Mission Design Drivers for Closed Brayton Cycle Space Power Conversion Configuration

Several forms of closed cycle heat engines exist to produce electrical energy suitable for space exploration or planetary surface applications. These engines include Stirling and Closed Brayton Cycle (CBC). Of these two, CBC has often been cited as providing the best balance of mass and efficiency for deep space or planetary power systems. Combined with an alternator on the same shaft, the hermetically sealed system provides the potential for long life and reliable operation. There is also a list of choices for the type of alternator. Choices include wound rotor machines, induction machines, switched reluctance machines, and permanent magnet generators (PMGs). In trades involving size, mass and efficiency the PMG is a favorable solution. This paper will discuss the consequences of using a CBC‐PMG source for an electrical power system, and the system parameters that must be defined and controlled to provide a stable, useful power source. Considerations of voltage, frequency (including DC), and power quality will be discussed. Load interactions and constraints for various power types will also be addressed. Control of the CBC‐PMG system during steady state operation and startup is also a factor.s

There is a need in the space science community for nuclear‐powered electric propulsion systems to enable high‐value, deep space and planetary exploration. Certain missions are driven by once‐in‐a‐lifetime or highly infrequent occurrences that require the near‐term development of a flight‐capable nuclear space power and electric propulsion system in order to take advantage of the scientific opportunity. The broader applicability of Brayton power systems to the commercial and military aircraft markets has provided fertile ground for the continued development and implementation of new technologies applicable to a closed Brayton cycle space Power Conversion Unit (PCU). One concept for effectively achieving a near‐term Brayton space power capability is based on the development work associated with the Integrated Power Unit (IPU). This unit embodies the state of the art in turbomachinery, generators, bearing systems and electric power management and distribution capability that can readily be evolved into a closed Brayton cycle PCU. This paper provides an overview of aircraft‐based Brayton power system technologies, their implementation into the IPU and one approach for leveraging this capability into a near‐term closed Brayton cycle space power conversion unit.

<div class="htmlview paragraph">The increasing need for higher electrical power levels for military and scientific space purposes has spurred development of energy conversion systems with higher efficiencies than the traditional solar array. Numerous nuclear options are currently under development. These fuels eliminate the need for large deployable solar arrays or concentrators, allow unrestricted pointing and maneuvering, and are inherently less susceptible to natural and threat environments; they are the only viable energy source for outer planetary exploration. Fission reactors are the most desirable nuclear source for electric power levels above 10 kW. Radioisotope sources have been used in conjunction with thermoelectrics up to 1 kWe for planetary exploration. However, for radioisotope systems in the 1 to 10 kWe range, a much higher conversion efficiency is required. For over 15 years, NASA and the Department of Energy (DOE) have been developing closed cycle turbo-alternator (dynamic) power conversion systems for space use. The Air Force and DOE are jointly developing such a system under the Dynamic Isotope Power System (DIPS) program.</div> <div class="htmlview paragraph">Two closed dynamic cycles are being evaluated for use as part of the DIPS system. They are the Closed Brayton Cycle (CBC) which uses an inert (single phase) gas as the working fluid. The other system is the Organic Rankine Cycle (ORC) which utilizes the phase change properties of the working fluid. Each has a significantly different effect on the design of a radiative heat rejection system, primarily because of the temperature variation of the CBC single phase heat rejection versus the nearly constant temperature two-phase (condensing) ORC heat rejection. The heat rejection system plays an important role, since the power conversion efficiency is strongly dependent on the heat rejection temperature. The lower the rejection temperature, the higher the efficiency. However, the radiator area requirement increases as the rejection temperature is reduced. On a weight optimization basis both systems are comparable, however, the ORC system requires more radiator area for the same cycle efficiency. With the design requirement that the radiator be mounted around the circumference of the vehicle, available area is limited. Therefore, the heat rejection system design may not be a clear-cut weight optimization but rather a matter of area reduction at minimum penalty to efficiency and weight.</div> <div class="htmlview paragraph">The paper presents the results of an optimization study performed on the heat rejection system for a space based ORC power system using an isotope heat source. The radiator sizing depends on the heat rejection temperature, radiator configuration, and radiator properties such as the fin effectiveness, emissivity, and absorptivity. The optimization analysis to evaluate the effect of each of these parameters on the system weight and area is presented.</div>

