NREL is operated by Midwest Research Institute • Battelle Contract No. DE-AC36-99-GO10337

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory

Innovation for Our Energy Future

Geothermal—

The Energy Under Our Feet

Geothermal Resource Estimates for the

United States

Bruce D. Green and R. Gerald Nix, 

National Renewable Energy Laboratory

Technical Report

NREL/TP-840-40665

November 2006

NREL is operated by Midwest Research Institute • Battelle Contract No. DE-AC36-99-GO10337

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory

Innovation for Our Energy Future
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

Operated for the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

by Midwest Research Institute • Battelle

Contract No. DE-AC36-99-GO10337

Geothermal—

The Energy Under Our Feet

Geothermal Resource Estimates for the 

United States

Bruce D. Green and R. Gerald Nix, 

National Renewable Energy Laboratory

Technical Report

NREL/TP-840-40665

November 2006
The Earth houses a vast energy supply 

in the form of geothermal resources. 

Domestic resources are equivalent 

to a 30,000-year energy supply at our 

current rate for the United States! In 

fact, geothermal energy is used in all 50 

U.S. states today. But geothermal energy 

has not reached its full potential as a clean, 

secure energy alternative because of issues 

with resources, technology, historically low 

natural gas prices, and public policies. These 

issues affect the economic competitiveness 

of geothermal energy

On May 16, 2006, the National Renewable 

Energy Laboratory (NREL) in Golden, 

Colorado hosted a geothermal resources 

workshop with experts from the geothermal 

community. The purpose of the workshop 

was to re-examine domestic geothermal 

resource estimates. The participating experts 

were organized into five working groups 

based on their primary area of expertise in 

the following types of geothermal resource 

Executive Summary 

Findings by Resource Category

Estimated Developable Resource*

Estimated 

Accessible 

Resource (MWe)

2006

(Actual MWe)

2015

(MWe)

2025

(MWe)

2050

(MWe)

Shallow Hydrothermal1

(Identified) >90 ̊C/194 ̊F

30,000 2,800 10,000 20,000 30,000

Shallow Hydrothermal1

(Unidentified) >150 ̊C/302 ̊F

120,000 TBD TBD TBD

Co-Produced & 

Geopressured2

>100,000 23 10,000 to 

15,000

70,000 >100,000

Deep Geothermal4 1,300,000 to 

13,000,000

0 1000 10,000 130,000

Thermal Uses (MWt) (MWt) (MWt) (MWt)

Direct Uses5 >60,000 620 1600 4,200 45,000

Geothermal Heat Pumps6 >1,000,000 7,385 18,400 66,400 >1,000,000

GHP6 Avoided Power 120,000 880 2,100 8,000 120,000

* Please note that these resource estimates represent a consensus of a group of experts who considered existing resource 

assessments (referenced on next page). There is considerable uncertainty in the estimates as many resources are hidden 

and exploration to date has been relatively limited. The figures shown above are not a resource assessment, but, even with 

uncertainty, clearly show that the U.S. geothermal resource is a very large and important domestic energy source.

or application: (1) Hydrothermal, (2) Deep 

Geothermal Systems, (3) Direct Use, 

(4) Geothermal Heat Pumps (GHPs), 

and (5) Co-Produced and Geopressured. 

Geothermal resources are categorized in 

several layers of accessibility and feasibility, 

from broadest criteria (i.e., total physically 

available), to criteria that includes 

technical and economic considerations. 

The total resource base is scaled downward 

to accessible resource, and finally to a 

category called developable resource 

(see p. 4 for explanation).

The table below shows estimates for the 

different geothermal resource categories, as 

compiled by the workshop experts. These 

estimates show the enormous potential 

of the U. S. geothermal energy resource.

New low-temperature electric generation 

technology may greatly expand the 

geothermal resources that can be developed 

economically today.


References

1. Assessment of Geothermal Resources of the United 

States – 1978, USGS Circular 790 (p. 41 and 157). 

