First, b-Si and Si immersed into 300 g of water were irradiated with solar light collected by a Fresnel lens made over the 600×600 mm 2 area out of polymethyl methacrylate (PMMA) manufactured by Japan Specialty Plastics (NTKJ, Co. Ltd.). A temperature change was measured by a thermocouple with a data logger and illuminance was measured at the plane of Fresnel lens Fig. (a). The results for temperature transients are shown in Fig. (b) for small and large lenses. For the small lens, the b-Si showed the fastest temperature increase.