Search

Transient response of the pinch point is crucial to the evaluations of the recuperator efficiency. A reheat and recompression cycle dynamic simulation system for supercritical CO2 is built, and a transient segmented model is developed to explore the dynamic characteristics of the recuperators. The operating conditions including inlet mass flowrate, temperature and pressure of the low temperature recuperator (LTR) change dramatically with the withdraw-mass variation, which causes different response characteristics of the pinch point. The pinch point gradually transfers from the cold end to the hot end of the LTR as the mass flowrate decreases. The higher pinch-point temperature difference leads to lower heat exchanger effectiveness, and the internal pinch point greatly weakens the recuperator performance. A new prediction model is proposed to quickly determine the pinch point location and analyze the operating conditions prone to cause internal pinch point. The range of split ratio in which the internal pinch point exists in the LTR is obtained. The lower-limit of split ratio variation is sensitive to the high- and low-pressures, the isentropic efficiency of the compressor, and the outlet temperature of the precooler, while the upper-limit of split ratio is only related to the pressures of the recuperator. [ABSTRACT FROM AUTHOR]

In this paper, the heat transfer characteristics of multiple pairs of vortex generators (VG) mounted on vertical plate fins of plate fin heat sink (PFHS) are evaluated using computational fluid dynamic simulations. The delta winglet pairs are used as longitudinal VGs, and they are mounted one after the other in the axial direction. Critical design parameters for the double pair of VGs (DPVGs) are identified, and a design of experiment-based simulations is carried out to develop a response surface model for Nusselt number and thermal hydraulic performance parameter. Results show winglet length is a critical design parameter compared with winglet height, and the axial inter VG distance between VG pairs plays a crucial role in improving the heat transfer characteristics. The optimum inter VG distance for a double pair of VG is evaluated, and it is 2.2 to 3 times the length of the delta-winglet. Thermal hydraulic performance of PFHS with double pair of VG is 1.49 times higher than that of plain PFHS. Finally, the variation of heat transfer characteristics against velocity is also evaluated for the PFHS with a DPVG. [ABSTRACT FROM AUTHOR]

The Earth-Air Heat Exchanger (EAHE) is a device where the external ambient air is cooled or heated when circulating inside ducts buried in the soil, which can bring benefits in terms of reducing energy consumption and providing thermal comfort conditions. This system has been employed in several countries in different climatic contexts, however, little explored in Brazil. The use of this technology in conjunction with bioclimatic strategies benefits buildings, increasing their efficiency. This study aims to evaluate the thermal performance of an EAHE alongside optimization parameters of the building envelope in two Brazilian bioclimatic zones and propose strategies that enhance energy efficiency in buildings. The study consists of the validation and verification of a computational model and the thermal evaluation of a Social Housing (SH), in which characteristics of solar orientation, thermal envelope, and the installation of an EAHE will be modified, combined, and investigated in the proposed models. These evaluations were conducted through dynamic simulation with the EnergyPlus™ software. The results demonstrate that the installation of an EAHE in SH brings improvements in the thermal performance of environments, thus contributing that future projects can rely on validated references and parameters. [ABSTRACT FROM AUTHOR]

• A dynamic model of civil aircraft air-cycle refrigeration system was developed • A dynamic solution method was introduced for dynamic mass transfer solution. • A near-logarithm heat transfer temperature difference function is presented. • A pressure-flow decoupling method is presented to obtain ACM pressure parameters. Dynamic simulation model of air-cycle refrigeration system s in civil aircrafts is essential for revealing the operation mechanism during flight process. In the present study, a dynamic model of three-wheel air-cycle refrigeration system was developed. A near-logarithm heat transfer temperature difference function is utilized to improve the stability of the heat transfer solution. A dynamic solution method based on continuity equation and theorem of momentum was presented as the framework of the dynamic mass transfer solution. A pressure-flow decoupling method is presented to solve the dynamic pressure in the high-pressure zone, resolving the absence of key pressure parameters. The model has been validated based on experimental data, with a pack discharge temperature deviation of 0.05K. Dynamic simulations have been conducted with actual flight data to obtain transient performance under airborne condition. In the takeoff and landing phase, the drastic change in the environment results in low discharge temperature, endangering the condenser with icing risk, which can be avoided by enabling the TCV. In the cruise phase, the discharge temperature slightly exceeds the upper limit for a couple of minutes. The optimization by slightly extending the length of heat exchanger channels succeeded in eliminating the excessive discharge temperature with minimal side effects. [ABSTRACT FROM AUTHOR]

The article studies the possibility of using the LabVIEW programming environment to develop real-time dynamic models of continuous processes. The principle of data flow implemented in the LabVIEW environment makes it possible to abandon the direct solution of differential equations when developing a dynamic simulation of a technological object. It is proposed to present the simulation object as a set of dynamic elements, the main of which is the integrating element. The block diagrams of the proportional, integrating, inertial, and delay elements are given. The applicability of the LabVIEW platform for modeling continuous processes is demonstrated on the example of a simple heat exchanger model, which hydrodynamic flows structure corresponds to the ideal mixing model. The schematic and block diagrams of the developed model are given. In order to test the heat exchanger model, a numerical experiment was carried out in static and dynamic modes. The results of testing of the model confirmed its consistency, sensitivity and validity, which are signs of adequacy. [ABSTRACT FROM AUTHOR]

In this paper, the heat pump distillation coupled heat exchange network system (MVRHP‐HEN) for acetic acid/acetic anhydride separation is optimized with the minimum total annual cost (TAC) and CO2 emissions as objective functions, meanwhile, the optimized process is simulated dynamically. Firstly, we proposed two optimization schemes: (a) the single objective and multi‐objective optimizations of the MVRHP system over the acetic acid separation column are firstly carried out and then constructed the heat exchange network, namely, MVRHP‐HEN‐1; (b) the heat exchange network is first constructed and then the single objective and multi‐objective optimization of MVRHP‐HEN is carried out, that is, MVRHP‐HEN‐2. The results show that MVRHP‐HEN‐2 obtains lower TAC and CO2 emissions compared with MVRHP‐HEN and MVRHP‐HEN‐1 and has better economic and environmental benefits. Then, the two‐point temperature control structure is built for the dynamic simulation of the optimized separation process. The results show that the proposed control structure has a strong anti‐interference ability. The research is expected to provide guidance for the development and design of advanced heat pump distillation technology. [ABSTRACT FROM AUTHOR]

