Applications Manual Bosch Geothermal Heat Pumps
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Bosch Thermotechnology Corp.
Data subject to change
has relatively good heat transfer ability as well. However,
it is extremely poisonous to humans and other animals in
any form and evaporates quickly. This process can result in
asphyxiation if an installer or other person fails to follow
all of the safety precautions for the substance. Methanol is
also highly ammable in concentrations greater than 25%,
and can cause an explosion.
Several states in the US have outlawed methanol use in
closed-loop ground heat exchangers buried deeper than
approximately 20 feet, and other states have outlawed
its use in all closed-loops in an effort to protect the
groundwater if the closed-loop should leak. However, even
with a closed-loop leak, methanol tends to biodegrade
rapidly.
Propylene glycol is used in the food preparation industry
and is considered non-toxic and non-corrosive. Only food
grade propylene glycol is recommended for geothermal
use to prevent the corrosion inhibitors (often present
in other mixtures) from reacting with local water.
Unfortunately, propylene glycol has exhibited some
viscosity problems at lower temperatures that limit its
use as antifreeze for owing uids with closed-loop
geothermal heat pump systems. It is however acceptable
in systems anticipating loops temperatures no colder than
approximately 40°F. These systems typically use antifreeze
because of low ambient conditions (outside plumbing,
etc.). When loop temperatures are below 40°F, the uid
becomes very difcult to pump and heat transfer decays.
Propylene glycol is relatively expensive and exhibits the
poorest heat transfer of all antifreezes for geothermal
use. It has also been known to form “slime-type” coatings
inside geothermal piping. Installers should always use
proper design principles when considering its use in
ground heat exchangers, as it is possible with poor design
for the uid ow to be too fast in the summer and too slow
in the winter.
Ironically, propylene glycol's low toxicity makes it the only
closed-loop antifreeze many states allow for ground heat
exchanger use. Many sources recommend propylene glycol,
but only for closed-loop ground heat exchangers that have
been properly sized and designed by a knowledgeable
ground heat exchanger designer or by using an industry-
accepted software program such as Bosch’s Geo
Solutions.
Ethanol or grain alcohol is also used by many installers
for geothermal heat pump closed-loop ground heat
exchangers. It has similar characteristics to methanol and
ows well. It also exhibits relatively good heat transfer
abilities, while providing good freeze protection down to
approximately 15°F in most situations. As with methanol,
ethanol is also very ammable in concentrations greater
than 10% and can cause explosions and asphyxiation. The
U.S. Bureau of Alcohol, Tobacco, and Firearms (ATF) limits
its distribution. Ethanol used for geothermal applications
must not be denatured with any petroleum based product.
Denaturing agents that are petroleum based can damage
polyethylene pipe.
Recommended levels of antifreeze solutions used in
geothermal heat pump applications are shown in Table 28.
Recommended Levels of Antifreeze Solutions
Type
Minimum Temp for Low Temp Protection
10°F
[-12.2°C]
15°F
[-9.4°C]
20°F
[-6.7°C]
25°F
[-3.9°C]
Methanol
25%
21%
16%
10%
100% USP food grade
Propylene Glycol
38%
25%
22%
15%**
Ethanol*
29%
25%
20%
14%
Tab. 28
* must not be denatured with petroleum based product.
** Not recommended without inhibitors
Antifreeze will change the properties of the closed-
loop ground heat exchanger uid. Adding antifreeze
will generally lower the thermal capacity of the uid
and increase the viscosity, which will increase pumping
requirements. Also, the “Reynolds number (Re)” (for
calculating ow – typically 2,100 minimum) will decrease
and raise the ow rate (GPM) at which laminar ow
begins. The “Reynolds number (Re)” expresses the ratio
of inertial (resistant to change or motion) forces to
viscous (heavy and gluey) forces. To offset these effects,
the design ow rate (GPM) may need to be increased.
Most closed-loop ground heat exchanger design software
programs (such as Bosch’s Geo Solutions) include a
range of antifreezes and take into account the change in
properties.
A potential negative effect of all geothermal heat pumps
is the release of antifreeze solutions to the environment.
These chemicals are generally mixed with water when
used as a heat exchange uid and can be released via
spills or corrosion of system components. Always check
with local codes and authorities concerning acceptable
antifreeze use and procedures.
The antifreeze mixture used in the Bosch geothermal heat
pump system also has an effect on the system economics,
including the cost of the antifreeze, the change in pipe
resistance, Bosch geothermal heat pump performance and
the change in pumping requirements.
All alcohols (methanol and ethanol) should be premixed
and pumped from a reservoir outside of the home if
possible, or introduced under the water level to prevent
fumes. The installer can calculate the total volume of uid
in the piping system by using the chart shown (Table 29).
Then use the percentage by volume shown in the previous
table for the amount of antifreeze necessary. Ground
heat exchanger software programs can also determine
the volume of uid in the piping system. Antifreeze
concentration should always be checked using a well
mixed sample and a hydrometer to determine the speci c
gravity and protection level.
