Shower water heat pump
 

I have been lurking the forums for a while and enjoy the cool DIY going on here. I'm always busy with some small projects here and there, but now I've found that I need some more input than just my own.

I want to start working on heat recovery from my shower water. In my house it is impossible to mount a vertical unit and horizontal units lack efficiency. Building a horizontal unit myself would not pay itself back for many years.

Therefore I was thinking about using a heat pump to recoup the heat from outgoing shower water. The outgoing water is about 35 degrees centigrade and incoming in winter about 10 degrees. At 6 litres a minute the amount of heat that would need moving is some 12.5 kW, back to the original 10 degrees.

For me there are 2 options, either a compressor heat pump or Peltier cells. Of course Peltiers are known for their low efficiency, but with no moving parts, low cost and small size very practical. Of course the question is, am I better off re-purposing an old dehumidifier/AC unit or going for the Peltier units. What kind of COP can I be expecting to achieve with either of the methods and is it worth starting this ?

Welcome to the site Ator.

Its an interesting idea you have. However, I am confused. You still need a heat exchanger to get the heat out of the drain water. How do you plan on doing that if a horizontal type won't work for you?

A normal horizontal heat exchanger only has about 20% efficiency, so then the water going through the drain is still some 30 degrees after the heat exchanger. I was hoping that a heat pump would be able to achieve a far better rate of recovery. So either adding an AC unit around the drain or peltier cells. The heat would then be added to the incoming cold water line, just like the commercial heat recovery units.

The Peltier doesn't have a high enough dT to make it worth while. I would make the HP. I have a friend who made a whole house drainwater HP 25 years ago and it worked well for many years in his low energy home. Taking heat out of the 10C water works well too. Lots of energy there but unfortunately, you have to design a system to work with poo, haha (I'm not fond of that part)

Why not use a normal heat exchanger ? If you use gas as an energy source, the COP should be very high in order to break even

What dT would be required for it to work ? Cheap peltiers I've seen can get up to 65-75 deg C and more expensive TEG cells can reach a dT of over 120 deg C, would this be comparable to a normal HP ? I've read some papers on peltiers and one paper claimed reaching a COP of 6-8 in a similar situation, what is the COP I can expect from a properly designed refrigeration cycle using an old AC unit? I'm just wondering why no one has really tried their luck at Peltiers yet; there might be some very sensible reasons not too of course.

A house drainwater project is not my cup of tea, I think I do not use enough water to justify such a big remodelling. I use about a 100m3 of water a year and after quickly crunching the numbers it wouldn't really make sense in my case. I do however like how far some people go in order to preserve energy !

Peltiers excel at high temperatures above the working range of conventional heat pumps, which your application isn't. Expect about half the efficiency of a conventional heat pump if you're lucky, 1/3 or less if you're not.

All Peltiers I have seen are used in the, for example, exhaust of an engine or wood stove where the input temps are quite high. Also the efficiency is very low so I doubt there is much real benefit at all.

Peltiers used to generate electricity are not very efficient and produce more power when the difference between the hot and the cold side is larger. So using the waste water to generate electricity would probably be less than 1% efficient. However I wish to use them as heat pumps and then a small dT would be most efficient in that case.


I do believe that a compressor will be more efficient, but the work and tools involved will probably make up for the improved efficiency. Since peltiers are cheap, I think I might order a couple to do some tests on them and report back on the forums. Seems like no one really has any hands on experience with them.

Peltier elements can work well in the right application.

I used some for cooling an overclocked pc processor several years ago, had a separate pc power supply just to feed the peltier which cooled the processor to around 5C with all the heat being dumped into a water cooling setup.

Very inefficient cooling setup, system would use just over 1KW when gaming and i'm sure the peltiers generated more heat than they actually moved.

It worked great for around 2 years until i used the pc and forgot to start the water pump, the acrylic plate on the water block melted and water leaked everywhere destroying the graphics card, sound card and motherboard :o

The point is, try to incorporate a fail safe control strategy to prevent freezing pipes or scalding hot water an peltiers can silently generate very undesirable temperatures.

Steve