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[en] Highlights: • Subcooled boiling first occurs in the inner side of helical coils under the studied test conditions. • ONB can appear in helical coils when mean wall temperature is lower than saturation temperature. • Effects of various parameters on wall temperature and HTC are studied in helical coils. • New correlations have been proposed for ONB and HTC of subcooled boiling in helical coils. - Abstract: Helical coils have been widely used in a variety of applications, such as heat recovery processes, power plants, cryogenic systems, etc, due to the practical importance of high efficiency heat transfer, compactness in structure, ease of manufacture and arrangement. Experiment investigation of heat transfer characteristics of subcooled flow boiling in helical coils with different inner diameters and coil diameters was performed in the present paper. The rise angles of these helical coils were all 6 degrees. The system pressure was in the range of 1.8 MPa and 7.8 MPa, mass flux ranged between 300 kg/(m2·s) and 1100 kg/(m2·s), and heat flux varied from 100 kW/m2 to 450 kW/m2. The experimental results showed that the onset of subcooled boiling was significantly influenced by heat flux and system pressure. A new correlation to predict the onset of subcooled boiling was proposed, correlating experimental results within ±20%. The effects of heat flux, mass flux and system pressure on heat transfer behavior in subcooled boiling region were discussed. A new correlation of subcooled boiling heat transfer coefficient in helical coils was developed, correlating experimental results within ±20%.
[en] Thermal energy recovery of pyrometallurgy slags is a worldwide problem that is widely concerned for decades. As chemical recovery method, molten slag cascade recovery method (MS-WHCR) is proposed in this work. As typical endothermic chemical reactions, pyrolysis, gasification, calcination and reforming reactions are applied in this method. Gasification–pyrolysis system, calcination–pyrolysis system, enhanced pyrolysis system (R-SEP) and fixed carbon gasification and sorption-enhanced pyrolysis system (CG–SEP) systems of MS-WHCR method are designed. Based on the first law of thermodynamics and second law of thermodynamics, enthalpy–exergy compass analysis method is applied to analyze the exergy efficiency, consumption of reactants and products of designed MS-WHCR method, compared with traditional water quenched (WQ) method and gravity bed waste heat recovery (GWHR) method. As calculation example, 1000kg copper slag is used in this paper. The results showed that the exergy efficiency and exergy loss of WQ method are 20.7% and −947 MJ respectively. By WQ method, energy quality of molten copper slag is discounted. Copper slag particles should be fast cooled during granulation process. Thus, lots of air is blown in to make enough heat transfer with copper slag particles, which generate some exergy loss. And exergy efficiency of GWHR method is 76.9%. Using chemical endothermic reactions, MS-WHCR method improves the exergy efficiency of molten slag waste heat recovery. There is a slight fluctuation of exergy efficiency by MS-WHCR method for four kinds of systems from 66.6 to 70.1%. Fixed carbon and combustible syngas are acquired by MS-WHCR. And enhanced pyrolysis process in proposed R-SEP and CG–SEP systems improves hydrogen contents in syngas.
[en] These specifications aim at describing the typical process and content of an assessment study of unavoidable heat potentials which are available within a specific territory, thus allowing deeper and more targeted studies to be performed for the implementation of unavoidable heat recovery projects. This concerns any effluent type, whatever its type (gas, liquid) and origin (industry, waste incineration plants, energetic valorisation units, data-centres, and so on) are. Four main phases are identified and discussed: inventory of unavoidable heat sources, assessment of the local valorisation potential, proposition of a strategy of actions, and study communication.
[en] Highlights: • An innovative exhaust air energy recovery system consisting of a BIPV/T and a TW. • Pre-heating/pre-cooling ambient air in winter/summer and producing electricity. • Genetic algorithm-based multi-objective optimization approach for BIPV/T-TW system. • Optimization leads to a 118.3% enhancement in the first-law efficiency of system. • Optimization leads to a 59.7% enhancement in the TW effectiveness. - Abstract: This paper presents a feasibility investigation of integrating an air-based photovoltaic/thermal (PV/T) system with a thermal wheel (TW) system for residential applications. The innovative system is capable of pre-heating/pre-cooling the ambient fresh air in winter/summer as well as producing electricity. The performance of the system is numerically evaluated and compared with the conventional building integrated PV/T (BIPV/T) and TW systems. Then, a multi-objective optimization approach is utilized to find the optimum values of geometric and operating parameters in order to maximize the annual average effectiveness of the TW and the first-law efficiency of the BIPV/T collector. The performances of the optimized and un-optimized BIPV/T-TW systems are compared for a complete year. The results demonstrated that the BIPV/T-TW system has a better thermal performance compared with the BIPV/T and TW systems, while it has a slightly lower electrical performance compared with the BIPV/T system. Furthermore, it was found that the annual average first-law efficiency and TW effectiveness of the optimized BIPV/T-TW system is 118.3% and 59.7% higher than that of the un-optimized system.
[en] Highlights: • The ULFM shows acceptable performance to predict CO for Sandia Flame D. • The ULFM shows good agreement with CO measurement at the HRSG stack. • Parametric study is performed on the influence of FCD and layout of activated burners in HRSG. • The CO emission is affected by the lean and rich limits for combustion of local mixture. - Abstract: Computational simulation is performed for flow field and carbon monoxide (CO) emission in an industrial scale heat recovery steam generator (HRSG) by ANSYS Fluent v13. The geometrical details are reproduced with burner holes and swirler blades simplified to avoid excessive computational burden. Turbulence-chemistry interaction is modeled by the steady laminar flamelet model (SLFM) and the unsteady laminar flamelet model (ULFM) through a lookup table without time consuming integration of stiff elementary reaction steps. The ULFM showed good agreement with measured CO mass fractions near the extinction limit for Sandia Flame D in Turbulent Nonpremixed Flame (TNF) Workshop. The proper trends of variation and the same order of magnitude of CO mass fractions were reproduced by the ULFM for the three reference cases of varying HRSG loads. Parametric investigations were performed to identify the factors influencing exhaust CO with respect to the number and layout of activated burners and flow correction device (FCD). Results showed two competing factors for CO emission, rich mixture by undermixing and lean mixture by overmixing, which may lead to local extinction below the flammability limit.
