title: Investigation into the effects of flow distribution on the photovoltaic performance of a building integrated photovoltaic/thermal solar collector

authors: Ghani, Faisal Abid

subjects: photovoltaic

abstract: The conversion of solar energy into usable forms of energy such as electricity and heat is attractive given the abundance of solar energy and the numerous issues recently raised in the consumption of fossil fuels. Solar conversion technologies may generally be categorised as either photovoltaic or solar thermal types capable of converting incidental sunlight into electricity and heat respectively. The photovoltaic cell is able to transform incidental sunlight into electricity via the Becquerel effect, however, the single junction crystalline silicon solar cell, the predominant cell type in today’s photovoltaic market is only able to utilise a small portion (less than 20%) of incidental sunlight for this purpose. A majority of the remaining portion is absorbed much like a traditional solar thermal collector and sunk as heat by the cell, elevating its operating temperature. Given the negative effect of temperature on photovoltaic cell operation, where a linearly proportional drop in conversion efficiency with elevated temperature can be expected, photovoltaic conversion can be reduced significantly particularly in areas of high irradiance and ambient temperatures. Based on the intrinsic absorption characteristics of the photovoltaic cell, a third type of solar panel referred to as the hybrid photovoltaic thermal collector (PVT) collector has been developed where fluid channels running along the underside of the photovoltaic panel transfer heat away from the cells to minimise this detrimental effect. Furthermore, heat captured from the cells may then be used for space heating or domestic hot water improving the overall collector efficiency.
In this study a unique building integrated PVT (BIPVT) collector is investigated consisting of an aluminium extrusion with structural ribs, fluid channels, and solar conversion materials. In order to evaluate this design, a mathematical model of the collector was developed in order to determine both thermal and electrical yield of the proposed design. The thermal analyses of the building integrated PVT collector in previous studies have generally adopted the approach applied to traditional solar thermal collectors where the distribution of coolant fluid flowing through the piping array is assumed uniform. For a conventional solar thermal collector this simplification may be reasonable under certain circumstances, however, given the temperature sensitivity of photovoltaic cells and their electrical connection scheme, this assumption may lead to significant modelling error. In order to further investigate this issue, a mathematical model has been developed to determine the photovoltaic yield of a BIPVT collector operating under non-homogeneous operating temperature as a result of flow maldistribution. The model is composed of three steps individually addressing the issues of 1) fluid flow, 2) heat transfer, and 3) the photovoltaic output of a BIPVT array. Fluid analysis was conducted using the finite element method in order to obtain the individual fluid channel flow rates. Using these values, a heat transfer analysis was then conducted for each module forming the BIPVT array to calculate the photovoltaic operating temperature for the constituent cells forming the array. During this step the finite difference method was utilised to approximate the fin efficiency of the building integrated collector, taking into account its irregular geometry. Finally the photovoltaic yield was calculated using a numerical approach which considered the individual operating temperature of the PV cells. During this step a new method was identified to determine the values of series and shunt resistances and also the diode constant required for the modelling of photovoltaic devices based on the multi-dimensional Newton-Raphson method and current-voltage equations expressed using the Lambert W-function. Experimentation was carried out to validate the new modelling methods.
These models were combined to quantify the detrimental impact of flow mal-distribution on photovoltaic yield for a number of scenarios. In the case where flow uniformity was poorest, only a 2% improvement in photovoltaic yield was obtained in comparison to a traditional photovoltaic panel operating under the same environmental conditions. For the case where flow uniformity was optimal however, photovoltaic output was improved by almost 10%.
This work has shown that the effects of poor flow distribution has the potential to have a substantial negative impact on the photovoltaic output of a building integrated solar collector especially given the variability in its physical geometry. The appropriate design of this technology should therefore consider the effects of this phenomenon. The methodology presented in this study can be used to approximate PV output for a BIPVT array with different array geometries and operating characteristics. Furthermore, the method to calculate solar cell modelling parameters developed in this study is not only useful for the analysis of hybrid PVT systems, but for the general analysis of photovoltaic systems based on crystalline silicon solar cells.