Includes identified and unidentified resources; 

2015 and later estimates are a consensus 

of the experts at the workshop. Estimated 

accessible figure includes identified (~30,000 

MW) and unidentified (~120,000 MW) (i.e., 

hidden or showing no surface manifestations) 

hydrothermal resources.

2. “Geothermal Electric Power Supply Possible 

from Gulf Coast, Midcontinent Oil Field Waters,” 

Oil and Gas Journal, September 5, 2005, and 

SMU Geothermal Laboratory Geothermal Energy 

Generation in Oil and Gas Settings Conference

findings, March 13 – 14, 2006, and USGS 

Circular 790. 

3. Based on Mafi Trench Unit on offshore platform 

now in operation. 

4. Energy Recovery from Enhanced/Engineered 

Geothermal Systems (EGS), Massachusetts 

Institute of Technology (MIT), September 2006. 

5. OIT Geo-Heat Center, using analysis based on 

USGS Circulars 790 and 892.

6. Geothermal Heat Pump Consortium, based 

on Energy Information Administration data 

and projections. The ‘avoided power’ figure 

represents the peak power not required or 

offset through use of GHPs. Thus, GHPs act as 

a proven demand and growth management 

option for utilities.

Dr. Roy Mink, DOE Geothermal Technologies Program 

manager, making opening comments.

Bruce Green, NREL, PIX 14846

The geothermal heat pump working-group during their 

resource deliberations. John Geyer (l), Wael El-Sharif (c), and 

Jack DiEnna (r) made up this working group.

Bruce Green, NREL PIX 14843
Introduction

The United States possesses vast underground stores of heat whose full potential has yet 

to be realized. The Earth’s interior reaches 

temperatures greater than 4,000°C (>7,200°F), and 

this geothermal energy flows continuously to the 

surface. The energy content of domestic geothermal 

resources to a depth of 3 km (~2 mile) is estimated 

to be 3 million quads, equivalent to a 30,000-year 

supply of energy at our current rate for the 

United States! While the entire resource base 

cannot be recovered, the recovery of even a very 

small percentage of this heat would make a large 

difference to the nation’s energy supplies. New lowtemperature electric generation technology may 

greatly expand the geothermal resources that can be 

developed economically today.

Geothermal resources could meet a substantial 

portion of the nation’s energy needs in the 21st

century. In fact, when including geothermal heat 

pumps (GHPs), geothermal energy is used in 

all 50 U.S. states today. The U.S. Department 

of Energy’s (DOE) Geothermal Technologies 

Program seeks to make geothermal energy the 

nation’s environmentally preferred baseload 

energy alternative. The Program’s mission is to 

work in partnership with U.S. industry to establish 

geothermal energy as an economically competitive 

contributor to the nation’s energy supply.

The purpose of the workshop was to re-examine 

domestic geothermal resource estimates. There 

were two guiding questions: what is the total 

potential accessible resource in the United States, 

and, given favorable circumstances and using 

existing practices with improved technology, and 

with institutional issues solved, how much of 

the resource is developable by 2015, 2025, and 

2050? Resource types include hydrothermal, deep 

geothermal systems, co-produced, geopressured, 

direct use, and GHPs. 

The goal of the workshop was to gather and 

summarize expert opinions about the potential 

of various geothermal resources for generation 

of electricity and utilization of heat energy. The 

workshop was not a formal assessment, but a 

recorded discussion by a group of experts who 

collectively state their opinions based on their 

experiences, knowledge, and interpretations 

of various detailed assessments (e.g., USGS 

Circular 790).

Figure 1. U.S. Energy and Geothermal Resources

Note: U.S. Total Resource Base from Characterization of U.S. Energy Resources and Reserves, December 1989, U.S. 

