Heat Recuperator Engineering for an ARL Liquid-Fueled 

Thermophotovoltaic Power Source Demonstrator

by William R Allmon, Dr C Mike Waits, and Dr Erik D Tolmachoff

ARL-TR-7113 September 2014

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Army Research Laboratory

Adelphi, MD 20783-1138

ARL-TR-7113 September 2014

Heat Recuperator Engineering for an ARL Liquid-Fueled 

Thermophotovoltaic Power Source Demonstrator

William R Allmon, Dr C Mike Waits, and Dr Erik D Tolmachoff

Sensors and Electron Devices Directorate, ARL

Approved for public release; distribution unlimited.
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4. TITLE AND SUBTITLE

Heat Recuperator Engineering for an ARL Liquid-Fueled Thermophotovoltaic 

Power Source Demonstrator

5a. CONTRACT NUMBER

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William R Allmon, Dr C Mike Waits, and Dr Erik D Tolmachoff

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US Army Research Laboratory

ATTN RDRL-SED-E

2800 Powder Mill Road

Adelphi, MD 20783-1138

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ARL-TR-7113

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

Direct thermal-to-electrical conversion shows promise for small-scale portable power sources using logistics and multiple other 

fuels. Some potential technologies include thermoelectric, thermophotovoltaic (TPV), and thermionic. For these technologies, 

thermal efficiency is critical to achieve high energy density and thermal-to-electric conversion efficiency. Combustion can be 

used to convert fuel to heat a surface, but more than 50% of the thermal power of the combustor can be lost to the exhaust. As a 

result, heat recuperation is absolutely necessary to reclaim this lost energy. This is accomplished by heating the inlet air and 

fuel going into the microcombustor using the hot exhaust gases from the microcombustor. The US Army Research Laboratory 

(ARL) is pursuing a concept demonstration to integrate key components of a combustion-based TPV power source including a 

microcombustor and heat recuperator. The heat recuperator employs counter flow and microchannel heat transfer to enable high 

transfer rates in compact form due to a high surface area-to-volume ratio, but challenges exist with pressure drop tradeoffs and 

fabrication complexity. This report focuses on identifying appropriate materials and fabrication limitations for a microchannel 

heat recuperator preliminary design. 

15. SUBJECT TERMS

Heat recuperator, micro channels, thermophotovoltaic, power source, microcombustor, microreactor, portable power

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William R Allmon

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iii

Contents

List of Figures iv

1. Background 1

2. Material Selection 2

3. Thermal and Fluidic Design Analysis 3

4. Fabrication Method 4

5. Weld Penetration Analysis 6

6. Conclusion 10

7. References 11

Appendix. Part and Assembly Fabrication Drawings 13

Distribution List 22
iv

List of Figures

Fig. 1 Primary components of thermophotovoltaic energy converter1 ..........................................2

Fig. 2 A heat recuperator with 3 rectangular channel openings for each direction of flow...........4

Fig. 3 The heat recuperator showing the assembly of the custom fabricated parts.......................5

Fig. 4 Solid Works Simulation model............................................................................................7

Fig. 5 The refined mesh .................................................................................................................8

Fig. 6 Von Mises stress..................................................................................................................9

Fig. 7 Deflection ..........................................................................................................................10
1

1. Background

Direct thermal-to-electrical conversion shows promise for small-scale portable power sources 

using logistics and multiple other fuels. Some potential technologies include thermoelectric, 

thermophotovoltaic (TPV), and thermionic. For these technologies, thermal efficiency is critical 

to achieve high energy density and thermal-to-electric conversion efficiency. Combustion can be 

used to convert fuel to heat a surface. High efficiency requires temperatures above 500 °C for 

thermoelectric and TPV. The exhaust gas will be above this temperature, but more than 50% of 

the thermal power of the combustor can be lost to the exhaust as it has a very low resistive path.

As a result, heat recirculation or heat recuperation is absolutely necessary to reclaim this lost 

energy. This is accomplished by heating the inlet air and fuel going into the microcombustor 

using the hot exhaust gases from the microcombustor. The US Army Research Laboratory

(ARL) is pursuing a concept demonstration to integrate key components of a combustion-based 

TPV power source including a microcombustor and heat recuperator.1

 

Figure 1 describes the primary components of a TPV system: a heat source, an emitter, and a 

photovoltaic converter. The heat source supplies thermal energy to the emitter, which radiates 

the energy across a gap to the photovoltaic cell or an array of photovoltaic cells. The 

photovoltaic cell(s) then converts the thermal radiation to electrical energy, which can be 

delivered to a load or conditioning circuitry. Optical filters between the emitter and the 

photovoltaic cell (not included in Fig. 1), as well as the reflectors deposited on the backside of 

the photovoltaic cell, are also common components. The optical cavity between the emitter and 

photovoltaic cell is often held under vacuum to minimize conduction and convective heat 

transfer.1
2

Fig. 1 Primary components of thermophotovoltaic energy converter1

For this concept demonstrator, the exterior of the heat recuperator and microcombustor will also 

be held at vacuum to minimize heat loss. The heat recuperator employs counter flow, where the 

microcombustor exhaust enters the recuperator at one end and the inlet air and fuel enters at the 

other end at ambient temperature. The heat recuperator employs microchannel heat transfer to 

enable high transfer rates in compact form due to a high surface area-to-volume ratio, but 

challenges exist with pressure drop tradeoffs and fabrication complexity. The heat recuperator 

design is focused on several factors to include the ability to integrate with the thin wide profile of 

the microcombustor, identify appropriate materials and fabrication limitations, validate analytical 

models used to determine performance, and explore liquid vaporization of fuel. This report 

focuses on identifying appropriate materials and fabrication limitations for a microchannel heat 

recuperator preliminary design. 

