EN 12975-2:2001

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Thermische Solaranlagen und ihre Bauteile - Kollektoren - Teil 2: Prüfverfahren (einschließlich Corrigendum EN 12975-2:2001/AC:2002)Installations solaires thermiques et leurs composants - Capteurs - Partie 2: Méthodes d'essaisThermal solar systems and components - Solar collectors - Part 2: Test methods27.160Solar energy engineeringICS:Ta slovenski standard je istoveten z:EN 12975-2:2001SIST EN 12975-2:2002en01-november-2002SIST EN 12975-2:2002SLOVENSKI
STANDARD



SIST EN 12975-2:2002



EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN 12975-2June 2001ICS 27.160English versionThermal solar systems and components - Solar collectors - Part2: Test methodsInstallations solaires thermiques et leurs composants -Capteurs - Partie 2: Méthodes d'essaisThermische Solaranlagen und ihre Bauteile - Kollektoren -Teil 2: PrüfverfahrenThis European Standard was approved by CEN on 19 January 2001.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the Management Centre or to any CEN member.Although this (draft) European Standard has been developed in one language only in accordance with Resolution BT 74/1997 related to theone language experiment, it exists in accordance with the CEN/CENELEC Internal Regulations in the three official versions (English,French, German).CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre: rue de Stassart, 36
B-1050 Brussels© 2001 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN 12975-2:2001 ESIST EN 12975-2:2002



Page 2EN 12975-2:2001Contents
PageForeword3Introduction31Scope42References43Terms and definitions54Symbols and units55Reliability testing of liquid heating collectors76Thermal performance testing of liquid heating collectors21Annex A (normative) Schematics for durability and reliability tests71Annex B (normative) Durability and reliability test report sheets80Annex C (normative) Stagnation temperature of liquid heating collectors105Annex D (normative) Performance test report for glazed solar collectors understeady state conditions107Annex E (normative) Performance test report summary for glazed solar collectors111Annex F (normative) Performance test report for unglazed solar collectors understeady state conditions114Annex G (normative) Performance test report summary for unglazedsolar collectors118Annex H (normative) Modelling of the coefficients c1 to c6 of the collector modelof 6.3121Annex I (infomative) Definintion of areas of glazed liquid heating collectors123Annex J (informative) Measurements of effective thermal capacity128Annex K (informative) Comparison of the collector model of 6.1 to the collectorModel of 6.3131Annex L (informative) Properties of water132Annex M (informative) Performance test report summary for quasi dynamic testmethod133Annex N (informative)
Comparison between EN 13975-2 and ISO 9806134Bibliography148SIST EN 12975-2:2002



Page 3EN 12975-2:2001ForewordThis European Standard has been prepared by Technical Committee CEN/TC 312 "Thermal solarsystems and components", the secretariat of which is held by ELOT.This European Standard shall be given the status of a national standard, either by publication of anidentical text or by endorsement, at the latest by December 2001, and conflicting national standardsshall be withdrawn at the latest by December 2001.The annexes A, B, C, D, E, F, G and H are normative. The annexes I, J, K, L, M, N and O areinformative.According to the CEN/CENELEC Internal Regulations, the national standards organizations of thefollowing countries are bound to implement this European Standard: Austria, Belgium, CzechRepublic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg,Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the United Kingdom.IntroductionThis standard specifies test methods for determining the ability of a liquid heating solar collector toresist the influence of degrading agents. It defines procedures for testing collectors under well-definedand repeatable conditions.This standard also provides test methods and calculation procedures for determining the steady-stateand quasi-steady-state thermal performance of glazed liquid heating solar collectors. It containsmethods for conducting tests outdoors under natural solar irradiance and natural and simulated windand for conducting tests indoors under simulated solar irradiance and wind.This standard also provides methods for determining the thermal performance of unglazed liquidheating solar collectors. Unglazed collectors are in most cases used for heating swimming pools orother low temperature consumers. In general the collectors are put together on-site, connectingabsorber strips with manifolds. Real absorber areas are mostly between ten to one hundred squaremeters. For unglazed absorbers, readily fabricated modules with a specific size are seldom used.Therefore, during the test, it is to check, that a realistic flow pattern and flow velocity is used.This standard also provides test methods and calculation procedures for determining the steady-state aswell as the all-day thermal performance parameters for liquid heating solar collectors, under changingweather conditions. It contains methods for conducting tests outdoors during whole days and understationary inlet temperature conditions and natural solar irradiance and natural and/or simulated windconditions. Important effects for the all-day performance of the collector, as the dependence onincident angle, wind speed, diffuse fraction of solar irradiance, thermal sky radiation and thermalcapacity are taken into account. Dependence on flow rate is not included in this standard.Some of the advantages of the proposed extension of the present steady-state test methods of all-daytesting are: A shorter and less expensive outdoor test, suitable for European climate conditions. A much wider range of collectors can be tested with the same method. At the same time, a much more complete characterisation of the collector is achieved. The collector model is still directly compatible with that of the present basic test standards, andonly correction terms are applied in this extended approach. All additions are based on long agreed collector theory. At any time, full backwards comparability to steady-state can be established by evaluating onlyperiods of the test days that correspond to steady-state test requirements. The same test equipment can be used as for stationary testing with only minor changes, which alsowill improve the accuracy of steady-state testing.SIST EN 12975-2:2002



