[Greenbuilding] Laundry day on the GB List

[nick pine]

RT wrote:

> In winter...  the dryer is vented indoors...

> The excess moisture and lint is dealt with by directing the exhaust into
> a bell jar-like receptacle made out of recycled 1,2 and 4 litre yoghurt,
> tofu and ice cream tubs.
>
> The tubs are configured so that the vapour condenses out of the exhaust...

This could help heat a house in wintertime... The tubs are filled with water?
Condensing 12,000 Btu (3.5 kWh) from a 12 lb load of towels which contain
an extra 12 pounds of water after washing and spinning, with a 10 F temp rise
(eg 70 to 80 F) would require 12000/10 = 1200 pounds or 545 liters of water
in an unstratified bell-jar-like receptacle. 

Stratification could help. If the dryer exhaust is 130 F, 5 C Btu/F containers
in series might end up at 120, 110, 100, 90, and 80 F. The first could store
(120-70)C = 50C Btu, the second could store 40C... and whole collection could 
store 150C, so C = 12K/150 = 80, with a total of 5x80 = 400 pounds of water, 
eg 182 2-liter bottles, eg 37 vertical bottles in a hex pattern, with 4 more
layers of 37 above that, in a sqrt(0.91x(4/12)^2x37) = 2' square x 5' tall
tube with baffles to help with stratification.

We might warm 70 F room air at 50% RH up to a 130 F dryer temp with a small
fraction of the 12K Btu, if we don't rehydrate the room air...

Can we cool the hot dryer output (130 F at 100% RH?) to 80 F at 100% RH,
then use the resulting heat to rewarm the 80 F air to 130 F at a lower RH
and circulate it through the dryer at 130 F until its RH rises to 100%?

Steven Tjiang (KZ6LSD) wrote:

>Such a dryer is commercially available.  The principle is as you describe though
the numbers are certainly different.  It's called a condensing dryer.  See http://goo.gl/kMpK

It looks like that dryer works like RT's. It's more efficient than a conventional dryer which
exhausts hot moist air to the outdoors, since all the heat ends up in the house and the house
does not have to warm up any cold incoming air in wintertime, but it still uses about 3.5 kWh
of electrical energy to evaporate 12 lb of water from clothes, and that energy is not useful
in summertime. (Altho it's more efficient than a water-cooled condensing dryer with no useful
wintertime heat.)  
 
Suppose we put the 5 groups of containers above into a well-insulated tube A and make another
tube B just like A, and connect the dryer exhaust to one end of tube A, and the output of A
into tube B and the output of B to the dryer inlet. On the next load, we swap A and B...

Like this:
                                         --------------------------
 ---------------->------|---------X---<>| 120  110  100   90   80  |<>---
|           | output         |           --------------------------      |
|           |                |                       A                   |     
|   dryer   |                |                                           |
|           |                |                       B                   |
|           |  input         |           --------------------------      |
 ----------------<------X---------|---<>| 120  110  100   90   80  |<>---
                                         --------------------------

At an average 100 F in a 4'x7'x7'-tall box with 210 ft^2 of R40 walls and a 5.25 Btu/h-F
conductance, the tubes might lose 24h(100-70)5.25 = 3780 Btu/day.
    
Is there a better way? In principle, clothes drying shouldn't require any energy, if we
reuse the heat, as in the combined cycle power generation in some old steam locomotives
or the 8-effect dehumidification achieved in a 1975 solar still in the Sonoran desert.

Nick
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