ASTM E905-87(2021)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E905 − 87 (Reapproved 2021)
Standard Test Method for
Determining Thermal Performance of Tracking
Concentrating Solar Collectors
This standard is issued under the fixed designation E905; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.7 Selection and preparation of the collector (sampling
method, preconditioning, mounting, alignment, etc.), calcula-
1.1 This test method covers the determination of thermal
tion of efficiency, and manipulation of the data generated
performanceoftrackingconcentratingsolarcollectorsthatheat
through use of this standard for rating purposes are beyond the
fluids for use in thermal systems.
scope of this test method, and are expected to be covered
1.2 This test method applies to one- or two-axis tracking
elsewhere.
reflecting concentrating collectors in which the fluid enters the
1.8 This test method does not provide a means of determin-
collectorthroughasingleinletandleavesthecollectorthrough
ing the durability or the reliability of any collector or compo-
a single outlet, and to those collectors where a single inlet and
nent.
outlet can be effectively provided, such as into parallel inlets
and outlets of multiple collector modules. 1.9 The values stated in SI units are to be regarded as
standard. The values given in parentheses after SI units are
1.3 This test method is intended for those collectors whose
provided for information only and are not considered standard.
design is such that the effects of diffuse irradiance on perfor-
1.10 This standard does not purport to address all of the
mance is negligible and whose performance can be character-
safety concerns, if any, associated with its use. It is the
ized in terms of direct irradiance.
responsibility of the user of this standard to establish appro-
NOTE 1—For purposes of clarification, this method shall apply to
priate safety, health, and environmental practices and deter-
collectors with a geometric concentration ratio of seven or greater.
mine the applicability of regulatory limitations prior to use.
1.4 The collector may be tested either as a thermal collec-
1.11 This international standard was developed in accor-
tion subsystem where the effects of tracking errors have been
dance with internationally recognized principles on standard-
essentially removed from the thermal performance, or as a
ization established in the Decision on Principles for the
system with the manufacturer-supplied tracking mechanism.
Development of International Standards, Guides and Recom-
1.4.1 The tests appear as follows:
mendations issued by the World Trade Organization Technical
Section
Barriers to Trade (TBT) Committee.
Linear Single-Axis Tracking Collectors Tested as
Thermal Collection Subsystems 11–13
System Testing of Linear Single-Axis Tracking Collectors 14–16 2. Referenced Documents
Linear Two-Axis Tracking and Point Focus Collectors
2.1 ASTM Standards:
Tested as Thermal Collection Subsystems 17–19
System Testing of Point Focus and Linear Two-Axis
E772Terminology of Solar Energy Conversion
Tracking Collectors 20–22
2.2 Other Standard:
1.5 This test method is not intended for and may not be
ASHRAE 93-86,Methods of Testing to Determine the
applicable to phase-change or thermosyphon collectors, to any
Thermal Performance of Solar Collectors
collector under operating conditions where phase-change
occurs, to fixed mirror-tracking receiver collectors, or to NOTE 2—Where conflicts exist between the content of these references
and this test method, this test method takes precedence.
central receivers.
NOTE3—Thedefinitionsanddescriptionsoftermsbelowsupersedeany
1.6 This test method is for outdoor testing only, under clear
conflicting definitions included in Terminology E772.
sky, quasi-steady state conditions.
1 2
This test method is under the jurisdiction of ASTM Committee E44 on Solar, For referenced ASTM standards, visit the ASTM website, www.astm.org, or
GeothermalandOtherAlternativeEnergySourcesandisthedirectresponsibilityof contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Subcommittee E44.20 on Optical Materials for Solar Applications. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Jan. 1, 2021. Published January 2021. Originally the ASTM website.
approved in 1982. Last previous edition approved in 2013 as E905 – 87 (2013). Available from the American Society of Heating, Refrigerating, and Air
DOI: 10.1520/E0905-87R21. Conditioning Engineers, Inc., 1791 Tullie Circle, N.E. Atlanta, GA 30329.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E905 − 87 (2021)
3. Terminology 3.2.8 rate of heat gain, n—the rate at which incident solar
energy is absorbed by the heat transfer fluid, defined math-
3.1 Definitions:
ematically by:
3.1.1 area, absorber, n—total uninsulated heat transfer sur-
˙
face area of the absorber, including unilluminated as well as Q 5 m˙C ∆t (1)
p a
illuminated portions. (E772)
3.2.9 response time, n—time required for ∆ t to decline to
a
10%ofitsinitialvalueafterthecollectoriscompletelyshaded
3.1.2 collector, point focus, n—concentrating collector that
from the sun’s rays; or the time required for ∆t to increase to
concentrates the solar flux to a point. (E772) a
90% of its value under quasi-steady state conditions after the
3.1.3 collector, tracking, n—solar collector that moves so as
shaded collector at equilibrium is exposed to irradiation.
to follow the apparent motion of the sun during the day,
3.2.10 quasi-steady state, n—refers to that state of the
rotating about one axis or two orthogonal axes. (E772)
collector when the flow rate and inlet fluid temperature are
3.1.4 concentration ratio, geometric, n—ratio of the collec-
constant but the exit temperature changes “gradually” due to
tor aperture area to the absorber area. (E772)
the normal change in solar irradiance that occurs with time for
clear sky conditions.
