Search

Search Results

• 11. [Article] Bi-Material Assembly Subjected to Thermal Stress: Propensity to Delamination Assessed Using Interfacial Compliance Model

  It is shown that an engineering stress model suggested about 30 years ago for the approximate evaluation of the interfacial stresses in adhesively bonded or soldered bi-material assemblies experiencing ...

  Citation × Citation Citation Abstract Copy Title: Bi-Material Assembly Subjected to Thermal Stress: Propensity to Delamination Assessed Using Interfacial Compliance Model Author: Suhir, Ephraim Year: 2016 It is shown that an engineering stress model suggested about 30 years ago for the approximate evaluation of the interfacial stresses in adhesively bonded or soldered bi-material assemblies experiencing thermal loading and based on the concept of interfacial compliance can be employed also for the assessment of the assembly’s propensity to delamination. The analysis is limited to the shearing mode of failure and to the elastic stresses. A probabilistic extreme value distribution (EVD) approach can be used to consider the random nature of both the actual and the critical stress-energy-release-rates (SERR). The step-wise nature of the typical failure oriented accelerated test (FOAT) loading in the electronics reliability field (such as, e.g., temperature cycling) is addressed. The general concepts are illustrated by numerical examples. Title: Bi-Material Assembly Subjected to Thermal Stress: Propensity to Delamination Assessed Using Interfacial Compliance Model Author: Suhir, Ephraim Year: 2016 It is shown that an engineering stress model suggested about 30 years ago for the approximate evaluation of the interfacial stresses in adhesively bonded or soldered bi-material assemblies experiencing thermal loading and based on the concept of interfacial compliance can be employed also for the assessment of the assembly’s propensity to delamination. The analysis is limited to the shearing mode of failure and to the elastic stresses. A probabilistic extreme value distribution (EVD) approach can be used to consider the random nature of both the actual and the critical stress-energy-release-rates (SERR). The step-wise nature of the typical failure oriented accelerated test (FOAT) loading in the electronics reliability field (such as, e.g., temperature cycling) is addressed. The general concepts are illustrated by numerical examples. Close

• 12. [Article] Quench Factor Characterization of Steel Hardening: A Review

  A quench factor, Q, has been derived that quantifies the overall severity of the steel hardening process—quench severity by interrelating steel hardenability with quenching variables including: the specific ...

  Citation × Citation Citation Abstract Copy Title: Quench Factor Characterization of Steel Hardening: A Review Author: Otero, Rosa Lucia Simencio, Otero, Walker Roberto, Totten, George E., Canale, Lauralice C.F. Year: 2014 A quench factor, Q, has been derived that quantifies the overall severity of the steel hardening process—quench severity by interrelating steel hardenability with quenching variables including: the specific quenchant being used, agitation, bath temperature, etc. In this analysis, quench severity is quantified by a so-called “quench factor” (Q) that is a single numerical value that effectively indicates the extent to which a given steel can be hardened. This article provides a description of the concept and an overall review of applications where it has been successfully applied. Title: Quench Factor Characterization of Steel Hardening: A Review Author: Otero, Rosa Lucia Simencio, Otero, Walker Roberto, Totten, George E., Canale, Lauralice C.F. Year: 2014 A quench factor, Q, has been derived that quantifies the overall severity of the steel hardening process—quench severity by interrelating steel hardenability with quenching variables including: the specific quenchant being used, agitation, bath temperature, etc. In this analysis, quench severity is quantified by a so-called “quench factor” (Q) that is a single numerical value that effectively indicates the extent to which a given steel can be hardened. This article provides a description of the concept and an overall review of applications where it has been successfully applied. Close

• 13. [Article] Engineered Mushing Cooker

  This project revolves around building methanol cookers for dog mushers in the Iditarod and for the Denali Park Ranger Kennel in Denali National Park, Alaska. Traditional cookers are bulky, slow to burn, ...

