FEDERAL ENERGY MANAGEMENT PROGRAM

Internet: http://www.eren.doe.gov/femp

Technology

Installation

Review

A case study on

energy-efficient

technologies

Prepared by the

New Technology

Demonstration

Program

No portion of this publication may be altered

in any form without prior

written consent from the

U.S. Department of Energy

and the authoring national

laboratory.

both residential and commercial/institutional

buildings. However, GHPs often have

higher first costs than conventional systems

making short-term economics unattractive.

This disadvantage can be magnified in commercial

buildings, many of which have much larger cooling

needs than heating needs, especially for buildings

located in climates typical of the southern United

States. For GHP systems using closed-loop vertical

ground heat exchangers, this load imbalance can result

in a ground temperature increase over time causing

system performance deterioration. Increasing the size

of the ground heat exchanger or increasing the distance

between adjacent heat exchanger boreholes can postpone the temperature increase problem but will also

result in higher system cost. An alternative, lower cost

approach for such applications can be use of a hybrid

GHP design. In hybrid GHPs, the ground heat

exchanger size is reduced and an auxiliary heat rejecter

(e.g., a cooling tower or some other option) is used to

handle the excess heat rejection loads during building

cooling operation. The extent to which the ground heat

exchanger size can be reduced in a hybrid GHP system

will vary with location and climate, but it

must be at least large enough to handle

the building heating requirements.

Hybrid GHPs can also be used

for sites where the geological

conditions or the available

ground surface will not

allow a ground heat

exchanger large enough

for the building cooling

loads to be installed.

A number of recent

reports and research

papers have been

published that deal

with both design of

hybrid GHPs and

operating experience

with a few installations.

ASHRAE (1995) and

Kavanaugh and Rafferty

(1997) both discuss advantages of hybrid GHPs and

present design procedures. The

former sizes the auxiliary heat

rejecter based on the difference

between monthly average heating and cooling needs of the building and offers general guidelines

Assessment of Hybrid Geothermal Heat Pump Systems

Geothermal heat pumps offer attractive choice for space conditioning

and water heating

Introduction

The purpose of this Technology Installation Review is

to provide an overview of hybrid geothermal heat

pump systems. It presents the results of recent research

on these systems, looks at system types, energy savings, maintenance considerations, and measured technology performance from several examples.

Using the ground as a thermal energy source and/or a

heat sink for heat pumps has long been recognized to

have a number of advantages over the similar use of

ambient air. Ground temperatures at about 3-ft depth

or lower are much less variable than ambient air temperatures. Further, soil or rock at these depths is usually warmer than ambient air during the coldest winter

months and cooler than ambient air during the summer

months. This fact leads directly to cooler condensation temperatures (during cooling operation) and

warmer evaporating temperatures (during heating) for

a heat pump with consequent improved energy efficiency. It also results in increased heating and cooling

capacity at extreme temperatures, thereby reducing or

eliminating the need for auxiliary heat.

Heat pump systems that make use of the

ground in this way are called groundsource or geothermal heat pumps

(GHPs). GHPs are also known by

a variety of other names: geoexchange heat pumps, groundcoupled heat pumps,

earth-coupled heat pumps,

ground-source systems,

ground-water source heat

pumps, well water heat

pumps, solar energy heat

pumps, and a few other

variations. Some names

are used to describe more

accurately the specific

application; however,

most are the result of marketing efforts and the need

to associate (or disassociate)

the heat pump systems from

other systems.

Why Hybrid GHPs?

The advantages of GHPs over conventional alternatives make them a very attractive

choice for space conditioning and water heating for

DOE/EE-0258

Matrix of

ground heat

exchangers

in vertical

bores shown

with optional

cooling tower

for hybrid

systems
2

closed loop is “conditioned” by exchanging heat with one or more geothermal heat

sources or sinks, such as the ground,

groundwater, surface water, wastewater

streams, or potable water supplies (where

allowed). Figure 1 illustrates the various

types of geothermal source/sinks that may

be used in GHP systems. Hybrid GHP system designs using outdoor air as a supplement heat sink can be configured with any

of these GHP source/sinks. Figure 2 is a

schematic illustration of a hybrid GHP

using a closed loop vertical ground heat

exchanger and a cooling tower as an auxiliary heat rejecter.

for integration of the heat rejecter into the

system piping. The latter bases heat

rejecter sizing on peak loads at design conditions and the difference between

required ground heat exchanger borehole

lengths for heating and cooling.

Kavanaugh (1998) revises and extends the

design procedures presented in the above

two publications. In addition, a control

method is proposed for balancing the cooling and heating loads on the ground heat

exchanger to limit long-term temperature

increase. The revised procedure is applied

to an office building in three climates,

and first cost and operating cost issues

are discussed.

Yavuzturk and Spitler (2000) present a

comparative study investigating several

control strategies for hybrid GHP systems.

The strategies investigated include set

point control (operating the auxiliary

rejecter whenever the heat pump entering

or exiting fluid temperature exceeds a set

temperature), differential temperature control (operating the auxiliary rejecter whenever the difference between heat pump

fluid temperature and ambient air temperature exceeds a set value), and operation of the

auxiliary rejecter to remove heat from the

ground heat exchanger field during nighttime hours. The purpose of the last strategy

is to use the tower to attempt to balance the

annual heat rejection and heat absorption

loads on the ground heat exchanger. A

20-year life-cycle cost analysis is conducted to compare each control strategy.

Thornton (2000) performed an analysis of

a hybrid GHP for a building at the U.S.

Navy’s Oceana Naval Air Station. Overall

system performance was compared for

similar control strategies as studied by

Yavuzturk for both 1-year and 10-year performance periods.

A few studies of actual hybrid GHP installations have been reported as well.

Phetteplace and Sullivan (1998) describe a

system installed in an administration building at the U.S. Army’s Fort Polk facility.

