The photovoltaic (PV) greenhouses are by now widely diffused in southern and central Europe1, but their optimal compromise between crop and electricity production is still under investigation. We already developed a prototype semi-transparent PV module to mitigate the crop shading caused by PVs installed on the greenhouse roofs2. However, this module was too small compared to the common dimensions of the greenhouse roof panes. In this study a new semi-transparent photovoltaic module was developed matching the size of greenhouse roof frames. The module was produced using 4800 spherical solar microcells with 1.2 mm diameter. The PV cells were composed by a p-type semiconductor as the inner core and an n-type semiconductor as the outer shell. The module size was 910 mm × 610 mm × 8 mm. The cells were sandwiched between 3-mm-thick glass plates after being embedded in 2-mm-thick transparent resin, and distributed across a 480 mm × 501 mm area (cell area) with 2 cells cm-2 density. The cross-sectional area of the cells covered 2.3% of the cell area, while 7.4% was covered with conductor wires. The remaining 90% area was transparent to sunlight, allowing the solar radiation to enter the greenhouse. The single cells are so small that they do not entirely eclipse the sun, when observed through the cells more than 0.13 m away from the PV module. The electrical and shading characteristics of the PV module were measured. The module was installed on the west-oriented roof of a disused greenhouse in the Shimane University campus (35°N, 133°E). The measured shading percentage of the module cell area was 25%. The peak power output of the module was 479 mW and 0.2% of impinged light on the cell area was converted to electricity. The energy produced by the STM is still insufficient to fulfill the greenhouse electrical loads. Further improvements of the technology and manufacturing process are necessary for a more competitive price and to increase the conversion efficiency and the light transmissivity of the module.
Development of a semi-transparent photovoltaic module based on spherical microcells for greenhouse-roof applications / Yano, Akira; Li, Zhi; Onoe, Mahiro; Cossu, Marco; Murgia, Lelia; Nakamura, Hidetoshi; Matsumoto, Toshinori; Nakata, Josuke. - (2015). (Intervento presentato al convegno International Symposium on New Technologies and Management for Greenhouses - GreenSys2015).