The global increase in the level of awareness regarding CO2 emissions has been a strong push for “clean” or “green” sources of energy. A recent study by Greenpeace International revealed that “it would only take 0.04% of the solar energy from the Sahara desert to cover the electricity demand of Europe”. Consequently, there has been a surge in implementing concentrator solar photovoltaic systems in order to generate free power from the sun by converting sunlight into electricity with zero emissions and no moving parts.
Concentrated photovoltaic (CPV) systems allow the conversion of sunlight into electricity with higher efficiencies than conventional flat plate collectors. This is mainly due to the use of highly efficient multi-junction PV cells and to the increasing conversion yield of chips as a function of irradiation. In addition, CPV systems offer cost advantages over flat plate collectors because the semiconductor area is reduced by the concentration factor of the lens, which is typically 500. However, the packaging of current commercial CPV systems is not yet mature. For example, current systems only collect electrical power and dissipate thermal power to the ambient surroundings, which causes a general problem of reduced efficiency due to high chip temperatures.
Nevertheless, the use of optical concentrators to obtain high optical intensities means that the solar cells located at the focal point must be cooled. Thus, the initial objective of the project is to improve the overall packaging of CPV chips such that the overheating problems are minimized, while the thermal waste energy is collected as a useful resource for a multi-effect boiling (MEB) desalination process. The first milestone of this project is to design a high-performance cooler and to optimize the thermal contact of the photovoltaic chip with this cooler. For that project, our group's expertise in processor chip cooling is leveraged to achieve high-performance liquid cooling of the CPV chip. Intermediate targets in this area are to demonstrate the removal of a 100-200W/cm2 heat load with ΔT<20ºC on a solar cell package and to verify the thermal cycling reliability for the PV cell–cooler assembly.
Another objective of this project is to optimize the design of a (non-imaging) concentrating optical system, which would then be integrated with the photovoltaic chip package in order to obtain higher optical concentration factors. Consequently, it is essential to optimize the conversion yield of the multi-junction photovoltaic chip and yet collect the remaining thermal energy at a high temperature level while keeping the chip's temperature low. Combining the electrical and thermal output will not only allow the overall efficiency of the system to be pushed beyond 50% (which outperforms the capabilities of the vast majority of competing solar technologies), but will also make the technology more profitable than current solar technologies.
- T. Brunschwiler, B. Smith, E. Ruetsche and B. Michel, "Toward Zero-emission Data Centers through Direct Reuse of Thermal Energy," IBM J. Res. Develop. 53(3), Paper 11 (2009). Available online.