ABSTRACT
Mankind has been running on renewable sources of energy until very recently. From a recent energy outlook prepared by ExxonMobil (Figure 12.1), it is possible to see that in the 1800s, biomass supplied more than 90% of the energy and fuel needs. From the late 1800s to the early 1900s, fossil fuels became the preferred energy resource because of their easy availability and low cost. Since then, energy consumption has been increasing due to industrialization and population growth. By the 1920s, coal and petroleum had largely replaced biomass sources in industrialized countries, although wood for home heating and hydroelectric power generation remained in wide use. By the end of the twentieth century, nearly 90% of the commercial energy supply was from fossil fuels; only very recently have other renewable and hydro resources been increasing their share (USEIA 2001; Klass 2004; BP 2012; IEA 2012; ExxonMobil 2013).