The lithium-cooled reactor with Brayton conversion power system mainly consists of fast reactor, intermediate heat exchanger, closed Brayton cycle (CBC) loops filled with Helium–Xenon (He–Xe) binary gas mixtures and heat rejection loop which radiate residual heat through heat pipes to outer space. In this study, a dynamic model of 100-kWe lithium-cooled reactor with Brayton conversion power system was established based on RESYS code, which include reactor core thermal-hydraulic model, six-group neutron kinetics model and decay heat model, heat exchanger model, Closed Brayton cycle model including turbine, compressor and rotating shaft, heat pipe radiator model etc. The nominal steady-state simulation is carried out to verify the correctness and accuracy of individual component model the integrated system model. Four typical transient conditions including accidental reactivity insertion, loss of coolant flow of primary loop, partial and total external load loss and a step reduction in the mass flow rate of heat rejection loop are simulated to analysis the transient characteristics and prove the safety features for those simulated scenarios. The results are helpful for the design, evaluation and operation of space liquid-lithium-cooled reactor power system with He–Xe closed Brayton cycle. [ABSTRACT FROM AUTHOR]

Fixed‐tube‐sheet shell‐and‐tube heat exchangers (FTHEs) are widely used in process plants to enhance energy efficiency and to control temperature levels of unit operations. Even moderate temperature differences between shell and tube bundle of FTHEs can lead to failure mechanisms induced by thermal stress. To avoid equipment failure, design codes require an analysis of the transient response of an FTHE during special operating conditions. This work presents a modular‐hierarchical approach for the dynamic simulation of FTHEs, which is applied to special operating scenarios such as a loss of tube‐side flow, a shell‐side steaming‐out (cleaning procedure to remove hydrocarbon residues with steam) and a reactor trip (sudden stop of exothermic reaction). [ABSTRACT FROM AUTHOR]

Dynamic simulation of thermal systems is crucial for their optimal control but facing the challenge of balancing calculation accuracy and efficiency, where numerically high-efficient and accurate heat exchanger (HX) modeling is a key. Herein, we propose a frequency domain model for HXs' dynamic simulation in the to meet this challenge. The governing equations of HXs are first converted to frequency domain through Fourier transform, and the analytical solution of the converted equations yields the frequency domain HX model in the form of transfer matrix that connects the inlet temperature variations and outlet temperature responses. Furthermore, integrating transfer matrices of HXs into the system's transfer matrix enables a high-efficiency thermal system simulation. Numerical cases of a single HX, a HX network, a district heating network, and a thermodynamic cycle are used to validate the developed method and demonstrate its efficacy by comparing it against finite difference/volume method. Results show that the proposed method could reduce the calculation time by 2–3 orders of magnitude, and the maximum deviation of node temperature is around 1 K. The proposed method can be a powerful tool for the analysis of thermal systems and potentially integrated energy systems. • Propose a dynamic frequency domain model for heat exchangers with Fourier transform. • Analytical solution of the dynamic model gives the outlet temperatures directly. • The proposed HX model can be directly integrated in thermal system simulation. • Calculation time is reduced by 2–3 orders of magnitude with the same accuracy. [ABSTRACT FROM AUTHOR]

Extracting thermal energy from medium-deep ground layer for building heating is a promising means for pushing forward cleaner heating initiative. There is no available analytical model that can provide fast, accurate and effective simulation of both fluid and soil temperature field of deep borehole heat exchanger (DBHE) under complex geothermal gradient and subsurface stratifications. In this study, a stratified segmented finite line source method (SS-FLS) is proposed as an main contribution and novelty. The proposed model is validated by comparing with finite element method, model degradation example, and actual engineering project data of DBHE. The accuracy and robustness of the SS-FLS is fully checked. Then the effects of stratification on soil temperature field and fluid temperature field are investigated. It is found that the shape of temperature contour and temperature distribution have direct link with stratification as well as heat extraction rate on the heat pump side. Moreover, two critical model parameters are discussed in details which poses an important guide for both model application and system analysis. [Display omitted] • Stratified segmented finite line source (SS-FLS) method is proposed. • SS-FLS model is accurate, fast and easy-implemented for DBHE applications. • Impacts radius of DBHE is enlarging from shallow to deeper layers. • The fluid in the coaxial tube of DBHE is cooled down at the same pace. [ABSTRACT FROM AUTHOR]

The subject of the study is mainly based on thermal reinforcement by an oily fluid containing nanometer particles of carbon. The study is carried out by the presence of discontinuous bars in two different shapes, i.e. flat and V, inside a horizontal heat exchanger. The study relies on simulations in thermal and dynamic terms from the literature. The turbulence effect is diagnosed by applying the k-e model, while the flow hydrothermal transport relationships are modeled based on the finite volume technique. Both the flow and heat-transfer aspects of all channel regions are studied and analyzed. The new heat-exchanger structure has been enhanced in the presence of these discontinuous bars by reducing the friction coefficient and eliminating stations with poor transfer of heat behind these deflectors. [ABSTRACT FROM AUTHOR]

Fouling in pre-heat trains of atmospheric crude distillation units is one of the major issues hindering the efficiency of oil refineries. This paper illustrates the benefits of using dynamic simulation, combined with plant measurements, to evaluate heat exchanger retrofit opportunities that provide the largest savings to the refinery. Once the fouling behavior of the network is well characterized and the most critical heat exchanger is identified, the performance of a retrofit design for that exchanger is assessed by re-running history using plant measurements for an extended period of operation. The comparison of the original design versus alternative ones, based on the potential reduction of fouling, provides an overall assessment of the impact on energy/economic savings for the whole network given by the proposed retrofits options. This enables the design engineer to use available plant data to evaluate and select the most beneficial design option, not simply based on expected operating conditions and fixed fouling factors, but on time-varying conditions and dynamic fouling build-up as it happens in the plant. The method is demonstrated with an industrial case study. The estimated savings in furnace duty were of 1.5–2.7 MW, resulting in expected economic benefits of $270–700 k per year. [ABSTRACT FROM AUTHOR]

