Luminescent Solar Concentrator Greenhouses for Concurrent Energy Generation and Lettuce Production in The United States

Agrivoltaic systems promote dual land use by strategically combining photovoltaics (PV) and agriculture. One application space is in greenhouses, where PV glazing can offset or completely meet high energy demands from greenhouse operations. However, fully opaque PVs generate electricity at the expense of light transmission, thereby eliminating crop photosynthesis. Luminescent solar concentrators (LSCs) have received growing interest in the agrivoltaics community as their higher transparency can benefit crop yield while concentrating light onto small-area PV cells. Integrating LSCs into greenhouse roofs presents a promising opportunity to provide clean energy without significantly compromising crop yield; however, the LSC design considerations that balance transmission and electricity generation need to be better understood.<br/><br/>This work presents a modeling framework to evaluate the tradeoff between light used for energy generation and light used for crop growth in LSC greenhouses. Our model incorporates solar resources, heat and energy, power generation, lettuce (cv. Rex) crop yield, and economic models. Using data pulled from NASA and NREL, we determine the hourly power generation and energy demands for operating LSC greenhouses in Arizona (AZ), Florida (FL), Minnesota (MN), and Pennsylvania (PA). We compare two periodic LSC roof structures: a small-area 8 x 8 cm² LSC film surrounded by 2 cm-wide PV cells (36% PV coverage) and a large-area 24 x 24 cm² film surrounded by 2 cm-wide PV cells (15% PV coverage), as well as a conventional glass greenhouse. We also study three non-toxic luminophores previously evaluated as LSC roof materials: copper indium sulfide/zinc sulfide quantum dots (CIS/ZnS), silicon quantum dots (Si), and Lumogen Red 305 molecular dye (LR305).<br/><br/>First, we determine the influence of the luminophore spectrum and concentration in the LSC films on lettuce growth. CIS/ZnS and Si quantum dot roofs absorb blue light, while LR305 roofs absorb green light. We find that in all cases, the quantum dot roofs lead to higher lettuce yields than the dye roofs due to the reduction in blue light fraction.<br/><br/>Next, we study the impact of luminophore concentration on energy generation and the potential for net zero energy (NZE) greenhouses, where the total energy demand is met by the power produced by the PV cells. In AZ and FL, all small-area LSC greenhouses are NZE as the higher PV coverage allows for more energy generation. No LR305 large-area LSC greenhouses are NZE due to reabsorption losses and a lower PV coverage. All MN and PA LSC greenhouses have an energy deficit due to the high heating demands in these cold climates. However, for small-area LSC greenhouses in PA, the PV cells could meet roughly 1/3 of the energy demand.<br/><br/>Lastly, we calculate the net present value (NPV) to evaluate the tradeoff between crop yield and energy generation. We find that crop yield is the most significant determinant for the NPV, with higher yields leading to more profitable greenhouses. LSC greenhouses in AZ and FL can be more profitable than conventional greenhouses. In MN and PA, small-area LSC greenhouses are less profitable than conventional glass greenhouses. Still, large-area LSC greenhouses can be more profitable in PA due to improvements in crop yield and energy offset provided by the PV cells. The profit from AZ and FL LSC greenhouses can be further improved through net metering, where surplus energy is sold back to the grid. Overall, this study furthers understanding of the balance between power generation and light transmission in LSC-based greenhouses.