Optics for Solar Cells for Portable Power

Duncan T. Moore

The Institute of Optics / University of Rochester

Abstract

For portable solar power applications such as those defined by the DARPA Very High Efficiency Solar Cell (VHESC) program, there are three requirements- charge a laptop or equivalent person electronic device in one hour, have a foot print that is no larger than the size of the largest side of the device (ie no fold out systems) and have no moving parts (and the device can not be moved after initially being set down). The first requirement implies that the efficiency be greater than 50%. With the best cells now at about 40% in the laboratory, a different system is needed. One way to accomplish this is to use spectral splitting- i.e., to use some optical method to separate the wavelengths so that different portions of the optical spectra impinge on different cells. In this case, the cells are used more efficiently. Three methods can be used-dichroics, diffraction gratings and prisms. The tradeoff of each will be discussed.

The requirement of no moving parts dictates that toric optics be used. Also the shape of the cells should be rectangular with the longer axis in the east-west direction. There are significant tradeoffs that can be made on the size of the cell and the time of charging.

Finally, in order to reduce costs, it is necessary to use concentrating photovoltaics (CPV). The basic tenet behind CPV is that the cost per square meter of optics should be much lower than that of the cell. With GaAs cells at prices of greater than $50K per square meter, this assumptions seems reasonable. If the concentration (the ratio of the area of the optics to the area of the cells)is 50, than the effective cost is $1000 per square meter for GaAs cells. For a typical laptop that would imply a cost of the GaAs portion to be about $100. One of the disadvantages of CPV is that the field of view is no longer 180 degrees as it is with flat panel systems. As the concentration increases, the field of view decreases. At very high concentrations as would be used in power plants, tracking then becomes a requirement. Further in a location like Rochester, NY or Seattle which are known for cloudy days, CPV has limited usefulness.

This paper will report some of the tradeoffs of optical design of these systems.

Biography

Dr. Moore is the Rudolf and Hilda Kingslake Professor of Optical Engineering, Professor of Biomedical Engineering, and Professor of Business Administration at the University of Rochester. In 2007, he was also appointed Vice Provost for Entrepreneurship at the University. In this role, he manages the Kauffman Campus Initiative ($10.6M over 5 years). From 2002 until 2004, he served as the President and Chief Executive Officer of the Infotonics Technology Center. From 1995 to 1997, Dr. Moore was Dean of Engineering and Applied Sciences at the University, and in 1996 he also served as President of the Optical Society of America.

The U.S. Senate confirmed Dr. Moore in the fall of 1997 as Associate Director for Technology in The White House Office of Science and Technology Policy (OSTP). In this position, which ended December 2000, he worked with Dr. Neal Lane, President Clinton's Science Advisor, to advise the President on U.S. technology policy.

Dr. Moore has extensive experience in the academic, research, business, and governmental arenas of science and technology. He is an expert in gradient-index optics, solar cell design, computer-aided design, and the manufacture of optical systems. He is also the founder and former president of Gradient Lens Corporation of Rochester, NY, a company that manufactures the Hawkeye boroscope.

Dr. Moore holds master's and Ph.D. degrees in optics from the University of Rochester, and a bachelor's degree in physics from the University of Maine.