• Energy Research

Abstract A new cooling technique for concentrator photovoltaic (CPV) systems is developed using various configurations of microchannel heat sinks. Five distinct configurations integrated with a CPV system are investigated, including a wide rectangular microchannel, a single layer parallel- and counter- flow microchannel, and a double layer parallel- and counter- flow microchannel. A comprehensive, three-dimensional thermo-fluid model for photovoltaic layers, integrated with a microchannel heat sink, is developed. The model is numerically simulated and validated using the available experimental and numerical data. Based on the results, the temperature contours on a plane located at the mid-thickness of the silicon layer are presented at different operating conditions and heat sink configurations. Accordingly, the maximum local temperature can be detected and temperature uniformity can be accurately estimated. Furthermore, at a concentration ratio of 20, the CPV system integrated with a single layer parallel- flow microchannel heat sink configuration (B) achieves the highest cell net power, electrical efficiency, and the minimum cell temperature. On the contrary, at the same operating conditions, the use of a single layer counter-flow microchannel heat sink configuration (C) is found to be the least effective cooling technique. The results of this study can guide industrial designers to adopt compact heat sink configurations and simple designs in the manufacturing process of hybrid CPV-thermal systems.

The contribution of renewable energy to the worldwide sustainable development and environmental preservation has been widely recognized nowadays. Concentrated photovoltaic (CPV) system, in particular, has received an extensive research effort as one of the most promising applications of solar energy. Due to the high concentration r1atio, a significant increase in the CPV temperature occurs. Consequently, the conversion efficiency deteriorates; thereby thermal regulation of a CPV system is of great importance. Therefore, a hybrid system including CPV, and phase change material (PCM) is considered as a single module to achieve higher solar conversion efficiency. Such a system provides a high-energy storage density at a constant temperature which corresponds to the phase transition temperature of the material. In the present study, a comprehensive model for CPV layers integrated with PCM was developed. This model was a coupled of a thermal model for CPV layers and fluid dynamic heat transfer model that took into account the phase-change phenomenon using enthalpy method, and the conversion of solar incident radiations. The effects of specific two variables on the solar cell temperature were investigated which were the PCM thickness of 50, 100, and 200 mm and concentration ratio (CR) from 5 to 20. It was found that the use of PCM could achieve a significant reduction of solar cell temperature. The solar cell temperature reduced from 180 °C to 38 °C by using PCM of thickness 200 mm at CR=5, while at the same PCM thickness, the cell temperature reduced from 510°C to 64°C at CR=20. Furthermore, the solar cell temperature was maintained at an average temperature of 38 °C for 8.4 hours using a 200 mm thickness of PCM at CR=5. In addition, at CR=20, the solar cell temperature was maintained at an average temperature of 64 °C for 2.0 hours using a 200 mm thickness of PCM. From the results, it was indicated that the use of PCM was an effective cooling technique since it attained a significant reduction in solar cell temperature, especially at high concentration ratio.

Abstract On the anode side of a direct-methanol fuel cell, carbon dioxide bubbles are generated as a result of the methanol oxidation reaction. The accumulation of such bubbles prevents methanol from reaching the diffusion layer (DL). Hence, a reduction in the reaction rate occurs, which limits the maximum current density of the cell. To keep carbon dioxide bubbles away from the diffusion layer surface, a new design of the anode flow channel besides wall surface treatment is developed. Such a design can introduce capillary actuation, which forces the carbon dioxide bubbles to move away from the diffusion layer due to capillary forces. This can be achieved by using a trapezoidal shape of the flow channel, as well as the combined effect of hydrophilic and hydrophobic surface treatments on the diffusion layer and top wall, respectively. To identify the optimal design of the anode flow channel, a three-dimensional, two-phase flow model is developed. The model is numerically simulated, and the results are validated with available measurements. Results indicated that treating the diffusion layer with a hydrophilic layer increases the area in direct contact with liquid methanol. Besides, the hydrophobic top channel wall makes it easier for the carbon dioxide bubbles to attach and spread out on the top surface. However, super-hydrophobic treatment of the top wall should be avoided, as it can cause difficulty in bubble extraction from the channel. The current findings create a promising opportunity to improve the performance of direct-methanol fuel cells.

