IEC TS 62862-3-3:2020

IEC TS 62862-3-3 ®
Edition 1.0 2020-02
TECHNICAL
SPECIFICATION
colour
inside
Solar thermal electric plants –
Part 3-3: Systems and components – General requirements and test methods
for solar receivers
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IEC TS 62862-3-3 ®
Edition 1.0 2020-02
TECHNICAL
SPECIFICATION
colour
inside
Solar thermal electric plants –

Part 3-3: Systems and components – General requirements and test methods

for solar receivers
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-7784-3

– 2 – IEC TS 62862-3-3:2020 © IEC 2020
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms, definitions, symbols and units . 9
4 Performance test of the receiver . 9
4.1 General . 9
4.2 Identification and geometry . 9
4.3 Manufacturer's instructions . 9
4.4 Calibration of testing instrumentation . 9
4.5 Heat loss test . 9
4.5.1 General . 9
4.5.2 Objective . 10
4.5.3 Receiver tube setup and location . 10
4.5.4 Inspection . 11
4.5.5 Test methodology – Resistance heating method . 11
4.5.6 Test methodology – Joule effect method . 15
4.5.7 Thermal emittance (optional) . 21
4.5.8 Heat loss and emittance curve models . 22
4.5.9 Test report . 24
4.6 Optical characterization test . 24
4.6.1 General . 24
4.6.2 Objective . 24
4.6.3 Method of non-destructive optical characterization (optional) . 27
4.6.4 Optical efficiency test – A transient method (optional) . 28
4.6.5 Optical efficiency test (optional) . 31
4.7 Durability tests for the receiver . 34
4.7.1 General . 34
4.7.2 Antireflective glass envelope coating durability tests . 34
4.7.3 Impact resistance test (optional) . 37
4.7.4 External and/or internal thermal shock test . 40
4.7.5 Thermal stability test of absorber selective coatings for full receiver
tube . 40
4.7.6 Thermal stability of selective absorber coatings for coated stainless-
steel samples . 41
4.7.7 Thermal cycling test . 43
4.7.8 Bellows test . 44
Annex A (informative) Test report form . 48
A.1 Description of receiver tube (supplied by the manufacturer) . 48
A.1.1 General specifications . 48
A.1.2 Size and construction parameters of the receiver tube tested . 48
A.1.3 Optical and thermal parameters (nominal temperature) . 48
A.1.4 Operating parameters . 48
A.1.5 Interfaces . 49
A.2 Test results – Heat loss . 49
A.2.1 Details of test SETUP . 49
A.2.2 Heat loss test . 49

A.2.3 Linear fit of heat loss results to absorber temperature . 50
A.2.4 Linear fit of emittance data to absorber temperature . 50
A.3 Test results, optical characterization test . 50
A.4 Test results, optical efficiency test . 51
A.4.1 General . 51
A.4.2 Details of test SETUP . 51
A.4.3 Solar simulator test (if relevant, depending on test method) . 51
A.5 Test results – Stationary abrasion resistance test . 51
A.5.1 Test conditions . 51
A.5.2 Results . 51
A.6 Test results – Condensation test . 51
A.7 Test results – Impact resistance test: Method 1 – Ice balls . 52
A.7.1 Test conditions . 52
A.7.2 Procedure . 52
A.7.3 Test results. 52
A.8 Test results – Impact resistance test: Method 2 – Steel balls . 52
A.8.1 Test conditions . 52
A.8.2 Procedure . 52
A.8.3 Test results. 52
A.9 Test results – Thermal stability test of absorber selective coating for full
receiver tube . 52
A.9.1 Test conditions . 52
A.9.2 Test results. 52
A.10 Test results – Thermal stability test of absorber selective coated stainless-
steel samples . 53
A.10.1 Instrumentation . 53
A.10.2 Test conditions . 53
A.10.3 Test results. 53
A.11 Test results – Thermal cycling test . 53
A.11.1 Test conditions . 53
A.11.2 Test results. 53
A.12 Test results – Bellows test . 53
A.12.1 Test conditions . 53
A.12.2 Test results. 53
Annex B (informative) Application notes: Temperature measurement in heat-loss test

with cartridge heater – Temperature measurement offset correction . 54
Bibliography . 55

