Temperature of solar cells with regard to photoactive and non-photoactive light absorption in concentrator PV modules

A new method has recently been proposed by us for accurate measurement of the solar cell temperature in any operational regime, in particular, at a maximum power point (MPP) of the I-V curve (T-p-n(MPP)). For this, fast switching of a cell from MPP to open circuit (OC) regime is carried out and open circuit voltage V-oc is measured immediately (within about 1 millisecond), so that this value becomes to be an indicator of T-p-n(MPP). In the present work, we have considered a practical case, when a solar cell is heated not only by absorption of light incident upon its surface (called "photoactive" absorption of power), but also by heat transferred from structural elements surrounding the cell and heated by absorption of direct or diffused sunlight ("non-photoactive" absorption of power with respect to a solar cell). This process takes place in any concentrator module with non-ideal concentrators. Low overheating temperature of the p-n junction (or p-n junctions in a multijunction cell) is a cumulative parameter characterizing the quality of a solar module by the factor of heat removal effectiveness and, at the same time, by the factor of low "non-photoactive" losses.

Aplicación de la energía solar térmica en una incubadora comercial de perdiz roja y supervisión de la actividad biológica mediante sensores inteligentes

La perdiz roja es la especie cinegética por excelencia en la península ibérica, cuya cría en cautividad y suelta controlada comenzó a regularse en los años 70 con la aparición del ICONA. La incubación controlada de huevos de perdiz es imprescindible, con fines cinegéticos y de preservación de la especie, y se desarrolla con incubadoras comerciales de pequeña y mediana escala, distribuidas en zonas rurales con acceso limitado y/o deficiente al suministro eléctrico. En nuestras latitudes el aporte de energía solar térmica se perfila como una posibilidad de mejorar la eficiencia energética de éstas y otras instalaciones y de reducir la dependencia energética exterior. Hay diversos factores físico-químicos que influyen en la calidad de la incubación: temperatura, humedad relativa, y concentración de gases, de los cuales sólo los dos primeros son habitualmente supervisados y controlados en este tipo de incubadoras. Esta Tesis surge en el marco de dos proyectos de cooperación con la AECID, y tiene como objetivos: la caracterización espacial de variables relevantes (temperatura (T), humedad relativa (HR)) en la incubadora comercial durante el proceso de incubación, la determinación de la relación existente entre la evolución de variables ambientales durante el proceso de incubación y la tasa de nacimientos (35-77%), así como el diseño y evaluación del sistema de apoyo solar térmico para determinar su potencial de utilización durante las incubaciones comerciales. La instalación de un número limitado de sensores permite la monitorización precisa del proceso de incubación de los huevos. Los resultados más relevantes indican que en incubaciones comerciales los gradientes de T y HR han sido despreciables (1ºC de diferencia entre las posiciones con mayor y menor T media y un 4,5% de diferencia entre las posiciones con mayor y menor HR), mientras que el seguimiento y ajuste (mediante modelos de crecimiento) de la concentración de CO2 (r2 entre 0,948 y 0,987 en las 5 incubaciones, para un total de 43315 huevos) permite valorar la actividad fisiológica de los huevos e incluso predecir la tasa de éxito (nacimientos), basándose en la concentración de CO2 estimada mediante modelos de crecimiento en el día 20 de incubación (r2 entre 0,997 y 0,994 según el modelo de estimación empleado). El sistema ha sido valorado muy positivamente por los productores (Finca Cinegética Dehesa Vieja de Galapagar). El aporte térmico se ha diseñado (con mínima intrusión en el sistema comercial) sobre la base de un sistema de enfriamiento de emergencia original de la incubadora, al que se han incorporado un colector solar, un depósito, un sistema de electroválvulas, una bomba de circulación y sensores de T en distintos puntos del sistema, y cuyo control ha sido automatizado. En esta Tesis se muestra que la contribución solar puede aportar hasta un 42% de las demandas de energía en nuestras condiciones geográficas para una temperatura de consigna dentro de la incubadora de 36.8ºC, sin afectar a la estabilidad de la temperatura. Además, el rendimiento del colector solar se ha acotado entre un 44% y un 85%, de acuerdo con los cálculos termodinámicos; valores que se mantienen dentro del rango aportado por el fabricante (61%). En el futuro se plantea evaluar el efecto de distintas estrategias de control, tales como controladores difusos, que incorporan el conocimiento experto al control automático. ABSTRACT The partridge is the quintessential game species in the Iberian Peninsula, which controlled breeding and release, began to be regulated in the 70s with the emergence of ICONA. The controlled incubation of eggs is essential, and takes place in commercial incubators of small and medium scale, distributed in rural areas with limited and/or inadequate access to power. In our latitudes the contribution of solar thermal energy is emerging as a possibility to improve the energy efficiency of the facilities and to reduce external energy dependence. There are various physicochemical factors influencing the quality of incubation: temperature, relative humidity and concentration of gases, of which only the first two are typically monitored and controlled in such incubators. This PhD comes within the framework of two cooperation projects with AECID and aims: the spatial characterization of relevant variables in a commercial incubator (temperature (T), and relative humidity (HR)), determining the relationships in the changes in environmental variables during incubation and birth rates (35-77%) as well as the design and evaluation of solar thermal support system to determine its potential use during commercial incubations; the installation of a limited number of sensors has allowed accurate monitoring of incubation of eggs. The most relevant results indicate that in commercial incubations, the gradients in T and HR have been negligible (1°C difference between the highest and lowest positions T and average 4.5% difference between the highest and lowest positions HR), while monitoring and fit using growth models of the concentration of CO2 (r2 between 0.948 and 0.987 in 5 incubations, for a total amount of 43,315 eggs) allows assessing the physiological activity of the eggs and even predict the success rate (hatchability), based on the estimated concentration of CO2 by using growth models on day 20 of incubation (r2 between 0.997 and 0.994 depending on the fit model).The system has been highly valued by producers (Finca Cinegética Dehesa Vieja de Galapagar). The hybrid heat system is designed (with minimal intrusion into the commercial system) based on an emergency cooling device, original in the incubator. New elements have been incorporated: a solar collector, a tank, a system of solenoid valves, a circulating pump and T sensors at various points of the system, whose control has been automated. This PhD shows that the solar contribution is responsible for up to 42% of energy demands in our geographical conditions for a setpoint temperature inside the incubator of 36.8ºC, without questioning the stability of the temperature. Furthermore, the efficiency of the solar collector has been bounded between 44% and 85%, according to thermodynamic calculations; values remain within the range provided by the manufacturer (61%). In the future it is proposed to evaluate the effect of different control strategies, such as fuzzy controllers, which incorporate the expertise to automated control.

