High temperature thermal energy storage system
High temperature thermal energy storage system comprised of solid-liquid phase change material that is fully encapsulated in helically grooved, sealed, thin-wall, metal tubes. Multitude of horizontally oriented, pressure-rated steam pipes or vessels filled with PCM tube capsules form a battery of thermal storage system. The steam pipes in the battery are connected through a common liquid-side distribution header system and a common steam-side distribution header system. Multitude of steam pipes stacked up to fill a block-shaped container space. This insulated enclosure provides the basis for simple fabrication, transport and filed-erection and forms the basic building block for a modular thermal storage system.
The present application generally relates to thermal storage systems and specifically to high temperature energy storage systems typically used in concentrated solar thermal plants.
BACKGROUND OF THE INVENTIONHigh temperature energy storage systems are particularly important for solar thermal applications because they provide continuity and stability of operation during daily solar cycles. Portion of the solar energy collected during the sunlit hours is stored at high temperatures and used during night of cloudy hours. Typical operating temperatures of such systems are between 200 to 1000 degrees Fahrenheit. A large variety of materials are used for thermal storage. Based on whether the materials change their physical state during the charge and discharge cycle, there are single-phase or phase-change processes. Most current technologies use single phase liquid or solid materials. Various salt mixtures in their molten state are applied for liquid storage, while solid state materials—such as high purity graphite and concrete—are not fully developed yet.
The most often used energy storage material in solar power systems is molten salt. This is a mixture of 60 percent sodium nitrate and 40 percent potassium-nitrate, commonly called saltpeter. It is used in liquid (single) phase to store thermal energy by increasing its temperature. It is utilized because it is possible to circulate it as a fluid, it is an efficient medium to store thermal energy and its operating temperatures are compatible with requirements of the currently applied high-pressure and high-temperature steam turbines. It is non-flammable, non-toxic and is commonly used in the chemical and metals industries as a heat-transfer fluid. There are two typical configurations of single-phase molten salt storage systems: In a single-storage tank system, stratification is used to separate the colder bottom from the hot upper layers. In two-tank systems the hot and “cold” liquids are kept in separate tanks.
Beside their benefits, there are considerable disadvantages of single-phase liquid-salt systems: Their melting temperatures are high (400-550° F.); therefore, to avoid freeze-up of critical components (piping, heat exchangers, pumps, valves etc) the complete system requires backup heating and constant circulation through most of the life cycle of the system. The liquid salt requires decoupling of typically three closed loops: primary heat transfer fluid (solar collection media) loop, heat storage media loop and power cycle working fluid loop. The decoupling of these high-temperature loops results in increased complexity and cost of fluid-handling as well as loss of efficiency caused by multiple stages of heat exchange. Another source of thermodynamic inefficiency results from the sliding temperature (or sensible heat) nature of the energy storage: The lowest temperature of the storage medium drives the power generation cycle efficiency.
Phase-change materials (PCMs) provide an alternative for thermal energy storage. PCMs have the potential of providing a more efficient means of energy storage as the temperature change is minimal or zero due to the latent heat of the phase change process. PCM systems typically operate near the solid-liquid equilibrium utilizing the latent heat of fusion (or melting). For high temperature storage required in concentrated solar applications, mostly inorganic salt mixtures are used because their melting temperature is in the desired range and because their relatively high phase-change enthalpy.
Single phase solid thermal storage—mostly concrete and purified graphite—systems are largely in a research and development phase. Their disadvantage is the difficult heat transfer between the storage media and the fluid of the primary loop as well as the working fluid.
Based on configuration and working principle, there are four main system arrangements: Single tank, Dual tank, Encapsulated and Solid state systems. Both the single and dual tank systems apply molten liquid storage materials. The single storage tank relies on stratification to separate the hot from the “cold” liquid layers in the tank. The tank is always full—the liquid level is constant. The hot liquid is charged and discharged from the top portion of the tank while the relatively colder liquid is pumped in and out from the bottom of the tank. In a dual tank system the liquid is pumped from the “cold” to the “hot” tank during the charging cycle and the direction of the flow is reversed during the discharge cycle. The tank levels vary during the cycle. The encapsulated systems use small pockets of thermal storage materials encapsulated and sealed-up in shells for permanent containment. The capsules are typically submerged in a heat transfer fluid. The solid state systems—as described above—have internal hollow or porous structures to facilitate the heat transfer between the thermal fluid and the solid storage material.
The currently known high temperature thermal storage systems have the following main challenges and disadvantages: The liquid, sensitive-heat systems are inherently complex and expensive because of the specialty equipment and materials required for fluid handling, flow-control and heat exchange. The encapsulated systems have a challenge caused by poor heat transfer from the primary heat transfer fluid to the thermal storage material encapsulated typically in spherical shells. The encapsulation process is also expensive as the capsule design needs to withstand high thermal expansions, high pressure and temperature conditions and at the same time provide high thermal conductivity. The low-cost, solid-state storage materials are typically poor thermal conductors and therefore they require high percentage of porous, hollow-space per active storage volume to improve the heat transfer rate. The high porosity results in low average density of the thermal storage system, thus lowering its efficiency, increasing size and cost.