Figure 1 – Solar receiver schematic sketch . 7
Figure 2 – Resistance heating method schematic sketch . 16
Figure 3 – Test bench for heat loss measurements of solar receiver tubes . 18
Figure 4 – Position of thermocouples and sections of voltage measurement . 19
Figure 5 – Example heat loss curve . 23
Figure 6 – Measurement principle . 27
Figure 7 – Test bench for optical efficiency – Transient method . 29
Figure 8 – Schematic of abrasion equipment . 35
Figure 9 – Drawing of receiver rotation in abrasion equipment for a tube or piece . 35
Figure 10 – Example of a test bench for testing bellows fatigue . 45

– 4 – IEC TS 62862-3-3:2020 © IEC 2020

Table 1 – Evaluation periods . 12
Table 2 – Stability requirements . 12
Table 3 – Evaluation periods . 19
Table 4 – Stability requirements . 20
Table 5 – Permitted deviation of measured parameters during a measurement period . 30
Table 7 – Ice ball test mass and speed . 38
Table 8 – Monitored parameters during waiting period . 41

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SOLAR THERMAL ELECTRIC PLANTS –

Part 3-3: Systems and components –
General requirements and test methods for solar receivers

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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6) All users should ensure that they have the latest edition of this publication.
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. In
exceptional circumstances, a technical committee may propose the publication of a Technical
Specification when
• the required support cannot be obtained for the publication of an International Standard,
despite repeated efforts, or
• the subject is still under technical development or where, for any other reason, there is the
future but no immediate possibility of an agreement on an International Standard.
Technical Specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC TS 62862-3-3, which is a Technical Specification, has been prepared by IEC technical
committee 117: Solar thermal electric plants.

– 6 – IEC TS 62862-3-3:2020 © IEC 2020
The text of this Technical Specification is based on the following documents:
Draft TS Report on voting
117/104/DTS 117/107/RVDTS
Full information on the voting for the approval of this Technical Specification can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 62862 series, published under the general title Solar thermal
electric plants, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
INTRODUCTION
The receiver is one of the most important and most sensitive components of Fresnel and
parabolic trough power plants. Large mirrors are aligned to concentrate solar radiation up to
80 times along the focal line of the mirrors onto the specially coated, evacuated receivers.
The generated heat is transported to a power generation unit, using a heat transfer fluid, and
converted to electricity.
The quality and long-term performance stability of the receiver has a decisive influence on
how effectively solar radiation can be converted into heat. For the power plant to achieve
maximum efficiency, the receiver has to absorb as much solar radiation as possible and
convert it into heat with minimized losses.
The solar receiver (see schematic in Figure 1) mainly consists of:
• a steel absorber tube: heat transfer fluid flows through the stainless-steel absorber tube. A
high-quality absorber coating converts the solar radiation into heat and minimizes infrared
heat loss at the same time;
• a glass cover tube: the cover is made from borosilicate glass and is coated with an
antireflective film to increase solar transmittance;
• evacuated space (annulus) or filled with noble gas between absorber tube and glass cover
tube: the vacuum between steel absorber and glass cover is essential to suppress gas
heat convection;
• bellows: the bellows are necessary to compensate for different rates of heat expansion of
the steel absorber and the glass cover. In contrast to the glass cover, the hot absorber
expands considerably when operating.

Figure 1 – Solar receiver schematic sketch

– 8 – IEC TS 62862-3-3:2020 © IEC 2020
SOLAR THERMAL ELECTRIC PLANTS –

Part 3-3: Systems and components –
General requirements and test methods for solar receivers

1 Scope
This document specifies the technical requirements, tests, durability and technical
performance parameters of solar thermal receivers for absorbing concentrated solar radiation
and transferring the heat to a fluid used in concentrated solar thermal power plants with
linear-focus solar collectors. The receivers addressed consist of an absorber tube and an
insulating glass envelope tube.
NOTE 1 Most of the test methods included in this document apply to solar receivers used both in solar thermal
electric plants with parabolic-trough and Fresnel collectors.
This document includes the definitions of technical properties and characterization of
geometry and performance parameters as well as the test methods for optical
characterization, heat loss, and durability.
NOTE 2 The experience accumulated so far regarding the different test methods currently available for receiver
tubes is not extensive enough to determine which test method is the best; this document describes all the different
methods currently available without defining one recommended method.
For the sake of clarity, it is stated here that the thermal loss tests described in this document
do not deliver the thermal loss of the receiver tubes when they are installed in commercial
solar fields.
Thermal losses obtained by indoor testing on a single receiver are significantly lower than the
thermal losses in outdoor, real operating conditions at commercial solar fields. However, the
indoor test procedures described in this document are suitable for receiver tube performance
comparison.
The thermal losses taken into account for solar field design are obtained by testing complete
collectors operating under real solar conditions.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC TS 62862-1-1, Solar thermal electric plants – Part 1-1: Terminology
ISO 6270-2:2017, Paints and varnishes – Determination of resistance to humidity – Part 2:
Condensation (in-cabinet exposure with heated water reservoir)
ISO 9806:2017, Solar energy – Solar thermal collectors – Test methods
ISO 9488:1999, Solar energy – Vocabulary
MIL-E-12397 – Eraser, Rubber-Pumice (for testing coated optical elements)