[en] Highlights: • An ORC modeling based on equivalent temperatures is described. • A reconstruction method of the thermodynamic cycle is presented. • The selection procedure of organic fluids based on various criteria is discussed. • A case study is presented for various source temperatures. - Abstract: Organic Rankine cycles (ORC) are part of heat recovery technologies, which allow thermal wastes to be converted into a mechanical power. These systems are suitable for various applications characterized by a large range of source and sink temperatures. Nonetheless, the selection of the organic working fluid is essential during ORC design processes; they have a major impact on the overall engine performance. Several works have been intending to develop appropriate methodologies to determine the optimal working fluid. Recently, an optimization approach independent of the organic fluid, has been proposed in the open literature, where optimal operating conditions are expressed in terms of equivalent temperatures and overall heat transfer surface areas. However, at the end, the primitive variables under which the real system must operate, should be determined. Within this framework, for a given working fluid, this paper presents a reconstruction method of a traditional thermodynamic cycle that allows actual thermodynamic variables (pressures, temperatures, mass flow rates) to be calculated. Furthermore, a methodology is also proposed for selecting the most appropriate organic working fluid, subjected to environmental and practical engineering design criteria. To emphasize the potential of the proposed approach, the entire procedure is then applied to a particular case study.
[en] Solar cell thermal recovery has recently attracted more and more attention as a viable solution to increase photovoltaic efficiency. However, the convenience of the implementation of such a strategy is bound to the precise evaluation of the recoverable thermal power and to a proper definition of the losses occurring within the solar device. In this work, we establish a framework in which all solar cell losses are defined and described. The aim is to determine the components of the thermal fraction. We therefore describe an experimental method to precisely compute these components from the measurement of the external quantum efficiency, the current–voltage characteristics, and the reflectivity of the solar cell. Applying this method to three different types of devices (bulk, thin film, and multi-junction), we could exploit the relationships among losses for the main three generations of PV cells available nowadays. In addition, since the model is explicitly wavelength dependent, we could show how thermal losses in all cells occur over the whole solar spectrum, and not only in the infrared region. This demonstrates that profitable thermal harvesting technologies should enable heat recovery over the whole solar spectral range.
[en] Highlights: • The operating performance of the metering pump for different strokes was analyzed. • The influence of the metering pump on the ORC system was conducted. • The maximum value of the specific speed was determined. • The effect of pump efficiency on net output power was discussed. - Abstract: In this paper, a performance test of a hydraulic diaphragm metering pump using R123 is conducted. The interaction relationships of key parameters of the pump and its influence on the performance of the ORC system are analyzed. The application feasibility of the pump has been proved by comparison to previous studies. The results indicate that the mass flow rate of the pump varies from 0.23 t/h to 2.06 t/h and is mostly independent of outlet pressure. Both power input and actual efficiency of the pump increase with stroke; the actual efficiency reaches up to a maximum of 88.27%. The specific speed decreases with the increase of outlet pressure. Moreover, the actual net power output and thermal efficiency of the ORC system increase with evaporating temperature. Actual net power output is more sensitive than thermal efficiency to stroke. The thermal efficiency presents a nonlinearly rapid decline trend with the increase of specific speed. Back work ratio (BWR) can reach up to a maximum of 0.93. Thus, the power input of working fluid pump is not negligible in the ORC system and the assumptions of actual efficiency of pump should be dependent on experimental results according to various operating conditions.
[en] Highlights: • Triplex loop heat pump system for ventilation heat recover is proposed. • Mass flow rate in heat pump system can be improved by triplex loop system. • COP of triplex loop is increase with the decrease of outdoor temperature. • The performance of triplex system is higher than traditional system in most cases. - Abstract: Ventilation heat recovery is an important means of effectively reducing the energy consumption of buildings. To improve the performance of a heat pump heat recovery system under large temperature difference conditions in winter, a triplex loop heat pump system, which contains three independent heat pump cycles, is proposed in place of its single loop counterpart. Operating characteristics and system performance were analyzed while indoor temperature was constant at 20 °C and as outdoor temperature dropped from 15 °C to −20 °C. Results showed that with the decrease of the outdoor temperature, the mass flow rate and temperature effectiveness of the triplex loop heat recovery system decreased whereas the heating capacity and the coefficient of performance (COP) increased. Under the experimental conditions, the COP of the triplex loop system had an advantage over the traditional heat pump system when the outdoor temperature was below 2.5 °C. When the outdoor temperature was −20 °C, the COP of the triplex system could reach 9.33, which was 23.1% higher than that of the traditional system.
[en] This study focused on the evaluation of the stress and fatigue life of the high-pressure evaporator of a heat recovery steam generator (HRSG). First, the piping system was analyzed, and the tube bundles were shown to satisfy the acceptance criteria under sustained and cyclic loads as per ASME B31.1. Next, an outlet header, a representative thick component, was evaluated as per the ASME Boiler and Pressure Vessel Code. In analyzing the header, the nozzle loads resulting from the analysis of the piping system were applied. As a result, the stress of the header under the design condition was found to satisfy the acceptance criteria. Finally, the temperature and thermal stress of the header during the transient operating conditions were analyzed, and the fatigue life at the nozzle and tube bores were evaluated. The header was found to be safe from fatigue failure.