Locomotives remain a fast and efficient way to move cargo overland. The low rolling resistance of steel-on-steel will always beat rubber on road, the ability to pull long chains of freight cars, linked closely together, gives trains a significant aerodynamic advantage as well. Not to mention the fact that the entire rail system is built around energy conservation, avoiding gradients where possible and conserving momentum by using the fewest possible stops.
But while some areas have largely managed to electrify their rail systems, others persist with diesel locomotives, and these are largely responsible for nearly half a percent of global greenhouse emissions – somewhere around 250 million tons of CO2 equivalent each year – according to OWID. This will need to be addressed in the race to net zero by 2050.
Fitting rail infrastructure with overhead lines might make sense in some areas, but in the US, where nearly all rail infrastructure is privately owned, distances are long, many areas are sparsely populated and the government has done little to encourage electrification, less than 1 percent of rail is electrified, and this kind of infrastructure seems like an impossibly difficult solution.
A new study in Nature Energy puts some numbers on an alternative: battery-electric locomotives that carry their own energy supply. These are already starting to pop up with initiatives like Wabtec's FLXDrive, the world's first 100 percent battery-powered locomotive, which completed a three-month trial earlier this year carrying 2.4 MWh of battery on board.
Since today's diesel locomotives typically run electric drive systems anyway, using the diesel engine as a generator, it's not difficult to convert them to DC battery power. You just need a big battery and the wiring to connect it to the drive system.
In the new study paper, the team proposes attaching dedicated battery boxcars behind the locomotive. Each boxcar, the team calculates, could pull as much as 14 MWh of energy storage in the form of long-cycle-life lithium ferrous phosphate batteries. You can run multiple locomotives, and/or multiple battery boxcars, to power a given train.
This study analyzes the energy use, practicalities and economics of a theoretical train operating in California, using four 3.3-megawatt locomotives pulling 100 boxcars and 6,806 tonnes of revenue-earning cargo, powered by a single battery boxcar.
It finds that this one large battery pack is enough to achieve a 241-km (150-mile) range, enough to meet the average distance between stops for US Class 1 freight trains. The weight of the battery car increases the train's energy use by about 5 percent, says the team, but the overall energy consumption of the train is about half what a typical diesel-electric train uses, thanks to the high efficiency of batteries and the ability to harvest some braking energy back into the system.
The battery cars could be charged at each stop during loading, unloading and crew changes, assuming a fast charger could be provided. This would take between 30-60 minutes using existing 2C charging technology. Alternatively, the battery car could potentially be swapped out for a fresh one – meaning little in the way of fancy infrastructure would be needed, and a spent battery could slowly charge up until it's next needed.
In terms of economics, the team calculated the total cost of ownership (TCO) for battery-electric and diesel-electric locomotives over 20 years, and found that battery-electrics should cost between US$6.47 to 8 million apiece, with the lion's share taken up by battery and charging infrastructure costs. Diesels will cost around $5.85 million each, the bulk of that taken up by fuel and maintenance.
But that's assuming there's no environmental damage costs – if these are levied "under the assumption of continued roll-out of the EPA Tier 4 rule," according to the researchers, this could take the TCO of a diesel locomotive up to $11.83 million.
Should the entire US rail sector electrify its fleets in this manner, the team points out that the country will effectively have some 200 gigawatt-hours of modular, mobile energy storage rolling around the country, portions of which can be dropped off to stricken areas suffering power loss from extreme weather events. This needn't cause the rail system to grind to a halt; the diesel engines are still on board, ready to go if needed. While connected to the grid for charging, they could also provide valuable fast-response load-balancing services, as well. Both of these could become revenue opportunities.
Obviously, there are other clean energy opportunities for rail as well, including hydrogen. A hydrogen fuel cell locomotive, like this one that debuted three years ago in Germany, could be rolled out at around half the cost of a battery train, say the researchers, since it could avoid the high early costs of batteries and the necessity to build charging infrastructure. But even under the most conservative assumptions, the TCO would end up about even with a battery train by 2050.
There is also the potential for diesel locomotives to be converted to run on ammonia, which would make the energy storage smaller and lighter than hydrogen or batteries. Ammonia would also simplify fueling logistics. On the other side of the coin, green ammonia will be more expensive than hydrogen fuel, and burning it will be less efficient than running hydrogen through a fuel cell. The study doesn't mention ammonia as an alternative or attempt to draw any price comparisons.
The study is open access in Nature Energy.
Source: Nature Energy