  Citation × Citation Citation Abstract Copy Title: Engineered Mushing Cooker Author: Ritter, Aimee Year: 2016 This project revolves around building methanol cookers for dog mushers in the Iditarod and for the Denali Park Ranger Kennel in Denali National Park, Alaska. Traditional cookers are bulky, slow to burn, and use up a lot of fuel. Our goal has been to revolutionize the way mushing cookers look and perform. Our cookers are built to pack well in the sleds, heat up to 3 gallons of snowmelt, perform in arctic climates, use methanol (specifically HEET) for fuel, and be lightweight. A traditional cooker would use 3 to 4 bottles or more of HEET (36-48+oz.) and take at least 1 hour to heat up 3 gallons of water to the temperature needed to warm up frozen dog food. Our design takes 30 minutes and uses 2 to 3 bottles of HEET (24-36 oz). Additionally, it is square, making it easier to pack in a sled. The picture below shows the final product of the cooker that we sent to the musher Lisbet Norris for use in the 2016 Iditarod. Title: Engineered Mushing Cooker Author: Ritter, Aimee Year: 2016 This project revolves around building methanol cookers for dog mushers in the Iditarod and for the Denali Park Ranger Kennel in Denali National Park, Alaska. Traditional cookers are bulky, slow to burn, and use up a lot of fuel. Our goal has been to revolutionize the way mushing cookers look and perform. Our cookers are built to pack well in the sleds, heat up to 3 gallons of snowmelt, perform in arctic climates, use methanol (specifically HEET) for fuel, and be lightweight. A traditional cooker would use 3 to 4 bottles or more of HEET (36-48+oz.) and take at least 1 hour to heat up 3 gallons of water to the temperature needed to warm up frozen dog food. Our design takes 30 minutes and uses 2 to 3 bottles of HEET (24-36 oz). Additionally, it is square, making it easier to pack in a sled. The picture below shows the final product of the cooker that we sent to the musher Lisbet Norris for use in the 2016 Iditarod. Close

• 14. [Article] Reversible Control of the Magnetization of Fe₃O₄ via Lithium Ions

  In this report, a reversible control of Fe₃O₄ saturated magnetization by Li ions is demonstrated. A miniature Li ion battery (LIB) was assembled using a Fe₃O₄ nanoparticle layer as the active cathode. ...

  Citation × Citation Citation Abstract Copy Title: Reversible Control of the Magnetization of Fe₃O₄ via Lithium Ions Author: Wei, Guodong, Wei, Lin, Wang, Dong, Tian, Yufeng, Chen, Yanxue, Yan, Shishen, Mei, Liangmo, Jiao, Jun Year: 2017 In this report, a reversible control of Fe₃O₄ saturated magnetization by Li ions is demonstrated. A miniature Li ion battery (LIB) was assembled using a Fe₃O₄ nanoparticle layer as the active cathode. A stable magnetism modulation is realized by a nondestructive electrochemical process in which the lithium insertion results in a valence change and partial redistribution of Fe cations in the crystal structure. The relation between the discharge voltage and the chemical phases were studied by ex situ X-ray diffraction and magnetic measurement. In a suitable discharge potential range, a reversible control of Fe₃O₄ saturated magnetization was obtained without structural damage to the magnetic electrode. The experimental results indicate by further optimizing the LIB's performance, a large reversible change in magnetization could be realized at room temperature, suggesting the potential for future practical applications. Title: Reversible Control of the Magnetization of Fe₃O₄ via Lithium Ions Author: Wei, Guodong, Wei, Lin, Wang, Dong, Tian, Yufeng, Chen, Yanxue, Yan, Shishen, Mei, Liangmo, Jiao, Jun Year: 2017 In this report, a reversible control of Fe₃O₄ saturated magnetization by Li ions is demonstrated. A miniature Li ion battery (LIB) was assembled using a Fe₃O₄ nanoparticle layer as the active cathode. A stable magnetism modulation is realized by a nondestructive electrochemical process in which the lithium insertion results in a valence change and partial redistribution of Fe cations in the crystal structure. The relation between the discharge voltage and the chemical phases were studied by ex situ X-ray diffraction and magnetic measurement. In a suitable discharge potential range, a reversible control of Fe₃O₄ saturated magnetization was obtained without structural damage to the magnetic electrode. The experimental results indicate by further optimizing the LIB's performance, a large reversible change in magnetization could be realized at room temperature, suggesting the potential for future practical applications. Close

