flexible aerogel as a superior thermal insulation for ...

FLEXIBLE AEROGEL AS A SUPERIOR THERMAL INSULATION FOR HIGH TEMPERATURE SUPERCONDUCTOR CABLE APPLICATIONS S. White, J. Demko, and A. Tomich Citation: AIP Conference Proceedings 1218, 788 (2010); doi: 10.1063/1.3422432 View online: http://dx.doi.org/10.1063/1.3422432 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1218?ver=pdfcov Published by the AIP Publishing Articles you may be interested in CRYOGENIC THERMAL PERFORMANCE TESTING OF BULK-FILL AND AEROGEL INSULATION MATERIALS AIP Conf. Proc. 985, 152 (2008); 10.1063/1.2908517 VACUUM-INSULATED, FLEXIBLE CRYOSTATS FOR LONG HTS CABLES: REQUIREMENTS, STATUS AND PROSPECTS AIP Conf. Proc. 985, 1343 (2008); 10.1063/1.2908492 Thermal conductivity measurements of aerogel-impregnated shuttle tile at cryogenic temperatures AIP Conf. Proc. 613, 1549 (2002); 10.1063/1.1472189 Aerogel beads as cryogenic thermal insulation system AIP Conf. Proc. 613, 1541 (2002); 10.1063/1.1472188 Thermal insulation performance of flexible piping for use in HTS power cables AIP Conf. Proc. 613, 1525 (2002); 10.1063/1.1472186

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions IP: 107.175.204.239 On: Sat, 19 Mar 2016 08:23:40

FLEXIBLE AEROGEL AS A SUPERIOR THERMAL INSULATION FOR HIGH TEMPERATURE SUPERCONDUCTOR CABLE APPLICATIONS

S. White1, J. Demko2, and A. Tomich1 1

Aspen Aerogels, Inc Northborough, MA, 01532, U.S.A 2

Oak Ridge National Laboratory. Oak Ridge, TN, 37830, U.S.A

ABSTRACT High temperature superconducting (HTS) cables are an advanced technology that can both strengthen and improve the national electrical distribution infrastructure. HTS cables require sufficient cooling to overcome inherent low temperature heat loading. Heat loads are minimized by the use of cryogenic envelopes or cryostats. Cryostats require improvement in efficiency, reliability, and cost reduction to meet the demanding needs of HTS conductors (1G and 2G wires). Aspen Aerogels has developed a compression resistant aerogel thermal insulation package to replace compression sensitive multi-layer insulation (MLI), the incumbent thermal insulation, in flexible cryostats for HTS cables. Oak Ridge National Laboratory tested a prototype aerogel package in a lab-scale pipe apparatus to measure the rate of heat invasion. The lab-scale pipe test results of the aerogel solution will be presented and directly compared to MLI. A compatibility assessment of the aerogel material with HTS system components will also be presented. The aerogel thermal insulation solution presented will meet the demanding needs of HTS cables. KEYWORDS: Aerogel, cryogenic thermal insulation, MLI replacement, flexible cryostat, HTS, energy savings INTRODUCTION CREDIT BE INSERTED ONwidely THE FIRST OF EACH An aging LINE and (BELOW) inadequateTO power grid is now seen PAGE as among the greatest FOR ARTICLES ON pp.in18–25, 26–33, States, 68–75, Europe and obstacles toPAPER efforts EXCEPT to restructure power markets the United 121–127, 136–142, 207–214, 246–253, 355–362, 388–395, 499– 506, 507–514, elsewhere. Utilities face several converging pressures, including steady load growth, 609–614, 780–787, 796–803, 804–811, 905–912, 1291–1300, 1301–1308, 1369–1376, 1581–1592, 1593–1600, and 1647–1651


CP1218, Advances in Cryogenic Engineering: Transactions of the Cryogenic Engineering Conference - CEC, Vol. 55, edited by J. G. Weisend II © 2010 American Institute of Physics 0-7354-0761-9/10/$30.00

