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re: att: nick pine  pls see inside
30 jan 2003
vlad   wrote:

>i am interested on energy conservation and i attached a great value
>to your contribution.

you have interesting ideas and projects as well.

>i would like to subscribe to other groups that have your contribution.
>for instance, what is, in your opinion, the best group to subscribe in
>order to discuss a system that recuperates the heat from the discarded
>water from a shower, to the cold water pipe that feeds the same shower.

there no usenet energy conservation group that i know of, nor a windpower
group. you might subscribe to alt.energy.renewable, or alt.solar.thermal
if the heat comes from the sun or alt.energy.homepower if its cogen or
misc.consumers.frugal-living if it's frugal. you might also enjoy misc.rural
or alt.solar.photovoltaic. pv people are frugal with electrical energy
("pay no attention to the propane tank behind the snow fence" :-)

>water heating is very energy consuming, specially  when water comes at
>such low temperature. in the order of 30 kwh a day for only 2 showers ???

well, 4 10 min showers per day and 20 minutes of dishwashing at 1.25 gpm
might involve heating 75 gallons of 55 f water to 110 with 8x75(110-55)
= 33k btu, about 10 kwh worth about $1. a $1,000 100% heat exchanger could
pay for itself in about 3 years. using hot water in say, 6 10 minute bursts
over 12 hours makes once-through heat exchange harder (something like 300'
of 3/4" copper tubing?) than storing significant volumes of greywater and
cold water with slow continuous heat exchange. 

>some of that heat will be transferred to the inside of house before exiting

a water-air heat exchanger in series with the greywater drain can move heat
from 40 c water to 20 c house air. maybe n 55 gallon plastic drums. with a
heat capacity flow rate of 8x75/24 = 25 btu/h-f and a 0 capacity rate ratio,
ntu = 25ft^2n(1.5btu/h-f-ft^2)/25 = 1.5n, and effectiveness e = 1-exp(-1.5n),
ie 78% for 1 drum, 95% for 2, and 99% for 3...

if the sewer drain is near the basement ceiling, we could avoid the initial
cost of a pump by hanging the heat exchanger near the ceiling. the operating
cost isn't much. i like attwood's v1250 12v bilge pumps (item 168226, $33.99
at boatus.com), which can move 1250 gph of cruddy water at 0 head or 500 gph
with a 6.7' head and a 3-year guarantee. their item 168228 float switch sells
for $20.99.  

>i have two pipes not far from each other. one carrying discarded
>water at about 40 c and another coming in with water at about at 5 c.
>is it practical to transfer that energy ?

we could save more cooling the water to 5 vs 20 c, especially if
(oil?) house heating is cheaper than (electric?) water heating...

greywater tends to foul heat exchangers with insulating layers of crud,
and we need a way to clean out or backflush holding tanks. i've thought
about a large galvanized tank inside a large unpressurized tank, a 4'x20'
horizontal pipe under the ceiling (which might come close to a counterflow
heat exchanger), gfx heat exchangers, or another water heater preheating
cold water next to a vertical thermosyphoning heat exchanger. a heat
exchanger with no external insulation can also act as a tempering tank,
preheating cold water with warm basement air long after hot water use. 

my basement has no sink. the septic pipe is 78" above the floor near the
shower drain and 60" above the floor near the washing machine. i could put
a $130 16" diam x 38" long 30 gallon galvanized tank inside a 2'x4'x3' tall
epdm-rubber-lined plywood box with drains from the shower and washer and
a sink on top and a bilge pump with a switch near the top of the box. 

how much money would that save per year, heating water at 6 cents/kwh?

nick




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