Plants do not harvest the whole solar spectrum equally at the different wavelengths. They indeed reflect green more and absorb dominantly red & blue thanks to the chlorophylls a-b. That’s why we see the plants green. The figure at the bottom illustrates that the solar spectrum is actually not optimized for energy conversion. However, incoming solar spectrum can be modified in order to shift less efficient wavelengths to more efficient one (like green to red) using fluorescent pigments or phosphors. There are various experimental studies that show implementation of fluorescent coatings increases the overall crop amount. In our previously published paper, we attributed the crop growth of these experimental studies to the diffusion of light rather than spectral amelioration. Because, regardless of the incoming light direction, phosphors emit the light isotropically, which results in waste of approximately 50% of the remitted light that reflects to the space. This phenomenon is illustrated at Figures at the bottom.
Therefore, we proposed to use nano-antennas in the vicinity of the phosphors to alter their emission direction of the light, so that the amount of light that is emitted to space can be minimized. The resulting proposed coating presented in Figure at the bottom.
During the design, we determined the proper materials to design the fluorescent coatings. As the host material, we chose silica (SiO2) due to its very low absorptivity (imaginary refractive index, k < 10-6) at the photosynthetically active region (PAR). We choose MgF2 as the anti-reflective material. During the study, we also suggested to use silica aerogel as the host material because of its superior properties for the proposed application (very low absorption at PAR and very low real refractive index which results in omitting the anti-reflective coating) but we avoid using aerogels since we could not find a prior attempt to implement nano-antenna and phosphors inside them to create a photonic structure. Since the most effective spectrum for the lettuce is given to be 600 nm, an ideal phosphor for the plant needs to emit around that spectrum. It also needs to be excited at the spectrum that the lettuce grows slower. We found phosphor (SrCa)AlSiN3:Eu suitable for this application. It has a peak emission wavelength of 607 nm, beside its high efficiency (Quantum Yield, QY >90%). Our preliminary study showed that a 21% effective light increase is theoretically possible. With the selected materials, it reduces to 14%. These values consider only the down-conversion of the fluorescent materials. Further increase is possible with up-converting phosphors which can make the non-utilized infrared light of the sun available to plants. However, this phenomenon is out of scope of this study and only the down-conversion is considered.
The studied plant should be popular in terms of human consumption and conducted research. We chose the plant as lettuce, Lactuca sativa. Lettuce is the most commonly produced crop in the greenhouses along with tomatoes and cucumber. The literature is rich and mature in terms of spectral light – growth relation for lettuce.
At a later stage of the study, we investigated the optimum antenna length, radius and distance to the phosphor, along with the optimum phosphor size. First, we found that, smaller the phosphor size, the better the emission redirection of the antenna. The antenna – phosphor interaction is inspected in the near field region of an electromagnetic field. As previously mentioned in the proposal, the researcher had not had a previous background in near-field optics to conduct this study himself. Thus, the researcher performed preliminary studies with his supervisors for knowledge transfer. He presented the outcomes at conferences in Italy ("Nanoscale and Microscale Heat Transfer VII" conference (Eurotherm seminar No 114)) and Germany (745. WE-Heraeus Seminar on "Photon, Phonon, and Electron Transitions in Coupled Nanoscale Systems" at the Physikzentrum Bad Honnef). They also wrote a paper about the topic, which is in a matured draft status. Greenhouses are structures that have been historically designed in order to protect fragile plants such as vegetables, flowers or fruit trees from the climatic contingencies (like freeze). They minimize the effect of climate, short or long term, and aim to maintain the plants growing inside in optimal conditions. With the development and emergence of new technologies and ideas, greenhouses can be engineered to present novel features. One of these is to improve the greenhouse lighting to significantly enhance the plants growth. Note that this proposal addresses 2 of the UN sustainable development goals (2: zero hunger, 12: responsible consumption and production) as well as the food security societal challenge of Horizon 2020 program and has therefore an important economic and societal potential.
Since the researcher resigned from the university, the study could not be further extended to the experimental stage. However, the presented outcomes had already proved that the use of nano-antenna in the vicinity of phosphor will provide a better illumination of crops compared to the fluorescent coatings without nano-antenna. Further studies from academia and industry that will reduce the production cost and increase the forward emitted light output will make the fluorescent coatings feasible. As a result, the fluorescent coatings can replace the ordinary greenhouse coatings especially at the climates where the light is restricting the plant growth.
Theoretical transmittance measurements
“Improving Photosynthetic Efficiency Using Greenhouse Coatings with Scattering and Fluorescent Pigments“, doi.org/10.1088/2053-1591/ab28b8
“Improving crop production in solar illuminated vertical farms using fluorescence coatings“, doi.org/10.1016/j.biosystemseng.2020.02.007
Related patent application
WO2020204858 - A LIGHTING SYSTEM FOR MULTI-LAYER GREENHOUSES