Quote:
Originally Posted by Student 07
If I understand you correctly, you're saying collect the heat from the stale air before it goes into the HX?
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No, collect the heat
after it has gone through the HX, ie the heat pump is between the two outgoing streams - one going out of the house (cooled stale air) and one going out of the HX into the house (warmed fresh air).
Look at it this way: Assume that the air in the house is at 20°C/68°F, while outdoor temp is 0°C/32°F. If the HX is 80% efficient, then the airstreams leaving it should have temps: 16°C/61°F (housebound) and 4°C/39°F (outbound). Now put a HP on those two to extract heat from the cold stream and cool it to 0°C/32°F, warming the housebound stream to 20°C/68°F. The HP will be pumping "against the temp gradient" to carry over those 4°C/7°F, so its COP won't be too high (though that can be slightly improved by increasing the size of the evaporator and condenser coils).
Now, imagine a HP which has to totally switch the temperatures of the two ingoing streams: It has to cool one stream from 20°C/68°C to 0°C/32°F and warm the other by the same amount. This may
seem like good potential for a higher COP, but that is only an illusion. It would be true if you were taking up to 10°C from the warm stream and feeding it to the cold, but you'll still be using electrical energy to do that - it won't flow for free unless you use a HX. Once the two streams' temperatures evened out you'd start pumping "against the temp gradient" again, only with each degree that gradient will get larger. At some point (assuming you are increasing the size of the heat pump) you'll have the warm, stale stream cooled to 4°C/39°F and the fresh stream warmed to 16°C/61°F. Now what's left is the same situation as in the HX case: a 4°C/7°F temperature difference against the gradient. So, summing it all up, you'll be doing all the work as in the first case,
plus lots of extra work just to cool the warm stream from 20°C/68°F to 4°C/39°F and to warm the cool stream by the same amount. At the beginning the COP may have been high for a short time, but in all you used much more energy just to do what a heat exchanger does for free.
The energy needed to counter extra pumping losses through the HX is negligible.