Department of Energy, DOE/CE-0279. Data for “Estimated Accessible Geothermal Resource” and “Estimated Developable 

Resource” are from Table 4 of this report.
Resource Definitions

The total resource base in the United States, 

both renewable and non-renewable, is very 

large, with an energy content of over 657,000 

billion barrels of oil equivalent (BBOE), or 

nearly 50,000 times the annual current rate of 

national energy consumption. Figure 1 shows 

graphically what the total resource base looks 

like, and in descending order, the values for the 

estimated accessible geothermal resource and 

estimated developable resource. 

Energy resources are traditionally classified according 

to the degree of certainty and the economic feasibility 

of exploiting the particular resource. The U.S. Geological 

Survey (USGS) and the U.S. Department of Energy 

(DOE) have used such identifying terms for resource 

classification. Following are simple definitions of these 

classification terms:

U.S. Total Resource Base – Resource base is all of a 

given material in the Earth’s crust, whether its existence is 

known or unknown and regardless of cost considerationsa.

Estimated Accessible Geothermal Resource– The 

accessible resource base for geothermal energy is that 

part of the resource base shallow enough to be reached by 

production drilling in the foreseeable futurea.

Estimated Developable Resource – This category is the 

subset of the accessible resource base that the workshop 

experts believe likely to be developed in future years.

a

Source: Assessment of Geothermal Resources of the 

United States – 1978, USGS Circular 790 (p. 4).

Geothermal resources are available throughout the entire U.S.

NREL, PIX03698
Hydrothermal

A hydrothermal system is defined as a 

subterranean geothermal reservoir that transfers 

heat energy upward by vertical circulation of 

fluids driven by differences in fluid density that 

correspond to differences in temperature (see 

Figure 2). Hydrothermal systems can be classified 

into two types—vapor-dominated and hot water—

depending on whether the fluid is steam or liquid 

water, respectively.

Most high-temperature geothermal resources 

occur where magma (molten rock) has penetrated 

the upper crust of the Earth. The magma heats 

the surrounding rock, and when the rock is 

permeable enough to allow the circulation of 

water, the resulting hot water or steam is referred 

to as a hydrothermal resource. Such resources 

are used today for the commercial production of 

geothermal power. They benefit from continuous 

recharge of energy as heat flows into the reservoir 

from greater depths.

Deep Geothermal Systems

Deep geothermal systems (a.k.a. enhanced 

geothermal systems or EGS) are defined as 

engineered reservoirs that have been created to 

extract heat from economically unproductive 

geothermal resources. The deep geothermal/EGS 

concept is to extract heat by creating a subsurface 

fracture system to which water can be added 

through injection wells. The water is heated by 

contact with the rock and returns to the surface 

through production wells, just as in naturally 

occurring hydrothermal systems.

Hydrofracturing and stimulation techniques are 

used widely in the oil and gas industry to extend 

production, and can be used to greatly extend and 

expand use of geothermal resources. Figure 3 gives 

a graphic idea of the domestic scope of geothermal 

resources at just 6 kilometers (3.7 miles), a 

nominal drilling depth in the oil and gas industry.

Description of Geothermal 

Resource Categories

Figure 2. llustration of a hydrothermal reservoir, showing the natural recharge, fractures, and heat source. 

Courtesy: Geothermal Education Office
Direct Use

Hot water from geothermal resources is used 

directly to provide heat for buildings, crop and 

lumber drying, industrial process heat needs, 

aquaculture, horticulture, ice melting on sidewalks, 

roads, and bridges, and district heating systems. In 

direct use applications, a well (or series of wells) 

brings hot water to the surface; a mechanical 

system— piping, heat exchanger, pumps, and 

controls—delivers the heat to the space or process. 

Often, direct use applications use geothermal 

fluids not hot enough for electricity generation. To 

improve efficiencies, used water from geothermal 

power plants can be ‘cascaded’ down for lower 

temperature uses, such as in greenhouses or 

aquaculture. Flowers, vegatables, and various fish 

species and alligators are examples of products 

from greenhouse and aquaculture systems.

