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| [[category:371 topics]] | | #REDIRECT [[Primary energy flow]] |
| [[category:Lecture 1B_Human_effects]]
| | [[Category:Done 2020-02-29]] |
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| [[File:sunlight2.jpg|400px|framed|right|Figure 1. Sunlight is a crucial part of life on Earth. It's also a flow of energy from the Sun to the Earth, providing most of the energy for other flows as well like wind and hydropower.<ref>Pixabay. (May 20, 2015). ''Sunlight'' [Online]. Available: http://pixabay.com/en/sunlight-sky-daylight-summer-422710/</ref>]]
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| <onlyinclude>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 service]]s such as [[home heating]], [[transportation]] and [[electrical generation]].</onlyinclude><ref>Wolfson, ''Energy, Environment and Climate,'' Second edition. New York, USA: W.W. Norton, 2010</ref>
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| Flows are one of two broad types of energy found in nature ([[primary energy]]), the other being [[fuel]]s. 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 | fuel vs. flow]]). When people talk about energy conservation, usually they mean using less of the energy from flows or [[fuel]]s (see [[fuel conservation]]).
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| Flows include harnessing [[wind power]], [[solar power]] (both [[photovoltaic system | photovoltaic]] and [[solar thermal power plant|solar thermal]]), [[hydropower]], [[energy from ocean waves|wave power]], [[tidal power]] and [[geothermal energy|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.
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| 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]].
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| The amount of [[solar energy|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.
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| 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.
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| <html><iframe class="data-visualization topic-Production offset-Flows" id='oecd-pie'></iframe></html>
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| 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 engine]]s have and flows are used almost entirely for electricity generation.
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| ==Potential confusion==
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| People occasionally confuse energy flows with other concepts that are related:
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| *[[Electricity]] is flowing energy, but it is '''not''' a flow in this sense. Electricity is produced through energy transformations from primary energy. It's not itself a primary energy form but an [[energy currency]].
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| *[[Heat]] is flowing energy. Specifically, it's thermal energy flowing from hot to cold. This is tricky, because there are times when heat can be considered a primary energy source, when taking geothermal energy from the ground. Likewise, when using the Sun's heat, that would be a flow. When something is merely hot and giving off heat, like a fire, that's not a flow.
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| *There are also [[material flow]]s, which aren't energy at all. This refers to how certain materials, like [[metal]]s, move through the world's economy. This isn't primary energy, so it's not a flow.
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| ==References==
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| {{reflist}}
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| [[Category:Uploaded]]
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