Overview of HCPV modules

Several designs of HCPV modules are developed using both planar Si cells as well as multijunction solar cells. A few initial models demonstrated on the field are explained.

In the design of HCPV module, the concentration is achieved via reflective or refractive optics. A reflective optics system is likely to use a central receiver, where an

array of cells densely packed close to each other receives the concentrated sunlight. The Dish concentrator developed by Solar Systems of Australia is shown in Figure 5.9. These concentrators utilize reflective optics to concentrate the light at 500 times the suns energy. The CS500 dish concentrator PV system has 112 curved reflecting mirrors mounted on a steel frame, which tracks the sun throughout the day. The combination of mirror profile, mounting framework and solar receiver are carefully designed to deliver concentrated sunlight energy to each PV module. The tracking mechanism maximises the amount of electricity produced. The critical part of the system is an array of close – packed high efficiency Triple Junction Solar Cells packed into a 36 cm2 actively cooled package that are located in the solar receiver, suspended above the focus of the mir­rors. The cells are mounted in a way that allows efficient dissipation of thermal energy as well as extraction of electricity. Since PV performance falls by around 1.7% for every 10°C rise in cell temperature and the sunlight is concentrated 500x, effective cooling is critical to achieve efficient performance. The module also incorporates elec­trical connections to deliver DC output as well as current and temperature sensors for real-time monitoring.

The control system keeps each dish pointing to the sun, monitors performance and adjusts the DC voltage to maximise electricity output. It also incorporates several failsafe systems to protect the CS500 from damage. This configuration is modular allowing Solar Systems to design different receiver configurations and sizes for both Dish and Heliostat solar applications. With active cooling, this design increases the reliability of the cells and produces the highest output power compared to other technologies, increasing cell life and providing more reliable operation. Further, this technology offers the advantage of lower capital investment cost to increase production capacity compared to conventional thin film or crystalline silicon production. This manufacturer, however, is presently not in production.

In the module developed by Amonix, a parquet of Fresnel lenses concentrates the light on individual cells mounted on a heat sink, as shown in Figure 5.10. This design allows passive cooling of the cells without raising the temperature much above the ambient, as the cells are spaced out from each other. The operating temperature of the

cell is typically 20-25°C above the ambient air. As with any passive cooling system, the temperature rise depends on the wind speed at the location. The Amonix module also uses the co-planar Si cell technology. There are currently over 700 kW of installed capacity operating in the states of Arizona and Nevada.

Other one is the micro dish module demonstrated by Concentrating Technologies (CT) which has utilised the multijunction cell technology in a grid-connected, high concentration module for the first time (Sherif 2005, Bett et al. 2006). The approach to the design of this module is a combination of reflective optics (Cassegrain like mirror optics) and the distributed location of small cells typical of refractive concentrators, thus avoiding the use of active cooling in a central receiver. This is accomplished by having a micro array of mirrors focusing the light on individual Power Conversion Units (PCUs). Each PCU has a secondary optical element to homogenize the light before it falls on the cells, and a heat sink attached to the back of the PCU to radiate heat to the ambient air.

Daido Steel of Japan has been active in the development of CPV systems since several years (Araki et al. 2003, 2004), and has developed a concentrating optic which uses a PMMA Fresnel dome lens and a glass kaleidoscope. Triple-junction cells manu­factured at Sharp are used in the modules. Two types of modules with a concentration ratio of 400 and 550 are designed and are subjected to intensive testing. Module efficiencies as high as 30% have been reported (Araki et al. 2005). These results are very promising for any III-V solar cell based CPV system. Further, this manufacturer considers reliability issues seriously which are not often addressed.

Several developers such as Pyron Solar, Green & Gold Energy, Isofoton and Sol3G have come up with CPV system structures which are briefly explained by Bett et al. (2006).

Recently, Zenith Solar (Israel) has developed a system which generates electric power as well as heat (Figure 5.11). The system consists of a mirrored dish that con­centrates the equivalent of 1000 suns onto a III-V multijunction solar cell. It will produce over 2 kW of electricity and the equivalent of 5 kW of solar hot water. The concentrator dish covers around 11 sq. m. of area, and the cell measures around 10 cm in each side. The high temperature created by the solar dish necessitate cooling the

solar cell with water instead of with passive metal heat sinks. The water is then run through heat exchangers to provide hot water to an industry or a commercial site. It is a modular and easily scalable HCPV.

The multijunction solar cell that can convert more than 40 percent of the sun’s energy into electrical energy is the heart of CPV technology. As seen already, the lattice-matched three-junction GaInP/GalnAs/Ge cell used in CPV is the result of inno­vation and refinement over the past decade. This cell structure is created by growing as many as 20 thin, single-crystal layers of III-V materials onto a single-crystal Ge wafer that is 100 mm in diameter using Metallorganic vapor phase epitaxy. The require­ments for crystal growth quality, thickness and doping concentration uniformity are extraordinarily high. With continued global research on advanced multijunction cell architectures by several ways such as incorporation of metamorphic semiconductor materials, increasing numbers of junctions and so on, it can be expected to increase practical concentrator cell efficiencies to 45 or even 50 percent. It will then become economical to deploy solar cell technology on a larger scale.