Figure 3. Estimated Earth temperature at 6-km (3.7-mile) depth. Courtesy: Southern Methodist University 

Geothermal Laboratory.
Geothermal Heat Pumps

Geothermal heat pumps (GHPs) use the Earth’s 

huge energy storage capability to heat and cool 

buildings, and to provide hot water. GHPs use 

conventional vapor compression (refrigerantbased) heat pumps to extract the low-grade heat 

from the Earth for space heating. In summer, the 

process reverses and the Earth becomes a heat 

sink while providing space cooling (see Figure 

5). GHPs are used in all 50 U.S. states today, 

with great potential for near-term market growth 

and savings.

Geopressured Resources

The geopressured resource consists of deeply 

buried reservoirs of hot brine, under abnormally 

high pressure, that contain dissolved methane. 

Geopressured brine reservoirs with pressures 

approaching lithostatic load are known to occur both 

onshore and offshore beneath the Gulf of Mexico 

coast, along the Pacific west coast, in Appalachia, 

and in deep sedimentary basins elsewhere in the 

United States. 

The resource contains three forms of energy: 

methane, heat, and hydraulic pressure. In the past, 

DOE conducted research on geopressured reservoirs 

in the northern Gulf of Mexico sedimentary basin, 

and operated a 1-megawatt (MW) power plant using 

the heat and methane from the resource (i.e., Pleasant 

Bayou, TX, 1989 – 1990). 

Co-Produced Geothermal Fluids

Sometimes referred to as the ‘produced water cut’ 

or ‘produced water’ from oil and gas wells, coproduced geothermal fluids are hot and are often 

found in waterflood fields in a number of U.S oil and 

gas production regions (See Table 1). This water is 

typically considered a nuisance to the oil and gas 

industry (and industry is accountable for proper 

disposal), but could be used to produce electricity for 

internal use or sale to the grid.

Like geopressured resources, co-produced 

geothermal resources can deliver near-term energy 

savings, diminish greenhouse gas emissions, and 

extend the economical use of an oil or gas field. New 

low-temperature electric generation technology may 

greatly expand the geothermal resources that can be 

developed economically today.

Figure 5 - Geothermal heat pump (GHP) illustration for 

a commercial application.

Figure 4. An illustration of the State Capitol geothermal district-heating system, Boise, Idaho.


Table 1. Co-Produced Geothermal Fluids

Estimated equivalent geothermal power from processed water associated with existing hydrocarbon 

production, using 140°C (285°F) as a nominal fluid temperature. 

State Total Processed Water, 2004 (bbl) Power, MW @ 140 ̊C (285) ̊F)

Alabama 203,223,404 47

Arkansas 258,095,372 59

California 5,080,065,058 1169

Florida 160,412,148 37

Louisiana 2,136,572,640 492

Mississippi 592,517,602 136

Oklahoma 12,423,264,300 2860

Texas 12,097,990,120 2785

Total 32,952,140,644 bbl 7,585 MW

Courtesy: Dr. David Blackwell, Southern Methodist University

Process Used

The participating experts were organized into 

five working groups based on their primary area 

of expertise in the following types of geothermal 

resource or application: (1) Hydrothermal, (2) 

Deep Geothermal Systems, (3) Direct Use, (4) 

Geothermal Heat Pumps, and (5) Co-Produced 

and Geopressured. Because of similarities 

(e.g., oil and gas industry involvement), the coproduced and geopressured resource working 

groups were combined. 

Through opening comments and guidance, 

Dr. Roy Mink of DOE and Dr. Gerry Nix of 

NREL introduced the participants to the task, 

purpose, goals, and approach of the workshop. 

The facilitator provided each working group 

with a set of guiding questions to help obtain 

results consistent with stated purpose, goals, 

and approach to this workshop. These included: 

what is the total potential accessible resource 

in the U.S., and, given favorable circumstances 

and using existing practices with improved 

technology, and with institutional issues solved, 

how much of the resource is developable by 2015, 

2025, and 2050? 

The working groups answered the guiding 

questions and formulated their responses, then 

presented their findings to the other participants. 

