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Heat Pump COP with solar-warmed inlet - Page 2

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Posted by psomdecerff on June 24, 2008, 10:15 pm
 
On Jun 8, 6:20am, -...@---.--- (Robert Scott) wrote:

Robert,
I have been thinking about the same thing.  I figure that since solar
panels tend to work better at lower temps and heat pumps work better
with higher outside temps (for heating), then why not couple them and
start taking baby-steps to a full-solar heating solution?

This page has some excellent graphs that should help answer your
question.  Basically the additional heat at the outside unit will bump
up the output indoors, but at least for the unit illustrated the ratio
of power in to power out (COP) only gets slightly better as the
temperature goes up.  It looks like to me that this COP would go from
8 or 10 to 12 or 13 over that range, but you'd have to plug in the
numbers for your unit.  However, since you'll be moving more heat
overall as well as moving it more efficiently you'll also change the
point at which supplemental heating is required, and by so doing you
should get heat-pump COP savings for a bigger fraction of your heating
hours, and you might even get by with a smaller unit.

http://www.arthurhewett.com/id26.htm#cop

If you have 75F water, though, it seems like you're getting close to
being able to use hydronic heating directly. That's a VERY high COP
indeed!

Posted by Robert Scott on June 25, 2008, 8:37 pm
 
On Tue, 24 Jun 2008 15:15:58 -0700 (PDT), psomdecerff@gmail.com wrote:


I think your math is a little off.  The data at the page you cited shows the
very best COP to be about 4.25 for a 65 degree outside air temp.  A COP of 8 to
10 is unreasonable.

But thanks for the reference.  Even though it is about an air-to-air unit and
mine is liquid-based, it does give me an idea about how COP is related to inlet
temperature.


Robert Scott
Ypsilanti, Michigan


Posted by Jeff on June 26, 2008, 12:08 am
 Robert Scott wrote:

  I think those are probably EER or SEER figures.

   Jeff


Posted by daestrom on June 25, 2008, 11:09 pm
 Robert Scott wrote:

The basic idea is sound.  But there is one very important issue.  What
happens if your heat pump moves heat out of the panel faster than the sun
can replace it?

In the worst case scenario, you cool the solar panel and storage down colder
than the outside and your heat pump works even harder (or not at all).

Or if you have a run of cloudy days and the storage isn't big enough to
'ride' through those days, the heat pump does the same thing.

So if the solar panels and storage are large enough to supply all your
heating needs while operating at some low but sustainable temperature, then
this idea will work and you should see improvements as you speculate.  But
if the overall panel heat collected is too small...

daestrom


Posted by Robert Scott on June 26, 2008, 2:45 am
 On Wed, 25 Jun 2008 19:09:19 -0400, "daestrom"


This solar assist would not replace my existing geothermal loops.  It would be
an additional alternate source.  So if the stored water starts to get too cold,
a valve will switch back to using the 2000 feet of tubing in the trench.


Robert Scott
Ypsilanti, Michigan


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