An oil company recently took out a full-page ad in the New York Times pointing out that the sunlight hitting Earth in one second could provide humans with enough energy to last many years.
And that, in a nutshell, is the whole problem.
There's no efficient way to capture the energy of the sunlight, or to convert the energy into work. Instead what the sunlight does is increase entropy. That is, it turns liquid water into water vapor, or ice into water.
Entropy is a complicated concept with a technical meaning. To simplify, entropy is the amount of energy "lost" in a system doing work. For example, gasoline is burned in a cylinder to make a car move. Some of the energy is translated into motion (work) while much is released as heat.
The heat is absorbed by air, and other objects, and cannot be recaptured. Almost everything humans do increases entropy: generating and transmitting energy, extracting and burning fossil fuels, cooking, blowing things up, heating our homes, going shopping.
Another way of thinking about entropy is the tendency of system to evolve from order into disorder. Which brings us back to sunshine.
Some things on Earth progress from disorder to order. That list includes giving birth, making chocolate, growing watermelons, and evolution. All of these activities that seem to be making order from disorder, are doing so at the expense of the sun.
The sun, and other stars, ultimately provide the energy that drives everything, either directly or through creation of heavy elements like uranium, which are cast off in supernovae and incorporated into planets.
The furiously turns order into chaos as it converts hydrogen into helium, sending radiation in every direction. No matter how we harness sunlight, the disorder of the sun increases. In fact, the disorder of this entire universe is also decreasing, as new stars are born and burn out.
Burning fossil fuels, fissioning uranium, or finding alternate energy sources are our relatively puny way of interrupting the grand flow of entropy.
Current cosmology suggests that the universe will ultimately turn into a cold, dark empty place. But that won't happen for a long, long time.
Still, it's something to look forward to.