• 15. [Article] Microstructures and Stabilization Mechanisms of Nanocrystalline Iron-Chromium Alloys with Hafnium Addition

  The low thermal stability of nanocrystalline metals severely limits their applications at high temperatures. In this study, we investigate the nanocrystalline stabilization mechanisms for Fe-14Cr alloys ...

  Citation × Citation Citation Abstract Copy Title: Microstructures and Stabilization Mechanisms of Nanocrystalline Iron-Chromium Alloys with Hafnium Addition Author: Xu, Weizong, Li, Lulu, Saber, Mostafa, Koch, Carl C., Zhu, Yuntian, Scattergood, Ronald O. Year: 2015 The low thermal stability of nanocrystalline metals severely limits their applications at high temperatures. In this study, we investigate the nanocrystalline stabilization mechanisms for Fe-14Cr alloys with 1, 2, and 4 at. pct Hf addition at 1173 K (900 °C). Microstructural characterizations using aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy reveal high density of HfO2 nanoparticles with sizes of ~4 nm dispersed throughout the ferritic matrix. This indicates that kinetic stabilization by HfO2 nanoparticle pinning is primarily responsible for the observed high thermal stability. In addition, some Hf and Cr segregation on grain boundaries is observed in the Fe-14Cr-4Hf, suggesting the existence of thermodynamic stabilization at high Hf content. Second-phase precipitations such as hafnium carbide, M23C6, and Fe-Cr-Hf intermetallic phase are also found in the Fe-14Cr-4Hf, but their large sizes and inter-spacing suggest that their contribution to stabilization is minimal. Title: Microstructures and Stabilization Mechanisms of Nanocrystalline Iron-Chromium Alloys with Hafnium Addition Author: Xu, Weizong, Li, Lulu, Saber, Mostafa, Koch, Carl C., Zhu, Yuntian, Scattergood, Ronald O. Year: 2015 The low thermal stability of nanocrystalline metals severely limits their applications at high temperatures. In this study, we investigate the nanocrystalline stabilization mechanisms for Fe-14Cr alloys with 1, 2, and 4 at. pct Hf addition at 1173 K (900 °C). Microstructural characterizations using aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy reveal high density of HfO2 nanoparticles with sizes of ~4 nm dispersed throughout the ferritic matrix. This indicates that kinetic stabilization by HfO2 nanoparticle pinning is primarily responsible for the observed high thermal stability. In addition, some Hf and Cr segregation on grain boundaries is observed in the Fe-14Cr-4Hf, suggesting the existence of thermodynamic stabilization at high Hf content. Second-phase precipitations such as hafnium carbide, M23C6, and Fe-Cr-Hf intermetallic phase are also found in the Fe-14Cr-4Hf, but their large sizes and inter-spacing suggest that their contribution to stabilization is minimal. Close

• 16. [Article] ZnO Nanosheet Arrays Constructed on Weaved Titanium Wire for CdS-Sensitized Solar Cells

  Ordered ZnO nanosheet arrays were grown on weaved titanium wires by a low-temperature hydrothermal method. CdS nanoparticles were deposited onto the ZnO nanosheet arrays using the successive ionic layer ...