The paper presents performance data over

a 22-month period. Singh and Foster (1998)

explore first-cost savings resulting from

hybrid GHP designs in two buildings—an

office building and an elementary school.

Both of these cases are examples of installations where site geological characteristics or surface area limitations precluded

use of a ground heat exchanger large

enough to handle the total cooling needs.

Technology Description

GHP system types

There are several types of geothermal heat

pump systems that can be used for building

space conditioning and water heating. The

common denominator is that water source

heat pumps exchange heat between indoor

air (for space heating or cooling) or water

(for heating or chilling water) and a liquid

(either water or a water-coolant mixture)

flowing in a closed loop. The liquid in the

FEDERAL ENERGY MANAGEMENT PROGRAM

Figure 1. Various geothermal source/sinks that can be applied to geothermal heat

pump systems in commercial or residential applications.

Commercial Applications:

Residential Applications:

Optional cooling

tower for

hybrid systems

Matrix of ground heat

exchangers in vertical bores
3

either desuperheaters on the heat pumps or,

where large amounts of hot water are

needed, dedicated water-to-water units.

GHP systems save money because they

use less energy and improve energy consumption patterns. The 4003 GHPs in

family housing at Fort Polk reduced the

summer electric peak demand of that city

of 12,000 people by 43% and increased

the annual electric load factor from 0.52 to

0.62. Federal sites can purchase electricity

at lower costs when their load characteristics improve so dramatically. (For more

details on the Fort Polk story see

www.eren.doe.gov/femp/financing/

tecspec.html. Click on “Geothermal Heat

Pumps.” Also see Hughes and Shonder

[1998].)

Maintenance Considerations—

Cooling Towers

For hybrid GHPs using cooling towers as

the auxiliary heat rejecter, tower maintenance is an issue that must be considered.

Cooling towers add additional maintenance time and cost to the HVAC system.

ASHRAE (1996) offers recommendations

for a tower maintenance program including daily and weekly visual inspections for

cleanliness, control component functioning, and sump water levels. These recommendations are intended to encourage

tower maintenance and operating practices

to reduce Legionella risks.

Geothermal heat source/heat sink

options

For the government to receive the best

value from GHP technology, installation

contractors and Federal site personnel need

to determine which geothermal heat

source/sink type or combination of types is

most economical for each site. The order

of preference is not universal, but it generally is as follows.

Groundwater already being pumped. Is

groundwater currently being pumped to

the surface? Some Federal sites pump

groundwater to the surface, treat it, and

re-inject it as a part of groundwater

remediation projects in areas near buildings. Tapping into such an already existing

heat source/sink may be economical. A

plate and frame heat exchanger can be used

to transfer heat between the groundwater

FEDERAL ENERGY MANAGEMENT PROGRAM

Figure 2. Hybrid GHP system schematic—tower isolated from building and ground loop.

The closed loop in a GHP system may

serve one or many heat pumps, depending

on the application. For example, military

family housing might be served with systems having one heat pump per living unit,

each with its own vertical ground heat

exchanger (GHX). Larger facilities might

have many heat pumps on a common loop

with a central variable-speed pumping station and one large vertical GHX. The schematic in Figure 2 is an example of a large

central system type. Individual water

source heat pump units provide heating

and cooling to each zone within the building. These units are connected to a common interior building pipe loop, which is

used as a heat source or sink as needed.

Dedicated water-to-water heat pumps may

also be connected to the loop to meet building water heating needs or to preheat or cool

ventilation air as shown in this example.

The building loop is connected to the

ground source/sink system (a vertical

ground heat exchanger in this case) via a

central pumping station. The pumping station may be configured with a few large or

several small single-speed pumps in parallel or with variable-speed pumps. Multiple

parallel pumps or variable speed pumps

offer the possibility of reducing pumping

power during periods when building load

is lower than the design values. In this

hybrid system example, the cooling tower

(or other heat rejection unit) is connected in

series with the ground heat exchanger and

is isolated from the building and ground

piping loops with a plate heat exchanger. A

wet or evaporative tower is shown in the

example, but dry towers may also be used.

Systems using dry towers would consume

more energy because the tower would have

to reject heat to the ambient air dry bulb

temperature rather than the lower wet bulb

temperature, as is the case with evaporative

towers.

Energy Saving Mechanisms

and Benefits

GHPs save energy and money because the

equipment operates more efficiently than

in conventional systems. The compressors

in the individual heat pump units of a GHP

system operate much more efficiently than

those in air source units because the geothermal source/sink temperature is far

more stable than that of outdoor air and has

much less severe high and low extremes.

In addition, air need be moved only on one

side of the GHP, and less power is needed

to move the liquid on the other side than

would be needed to move air. Unlike air

source units, GHP systems do not need

defrost cycles or backup electric resistance

heat at low outdoor air temperatures.

Common loop GHP systems recover heat

as part of their design. In cooler weather,

the heat pumps serving the building perimeter extract heat from the common loop to

provide space heating, while units serving

core areas are cooling space and rejecting

heat to the common loop. When the common loop is in balance, no net conditioning

is required from the geothermal source;

under some operating conditions, the offset

between heating and cooling units reduces

the thermal load on the source. Recovered

heat also can be used to heat water, using
4

and a common loop serving individual heat

pump units in nearby buildings. After

remediation is completed, the pumping on

the groundwater side of the heat exchanger

can be re-optimized for the HVAC application and continued using the same supply

and reinjection wells.

Stationary surface water. Are there large

volumes of stationary surface water on the

site, owned by the government and with

government use restrictions, near buildings

with significant heating and cooling loads?

Heat exchange with surface water

impoundments such as reservoirs, runoff

retention basins, reflecting pools, ponds,

and lakes, may be economical. A common

loop serving individual heat pump units in

nearby buildings can be submerged

directly into the body of water. If the loop

pipe material could be subject to damage

from local wildlife, or if the water is used

for recreational or other purposes that

might interfere with this approach, an onshore pump house with a plate and frame

heat exchanger and protected intake from

and discharge to the body of water could be

considered. The issue of sensitivity of

local fish to water temperature changes

may need to be considered as well.