3.1.5 quasi-steady state, n—solar collector test conditions
3.2.10.1 Discussion—It is defined by a set of test conditions
when the flow rate, fluid inlet temperature, collector
described in 10.1.
temperature, solar irradiance, and the ambient environment
have stabilized to such an extent that these conditions may be
3.2.11 solar irradiance, direct, in the aperture plane,
considered essentially constant (see Section 8).
n—direct solar irradiance incident on a surface parallel to the
collector aperture plane.
3.1.6 Discussion—The exit fluid temperature will, under
these conditions, also be essentially constant (see ASHRAE 3.2.12 solar irradiance, total, n—total solar radiant energy
incident upon a unit surface area (in this standard, the aperture
93-86).
of the collector) per unit time, including the direct solar
3.2 Definitions of Terms Specific to This Standard:
irradiance, diffuse sky irradiance, and the solar radiant energy
3.2.1 altazimuthal tracking, n—continual automatic posi-
reflected from the foreground.
tioningofthecollectornormaltothesun’sraysinbothaltitude
3.2.13 thermal performance, n—rate of heat flow into the
and azimuth.
absorber fluid relative to the incident solar power on the plane
3.2.2 area, aperture (of a concentrating collector),
of the aperture for the specified test conditions.
n—maximum projected area of a solar collector module
3.3 Symbols:
through which the unconcentrated solar radiant energy is
2 2
A =collector aperture area, m (ft ).
a
admitted,includinganyareaofthereflectororrefractorshaded
2 2
A =absorber area, m (ft ).
abs
by the receiver and its supports and including gaps between
2 2
A =ineffective aperture area, m (ft ).
reflector segments within a module. (E772) 1
C=geometric concentration ratio A /A , dimensionless.
a abs
3.2.3 clear-sky conditions, n—refer to a minimum level of −1 −1
C =specific heat of the heat transfer fluid,J·kg ·° C
p
−2 −2
direct normal solar irradiance of 630 W · m (200 Btu · ft · −1 −1
(Btu · lb ·°F ).
−1
h ) and a variation in both the direct and total irradiance of
E =diffuse solar irradiance incident on the collector
s,d
less than 64% during the specified times before and during −2 −1 −2
aperture, W · m (Btu · h ·ft ).
each test.
E =direct solar irradiance in the plane of the collector
s,D
−2 −1 −2
3.2.4 end effects, n—inlinearsingle-axistrackingcollectors, aperture, W · m (Btu · h ·ft ).
the loss of collected energy at the ends of the linear absorber E =directsolarirradianceintheplanenormaltothesun,
s,DN
−2 −1 −2
when the direct solar rays incident on the collector make a W·m (Btu · h ·ft ).
non-zeroanglewithrespecttoaplaneperpendiculartotheaxis E =globalsolarirradianceincidentonahorizontalplane,
s,2π
2 −1 −2
of the collector. W·m (Btu · h ·ft ).
E =totalsolarirradianceincidentonthecollectoraperture,
s,t
3.2.5 fluid loop, n—assembly of piping, thermal control,
−2 −1 −2
W·m (Btu · h ·ft ).
pumping equipment and instrumentation used for conditioning
f=focal length, m (ft).
the heat transfer fluid and circulating it through the collector
g=spacing between the effective absorbing surfaces of
during the thermal performance tests.
adjacent modules, m (ft).
3.2.6 module, n—the smallest unit that would function as a
K=incident angle modifier, dimensionless.
solar energy collection device.
L=length of reflector segment, m (ft).
l =length of receiver that is unilluminated, m (ft).
3.2.7 near-normal incidence, n—angular range from exact
r
−1
m =mass flow rate of the heat transfer fluid, kg · s (lbm ·
normal incidence within which the deviations in thermal
−1
h ).
performance measured at ambient temperature do not exceed
−1
˙
Q =net rate of energy gain in the absorber, W (Btu · h ).
62%, such that the errors caused by testing at angles other
−1
˙
than exact normal incidence cannot be distinguished from Q =rate of energy loss, W (Btu · h ).
L
errors caused by other inaccuracies (that is, instrumentation r=overhang of the receiver past the end of the reflectors, m
errors, etc.). (ft).
