The Greener Truth

I apologize for not posting recently.  As many of you may not know, I am a graduate student and I am defending my thesis in two weeks.  This has meant that work has taken precedence over the more interesting task of blogging.  Until I have more time, here is the beginning of the introduction to my dissertation that I thought would be interesting to the majority of you.  Hope you get something out of it, and in a few weeks, I will try to get back to a regular blogging schedule.

The Energy Crisis

Coal Power Plant

The motivation behind the search for cost effective, carbon neutral, renewable energy sources is readily apparent in all areas of our daily life.  Since the industrial revolution, the human need for high quality, inexpensive energy has driven both exploration and technology.  For most of that time, that need was filled by fossil fuels, which today comprise 85% of the U.S. energy supply.  Since their rise to power in the industrial revolution, fossil fuels have always been readily available and inexpensive.  The problem with fossil fuels that the cost of mining and distributing coal and oil does not take into account is their cost to the environment and their finite availability.  Because of these two missing factors, fossil fuels are sold for considerably less than their true costs, on a larger scale, meaning that other energy sources are not able to compete.  This has meant that for the last two hundred years, fossil fuels have had a monopoly on the energy market, while slowly changing the climate of our planet.  We have now come to expect an unlimited supply of relatively inexpensive energy, and therefore, any replacement must be cost effective, easily transportable and widely available.

Power of the Sun

Based on these criteria, there are a limited number of sources that are capable of providing the energy needed, and none that can do so for the same price that oil, coal and gas are sold for today.   The most obvious untapped energy source is harnessing the power from the sun.  More energy strikes the earth, in the form of sunlight, in a single hour (4.3 x 10²⁰ J) than humans consumed on the entire planet in 2001 (4.3 x 10²⁰ J). The current technology for collecting this energy is insufficient.  While fossil-fuel-derived energy can be produced for as little as $0.02-0.05 (kW-hr)^-1, the average cost of solar cells, based on a twenty year lifetime, are in the range of $0.21-0.46 (kW-hr)^-1 for a commercial utility scale power plant. Due to the large discrepancy between the costs of these two energy sources, incremental improvements to the cost of solar cells are not enough, and instead, new revolutionary ideas must be implemented that are not only more cost effective, but also more efficient.  This is the area of solar research categorized as third generation.

Solar Cells: Generating Electricity from Light

Thanks to Tree Hugger and the Pope

Solar cells are designed to absorb photons from sunlight and convert that energy into electricity by taking advantage of the photovoltaic effect and separating an electron and hole across the bandgap of the semiconductor.   If a photon does not have enough energy to excite an electron across the gap, it will either be transmitted or scattered.  On the other hand, if the photon has more energy than the bandgap of the semiconductor, the excess kinetic energy is lost to heat as the high-energy electron and hole cool to the bandedge.  Once the electron is separated from the hole, a built in asymmetry in the device (usually created with a junction of electrically different materials) causes the electrons to flow through an external circuit where they can be used to do electrical work.

The delicate balance of collecting the most photons possible while keeping the highest voltage based on the solar spectrum creates an upper limit to the theoretical efficiency of a traditional, single bandgap, terrestrial solar cell.  This detailed balance calculation, originally defined in 1961, leads to the Shockley-Queisser limit of 30-33% for solar cells with bandgaps in the range of 1-1.6 eV. While some of the energy loss is due to the transmission of low energy photons, the majority of the lost energy (almost 50%) is due to heat created as the high energy charge carriers cool to the bandgap.  By accessing this lost energy, the theoretical efficiency of a solar cell increases, and the goal of third generation solar is realized.