Moving surface water. Are large volumes

of reliable moving surface water (e.g.,

large rivers with reliable flow and modest

current), owned by the government and

with government use restrictions, on the

site near buildings with substantial space?

Use of this type of water for heat exchange

with a submerged pipe loop may be economical. If use of a submerged loop is not

feasible, an on-shore pump house with a

plate and frame heat exchanger and protected intake from and discharge to the

moving body of water could be considered. Issues such as historical high and low

water conditions, debris flow, sensitivity of

aquatic life to temperature changes, and

commercial and recreational traffic would

require serious attention.

Wastewater streams. Does the site have

large-volume, reliable flowing wastewater

streams near buildings with significant

square footage? If so, those streams may

offer economical heat exchange. A common

loop serving individual heat pump units in

nearby buildings could be conditioned by a

plate and frame heat exchanger in contact

with the wastewater. Heat exchanger

maintenance must be considered, as well

as the stability of the missions of the facilities that are the source of the wastewater.

Groundwater. Are large quantities of

groundwater available at a reasonable

depth at the site, as well as an acceptable

and economical means of disposal, near

buildings with significant heating and

cooling loads? Heat exchange with

groundwater may be economical even

when the project must bear the cost of

developing the supply and discharge wells.

The groundwater can be used with a plate

and frame heat exchanger to condition a

common loop serving individual heat

pump units in nearby buildings. Poor

water quality might require the use of

expensive heat exchanger materials, and

additional maintenance and aquifer

re-injection in some formations might be

expensive. Local environmental regulations on the use of groundwater need to be

considered. In some cases, a doublewalled heat exchanger may be required to

minimize the risk of contamination of the

ground-water with the fluid used in the

common building loop.

Standing column well. Standing column

well GHP systems are similar to standard

groundwater GHPs, but since water is recirculated between the well and the building, only one well may be required (larger

projects may have several wells in parallel). Standing column wells are feasible in

areas with near surface bedrock. Deep

bores are drilled, creating a long standing

column of water from the static water level

down to the bottom of the bore. Water is

recirculated from one end of the column to

the other. During peak heat rejection or

extraction periods, the system can bleed

part of the water to the surface (lake, pond,

stream, etc.) rather than reinjecting it all,

causing water inflow to the column from

the surrounding rock formation. This

cools the column during heat rejection

(building cooling), heats it during heat

extraction (building heating), and reduces

the required bore length.

Ground heat exchangers. Does the site

have sufficient land area to accommodate

ground heat exchangers near buildings

with significant square footage? If so, heat

exchange with the ground, using vertical or

horizontal loops, may be economical. A

central ground heat exchanger can condition

a common loop serving individual heat

pump units in nearby buildings. Alternatively, each heat pump, or small group of

heat pumps, can have its own ground loop.

Horizontal loops require considerably

more land area but may be less expensive

to install, depending on the types of soil

and rock formations encountered in drilling. Ground heat exchangers are an option

almost anywhere. They are placed last in

this list not because they are less economical than other geothermal options, but

merely to ensure that other options are considered where they exist.

Specific hybrid system

considerations

Sites should be examined to determine

whether a hybrid GHP system might be

advantageous. For applications where the

required geothermal heat sink capacity for

cooling is much greater than the required

heat source capacity for heating, hybrid

systems can lead to significantly reduced

installation costs. This is particularly true

for commercial or institutional buildings in

warm climates. A rule of thumb suggested

by Kavanaugh (1997) for defining a warm

climate location is one where the average

ground temperature at 30-ft depth exceeds

60°F. Hybrid designs may also have economic advantages in such buildings in

more moderate climates (30-ft ground temperature between 50-60°F) particularly for

buildings with low ventilation air requirements or where heat recovery units or other

conditioning means are used to reduce

ventilation air heat gains/losses. Hybrid

designs can also allow use of GHP systems

(and access to most of their advantages)

where site subsurface geological conditions

or surface area limitations prevent installation of geothermal source/sink systems

with sufficient capacity to fully meet the

building heat rejection needs for cooling.

Auxiliary heat rejecter location and maintenance requirements must be considered.

Technology Performance

Building 1562 – Fort Polk, LA

In 1993, Building 1562, a 24,000-ft2

administration building at Fort Polk, LA,

was renovated including conversion of the

existing HVAC system to a hybrid GHP system. The new system consisted of 14 water

FEDERAL ENERGY MANAGEMENT PROGRAM
5

source heat pumps (total cooling capacity

of about 120 tons) connected to a common

building loop that was conditioned by a

vertical ground heat exchanger grid with

seventy, 200-ft-deep boreholes on 10-ft

centers and a 78-ton wet cooling tower.

(Kavanaugh and Rafferty [1997] recommend that bore-to-bore spacing in vertical

GHX grids be at least 20-25 ft for applications that are cooling load dominated to

minimize long-term ground temperature

rise effects.) A hybrid system was selected

by the designer primarily because of limited

space available for borehole installation

and because high internal personnel and

computer loads together with the warm climate location led to design cooling loads

much greater than heating loads. Phetteplace

and Sullivan (1998) presented operating

data collected over a 22-month period.

Cooling tower operation was initiated

whenever the common loop fluid temperature exceeded 97°F and was terminated

when the loop temperature fell below 95°F.