E905 − 87 (2021)
R(θ)=ratio of the rate of heat gain to the solar power collector aperture due to decreased projected area (cosine
incident on the aperture, dimensionless. response) and other optical losses. The first effect is accounted
s=angle which the collector aperture is tilted from the for primarily in terms of the data generated for near-normal
horizontal to the equator, and is measured in a vertical N-S incidence thermal performance for a given collector. The
plane, degrees. cosineresponseportionofthesecondeffectisaccountedforby
t =ambient air temperature, °C (°F). the determination of the solar power incident on the plane of
amb
t =temperature difference across the absorber, inlet to the aperture. The departure of the optical response of the
∆ a
outlet, °C (°F). collector from the cosine response is determined by obtaining
t =temperature difference across the absorber inlet to the incident angle modifier data.The incident angle modifier is
∆ a,i
outlet at the time of initial quasi-steady state conditions, °C important in predicting such collector characteristics as all-day
(°F). thermal performance.
t =temperature difference across the absorber inlet to
∆ a,f
outlet at the time final quasi-steady state conditions are
5. Significance and Use
reached, °C (°F).
5.1 This test method is intended to provide test data essen-
t =temperature difference across the absorber inlet to
∆ a,T
tial to the prediction of the thermal performance of a collector
outlet at time T, °C (°F).
in a specific system application in a specific location. In
t =temperature of the heat transfer fluid at the inlet to the
f,i
addition to the collector test data, such prediction requires
collector, °C (°F).
validated collector and system performance simulation models
w=width of reflector segment, m (ft).
thatarenotprovidedbythistestmethod.Theresultsofthistest
β=solar altitude angle, degrees.
method therefore do not by themselves constitute a rating of
Γ(θ )=end effect factor, dimensionless.
||
the collector under test. Furthermore, it is not the intent of this
δ=solar declination, degrees.
test method to determine collector efficiency for comparison
θ=angle of incidence between the direct solar rays and the
purposes since efficiency should be determined for particular
normal to the collector aperture, degrees.
applications.
θ , θ = angles of incidence in planes parallel and
|| '
5.2 This test method relates collector thermal performance
perpendicular, respectively, to the longitudinal axis of the
to the direct solar irradiance as measured with a pyrheliometer
collector, degrees.
with an angular field of view between 5 and 6°. The prepon-
θ =maximum angle of incidence at which all rays incident
ι
derance of existing solar radiation data was collected with
on the aperture are redirected onto the receiver of the same
instruments of this type, and therefore is directly applicable to
module, degrees.
prediction of collector and system performance.
θ' =minimum angle of incidence at which radiation re-
c
flected from one module’s aperture is intercepted by the
5.3 This test method provides experimental procedures and
receiver of an adjacent module, degrees.
calculation procedures to determine the following clear sky,
φ=solar azimuth angle measured from the south, degrees.
quasi-steady state values for the solar collector:
5.3.1 Response time,
4. Summary of Test Method
5.3.2 Incident angle modifiers,
4.1 Thermal performance is the rate of heat gain of a
5.3.3 Near-normal incidence angular range, and
collectorrelativetothesolarpowerincidentontheplaneofthe
5.3.4 Rate of heat gain at near-normal incidence angles.
collector aperture. This test method contains procedures to
NOTE 4—Not all of these values are determined for all collectors. Table
measure the thermal performance of a collector for certain
1 outlines the tests required for each collector type and tracking arrange-
well-defined test conditions. The procedures determine the
ment.
opticalresponseofthecollectorforvariousanglesofincidence
5.4 This test method may be used to evaluate the thermal
ofsolarradiation,andthethermalperformanceofthecollector
performance of either (1) a complete system, including the
at various operating temperatures for the condition of maxi-
tracking subsystems and the thermal collection subsystem, or
mum optical response. The test method requires quasi-steady
(2) the thermal collection subsystem.
state conditions, measurement of environmental parameters,
5.4.1 Whenthistestmethodisusedtoevaluatethecomplete
and determination of the fluid mass flow rate-specific heat
system, the test shall be performed with the manufacturer’s
product and temperature difference, ∆t , of the heat transfer
a
tracker and associated controls, and thus the effects of tracking
fluid between the inlet and outlet of the collector. These
error on thermal performance will be included in the results.
quantitiesdeterminetherateofheatgain, m˙C ∆t ,forthesolar
p a
Linear single-axis tracking systems may be supplemented with
irradiance condition encountered. The solar power incident on
the test laboratory’s tracking equipment to effect a two-axis
the collector is determined by the collector area, its angle
tracking arrangement.
relative to the sun, and the irradiance measured during the test.
5.4.2 When evaluating a thermal collection subsystem, the
4.2 Two types of optical effects are significant in determin-
accuracyofthetrackingequipmentshallbemaintainedaccord-
ing the thermal performance: (1) misalignment of the focal
ing to the restrictions in 10.3.
zone with respect to the receiver due to tracking errors and
errors in the redirection of the irradiance intercepted by the 5.5 This test method is to be completed at a single appro-
collector, and (2) changes in the solar power incident on the priate flowrate. For collectors designed to operate at variable
E905 − 87 (2021)
TABLE 1 Required Tests for Each Collector and Tracking Arrangement
Test Method
Determination of
Determination of
Near-Normal Inci-
Incident Near-Normal Inci- Heat Gain at
Collector Type and Test Configuration
Response dence Angular
Angle Mod- dence (NNI) for Near-Normal
Time Range for Rate
ifier Tracking Accuracy Incidence
of Heat Gain at
Requirements
NNI
Linear Single-Axis Tracking Subsystem:
One-axis Tracking
Manufacturer’s × × × × ×
Laboratory’s × × × ** ×
Two-Axis Tracking
Manufacturer’s and Laboratory’s × × × ×
Laboratory’s only × × ** ×
Linear Single-Axis Tracking System:
One-Axis Tracking
Manufacturer’s only × × × ×
Two-Axis Tracking
Manufacturer’s and Laboratory’s × × ×
Linear Two-Axis Tracking and
Point Focus Subsystem:
Manufacturer’s × ^ ×
Laboratory’s × ×
Linear Two-Axis Tracking and Point Focus
System:
Manufacturer’s only × ×
× = Required.
