This article is part of our Innovation in America series, in which Foolish writers highlight examples of innovation going on today and what they see coming in the future.

If we look back at innovation in electrical energy, it has been progressing at a snail's pace for the past 130 years. The first coal power plant was built by Thomas Edison in 1882, and subsequent generations of coal plants still provide a large chunk of America's power. Nuclear power has proven to be expensive, hydro and wind can only provide so much to the grid, and the new leader, natural gas, is the best thing we've come up with so far -- not exactly a giant leap forward since Edison.

The holy grail of energy has always been our most abundant energy source: the sun. In less than one hour, enough solar energy hits the earth to power the world for an entire year. To put it another way, a square array of today's solar modules 300 miles on each side would generate more than enough energy for everyone in the U.S. I've even put together calculations here that show we could generate more than double our power needs from the Mojave Desert alone using high-efficiency modules.  

These are claims that other innovative renewable power sources can't claim; the physics simply won't allow it. There isn't enough hydroelectric, geothermal, biomass, or wind power to power the world. (Fellow Fool Sean Williams touched on these sources yesterday.) If innovation is going to have more than a marginal impact on our energy future, it will come from solar -- period.

So what are the main solar technologies, and what kind of innovations can we expect in coming years? I'll go over three technologies to watch below.

Crystalline silicon-based solar
Crystalline silicon solar cells power a vast majority of the solar modules in the world, and they have for decades. The process of making silicon-based modules starts by essentially melting high-grade silicon and forming it into an ingot or a block. GT Advanced Technology (Nasdaq: GTAT) is one of the main suppliers of polysilicon and photovoltaic, or PV, equipment to the industry, and its technology will be one of the drivers of increasing efficiency in the future. From here the output is cut into wafers, converted into an individual cell, and assembled into a complete module.

It isn't the sexiest technology, and people have been betting a better technology would emerge, but as someone said at the GTM Solar Summit earlier this year, "No one has bet against silicon and won."

There are a range of costs and efficiencies, but three general products are currently made. Multi-crystalline modules are lower-cost and lower-efficiency than their purer mono-crystalline brethren. Cell efficiency for mono cells has reached 21.1% in testing at Canadian Solar (Nasdaq: CSIQ), and it seems a new record is made every week. Modules made from this technology are typically around 15%, but they're approaching 17% at some manufacturers.

The other type of cell is being made by SunPower (Nasdaq: SPWR), which uses a back-contact cell and has reached 24% efficiency in production cells. It's making modules that exceed 20% efficiency.

Incremental improvements in efficiency have been a staple in crystalline silicon solar for a decade, but GT Advanced Technologies has a HiCz technology it says can increase cell efficiencies from 19% to 24% at similar costs to current production. If there's an innovation breakthrough coming in the next year or two to silicon-based modules, that may be it.

More than likely, incremental steps will be made to improve efficiency in crystalline silicon solar modules, but the real innovation will come on the cost side. Manufacturers are becoming more efficient, and costs are dropping quickly. Those who can stay ahead of the curve will survive; those who fall behind will be swept away.

Thin-film solar
Five years ago, thin-film solar was supposed to be the future. First Solar (Nasdaq: FSLR) was crushing everyone in the market with low costs, and new technology showed the promise of improved efficiency. Today, First Solar is floundering under declining margins and closing a major plant in Germany, and its CdTe technology hasn't come close to matching the efficiency of crystalline silicon competitors.

The new technologies that were supposed to change the face of solar also have yet to emerge. CIGS technology -- that's copper indium gallium selenide, if you're interested -- was supposed to be a game-changer, allowing lower costs, higher efficiency, and even flexible solar modules. But CIGS has proven harder than advertised to make, especially at costs that compete with crystalline modules. Amorphous silicon was also an emerging technology, but after Energy Conversion Devices went bankrupt, the industry saw that costs and efficiencies couldn't compete yet.

Hope isn't gone for thin-film solar, and 3M (NYSE: MMM) just completed a manufacturing expansion to make Ultra Barrier Solar Film -- a supposed replacement for glass in solar modules -- and allow a roll-to-roll manufacturing process.

If there is to be an eye-popping innovation made in solar over the next few years, it will probably be in thin-film, and continuing process capabilities could help dramatically lower costs. The question is whether it will be enough to compete with crystalline silicon modules, which are now selling at under $1 per watt. I wouldn't bet against it -- not yet. There are thousands of people toiling in labs around the world, trying to figure out a way to increase thin-film efficiency and lower costs. CIGS has supposedly made progress, and startups are still getting funding, so the venture community still has hope.

This is where the excitement will be in the next five years from an innovation standpoint, even if thin-film solar doesn't eventually win the battle with silicon-based cells.

Concentrated solar
One of the major problems with solar is the intermittent power it provides to the grid. A solution the industry has come up with is concentrated solar, a process that bounces light from mirrors to a power tower or tube with a liquid in it which then runs to a turbine and creates electricity.

This is currently much more costly per kW-hr than other solar technologies, but the advantage is energy storage. The heated material can be used to heat molten salt or other mediums that could provide power 24 hours a day.

Concentrated solar was the hot technology before the price of PV modules plunged, and cost has now become a major issue. Plants that were once designated as concentrated solar were converted to PV, a real sign of where the technology currently stands. This is the one technology that would be great for the grid, but innovation will need to take costs significantly lower to truly compete in solar.

A huge opportunity
Like I said at the start, there is no bigger opportunity in energy than solar. Innovation has happened at an extremely rapid pace over the last decade, resulting in higher efficiency and lower costs. If I have to bet on one technology winner, I'm sticking with crystalline silicon as the biggest player for the foreseeable future. Progress in this technology may not be fast, but it's steady and proven -- something the solar industry can't have enough of.

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Read more about innovation today and its future in America; head back to the series intro for links to the entire series.

Fool contributor Travis Hoium owns shares of SunPower in personal and managed accounts. You can follow Travis on Twitter at @FlushDrawFool, check out his personal stock holdings or follow his CAPS picks at TMFFlushDraw.

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