Characterization of spherical optics performance compared to other types of optical systems in a point-focus CPV system

In this paper the novelty is represented by comparison of optical and energy performances of spherical mirror, rarely used as primary optics in point-focus CPV systems, with those of refractive or parabolic reflective optics. First, concentration factor and optical efficiency are experimentally determined varying distance between Triple-Junction (TJ) cell and spherical mirror, also with secondary optics that increases acceptance angle reducing precision required to solar tracker. Successively, the TJ cell temperature and electrical power values are evaluated under maximum concentration obtaining daily electrical producibility of 98Wh at Fisciano (Italy). Producibility is evaluated for correct and incorrect solar tracking, showing after 750 s decrease of 38.0% and 50.3% respectively with and without secondary optics. Hence, equations linking optical parameters are obtained by experimental data collected for three months and are adoptable for similar point-focus configurations with same TJ cell dimensions and ranges of concentration factor and focal distance. Finally, spherical optics performances are theoretically compared with those of parabolic mirror and experimentally with those of Fresnel lens. Spherical optics is preferable to parabolic from technical-economic point of view because it is easier to produce. Moreover, spherical mirror allows concentration factor and optical efficiency values three times higher than Fresnel lens. The electric power unitary costs of both optics are near but spherical optics requires a CPV system area about three times smaller with same power output. Electrical power and temperature values reached by single TJ cell with spherical optics are respectively about 10 W and 30 ◦C higher than those of Fresnel lens.