Alkali and alkaline earth carbonate latent-heat storage salts, metallic containment materials, and thermal conductivity enhancement materials were investigated to satisfy the high temperature (704 to 871 C) thermal energy storage requirements of advanced solar-thermal power generation concepts are described. Properties of the following six salts selected for compatibility studies are given: three pure carbonates, K2CO3, Li2CO3 and Na2CO3; two eutectic mixtures, BaCO3/Na2CO3 and K2CO3/NaCO3, and one off-eutectic mixture of Na2CO3/K2CO3

Claar, T. D.

Petri, R. J.

English

NASA Technical Reports Server

HIGH-TEMPERATURE MOLTEN SALT THERMAL ENERGY STORAGE SYSTEMS 
FOR SOLAR APPLICATIONS 
Randy J. P e t r i  and T. D. Claar 
I n s t i t u t e  o f  Gas Technology 
PROJECT OUTLINE 
P r o j e c t  T i t l e :  High Temperature, Molten Sa l t -La ten t  Heat, Thermal Energy 
Storage Development f o r  Solar  Appl i  ca t ions  
P r i n c i p a l  I nves t i ga to r :  T. D. C l  aar 
Organizat ion: I n s t i t u t e  o f  Gas Technology 
3424 South S ta te  S t r e e t  . 
Chicago, I L  60616 
Telephone: (312) 567-3672 
P r o j e c t  Goals: Determi ne f e a s i b i l i t y  o f  us ing  carbonate s a l t s  as storage 
media fo r  h igh  temperature app l i ca t i ons  (700°C - 8700C). 
Review carbonate s a l t  p rope r t i es  and s e l e c t  s i x  s a l t s  as 
candi date media. 
I n v e s t i g a t e  methods t o  enhance heat t r a n s f e r  through s o l i  d 
s a l t .  
Conduct carbonate salt-containment ma te r i  a1 c o m p a t i b i l i t y  
s tud ies  t h a t  i nc lude  3000 hour screening tes ts .  
Measure thermophysical and t ranspor t  p rope r t i es  o f  two most 
promis ing s a l t s .  
P r o j e c t  Status: Review o f  p r o p e r t i e s  complete w i t h  the  f o l l o w i n g  s i x  s a l t s  
se lec ted  f o r  c o m p a t i b i l i t y  studies: t h r e e  (3 )  pure 
carbonates, K2CO3, Li2C03 and Na2C03; two ( 2 )  
e u t e c t i c  mixtures, BaCO /Na2C03 and 
K2C03/NaC03, and one (13 o f f  - e u t e c t i c  m ix tu re  o f  
N a2C03/K2C03. 
C o m p a t i b i l i t y  s tud ies  scheduled t o  be completed by mi d-1980. 
Contract  Number: DEN3-156 
Contract  Period: August 1979 t o  August 1980 
Funding Level : $1 36,840 
Funding Source: NASA Lewis Research Center 
https://ntrs.nasa.gov/search.jsp?R=19800014317 2020-03-21T18:14:54+00:00Z
HIGH-TEMPERATURE MOLTEN SALT THERMAL ENERGY STORAGE' SYSTEMS 
FOR SOLAR APPLICATIONS 
Randy J. Petri and T. D. Claar 
Institute of Gas Technology 
PROGRAM SUMMARY 
The objective of this program is to select,' test and develop alkali and 
alkaline earth carbonate latent-heat storage salts, metallic containment 
materials, and thermal conductivity enhancement (TCE) materials to satisfy the 
high-temperature (704" to 871°C; 1300' to 1600°F) thermal energy storage (TES) 
requirements of advanced solar-thermal power generation concepts. This will be 
accomplished by experimental screening of candidate salt/containment/TCE 
materials combinations in capsule compatibility tests employing both reagent- 
grade and low-cost technical-grade salts. The results of these compatibility 
tests will lead to the selection of the materials combinations that best meet 
the anticipated solar power system requirements. Needs for more reliable salt 
thermophysical and thermodynamic property data will be identified, and selected 
measurements will then be performed to support the future development and 
scale-up of solar-thermal TES subsystems. 
MATERIALS COMPATIBILITY TESTING 
Salt Selection 
Alkali and alkaline earth carbonate salts are being developed as latent- 
heat thermal energy storage materials for high temperature (704" to 871°C) 
applications, on the basis of their desirable thermophysical properties, good 
thermal chargeldischarge performances and endurance-stability exhibited by 
these materials in previous experimental programs at the Institute of Gas 
~ e c h n o l o ~ ~  .lr2 Table 1 is a list of the carbonate salts originally identified 
and studied under Contract No. NAS~-208062 as candidate storage media for 
Brayton or Stirking solar power TES applications. Table 1 includes pertinent 
thermophysical salt property data and thermal discharge characteristics deter- 
mined experimentally in 12.7 cm (5 in.) high X 7.6 cm (3 in.) diameter 
laboratory-scale TES modules having a 1.3 cm (0.5 in.) diameter heat exchanger 
tube. These modules were fabricated from AISI 304 and 316 stainless steels. 
The six salts identified by asterisks in this table have been selected as 
candidate salts for the compatibility screening tests to be conducted under 
the current program. Considerations in selecting these carbonate salt storage 
media included - 
Energydensity 