The temperature control loop of the actuating air following the heat exchanger is considered. The control loop consists of a heat exchanger, adjustable air intake, circulation fan, temperature sensor, and temperature controller. A dynamic model of the control loop that takes into account the automatic control system is created. Transient processes that arise in the control loop as a consequence of variations of the altitude of the aircraft are considered. Results of the variation in the actuating air temperature at a temperature control point are obtained. Partial optimization of the coefficients of the controller is performed. Optimal coefficients of the controller are selected. [ABSTRACT FROM AUTHOR]

This article describes the thermodynamical analysis of a hydrogen ( H 2 ) fueling station (HFS) with cryopump technology. A dynamic, object-oriented computer model of the HFS is developed in Matlab-Simulink. This model calculates, amongst other thermodynamic properties, the temporal and spatial variation of the H 2 temperature and the component temperatures within the HFS. The validation of the computer model with data from a series of measurements at a testing facility confirms a good accuracy of the model. The most important model object is the high pressure pipe through which the H 2 flows. The pipe is modeled and validated in different configurations. The thick-walled pipe material is discretized in radial and axial direction. In comparison to measurements the simulations show acceptable accuracy with an radial discretization length of s < 0.0026 m. In axial direction a discretization length of l < 1.18 m is found to deliver acceptable accuracy of the simulations compared to measurements. Based on the simulation results a new method of controlling the H 2 temperature by mixing two H 2 mass flows with different temperatures is assessed as practicable. The electrical power requirement of the electric heat exchanger in this HFS design is determined. Depending on the load cases it varies between 0.13 kWh el / (kg H 2 ) and 0.40 kWh el / (kg H 2 ). [ABSTRACT FROM AUTHOR]

• Flexible operation of air separation units causes wear on equipment. • Material fatigue can cause damage in plate-fin heat exchangers. • Large test rig is designed for proper validation of presented models. • Model results hint possible strategies for lifetime enhancement. Within the Kopernikus project "SynErgie", the sub-project "FlexASU" investigates an enhancement of the load flexibility of air separation units with the goal of synchronizing their power consumption with the availability of renewable energy sources. The main heat exchanger of these plants was identified as a key component for a dynamic plant operation, because startup, shutdown and load-change procedures can lead to thermal stress and an increased lifetime consumption of the apparatus. Aluminum-brazed plate-fin heat exchangers (PFHE) are used in these plants because of their high process integration, low production cost and compact design. The main focus of this work, which is an extended version of Haider et al. (2018a) , is the transient behavior of PFHE. The temperature profiles within a PFHE can be calculated, and a FEM-based model is available for investigation of the thermal stress induced by dynamic temperature changes in the apparatus. Hence, the reduction in lifetime due to certain dynamic plant operating scenarios can be estimated, and critical operating modes can be identified. For the further development of the model, a PFHE test rig is designed which is representative of extreme operating conditions in air separation units. A test scenario matching the conditions of plant startup after extended downtime in a cold state has been designed. The scenario leads to thermal stress that is expected to damage the PFHE within a few weeks of testing. The location of maximum stress in the FEM-model matches typical cracks observed in PFHE which have been exposed to severe thermal stress. [ABSTRACT FROM AUTHOR]

Based on a quasi 3-D steady model inside and a 2-D unsteady model outside of the borehole of ground heat exchanger(GHE), the relationship between the heat exchange rate per depth of borehole and main parameters (center-to-center distance of two branch pipes, radius and depth of the borehole) has been analyzed. The rational values of those design parameters with considering the heat exchange rate per depth of borehole are obtained. The dynamic simulations of underground temperature field (UTF) of single-pipe and pipe-group of U-tube GHE have been conducted by ANSYS. The change rules of UTF are obtained when ground source heat pump(GSHP) has experienced a complete cooling and heating cycle during one year. The simulation results of a single-pipe show that the action radius of borehole is 1.9 m, i.e., it is reasonable that the borehole center-to-center distance is 3.8 m. The simulation results of pipe-group indicate that temperature of the soil center encircled by four boreholes at the beginning of heating in winter is higher than the initial temperature of soil, but it is in opposite for cooling in summer, which is beneficial to the efficient operation of GSHP. This work can provide guidance for the rational design and operation of GSHP. • Relation of heat exchange rate per depth of borehole(HERPDB) with design parameters. • Rational values of design parameters with considering HERPDB. • Dynamic simulation of underground temperature field (UTF) of a single U-tube GHE. • Dynamic simulation of UTF of pipe-group of U-tube GHE round one year. • Efficient operation of GSHP with borehole center-to-center distance of 3.8 m [ABSTRACT FROM AUTHOR]

Highlights • A CCHP system primed by Maisotsenko combustion turbine cycle was proposed. • Performance comparison with that based on conventional gas turbine cycle was made. • Dynamic system simulations were conducted using the TRNSYS platform. • Different climate regions were considered with the results compared. • The loading profiles affected the relative performances of the two CCHP systems. Abstract Buildings are the significant primary energy consumption and pollutant emission contributors in a modern city. Distributed multi-generation systems such as combined cooling, heating and power (CCHP) systems are considered an alternative to fulfil the energy saving requirements in the building sectors. In this study, a CCHP system primed with Maisotsenko combustion turbine cycle (MCTC) was considered with its performance compared with that primed with a simple gas turbine with recuperator (SGTR). With dynamic models of the CCHP systems constructed using TRNSYS, a design optimisation was conducted using genetic algorithm to determine the system component size and number based on the best year-round cost saving. A study case which consisted of building complexes in a district scale was used for investigation under two climatic conditions. The simulation results showed that the relative performances of both CCHP systems depended on the load profiles which differed from the situations when both prime movers were used solely for power generation. In a heating-dominated application, the performance of the MCTC-primed CCHP system was better than the SGTR-primed one only during the summer period and that the year-round performance was inferior to the latter one. Last but not the least, a parameter study was conducted which showed the benefit reduction of CCHP system with the decreasing of public grid price and increasing fuel gas price. [ABSTRACT FROM AUTHOR]

