Practical Issues and Characterization of a Photovoltaic/Thermal Linear Focus $20\times $ Solar Concentrator

This paper presents some practical issues and experience acquired in the design and experimental verification of a small-sized solar concentrator producing both electrical and thermal power. The device is a low-concentration photovoltaic/thermal solar system with a 20 suns (20x) concentrator factor and is suitable for integration on building roofs. It consists of four small-sized (1930 x 250 x 166 L x W x H mm) linear focus semiparabolic mirrors mechanically linked and driven by a stepper motor and tracking the sun along a single axis. The sunrays are concentrated on strings of monocrystalline cells with conversion efficiency of about 18%. One of the main features of this device is that the parabola mirrors are designed to have their focus inside the mirror profile so that the solar receiver is installed inside the parabolic concentrator. As a result, each single concentrator height is as low as 166 mm. The solar energy that is not converted into electricity by the solar cells is recovered by a cooling medium flowing in an aluminum structure on which the PV cells are mounted. This paper presents an overall description of the device, an electrical layout of the bypass diodes of the solar receiver utilized to reduce the end losses, a method based on voltage measurement to derive the solar cell temperature inside the receiver, and a set of results derived from experimental validations performed on a device prototype. This paper also describes the monitoring system utilized for measurements on the solar concentrator and derives an overall efficiency of solar energy conversion in both electric and thermal generation as high as 61.5%. The predicted values were in good agreement with the experimental test results.