Natural gas is becoming increasingly popular as a source of electricity, and in many places—including Louisville—it’s replacing coal. But natural gas is also becoming more prevalent as fuel for vehicles.In Waste Management’sfacility off of Fern Valley Road, there’s a garbage truck parked after its daily route. On one side, so large it’s hard to miss, is a sign that says “Think Green, Think Clean”—a reference to the fact that this truck burns cleaner natural gas.Mike Fuller is the Senior District Manager for Waste Management. He’s showing off one of the 17 new natural-gas powered garbage trucks the company introduced in Louisville last month.“They burn much cleaner, not only from an air pollution, but also from a noise pollution standpoint,” he said. “They’re quiet. You still have the sound of the hydraulics, which you can’t minimize, but as far as the engine itself, it is night and day difference. It amazed me, the first time I heard it roll onto the lot, how quiet it was.”Waste Management has around ninety trucks on the road in Louisville every day. By the end of this year, nearly half of those will be powered by compressed natural gas, or CNG.And the trend in Kentucky isn’t limited to Waste Management, or even Louisville. CNG filling stations are planned across the state—in places like Somerset and Paducah. A company called Westport opened a facilityto manufacture fuel systems for Ford trucks that can run on conventional gasoline, or natural gas.“The natural gas market across Kentucky is mirroring what’s happening across the nation,” said Kentucky Clean Fuels Coalition director Melissa Howell.She says natural gas-powered vehicles were around in the 1990s, too. But a few things have changed since then: namely, the industry discovered the vast reserves of gas in the Marcellus Shale.Howell says back then, people and companies interested in natural gas powered vehicles generally couldn’t just get one off the line from an automaker.“Now that is the case. We have a lot to choose from with respect to vehicles. We have an abundant supply of natural gas. We have retailers that are very interested in natural gas. It’s about the economy. And it’s about what works on the bottom line.”Howell says the issue with so-called “alternative fuels”—natural gas included—is that if there’s nowhere to fill up, car manufacturers don’t make vehicles that can use the new fuel. And if car companies aren’t making vehicles, there’s no incentive to build filling stations.But she says it’s getting easier and easier to find a place to fill up a CNG vehicle. And that’s partly because of the price.“These advanced transportation technologies, whether it’s ethanol, biodiesel, electric, hybrid, there’s a niche for every one of these fuels and or technologies. And there is definitely a niche in the transportation sector for natural gas,” Howell said. “Specifically heavy duty fleets are ideal for natural gas. That business case can be made very quickly with respect to return on investment.”Waste Management built a public filling station on their property near the airport in Louisville. It’s open 24 hours, and looks pretty much like a regular gas pump. Except for the price: $2.39 for the equivalent of a gallon of gasoline. For companies that use big, gas-guzzling vehicles, those cost savings can add up.But for private citizens, the upfront costs of buying a CNG vehicle may not make it worthwhile. And the fuel does burn cleaner than diesel, but the use of natural gas in vehicles has been criticized due to the environmental damage caused by fracturing shale rock—or fracking—to extract the gas.Howell says Kentucky isn’t yet at a place where someone could buy a natural gas powered vehicle and easily find a place to fill up. But there are several projects in the works, and she expects interest to grow—especially among companies with large fleets.