^ = Required but method may not be practicable for point focus collectors—Safety precautions and technical precautions must be followed because of potential damage
to equipment and subsequent damage to personnel due to high levels of solar irradiance on the receiver support structure.
** = Optional test that may provide useful information on the effect of the accuracy of the manufacturer’s tracking equipment on thermal performance.
flowrates to achieve controlled outlet temperatures, the collec- 5.7.1.1 Increased or decreased reflectance, transmittance,
tor performance shall be characterized by repeating this test and absorptance at the concentrator and receiver surfaces, or
method in its entirety for more than one flowrate. These
5.7.1.2 Increased or decreased interception of the reflected
flowratesshouldbetypicaloftheactualoperatingconditionsof
or refracted solar radiant energy by the receiver.
the collectors.
5.7.1.3 That part of the decreased interception that is due to
5.6 The response time is determined to establish the time loss of collected energy at the ends of the absorber can be
required for quasi-steady state conditions to exist before each
calculated analytically from the collector geometry as an end
thermal performance test to assure valid test data, and to effects factor (see Appendix X1).
determine the length of time over which the quasi-steady state
5.7.2 The preferred procedure for determining the incident
performance is averaged.The response time is calculated from
angle modifier minimizes heat loss from the receiver by
transient temperature data resulting from step changes in
requiring that the working heat transfer fluid be the same as is
intercepted solar irradiance with a given flow rate. Initial
used in the rest of the test method, and that it be maintained at
quasi-steady state conditions are established, the irradiance
an inlet temperature approximately equal to ambient tempera-
level is then increased or decreased suddenly, and the final
ture. It is realized, however, that this procedure may not be
quasi-steady state conditions are established. For most collec-
practical to perform as specified, since some heat transfer oils
tors covered by this test method, the difference in the response
become too viscous near ambient temperatures to be pumped
timedeterminedbyeachofthetwoprocedureswillbesmallin
through the fluid test loop, or the fluid test loop cannot
terms of actual time. It is recognized that for some collectors,
practicably cool the working fluid sufficiently to approximate
particularly those with long fluid residence times, the differ-
the ambient temperatures that typically occur in the winter in
enceinthetwovaluesofresponsetimemaybelarge.However,
cold climates. In these cases, eitherAlternative ProcedureAor
the difference has not been found to influence the remainder of
Bmaybeusedatthediscretionofthemanufacturerorsupplier.
the test method.
Alternative Procedure A uses water as the working fluid at an
5.7 The incident angle modifier is measured for linear inlet temperature approximately equal to ambient to minimize
single-axis tracking collectors so that the thermal performance heat losses, but the procedure requires careful cleaning of the
at arbitrary angles of incidence can be predicted from the collector fluid passages, possibly use of a separate fluid test
thermal performance measured at near-normal incidence as loop, and may cause corrosion if the collector fluid passages
required in this test method. This is necessary because, during are incompatible with water.Alternative Procedure B uses the
actual daily operation, linear single-axis tracking collectors sameheattransferfluidasisusedintherestofthetestmethod,
will usually be normal to the sun only once or twice. but at an elevated temperature which is as close as practicable
5.7.1 At non-zero angles of incidence, the thermal perfor- toambient.AlternativeProcedureBinvolveshigherheatlosses
manceofalinearsingle-axistrackingcollectormaychangefor from the receiver which must be calculated and corrected for.
several reasons: An approximate correction for these heat losses is obtained in
E905 − 87 (2021)
Alternative Procedure B by determining the nonirradiated heat liometer on a separate sun-tracking mount. The opening angle
loss for the same fluid inlet temperature. oftheinstrument’sfield-of-viewshallbebetween5°arcand6°
arc. The instrument shall be a secondary reference or field use
5.8 Determination of the angular range of near-normal
pyrheliometer whose calibration is directly traceable to a
incidence is required to establish the test conditions under
primary reference pyrheliometer. Only the WRR scale is
which the measured thermal performance will adequately
permitted; in no case shall the IPS 1956 or other radiometric
represent the thermal performance at true normal incidence.
scalebeused.Theinstrumentshallberecalibratedatnogreater
NOTE 5—Measurement of angular range of the near-normal incidence
than six month intervals. After calibration, the instrument and
also provides data that can be used to evaluate the sensitivity of the
associated readout electronics shall be accurate to 61.0% of
thermal performance of the tracking accuracy.
the measured value. This accuracy may be met through
5.9 The thermal performance of the solar collector is deter-
application of correction factors for temperature and linearity,
mined under clear sky conditions and at near-normal incidence
if appropriate. The pointing error of the associated tracking
becausetheseconditionsarereproducibleandleadtorelatively
mount shall not degrade the accuracy of the direct component
stable performance.
measurement more than 0.5%.
