The simple physics of energy use

In industrially developed countries, energy is used primarily for three things-maintaining a comfortable environment in buildings, transporting people and goods and manufacturing products. Each accounts for about one-third of the total primary energy use. Controlling the indoor temperature accounts for most of the energy use in buildings. Therefore, this strongly depends on the local climate. Electricity accounts for a high proportion of the energy transfer in developed countries. The problem is that electricity cannot easily be stored, and that supply therefore has to match demand. This makes the use of intermittent renewables such as solar and wind particularly challenging. Transportation efficiency can be measured by the energy used to move a person or a tonne of freight over a given distance, but there is also the journey time to consider. Transportation, with the exception of trains, is constrained by the energy density and convenience of fuels, and it is hard to beat liquid hydrocarbons as fuels. Materials that are dug out of the earth are nearly always oxides, but we want the element itself. The reduction process inevitably uses energy and produces carbon dioxide. Even growing crops requires energy in addition to that provided by sunlight. A meat-based diet requires significantly higher energy inputs than a vegetarian diet. Growing crops for fuel is a poor use of land, the problem being that crops do not grow fast enough. Policy should ultimately be based on what works from a physics and engineering viewpoint, and not on legislation that mandates the use of favoured renewable energy sources.