METHODS AND SYSTEMS FOR THERMAL ENERGY STORAGE AND RECOVERY

BACKGROUND OF THE INVENTION
Field of the Invention

The present invention is generally directed to methods and systems for thermal energy storage and recovery.

Description of the Prior Art

Due to growing energy production from intermittent energy sources such as solar and wind, and due to the increasing skewedness in energy demand patterns, there have been several studies for understanding thermal energy storage systems. Recently, there have been numerous studies where the thermal behavior of the storage systems was studied during storage or recovery processes. These studies have been performed for solid as well as liquid storage media. However, in all of these studies the heat carrier or heat transfer fluid was in a single phase (i.e., in a gas or liquid state for all scenarios). Systems utilizing single phase heat transfer fluids have experienced unacceptably slow thermal energy gain, and thus the total energy storage achieved by such systems has been similarly unacceptable. Additionally, these systems have generally exhibited less predictable heat dispersion as the single phase fluid passes through the energy storage vessel non-uniformly, resulting in variable warm and cool spots with less efficient energy storage. Therefore, improved thermal energy storage methods and systems are needed.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, there is provided a method of storing thermal energy contained within a condensable fluid for later use. The method comprises feeding a gaseous heat transfer fluid into a vessel comprising a packed bed of inert particulates. The method further comprises contacting the gaseous heat transfer fluid with the inert particulates and condensing at least a portion of the heat transfer fluid on the inert particulates. The contacting and condensing step transfers at least a portion of the latent heat contained within the gaseous heat transfer fluid to the particulates. The method also comprises storing the portion of the latent heat within the particulates for a period of time until at least a portion of the stored latent heat can be recovered from the particulates.

In another embodiment, there is provided a thermal energy storage system comprising an evaporator adapted for vaporizing a fluid stream, a vessel comprising at least one packed bed of inert particulates and having at least one fluid inlet and at least one fluid outlet, and a conduit configured to direct the vaporized fluid stream from the evaporator to the at least one fluid inlet. The at least one fluid outlet is configured to remove a condensate of the vaporized fluid stream from the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of one embodiment of the present invention, wherein solid lines represent fluid flow during normal system operation and dashed lines represent fluid flow during heat recovery operation.

FIG. 2 is a schematic drawing of another embodiment of the present invention, wherein solid lines represent fluid flow during normal system operation and dashed lines represent fluid flow during heat recovery operation.

FIG. 3 is a is a schematic drawing of another embodiment of the present invention, wherein solid lines represent fluid flow during energy storage operation and dashed lines represent fluid flow during heat recovery operation.

FIG. 4 is a schematic drawing of an experimental setup of a packed bed heat sink.

FIG. 5 is a series of images showing the experimental packed bed vessel (top left), as well as X-ray images of the internal fluid flow (top) and thermal images of the external surface of the vessel (bottom) during the energy storage process.

FIG. 6 is a temperature plot showing the results for the slow injection steam experiments.

FIG. 7 is a temperature plot showing the results for the fast injection steam experiments.

FIG. 8 is a temperature plot showing the results for the air injection experiments.

FIG. 9 is a schematic drawing of drawing of one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Inventive methods and systems for storing thermal energy are provided herein. The inventive methods and systems utilize a thermal energy storage vessel comprising a packed bed of inert particulates. The particulates are capable of storing thermal energy transferred from a hot gaseous heat transfer fluid. Thermal energy can then be recovered from the particulates by passing a heat recovery fluid through the packed bed. The warmed heat recovery fluid can then be utilized in a number of applications requiring a source of heat.

One embodiment of the present invention is shown schematically in FIG. 1. As shown in FIG. 1, an evaporator 110, thermal energy storage vessel 120, and heat exchange unit 130 are provided in a system. These components are described in greater detail below. During normal operation, a heat transfer fluid stream 112 is fed to evaporator 110, which provides gaseous heat transfer fluid streams 114a and 114b. Stream 114b is fed to heat exchange unit 130 to provide necessary thermal energy for normal system operation. For example, in certain embodiments, heat exchange unit 130 may form a part of a building heating system or an absorber refrigeration system. The spent heat transfer fluid exits unit 130 as outlet stream 132. Stream 114a is fed to vessel 120, wherein thermal energy from the heat transfer fluid is transferred to solid particulates contained within vessel 120. As explained in greater detail below, at least a portion of the gaseous heat transfer fluid is condensed within vessel 120 thereby transferring latent heat to the particulates contained therein. The condensed fluid exits vessel 120 via outlet stream 122. This heat is stored by the particulates until it is desired to recover that energy for use within heat exchange unit 130, such as when evaporator 110 is offline. During heat recovery operation, the output of gaseous heat transfer fluid from evaporator 110 is reduced or ceased. In order to provide the necessary thermal energy to heat exchange unit 130, a heat recovery fluid stream 126 is fed to vessel 120, wherein thermal energy stored in the solid particulates is transferred to the heat recovery fluid, thereby forming a warmed heat recovery fluid stream 128. Warmed heat recovery fluid stream 128 is then fed to heat exchange unit 130 thereby ensuring a continuous supply of heat energy to the process or system of which unit 130 is a part.