SUMMARY OF THE INVENTIONThe present application thus describes one embodiment of the invention that may take the form of helically grooved, thin-wall, tubular encapsulation of phase change material (PCM). A permanently sealed, flexible, thin wall tube may be the containment shell of the encapsulated PCM. A helical grooving or “rifling” rolled into the thin wall of the tube may provide the radial and lateral-axial flexibility of the capsule to accommodate the thermal expansion and contraction during the charging and discharging process. The length of the tube may be significantly larger than its diameter. The tube may be completely filled with molten liquid PCM and sealed off at manufacturing. The thermal contraction induced torsion of the helically rifled tube will result in a fractured, graveled solidification of the PCM.
The present application further describes one embodiment of the invention that may take the form of pressure rated steam pipe or vessel that may be filled with PCM tube capsules. Three or more tubes may form a bundle to closely fit into the larger steam pipe. The tube-capsule bundle may fill the entire length of the steam pipe. The steam pipe may have one or multitude of liquid water ports and one or multitude of steam ports. The liquid port may be used for feedwater inlet or condensate drain outlet. The steam port may be used for steam inlet or outlet. Multitude of horizontally oriented steam pipes (filled with PCM capsule tubes) may be stacked on each other to fill the thermal storage container space. The liquid water ports may be connected to a common water header and the steam ports may be connected to a common steam header. The container housing the steam pipes and encapsulated PCM, is insulated to minimize heat losses. To increase the thermal storage capacity and the thermal conductivity, the space between the steam pipes may be filled with thermal oil during the commissioning of the installation.
The present application further describes the flow of steam in a charging and discharging operation mode. In charging mode the steam flows from the heat source—that may be a solar thermal steam generator—to the multitude of steam pipes (PCM container vessels) of the storage container. The steam flows through the space left between the tubular capsules, heating and melting the PCM in the tubes and thereby storing the thermal energy. As the latent heat of the steam transfers into the capsules, the condensed steam drains out of the steam vessels through the liquid port. In discharging operation mode, the feedwater is pumped through the liquid ports of the steam pipes and flows through the hollow space between the capsules. The hot molten PCM inside the tube-capsules, heats the feedwater and the evaporated steam leaves the steam pipes at the steam ports.
These and other features of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims.
Referring now to the drawings, in which like numerals indicate like elements throughout the several views,
Claims
1. A helically or axially grooved metal tube capsule wherein the ratio of the length to the diameter of the tube is from 6 to 300 and wherein the capsule may be formed from a thin wall tube and wherein the helical grooving along the length of the tubes may be formed by rolling or other metal-forming procedure and wherein the grooved tubes may resemble the rifled barrel of guns and wherein the profile of the groove is curved to facilitate radial and axial flexibility of the tube-capsule and wherein the number of the grooves on the tube may be from 2 to 12 and wherein the ratio of the depth of the groove to the diameter of the tube may be from 0.02 to 0.4 and wherein the angle between the tangent of the helix of the grooves and the axis of the tube may be from 0 to 30 degrees.
2. The helically or axially grooved metal tube capsule of claim 1 wherein the inside of the tube has heat transfer fins to promote the heat transfer in radial direction and wherein the fins extend in radial direction from circumference toward the center of the tube and extend in axial direction along the length of the tube and wherein the fins may be integral part of the wall manufactured by extrusion or similar process along with the wall of the tube or the fins may be inserted separately in the tube and pressed against the inside wall during the plastic deformation process of grooving.
3. The helically or axially grooved metal tube capsule of claim 1 wherein the tube is filled with a molten phase change material and hermetically sealed to complete the encapsulation and wherein the phase change material may or may not have eutectic properties and it may be a mixture of two or more molten-salt compounds such as Sodium Nitrate, Sodium Nitrite, Sodium Chlorate, Potassium Nitrate, Sodium Hydroxide, Potassium Bi-fluoride, Sodium Peroxide, Calcium Nitrate, etc.
4. The bundle of sealed PCM tube capsules of claim 1 filling a pressure rated steam-pipe or vessel wherein the free space between the grooved tube capsules is available for steam and condensate flow and wherein steam pipe or vessel has one or multiple steam ports and one or multitude of condensate/feedwater ports.
5. The multitude of pressure rated steam-pipes or vessels of claim 3 connected to form a battery wherein the steam pipes of the battery are connected through a common liquid-side distribution header system and a common steam-side distribution header system.
5. The battery of steam-pipes or vessels of claim 4 stacked up to fill a block-shaped container space wherein the battery is housed in a thermally insulated enclosure designed for structural integrity and wherein the battery enclosed in the housing forms one module of the high temperature thermal storage system.
Type: Application
Filed: Sep 3, 2010
Publication Date: Mar 8, 2012
Inventor: Peter Feher (Suwanee, GA)
Application Number: 12/807,320
International Classification: F28F 1/40 (20060101);