DLR QUARZ Test- and Qualification Center for Concentrating ... · Test- and Qualification Center...

DLR QUARZ

Test- and Qualification Center for Concentrating Solar Power Technologies

Deutsches Zentrum für Luft- und Raumfahrt German Aerospace Center Institute of Solar Research

51147 Köln, Germany Tel. +49 2203 601 3181

email: [email protected]

20.07.2012

Measurement of Parabolic Trough Receiver Thermal Loss Power and

Relative Optical Efficiency under Solar Simulator Light

Test Report

Johannes Pernpeintner

With contributions by Michael Anger, Niels Lichtenthäler, Philipp Ant, Christoph Happich, and Johannes Thoss

Document: Quarz_Receiver_1207_Schott_R3

Client: SCHOTT Solar CSP GmbH

Receiver type: SCHOTT PTR70, Type M, random sample from serial production

Number of receivers: 3

DLR QUARZ Evaluation Report Receiver

2 DLR-Quarz_Receiver_1207_Schott_R2

Contents 1. Executive Summary .......................................................................................................... 3

2. Scope ................................................................................................................................. 4

3. Random Sample Selection and Test Overview ............................................................... 5

4. Heat Loss Measurement ................................................................................................... 6

5. Measurement of the Relative Optical Efficiency at Ambient Temperature under Solar Simulator Light ......................................................................................................................... 9

6. Effect of Bellow Design and Absorber Thermal Expansion on the Optical Efficiency 11



1. Executive Summary Three parabolic trough receivers have been randomly selected from the production line by Mr. Daniel Kraus of TÜV Süd and have been provided to DLR for testing and evaluating their thermal and optical performance.

Receivers PTR-70 00356770, 00356772, and 00356801 show a heat loss at 350 °C absorber temperature of 153 W/m, 153 W/m, and 152 W/m.

Receivers PTR-70 00356770, 00356772, and 00356801 show an optical efficiency of 1.059 ∙ ηopt,rec (DLR 70-1, RT), 1.065 ∙ ηopt,rec (DLR 70-1, RT), and 1.062 ∙ ηopt,rec (DLR 70-1, RT). Typically receivers available on the market have an optical performance relative to ηopt,rec (DLR 70-1, RT) of a few percentage points above 1.

The analysis of the bellow design with a simple normal-incidence-model shows a negligible impact of the bellow design on the optical efficiency. The projected optical efficiencies at an absorber temperature of 350 °C are 1.059 ∙ ηopt,rec (DLR 70-1, RT), 1.065 ∙ ηopt,rec (DLR 70-1, RT), and 1.062 ∙ ηopt,rec (DLR 70-1, RT).



2. Scope DLR provides Schott with testing and evaluation on selected samples of receivers for application in solar parabolic trough collectors for the purpose of product evaluations for CSP projects. The characteristics of parabolic trough receivers are best described by steady-state heat loss power over the relevant operating temperature range, absorbed power under concentrated irradiance and geometric characteristics of the receivers.

The receiver samples for testing are of standard dimensions of 70 mm outer absorber tube diameter and nominal length of 4060 mm at room temperature.

Heat Loss Measurement

Heat loss testing is performed with electrical heating to steady state conditions at temperature levels of 250 °C to 400 °C. The temperature is measured with thermocouples at the inside of the receiver tube.

The test result is specific heat loss power over temperature according to existing standards for solar collectors up to the maximum test temperature. The result includes heat loss on the full length of the receiver including bellows and other elements. Parameters can be evaluated with a polynomial fit and can be applied in solar power plant models such as Greenius and SAM.

Optical Efficiency Testing

Absorption testing is performed in a solar simulator (ElliRec linear focus simulator) according to the method developed by DLR. This test consists of comparative enthalpy flow measurement of the cold water stream through an irradiated receiver tube at constant testing conditions. The test results in a relative combined optical efficiency value for the full length of the receiver, including properties of the glass and its coating, the absorber coating, and the effects of the bellow geometry.



3. Random Sample Selection and Test Overview Three receivers are chosen as a random sample selection directly from the production line independent controller Mr. Daniel Kraus of TÜV Süd. The certificate is appended to this report. These three receivers are sent to DLR. Table 1 gives a list of the performed tests documented in this report.