Cost 
Melting/solidification temperatures 
a Thermal cycling performance and endurance in laboratory-scale TES modules 
a Vapor pressure and high-temperature stability 
a Impurity levels in commercial-grade salts 
a Toxicity, safety, hygroscopicity, and handling considerations 
a Volumetric expansion on melting 
a Heat capacity 
The economic feasibility of latent-heat TES concepts requires the use of 
low-cost salt materials. Therefore, the potential corrosion problems, salt 
instabilities, and other limitations associated with using commercially avail- 
able (technical-grade) carbonates will be investigated in the capsule screening 
tests. Table 2 is a list of technical-grade alkali and alkaline earth car- 
bonates along with recent costs of each salt in bulk quantities. A number of 
companies have been identified and contacted as prospective suppliers of 
technical-grade carbonates. Chemical and physical property specifications and 
cost information on these carbonates are being obtained from chemical companies 
supplying these materials. Selection of the technical grade salts to be in- 
cluded in screening tests will be based on types of impurities present, impurity 
levels, and cost. 
Containment Materials Selection 
The work completed under Contract No. NAS3-20806 demonstrated the need to 
improve the long-term resistance to corrosion and thermal aging effects of 
containment materials in the elevated temperature regime of 704" to 871°C. The 
materials selected as candidate containment materials are generally required to 
possess superior resistance to - 
a Hot corrosion, oxidation, and carburization on the salt side 
a Intermetallic-phase precipitation, embrittlement, or other long-term 
aging effects caused by high-temperature (704O to 871°C).exposure 
a Thermal cycling effects on corrosion and strength 
a Interactions with working fluids (air, helium, and sodium) or their 
impurities. 
In addition, mechanical property data are being compiled and evaluated to 
assure structural compatibility for the proposed high-temperature TES applica- 
tions, including - 
a Stress-to-rupture at elevated temperatures 
a Thermal cycling fatigue behavior 
a Strain and thermal aging effects 
a Thermal expansion coefficients . 
Candidate containment materials are being selected primarily from among 
iron- and nickel-based stainless steels and high-temperature alloys. Most of 
these alloys contain a significant amount of chromium and thus form Cr203-rich 
oxidation product scales. Chromia scales are protective under air oxidation 
conditions, but A1203-containing protective films aregenerally-superior under 
molten carbonate conditions. Commercially available iron-nickel-chromium 
alloys, some containing approximately 2 to 5 wt % aluminum additons, are being 
considered. Application of a protective coating to iron-nickel-chromium alloy 
base materials by aluminizing techniques is also being evaluated. Varioussteel 
producers have been contacted as potential suppliers of these alloys, and 
available property data are being received. 
Thermal Conductivity Enhancement Materials Selection 
The incentive for TCE materials/configurations development arises from the 
fact that the thermal discharge performance of a TES subsystem is controlled 
largely by the rate at which the latent heat-of-fusion released at the solid/ 
liquid interface is transported to the heat exchanger surface through the grow- 
ing layer of solid salt. TCE selection criteria will thus include material 
thermal conductivity, expected chemical compatibility with the carbonate en- 
vironment, cost, availability, and ease of fabrication into a desirable TCE 
configuration. 
A preliminary list of candidate TCE materials is provided in Table 3. 
Aluminum is very attractive as a TCE material because of its high thermal 
conductivity of 220 W/m-K (127 Btu/hr-ft-OF) (%100X that of solid carbonates 
and 10X that of austenitic stainless steels). Under Contract No. NAS3-20806, 
a 95% porous retiiculated aluminum structure increased the discharge heat flux 
through the ternary eutectic Li2C03-Na2C03-K2C03 salt (m.p. 397OC) by approx- 
imately 45%. However, aluminum cannot be used for the 704" to 871°C solar 
thermal TES applications because of its 660°C (1220°F) melting point. While 
iron and nickel-based materials are more compatible at these temperatures, 
they are more costly and have much lower thermal conductivities than aluminum 
and copper, greatly reducing their attractiveness as TCE materials. Copper 
also has a very high thermal conductivity of 308 W/m-K (178 Btu/hr-ft-OF) and 
a reasonably high melting point of 1083°C (1981°F). Pyrolytic graphite, while 
still a high-cost developmental material, is also reported to have a high 