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unplanned additions of new generation capacity, rising reliability requirements, sharp price volatility resulting from new competitive forces, and stringent barriers to siting new facilities, particularly extra-high voltage equipment. One of the technologies with the greatest promise to address these concerns is high-capacity, underground HighTemperature Superconductor (HTS) cable, which is capable of serving very large power requirements at medium and high-voltage ratings [1]. HTS cables require sufficient cooling to overcome inherent low temperature heat loading. Heat loads are minimized by the use of cryostats or cryogenic envelopes. The successful application of HTS power cables requires improved flexible cryostats with respect to their thermal heat load between environment (300 K) and liquid nitrogen (LN2) (77 K) operating temperatures. Current cryostats use a high vacuum and “superinsulation” which consists of reflecting foils and spacer materials for thermal insulation. This high vacuum, multi-layer insulation (MLI) suffers from several issues including vacuum integrity, cumbersome installation, damage during installation, and degradation over time. If MLI is damaged during installation it often can go undetected until system-level testing. To overcome these challenges Aspen Aerogels is developing a compression resistant aerogel thermal insulation package to replace or enhance compression sensitive MLI in flexible cryostats for HTS cables. Aerogel thermal insulation packages will provide a more reliable alternative because they are robust, will not degrade significantly over time and are much higher performing in cases of vacuum loss versus MLI. Robust aerogel materials, such as those being developed at Aspen Aerogels, could easily replace or enhance MLI on sections of HTS cables where MLI is not effective, such as in cable bends, and on joints and valves. Aspen Aerogels has designed an aerogel thermal insulation package, tested aerogel material compatibility with HTS system components via an outgassing test (ASTM E1559), and fabricated and tested a prototype aerogel insulation package in a lab-scale pipe apparatus to measure the rate of heat invasion at Oak Ridge National Laboratory. Based on the outgassing data, it was determined that any outgassing species in the aerogel material that are associated with the manufacture of the material will not result in any contamination issues. The aerogel-based insulation package for the lab-scale pipe test consisted of 2 layers of Aspen’s fiber reinforced aerogel with a vapor barrier film on the surface of each layer. During this test the measured heat load for the aerogel package was compared with measurements for MLI for high vacuum cases and after letting the vacuum space up to atmospheric pressure. The MLI maintained lower temperatures under high vacuum due to the lower heat leak; however, once the vacuum was lost the aerogel insulated apparatus had a lower peak heat leak. The detailed set-up and results of this test will be described in this paper. Background into Aerogels Aerogels have fine pores of nanometer dimensions, extremely high porosities (generally between 90 and 99%), and very unique lattice structures. Because of their nanoporous, low-solid structures, there is a complex interrelationship between gas conduction, solid conduction, and radiation components of thermal conduction within aerogel structures making them excellent insulating materials. Aerogel-based insulations typically boast thermal conductivities two to three times lower than conventional insulation at room temperature and atmospheric pressure. Despite their tremendous thermal properties, commercialization of aerogels had been limited due to the inherent brittle nature and high cost of the material. However, Aspen Aerogels made a significant breakthrough in practical implementation of aerogel insulation