2. Material Selection

To handle the high temperature demands and retain a protective oxide coating under conditions 

of cyclic exposure to high temperature, a high nickel specialty alloy known as Inconel 600TM was 

selected for the heat recuperator. It is often used in furnace and heat treating fields, which require 

resistance to oxidation and to furnace atmospheres. We are currently using a similar Inconel 

grade known as HX for the microcombustor. Using the similar material for the microcombustor, 

heat recuperator, and the inlet and outlet tubing ensures thermal expansion compatibility. Unlike 
3

conventional metals such as carbon steel and 304 or 316 stainless steel, Inconel maintains a 

reasonable amount of strength and resists oxidation at these high temperatures. The strength of 

the material is of particular importance since the walls of the microcombustor and heat 

recuperator are very thin and create part of the vacuum wall. Other Inconel alloys are available,

but Inconel 600 is well characterized, readily available in tube, sheet, and bar in the required 

sizes, and available in small quantities for our low volume purposes. Inconel 600 is slightly more 

machinable than 304 stainless steel and slightly less machinable than 303 stainless. While the 

high temperatures seem well suited for ceramics, these materials are more brittle and fragile. 

3. Thermal and Fluidic Design Analysis 

The preliminary design was chosen as a tradeoff between performance and pressure drop. The 

channels were modeled using a ε-NTU method with a previously published Nusselt number.

2

The channel dimensions offered a tradeoff between pressure drop and fin efficiency, and the 

channel length was set to 1.97 inches and the thickness of the wall was set for machinability.

Due to thermal conduction and the short channel lengths for small-scale/portable power, the 

channel wall thickness plays an important part. Thus, a reasonable minimum thickness of 

0.020 inches was chosen. The details of the preliminary design are not the focus of this report 

and will be published in a future report. With the anticipated flow rate, COMSOL simulation 

suggests the effectiveness in the range of 0.6 to 0.7 can be achieved with this design.

3

Conducting a first-order optimization analysis of the geometry, a heat recuperator with 3

rectangular channel openings of 0.197 inches high and 0.039 inches wide for each direction of 

flow was found to provide good performance (Fig. 2).
4

.020

.020

.020

.039 TYP

.197 TYP

.020 TYP

Fig. 2 A heat recuperator with 3 rectangular channel openings 

for each direction of flow

4. Fabrication Method

To create the heat recuperator with this design geometry, an assembly of the following custom 

fabricated parts—base, cover, ends, and tubes—are required (Fig. 3). The most difficult part to 

fabricate is the base. It is fabricated using wire electric discharge machining (EDM) to create 

narrow deep cuts and thin features not possible using standard milling. Wire EDM does not put 

stress on the part, since it is a slow erosion process. In contrast, milling puts stress on the part as 

the tool moves and cuts away material. In addition, mill tooling small enough to cut these narrow 

channels is not long enough to reach to the bottom of these deep channels.
5

Base Cross-Section

Ends

Cover

Base

 .020 

 .020 

 .197 

 .039 

Fig. 3 The heat recuperator showing the assembly of the custom fabricated parts

Other methods of fabrication, such as direct metal laser sintering (DMLS), were considered.

DMLS does not provide the tolerance control required, making post-machining necessary prior 

to welding. In addition, parts designed for DMLS require material to be added in certain places 

to support the part during fabrication. This added material is removed through post-machining.

With this much post-machining, it seems more economical to fabricate the parts directly with 

wire EDM.

The parts are joined together to form the heat recuperator assembly. The thin features of the parts 

require precise welding to join them together. To accomplish this, electron beam welding (EBW) 

was selected. EBW is a fusion weld process where a beam of high velocity electrons is applied to 

the junction of two parts melting them together. The two parts must have gaps of less than 10% 

of thickness of the thinnest part (0.002 inch for this design). EBW is very controlled, can be used 

on very small assemblies, and can provide high levels of weld penetration. Since the base is 

fabricated using wire EDM, the EDM recast layer of carbonized material must be removed prior 

to welding. The welding contractor removes this recast layer mechanically or chemically.
6

Typically, removal of 0.0005 inch is adequate. The removal cannot be done with abrasive media 

such as sand or beads since the abrasive media tends to get embedded in the base material 

leading to weld joint contamination. 

