Heat
Earth-Coupled
PumpSystemsSelection,Design
E Performance
AEES.38
Universityof Kentucky
Collegeof Agriculture
CooperativeExtensionSeMce
Departmentof AgriculturalEngineering
L. W. Tumer, D.G.Colliverand W.E.Murphy
Risingenergycostshavecausedmanypeople
to lookfor moreefficientwaysto heatandcool
theirhomes.Oneof the mostenergy-efficient
systemsto comealongin recentyearsis the earthat the University
coupledheatpump.Pioneered
of
the earth-coupled
Kentuckyin the 1950s,
system
relieson the thermalcapacityof waterto transfer
heat,whereas
a conventional
heatpumprelieson
outsideair. ln an earth-coupled
system,a watersourceheatpumpis connected(or "coupled")to
pipesin thegroundor in wellsfor thesourceof
heatin winterandcoolair in thesummer.In this
way,heatingor coolingenergyis transferred
by
waterthroughthe system.
Promptedby the energycrisisof the 1970s,
manyKentuckians
haveinstalled
earth-coupled
systems.Sadly,not everyonehasrealizedthe
energysavingsthey had hopedfor becauseof a
poorlydesignedsystemnot suitedto theirhome
heatingandcoolingneeds.However,
when
properly,
installed
an earth-coupled
heatpump
systemoffersa high-efficiency
alternative
to the
morecommonair-source
heatpump.This
publication
is abouttheoperation'and
designof
the earth-coupled
heatpumpsystemas compared
to air-to-air
lt will helpyou,as a
systems.
homebuyer
or builder,recognize
the
characteristics
of a welldesignedsystem.
refrigeration
systemto anotherheatexchanger,
whichdelivers
the heat.Figure1 showsthetypical
wintertime
air-to-aircycleof operationin which l
heatis extracted
fromoutsideair anddelivered
to
the interiorheatedspaceby passingair overa
(auto-radiator-type)
finned-coil
heatexchanger.
Figure2 illustrates
the operation
cycleof
earth-coupled
heatpumpsystems.Insteadof an
air-typeheatexchanger,
the outdoorcoil is
replacedby a water-typeheatexchanger(tube-inpicksup
tubeor shellandtube).Thisexchanger
heatfrom waterwarmedby the earth. Thiscan be
waterin coilsof plasticpipe
doneby circulating
buriedin the soilor by drawingwaterdirectlyfrom
a well,pondor lake.
Therearetwo typesof earthcoupledsystems,
openandclosed-loop.
FIGURE1. Heat pump cycle lor heating,for an air-to-alr
heat pump system.
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Overviewof Systems
Earth-coupled
heatpumpsystemsoperateon
the samebasicprinciplesas air-sourcesystems.
Eachextractsheatusingsometypeof heat
The heatis thentransferred
exchanger.
by a
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O€VICE
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Publishedby the CooperativeExtensionSeMce in cooperationwith the Kentuctqy
Departmentof Energyas a part of the KentuctryEnergy
ConservationProgmm.
Qualityis
importantdesignconsiderations.
importantbecauseopen systemsexpose
equipmentto a continuoussourceof oxygenand
minerals.Therefore,steps must be taken to
preventcorrosionand removedepositsin the
water-typeheat exchanger.
Quantityof wateris importantbecausemost
water-sourceheat pumps specify a flow rate of
threeto five gallonsper minuteper ton of capacity'
A s s u m i n g1 Og a l l o n sp e r m i n u t ef o r a t w o - t o nu n i t ,
this would require7200gallonsin a 12-hourday of
operationor 1.8milliongallonsfor a 3000-hour
operationyear.Therefore,both abundantsupply
and efficientdisposalof water are very important
in open systems.
cooL WATER
F R O MS O U R C E
>AAM-
(METERING OEVICE
OR
EXPANSION VALVE
FIGURE2. Heatpumpcyclefor heating,for an earthcoupledheatpumPsYstem.
Closed-LoopSystems
arethosewhichuse
systems
Closed-loop
waterflowingthrougha buriedpipe to transferthe
heat.Wateris recirculatedthrough a sealedloop
of pipe and picksup or releasesheatdepending
upon the season.By recirculatingthe water
throughthe system,it is not dischargedor wasted.
