Nitin Goel

A Compact Falling Film Absorber

The paper presents a new design of a falling film absorber that could considerably reduce the absorber size without penalizing the vapor and coolant side pressure drops. The proposed design is based on the fundamental characteristics of an efficient absorber design: large liquid-vapor interfacial area and good wetting characteristics. A finite difference scheme is developed to numerically investigate the performance of the design. The results indicate that the proposed design is more compact and efficient than the horizontal tube-type falling film absorber.

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

A Review of Hydrogen Production Technologies

This paper describes an overview of the present status of the conventional hydrogen production technologies and some of the recent developments in the production of hydrogen using solar energy resources. It was found that conversion of fossil fuels and biomass, electrolysis of water using solar and wind energy, and direct solar conversion by thermochemical means are some of the most significant methods of H2 production. The technological status and economic analysis for commercial and near commercial technologies using renewable energy sources such as electrolysis using PV and solar thermal power, photochemical and photoelectrochemical hydrogen production, direct thermal decomposition of water, thermochemical cycles, and biological hydrogen production are outlined. Although fossil fuels are currently the least expensive and most widely used sources of hydrogen production, it is argued from an economic analysis that renewable sources of hydrogen are the most promising options for the future. Further, solar hydrogen becomes a storable fuel that is produced from this non-storable and intermittent source of energy.Copyright

Experimental Verification of a New Heat and Mass Transfer Enhancement Concept in a Microchannel Falling Film Absorber

This paper presents an experimental study of a new concept of using a screen mesh to enhance heat and mass transfer in a microchannel falling film absorber. Results of the experiments on the conventional and mesh-enhanced microchannel absorber designs are then reported. The experimental study shows that the absorber heat load for the mesh-enhanced design is about 17% ±3.4%-26% ±3.8% higher than a conventional microchannel design. The paper also presents a comparison of the experimental results with a numerical model. A finite difference scheme is used to model the heat and mass transfer processes in a falling film absorber. The numerical model agrees well with experimental results with some deviation at low temperature of coolant and high flow rate of weak solution.

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 300°C to 600°C 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.Copyright