  Citation × Citation Citation Abstract Copy Title: ZnO Nanosheet Arrays Constructed on Weaved Titanium Wire for CdS-Sensitized Solar Cells Author: Wu, Cuncun, Wei, Lin, Li, Yitan, Liu, Chang, Jiao, Jun, Chen, Yanxue, Mei, Liangmo Year: 2014 Ordered ZnO nanosheet arrays were grown on weaved titanium wires by a low-temperature hydrothermal method. CdS nanoparticles were deposited onto the ZnO nanosheet arrays using the successive ionic layer adsorption and reaction method to make a photoanode. Nanoparticle-sensitized solar cells were assembled using these CdS/ZnO nanostructured photoanodes, and their photovoltaic performance was studied systematically. The best light-to-electricity conversion efficiency was obtained to be 2.17% under 100 mW/cm/cm2 illumination, and a remarkable shortcircuit photocurrent density of approximately 20.1 mA/cm2 was recorded, which could attribute to the relatively direct pathways for transportation of electrons provided by ZnO nanosheet arrays as well as the direct contact between ZnO and weaved titanium wires. These results indicate that CdS/ZnO nanostructures on weaved titanium wires would open a novel possibility for applications of low-cost solar cells. Title: ZnO Nanosheet Arrays Constructed on Weaved Titanium Wire for CdS-Sensitized Solar Cells Author: Wu, Cuncun, Wei, Lin, Li, Yitan, Liu, Chang, Jiao, Jun, Chen, Yanxue, Mei, Liangmo Year: 2014 Ordered ZnO nanosheet arrays were grown on weaved titanium wires by a low-temperature hydrothermal method. CdS nanoparticles were deposited onto the ZnO nanosheet arrays using the successive ionic layer adsorption and reaction method to make a photoanode. Nanoparticle-sensitized solar cells were assembled using these CdS/ZnO nanostructured photoanodes, and their photovoltaic performance was studied systematically. The best light-to-electricity conversion efficiency was obtained to be 2.17% under 100 mW/cm/cm2 illumination, and a remarkable shortcircuit photocurrent density of approximately 20.1 mA/cm2 was recorded, which could attribute to the relatively direct pathways for transportation of electrons provided by ZnO nanosheet arrays as well as the direct contact between ZnO and weaved titanium wires. These results indicate that CdS/ZnO nanostructures on weaved titanium wires would open a novel possibility for applications of low-cost solar cells. Close

• 17. [Article] CdS Quantum Dot-Sensitized Solar Cells Based on Nano-Branched TiO2 Arrays

  Nano-branched rutile TiO2 nanorod arrays were grown on F:SnO2 conductive glass (FTO) by a facile, two-step wet chemical synthesis process at low temperature. The length of the nanobranches was tailored ...

  Citation × Citation Citation Abstract Copy Title: CdS Quantum Dot-Sensitized Solar Cells Based on Nano-Branched TiO2 Arrays Author: Liu, Chang, Li, Yitan, Wei, Lin, Wu, Cuncun, Chen, Yanxue, Mei, Liangmo, Jiao, Jun Year: 2014 Nano-branched rutile TiO2 nanorod arrays were grown on F:SnO2 conductive glass (FTO) by a facile, two-step wet chemical synthesis process at low temperature. The length of the nanobranches was tailored by controlling the growth time, after which CdS quantum dots were deposited on the nano-branched TiO2 arrays using the successive ionic layer adsorption and reaction method to make a photoanode for quantum dot-sensitized solar cells (QDSCs). The photovoltaic properties of the CdS-sensitized nano-branched TiO2 solar cells were studied systematically. A short-circuit current intensity of approximately 7 mA/cm2 and a light-to-electricity conversion efficiency of 0.95% were recorded for cells based on optimized nano-branched TiO2 arrays, indicating an increase of 138% compared to those based on unbranched TiO2 nanorod arrays. The improved performance is attributed to a markedly enlarged surface area provided by the nanobranches and better electron conductivity in the one-dimensional, well-aligned TiO2 nanorod trunks Title: CdS Quantum Dot-Sensitized Solar Cells Based on Nano-Branched TiO2 Arrays Author: Liu, Chang, Li, Yitan, Wei, Lin, Wu, Cuncun, Chen, Yanxue, Mei, Liangmo, Jiao, Jun Year: 2014 Nano-branched rutile TiO2 nanorod arrays were grown on F:SnO2 conductive glass (FTO) by a facile, two-step wet chemical synthesis process at low temperature. The length of the nanobranches was tailored by controlling the growth time, after which CdS quantum dots were deposited on the nano-branched TiO2 arrays using the successive ionic layer adsorption and reaction method to make a photoanode for quantum dot-sensitized solar cells (QDSCs). The photovoltaic properties of the CdS-sensitized nano-branched TiO2 solar cells were studied systematically. A short-circuit current intensity of approximately 7 mA/cm2 and a light-to-electricity conversion efficiency of 0.95% were recorded for cells based on optimized nano-branched TiO2 arrays, indicating an increase of 138% compared to those based on unbranched TiO2 nanorod arrays. The improved performance is attributed to a markedly enlarged surface area provided by the nanobranches and better electron conductivity in the one-dimensional, well-aligned TiO2 nanorod trunks Close

