Thermal‐hydraulic design features of a micronuclear reactor power source applied for multipurpose

Microheat pipe cooled reactor power source (HRP) designed for space or underwater vehicles meets the future demands, such as safer structure, longer operating time, and fewer mechanical moving parts. In this paper, potassium heat pipe cooled reactor power source system which generates 50 kWe electricity is proposed. The reactor core using uranium nitride fuel is cooled by 37 potassium high‐temperature heat pipes. The shields are designed as tungsten and water, and reactor reactivity is controlled by control drums. The thermoelectric generator (TEG) consists of thermoelectric conversion units and seawater cooler. The thermoelectric conversion units convert thermal energy to electric energy through the high‐performance thermoelectric material. A code applied for designing and analyzing the reactor power system is developed. It consists of multichannel reactor core model, heat pipe model using thermal resistance network, thermoelectric conversion, and thermal conductivity model. Then, the sensitivity analysis is performed on two key parameters including the length of the heat pipe condensation section and the cold junction temperature of the TE cell. Meanwhile, the steady‐state calculations are conducted. Results show that the maximum fuel temperature is 938 K located in the center of reactor core and the outlet temperature of coolant reaches 316 K. Both of them are within the limitation. It is concluded that the preliminary design of HPR design is reasonable and reliable. The designed residual heat removal system has sufficient safety margin to release the decay heat of the reactor. This research provides valuable analysis for the application of micronuclear power source.     

Comparative study on adsorbent characteristics for adsorption thermal energy storage system

The adsorption performance of the thermal energy storage (TES) system changes depending on the material properties of the adsorbent itself, but the change of the hardware structure can also substantially change the adsorption characteristics. In this study, a laboratory‐scale adsorption‐based TES system was constructed, and the adsorption performance of three adsorbents was evaluated in the same system to compare the adsorption performance between adsorbents. The adsorption characteristics of silica gel, zeolite 13X, and 4A, which are the most preferred adsorbents in the physical adsorption‐based TES system, were selected for evaluation. Experiments with each adsorbent were performed, including heat recovery to evaluate the heat transfer effect and the amount of heat recoverable in the actual TES system. Experimental results have identified several key characteristics of the adsorption and performance of each adsorbent in the TES system, as well as operating parameters that determine the influence of adsorption performance on the TES system. The actual energy storage density of the adsorbent is affected not only by the enthalpy of adsorption of the material itself but also by other factors. These factors include the difference in thermal conductivity that causes a difference in temperature distribution and the magnitude of mass transfer resistance due to the shape of the adsorbent particle and the actual TES system reactor structure. If the reaction heat generated during the adsorption reaction cannot be effectively released, the adsorption performance is significantly lowered due to the increased temperature of the reactor inside. This phenomenon was commonly observed in adsorbents examined in the present study. The uptake amount, X [g/g], was increased by allowing the inside of the reactor to be maintained at a lower temperature through heat recovery. In case of silica gel, the temperature rise during adsorption reaction is not high due to the difference of isotherm characteristics compared with zeolites, but it is possible to absorb more amount of adsorbate and to recover heat for a longer time. The energy storage density is affected by the temperature increase effect and the uptake amount of adsorbate during the adsorption reaction. The experimental results show that the energy storage density of zeolite 13X is 15% and 28.7% higher than that of silica gel and 4A, respectively, and the temperature rise due to heat generation during adsorption reaction is also high, which is advantageous in adsorption TES system performance.     

Dishwasher and washing machine heated by a hot water circulation loop

Electric energy (70–90%) used by electrically heated dishwashers and washing machines is used for heating the water, the crockery, the laundry and the machine and could as well be replaced by heat from other sources than electricity. This article evaluates prototypes of a dishwasher and a washing machine, where the machines are heated by a hot water circulation loop and the heat is transferred to the machines via a heat exchanger. The machine therefore uses water from the cold water pipe. Measurements and simulations have been performed showing that all energy for heating can be replaced if the supply water temperature is 65–70 °C. An alternative and common way to save electricity is to connect the machines to the domestic hot water pipe, but the electrical savings with this measure are much smaller, especially for the dishwasher. Computer modelling has been performed and the model has proved to have a high agreement with measured data. However comparison with manufacturers’ data indicates that the computer models overestimate the energy demand by about 10%.     