Table 1: Completed Tests

Type ID Heat Loss Test Optical Test

PTR-70 00356770 yes yes PTR-70 00356772 yes yes PTR-70 00356801 yes yes



4. Heat Loss Measurement Heat loss measurement is performed with the ThermoRec test bench. The test bench and the measurement methodology are described in [1]. Measurement conditions are provided in the ThermoRec Measurement Summary enclosed to the report.

No shields are provided with the receivers. Hence original Shields from Schott from previous campaigns are used for the thermal test. These Shields have a length of 60 mm and a height of 10 mm above the glass. This type of shields is used at the end of collectors.

The measurement results are given in Table 2, Table 3, Table 4, Figure 1, Figure 2, and Figure 3. In the tables Tabs, Tamb, and Tglass are the mean temperatures of absorber, ambient and glass. Pth,loss and pth,loss are heat loss and length specific heat loss of the receiver. dTabs,fit is the exponential fit correction, which considers, that the steady fit has not been reached perfectly in the measurement.

Receivers PTR-70 00356770, 00356772, and 00356801 show a heat loss at 350 °C absorber temperature of 153 W/m, 153 W/m, and 152 W/m.

Table 2: Heat loss measurement results of receiver PTR-70 00356770, date 9th of July 2012

Tabs Tamb Tglass Pth,loss pth,loss dTabs,fit in °C in °C in °C in W in W/m in K

412 23 54 1035 255 1.8 359 23 46 668 164 1.7 308 23 39 428 105 1.5 229 23 32 198 49 1.3

250 60 300 98 350 153 400 233



410 23 61 1018 251 1.5 359 23 51 670 165 1.3 309 23 43 430 106 1.4 250 23 37 248 61 1.4

250 61 300 98 350 153 400 231





414 24 60 1034 255 2.0 360 24 51 667 164 1.7 309 23 43 426 105 1.5 251 23 36 246 61 1.5

250 60 300 97 350 152 400 229

Figure 3: Specific heat loss of receiver PTR-70 00356770



5. Measurement of the Relative Optical Efficiency at Ambient Temperature under Solar Simulator Light

The receiver is tested for the relative optical efficiency in the ElliRec solar simulator test bench. The optical efficiency measurement is performed after the thermal measurement. The test bench and the measurement methodology are described in [2]. Optical efficiency includes the effects of glass transmission, absorption and geometrical effects due to a geometrically reduced absorber length due to the bellows1. An absolute calibration of the test bench has not yet been available with the required high accuracy; hence the absorbed power is used as a relative measure of the optical efficiency. The sample is compared to the optical efficiency of the reference receiver DLR 70-1 measured at room temperature (RT) ηopt,rec (DLR 70-1, RT). This reference optical efficiency is measured at the same day.

No shields are provided with the receivers. Hence original shields from Schott from previous campaigns are used for the optical test. These shields have a length of 60 mm and a height of 10 mm above the glass. This type of shield is used at the end of collectors. In the older receiver design of ~ 2008 these shields align with the end of the bellow, when they are installed as intended. However, the receivers in this test have a new bellow design, where the active length is increased at both ends for ~ 7 mm. Hence the older shields are used but moved backwards for ~ 7 mm, so that they align with the end of the bellow, see Figure 6.2

Figure 6: Shield mounting in optical test

1 In contrast to [3] in this report optical efficiency includes geometrical properties of the receiver. 2 In the optical test bench shields are necessary for the protection of the bellows and the glass-to-metal seal from concentrated radiation. If shields are provided with the receiver, these shields are used as intended by the manufacturer. If no shields are provided together with the receiver, it is standard procedure to use DLR-Shields. These shields are aligned with that border - near the bellow or the glass-to-metal seal - which defines the active aperture.



Detailed measurement conditions are provided in the measurement sheets appended to the report. Table 3 shows the results.

Table 3: ElliRec measurement and evaluation results

Date ID Type Absorbed

power in kW

Optical Efficiency in ηrec,opt (DLR 70-1,RT)

10.07.2012 DLR 70-1 6.107 1

10.07.2012 PTR-70 00356770 6.468 1.059 10.07.2012 PTR-70 00356772 6.502 1.065 10.07.2012 PTR-70 00356801 6.483 1.062

Receivers PTR-70 00356770, 00356772, and 00356801 show an optical efficiency of 1.059 ∙ ηopt,rec (DLR 70-1, RT), 1.065 ∙ ηopt,rec (DLR 70-1, RT), and 1.062 ∙ ηopt,rec (DLR 70-1, RT). The mean optical efficiency of all three receivers is 1.062 ∙ ηopt,rec (DLR 70-1, RT). Typically receivers available on the market have an optical performance relative to ηopt,rec (DLR 70-1, RT) of a few percentage points above 1.