<div class="htmlview paragraph">To support the Space Exploration Initiative, studies were performed to investigate and characterize Dynamic Isotope Power System (DIPS) alternatives for the surface mission elements associated with a lunar base and subsequent manned Mars expedition. A key part of this characterization was to determine how the mission environment affects system design. The impact of shielding to provide astronaut protection from power system radiation was also examined. Impacts of mission environment and shielding were examined for two representative DIPS types (closed Brayton cycle and Stirling cycle converters). Mission environmental factors included: (1) thermal background; (2) dust and atmospheric corrosion; (3) meteoroid damage; and (4) presence of an atmosphere on Mars.</div> <div class="htmlview paragraph">Physical effects of these factors on thermal power systems were identified and their parametric range associated with the mission and mission environment were determined. The quantitative impacts of these factors on DIPS mass and radiator area were then estimated. Results of the study showed that the greatest impacts result from integration with the user.</div>

This paper discusses power level control of closed cycle gas turbines using a throttle valve. It also discusses related reasons why radial compressors are preferable to axial compressors for closed cycle gas turbines. In nuclear powered steam engines, power output is controlled by controlling steam flow. Operators can increase or decrease the amount of power demanded from the reactor simply by adjusting a throttle valve. Because core reactivity is controlled by plant temperature, the reactor responds to the change in power demand without further action from the operator. The same principle can applied in nuclear heated gas turbine, but gas turbine designers have traditionally avoided throttle valves. It is widely thought that throttle valves impose an efficiency cost that can not be justified. Axial flow turbines are normally chosen for combustion gas turbines because they have higher peak efficiencies than radial compressors. This paper explains why closed cycle, nuclear heated gas turbine systems with throttle valve control systems and radial compressors are economical. Since the mid 1940's, there have been systems designed to combine the simplicity of Brayton cycle machinery with the fuel advantages offered by with nuclear power. Projects like the Aircraft Nuclear Propulsion Program, the U. S. Army's ML-1, and the Maritime Gas Cooled Reactor Project were all based on using nuclear-heated closed Brayton cycles. While there are a variety of reasons that the programs ultimately Eailed, a major technical problem is that there was no simple means of controlling the power level. Figure 1 is a schematic drawing of a closed Brayton cycle system. Coolant Inlet Coolant Outlat Figure 1. Closed Brayton Cycle Line Drawing A useful propulsion power source must be able to rapidly change power and operate efficiently at various power levels. Unlike baseload power stations, propulsion engines spend a considerable percentage of their time operating at less than their rated capacity. Several different schemes have been proposed and even patented to adjust the power output of closed cycle Brayton systems. Each of them have had serious limitations. As described in ~ c ~ o n a l d ' , many combustion closedcycle gas turbines control power level by varying the the coolant system pressure by using a series of valves and storage flasks. This system is normally referred to as inventory control. The changes in pressure are relatively slow and the range of power that can be produced is limited by the volume of the storage flasks. The flasks consume valuable space and the valving and piping systems are complex. The efficiency, however, is relatively constant over the allowed range of power level. With bypass control, valves are used to direct compressed gas around the heater to the turbine or past the turbine into the heat sink. This means of control is rapid and the equipment is simple, but efficiency can be lowered by a factor of four or more at low power levels when compressed gas is not heated or when heated gas is dumped directly to a heat sink. Copyright c 1994 by Rodney M. Adams. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission.

