Shapagat Berdibek BACHELOR OF SCIENCE IN NUCLEAR SCIENCE AND ENGINEERING AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY JUNE 2015

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1 MEASUREMENT OF OPTICAL PROPERTIES OF MOLTEN SALTS AND METALLIC COMPONENTS FOR ADVANCED SOLAR AND NUCLEAR SYSTEMS By Shapagat Berdibek SUBMITTED TO THE DEPARTMENT OF NUCLEAR SCIENCE AND ENGINEERING IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF BACHELOR OF SCIENCE IN NUCLEAR SCIENCE AND ENGINEERING AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY JUNE 215 Shapagat Berdibek. All rights reserved The author hereby grants to MIT permission to reproduce and to distribute publicly Paper and electronic copies of this thesis document in whole or in part. Signature of Author: Certified by: Certified by: Signature redacted Shapagat Berdibek Department of Nuclear Science and Engineering Signature redacted May 8,215 E Charles W. Forsberg Senior Research Scientist, Nuclear Science and Engineering / Thesis Supervisor Signature redacted Thomas J. McKrell Research Scientist, Nuclear Science and Engineering Thesis Co-Supervisor Acepedby Signature redacted MASSCHUSteSdNSTTU: MASSACUET ILNSTITUTE OF TECHNOLOGY SEP LIBRARIES ARCWVES Michael P. Short Assistant Professor of Nuclear Science and Engineering Chairman, NSE Committee for Undergraduate Students 1 The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created.

2 Measurement of optical properties of molten salts for advanced solar and nuclear systems By Shapagat Berdibek Submitted to the Department of Nuclear Science and Engineering on May 8, 215 In Partial Fulfillment of the Requirements for the Degree of Bachelor of Science in Nuclear Science and Engineering ABSTRACT Renewable energies can reduce the dependence on fossil fuels. Solar thermal systems designed to use molten salts to directly absorb the solar heat are promising due to (1) potentially higher efficiency in capture of sunlight and (2) use of the salt to simultaneously capture sunlight and store heat in the salt. The optical properties of the molten salts are crucial for the design of such thermal systems because they determine the depth of the salt required to absorb sunlight and allow modeling of the performance of such systems. Molten salts are also being developed as coolants for high temperature reactors. Optical properties are also required to determine the radiative heat transfer of the coolant. The objectives of this thesis were to build a better system to measure these properties and measure the optical properties of the proposed salt for a direct absorption concentrated solar thermal system. The attenuation coefficient of light in a binary nitrate salt mixture (KNO 3 -NaNO 3, 4-6 wt%) was measured over the wavelength range nm and the temperature range 3-4'C. This salt is the leading candidate for the first generation of a proposed concentrated solar power on demand (CSPonD) concept [3]. The relevant data was obtained using a FTIR spectrometer and an experimental apparatus designed for semitransparent liquids. The apparatus was validated using the published data for the attenuation of light in deionized water. The attenuation coefficients of the binary nitrate salt mixture for the lower wavelengths matched the data obtained by Passerini [1]. For the longer wavelengths, the attenuation coefficient peaked around 2.5 Pm as predicted by Drotning [2]. Since certain metallic components of solar and nuclear systems are exposed to the molten salt, it is important to characterize the behavior of their reflectivity in the presence of the molten salt. The reflectivity of 34L stainless steel was measured for the wavelength range 6-5 nm at incident angles of light 1, 4, and 7 after an 8-day molten salt immersion test. The reflectivity was measured to be less than 1% for the solar spectrum. Thesis Supervisor: Charles W. Forsberg Title: Senior Research Scientist, Nuclear Science and Engineering Thesis Co-Supervisor: Thomas J. McKrell Title: Research Scientist, Nuclear Science and Engineering 2

3 Acknowledgments I would like to express my gratitude to Prof. Jacopo Buongiorno for the opportunity to work in the Green Lab. I want to thank Dr. Thomas McKrell for his guidance and support during my time working on the CSPonD project. I am equally grateful to Dr. Charles Forsberg for his valuable input and suggestions in writing this thesis. 3

4 Contents 1 Introduction 8 2 Background Concentrated Solar Power on Demand Solar spectrum Light attenuation Methodology Measurement of the attenuation coefficient of the KNO 3 -NaNO 3 salt mixture W ater validation Measurement of the reflectivity of 34L stainless steel Results and Discussion Attenuation of light in the KNO 3 -NaNO 3 salt mixture Reflectivity of 34L stainless steel Conclusions 26 6 Appendix: Attenuation coefficient of the KNO 3 -NaNO 3 (4-6 wt%) salt mixture in tabular form 3 4