The observed data show that, over the

period of monitoring, the amount of heat

rejected to the ground (in cooling) was

about 43 times as great as that extracted

during heating—an indication that the

tower was not used effectively to reduce

the thermal load on the ground in this

installation. Phetteplace and Sullivan

noted that some increase in ground temperature occurred between the two summers

over which the data were taken. This is

attributed to the large imbalance between

cooling and heating loads on the ground

loop and the close spacing of the boreholes. They recommended that the tower

setpoint controls be revised to either initiate tower operation at a lower fluid

setpoint temperature or to operate the

tower whenever possible to minimize the

ground heat rejection load. Even though

this would result in increased tower energy

consumption, it is estimated that system

energy consumption would be reduced

because the heat pumps would operate at

higher efficiency (due to lower entering

fluid temperatures). The data showed that

the tower fan and pump only accounted

for 4% of the system energy over the

22-month monitoring period while the heat

pumps used 77% of the total. Thus,

improved efficiency of the heat pumps

could more than compensate for even large

increases in tower energy.

It was also noted that the building loop circulation pumps accounted for 19% of the

total system energy use over the monitored

period. Loop circulation is provided by

two 15-hp pumps, one of which operates

continuously. The system could be modified

to use solenoid valves at each heat pump

(to shut off fluid flow when the heat pump

is off) and variable speed loop pumps to

reduce loop flow during low load periods.

Phetteplace and Sullivan estimated that

such a pump control strategy could have

reduced the loop pumping power by 45%

over the study period resulting in a 8.5%

reduction in system energy use.

As of this writing, the system at Ft. Polk’s

building 1562 has not been modified to

incorporate any of the recommendations

made by Phetteplace and Sullivan. However, they are preparing to retrofit a group

of their buildings with GHP systems under

FEMP’s new GHP Super ESPC (see

sidebar below) and this may include

FEDERAL ENERGY MANAGEMENT PROGRAM

GHP Super ESPC

The Federal Energy Management Program (FEMP) has implemented a “Super ESPC” to streamline the process of procuring GHP-centered

energy saving retrofit projects for Federal facilities. In an ESPC (energy savings performance contract), an energy services contractor (ESCO)

bears the costs of implementing energy-saving measures in exchange for fixed (annual or monthly) payments from the resulting cost savings.

Under the Super ESPC, a GHP system must be the major focus but other energy saving measures (e.g., lighting improvements) can be

included if they make the project more economical. FEMP has selected and preapproved a pool of ESCOs with which Federal agencies can

contract. Under the Super ESPC, delivery orders can be awarded quickly, and facility managers have the assurance that all of the selected

ESCOs are qualified to deliver top-quality GHP-centered energy efficiency projects.

The following contractors have been selected for this Super ESPC:

• Constellation Energy Source, Baltimore, MD; www.cesource.com

• Duke Solutions, Charlotte, NC; www.dukesolutions.com

• Energy Performance Services, Inc., King of Prussia, PA; www.energy-assets.com

• Enron Energy Services Operations, Inc., Alpharetta, GA; www.enron.com

• Trane Company Asset Management Services, St. Paul, MN; www.trane.com

Among the advantages of GHP-centered projects under the Super ESPC are:

• Alignment with ESPC statutory authority and full compliance with all Federal procurement regulations is assured.

• New GHP-based HVAC and water heating systems can be acquired at no capital cost to the Federal facility.

• Super ESPC contracts were awarded to large financially stable ESCO teams that can offer financing at low rates. In addition, these ESCOs

had to demonstrate their GHP capabilities through past performance and a specific proposal for a large initial project.

• ESPC project cost savings are guaranteed to exceed payments to the ESCO for services and debt.
6

upgrading the controls of the existing

hybrid GHP system (Phetteplace 2001).

Paragon Center – Allentown, PA

The Paragon Center in Allentown, PA, is

an 80,000-ft2

, four-story office building. It

was intended to have a full GHP system for

building HVAC with a ground heat

exchanger consisting of fifty-five, 500-ft

deep boreholes to meet a 200-ton design

capacity (Singh and Foster 1998). Although

an early test bore indicated that the site

could accommodate the deep bores, subsequent drilling in the intended ground loop

location was unable to go lower than about

110-125 ft due to high water flow in a limestone strata. Drilling at that location would

require casing the boreholes. For the original

500-ft borehole design casing would have

increased installation costs beyond the

allowable budget. It was determined that a

hybrid GHP design with eighty-eight,

125-ft cased boreholes and a 120-ton cooling tower could be built within the budget

allowing the building to enjoy most of the

advantages of GHP technology. Singh and

Foster note that the building HVAC system

also employs heat recovery units to reduce

heat losses and gains from ventilation air

and uses variable speed pump control to

limit pump energy at off-peak conditions.

The building operating energy cost is

reported to be less than $1.00/ft2

 annually,

including demand charges.

Elementary School – West Atlantic

City, NJ

Singh and Foster (1998) also discussed a

planned expansion of an elementary

school in West Atlantic City, NJ. The

expanded structure will have 85,000 ft2

 of

conditioned area and will be occupied

9 months of the year. Total cooling load

including ventilation air is 275 tons. Heat

recovery equipment included in the HVAC

design covers 25 tons of the load, leaving

250 tons to be covered by the main HVAC

system. It was intended to use a full GHP

system with ninety, 400-ft bores. However, the relatively small site (210 ft by

580 ft) was not large enough to fit all of the

boreholes needed. A hybrid GHP will be

installed with a ground heat exchanger

system of sixty-six, 400-ft bores covering

133 tons of the design load and a 117-ton

cooling tower to make up the difference.

Ground heat exchanger installed cost is

about $9.70 per bore ft ($255,600 total).

No operating data is reported for the system, but Singh and Foster estimated that

the hybrid GHP would generate annual

energy cost savings of about $4500 and

maintenance savings of $5000 compared

to a conventional boiler/tower water loop

heat pump system. The estimated installation cost of the hybrid GHP is $1,139,100

compared to $1,118,300 for the conventional alternative, so the estimated simple

payback is 2.2 years.

