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(1995)Journal ArticleThe `passivated emitter and rear locally diffused` (PERL) silicon solar cell structure presently demonstrates teh highes terrestrial performance of any silicon-based solar cell. This paper presents a detailed investigation of the limiting loss mechanisms in PERL cells exhibiting independently confirmend 1-sun efficiencies of up to 23.0%. Optical, resistice, and recombinative losses are all analyzed under the full range of solar cell operating conditions with the aid of two-dimensional (2D) device simulations. The analysis is based on measurements of the reflectance, quaantum efficiency, dark and illuminated current-voltage (I-V) characteristics, and properties of the Si-SiO2 interfaces employed on these cells for surface passivation. Through the use of the 2D simulations, particular attention has been paid to the magnitudes of the spatially resolved recombination losses in these cells. Itis shown that approximately 50% of the recombination losses at the 1-sun maximum power point occur in the base of th cells, followed by the recombination losses at the rear and front oxidised surfaces (25% and <25%, respectively). The relativerly low fill factors of PERL cells are princip[ally a result of resistive losses; however, the recombination behavior in the base and at the rear surfacealso contributes. This work predicts that the efficiency of 23% PERL cells could be increased by about 0.7% absolute if ohmic losses were eliminated, a further 1.1% absolute if there were no reflection losses at the nonmetallised front surface regions, about 2.0% by introducing ideal light trapping and eliminating shading losses due to the front metallisation, and by about 3.7% absolute if the device had no defect-related reconbination losses. New design rules for future efficiency improvements, ev
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(1995)Journal Article
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(1995)ThesisThe aim of this study is to investigate the behaviour of fragmenting coals during devolatilization in conditions relevant to fluidized bed combustion. To achieve this objective, a mathematical model has been developed for devolatilization of a single fragmenting coal particle. This model incorporates heat transfer to and within the coal particle, devolatilization reactions, mass transfer of volatiles, pressure build up inside the coal particle, and devolatilization-induced fragmentation. Prior to the development of the model for a fragmenting coal particle, two simpler models for coal devolatilization have also been developed, namely the simple model for a fragmenting coal particle and the model for a nonfragmenting coal particle. The first model was used for a factorial evaluation and the second model was used to select parameters for the model for a fragmenting coal particle by comparing with experimental data on nonfragmenting coals. It has been found that factorial experimental design can be used to select significant factors affecting devolatilization of fragmenting coals. In order of significance, factors affecting the devolatilization time are coal particle diameter, internal heat transfer, bed temperature and chemical kinetics; while factors affecting the number of fragmentations are diameter of coal particle, diameter of coal pores and volatile viscosity. This agrees with the results of a parametric study using the model for a fragmenting coal particle. The model for a fragmenting coal particle has fitted well the experimental data on devolatilization time and the timing of fragmentation. A small effect of fragmentation on devolatilization time, in agreement with experimental observations, was explained by fragmentation which occurs at the later stage of devolatilization. Model predictions suggest that the probability of fragmentation is influenced by the diameter of coal pores. Due to limited data, this study recommends further experimental investigation on fragmentation probability and the distribution of coal pore sizes.
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