Using exhaust air after heat exchange (or not) as heat pump input?

[Garald]

If I understand correctly, a heat exchanger in MVHR simply uses the fact that heat naturally flows from a hotter to a colder body; in winter, even after heat exchange, the air exhausted is warmer than the outside air (in fact, it will be warmer than the air that flows into the house after going through the heat exchanger).

 

Why not use that air being exhausted as part of the input to a heat-pump? If it is the house's main heat pump, I suppose the best one can do is have the exhaust be close to the heat-pump's input, and then much of it will be lost. But what if one uses a separate heat-pump for hot water, as in a so-called thermodynamic water heater?

(Also, why not have ducting so that the said water heater exhausts its output outside during the winter, and has both its input and its output ducted indoors during the summer?)

 

I imagine this is either (a) a naïve proposal that has a fatal flaw, (b) something that is sound in principle but too expensive to implement in practice for some reason I don't know, (c) something that is actually in the market (or set up by enterprising installers) and I just haven't heard about it.

[SteamyTea]

You can get Exhaust Air Heat Pumps to for hot water.  @Gone West had one I think.

 

Down to the more fundamental point though, it is down to the amount of air you have to shift.

0.85 m³ of air weights a kilogram (ish) and has a specific heat capacity of 1 kJ.kg^-1.K^-1 (ish).  That is 0.00027778 kWh.kg^-1.K^-1.

 

So say you need 3 kWh of hot water and there was a 1K ΔT.

 

3 [kWh] / 0.00027778 [ kWh.kg^-1.K^-1] = 10,800 kg

Even of there was a 5 ΔT then that is still 2,160 kg, 2.2 tonnes of air to shift out your building, via the MVHR and into a heat exchanger on the ASHP.

If that was done at a constant rate, day and night, it is 92 kg.h^-1 and could be a lot higher in reality if you need a more domestic hot water or a faster heat up time.

Would work in a larger volume house, but not so great for a smaller place.

[Gone West]

36 minutes ago, SteamyTea said:

@Gone West had one I think.

Our EASHP was part of the MVHR system, so extracted heat from the MVHR exhaust, which got very cold in winter. The energy was used to heat the DHW and warm air heating through the MVHR ducts.

[SteamyTea]

Just now, Gone West said:

Our EASHP was part of the MVHR system

Your old place was quite large I seem to remember, is that right?

 

I like the idea of energy scavenging, but it is just that, a scavenge, and a rule of diminishing returns.

 

Also, to get the right efficiency gains reported, one must use the absolute temperature scale, not the celsius one.

[Gone West]

Just now, SteamyTea said:

Your old place was quite large I seem to remember, is that right?

No, it was only 125m2 IIRC and the version of the Genvex 185 we had was the larger one and only worked in our house because the house was small and was very well insulated. They are really designed for flats I think.

[SteamyTea]

Just now, Gone West said:

Genvex 185

Did it cause and extra noise in the MVHR vents.

 

I have wondered about one of them for my place (50m²), but was thinking it was probably oversized.

Though down here a bit of extra ventilation is not such a problem as the mean winter OAT is pretty high.

[saveasteading]

I once discussed the harvesting of energy from chiller plants, with a German boffin. The heat thrown away is ( I can't think of the right word for a measure of lots of heat).

He said that it was very difficult and inefficient, otherwise he'd be selling the solution. 10% recovery at best, using very expensive equipment he said.

I could never get installers to even divert chill store waste heat to adjoining offices.

[SteamyTea]

1 minute ago, saveasteading said:

I can't think of the right word for a measure of lots of heat

Negative exergy maybe.

[saveasteading]

I meant  the amount of heat wasted is huge, enormous, big, lots. None seems appropriate.

[Garald]

2 hours ago, saveasteading said:

 

3 hours ago, Gone West said:

No, it was only 125m2 IIRC and the version of the Genvex 185 we had was the larger one and only worked in our house because the house was small and was very well insulated. They are really designed for flats I think.

