Optimization of Closed Brayton Cycles for Space Power Generation

<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>

<div class="section abstract"><div class="htmlview paragraph">The present work discusses a novel oxy-fuel combustion cycle utilized in compression ignition internal combustion engine. The most prominent feature of this cycle is that the air intake is replaced by oxygen; therefore nitric oxide (NO<sub>X</sub>) emission is eliminated. The enrichment of oxygen leads to higher flame speed and mass fraction consumption rate; on the other hand, the high concentration of oxygen presented during combustion will result in intense pressure rise rate which may cause severe damage to engine hardware. As water injection is already utilized in gasoline engine to control knocking, the utilization of water injection in optimizing oxy-fuel combustion process has been tested in this study. To understand the relationship between water injection strategy and cycle efficiency, computational fluid dynamics (CFD) simulations were carried out. The model was carefully calibrated with the experimental results; the errors were controlled within 3%. The operation parameters such as speed, oxygen fraction, water injection pressure, and temperature were set according to experiment data. By doing so, this study investigates the effect of water injection strategy in improving cycle efficiency based on the calibrated model. The simulation results show that the injected water absorbs combustion heat to evaporate; therefore increased amount of working fluid leads to higher cycle efficiency. As water injection quantity increases, the water consumes too much heat and thus deteriorates combustion leading to lower cycle efficiency. On the other hand, different injection timing and temperature were selected to study its effect; the results show that earlier injection timing will cause the direct interaction between flame and water that leads to lower combustion efficiency and late injection timing will provide insufficient in-cylinder temperature for high efficient evaporation rate. In this case, the optimal injection strategy is 1:1.11 fuel-to-water ratio, 365°CA timing, and 160°C temperature; cycle efficiency can be improved by 5.2% under this strategy.</div></div>

<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>

Development of Transient Pulse Detonation Engine Cycle Analysis and Performance Prediction (PDE-CAPP) Code

<div class="htmlview paragraph">NASA's Lewis Research Center in Cleveland, OH utilizes a broadband LAN for a wide variety of office, engineering, and research purposes. Increasing use of the facility led to capacity and throughput concerns, focused on the terminal support channels. Henderson State University in Arkadelphia. AR developed PC software to monitor errors and utilization. The software presents real-time performance graphs, issues alerts for error conditions and logs performance data for analysis.</div> <div class="htmlview paragraph">The paper explores both the reasoning behind developing the monitor program and the results of a year's operation.</div>

<div class="htmlview paragraph">The NASA-LeRC under the direction of DOE is responsible for the test and evaluation of electric and hybrid vehicle propulsion systems and components. In September 1978, a contracted effort was undertaken with the General Electric Company to design, fabricate, and deliver a propulsion system breadboard of the GE-NTEV (ETV-1). This breadboard is currently under test in the LeRC Road Load Simulator and its operational characteristics are the subject of this paper.</div> <div class="htmlview paragraph">THE ELECTRIC TEST VEHICLE-ONE (ETV-1) was built by General Electric Company and Chrysler Corporation for the U.S. Department of Energy (DOE) to represent an electric vehicle designed and built from the ground up with present state-of-the-art technology. Two vehicles were built and are presently being evaluated by NASA's Jet Propulsion Laboratory (JPL). Some results of these JPL tests are also being presented at this conference. A duplicate set of propulsion system components were built, mounted on a breadboard, and delivered to NASA's Lewis Research Center for testing on the newly acquired Road Load Simulator (RLS). Lewis Research Center is responsible for the development, test, and evaluation of electric and hybrid vehicle propulsion systems and components for the Department of Energy. The breadboard is currently under test. This paper will describe driving cycle tests that have been completed on the system.</div>

<div class="htmlview paragraph">A major effort at the NASA Lewis Research Center in recent years has been to develop electromechanical actuators (EMA's) to replace the hydraulic systems used for thrust vector control (TVC) on launch vehicles. This is an attempt to overcome the inherent inefficiencies and costs associated with the existing hydraulic structures. General Dynamics Space Systems Division, under contract to NASA Lewis, is developing 18.6 kW (25 hp), 29.8 kW (40 hp), and 52.2 kW (70 hp) peak EMA systems to meet the power demands for TVC on a family of vehicles developed for the National Launch System. These systems utilize a pulse population modulated converter and field-oriented control scheme to obtain independent control of both the voltage and frequency. These techniques allow an induction motor to be operated at its maximum torque at all times. At NASA Lewis, we are building on this technology to develop our own in-house system capable of meeting the peak power requirements for an expendable launch vehicle (ELV) such as the Atlas. Our EMA will be capable of delivering 22.4 kW (30 hp) peak power with a nominal of 6.0 kW (8 hp). This system differs from the previous ones in two areas: 1) the use of advanced control methods, and 2) the incorporation of built-in-test. The advanced controls are essential for minimizing the controller size, while the built-in-test is necessary to enhance the system reliability and vehicle health monitoring.</div> <div class="htmlview paragraph">The ultimate goal of this program is to demonstrate an EMA which will be capable of self-test and easy integration into other projects. This paper will describe the effort underway at NASA Lewis to develop an EMA for an Atlas class ELV. An explanation will be given for each major technology block, and the status of the overall program will be reported.</div>