6. Effect of Bellow Design and Absorber Thermal Expansion on the Optical Efficiency

The optical efficiency generally changes with temperature due to the bellow expansion or compression and hence loss or gain in effective absorber area. As the receiver is measured in the sun simulator test bench only at ambient temperature, in this chapter this impact of receiver expansion on optical efficiency is discussed. However, the discussion in this chapter is only valid, if no reflectors at the bellows are used together with the receivers. These reflectors are offered by Schott. Their impact on the optical efficiency of the receiver must be considered in a separate analysis.

The optical efficiency of the receiver 𝜂𝑜𝑝𝑡,𝑟𝑒𝑐 is proportional to the geometric net area factor 𝜓𝑛𝑒𝑡, which is the ratio of receiver aperture length to receiver length [3]. In Figure 7 the operating principle of the shields of the Schott receiver is shown. In this case, the ends of the bellows are always aligned with the ends of the shields, regardless of the temperature. Hence the change in aperture will be nearly identical to the receiver thermal expansion.

shield receiver at ambient temperature

receiver atoperating temperature

Figure 7: Operating principle of the shields of the Schott-receiver

In Table 4 the results of the prediction of change of receiver optical efficiency ηopt,rec due to effects at the bellow is shown. For this calculation an aperture length for all receivers of 3924 ± 3 mm, and a receiver expansion of 26 mm at 400 °C compared to room temperature are assumed.



Table 4: Geometric impact of receiver expansion on net area factor and optical efficiency

T lrec lactive ψnet ηrec,opt in °C in mm in mm - in ηrec,opt (DLR 70-1,RT)

00356770 00356772 00356801

23 4060 3924 0.967 1.059 1.065 1.062 300 4079 3943 0.967 1.059 1.065 1.062 350 4083 3947 0.967 1.059 1.065 1.062 400 4086 3950 0.967 1.059 1.065 1.062

The calculations show that the change in optical efficiency due to bellow design is negligible.1

1 In Quarz-Receiver reports of 2009 the bellow analysis showed an increase in optical performance of the Schott receiver at higher temperature. The apparent contradiction to the findings of this report is due to a change in methodology. In the reports of 2009 the changes at higher temperature was evaluated relative to the absorbed power of the reference receiver at ambient temperature. Now it is evaluated relative to the optical efficiency of the reference receiver at ambient temperature. While the old approach is still considered a valid approach, this nevertheless is now considered counter-intuitive.



References

[1] Test Description – ThermoRec Heat Loss Test Bench v5_3 v6_3

[2] Test Description – ElliRec Elliptical Solar Simulator Test Bench v3

[3] Lüpfert et. al: Towards standard performance analysis for parabolic trough collector fields, Proceedings of SolarPACES Conference, Oaxaca, Mexico, 2004

ThermoRec Measurement Summary

Measurement Information

Receiver ID 00356770 System border front face

Manufacturer Shields installed

Campaign 1207_Schott Absorber length 4060 mm (specified)

Date, meas. 2012-07-09 Absorber Øouter 70 mm (specified)

Date, eval. 2012-07-11 Absorber wall 1.9 mm (specified)

Operator, meas. JT, NE Indicator before bright circle

Operator, eval. JT, JP Indicator after bright circle

Version, meas. v5_3 Comment

Version, eval. v2_22_jp

Result Overview

T abs p loss T amb T glass T shields r loss,ends dT abs,fit σ(T abs) Comment

in °C in W/m in °C in °C in °C in % in K in K

411.6 255.0 23.3 53.6 77.7 2.1 0.4 1.8

358.8 164.4 23.4 46.3 67.0 2.8 -0.3 1.7

308.3 105.5 22.7 39.2 57.8 3.5 -0.4 1.5

229.3 48.7 23.1 32.0 45.6 4.6 -1.0 1.3

250 59.8 interpolated value




c1 = W/°C c2 = W/(°C)4 c1 = W/(m °C) c2 = W/(m (°C)

4)

T abs absorber temperature p loss specific heat loss within system border

T amb ambient temperature r loss,ends ratio of axial heat loss and total heat loss

T glass glass temperature dT abs,fit absorber temperature correction applied by steady state fit

T shields temperature at the bellow shields σ(T abs) standard deviation of absorber temperatures

