The Savonius vertical axis wind turbine offers several benefits as simple construction, low manufacturing, installation and maintenance costs, low operating wind speed, and independence of wind direction. However, the conventional single-stage Savonius rotors have negative torque values at a certain range of rotor angles and large torque variation over the complete cycle which led to reduction of the power coefficient and the self-starting abilities. To overcome these drawbacks, a multi-stage Savonius wind rotor is proposed to obtain the optimum number of stages that enhances both the power coefficient and the self-starting capabilities. In the current study, one-, two-, three-, and four-stage Savonius wind rotors with straight semicircular blades are investigated. In two stages rotor, one single-stage rotor is mounted over another single stage with a phase shift of 90°. In the three stages rotor, the three singlestage rotors are mounted one above the other with a phase shift of 60° relative to one another while with a phase shift of 45° for the four stages. All rotors have overall rotor diameter (D) of 200 mm, a thickness of 2 mm, a stage aspect ratio of 1.0, and an overlap ratio (δ) of 0.0. The diameter of the circular end plates (Do) attached to the rotor tips is 1.1 of the rotor diameter (D). All studies are performed at a wind speed (V) of 6 m/s. The variations of torque and power coefficients are estimated for all the studied multi-stages rotors. Therefore, 3D incompressible unsteady Reynolds-Averaged Navier-Stokes equations along with the k-ω shear-stress transport turbulence model is developed and numerically simulated using ANSYS Fluent. The numerical model is validated using the available measurements and numerical results. The predicted flow field characteristics such as streamlines and pressure fields around the studied rotors with various numbers of stages are presented and analyzed. Accordingly, results indicated that with the increase in the number of stages, a significant enhancement of the torque and the power coefficients is attained. In addition, the rate of percentage gain in the torque and the power coefficients is higher for two stages rotor than that of three and four stages rotors. The maximum torque and power coefficients for the two stages rotor are 0.336 and 0.194, respectively. The power coefficient gain obtained by using the two stages Savonius rotor is 17.5 % compared to the conventional single stage which has a power coefficient of 0.165 at a wind velocity (V) of 6 m/s. Furthermore, using a multi-stage rotor significantly smooths the variations in the torque coefficient and produces positive static torque values at all rotational angles resulted in enhancing the self-starting capabilities of the Savonius rotor.

Wind energy comprises one of several renewable resources of energy engineered to contain the global energy crisis. Although horizontal axis wind turbines (HAWTs) have proven to be effective in low turbulence and steady wind conditions, vertical axis wind turbines (VAWTs) potentially have the advantage in highly variable and turbulent regions. The Savonius vertical axis wind turbine has several advantages such as simple design, low manufacturing costs, low operating wind speed, low noise, and Omni-directional capability. However, the Savonius rotor requires further design optimization to improve its aerodynamic performance before becoming competitive with other turbine designs. Thus, the main objective of the current study is to numerically investigate the aerodynamic performance of a multistage Savonius rotor to enhance the power coefficient and the ability of self-starting. In the current study, one-, two-, three-, and four-stage Savonius rotors with twisted blades are investigated. In a two-stage rotor, one single-stage rotor is mounted over another single-stage with a phase angle of 90°. In a three-stage rotor, the three single-stage rotors are mounted one above the other with a phase angle of 60° relative to one another while with a phase angle of 45° for the four stage-rotor. The blades of the studied Savonius rotor are twisted with a twist angle (φ) of 45°. This is the first contribution to understand how multi-stages influence the aerodynamic performance of the twisted-bladed Savonius rotor. Moreover, variations of torque and power coefficients are computed for all the studied rotors with various numbers of stages. The developed numerical model is simulated using ANSYS Fluent and validated using the available experimental and numerical results. Results showed that the coefficients of torque (C T ) and power (C P ) increase with rising the number of stages. Increasing the number of stages from 1 to 2 significantly increases the C T and C P of the rotor. However, with a further increase in the number of stages to 3 and 4 stages, both the C T and C P remains almost the same as the rotor with 2 stages. The maximum coefficient of torque (C T, max ) and power (C P, max ) for a two-stage rotor are 0.42 and 0.253, respectively. The gain in the coefficient of power obtained by using the two-stage Savonius rotor with twisted blades is 53.5% compared to the conventional single-stage which has a coefficient of power 0.165 at a wind velocity of 6 m/s. Moreover, using multi-stages and twisted blades significantly smooth the variations in the generated torque and produce positive values at all rotor angles resulted in improving the self-starting ability of the Savonius rotor.