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materials by using fiber-batting reinforcement and reducing the manufacturing process and thus cost. The materials are now being made in large quantities as a flexible composite with equivalent insulation properties to the monolithic materials. Typical thickness of Aspen’s aerogel blankets is 2 – 10 mm and they can be stacked, if necessary, to meet thermal conductivity requirements for various applications. Currently, there are two commercially available thermal insulation product families available through Aspen Aerogels, including Pyrogel® and CryogelTM, that can be used across a range of temperatures spanning from -273 ºC to 650 ºC, depending on the application requirements. Aspen Aerogels has the capability to manufacture ~40 million square feet of these materials per year. Pyrogel is a high temperature insulation for applications such as exhaust ducts, steam lines and boilers. Cryogel is a low temperature thermal insulation and was the target material used in the cryostat test described in this paper. Some areas where the benefits of flexible aerogel thermal insulation blankets as a superior insulation solution for high temperature superconducting cables include: • Thermal Performance - Aerogel is not significantly conductive in the plane of the insulation layer compared to the through thickness direction. Therefore any unexpected heat gain to a given layer will not be conducted along the layer. The thermal performance of the aerogel is predictable and is essentially unaffected by compression loads consistent with handling, installation and inadvertent “tool drop” impact. • Installation Time - Aerogel insulation installation is not labor intensive. Fewer layers are required (~3 layers of aerogel = ~30 layers of MLI) for equivalent thermal insulation performance. Since aerogels have lower thermal conductivity at reduced vacuum levels, their use in long length cryostats would require fewer joints (vacuum breaks). • Minimal Risk - Aerogels have robust, reproducible performance. Aerogel composites have been flexed tens of thousands of cycles with minimal performance degradation. HIGH PERFORMANCE CRYOSTAT TEST A high performance vacuum insulated flexible cryostat design was fabricated and initial testing conducted to measure thermal performance of a new aerogel insulation system. The insulated test pipe is shown in FIGURE 1. The test cryostat was instrumented with five thermometers inside the vacuum space to measure the thermal profile developed in the insulation as shown in FIGURE 2. The cryostat consisted of a 3 inch NPS corrugated inner pipe covered with a braid and a 5 inch NPS corrugated outer pipe covered with a braid. The original design called for three layers of Cryogel to be installed. However, the annular gap between the two sections was smaller than anticipated and only two layers of the aerogel blanket could be installed. At one end of the cryostat a getter packet was installed which limited the section of about 8 inches to only a single layer of aerogel blanket. The heads at the end of the test section were covered with MLI. Five resistance thermometers (RTD’s) were installed as shown in FIGURE 2. Two of the RTD’s were on the rigid pipe section near the heads on the inner pipe. One was placed on the outside of the braid at the midpoint of the inner pipe. One RTD was place on the outside of the aerogel blanket. A small cut was made at the seam between the two blankets and the last RTD was inserted underneath the top layer of the aerogel blanket.


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FIGURE 1. Complete aerogel insulated test pipe.

Tin

T3

T5

Outside of blanket

T4

Under outer layer

T 2 On Braid

T1

Tout

LN2 Flow FIGURE 2. Schematic of instrumented flexible cryostat.

Three tests were run on the aerogel insulated cryostat. The first test was conducted to measure a baseline heat load for the cryostat. A photograph of the test cryostat is provided in FIGURE 3. Two test runs in which the insulating vacuum was opened to atmosphere were conducted following the initial test. These tests are described below. Test 1 – Atmospheric Subcooler High Vacuum A baseline case was first run with the test cryostat as received from the fabrication shop after the instrumentation was installed (RTD’s described above). This served as a check-out of the instrumentation prior to the first vacuum break run. During all tests the temperatures for the RTD’s identified in FIGURE 2 were monitored, along with the pressure of the supply and return liquid nitrogen. The temperature rise between the inlet and outlet temperatures during this test was 0.273 K and corresponds to an average heat load of 79.6 W. Unfortunately this heat load is higher than a perfectly lofted MLI system, which had a baseline heat load of 29.4 W [2]. While there is room for improvement in an aerogel based insulation package for high vacuum conditions the real benefit for using aerogel was anticipated for cases where the vacuum is lost. This is described in the following section for Test 2. Test 2 – Atmospheric Subcooler with Loss of Vacuum The first loss of thermal insulating vacuum was conducted during the second test run. The baseline heat load was 72.9 W, the peak heat load after opening to atmosphere was 503.4 W, and the stable heat load was 300.4 W.


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FIGURE 3. Aspen aerogel insulated cryostat installed in ORNL test facility undergoing initial testing.

