Antonio Ramos Archibold

Multi-Objective Optimization of a Combined Power and Cooling Cycle for Low-Grade and Midgrade Heat Sources

Optimization of thermodynamic cycles is important for the efficient utilization of energy sources; indeed it is more crucial for the cycles utilizing low grade heat sources where the cycle efficiencies are smaller compared to high temperature power cycles. This paper presents the optimization of a combined power/cooling cycle, also known as the Goswami Cycle, which combines the Rankine and absorption refrigeration cycles. The cycle uses a special binary fluid mixture as the working fluid and produces power and refrigeration. In this regard, multiobjective genetic algorithms (GA) are used for Pareto approach optimization of the thermodynamic cycle. The optimization study includes two cases. In the first case the performance of the cycle is evaluated as it is used as a bottoming cycle, and in the second case as it is used as a top cycle utilizing solar energy or geothermal sources. The important thermodynamic objectives that have been considered in this work are, namely, work output, cooling capacity, effective first law and exergy efficiencies. Optimization is carried out by varying the selected

Performance Analysis of a Rankine Cycle Integrated With the Goswami Combined Power and Cooling Cycle

Improving the efficiency of thermodynamic cycles plays a fundamental role in reducing the cost of solar power plants. These plants work normally with Rankine cycles which present some disadvantages due to the thermodynamic behavior of steam at low pressures. These disadvantages can be reduced by introducing alternatives such as combined cycles which combine the best features of each cycle. In this paper, a combined Rankine–Goswami cycle (RGC) is proposed and a thermodynamic analysis is conducted. The Goswami cycle, used as a bottoming cycle, uses ammonia–water mixture as the working fluid and produces power and refrigeration while power is the primary goal. This bottoming cycle, reduces the energy losses in the traditional condenser and eliminates the high specific volume and poor vapor quality presented in the last stages of the lower pressure turbine in the Rankine cycle. In addition, the use of absorption condensation in the Goswami cycle, for regeneration of the strong solution, allows operating the low pressure side of the cycle above atmospheric pressure which eliminates the need for maintaining a vacuum pressure in the condenser. The performance of the proposed combined Rankine–Goswami cycle, under full load, was investigated for applications in parabolic trough solar thermal plants for a range from 40 to 50 MW sizes. A sensitivity analysis to study the effect of the ammonia concentration, condenser pressure, and rectifier concentration on the cycle efficiency, network, and cooling was performed. The results indicate that the proposed RGC provide a difference in net power output between 15.7% and 42.3% for condenser pressures between 1 and 9 bars. The maximum effective first law and exergy efficiencies for an ammonia mass fraction of 0.5 are calculated as 36.7% and 24.7%, respectively, for the base case (no superheater or rectifier process).

Parametric Investigation of the Melting and Solidification Process in an Encapsulated Spherical Container

A comprehensive parametric investigation on the fluid flow and heat transfer in a Latent-heat based energy storage/release system is explored for the axisymmetric melting and solidification process inside an encapsulated spherical container of 10, 20, 30 and 40mm in diameter. A numerical solution is developed using the finite-volume method and the enthalpy-porosity technique to solve Navier–Stokes and energy equations for natural convection coupled to a solid-liquid phase change. The study focused on Phase Change Materials (PCMs) with a melting temperature lying in the practical range of operation of concentrated solar thermal power generation (573.15K to 673.15K). Numerical calculations are performed in order to compute the evolution of the melting front and the velocity and temperature fields for different Grashof, Stefan and Fourier numbers. Also the effect of different metal coating materials subjected to a uniform wall temperature from 5K to 11K above the mean melting temperature of the PCM is presented. Simulation results show that a recirculating vortex is formed between the top region of the solid phase and the inner wall of the capsule that causes a more intense melting process in the upper part of the solid phase compared to the bottom region. The location of the eye of the recirculation pattern is observed to be dependent on the Grashof number and moves toward the unmelted portion of the PCM as natural convection is intensified.Copyright