Another embodiment of the present invention is shown in FIG. 2. The embodiment of FIG. 2 operates similarly to the embodiment of FIG. 1, except for the arrangement of process equipment. During normal operation, a heat transfer fluid stream 212 is fed to evaporator 210, which generates gaseous heat transfer fluid stream 214. Stream 214 is fed to thermal energy storage vessel 220, wherein thermal energy from the heat transfer fluid is transferred to solid particulates contained within vessel 220 resulting in the at least partial condensation of stream 214. Although a certain amount of thermal energy is transferred to the particulates and stored within vessel 220, a substantial amount of useful thermal energy may remain in the heat transfer fluid and be passed through vessel 220. The heat transfer fluid exits vessel 220 and is fed to heat exchange unit 230 via stream 222 to provide necessary thermal energy for normal system operation. The spent fluid exits heat exchange unit via stream 232. During heat recovery operation (i.e., when evaporator 210 is offline), the necessary thermal energy required by heat exchange unit 230 is provided by feeding a heat recovery fluid stream 226 to vessel 220. Similar to the process of FIG. 1, the thermal energy stored in the solid particulates is transferred to the heat recovery fluid, thereby forming a warmed heat recovery fluid stream 228. Warmed heat recovery fluid stream 228 is then fed to heat exchange unit 230.

Yet another embodiment of the present invention is shown in FIG. 3. The embodiment of FIG. 3 operates similarly to the embodiment of FIG. 2, except that the arrangement of the thermal energy storage vessel and the heat exchange unit are reversed. During normal operation, a heat transfer fluid stream 312 is fed to evaporator 310, which generates gaseous heat transfer fluid stream 314. Stream 314 is fed to heat exchange unit 330 to provide necessary thermal energy for normal system operation. Although a certain amount of thermal energy may be removed from the fluid in heat exchange unit 330, a substantial amount of useful thermal energy may remain in the heat transfer fluid, particularly if the heat load of unit 330 is relatively low. The heat transfer fluid is then passed through heat exchange unit 330 and exits via stream 332. Stream 332 is fed to thermal energy storage vessel 320, wherein thermal energy from the heat transfer fluid is transferred to the particulates and stored within vessel 320, resulting in at least partial condensation of the gaseous heat transfer fluid, which exits vessel 320 via outlet stream 322. During heat recovery operation, the necessary thermal energy required by heat exchange unit 330 is provided by feeding a heat recovery fluid stream 326 to vessel 320, transferring thermal energy stored in the solid particulates to the heat recovery fluid (thereby forming a warmed heat recovery fluid stream 328), and feeding warmed heat recovery fluid stream 328 to heat exchange unit 330. The spent heat recovery fluid exits unit 330 via stream 334.

Although certain preferred embodiments are described above, it should be understood that these embodiments are not intended to be limiting and that other process equipment and arrangements may be utilized within the scope of the present invention. That is, one of ordinary skill in the art would understand that the thermal energy storage vessel (or multiple thermal energy storage vessels) and heat exchange unit (or multiple heat exchange units) may be provided in parallel or in series in a variety of combinations. It is also understood that the system equipment described here is operably connected using various conduits for transporting the working fluids.

The heat transfer fluid may be any of a number of fluids capable of conducting and transferring thermal energy through the system. As described in more detail below, preferred heat transfer fluids are capable of being vaporized in the evaporator and at least partially condensed when contacted with the particulates in the thermal energy storage vessel in order to take advantage of latent heat energy released during condensation. Preferred heat transfer fluids will have a normal boiling point above about 75° C. In certain preferred embodiments, the heat transfer fluid has a normal boiling point temperature from about 75° C. to about 200° C., preferably from about 85° C. to about 150° C., more preferably from about 95° C. to about 125° C., and most preferably about 100° C. In certain embodiments, the heat transfer fluid may comprise refrigerants, such as R134a or others that undergo phase change, organic volatile solvents including glycol-based fluids, such as ethylene glycol and propylene glycol, and steam, with steam being particularly preferred. Depending upon system configuration (e.g., thermal storage occurring upstream or downstream of the heat load) the steam may be in the form of superheated steam or saturated steam. However, it is preferred that steam is supplied to the thermal energy storage vessel as saturated steam.