FIGURE 4 shows a comparison of the instantaneous temperature rises for the aerogel insulated cryostat and the MLI insulated cryostat with the vented subcooler. The peak heat load for the MLI was 538 W in comparison to aerogel at 503.4. It should also be noted that a significant amount of frost formed on the MLI insulated cryostat as a result of the poor insulation level after losing vacuum in comparison to the aerogel insulated test pipe. Test 3 – Subatmospheric Subcooler with Loss of Vacuum For the third run, a subcooler was vacuum pumped to lower the temperature of the circulating nitrogen stream. The minimum average temperature reached was around 73.6 K for the high vacuum case. Air condenses at 78.3 K at atmospheric pressure. So when the cryostat is opened to atmosphere, air can condense on the cold surfaces and increase the heat transfer to the cryostat. The baseline heat load for run 3 was only 46.8 W at high vacuum, this is quite a bit lower than what was observed in the first two runs (79.6 W and 72.9 W). The peak heat load after opening to atmosphere was 1314.4 W and it stabilized at 911.6 W. While the MLI achieved lower temperatures under high vacuum due to the lower heat leak, once the vacuum is lost, the aerogel blanket had a lower peak heat leak 1314.4 W vs. 1733 W). The instantaneous temperature rise is compared in FIGURE 5. The aerogel insulated test pipe reached a much lower peak. This could be in part due to the slightly higher temperature of the nitrogen flowing through the cryostat. ANALYSIS AND DISCUSSION The measured heat load for the aerogel blankets are compared with measurements for MLI in FIGURE 6 for high vacuum cases and FIGURE 7 for after letting the vacuum space up to atmosphere. Air condenses at 78.3 K at 1 atmosphere so the relative temperature to this point influences the heat transfer when the vacuum is opened to atmosphere as shown in FIGURE 7. The X-axis in both FIGURE 6 and FIGURE 7 is the difference between the saturated air temperature at one atmosphere (= 78.3 K) and the inlet temperature. This is the temperature where the components of air (oxygen, nitrogen) would liquefy. FIGURE 6 illustrates that the high vacuum heat load is the same for the relatively narrow temperature change ((Tsat air-T) between -1 and 5 K). In FIGURE 7, the four points are the peak and the stabilized value after the peak. This figure shows that for the catastrophic loss of vacuum the temperature has a significant effect on the heat load. If the temperature is above 78.3 K, the heat load is small because Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions IP: 107.175.204.239 On: Sat, 19 Mar 2016 08:23:40

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the air does not condense/liquefy. The colder it gets there are enhancements to the heat transfer because the air liquefies and the heat transfer coefficient from that process is very high. The thermal energy is deposited quickly at first. As time goes by, the cryostat wall and insulation closest to the cryostat increase slightly in temperature and there is a decrease in the heat transfer to the nitrogen. An estimate may be obtained regarding the heat loads from the ends and supports from analysis of the insulation system using the design tool for cryogenic insulation systems (TISTOOL) [3]. For the calculation of the MLI cryostat, a layer-by-layer model of the MLI available in TISTOOL was used to determine the ideal heat load for the cryostat. For the aerogel insulated cryostat, TISTOOL contains data for aerogel prototype package used in this test that was measured with boundaries at 77 K and 293 K as a function of vacuum level. This was assumed to have similar thermal conductivity to the blanket material used in these tests. The highest vacuum data was applied in the analysis. TABLE 1 and TABLE 2 show the estimated contribution from the ends and supports for the high, low, and average measured heat load cases using the calculated ideal insulation heat load. For both systems the contribution of the ends and supports is a substantial part of the total cryostat heat load. 1.8

MLI (11.7 l/min)

Temperature Rise (K)

1.6 1.4 1.2 1 0.8

Aerogel (11.1 l/min)

0.6 0.4 0.2 0 0

100

200

300

400

500

600

Time (min)

FIGURE 4. Comparison of the instantaneous temperature rises for the aerogel insulated cryostat and the MLI insulated cryostat with the vented subcooler. 7

Temperature Rise (K)

6

MLI

5 4

Aspen Aerogel

3 2 1 0 0

100

200

300 400 Time (min)

500

600

700

FIGURE 5. Comparison of the instantaneous temperature rises for the aerogel and MLI insulated cryostat for the low temperature case (vacuum pumped subcooler).