Page 4EN 12975-2:2001 Commonly available standard PC software can be used for the parameter identification, such asspreadsheets or more advanced statistical packages that has Multiple Linear Regression (MLR) asan option.An accurate long term prediction of the collector performance (not included in this standard) can be anintegral part of this test method, as the same collector model and parameters can be used for bothtesting and prediction.1ScopeThis European Standard specifies test methods for validating the durability, reliability and safetyrequirements for liquid heating collectors as specified in EN 12975-1. This standard also includes threetest methods for the thermal performance characterisation for liquid heating collectors.It is not applicable to those collectors in which the thermal storage unit is an integral part of thecollector to such an extent that the collection process cannot be separated from the storage process forthe purpose of making measurements of these two processes.It is not basically not applicable to tracking concentrating collectors, however thermal performancetesting as given in clause 6.3 (quasi dynamic testing) is also applicable to most concentrating collectordesigns, from stationary non-imaging concentrators as CPCs to high concentrating tracking designs.Parts of the solar radiation measurement have to be adjusted in case of a tracking collector and in casea pyrheliometer is used to measure beam radiation.Collectors that are custom built (built in; e.g. roof integrated collectors that do not compose of factorymade modules and are assembled directly on the place of installation) cannot be tested in their actualform for durability, reliability and thermal performance according to this standard. Instead, a modulewith the same structure as the ready collector may be tested. The module gross area shall be at least2m2.2ReferencesThis European Standard incorporates, by dated or undated reference, provisions from otherpublications. The normative references are cited at the appropriate places in the text and thepublications are listed hereafter. For dated references, subsequent amendments to or revisions of any ofthese publications apply to this European Standard only when incorporated in it by amendment orrevision. For undated references the latest edition of the publication referred to applies.ISO 9060Solar energy - Specification and classification of instruments formeasuring hemispherical solar and direct solar radiation.ISO 9806-1Test methods for solar collectors – Part 1: Thermal performanceof glazed liquid heating collectors including pressure drop.ISO 9806-2Test methods for solar collectors – Part 2: Qualification testprocedures.ISO 9806-3: 1995Test methods for solar collectors - Part 3: Thermal performance ofunglazed liquid heating collectors (sensible heat transfer only)including pressure drop.ISO 9846Solar energy - Calibration of a pyranometer using a pyrheliometer.ISO 9847Solar energy - Calibration of field pyranometers by comparison toa reference pyranometer.ISO/TR 9901EN ISO 9488Solar energy - Field pyranometers - Recommended practice foruse.Solar Energy - Vocabulary
(ISO 9488:1999)SIST EN 12975-2:2002