File Identification

Measurement Log File 20120709_00356770.txt

Evaluation Init File G:\TT-SF-QF\Mess\QUARZ-Receiver\ThermoRec\Auswertungen\1207_Schott\20120709_00356770\init_Rec_v2_22_jp_20120709_00356770.m

PDF Filename G:\TT-SF-QF\Mess\QUARZ-Receiver\ThermoRec\Auswertungen\1207_Schott\QUARZ_Thermal__00356770_20120709.pdf

QUARZTest and Qualification Center for CSP Technologies

Institute of Solar Research, Cologne

pern_jo

Schreibmaschinentext

Schott




Manufacturer Schott Shields installed




Operator, meas. MA, NE Indicator before bright circle

Operator, eval. MA Indicator after bright circle


Version, eval. v2_22

Result Overview



410.4 250.9 23.2 60.7 76.3 2.1 0.3 1.5

359.3 165.0 23.0 51.0 66.0 2.6 -0.3 1.3

308.9 106.0 22.9 42.8 57.3 3.2 -0.3 1.4

250.3 61.1 23.3 36.7 49.5 4.4 -2.3 1.4

250 60.9 extrapolated value




c1 = W/°C c2 = W/(°C)4 c1 = W/(m °C) c2 = W/(m (°C)

4)





File Identification


Evaluation Init File G:\TT-SF-QF\Mess\QUARZ-Receiver\ThermoRec\Auswertungen\1207_Schott\20120704_00356772\init_Rec_v2_22_20120704_00356772.m

PDF Filename G:\TT-SF-QF\Mess\QUARZ-Receiver\ThermoRec\Auswertungen\1207_Schott\QUARZ_Thermal_Schott_00356772_20120704.pdf






Manufacturer Shields installed




Operator, meas. MA, JT Indicator before bright circle

Operator, eval. JT, JP Indicator after


Version, eval. v2_22

Result Overview



413.8 254.6 23.5 60.4 79.3 2.3 0.6 2.0

359.6 164.3 23.6 51.0 68.4 2.9 -0.4 1.7

308.7 105.0 22.8 42.5 58.9 3.8 -0.5 1.5

251.3 60.6 22.9 35.9 50.1 4.9 -0.9 1.5

250 59.8 extrapolated value




c1 = W/°C c2 = W/(°C)4 c1 = W/(m °C) c2 = W/(m (°C)

4)





File Identification


Evaluation Init File G:\TT-SF-QF\Mess\QUARZ-Receiver\ThermoRec\Auswertungen\1207_Schott\20120706_00356801\init_Rec_v2_22_20120706_00356801.m

PDF Filename G:\TT-SF-QF\Mess\QUARZ-Receiver\ThermoRec\Auswertungen\1207_Schott\QUARZ_Thermal__00356801_20120706.pdf



pern_jo

Schreibmaschinentext

Schott

ElliRec Measurement Summary


Receiver ID PTR70-00356770 Shields Schott-ShieldsManufacturer Schott Shields border 68 mm (measured)Campaign 1207_Schott Absorber length 4060 mm (specified)Date, meas. 2012-07-10 15:57 Absorber Øouter 70 mm (specified)Date, eval. 2012-07-10 Absorber Øinner 66 mm (measured)Operator, meas. Ant Reference DLR70-1Operator, eval. N. Lichtenthäler Date, meas. 2012-07-10 10:30Version, meas. v3.0 CommentVersion, eval. v3.2.4

Result

η opt,rec = 1.059 ηopt,rec (DLR 70-1,RT)

Measurement Conditions

Receiver P abs Qwater T amb T ellirec T water dT water,max Comment# in kW in l/h in °C in °C in °C in K

6.468 851.8 24.2 31.6 24.273 0.065DLR70-1 6.107 846.7 23.6 30.5 24.231 0.057

ηopt,rel mean optical efficiency relative to DLR 70-1 Twater mean water temperature

Pabs mean absorbed power dTwater,max maximum temperatuere deveation of water inlet

Tamb mean ambient temperature Qwater mean volume flow

Tellirec mean temperature in ElliRec

File Identification

Measurement Log File, Sample ...gen\2012\2012.07.10_4_PTR70-00356770.txt

Measurement Log File, DLR 70-1 ...\Messungen\2012\2012.07.10_1_DLR70-1.txt

Evaluation Init File 2012.07.10_3_PTR70-00356772_R1.xls

PDF Filename 2012.07.10_3_PTR70-00356772_R1.pdf

QUARZTest and Qualification Center for CSP TechnologiesInstitute of Solar Research, Cologne