An integration study was performed by Rocketdyne coupling an SP‐100 reactor to either a Brayton, Stirling, or K‐Rankine power conversion system. The application was for a surface power system to supply power requirements to a lunar base. A power level of 550 kWe was selected based on the National Aeronautics and Space Administration (NASA) Space Exploration Initiative 90‐day study. Reliability studies were initially performed to determine optimum power conversion redundancy. This study resulted in selecting three operating engines and one standby unit. Integration design studies indicated that either of the three power conversion systems could be integrated with the SP‐100 reactor. From a performance consideration, the Brayton and Stirling mass was approximately 45% higher than the K‐Rankine. The K‐Rankine radiator area was 45% of the Stirling, which in turn was about 40% of the Brayton.

<div class="section abstract"><div class="htmlview paragraph">Power and efficiency characteristics of a hybrid cycle combining an electrochemical device (Fuel-Cell) and an internal combustion engine (ICE) were analyzed using the low-dissipation model. The low-dissipation model links energy dissipation with the energy transfer rate through the cycle. In the considered cycle, the electrochemical device transforms chemical potential of the fuel to electrical work, and the ICE uses the heat rejected by the electrochemical device and its exhaust effluent for mechanical work production. The cycle efficiency was calculated as a function of the hybridization level. The latter is defined as the electrical work fraction in the total cycle work. The results of the study show that the cycle efficiency is growing with the electrical work fraction increase. On the other hand, maximum power of the cycle is attained at an intermediate hybridization level. Moreover, power to weight ratio and power density of the cycle have maxima at different hybridization level. Cycle cooling losses are modeled as heat leak to the ambient that depends on the temperature and the duration of the cycle. Cooling losses are found to be the most influential parameter in optimization of the hybridization level for maximum power. In the extreme case of zero cooling losses, maximum power could be attained with ICE operation alone without the electrochemical reaction. The latter finding might be of interest for aerial propulsion systems. However, if efficiency is more important - for example for ground propulsion systems - the hybrid cycle is beneficial.</div></div>

Future space power requirements will vary from the subkilowatt range for deep space probes, to the hundreds of kilowatts range for a lunar base, to the multimegawatt range for interplanetary propulsion systems. Closed Brayton cycle (CBC) power conversion has the flexibility to be used in all these power ranges and with a variety of heat source options such as isotope, solar, and nuclear. Each of these types of heat sources has different characteristics that make it more appropriate for particular mission profiles and power output ranges. Heat source characteristics can also be major design drivers in the closed Brayton cycle design optimization process. This paper explores heat source selection and the resulting CBC system designs, and discusses optimization methods as a function of the main design drivers. Such power system requirements as power level, man-rated radiation shielding, fuel costs, eclipse/darkness duration, system mass, radiator area, reliability/mission duration, and insolation level are evaluated through several CBC parametric case studies. These cases include: (1) a 500 We power system for deep space probes; (2) a 50 kWe solar dynamic system for earth orbit and other applications; (3) a 100 kWe man-rated lunar/Mars stationary /rover power system; (4) a 200 to 825 kWe power system for the lunar outpost; and (5) 3300 kWe modules for interplanetary propulsions.

<div class="section abstract"><div class="htmlview paragraph">Reduction of carbon dioxide (CO₂) emission, which causes global warming, is an important guideline for vehicle engine development. There are two types of methods for reducing the CO₂ emission of a vehicle engine. The first involves improving engine efficiency. The second involves the use of a low-carbon fuel, i.e., fuel with high hydrogen to carbon ratio. Hydrogen-compressed natural gas blend (HCNG) has been researched as a low-carbon fuel. Given that thermal efficiency of an engine cycle increases with its compression ratio (CR), an HCNG engine with high compression ratio not only has high efficiency but also low CO₂ emission. However, unexpected combustion such as knock could occur owing to the increased CR.</div><div class="htmlview paragraph">In this study, we investigated the knock and emission characteristics of an 11-L heavy-duty HCNG engine with a modified CR. A conventional CNG engine was fuelled with HCNG30 (CNG 70 vol% and hydrogen 30 vol%). The engine was operated at 1,260 rpm, under the wide open throttle condition, and with CRs of 10.5 and 11.5. The results showed that under a CR of 11.5, the engine's knock margin decreased because of higher in-cylinder pressure and temperature. Thermal efficiency improved with the increased CR, which caused a significant decrease in CO₂ emission. However, NO<sub>x</sub> emission increased slightly at a certain excess air-ratio condition.</div></div>