Coronary artery disease is the abnormal contraction of heart supply blood vessel. It may lead to major consequences such as heart attack and death. This narrowing in the coronary artery limits the oxygenated blood flow to the heart. Thus, diagnosing its severity helps physicians to select the appropriate treatment plan. Fractional Flow Reserve (FFR) is one of the most accurate methods to pinpoint the stenosis severity. The advantages of FFR are high accuracy, immediate estimation of the severity of the stenosis, and concomitant treatment using balloon or stent. Nevertheless, the main disadvantage of the FFR is being an invasive procedure that requires an incision under anesthesia. Moreover, inserting the guidewire across the stenosis may result in a ‘tight-fit’ between the vessel lumen and the guidewire. This may cause an increase in the measured pressure drop, leading to a false estimation of the blood flow parameters. To estimate the errors in diagnosis procedures, a comprehensive three-dimensional model blood flow along with guidewire is developed. Reconstructed three-dimensional coronary artery geometry from a patient-specific scan is used. Blood is considered non-Newtonian and the flow is pulsatile. The comprehensive model is numerically simulated using boundary conditions. Based on the predicted results, the ratio between pressure drop and distal dynamic pressure (CDP) is studied. The predicted results for each case are compared with the control case (the case without guidewire) and analyzed. It was found that simulating the model by placing the guidewire at a full position prior to the simulation leads to an overestimation of the CDP as it increases by 34.3%. However, simulating the procedure of guidewire insertion is more accurate. It shows that the CDP value increases by 7%.

Atherosclerosis is a major arterial disease characterized by the thickening of the arteries’ walls. The development of stenosis at the carotid bifurcation affects the local variations in blood flow dynamic factors. The carotid artery dynamic factors: including the wall shear stress (WSS), time-averaged wall shear stress (TAWSS) and pressure gradient affect the rate of progression of the stenosis. It is essential to analyze the flow in three-dimensional reconstructed patient-specific geometries with realistic boundary conditions to estimate the blood flow dynamic factors. Hence, a three-dimensional comprehensive model is developed including the non-Newtonian blood flow under pulsatile flow conditions. The model is numerically simulated using computational fluid dynamics solvers along with the medical imaging to investigate the effect of stenosis locations on its progression. The numerically predicted blood flow dynamic factors are analyzed. It was found that the blood flow dynamic factors have the importance to influence the diagnosis and prediction of asymptomatic carotid artery stenosis progression. Based on results, the value of TAWSS at the stenosis in the stenotic Common Carotid Artery (CCA) is 46.68 Pa comparing to 19.24 Pa and 10.049 Pa in Internal Carotid Artery (ICA) and External Carotid Artery (ECA) respectively. Also, it was found that the maximum value of WSS in the healthy artery at the bifurcation with 3.829 Pa. However, in stenotic arteries the maximum value for WSS located at the stenosis throat which was found to be 102.158 Pa for CCA comparing to 46.859 Pa in ICA and 33.658 Pa in ECA.

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 gas diffusion layer. Hence, a significant reduction in the reaction rate occurs, which limits the maximum current density of the cell. To keep carbon dioxide bubbles away from the gas diffusion layer interface, a new design of the anode flow channel besides wall surface treatment is developed. Such a design can introduce the Concus-Finn phenomena, which forces the carbon dioxide bubbles to move away from the gas 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 hydrophobic and hydrophilic surface treatments on the gas-diffusion layer and channel walls. 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 results are validated with available measurements. Results indicated that treating the gas-diffusion layer with a hydrophilic layer increases the area in direct contact with liquid methanol. Besides, the hydrophobic top channel surfaces make it easier for the carbon dioxide bubbles to attach and spread out on the channel top surface. The current findings create a promising opportunity to improve the performance of direct methanol fuel cells.

Coronary artery disease is the progressive narrowing (stenosis) of the heart supply arteries. Heart attack could occur as a consequence of severe blockage in the artery. A definitive diagnosis of its severity is of great importance for physicians to decide the best treatment plan. Fractional flow reserve (FFR) is a procedure that adopts a sensor tipped guidewire to measure upstream and downstream pressures of the stenosis. It is considered the most accurate method to pinpoint the stenosis severity. In this procedure, the centerlines of the guidewire and blood vessel should be aligned together. Due to the long distance between its insertion point and the stenosis, a possible inclination of the guidewire can occur. The aim of the present study is to investigate the effect of guidewire inclination on pressure measurements. Three different degrees of severity are modeled with placing the guidewire at different inclination angles. Continuity and momentum equations of blood flow are stated and computationally simulated. Comparisons between calculated and available measured values show a good agreement. The ratios between pressure drop and distal dynamic pressure (CDP), and between pressure recovery coefficient and the area blockage (η) are calculated. The predicted results for each case are compared with the control case (without guidewire) and analyzed. The results aid in improving the FFR accuracy in diagnosis of stenosis severity.

