4 ms·

If the heat is already higher temperature than the target environment, then a heat pump is just a fancy resistor. Plus this doesn't obviate the need for a heat

If the heat is already higher temperature than the target environment, then a heat pump is just a fancy resistor. Plus this doesn't obviate the need for a heat exchanger. Better to just have a filter on the drain similar to a dishwasher.

benj111 4y ago
Is it a fancy resistor? If you have a cop above 1, you're extracting more energy than could be got just using straight heat exchanger. I don't see why that would break just because you go above an arbitrary temperature (arbitrary because the output temp of the heat pump isn't the ambient temperature of the room) Or am I missing something?
- Schroedingersat 4y ago
  The heat is conserved. If the waste water is hotter than the incoming water it will conduct on its own (and if they're arranged in back flow the equilibrium will be pretty close). Hurrying it up with a heat pump only makes it happen faster and adds the energy you put into the heat pump. You could cool the waste below ambient, but that's no different to cooling anything else outside. For simplicity, if your hot water system was already a heat pump, you could run it past/into the cold side. No real efficiency gain vs exchanging it with the cold water (actually a loss in theory) but might be cheaper.
  - benj111 4y ago
    Ok I've worked it out myself. I was inventing a magic free energy machine. I was thinking a cop of 4 would be able to extract 4 units of energy to the heat exchangers 1. But there's only 1 unit of energy in it to extract, which the heat exchanger is already extracting basically for free.
namibj 4y ago
No, the source is the waste water, the target is the fresh water. You could either use a heat pump to move the heat from the colder waste water to the hotter fresh water, or use a direct heat exchanger to pre-heat the freshwater while chilling the waste water down, to then put a resistance heater between the heat exchanger's freshwater outlet and the shower head's input. But it should be more efficient to run a heat pump against the heat exchanger's inefficiency/delta then to let it happen and compensate with an ohmic heater. Yes, the wastewater discharge with a 100% efficient heat pump might actually leave colder than the fresh water comes, due to heat loss to the room exceeding the work required to pump the heat against the exchanger's losses.
- Schroedingersat 4y ago
  Ambient temperature water is entering the system somewhere. You'd put your heat exchanger there. The replacement for the lost heat would happen to the equilibriated water and could happen with a heat pump. In principle (if your heat pump and exchanger is perfect) this generates no entropy so is more efficient than putting things with finite temperature delta in contact. In practise it's probably just simpler and requires a smaller heat pump. Whether you use the waste water (which will still be a degree or two above as exchangers aren't perfect) or whatever your normal source is (ground source may be warmer) would be about cost and making the heat pump work for not-the-shower too.

benj111 4y ago

Is it a fancy resistor? If you have a cop above 1, you're extracting more energy than could be got just using straight heat exchanger. I don't see why that would break just because you go above an arbitrary temperature (arbitrary because the output temp of the heat pump isn't the ambient temperature of the room) Or am I missing something?

Schroedingersat 4y ago
The heat is conserved. If the waste water is hotter than the incoming water it will conduct on its own (and if they're arranged in back flow the equilibrium will be pretty close). Hurrying it up with a heat pump only makes it happen faster and adds the energy you put into the heat pump. You could cool the waste below ambient, but that's no different to cooling anything else outside. For simplicity, if your hot water system was already a heat pump, you could run it past/into the cold side. No real efficiency gain vs exchanging it with the cold water (actually a loss in theory) but might be cheaper.
- benj111 4y ago
  Ok I've worked it out myself. I was inventing a magic free energy machine. I was thinking a cop of 4 would be able to extract 4 units of energy to the heat exchangers 1. But there's only 1 unit of energy in it to extract, which the heat exchanger is already extracting basically for free.

namibj 4y ago

No, the source is the waste water, the target is the fresh water. You could either use a heat pump to move the heat from the colder waste water to the hotter fresh water, or use a direct heat exchanger to pre-heat the freshwater while chilling the waste water down, to then put a resistance heater between the heat exchanger's freshwater outlet and the shower head's input. But it should be more efficient to run a heat pump against the heat exchanger's inefficiency/delta then to let it happen and compensate with an ohmic heater. Yes, the wastewater discharge with a 100% efficient heat pump might actually leave colder than the fresh water comes, due to heat loss to the room exceeding the work required to pump the heat against the exchanger's losses.

Schroedingersat 4y ago
Ambient temperature water is entering the system somewhere. You'd put your heat exchanger there. The replacement for the lost heat would happen to the equilibriated water and could happen with a heat pump. In principle (if your heat pump and exchanger is perfect) this generates no entropy so is more efficient than putting things with finite temperature delta in contact. In practise it's probably just simpler and requires a smaller heat pump. Whether you use the waste water (which will still be a degree or two above as exchangers aren't perfect) or whatever your normal source is (ground source may be warmer) would be about cost and making the heat pump work for not-the-shower too.