Hassan Abbas Khan

Modeling and analysis of the spectral response for AlGaAs/GaAs HPTs for short wavelength optical communication

Detailed spectral response (SR) modeling for heterojunction bipolar phototransistors (HPTs) is presented in this work. All the related physical parameters are taken into account for the resolution of photogenerated excess minority carrier continuity equations in the active layers of the HPT. The layer dependence of the optical flux absorption profile at near-bandgap wavelengths is also investigated and its generalization as a single-exponential has been refuted for HPTs based on GaAs material systems (InGaP-GaAs/AlGaAs-GaAs). The variation in the responsivity of the device with changing base width is analyzed at various wavelengths and a detailed experimental setup for optical characterization of HPTs is also provided. The measured results at 635, 780, 808, and 850 nm show good agreement to the modeled data, validating the newly developed theoretical model.

Photoresponse Modeling and Analysis of InGaP/GaAs Double-HPTs

Photoresponse of GaAs-based double-heterojunction phototransistors (DHPTs), in surface-illuminated orientation, has been analyzed with a modified small-signal model. The effect of incident optical illumination on various intrinsic parameters has been discussed for In0.49Ga0.51P/GaAs N+p+N- DHPT. Since the primary detecting material is GaAs, the device is optimized to detect short wavelength at 850 nm. A novel formulation for optical flux absorption in DHPTs is also provided along with its comparison with single-heterojunction phototransistors. The analysis of DHPTs presented in this paper can be utilized for performance enhancement through device optimization in sensors, photoreceivers in optical networks, and remote sensing applications employing integrated circuits.

Low irradiance loss quantification in c-Si panels for photovoltaic systems

Performance of bulk Si based solar photovoltaic (PV) panels deteriorate in weak light conditions. This generally affects the efficiency of associated power electronic components and compounds the overall loss in the yield of a PV system. It is, therefore, imperative to carefully plan the PV sizing and quantify this inherent loss in the efficiency of PV panels. In this work, the impact of varying solar irradiation on the efficiency of crystalline Si based solar panels has been quantified through a novel mathematical formulation, which models the deterioration in performance based on actual insolation and temperature profiles. This loss phenomenon is attributed mainly to a reduction in open circuit voltage and fill factor of a panel which in turn lowers the conversion efficiency. An empirical relationship is also derived which relates the loss in energy harvested to the actual peak-sunlight hours of a region. Results indicate that this low irradiance loss has approximately a linear dependence on equivalent peak sunlight hours (EPSH). For instance, the loss of 3.8%, 4%, 4.5% for Austin, Washington and Seattle was observed for EPSH of 4.4, 4, and 3.4, respectively. Moreover, the loss variation from one year to another for a region has been approximately constant. Finally, a comparison was performed for predicted energy harvested to the measured yield and a good fit has been observed between predicted and actual values.

Low cost, robust and efficient implementation of MPPT based buck-boost converter for off-grid PV applications

Rapidly growing solar PV energy sector demands development of low cost, reliable and energy efficient systems to continue its growth globally. Most PV systems deployed in the developed countries are based on grid-tied topology as the grid in these countries is largely stable. However, most of the developing world has an intermittent grid or a bulk of the population is not connected to the grid in the first place. In this context, this paper proposes a design and implementation of a low cost, efficient and robust maximum power point tracking based charge controller suited for stand-alone or backup PV applications. The charge controller incorporates a buck-boost topology that ensures a continuous battery charging in various conditions. The implementation includes a hardware protection against breakdown of converter switches in addition to a software based protection approach derived from converter analysis. The software algorithm, all programmed into a low cost 8 bit microcontroller, also incorporates a three state charging capability to charge battery under optimal conditions. The designed system was tested at 150W (output power) and performed optimally.

Detailed Analysis on the Spectral Response of InP/InGaAs HPTs for Optoelectronic Applications

We analyze an analytical spectral-response model for heterojunction phototransistors (HPTs) in order to understand the behavior of lattice-matched InPZIn0.47Ga0.53As HPTs with changing device and material parameters. The preliminary modeling of the spectral response lead to a good agreement between theoretical and experimental results for incident wavelength radiations at 980, 1310, and 1550 nm. We then performed several simulations in order to determine the individual influences of several parameters, such as the base-layer thickness and the surface-recombination velocity on the responsivity of the device. A decreasing trend for the surface-recombination parameter with increasing wavelengths was observed, and it is attributed to the greater recombination rate for high-energy photons generated near the surface.