• 18. [Article] Microstructural Evolution of SAC305 Solder Joints in Wafer Level Chip-Scale Packaging (WLCSP) with Continuous and Interrupted Accelerated Thermal Cycling

  Four high-strain design wafer level chip scale packages were given accelerated thermal cycling with a 10°C/min ramp rate and 10 min hold times between 0°C and 100°C to examine the effects of continuous ...

  Citation × Citation Citation Abstract Copy Title: Microstructural Evolution of SAC305 Solder Joints in Wafer Level Chip-Scale Packaging (WLCSP) with Continuous and Interrupted Accelerated Thermal Cycling Author: Zhou, Quan, Zhou, Bite, Lee, Tae-Kyu, Bieler, Thomas Year: 2016 Four high-strain design wafer level chip scale packages were given accelerated thermal cycling with a 10°C/min ramp rate and 10 min hold times between 0°C and 100°C to examine the effects of continuous and interrupted thermal cycling on the number of cycles to failure. The interruptions given two of the samples were the result of periodic examinations using electron backscattered pattern mapping, leading to room temperature aging of 30 days–2.5 years after increments of about 100 cycles at several stages of the cycling history. The continuous thermal cycling resulted in solder joints with a much larger degree of recrystallization, whereas the interrupted thermal cycling tests led to much less recrystallization, which was more localized near the package side, and the crack was more localized near the interface and had less branching. The failure mode for both conditions was still the same, with cracks nucleating along the high angle grain boundaries formed during recrystallization. In conditions where there were few recrystallized grains, recovery led to formation of subgrains that strengthened the solder, and the higher strength led to a larger driving force for crack growth through the solder, leading to failure after less than half of the cycles in the continuous accelerated thermal cycling condition. This work shows that there is a critical point where sufficient strain energy accumulation will trigger recrystallization, but this point depends on the rate of strain accumulation in each cycle and various recovery processes, which further depends on local crystal orientations, stress state evolution, and specific activated slip and twinning systems. Title: Microstructural Evolution of SAC305 Solder Joints in Wafer Level Chip-Scale Packaging (WLCSP) with Continuous and Interrupted Accelerated Thermal Cycling Author: Zhou, Quan, Zhou, Bite, Lee, Tae-Kyu, Bieler, Thomas Year: 2016 Four high-strain design wafer level chip scale packages were given accelerated thermal cycling with a 10°C/min ramp rate and 10 min hold times between 0°C and 100°C to examine the effects of continuous and interrupted thermal cycling on the number of cycles to failure. The interruptions given two of the samples were the result of periodic examinations using electron backscattered pattern mapping, leading to room temperature aging of 30 days–2.5 years after increments of about 100 cycles at several stages of the cycling history. The continuous thermal cycling resulted in solder joints with a much larger degree of recrystallization, whereas the interrupted thermal cycling tests led to much less recrystallization, which was more localized near the package side, and the crack was more localized near the interface and had less branching. The failure mode for both conditions was still the same, with cracks nucleating along the high angle grain boundaries formed during recrystallization. In conditions where there were few recrystallized grains, recovery led to formation of subgrains that strengthened the solder, and the higher strength led to a larger driving force for crack growth through the solder, leading to failure after less than half of the cycles in the continuous accelerated thermal cycling condition. This work shows that there is a critical point where sufficient strain energy accumulation will trigger recrystallization, but this point depends on the rate of strain accumulation in each cycle and various recovery processes, which further depends on local crystal orientations, stress state evolution, and specific activated slip and twinning systems. Close

