Batteries, Supercapacitors, Hydrogen, Hydraulic Hybrids, Compressed Air...
Personal transportation is a big part of our lives, but unfortunately the current system pollutes the air and disrupts the climate. Walkable neighborhoods, bikeable cities, and fast public transit can greatly improve things, but cars will also probably stick around for the foreseeable future. Engineers and scientists are working on new ways to make the green cars of the future go (the main challenge is with power storage - power generation is also crucial, but it's a whole other article). Let's look at some of the most promising technologies.
T-shaped GM Volt lithium-ion battery pack made by LG Chem. Photo by Michael Graham Richard
Chemical batteries are the most obvious ways to store electrical power, which can then be used to move a car with one or many electric motors. A lot of progress has been made in the past few decades, from lead-acid batteries to NiMH to Lithium-ion, but that improvement has had a bigger real-world impact on the portable electronics industry than the transportation sector. The Tesla electric Roadster is probably the best-known vehicle to use lithium-ion batteries, and all the gasoline-electric hybrids made by major automakers use NiMH batteries (the new Toyota Prius, new Honda Insight, Ford Fusion hybrid, Nissan Altima hybrid, etc).
While making batteries certainly has a non-negligible environmental impact, it still seems to be a big improvement over burning thousands and thousands of gallons of fossil fuels over the life of one cars. Thankfully, it also seems like the new battery chemistries are also greener than the old ones: Lithium-ion batteries, unlike NiMH or lead-acid batteries, "are mostly made of lithium metal oxides with zero lead, mercury, cadmium, hexavalent chromium, PBBs or PBDEs. In fact, there no heavy metals, nor any toxic materials." For more information about how they are recycled, see What Happens to a Tesla Electric Car Battery at the End of its Life. Another benefit of lithium-ion batteries is that there are many, many possible chemical combinations and that there's still lots of room for improvement in reducing cost and improving power storage capacity. Many labs, including MIT's battery lab, are working on just that, and even Google is investing in battery makers.
Capacitors. Photo: Flickr, CC.
Supercapacitors (sometimes also called hypercapacitors) are devices that can store electricity, a bit like batteries, but their different characteristics mean that they have different pros and cons. Unlike batteries, supercapacitors don't have 'memory'. They can be charged and discharged an unlimited number of times without degradation of storage capacity, which means that you can charge and discharge them completely (batteries in hybrids and EVs are usually only cycled within a fraction of their full charge to extend their lives). They can also be charged and discharged much faster than batteries (so that very fast recharge stations for EVs would be possible). The main downside is that so far their energy density has been much lower than chemical batteries, so they store only a fraction of the power.
A supercapacitor able to store close to as much power (or more!) as a chemical battery at a reasonable cost would be the holy grail of electric transportation. Many universities and companies are working on that. MIT is working on nanotube-enhanced supercapacitors and the secretive EEStor, which has recently gotten some patents, claims to have made big breakthroughs, though we're still waiting to actually see one of their Electrical Energy Storage Units (EESU) put to some real-world tests. Another promising research field is graphene ultracapacitors.
Hydrogen pump. Photo: Flickr, CC.
Ah! Hydrogen. Everybody knows about it, probably in good part because it has such a great angle for the media: "Only water comes out of the tailpipe!"
The main problems with it are that it takes energy to get hydrogen in its pure form (f.ex. via electrolysis of water), so it's not actually a fuel but rather a way to store energy that is produced some other way, we don't currently have a hydrogen infrastructure, and fuel cells are expensive.
Lots of very smart people are working on solutions to all of these problems. For example, new ways to purify hydrogen and generate it from microbes are under development, fuel cells are becoming incrementally cheaper (though still expensive), and Honda has started leasing some hydrogen-powered FCX Clarity cars in California where there are a few hydrogen refueling stations. But this probably won't be available on a large scale for a while...