The heat recovery fluid may similarly be any of a number of fluids capable of conducting and transferring thermal energy through the system. In certain embodiments, the heat recovery fluid may be the same as or different from the heat transfer fluid. However, in particularly preferred embodiments, the heat recovery fluid is different from the heat transfer fluid. In certain aspects of the present invention, it is important to utilize a heat recovery fluid that will not undergo a phase change when brought into contact with the heated inert particulates. Thus, the heat transferred to the heat recovery fluid is sensible heat. In certain embodiments, the heat recovery fluid is selected such that it will enter and leave the vessel entirely in the gaseous state, but should not be taken as excluding the use of liquid heat recovery fluids. In particularly preferred embodiments, the heat recovery fluid is air. However, it is also within the scope of the present invention that the heat recovery fluid be any other heat transfer fluid that can recover energy in the form of sensible heat, latent heat, or a combination of both, such as water, DOWTHERM™, heating oils, refrigerants, and others.

The evaporator is capable of supplying sufficient energy to the heat transfer fluid so as to vaporize the fluid. For example, in preferred embodiments, the evaporator is a renewable steam generator, meaning that the energy required to vaporize the fluid is obtained from a renewable source. Renewable steam generators include, for example, solar steam generators, biomass steam generators, and wind-powered steam generators.

However, it is understood that non-renewable evaporators may also be used if desired or required by a particular application. For example, the heat transfer fluid may be heated using a product stream from an exothermic reaction process or a heated flue gas stream. The heat exchange unit is capable of supplying heat to any number of processes. For example, in particularly preferred embodiments, the heat exchange unit may be a component in an indoor heating system, absorption refrigeration system, or any other domestic or industrial process heat, power generation, or cooling application.

The thermal energy storage vessel may be constructed in a variety of geometries, using a variety of materials. The vessel generally comprises an outer housing, one or more fluid inlets, and one or more fluid outlets. In certain embodiments, one or more of the fluid inlets may also be utilized as a fluid outlet, so long as the vessel is configured such that the heat transfer fluid may be passed through the vessel during normal operation and that the heat recovery fluid may be passed through the vessel during heat recovery operation. The vessel further comprises at least one packed bed containing the inert particulates. In certain embodiments, the packed bed comprises a cylindrical chamber and the inert particulates comprise a plurality of solid particles. The particles may be a variety of shapes, including, for example, spherical, cylindrical, elliptical, irregularly shaped, or combinations thereof. In preferred embodiment, the particles have an average diameter of from about 0.1 mm to about 8 mm, preferably from about 0.5 mm to about 6 mm, and more preferably from about 1 mm to 3 mm. Regardless of the particle shape, as used herein the particle diameter refers to the linear distance across the particle as taken across its largest dimension. In particularly preferred embodiments, the particles are spherical particles having an average diameter of less than 3 mm. The particles may comprise a variety of materials. However, the materials selected should generally be chemically inert and exhibit high thermal conductivity. In certain embodiments, the particles comprise at least one of alumina, graphite, silica, quartz, ceramic, or rock (e.g., pea gravel). In particularly preferred embodiments, the particles comprise alumina and/or pea gravel. The vessel may also be insulated using a variety of methods and materials known in order to minimize heat loss to the surrounding environment during energy storage. Exemplary thermal energy storage vessels are described in U.S. Application Publication No. 2014/0202157 and U.S. Application Publication No. 2014/0299306, both of which are incorporated by reference in their entireties, herein.

