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Ground Loop Heat Transfer Question

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Posted by Alan C37 on August 30, 2006, 2:14 pm
 
  I am trying to design a ground loop water circulation system to warm (or
cool) air for an air source Heat Pump.  Here I am concerned mainly with the
heating function.  I am having problems with some theoretical details
concerning heat transfer rates and would appreciate any advice anyone cares
to offer.
  The hypothetical situation is :-
  A modern Air Source Heat Pump with the external unit located in a fairly
large (I have little idea of how large it must be yet!) insulated enclosure,
during the heating season.  The manufacturer claims that the unit will give
a gain in heating with air temperatures as low as 10 deg F.  I also
understand that putting the unit in an enclosure like this would invalidate
the maker's warranty but I might take the risk.  The unit will be sized to
provide up to 12000 Btu/hr, or about 3.5 kW of heating
  A long (perhaps 300 ft) loop of 4 inch diameter pipe buried about 4 ft in
sandy soil which may be somewhat damp during the heating season.  Water (or
antifreeze) will be pumped around this loop, warmed by heat in the ground
and then passed through an array of finned pipes located on the air inlet
side of the blower in the heat pump outside unit.
  The idea, of course, is to raise the air temperature enough to extend the
range of outside temperatures at which I can get useful heating performance
for the system and reduce my use of back-up baseboard heaters (reverse in
summer but that's another, easier, problem).  The water in the pipe will be
warmed by the heat in the ground (at steady state) to perhaps 55 deg F, in
the area in which I live, even in the coldest month.  I want the heat pump
input air to be at a minimum temperature of 30 deg F (preferably higher if
possible) since the heating performance will fall off quite rapidly at lower
temperatures.  The output air might be significantly lower than this but
will have to be warmed up again by heat from the ground loop to 50 deg F or
so quickly enough to be capable of being recalculated at the higher
temperature.  Given an input water temperature of 50 deg F I would expect an
outlet water temperature of about 35 deg F and the water to be circulating
at about 1 gallon (Imperial) per minute.  This suggests that water would be
underground and moving along the pipe for about 2.5 hrs.  There are other
problems but the bits of theory I am stuck on at present are:-
  What volume of air do I need in the enclosure?
  What rate must I pump the water at (the 1 gall per minute given above is
merely a guess at present)?
  What is the minimum length of ground loop I could get away with?
  Will the soil around the pipe have sufficient time to recover from the
cooling effect of the water in the pipe before the water returns to the heat
exchanger in the enclosure?  This is my main problem and having no idea of
how heat would flow in the ground to the water in the pipe I find it
impossible to proceed with the design.  I am seriously considering building
a test installation and actually pumping water around it over the coming
winter and acquiring adequate test data with various water flow rates.  If
anyone can point me towards any existing valid theoretical or test data I
would be most grateful as it seems to be a bit more than my ageing brain can
deal with.
  I should probably say that I am aware of the existence of actual ground
source heat pumps and have seen claims about their performance which look
quite good.  I have had no useful responses from people who make these units
(maybe I haven't yet found the right manufacturer yet).  I am convinced that
modern air source heat pumps are readily available and have very good
performance figures, reasonably low prices and are not expensive to install.
Even if I do not go ahead with the enclosed unit idea it will still be
economically well worthwhile to install an air source heat pump and accept
that my existing baseboard heaters will be used more often than I would
like.  Other people's experience with these thing indicate that I should be
able to reduce my annual electrical costs from about $000 to $000 or less.
Adding the ground loop shows reasonable promise of significantly more
savings.
  Thanks.
   Alan C
--
acombellack@nspmsympatico.ca



--
Posted via a free Usenet account from http://www.teranews.com


Posted by Alan C37 on September 3, 2006, 1:16 pm
 
 I thought I had sent this yesterday but I don't see it on the NG so I am
trying again.  Alan C

