May 30, 2014

Geothermal System Basics

and

Epic’s Geothermal System

Daniel A. Rehbein, P.E., LEED™ AP

exp U.S. Services, Inc.

Department Manager of the mechanical

group and a Principal in the Milwaukee

office

25 years of mechanical engineering

consulting experience in Wisconsin

Experience with healthcare, corporate,

commercial, industrial and institutional

projects of all sizes

Graduate of the Milwaukee School of

Engineering

• Fundamentals of Geothermal Systems

• Design Considerations

• Geothermal System

Installation/Construction

Epic Geothermal System

• Bore Fields and Pond

• Distribution System

• Central Plant Equipment

Epic Deep Space Auditorium

Geothermal System Basics

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What is a

Geothermal

HVAC System?

Geothermal

System

Basics

##

Geothermal HVAC is the use of the natural

temperature of the earth as a heat source or heat

sink for heating and cooling.

Requires the use of a heat pump to

move energy between different relative

temperatures

Really a GeoExchange

system. Energy is

transferred between the

earth as the source and

the load; which is either

air or water.

Not to be confused

with Geothermal

energy.

Geothermal

energy is thermal

energy generated

and stored in the

Earth.

Wisconsin has a mean

“undisturbed” ground temperature

between 45-50 deg. F, depending

on the location in the state. This is

the soil temp found well below

ground level (20+ feet deep).

• A geoexchange heat pump system uses the

natural temperature of the earth near the surface

as a heat source or sink, depending on the time

of year

• “Surface” as defined as the top 400-500 feet

below grade

• Deeper can actually mean warmer temperatures.

Also the cost of drilling deep vertical bores can

be expensive.

##

Graph of a test location in Indiana

The ground temperature fluctuates more

the closer you are to grade level

##

Why use a geothermal heat

pump system?

Geothermal systems have a Coefficient of

Performance (COP) greater than 1. More usable

heating energy is obtained than the energy input

to the system

Heating System Type Typical Efficiency COP

Oil Burning Furnace 70-80% 0.7 - 0.8

High Efficiency Gas Furnace 90% 0.9

Electric Resistance Heat 100% 1.0

Ground Source Heat Pump 300 – 400% 3.0 – 4.0

Examples of heating system efficiencies:

Types of geothermal systems

Open loop

systems - ground

water with

reinjection well

Open loop

systems -

surface water

system

Types of geothermal systems

Closed Loop Systems

Horizontal Loops

Closed Loop Systems

Vertical Wells (Bores)

Closed Loop Systems

Pond Systems

What are the basic

components of a

geothermal

system?

GeoExchange source, being either a

loop field or bore field, or a geo pond

Circulation Pump(s)

Heat pump unit (sometimes

referred to as a ground source

heat pump, or a WaterFurnaceTM)

Heated or Cooled Load

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Geothermal

Design

Considerations

Geothermal

System

Basics

##

Geothermal Piping

High Density Polyethylene Piping (HDPE)

Continuous looped pipe with U-bend

for vertical bores or coiled pipe for

horizontal fields or ponds

##

Geothermal Loop Piping Material

• Piping is connected using butt fusion welding

• Provides a strong, seamless pipe joint

• Loop and bore piping typically ranges from ¾”

up to 1 1/4” OD.

• Coil lengths up to 1000 feet available in

smaller diameter piping, coils up to 4” OD

available.

• Straight HDPE sticks available in 48” OD

and larger.

##

HDPE piping wall thickness measured

using SDR = Standard Dimension Ratio

SDR is the ratio of the pipe diameter to wall thickness

SDR = D / s

where

D = pipe outside diameter (mm)

s = pipe wall thickness (mm)

• An SDR of 11 means that the outside diameter of the pipe is eleven times the

thickness of the wall of the pipe.

• The lower the number, the thicker the wall and the higher the pressure rating

• The higher the number, the thinner the wall and the lower the pressure rating

• Most geothermal loop piping is SDR-11.

• Headers and distribution piping from the building to the loop field can be higher

SDR pipe

Vertical Heat Exchanger (VHE) Design

• Size of the borefield is based on a

combination of peak building cooling and

heating loads and annual energy usage for

both heating and cooling

• Commercial buildings generally are cooling

dominated. This means the bore field sized is

based primarily on the amount of heat that

needs to be rejected to the earth due to the

building’s cooling needs.

Vertical Heat Exchanger (VHE) Design

• Vertical bores are generally spaced at 20 foot on

center, but can also be 15 or 25 foot OC.

• Vertical bore depth is typically in the range of

400 feet, and can be deeper or shallower

depending on design factors

• Bore diameter is generally in the 4”- 6” range.