Embodiments of the present invention demonstrate improved thermal energy transfer, storage, and recovery over prior art methods and systems due to the unique heat transfer phenomena taking place within the thermal energy storage vessel (and particularly within the packed bed portion of the vessel) during normal system operation and subsequent heat recovery operation. In contrast to prior art methods which used single-phase heat transfer fluids to transfer thermal energy to storage systems, the present invention preferably uses a condensable fluid that exists in both the vapor and liquid phase (e.g., saturated steam) during transport through the packed bed. It has been discovered that the use of such condensable fluids as the heat transfer fluid during thermal the initial energy transfer phase provides faster and more predictable heat transfer conditions. Without being bound by any theory, it is believed that the improved heat transfer behavior is due to the combination of two types of thermal energy transfer occurring within the vessel during this operation. First, when the vaporized heat transfer fluid condenses on the packed bed particulates, latent heat is released by the fluid and transferred to the particulates. The term “latent heat” is generally the change in internal energy experienced by a body or thermodynamic system with no change in temperature. Thus, the latent heat transfer aspect is due to the phase change (condensation) without a change of fluid temperature. This transfer of latent heat provides a rapid and more predictable transfer of a greater quantity of heat to the solid particulates as compared to a transfer of mere sensible heat from the heat transfer fluid. Second, when the hot vapor or liquid contacts the cooled particulates, sensible heat is transferred from the fluid to the particulates due to the difference in temperature. The term “sensible heat” is generally the change in internal energy experienced by a body or thermodynamic system as measured by the temperature change. Thus, the sensible heat transfer aspect is due to the temperature difference between the fluid and the particulates.

Based on the above-described principles, embodiments of the present invention comprise contacting a gaseous heat transfer fluid with inert particulates within the packed bed. During the initial energy transfer operation, the particulates are generally cooler than the fluid. Due to this temperature difference, sensible heat is transferred from the fluid to the particulates, and thus the fluid temperature decreases. When the fluid temperature reaches the boiling point temperature within the packed bed, at least a portion of the fluid condenses on the particulates, and latent heat is transferred from the fluid to the particulates. While the fluid condensation does not change the fluid temperature, the temperature of the particulates and the packed bed increases significantly during this step. After condensation occurs, some additional sensible heat transfer may continue to occur if there is a temperature difference between the fluid and the particulates. Advantageously, it has been discovered that the use of a condensable heat transfer fluid (e.g., steam) in the packed bed configurations of the present invention allows for improved cross-sectional uniformity as the fluid is fed through the packed bed and energy is transferred to the particulates. Thus, an observable and predictable thermal front is exhibited as the heat transfer fluid is fed into the packed bed and heat is being stored. Accordingly, during as heat transfer fluid is being fed to the energy storage vessel, a sufficiently steep temperature gradient is maintained along the flow direction, which discourages exergy losses due to thermal dispersion. The rate at which the gaseous heat transfer fluid is fed to the packed bed will influence that rate at which energy is transferred to the particulates. At higher feed rates, faster energy transfer is typically observed. However, the feed rate is limited by the fact that the heat transfer fluid should have a sufficient residence time within the packed bed such that the fluid can condense and release latent heat. Generally, the residence time (and related flow rate) should be selected based on other design parameters (e.g., particulate size, particulate material, and type of heat transfer fluid) such that the particulates have corresponding Biot numbers less than 0.1. The heat transferred to the packed bed particulates is then stored within the particulates for a period of time until recovery is desired (e.g., during times of reduced heated vapor generation).

During heat recovery operation of the system, the heat recovery fluid is fed into the thermal energy storage vessel and passed through the packed bed containing the heated particulates. The heat recovery fluid should have a cooler temperature than the heated particulates but should also be fed to the vessel in a single, preferably gaseous, phase. In contrast to the heat transfer fluid, the heat recovery fluid preferably passes through the packed bed in a single phase. For example, a gaseous heat recovery fluid is fed into the vessel and the fluid remains in the gaseous phase throughout the heat recovery operation in the vessel with little or no phase change. As the recovery fluid does not experience a change in phase, there is no energy transfer due to latent heat. Rather, thermal energy is transferred to the heat recovery fluid only in the form of sensible heat. This has the advantage of providing a longer duration of heat supply, as the particulates do not lose the stored thermal energy as quickly as they obtained it. Therefore, in particularly preferred embodiments, the heat recovery operation occurs over a greater period of time than the initial heat transfer operation.

