Thermal Ratchet Stopping Shovel Wall

Abstract

The thermal ratchet stopping shovel wall solves the problem of thermal ratcheting in packed bed heat storage units. It uses protrusions from the wall containing the packed bed to stop net downward rearrangement of the bed's solid granules. The protrusions limit the downward movement of packed bed granules during one part of a thermal cycle and, in another part of the cycle, the protrusions, with movement relative to the packed bed, “shovel” fallen granules back up to near their positions one cycle earlier.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to thermal ratcheting in granular media, packed bed heat storage units.

2. Description of Related Art

Regenerative heat storage units may be used in electricity generating plants and industrial manufactories such as blast furnaces, chemical processing plants, industrial plants needing exhaust air pollution control and some other industrial plants.

Regenerative heat storage units go through thermal charging and discharging cycling of thermal media with sensible heat. Sensible heat is heat that effects changes in temperature of material, in contrast to latent heat which effects changes in material phase (for example, solid, liquid and gas). Heat is always sensible heat in the context of this patent. Thermal media is material in which heat can be stored. Thermal charging and discharging cycling goes from a heat charged state, in which a heat storage unit's thermal media is relatively hot, to a heat discharged state, in which this media is relatively cold, with periods of heat charging and discharging in between.

Definitions Granule, Granular Solid material, Granular Media:

A granule is a compact particle of solid substance with geologist diameter less than fifty millimeters. A geologist diameter of a particle or granule is the greatest straight linear distance between any two points on its outer surface. Granular solid material and granular media are synonymous and are both piled collections of granules.

When a heat storage unit's thermal media is made of a packed bed of granular solid material, heat is transferred to the granular solid material during charging through contact with relatively hotter heat transfer fluid that flows through interstitial passages between the piled granules. During discharging of such a heat storage unit, heat is transferred from the granular solid material (cooling it) through contact with relatively cooler heat transfer fluid flowing through interstitial passages between the granules.

When a heat storage unit's thermal media is made of a packed bed of granular solid material, thermal charging and discharging cycling drives vertical settling of the granules making up the packed bed of granular media. Vertical settling can occur at a packed bed's wall boundaries and this potentially leads to destructive thermal ratcheting.

Destructive thermal ratcheting of packed bed heat storage units is (a) the gradual downward rearrangement (through many thermal charging and discharging cycles) of loose granules of the bed at the bed's wall regions, that (b) causes increased thermal contraction stresses on the storage unit's walls to the point that (c) these increased stresses cause structural failure of the walls. Downward rearrangement of loose bed granules during thermal ratcheting occurs by (i) gap spaces opening up between the bed and its wall due to differences in the thermal expansions of the bed and its wall and (ii) loose bed granules settling downward into gap spaces.

SUMMARY OF THE INVENTION

Technical Problem

Destructive thermal ratcheting of granular media, packed bed heat storage units.

Solution to the Problem

A solution to the problem of thermal ratcheting in packed bed heat storage units is illustrated in FIG. 1a and FIG. 1b. The invented solution uses protrusions from the wall containing a packed bed to stop net downward rearrangement of the bed's solid granules. The protrusions limit the downward movement of packed bed granules during one part of a thermal charging and discharging cycle and, in another part of the cycle, the protrusions, with movement relative to the packed bed, shovel-like, push fallen granules back up to near their positions one cycle earlier.

The invented solution acts while thermal charging and discharging cycling in the heat storage unit drives responsive thermal cycling of the storage unit's walls by contact between the wall and the unit's granular thermal media and heat transfer fluid and by thermodynamics. This coupled cycling in conjuction with any differential thermal expansions of the walls and the granular media packed bed drives another coupled cycling process in which gaps between the walls and the packed repeatedly widen and narrow. When these gaps next to the walls are wide granules from the packed bed can fall or settle downwards. However, the invented solutions protusions limit the extent of the downward movement of granules during periods of wide gaps. Later in the wall gap cycling, when the gaps narrow, the protusions are pushed (relatively) further into the packed bed. As this happens the protrusions push granules upwards (like a shovel-face pushed into a pile of pebbles pushes pebbles upwards). Over multiple cycles of limited settling/falling downwards of granules with following shovel-like pushing upwards of granules there is little or no net vertical movement of the granules.

