It's pretty obvious that the Earth gets a lot of sunlight. The solar energy received by the Earth in 40 minutes is equivalent to world energy use for a year. So why aren't we using more of this bountiful and free resource? Of course there are many reasons, including the fact that it's dark at night, skies are often cloudy, and the technology to convert solar energy to electricity is still relatively expensive and inefficient.
There are solutions to these problems, for example, orbiting (probably GEO) solar power satellites (SPS) could solve the night and cloud problems, converting constant sunlight to electricity which would be transmitted in concentrated microwave beams down to arrays of antennas on Earth for conversion back to electricity. We could even eventually use lunar materials for SPS construction outside of Earth's deep gravity well. I'm a big fan of space development, but it's clear that there are a lot of problems to solve before we can put huge SPS systems into space. The heavy lift infrastructure just isn't there yet. Could space elevators help? Sure, but even if they turn out to be feasible, development and deployment are certainly years away. Isn't there something we can do with the sunlight reaching the Earth's surface, using what we already know how to do?
According to this fascinating article in the January 2008 Scientific American, there is. The "grand solar plan" provides specific details on how we could build a practical solar energy infrastructure based on known technologies with modest and expected improvements over the next 40 years. According to the authors, this plan could provide 69% of the U.S.'s electricity and 35% of its total energy by 2050. It would require construction of very large ground-based solar arrays in the American southwest and utilize compressed air storage in underground caverns to store energy for use at night. It would also require building long distance HVDC (high voltage direct current) power lines to distribute the power efficiently from the southwest to other regions.
This plan would require the federal government to invest more than $400 billion over the next 40 years to complete the plan, but this investment would have many benefits including greatly reduced dependence on foreign oil and gas supplies. Developing the political will to follow such a plan is perhaps the biggest hurdle, but the authors point out that the annual expense would be less than the current U.S. Farm Price Support Program. The Iraq War has already cost us more in dollars alone.
There's much more to it than this, and I urge you to read the full article and look at the accompanying graphics and diagrams. It's a well thought-out plan, and while I'm sure there are pitfalls and complications beyond those discussed in the article, it certainly seems feasible. It's not a Manhattan Project or an Apollo that depends on numerous technology breakthroughs to succeed. It's really an infrastructure development project that builds on existing technologies and industries and makes wonderful use of one of the few things that are free in this world: sunlight!