Given our mobile lifestyle and addiction to driving we just might one day, when the technology improves enough to turn the personal transportation into a carbon negative process.
Utopian approach or not, we will now try to explore some possible concepts.
From energy efficiency perspective there are different requirements from a city car compared to a extra urban / motorway car.
Therefore for the optimal results we will assume the two car per family model, where one is used mostly for short local journeys, and another one for the extra urban / motorway journeys.

City Cars

The all-electric cars are now beginning to emerge and despite the limitations associated with their batteries, these cars already gained some popularity.
These teething problems will be solved probably within few years and the battery prices will eventually come down, once their mass production commences.
Given that the all-electric cars are carbon neutral, the power plants that provide electricity to charge up those cars are not, therefore the full energy cycle in still carbon positive.

A sufficiently large Ecobox can one day absorb all the CO2 produced by a coal fired power plant, meaning that the electricity it produces becomes green and renewable, while the full energy cycle of the city personal transportation at that point becomes carbon neutral.
For the time being Origo Industries is testing the prototype at John Lenon airport, and we yet have to see how efficient their technology is.
Additionally by applying these CO2 capturing algae to the power plants fumes that burn fast-growing willow rather than coal, the produced electricity could become super green (carbon negative) and finally we could have our planet saving electric city car.
The rationale behind this concept is that thanks to Ecobox none of the CO2 is released to the atmosphere, and yet the willow absorbs CO2 as it grows. The electricity production would stay carbon negative as long as not all of the biofuel produced by Ecobox is burned up (thus continuously locking away some carbon from the cycle). Alternatively if all of the produced biofuel is used up, but the CO2 is again recaptured, the cycle becomes carbon neutral and renewable.
For more about the carbon cycle see our carbon dioxide page.

Extra Urban Electric Cars

Designing a long range vehicle with carbon neutral footprint might be a more challenging task. If the car battery technology becomes advanced enough to propel the car for 400 miles on a single charge, and if the weight of those batteries doesn't exceed lets say 20% of the car's weight, then such car becomes usable for driving on long motorway journeys, and would help to save the planet. That is assuming that a form of renewable electricity is used for charging.
The challenging bit here is of course to mass produce such batteries.

Another battery related difficulty is the fact that they can't be charged quickly enough, but that could be overcome by arranging the batteries in a cartridge that is swapped with a charged cartridge at the refuelling station. For a scheme like this to take off, the total cost of ownership has to be comparable to conventional cars.
The wisdom of the electric vehicle industry has always been that the battery is part of the car. According to Shai Agassi the battery is the fuel of an electric car, and you buy fuel when you need it, not when you first buy the car. Agassi is certainly going to the right direction and we can only hope that his business model eventually succeeds.
Here is a video trailer presenting Project Better Place that shows the battery replacement station in action.
Even better yet the world's fastest production car company Shelby SuperCars plans to develop a electric car that charges in 10 minutes, and that is just enough time to have a cup of coffee.

Extra Urban Biofuel Cars

In this concept, a car with the internal combustion engine would use biofuel, where the CO2 is recaptured from exhaust.
Origo Industries seams to have a solution for that as well, since the company built a version of their CO2 capturing device combined with cartridges that can fit into a car, to form a closed loop carbon cycling system. Depending on the efficiency of their carbon capturing device the entire process could be carbon neutral in theory.
Right now the Ecobox used this way seams to be around 50% efficient in re-capturing the CO2. So, if such Ecobox is fitted to a green car like the third generation Toyota Prius hybrid, burning unleaded petrol would result in carbon footprint lower than an electric car charged from a non-renewable source of power, and yet achieve the much needed range.

Renewable energy sources

If some of the above concepts sound unrealistic and utopian, then there is even more to come. In absolute terms the Earth is overheating, therefore it basically suffers from surplus energy! That surplus energy could be used to power cars, only if we know how to harness it cost effectively.
At the end of the day, any energy generated from renewable source ultimately originates from energy radiated out by our Sun, and this continuous supply is what is making it renewable. Therefore to conclude, any renewable supply of energy effectively taps in to the above mentioned Earth's surplus energy.
For the time being the only renewable source of energy for UK at Terawatt levels that is effectively required to satisfy the national energy demand, seams to be the offshore windpower. While the same at European level would be the solar-PV electricity generated in North Africa.

On the energy storage side of things the Lithium Ion Batteries are currently the most advanced readily available battery technology, but their mass production is not sustainable in long term due to limited supplies of raw Lithium. In that respect an environmentally friendly virus batteries might look more promising, but they are still in early stage of development.

Perhaps in next 10 years (but more likely in 20 years) these emerging technologies may mature enough to prove that electric cars are viable solution for our warming planet, while at the same time the hydrogen cars being the close rivals to electric cars, are already on steady march. So far the opponents of the hydrogen cars fiercely criticised the subject of hydrogen generation, as being more carbon intensive then the petrol refineries and internal combustion engines coupled together. But not any more, since Honda who also makes the fuel cell car Honda FCX Clarity, made a recent breakthrough by developing their Honda Solar Hydrogen Station 6, that is intended for use as a home refuelling appliance capable of an overnight hydrogen refill. However for a sustainable production of fuel cells for cars there are still further problems, as for efficient performance of fuel cells about 0.8 oz (22.64g) of platinum is required, while that will have to come down, and it already did to some 2.5 times to achieve power density of world's best 2.5 kW per liter.