The boom in electric vehicles has brought a lot of attention to batteries for applications like energy storage and transportation. But it's not the only energy storage medium that's growing at a rapid rate. 

According to the U.S. Energy Information Administration (EIA), the fuel cell industry nearly doubled in 2014 to $2.2 billion with more than 50,000 fuel cells shipped. Here's what you need to know about this important industry. 

What are fuel cells?
Here's how the EIA describes fuel cells: 

Fuel cells are devices that electrochemically combine hydrogen and oxygen to produce electricity, water, and heat. Unlike batteries, fuel cells continuously generate electricity as long as a source of fuel is supplied. Fuel cells do not burn fuel, making the process quiet, pollution-free, and two to three times more efficient than combustion. A fuel cell system can be a truly zero-emission source of electricity when hydrogen is produced from nonpolluting sources. 

In short, a fuel cell is an engine that consumes hydrogen to produce electric energy and water. The liquid coming from the tailpipe of a fuel cell vehicle is likely more pure than the water coming from your faucet. 

The reason fuel cells are worth watching for investors is the incredible market opportunity they present. There's almost no energy market that fuel cells couldn't disrupt, meaning this is potentially a multitrillion-dollar market. 

More markets love fuel cells
The biggest reason fuel cells have a higher ceiling than batteries is because they can run continuously. While a battery has to be charged and discharged, as long as a fuel cell doesn't run out of fuel, it could theoretically run forever. 

This is what Toyota (TM 3.24%) thinks will give it an advantage in refueling fuel cells over electric vehicles. The thesis behind investing in cars like the Toyota Mirai is that a five-minute stop to refuel will be more attractive than hours to charge an electric vehicle. And considering that a fuel cell creates electricity to power the vehicle, there doesn't need to be a drop-off in performance. That advantage grows when you move into other fuel cell applications. 

Plug Power (PLUG -8.08%) and Ballard Power Systems (BLDP -2.61%) have built their fuel cell growth on the back of forklifts in warehouses, which need to run 24/7. Forklifts have run on batteries traditionally, but it saves time and money to fill them with hydrogen fuel rather than charging a bank of batteries all day long. According to the EIA, more than 2,500 fuel cells for material handling vehicles were sold in 2014, and that figure will likely grow in coming years. 

The other major market fuel cells could disrupt is electric energy. Some buildings are starting to install fuel cells as a form of back-up electricity, but that could just be the start of the use of hydrogen in the electric industry. Europe is testing using electrolyzers to create hydrogen straight from renewable energy plants. From there, it can go into a hydrogen vehicle, or into the gas grid and stored for later use. 

More advantages of hydrogen over batteries
For vehicles or materials handling, the shorter refill times are an advantage over batteries, but in energy storage, hydrogen holds a massive advantage. Batteries are great at storing a limited amount of energy for minutes or hours, but they aren't good for storage on a massive level. 

With enough storage tanks, a hydrogen-producing station could produce clean hydrogen and store it for weeks or months at a time. This dynamic gives energy storage options to maximize value creation and potentially eliminate the intermittency challenge with renewable energy. 

The fuel cell market becoming a multibillion-dollar market means costs will likely start coming down, and eventually, energy storage using hydrogen could be an economical reality. 

Hydrogen has a big upside
Electric vehicles have been all the rage for a couple of years now, but when we look out decades, I think there's far greater upside for fuel cells. At just $2.2 billion in the U.S., it's still a small market, but it's growing rapidly, and with so many potential uses, fuel cells will likely lower costs and become more attractive in the multitrillion-dollar energy market. 

With the potential ability to disrupt transportation, energy storage, and back-up power, I think the sky is the limit for hydrogen, and it's time for investors to start paying attention to this opportunity.