Exposure Testing of Solar Absorber Surfaces

The Los Alamos National Laboratory has been involved in supporting, monitoring and conducting exposure testing of solar materials for approximately ten years. The Laboratory has provided technical monitoring of the IITRI1, DSET2, Lockheed3, and Berry4 contracts and has operated the Los Alamos exposure Facility for over five years. This report will outline some of the past exposure testing, the testing still in progress, and describe some of the major findings. While this report will primarily emphasize solar absorber surfaces, some of the significant findings relative to advanced glazing will be discussed .


Introduction
The determination of materials durability is equally if not more important than initial performance or cost in establishing overall cost effectiveness.
For most applications, rates and degree of degradation are important factors in determining cost effectiveness.Although past exposure testing has included both mechanical and optical property evaluations, recent funding limitations have created focus on the most eas,'ly obtained, i.e., optical testing and visual effects.This paper will concentrate on the optical results obtained.
Many developmental materials were generated under funding by the DOE Program Research and Development Announcement (PRDA) and Request for Prcposal (RFP) processes.Many of these materials and products have been included in the Los Alamws exposure testing program in an attempt to identify those that mig))t evolve into durable commercial products.Other materials being exposed and evaluated arz continuations of the Los Alamos efforts to develop inexpensive selective absorber coatings, like the chemical conversion coatings or selective paints.Promising advanced coatings are being included as they are developed.
Long term real time data is also needed in which to compare accelerated test methods.The Los Alamos project will attempt to provide the long term exposure data upon which failure mechanisms and accelerated testing correlations can be based.riistory of types of test~?g --.-.-There has been a wealth of durability testinq conducted on solar materials over the past ten yea?s.Table 1 describes, in matrix form, some of the investigators and the types of tests that have been conducted, This is not intended to be a complete list., however, it does cover many of the major contributors that have received DOE support.Flnny additional tests have been conducted by commercial organizations or test~nq that has hoen contracted for and done by commercial testing laboratories that is not available to II)(J general public.
Much effort hag been exerted to bring solar standards and ~tandard test methods into effect.The ASTM Subcommittee E-44.04 has been the forerunner in this activit,l.

L08 Alamos testing program
The Los Alamou Exposure Teat Program is presently focused on outdoor real time exposure testinq and the determination of ot)tica] changeu on physical changes find failures.

Los Alamos
performs two basic functions in exposure tet3tinq: 1. research and development.matorialu 2, and commercial materials exposure, * 'l'his work was !~upport.mlby t.h~11.!; Tet.hnoIc)q {nn, The research and development role is a continuation of our efforts to determine the long-term durability of some of the more promising materials developed in DOE-funded Droarams.
In addition to determining durability effects and Drovidina manufacturer's guidance for these materials, for exposure and evaluation.Elmer Far-In~rared Detection Systems.
Although the majority of the data presented in this report is cantered around integrated measurements, spectral distributions have been stuc!~ed for many of these materials.
Much of this datn is described in more detailed reports5, Detoiied chanqen can normally first be detect~,d from the spectral diotributlons, however, it '% thĩ ntegrated data that defines the materials solar performance capabilities.

7.
8. range to 0.10 making it a better coating since the absorptance appears to be relatively constant.
Only additional testing will determine if other problems will begin appearing.
The Ferro porcelain enamels are quite stable after three years of exposure.The Sn02 overcoated sample having initial selectivity of a5/c of 0.945/0.204has shown a very slight chanqe to 0.934/0.211after three years of exposure.A slight degradation was noted early in the sample that appeared to be a spalled area.This defect has not increased in size or changed appreciably in properties.
The Berry developmental coatings on aluminum have shown rather insignificant changes in optical properties in the glazed exposure boxes.Only very small reductions in as, approximately two percent, have been noted in an unglazed exposure box.
The ULVAC coating has now shown an approximately two percent decrease in as in the single glazed selective surface exposure box after two years of exposure.