Technology Demonstration

Detailed comparative analyses have been

performed on full and hybrid GHP systems

by Yavuzturk and Spitler (2000) on a small

office building in two locations and by

Thornton (2000) on a Navy training

center building.

Yavuzturk and Spitler consider a small

office building (14,205-ft2

 conditioned

space) in Houston, TX, and Tulsa, OK, climates. Annual loads for this building in

Houston were calculated to be 7.5 million

Btus heating and 181.6 million Btus cooling. For the Tulsa location, the calculated

annual loads were 50.1 million Btus heating and 133.8 million Btus cooling. Design

cooling loads for the building were estimated to be about 15 tons in both locations.

The hybrid GHP system layout is based on

the schematic in Figure 2 where the cooling tower is isolated from the ground heat

exchanger and building loops with a plate

heat exchanger. The plate heat exchanger

is plumbed in series with the ground heat

exchanger. The ground loop is modeled

using a short time step approach developed

by Yavuzturk and Spitler (1999). Total

system simulation for both full and hybrid

GHP cases is done using the TRNSYS

code (Klein et al. 1996) with the ground

loop model incorporated into a TRNSYS

module. Eight different hybrid GHP

system control strategies within three main

categories were investigated to determine

their impacts on system energy consumption and overall system operation. The

specific strategies within each main

category that yielded the lowest system

(building heat pumps plus tower pump

and fan) energy use and life-cycle cost are

summarized in this review. System

maintenance costs were not quantified for

the life-cycle cost estimates in these studies

and no annual electricity escalation rate

was considered.

Hybrid Case 1. The cooling tower is activated when the fluid temperature leaving

the heat pumps exceeds a specified set

point. In this study, the set point temperature was 96.5°F.

Hybrid Case 2. This approach uses the

difference between the heat pump fluid

exit temperature and the ambient wet-bulb

temperature. Cooling tower operation is

initiated whenever the temperature difference exceeds 3.6°F and is stopped when

the difference falls below 2.7°F .

Hybrid Case 3. This strategy is based on

using the cooling tower to reject heat from

the ground to avoid long-term temperature

rise. This effect is achieved by operating

the tower for six hours daily (from midnight to 6 a.m.) year-round. To avoid high

loop temperatures, a secondary set point

control is included to operate the tower

whenever the fluid temperature entering

the heat pumps exceeds 96.5°F.

Summary results for the subject building in

the Houston and Tulsa locations are given

in Tables 1 and 2, respectively, and also in

Figures 3 and 4. Case 1 results in the least

usage and energy consumption for the

cooling tower of the three hybrid control

approaches. Case 2 yields the lowest values for annual system energy use and lifecycle cost for both locations while case 3

has the lowest system first cost. The case 2

strategy operates the tower a very large

number of hours and generally under

advantageous outdoor ambient (lower

temperature) conditions. This approach

tended to cool the ground over time resulting in lower ground loop fluid temperature.

This resulted in lower heat pump energy

use due to more efficient heat pump operation in the cooling season. Tower usage

and energy consumption is greatest for this

approach, but overall system energy use

compared to the full GHP design is 18%

lower for the Houston location and 6%

lower for the Tulsa location. Clearly, the

hybrid approach yields greater energy saving benefits in the warmer (higher cooling

load) location. Energy use for the ground

loop and building loop circulation pump is

FEDERAL ENERGY MANAGEMENT PROGRAM
7

also a significant factor in total system

energy consumption as noted above in

discussion of the Ft. Polk system. In analysis of the Yavuzturk and Spitler case

study for this review, however, it is assumed

to be the same for the full GHP and the

hybrid GHP cases. The building heat

pump flow requirements will be the same

for all systems and the total pressure loss

for the GHX and tower loops is assumed

not to vary significantly among the four

cases studied.

Figure 4 shows the comparison in system

first cost between full and hybrid GHP

designs for both locations. First cost of the

ground heat exchanger plus auxiliary heat

rejecter for the hybrid cases is lower than

for the full GHP ground loop in both locations. This difference is most dramatic in

Houston where the hybrid system reduces

installation costs by more than 50%.

FEDERAL ENERGY MANAGEMENT PROGRAM

Table 1. Summary of Hybrid GHP case study for 14,025 ft2 office building in Houston, Texas (from Yavuzturk 2000)

Heating degree-days = 1434; Cooling degree-days = 2889* Annual heating load = 7.5 million Btus Annual cooling load = 181.6 million Btus

Base case—no Hybrid Hybrid Hybrid

cooling tower Case 11 Case 22 Case 33

Number of boreholes in ground heat exchanger 36 @ 250 ft 12 @ 250 ft 12 @ 250 ft 12 @ 250 ft

Cost of ground heat exchanger4 $54,000 $18,000 $18,000 $18,000

Maximum fluid temperature entering heat pumps in 20-year period (°F) 96.6 96.3 80.5 96.0

Minimum fluid temperature entering heat pumps in 20-year period (°F) 71.3 67.3 40.5 54.1

Design capacity of cooling tower (tons) 22.5 11.5 8.5

Cost of cooling tower and plateheat exchanger including controls

and auxiliary equipment5 - $8,662 $4,427 $3,272

Total cost of ground heat exchanger and cooling tower equipment $54,000 $26,662 $22,427 $21,272

Present value of 20 years of electricity costs6 $19,611 $19,413 $16,011 $20,573

Present value of total costs $73,611 $46,075 $38,438 $41,845

Annual energy use (kWh)

 Heat pumps 24,425 23,877 17,792 24,453

 Cooling tower fan - 260 1,847 1,006

 Cooling tower pump - 42 302 164

 Total system 24,425 24,179 19,941 25,623

* From Air Force Manual AFM 88-29, “Engineering Weather Data,” July 1978. 1 Set point control to limit fluid temperature exiting heat pumps to 96.5°F or less.

