Characterisation and mismatch losses of building integrated photovoltaic generation

Braid, Robert Michael (2005) Characterisation and mismatch losses of building integrated photovoltaic generation. University of Southampton, Doctoral Thesis.

Abstract

The aim of this research was to characterise the operational behaviour of a building integrated photovoltaic (BIPV) generator and its inherent losses, in particular operational mismatch losses. Losses within PV arrays are complex, with many factors influencing overall energy output. Variations in the solar cell manufacturing process and conditions of operation may result in a reduction of the maximum power output of the generator, which is termed mismatch loss. Mismatch is defined as 'the power available from a combination of cells or modules being less than the sum of power of the individual cells or modules'. Mismatch is more likely in BIPV systems due to the difficulty in aligning PV arrays into a position where the best use of available sunlight can be made.

The 7.2kW BIPV facade installed at Building 2 in the University of Southampton was commissioned in June 1998. The PV installation comprises of 96, BP275 solar modules (nominally rated individually at a peak power of 75 Watts). The array is fully integrated into the Building 2 facade and connected to the utility grid via a number of different inverter types. This thesis describes the development of a post-installation monitoring regime and evaluation of the acquired data. This work was crucial to the overall assessment of generator performance, hi addition the monitoring of site environmental conditions, loss quantification and system yield optimisation are discussed.

In this study, monitoring of I-V characteristics for a variety of module and inverter connection schemes was carried out. This novel analysis studied a PV installation operating under reduced solar radiation due to non-optimal array orientation. Strategies for minimising losses and in particular mismatch losses were developed through experimental matching of modules and connection techniques using string inverters. Compared with a single, central inverter, multiple inverter (string) systems are considered to reduce mismatch losses within the series connection of PV modules producing higher system energy yields. Significant results on operation of string inverter systems and their influence on the energy yield of the PV system are presented in this thesis. Reduction of series string and hence inverter size can be counter productive on an un-shaded BIPV array where correct string configuration and inverter choice is an essential consideration.

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