Abstract The present study investigates the effects of the orifice nozzle number and the inlet pressure experimentally on the cooling performance of the counter flow type vortex tube. The energy generation has been conducted using a generator of type stream–tek generator model GNMD-KIT with the number of nozzles (2, 3, and 6), and aspect ratio 1.6 and, an inner diameter of 7.5 mm. In the experiments, for each of the orifices, inlet pressures have been adjusted from 200 kPa to 600 kPa. The energy separation to be investigated here focuses on the cold temperature difference and coefficient of performance for cooling. The experimental results concluded in this paper proved that the higher effect of nozzle number is for nozzle number three and hence that nozzle number could affect the energy separation efficiently. A comparison of the present experiments with other published works has been conducted. An analytical study of the characteristics equation has been carried out to evaluate the best correlation of the ratio of cold temperature difference to the inlet temperature as a function of pressure, cold mass fraction and nozzle numbers. The goodness of fit of the regression model is inspected using the analysis of variance and t -test. The values of R 2 are adjusted close to 97% which show a very good agreement between the experimental and predicted values.

Abstract Modifying the polycrystalline silicon solar cell by reducing the thermal resistance of the ethylene–vinyl acetate (EVA) layer is essential to enhance the thermal management process. This modification will improve the heat dissipation process from the silicon wafer especially at a higher solar concentration ratio (CR) and in return enhance the solar cell performance and output power. Thus, a modified design of a solar cell integrated with a microchannel heat sink is developed. In this new design, variations of the Ethylene-Vinyl Acetate (EVA) upper- and lower-layer thickness along with the interval width between the two consecutive silicon layers are investigated. To determine the effect of varying the design parameters on the cell temperature at various solar concentration ratios and coolant mass rates, a three-dimensional comprehensive model for the solar cell integrated with a heat sink is developed. The model is simulated and validated with numerical results and measurements. Results indicate that reducing the EVA lower layer thickness has a remarkable effect on the solar cell temperature. At a solar concentration ratio of 20, varying the EVA lower layer thickness from 1.0 mm to 0.2 mm results in decreasing the maximum cell temperature from 102.3 °C to 69.3 °C. With further increase of the concentration ratio up to 30, the maximum cell temperature reduces from 138.3 °C to 87.4 °C. It is found that at a coolant rate of 2000 gm/min, and a concentration ratio of 20, maximum temperature in the modified and conventional solar cell with a lower EVA thickness of 0.2 mm and 0.5 mm, reaches 69.3 °C and 81.5 °C, respectively. Furthermore, the conventional cell efficiency is about 8.90%, while the modified one achieves 9.6%. At a CR = 30, the maximum temperature of the modified cell is 87.4 °C, while it is beyond the permissible temperature for the conventional cell. However, the solar cell temperature was not affected by varying the EVA upper layer and the interval width between the silicon layers. The finding of the current results provides another direction for researchers to utilize a higher concentration ratio with polycrystalline silicon solar cells.

The present study investigates the effects of the orifice nozzle number and the inlet pressure experimentally on the cooling performance of the counter flow-type vortex tube. The energy generation ...

