Michael F. Piszczor, Jr.

Glenn Research Center, Cleveland, Ohio

Robert P. Macosko

Analex Corporation, Brook Park, Ohio

A High-Efficiency Refractive Secondary

Solar Concentrator for High Temperature

Solar Thermal Applications

NASA/TM—2000-208401

July 2000
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Michael F. Piszczor, Jr.

Glenn Research Center, Cleveland, Ohio

Robert P. Macosko

Analex Corporation, Brook Park, Ohio

A High-Efficiency Refractive Secondary

Solar Concentrator for High Temperature

Solar Thermal Applications

NASA/TM—2000-208401

July 2000

National Aeronautics and

Space Administration

Lewis Research Center

Prepared for the

Space Technology and Applications International Forum (STAIF–98)

sponsored by the University of New Mexico’s Institute for Space Nuclear Power Studies

Albuquerque, New Mexico, January 25–29, 1998
Acknowledgments

The authors wish to acknowledge the technical expertise and support of the entire

NASA Glenn/Analex Corporation/ADF Corporation team, whose dedication to this project made these

advancements possible. The authors also wish to extend their gratitude to the NASA Marshall

“Shooting Star Experiment” management and team for their technical support and dedication to the

implementation of this technology.

Available from

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7121 Standard Drive

Hanover, MD 21076

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National Technical Information Service

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Price Code: A03

Note that at the time of research, the NASA Lewis Research Center

was undergoing a name change to the

NASA John H. Glenn Research Center at Lewis Field.

Both names may appear in this report.
NASA/TM—2000-208401 1

A HIGH-EFFICIENCY REFRACTIVE SECONDARY SOLAR CONCENTRATOR

FOR HIGH TEMPERATURE SOLAR THERMAL APPLICATIONS

Michael F. Piszczor, Jr.

NASA Glenn Research Center

21000 Brookpark Road

Cleveland, OH 44135

(216) 433-2237

Robert P. Macosko

Analex Corporation

3001 Aerospace Parkway

Brook Park, OH 44142

(216) 433-6161

Abstract

 A refractive secondary solar concentrator is a non-imaging optical device that accepts focused solar energy from a

primary concentrator and redirects that light, by means of refraction and total internal reflection (TIR) into a cavity

where the solar energy is used for power and/or propulsion applications. This concept offers a variety of advantages

compared to typical reflective secondary concentrators (or the use of no secondary at all): higher optical efficiency,

minimal secondary cooling requirements, a smaller cavity aperture, a reduction of outgassing from the cavity and

flux tailoring of the solar energy within the heat receiver. During the past 2 years, NASA Lewis has been

aggressively developing this concept in support of the NASA Marshall Shooting Star Flight Experiment. This paper

provides a brief overview of the advantages and technical challenges associated with the development of a refractive

secondary concentrator and the fabrication of a working unit in support of the flight demonstration program.

INTRODUCTION

 Over the years, NASA Lewis has developed a variety of technologies that support the use of solar thermal power

for space systems. The refractive secondary solar concentrator described within this paper is a key element of that

technology thrust. The secondary concentrator unit, when used in conjunction with a large primary solar

concentrator, focuses the sunlight into a heat receiver cavity. The energy absorbed within this cavity can then be

directly converted into electrical energy for use by the spacecraft or used to heat a propellant to provide thrust for

orbit transfer and on-orbit maneuvering. This concept of solar thermal propulsion is currently being developed

under NASA and US Air Force programs. Under the auspices of the NASA Marshall program, the “Shooting Star

Experiment” (SSE) is a short-term space flight demonstration program to study and quantify the benefits and

operational implications of solar thermal propulsion (Curtis 1998). In cooperation with NASA Marshall, the NASA

Lewis Refractive Secondary Concentrator Team is fabricating a prototype secondary concentrator for ground test

and evaluation. Successful testing of this prototype unit should allow for its inclusion in the final flight hardware

design. This paper will briefly describe the progress made on the design, fabrication and testing of this first

prototype.

 For solar thermal energy applications in space, the secondary concentrator provides major benefits. It maximizes

the input of solar energy within the heat receiver cavity, while minimizing the size of the aperture necessary to

capture the light. This is important since a larger aperture increases the amount of heat lost through radiation. This

is extremely critical for cavities operating at high temperatures, i.e. 1500K to 2500K. Most large-scale solar thermal

designs have proposed using reflective secondary concentrators. A single crystal refractive secondary provides a

number of advantages over the traditional hollow reflective compound parabolic concentrator (CPC). The primary

advantages are higher efficiency, higher concentration ratio, flux tailoring, and the ability to function without

requiring elaborate cooling features. These advantages and a more detailed analysis of the design have been

described in a previous publication (Soules 1997).

