23The spherical micro-cells are a semi-transparent photovoltaic (PV) technology which can 24 contribute to improve the sustainability of greenhouse systems. Previous prototypes were 25 tested in laboratory conditions, but the size was not suitable for the greenhouse roof 26 application. In this work, a new prototype has been developed and tested on a real 27 greenhouse roof. The semi-transparent PV module (STM) was composed by 4800 28 spherical silicon micro-cells (1.2 mm diameter) sandwiched between glass plates and 29 integrated on a greenhouse roof with 26.5° slope. The STM was 910 mm long and 610 mm 30 wide to match the size of the greenhouse framework. The percentage of the STM area 31 covered with micro-cells was 2.3%, reaching 9.7% considering the metallic conductors. 32 The cell density was 2 cells cm -2 and the measured perpendicular light transmissivity of the 33 semi-transparent area was 73%. The characteristics of the prototype were compared with 34 those of a conventional planar multi-crystalline silicon module (CPM). The module 35 conversion efficiency was steadily around 0.2% over wide incident sunlight angle. The 36 micro-cells never completely eclipse the incident sunlight when observed from more than 1 37 m distance from the roof, keeping the eclipsing level at 9.7%. The yield factor of the STM 38 © 2015. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 was slightly higher than the CPM because of the isotropic properties of the spherical cells, 39 which are able to use both the sky-incident and the ground-reflected irradiation for energy 40 production, irrespective of the module slope. The prototype STM is promising for 41 greenhouse roof applications and its performance can be improved by increasing the 42 conversion efficiency. 43 44

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A novel “photovoltaics (PV) + electrolyzer” concept is presented using a simple, small, and completely stand-alone non-biased device for solar-driven overall water splitting. Three or four spherical-shaped p-n junction silicon balls were successfully connected in series, named “SPHELAR.” SPHELAR possessed small projected areas of 0.20 (3PVs) and 0.26 cm2 (4PVs) and exhibited working voltages sufficient for water electrolysis. Impacts of the configuration on the PV module performance were carefully analyzed, revealing that a drastic increase in the photocurrent (≈20%) was attained by the effective utilization of a reflective sheet. Separate investigations on the electrocatalyst performance showed that non-noble metal based materials with reasonably small sizes (<0.80 cm2) exhibited substantial currents at the PV working voltage. By combining the observations of the PV characteristics, light management and electrocatalyst performance, solar-driven overall water splitting was readily achieved, reaching solar-to-hydrogen efficiencies of 7.4% (3PVs) and 6.4% (4PVs).

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