From: davidm@meaddata.com (David Miller)
Subject: Re: Ground source heat pump
Organization: Mead Data Central, Dayton OH
Date: Wed, 29 Jul 1992 17:37:22 GMT
Distribution: usa

In article <1992Jul29.132627.14427@osf.org>, wwang@osf.org (Weidong Wang) writes:
|> 
|> Just heard on the radio (Paul Harvey News and Comments) this morning:
|> 
|> There is this so called ground source heat pump available now for new houses
|> which will pump the ground air (cooler in the summer and warmer in the winter)
|> to your house and which can save you at least 1/3 of your heating and air conditioning cost.
|> 
|> Has anyone had any experience with it? Just curiouse to know.
|> 
|> -- 
|>     +--------------------------------------------------------------------+
|>     |  Weidong Wang                   |  Open Software Foundation        |
|>     |  wwang@osf.org                  |  11 Cambridge Center             |
|>     |  (617) 621-7272                 |  Cambridge, MA 02142             |
|>     +--------------------------------------------------------------------+
|> 
|> 

A heat pump uses freon gas (or some other equivalent) to exchange heat.  It
is an air conditioner that works both ways.  For cooling, it compresses the
gas, releasing the excess heat outdoors, and evaporates to a gas and absorbs
heat indoors.  The process is reversed for heating.  Normally, the outdoor
heat exchanger is a set of coils with a blower to pull air across the coils.
When it is extremely hot outside, the unit must work much harder to release
even more heat, likewise, if it is extremely cold out, the unit must work
harder to absorb heat.  

Because of the extremes in outside air temperature, it makes a poor choice
for the heat exchange medium.  However, it is the easiest/cheapest.  

There are two geothermal variations using water as a heat exchange medium.  
One type (the most common I believe) uses two wells, and pumps ground water
from one well, through a heat exchanger, and back down the other well.  The
second type places the heat exchanger in contact with the water source (such
as a lake or underground stream).  In both cases, the water temperature 
variations are much less than air, plus water is a more efficient heat
exchange medium, so operation costs are reduced.

A newer form of geothermal unit takes the coils that use to be exposed to the
air and buries them below the frost line in the ground.  Since the ground
does circulate like water does, a larger amount of contact area is required.
Hence, a lot of copper coil, and more freon is required.  Again, the same
benefits as water are gained.

All heat pumps just warm the air while heating - regardless of heat exchange.  
The better the heat exchange on the outside, the less energy required.  
Also, the air coils require periodic defrosting, which is wasted energy.  
And, if the outside temperature is really cold, the auxillary heating device 
(usually resistence heat which is inefficient) is required.

The geothermal units require a larger initial investment (at least 5K more for
ground exchange in my area) than the air units.  The payback depends largely
on climate and usage.  Here in Ohio, the extremes aren't too bad so the payback
is longer.  In the UP of Michigan, an air exchange unit would probably be nearly
worthless as a heating device, but a geothermal unit would function fairly
economically.  Same things goes for cooling - a geothermal unit in a hot
climate would provide a much faster payback.

Hope this helps answer your question as well as some others I've seen recently.


-- 
David James Miller                        Architecture&Systems Evolution
Mead Data Central, B6F3                   (513) 865-7441
PO Box 933                                davidm@meaddata.com
Dayton Ohio 45401                         I speak for me, not MDC