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Review Article

J. Sol. Energy Eng. 2019;141(5):050801-050801-14. doi:10.1115/1.4043126.

Energy storage helps in waste management, environmental protection, saving of fossil fuels, cost effectiveness, and sustainable growth. Phase change material (PCM) is a substance which undergoes simultaneous melting and solidification at certain temperature and pressure and can thereby absorb and release thermal energy. Phase change materials are also called thermal batteries which have the ability to store large amount of heat at fixed temperature. Effective integration of the latent heat thermal energy storage system with solar thermal collectors depends on heat storage materials and heat exchangers. The practical limitation of the latent heat thermal energy system for successful implementation in various applications is mainly from its low thermal conductivity. Low thermal conductivity leads to low heat transfer coefficient, and thereby, the phase change process is prolonged which signifies the requirement of heat transfer enhancement techniques. Typically, for salt hydrates and organic PCMs, the thermal conductivity range varies between 0.4–0.7 W/m K and 0.15–0.3 W/m K which increases the thermal resistance within phase change materials during operation, seriously affecting efficiency and thermal response. This paper reviews the different geometry of commercial heat exchangers that can be used to address the problem of low thermal conductivity, like use of fins, additives with high thermal conductivity materials like metal strips, microencapsulated PCM, composite PCM, porous metals, porous metal foam matrix, carbon nanofibers and nanotubes, etc. Finally, different solar thermal applications and potential PCMs for low-temperature thermal energy storage were also discussed.

Commentary by Dr. Valentin Fuster

Research Papers

J. Sol. Energy Eng. 2019;141(5):051001-051001-8. doi:10.1115/1.4043082.

This paper presents the modeling theory and results of an innovative thermal energy storage (TES) facility, ideated, realized, and tested by ENEA (Italy). This prototype enabled the thermocline storage with molten salts in a novel geometry ideated for small-medium scale decentralized solutions, which includes two vertical channels to force the circulation through two heat exchangers, respectively, and realized for charging and discharging phases (in a single tank). A thermophysical model was built and tested properly for this particular geometry in order to analyze the temperature distribution along the radius. The numerical results well reproduced the experimental values. Furthermore, the analytical solution provided a short-cut methodology able to evaluate the thermocline distribution (along the vertical axis) depending on both the time and the radius values. Hence, the influence of the radial position (r) on the thermocline degradation was studied finding that, at the edges (r → 1), the thermocline remains unchanged for longer (around ten times more) than at the center of the tank (r → 0). The obtained numerical modeling and the analytical correlation can be useful for the process analysis to scale-up the thermal storage system and to evaluate the system reliability for industrial plants.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(5):051002-051002-6. doi:10.1115/1.4043125.

Using solar energy for space heating is an efficient and simply way to satisfy the energy demands of buildings. In this study, a typical office building is selected as a case model to obtain indoor air temperature characteristics with dual heat storage devices. By analyzing our solar heating system, a mathematical model of the system working process is set up. Using the software matlab/simulink for simulation, the indoor air temperature characteristics in 1 day are obtained. Simulation and experimental results show good consistency. And using the mathematical model, the storage tank size is optimized to search for the minimum size for the fixed building. Based on our analysis, the optimum ratio of storage tank A volume and collector field area is 0.11 m. This research can be a good reference for the design of the solar heating system.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(5):051003-051003-12. doi:10.1115/1.4043127.

A new three-zone heat extraction system and its analytical model for maximizing the thermal power output of salt gradient solar ponds against a given volume is proposed. The present study considers internal heat exchangers installed within the non-convective zone (NCZ), lower-convective zone (LCZ), and the ground below the pond. The work is validated against a simplified version of the model (eliminating ground and bottom-zone heat extractions) available in the existing literature. Contrary to the conventional practice of optimizing only the middle-zone pond thickness, here, the newly proposed expression is used to find ideal values of both the middle- and bottom-zone thicknesses of the pond along with its cross-sectional area. The present work acknowledges that although the three-zone heat extraction system is the best, yet if a choice for two-zone heat extraction is to be made between the NCZ–LCZ and ground–LCZ, then the former is a better alternative. The power output is observed to increase asymptotically with mass flow rates of the three heat exchangers. However, their values must lie much below their theoretical asymptotic limits and their selection is regulated by constructional and operational constraints. These involve a minimum pond depth to offset surface evaporation, ground seepage water loss, and constraints preventing turbulent flow in heat exchangers to reduce friction loss and pumping power. This work recommends using three heat exchangers instead of either one or two and provides cardinal guidelines to extract heat in an ideal manner for a fixed solar pond volume.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(5):051004-051004-8. doi:10.1115/1.4043130.

