EXPERIMENTAL RESEARCH OF STAGNATION IN SOLAR THERMAL SYSTEMS
Konrad Lustig, Matthias Rommel and Dirk StankowskiFraunhoferInstitutefor SolarEnergysystems,Oltmannsstrasse5, Freiburg, D 79100,Germany, Phone:++49-761-4588-357,Fax:
++49-761-4588-100,e-mail: [email protected]
Abstract - Thispaperdescribestheinvestigationsof stagnationbehaviour of two differenttypesof solardomestichotwatersystems.Thereforethissystemswerefittedwith precisetemperaturesensorsandhightfrequency pressuredatalogging.Theaimof theresearchis to getdetailedinformationof theongoingprocessesduringstagnation,aswell asgettinginformationaboutthermalstressandthesizeof thepressurepeaks.The testsystemsaredescribedandexemplarymeasurementsareshown. Furthermoresimulationresultsof thesystempressureandof differenttemperaturesarepresented.
1 Introduction
Todayanincreasingnumberof solarsystemsareinstalledwhicharenotonly usedto warmupdrinkingwaterbut alsocoverpartofthespaceheatingenergy requirementsof buildings. In Germanythe numberof this so calledcombinedspaceheatingsystemsissteadilyrising. In Austria,theoutriderinstallingthis typeof sys-tems,it is estimatedthat about40 % of new installedcollectorareais usedto cover part of thespaceheatingdemand.It is ob-viousthattheneededcollectorareahasto bedimensionedlarger(comparedto thestoragevolume)asin systemsonly heatingupdrinking water. In summerthesesystemsoften reachstagnationconditionsinceon a bright day the storagereachesratherearlythemaximumtemperature(e.g.95
�C). In thiscasetheCollector
circuit pumpis switchedoff by thecontroller. Now, thetempera-tureof theabsorberis goingup rapidly andreachesthesocalledstagnationtemperaturewhich is for flat platecollectorsat about180to 200
�C andfor vacuumtubecollectorsin therangeof 220�
C and300�C. Thecoolingmediumof thecollectorloop (nor-
mally water-glycol mixture) is forcedto evaporateat thesehightemperatures.Later-on thefluid is condensingwhile the collec-tor coolsdown in theevening.Duringstagnation,therearewaterhammerscomingup, which causespressureshocksfor all sys-temcomponents.This is thereasonthattheGermanandAustriancompaniesSolvis,Sonnenkraft,TechnischeAlternative,TyforopandScherzingerin cooperationwith FraunhoferISE (Germany)andArbeitsgemeinschaftErneuerbareEnergien(AEE Gleisdorf,Austria) arecarryingout an Europeanfoundedprojectto deter-minestagnationbehaviour of solarsystems.Two in detail inves-tigatedcollectorsystems(flat plateandvacuumtube)with differ-ent internalpiping arebuilt up at FraunhoferISE andequippedwith precisetemperatureandhighfrequency (40 to 250Hz) pres-suresensors.At AEE threefield systems(alreadyexisting sys-tems)usedfor solarassistedspaceheatingwereadditionallyfit-ted with temperatureandpressuresensors.The aim of this in-vestigationis to measurethe occurringtemperatureof all com-ponentsof thecollectorloop aswell asoccurringpressurepeaksin the collectorcircuit. A betterunderstandingof the processesin thesystemduringstagnationshouldbeattained.Thedetailedmeasuredsystemswill be presented.Selected,exemplarymea-surementof the temperatureduring evaporationand stagnationwill beshown andinterpreted.Theconclusionsof understandingthis processwill begiven.