<div class="section abstract"><div class="htmlview paragraph">With the development and maturity of new generation digital technologies such as artificial intelligence, Internet of Things, and 5G mobile communication, their integration with physical products is becoming increasingly seamless. Automobiles serve as a prime example in this regard. In recent years, automated vehicle (AV) technologies have emerged as a prominent focal point, witnessing an escalating acceptance in the market and a growing number of self-driving vehicles on the roads, existing roads are primarily designed for traditional human-driven vehicles (HVs). Due to the differences in perception between automated systems and human drivers, it is essential to assess AVs' feasibility to current road infrastructure. This paper analyzes the safety and comfort of automated vehicles equipped with adaptive cruise control systems (ACC-AVs) on longitudinal road profiles from the perspective of vehicle dynamics. Firstly, a co-simulation platform integrating PreScan, CarSim, and Simulink software is established, providing a comprehensive environment for simulating AV behavior. Secondly, an evaluation system is developed to assess AV’s feasibility on longitudinal roads, based on safety indicators (rear-end collisions occurrence) and comfort indicators (axial acceleration, vertical acceleration, and vertical acceleration change rate). Lastly, the feasibility of ACC-AVs on existing longitudinal profile roads is simulated and evaluated. The results indicate that, on straight slope section, ACC-AVs may experience rear-end collisions on downhill sections with design speeds below 100 km/h; when safety requirements are met, both uphill and downhill sections exhibit good comfort levels. For the vertical curve section, comfort is also favorable in segments with low design speeds and large vertical curve radii, however, as curve radii decrease or design speeds increase, comfort deteriorates. The findings of this study provide a reference for optimizing highway profile design for AVs.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Restrictive future CO<sub>2</sub> emissions regulations are incentivizing evaluation of carbon-free fuels. This is particularly true in the difficult to electrify heavy commercial vehicle segment. The reemergence of hydrogen internal combustion (H<sub>2</sub> ICE) for large displacement engines can both expedite hydrogen adoption and reduce total cost of ownership. This paper covers how the application of various boosting architectures can address challenges unique to H<sub>2</sub> ICE. The research presented is derived from simulations conducted by AVL List GmbH and SuperTurbo Technologies on a 13L H<sub>2</sub> ICE. The GT Power model was calibrated from dyno testing at AVL of an operational engine and then modified with different boosting systems. The primary H<sub>2</sub> ICE challenge that is addressed is the requirement for the engine to maintain a lean-burn combustion strategy through transient operation in order to control NO<sub>x</sub> formation and minimize aftertreatment requirements. This high lambda requirement can create challenges for turbochargers when available turbine power is insufficient for the desired compressor power. The simulation includes both conventional turbochargers – VGT and 2-stage - and driven turbochargers – SuperTurbo, E-boost, and E-turbo. Each system was evaluated through the WHTC drive cycle and load step transients to show instantaneous and cumulative NO<sub>x</sub>, transient response capability and cycle efficiency. The goal of the research is to show the capability of different boosting systems related to the unique challenges for hydrogen combustion to maintain performance and drivability, while controlling engine out NO<sub>x</sub> emissions to minimize aftertreatment NO<sub>x</sub> conversion requirements.</div></div>

<div class="htmlview paragraph">The modern day quest for speed in ground vehicle design utilizes advanced aerodynamic simulation methods using high-performance computers and calls for experienced engineers to interpret the results. The current widespread use of computational fluid dynamics analysis (CFD) in particular, is helping produce aerodynamically and thermally correct racing cars that can take full advantage of complex ground effects aerodynamic technology. The same CFD technology can greatly improve high-performance engine fuel efficiency and combustion as well as assist to develop the ultimate in under-the-hood thermal management systems.</div> <div class="htmlview paragraph">The use of these tools owes much to a related quest for speed that developed in the 18th Century by supply ships sailing from the western shores of Europe, to the eastern shores of the America's. In that quest for speed, mathematicians at some of Europe's leading Universities sought to resolve the underlying physical and mathematical models for the motion of such vehicles.</div> <div class="htmlview paragraph">Of particular interest was the fundamental work done to describe the decaying motion of a pendulum swinging through air lead directly leading to understanding the concept of viscous drag. This work, conducted by Par Navier (1827) at the University of Paris, and Sir Gabriel Stokes (1845) at Cambridge University, England, culminated in the famous Navier-Stokes equations that described virtually all fluid motions to a degree that still withstands the highest modern scrutiny. These equations have had their greatest impact in modern day aerospace design, including both airframes and turbine engines.</div>