ASTM G173 – 03 – Standard Tables for Reference Solar Spectral Irradiances: Direct Normal
and Hemispherical on 37º Tilted Surface
3 Terms, definitions, symbols and units
For the purposes of this document, the terms, definitions, symbols and units contained in
ISO 9488 and IEC 62862-1-1 apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
4 Performance test of the receiver
4.1 General
As receivers are one of the most important components in the solar field, they have a big
impact on the performance of the entire solar field. In order to be able to best simulate the
lifetime performance of the receiver as well as that of the solar field, it is crucial to perform
tests that characterize the receiver and its performance.
4.2 Identification and geometry
The receiver usually has a brand name for the product and is also defined by the outer
diameter of the stainless-steel tube. Another important identifier/parameter is the length of the
receiver, which may vary depending on the trough for which it is designed. Additional
parameters such as absorptance, emittance, transmittance, vacuum pressure, stainless-steel
material, design temperature and pressure and the Heat Transfer Fluid for which it is
designed, can be obtained from the manufacturer. These parameters/characteristics should
be noted as part of the report for the receiver being tested.
4.3 Manufacturer's instructions
In addition to the parameters in 4.2, the manufacturer may have additional instructions for the
use/preparation of the receiver or parts thereof for testing, for example, the cleaning of
samples prior to carrying out optical measurements. These instructions shall be noted as part
of the test procedure in case they have an effect on the results.
4.4 Calibration of testing instrumentation
Unless otherwise indicated by the manufacturer of the testing device, all instruments used
should be calibrated at least once a year. In the event that a device is used that requires
special calibration (i.e. spectrophotometer requiring calibration using a "golden sample"), this
should be noted in the report including the date of calibration and the specimen used.
4.5 Heat loss test
4.5.1 General
NOTE In 2016, a round robin test was carried out within the European project STAGE-STE (European Union
Seventh Framework Program FP7 (2007-2013) under grant agreement ID 609837) with five different tubes from
different manufacturers. From this round robin, the heat loss testing results showed standard deviations in the
order of 6 % to 12 % for most temperatures and receivers (see [1] ).
____________
Numbers in square brackets refer to the Bibliography.

– 10 – IEC TS 62862-3-3:2020 © IEC 2020
These differences should be considered when comparing results by different laboratories
using different methodologies. It is recommended when trying to compare receivers from
different manufacturers to test all receivers in the same method at the same institute, to get
comparable results.
For the thermal loss test, one of the following methods shall be used (both are described
below):
• resistance heating method;
• Joule effect method.
4.5.2 Objective
The purpose of this test is the thermal characterization of a solar receiver tube including
determination of the heat loss curve and calculation of thermal emittance (optional) based on
test data.
The application of this test is associated with the needs of solar thermal power plant projects
using parabolic-trough technology and, by analogy, also Fresnel technology. It is applicable to
solar receiver tubes forming part of a parabolic-trough collector or a Fresnel collector.
4.5.3 Receiver tube setup and location
4.5.3.1 General
The way in which the receiver tube is set up on the test bench has a determining influence on
the heat loss test results, so it is therefore recommended that the test bench is set up as
described in the following subclauses of 4.5.3. The tests shall be performed indoors to
minimize any possible environmental influence on the test samples. In addition, the space
around the receiver, equal to at least 50 cm on all sides, should be free from big obstacles to
avoid limiting natural air circulation.
In the event that the test is carried out under vacuum conditions, the surroundings are not
important as air circulation is negligible.
The general principle of measurement is based on the conversion of electricity into heat under
stationary conditions. Under these conditions, the heat loss is equivalent to the power
necessary to maintain the receiver at a constant temperature. Determination of the power
required at different temperatures leads to the heat loss curve characteristic of the receiver
test sample.
There are different methods for heating the receiver. For example, by Joule effect, by
electrical heated elements or by IR resistance attached to a copper tube inserted inside the
absorber tube (see example in [2]).
4.5.3.2 Receiver tube setup and initial inspection
The receiver tube to be tested shall be placed horizontally in the test support frame holders.
The absorber tube shall be visually inspected, and any damage or modification observed shall
be recorded in the test report.
4.5.3.3 Temperature measurements
The receiver tube temperature shall be measured with temperature sensors touching the
absorber from the inside in at least six positions along it, arranged symmetrically from the
centre, with a gap of no more than 1 m between sensors. Additional sensors shall be located
near the ends of the tube and be in contact with the tube, in order to control gradients towards
the ends. Three sensors are recommended for measuring the glass surface temperature.

