Thermal ratcheting mitigation system for packed bed thermal energy storage

Abstract

A stratified, packed bed thermal energy storage system having a vertically oriented tank that is separated into different zones using a plurality of vertically oriented tubes in and around which heat storage pebbles are placed. The plurality of tubes creates separated zones of heat storage pebbles that mitigate the effect of thermal ratcheting which consequently results in reduced stress on the tank's outer wall; the remaining space that does not contain heat storage pebbles will contain a liquid, which will be used as a heat transfer fluid that flows through and around the vertical tubes from top to bottom when entering the vessel hot, transferring heat to the heat storage pebbles (charging cycle); the liquid flows from bottom to top when cold, withdrawing heat from the heat storage pebbles (discharging cycle); this flow direction is important to maintain stratification and separation of thermal boundary layers due to natural convection.

Description

[0001] THERMAL RATCHETING MITIGATION SYSTEM FOR PACKED BED

[0002] THERMAL ENERGY STORAGE

[0003] FIELD OF THE INVENTION

[0004] This application pertains to thermal energy storage methods and systems.

[0005] BACKGROUND OF THE INVENTION

[0006] With the transition from fossil to sustainable energy there is a demand for sustainable energy generated in proximity to the user. In addition, due to the large amounts of intermittent sustainable energy, there is a growing demand for storage of such sustainable energy. Sustainable solutions with a bed of heat storage pebbles surrounded by a heat transfer fluid are particularly attractive due to the replacement of relatively cost driving fluids with relatively more cost-effective heat storage pebbles with higher volumetric energy densities than the fluids.

[0007] A technical challenge that arises from this storage concept is the settling of the heat storage pebbles due to the difference in expansion coefficients of the tank’s main structure and the heat storage pebbles. This process is called thermal ratcheting. Over the course of various thermal cycles, this settling effect leads to increasing stresses in the tank’s walls, ultimately leading to a serious risk of tank rupture during the tank’s lifetime. Current-day thermal energy storage studies recognize this issue.

[0008] There is a need to cost-effectively solve the problem of thermal ratcheting to make the high

[0009] SCE-103 / PCT 1 / 14 potential concept of a packed bed sensible thermal energy storage (explained later in this document in the Definitions section) a commercial success. The invention addresses the challenge of cyclic increasing stresses on the tank’s wall by the heat storage pebbles, whilst also enhancing the effectiveness of high-temperature heat utilization, which is significantly more valuable in heat transfer processes.

[0010] SUMMARY OF THE INVENTION

[0011] The thermal energy storage solution has a vertically oriented storage tank that enables thermal stratification using the effect of natural convection. The tank’s internal structure will contain an arrangement of tubes that are placed in the same vertical orientation as the tank. The arrangement of tubes contains a plurality of tubes, depending on the diameter of the tank and the size of the heat storage pebbles in the tank. In one example, up to 50 tubes could be used. In one example, a minimum of three tubes can be used. In still another example, a minimum of 10 or 15 could be used. The tubes and the space between the tubes contains a volume of heat storage pebbles. The bottom of each tube contains a perforated plate or wire mesh that allows the heat transfer fluid to travel through the bottom of the tube, hence not disturbing the thermocline, but keeping the heat storage pebbles in the tubes in the vertical direction. The arrangement of tubes is connected using one or more perforated plates. These plates will contain holes in which the vertical tubes can be placed. A rigid connection between the plate and the wall of the tubes will ensure that the tubes remain in position. The tubes will be filled and surrounded by heat storage pebbles in which heat is stored. These heat storage pebbles are added to achieve a higher total volumetric energy density than could be achieved with a tank filled solely with heat transfer fluid. The tubes lead to a reduction of stresses on the tank walls caused by thermal ratcheting by the bed of heat storage pebbles and the tank. The remaining space in the tank is (partially) filled by a fluid that adds heat

[0012] SCE-103 / PCT 2 / 14 from a heating source outside of the storage into the heat storage pebbles (charging), or withdraws heat from the heat storage pebbles to a heat demanding source outside the tank (discharging). The fluid will add heat to the storage (charging) by flowing through the tank from top to bottom, exchanging its heat with the heat storage pebbles while travelling downward. This results in a thermal front with a hot upper side and a cold bottom, separated by a transition zone; which is called a thermocline. As the hot flow enters from the top, the thermocline will move downward through the tank. On the contrary, the fluid will withdraw heat from the storage (discharging) by withdrawing heat from the heat storage pebbles, and leaving as hot fluid from the top to supply heat to a destination outside of the storage. When having transferred its heat, the colder fluid again enters the tank at the bottom. In this case, the thermocline will move upwards. The thermal energy storage solution is always integrated within a broader process that includes both a heat source and a heat demand, as later shown in FIG. 1. The charging and discharging rate of the Thermal Energy Storage is controlled by regulating the mass flow rate from and to the Thermal Energy Storage by one or more pumps, control valves and temperature sensors. These respond dynamically to match the charging or discharging speed of the Thermal Energy Storage with the heat supply or heat demand.