6. Interferences
7.1.1 The global solar irradiance shall be measured using a
6.1 Alignment error, tracker pointing error, and the distort-
pyranometer mounted in a horizontal orientation with the
ing effects of wind and gravity on the reflector and receiver
detector surface leveled. The instrument location shall be free
may contribute to decreased thermal performance by decreas-
from obstruction or enhancement of solar radiation due to
ing the fraction of solar radiation incident on the collector
nearby structures.The instrument may be a reference or a field
aperture that strikes the absorber. The degree to which these
use pyranometer, but its calibration shall be directly traceable
errors affect collector thermal performance depends on the
to a primary reference pyrheliometer. Only the WRR scale is
incident angle to the collector and the limits of the tracker,
permitted. The instrument shall be recalibrated at no greater
collector position and orientation relative to wind direction,
than six-month intervals. After calibration, the instrument and
wind speed, structural integrity of the collector and its support
its associated readout electronics shall be accurate to 62.0%
system, and so forth. Warping and sagging of the reflector due
of the measured value. This accuracy may be met through
to heat have been observed, particularly in the case of linear
application of correction factors for temperature, linearity, and
trough concentrating collectors, also causing a decrease in the
cosine response, if appropriate.
ability of the concentrator to direct the incident solar radiation
7.1.2 It is also recommended that total irradiance be mea-
to the absorber. Thermal expansion of the receiver may also
sured in the plane of the aperture with a pyranometer mounted
occur under operating conditions of concentrated solar energy,
to the collector on a suitable part of the tracking mechanism
and could cause damage to the receiver or the seals, possibly
such that the total irradiance measured is indicative of that to
resulting in increased heat losses.
which the collector is exposed.The pyranometer and its mount
6.2 Soiling of the collector surfaces (reflector/refractor,
shall not shade or block the collector. The instrument may be
absorber cover, etc.) may effectively reduce the solar energy
a reference or a field use pyranometer, but its calibration shall
available to the collector, in a way that is neither quantifiable
be directly traceable to a primary reference pyrheliometer.
nor reproducible.
Only the WRR scale is permitted. The instrument shall be
6.3 Small variations in the level of solar irradiance during
recalibrated at no greater than six-month intervals. After
testing may cause considerable difficulties in maintaining
calibration, the instrument and its associated readout electron-
quasi-steady state as required in 10.1.
ics shall be accurate to 62.0% of the measured value. This
6.4 Variations in the quality of the direct irradiance, com-
accuracy may be met through the application of correction
prising solar and circumsolar radiation, may give rise to
factors for temperature, linearity, cosine response, and tilt, if
irreduciblefluctuationsinthethermalperformancebecausethe
appropriate.
angular responses of the collector and of the pyrheliometer
7.2 (m˙C ), Product Determination—The determination of
differ.Thewideavailabilityofstandardpyrheliometersandthe p
the (m˙C )-productfortheheattransferfluidshallbeaccurateto
difficulty of making custom instruments make it impractical to p
62.0% for each data point. This requirement holds whether
test each collector relative to a pyrheliometer with the same
the mass flow rate and specific heat are determined separately,
angular response as the collector.
or their product is determined using a reference heat source or
6.5 Variations in the level of diffuse irradiance may affect
other technique. The fluid temperature to be used in each
the measured thermal performance, particularly for lower
determination shall be the average of the fluid temperature at
concentration ratio collectors. Therefore total (global) solar
the inlet and outlet of the collector.
irradiance measurements are to be made to indicate the
conditions under which the tests are performed, and to allow
7.3 Temperature and temperature difference measurements
comparisons to be made with available meteorological data.
shall be made in accordance with ASHRAE 93 and meet or
exceed its requirements for accuracy and precision.
7. Apparatus
7.4 All angular measurements except measurement of wind
7.1 Solar Irradiance Instrumentation—The direct compo-
nent of the solar irradiance shall be measured using a pyrhe- direction shall be accurate to within 60.1°.