The ambient temperature sensor shall be placed not more than 2 m from the sample, in a
location where it is not affected by hot spots or air currents. The positions of the sensors shall
be recorded in the report.
The temperature sensors shall be calibrated according to the temperature range to be tested.
The expanded uncertainties of the average temperatures of each sensor shall be less than:
• ±2 °C for the absorber tube;
• ±2 °C for the glass envelope tube;
• ±1 °C for ambient or surrounding air.
Uncertainty calculations are determined in accordance with ISO 9806:2017, Annex D.
The contact shall be suitable to ensure correct measurement. Correction methods for
compensating the influence temperature gradients near the sensors might be applicable.
Pressing temperature sensors on the absorber or glass often leads to inadequate
measurements as temperature sensors are influenced by air temperature and radiation
temperature in the annulus and heat conduction along the thermocouple wires. Reference
measurements can be used to ensure the validity of the measurement setup or to correct the
measurements, see Annex B. The temperature corrections, if done, shall be noted in the test
report.
4.5.3.4 Power measurements
The measurement equipment for electric heating power shall have an accuracy of at least 3 %
of the actual reading.
4.5.3.5 Data acquisition frequency
Power sensors used in the test shall be calibrated. Data recording frequency shall be no less
than 1 recording per 20 s and the minimum number of records shall ensure the statistical
representativeness of the test.
4.5.4 Inspection
When the test has finished, the receiver tube shall be inspected and note taken of any change
observed. Changes observed shall be recorded in the test report; photographs can be added.
4.5.5 Test methodology – Resistance heating method
4.5.5.1 Measurements
At least the following data shall be measured.
Before the test:
• the length of the metal receiver absorber tube measured at ambient temperature using a
measuring instrument (e.g. measuring tape) with an uncertainty of 1 mm. The instrument
should be inserted into the tube and measured from one end to the other;
• diameter (inner and outer) of the absorber tube measured at ambient temperature using a
caliper with an accuracy of at least 1/10 mm;
• position of temperature sensors using a measuring instrument (e.g. measuring tape) with
an uncertainty of at least 1 mm, with reference to a specific end/position on the absorber
tube.
– 12 – IEC TS 62862-3-3:2020 © IEC 2020
During the test:
• temperatures of the absorber tube;
• temperatures of the glass envelope tube (optional);
• temperature of the surrounding air;
• electrical power supplied by the resistance heaters or any other element used.
4.5.5.2 Procedure and test time
The measurement is performed at a steady state of stable absorber temperature and heating
power. Therefore, the following arithmetic means of evaluation of the measured quantities
over a longer evaluation period, which will also be called measurement point, are used.
Minimum evaluation periods are given in Table 1.
Table 1 – Evaluation periods
Absorber tube temperature in °C Minimum evaluation period in min
100 to 200 240
200 to 300 120
300 to 400 60
400 to 500 30
> 500 15
Table 2 shows the criteria for stability and homogeneity that shall be met for the duration of
the evaluation period.
Table 2 – Stability requirements
Parameter to be monitored Stability requirement
Absorber tube temperatures ±0,5 ºC
Absorber tube temperature homogeneity S < 0,04 T
TH abs,mean,°C
Heat loss (Equation (2)) ±1 %
Ambient temperature 20 °C ± 10 °C

