Performance prediction model of multistage centrifugal Pumps used as Turbines with Two-Phase Flow

Abstract Pump as Turbines (PaTs) can be used not only in hydraulic power generation but also in chemical processes, such as refinery, where fluids containing dissolved or undissolved gases or volatiles can be expanded from a higher to a lower pressure level for energy recovery. As the gas contained in the fluid is released from the solution during expansion, the flow rate increases and additional energy is delivered with respect to the case of incompressible flow. This higher power output is very attractive. In this work, a theoretical approach is proposed in order to predict the PaT performance with a two-phase flow whose expansion characteristics are known

Performance optimization of a gas-steam combined power plant partially fed with syngas derived from pomace

Abstract In this paper a gas-steam combined-cycle, partially fueled by syngas (produced in an embedded downdraft gasifier fed with pomace), is considered. In addition, an auxiliary combustion system is directly fed by ligno-cellulosic biomass. The thermodynamic model of the entire system is developed by means of the Cycle-Tempo software. The gasification process is supposed to occur at ambient pressure and air is used as gasifying agent. An optimization process has been introduced by means of the Design of Experiment ( DoE ) technique. The design variables and their corresponding ranges have been chosen by using a heuristic criterion. The power plant performance is represented by the thermal efficiency, _ η I , the exergetic efficiency, η II , the cost of electricity, COE, and the net return, R net . The DoE technique provided the so-called Pareto barrier, which isolates all the non-dominated solutions

Numerical Simulations of the flow field and chemical reactions of the Storage/Oxidation process within a NSC Pt - BaO Catalyst

A NOxStorage Catalyst (NSC) has been studied by means of reactive CFD simulations. In the scenario of automotive pollutant emission reduction, due to the stringent regulation, the detailed description of the chemical and physical phenomena within catalysts represents a key point in order to improve their conversion efficiency. The active part of the catalyst has been simulated as a porous medium. In this zone, surface reactions take place, which are modelled by means of an Arrhenius chemical kinetic approach, involving the Pt and BaO sites on the active surface of the matrix. Actually, two chemical mechanisms are considered, the simplest involves only BaO site, the other one includes both BaO and Ptsites. Both models are validated against data available in the literature and then applied to a real automotive catalyst geometry. Thus, a detailed description of the spatial distribution of the species is provided for both models. Lean condition is simulated in order to check the catalyst behaviour according to experimental results

Off-design and annual performance analysis of supercritical carbon dioxide cycle with thermal storage for CSP application

Supercritical Carbon Dioxide (sCO2) cycles can achieve higher efficiency compared to steam-Rankine or Air-Brayton cycles, therefore they are promising for concentrated solar power applications. Although sCO2 cycles show higher design efficiency, the off-design efficiency is highly sensitive to the ambient conditions, impacting the power block net-power and heat input. In the present work a recompression sCO2 cycle is connected to a central-tower solar field with two-tank thermal storage delivering molten chloride salt at 670 °C. The temperature of the molten-salt exiting from the power block and returning to the cold storage tank increases by 46 °C with respect to the design value when the compressor inlet temperature is raised by 13 °C relative to the design condition of 42 °C, which implies that the capacity of the thermal storage reduces by 25%. The main focus of this work is to investigate the off-design performance of a sCO2 recompression cycle under variable ambient temperature, molten-salt inlet temperature and molten-salt flow rate. Multi-objective optimisation is carried-out in off-design conditions using an in-house code to explore the optimal operational strategies and the Pareto fronts were compared. Since the power cycle can either be operated in maximum power mode or maximum efficiency mode, this study compares these two operational strategies based on their annual performance. Results indicate that the capacity factor of the concentrated solar power can be increased by 10.8% when operating in maximum power mode whilst the number of start-ups is reduced by about 50% when operating in maximum efficiency mode

Thermo-economic analysis, optimisation and systematic integration of supercritical carbon dioxide cycle with sensible heat thermal energy storage for CSP application

Integration of thermal energy storage with concentrated solar power (CSP) plant aids in smoothing of the variable energy generation from renewable sources. Supercritical carbon dioxide (sCO2) cycles can reduce the levelised cost of electricity of a CSP plant through its higher efficiency and compact footprint compared to steam-Rankine cycles. This study systematically integrates nine sCO2 cycles including two novel configurations for CSP applications with a two-tank sensible heat storage system using a multi-objective optimisation. The performance of the sCO2 cycles is benchmarked against the thermal performance requirement of an ideal power cycle to reduce the plant overnight capital cost. The impacts of the compressor inlet temperature (CIT) and maximum turbine inlet temperature (TIT) on the cycle selection criteria are discussed. The influence of the cost function uncertainty on the selection of the optimal cycle is analysed using Monte-Carlo simulation. One of the novel cycle configurations (C8) proposed can reduce the overnight capital cost by 10.8% in comparison to a recompression Brayton cycle (C3) for a CIT of 55°C and TIT of 700°C. This work describes design guidelines facilitating the development/ selection of an optimal cycle for a CSP application integrated with two-tank thermal storage