There are two main types of closed-loop
systems:verticaland horizontal.The vertical
systemcallsfor drillinga 250-to 300-foothole and
placinga pipe loop within an outer casing (Figure
4). For the horizontalloop system,pipe usuallyis
OpenSystems
systemsarethosein
Openearth-coupled
duringsomeportion
is
loop
open
whichthewater
systems
includes
This
cycle.
of thecirculation
well'b) a
in
a
single
is
recirculated
where:a) water
a
single
c)
or
are
used,
well
supplyanda return
withopen
supplywellis usedin conjunction
thesesystems,
of
In
any
3).
(see
Figure
discharge
waterare
quantity
available
of
quality
and
the
FIGURE3. Openearth-coupledheatpumpInstallallons:a) singlewell,recirculating,b) two-wellsyslem,c) single
well,opendischarge.
SUPPLY
W EL L
srr.rGLE
W EL L
RETURiI
TO HEAT
ExCHAI'IGER
-il
WELL
HE A T
E X C H AN G ER
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WELL
HEAT
N
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il
ltl l{-,,,*,
Il
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SUPPLY PUMP
(o)
S U P P L YP U M P
(b)
AEES38,p.2
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(c)
(b)
PIPE CROSSSECTION
(o)
FIGURE
4. Closed-loop
earth-coupled
healpumpinstallations:
a) verllcal loop, b) horizontalloop.
laid in a four-foot-deep
trench.The lengthof pipe
requiredvariesbetween300to 800feetper ton of
capacity,dependingupondepth,climate,soil type
and drainage.Waterflow ratesarethe samefor
closed-loopheatpumpsystemsas for open
systems.
SourceTemperatures
The advantages
of usingthe nearlyconstant
temperature
of wellwateras opposedto air as a
gained
heatsourceincludeimproved
efficiency
froma morefavorable
sourcetemperature
during
eachseasonand the greaterthermalcapacityof
wateras comparedto air.The temperature
of well
waterin mostlocationsis fairlyconstantat depths
of 50 feetor below.For Lexington,Kentucky,this
valueis 58 degreesF. In comparison,
West
Lafayette,
Indiana,hasa valueof 54 degreesF and
Florida,hasa valueof 74 degreesF.
Gainesville,
groundloopsalso
Horizontalearth-coupled
offerthe sameadvantages
as systemsusingwell
water,althoughgroundtemperature
variesmore
withseason(Figure5). In Lexington,
for example,
the monthlyaveragetemperature
of the ground
surfacevariesfrom 36 degreesF up to 83 degrees
F. However,
at a depthof fivefeet,the ground
temperature
variesonlyfrom about47 degreesF
up to 69 degreesF. Suchlimitedvariationprovides
an advantage
for the earth-coupled
unit as
opposedto usingairto supplyor discardheat.
AEES38,p.3
FIGURE5. Averagemonlhlylemperaturevariatlonfor a
partlcularsoll In Lexlnglon,KY.
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MAMJJASOND
TIME ,.MONTHS
Performanceof
Earth-Coupled
Systems
Fora constantwater-sourcetemperatureof
around52 degreesF, an earth-coupled
systemhas
a seasonalperformance
factor(SPF)of 3. This
meansit suppliesan averageof aboutthreetimes
as muchheatenergyas an electricresistance
heatingsystem,suchas an electricbaseboard
unit,for everykilowatthourof electricity
purchased.
In comparison,
mostair-sourceheat
pumpsystemshaveSPFvaluesof 1.25to 2.0,that
is,theygenerate1.25to 2.0timesmoreheatfor
everykilowatthourof inputthanan electric
resistance
heatingsystem.Operatingthe watersourceunit is thereforemoreeconomicalthanan
air-sourceheatpump,as longas the systemis
properlysized.The water-source
unit shouldbe
largeenoughto eliminate
the needfor
supplemental,
back-upheatto achieve
comfortabletemperatures,
exceptwhen it is very
cold outdoors.
Oneimportantaspectto considerin
comparingair-sourceto earth-coupled
systemsis
the factthatthe heatingor coolingcapacityof an
air systemchangeswith outdoortemperature.
However,
whena well is the watersource,the
capacityof an earth-coupled
systemis nearly
constant.Figure6 illustrates
this principle.lf both
systemsare balancedto meetthe heatingloadat a
certainoutdoorair temperature,
thenat air
temperatures
abovethe balancepointthe capacity
whilethe
of the air-sourceheatpumpincreases
water-source
heatpump'scapacityis constant.
