Brayton Cycle Power Conversion System Research Articles

On the dynamic modeling of Brayton cycle power conversion systems with the CATHARE-3 code

As the share of intermittent energy sources is increasing, the dynamic modeling of thermal power plants including their power conversion system gets researchers' attention. Recent applications require transient simulations of high-pressure nitrogen closed Brayton cycle for which the ideal gas assumption is no longer verified. In this work, the REFPROP real gas equation of state and pressure drop correlations suitable for high Reynolds numbers have been implemented in the CATHARE-3 code. Moreover, new real gas turbomachinery and sonic flow models have been developed and integrated in the code. Relative errors obtained for the nominal state of a high-pressure nitrogen closed Brayton cycle are in the range −3%/+0.8%. A detailed analysis of real gas effects is carried out on the heat exchangers heat flux and the piping pressure losses. The new sonic flow computed during a loss of coolant accident is in the best possible agreement with literature experimental results. With regard to the turbomachinery, the new real gas model creates a pressure dependence that brings compressors closer to the choke region when the pressure drops in the cycle. This work is expected to provide an efficient and reliable simulation tool for transient analysis of real gas closed Brayton cycles.

소듐과 이산화탄소 반응에 의한 소듐유로막힘 및 재료손상 현상 연구

본 논문의 목적은 소듐냉각고속로(sodium cooled fast reactor, SFR)와 초임계 <TEX>$CO_2$</TEX> Brayton cycle의 연계 시, 원자로 열수송 계통과 동력변환 계통의 압력 경계를 형성하는 회로인쇄형 열교환기의 경계면에 균열이 발생해 고압(약 200 bar)의 <TEX>$CO_2$</TEX>가 상압 수준의 액체소듐유로 측에 유입되었을 때의 물리/화학적 현상을 파악하여 열교환기 설계에 활용 가능한 실험 자료를 생산하는 것이다. 열교환기의 소듐-<TEX>$CO_2$</TEX> 경계면 균열 현상은 경계면의 균열 크기에 따라 미세 균열에 의한 소듐유로막힘(plugging) 현상과 상대적으로 큰 균열에 의한 열교환기 재료손상(wastage) 현상으로 나뉜다. Plugging 실험결과, 소듐유로 직경이 3mm일 때 <TEX>$CO_2$</TEX> 주입 즉시 소듐 흐름이 정지한 반면 소듐유로 직경이 5 mm일 때는 유량이 감소되기 시작하는 시점은 3 mm의 경우와 유사하게 <TEX>$CO_2$</TEX> 주입 즉시 나타났지만 소듐의 흐름이 완전히 정지할 때까지는 상대적으로 오랜 시간이 소요되었다. 이러한 실험결과는 실제 열교환기의 소듐-<TEX>$CO_2$</TEX> 경계면에서 미세균열이 발생했을 때, 소듐유로 직경이 3 mm로 좁을 경우 균열 발생과 동시에 해당 소듐유로가 반응생성물에 의해 막혀 해당 유로 외의 유로들로 지속적인 열교환기 운전이 가능하지만, 소듐유로의 직경이 5 mm로 넓어질 경우 소듐유로가 고체생성물에 의해 즉시 막히지 않고 생성물이 소듐유로를 따라 계통 내부를 이동하다 일정 농도 이상이 되어야 소듐유로를 막게 할 것으로 예상할 수 있는 결과이다. Wastage 실험결과, 열교환의 재질(STS316, Inconel600, G91 합금강), 운전온도(<TEX>$400{\sim}500^{\circ}C$</TEX>), 노즐직경(0.2~0.8 mm), 시편-노즐 거리(2~6 mm)와 무관하게 고압(약 200~250 bar)의 <TEX>$CO_2$</TEX> 분사에 의한 시편의 물리적 손상(erosion) 현상은 발생하지 않았다. 노즐에서의 분사되는 <TEX>$CO_2$</TEX>의 분사속도는 마하 0.4~0.7인 것으로 확인되었다. 본 연구의 실험결과는 열교환기 파손 대처 설계에 배경 실험 자료로 활용될 것으로 기대된다. We investigated the physical/chemical phenomena that a slow loss of <TEX>$CO_2$</TEX> inventory into sodium after the sodium-<TEX>$CO_2$</TEX> boundary failure in printed circuit heat exchangers (PCHEs), which is considered for the supercritical <TEX>$CO_2$</TEX> Brayton cycle power conversion system of a sodium-cooled fast reactor (SFR). The first phenomenon is plugging inside narrow sodium channels by micro cracks and the other one is damage propagation referred to as wastage combined with the corrosion/erosion effect. Experimental results of plugging shows that sodium flow immediately stopped as <TEX>$CO_2$</TEX> was injected through the nozzle at <TEX>$300{\sim}400^{\circ}C$</TEX> in 3 mmID sodium channels, whereas sodium flow stopped about 60 min after <TEX>$CO_2$</TEX> injection in 5 mmID sodium channels. These results imply that if pressure boundary of sodium-<TEX>$CO_2$</TEX> fails a narrow sodium channel would be plugged by reaction products in a short time whereas a relatively wider sodium channel would be plugged with higher concentration of reaction products. Wastage by the erosion effect of <TEX>$CO_2$</TEX> (200~250 bar) hardly occurred regardless of the kinds of materials (stainless steel 316, Inconel 600, and 9Cr-1Mo steel), temperature (<TEX>$400{\sim}500^{\circ}C$</TEX>), or the diameter of the <TEX>$CO_2$</TEX> nozzle (0.2~0.8 mm). Velocities at the <TEX>$CO_2$</TEX> nozzle were specified as Mach 0.4~0.7. Our experimental results are expected to be used for determining the design parameters of PCHEs for their safeties.