Page 5EN 12975-2:2001EN 12975-1:2000Thermal solar systems and components - Solar collectors - Part 1:General requirements3Terms and definitionsFor the purpose of this standard, the Terms and definitions given in EN ISO 9488 apply.4Symbols and unitsa1algebraic constant, reference to T*mWm-2K-1a2algebraic constant, reference to T*mWm-2K-2AAabsorber area of collectorm2Aaaperture area of collectorm2AGgross area of collectorm2AMoptical air massbucollector efficiency coefficient (wind dependence)m-1 sb1collector efficiency coefficientWm-2K-1b2collector efficiency coefficientWsm-3K-1c1heat loss coefficient at (Tm - Ta)=0Wm-2K-1c2temperature dependence of the heat loss coefficientWm-2K-2c3wind speed dependence of the heat loss coefficientJm-3K-1c4sky temperature dependence of the heat loss coefficientWm-2K-1c5effective thermal capacityJ m-2K-1c6wind dependence in the zero loss efficiencysm-1cfspecific heat capacity of heat transfer fluidJkg-1K-1Ceffective thermal capacity of collectorJK-1DdateYYMMDDELlongwave irradiance ( >3m)Wm-2Elongwave irradiance on an inclined surface outdoorsWm-2Eslongwave irradianceWm-2Fradiation view factorF´collector efficiency factorGhemispherical solar irradianceWm-2G*hemispherical solar irradianceWm-2G''net irradianceWm-2Gbdirect solar irradiance (beam irradiance)Wm-2Gddiffuse solar irradianceWm-2LTlocal timehKincidence angle modifierKbincidence angle modifier for direct radiationKdincidence angle modifier for diffuse radiationmthermally active mass of the collectorkg.mmass flowrate of heat transfer fluidkgs-1Q.useful power extracted from collectorWQ.Lpower loss of collector Wttimestaambient or surrounding air temperature°Ctdpatmospheric dew point temperature°CSIST EN 12975-2:2002



Page 6EN 12975-2:2001tecollector outlet (exit) temperature°Ctincollector inlet temperature°Ctmmean temperature of heat transfer fluid°Ctsatmospheric or sky temperature°Ctstgstagnation temperature°CTabsolute temperatureKTaambient or surrounding air temperatureKT*mreduced temperature difference ( = (tm – ta)/G*)m2KW-1Tsatmospheric or equivalent sky radiation temperatureKUmeasured overall heat loss coefficient of collector,with reference to T*mWm-2K-1ULoverall heat loss coefficient of a collector with uni-form absorber temperature tmWm-2K-1usurrounding air speedms-1Vf fluid capacity of the collectorm3ppressure difference between fluid inlet and outletPattime intervalsTtemperature difference between fluid outlet and inlet(te - tin)Ksolar absorptancetilt angle of a plane with respect to horizontaldegreesazimuth angledegreeshemispherical emittancesolar hour angledegreesangle of incidencedegreeslatitudedegreeswavelengthµmcollector efficiency, with reference to T*mo zero-loss collector efficiency ( at T*m = 0), reference to T*mStefan-Boltzmann constantWm-2K-4density of heat transfer fluidkgm-3ccollector time constantstransmittance()eeffective transmittance-absorptance product()edeffective transmittance-absorptance product fordiffuse solar irradiance()eneffective transmittance-absorptance product fordirect solar radiation at normal incidence()eeffective transmittance-absorptance product fordirect solar radiation at angle of incidence NOTE 1
In the field of solar energy the symbol G is used to denote solar irradiance, rather thanthe generic symbol E for irradiance.NOTE 2
C is often denoted (mC)e in basic literature (see also Annex H)NOTE 3
For more information about thermal performance coefficients (parameters) c1 to c6,see Annex H.SIST EN 12975-2:2002



Page 7EN 12975-2:20015Reliability testing of liquid heating collectors5.1 GeneralThe tests shall be carried out in the sequence shown in table 1. Changes in the test sequence, uponagreement of all parties involved or at discretion of the test laboratory, shall be reported with the testresults and the reasons for deviation shall be given.For some qualification tests, a part of the collector may have to be tampered with in some way, forexample a hole may have to be drilled in the back of the collector to attach a temperature sensor to theabsorber. In these cases care should be taken to ensure that any damage caused does not affect theresults of subsequent qualification tests, for example by allowing water to enter into a previouslyraintight collector.NOTE
This sequence has been determined with a view to minimise test costs while ensuringthat the possible effects of each degrading influence are likely to be evaluated in a later test.(For example, rain penetration may result if a collector is distorted by exposure to hightemperatures).Table 1 - Test sequenceSequenceSubclauseTest15.2Internal pressure25.3High-temperature resistance 1)35.4Exposure45.5External thermal shock 2)55.6Internal thermal shock 2)65.7Rain penetration 3)75.8Freeze resistance 4)85.2Internal pressure (retest)95.9Mechanical load106.1-6.2-6.3Thermal performance115.10Impact resistance125.11Final inspection1)For organic absorbers, the high-temperature resistance test shall beperformed first in order to determine the collector stagnation temperatureneeded for the internal pressure test.2)The two external and internal thermal shock tests may be combined with theexposure test or the high-temperature resistance test.3)The rain penetration test shall be carried out only for glazed collectors.4)
The freeze resistance test shall be carried out only for collectors claimed tobe freeze resistant.SIST EN 12975-2:2002