This paper proposes an efficient single-stage photovoltaic (PV) fed permanent magnet brushless DC (PMBLDC) motor water pumping system with grid power interface. The power conditioning system used in the conventional two-stage system is eliminated, and the maximum power point tracking (MPPT) scheme is incorporated within the inverter used for the electronic commutation of the BLDC motor. The BLDC motor pumping is advantageous than the conventional induction motor-aided water pumps. The proposed system is the first of its kind to employ a grid power export scheme utilizing the same inverter drive which in turn reduces the complexity of adding new inverter circuitry and thereby making the system economically viable. The BLDC pump and grid power export scheme is realized in both simulation and hardware environment for a power rating of 1 kW. The results reveal that the proposed system is highly viable.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(5):051005-051005-8. doi:10.1115/1.4043438.

In this paper, a continuous tracking strategy for the heliostat in the James S. Markiewicz Concentrated Solar Energy Research Facility at Valparaiso University is developed. A model of the nonlinear dynamics of the heliostat motion is developed and the open-loop control strategy is presented. Asymptotic stability of the heliostat control using the Lyapunov and LaSalle’s theorems was proven. Simulations using the nonlinear dynamic model are presented and interpreted to identify the feedback gain that maximizes the time response of the heliostat without introducing oscillations in its motion. Finally, the control strategy is put to the test during summertime operation. The data presented show that the tracking strategy has an root mean square (RMS) tracking error of 0.058 mrad, where the error is defined as the difference between the desired and actual heliostat positions. Images of the aperture of a high-temperature solar receiver over 8 h of testing are also presented to qualitatively demonstrate the success of the tracking strategy.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(5):051006-051006-15. doi:10.1115/1.4043515.

The aim of this study was to conduct thermodynamic and economic analyses of a concentrated solar power (CSP) plant to drive a supercritical CO2 recompression Brayton cycle. The objectives were to assess the system viability in a location of moderate-to-high-temperature solar availability to sCO2 power block during the day and to investigate the role of thermal energy storage with 4, 8, 12, and 16 h of storage to increase the solar share and the yearly energy generating capacity. A case study of system optimization and evaluation is presented in a city in Saudi Arabia (Riyadh). To achieve the highest energy production per unit cost, the heliostat geometry field design integrated with a sCO2 Brayton cycle with a molten-salt thermal energy storage (TES) dispatch system and the corresponding operating parameters are optimized. A solar power tower (SPT) is a type of CSP system that is of particular interest in this research because it can operate at relatively high temperatures. The present SPT-TES field comprises of heliostat field mirrors, a solar tower, a receiver, heat exchangers, and two molten-salt TES tanks. The main thermoeconomic indicators are the capacity factor and the levelized cost of electricity (LCOE). The research findings indicate that SPT-TES with a supercritical CO2 power cycle is economically viable with 12 h thermal storage using molten salt. The results also show that integrating 12 h-TES with an SPT has a high positive impact on the capacity factor of 60% at the optimum LCOE of $0.1078/kW h.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(5):051007-051007-12. doi:10.1115/1.4043516.

Savonius rotor, a class of drag-driven vertical axis wind turbine, has been extensively investigated mainly to calculate the torque and power coefficients (CT and CP) by various investigators. Hitherto, studies related to lift and drag characteristics are very few and have mainly been restricted to a semicircular-bladed rotor. A deeper investigation into the drag and lift coefficients (CD and CL) can result in the better design of rotor blades leading to an increment in CT and CP. In view of this, in the present investigation, CD and CL of an elliptical-bladed rotor with vent augmenters have been studied numerically. Initially, two-dimensional (2D) unsteady simulations using an ansys fluent solver is carried out to estimate the instantaneous CD and CL. The shear stress transport (SST) k–ω turbulence model is selected to solve the Reynolds averaged Navier Stokes (RANS) equations. Finally, three-dimensional (3D) unsteady simulations are carried out for the vented elliptical-bladed rotor. The unsteady simulations are performed for the nonvented elliptical- and semicircular-bladed rotors at the identical condition in order to have a direct comparison. From the unsteady simulations, the average CD for the vented elliptical profile is found to be 1.45; whereas, the average CD for the nonvented elliptical and semicircular profiles is found to be 1.43 and 1.35, respectively.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(5):051008-051008-12. doi:10.1115/1.4043517.