2 System Description
2.1 Flat Plate Collector System
V
T
T
T
T
T
TT
TT
T
TT
T
T
T
T
P
P
T
V
T
P
flat plate collector
exhaust valve
safety valve
expansion vessel
strorageclamp-on temperature sensor
fluid temperature sensor
pressure sensor
MID flow meter
aux. heating
Figure 1: Measurementconceptof theflat platecollectorsystemwith 5 m
�collectorareaand350l stratifyingstorage
Thefirst of two in detail investigatedsystemsis a flat platecol-lectorsystemwith a5 m
�collectoranda350l stratifyingstorage
producedby thecompany SOLVIS. Theabsorberof thelow-flow(�������� ������ ) collector is meandershaped.Due to the small
diameterof thepipingandtheresultinghigherpressuredrop,thecollectorcircuit is drivenby a specialdisplacementpumpof thecompaniesWILO andSCHERZINGER.Thebuffer storageis fit-tedwith aninternalsolarheatexchangerandanexternalservicewaterheatexchanger. As shown in Figure1 thecollectoris con-nectedin reversedmanner, thusthecollectorinlet is atthetopandtheoutletat thebottom. This reversedconnectiontogetherwiththe non returnvalve like behaviour of the pumpleadsto a pos-itive behaviour during stagnationconditions. The safetygroupconsistsof a 12 l expansionvessel,anexhaustvalve anda safetyvalve with 4 bar. In orderto obtainstagnationconditionsmoreoftenthereis anauxiliary heatingincludedwhich heatsthestor-ageupto nearlymaximumtemperature.Thesystemis fittedwithvarioustemperatureandpressuresensors.Thecollector, themaininvestigationobject, is suitedwith several clamp-onsensorsontheabsorberfin, theglazing,the insulationandthebackside.Inadditiontherearefluid sensorsat the inlet andoutletof thecol-lector. The temperaturesensorsarespeciallycalibratedfor hightemperaturerange(30 to 250
�C), so it is possibleto carry out
exact energy balancing(e.g. over the absorberplate). Thereisonepressuresensorinstalledat thecollectoroutletandoneat theexpansionvessel.This two sensorsareloggedwith a frequency
1
of 250Hz which meansevery 4 milliseconds(!) a measurementis taken. Themainreasonof measuringwith this high frequencyis to visualisethepressurepeaksoccurringduringevaporationincaseof stagnationcondition. Additionally installedtemperaturesensorsat the solarheatexchangerin- andoutlet, at the pumpandat the expansionvesselarereserved for the investigationofthe thermalstressof thesecomponentswhile normal operationandwhile stagnation.
2.2 Reduced Collector System
T
V
T
P
clamp-on temperature sensor
fluid temperature sensor
pressure sensor
ultra sonic flow meter
exhaust valve
expansion vessel
non-return valve
exhaust valve
pump
vacuum tube collector
safety valve
T
T
TP
T
T
P
T
T
T
T
V
Figure 2: Measurementconceptof thereducedcollectorsystemwith 2.94m
�collectorareaandwith no storage
Thesecondinvestigatedsystemconsistsof avacuumtubecollec-tor with 16 tubes(two bundlesof eight parallel tubesin series)with a total collectorareaof 2.94 m
�. The componentsof this
systemarefrom thecompany SONNENKRAFT. Figure2 showsthe sketch of this system. Onecanseethat thereis no storageincluded,sothissystemis mainly investigatedin permanentstag-nation. Thereareseveralclampon sensorsplacedat in differentheightsof oneabsorbertube,at the pumpandat the expansionvessel.Additionally thereis againonepressuresensorat theout-let of thecollectorandoneat theexpansionvessel.Thepressuremeasurementagainis takenwith a frequency of 250Hz. In dif-ferenceto the othersystem,thesafetyvalve is adjustedto openat a pressureof 6 bar. The usedpumpis a standardcentrifugalpump.Thespecialheattransferfluid for thermallyhighly stressedcol-lectorsystems,TYFOCORLS, of thecompany TYFOROPwasusedin bothsystems.During themeasurementperiodthephysi-cal andelectrochemicalpropertiesof thefluid wereanalysedre-peatedly.
3 Variation of Operation Condi-tions, Temperature Stress of theSystem Components
In order to get a statementof the thermalstressof eachsystemcomponentfor differentoperationconditions,thefollowing vari-ationof the testconditionswererealizedon dayswith high irra-diation:
� Continuousoperationof the solar circuit pump withoutload(unfilledstoragein system1)
� Stagnationwithout load(unfilledstoragein system1)
� Stagnationwith filled storage,solarcircuit pumpmanuallyswitchedoff (system1 only)
� Stagnationafter cutoff of the solar circuit pump by thecontroller (Storageat maximumtemperature)(system1only)
� Restartof thesolarcircuit pumpafterstagnationwhile un-abatedhigh irradiation
All theoperationconditionsmentionedabove weredrivenwith-out causingany problems.The temperatureof thesystemcom-ponentsin the cellar during stagnationwere lower than whilenormaloperation. The incidenceof steamwasrestrictedto therangeof the collector. The systempressurenever reachedthepressureto releasethesafetyvalve.The temperaturestressesof all componentswere analysedand statistic curves such as shown in Figure 3 were ob-tained. Figure 3 shows exemplarily the cumulative percent-age of the temperaturesover the whole measurementperiod(July - September1999). The maximumtemperaturewas be-low 220
�C but this high temperatureswere only at a short
period of time. The temperaturestressduring stagnationcanbe seenthrough an accumulationbetween120 and 180
�C.