5 List of Figures 1 Proposed positions of the molten salt container relative to the hill. Image adapted from [3] CSPonD storage system at various stages of stored energy. Image adapted from [4] ASTM G173-3 reference spectra. Data adapted from [5] Schematic of the experimental apparatus FTIR and the framing of the experimental apparatus Elements of the framing of the experimental apparatus Alignment of the outer cuvette Mounting of the inner cuvette and installation of the furnace KNO 3 -NaNO 3 salt mixture melted for attenuation measurements Attenuation of light in DI water for the first optical setup [6] Attenuation of light in DI water for the second optical setup [6] Reflectometer accessory inside the sample compartment of the FTIR spectrom eter L stainless steel strips before the 8-day immersion test in the binary nitrate salt mixture. (a): A - below salt-air interface, B - above salt-air interface Attenuation of the binary nitrate molten salt for wavelengths between 1 nm and 25 nm at T=3 C Attenuation of the binary nitrate molten salt for wavelengths between 1 nm and 25 nm at T=35 C Attenuation of the binary nitrate molten salt for wavelengths between 1 nm and 25 nm at T=4 C Attenuation of the binary nitrate molten salt for wavelengths between 4 nm and 25 nm at T=3 C Attenuation of the binary nitrate molten salt for wavelengths between 4 nm and 25 nm at T=35 C Attenuation of the binary nitrate molten salt for wavelengths between 4 nm and 25 nm at T=4 C Attenuation of the binary nitrate molten salt for wavelengths between 4 nm and 25 nm at three temperatures Attenuation of the binary nitrate molten salt for wavelengths between 4 nm and 22 nm at three temperatures L stainless steel strips after the 8-day immersion test in the binary nitrate salt mixture. (a): A - below salt-air interface, B - above salt-air interface Reflectivity of the control 34L stainless steel strip for wavelengths between 6 nm and 5 nm Reflectivity of the 34L stainless steel strip above the salt-air interface after the 8-day immersion test in the binary nitrate molten salt for wavelengths between 6 nm and 5 nm Reflectivity of the 34L stainless steel strip below the salt-air interface after the 8-day immersion test in the binary nitrate molten salt for wavelengths between 6 nm and 5 nm

6 26 ASTM G173-3 solar spectra and the attenuation coefficient of the binary nitrate molten salt for the wavelength range 4-25 nm at three temperatures 27 6

7 List of Tables 1 Detailed description of the optical setups used for water validation Detailed description of the optical setups used for reflectivity measurements Attenuation coefficient of the binary nitrate mixture at T=3"C Attenuation coefficient of the binary nitrate mixture at T=35 C Attenuation coefficient of the binary nitrate mixture at T=4 C

8 1 Introduction With ever growing demand for energy and diminishing reserves of fossil fuels, there is a need for alternative sources of energy. Molten salts are being considered as heat transfer fluids in advanced solar and nuclear systems because of their good heat transfer and optical properties. In the Concentrated Solar Power on Demand (CSPond) system light from a field of mirrors is focused on a pool of salt that absorbs the sunlight, stores heat, and acts as the heat transfer fluid for the power cycle [3]. Molten salts can also be used as a coolant in a nuclear reactor. Advanced nuclear reactors such as the Fluoride-salt-cooled High-temperature Reactor (FHR) have operating temperatures between 6"C and 7 C [7]. At these temperatures radiation heat transport through semitransparent salts starts to become an important heat transfer mechanism. The salt storage tank in the CSPonD system and the reactor core in the FHR are in contact with high temperature molten salts which alter the optical properties of the metallic components over time. At higher temperatures, radiation heat transfer becomes the dominant heat transfer mechanism and must be accounted for in the heat transfer analysis [7]. Therefore, it is important to analyze the surface optical properties of the metallic components for wavelengths beyond the solar spectrum where the black body spectrum of high temperature molten salts are peaked. In order to bring these projects into reality, the optical properties of molten salts and metallic surfaces must be measured. The work herein measured optical properties applicable to the CSPonD and further improved experimental methods that will be applicable to measuring the higher temperature salt properties for the FHR. 2 Background 2.1 Concentrated Solar Power on Demand The CSPonD is a new system that involves concentrated sunlight being absorbed by a semitransparent molten salt in an insulated tank [3]. A small opening for the incoming light minimizes heat loses by the collector. The salt acts as the light absorber as well as the heat storage medium, and the heat transfer fluid to the power cycle. Figure 1 shows a system with numerous heliostats mounted on a hillside reflects sunlight (A) directly into the opening of the container of the molten salt at the base of the hill, or (B) into a one-bounce system with the container at the top of the hill [3]. An insulated horizontal divider plate is used in the salt storage tank for continuous operation of the system. The divider plate separates the molten salt physically and thermally into two zones. The salt in the upper zone is heated with sunlight entering through the aperture. The heated salt as a heat source is extracted by the heat exchanger (HX) for the power cycle and returned to the lower zone of the tank at a lower temperature (Figure 2). The temperature stratification in the salt is maintained by varying the position of the divider plate. The vertical position of the divider plate controls the total volume of hot and cold salt inside the tank. The variable volumes of hot and cold salt allow for the 8