During the final stage of the workshop, the 

participants discussed data integration and 

related issues. See Table 2 for the findings of the 

working groups. Workshop participants during data integration discussions – 

Dr. John Lund makes a point.

The Gulf Coast region of the U.S. is rich in geopressured and 

co-produced energy resources.

Bruce Green, NREL, PIX 14845
Table 2. Findings by Resource Category

Estimated Developable Resource*

Estimated 

Accessible 

Resource (MWe)

2006

(Actual MWe)

2015

(MWe)

2025

(MWe)

2050

(MWe)

Shallow Hydrothermal1

(Identified) >90 ̊C/194 ̊F

30,000 2,800 10,000 20,000 30,000

Shallow Hydrothermal1

(Unidentified) >150 ̊C/302 ̊F

120,000 TBD TBD TBD

Co-Produced & Geopress2 >100,000 23 10,000 to 

15,000

70,000 >100,000

Deep Geothermal4 1,300,000 to 

13,000,000

0 1000 10,000 130,000

Thermal Uses (MWt) (MWt) (MWt) (MWt)

Direct Uses5 >60,000 620 1600 4,200 45,000

Geothermal Heat Pumps6 >1,000,000 7,385 18,400 66,400 >1,000,000

GHP6 Avoided Power 120,000 880 2,100 8,000 120,000

* Please note that these resource estimates represent a consensus of a group of experts who considered existing 

resource assessments (referenced below). There is considerable uncertainty in the estimates as many resources 

are hidden and exploration to date has been relatively limited. The figures shown above are not a resource 

assessment, but, even with uncertainty, clearly show that the U.S. geothermal resource is a very large and 

important domestic energy source.

Table 3. Co-Produced and Geopressured Resources (Estimated Developable Resources)2

2015 2025 2050

Co-Produced 5,000 MWe 10,000 to 20,000 MWe 30,000 to 40,000 MWe

Geopressured 5,000 to 10,000 MWe 50,000 to 60,000 MWe 70,000 to 80,000 MWe

Chena Hot Springs Resort (Alaska) geothermal power plant dedication in August 2006. New technology such as 

that being used at Chena could greatly extend the feasibility of using geothermal resources.

Bruce Green, NREL, PIX 14847
References

1. USGS Circular 790 (p. 157), includes identified 

and unidentified resources; 2015 and later 

estimates are a consensus of the experts at the 

workshop. Estimated accessible figure includes 

identified (~30,000 MW) and unidentified 

(~120,000 MW) (i.e., hidden or showing no 

surface manifestations) hydrothermal resources.

2. USGS Circular 790, Geothermal Electric Power 

Supply Possible from Gulf Coast, Midcontinent 

Oil Field Waters, “Oil and Gas Journal,” 

September 5, 2005, and SMU Geothermal 

Laboratory Geothermal Energy Generation in Oil 

and Gas Settings Conference findings, March 13 

– 14, 2006. 

3. Based on Mafi Trench Unit on offshore platform 

now in operation. 

4. Energy Recovery from Enhanced/Engineered 

Geothermal Systems (EGS), Massachusetts 

Institute of Technology (MIT), September 2006.

5. OIT Geo-Heat Center, using analysis based on 

USGS Circulars 790 and 892.

6. Geothermal Heat Pump Consortium, based on 

Energy Information Administration data and 

projections. The ‘avoided power’ figure represents 

the peak power not required or offset through use 

of GHPs. Thus, GHPs act as a proven demand and 

growth management option for utilities.

Energy Comparison Information

U.S annual energy consumption equals about 

100 quads (EIA, 2004).

U.S annual electricity production equals about 

40 quads (EIA, 2004).

U.S. petroleum demand equals about 21 million 

bbl/day, 7.67 billion bbl/yr (EIA, 2004).

World petroleum demand equals about 84 million 

bbl/day, 30 billion bbl/yr (EIA, 2004).

