6 ms·

What is the exact criteria by which energy ends up as rejected? That seems very difficult to define and I'm afraid this "America is so inefficient" rant is som

What is the exact criteria by which energy ends up as rejected? That seems very difficult to define and I'm afraid this "America is so inefficient" rant is some misunderstanding of what is an imperfect measure.

ac29 9y ago
Things like transmission system losses (for electricity) and ICE efficiency (for vehicles, only about 20% of the energy in fuels is converted to useful things like motion). More or less rejected means "lost as heat before we can do something useful with it".
- revelation 9y ago
  Sure, but where do we stop? At the customers electricity socket? Behind all power supplies, but before the specific device? Computers will turn essentially all power entering them into heat. But then by that measure we can never make an efficient computer but would have to look at something like flops/W to give us actionable information. What is a measure we can apply consistently to computers and cars and get actionable information?
  - dredmorbius 9y ago
    Reports such as this address the energy provisioning system: generation and distribution. What happens "past the meter" is a consumption question. Generally.
diafygi 9y ago
Whenever you burn something to do work, you are limited by a principle in thermodynamics called the Carnot cycle[1]. It basically says that converting heat to work has a maximum efficiency (depending on the situation). So the rejected energy in this sankey diagram is partly just the consequence of the thermodynamic efficiency limits of the situations. Also, since there are always heat and resistance losses (the real world is not a frictionless surface), the efficiency of converting energy into work is further decreased. All in all, we're actually pretty good at covering heat into energy. The rejected energy here is simply a consequence of the situation and thermodynamics. For other situations that don't involve heat-to-work, such as wind and photovoltaics, the Carnot cycle doesn't apply, and you can have much less rejected energy. [1]: https://en.m.wikipedia.org/wiki/Carnot_cycle https://en.m.wikipedia.org/wiki/Carnot_cycle
- usmeteora 9y ago
  You are correct, but just as a point out for people, wind I am not sure, but Photovoltaics I am positive are accumulate per panel, or per unit for newer panels redesigned as solar roofing materials, intead of panels on roofing materials, utilize inverters, which are some of the most lossy circuit designs for power conversion right now. Until inverters are made to be more efficient, this will always be a huge bottleneck for dynamic energy input and output for the solar industry, and should be considered as apart of the losses for a solar unit.
  - epistasis 9y ago
    >Until inverters are made to be more efficient, this will always be a huge bottleneck for dynamic energy input and output for the solar industry, That seems pretty unlikely, do you have any literature on that? Inverters do sometimes become less efficient when there's less power coming out of the panels, but getting <85% efficiency out of an inverter in any common scenario would probably just mean that the install had been poorly designed. I guess a case could be made for not capturing the thermal energy from the sun with panels and using that, but it's a strange case to make. (Probably about as strange as getting more than Carnot limits out of petroleum. Though if the waste heat is used in, say, steam pipes for distribution in a city, maybe not so strange...)
phonon 9y ago
Power plants are about 35% efficient, and then you lose another ~10% of what gets sent out from transmission losses...so your total efficiency for electrical generation is around 30%. [1] Ideally, that waste heat would be put to some use...that's called co-generation [2] [1] http://blog.schneider-electric.com/energy-management-energy-efficiency/2013/03/25/how-big-are-power-line-losses/ http://blog.schneider-electric.com/energy-management-energy-... [2] https://en.wikipedia.org/wiki/Cogeneration https://en.wikipedia.org/wiki/Cogeneration
- VLM 9y ago
  Its also worth pointing out the scale of the problem, with a simple one stage plant design, like a very stereotypical nuke or coal plant, a cold side around outdoor temps and a hot side around 1000F means 30-something percent efficient. And at 1000F or so, the steam pressure will be handwavy a bit less than 2000 PSI. So simply running hotter is not a realistic solution to get above 30 or so percent, not at current technology levels. That's where strange staged and cogeneration schemes are applied to boost efficiency. Low grade process heat is surprisingly useless in practice, unfortunately. Also economics and politics smack up against engineering ideals, where massive vertical integration would result in higher efficiency but short of nationalizing all industry or allowing massive monopolies its hard to integrate a coal generating plant with a steel mill heat treatment plant and a sardine canning plant and a greenhouse all under the same roof. Maybe with infinite scaling nano-technology all industrial plants could do all things to really boost efficiency.
  - coredog64 9y ago
    Carnot efficiency is 1 - Tcold/Thot. You must also remember to convert temps to an absolute scale. So from your example, that is 1 - (535/1500) or 66% efficient.
    - ultrafilter 9y ago
      The Carnot cycle is a poor model for steam turbine electric power plants. Rankine cycle efficiency of ~40% is the more relevant theoretical upper limit. https://en.wikipedia.org/wiki/Rankine_cycle https://en.wikipedia.org/wiki/Rankine_cycle
elihu 9y ago
It would be nice if we knew what constituted "rejected" when they were making the diagram. Are they taking into account Carnot efficiency, line loss, the efficiency of light bulbs to create light instead of heat, does heat loss from light bulbs count as waste in the winter time when the heat is a desirable side effect, etc.. We can guess what rejected energy meant when this was drawn (presumably, Carnot efficiency was taken into account, since that's the most probable explanation for the very low efficiency of transportation), but it's hard to interpret without knowing for sure. (I assume the LLNL has a document somewhere that breaks this all down, but it would be nice to have more details in the article.)