An exemplary energy storage and recovery system is shown in FIG. 9, and its operation utilizing steam as the heat transfer fluid, in accordance with one embodiment of the present invention, is described below. The embodiment described herein uses water as the heat recovery fluid in order to recover steam, but it should be understood that other heat transfer fluids and heat recovery fluids can also be used within the scope of the present invention. During energy storage operation, valve 21 opens to allow steam from an evaporator (not shown) to flow through fluid inlet 22 and into vessel 20. As the steam contacts the particulates 29 within vessel 20, the steam condenses, thereby transferring latent heat energy to the particulates 29. The condensed steam (water) then exits vessel 20 through fluid outlet 24 and valve 25, and the heat is stored in the packed bed of particulates 29 until energy recovery is desired. Preferably, vessel 20 is insulated or otherwise configured to minimize or eliminate heat loss through the outer surface of the vessel during heat storage. During energy recovery operation, valves 21 and 25 are closed, and water is introduced into vessel 20 through water inlet 26 and valve 27. When the water contacts the hot particulates 29, heat is transferred from the particulates 29 to the water, thereby vaporizing the water into steam. The steam then exits vessel 20 through fluid outlet 28 and valve 23 as recovered thermal energy.

Example

The following example sets forth radially and azimuthally symmetric steam condensation heat transfer experiments using a packed bed of alumina particles. It is to be understood, however, that these examples are provided by way of illustration and nothing therein should be taken as a limitation upon the overall scope of the invention.

Introduction

Conventional thermal energy storage systems have been largely ineffective due to complete mixing in the storage medium as the heat transfer fluid is introduced. When heat transfer fluid is introduced via mixed flow, the temperature of the energy storage medium increases; however, the temperature profile throughout the energy storage medium and storage vessel at any particular point in this process is relatively uniform. Such mixed flow methods require that in order to bring the temperature in entire bed from cold to hot will require the discharge of warm to hot fluid during the entire storage process (i.e., wasting large amounts of energy). As described in more detail below, a more preferred scenario is to achieve a plug flow (i.e. no mixing downstream) which enables steep temperature gradient. This is because bringing the entire bed from cold to hot temperature in such a scenario would not require any discharge of warm or hot fluid, and thus any fluid leaving the storage vessel during the storage process is always cold.

Another physical process which limits the ability to achieve plug flow is temperature diffusion within the storage medium. This occurs, for example, when heat transfers within the storage medium ahead of the heat transfer fluid. Although some prior art storage solutions have been able to achieve some gradient in the temperature profile of the storage medium during storage operation, this gradient is relatively gradual from inlet to outlet. Accordingly, these solutions have been unable to achieve the steep temperature profiles associated with plug flow due to the temperature diffusion within the storage medium. Such solutions have been shown to achieve thermal energy storage efficiencies not more than 50-60%.

Design Objectives

Previous studies assumed that a packed bed configuration enables the uniform flow and temperature distribution in the radial direction (i.e., in the direction normal to the flow). However, due to wall heat losses, the temperature of the wall and the bed were different from each other. Thus, accurate analysis required simultaneous wall temperature measurements. One of the primary objectives is to understand the axial dispersion of the temperature front upon steam injection, temperature must be measured at various locations in the axial direction without interfering significantly with the system behavior. This experiment focuses on steam condensation at atmospheric pressure, which simplifies the design of the vessel and fittings. One of the difficulties in understanding the steam condensate process is associated with the uncertainties in the flow conditions and thickness of the two phase (liquid-vapor) zone inside the packed bed. Moreover, effective thermal conductivity inside the bed is strongly dependent upon instantaneous liquid hold-up and interface location. Therefore, the condensate flow rate was measured or estimated throughout the experiment to correlate with the temperature response.

Experimental Setup

All experiments were performed in a cylindrical quartz tube randomly packed with spherical particles. The size of the cylindrical vessel was 6 inches (15.24 cm) tall and 2.5 inches (6.35 cm) diameter. The cylinder diameter was the only size limitation because of the standard ceramic flanges used to seal the ends. The diameter of the spherical alumina particles were 3-mm in order to provide the largest available ratio between the sphere diameters and the cylinder tube diameter. Having a ratio of tube to particle diameter greater than 20 also allowed for the plug-flow assumption or radially uniform dispersion of the thermal front.

The spherical alumina particles were procured from Saint-Gobain NorPro under the commercial name Denstone® 99. These commercially available particles were chosen because they allowed for uniform isotropic heating and have been previously tested for chemical inertness and robust thermo-mechanical behavior with steam. Alumina particles have high heat capacity, high thermal conductivity, and chemical inertness, which allows the rapid localized equilibration of thermal energy between the fluid phase and solid phase. Alumina is also non-degradable, allowing it to last a relatively long time and to remain stable through multiple heating and cooling cycles. Large heat transfer surface area was achieved due to considerably smaller particle or packing size as compared to the overall bed dimensions. This made the thermal front propagation more predictable and along the flow direction. With saturated steam as the heat transfer fluid, the rate of heat rejection is much faster during the condensation process. Therefore, if the media has sluggish response to absorb heat, this will lead to very complex energy balance in three phases. Therefore, using alumina particles achieves the crucial design objective of performing reproducible thermal behavior tests.

