As a youth, I remember participating in a German New Year's tradition called das Bleigiessen or "lead pouring." In this activity, you melt a small amount of lead in a tablespoon, drop it into a bowl of water, and divine your future based on the shape into which the lead solidifies.

The fact that I played with lead as a child may explain a lot. Nevertheless, I still enjoy a good forecast. Topping my recommended reading list this year is George Friedman's The Next 100 Years. (Space-based solar by 2080? Yeah, I guess I can see that happening.)

While looking slightly less far ahead, the energy industry is still taking a much longer view than most year-end prognosticators. We've recently seen some reports with highly divergent views on our possible energy future, and I thought I would share a few highlights.

Farewell, fossil fuels
In the November issue of Scientific American, two professors presented a plan to put the world on 100% renewables (i.e. wind, water, and solar) by 2030. This makes other recent studies on how to get the U.S. to 20% wind or 10%-20% solar within the same time frame look pretty unambitious. More to the point, those other plans make this one pretty hard to take seriously.

The authors call for 3.8 million 5-megawatt (MW) turbines, 1.7 billion rooftop photovoltaic systems, and 40,000 300MW photovoltaic power plants, among other installations. I guess it's conceivable that the world could build and install this much equipment, but I don't think we would get much sleep.

Take those 3.8 million turbines. That averages out to 190,000 per year over two decades, and over 500 per day. A-Power Energy Generation Systems (NASDAQ:APWR) is only geared to crank out 300 2.7-megawatt turbines per year at its new plant in Shenyang, China. Siemens (NYSE:SI), which has an actual track record in this area, has delivered fewer than 8,000 wind turbines of all sizes since 1980.

There are obvious material and manpower constraints involved here. Environmental opposition to utility-scale PV plants in places like California also appears formidable, as First Solar (NASDAQ:FSLR) is discovering. But let's get right to the punch line, which is the price tag: $100 trillion in construction costs, not including transmission.

I appreciate the spirit of this plan, but with several of the world's leading economies already leveraged to the hilt (i.e. America, the UK, and Japan), such a level of spending seems deeply implausible. A World War II-style mobilization of the global economy to green our energy infrastructure is an appealing analogy, but developed nations didn't then face the staggering fiscal burden of supporting an aging population as they do now.

A multi-dimensional approach
It may surprise some readers that the theme of ExxonMobil's (NYSE:XOM) Outlook for Energy: A View to 2030 is not "Drill, baby, drill!" The company's vision, as spelled out in a report earlier this month, is far more nuanced than that.

An obvious point is that world energy demand is going to grow over the 2005-2030 period in question. Also unsurprising is that this demand will be driven by non-OECD countries, whose energy usage will rise by some 65%. But Exxon sees OECD energy demand actually dropping slightly over this period.

How can this be, with GDP growth in the OECD countries projected at 50%? Efficiency gains are the key. Exxon calls these "the greatest source of energy in the future." (Take that, SunPower (NASDAQ:SPWRA)). I tend to agree on this point, and would love to bring an energy efficiency play into my portfolio. WaterFurnace Renewable Energy (Indiana-based but traded on the Toronto exchange) makes a killing on ground source heat pumps, and is one stock on my watch list.

Exxon is realistic about coal, which will not be dethroned as the top source of electric generation any time soon. But the company also sees nuclear and renewables like wind generating 40% of the world's electricity by 2030. Utilities like FPL Group (NYSE:FPL) will see to that. These generation sources benefit strongly from a $60 per ton carbon price, which Exxon has baked into its outlook. The company is unimpressed with the economics of both carbon capture and storage (CCS) and solar over this period, however.

As for natural gas, Exxon sees a very bright future, as underlined by its recent purchase of XTO Energy (NYSE:XTO). Gas will be the fastest-growing hydrocarbon, with global demand rising 55%, or 1.8% per year (compared to oil at just 0.8%/year). Over half of U.S. natural gas demand will be met by shale and other "unconventional" sources by the end of the period.

In 2030, Exxon sees fossil fuels continuing to dominate the energy mix, at around 80%. If you accept that crucial premise, then you'll see why oil and gas continue to command my attention, and solar power, even with its explosive growth, remains a sideshow.

First Solar is a Rule Breakers pick. See what other stocks our advisors see taking the future by storm with a free 30-day trial.

Fool contributor Toby Shute doesn't have a position in any company mentioned. Check out his CAPS profile or follow his articles using Twitter or RSS. The Motley Fool owns shares of XTO and FPL Group. The Fool has a disclosure policy.