Criteria in Table 2 refer to simple moving means (without weights) over 1 min for the
quantities of interest. The criterion for absorber tube temperatures refers to the temporal
stability for each temperature sensor.
Absorber tube temperature homogeneity S (t) at time, t, is defined as the difference of the
TH
highest and smallest measured temperature at t divided by the mean temperature:
TT−
abs,max abs,min
(1)
S =
TH
T
abs,mean
The maximum difference measured during one measurement between the different absorber
temperatures along the tube is important as it may indicate the uniformity of the coating.
The value of uniformity of S should be mentioned in the report. In addition, if during the
TH
measurement it was found that S > 2 % a warning should be mentioned in the report.
TH
• Method to achieve steady state:
PI or PID controllers can be used to control electric power input to reach target temperature
and a steady state. Steady state is achieved when heater set points do not change and the
centre-of-glass and absorber temperatures vary by less than 0,5 °C for a period of at least
15 min.
• This criterion of uniformity shall be followed throughout the steady-state period.
Once the desired absorber tube test temperature is reached and the adiabaticity of the
process at the ends of the tube is verified, measurements for steady-state periods shall be
performed. A measurement steady-state period shall have a duration of 15 min during which
the stability conditions listed in Table 2 shall be verified. Before each steady-state period, a
30-min period shall verify the same stability conditions listed in Table 2.
4.5.5.3 Heat loss (HL) calculations
Calculations shall be done using measurement point means.
The uncertainty of measurement shall be calculated in accordance with ISO 9806:2017,
Annex D.
The coefficient of loss in a receiver tube is defined as:
Pow + HL
∑ i ends
i
(2)
HL =
L
HCE()25 °C
where
HL is the heat loss of the tube [W/m];
Pow is the electrical power consumed by heating element i [W];
i
HL is the heat loss at the tube ends [W].
ends
Example of a test bench with a copper tube as heater type:
For this type of test bench, only the ends are insulated; the heat loss of the ends should be
calculated, for example, in the following way:
kA kA
HL (T ––T )+ (T T )
(3)
ends 1 2 NN –1
∆∆xx
where
T indicates the temperatures measured by sensors at the ends [°C] assuming N
1/2/N/N-1
sensors. In the case of a test bench based on electrical heating elements, the
sensors at the ends of the copper tube would be taken;
k is the coefficient of conductivity of copper [W/m °C];
A is the surface area of copper tube (heater) [m ];
∆x is the distance between sensors at ends [m];
L is the length of receiver tube at ambient temperature (25 °C ± 10 °C) [m]. The
HCE(25 °C)
length of the receiver tube aperture shall be measured.
The mean temperature of the absorber is calculated by weighting the distance that each
sensor covers along the absorber tube:
=
– 14 – IEC TS 62862-3-3:2020 © IEC 2020
Tp
∑ abs i
i
i
T =
[°C] (4)
abs
p
i
∑
i
where
T
is the absorber tube temperature measured by sensor i [°C];
abs
i
p
is the weight applied [-].
i
The mean temperature of the glass envelope tube is:
Tp
∑ gl,o i
i
i
T =
(5)
gl,o
p
∑ i
i
where
is the outer temperature of the glass envelope tube measured by sensor i [°C];
T
gl,o
i
p
is the weight applied [-].
i
Weights used to find the absorber and glass temperatures are calculated by the following
equations:
A
i
P = (6)
i
A
abs
where
A d × l , RT (7)
abs abs abs
is the area of the absorber at room temperature and A is that fraction of the absorber area for
i
which the temperature sensor i is the closest sensor.
Optional: In general the result of a measurement point is a pair of heat loss HL and mean
absorber temperatures, T . Both have associated uncertainties u(HL) and u(T ).
abs abs
Interpreting heat loss HL as a function of temperature T it is beneficial to have only HL
abs
associated with uncertainty. Uncertainty of temperature u(T ) and uncertainty of heat loss
abs
u(HL) can be merged to form a combined uncertainty of heat loss u (HL) by:
c
 ∂HL T 
( )
22 abs 2
(8)
u (HL) u (HL)+ × u (T )
 
c abs
 
∂T
abs
 
The partial derivative can be determined from the interpolation polynomials,
∂∂HL T / T
( )
abs abs
for example.
4.5.5.4 Thermal emittance calculation (optional)
Emittance can only be calculated if the annulus between the steel tube and glass cover is
evacuated (not filled with noble gas).
=
=
Emittance is calculated (optional) in several steps starting with the test results.
First the outside absorber tube temperature is calculated:
 