<div class="htmlview paragraph">The design criteria for an undersea powerplant application are reviewed. A closed Brayton cycle power system utilizing the catalytic combustion of hydrogen and oxygen as a heat source is described. System operation is at a design depth of 20,000 ft for a 50-kW rated output. The total system weight and volume is shown for various endurance missions, as well as a weight and volume breakdown for the closed Brayton cycle plant, reactants, pressure vessels, and buoyancy material. An alternative closed Brayton powerplant design, utilizing a high frequency output alternator, is compared with the reference 400 Hz design. By utilizing a higher shaft speed, the closed Brayton loop pressure level can be increased, which will reduce the size and weight of the closed loop heat exchanger components. The rotating unit size is also significantly reduced.</div>

<div class="htmlview paragraph">The Space Exploration Initiative, (SEI), has renewed interest in examining various technologies for generating electrical power in space as well as on the surface's of the Moon and Mars. One technology that is of interest is the Closed Brayton Cycle, (CBC). Recent work has further enhanced the capability to perform accurate preliminary design and optimization of CBC power conversion systems for space applications. The basic building block of the computer codes used for this work has been the Closed Cycle Engine Performance, (CCEP), code developed by John Klann at NASA Lewis Research Center. This model was originally developed for the preliminary design and performance analysis of the Space Station Freedom dynamic power conversion system.</div> <div class="htmlview paragraph">Improvements to the model have included linking CCEP with the public domain optimization code COPES/ADS. This linkage has greatly enhanced the utility of the program and streamlined the design process. The designer now has the capability of defining design variables, design constraints and optimization functions. This facilitates studies that explore the interrelationship between system mass, cycle efficiency and radiator area. In a recent study the relationship between system mass, radiator area, cycle efficiency and recuperator effectiveness was examined.</div> <div class="htmlview paragraph">In addition to coupling the design codes with the optimizer, additional modifications have been made to enhance the versatility of the model. A subroutine has been added to allow for an isotope heat source. The radiator code has been modified to allow for the preliminary sizing of the radiator using a closed form solution to the governing equations. To take advantage of the Brayton Cycle's ability to produce significantly more power than when operating at design speed and inlet pressure, the alternator sizing algorithms have been modified to allow for off-design sizing of the alternator. This capability allows the designer to specify the fraction of alternator design output that will be required at the turbomachinery design point. Off-design operation can then be examined at significantly higher power levels.</div> <div class="htmlview paragraph">The final modification that will be discussed is the linking of the finite-element pumped-loop radiator design code with the optimization code COPES/ADS. The radiator code is run externally to the main design routines simply due to the execution time required to converge. Output from the optimized radiator designs are used to update the inputs for the radiator specific mass and fin effectiveness used in CCEP.</div>

Parametric study and optimization of closed Brayton power cycle considering the charge amount of working fluid

In the power generation field, simple cycle gas turbines are dominant, with heat exchanged variants only selected based on particular user’s requirements. For the lesser known closed Brayton cycle (CBC) power plant, heat exchangers are mandatory. The following three categories of heat exchangers are addressed in this paper, 1) heat input to the closed cycle from an external source; for example the heat exchanger in a fluidized bed combuster in the case of a fossil-fired plant, or an intermediate heat exchanger (IHX) in the case of an indirect cycle nuclear gas turbine, 2) recuperator in the system to enhance efficiency, and 3) exchangers (i.e., precooler and intercooler) for heat rejection from the system. The influence that these heat exchangers have on the selection of system parameters, and plant performance is discussed. Heat exchanger technology state-of-the-art for CBC systems is highlighted.

As the depth of mines continues to increase, the problems of high temperature and potential heat damage become more prominent. In this study, the characteristics of natural and industrial heat sources in mines were reviewed, and then mainstream heat hazard governance technologies and corresponding utilization methods were discussed and compared. The first category of technologies comprises the optimization of ventilation systems, the insulation of roc heat, and artificial refrigeration. These cooling approaches are limited because the heat resources cannot be recovered. The second category is the utilization of waste industrial heat in mines, including the use of waste heat from the air compressors, drainage water, and foul airflow, but the current applications of these approaches have limited effectiveness in cooling the underground space. The third category is the application of geo-structures to recover natural heat in mines. Based on the principles of the chiller/heat pump cycle and the characteristics of heat sources and sinks in mines, the potential and constraints of each technology were discussed and summarized. This study provides a scientific reference for the selection of suitable heat governance and utilization technologies.