 

 

My place is 120m^2 loi Carrez (i.e. space you can stand on and that is not stair space; has high ceilings except in the attic) plus of course there are the sides of the attic, the stairs, the entryway, etc. So your experience is very much relevant. Do you think the system eventually paid for itself?

 

SteamyTea: by 5 Delta T, you mean the difference in temperature between the exhaust air and outside temperature?

 

Saveasteading: well, the difference in COP at higher and lower temperatures means a heat pump is a plausible tool for exploiting exhaust heat (even after heat exchange). Shouldn't be hard to figure out the maximum theoretical efficiency.

[SteamyTea]

1 minute ago, Garald said:

 

SteamyTea: by 5 Delta T, you mean the difference in temperature between the exhaust air and outside temperature

Yes.

It was just a guess, no idea what it really is, though a few people on here have data from their MVHRs that could work out a seasonal average.

 

2 minutes ago, Garald said:

Shouldn't be hard to figure out the maximum theoretical efficiency

It is part load that can be a killer to efficiency, not so much of a problem with DHW heating though.

 

 

[Gone West]

15 hours ago, SteamyTea said:

Did it cause and extra noise in the MVHR vents.

When the system decided heat was needed it ramped up the fan speed to maximum for around 30 seconds, which was noticeable, and then dropped down to normal which was very quiet.

[saveasteading]

12 hours ago, Garald said:

heat pump is a plausible tool for exploiting exhaust heat (even after heat exchange). Shouldn't be hard 

I'd have thought it was easy to divert the hot carrier liquid away from the radiator and fan, and off to a space that needs heating. Maybe it exists and the installers don't know or don't care.

[Gone West]

12 hours ago, Garald said:

Do you think the system eventually paid for itself?

I don't know how I could have worked that out. The only heating in the house was provided by three electric towel rails in the bathrooms and 4m2 of electric UFH in the kitchen. The Genvex unit only pumped out warm air when it was very cold outside. One advantage of the Genvex was that the MVHR supply air to the rooms was not colder than the room air temperature, unlike conventional MVHR where the supply air temperature is usually a couple of degrees colder. The house was designed not to need a conventional central heating system and had very low U factors, 0.095W/m2K, and good air tightness, 0.47ACH. We kept the whole house at 23C all the time.

[JohnMo]

18 hours ago, Garald said:

Why not use that air being exhausted as part of the input to a heat-pump?

I did do some calcs for something similar, and a 2 Deg uplift in temperature is worth approximately 0.3 CoP increase. Looking at my inlet and out temperature difference on the MVHR there is only 1.9 degs difference. So we are looking at a CoP uplift of 0.3 at best.

 

There are a few practical issues

The condenser wraps it's self around the ASHP in an L shaped, so the MVHR outflow would only hit some some parts of the condenser not all. So even if the air was hotter than outside air the impact on the condenser maybe much smaller than anticipated.

 

Getting the MVHR outlet and heat pump inlet in the same space may be difficult or not practical. Or just look a mess.

 

My conclusion was it really wasn't worth the effort for very marginal gains.

[Originaltwist]

The MVHR exhaust air could be just one input to the heat pump plenum greenhouse shed thing. The others being:

Sun (stored in the black drums), ground air pipes (circa +5c lift I believe). Obvs the more extra energy you can get in the fewer defrost cycles and the better COP.

[Garald]

4 hours ago, JohnMo said:

I did do some calcs for something similar, and a 2 Deg uplift in temperature is worth approximately 0.3 CoP increase. Looking at my inlet and out temperature difference on the MVHR there is only 1.9 degs difference. So we are looking at a CoP uplift of 0.3 at best.