text: Investigation into the effects of flow distribution on the photovoltaic performance of a building integrated photovoltaic/thermal solar collector The conversion of solar energy into usable forms of energy such as electricity and heat is attractive given the abundance of solar energy and the numerous issues recently raised in the consumption of fossil fuels. Solar conversion technologies may generally be categorised as either photovoltaic or solar thermal types capable of converting incidental sunlight into electricity and heat respectively. The photovoltaic cell is able to transform incidental sunlight into electricity via the Becquerel effect, however, the single junction crystalline silicon solar cell, the predominant cell type in today’s photovoltaic market is only able to utilise a small portion (less than 20%) of incidental sunlight for this purpose. A majority of the remaining portion is absorbed much like a traditional solar thermal collector and sunk as heat by the cell, elevating its operating temperature. Given the negative effect of temperature on photovoltaic cell operation, where a linearly proportional drop in conversion efficiency with elevated temperature can be expected, photovoltaic conversion can be reduced significantly particularly in areas of high irradiance and ambient temperatures. Based on the intrinsic absorption characteristics of the photovoltaic cell, a third type of solar panel referred to as the hybrid photovoltaic thermal collector (PVT) collector has been developed where fluid channels running along the underside of the photovoltaic panel transfer heat away from the cells to minimise this detrimental effect. Furthermore, heat captured from the cells may then be used for space heating or domestic hot water improving the overall collector efficiency.
In this study a unique building integrated PVT (BIPVT) collector is investigated consisting of an aluminium extrusion with structural ribs, fluid channels, and solar conversion materials. In order to evaluate this design, a mathematical model of the collector was developed in order to determine both thermal and electrical yield of the proposed design. The thermal analyses of the building integrated PVT collector in previous studies have generally adopted the approach applied to traditional solar thermal collectors where the distribution of coolant fluid flowing through the piping array is assumed uniform. For a conventional solar thermal collector this simplification may be reasonable under certain circumstances, however, given the temperature sensitivity of photovoltaic cells and their electrical connection scheme, this assumption may lead to significant modelling error. In order to further investigate this issue, a mathematical model has been developed to determine the photovoltaic yield of a BIPVT collector operating under non-homogeneous operating temperature as a result of flow maldistribution. The model is composed of three steps individually addressing the issues of 1) fluid flow, 2) heat transfer, and 3) the photovoltaic output of a BIPVT array. Fluid analysis was conducted using the finite element method in order to obtain the individual fluid channel flow rates. Using these values, a heat transfer analysis was then conducted for each module forming the BIPVT array to calculate the photovoltaic operating temperature for the constituent cells forming the array. During this step the finite difference method was utilised to approximate the fin efficiency of the building integrated collector, taking into account its irregular geometry. Finally the photovoltaic yield was calculated using a numerical approach which considered the individual operating temperature of the PV cells. During this step a new method was identified to determine the values of series and shunt resistances and also the diode constant required for the modelling of photovoltaic devices based on the multi-dimensional Newton-Raphson method and current-voltage equations expressed using the Lambert W-function. Experimentation was carried out to validate the new modelling methods.
These models were combined to quantify the detrimental impact of flow mal-distribution on photovoltaic yield for a number of scenarios. In the case where flow uniformity was poorest, only a 2% improvement in photovoltaic yield was obtained in comparison to a traditional photovoltaic panel operating under the same environmental conditions. For the case where flow uniformity was optimal however, photovoltaic output was improved by almost 10%.
This work has shown that the effects of poor flow distribution has the potential to have a substantial negative impact on the photovoltaic output of a building integrated solar collector especially given the variability in its physical geometry. The appropriate design of this technology should therefore consider the effects of this phenomenon. The methodology presented in this study can be used to approximate PV output for a BIPVT array with different array geometries and operating characteristics. Furthermore, the method to calculate solar cell modelling parameters developed in this study is not only useful for the analysis of hybrid PVT systems, but for the general analysis of photovoltaic systems based on crystalline silicon solar cells.

