**4. Conclusions and Outlook**

The practical integration of advanced solar energy-harvesting technologies into various elements of urban landscapes, including building windows, is rapidly becoming a mainstream trend. Substantial advances have been reported in recent years both in laboratory trials, and also in commercial demonstrations of the various semi-transparent solar cell types and solar window devices. A wide range of established semi-transparent PV and BIPV technologies exists currently, providing architects and building designers with multiple choices regarding the balance between the system aesthetics, degree of transparency (or colouration type), and power generating capacity. Multiple next-generation transparent solar-cell technologies, including dye-sensitised solar cells, patterned solar panels, organic polymer-based, and perovskite-based systems remain in active stages of development and continue to demonstrate new milestones in efficiency. Highly transparent, and colour-unbiased concentrator-type solar window systems are only beginning to make their entry into industry-wide acceptance. They now provide a previously unavailable combination of up to 70% in total visible light transmission and power conversion efficiency near 2.5%, based on systems demonstrated in 2017.

Despite the fundamental trade-offs between the required control over the visual appearance, degree of transparency, and the power generating capacity intrinsic to the design of advanced BIPV, their strong potential for transforming urban landscapes and providing substantial distributed generation capacity is certain. Developments in the materials science of advanced luminophores, coupled with novel designs of LSC-type semitransparent concentrator structures add continually to the possibilities of obtaining increased power conversion efficiencies. At the same time, a substantial energy-saving potential exists, provided by solar windows, which can also control the solar heat gain in buildings and the associated thermal insulation properties. The new trends in the local utilisation of the energy generated in the distributed way by the building components include using advanced windows with active transparency control, which can contribute substantially to both personnel comfort and climate control-related energy savings. It is currently expected that multiple commercial building-based trials of the latest transparent BIPV technologies will soon be conducted, uncovering their true practical applications potential.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1073/12/6/1080/ s1, Video S1: ECU-ClearVue power generating window prototypes 2016.mp4.

**Author Contributions:** All authors (M.V., M.N.A. and K.A.) have contributed to the conceptualisation of this review article and data collection; M.V. analysed the data and prepared the manuscript; all authors discussed the data, graphics, and the presentation; M.N.A. contributed substantially to the data curation and the original draft preparation; M.V. and K.A. further reviewed and edited the manuscript.

**Funding:** This research was funded by the Australian Research Council (grants LP130100130 and LP160101589) and Edith Cowan University.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
