A very good overview for renewables/alternatives and their related challenges. I would like to add a few bits that appear to have been overlooked. Certainly not the fault of the researcher as it is a very broad subject area:

1. Solar generation here is primarily broken down into passive solar (solar architecture) and photovoltaic. When taken in total, it's actually a small picture of all of solar energy. On the one hand, solar architecture could be considered an added efficiency as it generates no net energy in itself but reduces or eliminates the need for outside heating and cooling inputs.

a. Solar photovoltaic itself can be broken down into numerous segments.
1a. Traditional photovoltaic energy using silicon.
2a. Thin film photovoltaic energy using silicon.
3a. Thin film photovoltaic using other common materials.
4a. Thin film photovoltaic using nanotechnology materials.

GALLIUM NOT USED IN SILICON CELLS
It's important to note that gallium is NOT USED in silicon solar generation. Gallium is only used as part of second generation thin film photovoltaic technology. So far, only silicon production has proven to be a limiting factor in photovoltaic production. As there has now been a massive overbuild in silicon production to support the solar industry, this is not likely to be a problem for at least the next few years.

DIVERSE MATERIALS BASE
Cadmium, telluride, and carbon nanotubes are all substances that have been used for solar energy generation. In the realm of photovoltaics alone, the materials base is quite diverse and options are continuing to expand.

2. CONCENTRATED SOLAR POWER (CSP) is not addressed in the above article. CSP is a form of electricity generation that uses mirrors and lenses to reflect and concentrate solar radiation onto boilers, towers, or specially designed heat engines that collect the energy and, through mechanical process, turn it into electricity. Though not as large a production base as photovoltaics and primarily useful only in very sunny parts of the world, very large CSP power plants have been build and are under construction in areas like Arizona, California, and Spain.

MATERIALS FOR CONCENTRATED SOLAR POWER are common and do not require exotic inputs.

3. STORAGE FOR SOLAR POWER ALSO INCLUDES MOLTEN SALT and non-lead acid battery storage. An ongoing revolution in battery technology is providing a number of high capacity, high cycle, batteries for energy storage and for the automobile industry. Ferrus Iron batteries made by BYD systems for cell phones can cycle more than 3,000 times and offer a much higher charge density than lead acid batteries. Lithium Ion batteries with carbon nanotube storage are also being designed for the Chevy Volt and are now in use in Prius after market plug in electric upgrades. These batteries, designed by A123 systems, can cycle over 8,000 times and have an even higher charge density than BYD's offering.

RAPIDLY GROWING, BUT SMALL, PRODUCTION CAPACITY FOR SOLAR ENERGY.
The current 'nameplate' production capacity for the world's solar industry is about 13 Gigawatts -- including CSP but not including efficiency designs like solar architecture. If the entire 2008 market were to be utilized on a year on year basis, it would take 1000 years to build enough generation capacity to power the entire planet. That said, 2009 production capacity is expected to reach 18 Gigawatts, and 2010 capacity is expected to hit 24 Gigawatts. This approximate doubling every few years will have a massive effect come around 2015-2020 where new solar energy builds could represent 2-3 percent of world capacity EACH YEAR if the current rate of expansion is maintained.

CURRENT SOLAR ENERGY GENERATION CAPACITY IN THE UNITED STATES IS 3.8 GW. This represents enough energy to power 2.4 million households or about 2% total electricity demand. Figures on solar energy production have lagged while the industry has surged. Average growth rate in the US alone is 48% year on year since 2002. At the current rate of growth, solar energy will represent 10%+ of total US energy use within ten years.

CHALLENGES
Materials supply chains need to be built and expanded to support solar infrastructure. New materials will be needed to increase grid capacity as loading increases from new solar systems. New energy regimes will have to be established as states begin to share or build capacity across borders. For example, Virginia is currently building wind generation capacity out of state for in state use. They are paying to have the electricity transported via grid but this is still less expensive and politically troubling than building a massive number of new coal plants. I think, in the future, power sharing arrangements will also be made with sunny states and states that lack solar generation resources. Solar energy generated in Spain, for example, could help keep the lights on in other parts of Europe. As transportation moves increasingly to grid support, you will have to have a considerable overbuild in multiple generating areas -- solar, wind, nuclear + other. Storage will add some costs but result in net energy costs much lower than those for current transportation systems. In the end we could have a much better and more democratic energy system than the one we started with. But getting to that point is going to take a lot of ingenuity, resolve, and creativity. Greed will not get us there and we may well have to allocate FF resources to build the new infrastructure while rationing its use by consumers. In an orderly society this is certainly possible. But in the absence of salient leadership, things can break down very fast.

My additions on solar are not meant to divert from wind, nuclear or other energy sources. It just seems I had more to add in this area.

http://www.nextenergynews.com/news1/next-energy-news12.28d.html
http://en.wikipedia.org/wiki/Solar_cell
http://www1.eere.energy.gov/solar/csp.html
http://www.sciam.com/article.cfm?id=solar-power-lightens-up-with-thin-fi...
http://www.wired.com/science/planetearth/news/2005/11/69528
http://www.salon.com/news/feature/2008/04/14/solar_electric_thermal/
http://solar-in-china.blogspot.com/2007/12/qiangsheng-to-invest-400-mill...