The quartz tube was sealed with ceramic flanges at both ends to provide lowest possible energy dispersion effects from the boundaries. The quartz tube was chosen because it provided for visual inspection of movement of the liquid-vapor interface and the known constant emissivity value of quartz material in temperature range of 25° C.-100° C. allowed for easy measurement of wall temperatures with an IR camera.

The measurement setup to attain the temperature values along the outside wall of the heat sink vessel was a forward looking infrared (FLIR) camera. The packed bed temperatures were measured with an Omega® multi-point thermocouple tube to record the temperature at six axial locations in the bed. The multi-point thermocouple was positioned as close to the center of the bed as possible using a fitting screwed into the top flange of the vessel. Each thermocouple was numbered respective to its position from the inlet of the test chamber.

In-house steam supply was used in the experiments. Before starting the experiments, it was ensured that steam supply pressure and flow did not change for the duration of the individual experiments. This was done by allowing the steam to condense in a cylindrical flask with cold water and monitoring the change in the level with time during steam flow. In addition, during the actual experiments of steam injection in the packed bed condensate, flow was monitored to take into account the uncertainties, if any. Steam was supplied from the top of the test chamber after passing through a pressure regulator holding the back pressure constant for all experiments. A globe valve was situated just before the entrance to the test vessel to allow control of steam after the supply valve was opened. This combination allowed the evaluation of the system's response to a step input of constant pressure steam. The downstream end of the test chamber was connected to a tube-in-tube heat exchanger where any remaining vapor was condensed. This extra step allowed for the total mass of steam that passed through the chamber to be collected and measured, enabling a value for the total amount of energy input into the system to be obtained.

A simplified schematic of the experimental setup is shown in FIG. 4, with vessel 10 comprising a packed bed of particulates 12, fluid inlet 14, fluid outlet 16, and multi-point thermocouple 18 having thermocouples (TC-1-TC-6).

Experimental Procedure

The experiments were performed in two sets. The first set of experiments were the slow-injection tests, wherein the steam was slowly and gradually injected into the packed bed. In these experiments, a throttled steam supply was further regulated by a globe valve to nearly atmospheric pressure before being injected into the top of test section. This throttled condition was confirmed by the fact that the maximum temperature of the steam at the top of the cylinder did not rise significantly above 100° C. Prior to each test run, the system was flushed with dry cold air to ensure uniform temperature and no vapor content in the system. Steam was then continuously injected through the cylinder until such point as the thermographic camera registered that the wall of the cylinder had achieved steady state conditions.

The second set of experiments were the fast-injection tests. In these experiments, the globe valve was left completely open and flow injection was initiated with a butterfly gate valve. After the correct initial conditions were achieved (i.e., the alumina particles were both dry and at room temperature), and after setting the desired steam inlet conditions, the butterfly gate valve was quickly opened to inject the steam into the system. The steam was continuously injected into the system at the set pressure (i.e., atmospheric pressure) until the thermographic camera displayed a uniformly heated outer wall. While substantially all of the steam was condensed in the tube before being discharged in the slow injection tests, in the fast injection experiments the steam was allowed to flow through the packed bed and exit out the bottom into a discharge pipe.

The injection flow rate of the steam for each case was determined by condensing the steam, collecting the condensate, and timing how long the valve was open. Multiple experiments were run to ensure repeatability and consistent flow rate values for both the slow and fast injection cases. The average condensate collection flow rates for the slow and fast were measured to be 1.25 cm³/s and 45 cm³/s. In the following section, instead of exact flow rates, discussion will be made using the terms slow and fast injection. For each experiment it was found that uncertainty in the measurement of condensation collection flow rate is within 5% of the numbers stated above.