CONCEPT DESIGN

DTIRC/Flux Extractor

 There has been extensive research over the past 30 years in the design and application of secondary concentrators.

(Winston 1995). One specific type of secondary is the Dielectric Total Internally Reflecting Concentrator (DTIRC).

(Ning 1987). The secondary concentrator being developed under this program is based on the DTIRC concept. The

DTIRC by itself cannot efficiently pass all of the solar energy that it accepts into a lower index media (i.e. a heat

receiver cavity in air or vacuum). A DTIRC that is shaped for maximum concentration ratio will reflect, at the exit
NASA/TM—2000-208401 2

surface of the DTIRC crystal, as much as 50% of the solar energy that it accepts at the inlet surface. In order to

achieve a high optical efficiency, the secondary is connected to a flux extractor. The flux extractor is a dielectric rod

with facets of various size, shape, quantity and finish that is attached to the exit of the secondary. It projects into the

heat receiver cavity and permits nearly complete extraction of the solar energy from the DTIRC. It is typically made

of the same material as the concentrator but may be made from another material with a different index of refraction

(preferably higher). A sketch of a refractive secondary concentrator and flux extractor, as it would be installed in a

typical solar thermal propulsion engine, is shown in Figure 1.

Flux Tailoring

 The flux extractor, through changes in the design shape and surface, can provide variation in the solar flux

distribution within the cavity. This feature may be desired for certain applications, such as solar thermal propulsion,

where maintaining a proper thermal profile is important for efficient operation. Figure 2 shows a 3D trace for a

finite number of randomly distributed rays that enter the DTIRC within the design acceptance angle. As shown in

the ray trace, the majority of the light passing through the DTIRC stays in the flux extractor via TIR until it can exit

the extractor by striking the facets at angles that are less than the TIR angle. The corresponding flux distribution

pattern for a specific design is presented in Figure 3. It has been shown by Opticad analysis that the flux pattern can

be controlled by varying the extractor facet length, number and surface finish (specular or diffuse). Analysis and

testing is in progress to identify the surface treatments and shape alternatives possible that would provide efficient

solar flux distribution for a typical solar thermal propulsion engine.

Material Selection

 Currently, four “man-made” single crystal materials have been identified as possible candidates for use in this

application. Each material has temperature, environmental and/or fabrication limitations that would constrain its use

for certain mission application. This data is summarized in Table 1. Since the “Shooting Star” flight experiment is

planned for temperatures of 2000K or below, sapphire or YAG are presently the material of choice. Further material

evaluation and coating analysis is necessary to identify the best material (or combinations of materials) for operation

at cavity temperatures of 2500K and above.

TABLE 1. Secondary Concentrator Material Characteristics

Material Size

Availability

Melt Point Index of

Refraction

Est.Thermal

Conductivity

(Watts/cm2

•K)

Optical

Absorption

Cutoff

Chemical

Stability

Al2O3

sapphire

33 cm dia. x

15 cm long

boules

~2300K 1.76 100 @ 20K

0.25 @ 300K

0.1 @ 1000K

0.06 @ 2300K

~5μ Stable at high

temp. in air or

vacuum

MgO

magnesium

oxide

10 cm dia. x

15 cm long

irreg. shapes

~3000K 1.76 30 @ 20K

0.6 @ 300K

0.08 @ 1500K

~7μ Reduces in air

due to H2O,

stable at high

temp. in vacuum

ZrO2

zirconia

10 cm dia. x

15 cm long

irreg. shapes

~3000K 2.16 0.1 @ 300K ~6μ Reduces at high

temp. in

vacuum,

stable in air

Y3Al5O12

yttrium/aluminum/garnet

(YAG)

13 cm dia. x

20 cm long

boules

~2240K 1.82 0.1 @ 20K

0.08 @ 1000K

0.07 @ 1500K

>4μ Stable at high

temp. in air or

vacuum
NASA/TM—2000-208401 3

PROTOTYPE HARDWARE FABRICATION & TESTING

Design Implementation

 Current refractive secondary concentrator and flux extractor designs have focused on solar thermal propulsion

engine applications. Under operational conditions, these engines will operate at temperatures approaching 2500K.

Figure 1 shows the “Shooting Star Experiment” engine concept discussed earlier. While the engine is designed for

use with hydrogen propellant at 2500K, the short-term SSE flight will heat nitrogen as a propellant at temperatures

approaching 2000K. The ultimate goal of the Refractive Secondary Concentrator Program is to develop the

technology necessary to support the high temperature applications envisioned for future space systems; however the

program’s immediate concern is the initial demonstration and testing of this concept within the confines of SST

operational conditions.

