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Absorption heat pump? - Page 2

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Posted by Bruce Richmond on February 10, 2008, 9:05 pm
 

Thank you for the link but I asked about heat pumps because I need
heat.  I can get along fine without AC.  I thought by using a ground
loop and heat pump with a COP of 1.5 to 2 I could get more out of the
fuel I use.  The absorption unit seemed to make more sense than paying
the high price of electric so that they could dump waste heat and send
me just the electricity.  I don't think solar would do too well on our
long winter nights with the temp below 0 F degrees.

Bruce

Posted by George Cornelius on February 11, 2008, 6:50 am
 

Well, it is theoretically possible.

Imagine a Carnot engine operating from 1000K to 300K.  For an input
of dQ, you get 300 dQ / 1000 or 0.3 dQ at the lower temperature
sink (the home), and (1000-300) dQ / 1000 or 0.7 dQ at the shaft
output.

Feed that 0.7 dQ into a reverse Carnot cycle between 300K and 275K
to add additional heat to the home from a low temperature source
and you get 0.7 dQ * 300 / (300-275) or 8.4 dQ at the higher temp
and 0.7 dQ * 275 / (300-275) or 7.7 dQ from the lower temp source.

Counting the original 0.3 dQ waste heat, you get 8.7 dQ heating from
1.0 dQ input, a 770% gain.

[Ideal Carnot calculations are based on heat flow at a source/sink being
proportional to absolute temperature of the sink, the energy flow to/from
the shaft then being the difference between the two thermal flows].

In a real world situation, if the first engine is 20% efficient, you
only get 0.2 dQ into the shaft, and the second phase, if it has a COP
of 1.5, only gets 0.3 dQ from the low temperature source.  So, from
the original source and the low temperature source you get 1.3 dQ to
heat the home from an input of 1.0 dQ.

Note that in the second case we merely assume an efficiency for the
heat engine and no longer make any assumption about the temperature
of the original heat source.

There is a 6:1 ratio between the heat outputs in the two cases.  One
would assume that there _must_ be a way to do better than my second,
'real world', scenario, but I don't know if an absorption unit is it.

--
George Cornelius              cornelius ( A T ) eisner.decus.org

Posted by George Cornelius on February 12, 2008, 12:00 am
 corneliusNOTREAL@DECUServe.org (George Cornelius) writes:

     ^^^
     2.5 for heating mode

[I was thinking of a refrigeration unit's effectiveness rating, heat flow
on the cold side as a ratio K to shaft power in.  A heat pump in heating
mode is essentially a refrigeration unit arranged as if attempting to cool
the outside, and the heating COP would be the ratio of the heat flow at
the hot side to shaft power in, i.e., COP = K + 1 ].

--
George Cornelius              cornelius ( A T ) eisner.decus.org

Posted by Roger Thorpe on February 12, 2008, 10:17 am
 George Cornelius wrote:

  /snip..../


Some of the reasons that absorption and adsorption machines are so well
below Carnot stem from exergy losses (or entropy generation if you
prefer) due to:
* Finite sized heat exchangers with significant delta T
* Non - equilibrium processes (diffusion for example)
* Temperature and capacity flowrate mismatch between fluids in the heat
recovery components.
* Demand for driving heat by the working pair across a range of rather
than single temperature.
* Other stuff that I've forgotten.

  Some of these seem like fundamental limitations, but there's work
going on. Currently there are efforts being made to improve the COP with
novel pairs, cascades, multiple effect and improved heat transfer. The
solar heat pump may not come, but the solar air conditioner does seem to
be on its way. Reducing production costs will probably be key to this.
Roger thorpe

Posted by Neon John on February 10, 2008, 5:13 pm
 

How can you be a fan of something that doesn't exist in any meaningful way in the
civilian market?

Absorption ACs are commonly used where waste heat of the appropriate quality is
available.  Petrochemical industries, for example.

Conventional absorption ACs achieve a COP of less than 1.  That is, they use more
energy to operate than they transport.  Contrast that to a conventional AC that
has a
COP of from 3 to 5.


Sorry, but gotta call a bullshit foul on this one, at least based on what they
present.  It smells like the gentle Scandinavian version of a scam.

A few clues.  First, no meaningful technical or performance information at all.
If
the thing works then present ASHRAE-standard performance data.  Second, though
they
claim to have patented this system, I can find no evidence of it, searching for
the
company name, "triple-phase absorption" or "absorption air conditioning".  Third,
what little technical information there is, is presented in a deceptive manner.
Here,
for instance,

http://www.climatewell.com/index.php?pageId4

Near the bottom of the page.  They've made up something they call "electrical
COP"
which has no practical meaning.  They claim it to represent the cooling output
divided by the electrical input.  Only problem is, electricity does not
participate
in the refrigeration process.  If they drove the circulation pump using a
Sterling
engine powered by the solar heat then the "electrical COP" would be infinity.
bullshit!

The actual definition of COP has no connection to electricity.  It is strictly
energy
moved divided by energy consumed.  The natural gas fired ACs that have been
available
at various times that used IC engines and conventional refrigeration cycles had
COP
specification even though the only electrical use was for the thermostat.

Once my BS alarm goes off, it takes a lot to reset it.

The other "spec" on that page, the "thermal efficiency" (whatever that means)
might
be closer to a COP.  A COP of 0.68 for an absorption process using low quality
heat
would seem reasonable.  I suppose that this is OK if the site has lots of solar
energy available AND the owner can afford the collector array.  It would REALLY
suck
if one were using it on a district heating system (one of their claimed
applications)
where one pays for the BTUs.  Even on the heating side, the COP looks to be less
than
1 which is what simple resistance heating achieves.

An indication of just how inefficient this system is can be seen on the
"contacts"
page (why it is there, who knows?)

http://www.climatewell.com/index.php?pageId0

Look at the size of those collectors, especially for such a small house in a very
temperate climate.  I'd hate to see the size of the collector for a 2000 sq ft
house
in Atlanta!

I do have to admit that seeing a blonde haired chick wielding a MIG torch does
have
some appeal.... :-)  http://www.climatewell.com/index.php?pageId=3

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
A few harmless flakes working together can unleash an avalanche of destruction.


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