Martin Bliss

Analysis of Commercial Trench Power MOSFETs' Responses to

This paper presents a detailed analysis of commercial trench power MOSFET responses to Co60 irradiation of all key d.c. electrical test parameters. Charge trapping in the gate oxide causes the threshold voltage (V TH) to shift, which, in turn, accounts for most of the degradation exhibited by the device after irradiation.

{\rm Co}^{60}

Advances in photovoltaic technology resulted in increased complexity of device calibration, largely being affected by deviations of test spectrum from natural spectra. While the output spectrum of some solar simulators is adjustable, generally only light intensity and module temperature can be varied. This is due to the light sources used in current simulators. LEDs offer an additional degree of freedom, when using an appropriate combination of wavelengths. This paper presents the advantages of this lighting technology for solar simulation and backs these up through results of the prototype unit developed at the Centre for Renewable Energy Systems Technology. The ability to keep LEDs stable for a long time and dim them with minimal changes in the spectrum allows generation of a spectrum closely matched to AM1.5G standard test spectrum or indeed even realistic variations of the outdoor spectrum. LEDs can be controlled very fast within microseconds or operated continuously, combining a steady state and a flash solar simulator with additional functions such as variable flash frequencies and flash shape. Combined with the life expectancy exceeding 50.000h, LEDs are a strong candidate for solar simulator light sources introducing a significant improvement in calibration lifetime as well as significantly reduced running cost. The usage of LEDs can enhance todays characteristic measurement functions and even opens possibilities to fully characterise solar cells indoors within a much shorter time than is possible today, over a range of conditions previously only available through outdoor characterisation.

Irradiation

This paper presents photovoltaic-oriented nodal analysis (PVONA), a general and flexible tool for efficient spatially resolved simulations for photovoltaic (PV) cells and modules. This approach overcomes the major problem of the conventional Simulation Program with Integrated Circuit Emphasis-based approaches for solving circuit network models, which is the limited number of nodes that can be simulated due to memory and computing time requirements. PVONA integrates a specifically designed sparse data structure and a graphics processing unit-based parallel conjugate gradient algorithm into a PV-oriented iterative Newton-Raphson solver. This first avoids the complicated and time-consuming netlist parsing, second saves memory space, and third accelerates the simulation procedure. In the tests, PVONA generated the local current and voltage maps of a model with 316 × 316 nodes with a thin-film PV cell in 15 s, i.e., using only 4.6% of the time required by the latest LTSpice package. The 2-D characterization is used as a case study and the potential application of PVONA toward quantitative analysis of electroluminescence are discussed.

ADVANTAGES IN USING LEDS AS THE MAIN LIGHT SOURCE IN SOLAR SIMULATORS FOR MEASURING PV DEVICE CHARACTERISTICS

The first three-dimensional performance matrix for use in photovoltaic (PV) energy rating is reported utilizing a novel energy rating solar simulator based on LEDs. Device characteristics are measured indoors at varying irradiance (G), temperature (T) and spectrum (E). This opens the possibility for a more accurate measurement system for energy yield prediction of PV devices, especially for devices with high spectral dependence such as wide bandgap solar cells as they take into account spectral changes in the light. The main aspects of the LED-based solar simulator used are briefly described. A measurement method is developed and detailed in the paper, which takes into account the current imperfections in the achievable spectrum. Measurement results for a crystalline silicon solar cell are used to demonstrate the measurement approach. An uncertainty analysis of the measurement system is given, resulting in an overall absolute uncertainty of 4.3% (coverage factor k = 2) in maximum power measurements at 765 W m−2 irradiance with scope for further improvements.

Accelerated Spatially Resolved Electrical Simulation of Photovoltaic Devices Using Photovoltaic-Oriented Nodal Analysis

This paper presents an initial analysis of a large scale PV system monitoring campaign. Ongoing project aims to be a detailed inter-comparison of different modules technologies installed in a different types of climates, identifying optimal configurations in different climatic zones. Detailed description of created plants and developed monitoring facilities was shown. Appropriate performance indicators are discussed and applied to the measurements of two sites. The resulting performance analysis of different modules technologies, is presented and discussed. Differences between crystalline and thin film technologies were marked, with thin film technologies not performing as well as expected. This is most likely due to installation issues and will be rectified in the near future‥ Further investigations will be undertaken and reported in close future.

