Numerical Comparison of NASA’s Dual Brayton 

Power Generation System Performance Using CO2

or N2 as the Working Fluid

NASA/TM—2010-216949

December 2010

Albert K. Owen

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

Thomas M. Lavelle

Glenn Research Center, Cleveland, Ohio

David S. Hervol

QinetiQ North American, Cleveland, Ohio

U.S. ARMY

RESEARCH LABORATORY
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Numerical Comparison of NASA’s Dual Brayton 

Power Generation System Performance Using CO2

or N2 as the Working Fluid

NASA/TM—2010-216949

December 2010

Albert K. Owen

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

Thomas M. Lavelle

Glenn Research Center, Cleveland, Ohio

David S. Hervol

QinetiQ North American, Cleveland, Ohio

U.S. ARMY

RESEARCH LABORATORY

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2010-216949 1 

Numerical Comparison of NASA’s Dual Brayton Power Generation 

System Performance Using CO2 or N2 as the Working Fluid 

Albert K. Owen 

U.S. Army Research Laboratory 

Glenn Research Center 

Cleveland, Ohio 44135 

Thomas M. Lavelle 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

David S. Hervol 

QinetiQ North American 

Cleveland, Ohio 44135 

Abstract 

A Dual Brayton Power Conversion System (DBPCS) has been tested at the NASA Glenn Research 

Center using nitrogen (N2) as the working fluid. This system uses two closed Brayton cycle systems that 

share a common heat source and working fluid but are otherwise independent. This system has been 

modeled using the Numerical Propulsion System Simulation (NPSS) environment. This paper presents 

the results of a numerical study that investigated system performance changes resulting when the working 

fluid is changed from gaseous (N2) to gaseous carbon dioxide (CO2). 

Introduction 

As part of the Exploration Technology Development Program’s Fission Surface Power project (FSP), 

the NASA Glenn Research Center procured and tested a Dual Brayton Power Conversion System 

(DBPCS). Testing of the DBPCS has previously been reported (Ref. 1). The DBPCS has also been 

modeled extensively using the Numerical Propulsion System Simulation (NPSS) (Refs. 1 to 4) 

environment. 

The DBPCS tested at the NASA Glenn Research Center used nitrogen (N2) as the working gas; 

however, the use of carbon dioxide (CO2) had been suggested due to its higher molecular weight. 

Unfortunately, the DBPCS test program has been terminated due to a change in the power requirements 

anticipated for space power generation in the foreseeable future. Nevertheless, the availability of a 

simulation of the system allowed the qualitative exploration of a change in the working fluid. The Closed 

Cycle System Simulation (CCSS) software provided a component level representation of the system that 

allowed a qualitative study of the differences of operation of the system using N2 and the system using CO2. 

Dual Brayton Power Conversion System Description 

An isometric drawing of the DBPCS tested at the Glenn Research Center is shown in Figure 1. All 

components in the system were commercially available but required modifications to create a closed cycle 

test rig. The core of the system consisted of two Capstone C30 open-loop microturbine engines where the 

combustion chamber had been modified to allow the working fluid to be heated by an external heater. 

This electric heater, shown in Figure 1, was the only common component in the system. Also shown in 

Figure 1 and provided in Table 1 are the pressure and temperature measurement locations for data 
NASA/TM—2010-216949 2 

TABLE 1.—DBPCS MEASUREMENT STATIONS (REF. 5) 

Measurement Location, Definition 

P100, T100 Heater outlet 

P200, T200 Turbine outlet 

P300, T300 Recuperator outlet (Cooler inlet side) 

P400, T400 Cooler outlet (Compressor intake side) 

P500 Recuperator inlet (Compressor outlet side) 

P600, T600 Heater inlet (Recuperator outlet side) 

acquired during testing. Here the “Pxxx” indicate static pressures and “Txxx” designate static temperature 

measurements. Instrumentation was symmetrical for both “sides” of the DBPCS. All high temperature 

sections of the test rig were covered with thermal insulation blankets to reduce heat transfer to the 

ambient environment. 

Unfortunately, physical limitations prevented the placement of thermocouples at the compressor and 

the turbine exits resulting in a lack of temperature information. The lack of these thermocouples 

prevented the direct measurement of either the compressor or turbine performance. 

Closed Cycle System Simulation 

Closed Cycle System Simulation was developed within the NPSS software architecture. NPSS is an 

aviation industry cycle simulation developed through a NASA/industry consortium that included the 

major airframe and gas turbine engine manufacturers (Ref. 6). This one-dimensional performance model 

is characterized by flexibility, adaptability, and ease of use. Johnson and Hervol (Ref. 3) present a 

detailed explanation of the component models. Although it was developed principally for performance 

modeling of gas turbine applications, the environment has been used to develop fuel cell, altitude wind 

tunnel, and closed cycle models. The source code for this model originates from the NASA Closed Cycle 

Engine Program (CCEP) (Ref. 7), an in-house legacy code. 