[0013] The energy flow is described by the following equation:

[0014] Qheat TH. ' Cp • AT

[0015] Where Qheat is the charging or discharging power in [W], m is the mass flow rate in [kg / s] of the heat transfer fluid, cpis the specific heat capacity in [J / kg / K] and AT the difference in temperature of the heat transfer fluid flowing into the inlet of the thermal energy storage and out of the outlet

[0016] SCE-103 / PCT 3 / 14 of the thermal energy storage in [°C] or [°K],

[0017] The temperature difference between the cooler fluid and the hotter fluid in the tank causes local difference in fluid densities, with the hotter fluid being less dense and the cooler fluid being denser. Gravity ensures that the less dense fluid remains above the denser fluid, resulting in a separation between the hotter and cooler regions in the tank, known as thermal stratification. Reversing the flow directions to insert the hot liquid at the top and the cold liquid at the bottom aligns with these natural convection principles. The heat storage pebbles reduce the effect of forced convection; the mixing of the cold and hot fluid caused by the flow speeds of the liquid at insertion in the tank, increasing the thermocline effect.

[0018] In one embodiment, the invention is characterized as a method of mitigating thermal ratcheting in a packed bed thermal energy storage system. In this method, one would have a vertically oriented storage tank with a tank wall and at a top of the tank wall having one or more top in / out-lets and at a bottom of the tank wall having one or more bottom in / out-lets. The vertically oriented storage tank has a tank storage volume enclosed by the tank wall, and further has a vertical tank body. A plurality of tubes is arranged in the tank storage volume while leaving space in between the plurality of tubes. The plurality of tubes is arranged vertically in line with the vertical tank body. Heat storage pebbles are arranged within each of the plurality of tubes and in the space left in between the plurality of tubes within the tank storage volume.

[0019] The embodiment then has, in a charging cycle, the step of flowing a liquid at a charging temperature ranging from 0 degrees Celsius to 600 degrees Celsius through one or more of the top inlets to the one or more bottom outlets. The liquid passes the heat storage pebbles by flowing

[0020] SCE-103 / PCT 4 / 14 through the tank storage volume filling the plurality of tubes and the space between the plurality of tubes. A charging heat transfer takes place between the liquid and the heat storage pebbles, and the liquid leaves the one or more bottom outlets at a relatively lower temperature than the charging temperature.

[0021] The embodiment then also has, in a discharging cycle, the step of flowing the liquid at a discharging temperature ranging from 0 degrees Celsius to 600 degrees Celsius through one or more of the bottom inlets to the one or more top outlets. The liquid passes the heat storage pebbles by flowing through the tank storage volume filling the plurality of tubes and the space between the plurality of tubes. A discharging heat transfer takes place between the liquid and the heat storage pebbles, and the liquid leaves the one or more top outlets at a relatively higher temperature than the discharging temperature. The charging and discharging temperature top range can also be 345, 400, 450, 485, 500 or 550 degrees Celsius.

[0022] The charging cycle and the discharging cycle is repeated in a cyclical manner resulting in a thermal heating expansion and a cooling contraction of the tank wall resulting in a settlement of the heat storage pebbles, where a vertical tank body stress is caused by the settlement of the heat storage pebbles, and where the arrangement of the plurality of tubes in line with the vertical tank body reduces settlement of the heat storage pebbles and prevents a vertical tank body stress failure.

[0023] The plurality of tubes and its arrangement reduces the radial freedom of motion of the heat storage pebbles, as they cannot cross the walls of the tubes, and consequently also reducing their likelihood of settling vertically in the radial gap created due to a difference in thermal expansion of the tank wall and the heat storage pebbles, thereby mitigating the thermal ratcheting failure

[0024] SCE-103 / PCT 5 / 14 mode.

[0025] In a variation of this embodiment, the fluid flow, for both the charging cycle and discharging cycle the direction, can be reversed resulting in the charging cycle the fluid flowing through one or more of the bottom inlets to the one or more top outlets, and in a discharging cycle, the fluid flowing through one or more of the top inlets to the one or more bottom outlets.

[0026] Embodiments of the invention are described in open-ended, comprising language, but can also be described in consisting essentially of, consisting of, or wherein the improvement comprises of language.

[0027] BRIEF DESCRIPTION OF THE DRAWINGS

[0028] References number to the drawings

[0029] 1. Tubes containing the heat storage pebbles and heat transfer fluid.

[0030] 2. Perforated plates to support the tubes and heat storage pebbles whilst allowing the heat transfer fluid to pass.

