3 ms·

> Those could be installed everywhere from .. water heaters .. to scavenge some of the 65% of fossil fuel energy that winds up as waste heat. Modern condensing

> Those could be installed everywhere from .. water heaters .. to scavenge some of the 65% of fossil fuel energy that winds up as waste heat. Modern condensing gas boilers for domestic use are better than 90% efficient, the exhaust gas is very cool so I don't think that thermoelectric devices have much to offer there.

NohatCoder 5y ago
That is because the 90% is heat efficiency, this is for extracting electricity, which is much harder.
- IanCal 5y ago
  Also, that's not necessarily more efficient for heating when compared to heat pumps. With a SCOP of 4, converting gas to electricity at 35% efficiency is better than using it for heat at 100%.
yetihehe 5y ago
If you had CHEAP device, which uses exhaust at 70C to still generate some energy, this could be useful. This way, your exhaust could be lower temperature than your heated water. Currently this is not feasible, because such devices are not cheap enough.
lizknope 5y ago
This Old House on PBS routinely installs systems over 95%. There are systems here listed at 98.5 AFUE https://www.energystar.gov/products/most_efficient/furnaces https://www.energystar.gov/products/most_efficient/furnaces
- londons_explore 5y ago
  Fossil fuel systems 'cheat' a little in efficiently metrics by using the 'Lower Heating Value' of burning the fuel. That's the amount of energy the fuel contains if you assume that the resulting water produced in combustion is released as water vapor. However in real boilers, there is no need for this - by condensing that water vapor back into liquid water you can get more energy out. That's the 'Higher heating value'. When measuring efficiency, we always use the lower heating value, whereas we ought to use the higher heating value. Therefore a "98% efficient furnace" is actually more like 88% efficient when considering the higher heating value.
enkid 5y ago
If I understand the article correctly, you're talking about two different things. Something can be efficiently heated but if it cools down all of that energy is effectively lost. This device would allow you to recapture some of that energy that is lost to then reheat the water. Think of it more as active insulation than a better heating element.
- leoedin 5y ago
  I don't think that's quite how it works. This device (or any thermoelectric generator) converts a fraction of the heat energy flowing through it into electrical energy. That means energy has to flow through it for it to work - essentially the opposite of insulating. Using this device, by definition, will result in the original hot thing being cooler, and the "cold end" (normally the outside world) being ever so slightly warmer. The place this would be useful is in capturing otherwise unusable energy from low grade heat waste. For example, the output water of a steam turbine might still be "hot", but there's very little you can do with it - it's in the wrong place for heating houses, it's too cold for steam turbines. Generally that energy is lost to the air.
makomk 5y ago
Yeah, I think the more conventional way to integrate heat-to-electricty into heating applications is the reverse of that: feed the high-grade heat from fuel combustion into the converter, and use the rejected waste heat from it for heating. This gives a big enough thermal gradient to get useful amounts of electricity out, and the waste heat can still be plenty hot enough to be used for heating.
marktangotango 5y ago
Isn't the minimum exhaust temperature the same as the temperature of the heated water? How can it be less?
- dhajzhz 5y ago
  If the exhaust is too cold the water vapor condenses out left with a puddle of acid (the acid is from the trace sulfur in the Nat gas) Also, cold exhaust doesn't exhaust (since it cant rise through a density change). So, if your exhaust is too cold, it will collect in your basement (CO2 should be a bit denser than air, so absent any drafts it'll collect) The previous owners of my house installed my high efficiency stove improperly using the old brick flue. The fumes eventually ate away at the mortar and a few bricks fell I'm clogging it.
  - yetihehe 5y ago
    > Also, cold exhaust doesn't exhaust Unless you have a fan blowing smoke into flue. Without fan, my furnace would not work, exhaust is only slightly above water temperature and furnace is designed so that condensate pools in special container, which you have to empty or connect to sewage.
  - kwhitefoot 5y ago
    Most new and recently installed domestic gas boilers in the UK use balanced flues not chimneys and are condensing boilers. The exhaust is fan assisted and the temperature is very low. See https://en.wikipedia.org/wiki/Condensing_boiler#Exhaust https://en.wikipedia.org/wiki/Condensing_boiler#Exhaust
- namibj 5y ago
  intake air preheating and cold return water preheating come to mind

NohatCoder 5y ago

That is because the 90% is heat efficiency, this is for extracting electricity, which is much harder.

