For the first time in a century there is a popular debate in the United States about what personal transportation will look like in the near future.

Internal combustion engine cars powered by petroleum-based fuels, which have long dominated transportation, are beginning to come under fire from electric vehicles (EVs). There is a growing sentiment that EVs and plug-in hybrids (PHEVs) will become the most attractive forms of personal transportation in the next few years.

The future of EVs looks bright for both economic and environmental reasons.  The economic side is driven by the prices of both oil and national security, which have risen and are inherently unstable. On the other hand, the per mile fuel cost of plugging your car into an outlet is only a few cents. EVs are still expensive to manufacture but the potential is there, as demonstrated by Aptera’s $27,000 2e.

EVs have an environmental advantage over petroleum powered cars because they do not produce localized emissions and pollute less compared to gasoline powered cars. Most of the electricity generated in the United States still comes from fossil fuels, however, so EVs are far from emissions and pollution free. As more renewable energy is incorporated into the grid the footprint of EVs will decline. Smart grid implementation could also reduce the environmental impact of EVs.

While EVs are a rapidly developing technology with many advantages over petroleum-based fuel, they both share one weakness: a limited and politically sensitive resource. EV batteries rely on the mineral lithium, a light weight metal. Around half the world’s supply of this resource is located in Bolivia, with other sizable reserves in Chile and Tibet. Bolivia–known as the Saudi Arabia of lithium–is ruled by nationalist Evo Morales, who is not expected to make it easy for foreign firms to extract lithium from the Bolivian salt flats.

Political issues aside, lithium is a finite resource which, like petroleum, will become more expensive as demand rises and then eventually run out.

How can we break this resource trap and create sustainable personal transportation? The answer is simple: grow our own fuel. Energy cannot be created, only transformed, but the sun provides earth with far more energy than we consume.

The good name of biofuels has been sullied by ill fated attempts to push corn ethanol in the US. Ethanol itself is not an ideal transport fuel due to its chemical makeup, and corn is possibly the least efficient source of ethanol.

The most promising biofuels and the potential face of personal transportation in the future are made from algae, fitting since petroleum comes from prehistoric algae remains. Algae-based biofuel is the only transportation fuel with the potential to rate highly on all three of the economic, environmental, and national security scales by which transport fuels must be judged.

The resources necessary to produce algae are relatively limited. It can be produced on non-arable land using non-potable (saline or brackish) water, sunlight, and CO2. Algae populations grow exponentially under the right conditions, producing crop yields that can be up to 10 times greater than food crops. Land and drinking water are increasingly valuable resources for society as well as the environment, so algae’s relatively light use of them is a strength.

Conversely, CO2 is increasingly prevalent in the atmosphere. Exhaust from power plants and factories can even be piped in to  spur algae growth. Algae biofuel ultimately results in a carbon neutral cycle: absorbing CO2 while it grows and releasing CO2 when it is burned.

So why aren’t you pumping up your car with green slime right now if it’s such an attractive alternative to petroleum? (Well, technically, you are using prehistoric slime…) The technology has not yet evolved to the point where algae-based biofuels are commercially viable on an industrial scale. To produce what is known as “green crude” algae must be grown and harvested, and extracts must be removed. This green crude must then be refined, like petroleum, to produce jet fuel, diesel, and gasoline.

Every stage of this process is still being optimized (bringing capital costs down) or monetized (made profitable). The ideal strains of algae must be bred to meet certain conditions and production facilities must be perfected. There is still not even a consensus on whether it is ideal to grow algae for biofuels in open pools or enclosed bioreactors, or on whether biofuels should be extracted from the algae themselves or oils they secrete. Once this process is perfected, however, this fuel will meet the highest environmental, economic, and security standards.

EV technology is at the point where these vehicles can be commercialized and compete with traditional cars in the next few years, offering some strong advantages over petroleum. In the medium-term, however, expect the internal combustion engine to make a comeback, powered this time by modern slime rather than the remains of prehistoric slime.