Active direct-methanol fuel cells operate on a liquid supply of reactants to the anode flow channels. Gaseous carbon dioxide is produced during operation forming large bubbles on the top side of diffusion layer, limiting the transport of reactants to the functional layer. This causes a significant drop in the rate of reaction and therefore limits the maximum current density. To collect CO2 bubbles away from the diffusion layer, a new design is proposed. It includes a degassing channel placed at the top of the main trapezoidal anode channel. The wettability of the degassing channel and the dihedral angle of the anode channel are investigated. To assess the effect of these parameters, a three-dimensional, two-phase flow model is developed and numerically simulated. Results show that adding the degassing channel is advantageous in terms of bubble collection. A trapezoidal main channel achieves a significantly higher rate of bubble actuation compared to a rectangular channel. In addition, using a dihedral angle of 20° causes a decrease in the pumping pressure, which reduces pumping losses. Moreover, a contact angle of 100° for the degassing channel provides the best compromise in terms of actuation rate, extraction rate out of the channel, and pressure drop along the channel. However, degassing channels can yield up to three times longer bubbles, which are around 75% slower. These findings create the opportunity to improve the performance of direct-methanol fuel cells by enhancing/optimizing the mass transport of reactants on the anode side.

The objective of this work is to develop a detailed numerical simulation of solar photovoltaic cells in one, two, and three-dimensions. Such kind of numerical simulation can be used as a flexible research tool for the design and analysis of solar cells. The developed in-house simulation code has the advantage of conducting modifications of the suggested configurations to include effects not covered by the commercial simulation models. In addition, this tool is to serve as a test-bed simulator for the development of solar cells modeling and to design new material models. The photovoltaic solar cells governing equations are Poisson’s equation, the hole and electron continuity equations. Poisson equation is generally used to get the voltages across the device. However, in the present work, it is used to obtain the value of the electrical charge. The governing equations along with the appropriate boundary conditions are solved numerically using a finite difference based method. The resulting system of coupled nonlinear equations is then solved using Newton method for nonlinear systems. The predicted results include illuminated current-voltage characteristic, and dark current-voltage characteristics of photovoltaic module. Comparisons between predicted results and corresponding measured values by manufacturer are conducted in order to validate the numerical simulation. A good agreement between predicted and measured results was prevailed.

Abstract In the current study, three-dimensional theoretical model of the photovoltaic (PV) panel coupled with a heat spreader is carried out. A thermal model is constructed and solved mathematically by using ANSYS software. The effect of coupling the heat spreader with the PV and the heat spreader dimensions on the PV cooling and performance is studied. Also, the effect of solar radiation intensity and weather conditions (wind speed and ambient temperature) on the performance of PV with heat spreader system is considered. The model is validated with the previous results found in the literature. Moreover, the temperature distribution of the PV with the heat spreader is presented. The results show that the optimum thickness and cross sectional area of the heat spreader for PV dimensions of 125 × 125 mm are 10 mm and 0.3 m2. The cell temperature is decreased by 15 °C when the heat spreader is used with the PV. Also the average power output and efficiency of the PV module are increased by 9% when the heat spreader is used.

Abstract Direct methanol fuel cells (DMFCs) are primary candidates to power portable electronics. However, carbon dioxide bubbles that are generated on the anode side block the flow of reactants and negatively affect the cell’s performance. Therefore, a new secondary hydrophobic degassing channel is attached to the top of the main anode channel. The proposed design introduces an upward capillary force on CO2 bubbles toward a degassing channel that collects the actuated bubbles and leads to reactant flow without bubbles in the main channel. In the current study, parallel and serpentine flow fields are investigated to assess the effect of the new design on bubble removal and overall cell performance. Thus, an experimental study is performed on a transparent DMFC using both optical observations and electrochemical characterization methods. Results show that the new design significantly enhances the cell’s performance at high current densities. In addition, the new design is advantageous in terms of removing bubbles away from the diffusion layer, especially for the parallel flow field configuration. A comparison between the output power of the new designs and the original ones indicated that the parallel flow field with the new design outperformed both the plain serpentine and the plain parallel flow field with over 14% and 23%, respectively. The advantage of using a parallel flow field is minimizing the pumping pressures required at the anode inlet, which decreases the parasitic power consumption. Based on the current findings, anode blocking can be prevented using the new design, which improves the performance of DMFCs in the mass transport region, and allows for a higher output power per cell.