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SDHW Pump Selection - Page 3

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Posted by DJ on February 5, 2005, 1:16 am

I get what you mean. There has to be a sweet spot between fast and slow
pumps, hot and warm return lines.

If you run the system 'fast', the incoming water will be not be any
than the tank.  While the fluid is not in the collector for very long,
still picks up heat from the collector.  So it returns to the tank
than when it left.  With high velocity through the tank coil, there is
heat transfer, and heat is effectively being moved from the collector
to the

Thing is, the point is not to get the water in the tank to ambient
temperature, although that would save some energy. The point is to
potentially replace ALL fossil fuel expending, so the feed tank must
end up north of 60, which means the glycol, say, must be a fair bit
warmer than that. And to make it that hot, it has to stay in the box
for a while...

If you run the system 'slow', the incoming water will be the same
temperature as before.  The fluid will be in the collector longer and
up more heat per pound of fluid.  So the average fluid temperature and
collector temperature will be higher.  That's bad as it means more
losses from the hot collector.  Some may argue that this hotter fluid
up the tank 'better'.  But it does not.

No, it does not, from an efficiency viewpoint. But two things again:
one, especially in northern climes, letting the "box" get hot isn't a
bad idea; it'll melt the snow and ice off of it. These things, although
they don't work perfectly, do work well into the season where the white
stuff and the freezing rain is falling. So true, greater losses, but
quite often, those losses are useful! And besides, heat loss from solar
energy doesn't generate a bill at the end of the month, unlike that
from an electric hot water heater ;-).
Second, as I said, the purpose of these systems, as I understand them,
is to not heat water purely efficiently, but to heat it to a high
temperature as efficiently as possible, to replace the powered hot
water heater.

Another group I frequent, the moderator has a sig stating that the
internet is like a large circle of men with clubs, standing around
beating a spot on the ground where, years ago, a dead horse lay ;-).

Fact is, TD and the other manufacturers of these systems use slow
pumps, after presumably years and mucho bucks to come to that
conclusion. True, they could have been wrong, but it's not likely, is
it? Mind you, blindly accepting that fact is rather dull, and not very
stimulating ;-).

I say a dead horse is always worth beating if somebody learns something
from it. Daestrom, Nick, a few others, and I seem to have a fair handle
on  fluid dynamics and heat transfer, and anybody reading this thread
has a better handle on it now, too, including myself.

Well done, gents!

Now, to my boots and maybe a beer for the walk ;-)... gonna see if
Orion is in the sky yet...


Posted by nicksanspam on February 5, 2005, 3:09 pm

It might be, on a cold hazy day, or in early morning or late afternoon. If
it's 30 F outdoors and 100 Btu/h of sun (vs 250 Btu/h "full sun") enters
a 1 ft^2 collector with an R1 cover and perfect back insulation and P lb/h
of Ti (F) water flows in and To water flows out, the average collector temp
Ta = (Ti+To)/2, and the useful heat output q = p(To-Ti) Btu/h, right?

If the sun power that enters the collector equals the heat power that leaves,
100 = (Ta-30)1ft^2/R1+q = (To+Ti)/2-30+p(To-Ti), so To = (260-Ti(1-2p))/(1+2p)
and q = 2p(130-Ti)/(1+2p), right?

If Ti>130 F, this collector will lose heat, regardless of flow rate.

If Ti0, p=0 lb/h makes To0 F with q=0 Btu/h, p=0.1 makes To0 with q=5,
p=1 makes To0 with q , p=2 makes To2 with q$, and so on. So faster
pumping collects more heat at a lower temperature, but we can slow the pump to
make collector output temps higher than 130 F, with diminishing heat gains, and
collect some more heat at a higher return temp when the collector supply temp
is less than 130 F and the upper (stratified) storage tank temp exceeds 130 F.

Or possibly cooler. I don't see "residence time" as a useful concept here.

We'd like it warmer than that...

More than 60 C, ie 140 F...

It can be useful to slow down the pump to raise the collector output temp.
You might say that has someting to do with residence time.

Sounds like a good idea for PVs as well.

"Yearly backup fuel consumption" may be a nice measure of overall efficiency.