Stenosis is abnormal narrowing of blood vessels that causes a shortage in blood supply and a blockage of an artery Diagnosis of its severity guides the physician to determine the most appropriate treatment plan. Fractional flow reserve (FFR) is currently the most accurate procedure in stenosis diagnosis. It is a guidewire based technique that uses a small sensor on the tip of the wire to measure proximal and distal pressure of the stenosis. The difficulty of using such method is placing the guidewire precisely in centerline of blood vessel. Therefore, the main objective of the current study is to investigate how the measured pressure varies with the guidewire position. Accordingly, three different positions from the blood vessel centerline along with three degrees of severity are considered. The governing equations for blood flow are obtained and numerically simulated. Numerical results are validated using the available experimental and numerical data. A good agreement between predicted and measured values are obtained. Based on the predicted results, pressure drop coefficient (CDP) and pressure recovery factor (η) are computed. The predicted results with and without the effect of existing guidewire at different location are analyzed and the certainty of fractional flow reserve is reported and discussed. The current method is very helpful to increase the accuracy of fractional flow reserve procedure in stenosis severity estimation.

In this study, the performance of concentrator photovoltaic (CPV) cell enhanced by using double layer microchannel heat sink (DL-MCHS) with nanofluid is investigated. Pure ethanol and 0.2 % Vol. Al 2 O 3 -ethanol are utilized to reduce the solar cell temperature under indoor solar concentration ratio of 5.7 Suns. The designed DL-MCHS is monolithically fabricated from Maraging steel using 3D metal printer. The experimental results showed that using parallel flow (PF) operation mode of the designed DL-MCHS is favourable for cooling the CPV system compared with the counter flow (CF) operation mode. In the cooled CPV using PF mode, the open circuit voltage enhancement is about 12.7% in comparison to the uncooled case. The nanofluid results also showed a reduction in the solar cell temperature in comparison with the pure coolant. The current results can be used as a validation step for accurate numerical modelling of nanofluid applications in CPV system cooling.

In this study, a new design of double layer microchannel heat sink (DL-MCHS) has been monolithically fabricated using 3D metal printer and experimentally examined as a heat sink for concentrator photovoltaic (CPV) systems. Single phase liquid cooling using ethanol and flow boiling cooling using NOVEC-7000 coolant in the designed DL-MCHS are experimentally compared. The results proved that using the flow boiling cooling technique for the CPV systems attained a lower solar cell temperature with high temperature uniformity. In more details, flow boiling in counterflow (CF) operated DL-MCHS, attained a very low solar cell temperature close to the NOVEC-7000 boiling point with temperature uniformity of 0.2 °C over a wide range of coolant flow rate from 25–250 ml/hr.

In this study, an efficient cooling technique for concentrator photovoltaic (CPV) cells is proposed to enhance the system electrical efficiency and extend its lifetime. To do this, a comprehensive three-dimensional conjugate heat transfer model of CPV cells layers coupled with the heat transfer and fluid flow model inside jet impingement heat sink is developed. Four different jet impingement designs are compared. The investigated designs are (A) central inlet jet, (B) Hypotenuse inlet jet, (C) staggered inlet jet, and (D) conventional jet impingement design with side drainage. The effect of coolant flowrate on the CPV/T system performance is investigated. The model is numerically simulated and validated using the available experiments. The performance of CPV system is investigated at solar concentration ratios of 20 and coolant flowrate up to 6000g/min. It is found that increasing the flowrate from 60 g/min to 600 g/min decrease the maximum cell temperature by 31°C for the configuration D while increasing the flowrate from 600 g/min to 6000 g/min reduce the cell temperature by 20.2°C. It is also concluded that at a higher flowrate of 6000g/min, all the investigated configurations relatively achieve better temperature uniformity with maximum temperature differences of 0.9 °C, 2.1 °C, 3.6 °C, and 3.9 °C for configurations A, B, C, and D respectively.

