Investigation on mismatch loss caused by shading in building-integrated photovoltaic systems

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Investigation on mismatch loss caused by shading in building-integrated photovoltaic systems


Author: Cheng, Wai-ching
Title: Investigation on mismatch loss caused by shading in building-integrated photovoltaic systems
Degree: M.Eng.
Year: 2007
Subject: Hong Kong Polytechnic University -- Dissertations.
Building-integrated photovoltaic systems -- China -- Hong Kong.
Photovoltaic power systems -- China -- Hong Kong.
Department: Dept. of Building Services Engineering
Pages: xvi, 148 leaves : col. ill. ; 30 cm.
Language: English
InnoPac Record:
Abstract: Building-integrated photovoltaic (BIPV) systems in Hong Kong are often subjected to shading cast by surrounding buildings and obstructs. The non-uniform solar irradiation distribution on BIPV systems results in inconsistent performance of PV modules which is known as mismatch effect. This dissertation presents the findings of an investigation into the possible mismatch effect caused by shading in BIPV systems in Hong Kong. It aims at responding to the need for knowledge and information about the effect and its impacts on local BIPV systems. The monitored data collected from a BIPV system were applied to examine the mismatch effect and experimental data were gathered to explore electrical characteristics of mismatch loss. Simple series-parallel (SP), bridge-linked (BL) and total-cross-tied (TCT) arrays were tested to obtain the configuration comparatively less susceptible to the mismatch effects. The results indicate that the mismatch loss of a PV module could reach a significant high value in laboratory tests. The serial mismatch effect on a single PV module and the tested miniature system was found to be dominant whereas the parallel mismatch loss only accounted for a small ratio of the loss. It appears that the PV module may be damaged by overheating when small portion of area is shaded provided that no are bypass diodes installed. Among the three configurations investigated, the evidence suggested that the TCT array was found to be the best configuration to diminish the mismatch effect. It was found that the BIPV system on the roof of The Shaw Amenities Building performed well during 11:00 to 16:00. However, the system was suffered from shading produced by a neighbouring building in the morning and self shading problem in the afternoon. The morning shading problem dominated the system power production. The whole shadowed system resulted in small mismatch effect as the solar irradiation levels were uniform in most of the times in the morning. On the contrary, mismatch effect was more dominant in the afternoon self shading effect which PV modules arrays cast shadow on other PV arrays. The mismatch power decreased by self shadowing could go up to 10% which is twice of the value specified in PV system guides. This further shown that guides tend to assume that mismatch loss only caused by small portion of PV module being shaded. Generally, the results suggest that shadow may lead to great decline of power generation in dense cities like Hong Kong. However, such loss can be minimised by placing emphasis on configuration and orientation during design stage. These designs measures are more economical than restructuring after installation. Mismatch effect throughout a year, mismatch due to uneven temperature distribution and exploration of reduction methods on mismatch are recommended for future study.

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