 
NASA/TM—2010-216949 3 

A schematic depicting one half (side) of the simulated system is shown in Figure 2 (Ref. 1). With the 

exception of the heater, the simulation duplicates this schematic twice, once for each side of the DBPCS. 

Relative locations of the DBPCS measurement stations are also shown. 

Development of the model required definition of the physical dimensions of the components (pipes, 

bellows, etc.), turbomachinery performance maps, and recuperators’ and coolers’ performance, working 

fluid properties, and the components’ layout (bends, etc.). Inputs required to run the models include 

charge pressure (constant, 90 kpa), heater exit temperature (840 to 950 K), water coolant temperature 

(294 K), coolant mass flow, and shaft speed (50 to 90 krpm) (Ref. 1). An important software modification 

for the present study was the addition of CO2 gas property tables. The inclusion of these tables allows an 

easy transition between N2 and CO2 modeling. 

Turbine and compressor maps are required for this software. They can be provided in a number of 

ways but for this project, the maps were generated analytically using Centrifugal Compressor Off Design 

Code (CCODP) (Ref. 8) and Radial Turbine Off Design Code (RTOD) (Ref. 9). These codes, written at 

the Lewis Research Center (now Glenn Research Center) use simple one-dimensional calculations and 

correlations of experimental data to provide off-design performance of centrifugal compressors and radial 

turbine. Inputs include operating conditions, working fluid and machine geometry. 

Modifications to CCSS 

Modifications made to CCSS to allow the software to run CO2 were relatively minor and consisted of 

the following four general areas: 

1. Insertion of the turbomachinery maps for CO2, 

2. Modifications to the element that describes ducts in the software, 

3. Modifications to the model and run files that allow an easy change between working fluids, 

4. Removal of the N2 specific correction factors that improved the correlation between the 

simulation and the experiment. Unfortunately, no experimental data was available to develop CO2

correction factors. 
NASA/TM—2010-216949 4 

Insertion of the turbomachinery maps was straightforward. However, the maps provided did not have 

the same component inlet reference conditions. The N2 compressor pressure ratio map was provided as 

inlet pressure over outlet pressure while the CO2 map was given as outlet pressure over inlet pressure. The 

software was modified to accommodate these types of differences between turbomachinery performance 

maps. 

The duct element originally used in CCSS was upgraded with modifications to the calculations of 

duct turning losses and Reynolds number. In addition, losses for sudden expansions and contractions were 

added. The duct between the turbine exit and the cooler inlet was split into two ducts to allow more 

accurate modeling of the frictional losses in the bellows. Finally, the model representation was improved 

by adding the ability to model the splitting of ducts into a set of parallel ducts (such as between the heater 

and the turbine). These modifications did not significantly change the results for the ducts in this 

simulation. Total pressure difference     original revised revised Pt  Pt / Pt that resulted from the more 

detailed loss representations for any particular duct in the system was less than 5 percent. 

Changes to the model and run files were straightforward and simple. A switch was added to specify 

the working gas that directed the model to read the correct turbomachinery maps, gas tables, and 

molecular weight. 

Finally, N2 correction factors previously added to the DBPCS model were removed. As has been 

previously noted (Ref. 1), these correction factors had been added to the simulation to make it more 

closely predict the actual DBPCS performance. This allowed a more realistic comparison between N2 and 

CO2 predictions. It is unlikely that the same correction factors would apply to both working fluids. 

Furthermore, no experimental data exist for DBPCS operation with CO2, thus no CO2 specific correction 

factors could be created. 

The DBPCS was one of the most complex systems ever modeled within the NPSS environment. 

Several new modules needed to be developed. Nevertheless, the final representation proved to be robust 

and representative. The NPSS environment proved to be very adaptable. 

Comparison Between Experimental Data and Unscaled Predicted Results 

Figures 3 and 4 compare unscaled predicted station temperatures and measured station temperatures 

for the 80000 rpm, 950 K operating condition (temperature set at the heater exit). Here, the most notable 

observations are that there exists in the heater a pressure drop that is not accounted for in the model and, 

more importantly, the measured pressure drop—and energy taken out of the flow by the turbine—is 

significantly lower in the measured data than in the predicted results. The temperature plot shows the 

temperatures out of the recuperator are significantly higher than the model predicts, while the pressure 

drop is lower. Thus the model under predicts recuperator thermal performance and appears to over predict 

turbine performance. Unfortunately, the lack of experimental compressor and turbine exit temperature 

data make it impossible to identify overall system performance prediction errors with certainty. 