Homogeneous charge compression ignition (HCCI) is a combustion technology which has received increased attention of researchers in the combustion field for its potential in achieving low oxides of nitrogen (NO x ) and soot emission in internal combustion (IC) engines. HCCI engines have advantages of higher thermal efficiency and reduced emissions in comparison to conventional internal combustion engines. In HCCI engines, ignition is controlled by the chemical kinetics, which leads to significant variation in ignition time with changes in the operating conditions. This variation limits the practical range of operation of the engine. Additionally, since HCCI engine operation combines the operating principles of both spark ignition (SI) and compression ignition (CI) engines, HCCI engine parameters such as compression ratio and injection timing may vary significantly depending on operating conditions, including the type of fuel used. As such, considerable research efforts have been focused on establishing optimal conditions for HCCI operation with both conventional and alternative fuels. In this study, numerical simulation is used to investigate the effect of compression ratio on combustion and emission characteristics of an HCCI engine fueled by pure biodiesel. Using a zero-dimensional (0-D) reactor model and a detailed reaction mechanism for biodiesel, the influence of compression ratio on the combustion and emission characteristics are studied in Chemkin-Pro. Simulation results are validated with available experimental data in terms of incylinder pressure and heat release rate to demonstrate the accuracy of the simulation model in predicting the performance of the actual engine. Analysis shows that an increase in compression ratio leads to advanced and higher peak incylinder pressure. The results also reveal that an increase in compression ratio produces advanced ignition and increased heat release rates for biodiesel combustion. Emission of NO x is observed to increase with increase in compression ratio while the effect of compression ratio on emissions of CO, CO 2 and unburned hydrocarbon (UHC) is only marginal.

Concentrated photovoltaic (CPV) integrated with phase-change material (CPV-PCM) system is considered as a single module to reduce the CPV temperature rise and achieve higher solar conversion efficiency. For low concentration ratios (CRs), up to 20, a larger PCM thickness is needed to absorb much more heat and prolong the thermal regulation time of CPV systems. As a result, the heat absorbed in the PCM is not released efficiently to the ambient during the night time. Therefore, active heat dissipation from the CPV-PCM system is required during that time to attain full transition to solid state at the starting of each period of insolation. Thus, a hybrid CPV-PCM water system including various designs of the PCM heat sink is proposed. Such system provides a high-energy storage density during the daytime and enhances the heat extraction from PCM during the night time. To predict the thermal and electrical performance of the hybrid CPV-PCM water system, a comprehensive 2-D model for CPV layers integrated with both PCM, and water flow is developed. The model couples thermal models for CPV layers and thermo-fluid model that considers the phase-change phenomenon and water flow. Numerical simulations of the developed models are performed to determine the instantaneous liquid-solid interface evolution and the transient temperature distribution within the hybrid CPV-PCM water system. Results indicate that the hybrid CPV-PCM water system achieves a significant reduction in the CPV temperature during the daytime and improves the heat dissipation from PCM during the night time.

To achieve reliable and efficient operation of generic polycrystalline silicon solar cell under concentrated sunlight, a novel structure of the cell layers is proposed along with effective cooling technique using microchannel heat sink (MCHS). In the novel structure, Boron Nitride with the volume fraction of 20%, 40%, and 60% as a filler is incorporated in the Ethylene Vinyl Acetate (EVA) matrix to form a new composite. The new composite is used instead of the conventional EVA layer in the solar cell. Various solar cell structures integrated with MCHS are studied and compared with the conventional structure. To determine the performance of the developed concentrated photovoltaic thermal (CPVT) system, a comprehensive three-dimensional model of the solar cell with heat sink is developed. The model is numerically simulated and validated. Based on the validated results, it is found that the novel structure with EVA-60% BN composite along with aluminum foil back sheet attains 30% increase in the gained solar cell electric power with 10.9 % reduction in the cell temperature compared with the conventional solar cell structure at the same cooling mass flow rate of 50 g/min and concentration ratio of 20. However at CR = 20, V w = 1m/s and T a = 30°C a significant damage of the conventional solar cell structure will occurs if no effective cooling technique is used. Moreover, the developed design of solar cell achieves a higher CPVT-system thermal efficiency compared with the conventional one.

The high incident heat flux on the concentrated photovoltaic (CPV) system causes a significant increase in the cell temperature and thus reduces the system efficiency. Therefore, using an efficient cooling technique is of great importance for those systems. In the present study, a new technology for concentrated photovoltaic systems is introduced using a truncated-double layer microchannel heat sink. A comprehensive three-dimensional thermo-fluid model for the photovoltaic layers integrated with a microchannel heat sink was developed. The proposed model was simulated numerically to estimate the solar cell temperature, temperature uniformity, cooling system pumping power, electrical efficiency and thermal efficiency of the CPV system. The numerical results were validated with the available experimental, analytical and numerical results in the literature. In the designed heat sink, various design parameters are investigated such as the truncation length, cooling mass flow rate, concentration ratio, and converging width ratio of the flow channel. Results indicate that increasing the truncated length leads to an increase of solar cell temperature at a constant coolant mass flow rate. The cell temperature varies between 80.1°C and 146.5°C as the truncation length ratio increases from 0 (i.e. single layer microchannel) to 1 respectively at a concentration ratio (CR) of 40 and a cooling mass flow rate (ṁ) of 26.6 g/min. Using the double layer microchannel reduces the consumed pumping power at the same total mass flow rate compared to the single layer microchannel. The Double layer configuration with a truncation length ratio ( l/l sc ) equal to unity achieves a lower pumping power and solar cell temperature uniformity in comparison to the single layer microchannel.

