Sandra Bermejo

Accurate and Wide-Field-of-View MEMS-Based Sun Sensor for Industrial Applications

This paper describes the design, fabrication, simulation, and experimental results of an improved miniaturized two-axis sun sensor for industrial applications, created by adapting a technology used previously in satellite applications. The sensor for each axis is composed of six photodiodes integrated in a crystalline-silicon substrate and a layer of cover glass, which is used to protect the silicon and to hold the windows. The high precision is obtained by the subdivision of the field of view, which is ±60°, with a resolution of 0.1°. Each region is controlled by an independent high-precision solar sensor. The sensor can be used for sun-tracking applications in a photovoltaic system, heliostat concentration plants, and lighting applications.

Determination of spatially resolved trapping parameters in silicon with injection dependent carrier density imaging

We present spatially resolved and injection dependent excess carrier lifetime measurements on silicon. At low level injection conditions an anomalous increase often interferes in such measurements. The origin of the anomalous increase is discussed. Assuming trapping as the origin, highly resolved images of trap parameters together with low level injection recombination lifetimes with strongly reduced trapping effects have been obtained. A theoretical model for infrared lifetime imaging based on the Hornbeck and Haynes model [J. A. Hornbeck and J. R. Haynes, Phys. Rev. 97, 311 (1955)] is presented which describes the trapping effect on the measured lifetime. This model is fitted to experimental spatially resolved data to extract trapping parameters, particularly trap density and the trap-escape ratio, i.e., the ratio between escape rate of minority carriers from the trap level into the minority band (detrapping) and trap rate from the minority band into the trap level (trapping). An upper bound for the low...

Colloidal crystals by electrospraying polystyrene nanofluids

This work introduces the electrospray technique as a suitable option to fabricate large-scale colloidal nanostructures, including colloidal crystals, in just a few minutes. It is shown that by changing the deposition conditions, different metamaterials can be fabricated: from scattered monolayers of polystyrene nanospheres to self-assembled three-dimensional ordered nanolayers having colloidal crystal properties. The electrospray technique overcomes the main problems encountered by top-down fabrication approaches, largely simplifying the experimental setup. Polystyrene nanospheres, with 360-nm diameter, were typically electrosprayed using off-the-shelf nanofluids. Several parameters of the setup and deposition conditions were explored, namely the distance between electrodes, nanofluid conductivity, applied voltage, and deposition rate. Layers thicker than 20 μm and area of 1 cm2 were typically produced, showing several domains of tens of microns wide with dislocations in between, but no cracks. The applied voltage was in the range of 10 kV, and the conductivity of the colloidal solution was in the range of 3 to 4 mS. Besides the morphology of the layers, the quality was also assessed by means of optical reflectance measurements showing an 80% reflectivity peak in the vicinity of 950-nm wavelength.

Wettability increase by "corona" ionization

Experiments showing an increase in the wettability of a hydrophobic surface when using corona air ionization are shown. Photoluminiscence observations support the predictions of charge accumulation at the triple line and confirm previous experiments. In all of the experiments, the contact angle was in the saturation regime at a value smaller than that predicted by the condition of a zero value for the solid-liquid surface tension. The PDMS did not show any deterioration due to the corona exposure under the experimental conditions used. The contact angle is shown to increase with humidity.

Very low recombination phosphorus emitters for high efficiency crystalline silicon solar cells

This work studies low recombination phosphorus emitters on c-Si. The emitters are fabricated by diffusion from solid sources and then passivated by thermal oxide yielding sheet resistances between 15 and 280 Ω/sq. Emitter saturation current densities lie in the 2.5–110 fA cm−2 range, leading to implicit open-circuit voltages between 674 and 725 mV. Bulk lifetime is limited by intrinsic recombination mechanisms. Surface recombination velocities between 80 and 300 cm s−1 have been obtained, appearing among the lowest reported in this range of emitter sheet resistances.