The condensate was collected and measured for the experiments using an air supply to remove all of the liquid before and after the run and a beaker to collect all of the condensation during and after a run. The velocity was assumed constant, and thus only flow rates were measured for experiments where the bed was completely filled. After observing the experimental results trends for the slow-injection case, it was decided to only fill the bed partially and measure the different condensing flow rates. After testing this, it was found that the velocity did change. Specifically, it was noticed that the velocity had a negative linear slope as the bed was being heated. Using this varying velocity data, the accuracy of the models' solutions was improved, as presented in the results sections. To compare the steam injection experiments to a single phase heat transfer fluid, air was heated and sent through the randomly packed bed. The flow rate of the air was controlled with an air compressor and its outlet nozzle. The air was allowed to reach a steady state temperature by bleeding out through a valve located just before the packed bed, and once it had reached the steady temperature, the inlet and bleed valves were slowly opened and closed, respectively. This allowed the pressure to remain relatively unchanged so that the steady state air would enter the packed bed without any uncertainties. This allowed the experiments to be easily reproduced and led to accurate data and results.

Results

Thermal Imaging.

To observe the uniform heat rejection process in the bed with radial symmetry, thermal images of the cylindrical quartz walls were captured with frequency 6.25 fps. X-ray images of the interior of the vessel and thermal images along the wall of the packed bed for one of the experiments are shown in FIG. 5. Without the solid media in the vessel, the steam would circulate through the containment and condense randomly along the walls. With the solid media present, the thermal front propagation images at different times shown in FIG. 5 reveal that the steam condensation over the packed bed is radially uniform, indicating plug flow. Therefore, qualitative and quantitative analyses using the data obtained can be done in axial direction (1-D) i.e. direction of steam flow.

Vapor Flow Modeling.

Steam or vapor flow in hot porous media has been studied previously using convection diffusion models. Radially symmetric time dependent one-dimensional convection-diffusion equation can be written as

$\begin{matrix} \frac{\partial T}{\partial t} + κ v \frac{\partial T}{\partial x} = α \frac{\partial^{2} T}{\partial x^{2}} & (1) \\ κ = \frac{{ρ C_{p}}_{f}}{{ρ C_{p}}_{b + f}} & (2) \end{matrix}$

where T is the local temperature of bed and fluid stream, x and t are the axial dimension and time variable respectively, {ρC_p} is the energy density per unit time, subscripts f and b denote fluid and bed respectively, v is the fluid stream velocity and a is thermal diffusivity of the bed. Although at inlet condition, the fluid stream is saturated steam and possesses latent heat, for simplified discussion in mathematical form this can be considered as specific heat spread over the small temperature difference around the evaporation temperature. This convection-diffusion equation can be used to provide a simplified qualitative analysis of the vapor flow. The

$κ v \frac{\partial T}{\partial x} and α \frac{\partial^{2} T}{\partial x^{2}}$

terms in Equation 1 are the advection and conduction terms, respectively. Assuming the temperature is zero at the exit or bottom of the bed, in the direction of the motion of the fluid, the temperature gradient,

$\frac{\partial T}{\partial x},$

is negative, and the term

$\frac{\partial^{2} T}{\partial x^{2}}$

is positive. Therefore at any axial location in the bed, both of these terms will lead to positive rate of change of temperature. In the following subsections, the impact of slow and fast injections of steam on the temperature front progression at the points of measurement is described.

Slow Steam Injection.

The characteristic thermal response of the packed bed system at different times upon slow injection of steam is highlighted in this discussion with explanation of results. In the case of slow injection, there are two thermal transport mechanisms—advection and conduction modes at different spatial locations and different time frames. Near the steam entry port, the temperature of the bed and fluid streams became almost equal to the steam inlet temperature or saturation temperature in a very short period of time. As steam supply was continuously available, irrespective of injection rate, the constant temperature conditions at the inlet implies that the bed temperature at the top was always maintained at saturation temperature. This constant bed temperature at the top (inlet) conducts heat from the top to bottom of the bed due to non-negligible thermal conductivity of the alumina particles and water in the bed (i.e., conduction mechanism). Simultaneously, the steam injected into the bed was also carrying some amount of energy as it moved in the bed (i.e., advection mechanism).

Due to the slow injection rate, an initial rise in the temperature at axially farther locations was dominated by the conduction mechanism. As the steam or two phase mixture front, which is at a temperature near the saturation temperature, reaches those regions located far away from injection point, there was a sudden change in the temperature, as shown in FIG. 6, with temperature measurements at different times and different locations. The rate of increase of temperature at different locations is divided into two distinct regimes with two distinct slopes for the last four (3-6) locations. The initial regimes, which show lower slope, are governed by conduction mechanism. The later regimes with higher slope are governed by advection. These conclusions are substantiated with the observation that conduction dominated for longer times for the locations farther from the injection point.

Fast Steam Injection.