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90 Aerogel MLI

80

Heat Load (W)

70 60 50 40 30 20 10 0 -1

0

1

2

3

4

5

6

7

(Tsat air - T) (K)

FIGURE 6. Measured heat loads for the cryostat under vacuum. 2000 1800 MLI Aerogel

Heat Load (W)

1600 1400 1200 1000 800 600 400 200 0 -1

0

1

2

3

4

5

6

7

(Tsat air - T) (K)

FIGURE 7. Heat loads for the cryostat after opening the vacuum space to atmosphere. TABLE 1. Estimated heat load due to end effects and supports in vacuum MLI cryostat. MLI Measurements [W] Calculated Ideal [W] Ends and Supports [W] High Low Average Standard Deviation

13.2 29.4 19.2 6.0

0.815 NA

TABLE 2. Estimated heat load due to end effects in aerogel insulated cryostat. Aerogel Measurements [W] Calculated Ideal [W] High Low Average Standard Deviation

46.8 79.6 66.4 17.3

22.8 NA

12.35 28.61 18.40 NA

Ends [W] 23.98 56.80 43.63 NA

SUMMARY A summary of the heat loads for the vacuum condition and the peak after the vacuum was broken are given in TABLE 3 for the aerogel insulated cryostat and TABLE 4 for the


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MLI insulated cryostat. The MLI insulated test cryostat achieved lower temperatures under high vacuum due to the lower heat leak; however, once the vacuum was lost the aerogel insulated apparatus had a lower peak heat leak (24%). The results of a test program of a cryostat insulated with aerogel have been presented and compared to the same cryostat with vacuum multi-layer superinsulation. The thermal responses have been measured for the flow stream and the insulation blankets. The contributions to the heat load from the ends and supports (in the MLI) are a significant portion of the total measured heat load. Future cryostat designs and tests should give more consideration on how to minimize these contributions. TABLE 3. Summary of Heat Loads for Aerogel Insulated Cryostat. Inlet T (K)

∆T with Vacuum (K)

Baseline Q (W)

Peak ∆T (K)

Peak Q After Loss of Vacuum (W)

Baseline Loss of Vacuum (atmospheric)

78.9 79.0

0.273 0.238

79.6 72.9

N/A 1.647

N/A 503.4

Loss of Vacuum (subatmospheric)

73.6

0.156

46.8

4.431

1314.

TABLE 4. Summary of Heat Loads for MLI Insulated Cryostat [2]. Inlet T (K)

∆T with Vacuum (K)

Baseline Q (W)

Peak ∆T (K)

Peak Q After Loss of Vacuum (W)

Baseline Loss of Vacuum (atmospheric)

78.8 78.9

0.056 0.091

18.1 29.4

N/A 1.662

N/A 538.

Loss of Vacuum (subatmospheric)

73.0

0.044

13.2

5.84

1733.

ACKNOWLEDGEMENTS This material is based upon work supported by the Department of Energy under Award Number DE-FG02-08ER85189. REFERENCES 1 2. 3.

http://www.nexans.com/eservice/CentralAmericaen/navigate_191449/Superconducting_Cable_Systems.html#top J. A. Demko, R. C. Duckworth, M. Roden, and M. J. Gouge, “Testing Of A Vacuum Insulated Flexible Line With Flowing Liquid Nitrogen During The Loss Of Insulating Vacuum”, Advances in Cryogenic Engineering , American Institute of Physics, Vol. 53, pp. 160-167, 2008. J. A. Demko, J. E. Fesmire, S. D. Augustynowicz, “Design Tool For Cryogenic Thermal Insulation Systems”, Advances in Cryogenic Engineering, American Institute of Physics, Vol. 53, pp. 145-151, 2008.


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