Computational analysis of supercritical CO2 Brayton cycle power conversion system for fusion reactor

The Optimized Supercritical Cycle Analysis (OSCA) code is being developed to analyze the design of a supercritical carbon dioxide (S-CO2) driven Brayton cycle for a fusion reactor as part of the Modular Optimal Balance Integral System (MOBIS). This system is based on a recompression Brayton cycle. S-CO2 is adopted as the working fluid for MOBIS because of its easy availability, high density and low chemical reactivity. The reheating concept is introduced to enhance the cycle thermal efficiency. The helium-cooled lithium lead model AB of DEMO fusion reactor is used as reference in this paper.

Finite-thrust optimization of interplanetary transfers of space vehicle with bimodal nuclear thermal propulsion

The nuclear thermal rocket (NTR) propulsion is one of the leading promising technologies for primary space propulsion for manned exploration of the solar system due to its high specific impulse capability and sufficiently high thrust-to-weight ratio. Another benefit of NTR is its possible bimodal design, when nuclear reactor is used for generation of a jet thrust in a high-thrust mode and (with an appropriate power conversion system) as a source of electric power to supply the payload and the electric engines in a low-thrust mode. The model of the NTR thrust control was developed considering high-thrust NTR as a propulsion system of limited power and exhaust velocity. For the proposed model the control of the thrust value is accomplished by the regulation of reactor thermal power and propellant mass flow rate. The problem of joint optimization of the combination of high- and low-thrust arcs and the parameters of bimodal NTR (BNTR) propulsion system is considered for the interplanetary transfers. The interplanetary trajectory of the space vehicle is formed by the high-thrust NTR burns, which define planet-centric maneuvers and by the low-thrust heliocentric arcs where the nuclear electric propulsion (NEP) is used. The high-thrust arcs are analyzed using finite-thrust approach. The motion of the corresponding dynamical system is realized in three phase spaces concerning the departure planet-centric maneuver by means of high-thrust NTR propulsion, the low-thrust NEP heliocentric maneuver and the approach high-thrust NTR planet-centric maneuver. The phase coordinates are related at the time instants of the change of the phase spaces due to the relations between the space vehicle masses. The optimal control analysis is performed using Pontryagin’s maximum principle. The numerical results are analyzed for Earth–Mars “sprint” transfer. The optimal values of the parameters that define the masses of NTR and NEP subsystems have been evaluated. It is shown that the low-thrust NEP subsystem with Brayton cycle power conversion system is preferable in comparison with NEP subsystem with thermoemission power conversion system.

