Even when most part of the solar radiation is reflected the 0.1% is used and confined by the photosynthesis. Humans have used this confined energy as coal and firewood. The firewood constitute about 15% of the world energetics. In contrast with gas, coal and oil, firewood is a renewable natural resource. Now a day it's possible to make use of this energy by the anual growth of the plants, hundreds of years are not needed to use this energy source.

There are different products that directly or indirectly come organic matter; they can be considered as alternative energy sources and can be found as solid, liquid or gas.

The most direct way to obtain energy from plants is using them as fuel, being the firewood one of the most ancient and very important in some regions; the traditoinal way used to burn these materials has a very low efficiency because heat is lost with the air. Actually new technologies have been developed to improve the use of the energy.

There is a process used to obtain fuel from a gassy mixture of CO and H₂ using plants under high pressure with air and vapor steam.

The sugarcane gives fuel when its juice, with high sugar levels, is fermented to make alcohol just as wine is produced.

In some countries, alcohol is obtained from grains and mixed with gasoline to produce gasohol in the Anglo-Saxos region. If the alcohol is suministrated in amounts of less than 10%, can be used as automobile fuel without a big change in the engine. And the idea of a car using just alcohol has come to life.

One of the problems obtaining alcohol from plants is its low efficiency, so it's necessary to sow great areas to obtain enough alcohol. It will be necessary to sow all the Arizona surface in USA to cover just 10% of the fossil fuels needs in the United States. Another disadvantage is the generation of CO₂ that, in the case of gasohol, is the same as gasoline, and in the case of firewood is greater.

Lilies might be a solution because they produce a rich energetic gas directly. This method uses water in rivers or lakes in tropical zones.

In 1973, the oil embargo led to the production of gasohol, a mixture of 10% ethanol and 90% gasoline. Ethanol is relatively expensive because it is generally made from grains such as corn. Methanol is the only fuel used in race cars at the Indianapolis. For economical and environmental reasons, methanol is attracting considerable attention as a gasoline substitute.

Molecules making up gasoline are complex hydrocarbons, but a methanol molecule includes oxygens as well as hydrogen and carbon. It's chemical formula is CH₃OH. The complete combustion of methanol is:

2CH₃OH + 30₂ -> 2CO₂ + 4H₂O

Hydrocarbons emitted in incomplete combustion enter into ozone-producing photochemical reactions at a much slower rate than those produced using gasoline.

The octane rating of methanol is higher than that of gasoline or diesel fuel.

Natural Gas

Clean burning natural gas is the favorite fuel for household heat, and this makes it attractive as an automobile fuel. It is relatively easy to convert a conventional car to use natural gas, being most of the cost for a bulky fuel tank that hold the fuel as a liquid under high pressure.

Natural gas is more than 50% cheaper than gasoline, therefore, the cost of converting a conventional car to use natural gas is offset by reduced fuel prices.