Heat recovery from waste water

[Cyril Rutledge]

Has any one any experience of these systems?

I came across the following on the net http://gfxtechnology.com/contents.html.  Looks interesting but is quite expensive and has to be imported from the USA.  Maybe another potential product for Ivan to stock!

[wyleu]

Seems unlikey that you could extract as much heat as they claim from a drain down pipe. Unless you had a high volume of water in which case your drains would overflow. But this is only my impression. Has anyone actually seen one?

[snozzer]

I see the case studies are all hotels, hostels and  housing projects (blocks of flats).  I would imagine that the combined waste water in these installations would be far greater than in a single domestic house (500 students all go for their monthly shower on the 1st of the month would probably heat South Carolina for a week).

Its like many RE systems, the science is very achievable until you apply it to a real world scenario.

Now, why not install a bigger version in a mains sewer and reclaim the energy to heat a school?  Although I suppose the mix of emptying hot baths and showers would be cooled by the great quantity of flushed toilets reducing the temperature of the waste water.

Snoz

[Much]

Just like a ventilation heat exchanger, but for water.
Actually; I can see it having a useful input in pre-heating shower water (as in the diagram on the site) for instant / combi shower sources;  but if you're so keen on efficiency, you probably would have a more efficient system installed in the first place!

I really can't imagine how I would use one of these in my house; the output isn't going to be usably hot (just no longer cold), and will need piping / pumping to where it might be used.

[Ork-NAK]

Interesting link, so thanks for that..... not sure about wanting to take my toilet water through it though!

In terms of whether it is worth it: It looks to me as though this will only work for water uses where the disposal takes place at the same time as delivery of water. i.e. The water sent to the sewer from a bath would not be usable because the cold water that flowed into the house to replace the hot water taken from the cylinder had come in when the bath was drawn, not drained. SO the heat could not be salvaged. It would work for  showers, as the import of water is at the same time as the export. I assume the same is true for the washing machine, and also wash basin to some extent.

It strikes me that there would be some sort of advantage in capturing the heat from waste water into the building envelope. i.e. letting the water cool within the insulated box of the house rather than sticking hot water into the sewer.

If you took water in at 10 deg C and heated it to 60 in the tank, you'd generally throw out when it is around blood temperature (39 deg C.. call it 40). So that means that there is still 40-10 degrees of heat in it, having 'used' the 60-40 degrees. i.e. you have thrown away over half of the energy put in. So for a 150 litre tank of that is around 4kWh of power 'lost'.

Makes me think.....

[ecogeorge]

Channel all your "grey" water (not sewerage !) down the same trench that you dug to take your ground source heatpump pipes. If you backfilled the trench  with gravel / sand then you have a useful soakaway as well as heating up the slinky coils !! clever idea eh !
rgds George.

[kent]

This is been developed in the UK have a look at http://www.lowheat.iphe.org.uk/index.html

[Ork-NAK]

Kent,

WELL DONE for finding this link. I'll see if I can get some more information on this and let the forum know how I get on.

Neil

Footnote: I have come across a crowd in the states with a device at http://gfxtechnology.com/contents.html

In effect it looks like a copper coil round a metalic section of drain pipe. Can't be that hard to manufacture Id have thought, so will probably give that a go.

A few thoughts come to mind:
1. Interface between pipe and coil will be critical, but there must be some sort of highly conductive grease that will do the job.
2. Not sure if there is a need to mount it vertically. Obviously there is less chance of air locks with a vertical pipe, but horizontal should work OK I'd have thought.
3. Lagging the whole assembly would make sense


Anyway, glad of any practical experiences with this before I set my local engineering shop off with a pipe bending challenge!

[Antman]

Surely a 'tube within a tube' design would be far more effective at transferring heat?  It is also easier to manufacture.
Using a coil around the waste pipe means that only the 'point contact' (of the circumference) of the coil tube is able to absorb the thermal energy. Also the manufacturing method must ensure that the coil is held in direct (tight) contact to the waste pipe throughout its length.

Antman

[Ork-NAK]

On a different tack: Has anybody had experience of mechanically recovering the heat from bath water? If so what sort of efficiencies would one obtain?

The simplified model in my head is that water drops into what would amount to being a chest freezer. The water would then be cooled to ground temperature (say 10 deg C) and then released to sewer. The coil on the back of the 'freezer' would be placed in a water jacket, so transferring heat from waste water to future bath water.

So is there anything out there doing this already?

A few thoughts on other approaches:

1. Letting the water go 'cold' in the bath to leave the energy within the building envelope. Simple, but may mean more bath cleaning! Water will only drop to ambient at around 20C, so energy imparted to get from inflow (10C) to 20C is lost. Bathwater becomes old bathwater at about blood temperature (39C). Energy released = (39-20)*4200*bath volume <Joules>

2. Run through a flow heat exchanger as discussed above. Works for showers, but not for baths unless there is a more complex water handling system. i.e batch process of heating water, so old bath water can be set to pre-heat incoming water before it is heated properly.

The bit that worries me is the need to have a tank of used bath water being chilled and whether there would be biological slime build up in the nutrient rich waste water. (I had experience of a grey water recycling ststem and it went foul due to chlorination failures.... I'm not doing THAT again)

What would be the power requirements versus the payback for running a compressor?

So, any pointers/experiences gratefully received.

[robertsathome]

Hi,

I used to work for Pera and have written the grant proposals for several renewable energy projects including involvement in the Lowheat one you feature in one of the links in this string. It is designed primarily for domestic appliances and showers etc, as you have noted-timing is critical-ie near simultaneous.

However, your idea about the chest freezer got me thinking about a simple thermal store system where all grey water is stored in a well insulated tank and the cold feed to hot water passes through this tank, possibly using a high gain shower coil type exchanger used in conventional thermal stores. Filtration of soap scum & hair etc would be necessary, and require regular clean out.

If anyone has a really good renewable idea they want to commercialise, but do not have the funds, then I may be able to get a grant. So email me~ chris.roberts@mitconsultants.co.uk

I am currently a business partner in another renewable energy project and I am a small scale property developer, so I am very active and deeply interested in affordable renewable technology - hey that rings a bell, anybody heard of a great company called ....Navitron!!!