Designs That Work
Hot-Humid Climate
Advanced Technologies - Geothermal Heating and Cooling
Geothermal (Ground Source) Heat Pumps work similar to air source heat
pumps, except the energy is transferred to the ground instead of to the
atmosphere. The higher efficiency that can be achieved from a geothermal
heat pump is due to the relatively stable ground temperature during the
heating and cooling seasons compared to the variable air temperature of
air source heat pumps. While these systems are more efficient the standard
air source heat pumps or air conditioning units, they are also more
expensive to install and will run upwards of $5,000 to $10,000 for the
installed system.
Figure 30: Schematic of a Ground Source Heat Pump
There are three types of geothermal heat pumps, open loop, closed loop,
and direct exchange (DX).
Open Loop
In an open loop system, ground water is used as the heat exchange fluid
between the ground and the refrigerant loop. Water is drawn out of the
ground and circulated through a heat exchange tank containing the
refrigerant line. This is not a very common system due to the potential
problems with dirt and debris and general water quality issues that may be
encountered from using the natural ground water.
Closed Loop
The closed loop system is the most common system used with geothermal
heat pumps. This system uses plastic tubing that is run through either
vertical or horizontal wells to transfer the energy to the ground. While
similar to an open loop system in that the water is circulated through a
heat exchange tank containing the refrigerant line of the heat pump, in
this system the water used is not connected to the ground water, but
instead run in the plastic tubing. This controls the water quality used in
the system, and therefore reduces the potential for problems and
maintenance. In heating climates, there is a concern for freezing of the
system and therefore some form of anti-freeze will need to be added to the
ground loop system.
Direct Exchange
Direct Exchange systems run the refrigerant line directly into the
ground, eliminating the heat exchange tank. Because this extra heat
transfer step is eliminated from the design, the system should be more
efficient. In this system, copper lines are installed into the ground and
the refrigerant of the system is circulated through them. Copper, due to
its higher thermal conductivity is better able to exchange the heat with
the ground when compared to a water circulated system with plastic pipe.
While more efficient, there are some considerations that need to be made.
The cost of the system is higher due to the use of copper tubing
instead of plastic tubing. Depending on the number and depth of the wells
required, this can create a significant cost to the system. The system
also has to be site charged with refrigerant, so unlike factory built
closed loop GSHP systems, the efficiency is based on the quality of the
installation.
Design Considerations
In order for the system to perform properly there must be adequate heat
exchange with the ground. The heat exchange is through either vertical or
horizontal wells in which the heat exchange fluid is circulated. A general
rule of thumb is that a 200 ft ground well is required for each ton of
cooling needed. Therefore, for a 3 ton cooling load, three 200 ft wells,
would be needed.
From the heat pump side, there are generally two systems currently
being used on the market, a packaged system and a split system.
In the packaged system, the compressor and heat exchange tank to the
interior of the house and integrated into the air handler. The benefit of
this system is that the charging of the refrigerant line is all done in
the factory under controlled conditions and it is a fairly simple
installation and connection to the ground loops at the site. On the other
hand, the compressor is now inside the house, and issues with noise can
sometimes occur.
Split systems place the compressor and heat exchange tank on the
exterior and the refrigerant line is run to the air handler as in more
conventional air source heat pumps. This reduces the noise inside the
house; however the refrigerant charge must now be determined on site by a
mechanical contractor.
Energy Model Results
The system used in the energy model is based on the specifications of a
ClimateMaster Genesis Packaged Unit. The efficiency of the system is based
on the entering water temperature. Therefore the performance of the system
used in the energy model was based on the expected entering (returning)
water temperature in the both the heating and cooling seasons. This
entering water temperature is a function of the average ground temperature
and the heat transfer efficiency of the ground. This resulted in a 17 EER
for cooling and a 4.0 COP for heating. The resultant incremental whole
house energy consumption reduction was 3.1%. |