In the current work, a hybrid system including Concentrated photovoltaic (CPV) and phase change material (PCM) as a heat sink is considered as a single module to achieve high solar conversion efficiency. The main objective is to accelerate the thermal dissipation with a longer thermal regulation period. Thus, a new CPV-PCM system using various configurations of the PCM heat sink and different combinations of PCMs is investigated. This study presents a numerical simulation of the effects of PCM materials and designs on the CPV-PCM system performance. To estimate the thermal performance of the new CPV-PCM system, a comprehensive 2-D model for CPV layers integrated with PCMs is developed. This model couples a thermal model for CPV layers and a thermo-fluid model that considers the phase-change phenomenon using the enthalpy method. The model is numerically simulated at different configurations and combinations of PCM with various ranges of phase transition temperatures. Three different configurations of PCMs are investigated: one with a single cavity, and two with parallel arrangements including three and five cavities. Results indicate that the use of PCM heat sinks with three and five cavities increases the heat transfer inside the PCM and achieves a significant reduction of the solar cell temperature compared with a single cavity CPV-PCM system. Furthermore, thermal regulation effect and temperature uniformity of the CPV-PCM system is enhanced by using various combinations of PCMs.

The photovoltaic output power is directly proportional to the solar radiation and inversely with the cell temperature. The higher the photovoltaic temperature is, the lower the electrical efficiency is with possible damage to the cell. To improve the electrical efficiency and to avoid the possible damage, a concentrating PV system associated with an effective cooling technique is of great importance. In the present study, a new cooling technique for concentrated photovoltaic (CPV) systems was introduced using various designs of micro-channel heat sinks. The suggested configurations included parallel flow, counter flow single and double layer micro-channels, and single layer flat micro-channel integrated with CPV system. A comprehensive three-dimensional thermo-fluid model for photovoltaic layers integrated with microchannel heat sink was developed. The model was simulated numerically to estimate the solar cell temperature. The numerical results were validated with the available experimental and numerical results. In the meantime, the effects of different operational parameters were investigated such as solar concentration ratio and cooling mass flow rate. Performance analysis of CPV using different microchannel configurations was implemented to determine the average and local solar cell temperature, pumping power, and temperature uniformity. Results indicated that the use of microchannel heat sink was a very effective cooling technique which highly attained temperature uniformity, viz., eliminated the hot spots formation with a significant reduction in the average temperature of CPV. The single layer parallel flow achieved the minimum solar cell temperature while the counter flow attained the most uniform temperature distribution compared with other configurations. Furthermore, the double layer parallel flow microchannel attained the minimum pumping power for a given cooling mass flow rate.

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 r 1 atio, 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.

The objective of the present work is to develop a dynamic electro-thermal model for photovoltaic cells to optimize both electrical and thermal performance of such systems. The thermal model is developed by generating the equivalent RC network (resistance-capacitance) parameters. Then, the thermal model is combined with electrical model and implemented by PSIM simulation program to evaluate performance parameters for any predefined operating condition. The electro-thermal model predicts instantaneous temperature of photovoltaic device at the actual circuit working conditions. Consequently, the temperature rise during the system startup and during load transient is investigated. The thermal model predicts the junction temperature based on transient heat dissipation calculated from the electrical model. The calculated junction temperature is used as an input to the electrical model. The factors that control the junction temperature are module reaching irradiance, optical properties of the photovoltaic cell, photovoltaic conversion efficiency, heat transfer and electrical characteristic of the load. If the junction temperature exceeds a certain limit, it causes hotspot on the photovoltaic module surface. Consequently, the formed hotspots result in reducing the system efficiency and life time. The predicted dynamic behavior of photovoltaic cell is compared with theoretical predictions and other reported data in order to validate the developed model. The obtained results show a good concurrence with the predictions of other works. Detailed comparisons between predicted and measured results are reported and discussed.