This innovative solar module is designed to be integrated with the greenhouse roofs. As illustrated in the picture shown left, the module is designed such that a bi-facial Si panel was placed at the top of the module and a dichroic reflector was placed under the bi-facial panel. The dichroic reflector, a key component of the design, splits the light from 725nm to 1100nm: passing light with a wavelength that is shorter than 725nm and a wavelength that is longer than 1100nm but reflects ALL the light between 725nm and 1100nm. The shape of the reflector is designed to reflect and concentrate NIR light to the backside of the bi-facial Si panel. Therefore, when the incoming sunlight shines on the module a small portion of sunlight hits the front of the Si panel and generates a photocurrent. A larger portion passes through the open areas by the side of the Si panel and hits the dichroic reflector underneath where the NIR light is reflected to the backside of the Si panel generating more photocurrent. Hence, this solar module passes a majority of visible light while at the same time, harvesting more photocurrent.
Terminology: Visible light has a wavelength from 400-725nm, aka photosynthetically active radiation (PAR), NIR light has a wavelength from 725-1100nm.
The fundamental reason why a conventional opaque Si PV panel can’t be put on the roof of a greenhouse is that it competes with plants for sunlight, particularly the PAR. The chlorophylls in the leaves absorb both blue and red light to drive the photosynthetic reactions. The green light, even though not heavily absorbed, still plays a role in driving photosynthesis deeper within the leaves. It also plays an important role during fructification. Thus, the whole visible light band coincides well with PAR. The conventional Si panel absorbs sunlight up to 1100nm, which covers the PAR and the NIR. Clearly, Si panels take away all the PAR that crops need to thrive, thereby impeding agricultural activities.
The VIS and NIR light bands are illustrated on an AM1.5G solar spectrum. The VIS photons are more energetic than the that in the NIR. They account for about 44% of the total solar irradiance while the NIR photons account for another 31%. However, the number of photons in both bands are almost equal. Therefore, they are equivalent from a photovoltaic point of view, although the low-energy NIR photons cannot drive the photosynthesis process because their energy is below the threshold of chlorophyll’s absorption edge, while VIS photons have enough energy to drive the process. However, the VIS and NIR photons generate the same photocurrent in the Si PV panel, thereby giving an opportunity to split the photons off for different uses. Ideally, we want to let VIS pass for photosynthesis but intercept NIR for electricity. Directing NIR photons to Si PV panels also reduces thermalization loss as they are low energy photons, thereby improving overall solar energy utilization efficiency.
The Agri-Solar module is static on the roof structure. Therefore it is important for the designer to consider the Sun’s motion in the sky. With the help of our collaborators, an optical simulation was performed with a set of geo-parameters. The simulation predicts the design will have +/- 25° of the field of view to compensate for Sun’s elevational change (left Figure).
The test results agreed with the simulation well (right Figure). The red line in the Figure is the power without NIR collection. We can see a max of 70% power boost by NIR collection (the blue line).