Indoor measurement of photovoltaic device characteristics at varying irradiance, temperature and spectrum for energy rating

Light Beam Induced Current (LBIC) measurements are a useful tool in photovoltaic (PV) device characterisation for accessing the local electrical properties of PV devices. The main disadvantage of a typical LBIC system is measurement time, as a raster scan of a typical silicon solar cell can last several hours. The focus of this paper is the reduction of LBIC measurement time by means of compressed sensing (CS). The CS-LBIC system described in this paper can theoretically reduce measurement time to less than 25% of that required for a standard LBIC raster scan. Measurement simulations of a CS-LBIC system are presented as well as a practical demonstration using a digital micro-mirror array, which further reduces the measurement time by an order of magnitude. Instead of a raster scan, the PV device under measurement is sampled by a series of patterns and the current map is reconstructed using an optimization algorithm. Simulations of CS-LBIC measurements using the 2D spatially-resolved PV-Oriented Nodal Analysis (PVONA) model developed at CREST are used as a tool to explore the capabilities and verify the accuracy of this measurement technique as well as its ability to detect specific defects, such as cracks and shunts. Simulation results confirm that the CS sampling theory can be applied as an effective method for significantly reducing measurement time of current mapping of PV devices. An initial CS-LBIC system prototype has been built at the National Physical Laboratory (NPL) and measurements of small area devices (1cm x 0.8cm) using this system are given. The current maps are created using a Digital Micromirror Device (DMD) kit as a pattern generator. The response time of the micro mirror array is less than 20μs. This is another factor in the reduction of measurement time, as the movement time of an x-y translation stage is considerably slower. Initial measurement results show that current maps of PV cells can be acquired with 75% fewer measurements which, combined with the fast response of the pattern generator, can reduce LBIC measurement time by an order of magnitude.

Large scale PV system monitoring - modules technology intercomparison

The accuracy of the characterisation of photovoltaic devices may be affected by the method of extraction of the performance parameters. This paper investigates potential uncertainties in extracted short circuit current (Isc), open circuit voltage (Voc) and maximum power (Pmpp) from current-voltage (I-V) data of a photovoltaic (PV) device. Different interpolation methods are compared. Linear interpolation, straight line regression and polynomial regression methods are compared in different scenarios in which the curve point density and point selection for extraction are varied. The comparison shows that regression approaches can produce more accurate results when appropriate curve point density and point selection are selected. When attempting to extract Isc. Voc and Pmpp from noisy or irregular I-V data, linear interpolation is less robust than regression because of the irregular point distribution on the I-V curve.

Fast Current Mapping of Photovoltaic Devices Using Compressive Sampling

This paper demonstrates photovoltaic (PV) device performance measurements for energy rating and energy yield calculation derived indoors with an LED-based solar simulator prototype under varying irradiance (G), temperature (T) and spectrum (E), opening the possibility for much faster and more accurate energy yield prediction than previously possible from measurements acquired either indoors or outdoors, with the additional inclusion of spectral influences. The main aspects of the LED-based solar simulator used are described briefly and the measurement method with its requirements is explained in detail. Also presented are the first performance measurements made with an amorphous silicon solar cell; measuring the spectral effects reported in outdoor measurements for the first time in the laboratory. Results show a good agreement with previously reported spectral effects from outdoor measurements and underline the importance to consider all three environmental vectors (irradiance, spectrum and device temperature) for energy yield focused measurements.

Analysis of key performance parameter extraction from current voltage measurements of photovoltaic devices

A new photovoltaic (PV) device current mapping method has been developed, combining the recently introduced compressed sensing (CS) sampling theory with light beam induced current (LBIC) measurements. Instead of a raster scan, compressive sampling is applied using a digital micromirror device. The aim is to significantly reduce the time required to produce a current map, compared to conventional LBIC measurements. This is achieved by acquiring fewer measurements than a full raster scan and by utilizing the fast response of the micromirror device to modulate measurement conditions. The method has been implemented on an optical current mapping setup built at the National Physical Laboratory, U.K. Measurements with two different PV cells are presented in this paper and an analytical description for realization of an optimized CS current mapping system is provided. The experimental results illustrate the feasibility of the method and its potential to significantly reduce measurement time of current mapping of PV devices.

Performance measurements at varying irradiance spectrum, intensity and module temperature of amorphous silicon solar cells

The Compressed Sensing (CS) sampling theory has been combined with the Light Beam Induced Current (LBIC) method, to produce an alternative current mapping technique for photovoltaic (PV) devices. Compressive sampling of photocurrent is experimentally implemented using a Digital Micro-mirror Device (DMD). The main advantage of this new method for current mapping is that measurement time can be significantly reduced compared to conventional LBIC measurement systems. This is achieved mainly by acquiring fewer measurements than a raster scan would need and by utilizing the fast response of the micro-mirror array. Two different experimental layouts are considered in this work. The first is a small area optical set-up based on a single wavelength laser source. The second layout utilizes a commercial Digital Light Processing (DlP) projector through which compressive sampling is applied. Experimental results with both experimental schemes demonstrate that current maps can be produced with less than 50% of the measurements a standard LBIC system would need. The ability to acquire current maps of individual cells in encapsulated modules is also highlighted. The advantages and drawbacks of the method are presented and its potential to significantly reduce measurement time of current mapping of PV cells and modules is indicated.