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2020 ASHRAE TECHNOLOGY AWARD CASE STUDIES

BY BENJAMIN D. HOBBS, MEMBER ASHRAE

DCV, Geothermal Systems Drive Building Design

PHOTO COURTESY FRANK DÖRING

The Lexington, Kentucky, high school currently operates with an energy use intensity of 18.3 kBtu/ft2 (207.8 MJ/m2), 60% lower than the average K–12 school.

©ASHRAE www.ashrae.org. Used with permission from ASHRAE Journal at www.cmta.com. This article may not be copied nor distributed in either paper or digital form without ASHRAE’s permission. For more information about ASHRAE, visit www.ashrae.org.

SECOND PLACE | 2020 ASHRAE TECHNOLOGY AWARD CASE STUDIES

Building at a GlanceFrederick Douglass High SchoolLocation: Lexington, Ky.

Owner: Fayette County Public Schools

Principal Use: Educational facility

Gross Square Feet: 287,125

Conditioned Square Feet: 287,125

Substantial Completion/Occupancy: July 2017/August 2017

Architect Tate Hill Jacobs/Perkins+WillBenjamin D. Hobbs is a mechanical engineer at CMTA, Inc., Lexington, Ky.

The new Frederick Douglass High School is a two-story, 287,125 ft2 (26 675 m2) facility serving 1,800 students and 150 staff in the Fayette County Public School District in Lexington, Kentucky. The facility added the capacity required in the rapidly growing school district with the intent of being the district’s first net zero energy–ready high school. It also is home to the Carter G. Woodson Academy, a 6th–12th grade academy focused on high-potential minority males in the district.

Energy EfficiencyThe high school was designed

through parametric energy analysis

based on accumulated metered data

and knowledge of ANSI/ASHRAE/

IES Standard 90.1 Energy Standard for

Buildings Except Low-Rise Residential

Buildings, design parameters to

select highly efficient systems and

equipment.

A geothermal field under

the parking lot consists of

452, 300 ft (91 m) deep bores on

20 ft (6 m) centers and serves as the

building’s heat source/sink for the

heat pump system. The earth either

absorbs heat from or rejects heat to

the system’s building loop piping

as required to keep the loop within

design parameters, negating the

need for a boiler or cooling tower.

Most of the system’s heat pumps are

two-speed so that the units can cycle

to low speed at part-load condition.

All of the heat pumps have an EER

over 21. The two-way valve on each

heat pump closes when the com-

pressor is off to minimize pumping

energy.

A single centralized geothermal

pump with a 100% backup pump

circulates the geothermal water

through the field and building.

Three pressure sensors located in

remote parts of the piping circuit

measure the differential pressure

between the heat pump supply and

the heat pump return to regulate

the variable frequency drive on the

main centralized pump (Figure 1).

High-volume low-velocity fans

were provided to supplement air

circulation in the two-story cafeteria,

gymnasium, and auxiliary gymna-

sium. These fans draw only 5 A at

120 V, and the increased air velocity

allows for a higher space tempera-

ture setpoint during the warm sea-

sons, thus saving compressor cooling

energy. The fans alone have proven

to keep the gymnasium and auxiliary

gymnasium sufficiently cool during

after-school and weekend activities,

saving additional energy.

With the use of efficient systems

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ASHRAE TECHNOLOGY AWARD CASE STUDIES20

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Energy Performance EUI (kBtu/ft2)

Average Fayette County High School 89.9

Best Performing Fayette County High School (2017) 57.2

Average K-12 School 48.5

High School with 75 Energy Star Score 35

Frederick Douglass High School 18.3

FIGURE 1 Hydronic piping schematic.

Geothermal Vault With Purge Ports

Geothermal Well Field

Air Separator

Pumps with VFD

Two-Way Control Valve

Water Source Heat Pump

Differential Pressure Sensor

10Ӯ GS10Ӯ GS

10Ӯ HPS10Ӯ HPR

and equipment, the school currently

operates at an energy use intensity

(EUI) of 18.3 kBtu/ft2 (207.8 MJ/m2).

It has achieved a 100 Energy Star rat-

ing, making it the first perfect score

Energy Star high school in Kentucky

and the most energy-efficient high

school in the state. This outcome has

exceeded the design team’s expecta-

tions and projected energy perfor-

mance compared not only to the

district’s previously best-performing

high school but also to the average K–12 school and the

average Energy Star high school score of 75. According to

Energy Star, the average site EUI for K–12 schools is 48.5

kBtu/ft2 (550.8 MJ/m2), so this building represents a 60%

savings over the average.