9 Figure 1: Proposed positions of the molten salt container relative to the hill. Image adapted from [3]. Insulating Ud (A) Aperture door (B) ToHX Cold salt Divider plate From HX Figure 2: CSPonD storage system at various stages of stored energy. Image adapted from [4]. adjustment of the total amount of heat present in the tank at any given time. Therefore, the divider plate and molten salt function as a thermal storage system. Figure 2 describes the procedure of continuous operation. During daytime, (A) the energy is stored in the upper zone by lowering the divider plate as the salt absorbs the incoming light. In the absence of sunlight, (B) the stored energy is used for power generation and t he divider plate is raised as hot salt is extracted by the heat exchanger. 2.2 Solar spectrum The spectrum of the electromagnetic radiation emitted by the sun spans over the ultraviolet, visible, and infrared regions. This spectrum is similar to the spectrum of a blackbody with a temperature of around 58 K. The spectrum of the sunlight collected on the surface of the earth, however, is not identical to the spectrum of this blackbody due attenuation of certain spectral lines by gases in the atmosphere. The solar spectrum at sea level depends on factors such as the density and composition of the atmosphere and the altitude of the location where the spectrum is 9

10 measured. The spectra in Figure 3 correspond to solar irradiance spectra measured according to the ASTM G173-3 standards. The black curve represents the solar spectrui measured above the earth's atmosphere. The blue curve corresponds to the measured solar spectral radiation from the slin and reflected from the ground on south facing surface tilted 37 from the horizontal. The red curve shows the spectrum of the direct normal irradiance and the spectral irradiance within a 2.5 field of view centered on the solar disk. 2. -Etr W*m -2*nm-1 E c E Global tilt W*m-2*nm-1 Direct+circumsolar W*mn-2*nm-1 L).5. ( 5(x Wavelength nm Figure 3: ASTM G173-3 reference spectra. Data adapted from [5]. The last solar irradiance spectrum is considered in this thesis. Most of this spectrum spans over wavelengths between and 25nin. The binary nitrate salt mixture is opaque in the ultraviolet region and semitransparent in the visible range [1]. The attenuation coefficient of the molten mixture is predicted to be high in the infrared region around 2.5 pm [2]. 2.3 Light attenuation The attenuation coefficient of a binary nitrate salt mixture was measured in this study. The mixture consists of 6 wt% NaNO3 and 4 wty(, KNO. 3. Its melting point is at 222'C and operating temperature is up to 5-6'C [2]. The binary salt mixture is a candidate for the first generation of the CSPonD. The previous works have characterized the attenuation of light in the binary mixture. Passerini measured the attenuation coefficient in the visible region (4-8 1m) [1]. The binary nitrate salt mixture is semitransparent in the visible region and opaque in the ultraviolet region. Drotning observed high attenuation at 2.5 pm for a ternary mixture of 1

11 KNO 3, NaNO 2, and NaNO 3 ( wt%) at T=2' [2]. Since at higher temperatures NaNO 2 decomposes to NaNO 3, the binary nitrate salt mixture can be expected to be opaque at 25 nm [2]. The attenuation coefficient of a semitransparent liquid can be calculated using the Beer-Lambert law: T - - (1) 1 where T is the transmittance, I is the intensity of the transmitted light, Io is the intensity of the incident light, 3 is the attenuation coefficient of the sample, and x is the thickness of the sample. The transmission method is a preferred procedure for measuring the attenuation coefficients of liquid samples with neither too small nor too large optical depths. Therefore, a thick sample of a low attenuation liquid and a thin sample of a high attenuation liquid should be used for accurate measurements. In order to account for the reflection of light at the interfaces of the liquid and other experimental losses, the transmittance of the liquid can be determined by measuring the ratio of the transmitted light for different optical paths: I(Xref + Ax) _ A (2) I(Xref) where Xref is the reference thickness of the sample, and Ax is the change in the thickness of the sample. Using Equation 2, the attenuation coefficient of the liquid: 3 Methodology 1 I(Xref + AX) =In -- (3) AX I(Xref) 3.1 Measurement of the attenuation coefficient of the KNO 3 -NaNO 3 salt mixture The experimental setup for this study consists of two parts. The first part is a Bruker Vertex 7 Fourier transform infrared spectroscopy (FTIR) spectrometer which was used to measure the transmittance of light. The spectrometer operates in the visible and infrared regions using multiple light sources, detectors and beam splitters. The second part of the setup is an electrically heated cylindrical furnace. Salt samples placed in a cylindrical quartz cuvette were heated inside the furnace to reach the operating temperatures. The attenuation coefficient of molten salts was determined by coupling the spectrometer and the furnace as shown in Figure 4. A light source inside the spectrometer generates a light beam directed at the furnace. A parabolic mirror reflects the light beam upward along the central axis of the furnace. The furnace heats the salt sample. The thickness of the molten salt inside an outer cuvette is adjusted by moving an inner cuvette with a smaller diameter. This technique was adopted from Passerini because it allows for a 11