E905 − 87 (2021)
7.5 Any tracking system other than the manufacturer’s stowed so that solar radiation is still incident on the collector
tracker used by the test lab shall limit the aperture normal aperture and at some point is focused on a part of the receiver
tracking error to 0.1° in all principal tracking axes required by support structure, for example.
the collector. 8.2.2 Damage to the tracker and any piping, wires, etc.
attached to the collector may occur in attempting to achieve
7.6 Irrespective of the means of collecting data for the
certain angles of incidence during testing, if precautions have
determination of thermal performance (see 7.7) irradiance and
not been taken to stay within the collector’s operational limits.
fluid temperature shall be monitored at not greater than 10-s
8.2.3 Most concentrating solar collectors require very
intervals such that variations in irradiance and fluid tempera-
steady irradiance in order to maintain quasi-steady state
ture stability can be assessed during all periods of quasi-steady
conditions. Therefore, a two-axis tracking arrangement is
state, before and during testing.
preferred for testing, such that the collector is constantly
7.7 A data point for any variable shall be the average of at
directed at the sun for near-normal incidence testing, or is
least10observationstakenatintervals(scanrate)ofnogreater
maintained at a given angle of incidence, unless such posi-
than 30 s. Each data point must meet all the requirements for
tioning would subject the collector to conditions for which it
quasi-steady state conditions, as listed in 10.1, where the
was not designed. (Such conditions must be specified by the
allowable variation in any variable refers to the difference
manufacturer.) The testing laboratory’s tracking devices may
between the maximum and minimum observed values.
be used to supplement the collector’s tracking mechanism to
achieve two-axis tracking. If a two-axis tracking arrangement
8. Precautions
is not used, then the collector shall be allowed to track
normally.Atwo-axistrackingarrangementmayberequiredfor
8.1 Safety Precautions—Potential hazards in operating con-
testingcollectorswithlongresponsetimesinordertomaintain
centrating solar collectors include high pressures and high
quasi-steady state conditions.
temperatures; toxic, flammable, and combustible materials;
mechanical and electrical equipment; and concentrated solar
9. Preparation of Apparatus
radiation.
8.1.1 Pressurized fluids can be released if a rupture occurs 9.1 The collector shall be installed and aligned properly
orifareliefvalveopens.Flashingoftheheattransferfluidmay according to a test method approved by the manufacturer.
occur. Inspection for leaks and any potential hazards should be
9.2 Collector surfaces exposed to the environment shall be
conducted frequently.
cleaned at the beginning of each test day according to the
8.1.2 Cautionshouldbeexercisedagainstaccidentalcontact
manufacturer’s recommended procedures. The test method
or exposure to components with elevated temperature. Protec-
used for cleaning shall be reported in full.
tiveglovesshouldbewornwhentouchinganyheatedsurfaces,
9.3 Thegeographicallocation(latitudeandlongitude)ofthe
including valves which are subject to being heated.
collector shall be determined and reported to an accuracy of
8.1.3 Materials soaked with heat transfer oils are a potential
60.1°.Where applicable, the orientation of any fixed collector
fire hazard and may even undergo spontaneous combustion
axis shall be measured to an accuracy of 60.1% and reported.
whenexposedtotemperaturesbelowtheflashpointofthefluid
9.4 The pyrheliometer and pyranometer shall be inspected
(approximately 150°C for some oils). These fluids should be
cleaned up immediately should a spill occur, and the materials at the beginning of each day at which time the outer glass
surface shall be cleaned and dried if dirt or moisture are
properly disposed of. Chemicals used for fluid treatment or for
present. Any evidence of moisture or debris in the interior of
solvents have potentially toxic effects. Gloves, eye protection,
the instrument shall be cause to remove it from service.
and aprons should be worn when handling these chemicals.
8.1.4 Moving elements associated with collector tracking
9.5 Thepyrheliometertrackershallbecheckedandadjusted
may pose entanglement hazards while the collector is under
for proper alignment periodically throughout the test day.
test. If necessary, considerations should be given to shielding
these moving elements and providing safety override/controls
10. Test Conditions
interlocks. General precautions applicable to the operation of
10.1 Since measurements for determining the rate of heat
electrical systems should be followed.
gain are not made simultaneously at the inlet and outlet of the
8.1.5 High levels of solar radiation that exist during collec-
collector and hence not on the same element of fluid, quasi-
tor testing present a high-temperature hazard to exposed skin
steady state conditions are required to ensure valid results.
and also an intense light hazard to the eyes. Therefore,
Except where noted, these conditions must exist for a time
concentrated solar radiation should be avoided whenever
period equal to two times the response time before each test,
possible.Whenmaintenanceisrequiredonthereflectorsideof
and for the duration of each test, which shall be the longer of
the collector, the collector should be positioned so that the
5 min or one-half the response time. Quasi-steady state
reflective surface is shadowed.
conditionswillbesaidtoexistwhentherequirementsin10.1.1
8.2 Technical Precautions: through 10.1.6 are met.
8.2.1 Damage to equipment can occur very quickly if for 10.1.1 Inlet temperature to the collector, t , shall vary less
f,i
any reason concentrated solar radiation is focused on parts of than 60.2°C (60.4°F) or 61.0% of the value of ∆ t ,
a
the collector other than the receiver. This may occur when the whicheverislarger,duringthespecifiedtimebeforeandduring
collectorisnottrackinginnormaloperation,butisnotproperly each test.