Analytical Modeling of the Spectral Response of Heterojunction Phototransistors

Spectral response model for heterojunction phototransistors (HPTs) is developed from resolution of continuity equations that govern the excess optically generated minority-carrier variation in the active layers of the HPT taking into account the related physical parameters. Realistic boundary conditions have been considered for efficient device operation, and a detailed optical-power absorption profile is constructed for accurate device modeling. The analysis is performed for GaAs-based HPTs, and the measured results at 635, 780, and 850 nm show a good agreement with theoretical calculations.

Solar PV-Based Scalable DC Microgrid for Rural Electrification in Developing Regions

In this paper, we detail the design, analysis, and implementation of a highly distributed off-grid solar photovoltaic dc microgrid architecture suitable for rural electrification in developing countries. The proposed architecture is superior in comparison with existing architectures for rural electrification because of its 1) generation and storage scalability, 2) higher distribution efficiency (because of distributed generation and distributed storage for lower line losses), 3) ability to provide power for larger communal loads without the requirement for large, dedicated generation by extracting the benefit of usage diversity, and 4) localized control by using the hysteresis-based voltage droop method, thus eliminating the need for a central controller. The proposed microgrid architecture consists of several nanogrids capable of the self-sustained generation, storage, and bidirectional flow of power within the microgrid. Bidirectional power flow and distributed voltage droop control are implemented through the duty cycle control of a modified flyback converter. A detailed analysis in terms of power flow, loss, and system efficiency was conducted by using the Newton–Raphson method modified for dc power flow at varying distribution voltages, conductor sizes, and schemes of interconnection among the contributing nanogrids. A scaled-down version of the proposed architecture with various power sharing scenarios was also implemented on hardware, and yielded satisfactory results.

A practical perspective on Grid-tied PV systems in low reliability grids

The percentage of solar PV in the global energy mix has considerably increased in the past decade. Grid-tied systems, generally, have a better return-on-investment compared to standalone/backup off-grid systems. However, the implementation of grid-tied systems largely depends on the grid and the state of captive generation. Unfortunately, the developing countries mostly have intermittent grids. In places where grids are supplemented by captive generation, these systems can be deployed. We have analyzed one such system in Pakistan with overall 42.24 kWp capacity modularly implemented with three independent inverters. We analyze the effect of partial shading losses along with several other unconventional effects which are particular only to weak grids. The practical perspective from this installation can be highly useful for future installations within Pakistan and other developing countries to reduce their dependence on national grid.

Smart DC microgrids: Modeling and power flow analysis of a DC Microgrid for off-grid and weak-grid connected communities

This paper presents a modular, scalable and viable architecture for a DC Microgrid to supply electrical power to off-grid communities, particularly in developing countries, where grid electricity is unavailable or is largely intermittent. We have modeled a system for a 40 house village where each house generates (through PV), stores and can share electrical energy with the community for an optimized system. The concept is then extended to an islanded DC Microgrid model with scalable renewable generation and smart power regulation. This paper also includes the operational analysis of the proposed microgrid architecture using Newton Raphson method modified for DC grid analysis. This analysis is used to optimize the voltage level for regulation, efficiency and safety. The results are then verified through an experimental setup in the laboratory. Lastly, this paper also proposes a protection scheme for the DC microgrid and the contributing households.

Analysis on central and distributed architectures of solar powered DC microgrids

In this work, central and distributed architectures of DC microgrids for rural electrification are analyzed under various operating conditions. In the proposed scheme, a single household consumer forms the atomic nanogrid unit which may integrate its resources in a scalable model with the community to form a microgrid, without dependence of the national grid. The flow of power between houses and the microgrid is implemented through a bidirectional flyback converter. The operation of proposed scheme for two different architectures, i.e. distributed generation distributed storage architecture (DGDSA) and centralized generation centralized storage architecture (CGCSA) is evaluated at various distribution voltage levels and conductor sizes. Modified Newton Raphson Method based analysis is performed for both architectures which show that distributed architecture has significant advantages over central architecture due to higher efficiency, low voltage drop and lower line losses. Further, the scalable nature with minimum installation cost for distributed architecture makes it more favorable for rural electrification applications in comparison to central architecture. The simulated results are also verified using a scaled down version of hardware implementation.