These differences do not explain the differences between the predicted power output for this 

condition of 26.54 kW and the measured power output of 18.15 kW. This represents a substantial 

difference in power outputs. Possible causes for this discrepancy are currently being examined. 
NASA/TM—2010-216949 5 
NASA/TM—2010-216949 6 

Comparison Between Predicted Results Using Nitrogen 

and Carbon Dioxide Working Fluids 

Turbomachinery maps for both N2 and CO2 were provided by Jeff Noall of Barber Nichols Inc. 

(Ref. 10) using NASA legacy codes CCODP (Ref. 8) and RTOD (Ref. 9). These maps were generated 

using CCODP for the compressor and RTOD for the turbine. The design point for the Capstone 

compressor is 92000 rpm. However, the compressor performance prediction code was unable to find a 

solution for the given compressor geometry above 75000 rpm. Therefore, 73000 rpm was arbitrarily 

selected as 100 percent design speed, allowing a 2 percent overspeed limit for the compressor. Certainly 

turbomachinery specifically designed with CO2 as the working fluid would have allowed operation at 

higher speeds. 

Figure 5 shows the predicted compressor pressure ratio and efficiency. The compressor operation is 

acceptable at lower speeds with the different working fluid. As would be expected, performance using 

CO2 begins to drop off above 60 k rpm and is significantly worse with efficiency at 75000 rpm being 

almost 4 percent worse than at 60 k rpm and 2 percent lower than the equivalent N2 efficiency. Figure 6 

indicates the turbine is operating poorly throughout the operating range and is badly designed for 

operation with CO2 in this system. 

 
NASA/TM—2010-216949 7 

 

The turbine configuration is not optimum as the efficiency of the turbine drops almost 14 percent as 

the speed of operation increases from 30000 to 75000 rpm. Should operation of the DBPCS be desired, 

the turbine and compressor would need to be redesigned for optimum performance. 

Figure 7, the system efficiency shows that the system runs significantly worse with CO2 as the 

working fluid with the cycle efficiency, at best, around 2.5 percent lower and at 75 percent rpm the cycle 

efficiency almost 8 percent lower. This is despite improved recuperator effectiveness (~10 percent) while 

using CO2 (Fig. 8). 

The lower efficiency for CO2 operation is a result of the higher molecular weight of CO2 (MW 44 for 

CO2 versus 28 for N2). The cycle efficiency is calculated using the formula: 

  W Ho  Hi

  *

Poweralternator

cycle (1) 

where cycle is the cycle efficiency, W represents massflow and (Ho–Hi) calculated the specific enthalpy 

change across the heater. The significantly larger molecular weight will reduce efficiency without a 

corresponding increase in power output or change in H. For example, at 50000 rpm CO2 develops 

~35 percent more power but requires ~57 percent more massflow. This coupled with a slightly higher H

(~7 percent) results in the significantly lower recuperator effectiveness (rc), defined as: 

 

  c h c

h h h rc cap Tin Tin

cap Tin Tout



  

*

* (2) 

where cap is the heat capacity, Tin is the inlet temperature, and Tout is the outlet temperature. The h and c

subscripts represent the hot and cold flows, respectively. The hot side and cold side heat capacities will 

normally be the same. 

This much larger massflow is likely the result of the difference in molecular weight. Certainly at 

higher speed where both turbine and compressor move significantly away from optimal operation 

(Figs. 9(b) and 10(b)), the cycle efficiency for CO2 operation decreases notably. 

Figure 9 provides the compressor maps for the system operating with N2 (Fig. 9(a)) and CO2

(Fig. 9(b)). These figures also provide the predicted operating line for the components at speeds between 

30 and 75 k rpm. The operating line for N2 was to the left of the map, beyond the predicted surge line. 

The predicted operating line for CO2, however, is well to the right of the surge line. The maps provided 

reflect the conservative predictions of the CCODP software, which will likely underpredict the true 
NASA/TM—2010-216949 8 

operating range of a tubomachinery configuration. Since stable operation beyond the surge line is not 

possible, the CCSS solution algorithms will extrapolate the stable region of operation of the 

turbomachinery components to find a converged solution. Figure 9(a) shows the extended compressor 

map for N2. It is clear that compressor operation on N2 remains in the higher efficiency regions of the map 

throughout the range of interest while compressor efficiencies drop are lower during operation with CO2

at both low and high rpm. 

Figure 10 provides the turbine maps for N2 and CO2. As with the compressor maps, the turbine 

operates in high efficiency regions with both gases except with CO2 at higher speeds. 