[0031] 3. Hot fluid in / out-let, which can be located anywhere at the top. The tank can contain one or more hot fluid in / out-lets. The drawings for clarity purposes only show one in / out-let.

[0032] 4. Open space partially or fully filled with liquid. Noted is that also the storage pebbles are in between the tubes.

[0033] 5. Cold liquid in / out-let, which can be located anywhere at the bottom. The tank can contain one or more cold fluid in / out-lets. The drawings for clarity purposes only show one in / out- let.

[0034] 6. Insulation of thermal energy storage tank.

[0035] SCE-103 / PCT 6 / 14 7. Tank supports to ground.

[0036] 8. Heat storage pebbles.

[0037] 9. Wall of tubes.

[0038] 10. Mesh or perforated plate at bottom, top and potentially other locations of the plurality tubes.

[0039] 11. Thermal Energy Storage tank.

[0040] 12. Hot fluid piping connected to top of tank.

[0041] 13. Heat source (charging process) or heat demand (discharging process), including pump to transfer the fluid.

[0042] 14. Colder fluid piping connected to bottom of tank.

[0043] 15. Vertical tank body.

[0044] FIG. 1 shows according to an exemplary embodiment of the invention a schematic process flow diagram of the charging and discharging cycles of the Thermal Energy Storage 11 and the heat source or demand 13.

[0045] FIG. 2 shows according to an exemplary embodiment of the invention a cross section of a vertically oriented packed bed Thermal Energy Storage (TES) containing the vertically oriented plurality of tubes and heat storage pebbles.

[0046] FIG. 3 shows according to an exemplary embodiment of the invention a side view of the inner structure containing the plurality of tubes 1 and two of the perforated plates 2 for structural integrity holding them in place removed from the TES.

[0047] FIG. 4 shows according to an exemplary embodiment of the invention a bottom view of the plurality of tubes 1, one of the perforated plates for structural integrity 2 and the mesh or perforated plates 10 at the end of the tubes.

[0048] SCE-103 / PCT 7 / 14 DETAILED DESCRIPTION

[0049] Definitions

[0050] Thermal Energy Storage (TES): a Thermal Energy Storage is a system which stores energy in the form of heat or cold for later use, enabling the energy to be stored during periods of surplus and released during periods of demand.

[0051] Sensible Heat Storage: Sensible heat storage is a method of thermal energy storage in which heat is retained by raising the temperature of a solid or liquid medium without undergoing a phase change.

[0052] - Packed Bed Thermal Energy Storage: a type of sensible heat storage in which thermal energy is stored by circulating a heat transfer fluid (such as air or thermal oil) through a bed of solid granular materials (such as rocks, pebbles, or ceramic spheres), which absorb or release heat from or to the heat transfer fluid.

[0053] - Heat storage pebbles: the granular solid material that constitutes the packed-bed and serves as solid sensible heat storage medium. It can be pebbles, spheres, irregular and / or regular granules, or other solid forms of varying sizes.

[0054] Thermal Ratcheting: a mechanical degradation phenomenon that can occur in packed bed thermal energy storage systems. It is the progressive and irreversible movement (settling) of heat storage pebbles in a packed bed due to repeated thermal expansion (heating) and contraction (cooling) cycles and a difference in thermal expansion coefficients of the tank material and the heat storage pebbles, leading to mechanical stress and potential structural damage on the tank walls over time.

[0055] - Natural Convection: Natural Convection is the movement of fluid driven by buoyancy forces that arise from temperature-induced density differences within the fluid, without

[0056] SCE-103 / PCT 8 / 14 external mechanical input.

[0057] - Forced Convection: Forced convection is the transport of heat within a fluid that is actively moved by an external source such as a pump or fan, enhancing heat transfer rates.

[0058] Thermal Expansion: Thermal Expansion is the increase in volume or linear dimension of a material as its temperature rises.

[0059] Thermal Expansion Coefficient: The thermal expansion coefficient is a material-specific parameter that quantifies the rate at which a material's dimensions change in response to temperature variations, typically expressed as the fractional change in length or volume per degree of temperature change.

[0060] Stratification: Stratification is the formation of distinct thermal or density layers within a fluid medium, typically caused by temperature gradients that inhibit mixing and result in stable separation of fluid zones.

[0061] Stress: unless stated otherwise, the term 'stress' as used in this document refers to mechanical stress within a material, resulting from thermal and / or structural loading.

[0062] Volumetric Energy Density: Volumetric energy density is the amount of energy stored or delivered per unit volume of a material or system, typically expressed in joules per cubic meter (J / m3).

