The interest in the use of light-cascade greenhouses stems from the desire to increase harvested mass without altering quality or modifying the growing period.
In addition to fertile ground, water, and sufficient CO2, plant physiology requires one other very important input—light—which must be available in the correct amount and at the correct wavelength. The action spectrum for the majority of green plants peaks between 400 to 510 nm and 590 to 750 nm, which correspond to the absorption of the chromoproteins cryptochrome and phytochrome, respectively (see figure below). The products of the light-dependent reactions are ATP from photophosphorylation and NADPH from photoreduction. These products provide the energy that drives the light-independent reactions which convert CO2 and other compounds into glucose.
Figure 1: Absorption spectrum of chlorophyll a (gray curve), chlorophyll b (green curve) and bacteriochlorophyll a (red curve). Also shown is the solar spectrum (yellow curve).
Thus, plant physiology does not exploit the green and UV portions of the solar spectrum for photosynthesis. To exploit these portions of the spectrum, the light cascade principle can be applied to greenhouse covers to concentrate the solar energy in the portion of the spectrum that is most efficient for photosynthesis.
The LPRL has doped greenhouse-cover materials such as PEBD/EVA by optically active molecules (OAM) that redshift the incoming radiation. The majority of the optically active molecules used are organic molecules due to their solubility in organic host matrices such as PEBD, PMMA, Polycarbonates, Polyethylene, Acrylic, etc. The photo below shows the LPRL greenhouse covers in use.
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Figure 2: Photos of two field tests of LPRL greenhouse covers.
The goal of applying light cascade materials to greenhouse covers is
1. To absorb UV radiation between 360 to 400 nm and to re-emit the energy in the blue band from 400 to 490 nm. This energy is added to the natural solar energy already present in this band.
2. To absorb radiation in the band from 510 to 580 nm and re-emit in the red band from 650 to 720 nm. This energy is added to the natural solar energy already present in this band.
3. To assure an optimum diffusion of solar radiation to limit the effects of shading that reduce the photosynthetic efficiency towards the beginning and the end of the diurnal period.
4. To improve the thermal characteristics of the greenhouse by limiting the loss of thermal radiation from the soil (especially important in winter) and, conversely, to reduce the diurnal temperature during the summer period by limiting influence of solar IR radiation.
The first tests were realized at the Plant Physiology Laboratory of the CNRS (Laboratoire de Physiologie Végétale) on tomato plants. The results, compared to control plants grown in a nontreated greenhouse, are given below:
1. Leaf mass : 73% increase with respect to nontreated greenhouse cover
2. Leaf surface : 36% increase with respect to nontreated greenhouse cover
Tests carried out at the Institute ENTIH at Angers on green-bean varieties also showed an increase in the harvested mass. The result for the CRISTAL variety is shown below.
Figure 3: Yield of CRISTAL variety of green beans in normal greenhouse vs in a greenhouse whose cover was modified by LPRL light-cascade technology.
Combining Photovoltaics with Greenhouse Covers
In partnership with Greenelec, a specialist in photovoltaic greenhouses, the LPRL, through Greenhouse-Integrated Photovoltaics (GIPV), is developing new greenhouse designs that serve the dual purposes of
- increasing the harvested mass and
- efficiently generating electrical power through integrated photovoltaic modules.
The
surface area of agricultural greenhouses worldwide is estimated at over
a million hectares, which is comparable to the worldwide surface area
of commercial roofing. The photovoltaic market, and in particular the
building-integrated-photovoltaic (BIPV) market, has enjoyed a growth
rate of 35% over the last 15 years, and this growth is projected to
continue, or even increase, through the year 2030.
GIPV is
developing a new generation of photovoltaic greenhouses based on a
unique technology and has demonstrated the capacity to modify and adapt
the solar spectrum by using optically active materials. These materials,
which are proprietary to GIPV, can be directly integrated into the
roofing system of photovoltaic greenhouses. Starting from the solar
spectrum, these materials generate light whose spectrum is optimized for
conversion by photovoltaic cells or by photosynthetic organisms. Thus,
both system-efficiency parameters are improved in a synergistic manner
These
unique greenhouse-photovoltaics systems address three main markets:
rural greenhouses, urban agriculture, and remote greenhouses (off-grid)
that require energy self-sufficiency. This technology is thus of
particular interest for agricultural development in Latin America and
Africa.
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Figure 1: Left panel shows a possible design of a greenhouse roof. Right panel shows an expanded view of the proposed greenhouse roof design.
For more information, please send us an email at contact@lprl.org.