E905 − 87 (2021)
10.1.2 The temperature difference between the inlet and the the linear single-axis tracking collection subsystem, under
outlet to the collector, ∆t , shall vary less than 60.4°C clear-sky, quasi-steady state conditions. In addition, determi-
a
(60.8°F) or 64% of the value of ∆t , whichever is larger, nation of the near-normal incidence angular range may be
a
during the specified times before and during each test. required, depending on the tracking system used (see Table 1).
10.1.3 The measured value of the (m˙C )-product shall vary
p
12.2 Either the test laboratory’s tracking system or a track-
less than 61.0% during the specified times before and during
ing system supplied to the test laboratory for the purpose of
each test.
testing the collector (herein called “manufacturer’s tracker”)
10.1.4 The variation in both the direct and global irradiance
may be used to move the collector about its normal tracking
shall be less than 64% during the specified times before and
axis, but the tracking accuracy must be maintained according
during each test.
to the requirements in 7.5 and 10.3.
10.1.5 The maximum allowable variation in ambient tem-
perature for quasi-steady state conditions shall be 62.0°C
13. Procedure
(3.6°F).
13.1 Response Time—In either of the following alternative
10.1.6 Average wind speed across the collector shall be less
−1
procedures for measuring the response time, the heat transfer
than 4.5 m · s (10 mph) throughout the quasi-steady state
fluid used shall be the same as that used to measure the rate of
conditions, unless it can be shown that the effects of winds in
heat gain at near-normal incidence (Section 13.5).
excess of this requirement are indistinguishable from other
13.1.1 Procedure A—Theresponsetimeshallbedetermined
measurement inaccuracies.
by shading an irradiated collector as follows:
10.2 Minimum direct normal solar irradiance averaged over
13.1.1.1 Adjust the inlet temperature of the heat transfer
−2 −1 −2
each test period shall be 630 W · m (200 Btu· h ·ft ), and
fluid, t , to within 610.0°C (618.0°F) of the ambient
f,i
the difference between the maximum and minimum irradiance
temperature, or to the lowest possible operating temperature,
−2
values shall be less than 200 W· m .
whicheverishigher,whilecirculatingthetransferfluidthrough
NOTE 6—Since the thermal performance of some concentrating collec-
the collector at the flow rate specified and maintaining quasi-
tors is sensitive to the level of solar irradiance, it may be desirable to
steady state conditions as specified in 10.1. While maintaining
repeat the “Rate of Heat Gain at Near-Normal Incidence” test (see 13.5)
themassflowrateandmeasuringthetemperaturedifferenceof
at more than one range of irradiance values in order to fully characterize
the heat transfer fluid between the inlet and outlet to the
thecollector.Ifthisisdone,theminimumlevelofirradiancemaybelower
−2 −1 −2
than 630 W · m (200 Btu · h ·ft ), as long as all other quasi-steady
collector, abruptly reduce the incident solar energy to approxi-
state conditions are met. The difference between the maximum and
mately zero by shielding the collector from the sun. This may
minimum values of irradiance for testing at each desired level of
be accomplished by stowing the collector face down; by
irradiance may need to be further restricted if testing is done at more than
turningthecollectorawayfromthesun(onamovablemount);
one level.
shading the collector with a white, opaque cover; intercepting
10.3 When evaluating a thermal collection subsystem using
the reflected radiation; or defocusing the collector so that the
any manufacturer’s tracking equipment, the tracking accuracy
reflected radiation is no longer incident on the receiver. If a
of such equipment shall be maintained such that the tracking
cover is used, it should be suspended off the surface of the
error is shown to be less than the error allowed by the
collector so that ambient air is allowed to pass over the
near-normal incidence tracking accuracy requirement. This
collectoraspriortothebeginningofthetransienttest,andcare
requires that the procedure in 13.4 be followed, and that the
should be taken to avoid excessive temperature. Turning the
tracking errors of the collector during testing be measured and
collector shall not alter or interrupt the operation of the
reported.Thedeviceusedtomeasurethetrackingerrorshallbe
collector in any manner (such as changing or stopping flow
in place throughout the test to verify that the tracking accuracy
through the collector), nor shall it disturb the instrumentation
required by 13.4 is maintained. The device with which this
necessary to perform the test. If the reflected radiation is
measurementistobemadeisnotspecifiedinthismethod.Any
intercepted, care must be taken to avoid reradiation to the
testlaboratory’sequipmentusedshallmeettherequirementsof
receiver.Ifthecollectorisstowedorturnedawayfromthesun,
7.5.
the response time shall be measured relative to the time at
10.4 This test method is to be completed at a single
which the movement was initiated. Because of possible time
appropriate flow rate unless an exception is specifically noted,
delaysandrelativelyslowmotionofthecollector,theresulting
as in 13.2.2.
