Combating climate change is one of the most important challenges of our time. A sustainable energy supply via renewable energy sources is widely considered to play a crucial role in meeting this challenge. Among various renewable energy concepts, solar energy shows outstanding potential for success on a global scale. However, the world production of photovoltaics (PV) modules covers less than 1 % of human power consumption. Thus, for very large area scales different approaches are sought. One of these approaches is concentrated PV (CPV). Here, light is focused by a concentration factor of C. That means that the module area collecting the sunlight is C >100 times larger than the area of the solar cells in the focus, and therefore the received power is C >100 times larger. In addition to the significant reduction of the needed solar cell area, CPV modules are also more efficient than standard PV. On the other hand, the drawbacks of CPV are an extreme increase in bulkiness, peripheral cost, and maintenance compared to standard PV.
The project ConPhoNo focused on how to overcome these drawbacks of CPV. A node mirror concentrator was proposed that enables concentration of sunlight without any focus-length. While state-of-the-art CPV modules show a thickness of about 10 cm, node mirror concentrators can be as thin as 500 µm. As these novel concentrators require perpendicular incidence, a combination with a steering layer was intended. Beam steering devices based on electro-wetting have been studied for almost twenty years. They make it possible to steer the incident light beam, thus rendering it unnecessary to mechanically move the entire CPV module. Their tremendous development had raised the hope that first large-area beam steering layers are soon going to be ready for market launch.
During the project, PMMA based prototypes of node mirror concentrators were fabricated. Grey scale lithography and 3D electron-beam lithography were applied for the fabrication of the circular mirror structures. With the latter technique provided by our associated partner, structures of excellent quality could be created. However, once imprinted into the PMMA layers the local mirror angles showed slight variations which reduced the optical efficiency of the concentrators. This had not been anticipated beforehand. Different approaches have been developed to eliminate this issue in the future.
On the other hand, the development of large-area beam steering layers did not meet the expectations. Being a central piece of the solar concentrators, it weighs down the profitability of the foil to nearly zero. Too much time and money is needed to develop it to a usable stage. Even though the fabrication method for the waveguide is possible as intended, the efficient fabrication of suitable mirrors is still ongoing. In combination with the low profitability of the steering layer, this concept is not as promising as originally expected.
During the course of the project we developed a new opto-electrical device. Light inside this novel waveguide geometry can be electrically switched between radiative and non-radiative states. In the non-radiative state, light is bound in the system and transported nearly without losses. With a grating structure placed in the middle of the waveguide its interaction with light can be electrically tuned. The grating can be “hidden”, so that diffraction only occurs at controllable switching positions. This way, for instance, a concentrator can follow a moving focus point. Furthermore, not only the coupling between free space modes and waveguide modes is possible, but also controlled deflection of the light in the waveguide.
This new approach enables a large variety of new optical systems without the need for mechanical components. In the field of CPV, it allows the combination of concentration and sun-tracking without moving parts. Such systems would only require grating structures. They would not rely on technologically challenging volumetric angles or additional beam steering devices. In that sense, the two critical issues that occurred during the project could be eliminated.
In summary, at least at this point in time, the proposed concept is not yet ready for the market. The research conducted in the project ConPhoNo has, however, contributed to novel ideas that are very likely to eliminate the issues that appeared during the course of this project.