Figure 11 shows the power output comparison between N2 as the working gas and CO2. Table 2 

provides specific information at an operating speed of 75000 rpm. Note that the system efficiency 

provided in Table 2 is simply the electric power provided by both generators divided by the electric 

power provided to the electric heater. 
NASA/TM—2010-216949 9 

TABLE 2.—OPERATING PARAMETERS AT 75000 rpm 

Parameter CO2 N2

Shaft speed, rpm 75000 75000 

Turbine power, kW 44.679 31.564 

Compressor power, kW –36.023 –19.342 

Power output, kW 7.614 11.425 

Compressor pressure ratio 2.896 2.268 

Turbine pressure ratio 2.782 2.212 

Recuperator effectiveness 0.843 0.789 

Turbine efficiency 0.764 0.906 

Compressor efficiency 0.741 0.763 

System efficiency 0.023 0.047 

Cycle efficiency 0.041 0.113 

It is interesting to note that even though the turbomachinery performance with CO2 as the working 

gas is consistently and significantly worse than that of N2, overall power output is higher until very high 

operating speeds. This result suggests that the use of CO2 as the working fluid in closed Brayton power 

generation units should be explored experimentally in the future. 

Results and Conclusions 

The following results and conclusions can be made: 

 A closed cycle dual Brayton power system, designed to be operated with N2, can successfully use 

CO2 as the working fluid. 

 Turbomachinery designed with N2 (or air) as the working fluid will not operate as efficiently with 

CO2 as the working fluid. Compressor to rotational speed will be limited and the turbine 

operating points will likely be far from optimum operating conditions. 

 Simulations indicate that power production using CO2 can be higher despite lower operating 

efficiencies but require turbomachinery designed for the operating fluid to achieve optimum 

performance. 

 
NASA/TM—2010-216949 10 

 This higher power production from the same physical system using CO2 as a working fluid 

suggests that systems where space and/or weight are constrained, CO2 Brayton cycle systems may 

offer significant advantages. 

 The NPSS environment proved readily adaptable to the development of a system that is far from 

the type of systems that NPSS was originally developed for. 

References 

1. Hervol, David S. et al., “Experimental and Analytical Performance of a Dual Brayton Power 

Conversion System,” NASA/TM—2009-215511, AIAA–2008–5735, February, 2009. 

2. Johnson, P.K., and Mason, L.S., “Initial Results of a Dual Closed-Brayton-Cycle Power Conversion 

System,” NASA/TM—2007-214981, December 2007. 

3. Johnson, P.K., and Hervol, D.S., “Experimental Validation of a Closed Brayton Cycle System 

Transient Simulation,” Proceedings of the Space Technology and Applications International Forum, 

edited by M. El-Genk, American Institute of Physics, vol. 813, New York, 2006, pp. 673–681, also 

NASA/CR—2006-214239. 

4. Johnson, P.K., and Mason, L.S., “Performance and Operational Characteristics for a Dual Brayton 

Space Power System with Common Gas Inventory,” Proceedings of the 4th International Energy 

Conversion Engineering Conference, vol. 4, AIAA, San Diego, CA, 2006, Also NASA/TM—2006-

214393, November 2006. 

5. Fuller, Robert, “Operating Manual BNPM-08-000 Dual Loop Closed Brayton Cycle Power System,” 

Barber Nichols Inc. 

6. Binder, M., “Numerical Propulsion System Simulation Introduction,” Multimedia CD available from 

the NASA Glenn Research Center, 2000. 

7. Klann, John L., “Closed Cycle Engine Program Operational Manual,” Unpublished Used Manual, 

Lewis Research Center, April 1991. 

8. Wood, J.R., “Computer Code for Off-Design Performance Prediction of Centrifugal Compressors,” 

Unpublished User Documentation, Lewis Research Center, 1995. 

9. Meitner, P.L., and Glassman, A.J., “Computer Code for Off-Design Performance Analysis of Radial 

Inflow Turbines With Rotor Backsweep,” NASA TP-2199, 1983. 

10. Noall, Jeff, Private Communication, Barber Nichols Inc., 2008. 
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14. ABSTRACT

A Dual Brayton Power Conversion System (DBPCS) has been tested at the NASA Glenn Research Center using Nitrogen (N2) as the 

working fluid. This system uses two closed Brayton cycle systems that share a common heat source and working fluid but are otherwise 

independent. This system has been modeled using the Numerical Propulsion System Simulation (NPSS) environment. This paper presents 

the results of a numerical study that investigated system performance changes resulting when the working fluid is changed from gaseous 

(N2) to gaseous carbon dioxide (CO2). 

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