Page 8EN 12975-2:20015.2 Internal pressure tests for absorbers5.2.1 Metallic absorbers5.2.1.1 ObjectiveThe absorber shall be pressure-tested to assess the extent to which it can withstand the pressures whichit might meet in service.5.2.1.2 Apparatus and procedureThe apparatus, shown in figure A.1, consists of a hydraulic pressure source (electrical pump or handpump), a safety valve, an air-bleed valve and a pressure gauge with an accuracy better than 5%. Theair-bleed valve shall be used to empty the absorber of air before pressurisation. The metallic absorbershall be filled with water at room temperature and pressurised to the test pressure for the test period(see 5.2.1.3.2). This pressure shall be maintained while the absorber is inspected for swelling,distortion or ruptures.5.2.1.3 Test conditions5.2.1.3.1 TemperatureMetallic absorbers shall be pressure-tested (see 5.2.1.3.2) at ambient temperature within the range 5°Cto 30°C.5.2.1.3.2 PressureThe test pressure shall be 1,5 times the maximum collector operating pressure specified by themanufacturer.The test pressure shall be maintained for 15 min.5.2.1.4 ResultsThe collector shall be inspected for leakage, swelling and distortion. The results of this inspection shallbe reported together with the values of pressure and temperature used and the duration of the test.5.2.2 Absorbers made of organic materials (plastics or elastomers)5.2.2.1 ObjectiveThe absorber shall be pressure-tested (see 5.2.1.3.2) to assess the extent to which it can withstand thepressures, which it might meet in service while operating at elevated temperature. The tests shall becarried out at elevated temperatures, because the pressure resistance of an organic absorber may beadversely affected as its temperature is increased.SIST EN 12975-2:2002



Page 9EN 12975-2:20015.2.2.2 Apparatus and procedure5.2.2.2.1 GeneralThe apparatus consists of either a hydraulic or a pneumatic pressure source, and a means of heating theabsorber to the required test temperature.The characteristics of a solar irradiance simulator shall be the same as those of the simulator used forefficiency testing of liquid heating solar collectors.A temperature sensor shall be attached to the absorber to monitor its temperature during the test. Thesensor shall be positioned at two-thirds of the absorber height and half the absorber width. It shall befixed firmly in a position to ensure good thermal contact with the absorber. The sensor shall beshielded from solar radiation.The test conditions specified in 5.2.2.3 shall be maintained for at least 30 min prior to test and for thefull duration of the test.The pressure in the absorber shall be raised in stages as specified in 5.2.2.3, and the absorber shall beinspected for swelling, distortion or rupture after each increase in pressure. The pressure shall bemaintained while the absorber is being inspected.For safety reasons, the collector shall be encased in a transparent box to protect personnel in the eventof explosive failure during this test.5.2.2.2.2 Organic absorbers for use in unglazed collectors (test temperature < 90°C)Where the maximum test temperature is below 90°C, absorbers may be submerged in a heated waterbath and pressure-tested. The pressurised fluid supply to the absorber shall be fitted with a safety valve,air-bleed valve (if required) and pressure gauge having an accuracy better than 5%. The apparatus isshown in figure A.2.5.2.2.2.3 Organic absorbers for use with oil-based fluids (test temperature > 90°C)When the test temperature exceeds 90 °C, the absorber may be connected to a hot oil circuit. Theabsorber and hot oil circuit are then pressurised. The hot oil circuit shall be fitted with a safety valve,air-bleed valve and pressure gauge having an accuracy better than 5%.The absorber may be heated by any of the following methods:a) connecting a heater in the oil circuit (see figure A.3);b) heating the whole collector in a solar irradiance simulator (see figure A.4);c) heating the whole collector outdoors under natural solar irradiance (see figure A.4).Safety measures should be taken to protect personnel from hot oil in the event of explosive failureduring this test.5.2.2.2.4 Organic absorbers - high temperature pneumatic pressure testThe absorber may be pressure-tested using compressed air, when heated by either of the followingmethods:a) heating the whole collector in a solar irradiance simulator (see figure A.5);b) heating the whole collector outdoors under natural solar irradiance (see figure A.5).The compressed air supply to the absorber shall be fitted with a safety valve and a pressure gaugehaving an accuracy better than 5%.SIST EN 12975-2:2002