A5
industrial firms began their investigations into these coatings, their prototype products were also brought into the program.The original coating investigations were centered around aluminum for a ~,ubstrate materials, Collector manufactures using other substrate materials, i.e., copper and stainless steel, and showing interest in the low cost selecti-Je paints brought about the durability testing of these materials.
Tables 4 through 6 describes the long term effects of exposing the ~elective paints to outdoor exposure testing.Most of these materials were removed and measured at three month, six month, one year, two year, and three year intervals.For simplification only the highest exposure results, the three year data, are tabulated for comparison with the original base line dat>, These tables describe the exposure results for the three year paint samples and so~.,e more recently added samples that have only accrued L6 and 24 months of exposure.
More detail of the selective paint proqrams can be found in Sular Enerqy Materials8,~.
Those produced earlier by the reverso roll coat prrccss show a thr~e to five percent increiise in emittanccs, The FI?Povercoated material also appears to be very The Zuel cotitinqs for oil thicknons{?~hnvc nhown qc>od stability in all of th~cx~>nnuro boXQs .'1'he only c+iqnlfi{'~int c:hango 15 n 3% decrnano {n 18 for the liqht continq in the upon unglazed exposure l~ox, 6.An early selective paj.nt by Solec, Hi/Sorb-I has shown a durability p~oblem with heavy coatings and considerable change in medium and light coatings.This coating was cha.>ged and improved with a later product.
7. The Solkote Hi/Sorb-11 has now been exposed for 16 months and is showing good stability on aluminum and stainless steel substrates.This coating on copper is showing changes in both as and c , indicating that diffusi~,-, be influencing the properties.LA-9842-YS10 describes the various coatings investigated.
Base line materials and some of the more promising coatings that evolved from this program were incorporated into the exposure test program.The objective is to determine their durability and ultimately determine if any have commercial potential.
Samples of substrate materials anfithe completed converted samples were exposed in order to determine exposure effects.Some coatings may offer additional protection for marginally durable substrates.Some of the coatings being tested and the optical results are shown in Table 7

Aluminum Substrate ,4aterials
The substr~i.eresults show that the oxide build-up in the open box is having only a small effect on the as received aluminum, The etched aluminum sample?likewise are changing only a small amount in the SGSS exposure box.The A-15 coating is highly degraded in the open box, but has reasonably good stability in tie co''ered boxes, snowing only about two perc~nt decrease in us.

copper Substrate Materials
All samples are showing considerable degradation in properties.The Vestar overcoat appears to be stabilizing the coating from an emittance standpoint: however, lar9e decreases in as are occurring.

Transparent materia~~ex~osure testing
Potential solar glazing materials are being exposed at the LOS Alamos exP=ure f~cilitY in two forms, i.e., small samples exposed to direct LIV or outdoor weathering and full size glazings or collectors.
Spectral distributions are measured for the small samples and their wei~hted air mass 1.5 solar transmittances determined from these distributions.Total solar transmittances are determined for the full size gl.zi.ng materials using the pyranomater method.

Figure 2 .
Figure 2. DSET experiments at the Los Alamos exposure facility q XP05UIJEt4A;:::AL stable.The open exposure box tests for one sample has shown seven pcrccnt emittance increases, while remaining stable for another sot.The samples produced witlt he Cargill 6247 resin and exposed under glazing or film show decreases in ( , which makes it a better coating, however, this may b~signaling a long term de~raclntion problem.The open box paints ('I'ISP) have shown one to two pc!rccnt decreases in (Is while r has remained relatively constant, The Thermo-250 paints on stainless steel and on aluminum appear to have only mlnoc hanges in properti.~s,none that reduce the performance of tl)ecoating.Dulito contings exposed under glass also appear to bc quite ~:tnble for th@ hcnvy o]" medium coating thickne~se~.Thp light thicknofis c!oatingc arc showing a cliqht reduct~on in (Is.Paint nnmp]c?n expos~d in t,ho opon box hmd tho painted surfac:o n(~{trly compLratcly removed by a ht~ilstorm, and oxidation of the copper substrate may 8.The Colt selective paint on copper has shown a large increase in c in the open exposure box and a slight increase in the glazed boxes.Chemicall~onverted coatings ---_._____ As part of a Los Alamos program to develop and investigate low cost selective surfaces, selective surfaces produced by chemical co~version were investigated. Coating becoming powdery and difficult to handle.(c) Spalled a.ea (d) Two-year exposure Paint Process Better properties could probably have been obtained with greater optimization and better process ~cntrr,l; however, these samples were used to determine changes that may have been occurring.Dip-coated samples produced aS/C of 0.950/0.694.

Table 2 .
Ergenics Maxorb has shown exceptional stability under glass or cover films.When exposed in an open box it has shown an approximately 20 percent reduction in cis.Ergenics Skysorb has shown good stability in all exposure boxes, including the open box .It is unfortunate that jnitial as and c are not better, i.e. as.
through 10 for zinc, galvanized and zinc/aluminum, nickel, aluminum and copper substrates.Some of the important results are: 1. Zinc, galvanized and zinc/aluminum substrate materials The sub~;trate materials results are showing relative large increases in as without too great an increase in c when exposed in the open box.It is doubtful if they will everi reach the point where they are relatively good intrinsic selective absorbers, however, this appears to be the trend.This is particularly true for Galvalume smoothed and large spangle.Most zinc base selective coatings are not showing good stability in the open box, however, some zre showinq good '.tability in the covered boxes.
2. Nickel Substrate MaterialsGood stability is ~hown for the Patclin Chemical Co.Pa50x #1 conversion coatings and the Metex Black Nickel on nickel sulfamate in the covered b~xes.All show poor stability in the open exposure box.

Table 3 .
Developmental or Advanced Selective Coatir,gs