Light Environment Analysis and Production of Red Leaf Lettuce in Greenhouses with Flexible Solar Cells

Recently, agrivoltaic systems, which utilize a solar-sharing conception by having solar cells installed on top of the cultivation area, using the extra solar light for photovoltaics, have been attracting attention. However, most applications are reported in open-field conditions, and some issues have been raised about the installation of these structures. Accordingly, we conducted a cultivation experiment in which an agrivoltaic system was applied to protected horticulture with flexible solar cells. Red leaf lettuce was used in the experiment and was cultivated in three single greenhouses with the system installed either on one side of the greenhouse ceiling, denoted as photovoltaic I, or on both sides denoted as photovoltaic Ⅱ. The control system used no agrivoltaic system. A ray-tracing simulation was also conducted to determine the light distribution of the total greenhouse floor area and daily difference according to solar altitude changes. It was found that the monthly amounts of energy generated in the greenhouses were 156.85 and 323.34 kWh for photovoltaic Ⅰ and photovoltaic Ⅱ, respectively. Due to the area shaded from light by the solar cells, daily light integral (DLI) values were 86% of the control for photovoltaic Ⅰ and 71% for photovoltaic Ⅱ. In the results of the ray-tracing simulation under identical conditions, the root mean square error between the calculated DLI and the measured DLI was 14.8 µmol·m-2·day-1. The shoot fresh weight, leaf area, SPAD and anthocyanin content of red leaf lettuces showed no significant differences according to the treatment. Through this study, it was confirmed that greenhouses with flexible solar cells can be applied for the production of leafy vegetable such as lettuce with low light saturation points.