Page 10EN 12975-2:20015.2.2.3 Test conditions5.2.2.3.1 TemperatureFor absorbers made of organic materials, the test temperature shall be the maximum temperature whichthe absorber will reach under stagnation conditions.The reference conditions given in table 2 shall be used.The calculations employed to determine the test temperature are included in Annex C and shall either:- use measured collector performance characteristics, or- extrapolate from average values, measured in the high-temperature resistance test (see 5.3.3), of theglobal solar irradiance (natural or simulated) on the collector plane, the surrounding air temperatureand the absorber temperature.Table 2 - Climate reference conditions to determine test temperatures for internal pressure testof organic absorbersClimate parameterValue for all climate classesGlobal solar irradiance on collector plane, G in W/m21000Surrounding air temperature, ta in °C305.2.2.3.2 PressureThe test pressure shall be 1,5 times the maximum collector operating pressure specified by themanufacturer.For absorbers made of organic materials, the pressure shall be raised to the test pressure in equal stagesof 20 kPa (approximately) and maintained at each intermediate pressure for 5 min. The test pressureshall then be maintained for a least 1 h.5.2.2.4 ResultsThe collector shall be inspected for leakage, swelling and distortion. The results of the inspection shallbe reported.Full details of the test procedure used, including the temperature, intermediate pressures and testperiods used, shall be reported with the test results.5.3 High-temperature resistance test5.3.1 ObjectiveThis test is intended to assess rapidly whether a collector can withstand high irradiance levels withoutfailures such as glass breakage, collapse of plastic cover, melting of plastic absorber, or significantdeposits on the collector cover from outgassing of collector material.SIST EN 12975-2:2002



Page 11EN 12975-2:20015.3.2 Apparatus and procedureThe collector shall be tested outdoors, or in a solar irradiance simulator. A schema for testing is shownin figure A.6.The characteristics of the solar irradiance simulator to be used for the high-temperature resistance testshall be those of the solar irradiance simulator used for efficiency testing of liquid heating solarcollectors.The collector shall be mounted outdoors or in a solar simulator, and shall not be filled with fluid. Oneof its fluid pipes shall be sealed to prevent cooling by natural circulation of air, but the other shall beleft open to permit free expansion of air in the absorber.A temperature sensor shall be attached to the absorber to monitor its temperature during the test. Thesensor shall be positioned at two-thirds of the absorber height and half the absorber width. It shall befixed firmly in a position to ensure good thermal contact with the absorber. The sensor shall beshielded from solar radiation.NOTE 1
When testing collectors, such as evacuated tubular collectors, for which it is notappropriate to measure the stagnation temperature at the absorber, the temperature sensorshould be placed at a suitable location in the collector, and this location should be clearlydescribed with the test results.NOTE 2
In some cases, such as evacuated collectors, it may be difficult to attach athermocouple to the absorber. In such cases, instead of attaching a thermocouple to theabsorber, the testing laboratory may partially fill the absorber with a special fluid, seal theabsorber and measure the pressure in the absorber. The relationship between the internalpressure in the absorber and its temperature should be known from the standard vapourpressure/temperature relationship for the fluid.The test shall be performed for a minimum of 1 h after steady-state conditions have been established,and the collector shall be subsequently inspected for signs of damage as specified in 5.3.4.5.3.3 Test conditionsThe set of reference conditions given in table 3 shall be used for all climate classes.Table 3 - Climate reference conditions for high-temperature resistance testClimate parameterValue for all climate classesGlobal solar irradiance on collector plane, G in W/m2>1000Surrounding air temperature, ta in °C20 - 40Surrounding air speed in m/s 15.3.4 ResultsThe collector shall be inspected for degradation, shrinkage, outgassing and distortion.The results of the inspection shall be recorded together with the average values of solar irradiance(natural or simulated) on the collector plane, surrounding air temperature and speed, and absorbertemperature (and the pressure of the special fluid in the absorber, if that method is used) recordedduring the test.SIST EN 12975-2:2002



