On the 6th of April, at Wageningen UR Greenhouse Horticulture in Bleiswijk held a workshop on the possibility of applying fluorescent solar concentrators in horticulture. During the meeting there was a lively exchange of knowledge between the 30 participants including researchers from Wageningen UR, TU Eindhoven, TU Delft, Utrecht University and the Energy Research Centre of the Netherlands with a variety of representatives from the (horticultural) industry.
The energy balance of a Greenhouse
Over the year a greenhouse receives more energy from the sun than needed in the greenhouse itself. The reason why a lot of heat energy is still needed to operate a greenhouse arises from a mismatch in the seasonal supply and demand, additional heat is needed in winter and excess heat from the sun is ventilated out in summer. Theoretically, this means that it should be possible to make a greenhouse energy neutral and even energy (heat or electricity) generating, attempts have shown, however, in the past that this is technically difficult and is a challenge in economic terms.
Electricity producing glass
It would be desirable if the windows in a greenhouse itself could generate energy. A possibility to realize this is by placing thin strips of solar cells at the edges of the glass and concentrate light there by applying a fluorescent coating on top of the glass, then light will be absorbed and re-emitted and the emitted light remains partly trapped in the glass and is wave guided to the edges to be converted into electric power.
Currently this process still lacks in efficiency and during the workshop different approaches to improve this have been discussed and explained. Examples include the use of liquid crystal to reduce surface losses at the TU/e or the use of Thulium doped halides to reduce losses related to self-absorption currently investigated at TU Delft.
Another approach is to simply accept the disadvantage of a low efficiency together with the advantages like 1: the aesthetic value as applied in the electrical Mondrian at Utrecht University; or 2: the low material cost and the ease with which this technique can be integrated into the built environment such as applied in the Solar Noise Barrier about which ECN presented the use of a fluorescent solar concentrator with a few percent efficiency integrated in a noise barrier which had to be built anyway and could be cheaper than covering them with solar cells.
Another advantage of this technique for the horticulture is that it can be, for example, very selectively used to convert UV-radiation into PAR-light and electricity, or a removable fluorescent layer could possibly be developed and applied during the summer months. Currently we are not near this stage, but during the meeting on April the 6th, the possibilities for the future in the horticultural were made very clear and several parties indicated that work will surely continue on this topic.
This workshop has been made possible by the contribution from the program Greenhouse as Energy source, innovation and action program of the Ministry of Economic Affairs and LTO Glaskracht Netherlands and co-financed by the Foundation Program Fund Greenhouses.