• Basic rule of thumb for geothermal field sizing is

150 feet of bore per ton of building cooling load

The vertical bore

hole is created

just like drilling a

water well,

without the well

casing and pump

Drill rig bores a

vertical hole and

then the U-bend

loop is dropped

down the bore

shaft

##

The vertical bore

hole is then filled

with thermally

enhanced grout.

• Grout is a mixture of Bentonite and sand

mixed with water

• Conducts heat at a high rate

• The grout does not become hard like

concrete, but sets firm and dense

##

Soil Heat Transfer Characteristics

Thermal conductivity (k) is measured in

BTU/(hr⋅ft⋅°F) and is the ability of the soil to

conduct heat

Thermal diffusivity (α) is the ratio of the soil

to conduct thermal energy relative to its ability

to store thermal energy

Geothermal heat exchange effectiveness is

dependent on 2 main factors:

• Thermal Conductivity

• Thermal Diffusivity

##

The size of the VHE is dependent on the

thermal conductivity of the soil where

the borefield will be located, therefore;

Understanding the actual thermal

conductivity of the surrounding soil is

critical!

Designer must arrange for a soil

thermal conductivity test to

understand local conditions

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Geothermal System

Geothermal

System

Basics

Installation/Construction

Borefield Configuration

The individual bores are constructed by dropping a looped

pipe down the borehole

The bores are then interconnected to form individual circuits.

Circuits are generally piped in a reverse return arrangement

to balance flow between the individual bores.

##

On larger systems, each

circuit is brought back to

a distribution manifold.

This allows for individual

loop isolation and

balancing

Multiple manifolds can then

be interconnected to the

piping main and brought

back to the geothermal

system pumps

##

HDPE Pipe Joining Methods

Fusion joint types

•Butt Weld (most typical)

•Socket Joints

•Saddle Joints

Electrofusion fittings are typically

used for the 2 latter joints

##

Butt weld fusion joint construction

Fusion machine

##

Fusion in Process

Amount of rock encountered

during drilling Fissures and voids in the

substructure

Potential Geo installation issues and

considerations

Initial pressure testing of vertical loops

Pressurization of loops during grouting

Final loop pressure testing before

interconnection

Sub-surface conditions

during drilling

Vertical loop installation

Potential Geo installation issues and considerations

Avoiding course and sharp backfill

Proper pipe bedding

Pipe identification (tracer tape)

Documenting as-built pipe locations

Proper fusion pressure

Ensuring flat and clean mating surfaces

Correct melting temperature

Proper joint heating time (heat soak)

Pipe ovality

Pipe alignment

Piping laying, bedding, backfill

HDPE fusion joints

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Facts and Figures

Epic’s

Geothermal

System

Currently has 3500 vertical bores installed with 2500

more under construction

Shortest bores are 300 feet with deepest at 500 feet.

System includes a 10 acre geothermal pond that can

reject approximately 1300 tons of heat during cooling

operation in summer

The geo pond has 1,296 loops that are 600 feet long

of 1” OD piping.

Main system pumphouse has 12 pumps, totalling

5,400 HP and can accommodate 4 more pumps

Geothermal flow capacity to provide close to 80,000

GPM of pumping capacity in a primary/secondary

configuration (160,000 GPM total circulation rate)

System can expand to provide over 30,000 tons of

cooling, enough for 10,000,000 GSF of building

Geothermal mains to the borefields and distribution

piping to the buildings is mostly 36” pipe

All buildings on the campus are served with

geothermal water, including the new Deep Space

Auditorium and the City of Verona’s

well/pumphouse building

Total length of HDPE piping installed on the campus

will exceed 1000 miles of piping (5.3 million feet).

This is the distance between Madison, WI and

Gainesville, FL or Galveston, TX.

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Borefield

Installation and

Construction

Epic’s

Geothermal

System

Borefield Installation and Construction

Well Drilling

Borefield Installation and Construction

Loop Installation

Loop Backfilling

Borefield Installation and Construction

Campus 2 Vault

interior

Campus 2 Vault

exterior

Borefield Installation and Construction

Later Vault Construction

Geothermal Pond Construction

Pond looking West

Geothermal Pond Construction

Pond looking toward manifold vault

Pond slinky loop assembly

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Distribution

System

Construction

Epic’s

Geothermal

System

##

Distribution System

Piping mains in trench

##

Distribution System

Pipe run across field

Fusion in Winter

##

Distribution System

Distribution System

Valve Vault Construction

Distribution System Valve

Vault Construction

##

Distribution System

Distribution System Valve Vault Interior

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Reference and Resources

• ASHRAE

• GeoExchange http://www.geoexchange.org/

• International Ground Source Heat Pump Association

http://www.igshpa.okstate.edu/

• Energy.gov – Geothermal heat pumps

http://energy.gov/energysaver/articles/geothermal-heat-pumps