Geothermal In Detail
In cooling mode warm air from the building exchanges heat with water that runs through a series of underground looping, the earth heat exchanger (EHX). As the water runs through the EHX it exchanges heat with the earth, resulting in a moderate water temperature. The heat pump uses a refrigeration cycle to cool the building’s air. Because the water enters the heat pump from the EHX with a moderate temperature, the temperature difference between the water and the ambient air is reduced compared to conventional systems that use the ambient air as a heat transfer medium. The reduced delta T eliminates the need for excess energy, resulting in energy efficiencies ranging from 300%-600%. In heating mode the heat pump uses a reverse refrigeration cycle.
In cooling mode, warm air from a building transfers heat to a refrigerant within the heat pump, while processing through a refrigeration cycle. The cool, comfortable air is then exhausted from the heat pump and distributed to the building through a series of air ducts. The now heated refrigerant transfers heat to the fluid in the earth heat exchanger (EHX). The EHX fluid, which is generally water, is then circulated from the heat pump through a series of underground piping, the earth heat exchanger. As the EHX fluid travels through the underground looping, the earth is used as a heat sink, and heat is transferred from the fluid to the earth. This enables the fluid to reenter the heat pump at a much cooler temperature, resulting in a highly energy-efficient cooling system.
In heating mode, the heat pump uses a reverse refrigeration cycle, but the results are the same. The air distributed to the building is at a comfortable temperature, while the system remains highly energy-efficient. The differences lie in which way the heat travels. Heat is transferred to the building’s air, and the earth is used as an energy (heat) source.
GHP Systems can be broken down into three separate parts or sections, the Heating/Cooling Distribution System, Heat Pump, and the Ground Heat Exchanger.
Heating/Cooling Distribution System
This section of the GHP System distributes heated or cooled air/water from the heat pump to the building. There are two ways heat is transferred from the heat pump to the building, water-to-air and water-to-water. The most common method is water-to-air in which air from the building is circulated through the heat pump and is either heated or cooled and then is distributed throughout the building. This method uses a series of air ducts to provide air to each zone of a building. In large systems multiple heat pumps are used, usually one heat pump per building zone. This type of air distribution system is the same as those used for conventional HVAC systems. Because of this fact, retrofit projects involving a new GHP System will already have the existing framework for the air distribution system.
The other method, water to water, is less common, and involves water loop systems in a buildings flooring or ceiling. Water travels through the heat pump, and exchanges heat with the refrigerant. As the water exits the heat pump it travels through the water loop system expelling heat into the building.
Heat Pump
The heat pump uses a refrigeration cycle, which requires compression and expansion of a refrigerant to drive heat transfer between the building and the earth heat exchanger (EHX). Taking a look at the heat pump during heating mode, the heat pump works as described below:
Heat from the earth is transferred to a fluid, usually water, in the EHX.
As the fluid travels through the EHX to the EHX-to-refrigerant heat exchanger
(The Evaporator), a refrigerant on the other side of the evaporator is
at a cooler temperature than the fluid from the EHX. Heat is transferred
from the EHX fluid into the refrigerant via the 2nd law of thermodynamics,
causing the refrigerant to evaporate. The fluid in the EHX passes through
the series of underground loops raising its temperature to its initial
state at the evaporator. The now gaseous refrigerant enters a compressor,
resulting in a rise in temperature and pressure of the refrigerant. The
refrigerant continues its course through a second heat exchanger (The
Condenser). A fan blows cool air from the building through a coil on
the other side of the condenser, and heat is then transferred from the
refrigerant to the air until the air is at a comfortable temperature.
The air is then distributed to the building through the heating/cooling
distribution system. The refrigerant, now at a cooler temperature, enters
an expansion valve, which reduces its pressure and temperature to its
initial state at the evaporator. The cycle then repeats itself.
The above describes heating mode for water-to-air heat pumps which is the most common heat pump used in GHP Systems. GHP Systems are unique in that they can also run in cooling mode. A reversing valve within the heat pump reverses the cycle, and the evaporator then becomes the condenser, while the original condenser becomes the new evaporator.
Earth Heat Exchangers
There are four main types of earth heat exchangers: Vertical Loop, Horizontal Loop, Water Source, and Open Loop. The most common of which is the vertical loop earth heat exchanger.
Vertical Loop heat exchangers consist of a series of boreholes drilled to a depth ranging from 150ft to 500ft depending on a variety of variables (Load requirements, land availability, geological features, fluid entering/exiting temperatures, ground temperature, etc.). Each borehole consists of one or two loops of high-density polyethylene piping with a u bend at the bottom, and are connected by fusion-bonded pipe connections. The boreholes are drilled by rigs used for well drilling (EnLink uses its patented Sonic Drill (Link) when conventional drilling practices won’t suffice). Once a borehole is drilled, the pipe looping is inserted and backfilled with grouting to prevent ground or surface water from seeping into the bore (Enlink’s Coil Tubing Unit (Link) ensures faster loop insertion and proper grouting, saving time and money compared to conventional methods). The series of vertical piping is then connected underground to supply/return header pipes, which carry the fluid in the EHX to and from the heat pump. The fluid within the EHX is usually water or a solution of primarily water with a small percentage of non-toxic refrigerants.
Vertical Loop heat exchangers are the most common earth heat exchangers for a variety of reasons. Vertical EHXs are deep underground where the ground temperature is constant, while Horizontal EHXs are only a few feet below and are subject to seasonal temperature fluctuations, resulting in Vertical EHXs to be more energy efficient. While Water Source EHXs are sometimes preferable, finding available surface waters to install these EHXs can prove to be highly difficult. As well, when land availability is limited Vertical Loop EHXs can be well suited to fit the required spacing.
More information on the different types of heat pumps and earth heat exchangers can be found at the following websites:
U.S. Department of Energy, Energy Efficiency & Renewable Energy: Geothermal Heat Pumps
Oak Ridge National Laboratories Geothermal Program
International Ground Source Heat Pump Association
National Groundwater Association