I am somewhat surprised that there has been no response to my post.  I am
well aware that this is complicated and have only rudimentary ideas about
how to answer my own questions so may I rephrase my main problem a little?
  Consider a loop of 4 inch diameter plastic pipe buried 4 ft deep in sandy,
snow covered soil.  The initial temperature of the soil is 55 deg F.  The
pipe is full of water which is being pumped around the loop at 0.16 cubic
feet per minute.  The temperature of the water when it enters the loop is 35
deg F.  I assume it has been circulating for a full day or more and the
system has reached a steady state.  The question I need an answer to is
What will be the temperature of the water at the output end of the loop?  I
doubt that it will be as high 55 deg F but I hope it will be significantly
more than 35 deg F.  Any ideas would be gratefully received as I am having
problems even imagining how heat will flow under these circumstances.
  Thank you,
  Alan C


Posted via a free Usenet account from http://www.teranews.com


Posted by daestrom on September 3, 2006, 3:58 pm
 

What you are describing is a ground-source system with an intermediate air
exchange.  Quite frankly, because of the intermediate air-exchange, it is
bound to be less effective than a direct ground-source unit.  Unless the
price difference between ground-source and air-source-with-ground-loop is
really large, I would go with the direct ground-source unit.

As far as how much water you would need to circulate, you can get an
estimate by this calculation...

Q = Mrate * Cp *(Tin-Tout)
Where
Q is BTU/hr
Tin & Tout are degrees F
Cp = 1 BTU/lbm-F for water, about 0.7 for 50/50 antifreeze
Mrate is in lbm/hour

For water....
12000 = Mrate * 1.0 *(50 -35)
Mrate = 800 lbm/hr
800 lbm/hr is about 1.6 gpm

For antifreeze/water
12000 = Mrate * 0.7*(50-35)
Mrate = 1142 lbm/hr
1142 lbm/hr is about 2.3 gpm

But 12000 BTU/hr seems like a very low number for climates that get below
freezing for much of the winter.  Are you sure about that number?  My
conventional furnace is 80,000 BTU/hr.  To replace it with a heat-pump that
runs the same amount of time, I would need water flow of...

80000 = Mrate * 1.0*(50-35)
Mrate = 5333 lbm/hr
5333 lbm/hr is about 10.7 gpm

And because my ground temperature at *six* feet down is only about 40F....
80000 = Mrate * 1.0*(40-35)
Mrate = 16000 lbm/hr
16000 lbm/hr is about 32 gpm


The only difficulty with ground loops (whether direct-ground-source heat
pumps, or your design) is calculating the amount of heat that can be
extracted on a sustained basis.  Yes, the average ground temperature might
be as high as 55F where you are (in winter here in NY it is decidedly colder
than that at 4 feet), but it won't *stay* at 55F for days on out with you
extracting a lot of heat from it.  It will start to develop a sort of curved
profile with the temperature at 4 ft in your neighbors land still at 55.
But as you get closer and closer to your buried line it will start to drop
off and get colder as it approaches the line.  How 'steep' this profile gets
is a function of the type of soil (how well it conducts heat from
surrounding soil, ground-water such as you mentioned can help a lot) and how
much heat is being extracted on average per day per linear foot.  (spacing
between adjacent 'runs' of tubing is also important).

If there is 'moving' ground water, it can be really beneficial as the ground
water can carry a lot of energy passed your system.  Water from much deeper
can be a lot warmer and is often used in this way (open-loop, deep-well heat
pumps).

I suspect that manufacturers of ground-source systems haven't been too
responsive because of this variation.  The success or failure of such a
system is largely determined by the ground-loop field.  Deep-well systems,
'slinky-loops', straight tubes, in wet soil, dry soil,  wells drilled
through solid rock, there are just so many variations that the manufacturer
that just puts together the heat-pump and heat-exchanger has little control
over.

It would be best if one could somehow measure the performance to expect from
a given soil condition/installation without the expense of burying a long
length of tubing.  Then one could get an accurate idea of how much heat can
be extracted from some ground-loop installation and how big of an
installation is needed.


One thought about your idea.  If a conventional ground-source heat pump has
a ground loop that underperforms, the whole unit has to switch to resistive
heating or some other back up when the temperature of the ground loop falls
too low.  This can *really* hurt overall performane costs.  With your
system, if you leave some provision for opening the enclosure, then *if* the
ground loop performance is not up to the task, you can open the enclosure
and run the unit as an air-source.  Admittedly this may still be a bit
costly (poor ground loop performance is most likely in cold weather), but
the whole system isn't completely out-of-commission.  Then the next year,
funds permitting, the ground loop could be expanded/updated.