0
50
100
150
200
250
300
0 20 40 60 80 100 120 140 160 180 200 220 2400
5
10
15
20
25
30
Sha
re o
f mea
sure
men
t per
iod[
h]
measurement periode=986.26 h
Sha
re o
f mea
sure
men
t per
iod[
%]
Temperature[°C]
Figure 3: Cumulativepercentage of thetemperaturesat theab-sorberof thevacuumtubecollectorover thewholemeasurementperiod
2
4 Description of the Processes whileStagnation
4.1 Flat Plate Collector System
Due to the serial connectionof all components(including theabsorber)in this systemthe processof stagnationis more eas-ily be understoodthanat a systemwith paralleldisposedcom-ponents. In caseof a configurationshown in Figure 1 (e. g.collector inlet at the top, deflationto the bottom) the collectoris deflatedin the first momentof evaporation,which take placeat oneof the uppertubesof the absorber. During the deflationof the collector, the fluid contentof the collectoris pushedintothe expansionvesseland the pressureincreasesrapidly (seeinFigure 4). This again resultsin an increaseof the saturationtemperature.After thecollectoris depletedpartof thecollector(connectionpipesandheader)is refilled again,andthepressuredecreasesuntil an equilibrium is reached.The steamin the ab-sorberis overheatedandreachedat this daynearly180
�C. After
deflation there is no energy transportedto the inlet and outletsensorsso the temperaturestaysbelow the saturationtempera-ture.With decreasingirradiationatlateafternoonthetemperatureof the absorberdecreasesandthe collector is refilled smoothly.
2.42.62.8
33.23.43.63.8
2.42.62.833.23.43.63.8
20
40
60
80
100
120
140
160
180
200
13 13.5 14 14.5 15 15.5 16 16.5 1720
40
60
80
100
120
140
160
180
200
Collector inletCollector outlet
Absorber tubeAbsorber fin
Saturation temp.
Tem
pera
ture
Time
Pre
ssur
e [b
ar]
pump on pump off
presure at expansion vesselpresure at collector outlet
Figure 4: Stagnation processof the flat plate collector at abright day
4.1.1 Simulation of Pressures and Temperatures
This specialtype of solarsystemcanbe describedwith simplemodels,becausethedeflationtakesplacevery rapidly andwith-out any kind of instabilities. The pressureis dominatedby thevolumeof the systemandthe lay out volumeof the expansionvessel.Thesaturationtemperatureitself dependson thepressureagain.For thesimulationthepropertiesof waterwereappliedtothemodel,sincetherearecompleteC-programminglibraries[1]available.Thesimplifiedmodelwasincludedinto thesimulationenvironmentColSim [2]. Theresultsof thesimulationis shownfor oneexemplaryday in Figure5. The following assumptions
weremadefor thesimplifiedsimulation:� Theabsorbersurfacehasthesametemperature������� over
thewholearea� Theevaporationstartsin caseof � ����� � � ������ Thedeflationof thecollectoris underconstantmassflow
rate(e. g. independentof theirradiation)� Only onecollectorvolumecanbedeflated� Thesystempressuredependson thechargedpressure,the
occupancy andthegastemperaturein theexpansionvessel
As one can see, the correlation between the measure-ment and the simulation is good, but is only valid forthe special connected flat plate collector system. Fur-ther works on simulation models will allow to simulatemore complex connected absorber and absorber systems.
simulated absorber temperaturesimulated temperature at exp. vesselmeasured absorber temperaturemeasured temperature at exp. vessel
2.4
2.5
2.6
2.7
2.8
2.9
3
Pre
ssur
e
simulated pressure
measured pressure0
20
40
60
80
100
120
140
160
180
8
10
12
14
16
18
20
Tem
pera
ture
Time
Figur e
5:
Simulationof
pr
essureandT
emperaturesat
theab-sorberande
xpansionvessel4.2
ReducedCollectorSystem
The
internalconnectionof
theused
v
acuum
tubecollectorin
this
system
is
a
compound
of
tw
o
times 8
parallel
tubes
in
series.
