Using Earth's Energy to Cut Heating and Cooling Costs

A ground source heat pump (GSHP) is a heating and cooling system that transfers heat to or from the ground, using the ground as a heat sink in the summer and heat source in the winter.  Like air-source heat pumps, GSHPs use a reversible refrigeration cycle to provide heating and cooling. A ground source heat pump can be significantly more energy efficient than an air source heat pump because outside air temperatures vary by as much as 100°F from winter to summer.  At either temperature extreme, an air source heat pump is not efficient in transferring heat.

In comparison, ground temperatures may vary by as little as 10°F from winter to summer.  This temperature stability means that system efficiencies remain steady during times of significant heating or cooling demand, in contrast to air source heat pumps whose efficiencies can drop during extreme weather events when demand is the greatest.

Figure 1 - Ground Source Heat Pump
This diagram depicts a ground source heat pump in cooling mode. GSHPs use a reversible refrigeration cycle to provide either heating or cooling to a building.

Careful planning and engineering design needs to be performed to determine if a building and site are appropriate for a ground-source heat pump.  A geotechnical engineer should evaluate subsurface conditions on the site.  Test borings may be used to identify soil conditions, rock strata, and the presence of water. The heat transfer properties of these materials must also be evaluated so that the system can be designed to function properly for many years.  The geotechnical engineering and the heat exchanger installation are significant reasons why ground source heat pump systems can have much higher installation costs than competing systems.

Figure 2 - Ground Source Heat Pump Energy Comparison Chart

The U. S. Department of Energy report notes that "the modeled 2.9 coefficient of performance (COP) heating efficiency of the air-source heat pump is halfway between the cold weather (17°F) and standard, mild weather (47°F) rating conditions of a new high-efficiency (FEMP-recommended and ENERGY STAR model). Similarly, the modeled cooling efficiencies of the air-source heat pump, gas furnace, and air-source air conditioner all represent models that just meet the FEMP-recommended and ENERGY STAR qualifying levels." Original site energy use data converted to source energy use per EPA national conversion factors of 3.34 source kBtu per site kBtu for electricity use and 1.047 source kBtu per site kBtu for natural gas use.

Basic Concepts

• Geothermal energy is a term which describes the "heat from the earth".  At 10 feet below the earth's surface, the ground temperature remains nearly constant all year.  In winter the earth is warmer than the outside air temperature, and in summer the earth is cooler than the outside air temperature.
  
  
• The term "ground source heat pump" (GSHP) describes various types of vapor compression-based space conditioning equipment that use a geothermal resource -  such as the earth, ground water, or surface water - as a source of heat in winter and a sink for heat in the summer.  Ground source heat pumps use reversible refrigerant cycles to provide either heating or cooling as required by the building.
  
  
• The components of a GSHP system are the heat exchanger that is surrounded by earth or water, the compressor, and the heat exchanger in the air distribution system.  In a direct exchange system, the heat exchanger in the ground, typically comprised of copper tubing, is used as an evaporator in the heating mode and as a condenser in the cooling mode. Similarly, the coil in the air distribution system is a condenser during heating and an evaporator during cooling. Direct exchange systems are more efficient than indirect systems that use water loops since the refrigerant directly exchanges heat with the ground without the need for a refrigerant-water heat exchanger.  Most systems are indirect and consist of two loops: a refrigerant loop that is contained within the heat pump cabinet and a secondary water loop that circulates a water-antifreeze mixture through the ground heat exchanger.  Indirect systems are less efficient at transferring heat and require larger land areas for the ground heat exchanger than direct systems, but piping costs are lower than direct system piping costs since they generally use some type of plastic piping rather than copper.

Cost Savings and Environmental Benefits

The benefits of a ground source heat pump system are illustrated by a U. S. Department of Energy analysis of a 25,000 square foot office building in Washington, DC. The analysis concluded that "the biggest benefit of ground source heat pumps is that they use 25%-50% less electricity than conventional heating or cooling systems. This translates into a GSHP using one unit of electricity to move three units of heat from (or to) the earth. According to the EPA, geothermal heat pumps can reduce energy consumption-and corresponding emissions-up to 44% compared to air-source heat pumps and up to 72% compared to electric resistance heating with standard air-conditioning equipment."

The Figure 2 GSHP Energy Comparison Chart was developed from data included in the U. S. Department of Energy analysis.  The chart provides a comparison of annual energy use for an Air-Source Heat Pump (ASHP), a Gas Furnace with Air Conditioning, an "energy-efficient" Ground Source Heat Pump, and a "Best Available" Ground Source Heat Pump.