Numerical Solution of Heat Transfer During Solidification of an Encapsulated Phase Change Material

Macro encapsulation techniques have gained considerable attention in latent heat storage systems for solar energy applications in order to improve the overall energy conversion efficiency in solar thermal power plants. However the heat transfer mechanisms that govern the charging and discharging processes at high operating temperatures are still under development and represent an important aspect in the thermal energy storage design process. This study presents a numerical solution of the heat transfer and phase change that occurs during the solidification process of a phase change material (PCM) encapsulated in a spherical container. A transient two-dimensional axisymmetric mathematical model was solved using the control volume discretization approach along with the enthalpy-porosity method to track the melting front. A spherical shell of thickness t, under the gravitational field is completely filled with liquid PCM. For time t>0, a constant temperature boundary condition Tw, which is lower than the phase change temperature of the PCM, is imposed at the outer surface of the shell. A comprehensive analysis is presented in order to assess the role of the capsule size, buoyancy-driven flow in the liquid phase, and shell outer surface temperature on the thermal performance of the system. Results show that with the increase of Stefan number the solidification rate is enhanced. A reduction of 39.25% in total solidification time is predicted when the Stefan number changed from 0.095 to 0.143. Finally a generalized correlation for the solid mass fraction during solidification is obtained based on a combination of Fourier and Stefan numbers and a dimensionless material parameter.Copyright

High Temperature Latent-Heat Thermal Energy Storage Module With Enhanced Combined Mode Heat Transfer

A numerical solution of the melting problem of a semitransparent gray, medium contained in a closed heated spherical shell is presented in this study. The influence of all the fundamental energy transfer mechanisms on the melting dynamics of the phase change medium (PCM) has been analyzed, in order to extend the convectional natural convection-dominated model and to expand the limited literature in the thermal energy storage (TES) area at high operating temperatures (>800°C). A two-dimensional, axisymmetric, transient model has been solved numerically. The discrete ordinate method was used to solve the equation of radiative transfer and the finite volume scheme was used to solve the equations for mass, momentum and energy conservation. The effect of the optical thickness of the medium on the melt fraction rate, total and radiative heat transfer rates at the inner surface of the shell has been analyzed and discussed. Also the influence of thermal radiation has been quantified by performing comparisons between the pure conduction and the simultaneous conduction and radiation models. The results showed that the presence of thermal radiation enhances the melting process, particularly during the solid phase sensible heating process in the multi-mode heat transfer model. Also, it was found that the contribution of the radiant energy exchange is one order of magnitude smaller than the convective transport process.© 2014 ASME

PERFORMANCE ANALYSIS OF A RANKINE-GOSWAMI COMBINED CYCLE

Improving the efficiency of thermodynamic cycles plays a fundamental role in reducing the cost of solar power plants. These plants work normally with Rankine cycles which present some disadvantages due to the thermodynamic behavior of steam at low pressures. These disadvantages can be reduced by introducing alternatives such as combined cycles which combine the best features of each cycle. In this paper a combined Rankine-Goswami cycle (RGC) is proposed and a thermodynamic analysis is conducted. The Goswami cycle, used as a bottoming cycle, uses ammonia-water mixture as the working fluid and produces power and refrigeration while power is the primary goal. This bottoming cycle, reduces the energy losses in the traditional condenser and eliminates the high specific volume and poor vapor quality presented in the last stages of the lower pressure turbine in the Rankine cycle. In addition, the use of absorption condensation in the Goswami cycle, for regeneration of the strong solution, allows operating the low pressure side of the cycle above atmospheric pressure which eliminates the need for maintaining a vacuum pressure in the condenser. The performance of the proposed combined Rankine-Goswami cycle, under full load, was investigated for applications in parabolic trough solar thermal plants for a range from 40 to 50 MW sizes. A sensitivity analysis to study the effect of the ammonia concentration, condenser pressure and rectifier concentration on the cycle efficiency, network and cooling was performed. The results indicate that the proposed RGC provide a difference in net power output between 15.7 and 42.3% for condenser pressures between 1 to 9 bars. The maximum effective first law and exergy efficiencies for an ammonia mass fraction of 0.5 are calculated as 36.7% and 24.7% respectively for the base case (no superheater or rectifier process).Copyright

