fluid inlet temperature of 30 °C that fluid velocities should be below 0.1 m/s and Reynolds numbers below 3000–5000 at the outlet of the diffusor in order to avoid the disturbance of a hotter 50 °C zone above the inlet. Furthermore, the fluid path within the diffusor must exceed a minimum length that corresponds to three to four times the hydraulic diameter of the diffusor.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Sol. Energy Eng. 2016;138(5):054501-054501-8. doi:10.1115/1.4033793.

Three eutectic salts from a system of halide salts NaCl–KCl–ZnCl2 were chosen for detailed study of thermal and transport properties with the objective of developing a next generation high-temperature heat-transfer fluid (HTF) for concentrated solar thermal power (CSP) technology. The acceptable range of the working temperatures for the HTF is from below 250 °C to at least 800 °C. The tested properties are presented here for the three candidate eutectic salts, including melting point, heat of fusion, heat capacity, vapor pressure, viscosity, density, and thermal conductivity. Data-fitted equations are provided for all the measured properties for convenience in engineering application. It is concluded that the three eutectic salts can satisfy the needs for a high-temperature HTF and thus are recommended as a new generation high-temperature HTF.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(5):054502-054502-5. doi:10.1115/1.4033985.

The behaviors of tip displacement, maximum stress, and natural frequency of vibration as a function of blade length are investigated for extendable wind turbine blades. A three-dimensional linear elasticity finite-element model of the blade is used along with a typical profile and representative material properties. The quasi-linear response and free vibration behavior are investigated for a sequence of blade geometries. These estimates are intended to give approximate measures of expected changes in the elastic and dynamic field as the operating length changes and provide preliminary guidelines for this novel class of structure.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(5):054503-054503-8. doi:10.1115/1.4034071.
OPEN ACCESS

To address the lack of knowledge of local solar variability, we have developed and deployed a low-cost solar variability datalogger (SVD). While most currently used solar irradiance sensors are expensive pyranometers with high accuracy (relevant for annual energy estimates), low-cost sensors display similar precision (relevant for solar variability) as high-cost pyranometers, even if they are not as accurate. In this work, we present evaluation of various low-cost irradiance sensor types, describe the SVD, and present validation and comparison of the SVD collected data. The low cost and ease of use of the SVD will enable a greater understanding of local solar variability, which will reduce developer and utility uncertainty about the impact of solar photovoltaic (PV) installations and thus will encourage greater penetrations of solar energy.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(5):054504-054504-6. doi:10.1115/1.4034202.

In this paper, a performance evaluation technique using most frequent conditions (MFC) for accurate design of photovoltaic systems, based on energy rating and site-specific standards is reported. Most frequent conditions are estimated for the three different technologies: multicrystalline silicon (mc-Si), amorphous silicon (a-Si), and hetero-junction with intrinsic thin layer (HIT) for the site based on air-mass, module temperatures, incident in-plane irradiance, and power output. The performances are analyzed over a period of 3 years by evaluating changes in the performance ratio, the energy yields, and the percentages of occurrence of data points corresponding to standard test condition (STC), nominal operating cell temperature (NOCT), and MFC. For MFC, performance ratio (PR) values are ranging from 0.70 to 0.83, 0.70 to 0.86, and 0.70 to 0.90 for mc-Si, a-Si, and HIT, respectively. The total energy yield of HIT is the highest followed by a-Si and mc-Si modules for this climatic zone.

Commentary by Dr. Valentin Fuster

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