Based on the explanations provided in the previous subsection, it was expected that the advection term would be much higher as compared to conduction term throughout the fast injection experiment. The higher advection term implies that total amount of influx enthalpy carried by the steam or two-phase mixture is much higher and thus, as the fluid stream moves through the bed it is equilibrating the bed to the saturation temperature at almost constant rate at all spatial locations. Due to much higher rate of enthalpy injection in the bed due to advection term, the effects of conduction were not expected have much impact on rate of temperature increase in the bed. The results in FIG. 7, for fast injection, confirm this. In the slow steam injection case, a short steep injection front was observed between the infiltrating steam and the run off condensation. While this phenomena is prominent in the slow injection case, fast injection showed simultaneous heat transfer from both advection and diffusion throughout the bed. As the steam front traveled through the bed at much quicker pace, the conduction in the bed did not play a major role in transporting this heat to points in the bed farther downstream.

The bottom thermocouple in the fast case was unusable because of how fast it responds to temperature change compared to the other five thermocouples. The reason for this is not clear, so it was left out of the plots intentionally for the fast temperature cases. But for the slow cases, the temperature change at this location was slow enough that this effect was unnoticeable. In comparing slow injection and fast injection, the cases showed a considerable difference in the heat transfer and thermal front propagation through the packed bed. The most noticeable difference was the reduced amount of time for the bed to reach peak temperature throughout the bed in the fast injection case.

Air Injection.

Only one set of results are shown for the case wherein air was used as the heat transfer fluid, as the difference in flow rate caused no profound difference in the thermal front propagation in the packed bed. The slow injection air case is similar to the fast injection air case because no significant difference was demonstrated using the experimental setup. However, one noticeable difference was that the slower case had a slightly higher temperature due to more heat transferred to the slower moving air. This allowed the air to reach higher temperature before entering the packed bed. The top thermocouple was bypassed by the air stream, and thus this reading was unreliable for assessing the temperature data and was not used. The packed bed at each vertical position was at a lower temperature going from the inlet to exit of the vessel. This results in a more elongated thermal front as compared to the case with slow steam injection. The experimental data from an air injection case is shown in FIG. 8.

Comparing the results of the experiments using air to those using steam as the heat transfer fluid, there are some very distinct observations which can be made. Due to the low energy density of air, it requires a greater amount of time to heat the packed bed. Even in the steady state situation, the rate of heat loss through the bed walls were comparable to the rate of heat input injected in the bed. This led to temperature gradients within the bed, even after a continuous steady state was achieved, as can be seen in the time series plots of different thermocouples. The plots show that lower end thermocouples remain at lower temperatures with steady state condition. In contrast, the high energy density of steam (because of the latent heat) allows it to saturate the packed bed to top temperature more quickly, as shown in FIGS. 6 and 7.

Conclusion

The IR camera images of experimental runs show that steam condenses with cross-sectional uniformity over the packed bed of spherical particles in the directional plane normal to the steam flow, justifying the design basis. The experiments were conducted with two modes of steam injection—fast and slow mode. Thermal response of the bed was found to be distinct in both of these cases. In case of slow injection mode, the temporal behavior of the bed was found to be divided into two spatial zones, advection driven temperature rise in the bed near the steam injection point and conduction driven temperature rise for the regions far from the injection point. As the slow moving steam front reached the far zone, a steeper rise in the temperature was seen during later stages of the experiment. Fast injection mode involved high enthalpy flux penetrating and equilibrating the bed quickly, and thus only advection driven temperature rise was observed at all spatial locations. Notably, as the steam condensed with cross-sectional uniformity and the latent heat was transferred to the particulates for storage, the packed bed design was able to maintain a sufficiently steep temperature gradient along the flow direction to discourage energy losses due to thermal dispersion. These tests show that efficiency of energy storage is close to 99%. Advantageously, the system can achieve an overall efficiency of energy storage and recovery (i.e., round trip efficiency) between 95% and 98%, depending upon the type of recovery fluid and duration of storage.

As shown in the results above, different heat transfer fluids also exhibited distinct heat transfer behavior. The use a condensable fluid (steam) demonstrated much faster heat transfer to the packed bed due to the latent heat transferred during condensation of the steam. In contrast, the use of air as a heat transfer fluid exhibited much slower heat transfer, as the heat transfer in that case was only achieved using sensible heat transfer.

METHODS AND SYSTEMS FOR THERMAL ENERGY STORAGE AND RECOVERY

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