Some of you may remember a thread that I started a month or two ago about
finding a potable water pump for a TEC based water cooler I wanted to build.
I  received many useful and interesting replies. Those, of course, are two
overlapping, distinct, subsets of all the replies :-)


Well, I have progressed in my thinking and want to expose my current
thoughts for comment. See the section after the line "******" for specific
areas where I am looking for ideas.

As currently designed, my system would consist of three primary components:

1: A cold water reservoir. This is fed by the output of the water filter, at
house mains water pressure, and the output is to the separate "good water"
tap at the sink.

2: A cooling loop. This consists of a water-to-water heat exchanger within
the cold water reservoir (just a coil of copper tubing), a small pump, and a
waterblock mounted to the TEC for extraction of heat from the system.

3: A heat sink. This would consist of a large container of water thermally
connected to the hot side of the TEC.

I expect to fabricate the cold water reservoir from a vertical section of
large diameter PVC pipe. This is both pressure-safe and safe for drinking
water. The reservoir will have two ports: (a) at the top, via a simple
diffuser that forces the water to spread out horizontally for the incoming
water, and (b) at the bottom for the cold water draw off to be sent to the
tap at the sink. I need to complicate things a little bit, though, because I
have to run a loop back from the tap which returns to the top port. This
allows me to keep the water in the loop at the cold reservoir temperature so
that it is immediately available at the tap. (Otherwise I'd have a problem
similar to waiting forever for the hot water when at a distance from the
heater and waste a lot of my cold water just filling the tubing)

The cooling loop is a separate closed loop for several reasons:
1: Its components do not have to be safe for drinking water.
2: It can operate at atmospheric instead of house mains pressure.
3: Its cooling liquid can contain additions to inhibit bacteria, mold and
corrosion as necessary.
4: Its cooling liquid can run below 0 C to provide more efficient heat
transfer to the reservoir

My heat sink will just be a large container of room temperature water (in
the basement in my case) thermally connected to the hot side of the TEC. I
can make a lot of cold water before the heat sink will get too hot, and if
it does the controller can just shut the system down, or refill the heatsink
with new tap water :-)

To maximize efficiency, the cooling loop (not including the pump motor of
course), the cold water reservoir, and the loop to the tap (about 2 m long)
would all be 'hyperinsulated'. Most of it would be inside several inches of
foil-faced polyisocyanurate foam (best R for given thickness, easy to work
with) sealed with foil tape. Both the up and down runs of tubing for the
loop to tap would be enclosed at the center of a single assembly of two or
three layers of standard foam pipe insulation wrapped with foil and sealed
with foil tape. With this construction, my computations show that I can
limit loses to 1 or 2 Watts or so, not counting whatever loss I get by
reverse conduction through the unpumped liquid in the loop from the heat
sink when the TEC is off.

Just a couple of numbers from my thinking:

Desired dispense temperature: 5 C
Recovery rate: 2L per hour assuming incoming feed at 20 C
Heat flow required for recovery above: 35 Watts
Reservoir capacity: 4L
Reservoir size: about 50 cm of  4" PVC pipe (sorry for the mixed units but I
really know nothing about metric pipe sizes).

********

So finally, questions/requests:

1: Any general comments, including criticisms please (if couched politely).

2: I need a way to get the water in the reservoir and the loop to the tap to
recirculate for two reasons: (a) to keep cold water at the top of the loop
near the tap, and (b) to mix the water in the reservoir so that it is all
cooled by the heat exchanger. I don't know if natural convection can do it
for me, but I don't think so without allowing for more heat loss to set up a
bigger temperature differential and I don't want that. Any good ideas here?
I don't want or need a real pump here, just something to keep the water
drifting along. Also this is something that I don't want to have in action
when I am in a drawdown state (i.e.: water has been used and I haven't yet
recovered from it), because I am hoping that the water will remain
stratified in the reservoir (warm on top where it comes in and cold on the
bottom where it is drawn). Note, by the way, that a checkvalve must be
included between the loop return and the top of the reservoir to prevent
the tap from trying to draw from the top instead of the bottom of the
reservoir. The recirculation could be driven in the tap loop, or in the
reservoir itself. Perhaps something magnetically coupled. Lack of wear
points would be great.

3: Any good insulation ideas. Especially those that would allow for easy
access for rework, repairs, etc.

4: Any good way to thermally isolate the heatsink / TEC from the cooling
loop when the TEC is not running to avoid thermal loss. (One of my prime
goals here is to make the 'idle efficiency' of this thing ridiculously
good - just for the challenge of it. The construction here is currently
planned as follows: (a) Large container with thermal 'slug'
bolted/soldered/epoxied/welded to it, (b) Slug/TEC/waterblock stack bolted
together, (c) waterblock plumbed into cooling loop. For details on the
waterblock I have see:
http://www.polarflo.com/index.asp?PageAction=VIEWPROD&ProdID=125&HS=1

5: I need cheap, simple temperature sensors to connect to PIC. Working
temperature range from 0 C to 30 C or so. Accuracy not as important as
precision, although it would be nice if they 'tracked' each other very well.
Response time doesn't have to be particularly fast. Linearity is not a big
deal if the correction function is known. They should be easy to attach to
various components of system. They will all be within the 'enclosure' of
this device, at most 0.75 m or so from the controlling PIC. [Hey, is that
enough PIC related to let me change the topic of this message? :-)]

6: Good ideas for diffusers for the reservoir. The goal is to keep the water
stratified when in the drawdown state so that consistently cold water is
available for as long as possible.

7: Good ideas for bringing the cooling loop lines, and several termperature
sensor lines, out of the reservoir without compromising the pressure
integrity of the reservoir.

8: A simple way to sense when water is being drawn from the system.

n: Any general comments, including criticisms please (if couched politely).

===============

Addendum:

Expected user interface: one tricolor LED and one 'touch button'
(capacitively coupled input on sink deck). Touch button operates small
valves mounted directly below tap to select between output of cooler and
direct output of filter (no sense in filling the tea kettle with chilled
water). LED is green when valve has selected room temperature water, blue
for cold, red is reserved for potential future 'instant hot water' mode.
Unit reverts to cold water mode after a timeout. If unit is in recovery mode
the blue LED blinks, mostly on when nearly back to full capacity, lower duty
cycle when further to go.

Comments on this?

Thanks!!!

Bob Ammerman
RAm Systems

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