PTR70-00356770




Result




6.502 851.8 24.3 31.4 24.231 0.142DLR70-1 6.107 846.7 23.6 30.5 24.231 0.057





File Identification






PTR70-00356772




Result




6.483 852.3 24.5 31.4 24.189 0.040DLR70-1 6.107 846.7 23.6 30.5 24.231 0.057





File Identification






PTR70-00356801

DLR QUARZ


Deutsches Zentrum für Luft- und Raumfahrt German Aerospace Center

Institute of Technical Thermodynamics – Solar Research 51147 Köln, Germany

Tel. +49 2203 601 3983 e-mail: [email protected]

20.07.2012

ThermoRec - Heat Loss Test Bench

Test Description

Versions:

v5.3

v6.3

Michael Anger


DOC: DLR-Quarz_Receiver_Test_Description_-

_ThermoRec_Heatloss_Test_Bench_v5_3_v6_3_120720.doc

DLR QUARZ Test Description Receiver

2 Thermorec_v5.3 and 6.3

Contents Thermal Loss Measurement at the Thermorec Heat Loss Test Bench ................................. 3

0 Hardware Configurations .................................................................................................. 3

1 Measurement Principle ..................................................................................................... 3

2 Energy Balance ................................................................................................................. 3

3 System Borders ................................................................................................................. 4

4 Temperature ...................................................................................................................... 4

5 Axial Heat Loss on Insulation Caps ................................................................................. 5

6 Interpolation of Specific Heat Loss at certain Absorber Temperatures ........................ 6

7 Measurement Uncertainty ................................................................................................. 6



Thermal Loss Measurement at the ThermoRec Heat Loss Test Bench

0 Hardware Configurations

Version v5.3 v6.3

Description high accuracy high temperature

Heating elements 1 main heating element 5 kW [4060x11] + 2 additional heating pairs 320 W

1 main heating element 5 kW [4060x11] + 2 additional heating pairs 320 W

Homogenisation tube Brass [4060 x 50 x 5]mm None

Mounting / Centring 10 pairs spring steel bars 5 ”quads” spring steel bars

# TC absorber 5 pairs (top and bottom) + 2 top

5 pairs (top and bottom)

# TC glass 3 pairs (top and bottom) 3 pairs (top and bottom)

# TC shield 2 pairs (top and bottom) 2 pairs (top and bottom)

# TC insulation cap 2 pairs (cylinder and face) 2 pairs (cylinder and face)

1 Measurement Principle Thermal loss measurement at the ThermoRec is realized by inserting electrical heating elements into the absorber tube. At steady heating power and steady absorber temperature heat loss can be determined by measuring the electrical power of the heating elements and the thermal losses at the ends of the receiver. A LabView based computer program controls the heaters either by a schedule of constant power steps or of mean temperature steps.

Evacuated parabolic trough receivers are tested with the evacuation nipple facing upwards, unless a getter holder is mounted on the absorber, then the getter faces upwards.

2 Energy Balance At the ThermoRec a heating element of 4060 mm in active length provides the bulk of the heating power Pheat,main. The main heater is enclosed by a homogenization brass tube (v5) of the same length or centered by brass rings (v6) and inserted into the absorber tube. On both ends of the inner tube two 40 mm heating elements are inserted adding power Pheat,ends. They are PI-controlled in order to achieve a homogenous longitudinal absorber temperature profile. Their heating power contribution is typically 2 % to 5 % of the overall heating power. Two cylindrical insulation caps are mounted at the ends of the receiver. Each has a diameter of 180 mm and a length of 150 mm. They minimize axial thermal flow and thus, thermal losses of the receiver in axial direction to again 2 % to 5 % of the overall heating power. The surface temperature of the insulation caps is monitored, and the axial end loss Ploss,ax is calculated by a model regarding convection and radiation loss, which is described in A3. The term of interest is heat loss in radial direction, named Ploss,rdl in the appendix. It is equal to Ploss in the main section and calculated by



axloss,ends heat,mainheat,rdlloss, PPPP −+= . (1)

For the calculation of specific heat loss ploss,rdl by

lP

p rdlloss,rdlloss, = (2)

the nominal length l of 4060 mm is used for all temperatures neglecting absorber expansion.