thermal conductivity (208 W/m-K; 120 Btu/hr-ft-OF) and has a good thermal 
stability in non-oxidizing environments. Both copper and graphite would be 
severely oxidized in a high-temperature molten carbonate environment having 
access to the air environment. However, if gaseous oxidants are excluded by 
use of sealed containment or an inert cover gas, oxidation of copper and 
graphite TCE materials should be acceptably low. Candidate TCE materials will 
be corrosion tested in bulk form, or if available, as porous reticulated 
structures. 
Screening Tests 
Compatibility screening tests of the candidate salt/containment/TCE 
materials combinations will be conducted in welded containment capsules (2.54 cm 
diameter by 10.2 cm long) containing the salt and TCE material. The compati- 
bility of 5 containment and 2 TCE materials will be evaluated with both reagent- 
and technical-grades of 6 candidate salt compositions. The first phase will be 
a 1000 hour test of the salt/containment/TCE matrix (minimum of 70 capsules). 
Based on the post-test examination results of the 1000 hour test, the 30 most 
promising materials combinations will then be evaluated in a 3000 hour compati- 
bility test. Compatibility evaluations will be based on - 
Depth and nature of salt-side corrosion of containment and TCE 
materials 
Air-side containment oxidation 
SEMIEDAX analysis of selected areas 
Containment alloy thermal aging effects 
Weld integrity in salt environment 
Gravimetric analyses of containment alloy and TCE coupons 
Chemical and physical analyses of salt 
PROPERTY MEASUREMENTS 
The design and scale-up of a latent-heat TES subsystem require accurate 
salt property data, such as melting/solidification temperatures, heat of fusion, 
volumetric expansion upon phase change, heat capacity and thermal stability. 
However, many of these salt property data for salts in the 704" to 871°C 
temperature range are either unavailable or subject to large error and incon- 
sistency. Based on the results from previous experimental testing in laboratory 
scale TES modules and the 1000 and 3000 hour compatibility tests, at least two 
carbonate salt compositions most suitable for anticipated solar thermal energy 
storage applications will be identified. Available thermodynamic'and thermo- 
physical property data for these salts will be critically reviewed and critical 
gaps will be identified. The required property measurements will then be per- 
formed on a maximum of six salt composition-purity level combinations. The 
results from this property measurement activity will subsequently be utilized 
in future carbonate TES development efforts. 
REFERENCES 
1. Maru, H. C., Dullea, J. F., and Huang, V. S., "Molten Salt Thermal Energy 
Storage Systems: Salt Selection", COO-2888-1, Institute of Gas Technology, 
Chicago, August 1976. 
2. Maru, H. C. et al., "Molten Salt Thermal Energy Storage Systems: System 
Design," COO-2888-2, Institute of Gas Technology, Chicago, February 1977. 
3. Petri, R. J., Claar, T. D., and Marianowski, L. G., "Evaluation of Molten 
Carbonates as Latent Heat Thermal Energy Storage Materials," 14th Inter- 
society Energy Conversion Engineering Conference, Boston, August 5-10, 1979. 
4. Petri, R. J., Claar, T. D. and Tison, R. R., "High Temperature Molten Salt 
Thermal Energy Storage Systems", Contract No. NAS3-20806 Final Report, 
Institute of Gas Technology, Chicago, to be published. 
Table 1, CANDIDATE CARBONATE COMPOSITIONS TESTED UNDER 
CONTRACT NAS3-20806 
Composl t  i o n  
Vt X n01 r 
-- 
44.3-55.7 37-63 
52.2-41.8 37-63 
48.8-50.0- 44-55-1 
1 . 2  
50-50 44-56 
100 100 
81-3-18.7 85-15 
L l c e r a t u r r  - - 
H r l t l n g  I'oint 
=c *P 
-- 
Thermal 
C o n d u c t i v i t y  
W/m-K - 
tleac 1 :u l~dcl ty  
a t  H r l t i n g  P o i n t  D i s c h a r g e  - 
J/kg-K D e n s i t y  S o l i d i f i c a t i o n  Q cup + 50'C ( 9 0 ' ~ )  No. 
"(a) C P ( ~ )  a t  25-C Range mp - 50°C (90°F) o f  kg/m3 'C -t OC u/ m2 
- 
C y c l e s  
1 - - - 662 - 657 40 ,420 2  5  
Hours a t  
O p r r a t  i o n a l  
T n p e r a t u r e  
312 
984 
528 
* S e l r c t c d  a s  c a n d i d a t e  s a l t  f o r  c o m p a t i b i l i t y  s c r e e n i n g  t e s t s .  
Table 2. ESTIMATED COSTS OF TECHNICAL GRADE ALKALI 
AND ALKALINE EARTH CARBONATES* 
Salt - Description 
1. Li2C03 Powder, bags 
2. Na2C03 Powder 
3. K2C03 Granulated, purified 
Calcined 99 to 100% K2CO3 
4. MgC03 Powder, bags 
Ultrafine, USP bags 
Natural dry-ground air-floated, -325 
mesh, bags 
Glass ground, bags 
Precipitated, bags 
Photo grade, bags 
Electronics grade, bags 
Cost, 
$/kg Sllb 
* 
Chemical Marketing Reporter, September 19, 1979. 



High-temperature molten salt thermal energy storage systems for solar applications

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