Showing papers by "Terrence W. Simon published in 2014"

Spray-cooling concept for wind-based compressed air energy storage

Abstract: Wind turbine output energy varies over time with local wind speed and is typically inconsistent with grid power demand. Without energy storage, the resulting difference between rated (peak) power and average power output leads to over-sizing of electrical generator and transmission lines. This conventional arrangement can be avoided if wind turbines can be coupled with energy storage to eliminate the output variations and instead produce their average power on a continuous basis. This would allow a smaller, lower-cost, constant-speed generator and a reduced capacity transmission system sized only for average power output. To accomplish this goal, this study discusses a concept for a storage system for a 5 MW off-shore wind turbine, which integrates a spray-based compressed air energy storage with a 35 MPa accumulator. The compressor employs a liquid piston for air sealing and employs water spray to augment heat transfer for high-efficiency. The overall compression is proposed in three stages with pressure ratios of 10:1, 7:1, and 5:1, all operated at 1 Hz to maintain moderate liquid surface acceleration. Based on a simple and fundamental description of the system, compression efficiency was found to be strongly dependent on droplet surface area, which can be achieved through either high mass loading or small drop sizes. The simulations also show that direct injection spray can increase overall three-stage compression efficiency to as high as 89%, substantially better than the 27% associated with a conventional adiabatic compression at the same pressure ratio. In addition, this study introduces a key performance parameter, termed the Levelization Factor, which can be used to quantify the impact of storage on wind energy systems. However, experiments and simulations based on 3-D geometries with design details are needed to determine the potential of this concept.

Numerical analysis of heat exchangers used in a liquid piston compressor using a one-dimensional model with an embedded two-dimensional submodel

Abstract: The present study presents a one-dimensional liquid-piston compressor model with an embedded two-dimensional submodel. The submodel is for calculating heat conduction across a representative internal plate of a porous heat exchanger matrix within the compression space. The liquid-piston compressor is used for Compressed Air Energy Storage (CAES). Porous-media-type heat exchangers are inserted in the compressor to absorb heat from air as it is compressed. Compression without heat transfer typically results in a temperature rise of a gas and a drop in efficiency, for the elevated temperature leads to wasted thermal energy, due to cooling during subsequent cooling back to ambient temperature. The use of heat exchangers can reduce the air temperature rise during the compression period. A typical numerical model of a heat exchanger is a one-dimensional simplification of the two-energy-equation porous media model. The present authors proposed a one-dimensional model that incorporates the Volume of Fluid (VOF) method for application to the two-phase flow, liquid piston compressor with exchanger inserts. Important to calculating temperature distributions in both the solid and fluid components of the mixture is heat transfer between the two, which depends on the local temperature values, geometry, and the velocity of fluid through the matrix. In the one-dimensional model, although the axial temperatures vary, the solid is treated as having a uniform temperature distribution across the plate at any axial location. This may be in line with the physics of flow in most heat exchangers, especially when the exchangers are made of metal with high thermal conductivity. However, it must be noted that for application to CAES, the gas temperature in the compression chamber rises rapidly during compression and the core of the solid wall may heat up to a different temperature than that of the surface, depending on the geometry, solid material of the exchanger and fluid flow situation. Therefore, a new, one-dimensional model with embedded two-dimensional submodel is developed to consider two-dimensional heat conduction in a representative solid plate. The VOF concept is used in the model to handle the moving liquid-gas interface (liquid piston). The model gives accurate solutions of temperature distributions in the liquid piston compression chamber. Six different heat exchangers with different length scales and different materials are simulated and compared.

Heat and mass transfer model of a packed-bed reactor for solar thermochemical CO2 capture

Abstract: A 1 kWth dual-cavity solar thermochemical reactor concept is proposed to capture carbon dioxide via the calcium oxide based calcination−carbonation cycle. The reactor is oriented beam-up wherein concentrated solar energy from a heliostat field enters an aperture located at the bottom of the reactor. In the endothermic calcination step, concentrated solar radiation is captured by the inner cavity and transferred by conduction through a diathermal cavity wall to the particulate CaCO3 medium located in the annular reaction zone. The liberated CO2 is removed from the reactor to external storage. In the exothermic carbonation step, a CO2-containing gas flows through a bed of CaO particles in the reaction zone, forming CaCO3, while CO2-depleted gas leaves the system. The reactor design is refined using a numerical heat and fluid flow model for the calcination step. The Monte Carlo ray-tracing method is employed to solve for radiative exchange in the inner cavity, coupled with a computational fluid dynamics analysis to solve the mass, momentum, and energy equations in the concentric reaction zone modeled as a gas-saturated porous medium consisting of optically large semitransparent particles. The cavity diameter and length-to-diameter ratio are varied to study their effects on pressure drop, temperature distribution, and heat transfer in the reactor. Increasing the cavity diameter and length-to-diameter ratio decreases the radial temperature gradients across the cavity wall and within the reaction zone. However, it also results in increased pressure drop and reduced heat transfer to the reaction zone.

Numerical Modeling of Three Dimensional Heat Transfer and Fluid Flow Through Interrupted Plates Using Unit Cell Scale

Abstract: Interrupted-plate heat exchangers are used as regenerators for absorbing and releasing thermal energy such as in a Compressed Air Energy Storage (CAES) system in which the exchanger absorbs energy to cool the air being compressed. The exchanger features layers of thin plates in stacked arrays. In a given layer, the plates are parallel to one another and parallel to the exchanger axis. Each successive layer is rotated to have its plates perpendicular to the layer below but still parallel to the exchanger axis. As flow passes from one layer to the next, new thermal boundary layers develop, beneficial to effective heat transfer. The interrupted-plate heat exchanger, also seen as a porous medium, demonstrates strong anisotropic behavior when flow approaches the plates other than axially. Pressure drops and heat transfer coefficients are dependent upon the attack angle. Mathematical models for anisotropic pressure drop and heat transfer are proposed based on numerical experiments on a Representative Elementary Volume (REV), which represents a unit cell of the interrupted-plate medium. The anisotropic pressure drop is modeled by the traditionally used Darcy and inertial terms, with the addition of another term representing mixing effects. Heat transfer between fluid and plates is formulated in terms of Nusselt number vs. Reynolds number and mean flow angle. These models can be used when solving the volume-averaged Navier-Stokes equations for global-scale flow through the interrupted-plate arrays, by assuming that the porous medium region is a continuum. The global-scale analysis is used for the design and optimization of the medium.