12 Parabolic mirror Detector Inner cuvette Outer cuvette Molten salt Furnace Support tube FTIR Parabolic mirror Figure 4: Schematic of the experimental apparatus. quick adjustment of the sample thickness and elimination of vibration issues associated with the free surface of the sample [1]. After the light beam passes through the molten salt, the transmitted light is focused by another parabolic mirror and collected by a detector. Subsequently, the FTIR displays the transmittance of light for the specified wavelength range. The cuvettes were made by fusing fire polished quartz windows to the bottom of quartz tubes produce by QSI. The measurement of the attenuation coefficient of the binary nitrate salt mixture was possible due to the wide transmission range ( pm) and the high melting point (147'C) of quartz [8]. The attenuation coefficient was measured for the wavelengths range nm. The wavelength dependence of the attenuation coefficient of the binary salt mixture over the entire solar spectrum was acquired by coupling the data for the wavelength range 4-8 nm measured by Passerini with the data obtained in this study [1]. The experimental apparatus was built specifically for attenuation measurements of high temperature semitransparent liquids to incorporate improvements based on what was learned by Passerini. Figure 5 shows the FTIR and the framing of the apparatus. In order to obtain accurate results, many parts of the apparatus were designed to minimize the errors associated with imprecision. Figure 6 presents a close view of the lower parabolic mirror, leveling plate, and precision slide with millimeter divisions. The parabolic mirror mounts have three fine resolution screws that provide precise tilting of the mirror in two directions. The leveling plate was used to level the support tube by adjusting the nuts on three T-bolts that are mounted on the apparatus frame. The precision slide was used to accurately measure the thickness of the liquid sample between the outer and inner cuvettes. Since the parabolic mirror reflects the incident light in the vertical direction, it was 12

13 Figure 5: FTIR and the framing of the experimental apparatus. important to place the outer and inner cuvettes concentrically and parallel to the reflected light in order to ensure that the light does not hit the cuvette walls. This was achieved by leveling the cuvettes separately. Figure 7 shows the support tube fixed to the leveling plate with a plastic clamp. The support tube was leveled by adjusting the positions of the nuts on the T-bolts. A washer was used to connect the support tube and the outer cuvette. The inner cuvette shown in Figure 8 was connected to the precision slide using a utility clamp with high temperature resistant fiber glass sleeving. The clamp was adjusted so that the inner cuvette was level and concentric with the outer cuvette. Proper alignment of the cuvettes was verified by comparing the counts received by the detector at various positions of the inner cuvette. The assembly of the apparatus was completed by covering the cuvettes with an electrically heated furnace insulated by a refractory material. The attenuation coefficient of the KN 3 -NaN 3 salt mixture was measured at 3 C, 35 C, and 4 for the wavelength range nm. The temperatures were chosen in order to avoid freezing at lower temperatures and bubbling at higher temperatures. The optical setup of the FTIR consisted of an external water cooled NIR light source, a quartz beam splitter and a room temperature InGaAs detector that operates in the wavenumber range 4-12 cm- 1 or in the wavelength range nm. At each temperature, the transmission of the incident light was measured twice for seven different thicknesses of the sample incremented by 1 cm. The measured transmission data for each optical path was the average of the two measurements. The instrumentation error was decreased by configuring the FTIR to run 32 scans for each measurement. Six values of the attenuation coefficient were obtained using Equation 3 and setting the transmission value for the smallest sample thickness to be the reference transmission value. The final value of the attenuation coefficient was calculated by averaging the six values. The dimensions of the support tube and cuvettes were carefully chosen in order to increase the accuracy of the results. The furnace is 3.48 cm (12 in) high and its inner diameter is 12.7 cm (5 in). Due to heat loss at the ends, the temperature gradient is smaller in the middle of the furnace. Therefore, the 7.62 cm (3 in) support tube was used such that the liquid sample was in the middle of the furnace. However, when the inner 13