year: 2013

title: Stories of happiness, emotional goals and identity in 21ˢᵗ century teachers

authors: Hooker, Tom

subjects: Happiness

abstract: Happiness may be the key to keeping teachers in the profession. Rarely, however, does teaching tend to be associated with moments of happiness in educational research or in the popular imagination.
In fact, quite the opposite view is presented in the significant body of literature that describes the emotional travails, stress and burnout of teachers. Consequently, there are recruitment and retention crises in schools throughout the Western World. Teacher identity is also in crisis, undermined since the late-1980s by economistic reforms which view teachers as producers and achievement as currency in an educational marketplace. Running counter to the more holistic, altruistic educational philosophies held by many in the profession, the reforms have attached a vulnerability to the identities of many teachers. Teaching culture has been described as one of unhappiness.
This study aims to shine a light on the happy moments which exist in teaching life. In doing so, it wonders if others were to do the same, whether an embattled but noble profession could find new life and allow the people within it greater agency in their identities. With little in the way of preceding research, the thesis is exploratory. Sitting in the interpretive paradigm, it utilises a narrative methodology to capture stories of happiness from teaching life. Interviews were employed to elicit the stories from eight participants, with the aim of providing thick, socially-situated description of happy episodes in the classroom. Supporting, scene-setting data comes from two focus groups while participants also completed a diary to document their daily emotion.
The study shows that teachers mainly experience eudemonic happiness, a type of positive emotion associated with self-actualisation after meeting long-held, personally important goals. It finds that a necessary precursor to these experiences is for participants to emphasise these long-held, altruistic goals in the narrative construction of their personal-professional identities, their teacherly selves. Furthermore, positive aspects of the teacherly self emerge from happy experiences in teaching because of self-actualisation occurring in the experience of eudemonia. Additionally, many of the teacher participants found particular happiness in the positive narrative arc of an underdog. Positive professional identity is important in teaching because it is associated with quality teaching practice.
Therefore, there are implications for teachers who want to improve their happiness and also their practice. There are also implications for educational leaders and teacher educators to make space for happiness and the positive identity formation of teachers. This is because students are likely to benefit from teachers’ happy episodes if such episodes improve practice, although, further research into the connection between teacher happiness and student outcomes is suggested. That teachers may be particularly enlivened by an underdog story is good for students who require the greatest support. It is recommended that researchers address the connection between eudemonia and positive identity formation. In particular, it is important to determine if similar phenomena can be located across a range of settings and cultures.

text: Stories of happiness, emotional goals and identity in 21ˢᵗ century teachers Happiness may be the key to keeping teachers in the profession. Rarely, however, does teaching tend to be associated with moments of happiness in educational research or in the popular imagination.
In fact, quite the opposite view is presented in the significant body of literature that describes the emotional travails, stress and burnout of teachers. Consequently, there are recruitment and retention crises in schools throughout the Western World. Teacher identity is also in crisis, undermined since the late-1980s by economistic reforms which view teachers as producers and achievement as currency in an educational marketplace. Running counter to the more holistic, altruistic educational philosophies held by many in the profession, the reforms have attached a vulnerability to the identities of many teachers. Teaching culture has been described as one of unhappiness.
This study aims to shine a light on the happy moments which exist in teaching life. In doing so, it wonders if others were to do the same, whether an embattled but noble profession could find new life and allow the people within it greater agency in their identities. With little in the way of preceding research, the thesis is exploratory. Sitting in the interpretive paradigm, it utilises a narrative methodology to capture stories of happiness from teaching life. Interviews were employed to elicit the stories from eight participants, with the aim of providing thick, socially-situated description of happy episodes in the classroom. Supporting, scene-setting data comes from two focus groups while participants also completed a diary to document their daily emotion.
The study shows that teachers mainly experience eudemonic happiness, a type of positive emotion associated with self-actualisation after meeting long-held, personally important goals. It finds that a necessary precursor to these experiences is for participants to emphasise these long-held, altruistic goals in the narrative construction of their personal-professional identities, their teacherly selves. Furthermore, positive aspects of the teacherly self emerge from happy experiences in teaching because of self-actualisation occurring in the experience of eudemonia. Additionally, many of the teacher participants found particular happiness in the positive narrative arc of an underdog. Positive professional identity is important in teaching because it is associated with quality teaching practice.
Therefore, there are implications for teachers who want to improve their happiness and also their practice. There are also implications for educational leaders and teacher educators to make space for happiness and the positive identity formation of teachers. This is because students are likely to benefit from teachers’ happy episodes if such episodes improve practice, although, further research into the connection between teacher happiness and student outcomes is suggested. That teachers may be particularly enlivened by an underdog story is good for students who require the greatest support. It is recommended that researchers address the connection between eudemonia and positive identity formation. In particular, it is important to determine if similar phenomena can be located across a range of settings and cultures.