Since the heat recuperator is part of the vacuum wall, it is critical the welds provide adequate 

sealing and structural integrity. The EBW contractor will leak test the assembly by pressurizing 

the interior with helium to 14.7 psig and sniffing the exterior with a helium leak detector with the 

leak threshold set at 1 x 10-9 std cc/s. This allows leaks to be located and welds to be reworked 

as necessary.

5. Weld Penetration Analysis

As part of the design, a stress analysis was performed to determine the weld penetration required 

to maintain structural integrity of the heat recuperator assembly under vacuum. The analysis was 

performed using Solid Works Simulation. The assembly worse case was modeled in simplified 

form as a shelled out chamber with a wall the thickness equal to the weld penetration. The case 

analyzed had a wall thickness of 0.0025 inches and outside dimensions of 0.200 x 0.225 inches

—a worst-case equivalent model of the upper or lower 3 channels. Since the cover is only 

welded to the outer fins and the interior is pressurized, the center fins provide no support.

Figure 4 shows the model used in the analysis that is the entire width of the recuperator and the 

height of one 3-channel side. A pressure of 14.7 psi was applied to all the inside surfaces and one 

small end was fixed to allow the solver to function. The following material properties provided 

by Special Metals Publication SMC-027 for Inconel 600 operating at 1600 °F (871 °C) were 

used:

Name: Inconel 600 at 1600 °F

Model type: Linear Elastic Isotropic

Default failure criterion: Max von Mises Stress

Yield strength: 17000 psi

Tensile strength: 22000 psi

Elastic modulus: 2.28e+007 psi

Poisson's ratio: 0.329 

Mass density: 0.306 lb/in3

Shear modulus: 8.58e+006 psi

Thermal expansion 

coefficient:

9.1e-006 /Fahrenheit
7

 2.360 

A

A

SECTION A-A SCALE 12 : 1

 .225 

 .200 

 .0025 

Fig. 4 Solid Works Simulation model 

Figure 5 shows the refined mesh of the walls following several meshes and solving iterations.
8

Fig. 5 The refined mesh 

From Fig. 6, the maximum von Mises stress is in the corners and in the middle of the surface. It 

is shown as bright green. Using the scale on the right, bright green is near the yield strength of 

the Inconel 600 at 1600 °F (871 °C). It is believed this case is worse than will be seen in service 

since the temperature is likely not to be 1600 °F for the whole assembly. In addition, the wall 

thickness is 0.020 inches. Only at the weld will it be 0.0025 inches. As a result, less deflection 

and thus less stress is likely throughout the assembly. To add a factor of safety, the weld 

penetration call out in the weldment drawing is a minimum of 0.005 inches.
9

Fig. 6 Von Mises stress

Figure 7 shows the deflection of the model. The deflection shown in red is a maxium at the 

center of the surface and is 0.002 inches.
10

Fig. 7 Deflection 

Complete detailed fabrication drawings showing the parts and assembly with dimensions and 

notes can be found in the appendix. 

6. Conclusion

ARL created a preliminary design of a high temperature heat recuperator for use with a 

microcombustor as part of a TPV power source. The heat recuperator is critical to the efficiency 

of such a system through reuse of the heat that would otherwise be lost to ambient. The 

recuperator is made of a high nickel specialty alloy known as Inconel 600 and fabricated using 

wire EDM and EBW. An analysis of the required weld penetration was conducted to ensure 

sealing and structural integrity of the heat recuperator since the exterior is in vacuum and the 

interior is at or near ambient pressure.
11

7. References

1. Waits MC. Thermophotovoltaic energy conversion for personal power sources. Adelphi

(MD): US Army Research Laboratory (US); February 2012. Report No.: ARL-TR-5942.

2. Smith AN, Nochetto H. Laminar thermally developing flow in rectangular channels and 

parallel plates: uniform heat flux. Heat Mass Transfer. DOI 10.1007/s00231-014-1363-8, 

2014.

3. Smith AN, Nochetto H, Waits CM. Influence of axial conduction in the design of a compact 

recuperator for catalytic combustor based portable power generation. Proceedings Comsol 

Conference 2014.
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INTENTIONALLY LEFT BLANK.
13

Appendix. Part and Assembly Fabrication Drawings
14

This appendix includes detailed part and assembly fabrication drawings with dimensions, 

tolerances, material, finish, and other notes.

The order of drawings is as follows:

• Heat Exchanger ASSY-V2

• Heat Exchanger WELDMENT-V2

• Heat Exchanger Base-V2-R1 

• Heat Exchanger Cover-V2-R1

• Inlet-Outlet Ends-3-V2-R2

• Tube-2-Bends-V2-R1

• Tube-Straight-V2-R1
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1 DEFENSE TECHNICAL

(PDF) INFORMATION CTR

DTIC OCA

2 DIRECTOR

(PDF) US ARMY RESEARCH LAB

RDRL CIO LL

IMAL HRA

1 GOVT PRNTG OFC

(PDF) A MALHOTRA

3 DIRECTOR

(PDF) US ARMY RESEARCH LAB

RDRL SED E

WILLIAM R ALLMON

DR C MIKE WAITS

DR ERIK D TOLMACHOFF