3 System Borders Receiver heat loss is measured with - or without installed radiation protection shields at the bellows. Receiver heat loss is the sum of all heat flux over all surfaces of a receiver, except the heat loss via the end face Figure 1. The end faces are considered adiabatic as in the field these surfaces are connected to neighboring receivers at nearly identical temperature. In order to measure shield losses, the shield surface temperatures are measured, heat flux over the shields is calculated similarly to Eq.5 and the total heat loss corrected by an additional negative term Ploss,blw in equation 1.

Figure 1: System Borders at receiver ends; heat loss measurement can include bellow heat

loss (a) and exclude bellow heat loss until glass to metal seal (b)

4 Temperature Absorber temperatures are measured with 10 to 16 internal thermocouples. They are attached to the brass tube (v5) or to brass rings (v6) and located at various positions along the receiver axis (x-position). Furthermore thermocouples measure different positions on the circumference of the absorber, e.g. directly upwards (0°) or downwards (180°). Good thermal contact of the thermocouples to the absorber tube is insured by steel spring leafs. All spring leafs are tested before and after each measurement to apply a force of at least 8 N to the inner absorber wall. In case of a spring leaf with a force < 5 N or unusual behavior in T(t), the respective thermocouple is excluded of evaluation. The mean temperature of the absorber Tabs,m is calculated assuming only radiative losses by weighting the thermocouple measurements Tabs,i to the 4th power and with their corresponding lengths li



∑ 4iabs,i

4mabs, TlTl = . (3)

There the absorber length l is the sum of the corresponding lengths l = Σ li and the borders of the corresponding length li of a thermocouple i is defined as running half way to the nearest neighbors on both sides, or to the end of the receiver (at the outermost thermocouples). Top and bottom side thermocouples are considered independently. The resulting mean temperature of the whole absorber is again calculated by using equation 3, considering each side to represent half of the absorber.

Glass temperature is measured with six thermocouples, three on top (0°) and three on the bottom (180°) of the glass envelope. Additional thermocouples measure the temperature on top and bottom at both glass-to-metal seals or protection shields, respectively. Convection at the outer envelope leads to noise in the temperature measurement, hence each measurement of glass and shield temperature is averaged over 5 minutes. The mean glass temperature Tglass,m is calculated by the arithmetic mean of n thermocouples as free convection is a major contributing loss mechanism

∑n

1iiglass,mglass,

=

= TTn . (4)

Similarly, the mean shield (or glass-to-metal seal) temperature is calculated by

∑n

1iishield,mshield,

=

= TTn . (5)

5 Axial Heat Loss on Insulation Caps An insulation cap is a cylinder with flat end sides. For each cap the cylindrical surface, the front face and the ring face are considered for the calculation of the axial end losses. The cylinder surface is horizontally oriented. At each cap one thermocouple is placed at the upper center of the cylinder representing the mean cylinder temperature Tcyl and another one approximately at the mean distance between center and edge of the front face representing the mean face temperature Tface. An investigation showed that the cap temperatures are measured within ± 2 K difference to the mean temperature.

The end loss model uses the following equations for the calculation of convective losses Pconv,i, taken from VDI Heat atlas 2006

im

ambiiconv, Aα

TTP

-= (6)

with the temperature Ti (Tcyl or Tface) and area Ai of the corresponding surface, the ambient temperature Tamb and the mean heat transfer coefficient αm

Lλα Nu

m = . (7)

The Nusselt number Nu for horizontal cylinders is

261

3 )(Pr))fPrGr((387.0752.0Nu += . (8)

The Nusselt number Nu for vertical surfaces (front side surface) is



261

3 )(Pr))fPrGr((387.0825.0Nu += . (9)

The Grashof number Gr is

2ambi

3 )(Grν

TTβLg -= , (10)

where β is the thermal expansion coefficient, which is for an ideal gas of ambient temperature β = 1/ Tamb. Radiation losses at the surface are calculated by

)()ε(σ 4amb

4iiiirad, TTTAP -= . (11)

The inserted values are given in Table A1.

Table A1: Summary for end cap heat loss calculation at 20 °C

Name Variable Unit

Prandtl number Pr 0.708 Aux. function 1 f1(Pr) 0.345 Aux. function 3 f3(Pr) 0.325 Gravitational acceleration g 9.81 m/s2

Diameter D 0.180 m Characteristic linear dimension (circular surface) L D/4 m Characteristic linear dimension (cylindrical surface) L π D/2 m Specific volume of air at 20 °C ν 15.32·10-6 m3/kg Emissivity of insulation caps (measured) ε 0.40 ± 0.04 Stefan-Boltzmann-constant σ 5.67·10-8 W/m2

The resulting axial heat loss Ploss,ax is the sum of radiation and convection losses on both cylinders and front surfaces of the insulation caps.

