|Fig. 1: Solar cell production over the past ten years by region.|
When talking about new sources of energy, the buzz word that makes its way around to conversations seems to always be "solar". This comes to no surprise when one realizes that the capacity of the photovoltaic market (that is the yearly megawatt contribution from solar panels) has experienced an average growth of 37% during the past 10 years.  In 2010 alone, 17.5GW of installations exceeded the 2009 installations of 7.5GW by 141%. 
The rapid market growth has inspired many companies to invest in different kinds of solar technologies. Currently, there are two well-established ways of converting solar energy into power. The first is known as photovoltaic (PV), in which incident sunlight is converted directly to electricity. The second approach is known as concentrated solar power (CSP), and it uses mirrors to concentrate sunlight and take advantage of the thermal energy. The thermal energy is most commonly used to drive heat cycles or to heat up liquids which can retain the energy for longer periods of time. Finally there is a third technology called Photon-Enhanced Thermionic Emission (PETE), which combines the power of PVs and CSPs to increase the overall efficiency of conversion. This approach however, is still in the research stage.
The PV approach is the most actively researched at the moment, as different materials might provide widely different results in terms of cell cost and efficiency. Due to cost-related issues, most research is based on single-junction devices, which have a theoretical limit of 32%.  The total intensity of the AM1.5 solar spectrum is very close to 1000 W/m2, which means that with single-junction devices, we can hope for a maximum of roughly 320 W/m2.  The current price for silicon based solar cells is about $1.40/Watt, which is very close to becoming competitive in the energy business. 
Concentrated solar power has been around for a longer time as a utility-scale solution to power generation. For example, the Solar One project (with a maximum rate of 42 MW) in Daggett, CA was completed in 1981. Although this was built as a proof-of-concept, it remained operational until 1986. This approach inspires research on the concentrator technology, specifically in the field of nonimaging optics.
A third technology, known as PETE (Photon-Enhanced Thermionic Emission) promises to surmount the barrier of efficiency that haunts the two previously described technologies. Using a standard PV conversion efficiency of 22% along with a thermal-to-electricity efficiency of 31.5%, the total conversion efficiency could exceed 53%.  As aforementioned however, this technology is still in the early stages of research.
Due to availability of data, only numbers for PV and CSP technologies are presented here. It is estimated that the world-wide power available using PV technology is of 6500 TW. Placing various restrictions on this number and taking into account PV systems in likely-developable locations, the power becomes 340 TW. The number for CSP, in contrast is of 240 TW. In order to provide some grounds for comparison, we can consider the number provided for wind power, which is around 40-85 TW. 
Currently, the PV market is a rapidly growing one; in 2010 alone, 17 GW of capacity was added worldwide, bringing the total capacity up to about 40GW.  In 2009, 604MW of CSP plant capacity was in operation globally, 761MW were in construction and 5780MW were in the planning phase. 
Solar power is a very attractive source of energy for many reasons. For now, the biggest factor in choosing one type of technology over the other might be price as PV is a more expensive than CSP. However, as PV prices drop due to the economies of scale and the advancement of research, this factor might become less important.
It should be noted that these technologies are not cheap enough to compete with other sources of energy as it stands. Breakthroughs in PV technology and manufacturing processes promise competitiveness in the years to come. Different routes, like PETE for example, might also offer improvements in the cost of converting solar energy to electrical power.
© Alvaro Aguilar. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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