[0063] FIG. 1 shows a schematic process flow diagram of the charging and discharging cycles of the Thermal Energy Storage system. In the charging cycle the heat transfer fluid flows from the heat source 13 through the hot fluid piping 12 to the top of the Thermal Energy Storage tank 11 where it transfers the heat to the heat storage pebbles and leaves at a lower temperature compared to the inlet temperature at the bottom of the tank. From there, it flows through the colder fluid piping 14 to the heat source 13 again where the cycle is repeated. In the discharging cycle this flow is

[0064] SCE-103 / PCT 9 / 14 reversed, where the colder fluid flows from the heat demand 13 through the colder fluid piping 14 to the bottom of the Thermal Energy Storage tank 11. Here it extracts the heat from the heat storage pebbles and leaves the Thermal Energy Storage tank 11 at the top at a relatively higher temperature than the inlet temperature, through the hot fluid piping 12 to the heat demand 13. There it transfers the heat to the demand, and then enters the colder fluid piping 14 to repeat the cycle.

[0065] FIG. 2 shows a cross section of a packed bed Thermal Energy Storage (TES) containing the plurality of tubes 1 and the heat storage pebbles 8 which store the thermal energy. The top and bottom in / out-lets 3 and 5 can be located at any arbitrary position at the top or bottom of the tank and can be either one or multiple. The spaces 4 between the tubes and at the top and bottom of the tank are fully or partially filled with the fluid. At the top space 4, some space might be left vacant for the thermal expansion of the fluid. The insulation 6 around the tank reduces the heat losses of the Thermal Energy Storage to the ambient. The tank supports 7 makes sure the tank stays mounted in place during operation. The perforated plates 10 provide structural integrity to the plurality of tubes 1 in the TES.

[0066] FIG. 3 shows an elevated front view of the inner structure containing the plurality of tubes 1 and the perforated plates 2 for structural integrity holding them in place removed from the TES. There can be two or more perforated plates for structural integrity 2, depending on the height of the TES. The plurality of tubes can have two or multiple tubes (in one example 50), depending on the diameter of the TES and the heat storage pebbles’ size.

[0067] FIG. 4 shows a bottom view of the plurality of tubes 1. The bottom perforated plate for structural

[0068] SCE-103 / PCT 10 / 14 integrity 2 keeps the plurality of tubes in place. The mesh or perforated plates 10 at the end of the tubes make sure the heat storage pebbles remain in the tubes and in the space around the tubes when the liquid flows through.

[0069] SCE-103 / PCT 11 / 14

Claims

CLAIMSWhat is claimed is:

1. A method of mitigating thermal ratcheting in a packed bed thermal energy storage system, comprising:(a) having a vertically oriented storage tank with a tank wall and at a top of the tank wall having one or more top in / out-lets and at a bottom of the tank wall having one or more bottom in / out-lets, wherein the vertically oriented storage tank has a tank storage volume enclosed by the tank wall, and wherein the vertically oriented storage tank has a vertical tank body;(b) having an arrangement of a plurality of tubes in the tank storage volume while leaving space in between the plurality of tubes, wherein the plurality of tubes is arranged vertically in line with the vertical tank body;(c) having heat storage pebbles within each of the plurality of tubes and in the space left in between the plurality of tubes within the tank storage volume;(d) in a charging cycle, flowing a liquid at a charging temperature ranging from 0 degrees Celsius to 600 degrees Celsius through one or more of the top inlets to the one or more bottom outlets, wherein the liquid passes the heat storage pebbles by flowing through the tank storage volume filling the space in the plurality of tubes and the space between the plurality of tubes which is not occupied by the heat storage pebbles, wherein a charging heat transfer takes place between the liquid and the heat storage pebbles, and wherein the liquid leaves the one or more bottom outlets at a relatively lower temperature than the charging temperature;(e) in a discharging cycle, flowing the liquid at a discharging temperature ranging from 0 degrees Celsius to 600 degrees Celsius through one or more of the bottom inlets to theSCE-103 / PCT 12 / 14one or more top outlets, wherein the liquid passes the heat storage pebbles by flowing through the tank storage volume filling the space in the plurality of tubes and the space between the plurality of tubes which is not occupied by the heat storage pebbles, wherein a discharging heat transfer takes place between the liquid and the heat storage pebbles, and wherein the liquid leaves the one or more top outlets at a relatively higher temperature than the discharging temperature; and(f) repeating in a cyclical manner the charging cycle and the discharging cycle resulting in a thermal heating expansion and a cooling contraction of the tank wall resulting in a settlement of the heat storage pebbles, wherein a vertical tank body stress is caused by the settlement of the heat storage pebbles, and wherein the arrangement of the plurality of tubes in line with the vertical tank body reduces settlement of the heat storage pebbles and thereby prevents a vertical tank body stress failure.

2. The method as set forth in claim 1, wherein the fluid flow, for both the charging cycle and discharging cycle the direction, is reversed resulting in the charging cycle the fluid flowing through one or more of the bottom inlets to the one or more top outlets, and in a discharging cycle, the fluid flowing through one or more of the top inlets to the one or more bottom outlets.SCE-103 / PCT 13 / 14

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