2 Differential temperature control activates tower when difference between heat pump exiting fluid temperature and air wet-bulb temperature exceeds 3.6°F and turns

tower off when this difference falls below 2.7°F. 3 Tower operated between midnight and 6 a.m. year-round to cool ground heat exchanger field (attempt to balance heating and cooling loads on ground); secondary set

point control to limit fluid temperature entering heat pumps to 96.5°F or less. 4 Estimated at $6.00/ft of borehole, including horizontal runs and connections (Kavanaugh and Rafferty 1997).

5 Estimated at $385/ton of tower design capacity based on data from Means (1999).

6 Assumes $0.07 per kWh cost of electricity; no price escalation rate assumed. A 6% discount rate is used for present value computation.

Thornton (2000) performed a comparative

analysis of hybrid and full GHP systems

for Building 137 at the U.S. Navy Oceana

Naval Air Station. Building 137 is a training facility with about 21,000 ft2

 of office

and classroom space. In addition, there are

a number of large, open-bay hangars

attached to the building. The building is

presently heated and cooled using a combination of steam from a central base steam

generating station (for the hangar areas)

and water source heat pumps connected to

a common building water loop (for the

classroom/office area). When the building

is being cooled, condenser heat from the

heat pumps is rejected through a 100-ton

cooling tower. The HVAC system for the

office/classroom portion of the building

will be renovated to a GHP-based system

under an energy savings performance contract (ESPC). This will be one of the first

such arrangements to be put in place under

the GHP “Super ESPC” recently approved

by FEMP (see sidebar, page 5).

In Thornton’s analysis, the TRNSYS code

was used to model the building and all

HVAC system components. The annual

heating load for the office/classroom portion of the building was calculated to be

199.6 million Btus while the annual cooling load was 439.1 million Btus. There are

21 individual water source heat pumps

used to provide heating and cooling with a

total nominal cooling capacity of about

75 tons. Total length of the ground heat

exchanger for the full GHP baseline case

was 11,400 ft. The ground heat exchanger

was simulated using the DST algorithm

(Pahud and Hellstrom 1996) which was

cast as a TRNSYS module. Thornton considered several different hybrid system

configurations and control strategies based

on using the existing cooling tower as the
8

Table 2. Summary of Hybrid GHP case study for 14,025 ft2 office building in Tulsa, Oklahoma (from Yavuzturk 2000)

Heating degree-days = 3680; Cooling degree-days = 1949* Annual heating load = 50.1 million Btus Annual cooling load = 133.8 million Btus

Base case—no Hybrid Hybrid Hybrid

cooling tower Case 11 Case 22 Case 33

Number of boreholes in ground heat exchanger 16 @ 240 ft 9 @ 240 ft 9 @ 240 ft 9 @ 240 ft

Cost of ground heat exchanger4 $23,040 $12,960 $12,960 $12,960

Maximum fluid temperature entering heat pumps in 20-year period (°F) 96.4 96.9 79.0 97.9

Minimum fluid temperature entering heat pumps in 20-year period (°F) 50.2 39.8 24.2 39.2

Design capacity of cooling tower (tons) - 17.0 11.0 5.5

Cost of cooling tower and plate heat exchanger including controls

and auxiliary equipment5 - $6,545 $4,235 $2,118

Total cost of ground heat exchanger and cooling tower equipment $23,040 $19,505 $17,195 $15,078

Present value of 20 years of electricity costs6 $15,999 $15,988 $14,976 $17,595

Present value of total costs $39,039 $35,493 $32,171 $32,672

Annual energy use (kWh)

 Heat pumps 19,927 19,813 16,463 20,769

 Cooling tower fan - 86 1,882 984

 Cooling tower pump - 14 308 161

 Total system 19,927 19,913 18,653 21,914

* From Air Force Manual AFM 88-29, “Engineering Weather Data,” July 1978. 1 Set point control to limit fluid temperature exiting heat pumps to 96.5°F or less.

2 Differential temperature control activates tower when difference between heat pump exiting fluid temperature and air wet-bulb temperature exceeds 3.6°F and turns

tower off when this difference falls below 2.7°F. 3 Tower operated between midnight and 6 a.m. year-round to cool ground heat exchanger field (attempt to balance heating and cooling loads on ground); secondary set

point control to limit fluid temperature entering heat pumps to 96.5°F or less. 4 Estimated at $6.00/ft of borehole, with horizontal runs and connections (Kavanaugh and Rafferty 1997).

5 Estimated at $385/ton of tower design capacity based on data from Means (1999).

6 Assumes $0.07 per kWh cost of electricity; no price escalation rate assumed. A 6% discount rate is used for present value computation.

Tulsa, Ok Houston, TX

30,000

25,000

20,000

15,000

10,000

5,000

0

Full GHP Hybrid GHP Case 1 Hybrid GHP Case 2 Hybrid GHP Case 3

Annual system Energy Use (kWh)

19,927

24,425

19,913

24,179

18,653

19,941

21,964

25,623

Figure 3. Full vs. hybrid GHP system (heat pumps, tower pump, and tower fan) energy

use for 14,025-ft2 office building—Houston, Texas, and Tulsa, Oklahoma, locations.

FEDERAL ENERGY MANAGEMENT PROGRAM

auxiliary heat rejecter. The three that produced the lowest hybrid GHP system

(building heat pumps plus tower fan and

circulating pump) energy usage are summarized below.

Oceana Hybrid Case 1. For this system,

the tower is piped in series with the ground

heat exchanger and is not isolated from the

GHX loop as shown in Figure 5. The

tower is activated when the fluid temperature in the system fluid loop exceeds a

specified set point. In this case, the set

point temperature was 80°F. Ground heat

exchanger size for this case was 7,500 ft vs.

11,400 ft for the base case.

