Concentrators

Current Limitations

(A) GEN-1 Array from off axis.

(B) GEN-1 Array from on axis.

FIG 2.4. Total Internal Reflection Replaces Mirrors. (A) GEN-1 array seen from off axis is partially transparent because the conditions for TIR have not been met. The blue background behind the solar array is seen through the lenses. (B) The GEN-1 array seen form almost directly on the optical axis looks mirrored due to the conditions of TIR now existing. In these images the tiny secondary mirror is visible. This secondary mirror will be eliminated in the GEN-2 build, thus providing a completely mirrorless array.

The physical limits to concentrator performance are ohmic losses (heating) in the mirrors, ohmic losses in the lens material, reflection losses at air-glass or air-plastic interfaces, material changes due to the environment and etendue mismatches, which are due to attempting to squeeze too much light into a region without sufficient area and angular acceptance to allow passage through the optical system.

The manufacturing limits to concentrator performance are low accuracy molds, low precision molds, as well as unintended variations of concentrator material properties. These limitations manifest themselves as leakage losses and misfocused light losses. Specifically, we have found that most lens based fabrication techniques have a precision of about 10 times too small to allow the types of high-performance optics that are really needed in the solar industry to dramatically lower costs by increasing the amount of harvested light. The accuracy and precision issues of the mold may only be addressed by developing and introducing new fabrication techniques. Therefore, XE is developing new manufacturing techniques to overcome these limits using automated nano-scale metrology and fabrication to create ultra sharp facets and near perfect optical devices.

The environmental limits to concentrator performance are all the typical weather conditions. However, the most critical issue for mirrored concentrators is the ability to perform for decades of extreme temperature cycling. Cracking, pealing, scratches, and other degrading conditions have a strong potential to degrade the performance of the concentrator lens. This reduces the life expectancy of the concentrator and the total energy and dollars harvested over its working life.

Finally, large concentrator systems do not gain the advantage of economics-of-scale that are found in the electronics industry. Thus the tendency of the established solar industry to use large-scale optical systems, often huge and larger than a city bus in size, is not consistent with the highly refined electronics fabrication industry. We feel that this is a serious problem that is addressed by making the HCPV cells much smaller and utilizing the billions of dollars spent in packaging technologies from the electronics industry.