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Heat Pump COP with solar-warmed inlet

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Posted by Robert Scott on June 8, 2008, 11:20 am
The COP of a geothermal heat pump is given for inlet water temperatures in the
range of 30-50F as being from near 1.5 to about 3.0.  Does anyone have any data
on what the COP would be if the inlet temperature were raised to about 75F by
low-grade solar heat?

I currently have a "Water Furnace" brand heat pump with a closed-loop feed
system.  I am trying to decide if I should add solar heat using an independent
system, or if I should use the solar heat just to boost the efficiency of my
heat pump,  One of the advantages of the second approach is that I can make use
of lower temperature storage, as opposed to the direct method.  But the second
approach won't do me much good unless it significant raises the COP of the heat

Robert Scott
Ypsilanti, Michigan

Posted by Jim on June 10, 2008, 11:25 am
check out  http://www.bts-hvac.com/residential/#04

I am thinking of a heat pump in the Boston area, are they that much less
expensive to run ? Electrical cost ?
I have a high water table , will this decrease the amount of coils in the

Posted by AstickfortheMULE on June 10, 2008, 3:39 pm

You are basically want to use the ground as heat storage is that
correct?  Do you know what your inlet termpature is of your closed
ground loop?
Another question is what is how much energy are you using to heat your
house and heat hot water?

East Lansing averages around 400 btu's Nov-January for horizontal
insolation.  If you are using 300,000 btu's/day you want to augment
the ground energy use  by 200,000 btu's you would probably need about
400 sf of panels angled south.

I have 500 sf of solar panel in Seattle WA.  I have 4000 gallons of
heat storage that is burried and 800 gallon that is heated with a heat
pump.  My panels are sloped 20 degrees to the south.  Average
horizontal insolation for january is around 300btu's/sf.  With the
slope, lets call it 400 btu's/sf.

My panels are unglazed roofing system from Switzerland, called energie-
solaire  (www.energie-solair.com)  They are very efficient at lower
tempratures and might work very well for this type of system.

Technically I should recover up to 200,000 btu's per day in january.
I use about 3 therms a day for heating my 3,500 sf house (all SIPS
including roof, insulated basement with icf and 2" rigid under
concrete).  My heat storage is 4000 gallons and next winter I plan on
using my 3 ton water-water heat pump to pull heat out of the 4000
gallons and let the panels warm the heat storage.

Does this help?


Posted by Robert Scott on June 10, 2008, 4:28 pm
 On Tue, 10 Jun 2008 08:39:06 -0700 (PDT), AstickfortheMULE

I have two 2000 gal. septic tanks that I buried next to the house 30 years ago
just for this purpose, and there they have sat, unused and unconnected.  Talk
about a lack of follow through!  Anyway, I would use them to store the solar
heat from a liquid-type collector.  Then I would use that stored heat by
providing an alternate path for the closed loop liquid.  Ideally, I would have a
valve (either manual or automatic) that switched between the pipes in the
trench that I am using now and some copper tubing in the septic tanks.  Since I
want to protect the heat exchanger in the heat pump, I would not circulate the
water from the septic tank itself, thus the heat exchanger in the septic tanks.
Of course the other reason is that the septic tank is not pressurized, but the
close loop is (75 PSI, I think).

As for empirical data, I have been monitoring inlet and outlet temperatures in
the geotherman closed loop for 7 years now.  In the middle of the winter, after
a lot of use, the inlet temperature drops down to about 31F.  The outlet
temperature (in heating mode) is always about 7 degrees F lower than the inlet.

As for how much heat I need, back in the days when we had straight resistance
heat, I had a 10KW furnace, and in the middle of the winter it ran, worst case,
at a 60% duty cycle.  So we need 6 KW.

Yes, thanks for the case study.  But how do you use the stored heat to heat your
house?  Radiant floors?

Robert Scott
Ypsilanti, Michigan

Posted by News on June 17, 2008, 8:33 pm

Interesting.  The 800 gallon tank is DHW? A thermal store that gives DHW and
heating? Underfloor heating?

The 4000 gal underground tank. Insulated?  Only solar heated water water in
this tank?   How are they connected?

I have always thought that a pump, pumping through slinkies into an
underground heat store will keep the store at ground temp.  Then a heat pump
can extract heat from this.  Water stores far more heat than soil.  Freeze
protection of course would be needed.

Let us know how it performs when you do it.

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