14 (a) Lower parabolic mirror I (b) Leveling plate with 3 T bolts ( c) Scale with millimeter divisions Figure 6: Elements of the framing of the experimental apparatus. cuvette slides down to decrease the sample thickness, the displaced salt moves up between the cuvettes. This leads to a change in the bulk temperature of the salt. To minimize the change in temperature, a wide outer cuvette (Di=8 mm, D =84 mm) and a narrow cuvette ( di=45 mm, d =48) were ordered. Since the temperature of the salt did not change appreciably in the axial direction, the bulk temperature of the salt was measured by a K-type termocouple placed in the middle of the furnace and in contact with the outer cuvette. The thermocouple was not placed inside the salt in order to avoid contamination and because the temperatures measured outside and inside differed by less than 1 C. 14

15 (a) Support tube fixed to the leveling plate (b) Support tube leveled with the T-bolts ( c) Outer cuvette on a connecting washer Figure 7: Alignment of the outer cuvette. (a) Inner cuvette held with a utility clamp (b) Furnace covered with a refractory material Figure 8: Mounting of the inner cuvette and installation of the furnace. 15

16 (a) Binary nitrate salt at room temperature (b) Molten salt inside the heated furnace at T=3 C Figure 9: KN 3 -NaN 3 salt mixture melted for attenuation measurements The molten salt sample was obtained by melting the necessary amount of the binary nitrate salt mixture. To measure the transmission of light at 7 positions incremented by 1 cm, the volume of the molten salt was chosen so that its height was equal to 1 cm when the inner cuvette was above the salt-air interface. Therefore, since the density of the binary nitrate salt mixture is around 1.8 g/ crn 3, 1 kg of the salt mixture ( m N an o 3 =6 g, mk No 3 =4 g) was poured between the cuvettes. The salt column in Figure 9a is higher than 1 cm because the salt is not fully consolidated until it is melted. Finally, the salt mixture was melted and maintained at the specified temperatures by adjusting the power supplied to the electrical furnace (Figure 9b). The vertical gap in the furnace wall is open for illustrative purposes. During the attenuation measurements the gap was sealed with a refractory material to reduce the heat flow outside the furnace. 3.2 Water validation The validity of the experimental results is of immense importance. Therefore, the attenuation coefficient of water was determined before measuring the attenuation coefficient of molten salts. The light attenuation coefficients of water measured at various wavelengths was compared with the published attenuation coefficient [6]. Water was chosen for validation because it is nearly transparent in the visible region and it can be readily used in the experimental apparatus with small modifications instead of the binary nitrate salt mixture. The transmittance of light over two spectral regions was measured using two optical 16

17 Table 1: Detailed description of the optical setups used for water validation. Name Wavenumber Wavelength Light source Beamsplitter Detector range, c I range, n_ Setup NIR (External) Quartz Si-Diode Setup NIR (External) Quartz InGaAs setups (Table 1) at six positions with path lengths incremented by I cm. The position with the shortest path length was taken as a reference position. The transmittance of light in water was calculated by taking the relative path lengths of other positions as Ax in Equation 3. The attenuation coefficient of water was measured in two optical regions: nmim (7-14 cmn') and urn (8-25 cm-'). The attenuation coefficient of water with relatively high signal to noise ratio was calculated for wavelengths between around 5 inn and 133 nm. 12 * Published data ;ured datai Meas -j- 8- U c et I Wavelength [iu] Figure 1: Attenuation of light in DI water for the first optical setup [6]. Figures 1 and 11 show that the measured attenuation coefficients agree with the published data. The attenuation data for the entire solar spectrum cannot be obtained accurately due to noise. In addition, water cannot be used for the validation of the apparatus for wavelengths above around 134 nm because the published attenuation coefficients are very high and most of the incident light is absorbed in water. 3.3 Measurement of the reflectivity of 34L stainless steel The goal of the reflectivity measurements was to determine the influence of the high temperature binary nitrate molten salt on the reflectivity of as-rolled 34L stainless steel. The reflectivity of 34L stainless steel at the incident angles of 1", 4), and 7' was 17