Multi-objective sensitivity analysis of shell-and-tube LHTES performance

In the present paper a sensitivity analysis has been carried out concerning the charging/discharging time and the stored energy performances of a shell-and-tube LHTES with respect to the number of tubes and the tube internal radius. The aim of this analysis is to investigate how the design variables affect the LHTES performance. this could lead to determine the thermal storage optimal design. Thus, the sensitivity analysis has a key role in the selection of several acceptable solutions. The considered LHTES exhibits a cylindrical shell geometry characterized by constant height and diameter. This aspect has allowed to employ simplified theoretical models able to predict the charging/discharging time and the stored energy performance. These models consider a constant heat exchange wall temperature whereas the heat exchange area and the whole PCM volume vary according to the design variables. This analysis represents the first step to solve the multi-objective optimization of the thermal storage design problem and then to determine the best solutions in both design variables and thermal storage performance domains

A Numerical Investigation of VVA Influence on the Combustion Phase for Premixed Combustion Engine under Partial Load Conditions

Nowadays, the vehicle hybridization and the use of non-conventional fuels for heavy-duty applications brings to a new beginning in the use of spark ignition (SI) engines. For a standard intake system, the premixed fuel/ air mixture is controlled by the injection of fuel after the throttle valve. Then, the geometry of the intake system, with the intake duct, the intake valves and the cylinder head shape, influences the characteristics of the flow within the cylinder up to the combustion process. The new technology of fluidpower and electrical actuations gives the opportunity to decouple the intake and exhaust valve actuations with respect to the standard cam shaft distribution. The Variable Valve Actuation (VVA) concept is not new, but its application is now affordable and flexible enough to be applied to partial load conditions. In this work, the intake, compression and combustion processes of an SI engine are studied by means of a three-dimensional numerical approach based on a finite volume approach. In this model, the Unsteady Reynolds-Averaged Navier-Stokes (U-RANS) equations are solved together with a k-ε model for turbulence and an Extended Coherent Flamelet Model (ECFM) for combustion. The 4-valve engine is equipped with two symmetrical intake valves as well as two symmetrical exhaust valves. Two strategies are studied under partial load conditions: a standard valve lift profile for both intake valves, and a single intake valve lift profile, to provide the same overall fresh mass in the cylinder of the 2-valve opening. The valve timing has been kept constant for both strategies, with an Early Intake Valve Closing (EIVC) approach due to the partial load conditions. The intake flow characteristics and their influence on the combustion process are analyzed and a comparison between the two strategies is carried out. The results show flow structures quite different between the single valve opening and the standard 2-valve opening. The asymmetry of the intake flow, induced by the single valve approach, leads to an increase of the swirl ratio with respect to 2-valve opening. The highest swirl ratio of the single valve case is sustained till spark ignition occurs. At spark timing, the Turbulent Kinetic Energy (TKE) is greatly influenced by the valve strategy, leading to higher values for the single valve lift case with respect to the standard two valves lift. Moreover, the results show that single valve opening provides a faster combustion in lean mixture conditions than the standard lift

Convective Effects in a Latent Heat Thermal Energy Storage

Convection within a latent heat thermal energy storage (LHTES) shell-and-tube device filled with phase change material (PCM) has been studied by means of numerical simulations. Both, the heat transfer fluid and the PCM mass, momentum and energy equations are solved and coupled with a conjugate heat transfer model. The study highlights three specific zones within the PCM: the top convective-dominated part, the curvilinear solid–liquid interface, and the bottom conductive-dominated part. The PCM melts from the top to the bottom, therefore the main mechanism of melting appears to be confined in the top part of the solid PCM. However, the flow details reveal a convective cell that includes the whole melted PCM from the top to the bottom of the PCM enclosure. Even though the problem is widely studied by means of experiments and numerical simulations, here the convective flow has been studied quantitatively. During the melting phase the viscous and thermal boundary layers at the walls has been reported at different heights from the bottom of the device. Results show in detail the phenomenology of the melting process within a shell-and-tube LHTES supporting the development of design solutions that could enhance the heat transfer of such device