Neutron physics of the liquid‐fuel heat‐pipe reactor concept with molten salt fuel—Static calculations

A liquid‐fuel heat‐pipe reactor (LFHPR) is a novel fast heterogeneous reactor developed by Harbin Engineering University, China, on the basis of liquid‐fuel reactor designs and the heat‐pipe reactor concept. In the concept, the reactor abandons the graphite moderator and keeps neither fuel tubes arranged in the graphite nor fuel rings around the heat pipe. Instead, the reactor applies molten salt fuels, molten metallic eutectic fuels, or other fuels in liquid form. The heat generated in the reactor is removed by the heat pipes driven by liquid metals. With this change, an LFHPR is much more flexible in design and application and able to achieve several advanced features compared with conventional heat‐pipe reactors. In this paper, we describe the general reactor design of an LFHPR, discuss its potential advantages, and give a preliminary verification of the neutron physical feasibility for the reference case, which uses molten salt as the fuel, by using both Monte Carlo and deterministic methods. Results show that the LFHPR yields a hard neutron spectrum that brings a very good neutron economy and is a promising application for breeding. From our approach, we conclude that the proposed LFHPR has a very high power density and high negative temperature feedback coefficient.     

利用热管强化吸附床内的传热传质

为了强化吸附式制冷吸附床内的传热传质，设计了利用高效传热元件热管作为内翅片的吸附床。在能量守恒关系和吸附平衡方程的基础上建立了吸附床的数学模型，并对此模型用数值方法进行了求解。求解结果表明利用热管元件可以显著的改善吸附床内的传热传质过程，缩短了吸附式制冷的循环时间，提高了系统的效率，该数学模型为吸附床的设计参数的选择和优化等提供了依据。     

Dynamic simulation of a space gas-cooled reactor power system with a closed Brayton cycle

Space nuclear reactor power (SNRP) using a gas-cooled reactor (GCR) and a closed Brayton cycle (CBC) is the ideal choice for future high-power space missions. To investigate the safety characteristics and develop the control strategies for gas-cooled SNRP, transient models for GCR, energy conversion unit, pipes, heat exchangers, pump and heat pipe radiator are established and a system analysis code is developed in this paper. Then, analyses of several operation conditions are performed using this code. In full-power steady-state operation, the core hot spot of 1293 K occurs near the upper part of the core. If 0.4 $ reactivity is introduced into the core, the maximum temperature that the fuel can reach is 2059 K, which is 914 K lower than the fuel melting point. The system finally has the ability to achieve a new steady-state with a higher reactor power. When the GCR is shut down in an emergency, the residual heat of the reactor can be removed through the conduction of the core and radiation heat transfer. The results indicate that the designed GCR is inherently safe owing to its negative reactivity feedback and passive decay heat removal. This paper may provide valuable references for safety design and analysis of the gas-cooled SNRP coupled with CBC.     

Complete and reduced models for metal hydride reactor with coiled-tube heat exchanger

Thermal effects during hydriding/dehydriding have a significant influence on the performance of metal hydride hydrogen storage system. The heat exchanger is widely used in the metal hydride reactor in order to improve the efficiency of system. In this work, based on mass balance, momentum balance, energy balance equations, equation of reaction kinetics and equilibrium pressure equation, a two dimensional axisymmetric model of metal hydride reactor packed with LaNi₅ is developed on Comsol platform. The model is validated by comparing its simulation results with the experiment data and the simulation results from other works. Then, the straight pipe heat exchanger and the coiled-tube heat exchanger are taken into consideration in order to improve heat transfer from metal hydride reactor to ambient environment. The complete three dimensional model is developed for the metal hydride reactor equipped with the coiled-tube heat exchanger. The case with coiled-tube heat exchanger shows better efficiency than the other. In general, the temperature in central area is higher than others. In order to cool central area effectively, two designs of heat exchangers, including the combination of coiled-tube heat exchanger and straight pipe heat exchanger and the concentric dual coiled-tube heat exchanger, are studied. The results show that it is an effective method to improve the efficiency of metal hydride reactor by equipping dual coiled-tube heat exchangers. Reduced two dimensional model is applied to metal hydride reactor with coiled-tube heat exchanger to reduce computing time. The simulation results of reduced model generally agree with those of complete three dimensional model.     