18 2 I 'Published data Measured data 15 o 1 U C W~ Wavelength [nmn] Figure 11: Attenuation of light in DI water for the second optical setup [6]. measured on both sides of the samples for each angle and averaged for the wavelength range 6-5 ni by using three optical setups. Table 2 presents detailed information about the optical setups. The measurements were made by mounting a Seagull T 1' variable angle reflection accessory in the sample compartment of the FTIR shown in Figure 12. The reflectometer contains seven mirrors that direct the incident light onto a sample and guide the reflected light to the detector. Table 2: Detailed description of the optical setups used for reflectivity measurements. Name Wavenumber Wavelength Light source Beaisplitter Detector range, cm 1 range, inn Setup NIR (External) Quartz Si-Diode Setup NIR (External) Quartz InGaAs Setup MIR (Internal) KBr DLaTGS The wavelength range was increased to 5 pin because at high tenperatures radiation heat transfer becomes important and the black body spectrum at T-6'C is centered just below 5 pm. Since the binary nitrate salt mixture decomposes at temperatures around 5-6"C, the salt was maintained at T=5"C. Two 34L stainless steel strips that are 1.16 cm (4 in) long and 2.54 cm (1 in) wide were placed inside the inner cuvette containiig 3 g of the binary nitrate salt mixture. While one strip was iunmersed completely in the salt, the other strip was placed above the salt-air interface such that it was exposed to the fumes generated by the molten salt (Figure 13). To produce an appreciable effect, the test was run for 8 days at T=5'C. 18

19 Figure 12: Refiectometer accessory inside the sample compartment of the FTIR spectrometer. (a) 34L stainless strips before the 8-day test (b) Beginning of the 8-day immersion test Figure 13: 34L stainless steel strips before the 8-day immersion test in the binary nitrate salt mixture. (a): A - below salt-air interface, B - above salt-air interface. 4 Results and Discussion 4.1 Attenuation of light in the KN 3 -NaN 3 salt mixture The attenuation coefficient of the binary nitrate salt mixture was measured for the wavelength range nm at temperatures 3 C, 35 C, and 4 C. The attenuation 19

20 - - data was coupled with the data acquired by Passerini for the wavelength range 4-8 nn in order to cover the solar spectrum T=3" C u 8 6 a) Wavelength [nm] Figure 14: Attenuation of the binary nitrate molten salt for wavelengths between 1 nn and 25 in at T=3 C I c 12 1 u 8 c ' 6 c S4 2 F Wavelength [nm] Figure 1.5: Attenuation of the binary nitrate molten salt for wavelengths between 1 inn and 25 inn at T=35'C. Figures 14, 1.5, and 16 show the attenuation coefficient of the binary nitrate salt mixture in the infrared region. The salt mixture is semitransparent up to 17 mn and opaque for longer wavelengths. The attenuation coefficient peaks around 2.5 pm as 2

21 16 -T=46C 14 U.A m a) Wavelength [nm] Figure 16: Attenuation of and 25 nm at T=4'C. the binary nitrate molten salt for wavelengths between 1 inn preclicted. The error bars in all plots represent one standard deviation. The uncertainty is high close to the upper and lower limits of the wavelength range for two reasons. First, these wavelengths are close to the operating limits of the InGaAs detector. Second, close to 2.5 pm. the attenuation coefficient is very high and the salt absorbs all photons with these wavelengths. This results in a low signal to noise ratio I ---- Passerini, 21: T=3" C Measured: T=3" C 12 1 u 8 c 6 I S4 2 ni- -I I Wavelength [nm] 2 25 Figure 17: Attenuation of the binary nitrate molten salt for wavelengths between 4 nm and 25 nn at T=34C. 21

22 18 j Passerini, 21: T=35" C Measured: T=35" C 12 1 U 8 6 Q) 4 4 ni Wavelength [nm] 2 25 Figure 18: Attenuation of the binary nitrate molten salt for wavelengths between 4 1m and 25 nim at T=35'C. U a) I Passerini, 21: T=4" C Measured: T=4" C Wavelength [nm] 2 25 Figure 19: Attenuation of the binary nitrate molten salt for wavelengths between 4 inn and 25 nm at T=4"C. The attenuation coefficient over the solar spectrum was constructed in Figures , and 19 by coiplling the Passerini data for the wavelength range 4-8 nm1 with the data measured in this study. The KNO 3 -NaNO3 salt mixture is opaque closer to the edges of the solar spectrum and semitransparent for the intermediate wavelengths. The published data and the measured data intersect at 8 nm1. The attenuation coefficients at the borderline (1 not transition smoothly due to several reasons. The composition of the sample may not 22

23 be identical due to different levels of impurities inside the molten salt samples. In addition. measuring the attenuation coefficient for wavelengths close to the operating limits of the detector is less accurate C 8,.., ~ 6 C.) C 4 Measured: 3' C. Measured: 35" C - Measured: 4" C Passerini, 21: 3" C Passerini, 21: 35" C o Passerini, 21: 4" C * Pasrn,21: Wavelength [nm]n Figure 2: Attenuation of the binary and 25 inn at three temperatures. nitrate molten salt for wavelengths between 4 nm 2 15 Measured: 3" C * Measured: 35" C * Measured: 4" C * Passerini, 21: 3" C 1 Passerini, 21: 35" C * Passerini, 21: 4" C o 1 U e ED * * y C ) 5 E V, TT vy U 1 Ye IV TIV TV Wavelength [nn] 2 Figure 21: Attenuation of the binary nitrate molten salt for wavelengths between 4 nm and 22 ni at three temperatures. The temperature dependence of the attenuation coefficient is visualized in Figure 2 by including the attenuation data for the visible and infrared regions at temperatures 23