Thermal control of SUNRISE, a balloon-borne solar telescope

SUNRISE is a balloon-borne solar telescope flown with a long-duration balloon by NASA's Columbia Scientific Balloon Facility team from Esrange (Swedish Space Corporation), on 8 June 2009. SUNRISE has been a challenging mission from the thermal point of view because of its size and power dissipation. Thus, a dedicated thermal analysis has been carried out to find a solution that allows all the devices to be kept within their appropriate temperature ranges, without exceeding the allowable temperature gradients, critical for optical devices. In this article, the thermal design of SUNRISE is described. A geometrical mathematical model and a thermal mathematical model of the whole system have been set up for the different load cases in order to obtain the temperature distribution and gradients in the system. Some trade-offs have been necessary to fulfil all the thermal requirements. The thermal hardware used to achieve it is described. Finally, the temperatures obtained with the models have been compared with flight data.

Thermal control of SUNRISE, a balloon-borne solar telescope

SUNRISE is a balloon-borne solar telescope flown with a long-duration balloon by NASA's Columbia Scientific Balloon Facility team from Esrange (Swedish Space Corporation), on 8 June 2009. SUNRISE has been a challenging mission from the thermal point of view because of its size and power dissipation. Thus, a dedicated thermal analysis has been carried out to find a solution that allows all the devices to be kept within their appropriate temperature ranges, without exceeding the allowable temperature gradients, critical for optical devices. In this article, the thermal design of SUNRISE is described. A geometrical mathematical model and a thermal mathematical model of the whole system have been set up for the different load cases in order to obtain the temperature distribution and gradients in the system. Some trade-offs have been necessary to fulfil all the thermal requirements. The thermal hardware used to achieve it is described. Finally, the temperatures obtained with the models have been compared with flight data.

