An energy flow is a natural process that has energy associated with movement, this energy can be sometimes be harvested. Examples include solar radiation shining on the earth from the sun, or water flowing downstream in a river. These flows can be harnessed to provide energy services such as home heating, transportation and electrical generation. Flows usually get their energy from the sun (see conservation of energy) they are usually considered to be a primary energy source. When people talk about energy conservation, usually they mean using less of the energy from flows or fuels (see fuel conservation).
Flows include harnessing wind power, solar power (both photovoltaic and solar thermal), hydropower, wave power, tidal power and geothermal power. Because flows are taking advantage of moving energy that naturally exists, those flows tend to replenish their energy supply. This is why flows are usually thought of as renewable, and often sustainable (see renewable and sustainable energy). Despite this, it is possible to use flows unsustainably, which has specifically been a problem with geothermal energy.
Flows tend to be diffuse (low energy density) and are incorrectly perceived as having no environmental impact. However, it is difficult to compare the energy density and environmental impact of flows to fuels (see fuel vs. flow).
Electricity is flowing energy, but since its not a natural flow of energy to be harvested, it is not considered a flow (it's instead an energy currency). Materials, like metals in use in society are also referred to as flows, but in this encyclopedia the term material flow is used instead to be less ambiguous.
At the moment energy accounting only keeps track of flows that are used for harnessing power for power's sake like generating electricity. There's a fair amount of power taken from flows that don't get formally accounted for. Some flows contribute to energy for transportation like sailing ships using wind and many ships using tides (which is different from the electrical generation of tidal power). The largest flow used is of course sunlight, and the biggest way that's used is by keeping the Earth at a comfortable living temperature.
The amount of energy from sunlight is enormous, but mostly it goes towards keeping the atmosphere at a livable temperature, evaporating water for the hydrologic cycle and driving wind. Because this total flow of solar energy is thousands of times greater than the total energy use by humans many people advocate for using more solar energy. The difficulty with this energy is that it's delivered over a very wide area (it's very diffuse) and not easy (or cheap) to catch it all, but it is there.
By formal reckoning, flows provide only a small fraction (~5%) of today's primary energy, see figure 1. Almost all of this energy is going into electricity generation. Due to peculiarities of thermodynamics, flows generate practically no waste heat (all fuels do). This means that all of the energy that's successfully harvested from the flow is used for generating electricity (see Betz limit for why not all of the energy can be harvested for wind power). This means that despite being only ~5% of the primary energy, flows make up ~19% of electrical generation, the bulk of which is hydro (16.5% of electricity generation worldwide) and wind (~2% of electricity generation worldwide). Different countries have very different energy mixes, please see electricity generation for a detailed map of where different countries get their electricity from.
Figure 1. Only a small fraction, shown as the extracted pie pieces, of the world's energy comes from flows. Most of this energy comes from hydropower, although a small fraction is wind power, solar power, geothermal energy, and tidal power. The amount of electricity that comes from flows increases to about 19% (still mostly hydro) because flows don't have the same limitations of having a thermal efficiency's that heat engines have and flows are used almost entirely for electricity generation.