On the design of concentrator photovoltaic modules

High concentration photovoltaics (HCPV) promise a more efficient, higher power output than traditional photovoltaic modules. This is achieved by concentrating sunlight onto a small 1 cm2 triple junction (CTJ) InGaP/InGaAs/Ge cell by using precision optics. In order to achieve high performance, careful and informed design decisions must be made in the development of a HCPV module . This project investigated the design of a HCPV module and is divided into sections that concentrate on the optical design, thermal dissipation and electrical characterization of a concentration triple junction cell. The first HCPV module (Module I) design was based on the Sandia III Baseline Fresnel module which comprised of a Fresnel lens and truncated reflective secondary as the optical elements. The parameters of the CTJ cell in Module I increased with increased concentration. This included the short circuit current, open circuit voltage, power and efficiency. The best performance achieved was at 336 times operational concentration which produced 10.3 W per cell, a cell efficiency of 38.4 percent, and module efficiency of 24.2 percent Investigation of the optical subsystem revealed that the optics played a large role in the operation of the CTJ cell. Characterization of the optical elements showed a transmission loss of 15 percent of concentrated sunlight for the irradiance of which 66 percent of the loss occurred in wavelength region where the InGaP subcell is active. Characterization of the optical subsystem indicated regions of non-uniform irradiance and spectral intensity across the CTJ cell surface. The optical subsystem caused the InGaP subcell of the series monolithic connected CTJ cell to be current limiting. This was confirmed by the CTJ cell having the same short circuit current as the InGaP subcell. The performance of the CTJ cell decreased with an increase in operational temperature. A form of thermal dissipation was needed as 168 times more heat needs to be dissipated when compared to a flat plate photovoltaic module. The thermal dissipation was achieved by passive means with a heat sink which reduced the operational temperature of the CTJ cell from 50 oC to 21 oC above ambient. Cell damage was noted in Module I due to bubbles in the encapsulation epoxy bursting from a high, non-uniform intensity distribution. The development of the second module (Module II) employed a pre-monitoring criteria that characterized the CTJ cells and eliminated faulty cells from the system. These criteria included visual inspection of the cell, electroluminescence and one sun current-voltage (I-V) characteristic curves. Module II was designed as separate units which comprised of a Fresnel lens, refractive secondary, CTJ cell and heatsink. The optimal configuration between the two modules were compared. The CTJ cells in module II showed no form of degradation in the I-V characteristics and in the detected defects. The units under thermal and optical stress showed a progressive degradation. A feature in the I-V curve at V > Vmax was noted for the thermally stressed unit. This feature in the I-V curve may be attributed to the breakdown of the Ge subcell in the CTJ cell. Based on the results obtained from the two experimental HCPV modules, recommendations for an optimal HCPV module were made