Spectral study and classification of worldwide locations considering several multijunction solar cell technologies

Multi-junction solar cells are widely used in high-concentration photovoltaic systems (HCPV) attaining the highest efficiencies in photovoltaic energy generation. This technology is more dependent on the spectral variations of the impinging Direct Normal Irradiance (DNI) than conventional photovoltaics based on silicon solar cells and consequently demands a deeper knowledge of the solar resource characteristics. This article explores the capabilities of spectral indexes, namely, spectral matching ratios (SMR), to spectrally characterize the annual irradiation reaching a particular location on the Earth and to provide the necessary information for the spectral optimization of a MJ solar cell in that location as a starting point for CPV module spectral tuning. Additionally, the relationship between such indexes and the atmosphere parameters, such as the aerosol optical depth (AOD), precipitable water (PW), and air mass (AM), is discussed using radiative transfer models such as SMARTS to generate the spectrally-resolved DNI. The network of ground-based sun and sky-scanning radiometers AERONET (AErosol RObotic NETwork) is exploited to obtain the atmosphere parameters for a selected bunch of 34 sites worldwide. Finally, the SMR indexes are obtained for every location, and a comparative analysis is carried out for four architectures of triple junction solar cells, covering both lattice match and metamorphic technologies. The differences found among cell technologies are much less significant than among locations.

Optimizing phosphorus diffusion for photovoltaic applications: peak doping, inactive phosphorus,gettering, and contact formation

The phosphosilicate glass (PSG), fabricated by tube furnace diffusion using a POCl3 source, is widely used as a dopant source in the manufacturing of crystalline silicon solar cells. Although it has been a widely addressed research topic for a long time, there is still lack of a comprehensive understanding of aspects such as the growth, the chemical composition, possible phosphorus depletion, the resulting in-diffused phosphorus profiles, the gettering behavior in silicon, and finally the metal-contact formation. This paper addresses these different aspects simultaneously to further optimize process conditions for photovoltaic applications. To do so, a wide range of experimental data is used and combined with device and process simulations, leading to a more comprehensive interpretation. The results show that slight changes in the PSG process conditions can produce high-quality emitters. It is predicted that PSG processes at 860 °C for 60 min in combination with an etch-back and laser doping from PSG layer results in high-quality emitters with a peak dopant density Npeak = 8.0 × 1018 cm−3 and a junction depth dj = 0.4 μm, resulting in a sheet resistivityρsh = 380 Ω/sq and a saturation current-density J0 below 10 fA/cm2. With these properties, the POCl3 process can compete with ion implantation or doped oxide approaches.

Solar driven energy conversion applications based on 3C-SiC

There is a strong and growing worldwide research on exploring renewable energy resources. Solar energy is the most abundant, inexhaustible and clean energy source, but there are profound material challenges to capture, convert and store solar energy. In this work, we explore 3C-SiC as an attractive material towards solar-driven energy conversion applications: (i) Boron doped 3C-SiC as candidate for an intermediate band photovoltaic material, and (ii) 3C-SiC as a photoelectrode for solar-driven water splitting. Absorption spectrum of boron doped 3C-SiC shows a deep energy level at ~0.7 eV above the valence band edge. This indicates that boron doped 3C-SiC may be a good candidate as an intermediate band photovoltaic material, and that bulk like 3C-SiC can have sufficient quality to be a promising electrode for photoelectrochemical water splitting.

The segmental approximation in multijunction solar cells

The segmental approach has been considered to analyze dark and light I-V curves. The photovoltaic (PV) dependence of the open-circuit voltage (Voc), the maximum power point voltage (Vm), the efficiency (?) on the photogenerated current (Jg), or on the sunlight concentration ratio (X), are analyzed, as well as other photovoltaic characteristics of multijunction solar cells. The characteristics being analyzed are split into monoexponential (linear in the semilogarithmic scale) portions, each of which is characterized by a definite value of the ideality factor A and preexponential current J0. The monoexponentiality ensures advantages, since at many steps of the analysis, one can use the analytical dependences instead of numerical methods. In this work, an experimental procedure for obtaining the necessary parameters has been proposed, and an analysis of GaInP/GaInAs/Ge triple-junction solar cell characteristics has been carried out. It has been shown that up to the sunlight concentration ratios, at which the efficiency maximum is achieved, the results of calculation of dark and light I-V curves by the segmental method fit well with the experimental data. An important consequence of this work is the feasibility of acquiring the resistanceless dark and light I-V curves, which can be used for obtaining the I-V curves characterizing the losses in the transport part of a solar cell.