Different types of primary energy fall into two main categories: primary fuels and primary energy flows. Fuels like coal, natural gas, and uranium are dense stores of energy that are consumed when used. Primary flows are natural processes that have energy associated with movement. Using a primary flow means harnessing energy that comes from this movement like wind and tides. Once the energy is extracted from a fuel, it's gone, but extracting the energy from a primary flow is different. Natural processes put energy back into primary flows over time making them renewable.
Most of the world's primary energy (90-95%) comes from fuels (see Figure 1). Of these primary fuels like coal and natural gas are put into power plants. Crude oil is processed into secondary fuels, like gasoline, diesel and kerosene. Most cars are propelled by igniting a fuel like gasoline or diesel. Coal, natural gas, gasoline, and diesel are all considered to be fuels, because the energy we take from them is stored discretely inside them, and once we use it up, the fuel is gone. Fuels are dispatchable, which means the energy is available whenever it is needed.
Power sources like hydroelectricity, tidal power, wind power, and solar power take advantage of primary energy flows, where energy is harnessed from natural energy sources. A fuel is a substance that stores energy while an energy flow is a natural process that has energy that can be extracted from the process.
Both primary fuels and primary flows can be sustainable, but for the most part, only flows are renewable. For a discussion of why this distinction is important please see renewable and sustainable energy.
Public perceptions of these fuels and flows are complex, but the science is clear: each fuel and flow has its own advantages and drawbacks. Comparing which is the best is not straightforward, and depends on many parameters.
Primary energy flows almost always have fewer emissions and pollution than fuels (although nuclear fuels lead to very few emissions). This is especially true of the CO2 footprint, which has led to aggressive pursuit of flows, especially by environmental groups. While there is usually no direct waste from flow harvesting technologies life cycle analysis often shows that there is waste that should be taken into consideration. The carbon dioxide released from harvesting flows is almost always quite a bit less. The greenhouse gases from hydroelectricity are a problem, but tend to be less of a problem than fossil fuels. In theory it's possible to capture and sequester carbon dioxide from fossil fuel plants, but it appears expensive and difficult, see for example the |Boundary Dam project in Saskatchewan Canada.
Primary energy flows like wind, sunlight and tidal power are non-dispatchable and intermittent, meaning that there is little to no control over when the energy comes. Issues with energy storage make this a difficult challenge to overcome in using flows. Fuels are ready whenever they're needed. For example, both wind and tides have been used for years for sailing ships. The ships go when the flows take them, because "Time and the tide wait for no one."
While flows have been used for millennia (like daylighting for light in buildings, or milling using hydropower or wind), the technology necessary to get electricity out of flows tends to be harder to build. For example, it’s easier to develop the technology needed to make a coal-fired power plant than to manufacture a photovoltaic cell (let alone mass producing them). This means that in rapidly developing countries like China, coal tends to grow more rapidly than any of the primary energy flows (see world primary energy).
Hydroelectricity uses the flow of water to turn turbines, connected to generators, which create electricity. The potential energy put into this system (water evaporating, condensing, flowing in rivers), comes from the sun, hydro power plants harness the kinetic energy of an already-moving system.
Solar power uses the radiation from the sun in order to create electricity. Solar thermal power plants use the heat from the sun to spin turbines, whereas photovoltaic systems convert the energy directly.
Figure 1. Approximately 95% percent of the primary energy in the world comes from fuels like oil, coal and natural gas (all of which except nuclear fuels produce extensive greenhouse gases when used). Most of the rest of the world's primary 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 primary flows increases to about 20% (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.