Energy can be classified as non-renewable or renewable in terms of source, and primary or secondary in terms of process. For instance, heat and electricity are derived from the combustion process of primary and secondary materials, which in turn may come from non-renewable or renewable sources. Both are gifts of nature: the former is a one-time gift, the latter depends upon whether the sun shines, the wind blows or waves dance in their diurnal motion.
Historically, mankind relied on renewable sources for cooking, heating and lighting. Extensive use of non-renewable sources dates from about the mid-nineteenth century and was largely responsible for driving the Industrial Revolution and urbanization. The convergence of technologies ...