Oceana Hybrid Case 2. For this system,

the tower is isolated from the ground heat

exchanger with a plate heat exchanger as

shown in Figure 6. The auxiliary heat

rejection circuit (tower) operates when the

fluid temperature in the tower fluid loop
9

exceeds 70°F. Main loop flow was split

between the plate and ground heat

exchangers such that the total heat rejection to the ground in summer equaled the

heat extraction in winter. The required

split in loop flow to achieve ground thermal load balance for this case was 90% to

the plate heat exchanger. Ground heat

exchanger size for this case was 4,650 ft

vs. 11,400 ft for the base case.

FEDERAL ENERGY MANAGEMENT PROGRAM

Figure 4. Full vs. hybrid GHP system first cost for 14,025-ft2 office building—Houston,

Texas, and Tulsa, Oklahoma, locations.

Tulsa, Ok Houston, TX

60,000

50,000

40,000

30,000

20,000

10,000

0

Full GHP Hybrid GHP Case 1 Hybrid GHP Case 2 Hybrid GHP Case 3

System First Cost ($)

23,040

54,000

19,505

26,662

17,195

22,427

15,078

21,272

Oceana Hybrid Case 3. For this system,

the tower is isolated from the ground heat

exchanger with a plate heat exchanger as

shown in Figure 6. The auxiliary heat

rejection circuit (tower) operates when the

fluid temperature in the tower fluid loop

exceeds 70°F. Main loop flow was split

between the plate and ground heat

exchangers so that the maximum fluid

temperature entering the building heat

pumps was kept below 95°F. The required

split in loop flow for this case was 42% to

the plate heat exchanger. Ground heat

exchanger size for this case was the same

as for case 2.

Summary results for Thornton’s ten-year

analysis for the full GHP base case and the

three hybrid GHP cases are presented in

Table 3. As in Yavuzturk and Spitler’s

analysis, the setpoint control strategy of

case 1 results in the least tower usage and

energy consumption. Case 3 yields the

lowest value for both first cost and lifecycle cost. For this case study, the first cost

is taken to be the GHX cost plus the cost of

the plate heat exchanger and tower loop

circulation pump. Tower cost is not included

because a tower already exists for this

building. All other system costs (for the

internal building systems and controls,

GHX pump and controls, etc.) are assumed

to be the same for all systems examined.

For the Oceana site, annual energy use for

the building heat pumps is almost the same

for all cases (full and hybrid GHPs) in contrast to the results seen for the previous

case study assessment for Houston and

Tulsa locations. At Oceana, the cooling-toheating (C/H) load ratio for the building

studied was about 2.2—about 20% lower

than for the office building studied in Tulsa

(C/H ratio about 2.7) and much lower than

for the building studied in Houston (C/H

ratio about 24.2). Thornton’s Oceana

study results show that in the hybrid GHP

cases, the building heat pumps used more

energy in the heating season than for the

base case full GHP due to the lower minimum fluid temperatures. Cooling season

heat pump energy use ranged from slightly

lower to about the same for the hybrid

GHP systems. With the added energy use

of the tower fan and pumps, overall system

energy use for the hybrid GHP cases studied at Oceana ranges from 1% to 13%

higher than for the base case.

Figure 7 shows the comparison of first cost

between the full and hybrid GHPs for the

Oceana study. For case 1 (with tower not

isolated from GHX and building), the

GHX size and system first cost are both

reduced by about 34%. In cases 2 and 3,

with an isolated tower circuit, it is possible

Figure 5. Hybrid GHP system schematic—tower unisolated from ground and

building loop.
10

Figure 6. Hybrid GHP system schematic for Oceana analysis cases 2 and 3—tower

isolated from building and GHX and piped in parallel with GHX.

Table 3. Summary of Oceana Hybrid GHP case study - existing tower (from Thornton 2000)

Heating degree-days = 3639; Cooling degree-days = 1485* Annual heating load = 199.6 million Btus Annual cooling load = 439.1 million Btus

Base case—no Hybrid Hybrid Hybrid

cooling tower Case 11 Case 22 Case 33

Number of boreholes in ground heat exchanger - ten-year design 76 @ 150 ft 50 @ 150 ft 31 @ 150 ft 31 @ 150 ft

Cost of ground heat exchanger4 $92,454 $60,825 $37,712 $37,712

Maximum fluid temperature entering heat pumps in ten year period (°F) 94.9 84.6 92.5 95.1

Minimum fluid temperature entering heat pumps in ten year period (°F) 63.3 53.7 35.5 44.4

Existing cooling tower capacity (tons) - 100.0 100.0 100.0

Cost of plate heat exchanger and tower circuit pump5 - - $3,800 $3,800

Total cost of ground heat exchanger, plate heat exchanger,

and tower pump $92,454 $60,825 $41,512 $41,512

Present value of 10 years of electricity costs6 $29,069 $29,492 $32,898 $31,906

First-year electricity costs6 $3,950 $4,007 $4,470 $4,335

Present value of first costs plus electricity costs $121,523 $90,317 $74,410 $73,418

Annual energy use (kWh)

 Heat pumps 70,027 69,083 72,234 71,632

 Cooling tower fan - 573 3,115 1,379

 Cooling tower pump - 1,391 3,902 3,840

 Total system 70,027 71,047 79,251 76,861

* From Air Force Manual AFM 88-29, “Engineering Weather Data,” July 1978. 1 Unisolated tower; tower operated whenever loop temperature exceeds 80°F.

2 Isolated tower with plate heat exchanger; tower operated to achieve balance between heating and cooling loads on ground heat exchanger. 3 Isolated tower with plate heat exchanger; tower operated with setpoint control to keep maximum fluid temperature entering heat pumps at or below 95°F. 4 Actual cost of test bores drilled at Oceana site for ground thermal conductivity testing was $8.11 per ft of borehole, including horizontal runs and connections (personal

communication with B. Koshka, Trane Co. December, 2000).