Thisshowsone of the majordisadvantages
of an
air-sourcesystem:it mustbe sizedto meetthe
heatingloadat low outdoorair temperatures,
even
thoughit hasa highercapacityat higher
temperatures.
Therefore,
a largercapacity
is requiredthan is the casewith earthcompressor
coupledsystems.
Heatpumpsare ratedat different
temperatures,
dependinguponthe source.In the
heatingmode,the water-source
heatpumpsof an
earth-coupledsystemare ratedat 50 degreesF
enteringwatertemperature.
Becauseair
temperature
variesmuchmorethanwater
temperature,
air-sourceunitsare ratedat two airsourcetemperatures:
47 degreesF, and an
"application"ratingat 17 degreesF, a temperature
representative
of typicalcold-weather
operation.
In the coolingmode,water-source
earthcoupledunitsaretestedat 70 degreesF entering
temperature
whileair unitsaretestedat 95
Figure6. Heatlngand coolingload llnesfor a four-lon (48,000BTU/hr.)loadat balancepolnt temperatureof 30
degreesF heallngand 95 degreesF coollng. (Braud,1979.)
I000
Btu/hr.
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HEATING
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COOLING
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Outdoor
Temperature
AEES38,p.4
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4*
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t
ratingsdo not
degreesF. Notethat water-source
includethe electricityneededto pumpwater.
thisinputas 10 kilowatts
estimate
Someengineers
per millionBTU'sdeliveredfor an opensystem.
rating
Thiswouldserveto reducethe performance
are
curvessometimes
slightly.Performance
to enableyour
fromthe manufacturer,
available
to be usedin
particularsourcetemperature
designingthe system.A standardmethodof rating
all heatpumps,bothair-sourceand water-source,
and
hasbeendevelopedby the Air-Conditioning
RefrigerationInstitute.
System Design
Thekeyto designing
a workable
earthcoupledheatpumpsystemis propersizingand
installation
of thewellor groundcoil.This
water
dependsuponclimate,soilproperties,
qualityandquantity,
andheatingcapacity.
lmportant
considerations
alsoincludepumping
powerrequired,freezeprotectionand the
possibility
of addinga solarcollector.
$oilhoperties
The most importantfactors with regardto soil
propertiesare moisturecontent and dry density.
Figure7 indicatesthe variationof soils in their
resistanceto heat flow. For example,for a dry,
organicsoil,the pipe lengthrequiredto pick up
sufficient heat may be two to three times the
lengthrequiredfor a wet, granularsoil.
yariationin heal-flowresistance
FIGURE7. Qualitative
of soil with moistureconlenland soil type.
Coil or Well Design
systems
typically
Coillengthsfor ground-loop
rangef rom300feetperton of capacityfor a wet,
granularsoil up to 1000feet perton for a verydry,
areon the
organicsoil.Pumpingrequirements
orderof 10gallonsperminute.Pipelengthsof this
rcetemperature
magnitudetypically g ivewater-sou
changesof fiveto 10degreesF acrossthe outdoor
heatexchanger.
The pipeusedby mostinstallers
pipe.Fouris oneandone-halfinchpolybutylene
inchPVCpipealsohasbeenusedbut is not
recommended
becauseof poor heattransferand
Installation
costsfor the
short-life
characteristics.
piperangefrom $2.00perfoot up to $4.00per foot
for the horizontal-loop
systems.Thevertical
systemsusuallyemploya five-inchcasingwith
depthsof 300feet.Waterwell drillingcostsare
higherperfootandvarywith localgroundand
waterconditions
andamongcontractors.
Pumping Power, Freeze hotection
Fora deep-well,
opensystem,the pumping
powerrequiredis approximately
sevento 10
per
kilowatthours millionBTU.Typically
the
pumpingpowerfor a horizontal
closed-loop
open
systemwill be lessthanthat of a deep-well,
systembecausethereis no elevationto pump
against.Onlythe pipe'sfrictionmustbe overcome.
To functionproperly,
the pumpmustbe designed
to handlethe pressure
dropanticipated
for the
system.
A closed-loop
systemshouldbe protected
fromfreezes.