Highlights • Double U-tube borehole heat exchangers are studied by 3D finite element simulations. • Simulation results are validated by comparing 3D and 2D borehole thermal resistance. • Equations to evaluate the mean fluid temperature of a double U-tube borehole are given. • Results hold for any borehole length, shank spacing, conductivity and working condition. • Results improve thermal-response-tests evaluation and borehole-fields hourly simulation. Abstract The design of ground-coupled heat pump systems requires the knowledge of the mean temperature T fm of the working fluid in borehole heat exchangers. This quantity is usually approximated by the arithmetic mean of inlet and outlet temperature. The approximation yields an overestimation of the thermal resistance of the heat exchanger in thermal response tests, as well as errors in the estimation of the outlet fluid temperature in the dynamic simulation of ground-coupled heat pumps. Recently (Applied Energy 206, 2017, 1406–1415), Zanchini and Jahanbin determined, by finite element simulations, correlations that allow an immediate evaluation of T fm in any working condition, with reference to double U-tube boreholes with length 100 m and shank spacing 85 mm. In this paper, the analysis presented there is extended to double U-tube boreholes with any length, between 50 and 200 m, and any shank spacing, between 65 and 105 mm. The results hold for every thermal conductivity of the sealing grout between 0.9 and 1.6 W/(m K), every volume flow rate between 12 and 24 L per minute, every diameter of the BHE and every working condition (heating, cooling, thermal response test), both in quasi-stationary and in transient regime. [ABSTRACT FROM AUTHOR]

Abstract The present investigation is a continuation of a previous study conducted by the authors that presented a dynamic-simulation model of an Ericsson engine equipped with a gas-to-gas heat exchanger. This model is here used to investigate the effect of compressor and expander valve timing variation. The investigation begins with an un-optimized case and examines various improvements in terms of valve timings. This results to an optimized case that improves power and thermal efficiency. The optimized case serves as reference to study the effects of variations in the valve timings of the compressor and expander. The inlet valve closing timing of the expander (IVC e), which determines the intake duration of the expander, necessitates the readjustment of some of the other valve timings, since it significantly affects the pressure levels in all devices. The IVC e variation led to the potential of using a single heater with multiple compressor-expander pairs that are sequentially and periodically coupled to the heater. This increases the specific power output compared to using several compressor-heater-expander triplets, since it reduces the total volume of the engine. The number of compressor-expander pairs that can be coupled to a single heater is largely determined by the IVC e timing. Highlights • Valve timing effects are analyzed in terms of flow conditions and performance. • Expander inlet valve closing greatly affects pressure levels and performance. • Multiple compressor-expander pairs coupled to a single heater increase specific power. • The number of compressor-expander pairs depends on expander inlet valve closing. [ABSTRACT FROM AUTHOR]

A novel method combined signal flow graph of a single heat exchanger with the transfer function of streams is developed for the dynamic behaviors of heat exchanger networks problems, which are determinate factors of the process control and operation optimization in the processing industries. The transfer functions between any two nodes of heat exchanger networks including the inlet and the outlet are obtained based on the signal flow graph of the networks by block-diagram reduction, Mason's rule and the seeking-up method. The developed method is solved by a numerical inverse Laplace transform and the analytical solution to the dynamic behavior of heat exchanger networks is presented in the time domain. The numerical results demonstrate that the presented method is more efficient and more accurate for the dynamic behaviors of heat exchanger networks problems. [ABSTRACT FROM AUTHOR]

The transition in the energy market and the growing share of renewable energy sources have been boosting the research in new power cycles. For example, the concept of batch evaporation in the Misselhorn Cycle promises to increase the overall efficiency in low-temperature applications and therefore saves resources. In this paper, a dynamic evaporator model was extended in order to prove the feasibility of the Misselhorn Cycle despite its transient character. In this context, the thermal capacity of the wall material as well as the residence time of the heat source medium were added. The previous, underlying model predicted an improved system efficiency for the Misselhorn Cycle of about 50% compared to an Organic Rankine Cycle (ORC) at 100 C ∘ . Initially, the results of the extended model showed a negative influence of the inertial effects on the possible net power output (advantage over ORC only 10%). However, an unheated discharge phase and reduced dimensions of the heat exchanger could compensate these drawbacks and achieved results (about 40% better than ORC) in the same range as the previous, simple model predicted. These findings prove the general practical feasibility of the Misselhorn Cycle. [ABSTRACT FROM AUTHOR]

This paper presents the design of a novel high-temperature solar assisted triple-pressure level combined cycle power plant. The system includes innovative high-temperature flat plate evacuated solar thermal collectors, a double stage lithium bromide/water absorption chiller, pumps, heat exchangers, storage tanks, mixers, diverters, controllers and a triple-pressure combined cycle power plant. This novel layout uses high-vacuum non-concentrating flat plate solar thermal collectors driving a double effect absorption chiller, leading to an overall high coefficient of performance for the solar cooling section. The provided cooling energy is used to cool gas turbine inlet air to enhance system efficiency and electrical capacity. This effect is mainly performed during central hours of the day when the conventional gas-fired combined cycles dramatically suffer for efficiency and capacity reduction, as a consequence of the corresponding increase of external air temperature. Such increase is related to the higher availability of solar radiation. This technology may be a viable solution, especially for hot and dry areas, in terms of primary energy savings and increased revenues. This prototypal system was numerically analysed for a combined cycle with a rated electrical power of 99 MWe and an electrical efficiency of 56%, developing a dynamic simulation model and detailed thermo-economic optimizations. Special attention is also paid to the design of novel control strategies aiming at maximizing the solar cooling effect. The results of the dynamic simulations show a very high average thermal efficiency of the solar collectors, equal to 34%. Implications of the performed work allowed an increase of the power output up to 5.5% and a satisfactory payback period equal to 10.7 years. [ABSTRACT FROM AUTHOR]