Because GSHP systems better maintain their efficiency levels during temperature extremes compared to air source heat pumps, GSHP systems can also provide significant savings in electric utility demand charges compared to air source heat pumps and comparable A/C systems.

Ground-Loop Piping Considerations

Proper design of the piping system is critical to the success of the project.  Piping will vary depending on soil conditions and heat pump design.  Piping may include horizontal or vertical fields (vertical shafts or wells).  A lake, pond, or reservoir may also be a geothermal source, or sink, for heating or cooling a nearby facility.  Piping design may be a closed loop system, which circulates water, or water mixed with anti-freeze.  Piping may also be connected to an open well system that pumps ground water through the system and discharges it back to the ground or to a surface reservoir.

Figure 3 - Ground Source Heat Pump Coil

A Sample Installation A model two-story school with 100,000 square feet is situated adjacent to play fields which accommodate 88,000 square feet (approximately 2 acres) of horizontally coiled piping, which is covered by at least six feet of soil, which was stockpiled on the site. Installation costs were reduced by installing the piping as the site was graded in preparation for building construction. The model two-story school has a central air distribution system, a heat recovery system, and a two-pipe water distribution system. The heat recovery system collects heat from exhaust air before it is discharged from the building. This heat is then used to temper outside air before it is distributed to each space. A heat pump is provided for each classroom, and other larger spaces, and is mounted in the branch supply air duct, which serves that space. Water is pumped from the exterior field to the heat pumps to meet heating or cooling demands. Then, the water is re-circulated through the field again. The configuration of this system provides individual room temperature control for each major space in the building.

Photograph Credit: Air Solutions

 

The piping itself can be relatively inexpensive, such as high-density polyethylene (HDPE) tubing.  At the same time, vertical boring and horizontal excavation can be relatively expensive.  Vertical borings may be filled with a thermally conductive grout after the piping is installed.  This is an important detail to be considered in the design of the piping system.

The piping configuration is frequently influenced by the size of the building, the amount of land available, and surface improvements.  If the building footprint is relatively small, land is plentiful, and trenching can be accomplished without disturbing existing trees and shrubs, a horizontal field may be a relatively inexpensive solution.  If the building footprint is large, land is limited, and trees and shrubs limit trenching or excavation, deep vertical borings may be required around the building to accommodate vertical wells.  For instance, Oak Ridge National Laboratory estimates that 1500 ft² to 3000 ft² of land area is required per ton of cooling for horizontal fields, while vertical borings typically require 250 ft² to 300 ft² of land area per ton¹. Coordination of this work with site grading or foundation excavation is important to provide access for boring equipment and minimize installation costs.

The integration of ground source heat pumps with heating, ventilation, and air conditioning distribution systems may vary by building type and climate conditions.  The heat exchanger may be water-to-water or water-to-air.  Air systems are commonly used, but there are applications for hydronic systems also. The size and number of heat pumps is also dependent on the building type and the size of spaces being served.  The various options, combinations, and configurations are a topic for another discussion.

Training Highlights

California utilities offer outstanding educational opportunities that focus on the design, construction and operation of energyefficient buildings. Listed here are a few of the many upcoming classes and events; for complete schedules, visit each utility's website. Integrating Energy Efficiency and Renewables in Commercial Retrofits This course will focus on decreasing energy use through a whole-system approach to buildings, using the best combination of technologies to lower a building's energy footprint. The instructor will cover the science of building performance, energy efficiency measures, and renewable energy systems. PEC. June 15, 9am - 12pm, San Francisco. (Course also offered for Home Retrofits: June 11, 9am - 4:30pm, Santa Cruz Police Department.) register >

Kensington Clean Energy Festival This free public event features information on property assessed clean energy loans for the installation of solar electric, solar water heating systems, energy efficiency and water savings technologies. June 26, 10:30am - 2:30pm, 4121 Adams Avenue, San Diego. register > Save Energy, Save Money: An Introduction to Energy Efficiency and Rebates Learn how to save money on lighting, air conditioning, motors, controls, refrigeration, and other equipment. Course includes how to complete your own energy audit, ways to reduce energy usage and how to use rebates and incentives to help reduce costs. Southern California Edison, CTAC. July 13, 8:30am - 12:30pm, Irwindale. register >

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