A highly efficient, low-cost hybrid module, built on a full-size bifacial silicon module

By augmenting silicon PV cells with close to 50% efficient multi-junction cells, there is the potential to approximately double overall conversion efficiency of solar PV modules. In this paper we describe the design of a hybrid module built onto a commercial 1?m x 2?m bifacial PV module. This serves not only to harvest the energy of diffuse light, but also to carry and cool an array of small multijunction PV cells to harvest direct sunlight. The single-junction silicon PV cells of the bifacial are sealed between 2?mm thick glass windows, front and back. In our design, the MJ cells are attached to the bifacial module's front window via 5?mm square copper/ceramic substrates. Attached in front of the module is a glass sheet formed into an array of 34?mm square lenses which focus sunlight to the cells while also transmitting diffuse light to the module's silicon cells. An 8?mm diameter secondary ball lens is bonded in front of each 2?mm square MJ cell, yielding 290 x concentration, maintained at >95% geometric throughput for solar mis-pointing as far as 1° off-axis. To ensure good cooling of the MJ cell, despite its being mounted on glass, the copper wires used to carry electrical power to the sides of the bifacial module are configured to also carry heat away from the cells. The wires have 1?mm square cross section and are bonded along their full length to the glass, with thermally conductive adhesive. Thermal modeling shows that for DNI solar illumination of 1 kW/m2, the temperature drop between the MJ cells and the back convecting surface of the bifacial module is limited to 10°C. The lens array is imprinted into a full-size (1?m x 2?m), 6?mm thick sheet of glass, by a process in which the sheet is first heated to 850°C, then pressed between relatively cold molds. The focal length of these lenses is chosen to be 125?mm, slow enough at f/3.5 to limit total internal reflection losses of the diffuse light to ∼10%. To minimize shipping costs, the glass sheets and modified bifacial modules will be packed and shipped separately, for assembly on-site as complete hybrid modules, using reflective side walls. We envisage that dual axis tracking will be provided by augmentation of single axis E-W tracking now used for standard utility scale PV plants, with the addition of cross-axes to turn the modules also in the N-S direction.