This paper outlines the development of models of a solar field designed to provide thermal energy to a Multi-Effect Desalination (MED) plant. The dynamic model can be used both for simulation and control purposes. Some of these models have been developed based on static and dynamic energy and mass balances, and some others are based on step response methods (experimental tests). The solar field comprises a flat-plate collector field, an air cooler, a heat exchanger and the corresponding pipelines and interconnections. The main purpose of the solar field is to feed the MED unit with hot water within a specific temperature range using two thermal storage tanks as input buffers to the MED system. The main achievement of this paper is that the developed model provides an adequate tradeoff between complexity and performance. [ABSTRACT FROM AUTHOR]

A multi-variable extremum seeking control is proposed as a model-free strategy to maximize the energy efficiency in real time for a multi-functional variable refrigerant flow system. For the cooling-only, heating-only, cooling-dominated, heating-dominated, and heat-recovery modes, the manipulated inputs of extremum seeking control include compressor suction/discharge pressure set-point, outdoor unit fan speed, bypass flow valve opening, and superheat set-points for evaporating heat exchangers. The feedback of extremum seeking control is the total power of the compressor motor, outdoor, and indoor unit fans. The proposed control strategy is validated with a Modelica-based dynamic simulation model, in terms of both steady-state and transient performance. [ABSTRACT FROM AUTHOR]

The multi-functional variable refrigerant flow system enables simultaneous heating and cooling of the zones it serves by allowing the heat exchangers of both indoor units and outdoor units to operate as evaporators or condensers. To realize automatic and smooth switching between different modes, a mode switching strategy is proposed for a multi-functional variable refrigerant flow system. Whether to turn on or off an indoor unit is determined by the zone temperature and a preset hysteresis band about the temperature set-point. Based on a thermodynamic analysis, a decision variable for determining outdoor unit-heat exchangers mode switching is proposed as the outdoor unit-heat exchangers airside temperature differential normalized by the dimensionless outdoor unit fan speed. Two bumpless transfer strategies are applied to produce smooth mode switching. To evaluate the proposed strategy, a simulation study is performed with a Modelica-based dynamic simulation model of a four-zone multi-functional variable refrigerant flow system. Simulation results validate the effectiveness of the proposed strategy and the performance of the bumpless transfer strategies. [ABSTRACT FROM AUTHOR]

The performance of a vertical ground source heat pump system (GSHPS) largely depends on the fluid temperature leaving the borehole heat exchanger (BHE) that may be affected by the short-term behavior of the BHE. Although considerable research has been carried out to analyze the short-term transient response of the BHEs, few studies have investigated its impact on dynamic simulation of GSHPS. Therefore, this paper presents a numerical approach based on a transient BHE model to evaluate the performance of a residential GSHPS over short and long timescales. The numerical results are compared with the results of EnergyPlus software. It is shown that the proposed model can appropriately predict the dynamic behavior of the system. Moreover, effect of borehole thermal capacity on the performance of the GSHPS is investigated in comparison with a quasi-steady state model. It is found that including the borehole thermal capacity substantially affects the design borehole length. Using the transient model instead of the quasi-steady state model leads to a 16% reduction in the required borehole length. [ABSTRACT FROM AUTHOR]

In this article, the dynamic responses of heat exchanger networks to disturbance and setpoint change were studied. Various control strategies, including: proportional integral, model predictive control, passivity approach, and passivity-based model predictive control were used to monitor all outlet temperatures. The performance of controllers was analyzed through two procedures: 1) inducing a ±5% step disturbance in the supply temperature, or 2) tracking a ±5°C target temperature. The performance criteria used to evaluate these various control modes was settling time and percentage overshoot. According to the results, the passivity-based model predictive controllers produced the best performance to reject the disturbance and the model predictive control proved to be the best controller to track the setpoint. Whereas, the ensuing performance results of both the PI and passivity controllers were discovered to be only acceptable. [ABSTRACT FROM AUTHOR]

A physical model and dynamic simulation models of a solar phase-change heat storage heating system with a plate solar collector, phase-change material (PCM) storage tank, plate heat exchanger, and auxiliary heat sources were established. A control strategy and numerical models for each of seven different operation modes that cover the entire heating season of the system were developed for the first time. The seven proposed operation modes are Mode 1: free cooling; Mode 2: reservation of heat absorbed by the solar collector in the PCM storage tank when there is no heating demand; Mode 3: direct supply of the heating demand by the solar collector; Mode 4: use of the heat absorbed by the solar collector to meet the heating demands, with the excess heat stored in the PCM storage tank; Mode 5: use of heat stored in the PCM storage tank to meet the heating demands, Mode 6: combined use of heat stored in the PCM storage tank and the auxiliary heating sources to meet the heating demands; and Mode 7: exclusive use of the auxiliary heat sources in order to meet the heating demands. Mathematical models were established for each of the above seven operation modes, taking into consideration the effects of the outdoor meteorological parameters and terminal load on the heating system. The real-time parameters for the entire heating season of the system with respect to the different operation modes can be obtained by solving the simulation models, and used as reference for the optimal design and operation of the actual system. [ABSTRACT FROM AUTHOR]

This paper implements a sensitivity analysis of a vertical Geothermal Heat Exchanger model from a Ground Coupled Heat Pump system, simulated with the software TRNSYS. Afterward, the optimization software GenOpt is applied in an iterative calibration process, calibrating those factors that were proven to be the most important in the sensitivity analyses. A comparison is made between the calibrated simulation and the experimental data collected from the system, obtaining a significant reduction in the error. The results demonstrate that the methodology used is effective, which indicates that a sensitivity analysis is an appropriate method to determine which parameters should be calibrated, while avoiding biases of the modeler and decreasing the number of iterations required in the calibration by approximately 89%. [ABSTRACT FROM AUTHOR]