The first secondary concentrator ground test articles were made from zirconia. Zirconia was selected due to it’s size

availability, lower manufacturing cost, and high melt point temperature. Zirconia is ideally suited to demonstrating

and quantitatively measuring optical performance of the DTIRC/flux extractor unit. However, as indicated in Table

1, zirconia reduces at high temperature in vacuum (i.e. changes color with time) and therefore has a limited useful

life for space applications. Following discoloration in vacuum, the zirconia can be restored to it’s original color by

heat treating in air at elevated temperature. Tests have shown that zirconia retains its original qualities and can be

reused as a test article. This time/temperature relationship is being studied to determine ground test limitations for

the prototype hardware. Although the SST experiment is a short duration test, it remains to be seen if zirconia is a

suitable material for flight hardware demonstration.

Component Test Results

 Researchers at NASA Lewis have evaluated a variety of sample materials and coatings in an effort to optimize

system performance within the given optical, thermal and mechanical constraints. Coupons of sapphire, zirconia,

and magnesium oxide were fabricated and polished to “best effort” flatness in an attempt to achieve 1/20th wave or

better. Early analysis indicated that the contact surfaces between the flux extractor and concentrator would require

1/20th wave flatness or better (depending upon index of refraction of the DTIRC/flux extractor) in order to

efficiently transfer the solar energy from the concentrator into the extractor. To date the best efforts have produced

only 1/10th wave flatness.

 It was also believed, early in the program, that mechanically contacted optical surfaces would provide a thermal

resistance in vacuum that would reduce the conduction heat loss from the high temperature extractor to the cooler

concentrator. Contact resistance tests were conducted under contract to Texas A&M University using sapphire

wafers polished to ~ 1 wave and MgO wafers polished to 1/10 wave. No appreciable thermal resistance was

achieved. (Mirmira 1998). As a result of this testing, a decision was made to pursue diffusion bonding as a means

of joining the extractor to the concentrator. A successful diffusion bond should result in a lossless optical

connection. Coupons of all candidate materials are to be bonded and tested for optical and mechanical performance.

The samples will involve bonding like as well as dissimilar materials. Diffusion bonding of like materials is reported

to result in bonds as strong as the parent material. For unlike materials small differences in the coefficient of thermal

expansion (CTE) may have an adverse effect on the strength of the bond.

Prototype Hardware Fabrication & Assembly

 The fabrication of two sets of zirconia crystals was recently completed by the Optikos Corporation for use in

ground testing at NASA Lewis and at the University of Alabama, Huntsville (UAH) Solar Laboratory. The first

crystal set was designed to optimize optical performance based on specifications provided by NASA Marshall for the

primary solar concentrator. The first prototype unit is shown in Figure 4. The secondary concentrator DTIRC has

an 8.9 cm. inlet diameter, a 1.9 cm. exit diameter, and is 12 cm. long. The flux extractor is 15 cm. long and has 3

equilateral facets. The DTIRC accepts a light beam with a 22 degree entrance half angle. The second crystal set was

designed for a 42 degree entrance half angle. This design variation from the SST requirements allows the second

unit to be tested with primary concentrators at the Lewis Research Center’s Tank 6 Facility and at the Edwards Air

Force Base’s Solar Laboratory. In addition, the second prototype unit incorporates a 4 facet diamond shaped

extractor for improved efficiency. The DTIRC and flux extractor from the first crystal set will be diffusion bonded
NASA/TM—2000-208401 4

and then tested for throughput efficiency and high temperature operation at the UAH Solar Laboratory. Present

plans are to hold the second set as a spare until preliminary test results are acquired at UAH.

 The design of a crystal holder that will support the concentrator/extractor assembly at the DTIRC inlet, see Figure

1. is in process. The current design has the flux extractor cantilevered from the DTIRC into the solar thermal engine

cavity through an opening in the engine insulation. Launch load survival tests are planned to verify that the crystal

mass can be supported in this manner and survive the high G forces associated with a Space Shuttle launch.

SUMMARY

 Significant progress has been made in the past year in developing a refractive secondary concentrator design to

support the SST and other future solar thermal power and propulsion applications. The feasibility of this concept has

been demonstrated and design tools have been developed to assist in design optimization. Candidate materials have

been identified along with sources for raw materials and fabrication. Crystal material issues, specifically for high

temperature and long-term space applications, still need to be addressed. Two prototype DTIRC/flux extractor units

have been fabricated and are awaiting test. These units, fabricated from zirconia, will be used to verify predicted

performance (optical, thermal, and mechanical).

