As we step further into the 21st century, it’s clear that one of the defining challenges facing humanity is the need to wean ourselves from an unsustainable addiction to fossil fuels. With glaciers receding like my hairline, and energy prices straining wallets around the globe, the world may finally be ready to accelerate the wholesale adoption of alternative energy sources.

If the wave of recent research and discussion on this topic within Motley Fool CAPS is any indication, Fools are not only embracing the idea, but are positioning themselves to profit from emerging solutions. With so many technologies in various stages of development, the CAPS blogosphere is a great tool to help investors decide which of them may have the best prospects for gaining traction.

The No. 3 player in all of CAPS, rudolphsteiner, posted a terrific primer on alternative energy last week. The technical knowledge exhibited in his post, and in the comments that follow it, is illustrative of the collective expertise available within the CAPS community.

Solar thermal energy
CAPS All-Star binv271828 made alternative energy the focus of his first-ever CAPS blog post, crafting a far-reaching narrative that sparked a flood of constructive discussion. His personal pick for the most promising technology is solar thermal:

Solar Thermal is such an awesome technology, because it is so beautifully simple. You have a [lot] of parabolic mirrors (nothing fancy or special about the mirrors, very low tech) sitting in the desert to focus the heat up to a collector. The fluid in the collector is moved by the thermal gradient, this spins a turbine which turns a shaft, and voila! electricity.

While solar thermal is certainly an exciting concept, the industry has not yet provided retail investors with opportunities to get involved. In the photovoltaic industry, however, Fools have a daunting array of options.

More color on solar
In his recent Hitchhiker’s Guide to solar technology, CAPS All-Star AnomaLee rated a number of well-known solar players based on their prospects for competing in the technology race toward a sun-powered future. Let’s look at a few of these.

First Solar (NASDAQ:FSLR):

... If First Solar had a 50 multiple as of today I would say that it's a reasonable buy and hold, but with a P/E above 120 for a $20 Billion company? They're selling solar cells-not the cure for cancer. Although their semiconducting materials(CdTe) do not convert as effeciently as traditional "First Generation" silicon solar cells or as efficient as the CIGS thin film technology they are currently the most cost effective.

Fools with a taste for scorching growth from smaller companies may wish to focus on a mid-cap name like Michigan’s own ...

Energy Conversion Devices (NASDAQ:ENER):

... Sales have nearly tripled. Great technology. They manufacture a lightweight, flexible solar cell that can literally be rolled ... Their production is relatively cost effective but they suffer from generally poor conversion rates compared to other technology available ...

If those forward P/E ratios -- 46 for First Solar and 50 for Energy Conversion -- are outside your comfort zone, the following company combines the smaller market cap of $1.2 billion with a more earthbound 2009 P/E of 20.

Evergreen Solar (NASDAQ:ESLR):

... String Ribbon Technology is the only way "First Generation" technology production can compete in the future, and Evergreen solar is the market leader. While energy conversion is lower the savings in production costs are substantial and more economical ...

ETFs for diversified exposure
Fools looking to branch out from sun-based solutions might consider a stake in the Market Vectors Global Alternative Energy ETF (NYSE:GEX). Looking back to binv271828’s blog post, he shares why he prefers GEX over the Power Shares WilderHill Clean Energy ETF (AMEX:PBW):

... I think it is a little more diversified across these technologies. And its top holding is Vestas Wind System (which I am a huge fan on). So if you want to make a play for Vestas and get some alternative energy exposure, I would recommend GEX ...

While ETFs may be a relatively safe way to break into the sector, I personally would suggest that Fools instead use CAPS to identify one favorite company as their speculative horse in the alternative-energy race.

For further Foolishness: