Sarada Kuravi

Numerical Investigation of Flow and Heat Transfer Performance of Nano-Encapsulated Phase Change Material Slurry in Microchannels

Microchannels are used in applications where large amount of heat is produced. Phase change material (PCM) slurries can be used as a heat transfer fluid in microchannels as they provide increased heat capacity during the melting of phase change material. For the present numerical investigation, performance of a nano-encapsulated phase change material slurry in a manifold microchannel heat sink was analyzed. The slurry was modeled as a bulk fluid with varying specific heat. The temperature field inside the channel wall is solved three dimensionally and is coupled with the three dimensional velocity and temperature fields of the fluid. The model includes the microchannel fin or wall effect, axial conduction along the length of the channel, developing flow of the fluid and not all these features were included in previous numerical investigations. Influence of parameters such as particle concentration, inlet temperature, melting range of the PCM, and heat flux is investigated, and the results are compared with the pure single phase fluid.

Organic Fluids in a Supercritical Rankine Cycle for Low Temperature Power Generation

This paper presents a performance analysis of a supercritical organic Rankine cycle (SORC) with various working fluids with thermal energy provided from a geothermal energy source. In the present study, a number of pure fluids (R23, R32, R125, R143a, R134a, R218, and R170) are analyzed to identify the most suitable fluids for different operating conditions. The source temperature is varied between 125 C and 200 C, to study its effect on the efficiency of the cycle for fixed and variable pressure ratios. The energy and exergy efficiencies for each working fluid are obtained and the optimum fluid is selected. It is found that thermal efficiencies as high as 21% can be obtained with 200 C source temperature and 10 C cooling water temperature considered in this study. For medium source temperatures (125 150 C), thermal efficiencies higher than 12% are obtained.

Encapsulated Phase Change Material Slurry Flow in Manifold Microchannels

The heat transfer performance of water-based microencapsulated phase change material slurry (particle size 5 μm) flow inside manifold microchannels of hydraulic diameter 170 μm was experimentally and numerically investigated. Slurry performance was poorer compared with pure fluid due to the large size of particles used and lower thermal conductivity of slurry compared with water. A parametric study was performed with nanoencapsulated phase change material slurry flow (particle size of 100 nm) in microchannels of hydraulic diameters 170 and 47 μm. Two different base fluids were considered and the heat transfer enhancement of slurry with various particle mass concentrations compared with its base fluid was presented. For developing flows, the performance of phase change material slurry depends on various parameters such as base-fluid thermal conductivity, channel dimensions, amount of phase change material melted, and particle mass concentration. In the case of manifold microchannel heat sinks, where the microchannel flowpath is much shorter compared with traditional microchannels, using higher-thermal-conductivity phase change material, narrower channels, smaller particles, and optimum parameters will aid in obtaining better thermal performance of phase change material slurry compared with pure fluid.

Investigation of a High-Temperature Packed-Bed Sensible Heat Thermal Energy Storage System With Large-Sized Elements

A high temperature sensible heat thermal energy storage (TES) system is designed for use in a central receiver concentrating solar power plant. Air is used as the heat transfer fluid and solid bricks made out of a high storage density material are used for storage. Experiments were performed using a laboratory scale TES prototype system and the results are presented. The air inlet temperature was varied between 300C to 600C and the flow rate was varied from 50 CFM to 90 CFM. It was found that the charging time decreases with increase in mass flow rate. A 1D packed bed model was used to simulate the thermal performance of the system and was validated with the experimental results. Unsteady 1D energy conservation equations were formulated for combined convection and conduction heat transfer, and solved numerically for charging/discharging cycles. Appropriate heat transfer and pressure drop correlations from prior literature were identified. A parametric study was done by varying the bed dimensions, fluid flow rate, particle diameter and porosity to evaluate the charging/discharging characteristics, overall thermal efficiency and capacity ratio of the system. INTRODUCTION The use of renewable energy sources has become important in view of growing energy demands and continuous depletion of conventional sources of energy. Among renewable energy sources, solar energy is considered a feasible means of generating electricity. Concentrating solar power (CSP) plants can be easily coupled with thermal energy storage (TES) and back-up heat sources making them highly dispatchable. The stored energy can be utilized in the absence of solar radiation or under peak load conditions. Two-tank systems, thermocline systems and packed bed systems using either sensible heat, latent heat, or thermochemical reactions have been used or analyzed for use in CSP plants. When incorporated in a power plant, TES system level efficiency and performance is very important in determining the performance and cost of the plant. Hence, as with any other application, an efficient TES system design that can reduce the levelized cost of energy (LCOE) of the power plant is desirable. Packed Bed Storage Systems A packed bed energy storage system consists of solid storage materials such as rocks or encapsulated phase change materials (PCMs), packed into a storage tank, and a heat transfer fluid that is circulated through voids in the bed. Hot fluid flows from solar collectors into the bed from top to bottom, where thermal energy is transferred from hot fluid to storage material during the charging phase. For heat retrieval, cold fluid flows from bottom to top during the discharging phase. A sensible heat storage system in a packed bed of rocks is especially suitable when air is used as the heat transfer fluid (HTF) in the solar receiver [1]. The advantages of a packed bed system with air as the HTF are: 1) Operating temperature

THERMAL ENERGY STORAGE FOR CONCENTRATING SOLAR POWER PLANTS

Thermal energy storage for concentrating solar thermal power (CSP) plants can help in overcoming the intermittency of the solar resource and also reduce the levelized cost of energy (LCOE) by utilizing the power block for extended periods of time. In general, heat can be stored in the form of sensible heat, latent heat, and thermochemical reactions. This article describes the development of a costeffective latent heat storage TES at the University of South Florida (USF). Latent heat storage systems have higher energy density compared to sensible heat storage systems. However, most phase change materials (PCMs) have low thermal conductivity that leads to slow charging and discharging rates. The effective thermal conductivity of PCMs can be improved by forming small macrocapsules of PCM and enhancing convective heat transfer by submerging them in a liquid. A novel encapsulation procedure for high-temperature PCMs that can be used for thermal energy storage (TES) systems in CSP plants is being developed at USF. When incorporated in a TES system, these PCMs can reduce the system costs to much lower rates than currently used systems. Economical encapsulation is achieved by using a novel electroless deposition technique. Preliminary results are presented and the factors that are being considered for process optimization are discussed.

Investigation of Polyaniline Nanocomposites and Cross-Linked Polyaniline for Hydrogen Storage

One of the biggest challenges for the commercial application of existing hydrogen storage materials is to meet the desired high volumetric and gravimetric hydrogen storage capacity and the ability to refuel quickly and repetitively as a safe transportation system at moderate temperature and pressure. In this work, we have synthesized polyaniline nanocomposites (PANI-NC) and hypercrosslinked polyaniline (PANI-HYP) materials to provide structure and composition which could meet the specific demands of a practical hydrogen storage system. Hydrogen sorption measurements showed that high surface area porous structure enhanced the storage capacity significantly at 77.3K and 1atm (i.e., 0.8wt% for PANI-HYP). However at 298K, storage capacity of all samples is less than 0.5wt% at 70 bar. Hydrogen sorption results along with the surface area measurements confirmed that hydrogen storage mechanism predominantly based on physisorption for polyaniline.

Analysis of Transient Heat Transfer in a Thermal Energy Storage Module

The transient behavior of a thermal energy storage system was studied numerically. The storage system is composed of cylindrical tube containing the phase change material (PCM) surrounded by the heat transfer fluid (HTF) that flows along the axial direction of the tube. The melting of PCM was solved using specific heat capacity method. The heat transfer inside the tubes was analyzed by solving the energy equation, which was coupled with the heat conduction equation in the container wall. The velocity profile was obtained by solving the annular flow outside the tubes. The parameters that control the thermal behavior were identified. Several numerical simulations were performed to assess the effects of the Reynolds number on the heat transfer process of the system during the melting of a PCM.Copyright

Macroencapsulation of Sodium Nitrate for Thermal Energy Storage in Solar Thermal Power

Storage systems based on latent heat storage have high-energy storage density, which reduces the footprint of the system and the cost. However, phase change materials (PCMs) have very low thermal conductivities making them unsuitable for large-scale use without enhancing the effective thermal conductivity. In order to address the low thermal conductivity of the PCMs, macroencapsulation of PCMs is adopted as an effective technique. The macro encapsulation not only provides a self-supporting structure but also enhances the heat transfer rate.In this research, Sodium nitrate (NaNO3), a low cost PCM, was selected for thermal storage in a temperature range of 300–500°C. The PCM was encapsulated in a metal oxide cell using self-assembly reactions, hydrolysis, and simultaneous chemical oxidation at various temperatures. The metal oxide encapsulated PCM capsule was characterized using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The cyclic stability and thermal performance of the capsules were also studied.Copyright

Numerical Simulation of Heat Transfer in a Microchannel Heat Sink With Micro Encapsulated Phase Change Material (MEPCM) Slurry

Abstract : High heat flux removal from devices such as Insulated-Gate Bipolar Transistor (IGBT) and Monolithic Microwave Integrated Circuit (MMIC) will be important for future Navy ships. Micro encapsulated phase change material (MEPCM) slurry was used as a heat transfer fluid inside a microchannel instead single phase fluid. Presence of phase change material increases the effective heat capacity of the fluid. The performance of encapsulated phase change material (EPCM) slurry flow in microchannels was investigated using the effective specific heat capacity method. Lattice Boltzmann method was used to simulate the particle paths when the duct shape has different aspect ratios. For higher concentrations, a shear induced migration model was used to simulate the nonhomogeneous particle distribution. Results of the model were used to solve the temperature distribution inside the channels. Parametric study was carried out with water and PAO as base fluids in microchannels of two different widths, 101 urn and 25 urn. Parameters varied include particle concentration, inlet temperature of the fluid, melting range of PCM and base heat flux.

Review of Geothermal and Solar Thermal Power Plants and a Comparative Design Analysis

Thermodynamics indicates that the lower the temperature of a resource, the less energy that could be extracted from it due to lower maximum thermal efficiency. Geothermal resources exist in varying temperatures. The lowest ones (around 120°C), are too small for economic power production. On the other hand, concentrating solar power (CSP) can achieve high temperatures during the day (from 350 to 550°C, based on a Parabolic Trough CSP plant [1]) but once the sun is not shining, that temperature is reduced drastically.Transition to renewable energy systems is an environmentally friendly and potentially rewarding economic decision that society can make nowadays. This paper briefly reviews geothermal and solar thermal based plants in terms of energy growth or decay from one year to another (2012–2013). In addition, an example site location is chosen and the performance of both these types of power plants is analyzed in terms of capacity factor, Thermal Energy Storage (TES) hours, solar multiple, area requirement and Levelized Cost of Energy (LCOE) for a given set of environmental conditions. This analysis is performed using the System Advisor Model (SAM), on which simulation of parabolic trough, power tower, linear Fresnel, dish Stirling and geothermal (binary cycle) energy conversion systems are considered. At the same time, the analysis discussed will take place in a further study which will include economic viability for the two technologies running under the same combined cycle.Copyright