This paper presents a three-dimensional numerical analysis of a flat plate solar air heater in the presence of a pin fin array using the computational fluid dynamics (CFD) software tool ansys fluent 16.2. The effect of geometric parameters of pin fins as well as the flow Reynolds number (4000–24,000) on the effective efficiency is evaluated. The longitudinal pitch (PL) of pin fin array is varied as 30 mm, 40 mm, and 50 mm and the diameter (Dw) is varied as 1.0 mm, 1.6 mm, and 2.2 mm. The results show that the presence of pin fins generate considerable enhancement in fluid turbulence as well as heat transfer area to a maximum extent of about 53.8%. The maximum average increase in instantaneous thermal efficiency is found to be about 14.2% higher as compared with the base model for the fin diameter of 2.2 mm and a longitudinal pitch value of 30 mm. In terms of effective efficiency, the pin fin array exhibits significant enhancement, especially at lower flow rate conditions. Finally, the effective efficiency of the pin fin array is compared with the previous work of authors involving spherical turbulators and sinewave corrugations on the absorber plate. The results show that the pin fin array exhibits a relatively superior effective efficiency to a maximum extent of about 73% for lower flow rate conditions.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(5):051009-051009-17. doi:10.1115/1.4043518.

Due to their thermal storage capability, concentrated solar power (CSP) plants have flexibility on electricity dispatch, being able to participate in balancing power markets. The development of an optimum electricity delivery schedule should be fast to react to updated forecast and the dynamic electricity markets, apart from considering best operational practices, as it brings significant cost reductions and improvement in plant performance. Therefore, dispatch optimization tools should combine financial and operational optimization with acceptable computational time. In this context, an innovative dispatch optimization algorithm used to derive a CSP plant operation schedule is presented. FRED is a heuristic rule-based algorithm used to optimize financial income while considering plant best operational practices. Simulations performed with a CSP plant tower model following FRED optimization strategy show the possibility of improved financial results with fast dispatch planning, ensuring the importance of this technology in the pathway to a highly renewable energy mix.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(5):051010-051010-7. doi:10.1115/1.4043550.

The performance of the photovoltaic-thermoelectric (PV-TE) hybrid system was examined using three types of PV cells and a thermoelectric generator (TEG) based on bismuth telluride. The investigated PV cells are amorphous silicon (a-Si), monocrystalline silicon (mono-Si), and cadmium telluride (CdTe). The results showed that the TEG contribution can overcome the degradation of the PV cell efficiency with increasing temperature at the minimal working condition. This condition corresponds to the critical temperature difference across the TEG that guarantees the same efficiency of the hybrid system as that of the PV cell alone at 298 K. The obtained results showed that the critical temperature difference is 13.3 K, 44.1 K, and 105 K for the a-Si, CdTe, and mono-Si PV cell, respectively. In addition, the general expression of the temperature difference across the TEG needed for an efficiency enhancement by a ratio of r compared with a PV cell alone at 298 K was given. For an efficiency enhancement by 5 % (r = 1.05), the temperature difference required is 30.2 K, 61.3 K, and 116.1 K for the a-Si, CdTe, and mono-Si PV cells, respectively. These values cannot be achieved practically only in the case of the a-Si PV cell. Moreover, a TE material with a high power factor can reduce this temperature difference and improve the performance of the hybrid system. This work provides a tool that may be useful during the selection of the PV cell and the TE material for the hybrid system.

Topics: Temperature
Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(5):051011-051011-9. doi:10.1115/1.4043549.

The performance of a solar collector with wax as a phase change material (PCM) located in a set of staggered pipes configuration was simulated computationally in this work. For the solar radiation of Chile, the accumulation of heat in the PCM system and the heat release at different time intervals were analyzed during the process of energy capture in summer: (a) without wax and with airflow, (b) with wax and without airflow, and (c) with wax and with airflow. Furthermore, the effects of solar radiation (summer and winter) airflow in the collector were analyzed on the performance of the system. The simulation results show that the use of a PCM in a solar air heater allows to store greater amounts of energy and it helps to extend the period of time when the air coming out of the collector has an elevated temperature. By increasing the airflow rate, the efficiency of the system increases and also the energy released to the air to be absorbed.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(5):051012-051012-5. doi:10.1115/1.4043614.

The tubular light guides are devices allowing deliverance of solar light into deep interior rooms, offices, or underground spaces. Due to considerable costs of such systems, the reasonable assessment of their lighting performance is desirable. To predict accurately their efficiency, precise numerical computations have to be performed. Such computations may be strongly time consuming, mainly when mass calculations are required as it is in case of the so-called climate-based daylight modeling. This paper presents an analytical solution to the optical efficiency of cylindrical straight pipes that is applicable over a wide range of pipe’s parameters and under arbitrary sky luminance conditions. The proposed method gives results in good agreement with ray-tracing numerical simulations—the mean absolute percentage errors are less than 3%—but unlike them, the calculations are much faster. Therefore, it appears to be convenient for daylight modeling, which takes into account utilization of tubular light guide systems in buildings.

Commentary by Dr. Valentin Fuster

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