Author Topic: Buyers final Checklist -- Geothermal Heat Pumps  (Read 3208 times)

factminder

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Buyers final Checklist -- Geothermal Heat Pumps
« on: May 29, 2013, 01:28:13 AM »
 
Buyer's final Checklist -- Geothermal  Heat Pumps
 
 
. The  "Home Energy Audit Report & Manual J load" concluded:  A   4t/3.5COP  system would be required for the home.
   
 HP 4t/49k btu's consumes 5.1kw   and  the water-circulation pumps and the air handler -use additional 2.6kw of energy.
"Coefficient of performance" measures only  the energy required by the heat pump.
 
 
 * Urgent Question: (Total kwh electricity used per 100.000btu's of  usable heat produced: (Compressor/ water circulating pumps/fan  is then..............kwh?)
 
                             
 Please enlighten.
 Inquiring Minds want to know?

woodgeek

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Re: Buyers final Checklist -- Geothermal Heat Pumps
« Reply #1 on: May 30, 2013, 06:43:12 PM »
The conversion is 3.414(kBTU/kWh).  The COP*3.414 is called the heating seasonal performance factor.  A COP=3.5 figure is 3.5*3.414=~12, about the same as a minisplit air source heat pump that costs a lot less.

Your question regarding the pump power is astute...it does reduce the effective COP significantly in geo systems (esp poorly installed one with too large pumps or deeper wells).  In your case, it would reduce the COP by a factor ~5.1/(5.1+2.6) = 0.66, reducing the COP to ~2.3.  This is too pessimistic since the air handler energy would appear as heat at COP = 1, I'd guess the COP is ~2.5.

In this case 100,000 BTUs would cost 100/(2.5*3.414) = 11.7 kWh.

My conventional cheapo air source HP (tricked out with defrost controls I hacked myself) is giving me COP=2.6 in a Philly climate, better than your quoted geo!

In general, in climates outside the northerm tier of the lower 48 and inland Canada, air source HPs provide a better ROI/lifetime operating cost than geos.  Where are you?

check out:
http://homeenergypros.lbl.gov/forum/topics/geothermal-real-world-performance?xg_source=activity&id=6069565%3ATopic%3A105500&page=1#comments
« Last Edit: May 30, 2013, 06:47:58 PM by woodgeek »

Matte

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Re: Buyers final Checklist -- Geothermal Heat Pumps
« Reply #2 on: June 01, 2013, 11:11:25 PM »
I would agree with factminder on the statement that air source would most likely be better for you dollars wise.  unless you can build a geothermal field affordably I highly doubt its worth it, here in Canada at least the cost and markup on geothermal make it way too expensive to build make sense (10k wont even get your heat sink or field drilled). 

Big cost savings could be had if you have the appropriate terrain and can make your heat sink side yourself, and combine it with something like a pond or landscaping job (horizontal field) or have a well or similar to use.