An energy flow refers to natural processes with energy that can be extracted to be used. Energy flows include solar radiation shining on the Earth from the Sun, or water flowing downstream in a river. The energy in these energy flows can be harnessed to provide energy services such as home heating, transportation and electrical generation.
Flows are one of two broad types of energy found in nature (primary energy), the other being fuels. Just like with fuels, most of the energy in the Earth's energy flows come from the Sun (see Earth's energy flow). Flows tend to be diffuse (low energy density) and give the illusion of having no environmental impact. It is difficult to compare the energy density or environmental impact of flows to fuels (see fuel vs. flow). 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 take advantage of energy in Earth's systems, 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.
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.
People occasionally confuse energy flows with other concepts that are related: