Appearance, Performance and Reliability Assessment of Coloured Building-Integrated Photovoltaics

Building-integrated photovoltaics (BIPV) combines the energy-producing characteristics of PV modules with one or more building functions. It is considered one of the major technological innovations contributing to the green transition, as it allows for material savings by directly replacing cladding materials as well as power generation co-located with consumption, reducing the need for transmission infrastructure expansion. Recent years have shown an increase in colouring technologies for BIPV on the market, which allow for improved aesthetic integration into building roofs, façades and ancillary components. Nevertheless, the BIPV market sector is only slowly gaining in size, which can be attributed both to economic, regulatory and societal barriers as well as knowledge gaps concerning how the integration of PV modules in buildings as well as the introduction of colouring components affect their performance and reliability. This PhD project addresses aspects of appearance, (electrical) performance and reliability of coloured and uncoloured BIPV products. It outlines how structured glass surfaces can be used to minimize glare from PV modules while simultaneously avoiding increased transmission losses; it highlights characteristics of coloured BIPV which are essential for performance modelling, such as spectral mismatch gains or losses and parasitic heating in coloured layers; and finally discusses potential reliability issues in structural colour interlayers and glass-free PV modules. A major advantage of BIPV compared to building-applied photovoltaics (BAPV) is a facilitated architectural integration, improving the aesthetics of buildings. Chapter 2 presents a multidimensional evaluation method developed within IEA PVPS Task 15, which includes a semi-quantitative appearance assessment, which can be used for cross-sectional comparison of BIPV projects. One appearance-related aspect of PV installations concerns glare, which is especially relevant for BIPV installations due to their location in urban environments, where glare hazards potentially affect many people. While a comprehensive glare assessment is difficult to realize, a simplified method for evaluation the glare risk of differently structured glass surfaces is presented. Through it, it is shown that antireflective coatings alone are insufficient to eliminate glare risk from PV modules, while structured and especially satinated glass can reduce reflections below discomfort glare levels for all but high incidence angles. Chapter 3 addresses various aspects related to the electrical performance of BIPV installations and products. It includes the performance analysis of a BIPV test site containing uncoloured modules with different mounting configurations as well as coloured modules, monitored for operating temperature and power output. Observations show that even small air gaps for rear ventilation can lead to 40 % to 50 % lower differences between module and ambient temperatures compared to insulated modules. Later sections of this chapter go in-depth on a number of specific performance-related aspects, such as how the absorption of light in coloured interlayers contributes to module heating, leading to significantly higher operating temperatures in darker coloured modules than in lighter ones. It is therefore suggested to use the reflectance instead of transmittance to determine the effective irradiance for temperature modelling. The differences in the incidence angle modifier (IAM) between different colours are shown to measurably affect performance and cause modelling errors when disregarded, which is also true of spectral mismatch gains, which are shown to cause up to 10 % higher relative yields for red coloured modules than for uncoloured references in the afternoon. The losses and gains introduced through structured glass surfaces are also investigated, showing <1 % reflection losses at normal incidence angles for satinated glass. Effective transmittance exceeds that of flat glass at incidence angles >60◦ due to reduced reflection losses. The final category, which often causes concern for BIPV products, is related to reliability. On the one hand, different operating conditions, such as elevated temperatures, can contribute to accelerated degradation – on the other hand, the introduction of new (colouring) materials into PV laminates can lead to detrimental material interactions or disadvantageous material properties. Chapter 4 includes an investigation of how materials used in structural colours interact with different module encapsulants under accelerated ageing, as well as a study on the water vapour tranmission rate (WVTR) of various polymers used in glass-free PV modules. It demonstrates that proper coverage of fibres used for structural reinforcement of backsheets or composite structures is essential in order to achieve adequate moisture barrier properties. While this thesis only covers a small selection of aspects related to appearance, performance and reliability of (coloured) BIPV, it provides the basis for the improvement of existing PV performance models for use with BIPV and identifies some of the major challenges to be addressed. It ends with an outlook on future research required to further improve coloured BIPV modelling accuracy.