This paper describes a power system concept that provides the electric power for a baseline electric propulsion system for a piloted mission to Mars. A 10‐MWe space power system is formed by coupling an Enabler reactor with a simple non‐recuperated closed Brayton cycle. The Enabler reactor is a gas‐cooled reactor based on proven reactor technology developed under the NERVA/Rover programs. The selected power cycle, which uses a helium‐xenon mixture at 1920 K at the turbine inlet, is diagramed and described. The specific mass of the power system over the power range from 5 to 70 MWe is given. The impact of operating life on the specific mass of a 10‐MWe system is also shown.

In closed Brayton cycle power generation systems, sudden load disturbances can induce a compressor surge in turbine–alternator–compressor systems, posing significant risks to dynamic stability and operational reliability. To address this challenge, this study proposes a PID control strategy optimized via a genetic algorithm. A high-fidelity dynamic model of the turbine–alternator–compressor system under closed Brayton cycle conditions is developed in Simulink, incorporating surge boundaries derived from performance maps. Control parameters are tuned using a weighted multi-objective fitness function that integrates overshoot, rise time, and the integral of absolute error. Simulation results demonstrate that the proposed control scheme markedly enhances system responsiveness—achieving approximately a 70% improvement in rotational speed regulation—and effectively maintains the operating point outside the surge region. The proposed framework provides a practical and robust approach for improving the dynamic stability and reliability of closed Brayton cycle power generation systems.

Methodology to develop off-design models of heat exchangers with non-ideal fluids

Helium-xenon (HeXe)-cooled mobile nuclear reactors have promising potential in future low-carbon energy systems. However, there is currently a lack of fast and reliable tools for analyzing the complicated dynamic characteristics of such systems. In this study, we developed a comprehensive dynamic modeling approach for a HeXe-cooled nuclear power system coupled with a closed Brayton cycle (CBC). The system’s key components, including the reactor, printed circuit heat exchanger (PCHE), and turbomachinery, are lumped-modeled to capture their time-varying behavior. A step-solving algorithm that incorporates HeXe mass conservation iteration is designed. The verification results demonstrate that the dynamic program is robust and reliable, with each time step converging within 25 iterations and the HeXe mass remaining within the range of 3.755 ± 0.01 kg throughout the simulation meeting the law of mass conservation. Then, a 1500 s frozen start-up simulation for the coupled system is conducted, in which the CBC is started in the first 500 s by increasing the main shaft speed to 40% of the rated value, and then the reactor is started by inserting external reactivity between 500 and 800 s. Both the dynamic process and the steady-state performance after the start-up are analyzed. The results show that the system achieved a stable electrical output of 5.7 MWe with a thermal efficiency of 32.5%. This study lays a solid foundation for future work aimed at improving the overall efficiency and performance of HeXe-cooled nuclear power systems.

Design and performance evaluation of solar space Closed Brayton Cycle (CBC) is described in this paper taking into account the influence of orbital and solar conditions. With fixed external conditions (insolation, Tsink, power) overall performance and area of the plant are obtained and optimized (plant area minimization), while to evaluate plant mass a detailed and complete design of the plant components is carried out. Utilizing as the input the results obtained with fixed external conditions, plant transient orbital analysis (TOA) is performed taking into account modification of insolation, Tsink, and power to be generated versus orbit time, (quasi steady transient analysis). All these methods have been fully integrated — the common inputs are interchanged and the output of one code is directly input to the other codes — in a complete design procedure, named CBC-SPACE, suited for Low Earth Orbit (LEO) station power plant design. The most important results are presented and discussed, while the importance of this study is pointed out taking also into account the possibility to extend this analysis to SDCC (solar dynamic combined cycle) plant proposed by the author (Massardo, 1991).