Design,optimisation and conversion-efficiency determination of a line-focus parabolic-trough solar-collector (PTC)

PTCs are the preferred type of collector used for steam generation, due to their ability to work at high temperatures with high efficiencies. The results produced from a simulation program, showing the variation of collector's efficiency as a function of heat transfer fluid flux, pipe diameter, solar radiation intensity and active area of the PTC, are presented.     

Heat and mass transfer during adsorption of ammonia in a cylindrical adsorbent bed: thermal performance study of a combined parabolic solar collector, water heat pipe and adsorber generator assembly

In this paper we present the study of adsorption refrigerator which use an activated carbon-pair ammonia. The ability of activated carbons to adsorb large mass of ammonia makes them ideal for use in adsorption refrigeration and pump systems. These systems have not reasonable efficiency. In order to make these systems economically viable, their size must be reduced. This implies a need for a rapid heating and cooling the adsorbent/refrigerant pair. However, the main problems to be overcome is related to the poor heat transfer in the adsorbent bed. So, it is necessary to study and understand the heat and mass transfer within the bed and to improve it. A detailed model of heat and mass transfer into the generator has been developed. For a given heat flux, temperature and adsorbed mass have been computed in every point at each step time along the adsorbed bed (generator). Experimental installation simulating an adsorption machine working within a temperature ranging from 20 to 250 °C and pressure ranging from 0 to 2.5 × 10⁶ Pa, allows for identification of the generator's equivalent thermal conductivity and internal heat transfer coefficient. These two parameters are then used to simulate thermal performance of a design whose features include the insertion of stainless steel water heat pipe (HP's) condensers into the generator. The HP's evaporator heat input is of solar origin using a compound parabolic collector (CPC). Nominal Solar coefficient of performance, COPs =14.37% obtained through both Adimensional Exergy Loss (AEL), and COP study, shows the competitiveness of the proposed design.     

Enhancement of hydrogen charging in metal hydride-based storage systems using heat pipe

Heat transfer in metal hydride bed significantly affects the performance of metal hydride reactors (MHRs). Enhancing heat transfer within the reaction bed improves the hydriding rate. This study presents performance analysis in terms of storage capacity and time of three different cylindrical MHR configurations using storage media LaNi₅: a) reactor cooled with natural convection, b) reactor with a heat pipe on the central axis, c) reactor with finned heat pipe. This study shows the impact of using heat pipes and fins for enhancing heat transfer in MHRs at varying hydrogen supply pressures (2–15 bar). At any absorption temperature, hydrogen absorption rate and hydrogen storage capacity increase with the supply pressure. Results show that using a heat pipe improves hydrogen absorption rate. It was found that finned heat pipe has a significant effect on the hydrogen charge time, which reduced by approximately 75% at 10 bar hydrogen supply pressure.     

Dynamic characteristics of a novel adsorption refrigerator with compound mass‐heat recovery

A three‐effect heat pipe (heat pipe heating, heat pipe cooling and heat pipe heat recovery) adsorption refrigeration system using compound adsorbent (calcium chloride and activated carbon) was designed. The dynamic characteristics of mass and heat pipe heat recovery were studied. The results show that mass recovery and heat pipe heat recovery can improve (specific cooling power) SCP and (coefficient of performance) COP greatly. The averaged SCP of the cycle with mass recovery and the cycle without mass recovery is 502.9 W/kg and 436.7 W/kg at about 30 °C of cooling water temperature and ?15 °C of evaporating temperature. The corresponding COP is 0.27 and 0.24 respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

Heat and mass transfer in a non-isothermal fixed bed solid adsorbent reactor: a uniform pressure-non-uniform temperature case

A uniform pressure model is presented to describe the heat and mass transfer in a fixed bed of solid adsorbent in a finned reactor. This model neglects the resistance to mass diffusion but takes into account the resistances to heat diffusion through two coefficients: the heat conductivity of the adsorbent bed and the heat transfer coefficient between the adsorbent bed and the fins. An experiment has been conducted to validate this model and the two heat transfer coefficients are obtained by an identification technique. When the temperature of the closed reactor is modified on one side of the reactor, large temperature inhomogeneities inside the reactor are observed and mass transfer occurs through a heat pipe effect: the model explains that effect which is observed experimentally. That uniform pressure model is more adapted to describe the history of solid adsorbent reactors used in thermal processes than uniform temperature models proposed by other authors.     