• 19. [Article] Distribution of Mean Kinetic Energy Around an Isolated Wind Turbine and a Characteristic Wind Turbine of a Very Large Wind Farm

  An isolated wind turbine and a very large wind farm are introduced into large-eddy simulations of an atmospheric boundary layer. The atmospheric flow is forced with a constant geostrophic wind and a time-varying ...

  Citation × Citation Citation Abstract Copy Title: Distribution of Mean Kinetic Energy Around an Isolated Wind Turbine and a Characteristic Wind Turbine of a Very Large Wind Farm Author: Cortina, Gerard, Calaf, Marc, Cal, Raul Bayoan Year: 2016 An isolated wind turbine and a very large wind farm are introduced into large-eddy simulations of an atmospheric boundary layer. The atmospheric flow is forced with a constant geostrophic wind and a time-varying surface temperature extracted from a selected period of the CASES-99 field experiment. A control volume approach is used to directly compare the transfer of mean kinetic energy around a characteristic wind turbine throughout a diurnal cycle considering both scenarios. For the very large wind farm case, results illustrate that the recovery of mean kinetic energy around a wind turbine is dominated by the vertical flux, regardless of atmospheric stratification. Contrarily, for an isolated wind turbine, the recovery is dependent on the background atmospheric stratification and it is produced by a combination of advection, vertical flux, and pressure redistribution. The analysis also illustrates that during the unstable stratification periods vertical entrainment of mean kinetic energy dominates, whereas during the stable regime horizontal entrainment is predominant. Finally, it is observed that in both scenarios, the single wind turbine and the large wind farm cases, turbulent mixing is driven by the background convective stratification during the unstable period and by the effect of the wind turbine during the stable regime. Title: Distribution of Mean Kinetic Energy Around an Isolated Wind Turbine and a Characteristic Wind Turbine of a Very Large Wind Farm Author: Cortina, Gerard, Calaf, Marc, Cal, Raul Bayoan Year: 2016 An isolated wind turbine and a very large wind farm are introduced into large-eddy simulations of an atmospheric boundary layer. The atmospheric flow is forced with a constant geostrophic wind and a time-varying surface temperature extracted from a selected period of the CASES-99 field experiment. A control volume approach is used to directly compare the transfer of mean kinetic energy around a characteristic wind turbine throughout a diurnal cycle considering both scenarios. For the very large wind farm case, results illustrate that the recovery of mean kinetic energy around a wind turbine is dominated by the vertical flux, regardless of atmospheric stratification. Contrarily, for an isolated wind turbine, the recovery is dependent on the background atmospheric stratification and it is produced by a combination of advection, vertical flux, and pressure redistribution. The analysis also illustrates that during the unstable stratification periods vertical entrainment of mean kinetic energy dominates, whereas during the stable regime horizontal entrainment is predominant. Finally, it is observed that in both scenarios, the single wind turbine and the large wind farm cases, turbulent mixing is driven by the background convective stratification during the unstable period and by the effect of the wind turbine during the stable regime. Close

• 20. [Article] Measuring the Effect of Vegetated Roofs on the Performance of Photovoltaic Panels in Combined Systems

  Recent studies suggest that integration of photovoltaic panels with green roofs may improve the performance of both. While vegetation may provide a benefit by reducing the net radiation load on the underside ...