Page 12EN 12975-2:20015.4 Exposure test5.4.1 ObjectiveThe exposure test provides a low-cost reliability test sequence, indicating (or simulating) operatingconditions which are likely to occur during real service and which also allows the collector to "settle",such that subsequent qualification tests are more likely to give repeatable results.5.4.2 Apparatus and procedureThe collector shall be mounted outdoors (see figure A.7), but not filled with fluid. One of the fluidpipes shall be sealed to prevent cooling by natural circulation of air, while the other shall be left opento permit free expansion of air in the absorber.The air temperature shall be recorded to an uncertainty of 1 K and the global irradiance on the plane ofthe collector recorded using a pyranometer of class I in accordance with ISO 9060. Irradiation andmean air temperature values shall be recorded every 30 min and rainfall shall be recorded daily. Thecollector shall be exposed until the test conditions have been met.At the end of the exposure, a visual inspection shall be made for signs of damage as specified in 5.4.4.5.4.3 Test conditionsThe set of reference conditions given in table 4 shall be used.The collector shall be exposed until at least 30 days (which need not be consecutive) have passed withthe minimum irradiation H shown in table 4. The irradiation is determined by recording irradiancemeasurements using a pyranometer.The collector shall also be exposed for at least 30 h to the minimum irradiance level G given in table 4,as recorded by a pyranometer, when the surrounding air temperature is greater than the value shown intable 4. These hours shall be made up of periods of at least 30 min.NOTE
In regions where these conditions cannot be met during certain periods of the year,the 30-h exposure to high irradiance levels (table 4) can be conducted in a solar irradiancesimulator having characteristics identical to those of a simulator used for efficiency testing ofliquid heating solar collectors. The 30-h exposure test should be conducted after the collectorhas completed at least 10 days, but no more than 15 days, of the exposure to the minimumirradiation level (table 4).If the external and internal thermal shock tests are combined with the exposure test, the first externaland internal shocks shall be caused during the first 10 of the 30 h defined above, and the second duringthe last 10 of the 30 h.Table 4 - Climate reference conditions for exposure test as well as for external and internalthermal shock testsClimate parameterValue for all climate classesGlobal solar irradiance on collector plane, G in W/m2850Global daily irradiation on collector plane, H in MJ/m214Surrounding air temperature, ta in °C10NOTE
Values given are minimum values for testing.SIST EN 12975-2:2002



Page 13EN 12975-2:20015.4.4 ResultsThe collector shall be inspected for damage or degradation. The results of the inspection shall bereported together with a record of the climatic conditions during the test, including daily irradiation,surrounding air temperature and rain.5.5 External thermal shock test5.5.1 ObjectiveCollectors may from time to time be exposed to sudden rainstorms on hot sunny days, causing a severeexternal thermal shock. This test is intended to assess the capability of a collector to withstand suchthermal shocks without a failure.5.5.2 Apparatus and procedureThe collector shall be mounted either outdoors or in a solar irradiance simulator, but shall not be filledwith fluid. One of its fluid pipes shall be sealed to prevent cooling by natural circulation of air, whilethe other shall be left open to permit free expansion of air in the absorber (see figure A.8).A temperature sensor may be optionally attached to the absorber to monitor its temperature during thetest. The sensor shall be positioned at two-thirds of the absorber height and half the absorber width. Itshall be fixed firmly in a position to ensure good thermal contact with the absorber. The sensor shall beshielded from solar radiation.NOTE 1
When testing collectors, such as evacuated tubular collectors, for which it is notappropriate to measure the stagnation temperature at the absorber, the temperature sensorshould be placed at a suitable location in the collector, and this location should be clearlydescribed with the test results.NOTE 2
In some cases, such as evacuated collectors, it may be difficult to attach athermocouple to the absorber. In such cases, instead of attaching a thermocouple to theabsorber, the testing laboratory may partially fill the absorber with a special fluid, seal theabsorber and measure the pressure in the absorber. The relationship between the internalpressure in the absorber and its temperature should be kn
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