In this paper, a detailed numerical analysis of a Combined Cooling, Heating and Power system is presented, aiming at determining its optimal operating strategy in a real industrial application. The system layout includes a reciprocating engine, fuelled by natural gas, heat exchangers for waste-heat recovery, pumps, storage tanks, a single-effect water lithium bromide absorption chiller, a cooling tower, a back-up vapour-compression electric chiller, mixers and valves. A dynamic simulation model of the whole system was developed in TRNSYS. A case study was analysed, referred to a real industrial application, where the system under evaluation should be installed in the near future, providing electricity, mainly used for the production process, and space cooling. Real measured data were used to estimate the electric energy demand of the factory. A detailed building simulation model was used to calculate heating and cooling demands. A detailed economic analysis was carried out, aiming at evaluating: (i) the optimal size of the Combined Cooling, Heating and Power system; (ii) the optimum control strategy, from a thermo-economic point of view, comparing three different cases: Base-Load operation, electric load tracking and a new hybrid strategy based on the simultaneous tracking of electric and thermal-loads. The results showed that the optimal capacity of the system was lower than that selected by the designers of the real unit to be installed. The hybrid control strategy obtained the best profitability, achieving a simple pay-back period equal to 3.8 years, compared to 4.1 years achieved in case of electric-load tracking. [ABSTRACT FROM AUTHOR]

The aircraft air-conditioning system, which consumes the engine bleed air to provide stable and comfort environment for passengers, has a high demand of system reliability and energy efficiency. For analyzing system performance and control effect under a wide range of operating scenarios, this paper presents a dynamic model of the aircraft air-conditioning system. A pressure-node based method is proposed to decouple the system architecture, and a modelling framework is established with reflecting the interdependencies between component-level modules. Dynamic sub-models including pressure nodes, heat exchangers and the air cycle machine are built to predict all major dynamics in the system. The model is verified by the measured data of an airborne testing. The predicted temperatures show good agreement with the measured data, of which the average deviations at the compressor outlet and system outlet are 4.60 °C and 3.49 °C, respectively. The effectivity of the proposed model is investigated under various conditions, including different control signal inputs, standard pull up/pull down conditions and an entire flight case. The simulation results indicate that the model can be successfully applied for a wide range of aircraft operating scenarios, and provide insight towards control design and fault detection of the aircraft air-conditioning system. • A pressure-node based modelling framework for aircraft ACS is proposed. • Fully dynamic component-level models are built to predict all transient features. • Model predictions show good agreements with the airborne testing data. • System response characteristics are analyzed under various operation conditions. • Cabin temperature control effect is examined by simulating an entire flight case. [ABSTRACT FROM AUTHOR]

Fouling is a major problem in the operation of heat exchangers, resulting in increased capital, operational, and maintenance costs. Shell-and-tube heat exchangers are traditionally designed using fixed values of fouling resistances, ignoring that fouling rates depend on the exchanger geometry, rendering different fouling resistances for the same thermal service. This article discusses the use of fouling rate models in the design of shell-and-tube heat exchangers. We link a heat exchanger design algorithm to a dynamic simulation of the fouling rate. The proposed procedure is explored for the design of heat exchangers where fouling occurs in the tube side due to crude oil flow. Four examples illustrate how the utilization of the fouling rate model alters the solution of the design problem, including aspects related to a "no fouling" condition in the design, the impact of the duration of the operational campaign in the results, and how the uncertainty in the fouling prediction can be handled. [ABSTRACT FROM AUTHOR]

Dynamic performance simulation of a CFB boiler in a commercial-scale power plant is reported. The boiler system was modeled by a finite number of heat exchanger units, which are sub-grouped into the gas-solid circulation loop, the water-steam circulation loop, and the inter-connected heat exchangers blocks of the boiler. This dynamic model is an extension from the previously reported performance simulation model, which was designed to simulate static performance of the same power plant, where heat and mass for each of the heat exchanger units were balanced for the inter-connected heat exchanger network among the fuel combustion system and the water-steam system. Dynamic performance simulation was achieved by calculating the incremental difference from the previous time step, and progressing for the next time step. Additional discretization of the heat exchanger blocks was necessary to accommodate the dynamic response of the water evaporation and natural circulation as well as the transient response of the metal temperature of the heat exchanger elements. Presentation of the simulation modeling is organized into two parts; system configuration of the model plant and the general approach of the simulation are presented along with the transient behavior of the sub-models in Part I. Dynamic sub-models were integrated in terms of the mass flow and the heat transfer for simulating the CFB boiler system. Dynamic simulation for the open loop response was performed to check the integrated system of the water-steam loop and the solid-gas loop of the total boiler system. Simulation of the total boiler system which includes the closed-loop control system blocks is presented in the following Part II. [ABSTRACT FROM AUTHOR]

Despite many studies that were found in the literature regarding the modelling of the short-time-step (STS) behaviour of the borehole ground heat exchangers, reported investigations of its effect on the long-term dynamic simulation of a ground-source heat pump (GSHP) system were sparse. Hence, in this study, the dynamic performances of a GSHP system applied to an office zone in Hong Kong over a period of 10 years were compared with both the long-time-step (LTS) and STS approaches for the boreholes. It was found that with the adoption of the more appropriate STS approach, the simulated fluid temperature leaving the ground heat exchanger borefield was lower by at most 1.62 °C over a range of ground and grout thermal conductivities investigated. This also allowed the design borehole length to be reduced by at most 14.4% while the resulting energy consumption was still better than those based on the LTS approach. In this regard, the STS approach should be employed in the long-term dynamic simulation of GSHP systems. [ABSTRACT FROM AUTHOR]

