Geothermal Systems and GeoExchange Systems – What’s the Difference?

PART ONE 

General Principles

Traditional Geothermal

Geothermal technology and GeoExchange technology are often confused with one another in the mind of the public. This confusion is understandable as the term ‘Geothermal’ is often applied to both technologies – even by industry practitioners.

‘True’ Geothermal, also referred to as ‘hot rocks’, can be characterized as seeking a source of high – temperature heat from within the Earth with the goals of either producing electricity or providing space heating. Super-heated water or steam from Earth’s interior can be utilized to run turbines in a conventional power plant to generate electricity. Technologies currently in use include dry steam power plants, flash steam power plants and binary cycle power plants. Geothermal electricity generation is currently used in approximately 24 countries around the world. Geothermal space-heating involves harnessing a source of heat, most commonly hot springs, and distributing that source water to buildings via a piping network employing heat exchangers. Geothermal heating is globally in use in approximately 70 countries.
Geothermal power is considered to be sustainable because the heat extraction is minimal compared with the Earth’s heat content. The emission intensity of existing geothermal electric plants is on average approximately 122 kg of CO2 per megawatt-hour (MW/h) of electricity – approximately one-eighth that of a conventional coal-fired plant.

GeoExchange

In contrast to Traditional Geothermal technology, GeoExchange systems rely on solar radiation as an energy source. Each and every day the Earth is bombarded by solar radiation from the Sun. Approximately 47% of this radiation energy is absorbed into the Earth’s crust. This natural phenomenon results in relatively stable temperatures only a few feet below the surface of the ground or within a water body.

The term ‘GeoExchange’ is gradually becoming more commonplace and adopted as a differentiator to ‘True’ Geothermal. Examples of the range of terms that have been applied to GeoExchange include:
• Geothermal;
• ground – source heat pumps;
• geothermal heating and cooling;
• ground heat exchangers;
• ground-coupled low temperature geothermal;
• earth energy systems;
and many more which has led to confusion and mis-understanding.

How GeoExchange Systems Work

Generally, GeoExchange Systems can be divided into three basic sub-systems:
• Source system – thermal exchange coupling with the earth (commonly referred to as the ground heat exchanger or GHX) often consists of a network of drilled boreholes with closed-loop High Density Polyethylene (HDPE) heat exchange tubing installed within each borehole. Heat exchange fluid is circulated through the borehole piping network to absorb heat from the ground (winter) or dissipate heat into the ground (summer). Depending on site conditions, the GHX can take several different forms including heat exchange with groundwater. There are various types and configurations of Source Systems which are discussed below.
• Heat pump system – GeoExchange heat pumps use a refrigeration cycle (e.g. – the specific application of vapour compression principles) to transfer heat across the temperature gradient from the ground or water source to the heated or cooled space. The refrigerant cycle helps the GeoExchange system take advantage of two primary principles of heat transfer:
1. Heat energy flows from areas of higher temperature to areas of lower temperature.
2. The greater the difference in temperature between two adjacent areas, the higher the rate of heat transfer between them.
• Distribution system – system that conveys and distributes heat between the heat pumps and the building space (or to other demands for heat such as hot water heating or process heating).

Basic Relationship Between GHX Subsystems

Ground Heat Exchanger (GHX) Configurations
Below a certain depth – generally 6 to 15 feet (1.8 m to 4.5 m) – depending on local factors – the temperature of the undisturbed ground remains seasonally stable roughly approximating the mean annual outdoor ambient temperature. GeoExchange systems take advantage of the relatively warm ground in the Winter and relatively cool ground in the Summer to provide energy efficient heating and cooling.
The ground heat exchanger (GHX) can take many forms including:
 Closed – Loop Vertical systems;
 Closed – Loop Horizontal Trenched or Excavated Systems;
 Closed – Loop Water Body Systems (Lake, Pond, River, Ocean);
 Standing Column Well Systems;
 Open – Loop Groundwater Systems;
 Open – Loop Lake, Pond, River or Ocean Systems;

GeoExchange technology is recognized as one of the most efficient HVAC systems available with the following attributes:
• Provides the lowest cost heating & cooling, even when higher first costs are factored into analysis;
• Geothermal Heat Pumps have the lowest CO2 (greenhouse gas) emissions and the lowest over all environmental cost;
• Can be highly cost-effective for utility conservation programs;
• Provide strategic partnerships to promote advance space conditioning equipment – One appliance provides both heating and cooling, reducing maintenance compared to conventional fossil fuel and cooling tower systems;
• Flexible layout with a reduction in mechanical space;
• As the atmosphere is not used as a heat sink, bulky and noisy exterior equipment such as cooling towers and condensing units are not necessary;
• High coefficients of performance due to favorable ground temperatures leading to economical operating costs;
• Hot water for domestic or snow melting use can be scavenged any time the heat pump compressors are running;
• Chilled water is available, which has superior latent cooling capabilities; and
• Excellent zoning and part load performance.