Across the U.S., dozens of proposed solar, wind, and battery projectsencompassing thousands of gigawatts of potential powerare backlogged as they wait to be allowed to plug into the power grid. And, even in areas where renewable energy projects are already online, their output is often heavily curtailed. This clean energy bottleneck stems from the fact that, as demand for renewable energy rises, the U.S. isnt building new transmission lines fast enough to transport large amounts of clean energy from point A to point B.Now, theres a company looking to address that problem with a simple yet radical solution: Putting renewable energy into giant batteries and transporting those batteries by train.SunTrain is a San Francisco-based company founded by green energy developer Christopher Smith, who now serves as the companys president and chief technology officer. The idea, he explains, is to use the existing U.S. freight train systemwhich covers around 140,000 miles of terrainto bring renewable energy thats being curtailed by transmission bottlenecks to the areas that need it most.SunTrain is currently working on a pilot project that would run between Pueblo, Colorado, and Denver. If its approved by regulators, Smith says, he expects the pilot could be off the ground in just two years.[Photo: SunTrain]Current challenges to transporting renewablesClean energy is the fastest-growing source of electricity in the U.S. According to a report from American Clean power, 93% of the new energy capacity last year was solar, wind, and battery storage. The issue, Smith says, is that transmission line infrastructure lags far behind the rate of clean energy growth.The United States needs 300,000 miles of new transmission lines, like, right now. Thats the amount that we need immediately to keep up with current demand, Smith says. Its also estimated that, to reach 100% renewables, as well as the electrification demand that well have by 2050, well need over a million miles of new transmission lines by 2050. Currently, were building less than 1,000 miles a year.[Image: SunTrain]Building new transmission lines is challenging for a number of reasons, including environmental regulations, the time it takes, and the fact that any new lines would have to cross thousands of miles of privately owned land.In Colorado, for example, Smith says theres a lot of renewable energy in the states southeast corner, which flows through the grid to Pueblo. However, because theres not enough transmission line capacity between Pueblo and Denver, much of that power cant ultimately be used.Once that renewable energy gets to Pueblo, theres not enough transmission line capacity to get it into downtown Denver, Smith says. So that energy basically gets curtaileda fancy word for being wasted.Until now, the costly construction of new transmission lines has been the main solution thats available. But Smith says this discussion overlooks a resource thats been used to transport energy for almost 200 years: railroads.The freight railroad network already moves virtually every single form of energy known to man thats used in a real way: natural gas, coal, oil, ethanol, biomass, spent nuclear waste, various fossil fuels, and the list goes on and on, Smith says. So theres this huge amount of overlap of our great railroad network and our electrical systems already. There is no reason why we cannot be moving battery trains over the freight rail network like we move every other form of energy.[Photo: SunTrain]SunTrains solutionFor its pilot project, SunTrain is partnering with Xcel Energy, Colorados largest electric utility. Xcel owns a coal plant in Pueblo (Comanche Generating Station) and a natural gas plant in Denver (Cherokee Generating Station) that are both set to be decommissioned within the next several years. Through a collaboration with SunTrain, these plants could potentially be re-powered with battery stored energy.Smith says SunTrain would use the existing substation inside Comanche Generating Stationwhich already has extensive railroad infrastructure from its history as a coal plantto charge its batteries from Pueblos bottlenecked grid. During the day, Smith says, the energy will likely be 100% renewable. Then, the batteries would be transported to Denver and the energy offloaded at the Cherokee Generating Station onto Denvers grid. (Charging and discharging take between four and six hours each, and the 139-mile trip from Pueblo to Denver takes about five hours by train.)A substation can turn energy into a format that can cover long distances without losing much energy, Smith says. A substation can also turn electricity generated from a power plant into a format that can be used by local homes and businesses. For SunTrains purposes, the Pueblo substation allows us to get the electricity formatted properly for our batteries while also collectively accessing all the various renewable energy generators in the region.[Photo: SunTrain]SunTrains proposed railcars will be made of 20-foot shipping containers, each of which will hold about 40 tons of batteries. The company designed proprietary charging and discharging systems that allow the energy to flow right from where its generated, whether its a solar array or a substation, right under the batteries on the railcar, Smith says. Then, once the train arrives at its destination, the discharging system would similarly allow the energy to flow right off the batteries. The whole process is designed so that the batteries never actually need to be removed from the train.[Image: SunTrain]In an interview with the podcast In the Noco, Smith said SunTrains first generation railcars are designed to match the freight railroads existing standards for coal trains, to ensure that the system itself doesnt need to change anything in order for SunTrain to come to market. Based on those parameters, each train will be built at between 8,000 and 9,000 feet long, with the capacity to carry around two gigawatt hours of power in total. Thats enough to power a city of 100,000 for a full day.Smith says the team has already tested a proof-of-concept train on several trips amounting to more than 10,000 miles on the Union Pacific network, traveling from SunTrains San Francisco testbed to discharge locations across California, Nevada, and Colorado. Now, the company is waiting for Colorados Public Utilities Commission to approve Xcels expenditure of about $125 million to begin construction on the pilot project.We tested the technology, the feasibility, made sure the mechanical standards were there, Smith says. Our manufacturing partners can deliver entire unit trains of thesemeaning 200 rail cars of batteries that could carry about 1.75 gigawatt hours of energy. So this isnt something thats far away, coming in the pipeline, or needing some kind of technological breakthrough. This is an immediately executable idea. It just needs the capital.