In this work, a novel re-compressed adiabatic compressed air energy storage (RA-CAES) system is proposed to raise the operating pressure of the expansion train. In the discharging process, a compressor is employed to enhance the low-pressure air from storage vessels to the operating pressure. Off-design models for components have been established and experimental data available in the open literature are adopted for validation. Potential induced degradation (PID) control technology is employed to control the outlet temperature of the air in inter-cooling and after-cooling heat exchangers. The calculation results for the components have been presented and discussed. Meanwhile, the system performances of the proposed RA-CAES are compared with that of conventional A-CAES. The comparison results show the round trip efficiency (RTE) of RA-CAES is 69.89%, while it is only 65.23% for A-CAES, and the increment of RTE between A-CAES and RA-CAES is 4.66%. The influence of isentropic efficiencies of turbine and compressor is also investigated, the RTE of RA-CAES is always higher than that of A-CAES, and the minimum increment of RTE is near 4%. Overall, this study presents valuable insights into the design of A-CAES, the proposed RA-CAES is more efficient than the conventional A-CAES. • A novel re-compressed adiabatic compressed air energy storage system is proposed. • The discharge pressure can be raised up by an additional compressor. • Off-design models have been established and validated for the components. • The round trip efficiency of the proposed system is 4.66% higher than that of the conventional system. [ABSTRACT FROM AUTHOR]

• Finnish building energy system with deep coaxial boreholes studied. • Four-step modelling approach combining simulation and hourly optimization applied. • Improved economic feasibility compared to reference district heating configuration. • Economic benefits from increasing the borehole depth decreased nonlinearly. • Deeper boreholes enable increased operating flexibility. Deep coaxial borehole heat exchangers (CBHEs) have been considered to enable cost and energy savings in building energy systems. However, achieving improved performance requires optimal design and operation of the CBHE. This work presents a case study for a building energy system in Finland, for which a hydraulic diagram was developed to incorporate a number of deep CBHEs (800 m to 2400 m) with multiple operating modes. To evaluate the techno-economic feasibility with optimized operating strategies, a four-step methodology is introduced, combining steady-state modelling, dynamic simulation and system optimization. The systems with a CBHE were found economically feasible compared to a district heating connection, enabling lower annualized total system cost (18.3–26.3%) and operating costs (49.1–53.5%). With deeper CBHEs, the main cost reductions originated from improved heat pump performance, increased operating flexibility and decreased capacity requirements of process components. However, the benefits partly decreased non-linearly with increased CBHE depth. Economic feasibility displayed the highest sensitivity to the price of electricity, with a relatively smaller impact for deeper CBHEs. Furthermore, maximizing the heat pump performance was found economically more significant than achieving a high borehole heat extraction rate. [ABSTRACT FROM AUTHOR]

The supercritical cycle has a better variable-temperature match between the working fluid and heat source, potentially providing higher efficiency. Accurate simulation of the transient performance of supercritical heat exchangers is significant in advanced thermal system control. This paper proposes a dynamic model of supercritical heat exchangers based on the moving boundary method. The model redefines the concept of "interface" in the traditional moving boundary model. The new interfaces are determined by the thermophysical properties of the working fluid, called "virtual interfaces". A control-volume separation method is proposed based on the virtual interface. Additionally, the specific enthalpy is selected as the switching criterion for the configuration of the control volumes, so the model can dynamically adjust the number of control volumes. The integrity check results show that the simulation accuracy is at least equivalent to the finite volume model with 20 control volumes; however, the time cost is approximately 1/5 of that of the finite volume model with the same precision. An experimental study on the convective heat transfer of supercritical n -decane in an electrically heated vertical tube was conducted. The maximum absolute error of the steady-state outlet temperature is ±1.5 K, and the relative error is ±3% under different pressures. • Mass and energy exchange occur in heat exchangers in the thermal system. • Heat transfer has crucial influence on the dynamic performance of thermal systems. • Under supercritical pressures, the working fluid does not have a phase interface. • The proposed model shows a significant advantage in simulation efficiency. • The time cost of the model is approximately 1/5 of that of the FV model. [ABSTRACT FROM AUTHOR]

This paper sets out to make a dynamic analysis of a single-effect, lithium bromide–water (LiBr/H 2 O) absorption chiller. A mathematical model has been developed based on conserving mass, energy, and species, which considers the correlations of the convective coefficients of absorption refrigeration processes so as to determine the overall heat transfer coefficients of the heat exchangers of chillers. The implementation of this mathematical model was built on the Matlab platform, and resolved the system of non-linear equations by making explicit use of the finite difference method. The model has the ability to simulate and predict the behavior of internal and external parameters such as temperature, concentrations and pressures when these are subjected to disruptions in the power supply and thermal load. The largest absolute differences found when validating the model by means of the experimental numerical values in the water circuits were approximately 1.0 °C, 0.7 °C and 0.2 °C for hot, chilled and cold water, respectively. In terms of relative error, these divergences represent a maximum of 5% and a minimum of 0.3%. The model showed good results when subjected to disturbances in the load and power source, as it reproduced the behavior of the temperature parameters and the Coefficient of Performance (COP), in line with the comparisons made with the data provided by the manufacturer, the COP being approximately 0.7 for operating conditions. [ABSTRACT FROM AUTHOR]

Process studies for Toroidal Field Structure (TF ST) system with a dedicated Auxiliary Cold Box (ACB-ST) have been conducted under 15 MA baseline, including plasma disruptions. ACB-ST consists of two heat exchangers immersed in the Liquid Helium (LHe) subcooler, which are placed at the inlet/outlet of a Supercritical Helium (SHe) cold circulator (centrifugal pump). Robustness of ACB-ST is a key to achieve the stability of TF coil operation since it provides the thermal barrier at the interface of the TF Winding Pack (WP) with ST. The paper discusses the control logic for the nominal plasma operating scenario and for Mitigation to regulate the dynamic heat loads on ST. In addition, the operation field of a cold circulator is described in the case of plasma disruptions. The required performance of heat exchangers in the ACB-ST is assessed based on the expected operating conditions. [ABSTRACT FROM AUTHOR]

Modelling and simulation is presented for a finned cross-flow heat exchanger with the aim to heat cold air to be fed to air conditioning batteries for marine purposes. The model employed in this paper is finalized to simulate the dynamic behavior of air and water temperatures fed to the air conditioning batteries operating in cold environments, in order to predict possible troubles owing to the change in input parameters, such as unwanted flow rate variations due to system malfunctions. In the investigated model, heat balance equations are presented and discretized by Laplace transform, which has the advantage to easily account for the different structures of heaters used for the purpose of validation. The model was implemented in the Matlab-Simulink environment for its high capacity of dealing with dynamic systems. The results of the model are satisfactory, as the dynamic behavior of the air stream temperature is correctly reproduced, as compared to experimental data, providing a suitable parameter for malfunctions prediction. [ABSTRACT FROM AUTHOR]

