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Old 10-10-09, 12:44 PM   #16
AC_Hacker
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Default Domestic Water Heat Recovery...

I saw an interesting and stunningly simple shower heat recovery unit...

It used a coil of copper tubing which lay flat on the bottom of the shower, under a wooden grate.

Incoming cold water would run into the copper tube, get warmed by the shower water, then out to be mixed with hot water and on to the shower head.

I expect that there would be a brief period of temperature adjustment, until things stabilized, but, no massive drain replumbing, no serious problem with the heat recovery unit being fouled with shower mung. Just lift the grate and brush it off from time to time.

Using direct heat exchange has its limits because heat can only flow down hill, and the less the slope of the hill, the lower the rate of heat transfer. Things can be improved by using a vapor-compression device to, in effect, change the slope of the hill.

So, another direction for this discussion might be the use of a small re-purposed vapor-compression unit, such as those found in air conditioners, de-humidifiers, tiny refrigerators, etc, to extract heat from the drain water (COP > 4) and store it in an auxilliary water pre-heat tank, or a large super-insulated water tank, which would be a "thermal battery", or to send the heat into the ground via a polyethylene loop field, where it would, over time also act as a thermal battery.

There are companies in the USA making super-insulated water tanks, but their performance is well behind that of some of the European countries, where energy is 2x what it is here, and the expense of perpetual war is not draining the national economy.

I did some hasty calcs regarding how much water would be required in a water type thermal battery, to make a difference toward helping the heating of my small home (< 900 sq. ft.) and I figured it should be in the 500 to 1000 gallon size.

Once built, a thermal battery could store waste heat from a refrigerator/freezer, solar heat collector, recycled heat from domestic hot water, etc.

There was some work done on thermal water storage tanks at build it solar at this link and also at this link. And of course, there has been work done on building your own in-ground loop field for heat extraction and injection at this link.

Best Regards,

-AC_Hacker

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