ac29 9y ago

Things like transmission system losses (for electricity) and ICE efficiency (for vehicles, only about 20% of the energy in fuels is converted to useful things like motion). More or less rejected means "lost as heat before we can do something useful with it".

revelation 9y ago
Sure, but where do we stop? At the customers electricity socket? Behind all power supplies, but before the specific device? Computers will turn essentially all power entering them into heat. But then by that measure we can never make an efficient computer but would have to look at something like flops/W to give us actionable information. What is a measure we can apply consistently to computers and cars and get actionable information?
- dredmorbius 9y ago
  Reports such as this address the energy provisioning system: generation and distribution. What happens "past the meter" is a consumption question. Generally.

diafygi 9y ago

Whenever you burn something to do work, you are limited by a principle in thermodynamics called the Carnot cycle[1]. It basically says that converting heat to work has a maximum efficiency (depending on the situation). So the rejected energy in this sankey diagram is partly just the consequence of the thermodynamic efficiency limits of the situations. Also, since there are always heat and resistance losses (the real world is not a frictionless surface), the efficiency of converting energy into work is further decreased. All in all, we're actually pretty good at covering heat into energy. The rejected energy here is simply a consequence of the situation and thermodynamics. For other situations that don't involve heat-to-work, such as wind and photovoltaics, the Carnot cycle doesn't apply, and you can have much less rejected energy. [1]: https://en.m.wikipedia.org/wiki/Carnot_cycle https://en.m.wikipedia.org/wiki/Carnot_cycle

usmeteora 9y ago
You are correct, but just as a point out for people, wind I am not sure, but Photovoltaics I am positive are accumulate per panel, or per unit for newer panels redesigned as solar roofing materials, intead of panels on roofing materials, utilize inverters, which are some of the most lossy circuit designs for power conversion right now. Until inverters are made to be more efficient, this will always be a huge bottleneck for dynamic energy input and output for the solar industry, and should be considered as apart of the losses for a solar unit.
- epistasis 9y ago
  >Until inverters are made to be more efficient, this will always be a huge bottleneck for dynamic energy input and output for the solar industry, That seems pretty unlikely, do you have any literature on that? Inverters do sometimes become less efficient when there's less power coming out of the panels, but getting <85% efficiency out of an inverter in any common scenario would probably just mean that the install had been poorly designed. I guess a case could be made for not capturing the thermal energy from the sun with panels and using that, but it's a strange case to make. (Probably about as strange as getting more than Carnot limits out of petroleum. Though if the waste heat is used in, say, steam pipes for distribution in a city, maybe not so strange...)

phonon 9y ago

Power plants are about 35% efficient, and then you lose another ~10% of what gets sent out from transmission losses...so your total efficiency for electrical generation is around 30%. [1] Ideally, that waste heat would be put to some use...that's called co-generation [2] [1] http://blog.schneider-electric.com/energy-management-energy-efficiency/2013/03/25/how-big-are-power-line-losses/ http://blog.schneider-electric.com/energy-management-energy-... [2] https://en.wikipedia.org/wiki/Cogeneration https://en.wikipedia.org/wiki/Cogeneration

VLM 9y ago
Its also worth pointing out the scale of the problem, with a simple one stage plant design, like a very stereotypical nuke or coal plant, a cold side around outdoor temps and a hot side around 1000F means 30-something percent efficient. And at 1000F or so, the steam pressure will be handwavy a bit less than 2000 PSI. So simply running hotter is not a realistic solution to get above 30 or so percent, not at current technology levels. That's where strange staged and cogeneration schemes are applied to boost efficiency. Low grade process heat is surprisingly useless in practice, unfortunately. Also economics and politics smack up against engineering ideals, where massive vertical integration would result in higher efficiency but short of nationalizing all industry or allowing massive monopolies its hard to integrate a coal generating plant with a steel mill heat treatment plant and a sardine canning plant and a greenhouse all under the same roof. Maybe with infinite scaling nano-technology all industrial plants could do all things to really boost efficiency.
- coredog64 9y ago
  Carnot efficiency is 1 - Tcold/Thot. You must also remember to convert temps to an absolute scale. So from your example, that is 1 - (535/1500) or 66% efficient.
  - ultrafilter 9y ago
    The Carnot cycle is a poor model for steam turbine electric power plants. Rankine cycle efficiency of ~40% is the more relevant theoretical upper limit. https://en.wikipedia.org/wiki/Rankine_cycle https://en.wikipedia.org/wiki/Rankine_cycle

elihu 9y ago

It would be nice if we knew what constituted "rejected" when they were making the diagram. Are they taking into account Carnot efficiency, line loss, the efficiency of light bulbs to create light instead of heat, does heat loss from light bulbs count as waste in the winter time when the heat is a desirable side effect, etc.. We can guess what rejected energy meant when this was drawn (presumably, Carnot efficiency was taken into account, since that's the most probable explanation for the very low efficiency of transportation), but it's hard to interpret without knowing for sure. (I assume the LLNL has a document somewhere that breaks this all down, but it would be nice to have more details in the article.)