Slow pumps can cost less and use smaller pipes and less power. They are good
for stratified heat stores, esp. with evacuated tubes, which don't care much
about outdoor temps.


Posted by daestrom on February 5, 2005, 6:15 pm

Which isnt' really a problem with a proper controller.  With only 100 Btu/h
of sun and 30F outdoors, the collector will only reach 130F.  If the tank is
not at least 4 or 5 degrees cooler than the collector, the pump shouldn't
run at all.

Or keep the pump running at 'fast', and the whole system heats up to higher
temperatures faster.  Slowing the pump down makes for higher outlet
temperatures, but not higher heat collection.  And that's the whole point
isn't it, to collect more heat?

Of course the trade off is that higher flows require more pumping power.  So
there is a point of diminishing returns.

If you can 'stratify', then yes, we have a 'whole new ball game'.  Slower
flow rates can result in *some* of the water reaching the desired
temperature sooner.  But less water will actually be heated, much of the
stratified tank will still be cooler.  So if you just want a short burst of
hot water after the sun's been up for a couple hours, slower can be the way
to go.  But if you use a loop-tank heat-exchanger, then stratification is
harder to accomplish.  Unless you just put the coil in the top of the tank
to heat just the top portion.  But now you've 'wasted' a lot of storage tank
that never gets heated all the way.

Maybe a dual tank heating system, where a short coil in the top is heated
first, then as it's outlet temperature rises (indicating the top of the tank
is well heated in the morning), a couple of solenoid valves shift the flow
to a lower tank coil to heat the remaining water.  Also shift the pump speed
as well.  'Slow' in the morning while 'topping', to get the small amount of
water heated quickly, then 'fast' for bulk heating of the rest of the
storage tank.  Sort of like the two-stage electric elements used in some
electric heaters.

This would give quick 'topping' in the morning to make some water available
at the desired temperature, then shift to 'bulk' storage for the
accumulation of the solar heat the rest of the day.

Why so hot?  Unless you need 140F for a dishwasher, most domestic HW needs
don't need this high a temperature.  Easier to store a 100 gal at 120F than
80gal at 140F.  Mixed with a little raw 40F water, either one makes for just
as long/number of showers.


Posted by daestrom on February 5, 2005, 3:15 pm

Running 'fast' doesn't just warm the tank up to ambient.

Run 'slow', and the collector heats up fast, but the tank lags behind.  So
ambient losses from the collector rise quickly after the sun comes up and
the tank takes all day to warm up.

Run 'fast' and the collector and tank temperatures stay close to each other
and the two of them heat up slowly.  The collector doesn't warm up a lot
above ambient early in the day, so losses are kept lower.  Still takes
almost as long to warm the tank, but a bit less since the collector doesn't
lose as much.

A bogus argument.  Running the system 'fast' or 'slow', the collector is
warmer than freezing.  Otherwise, you'd be melting the snow with the warm
water from your tank/well.  If the collector isn't at *least* warmer than
the tank, you got no business running the circulating pump at all.

Running 'fast' or 'slow' will not change how fast the tank is warmed up.
That's the whole point.  Run it 'slow' and you have a 'slow' flow rate of
hot water into the tank.  So the tank heats up 'slow flow * high temp' =
'medium'.  Run it 'fast' and you have a 'fast' flow rate of 'luke-warm'
water into the tank.  So the tank heats up 'fast flow * luke-warm temp' =

The only difference is which one has more ambient losses.  Another argument
is if the return piping from the collector has a lot of distance to run, and
isn't super-insulated, the temperature to the tank will be significantly
less than the collector outlet if the system is 'slow'.  More losses there.

You just have to define 'slow' and 'fast' in the right way :-).  To get a
minimum of 4 ft/s in a 3/4 inch pipe is ~3GPM.  And that's what *I* think of
as 'fast'.  In a 4kw collector (peak energy input), 3 GPM would 'see' about
a 8.8 F temperature rise.

So are you an Aussie?  Orion's been in our sky here in New York for a couple
of months ;-)


Posted by DJ on February 6, 2005, 5:13 pm
 So are you an Aussie?  Orion's been in our sky here in New York for a
of months ;-)


Clouds and mountains, amigo ;-). And nope, not Aussie, Quebecois ;-).


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