This project won inthe category for

Alternativeand/or Renewable

Energy Use

1998ASHRAE

TechnologyAward

A SHRAE JOURNAL

5 2 A S H R A E J o u r n a l M a y 1 9 9 8

he Daniel Boone High School is a160,000-ft2 (14 864 m2) facility lo-cated in Washington County, Tenn.The school was completed in 1971

and serves approximately 1,100 students.This project involved replacing the heat-

ing and cooling system serving the entireschool. The original system used a two-pipechilled water system (R-11 chiller) for coolingand electric resistance heat. Lighting, build-ing envelope or other building modificationswere not within the renovations scope.

The school system was interested in com-fort, energy efficiency and low maintenancein the final project design. This project gavethe Technology Advancements group of theTennessee Valley Authority (TVA) an op-portunity to test and monitor variable flowpumping in a large geothermal heat pumpapplication.

Project DesignThe basic design concept is a central

closed-loop ground heat exchanger as a heatsink/source for the extended-range water-source heat pumps in each zone. The designcriteria included the ability to provide simul-taneous heating and cooling of individualzones. The building featured a pod designwith fixed ceilings and limited attic space thatrequired piping runs inside the finished space.

The following design options were evaluatedby TVA and considered by the owner:

Water loop heat pump (WLHP) with electricboiler.

WLHP with gas boiler. WLHP with electric thermal storage. Geothermal Heat Pump (GHP): A WLHP with

a closed loop geothermal heat exchanger (GHX). Four-pipe system using a natural gas en-

gine-driven chiller and boiler. Energy savings from variable flow pumping

were considered for all WLHP options.The GHP system with variable flow pumping

was chosen. The GHX consists of 320 boreholes,each 150 ft (46 m) deep. Each borehole contains300 ft (91 m) of 0.75 in. (19 mm) diameter polyeth-ylene pipe. The boreholes are placed in sectionsof 20 holes at 15-ft (5 m) centers, and 20 ft (6 m)spacing between sections. Each section is valvedto facilitate purging and allow isolation in theunlikely event of a leak. The 8 in. (203 mm) sys-tem supply and return lines enter the schoolthrough the existing mechanical equipment room.

The 160,000 ft2 (14 864 m2) school has a central core area surrounded by five circular pods.The geothermal heat exchanger is located in the lawn in the foreground.

Geothermal System for SchoolDavid R. Dinse, P.E.Member ASHRAE

T

About the AuthorDavid R. Dinse, P.E., is presently a projectmanager in Technology Advancements, TVAsresearch organization. He is a member ofASHRAEs Technical Committee 6.8, Geo-thermal Energy Utilization.

The following article was published in ASHRAE Journal, May 1998. Copyright 1998 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.

M a y 1 9 9 8 A S H R A E J o u r n a l 5 3

Controls and Innovative ApproachesParasitic pumping in WLHP and GHP systems has consider-

able potential for energy savings. Traditional designs incorpo-rate constant operation of circulation pumps, which can sub-stantially increase energy use. This system utilizes a pair oftwo-speed circulating pumps, each sized at approximately 80%of the system capacity. The circulation pumps are staged asfollows: Stage 1: one pump at 1150 rpm; Stage 2: one pump at1750 rpm; Stage 3: two pumps at 1750 rpm. To ensure adequatesystem flow and optimum performance, the pumps are con-trolled by a combination of loop flow and system differentialpressure using a programmable logic controller.

Each terminal heat pump unit uses a two-way valve to stopflow through the heat pump when not in use. (A small amountof bypass in the loop is maintained by eliminating the two-wayvalves on several small, strategically placed units.) As buildingload decreases, heat pumps cycle off, the flow rate decreasesand pressure differential increases until the controller reducesthe pumping by one stage.

As building load increases, flow rate increases, differentialpressure decreases, which brings on an additional pumpingstage. In all but peak conditions, one pump on high speed willcarry the building, providing acceptable system redundancy.The basic system schematic is illustrated in Figure 1.

Energy EfficiencyFor two years prior to the renovation, energy use averaged

3,481 MWh (kWh 103) per year. Annual energy use for the GHPsystem was projected at 2,232 MWh. The renovation was sched-uled for completion during the summer of 1995; however, a de-lay allowed for only two thirds of the GHX to be installed beforethe 199596 winter heating season. The system operated throughthe bitterly cold winter on the partial ground loop. The groundloop was completed in April 1996. Energy use for the 199697school year (July to June), the first year on the completed retro-fit, was 2,298 MWh. Figure 2 shows the relationship of energyusage to degree days for two years prior to retrofit, and twoyears after (including the one year with the partial GHX).

Cost EffectivenessWater loop heat pumps were chosen as a base case for the

retrofit conditions in order to provide simultaneous heatingand cooling with a two-pipe system. A base case was devel-oped using a boiler, a cooling tower and WSHPs. Analyses ofthe base case and alternate systems were accomplished usingan hourly analysis model. The model was calibrated to actualenergy use and weather data prior to the renovation. Energycosts for the base case were estimated at $164,000 per year. Theenergy costs for the installed system were estimated at $135,000per year, for an annual energy savings of $29,000.

A preliminary feasibility study estimated the maintenancecost for the GHP system to be $0.05/ft2 per year less than theboiler/tower design. This $8,000 savings would include boiler,cooling tower, and heat exchanger maintenance as well as towerchemicals and makeup water usage. Total annual savings wereestimated at $37,000.

Using the actual energy costs of $139,000 for the 199697

Figure 2: Annual energy use.

Figure 1: Basic system schematic.

school year, the annual savings would be $33,000 per year overthe base case. Based on the energy costs for 199697 of $139,000(with 4,455 heating degree days) the system should be able tomeet the original projection of $135,000 per year for a normalyear (4,143 degree days for the Bristol, Tenn. area). Figure 3shows the relationship of energy costs to degree days beforeand after retrofit. (Electric utility rates were constant for theperiod.)

The GHX cost was $451,000 including a $100,000 changeorder to cover unexpected borehole casing costs. TVA pro-vided $104,000 in direct funding plus the system monitoringcosts. The costs for a conventional boiler, cooling tower, plate-frame heat exchanger and associated pumping and controlswere estimated at $150,000. The incremental cost to the schoolsystem was $197,000. (The cost estimate does not include ex-tending the natural gas line to the facility if a gas boiler werespecified. This could have added substantial costs.)

Using the estimated savings of $8,000 and the actual 199697 energy costs savings (compared to a boiler/tower base case)

H E A T P U M P S

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Figure 3: Annual energy cost.

a simple payback of six years is achieved. Using the projectedoperation and maintenance costs for a normalized weather yearreduces the payback to a little more than five years. The systemis presently monitored to validate its operation costs and fine-tune its performance.

ConclusionThis project demonstrates the marriage of two energy-effi-

cient technologies: variable flow pumping and geothermal heatpumps. Also, the project introduced regional well drillers andmechanical contractors to the closed loop ground heat ex-changer and variable flow pumping concepts. The school sys-tem is so pleased that they have employed the technology intwo other locations. In a new construction application, a sig-nificant credit could be taken for substantially reducing themechanical equipment room requirements, which would furtherreduce the system payback.

AcknowledgmentsThe final project was a joint effort involving the Johnson

City Power Board, TVA, the Geothermal Heat Pump Consor-tium, the Washington County School Board, local consultants,architect, Tony Street and engineer, Ronald Shuttle.

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