Energy is the basis for modern human civilization, because of the significant increase in productivity it brings. It is a vital part for most, if not all, of human activities: agriculture, communications, trade, manufacturing, mining, education, health etc. Consequently, there is a correlation between energy consumption and wealth (Webber, 2013).
According to the first law of thermodynamics, which says that energy of an isolated system is constant, humans are basically transforming energy that is already in the system into energy more convenient to use. For example, a windmill transformed wind energy into mechanical energy used to mill grain. Replacing human muscle with wind energy increased enormously the efficiency of the process. Repeating this idea for thousand and thousand of processes led to the highly efficient and also highly energy-transformative economy we have today.
For dawns of civilization the main energy source used by humans was wood, later followed by coal and now we live in an era dominated by oil.
There are many forms of energy – kinetic (motion), thermal, chemical, nuclear, radiant (light), gravitational, etc – electricity is just a form of energy, used according to our needs. Electrical conversion is basically a transformation of various energies already existing in the system. For example, thermal energy (heat – which is basically kinetic energy at molecular level) from burning coal; or kinetic energy from wind motion; or gravitational energy from waterfalls is converted into electrical energy.
Energy sources have several classifications. For example, the International Energy Agency manual for energy statisticians (IEA, 2005), which shares harmonized definitions, units and methodology with Eurostat, the statistical office of the European Union, and the United Nations Economic Commission for Europe, considers that energy sources (called “commodities”) can produce primary electricity through direct use of natural resources, such as hydro, wind, solar, tide and wave power or they can produce secondary electricity, using thermal energy as intermediate step, such as from nuclear fission of nuclear fuels, geothermal heat and solar thermal heat, or by burning coal, wood, natural gas, oil, etc.
US Energy Information Administration (EIA, 2015) makes the distinction between primary energy sources and secondary energy sources. Primary energy sources are those forms of energy, such as oil, natural gas, coal, uranium, biomass, wind, that are used to convert energy into energy carriers. Energy carriers, called secondary energy sources, such as electricity and hydrogen, transport energy, which is later converted into other forms of energy that are useful for humans. Electricity is used because it is easy to transport and can be quickly transformed in other forms of energy we need (kinetic, e.g. for coffee maker, thermal, e.g. for light bulbs).
Other several classifications, more or less scientific are used. Conventional energy is used mainly to nominate energy production from fossil fuels, while alternative energy is basically any source other than fossil fuels. Renewable energy is energy derived from processes with a replenishment rate higher than consumption. Eurostat, however, considers biofuels and municipal waste as renewables. Green energy is any form of energy with small environmental impact at its end-use (IEA, 2015; Webber, 2013). However, all energy sources have an environmental impact (Webber, 2014, Sheldon 2014, Mayfield, 2015).
Energy is not the same as power, although similar in meaning. Energy is power over time. For example, a refrigerator has a power of 225 Watts, and in an hour it uses 225 Wh (energy.gov, 2015), which is a measure of energy.
From a long term energy strategy standpoint, the second law of thermodynamics, which says that entropy always increases or remains the same in a close system, is relevant. This means that Earth-based highly-ordered forms of energy, such as fossil fuels, will always have conversion loses, because Earth is a closed system (Sheldon, 2013; Webber, 2013). For example, overall efficiency for converting primary energy to light using a light bulb is just 1.6% (Tester et al, 2005, p.58). However sun radiance can be used at will, because Earth is not a closed system regarding this type of energy (Webber, 2013; Sheldon, 2013).
In other words, there is a lot of space for progress in energy production.