9. Biomass

Biomass is the oldest renewable energy used by mankind. It still accounts for a tenth of the world's primary energy consumption. Most biomass is plant-based. The photosynthesis process that enables plants to grow requires solar radiation, water and carbon dioxide. Biomass stores solar energy in chemical form, which can be used when needed. The yield is low, generally less than 1% of incident solar energy in temperate countries.

The primary use of biomass is as food, but it is also used to produce energy. It is then used in the form of biofuels.

• Wood has been used to generate heat since man first discovered fire, probably around 500,000 years ago. As it burns, wood emits many particles and volatile organic compounds. Depending on how it is burned, it can also emit carbon monoxide and nitrogen oxides. Burning industrial...