 An important area remaining to be pursued is the development of specialized high temperature coatings that can

enhance the overall performance of the secondary concentrator. One such coating is an infra-red (IR) block coating

that could be applied to the crystal inlet surface. Such a coating would reflect back the majority of IR radiation

losses from the hot cavity while rejecting a much smaller amount of the incoming solar radiation.

 The high temperature refractive secondary concentrator concept offers many advantages over other secondary

concentrator systems. The NASA Lewis Refractive Secondary Concentrator Team plans to continue the

development of this technology for a variety of solar thermal propulsion and power applications.

References

Curtis, L. A. and Toelle, R. G. (1998) “Solar Thermal Propulsion Shooting Star Experiment,” Space Technology &

Applications International Forum (STAIF-98), Albuquerque, NM, Jan. 25-29, 1998.

Soules, J. A., Buchele, D. R., Castle, C. H. and Macosko R. P. (1997) “Design and Fabrication of a Dielectric Total

Internal Reflecting Solar Concentrator and Associated Flux Extractor for Extreme High Temperature (2500K)

Applications,” Proceedings of the SPIE, Volume 3139, pp. 237-249, July 27-28, 1997.

Winston, Roland (ed.) (1995), selected papers on non-imaging optics, SPIE Milestone Series, Vol. MS 106.

Nin, X., Winston, R., and O’Gallagher, J. (1987) “Dielectric Totally Internally Reflecting Concentrators,” Applied

Optics, Vol. 26(2), pp. 300-305, January 15, 1987.

Mirmira, S. R., Fletcher, L. S. and Baker K. W. (1998) “Interfacial Contact Resistance of Single Crystal Ceramic

Solar Concentrator Materials,” 36th AIAA Aerospace Sciences Meeting & Exhibit, Reno, NV, Jan. 12-15, 1998.
Exhaust

Gap

Zirconium Oxide 

“DTIRC”

Walk-on 

Shield

Porous Heat 

Exchanger

Exhaust Zirconium Oxide 

“DTIRC”

Flux Extractor 

Zirconium Oxide

Propellant 

Inflow

Gap/Optical 

Contact

FIGURE 1. Schematic Drawing Showing a Refractive Secondary Integrated into a Solar Thermal

Propulsion Application Concept.

FIGURE 2. Ray Trace Showing Light Rays Entering DTIRC, Exiting Flux Extractor and Impinging upon

the Heat Receiver Cavity Wall.

NASA/TM—2000-208401 5
16.0%

14.0%

12.0%

10.0%

8.0%

6.0%

4.0%

2.0%

0.0%

0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0

Cavity Axial Position (cm)

Secondary Concentrator Demonstrator - Flux on Cavity

% of Total Energy Entering Cavity

FIGURE 3. Solar Flux Intensity Distribution within the Heat Receiver Cavity for the Current Secondary

Concentrator Design. (0.00 = Cavity entrance).

FIGURE 4. Secondary Concentrator and Flux Extractor Made from Single Crystal Zirconia.

NASA/TM—2000-208401 6
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A03

A High-Efficiency Refractive Secondary Solar Concentrator for High

Temperature Solar Thermal Applications

Michael F. Piszczor, Jr. and Robert P. Macosko

Space power; Solar power; Concentrators; Secondaries; Energy collection; Solar thermal

power; Solar thermal propulsion; Light collectors

Unclassified -Unlimited

Subject Categories: 16 and 20 Distribution: Nonstandard

Prepared for the Space Technology and Applications International Forum (STAIF–98) sponsored by the University of New

Mexico’s Institute for Space Nuclear Power Studies, Albuquerque, New Mexico, January 25–29, 1998.

Michael F. Piszczor, Jr., NASA Glenn Research Center and Robert P. Macosko, Analex Corporation, 3001 Aerospace

Parkway, Brook Park, Ohio 44142. Responsible person, Michael F. Piszczor, Jr., organization code 5410, (216) 433–2237.

A refractive secondary solar concentrator is a non-imaging optical device that accepts focused solar energy from a primary

concentrator and redirects that light, by means of refraction and total internal reflection (TIR) into a cavity where the solar

energy is used for power and/or propulsion applications. This concept offers a variety of advantages compared to typical

reflective secondary concentrators (or the use of no secondary at all): higher optical efficiency, minimal secondary

cooling requirements, a smaller cavity aperture, a reduction of outgassing from the cavity and flux tailoring of the solar

energy within the heat receiver. During the past 2 years, NASA Lewis has been aggressively developing this concept in

support of the NASA Marshall Shooting Star Flight Experiment. This paper provides a brief overview of the advantages

and technical challenges associated with the development of a refractive secondary concentrator and the fabrication of a

working unit in support of the flight demonstration program.