Advantageous Effects of the Invention

By stopping thermal ratcheting, it avoids the destruction of the containing walls of packed bed heat storage units using thermal media made of loose, granular solid material.

It allows thermal media to be made of cheap, non-self-supporting thermal media, such as beds of pebbles, in thermally cycling heat storage units.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a illustrates the thermal ratchet stopping shovel wall at one extreme of a thermal charging and discharging cycle.

FIG. 1 b illustrates the thermal ratchet stopping shovel wall at the other extreme of a thermal charging and discharging cycle.

FIG. 2 illustrates the thermal ratchet stopping shovel wall with a non-vertical wall.

FIG. 3 illustrates the thermal ratchet stopping shovel wall with a non-vertical wall.

DETAILED DESCRIPTION OF THE INVENTION

The invention works within granular media, packed bed heat storage units and the process of repeated cycles that such heat storage units are driven through.

A granular media, packed bed heat storage unit comprises at least (i) a packed bed of granular media of granules, (ii) heat transfer fluid wherein the fluid can flow in the interstitial passages between the granules and (iii) one or more walls wherein these walls contain and are in contact with both of the packed bed and the heat transfer fluid; in addition, one or more devices capable of forcing the heat transfer fluid to flow through the interstitial spaces between the granules need to be available for such a heat storage unit to be operated.

Definition of a Thermal Charging and Discharging Cycle:

A thermal charging and discharging cycle is a driven process of four steps operated in a consecutive order within a granular media, packed bed heat storage unit wherein the consecutive order of steps is (i) heat charging step, (ii) heat storing step, (iii) heat discharging step and (iv) heat discharged step. Wherein, a thermal charging and discharging cycle process may be driven by forcing flows of alternately relatively hot heat transfer fluid or alternately relatively cold heat transfer fluid through the interstitial passages between the granules of the granular media. Further wherein, a thermal charging and discharging cycle process may also be driven by either turning these forced heat transfer fluid flows off and on or by reducing or increasing any temperature differences between flowing heat transfer fluid and the granular media this fluid flows in. Further wherein, the following happens in the four steps:

• • 1. Heat Charging Step, a heat charging step raises the temperature of heat storage unit granular media by contact with hotter heat transfer fluid forced to flow through the interstitial passages between the granules of the granular media;
  • 2. Heat Storing Step, a heat storing step is a process in a time period, possibly as short as one second, wherein temperature gradients are smaller than ten degrees Kelvin per meter in directions perpendicular to heat storage unit granule surfaces in contact with heat transfer fluid and wherein this temperature gradient condition is either satisfied by turning off forcing of heat transfer fluid flows in the granular media and waiting for heat transfer fluid and granular media temperatures to approach equilibrium sufficiently closely to satisfy the temperature gradient condition or by otherwise altering the temperature of forced flowing heat transfer fluid so that any temperature differences between this heat transfer fluid and the granular media are so small that the small temperature gradient condition is satisfied;
  • 3. Heat Discharging Step, a heat discharging step lowers the temperature of heat storage unit granular media by contact with colder heat transfer fluid forced to flow through the interstitial passages between the granules of the granular media; and
  • 4. Heat Discharged Step, a heat discharged step is a process in a time period, possibly as short as one second, wherein temperature gradients are smaller than ten degrees Kelvin per meter in directions perpendicular to heat storage unit granule surfaces in contact with heat transfer fluid and wherein this temperature gradient condition is either satisfied by turning off forcing of heat transfer fluid flows in the granular media and waiting for heat transfer fluid and granular media temperatures to approach equilibrium sufficiently closely to satisfy the temperature gradient condition or by otherwise altering the temperature of forced flowing heat transfer fluid so that any temperature differences between this heat transfer fluid and the granular media are so small that the small temperature gradient condition is satisfied.

