Battery-electric vehicles, or BEVs, get a lot of credit for being "zero-emission" cars. However, that designation isn't entirely accurate. BEVs are only "zero-emission" if you disregard emissions from charging the battery and exclude manufacturing emissions. If you include those -- and you must, to factor in the overall climate impact -- BEVs, in large part, are worse for the climate than hybrids.

So what does that mean for the future of BEVs?

2011 F-CELL. Photo: media.mbusa.com. 

Science vs. what's popular
The push to find a "green car" is a worthy venture. Cars have a negative impact on the environment by producing a significant amount of greenhouse gas emissions. Right now, BEVs are seen as the next big thing, and on the surface, they're great. They don't use gas, thereby reducing our dependence on foreign oil. They can be charged with green energy -- where available. And when they're running, they don't produce emissions like a vehicle with an internal-combustion engine.

Unfortunately, if you look at the lifecycle of a BEV, you'll find some concerning facts. First, it's true that BEVs are only as green as what charges the battery. But that's only part of the picture, as battery manufacturing produces a 10,000- to 40,000-pound carbon debt, which, according to Climate Central, can only be overcome by charging the car on green energy, and driving it for tens to hundreds of thousands of miles. 

Second, according to the National Academies of Science, lithium-ion batteries are "the battery of choice for electric vehicles for the foreseeable future" because of technological difficulties facing other types of batteries. That's a problem, because lithium is an alkali metal and isn't naturally occurring. Consequently, it has to be refined after being extracted through salar brines (the most common approach) and mined hard rock.

Furthermore, the European Commission on Science for Environmental Policy states: "Although there is no immediate shortage of lithium, its continued use needs to be monitored, especially as lithium mining's toxicity and location in places of natural beauty can cause significant environmental, health, and social impacts."

What's the alternative?
Two possible solutions are to shift the entire grid into running on only green energy, or to find an alternative to batteries. There's already a push to use more green energy, such as solar, wind, and hydro power, which will definitely help cut down on CO2 emissions. But even if the grid did go entirely "green," it still wouldn't get rid of the initial carbon debt from batteries -- and there's still the issue of lithium.

The second solution -- finding an alternative to batteries -- has a number of promising options. Recently, the University of Colorado at Boulder developed a way to split water into its hydrogen and oxygen components using sunlight. That development is a major breakthrough for hydrogen, as it makes the process of obtaining the element much more efficient.

Bring on the hydrogen
The Department of Energy has estimated that the lifecycle emissions of future hydrogen fuel cell vehicles, or FCVs, powered by natural gas will be less than those of hybrid electric vehicles, or HEVs. Climate Central estimates that an HEV's lifecycle emission is less than a BEV. As such, it stands to reason that an FCV powered by hydrogen that's obtained through sunlight will have fewer lifecycle emissions than BEVs. More good news? A number of car manufacturers are actively pursing hydrogen fuel cell technology.

The National Renewable Energy Laboratory, or NREL, has teamed up with Toyota Motor (NYSE: TM) to test the Highlander fuel cell hybrid. Toyota also teamed up with BMW and announced that they'll unveil a new hydrogen fuel-cell car at the Tokyo auto show in November -- and it'll go on sale in 2015 with a price of less than $70,000. Meanwhile, Daimler AG's (NASDAQOTH: DDAIF) Mercedes-Benz and Ford (NYSE: F) have teamed up for a "practical approach" to fuel cells, and General Motors (NYSE: GM) and Honda Motor (NYSE: HMC) -- which have the most patents for fuel cells -- are working together to jump-start the hydrogen infrastructure. According to CNN, "virtually every major carmaker is preparing to launch hydrogen-powered models in the next few years," which means FCV may be a viable alternative to batteries sooner than you think.

A number of issues remain to be overcome with hydrogen, some of which are the lack of infrastructure, and the fact that hydrogen cells use a platinum catalyst -- making the fuel cell extremely expensive. However, technological advancements are proving that hydrogen has strong potential for future use.

Liquid air
Another strong alternative to batteries is Dearman Engine's piston engine that runs on liquid air. As such, its manufacturing emissions are nowhere near as high as a battery's manufacturing emissions. Even better, Toby Peters, Dearman's CEO, believes the engine could be made of plastic or resin because of the properties of liquid air. 

With proper insulation, liquid air could use existing infrastructures for distribution, and it could be stored in non-pressurized insulated tanks. As a bonus, energy stored as liquid air would allow "wrong time" energy produced by wind farms and other alternative sources to be stored for later use, instead of being wasted, and according to Green Car Congress: "A cryogenic engine such as the Dearman piston engine produces zero emissions at the point of use, has low greenhouse gas emissions provided the liquid air or nitrogen is produced from low carbon electricity, has energy and power density on a level with battery electric technology, and has the potential for rapid refueling."  

There are drawbacks to liquid air engines, and one is efficiency -- liquid air is 60% efficient with ambient and 70% with waste heat. However, the benefits outweigh the drawbacks. More pointedly, a proof-of-concept model has already been tested, and engineering company Ricardo, which built the McLaren racecar engine, has teamed up with Dearman and is building a commercial demonstration engine. 

The future of BEVs
Right now, BEVs are being touted as a "green car," and in relation to internal-combustion vehicles, they're a step in the right direction. But that doesn't mean they're the right step. Because of their battery, BEVs don't offer a significant reduction in greenhouse gas emissions -- which is a major point for "green" cars. So are BEVs doomed? Probably not. But it's unlikely they'll become the "green car of the future" without a significant improvement in battery manufacturing technology. Consequently, from an investing standpoint, there are better options for the green-car investor.

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Fool contributor Katie Spence has no position in any stocks mentioned. Follow her on Twitter: @TMFKSpenceThe Motley Fool recommends Ford and General Motors and owns shares of Ford. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.