Flexible conversion ratio fast reactors: Overview

Conceptual designs of lead-cooled and liquid salt-cooled fast flexible conversion ratio reactors were developed. The performance achievable by the unity conversion ratio cores of these reactors was compared to an existing supercritical carbon dioxide-cooled (S-CO 2) fast reactor design and an uprated version of an existing sodium-cooled fast reactor. All concepts have cores rated at 2400 MWt. The cores of the liquid-cooled reactors are placed in a large-pool-type vessel with dual-free level, which also contains four intermediate heat exchangers (IHXs) coupling a primary coolant to a compact and efficient supercritical CO 2 Brayton cycle power conversion system. The S-CO 2 reactor is directly coupled to the S-CO 2 Brayton cycle power conversion system. Decay heat is removed passively using an enhanced reactor vessel auxiliary cooling system (RVACS) and a passive secondary auxiliary cooling system (PSACS). The selection of the water-cooled versus air-cooled heat sink for the PSACS as well as the analysis of the probability that the PSACS may fail to complete its mission was performed using risk-informed methodology. In addition to these features, all reactors were designed to be self-controllable. Further, the liquid-cooled reactors utilized common passive decay heat removal systems whereas the S-CO 2 uses reliable battery powered blowers for post-LOCA decay heat removal to provide flow in well defined regimes and to accommodate inadvertent bypass flows. The multiple design limits and challenges which constrained the execution of the four fast reactor concepts are elaborated. These include principally neutronics and materials challenges. The neutronic challenges are the large positive coolant reactivity feedback, small fuel temperature coefficient, small effective delayed neutron fraction, large reactivity swing and the transition between different conversion ratio cores. The burnup, temperature and fluence constraints on fuels, cladding and vessel materials are elaborated for three categories of material – materials currently available, available on a relatively short time scale and available only with significant development effort. The selected fuels are the metallic U–TRU–Zr (10% Zr) for unity conversion ratio and TRU–Zr (75% Zr) for zero conversion ratio. The principal selected cladding and vessel materials are HT-9 and A533 or A508, respectively, for current availability, T-91 and 9Cr–1Mo steel for relatively short-term availability and oxide dispersion strengthened ferritic steel (ODS) available only with significant development.

Optimization of Advanced High-Temperature Brayton Cycles with Multiple-Reheat Stages

This paper presents an overview and a few point designs for multiple-reheat Brayton cycle power conversion systems (PCSs) using heat from high-temperature molten salts or liquid metals. All designs are derived from the General Atomics gas turbine-modular helium reactor (GT-MHR) power conversion unit (PCU). Analysis shows that, with relatively small engineering modifications, multiple GT-MHR PCUs can be connected together to create a PCS in the >1000 MW(electric) class. The resulting PCS is quite compact, and results in what is likely the minimum gas duct volume possible for a multiple-reheat system. To realize this, compact plate type liquid-to-gas heat exchangers (power densities from 10 to 120 MW/m3) are needed. Different fluids such as helium, nitrogen and helium mixture, and supercritical CO2 are compared for these multiple-reheat Brayton cycles. For turbine inlet temperatures of 900, 750, and 675°C, the net thermal efficiencies for helium cycles are 56, 51, and 48%, respectively, and corresponding PCU power densities are 560, 490, and 460 kW(electric)/m3, respectively. The very high PCU power densities could imply a large material saving and low construction cost, and bring down the specific PCU cost to about half that of the current GT-MHR PCS design.