The European Union introduced the concept of "Net Zero Energy Building" (NZEB) to promote competitive, sustainable, eco-friendly, and "decarbonised" cities. To these aims, mainly with reference to highly urbanized places, it is necessary to renovate existing buildings to achieve the NZEB target. Thus, this paper analyses the energy retrofit of an existing building considering two low enthalpy geothermal systems in the HVAC plants: the Ground-to-Air Heat eXchanger (GAHX) and the Ground Source Heat Pump (GSHP). These two plants are used in different retrofit strategies and compared from an energy, environmental and economic point of view. A dynamic simulation software based on EnergyPlus calculation engine is used. The results are very interesting and demonstrate how by using a low enthalpy geothermal plant, a very low value of primary energy requirement can be obtained (about 60 kWh/m2, i.e., −71% compared to the existing building, characterized by a gas boiler). Moreover, substantial savings on annual energy bills and reduction of CO 2 emissions are obtained. • Retrofit strategies of an existing building to obtain a NZEB are proposed. • Two low enthalpy geothermal energy systems are analysed (GAHX, GSHP). • A technical-economic evaluation is made by using dynamic energy simulation. • The performances of the GAHX and the GSHP are evaluated in order to obtain a NZEB. [ABSTRACT FROM AUTHOR]

• A microchannel heat exchanger made by the fluidic oscillators array was designed. • The oscillation modes of two parallel fluidic oscillators were studied. • The time-resolved pressure field and velocity field were analyzed. • The heat transfer performance of the fluidic oscillators array was evaluated. Impinging jet played an important role in cooling technology. The steady-state jet could be transformed into an oscillatory one based on the intrinsic flow instability mechanisms by using self-oscillating fluidic oscillators. In this paper, a high-performance microchannel heat exchanger based on a fluidic oscillator array was presented. The fluidic oscillators were tilted at an angle of 30° above the impinging surface to extend the influence area of the jets. The unsteady RANS turbulence models were adopted to study the fluid dynamic and heat transfer performance of the jet array. The time-resolved pressure field and velocity field were calculated to show that the parallel oscillators would produce three modes, including co-directional oscillation, reverse oscillation, and irregular oscillation, under synchronous and asynchronous output phases. The sweeping jets improved the heat removal performance by increasing the average Nusselt number and enlarging the influence range on the external flow field. The heat removal performance of the sweeping jets was almost equivalent under different oscillation modes. [ABSTRACT FROM AUTHOR]

Abstract: Waste heat recovery by means of a Rankine Cycle is a promising approach for achieving significant reductions in fuel consumption and, as a result, exhaust emissions of passenger car engines. This approach is already well established in industrial applications such as gas and steam power plants or ship propulsion systems. While these systems are mainly designed for stationary operation, the behaviour in highly dynamic operating conditions becomes more important when the principle is transferred to a passenger car engine. Knowledge of the dynamic response of the employed heat exchangers plays an important role in performance prediction and control system design of the steam cycle. Hence, a dynamic model of the exhaust gas heat exchanger employing the moving-boundary principle was developed and is presented in this paper. The model describes both design operation and the heat-up procedure of the component. For achieving high model accuracy in the resulting broad range of operating conditions, new approaches for modelling wall temperature distribution and zone switching were developed. Simulations of stationary operating points as well as the response to typical disturbances of the system’s input variables are in good agreement with test bench measurements. The model is used to develop a control system for dynamic operation on the test bench. Further studies of the operating characteristics reveal varying dynamic behaviour depending on the heat flow rate from exhaust gas to working fluid as well as coupling of evaporation pressure and outlet steam temperature. [Copyright &y& Elsevier]

Abstract: A 1 kW solid oxide fuel cell (SOFC) balance of power (BOP) system is analyzed using experiments and simulations to optimize the design and operating conditions. The system consists of an autothermal reformer, combined heat exchanger and afterburner, and dummy stack. Thermolib program is used for the simulations. The system exhibits stable operation during a two cycle on/off experiment. The simulated results obtained by zero-dimensional modeling are similar to the experimental results although a discrepancy appears early in operation owing to the simplified modeling. The system''s characteristics are also investigated in terms of the fuel utilization, presence or absence of internal reforming and external heat transfer coefficient. [Copyright &y& Elsevier]

The dynamic simulation of a sub-scale ITER Central Solenoid (CS)/STRucture (STR) cooling loop has been performed to investigate the cryogenic control strategies against pulsating heat loads. HELIOS facility has been modified for this particular study, which consists of a Liquid Helium (LHe) reservoir with two immersed heat exchangers, a circulation pump and an approximately 130 m long piping with three evenly distributed heated sections. The setup provides the equivalent thermo-hydraulic configuration of the ITER CS/STR, cooled by the forced-flow Supercritical Helium (SHe). The dynamic simulator, C-PREST, has been utilized to model the HELIOS test loop and to compare the results with the experimental data sets. This paper presents the dynamic simulation results and discusses the control strategy to minimize the cooling power requirements, to have high reliability. [ABSTRACT FROM PUBLISHER]

Abstract: With a view towards better efficiency in renewable energy utilisation, particularly solar energy, the authors study a long-term solar thermal energy storage based on water absorption by a lithium bromide aqueous solution. After a description of the process, the system dynamic simulation model is detailed and used to investigate the influence of certain parameters (heat exchanger size, solution flow rate, absorption percentage) and operating conditions (heat supply temperature to the building, crystallisation ratio, heat need) on the system performance (storage density, thermal efficiency, etc.). The analysis of simulations made for a low-consumption building in Chambéry shows that the solution flow rate is a critical parameter in the process performance. It also appears that crystallisation in the solution storage tank increases the storage density more than three times. The simulation results are used in the design of a prototype that is under experimentation for validation of the model. [Copyright &y& Elsevier]