 

This is a bit I don't understand. Say it's 0 C outside and 20 C inside. The air being output is at 20C, the air coming in is at 0 C. Since the volumes of air going in and going out are the same, we see that, after going through the heat exchanger, the air coming in is at most 10 C (of course it will be a bit less, unless the heat exchanger is magically perfect) and the air going out is at 10 C at the very least. So why wouldn't be talking about an uplift in temperature of at least 10 degrees?

[SteamyTea]

47 minutes ago, Garald said:

This is a bit I don't understand

Temperature is not energy.

 

[JohnMo]

49 minutes ago, Garald said:

 

This is a bit I don't understand. Say it's 0 C outside and 20 C inside. The air being output is at 20C, the air coming in is at 0 C. Since the volumes of air going in and going out are the same, we see that, after going through the heat exchanger, the air coming in is at most 10 C (of course it will be a bit less, unless the heat exchanger is magically perfect) and the air going out is at 10 C at the very least. So why wouldn't be talking about an uplift in temperature of at least 10 degrees?

 

Just looked at a photo of the temperature when it was 3 degs outside outflow are was at 4 ish. It's a cross flow heat exchanger so the outgoing air will be cooled to nearly incoming. Incoming air will be heated to nearly the same as the air being extracted from the house.

 

Actual temps were

OAT 2.9

Extract to outside 4.6

Room extract temp 19.8

Room supply temp 18.9

[Garald]

15 minutes ago, SteamyTea said:

Temperature is not energy.

 

 

Wait - surely temperature is average kinetic energy? And the density of hot air is if anything a bit lower than that of cold air?

 

More to the point, I'm surprised (in a good sense) by this:

 

13 minutes ago, JohnMo said:

Actual temps were

OAT 2.9

Extract to outside 4.6

Room extract temp 19.8

Room supply temp 18.9

 

How can you extract air at 4.6 while heating air to 18.9 without putting energy into the heat transfer? Either the heat exchanger needs power for that, or it is actually using the energy from the initial heat transfer from hot inside air (A) to cold outside air (B) to then keep transferring heat from A to B even once B is warmer than A. Is this the case? Sounds sweet (and then feeding exhaust air to a heat pump would be gilding the lily, though I suppose one can sometimes arrange matters so that the exhaust just happens to be close to the heat-pump's input).

 

Edited May 16 by Garald

[SteamyTea]

24 minutes ago, Garald said:

Wait - surely temperature is average kinetic energy? And the density of hot air is if anything a bit lower than that of cold air?

It is the mean, free path, velocity.

But only in an 'ideal gas'.

But as I said earlier, the kelvin scale must be used to get the real factional improvements in performance.

28 minutes ago, Garald said:

Either the heat exchanger needs power for that,

It does, but is usually added after the heat exchanger efficiency is calculated.

Personally it is not a calculation method I like.

 

Ideally all heat exchangers should be calculated the same way as heat engines. That way all the inputs and outputs are included.

[JohnMo]

35 minutes ago, Garald said:

How can you extract air at 4.6 while heating air to 18.9 without putting energy into the heat transfer?

Here is a typical schematic showing the flow paths. No heaters attached to my MVHR. But as I mentioned to gain 2 degs at best more likely 1 Deg or less) not worth the effort, for the marginal gain in CoP.

 

[SteamyTea]

https://www.engineeringtoolbox.com/heat-recovery-efficiency-d_201.html

 

 

[Garald]

2 hours ago, JohnMo said:

Here is a typical schematic showing the flow paths. No heaters attached to my MVHR. But as I mentioned to gain 2 degs at best more likely 1 Deg or less) not worth the effort, for the marginal gain in CoP.

 

Impressive. And this is done without putting in work? Then, right, the supply temp can't possibly be as high as the extract temp - but 2 deg is very little to lose.

What is the mechanism, roughly?

Edited May 16 by Garald

[JohnMo]

The only work being done is done by the fans to move the air.

 

25 minutes ago, Garald said:

What is the mechanism, roughly

https://www.open.edu/openlearn/nature-environment/energy-buildings/content-section-2.3.2