MIT PEBBLE BED REACTOR PROJECT

The conceptual design of the MIT modular pebble bed reactor is described. This reactor plant is a 250 Mwth, 120 Mwe indirect cycle plant that is designed to be deployed in the near term using demonstrated helium system components. The primary system is a conventional pebble bed reactor with a dynamic central column with an outlet temperature of 900 C providing helium to an intermediate helium to helium heat exchanger (IHX). The outlet of the IHX is input to a three shaft horizontal Brayton Cycle power conversion system. The design constraint used in sizing the plant is based on a factory modularity principle which allows the plant to be assembled “Lego” style instead of constructed piece by piece. This principle employs space frames which contain the power conversion system that permits the Lego-like modules to be shipped by truck or train to sites. This paper also describes the research that has been conducted at MIT since 1998 on fuel modeling, silver leakage from coated fuel particles, dynamic simulation, MCNP reactor physics modeling and air ingress analysis.

Optimized Helium-Brayton Power Conversion for Fusion Energy Systems

This paper presents an overview and a few point designs for multiple-reheat helium Brayton cycle power conversion systems using molten salts (or liquid metals or direct helium cooling). All designs are derived from the General Atomics GT-MHR power conversion unit (PCU). The important role of compact, offset fin heat exchangers for heat transfer to the power cycle helium, and the potential for these to be fabricated from carbon-coated composite materials that would have lower potential for fouling, are discussed. Specific links are made to the ITER TBM and laser IFE blanket design, and to Z-Pinch/HIF thick-liquid IFE.

Predicting the Behavior of Solar Dynamic Closed Brayton Cycle Power Conversion Systems

Since space power conversion systems must operate both in the sun and in the earth’s shadow, they seldom encounter design operating conditions. As a consequence, consideration of off-design performance is essential in the preliminary design of these systems. To illustrate the necessity and utility of an off-design system model, this paper presents the results of a study of the solar dynamic closed Brayton cycle power conversion system for use on the NASA Space Station.

Closed brayton solar dynamic power for the space station

The Rocketdyne Division of the Rockwell International Corporation has conducted a study of the electrical power system (EPS) for the NASA Space Station. The results of this study indicate that the EPS should be a hybrid system consisting of a combination of photovoltaic and solar dynamic power systems symmetrically mounted outboard on the transverse boom as shown in Figure 1. The selection of this power system combination has been previously reported by Rocketdyne (ref. 1, 2, 3). Garrett has supported the Rocketdyne study with designs of the solar dynamic closed Brayton cycle (CBC) power conversion system option. This paper describes the results of the study.

Closed Cycle Gas Turbine as a Proposed Undersea Power Source

Undersea application of compact and lightweight closed Brayton cycle power systems currently being developed for space is discussed. Design details of a 50 kwe hydrogen-oxygen fueled system for a short-duration undersea mission of 1000 kw-hr are presented. For this same mission, a new hydrogen-oxygen injection gas turbine cycle is described. Performance characteristics of undersea long-duration 10 to 150 kwe power systems, using radioisotope and nuclear reactor heat sources, are also presented. Development status of closed Brayton cycle power conversion systems is presented.

Integration of Large Power Systems into Manned Space Stations

Essential design factors and system characteristics are explored for integration of large power systems into manned space stations. The impact of the type of power system selected upon the space station is outlined, as is the impact of the mission requirements upon the selection of power systems. Criteria for resolving the selection/application/ integration problems are provided. Comparisons between systems are based on recently defined space-station models for 90-day to five-year mission durations in the 1970' s, with four-to nine-man crews. Power systems encompass power levels from 3 to 50 kWe and include solar cell/battery. fuel cell, hybrid fuel cell/solar cell, radioisotope, and nuclear reactor systems. Thermoelectric, Brayton cycle, organic Rankine, and liquid-metal Rankine power conversion systems are considered for the nuclear energy sources. Both rigid and roll-out photovoltaic array configurations are analyzed with respect to the solar energy source.