IanCal 5y ago
Also, that's not necessarily more efficient for heating when compared to heat pumps. With a SCOP of 4, converting gas to electricity at 35% efficiency is better than using it for heat at 100%.

yetihehe 5y ago

If you had CHEAP device, which uses exhaust at 70C to still generate some energy, this could be useful. This way, your exhaust could be lower temperature than your heated water. Currently this is not feasible, because such devices are not cheap enough.

lizknope 5y ago

This Old House on PBS routinely installs systems over 95%. There are systems here listed at 98.5 AFUE https://www.energystar.gov/products/most_efficient/furnaces https://www.energystar.gov/products/most_efficient/furnaces

londons_explore 5y ago
Fossil fuel systems 'cheat' a little in efficiently metrics by using the 'Lower Heating Value' of burning the fuel. That's the amount of energy the fuel contains if you assume that the resulting water produced in combustion is released as water vapor. However in real boilers, there is no need for this - by condensing that water vapor back into liquid water you can get more energy out. That's the 'Higher heating value'. When measuring efficiency, we always use the lower heating value, whereas we ought to use the higher heating value. Therefore a "98% efficient furnace" is actually more like 88% efficient when considering the higher heating value.

enkid 5y ago

If I understand the article correctly, you're talking about two different things. Something can be efficiently heated but if it cools down all of that energy is effectively lost. This device would allow you to recapture some of that energy that is lost to then reheat the water. Think of it more as active insulation than a better heating element.

leoedin 5y ago
I don't think that's quite how it works. This device (or any thermoelectric generator) converts a fraction of the heat energy flowing through it into electrical energy. That means energy has to flow through it for it to work - essentially the opposite of insulating. Using this device, by definition, will result in the original hot thing being cooler, and the "cold end" (normally the outside world) being ever so slightly warmer. The place this would be useful is in capturing otherwise unusable energy from low grade heat waste. For example, the output water of a steam turbine might still be "hot", but there's very little you can do with it - it's in the wrong place for heating houses, it's too cold for steam turbines. Generally that energy is lost to the air.

makomk 5y ago

Yeah, I think the more conventional way to integrate heat-to-electricty into heating applications is the reverse of that: feed the high-grade heat from fuel combustion into the converter, and use the rejected waste heat from it for heating. This gives a big enough thermal gradient to get useful amounts of electricity out, and the waste heat can still be plenty hot enough to be used for heating.

marktangotango 5y ago

Isn't the minimum exhaust temperature the same as the temperature of the heated water? How can it be less?

dhajzhz 5y ago
If the exhaust is too cold the water vapor condenses out left with a puddle of acid (the acid is from the trace sulfur in the Nat gas) Also, cold exhaust doesn't exhaust (since it cant rise through a density change). So, if your exhaust is too cold, it will collect in your basement (CO2 should be a bit denser than air, so absent any drafts it'll collect) The previous owners of my house installed my high efficiency stove improperly using the old brick flue. The fumes eventually ate away at the mortar and a few bricks fell I'm clogging it.
- yetihehe 5y ago
  > Also, cold exhaust doesn't exhaust Unless you have a fan blowing smoke into flue. Without fan, my furnace would not work, exhaust is only slightly above water temperature and furnace is designed so that condensate pools in special container, which you have to empty or connect to sewage.
- kwhitefoot 5y ago
  Most new and recently installed domestic gas boilers in the UK use balanced flues not chimneys and are condensing boilers. The exhaust is fan assisted and the temperature is very low. See https://en.wikipedia.org/wiki/Condensing_boiler#Exhaust https://en.wikipedia.org/wiki/Condensing_boiler#Exhaust
namibj 5y ago
intake air preheating and cold return water preheating come to mind