Chapter VIII - Advanced Approaches-II

Abstract

This chapter focuses on materials concerning the use of concentrated sunlight as the photonic input to solar cells. It also considers the electrical energy output for solar cells operating under concentrated sunlight, the thermal energy. Thus, a complete systems approach to producing energy from photovoltaic cells is developed. This chapter also considers various approaches to further improve the overall energy output (both electrical and thermal) from photovoltaic systems. Most of these systems involve modifying the spectral characteristics of the light that is used to illuminate the solar cells. The efficiency is improved as the large number of generated hole-electron pairs saturates the recombination mechanism resulting in an increased photocurrent, increased even beyond that dictated by an increase in photon input. The cost of the solar system comes down because a significant area of expensive solar cells is replaced by lenses (or mirrors). In second generation solar systems the incoming photons are concentrated, but their spectral distribution is of a solar type. In both second and third generation solar cell systems, the input is solar energy in the form of photons. Unlike first generation solar cell systems, there is enough thermal energy available in these concentrated light systems to permit consideration of the delivery of at least a part of this thermal energy to some end user. Thus, the overall system efficiency is a mixture of both electrical and thermal components. One method for improving the performance of solar cells is to insure that only a limited range, the optimum wavelengths, actually reach the solar cells. To accomplish this spectrum splitting dichroic mirrors are employed. Another improvement technique involves constructing a series of solar cells, one layered upon the other. This is known as a tandem cell and the top solar cell has the widest energy gap.