Energy Equivalents

1 Quad = 0.170 billion barrels of oil, 170 million 

barrels of oil

1 Quad = 45 million short tons of coal

1 Quad = 1 trillion cubic feet of dry natural gas

1000 KWh = 0.59 barrels of crude oil 

1000 KWh = 0.15 short tons (300 pounds) of coal

1000 KWh = 3,300 cubic feet of dry natural gas

1 barrel of crude oil = 1,700 kWh

1 barrel of crude oil = 5,600 cubic feet of dry natural gas

1 barrel of crude oil = 0.26 short tons (520 pounds) of coal

Table 4. Energy Equivalents by Resource Category (see appendices for calculation method)

Resource Category Estimated Accessible Resource Estimated Developable Resource – 2050

Shallow Hydrothermal1

(Identified) >90 ̊C/194 ̊F

0.81 Quads

135 million BOE

0.81 Quads

135 million BOE

Shallow Hydrothermal1

(Unidentified) >150 ̊C/302 ̊F

3.2 Quads

540 million BOE

–

Co-Produced & 

Geopressured2

2.7 Quads

450 million BOE

2.7 Quads

450 million BOE

Deep Geothermal4 35.1 to 351 Quads

5.8 to 58.5 BBOE

3.5 tp 35 Quads

0.58 to 5.8 BBOE

Direct Use5 0.88 Quads

150 million BOE

15 Quads

112.5 million BOE

Geothermal Heat Pumps 

(GHP)6

15 Quads

2.5 billion BOE

15 Quads

2.5 BBOE

GHP6 Avoided Power 1.8 Quads

300 million BOE

1.8 Quads

300 million BOE

10
Appendices

• Present Development 

• Benefits

• Strategic Value

• Profile of a Geothermal Project 

• List of Workshop Participants and Staff 

• How Energy Equivalent Calculations Were Made

• References and Supporting Documents

The Nation’s geothermal resources represent a huge and viable energy resource, providing the U.S. with 

various ways to use them and enhance national security, and economic and environmental health. 

11
Present Development

Today’s U.S. geothermal industry is a $2-billionper-year enterprise involving over 2800 MW of 

electricity generation capacity, about 620 MW of 

thermal energy capacity in direct-use applications 

such as indoor heating, greenhouses, food 

drying, and aquaculture, and over 7,300 MW of 

thermal energy capacity from geothermal heat 

pumps. The international market for geothermal 

power development could exceed a total of $25 

billion for the next 10 to 15 years. At the present 

time, U.S. technology and industry stand at the 

forefront of this international market.

Benefits

Geothermal energy benefits the nation by helping 

to solve three problems— energy reliability and 

security, economic development, and air quality. 

Geothermal resources can help address the 

shortage of new electricity generating capacity 

in the United States cited in the National Energy 

Policy Act of 2005. As a baseload generation 

source, geothermal energy is well proven and 

reliable. 

Geothermal power plants emit little carbon 

dioxide, very low quantities of sulfur dioxide, 

and no nitrogen oxides. U.S. geothermal 

generation annually offsets the emission of 22 

million metric tons of carbon dioxide, 200,000 

tons of nitrogen oxides, and 110,000 tons of 

particulate matter from conventional coal-fired 

plants. See Charts 1, 2, and 3 for details on 

comparative emissions. 

12
Strategic Value

Use of domestic geothermal energy 

resources has substantial strategic 

value for the nation. Geothermal 

resources can contribute to: Clean electricity 

generation

Baseload power production, 

having established high 

capacity factors

Distributed energy systems 

with modular and shorter 

development timeframe 

advantages

Direct-use 

for building 

energy needs

Ethanol and biodiesel 

production – thermal 

energy requirements

Mineral recovery, 

such as silica and 

zinc – both strategic 

minerals in short 

supply.