∑ irad,iconv,axloss, PPP += (12)

6 Interpolation of Specific Heat Loss at certain Absorber Temperatures Characteristic numbers summarizes the heat loss behavior of a receiver. Typically specific heat loss at 350 °C or the maximum operation temperature of 400 °C for oil and 550 °C for salt receivers are chosen. This number is not measured directly but calculated by performing cubic spline interpolation with a not-a-knot constraint.

7 Measurement Uncertainty Two measurement uncertainties have to be regarded at heat loss measurement, as the heat loss function pth,loss(Tabs) is sought: Uncertainty in power P measurement and uncertainty in temperature T measurement. The overall uncertainty is dominated by the temperature measurement, as the power measurement leads to a measurement uncertainty of ± 1 % and the temperature measurement to (+ 2 - 10) %.

The uncertainty in the heat loss power as calculated with equation 1 leads to ± 1 %, where all relevant partial uncertainties are included. The uncertainty of the main heating power is in the order of ± 1 W to ± 4 W, which equals ± 0.2 % to ± 0.5 % related to the radial loss power,



depending on power and measuring range. The uncertainty of the additional heating power is ± 1 % relative to its value, and thus << ± 0.1 % relative to the overall loss power. The uncertainty of loss power at the ends is dominated by the uncertainty of the measurement of the mean temperature on the insulation caps. Here, an uncertainty of ± 2 K is assumed, leading to ~ ± 2 W uncertainty, which equals ~ 0.5 % related to overall loss power at operating receiver temperatures.

The dominant part of the uncertainty in the heat loss measurement is the measurement of the absorber temperature. It is dominated by the uncertainty of the thermal contact from absorber tube to thermocouple, which is estimated to + 0 K, - 10 K. It is asymmetric as the absorber is the coldest part in the annulus.

The quality of the steady state is checked by performing an exponential least squares fit for each temperature measurement Ti (t). It is used to analyze the difference between the temperature that has been reached in the measurement and the temperature that is predicted as steady state temperature using the model function

)e1()+(1

i,i,fit

ii

ctτTT ∞ ±= , (13)

including fit parameters T∞, τ and c using ‘+’ for decreasing and ‘-‘ for increasing temperatures. The model is based on the assumption of a lumped heat model, constant heating power and heat loss depending linearly on temperature, which is adequately fulfilled for small temperature variations. If the measured temperature reaches T∞ at measurement time within ± 0.2 K, then the measured temperature is used. Otherwise the RMS difference between fit and original data must be < 0.1 K to ensure a sufficient agreement of the fit and parameter T∞ replaces measured temperature at the corresponding measurement point.

Uncertainty related to thermocouple drift, calibration and data processing error is calculated to ± 1.4 K. The overall uncertainty in the temperature measurement leads to (+ 2 – 10) K. The asymmetry is not corrected - as required by the GUM (Guide to the Expression of Uncertainty in Measurement) - as the correction is determined by the difficult estimate of thermo-contact uncertainty.

Due to the slope of the function pth,loss(Tabs), the uncertainty of + 2 K, - 10 K translates to approximately + 2 % - 10 % uncertainty in heat loss power at a given temperature. This means, heat loss measurement at the ThermoRec underestimates the heat loss. As this is not a statistical but a systematic effect, a relative comparison is more accurate.

A comparison of the ThermoRec to thermal loss test benches at Schott and NREL in work by Eichel et. al.: Heat Loss Measurements on Parabolic Trough Receivers, Proceedings of SolarPACES 2010, Perpignan could show good agreement with < 10 % differences in power at the same temperatures between the test benches in spite of significant differences in the methodology.