Multichip module photovoltaic miniarrays

In this work a novel concept of photovoltaic mini array assembly is described based on a flip-chip multichip module technology. A number of arrays composed of 15 series-connected 2 mm/sup 2/ c-Si photovoltaic cells have been fabricated, achieving a packaging density of 40 cells/cm/sup 2/. Measurements of the dark I-V characteristic confirmed that the resistive losses of the approach are small and do not degrade the electrical characteristics of the array. Nearly 0.5 mW/cm/sup 2/ delivered density power, and 7 V in open circuit conditions were typically measured under normalized solar spectrum (AM1.5 100 mW/cm/sup 2/), and around 6.3 mW/cm/sup 2/ and 8.5 V using a 150 W commercial lamp placed at 5 cm distance from the panel.

Crystalline silicon solar cells beyond 20% efficiency

This paper describes a fabrication process to obtain high efficiency c-Si cells (> 20%) based on the Laser Fired Contact Passivated Emitter Rear Cell (LFC-PERC) concept. Photovoltaic efficiencies beyond 20% have been achieved using thermal SiO2 as a rear passivation layer on 2 cm × 2 cm solar cells with 0.45 Qcm Fz c-Si substrates. Efficiencies up to 22% are expected for material resistivities in the 0.4–5 Ωcm using an optimized rear contact grid.

Energy production by a PV array

Publisher Summary This chapter focuses on the subject of energy production by a photovoltaic (PV) array. Studies show that PV arrays that track the sun can collect a higher amount of energy than those installed at a fixed tilt. The use of tracking is common for concentrator arrays which, at least for appreciable concentration ratios, collect only the direct radiation. This chapter begins with introducing concept, definition and illustration of peak solar hours. The chapter then elaborates the concepts of nominal array power, and temperature dependence of array power output. The chapter also discusses fixed tilt arrays, arrays with tracking, and statistical analysis of the energy production. The relationship between the annual solar radiation captured by a tracking system and a fixed tilt panel inclined at the angle of latitude for a number of locations across the world is illustrated in the chapter. It is seen that, on a yearly basis, the energy capture by a tracking flat-plate system is increased by more than 30% over a fixed array at latitude inclination. It is also noted, however, that a tracking concentrator system will collect more energy than a flat-plate system only in locations with predominantly clear skies. Practical methods of estimating the solar energy available to concentrator systems are also discussed in the chapter.

Optocoupler driving of MEMS electrostatic switches

The dynamics of novel optocoupler direct driving of a microelectromechanical systems (MEMS) switch is described by means of PSpice simulations and experimental measurements. The output circuit of an optocoupler array is used to drive an electrostatic MEMS switch. It is shown that the optocoupler drive provides built-in dc voltage step-up conversion with no ripple, galvanic isolation and switching time and energy control in a similar way as current drive does. PSpice models describe the actuation theory including parasitic elements, which can modify the actuation characteristics. Comparison between theory and measurements shows that although there is great impact of parasitic parallel resistance and capacitance, current drive benefits are mostly preserved.

Low-cost thermal Σ-Δ air flow sensor

This paper describes the design and optimization of a hot-wire air flowmeter. A low-cost design of the packaging allows good thermal contact with the airflow, as well as good thermal isolation between the hot and cold points. It is a compact solution which allows easy PCB mounting and adaptation to standard-size small air pipes. The design has been optimized for low-cost applications. The sensor is read out by a thermal sigma-delta modulator. The dynamic range of this modulator has been extended by adding a constant power offset to its output. The fractal nature of the modulator response at low-clock frequencies is also experimentally shown.This paper describes the design and optimization of a hot-wire air flowmeter. A low-cost design of the packaging allows good thermal contact with the airflow, as well as good thermal isolation between the hot and cold points. It is a compact solution which allows easy PCB mounting and adaptation to standard-size small air pipes. The design has been optimized for low-cost applications. The sensor is read out by a thermal sigma-delta modulator. The dynamic range of this modulator has been extended by adding a constant power offset to its output. The fractal nature of the modulator response at low-clock frequencies is also experimentally shown.