And since you're currently using resistance heating (electric baseboard),
*any* amount of heat-pump operation (spring/fall mild-winter weather) will
reduce energy costs.

daestrom
P.S.  I'd seriously think about using an antifreeze mixture.  Then you can
operate the loop at much colder temperatures and freezing the ground water
can provide substantial energy for a short term need (say, one particularly
cold day or two).


Posted by SJC on September 3, 2006, 4:22 pm
    I like combining solar thermal a with fluid source heat pump.
With a large water store, you can run the solar thermal collectors
at a very efficient point even on a cold day. If the air is 30F and
the water gets to 90F, then there is plenty of heat in say 1000 gallons
of water to extract with a heat pump. Running solar thermal collectors
at only 90F on a 30F day is efficient. Storing water at only 90F reduces
the loses in the tank. You would just have to find what kind of COP a
fluid source heat pump gets when you input warm water like 80-90F
instead of 50F water from a ground source.

You can find the efficiency of a flat plate solar thermal collector in the PDF
here.
http://www.sunearthinc.com/empire_series_flat_plate.htm
A 36F delta between air and water gets you over 50% of the available solar heat.

Here is one of many ground (fluid) source heat pump manufacturers.
http://www.hydronmodule.com/singlespeed.html
I would think that they have performance data like COP based on source
temperature.


cool) air for an air source Heat Pump.  Here I am

theoretical details concerning heat transfer rates

large (I have little idea of how large it must be yet!)

the unit will give a gain in heating with air

an enclosure like this would invalidate the maker's

12000 Btu/hr, or about 3.5 kW of heating

sandy soil which may be somewhat damp during the

warmed by heat in the ground and then passed through an

pump outside unit.

range of outside temperatures at which I can get

baseboard heaters (reverse in summer but that's another,

ground (at steady state) to perhaps 55 deg F, in the

air to be at a minimum temperature of 30 deg F

quite rapidly at lower temperatures.  The output air

heat from the ground loop to 50 deg F or so quickly

an input water temperature of 50 deg F I would

circulating at about 1 gallon (Imperial) per minute.

about 2.5 hrs.  There are other problems but the bits

merely a guess at present)?

cooling effect of the water in the pipe before the water

having no idea of how heat would flow in the ground

am seriously considering building a test

acquiring adequate test data with various water flow

data I would be most grateful as it seems to be a

source heat pumps and have seen claims about their

people who make these units (maybe I haven't yet found

pumps are readily available and have very good

Even if I do not go ahead with the enclosed unit

heat pump and accept that my existing baseboard

with these thing indicate that I should be able to

the ground loop shows reasonable promise of

exchange.  Quite frankly, because of the intermediate

unit.  Unless the price difference between

the direct ground-source unit.

by this calculation...

freezing for much of the winter.  Are you sure about

heat-pump that runs the same amount of time, I would

pumps, or your design) is calculating the amount of heat

temperature might be as high as 55F where you are (in winter

55F for days on out with you extracting a lot of

temperature at 4 ft in your neighbors land still at 55.

and get colder as it approaches the line.  How

conducts heat from surrounding soil, ground-water such as

per day per linear foot.  (spacing between adjacent

water can carry a lot of energy passed your system.

(open-loop, deep-well heat pumps).

responsive because of this variation.  The success or

Deep-well systems, 'slinky-loops', straight tubes, in

variations that the manufacturer that just puts

given soil condition/installation without the

idea of how much heat can be extracted from some

ground loop that underperforms, the whole unit has

the ground loop falls too low.  This can *really*

for opening the enclosure, then *if* the ground loop

as an air-source.  Admittedly this may still be a

the whole system isn't completely out-of-commission.

expanded/updated.

*any* amount of heat-pump operation (spring/fall

operate the loop at much colder temperatures and

(say, one particularly cold day or two).


Posted by Anthony Matonak on September 3, 2006, 5:35 pm
 SJC wrote:

I wonder if one couldn't create a solar thermal collector for the
ground over the geothermal loops simply by suspending sheets of
greenhouse plastic film an inch or two off the dirt. They would be
like cold frames or really flat greenhouses and as long as they
are kept clear of snow, they should warm the ground somewhat.

Anthony

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