- LEED Introduction
- Sustainable site
- Energy & Atmosphere
- Indoor Environment
- Water Efficiency
- Materials and Resources
Geothermal Heating and Cooling
Geothermal Heating and Cooling
LEED and Geothermal[i]
Using a Geothermal energy system in a home, depending on the system, can earn a home a substantial amount of points towards LEED certification.
Innovation and Design Process
Innovative or Regional Design: up to 4 points
Energy and Atmosphere
Optimize Energy Performance: up to 34 points
Enhanced Refrigerant Management: 1 point
Heating and Cooling Distribution System: 1-3 points
Space Heating and Cooling Equipment: 2-4 points
Renewable Energy System: up to 10 points
Residential Refrigerant Management: 1 point
Indoor Environment Quality
Energy Star with IAP- up to 13 points
Distribution of Space Heating and Cooling: 1-3 points
What is a Geothermal Heat Pump System and How does it Work?
A Geothermal Heating/Cooling system is a more energy efficient alternative to traditional central heating/air conditioning systems that utilizes the consistent and stable heat energy available in either the ground or water to heat and cool a home.
All heating systems rely on three basic scientific principles. 1.) All matter contains heat. 2.) Cold is the absence of heat and exists only in terms of relation between the differing amounts of heat contained in matter. 3.) Heat moves from higher temperature matter to matter of lower temperature by means of conduction, convection, and radiation. Engineers have been able to use these principles to develop “heat exchangers” which can transfer or extract heat from one source and deliver it to another.
Conventional “air to air” and geothermal heat pump systems both rely on the same basic principle of heat exchange. They use pressurized refrigerant to capture and move heat from indoors to out. “A refrigerant carries the heat from one area to another. When compressed, it is a high temperature, high-pressure liquid. If it is allowed to expand, it turns into a low temperature, low pressure gas. The gas then absorbs heat.”[ii] However, the systems differentiate in the medium from which they draw heat and into which they pump excess heat. Conventional systems rely on the outside air which has high fluctuations in temperature. Geothermal systems rely on the stable and consistent heat found in the ground beneath the home.
“Depending on latitude, ground temperatures range from 45°F (7°C) to 75°F (21°C). Like a cave, this ground temperature is warmer than the air above it during the winter and cooler than the air in the summer.”[iii]
A geothermal heat pump system is made up of a series of pipes, a “loop”, through which the refrigerant is pumped. This series of pipes is installed in the ground beneath the home and connected to a pump which circulates the refrigerant, an electrically driven compressor and a heat exchanger. The loop, typically made either of high-density polyethylene or copper pipes, carries the refrigerant used in the absorption of heat. Like conventional systems the compressor and heat exchanger distribute the heated or cooled air throughout the house via the ductwork.[iv]
In the winter the loop draws in heat energy from the earth through the refrigerant flowing through the pipes. The compressor and heat exchanger take the excess heat from the refrigerant and concentrate it. Then a fan disperses the higher temperature air through the ducts. The system reverses the process to cool the house in the summer. The loop draws heat from the air inside the house and pumps it into the ground.
Geothermal Heat Pump System Efficiency[v]
Geothermal Heat Pump efficiency is measured by the Coefficient of Performance (COP) for heating and the Energy Efficiency Ratio (EER) for cooling. The COP is the ration of heat produced in Btu per Btu of energy input (electricity). The EER is the ratio of heat removed in Btu per hour to the amount of electricity in watts required to run the system. The EPA’s Energy Star rating applies to geothermal heat pump systems with a COP of at least 3.3 and an EER of at least 14.1 for closed water-air systems and 3.6/16.2 for open water-air systems. For water-water systems it is 3.0/15.1 (closed systems) and 3.4/19.1. Over the next two years, the efficiency levels required for an Energy Star rating will slowly increase, challenging manufacturers to build better and better systems.
The use of the stable heat energy contained within the ground or water allows geothermal systems to provide heating and cooling 30-60% more efficiently than conventional systems. The level of efficiency of these systems varies based upon factors such as geology (soil and rock composition), hydrology (water source properties), and land availability. Energy Star rated systems claim to run at least 45% more efficiently than conventional systems.
Types of Geothermal Systems[vi]
Horizontal Closed Loop System
This is the system which is generally the most cost effective for installation in homes. The horizontal loop system requires trenches at least 4 feet deep and two feet wide to place the piping. The length and number of trenches is dependent on the size of the lot upon which the home is to be built. Certain techniques which are able to loop the piping in each coil allow for horizontal applications to be installed on smaller lots.
Vertical Closed Loop System
These systems are most commonly used for larger commercial buildings and schools. This is due to the large amount of land needed for a horizontal system to provide enough energy to be cost effective. A vertical system functions the same way as a horizontal. But instead of being installed in shallow trenches that run the length of the lot, the loop is placed in holes drilled 20 feet apart and 100-400 feet deep.
Pond/Lake Closed Loop System
For projects that are close to a substantial body of water a Pond/Lake system may be the most cost effective. In this type of system the closed loop piping is run from the building underground to the water source, at least 8 feet below the surface to avoid freezing. These systems function just like a ground loop system, except that it draws its heat energy from water and not earth.
Well Open Loop System
This type of system is referred to as “open” because it uses water from a well or aboveground source as the fluid used in heat exchange that flows through the heat pump system. The water is then returned to the source through the well, a recharge well, or a surface discharge. Like the pond/lake system, this system is only effective if there is an adequate water source.
Cost and Benefit of Geothermal
A typical geothermal heat pump system costs about $2500 per ton of capacity, not including drilling and installation. A typical residential unit will be a three ton capacity system costing $7500. Depending on the system, installation costs run from $10,000-$30,000. This translates into the up-front cost in comparison to conventional systems to be from 50-150% more expensive.[ix] Energy Star certified geothermal systems all qualify for up to a 30% federal tax rebate on all equipment and installation costs including labor.[x]
For a basic cost benefit analysis we will use $37,500 as the total cost of the system and installation to reflect the highest cost for a typical residential system. Assuming the system is Energy Star rated; it qualifies for the 30% tax rebate and runs at a minimum of 45% more efficiently than a conventional system. The tax rebate would bring the up-front cost of the system down to approximately $26250. If we assume that this system is 100% (two times) more expensive than what a conventional system would have been the initial investment for geothermal would be $13,125 more. At minimum efficiency for an Energy Star system (45%) on a home with $1,800 yearly energy bills, the geothermal system would save $810/year. At this rate it would take approximately 16 years for the system to pay off.
While 16 years is a long time for an investment to begin seeing positive gains, this number assumes maximum system costs and minimal efficiency for an Energy Star system. Obviously, a less expensive and more efficient system would pay for itself more quickly. There are also other factors which influence the cost-benefit analysis of a geothermal system. Geothermal heat pump systems have better longevity and less required maintenance than conventional systems. They also add to the value to and reduce the carbon footprint of the home.
While the financial value of a geothermal system can certainly be measured in the dollars and cents that a homeowner will save, what is perhaps its most important benefit cannot. The immediacy of the world’s environmental crisis must be taken into account in any discussion of alternative and renewable energy sources. Geothermal energy is an important step in transforming our society in a way that will help sustain life on our planet.