DLR QUARZ


Deutsches Zentrum für Luft- und Raumfahrt German Aerospace Center

Institute of Technical Thermodynamics – Solar Research 51147 Köln, Germany

Tel. +49 2203 601 3181 e-mail: [email protected]

23.04.2012

ElliRec Elliptical Solar Simulator Test-Bench - Measurement of Relative Parabolic Trough Receiver

Optical Efficiency under Simulated Sunlight

Test Description

Version:

v3.0


DOC: DLR-QUARZ_Receiver_Test_Description_-_ ElliRec_Elliptical_Solar_Simulator_Test_Bench_v3.0.doc


2 Ellirec_V.2011-10-06

ElliRec Elliptical Solar Simulator Test Bench

At the DLR QUARZ Center in Cologne the optical efficiency of parabolic trough receivers is assessed with the ElliRec linear focus solar simulator test bench. In this test bench the sample receiver is positioned in the linear focus and the enthalpy increase of water running through the receiver is measured at ambient temperature hence with negligible heat loss. The comparison to the enthalpy increase in a reference receiver yields a relative measure of the optical efficiency of the sample under simulated sunlight.

The optical efficiency of a receiver 𝜂𝑜𝑝𝑡,𝑟𝑒𝑐 is part of the energy balance of the receiver in the solar field of

𝑃𝑐𝑜𝑙𝑙 = 𝜂𝑜𝑝𝑡,𝑟𝑒𝑐 ∗ 𝑃𝑖𝑛 − 𝑃𝑡ℎ,𝑙𝑜𝑠𝑠, (1)

where 𝑃𝑐𝑜𝑙𝑙 is the enthalpy increase of the heat transfer fluid, 𝑃𝑖𝑛 is the flux on the receiver and 𝑃𝑡ℎ,𝑙𝑜𝑠𝑠 is the thermal loss. In the test bench thermal losses 𝑃𝑡ℎ,𝑙𝑜𝑠𝑠 are neglected as the water is at ambient temperature. As 𝑃𝑖𝑛 is not known, the enthalpy increase of the sample receiver is compared to the enthalpy increase of a reference receiver DLR 70-1. The ratio of enthalpy increases of sample and reference is equal to the ratio of optical efficiencies under simulated sunlight.

𝜂𝑜𝑝𝑡,𝑟𝑒𝑐,𝑠𝑎𝑚𝑝𝑙𝑒

𝜂𝑜𝑝𝑡,𝑟𝑒𝑐,𝐷𝐿𝑅70−1= 𝑃𝑐𝑜𝑙𝑙,𝑠𝑎𝑚𝑝𝑙𝑒

𝑃𝑐𝑜𝑙𝑙,𝐷𝐿𝑅70−1 (2)

For the test 4 HMI-lamps with each 4 kWe electrical input power are operated. The hemispherical spectral distribution of the lamps is shown in Figure 1.

Figure 1: Spectral distribution of the HMI lamps of the Ellirec test bench, source: OSRAM

The solar simulator lamps are situated in one focal line of an elliptical mirror trough. The sample receiver is situated in the second focal line of the elliptical mirror trough. Flux distribution along the focal line of the receiver varies ± 5 % in longitudinal direction.

0

0.5

1

1.5

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2.5

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0 500 1000 1500 2000 2500

wavelength in nm

radi

ant

flux

in a

.U.

HMI 4000 SE UV-Block ASTM 173d


3 Ellirec_V.2011-10-06

Water enthalpy increase is calculated from flow rate and inlet- and outlet temperature measurements. Temperature measurements are performed with three PT100 resistance thermometers at water inlet and water outlet side. Flow rate of ~0.85 m3 h-1 - in the case of the standard 70 mm receiver - is measured with a Siemens Sitrans F M Mag 6000. A displacement cylinder is inserted into the absorber tube in order to increase the heat transfer coefficient from absorber to the water. The displacement cylinder creates a gap of ~ 5 mm at the inside of the absorber. In order to further enhance water speed and homogenise temperature in circumferential direction guiding rods force the water on a corkscrew-like motion with a pitch of 200 mm. Bending of the absorber due to temperature differences are smaller than bending due to gravity.

After installation of the receiver and appropriate bellow shields, the receiver is cleaned with soft cotton cloth and desalinated water. Furthermore at every day of measurement the lamps and the mirrors are cleaned. After firing the lamps during a 30 minutes waiting period the flux from the lamps stabilise. Consequently the measurement period of 10 minutes duration begins. The calculated water enthalpy increase Pcoll is the mean enthalpy increase of the 10 minutes measurement period.

Repeatability of the measurement setup including mounting is in the order of ± 0.2 % (1σ).

DLR uses the reference receiver DLR 70-1 for receivers of 70 mm diameter and 4060 mm length. The reference is measured at the same day as the sample is measured. Based on published data of glass envelope transmittance and absorber coating absorptance and measured aperture length, the optical efficiency of the reference is estimated to (84 ± 2) %.

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