Indoor Air QualityThe building was designed around a demand-control

ventilation system. This system utilizes individual CO2

sensors in each occupied space that direct the outside

air supply based on the location of the occupants. Each

space has a variable air volume (VAV) box that modu-

lates to maintain a room CO2 level of 700 ppm above the

ambient CO2 level. This approach allowed the outside

air unit to be right-sized for the number of occupants

and square footage of the building and to deliver fresh

air directly where it is required, while still meeting the

requirements of ANSI/ASHRAE Standard 62.1-2010,

Ventilation and Acceptable Indoor Air Quality. A single

30,000 cfm (14 158 L/s) VAV air-handling unit provides

clean, filtered, neutral room temperature outside air

to every occupied space in the building. The building

relief air and exhaust are drawn back to this unit, and

the outgoing airstream’s sensible and latent energy is

reclaimed with a desiccant energy recovery wheel. A

positive building air pressure relationship to the outside

is maintained to eliminate infiltration of warm, moist

air to prohibit mold spore growth. This unit also uses the

geothermal field as its thermal energy source.

ANSI/ASHRAE Standard 55-2010 Thermal Environmental

Conditions for Human Occupancy, was utilized to ensure

occupant comfort within the building. Shortly after the

school was occupied, the school district enacted new

security protocols, one of which involved screening

all students through metal detectors as they enter the

school. This required that exterior doors in multiple

locations be held open for an hour each morning as stu-

dents lined up for screening. The occupants complained

of high temperatures and humidity every morning, so

the design team investigated. After reviewing several

space temperature trends, they found that the screening

process was allowing the building temperature to spike

right before the building went into occupied mode. An

adjustment was made to enter occupied mode thirty

minutes earlier, resolving the comfort issue.

InnovationTubular daylighting devices and clerestories are used to

daylight the interior corridors. The building’s north-south

orientation allows overhangs to control sunlight entering

the building, with particular attention given to window

exposure. The building has 26% glazing on the verti-

cal surfaces, with 80% of the glazing in the north/south

directions and 20% in the east/west directions to take

advantage of daylighting while minimizing heat gain and

glare. The building orientation also maximized the roof

area for photovoltaic panels, which are optimized with

southern sun exposure. This 240 kW solar array was bid

as an alternate for $491,000, but was not accepted on bid

day. When the array is approved, the roof design can sup-

port all projected panels with no supplemental structures

needed. The design team also took special care to avoid

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FIGURE 2 Life-cycle cost comparisons—HVAC systems.

37.7

33.7

29.7

25.7

21.7

17.7

13.7

9.7

5.7

1.7

Cost

(In M

illion

s)

0 5 10 15 20 25 30Years

WSHP—Geothermal 4-Pipe

WSHP—Boiler/Cooling Tower

equipment on the roof as much as possible and installed

spare conduits and panel capacity to facilitate the future

installation of these solar panels.

Each classroom has a “green button” as part of the

lighting controls to minimize energy consumption.

The teachers or students can select this button, which

resets the classroom lighting level from 50 footcandles

to 30 footcandles to reduce the illumination. It also

widens the room temperature dead band by 2°F (1.1°C)

up and 2°F (1.1°C) down. This meets the Kentucky

Department of Education building design guides while

allowing the staff and students to become part of the

energy-saving initiative.

A geothermal water-to-water heat pump produces

domestic hot water. The hot water recirculation pump

is turned off during unoccupied hours to reduce heat

loss through the piping. The kitchen cooler and freezers

do not have conventional air-cooled condensing units

but rather utilize the geothermal well field to reject

condenser heat. The cooler/freezer system also has a

dedicated 1/3 hp (0.25 kW) circulating geothermal loop

pump, backed up by emergency generator, to allow the

main centralized 60 hp (45 kW) pump to be shut off

during unoccupied hours and/or summer occupancy.

A domestic water connection was also provided to cycle

water through the compressors should the geothermal

loop need to be drained for maintenance.

Operation and MaintenanceTo maintain HVAC equipment, an additional 12 in.

(305 mm) was added to the above-ceiling space after

design development was complete. All service clearances

are shown on the HVAC equipment. During construc-

tion, engineers walked through the building with facili-

ties maintenance staff to confirm that service clearances

were maintained. The LED lights installed throughout

the school (along with LED theatrical lighting) not only

save energy but have lamp lives measured in decades

and not hours, so relamping is required rarely, if ever.

The electric panels serving lighting, HVAC, and plug

loads are separately submetered so the owner can moni-

tor and control energy usage.