Design and performance prediction of a novel double heat pipes type adsorption chiller for fishing boats

A novel double heat pipe type adsorber, which uses compound adsorbent of CaCl₂ and expanded graphite to improve the adsorption performance, is designed. The double heat pipes are integrated into the adsorbers in order to solve the problem of the corrosion between seawater and the steel adsorber in ammonia system and improve the heat transfer performance of the adsorber. There are two kinds of heat pipes integrated with the adsorber. One is the split type heat pipe for heating the adsorber in desorption phase, the other one is the two-phase closed thermosyphon heat pipe for cooling the adsorber in adsorption phase. The performance of two-adsorber adsorption chiller integrated with double heat pipes is predicted. The heat transfer performance of the heat pipes can meet the heat demands for adsorption/desorption of the adsorbent when the heating/cooling time is 720 s and mass recovery time is 60 s. When the exhaust gas temperature is 550 °C, the cooling water temperature is 25 °C, the inlet and outlet chilled water is −10 and −15.6 °C, respectively; the simulation results show that the cooling power and COP of this adsorption system are 5.1 kW and 0.38, respectively.     

Major applications of heat pipe and its advances coupled with sorption system: a review

Heat pipe utilizes continuous phase change process within a small temperature drop to achieve high thermal conductivity. For decades, heat pipes coupled with novel emerging technologies and methods (using nanofluids and self-rewetting fluids) have been highly appreciated, along with which a number of advances have taken place. In addition to some typical applications of thermal control and heat recovery, the heat pipe technology combined with the sorption technology could efficiently improve the heat and mass transfer performance of sorption systems for heating, cooling and cogeneration. However, almost all existing studies on this combination or integration have not concentrated on the principle of the sorption technology with acting as the heat pipe technology for continuous heat transfer. This paper presents an overview of the emerging working fluids, the major applications of heat pipe, and the advances in heat pipe type sorption system. Besides, the ongoing and perspectives of the solid sorption heat pipe are presented, expecting to serve as useful guides for further investigations and new research potentials.     

Thermofluidynamic modelling of hydrogen absorption in metal hydride beds by using multiphysics software

In this study, a performance analysis of metal hydride reactors (MHRs) based hydrogen storage during absorption process is presented. The study shows the effect of using heat pipe and fins for enhancing heat transfer inside MHRs at various hydrogen supply pressures. Three different cylindrical MHR configurations using LaNi₅ as a storage media were adopted including: i) reactor cooled by means of natural convection, ii) reactor equipped with a heat pipe along its central axis, iii) reactor equipped with finned heat pipe. A 3-D mathematical model is developed and utilized to simulate the thermofluidynamic behaviour of a metal hydride bed. The simulation study is conducted by solving simultaneously the energy, mass, momentum, and kinetic differential equations of conservation by using COMSOL multiphysics 5.2a software. Parameters such as hydrogen stored capacity, internal temperature distribution for the reactor, and their duration have been optimized. The model was validated against experimental result which have been previously published by the authors. The obtained results confirmed that the simulation and experimental results reasonably match where the maximum error vlaue was less than 8% at 10-bar hydrogen supply pressure, which proves that the model has efficiently captured the key experimental trends. On the other hand, the MHR design, which is equipped with a finned heat pipe is shown a superior performance as compared to all the other tested configurations in terms of charging time and storage capacity. Therefore, the model can be used as a helpful tool in the optimization of the MHR designs and performance.     

Solar test of an integrated sodium reflux heat pipe receiver/reactor for thermochemical energy transport

A chemical reactor for carbon dioxide reforming of methane was integrated into a sodium reflux heat pipe receiver and tested in the solar furnace of the Weizmann Institute of Science, Rehovot, Israel. The receiver/reactor was a heat pipe with seven tubes inside an evacuated metal box containing sodium. The catalyst, 0.5 wt% Rh on alumina, filled two of the tubes with the front surface of the box serving as the solar absorber. In operation, concentrated sunlight heated the front plate and vaporized sodium from a wire mesh wick attached to the other side. Sodium vapor condensed on the reactor tubes, releasing latent heat and returning to the wick by gravity. The receiver system performed satisfactorily in many tests under varying flow conditions. The maximum power absorbed was 7.5 kW at temperatures above 800°C. The feasibility of operating a heat pipe receiver/reactor under solar conditions was proven, and the advantages of reflux devices confirmed.     