5 Plate heat exchanger estimated at $25.50 per ton based on data presented by Kavanaugh and Rafferty 1997; tower circuit pump cost estimated at $1250. 6 Electricity cost $0.0564 per kWh (rate charged to Oceana buildings by U.S. Navy Public Works Center - Norfolk); no price escalation rate assumed. A 6% discount

rate is used for present value computation.

to operate with lower temperatures in the

GHX (using an antifreeze solution). The

GHX size is reduced about 59% and system first cost by about 55% in these cases.

The hybrid systems had life-cycle costs

from 26% lower (for case 1) to 40% lower

(for case 3) than that for the base case.

Summary. Based on the results from the

two case studies analyzed for this review,

the following observations are made.

• Hybrid GHP systems can significantly

reduce system first costs even when a

tower needs to be purchased. Costs can

be reduced by more than 50% for very

highly cooling dominated applications

such as the small office building in

Houston (cooling-to-heating load ratio

of 24:1).

FEDERAL ENERGY MANAGEMENT PROGRAM
11

FEDERAL ENERGY MANAGEMENT PROGRAM

Figure 7. Full vs. hybrid GHP system first cost comparison for Bldg 137 Oceana NAS,

Virginia (existing cooling tower).

100,000

80,000

60,000

40,000

20,000

0

Full GHP Hybrid GHP Case 1 Hybrid GHP Case 2 Hybrid GHP Case 3

System First Cost ($)

92,454

60,825

41,512

• For applications where a suitable tower

already exists (as at the Oceana study

site), a hybrid system can result in system cost reductions of more than 50%

even when the building is not overly

cooling load dominated.

• For heavily cooling dominated sites,

hybrid GHPs can result in heat pump

and system energy savings compared

to full GHPs when the supplementary

heat rejecter is operated enough hours

to reduce the average heat pump entering fluid temperature during the cooling season.

• The authors of both case studies point

out that none of the hybrid system

designs they examined have been optimized. A design optimization method

is needed to balance GHX size, supplemental heat rejecter size and type, control strategy, and electric rate structure

to achieve lowest life-cycle or first cost

designs for a given location.

References

ASHRAE. 1995. Commercial/institutional ground-source heat pumps engineering manual. Atlanta: American

Society of Heating, Refrigerating, and

Air-Conditioning Engineers, Inc.

ASHRAE. 1996. 1996 ASHRAE handbook—Systems and equipment, chapter

36. Atlanta: American Society of Heating,

Refrigerating, and Air-Conditioning Engineers, Inc.

Hughes, P.J., and J. Shonder. 1998. The

evaluation of a 4000-home geothermal

heat pump retrofit at Fort Polk, Louisiana:

final report. Report No. ORNL/CON-460

Oak Ridge: Oak Ridge National Laboratory.

Kavanaugh, S.P., and K. Rafferty. 1997.

Ground-source heat pumps: Design of

geothermal systems for commercial and

institutional buildings. Atlanta: American

Society of Heating, Refrigerating, and AirConditioning Engineers, Inc.

Kavanaugh, S.P. 1998. A design method

for hybrid ground-source heat pumps.

ASHRAE Transactions 104 (2): 691-698.

Klein, S.A., et al. 1996. TRNSYS Manual,

a transient simulation program. Madison:

Solar Engineering Laboratory. University

of Wisconsin-Madison.

Means. 1999. Means mechanical cost

data. Kingston: R.S. Means Co.

Pahud, D., and G. Hellstrom. 1996. The

new duct ground heat model for TRNSYS.

Eurotherm Seminar N* 49, Eindhoven,

The Netherlands, pp 127-136.

Phetteplace, G., and W. Sullivan. 1998.

Performance of a hybrid ground-coupled

heat pump system. ASHRAE Transactions

104 (2): 763-770.

Phetteplace, G. 2001. Personal

communication.

Singh, J.B., and G. Foster. 1998. Advantages of using the hybrid geothermal

option. The Second Stockton International Geothermal Conference, The Richard Stockton College of New Jersey.

(http://styx.geo-phys.stockton.edu/proceedings/hybri/singh/singh.PDF).

Thornton, J. 2000. A detailed TRNSYS

analysis of building 137; Oceana Naval Air

Station, Virginia Beach, Virginia. Project

report to Oak Ridge National Laboratory.

Yavuzturk, C., and J. Spitler. 2000. Comparative study of operating and control

strategies for hybrid ground-source heat

pump systems using a short time step simulation model. ASHRAE Transactions

106 (2).
Disclaimer

This report was sponsored by the United States Department of Energy, Office of Federal Energy Management

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the United States Government or any agency or contractor thereof. The views and opinions of authors expressed

herein do not necessarily state or reflect those of the United States Government or any agency or contractor thereof.

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About FEMP’s New Technology Demonstration Program

The Energy Policy Act of 1992, and subsequent

Executive Orders, mandate that energy consumption

in Federal buildings be reduced by 35% from 1985

levels by the year 2010. To achieve this goal, the U.S.

Department of Energy’s Federal Energy Management

Program (FEMP) is sponsoring a series of programs to

reduce energy consumption at Federal installations

nationwide. One of these programs, the New Technology Demonstration Program (NTDP), is tasked to

accelerate the introduction of energy-efficient and

renewable technologies into the Federal sector and to

improve the rate of technology transfer.

As part of this effort FEMP is sponsoring a series of

publications that are designed to disseminate information on new and emerging technologies. New

Technology Demonstration Program publications

comprise three separate series:

Federal Technology Alerts—longer summary

reports that provide details on energy-efficient, waterconserving, and renewable-energy technologies that

have been selected for further study for possible

implementation in the Federal sector. Additional

information on Federal Technology Alerts (FTAs) is

provided in the next column.

Technology Installation Reviews—concise reports

describing a new technology and providing case study

results, typically from another demonstration program

or pilot project.

Technology Focuses—brief information on new,

energy-efficient, environmentally friendly technologies of potential interest to the Federal sector.