In common low-e coatings, applied in double-glazing for buildings, often an extremely thin layer of silver is used. This virtually completely eliminates the thermal infrared emission of glass. This does result in a loss of light, according to the project ‘Without emission to high transmission’ in which low thermal emission properties (also called low-e) are integrated in an anti-reflective coating.
Bram van Breugel of Wageningen University & Research talks about it at Kas als Energiebron.
Transparent conductive oxides (TCO) exhibit similar properties when it comes to the reduction of thermal infrared emissions. The reduction with these materials is slightly lower, but a higher hemispherical transmission can be achieved.
What the research partners have presented so far are various TCOs from current production lines at glass manufacturer AGC and experimental TCOs, which are actively being developed for photovoltaic applications at Solliance/TNO.
The focus is now on creating TCOs with the lowest possible absorption in the PAR area and reducing the reflection of light. Work is also being done on the compatibility of the coatings with the glass-hardening process.
Looking at what can be done on an industrial scale in the short term, it seems that the hemispheric transmission will be slightly less than with double AR coated glass for the time being, but much better than with standard uncoated glass. At the same time, it is slightly more energy efficient than using a screen.
A low-e greenhouse cover with a single screen also saves a little more energy than a conventional two-screen glass greenhouse cover. It has been demonstrated that there is a price point at which this method of saving energy will be competitive. Whether the production at that price point is also profitable for the glass manufacturer is not yet known and cannot be answered unequivocally.
In addition, it has been mapped out what can already be made on a more experimental scale. The cooperation with Solliance was sought, because this project essentially takes advantage of developments in photovoltaic technology, where electrical conductivity and transparency for the electrodes are of essential importance.
The fact that these materials therefore also have low-e properties is a secondary consideration in the PV industry. In terms of light transmission, however, it seems that there is certainly still room for improvement. It is expected that at the end of this project a prototype can be presented, which allows at least a few percent more light to pass through than the industrial variant.
This project is financed by Kas als Energiebron, the innovation and action programme of the Ministry of Agriculture, Nature and Food Quality and Greenhouse Horticulture in the Netherlands. In addition, Solliance and glass manufacturer AGC also make contributions.