This
results
in
a
much
more
comple
x
beha
viourduring
thestagnation,
since
cross
flo
w
from
one
tube to
theother
may accrue.
The
tem-
perature
and
thepressure
curv
e
of thereduced
system
forone
bright
day
in
January
2000
issho
wnin
Figure
6.
The
absorber
temperatures
there
areobtainedfrom
dif
ferentheights
of
thelast
absorberpipe.
One
can
see that
after
deflation
of
thecollector
(again
represented
by
a
steep increase
of
thesystem
pressures),
themeasured
absorber temperatures
rapidly
rise
up
to
themaxi-mum
temperature of
about
205
�C.At
this
high
temperature, of
course,
all
liquidise
v
aporatedand
the steamiso
v
erheated.
The
inlet
and
outlettemperatures
stay
o
v
er
one
hourat
the saturation
temperaturebefore
theinlet
temperature startsdecreasing.
The
reasonis
that
this
sensor
is
placeda
little
bitapartfrom
thecollector
inlet
and
thespace
tubing
in
betweenw
as
notin-
sulatedv
ery
well.
The
sameappliesforthe
collector outlet
sensor
which
”sees”
alternately the steamand
liquid
phase
of
3
the collector fluid. This alternationis initiated by the coolingdown of the headerdue to the missing energy transportfromthe collector tubesto the header(sincethe tubesaredry). Thetemperatureof the headerthendropsbelow the saturationtem-peratureanddueto thecounterpressureof theexpansionvessela small portion of liquid is pushedback into the header. Thisportion comesinto the overheatedtubesand is evaporatedim-mediately. Thecollectoroutlet temperatureis rising againandapressureshockpassesthroughthesystem,thecollectorinlet tem-peraturedropsa little bit, sincecolder liquid is pushedinto thedirectionof thecollector. However, over thewholemeasurementperiod the pressurepeakswere in the rangeof � 1 bar; there-fore the pressurenever reachthe level to openthe safetyvalve.
20
40
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140
160
180
200
10 11 12 13 14 15 16 17
0
20
40
60
80
100
120
140
160
180
200
220Temperature
Time
Collector inlet
Collector outlet
Absorber tube midway
Absorber tube bottom
Absorber fin midway
Saturation temp.
presure at collector outlet presure at expansion vessel
3
3.2
3.4
3.6
3.8
4
4.2
4.4
4.6
3
3.2
3.4
3.6
3.8
4
4.2
4.4
4.6
Pressure
Figure 6: Stagnationprocessof thereducedcollectorsystemata bright day
5 ConclusionsIt hasbeenshown, thatfor bothinvestigatedsystemsthepressurelevel neverreachedcritical values.Thesafetyvalveneveropenedduring the measurementperiod.The serial connectedmeandershapedflat platecollectorshowedanrathersimplestagnationbe-haviour. Thereversedconnection(collectorinlet at thetop,outletat thebottom)leadsto afasterdeflation. Simulationcalculationsfor thiscasewereappliedsuccessfully. Thevacuumtubesystemsproduced,duringthestagnation,pressurealternationin therangeof
�
1 barat thecollectoroutlet.After thefirst investigationsthismaybedueto thecollectingheaderpipewhichcoolsdown belowthe saturationtemperatureand thereforeenablesliquidto entertheabsorberagain.Further, moredetailed,temperaturemeasure-mentswill investigatethisphenomena.
References[1] BernhardSpanghttp://chemengineer.miningco.comPropertiesof Water and Steamin SI-Units , 2nd RevisedandUpdatedPrinting,Springer1979,pp.175ff.[2] ChristofWittwer ColSim- SimulationvonRegelungssyste-menin aktivensolarthermischenAnlagenUniversitat Karlsruhe;Institut fur IndustrielleBauproduk-tion. Dissertation.1999.http://www.ubka.uni-karlsruhe.de/cgi-bin/psview?document=1999/architektur/1&format=04
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