Hydrogen production – 

via off-peak electrolysis

 Coproduction and 

enhanced oil 

recovery, thus 

gaining more oil 

Climate change mitigation – by offsetting 

needs for fossil-fueled power plants

Rural economic development 

– aquaculture and horticulture, 

and lumber drying

13
Profile of a Geothermal Project

Geothermal power and direct use development 

begins with exploration to locate an economic 

reservoir, using a variety of techniques. Wells 

are drilled to measure subsurface temperatures 

and flow rates and to produce and inject the 

hydrothermal fluid. Once the reservoir has been 

proven, the site is developed either for power 

generation or a direct use application.

Geothermal projects are capital-intensive, 

and the major expenses are incurred before 

the project produces revenue. Exploration 

represents only about 10% of the total cost 

of a successful project, but many projects 

can fail at this stage. A high degree of risk 

evolves from the need for success on the first 

wells drilled into the reservoir. The extent to 

which these wells produce hot fluids influence 

subsequent investment decisions. Although the 

most expensive element of a power generation 

project is surface plant construction, drilling 

to create a well field involves higher risk due 

to uncertainties in reservoir characteristics. 

Direct use applications are usually less costly 

than power generation, because the resource is 

shallower, the fluids are less difficult to manage, 

and the technology less complex.

Typically, geothermal power plants are baseload 

facilities, but they may be operated in a loadfollowing mode. Power conversion options 

include (1) the transformation (flashing) of hot 

geothermal fluids to steam which drives a turbine 

or (2) transfer of heat from the geothermal 

fluids to a secondary (binary) working fluid 

which drives a turbine. Geothermal plants have 

very high availabilities and capacity factors, 

exceeding 90%. Liquids produced from the 

reservoir are reinjected to sustain production 

pressures. 

Horticulture represents a rapidly growing domestic industry 

that lends itself well to rural economic development. 

Bruce Green, NREL/PIX13079

Residential application of a geothermal heat pump.

Courtesy: Geothermal Heat Pump Consortium

Clean, baseload, distributed electricity is being produced 

from geothermal resources today. Big Geysers, a 75-MW 

geothermal power plant located in northern California, is 

shown above.

Courtesy: GRC

Geothermal heat pump (GHP) systems use highly durable 

heat exchangers that are placed into the ground, a water 

source, or a well, and deliver heating and cooling very 

efficiently, as well as providing hot water. Commercial 

and institutional GHP installations are often costcompetitive today, achieving substantial energy savings 

for building owners and tenants. Once connected to the 

ground (or water), the GHP system is typically integrated 

with traditional HVAC equipment. Only accredited 

designers and installers (see: www.igshpa.okstate.edu) 

should be used for GHP installations. There are federal 

tax benefits for use of GHPs, and some states also offer 

incentives (see: www.geoexchange.org).

14
Courtesy- Calpine Corp. 

List of Workshop Participants 

and Staff

Hydrothermal

Jim Lovekin, GeothermEx, Inc., CA

Al Waibel, Columbia Geoscience, OR

Matthew Sares, Colorado Geologic Survey, CO

EGS

Susan Petty, Black Mountain Technology, WA

Direct Use

John Lund, Oregon Institute of Technology,

Geo-Heat Center, OR

Co-Produced (combined w/Geo-Press.)

David Blackwell, Southern Methodist University, 

Geothermal Laboratory, TX

Richard Erdlac, University of Texas

Permian Basin, TX

Mark Milliken, Rocky Mountain Oil Testing Center, WY

Geopressured

Chip Groat, University of Texas – Austin, TX

Geothermal Heat Pumps

Wael El-Sharif, Geothermal Heat Pump Consortium, 

D.C.

Jack DiEnna, Geothermal Heat Pump Consortium, PA

John Geyer, JG&A, Inc., WA

DOE, INL, and NREL Staff

Roy Mink, DOE

Ray Fortuna, DOE

Jay Nathwani, DOE

Sandy Glatt, DOE

Joel Renner, INL

Gerry Nix, NREL

Bruce Green, NREL

Observers

Colin Williams, USGS

Marshall Reed, USGS

Kermit Witherbee, BLM

Facilitators

Kathleen Rutherford, The Keystone Center

Jody Erikson, The Keystone Center

Bruce Green, NREL, PIX 14844

Several geothermal power plants are visible at The Geysers in 

northern California.