24 3 C, 35 C, and 4 C. Figure 21 shows that the binary nitrate salt is semitransparent for wavelengths between 5 nm and 17 nm, and highly absorbent for the rest of the solar spectrum. Passerini discovered that the absorption edges in the ultraviolet region shifts to the right as temperature increases. Figure 21 shows that the absorption edge in the infrared region also shifts to the right as the temperature of the molten salt is raised. Measured attenuation coefficients for wavelengths below 12 nm diverge as the wavelength decreases. This can be explained by the fact that the intensity of the light source is peaked around 2 µm and the signal decreases closer to the edges of the wavelength range of the measurements. 4.2 Reflectivity of 34L stainless steel The reflectivity of a control sample and two test samples of 34L stainless steel was measured over the wavelength range 6-5 nm. The 8-day immersion test changed the colors of the test samples and the molten salt. Figure 22 shows that the strip immersed in the salt turned dark brown and the molten salt became green. The fact that the mass of Strip A decreased by.23 suggests that the iron, nickel, and chromium contents in the strip diminished due to oxidation. Subsequently, the nickel and chromium oxides contaminated the salt and changed its color to green. The strip above the salt-air interface became charcoal color due to mass gain equal to.63 of its initial mass associated with the fumes. 8 (a) 34L stainless strips after the 8-day test (b) End of the 8-day immersion test Figure 22: 34L stainless steel strips after the 8-day immersion test in the binary nitrate salt mixture. (a): A - below salt-air interface, B - above salt-air interface. 24

25 I The reflectivity of the control sample and the test, samples at 1'. 4', and 7' are shown in Figures 23, 24, and 25. The reflectivity was measured for the wavelength range 6-5 nm using three optical setups described in Table 2. The reflectivity increases with angle as predicted by the Fresnel equations. The local peak of reflectivity around 8 nm shifts to the left as the incident angles increases. The control sample has relatively higher uncertainty in reflectivity due to its rough surface finish. However, oxide coating made the surface of tihe test samples smoother and the two measurements at each angle yield similar results. Compared to the control sample. the reflectivity of the test, samples after the 8-day test decreases significantly regardless of their position relative to the salt-air interface. Since the reflectivity of the test samples is below 1% for the solar spectrun, the metallic components of a solar power system in contact with the molten salt are highly absorbent to the sunlight that is not attenuated by the salt. The reflectivity at 1 n1 is discontinuous because the signal to noise ratio for Setup 1 is lower than those for Setup 2 and 3. This introduces large instrumental errors and leads to an abrupt change in reflectivity at 1 nm. For the wavelengths near the peak of the spectrum of a black body at 6"C, the reflectivity is less than 5% for the smaller angles but reaches 15% for the largest incident angle " 11 S4-.8-7" >.6 Setup 1 Setup 2 Setup Wavelength [nm] Figure 23: Reflectivity of the control 34L stainless steel strip for wavelengths between 6 nmi and 5 nm. 25

26 F " 4" -1 7".5 A:. 1 [Setup 1 Setup 2 Setup Wavelength [nm] Figure 24: Reflectivity of the 34L stainless steel strip above the salt-air interface after the 8-day imniersion test in the binary nitrate molten salt for wavelengths between 6 mrn and 5 m F 4" --- 7".5.1 Setup 1 Setup2 Setup Wavelength [ninj 5 Figure 25: Reflectivity of the 34L stainless steel strip below the salt-air interface after the 8-day immersion test, in the binary nitrate molten salt for wavelengths between 6 urn and 5 mn. 5 Conclusions The attenuation coefficient of the KNO 3 -NaNOj salt mixture was measured over the wav elength range nmi at three temperatures. This data when combined with the 26

27 I 2.) -- Global tilt W*m-2*nm-1 E C- E 1.51) - Direct+ci rcumsolar W*m-2*nm-1 1.5) N CO 1.51) ( 5(x) 1H) 15() 2() 25( 3(( 35(X) 41 Wavelength nrm (a) ASTM G173-3 reference spectra. Data adapted from [5]. L).2 m Etr W*m- 2*nm v Measured: 3' C * Measured: 35" C * Measured: 4" C Passerini, 21: 3" C o Passerini, 21: 35" C * Passerini, 21: 4" C eu 2 U Wavelength [nm] (b) Attenuation of the binary nitrate molten salt for wavelengths between 4 nm and 25 inn at three temperatures Figure 26: ASTM G173-3 solar spectra and the attenuation coefficient of the binary nitrate molten salt for the wavelength range n at three temperatures data for wavelengths between 4 1m and 8 inn acquired by Passerini quantify the temperatllre and wavelength dependencies of the attenuation coefficient over the solar spectrum [1]. 27

