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No, I'm not proposing reusing the dirty water. (Although it could be reused for a lavatory flush - thats a separate, baked idea)
I propose the use of a counter-current heat exchanger to scavenge heat from the waste water. This would necessitate the use of some extra plumbing, but I propose that much
of it (The tricky part, including the heat exchanger itself) could be in a prefabricated unit in commercial designs)
This would heat up the new cold water using the waste water. (See link - best diagram I could find. Note that counter-current exchangers are common in nature).
The water temperature would ideally be carefully regulated electronically to prevent a creep upwards. Shower temperature regulation is baked so I won't discuss it further.
Further to this, I propose that a cold water tank be situated above the shower. This would have a metal, sloping, corrugated base. The idea here is to remove steam and prevent 'steaming up' the whole bathroom, while also avoiding cold water drips on the occupant.
Cold water flowing into the shower would thus be:
mains > (water level regulator) > cold tank above shower > down past drain > up through heat exchanger > mixed with hot water > pass temperature sensor > out of shower head.
Diagram of counter-current heat exchanger
In some animal - don't let that put you off. [Loris, Aug 09 2002, last modified Oct 05 2004]
More on counter-current heat exchangers
elegant textual description [Loris, Aug 09 2002, last modified Oct 05 2004]
this has been baked for a while... [Freefall, Oct 04 2004, last modified Oct 05 2004]
(?) Baked in several models
retrieves 60% of drained water heat. [macrumpton, Oct 04 2004, last modified Oct 05 2004]
||Well, the "spent" shower water is maybe at 30 - 35 C, so with a feedwater temperature of 10 C and a reasonable retention time in the exchanger, you should recover a reasonable amount of low-grade heat.
The economics aren't clear-cut; you'd need a fairly large copper multiplate exchanger to ensure a significant heat transfer, and that would cost, both financially and in terms of energy for manufacture (most copper is electrolytically refined). But I'll give you a croissant for a practical and well-intentioned suggestion.
||In college for my heat transfer class, we had a project to design a counter-flow heat exchanger for dealing with a difference of about 30 C. We ended up with a simple tube-in-tube design around 6 or 7 feet long, with the inner tube being just copper and the outer tube of PVC with some added fittings on the side. Total cost, a few bucks for the pipe, PVC, and fittings. You could do the same thing with whatever existing plumbing is already installed.
||My clogged drain gave me a much easier idea. If you're heating your place's air, close the drain soon as the water gets hot. Let the water cool for hours in the tub, then drain it. Should transfer a lot of heat.
||I myself went trough the same idea some time ago.It makes a lot of sense here in Brazil. We usually do not heat water in furnaces, electricity is used instead in the lage majority of cases.
Pre heating the water to the electric heater would improve efficiency anyway.Also a very small head pump can be used ro recycle (not so dirty) water to flush toilets.
||I was hoping for something to make showering less tiring.....
||Even better, move heat from waste water to fresh water using a heat pump, instead of a mere heat exchanger.