Thermal Assessment of a Latent Heat Energy Storage Module Using a High Temperature Phase Change Material With Enhanced Radiative Properties

This paper presents a comprehensive analysis of the heat transfer during the melting process of a high temperature (> 800°C) PCM encapsulated in a vertical cylindrical container. The energy contributions from radiation, natural convection and conduction have been included in the mathematical model in order to capture most of the physics that describe and characterize the problem and quantify the role that each mechanism plays during the phase change process. Numerical predictions based on the finite volume method has been obtained by solving the mass, momentum and energy conservation principles along with the enthalpy porosity method to track the liquid/solid interface. Experiments were conducted to obtain the temperature response of the TES-cell during the sensible heating and phase change regions of the PCM. Continuous temperature measurements of porcelain crucibles filled with ACS grade NaCl were recorded. The temperature readings were recorded at the center of the sample and at the wall of the crucible as the samples were heated in a furnace over a temperature range of 700 °C to 850 °C. The numerical predictions have been validated by the experimental results and the effect of the controlling parameters of the system on the melt fraction rate, total and radiative heat transfer rates at the inner surface of the cell have been evaluated. Results showed that the natural convection is the dominant heat transfer mechanism. In all the experimental study cases, the measured temperature response captures the PCM melting trends with acceptable repeatability. The uncertainty analysis of the experiment yielded an approximate error of ±5.81°C.Copyright

Melting in Vertical Cylinders During Thermal Energy Storage

The present study numerically investigates the process of melting in a hollow vertical cylinder, filled with a phase change material (PCM). The PCM used is sodium nitrate, which expands upon melting. Therefore, the cylindrical shell is partially filled with the PCM and the remaining volume is occupied by air. The influence of cylinder shape on the melting and heat transfer rate is analyzed. The numerical model takes both conductive and convective heat transfer into account during the melting process. For the problem being considered here, the dimensionless numbers that characterized the process are the Grashof, Stefan, and Prandtl numbers. The Aspect Ratio (AR) is used to characterize the shape of the cylinder, which is defined as the ratio of the height to the diameter of the cylinder. In this study, a range of AR values from 0.25 to 10 are investigated. Cylinders with small AR, corresponding to high Grashof numbers, lead to lower melting times compared with cylinders with high AR. The partially melted PCM shape differed between the high and low AR cases. The molten PCM stream function was also influenced greatly by the variation in solid PCM shape.Copyright

Multi-Objective Optimization of a Combined Power and Cooling Cycle for Low-Grade and Mid-Grade Heat Sources

Optimization of thermodynamic cycles is important for the efficient utilization of energy sources; indeed it is more crucial for the cycles utilizing low grade heat sources where the cycle efficiencies are smaller compared to high temperature power cycles. This paper presents the optimization of a combined power/cooling cycle, also known as the Goswami Cycle, which combines the Rankine and absorption refrigeration cycles. The cycle uses a special binary fluid mixture as the working fluid and produces power and refrigeration. In this regard, multi-objective genetic algorithms (GA) are used for Pareto approach optimization of the thermodynamic cycle. The optimization study includes two cases. In the first case the performance of the cycle is evaluated as it is used as a bottoming cycle, and in the second case as it is used as a top cycle utilizing solar energy or geothermal sources. The important thermodynamic objectives that have been considered in this work are, namely, work output, cooling capacity, effective first law and exergy efficiencies. Optimization is carried out by varying the selected design variables; boiler temperature and pressure, rectifier temperature, and basic solution concentration. The boiler temperature is varied between 70–150 °C and 150–250 °C for the first and the second cases, respectively.Copyright