Thisinvolves
addingabout1.5
p o u n d so f c a l c i u mc h l o r i d ep e rg a l l o no f f l u i di n
the system.Otherfreezeprotectorsmightbe
considered
as well.
Solar Assist
Whensolarcollectorsareaddedto an earthis
coupledsystem,evenbetterperformance
realized,
as the earthcan storeextraenergy
to a
for thesystem.Whencompared
collected
conventional
heatingsystem-onethatdoesn'tuse
a heatpump-H.J.Braudfoundthatsolar-assisted
heatpumpsin SiouxCity,lowa,saved36 percent
of heatingcosts.Withoutsolarassist,onlya 9
percentsavings
wasrealized
overtheconventional
unit.Of course,theaddedcapitalinvestment
in the
in thedecision.
solarunit mustbe considered
Solar-assisted
systemsare mosteconomicalin
coldlocations
thathavequitea fewsunnydaysin
thewinter.
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MOTSTURE CONTENT, 7"
AEES38, p.5
Economics
earth-coupledheat pump systemsusing well water
saveoperatingcosts.Table 1 givesa comparison
of simulateddata for Sioux City, lowa, for air, well
water and solar-assistedfour-ton systems.For
example,the unit with an EER ratingof eight gave
a savingsof 1455kilowatt hours over the year. At
6.5 cents per kilowatthour,this is a savingsof
$94.58per year. However,for an estimatedlife of
15 years,the savingswould justifyan additional
investmentof only $733 over the air-to-airsystem.
It is possiblethat a unit with largercapacitycould
be used to reducethe supplementalheat required,
therebyincreasingefficiencyenoughto justifythe
added cost. Also, some power suppliersoffer
specialfinancialprogramsto encourage
installationof such systemsin order to bring about
lesswinterdemandfor energy.
There are two viewpointsin evaluatingthe
economicsof earth-coupledheat pump systems.
One viewpointis that of the homeowner,who
wants to pay a lower energy bill. The other view is
that of the power supplier,who also may gain
economicallythrough reduced demand for energy
during peak periods.Becauseof its improved
performanceat low temperatures,the watersource heat pump demandsless in terms of
kilowattsin cold weather.This could allow the
supplierto reduce power generationcapacity
during the peak heatingseason,resultingin a
capitalsavings.
For the homeowner,resultsof a computer
simulationstudy by Braud show how various
TABLE1.-Annual EnergyConsumptionfor Air- andWater-Source
Heal
Pumps,4-ton
Capaclty,
SiouxCily,lowa
Raling
(EER)
Source
AnnualKWH
Heating Cooling Suppl'
Total
7
Air
(52'F)
WellWater
Solar* WellWater
26972
25646
22302
4421
3999
3999
14937
13713
2478
46330
43358
28779
8
Air
(52oF)
WellWater
Solar* WellWater
21475
21518
19516
3913
3639
3639
14937
13713
2478
40325
38870
25633
9
Air
WellWater(52oF)
Solar* WellWater
17871
18621
17390
3512
3342
3342
14937
13713
2478
36320
35676
23210
10
Air
(52"F)
WellWater
Solar* WellWater
17293
18142
17024
3184
3093
3093
14937
13713
2478
35414
34948
22595
*Supplemental
AEES38, p.6
Summary
Reference
Earth-coupledheatpumpsystemsotfer a
high-efficiencyalternativeto the conventionalairto-air heatpumpsystem.They also requirea large
initialinvestment,
as muchas $1,500to $2,000
morethan an air-to-airsystem.To makethis
investment
worthwhile,
a properlydesigned
systemis imperative.For this reason,only a
qualifiedcontractorwho has receivedadequate
be
trainingshouldinstallit. This professionalwill
awareof the manyfactgrsdiscussedherethat
must be consideredin orderto obtainthe full
benefitof an earth-coupledheatpumpsystem.
Braud,H. J. 1979.Watersourceheatpumpsfor
agriculturalapplications.AmericanSocietyof
AgriculturalEngineersPaperNo.79-4562.
AEES38, p.7
For more detailedinfonnation:
"Design/Data
Manualfor Closed-Looped,
GroundAmerican
CoupledHeatPumpSystems,"
and Air
Societyof Heating,Refrigeration
Inc.,Atlanta,Ga.,
ConditioningEngineers,
198s.
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1M-3-87