  Citation × Citation Citation Abstract Copy Title: Measuring the Effect of Vegetated Roofs on the Performance of Photovoltaic Panels in Combined Systems Author: Ogaili, Hamid Hawi Kadham Year: 2015 Recent studies suggest that integration of photovoltaic panels with green roofs may improve the performance of both. While vegetation may provide a benefit by reducing the net radiation load on the underside of the photovoltaic (PV) panels, it may also affect convective cooling of panels, and consequently, panel efficiency. Both effects likely diminish with the height of the PV panel above the roof, although placing PV panels too close to the vegetation increases the risk of the plants growing over the edges of, and shading the PV panel. There is a gap in the literature with respect to evaluating these competing effects. The present study aims to fill this gap. Experiments were conducted over a two-month period during summer using two identical PV panels within an array of rooftop-mounted panels. These experiments were performed at two heights (18 cm and 24 cm) using three roofing types: white, black and green (vegetated). Results showed that the mean power output of the system in which the PV panel was mounted above a green roof was 1.2% and 0.8% higher than that of the PV-black roof and the PV-white roof at the 18 cm height. At the 24 cm height, the benefit of the green roof was slightly diminished with power output for the PV panel above a green roof being 1.0% and 0.7% higher than the black and white roof experiments, respectively. These power output results were consistent with measured variations in mean panel surface temperatures; the green roof systems were generally cooler by 1.5˚C to 3˚C. The panel surface mean heat transfer coefficients for the PV-green roof were generally 10 to 23% higher than for the white and black roof configurations, suggesting a mixing benefit associated with the roughness of the plant canopy. As expected, the results indicate that the best PV panel performance is obtained by locating the PV panel above a green roof. However, the relative benefits of the roof energy balance diminish with distance between the PV panel and the roof. Moreover, the results of this study showed that the mean power output of the PV panel above the white roof was 0.7% and 0.44% higher than that of the PV panel above the black roof at 18 cm and 24 cm heights, respectively. The results of the power output differences in all the experiments were statistically significant at the 95% confidence interval (P < 0.01). Title: Measuring the Effect of Vegetated Roofs on the Performance of Photovoltaic Panels in Combined Systems Author: Ogaili, Hamid Hawi Kadham Year: 2015 Recent studies suggest that integration of photovoltaic panels with green roofs may improve the performance of both. While vegetation may provide a benefit by reducing the net radiation load on the underside of the photovoltaic (PV) panels, it may also affect convective cooling of panels, and consequently, panel efficiency. Both effects likely diminish with the height of the PV panel above the roof, although placing PV panels too close to the vegetation increases the risk of the plants growing over the edges of, and shading the PV panel. There is a gap in the literature with respect to evaluating these competing effects. The present study aims to fill this gap. Experiments were conducted over a two-month period during summer using two identical PV panels within an array of rooftop-mounted panels. These experiments were performed at two heights (18 cm and 24 cm) using three roofing types: white, black and green (vegetated). Results showed that the mean power output of the system in which the PV panel was mounted above a green roof was 1.2% and 0.8% higher than that of the PV-black roof and the PV-white roof at the 18 cm height. At the 24 cm height, the benefit of the green roof was slightly diminished with power output for the PV panel above a green roof being 1.0% and 0.7% higher than the black and white roof experiments, respectively. These power output results were consistent with measured variations in mean panel surface temperatures; the green roof systems were generally cooler by 1.5˚C to 3˚C. The panel surface mean heat transfer coefficients for the PV-green roof were generally 10 to 23% higher than for the white and black roof configurations, suggesting a mixing benefit associated with the roughness of the plant canopy. As expected, the results indicate that the best PV panel performance is obtained by locating the PV panel above a green roof. However, the relative benefits of the roof energy balance diminish with distance between the PV panel and the roof. Moreover, the results of this study showed that the mean power output of the PV panel above the white roof was 0.7% and 0.44% higher than that of the PV panel above the black roof at 18 cm and 24 cm heights, respectively. The results of the power output differences in all the experiments were statistically significant at the 95% confidence interval (P < 0.01). Close