The design team explored three system types during

design: four-pipe hot/chilled water, water source heat

pumps with a boiler and cooling tower central plant,

and water source heat pumps with a geothermal cen-

tral plant. With a payback of six years compared to the

four-pipe system, the geothermal central plant was the

preferred option. Compared to the boiler and cooling

tower central plant, the geothermal central plant had a

payback of 9 years (Figure 2).

The school board hired a third-party commissioning

agent as part of the construction progress, but the design

team performed retro-commissioning as a follow-up

to the original agent. The retro-commissioning was

performed using an experimental method, with full

permission of the owner’s representative and school

principals. This allowed the design team to try multiple

approaches to energy reduction, and if one was found to

adversely affect the occupants, the design team scaled

back to a previous set point that made the occupants

comfortable. One tried approach was to reduce the dif-

ferential pressure set point on the main centralized

geothermal loop pump. The design team initially low-

ered the set point by 5 psi (34 kPa) and found that the

building operated smoothly. An additional decrease of

5 psi (34 kPa) was attempted, but it was quickly found

that the lower set point did not provide adequate flow to

the gymnasium heat pumps.

During the construction process, the building enve-

lope was pressure tested through a series of limited-area

tests. The design team identified a stairwell in the ini-

tial stages of construction that could represent a small

sample of construction methods, with roof, window, and

wall elements. This stairwell failed the initial pressure

test, and the contractor and design team collaborated to

find the problem areas and adjust construction meth-

ods to meet the leakage requirements. The stairwell

was tested three times before passing the pressure test,

and the lessons learned were applied throughout the

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2020 ASHRAE TECHNOLOGY AWARD CASE STUDIES

TOP The building’s energy efficient LED lighting was designed to mimic the pattern of the canopy ceiling at the student entry.

BOTTOM Each classroom has a “green button” as part of the lighting controls to minimize energy consumption. The teachers or students can select this button, which resets the classroom lighting levels and temperatures.

remainder of the construction. This

trial-and-error method of testing

and adjusting a small portion of the

building allowed the entire build-

ing to pressure test to just 0.152

cfm/ft2 (0.772 L/s·m2) of building

envelope at project completion. The

standard testing allows a maximum

of 0.25 cfm/ft2 (1.3 L/s·m2)of building envelope, which

would have allowed a total additional infiltration of over

46,000 cfm (21 710 L/s) to the building. The rigorous

testing procedure undertaken resulted in a 34% reduc-

tion in air infiltration. This process also ensured that the

design team properly met ASHRAE Standard 55-2010

thermal comfort requirements.

Cost EffectivenessThe cost of this energy-efficient school was $206/ft2

($2217/m2), which is within the average range for this

area. The utility cost for the past 12 months has been

$0.58/ft2 ($6.24/m2), which is the lowest cost of any

Fayette County Public Schools high school. The outside

air, LED lighting, and window glazing systems were

some of the design options that the design team took

into consideration to ensure initial cost and utility costs

were minimized. Some choices had a higher initial cost

but resulted in a lower life-cycle cost (Figure 2). The out-

side air system consists of a single external air unit with

air distributed throughout the entire building rather

than multiple outdoor air units, which increased the

initial cost. Individual CO2 sensors were used to control

the appropriate amount of outside air being condi-

tioned. Illuminating with LED light fixtures cost more

initially, but there is also an offset since the HVAC system

can be sized for the reduced heat output. The choice to

use LED lighting allowed for a designed lighting load

of 0.6 W/ft2 (6.5 W/m2), a 50% decrease from the 2012

International Energy Conservation Code allowance of

1.2 W/ft2 (13 W/m2). The cafeteria had vast expanses

of west-facing glass that could have caused significant

heat gain from the late-afternoon sun, so blinds were

installed. East- and west-facing glazing were minimized,

but where deemed architecturally necessary, as in the

large cafeteria, the windows were equipped with motor-

ized shades to allow the occupants to shield the space

from the heat during evening hours.

Environmental ImpactTraditionally, Fayette County Public Schools utilized

variable refrigerant flow (VRF) systems for heating and

cooling in all their buildings. The design team’s selection

of a geothermal system instead of a VRF system reduced

the total amount of refrigerant in the building. If the

HVAC system utilized VRF equipment, there would be

approximately 3,400 lb (1542 kg) of refrigerant in use for

the building. The total refrigerant in all heat pumps in

the geothermal system is approximately 970 lb (440 kg),

a reduction of 2,430 lb (1102 kg) of refrigerant. Utilizing

a central geothermal plant instead of cooling towers also

eliminated the use of any process water

for mechanical cooling.

The reduced energy use at Frederick

Douglass High School saves approxi-

mately 582 metric tons of CO2 annually. https://bit.ly/2V8P31I

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