r
abs,o
HL ln  
 
r
abs,i
 
TT −
(9)
abs,o abs,i
2π k
abs
where
HL is heat loss measured during the test at a given T [W/m];
abs
r is the outer radius of the absorber tube [m];
abs,o
r is the inner radius of the absorber tube [m];
abs,I
k is the thermal conductivity of the absorber [m/°C] – to be received from
abs
manufacturer;
T is the inner absorber temperature [°C] (also called T ).
abs,i abs
NOTE The radii, r and r are supplied by the manufacturer, as are those for the glass tube in Formula (10).
abs,i abs,o
Second, the temperature inside the glass envelope tube is calculated:
 r 
gl,o
HL ln  
 
r
gl,i
 
TT +
(10)
gl,,iogl
2π k
gl
where
r is the outer radius of the glass envelope tube [m];
gl,o
is the inner radius of the glass envelope tube [m];
r
gl,i
k is the thermal conductivity of glass [m/°C] – to be received from manufacturer;
gl
is the outer temperature of the glass envelope tube [°C].
T
gl,o
Finally, the absorber tube emittance is calculated using the Forristal equation (Equation (11)):
HL
ε =
abs
(11)
1− ε  
r
gl
abs,o
2πσr T −−T HL  
( )
abs,o abs,oK() gl,i()K
 
ε r
gl gl,i
 
where
ε glass emittance [-] (use value provided by manufacturer; if none provided, use 0,89);
gl
σ Stefan-Boltzman constant [].
4.5.6 Test methodology – Joule effect method
4.5.6.1 Testing apparatus schematic layout
The Joule effect heating method consists in using the steel tube itself as a heater in order to
provide the required power to bring the receiver to a certain temperature.
Its schematic layout is shown in Figure 2. The receiver ends are mechanically and electrically
connected with two pieces of well insulated steel tube (220 mm minimum length) with the
same outer diameter and thickness as the tube under test.
=
=
– 16 – IEC TS 62862-3-3:2020 © IEC 2020
The ends of these tube extensions are connected to a current (preferred) or voltage
generator. During the test the voltage differences in some critical points as well as the current
flowing into the circuit are measured in order to determine the exact amount of power
transferred to the tube in the different sections.

Figure 2 – Resistance heating method schematic sketch
4.5.6.2 Effective voltage measurements
The effective voltage on the receiver tube shall be measured with suitable probes touching
the absorber tube from the inside, where the steel tube is not accessible, and/or from the
outside in the other positions shown in Figure 2.
The effective voltage difference should at a minimum be measured in the following positions
using the low-voltage reference at one end of the testing equipment set-up: at point 1 in
Figure 2; close to the other current connection clamp at other end of the set-up (point 2); as
close as possible to both the receiver tube ends (points 3 and 4); at least in 3 other positions
along it (points 5 to 7), arranged symmetrically from the centre to evaluate the central HL
value.
The expanded uncertainties of each voltage sensor shall be less than ±1 mV.
Uncertainty calculations are determined in accordance with ISO 9806:2017, Annex D.
4.5.6.3 Current measurements
The current flowing into the receiver tube shall be measured with a suitable instrument to be
connected in series in the electric loop.
The expanded uncertainties of the current sensor shall be less than ±0,1 A.
Uncertainty calculations are determined in accordance with ISO 9806:2017, Annex D.
4.5.6.4 Power measurements
The equipment for measuring electric heating power (based on calculations from the voltage
and current measurements) shall have an accuracy of 2 % to 3 % of the actual reading.
4.5.6.5 Data acquisition frequency
Power sensors used in the test shall be calibrated. Data recording frequency shall be no less
than 1 recording per 20 s and the minimum number of records shall ensure the statistical
representativeness of the test.

4.5.6.6 Measurements
At least the following data shall be measured.
Before the test:
• the length of the metal receiver absorber tube measured at ambient temperature using a
measuring instrument (e.g. measuring tape) with an uncertainty of 1 mm. The instrument
should be inserted into the tube and measured from one end to the other;
• diameter (inner and outer) of the absorber tube, measured at ambient temperature using a
caliper with an acc
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