The “Energy Performance of Buildings Directive” requires that all new buildings in Europe must be nearly zero energy buildings by 31st December 2020 (public buildings by 31st December 2018). Consequently, this directive is one key driver for an evolving market for building integrated photovoltaic (BiPV) products. It is anticipated, that by 2020 the majority of newly constructed buildings in Europe will generate substantial amounts of photovoltaic electricity. Key needs for BiPV which are also explicitly addressed in the current SET-plan of the Solar Europe Industry Initiative are:
- Flexibility of design and improved aesthetics
- Optimisation of performance and improved optical appearance at reduced costs;
- Industrial low-cost manufacturing process for producing BiPV products with tailored design
There are several approaches to tackle the requirements listed above. One of them is to print onto the inner side of cover glasses. However, when trying to realize clear and bright colours that approach corresponds to efficiency losses of up to 50% for the PV module. The reason for printing on the inner side of the cover glass is related to reliability issues and abrasion of the used inks. Recently several printing inks and coating technologies for covering the outer side of glass became available which are already used in construction business but almost not for BiPV applications.
When analysing the reasons of the optical appearance of photovoltaics it becomes clear that its appearance is predominantly determined by the cover glass. In particular the reflections at the surface of the cover glass are substantially higher (more than twice) than those at the solar cell. From an optical design perspective this fact makes it difficult to tune the aesthetics of a photovoltaic module effectively only by changing the colour of the cells. More promising is the approach to tackle the surface which is mainly responsible for the design: The air-glass interface. Combining different kinds of coatings with various patterns of glass allow for new degrees of freedom in designing new BiPV solutions. In particular an issue could be tackled which turned out to be a road-block for some façade integrated BiPV projects -namely glare.
The target of the project is to develop BiPV module prototypes based on glass-glass technology and c-Si solar cells (including bi-facial cells) and applying novel glass coatings for the outer side (environmental side) of the cover glasses. Those module prototypes should show the following properties:
- Flexible and innovative design in terms of colour and surface texture;
- Minimum glare (less than 0.1% of specular reflection);
- At least 150 W/m² (STC) by exploiting back reflected light in bi-facial cells;
- Ageing and adhesion of surface coatings are investigated and reliable for at 30 years.
A further objective is to realize a prototype BiPV installation for demonstrating the feasibility of the prototypes, which will be in operation beyond the end of the project.