response time measurement will be conservative. Continue to
monitor the inlet and outlet temperatures as a function of time
LINEAR SINGLE-AXIS TRACKING COLLECTORS
(for example, on a strip chart recorder) throughout the test,
TESTED AS THERMAL COLLECTION SUBSYSTEMS
until final quasi-steady state conditions (Section 10.1 with the
11. Scope exception of 10.1.4) are reached.
13.1.2 Procedure B—Theresponsetimeshallbedetermined
11.1 This test method covers the determination of the
by suddenly irradiating a shaded collector as follows:
thermal performance of linear, single-axis tracking solar col-
13.1.2.1 Shade the collector in the same manner as de-
lectors tested as a thermal collection subsystem.
scribed in paragraph 13.1.1.Adjust the inlet temperature of the
12. Summary of Test Methods
heat transfer fluid, t , to within 610.0°C (618.0°F) of the
f,i
12.1 Theresponsetime,theincidentanglemodifier,andthe ambient temperature, or to the lowest possible operating
rate of heat gain at near-normal incidence are determined for temperature, whichever is higher, while circulating the fluid
E905 − 87 (2021)
through the collector at the flow rate specified until the procedure at additional, intermediate angles of incidence, the
collectorreachesandmaintainsquasi-steadystateconditionsas number of which is determined from the following table:
specified in 10.1. Then suddenly turn or uncover the collector
Minimum Number of Additional
K(θ ) Angles of Incidence
so that the collector aperture is fully irradiated. If the collector max
0.8–1.0 2
is stowed or turned away from the sun, the response time shall
0.6–0.8 3
be measured relative to the time at which the movement was
0.4–0.6 4
<0.4 5
initiated. Because of possible time delays and the relatively
slow motion of the collector, the resulting response time
13.2.1.2 The intermediate angles of incidence shall be
measurement will be conservative. Continue to monitor the
approximately equally spaced between normal incidence and
inlet and outlet temperatures as a function of time (for
θ .Itisrecommendedthatwhenincidentanglemodifierdata
max
example, on a strip chart recorder) throughout the test, until
are obtained on more than one day, the procedure be repeated
final quasi-steady state conditions (see 10.1) are reached.
for normal incidence on each of the test days in order to
minimize the effects of meteorological variations on the
NOTE 7—Procedure B is the more difficult procedure to complete since
results.
it requires stable irradiance, and establishing and maintaining stable
tracking conditions throughout the test period. 13.2.2 Alternative Procedure A—Follow the procedure of
13.2.1, but use water as the heat transfer fluid through the
13.2 Incident Angle Modifier—It is the intent of the follow-
collector. The mass flow rate must be altered such that the
ing procedure to generate sufficient incident angle modifier
(m˙C )-productisapproximatelyequaltothatusedintherestof
data, K(θ), to characterize the collector thermal performance p
thistestmethod. Warning—IfAlternativeProcedureAisused,
over the full range of actual operating angles that will be
and the heat transfer fluid to be used for the rest of this test
encountered. The range of angular data required is influenced
method is incompatible with water, then the incident angle
bythecollectortypeandorientation(forexample,north-south,
modifiermustbecompletedusingaseparatefluidloop,priorto
east-west, polar axis mount). Both the number and range of
filling the collector with the usual working fluid.
data points required are in part determined by the manner in
which K(θ) varies. A large, rapid decrease in K(θ)as θ
NOTE 9—If t is near or below 0°C (32°F), it may not be possible to
amb
increasesrequiresalargernumberofdatapointsthanagradual hold t = t 61.0°C(61.8°F),inwhichcasethisalternativeprocedure
f,i amb
may not be used.
decline.Therefore, the procedure provides for this K(θ) depen-
dence by requiring that the minimum number of data points be
13.2.3 Alternative Procedure B—Follow the procedure of
afunctionofthevalueof K(θ)atthemaximumoperatingangle
13.2.1,usingthesameheattransferfluidasusedin13.5ofthis
ofincidence.Ifthecollectorisopticallyasymmetric,thevalues
test method.The fluid inlet temperature shall be held within 6
of K(θ) are determined on both sides of the normal unless the
0.1°C (6 0.2°F) of the lowest possible fluid inlet temperature.
collector is restricted in actual use to only one operational
In addition, determine the nonirradiated collector heat loss for
orientation, in which case the K(θ) is obtained on the side
this same fluid inlet temperature by shielding the collector in
corresponding to the operational orientation. Preferred and
the same manner as prescribed in 13.1.1, and determining that
alternate procedures are defined. A two-axis tracking arrange-
final quasi-steady state conditions (10.1 with the exception of
mentispreferredformaintainingagivenangleofincidencefor
10.1.4) are reached. Measure the mass flow rate-specific heat
the duration of each test, and for maintaining the levels of
product (m˙C ) and the heat transfer fluid temperature differ-
p
irradiance required for quasi-steady state conditions.
ence between the inlet and outlet of the collector (∆t ).
a
13.2.1 Preferred Procedure—Determine the mass flow rate-
13.3 Determination of Near-Normal Incidence Angular
specificheatproduct(m˙C )andthetemperaturedifference,∆t ,
p a
Range for Determining the Rate of Heat Gain at Near-Normal
of the design heat transfer fluid between the inlet and outlet of
Incidence—“Near-normal incidence” shall be defined as tha
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