Members of the entire workshop group during discussions 

about resource data integration.

15
Electricity as the equivalent energy content of the produced electricity –

Barrels (bbls) of oil equivalent (BOE)

 [ (electricity production potential/MWe) (1000 kW/MW) (8760 hrs/yr.) (0.9 capacity factor)

 (3413 BTU/kW-hr) ] 1 bbl oil equivalent/6 x 106 BTU = BOE/yr. for a minimum of 30 years. 

Quads (Q)

 [ (electricity production potential/MWe) (1000 kW/MW) (8760 hrs/yr.) (0.9 capacity factor)

 (3413 BTU/kW-hr) ] 1/1015 BTU/Q

NOTE: If properly designed, plants are sustainable.

Thermal calculations assumed a duty cycle of 50% –

BOE

 [ (MWt) (1000 kW/MW) (8760 hrs/yr.) (0.5 duty cycle) (3413 BTU/kW-hr) ] 

 (1 bbl oil equivalent/6 x 106 BTU)

Quads (Q)

 [ (MWt) (1000 kW/MW) (8760 hrs/yr.) (0.5 duty cycle) (3413 BTU/kW-hr) ] 1/1015 BTU/Q

NOTE: Actual fuel displacement for alternative production sources is somewhat larger.

Supporting Documents

1. Assessment of Geothermal Resources of the 

United States – 1978, USGS Circular 790 

(p. 157), includes identified and unidentified 

resources; 2015 and later estimates are a 

consensus of the experts at the workshop.

2. Geothermal Electric Power Supply Possible 

from Gulf Coast, Midcontinent Oil Field 

Waters, “Oil and Gas Journal,” September 5, 

2005, and Geothermal Energy Generation 

in Oil and Gas Settings Conference findings, 

March 13 – 14, 2006, and SMU Geothermal 

Laboratory. Available at: http://www.smu.

edu/geothermal/publications/Oil&GASJ2005_

McKenna.pdf

3. Energy Recovery from Enhanced/Engineered 

Geothermal Systems (EGS), Massachusetts 

Institute of Technology (MIT), September 

2006. Available at: http://www1.eere.energy.

gov/geothermal/egs_technology.html

4. Characterization of U.S. Energy Resources and 

Reserves, December 1989, U.S. Department of 

Energy, DOE/CE-0279.

How Energy Equivalent Calculations Were Made

16
17
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1. REPORT DATE (DD-MM-YYYY)

10/2006 

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Technical Report 

3. DATES COVERED (From - To)

2006

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Geothermal -- The Energy Under Our Feet 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/TP-840-40665 

5e. TASK NUMBER

GT06.1000 

6. AUTHOR(S)

Bruce Green and Gerry Nix, National Renewable Energy Laboratory 

5f. WORK UNIT NUMBER

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National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

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NREL/TP-840-40665 

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13. SUPPLEMENTARY NOTES

NREL Technical Monitor: Gerry Nix 

14. ABSTRACT (Maximum 200 Words)

On May 16, 2006, the National Renewable Energy Laboratory (NREL) in Golden, Colorado hosted a geothermal 

resources workshop with experts from the geothermal community. The purpose of the workshop was to re-examine 

domestic geothermal resource estimates. The participating experts were organized into five working groups based on

their primary area of expertise in the following types of geothermal resource or application: (1) Hydrothermal, (2) 

Deep Geothermal Systems, (3) Direct Use, (4) Geothermal Heat Pumps (GHPs), and (5) Co-Produced and 

Geopressured. The workshop found that the domestic geothermal resource is very large, with significant benefits. 

15. SUBJECT TERMS

Geothermal; resources; hydrothermal; geopressured; co-produced, geothermal heat pump; GHPs; earth energy; 

deep geothermal systems; EGS; enhanced geothermal systems; direct use; and energy resources. 

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