Operation Characteristics of a High Temperature Special Shaped Heat Pipe Used in Solar Thermochemical Reactors

Solar energy can be transformed to storable chemical fuels through high temperature thermochemical processes driven by concentrated solar radiation. Solar thermochemical reactors (STRs) are the key equipment to these processes. An STR integrated with heat pipe technology is proposed. The core part of this reactor is a high temperature special shaped heat pipe (HTSSHP), which is composed of a flat disk-shape evaporator and multiple cylindrical condensers. A second generation HTSSHP with sodium as the working fluid was fabricated and tested in this work. The evaporating behaviors of sodium and isothermal characteristics were investigated at various operation conditions. A temperature pulsation phenomenon on heating surface was observed lasting for a short period during its startup when the heat flux beyond 47.2 kW/m², implying the generation of film boiling. Further, the operation characteristic of HTSSHP was experimentally proven steady under various heat fluxes and inclination angles, and it showed a good ability to spread temperature at the cooling side where the thermochemical processes take place. This work will facilitate the preliminary understanding of the operation characteristics in HTSSHP.     

Heat transfer design in adsorption refrigeration systems for efficient use of low-grade thermal energy

Adsorption refrigeration and heat pump systems have been considered as important means for the efficient use of low-grade thermal energy of 60–150 °C. Sorption systems are merely thermodynamic systems based on heat exchangers, and therefore a good design to optimize heat and mass transfer with reaction or sorption processes is very important, for which the notable technique is the use of expanded graphite to improve both heat and mass transfer in the chemisorption beds. Studies have also shown the need to enhance the heat transfer in adsorption bed by matching with the efficient heat transfer of thermal fluids. Heat pipes and good thermal loop design coupled with adsorption beds could yield higher thermal performance of a sorption system. A novel design with passive evaporation, known as rising film evaporation coupled with a gravity heat pipe was introduced for high cooling output. It has also been shown that the performance of traditional heat and mass recovery in the sorption systems is limited, and novel arrangement of thermal fluid and refrigerant may improve the performance of sorption systems. Based upon the above researches, various sorption systems have been developed, and high performances have been reached.     

A new type adsorber for adsorption ice maker on fishing boats

A new type of adsorber for an adsorption ice maker on fishing boats, which uses a compound adsorbent (activated carbon and CaCl₂) and ammonia working pair, is designed. This type of heat pipe adsorber solves the problem of incompatibility between ammonia, copper, seawater and steel. The heating/cooling power for the adsorption/desorption process of the adsorbent, which is required to be transferred by one heat pipe in the adsorber, is computed by the test results of the adsorbent, and the heat transfer performance of one heat pipe in the adsorber is simulated according to the theory of the two phase closed thermosyphon. The heat transfer performance of the heat pipe can meet the heat demands for adsorption/desorption of the adsorbent when the evaporating temperature is −15 °C and the cycle time is 10 min. A test unit is set up to test the heating/cooling performance of the heat pipe type adsorber, and the experimental results are coincident with the simulation. The performance of a two bed adsorption ice maker with heat pipe adsorbers is predicted, and the cooling power is about 17.1–17.8 kW at the evaporating temperature of −15 °C and cycle time of 10 min with mass recovery between two 29 kg compound adsorbent beds.     

Numerical simulation of heat-pipe and folded reformers for efficient hydrogen production through methane autothermal reforming

In order to intensify the methane autothermal reforming (ATR) process for efficient hydrogen production, novel heat-pipe and the folded reactors were designed and studied by numerical simulations. The reactor performance, such as methane conversion rate, hydrogen yield, and temperature difference between the reactor inlet and outlet, was computed and compared for both traditional tubular reactors and the novel reactors. Under the optimum operating parameters, compared with the tubular reactor, the heat pipe reactor and the folded reactor can help to increase methane conversion rate from 34% to 45% and 50%, while hydrogen productivity from 22.2% to 28.6% and 31.4%, respectively. The performance of the heat pipe reactor depends on the length and position of the heat pipe, while for the folded reactor, thicker intermediate plate with higher thermal conductivity material is more beneficial. Results were compared with previous experiments and could be used as a reference to guide new experimental and theoretical works toward the ultimate goal of designing and optimizing the ATR reactors for hydrogen production in the future.