Why are fossil fuels used so widely?

The primary reason is that fossil fuels are cheap and have a high energy density, so the combustion of a certain mass of a fossil fuel generates more energy than can typically be obtained from other energy sources, such as solar panels or wind turbines. Consider the application of energy densities to fuels in a vehicle.

##### Worked Example
1. Most modern vehicles are fueled by gasoline. The energy density of gasoline is about 3.4×104 kJ L-1. Suppose a small car has a fuel tank that can hold 50 L of gasoline: how much energy can be produced from one tank of gasoline?
2. Suppose that a new prototype car is able to use solar panels that store the energy needed to operate the vehicle (unfortunately, the storage of solar energy is currently a major technological problem). Assuming that the storage technology is in place, a solar panel with a power capacity of 200 W is able to generate about 1.7×104 kJ of energy per day. If the car is capable of carrying two solar panels, which have stored energy for 6 hours before use, how much energy is obtained? How does this compare to the energy obtained from the combustion of a tank of gasoline?
3. How long (in days) would it take the solar panels from part B to generate the same amount of energy that is obtained from the combustion of 50 L of gasoline?