28 Accurate measurements were obtained by building an experimental apparatus designed for attenuation measurements of semitransparent liquids at high temperatures. Two concentric and level cuvettes were used to readily change the thickness of the molten salt sample and to avoid interface effects. The large volume of the molten salt decreased the concentration of impurities in the salt sample and improved the accuracy of the results. The results of the attenuation measurements showed that the binary nitrate molten salt is opaque for wavelengths close to the edges of the solar spectrum and semitransparent for wavelengths between 5 nm and 17 nm. The absorption edges in the infrared and ultraviolet regions shift to the right with increasing temperature. Figure 26 summarizes the results of the attenuation measurements and provides a perspective of how the attenuation coefficient varies relative to the expected solar input. The reflectivity of a control and two test samples of 34L stainless steel was measured for the wavelength range 6-5 nm that covers the solar spectrum and the peak of a black body spectrum at 6 C. The reflectivity of the samples increased with the incident angle. The test samples reflected the incident light poorly over the solar spectrum, but reflected about 15% of the light around 5 pm. The molten salt sample inside a narrow cuvette changed its color from pale yellow to dark green after the 8-day immersion test. The contamination of the salt with oxides was fast because the ratio of the molten salt volume to the surface area of the sample below the salt-air interface is low (-2.8 cm). Therefore, a similar contamination of a large salt storage tank will take significantly longer time period. Nonetheless, options for keeping the salt clean should be considered because the oxides not only contaminate the molten salt but also change its optical properties. Additional measurements of corrosion with time should be undertaken to determine whether this is an initial burst of dissolution of oxides with formation of a stable oxide layer and limited further dissolution or a longer-term phenomenon. The results obtained in this study can be improved further with simple adjustments. The attenuation coefficient of the binary nitrate salt mixture was measured up to 2.5 Pm. To extend the upper limit in order to cover the peak of a black body spectrum at T=6"C, the quartz windows of the cuvettes should be replaced with sapphire windows that transmit light with wavelengths up to 5 tam. In addition, the reflectivity of 34L stainless steel in the wavelength range covered by Setup 1 can be measured more accurately by increasing the sensitivity of the detector to improve the signal to noise ratio. 28

29 References [1] Stefano Passerini. Optical and Chemical Properties of Molten Salt Mixtures for Use in High Temperature Power Systems. Master's thesis, Massachusetts Institute of Technology, 21. [2] William D Drotning and Sandia Laboratories. Optical Properties of Solar-Absorbing Oxide Particles Suspended in a Molten Salt Heat Transfer Fluid. Solar Energy, 2(4): , [3] Alexander H. Slocum, Daniel S. Codd, Jacopo Buongiorno, Charles Forsberg, Thomas McKrell, Jean-Christophe Nave, Costas N. Papanicolas, Amin Ghobeity, Corey J. Noone, Stefano Passerini, Folkers Rojas, and Alexander Mitsos. Concentrated solar power on demand. Solar Energy, 85(7): , July 211. [4] Daniel S. Codd. Concentrated solar power on demand. PhD thesis, Massachusetts Institute of Technology, July 211. [5] National Renewable Energy Laboratory. Reference solar spectral irradiance: ASTM G [6] Oregon Medical Laser Center. Optical absorption of water compendium. URL: [7] Charles Forsberg, Daniel Curtis, John Stempien, Ruarihd MacDonald, and Per Peterson. Fluoride-Salt-Cooled High-Temperature Reactor (FHR) Commercial Basis and Commercialization Strategy. Advanced Nuclear Power Program, MIT-ANP-TR-153(December), 214. [8] JANIS. Quartz (Si 2 ). URL: Quartz-SiO2_TransmissionCurveDataSheet.sfib.ashx. 29

30 6 Appendix: Attenuation coefficient of the KNO 3 -NaNO 3 (4-6 wt%) salt mixture in tabular form Table 3: Attenuation coefficient of the binary nitrate mixture at T=3'C. Wavelength, nm Attenuation coefficient, m- 1 Standard deviation, m

31 Table 4: Attenuation coefficient of the binary nitrate mixture at T=35"C. Wavelength, nm Attenuation coefficient, m- 1 Standard deviation, m

32 Table 5: Attenuation coefficient of the binary nitrate mixture at T=4'C. Wavelength, nm [Attenuation coefficient, m Standard deviation, m 1 l