For almost two decades, trains have been equipped with insulating glass in order to reduce energy losses. « These double-glazed windows contain a conductive metal coating that prevents either overheating by near-infrared solar radiation and/or heat loss by thermal radiation, » explains Andreas Schüler of the Nanotechnology Group for Solar Energy Conversion at EPFL (of the Solar Energy and Building Physics Laboratory LESO-PB). « This thin metal layer therefore makes it possible to control the interior temperature of train cars and save heating and/or cooling energy. »
Insulating glass, but impervious to waves
However, the glazing does have one drawback: it strongly attenuates telecommunication waves. « In addition to the metal envelope of modern railcars, this makes the trains completely opaque to mobile phone signals, » says Schüler.
About ten years ago, railways and mobile network companies found a solution to signal blocking in the form of signal amplifiers (commonly known as « repeaters »). Placed inside each rail car, these devices pick up and amplify the signal emitted by the telecommunication antennas placed along the rails. Although repeaters are effective, they have several disadvantages such as a limited life span (between 4 and 8 years, compared to about 30 years for a wagon), high cost (CHF 15,000) and additional power consumption (up to 700 watts).
A first solution developed at EPFL in 2016
This finding prompted Andreas Schüler and his colleagues to develop a new type of glazing in 2016 that is both energy efficient and transparent to the microwaves used in mobile telecommunications. How? « By modifying the metal layer with a specific laser treatment. Thus, the glazing can let the waves through, » says Schüler. The technology has won several awards and the windows have been installed in 34 regional trains operated by the Swiss transport company BLS.
New glass compatible with the latest telecommunication technologies
The rapid evolution of mobile communication generations and, above all, the growth in data traffic have led to the use of a new communication technology called MIMO (see definition in box). The MIMO technology is already used in 4G LTE (0.8, 1.8 and 2.6 GHz) and is of utmost importance for the implementation and coverage of the Swiss 5G (3.5 − 3.8 GHz). The antennas used in this technology are crossed and the electromagnetic waves now have two distinct polarization directions, with horizontal and vertical components.
« Swisscom approached us in 2019 to express interest in an energy-saving glazing specially adapted to MIMO technology in order to increase data throughput rates without further expensive renewal of the signal boosters, » says Schüler. This triggered a collaboration between Swisscom, SBB, LESO-PB (EPFL) and the company Flachglas on the MIMO friendly window project, which was developed as part of a 6-month internship with the support of the enable program offered by the Technology Transfer Office of EPFL.
« The innovative double glazing developed initially in 2016 has been developed further and adapted to MIMO technology, » explains Héloïse Delaporte, who has just completed her internship at the LESO-PB of EPFL. The first prototype, manufactured in collaboration with Flachglas, has been tested and the feasibility has been proven. « We are now in the final stages of optimization and hope to have the final prototype by the end of the year, » adds Schüler. « At the same time, a patent has been filed on behalf of EPFL and Swisscom”.