year: 2020

title: Outcomes in different ethnic groups of New Zealand patients with screen-detected vs. non-screen-detected breast cancer.

authors: Lawrenson, Ross

subjects: Breast screening

abstract: OBJECTIVE: To compare characteristics and survival of New Zealand European, Māori, and Pacific women with screen-detected vs. non-screen-detected breast cancer. 
METHODS: Women aged 45-69 diagnosed with invasive breast cancer between January 2005 and May 2013 were identified from the Waikato and Auckland Breast Cancer Registries. Patient demographics and tumour characteristics were described by detection mode and ethnicity. Kaplan-Meier method was used to estimate the five-year breast cancer-specific survival of women with stage I-III breast cancer by ethnicity and detection mode. 
RESULTS: Women with screen-detected cancers were older, had smaller tumours, fewer stage IV (0.8% vs. 7.6%), fewer high grade (16.8% vs. 39.0%), and fewer lymph node positive diseases (26.3% vs. 51.5%) than women with non-screen-detected cancers. There were more Luminal A (70.0% vs. 54.0%), fewer human epidermal growth factor receptor 2 positive non-Luminal (4.4% vs. 8.8%), and fewer triple negative cases (7.0% vs. 13.8%) in screen-detected than non-screen-detected cancers. If not screen detected, 22.7% of breast cancers in Pacific women were stage IV compared with 2.4% if screen detected. If not screen detected, the five-year breast cancer-specific survival was 91.1% for New Zealand European women, 84.2% for Māori women, and 80.2% for Pacific women (p-value <0.001). For screen-detected breast cancer, survival between different ethnic groups was similar.
CONCLUSIONS: Breast cancers detected through screening are diagnosed at an earlier stage and have a greater proportion of subtypes, with better outcome. Variations in survival for Māori and Pacific women are only found in women with non-screen-detected breast cancer.

text: Outcomes in different ethnic groups of New Zealand patients with screen-detected vs. non-screen-detected breast cancer. OBJECTIVE: To compare characteristics and survival of New Zealand European, Māori, and Pacific women with screen-detected vs. non-screen-detected breast cancer. 
METHODS: Women aged 45-69 diagnosed with invasive breast cancer between January 2005 and May 2013 were identified from the Waikato and Auckland Breast Cancer Registries. Patient demographics and tumour characteristics were described by detection mode and ethnicity. Kaplan-Meier method was used to estimate the five-year breast cancer-specific survival of women with stage I-III breast cancer by ethnicity and detection mode. 
RESULTS: Women with screen-detected cancers were older, had smaller tumours, fewer stage IV (0.8% vs. 7.6%), fewer high grade (16.8% vs. 39.0%), and fewer lymph node positive diseases (26.3% vs. 51.5%) than women with non-screen-detected cancers. There were more Luminal A (70.0% vs. 54.0%), fewer human epidermal growth factor receptor 2 positive non-Luminal (4.4% vs. 8.8%), and fewer triple negative cases (7.0% vs. 13.8%) in screen-detected than non-screen-detected cancers. If not screen detected, 22.7% of breast cancers in Pacific women were stage IV compared with 2.4% if screen detected. If not screen detected, the five-year breast cancer-specific survival was 91.1% for New Zealand European women, 84.2% for Māori women, and 80.2% for Pacific women (p-value <0.001). For screen-detected breast cancer, survival between different ethnic groups was similar.
CONCLUSIONS: Breast cancers detected through screening are diagnosed at an earlier stage and have a greater proportion of subtypes, with better outcome. Variations in survival for Māori and Pacific women are only found in women with non-screen-detected breast cancer.

year: 2019