4 ms·

> But if you are trying to heat your house to a livable temperature while it’s -20 outside, your efficiency is terrible. Amazingly (to me), there are now air s

by JackC 6y ago

> But if you are trying to heat your house to a livable temperature while it’s -20 outside, your efficiency is terrible. Amazingly (to me), there are now air source heat pumps that stay > 100% efficient down to -20C before regular electric heat takes over: https://www.nordicghp.com/2017/01/heat-pump-effective-temperature-range/ https://www.nordicghp.com/2017/01/heat-pump-effective-temper... Though apparently you'd be better off with a ground source heat pump if your average temperature during heating season was below freezing or so. [Edit: I mention this because I've heard that "you don't see heat pumps in the north" was true in the past but is now outdated -- they've made sense in warmer climates for a while but have only recently crossed over for colder climates, so it will take a while until they're common.]

dragontamer 6y ago
> heat pumps that stay > 100% efficient down Note: there are two popular "efficiency" measures, and its important to realize that they're incompatible. Air conditioners are commonly "energy moved / energy used", which can reach greater than 100%. If you move 150W of heat using only 100W of electricity, you have 150% "efficiency". --------- I don't know the term for the other kind of efficiency (I'm not a physicist), but lets call it "inverse engine efficiency". This is "energy out / energy in", which ends up being pretty close to "energy moved / (energy moved+electricity used)" Under this measurement of efficiency, 150W moved with 100W of electricity is 60% efficient. This follows the more standard physics rule of thermodynamics (you can never go above 100% efficiency: it will always take some number of energy to move heat around). Note: Car air-conditioners are funny systems. They use the heat from the combustion engine to move heat from inside the cabin to the outside world. So you are literally using heat to move other heat.
- greesil 6y ago
  Isn't this just the coefficient of performance?
- Reason077 6y ago
  > "Note: Car air-conditioners are funny systems. They use the heat from the combustion engine to move heat from inside the cabin to the outside world. So you are literally using heat to move other heat." Car air-conditioners are not powered by heat from the combustion engine. They are powered mechanically by a belt ("serpentine belt") connected to the engine. Or, in some cases such as battery electric vehicles, powered electrically with the compressor turned by an integrated electric motor. The extra heat produced is just a byproduct (thermal inefficiency) of producing and transmitting that mechanical/electrical energy.
  - dragontamer 6y ago
    > Car air-conditioners are not powered by heat from the combustion engine. They are powered mechanically by a belt ("serpentine belt") connected to the engine. But that belt is powered by the expansion of gas that takes place inside of a piston, due largely to the increase in heat from combusting gasoline. Ultimately, an ICE engine is a heat-engine (like a steam engine or sterling engine, but different).
    - Reason077 6y ago
      Yes, but the useful energy in a combustion engine comes from that expansion of gas driving the pistons. The heat is, for the most part, just a wasted byproduct. (It can be used for ancillary purposes like heating the cabin).
      - dragontamer 6y ago
        > Yes, but the useful energy in a combustion engine comes from that expansion of gas driving the pistons. Isn't that expansion of gas largely driven by the increase of heat? https://en.wikipedia.org/wiki/Internal_combustion_engine https://en.wikipedia.org/wiki/Internal_combustion_engine Wikipedia lists the ICE as a heat-engine. ------ With that being said: the octane combustion formula has 25 O2 as input and 16 CO2 + 18 H2O as output (and I assume the H2O is mostly water vapor). So that's 25 molecules of gas input -> 34 molecules of gas output. So it seems like more "CO2 + vapor" is created than the number of input O2 molecules. But that only accounts for 34% expansion of the volume of the stroke. (25 mols input -> 34 mols output). The rest of the stroke's power comes from the ideal-gas law: higher temperature means higher pressure and larger volume. Literally the heat generated by the chemical reaction.
- dredmorbius 6y ago
  SEER (seasonal energy efficiency ratio) and COP (coefficient of performance), measuring energy-moved/energy-used. https://en.wikipedia.org/wiki/Seasonal_energy_efficiency_ratio https://en.wikipedia.org/wiki/Seasonal_energy_efficiency_rat...

by JackC 6y ago

dragontamer 6y ago

> heat pumps that stay > 100% efficient down Note: there are two popular "efficiency" measures, and its important to realize that they're incompatible. Air conditioners are commonly "energy moved / energy used", which can reach greater than 100%. If you move 150W of heat using only 100W of electricity, you have 150% "efficiency". --------- I don't know the term for the other kind of efficiency (I'm not a physicist), but lets call it "inverse engine efficiency". This is "energy out / energy in", which ends up being pretty close to "energy moved / (energy moved+electricity used)" Under this measurement of efficiency, 150W moved with 100W of electricity is 60% efficient. This follows the more standard physics rule of thermodynamics (you can never go above 100% efficiency: it will always take some number of energy to move heat around). Note: Car air-conditioners are funny systems. They use the heat from the combustion engine to move heat from inside the cabin to the outside world. So you are literally using heat to move other heat.

greesil 6y ago
Isn't this just the coefficient of performance?
Reason077 6y ago
> "Note: Car air-conditioners are funny systems. They use the heat from the combustion engine to move heat from inside the cabin to the outside world. So you are literally using heat to move other heat." Car air-conditioners are not powered by heat from the combustion engine. They are powered mechanically by a belt ("serpentine belt") connected to the engine. Or, in some cases such as battery electric vehicles, powered electrically with the compressor turned by an integrated electric motor. The extra heat produced is just a byproduct (thermal inefficiency) of producing and transmitting that mechanical/electrical energy.
- dragontamer 6y ago
  > Car air-conditioners are not powered by heat from the combustion engine. They are powered mechanically by a belt ("serpentine belt") connected to the engine. But that belt is powered by the expansion of gas that takes place inside of a piston, due largely to the increase in heat from combusting gasoline. Ultimately, an ICE engine is a heat-engine (like a steam engine or sterling engine, but different).
  - Reason077 6y ago
    Yes, but the useful energy in a combustion engine comes from that expansion of gas driving the pistons. The heat is, for the most part, just a wasted byproduct. (It can be used for ancillary purposes like heating the cabin).
    - dragontamer 6y ago
      > Yes, but the useful energy in a combustion engine comes from that expansion of gas driving the pistons. Isn't that expansion of gas largely driven by the increase of heat? https://en.wikipedia.org/wiki/Internal_combustion_engine https://en.wikipedia.org/wiki/Internal_combustion_engine Wikipedia lists the ICE as a heat-engine. ------ With that being said: the octane combustion formula has 25 O2 as input and 16 CO2 + 18 H2O as output (and I assume the H2O is mostly water vapor). So that's 25 molecules of gas input -> 34 molecules of gas output. So it seems like more "CO2 + vapor" is created than the number of input O2 molecules. But that only accounts for 34% expansion of the volume of the stroke. (25 mols input -> 34 mols output). The rest of the stroke's power comes from the ideal-gas law: higher temperature means higher pressure and larger volume. Literally the heat generated by the chemical reaction.
dredmorbius 6y ago
SEER (seasonal energy efficiency ratio) and COP (coefficient of performance), measuring energy-moved/energy-used. https://en.wikipedia.org/wiki/Seasonal_energy_efficiency_ratio https://en.wikipedia.org/wiki/Seasonal_energy_efficiency_rat...