Substantial net heat exchanges can occur from the heat transfer fluid to the granular media during a heat charging step. While, during a heat discharging step substantial net heat exchanges can occur from the granular media to the heat transfer fluid. During a heat discharged step or a heat storing step there is little or no net heat exchange between the heat transfer fluid and the granular media.

Definition of Thermal Charging and Discharging Cycling:

Thermal charging and discharging cycling is a driven process wherein individual, driven thermal charging and discharging cycles are repeated one after another, such that, the heat discharged step of one cycle is followed by the heat charging step of the next cycle.

Definition of Responsive Driven Wall Thermal Cycling:

Responsive driven wall thermal cycling is a process of cycling changes in temperature of walls of a heat storage unit such that this heat storage unit is in the process of thermal charging and discharging cycling. Responsive driven wall thermal cycling occurs in response to the cycling changes in temperatures of the heat storage unit's granular media and heat transfer fluid since the walls are in contact with the granular media and heat transfer fluid and the laws of thermodynamics are obeyed.

FIG. 1 a and FIG. 1b each show vertical sections of the same boundary region 10 between a packed bed 20 of solid granules and a wall 30 containing the packed bed of a granular media, packed bed heat storage unit. The two figures, FIG. 1a and FIG. 1b, illustrate different time periods in a repeating thermally driven cycle which is here called the “wall gap cycle.” The orientation of gravity matters to the invention and this orientation defines vertical directions associated with terms such as “fall,” “up” and “lower” in the normally accepted way. Gravity's orientation is indicated for FIG. 1a and FIG. 1b by the arrow 100. FIG. 1a and FIG. 1b illustrate a detailed part of a larger heat storage unit 1 such that the right sides of the figures is the interior of the heat storage unit 1 and the packed bed 20 extends further to the right as well as up and down; likewise, the wall 30 is the containing boundary of the packed bed 20 and the wall 30 extends leftward to the outer wall surface (not shown) as well as up and down (so it contains extensions of the packed bed 20 and any other required enclosed part of the heat storage unit 1).

The wall gap cycle includes two half cycles that are referred to in this specification and following claims as “wall gap widening half cycles” and “wall gap narrowing half cycles,” each form a half of a wall gap cycle and they are alternately passed from one to the other as multiple wall gap cycles are completed. The time period illustrated by FIG. 1a is the transition time from the end of the gap narrowing half cycle to the beginning of the gap widening half cycle. Whereas, the time period illustrated by FIG. 1b is the transition time from the end of the gap widening half cycle to the beginning of the gap narrowing half cycle. During wall gap widening half cycles, relative movement between the wall 30 and the packed bed 20 widens gap spaces between the wall 30 and the packed bed 20 such that granules at the boundary of the packed bed can fall into the widened wall gap spaces. During wall gap narrowing half cycles, relative movement between the packed bed 20 and the wall 30 narrows gaps between the packed bed 20 and the wall 30. The wall gap cycle occurs due to (a) thermal charging and discharging cycling of heat storage unit granular media and heat transfer fluid, (b) responsive driven wall thermal cycling of these heat storage unit's walls, (c) differential thermal expansions of the materials making up these heat storage unit walls and the granular media packed beds contained in those walls and (d) the physical, particularly time, coupling of (a), (b) and (c).

FIG. 1 a and FIG. 1b can be used to describe two cases of embodiments of the invention that differ by relative sizes of thermal expansion coefficients. Case (i), smaller coefficient, the packed bed granules have a smaller coefficient of thermal expansion than the materials the wall is made out of. In this case FIG. 1a shows a section of a boundary region 10 at its lowest temperature within a cycle with the wall 30 most contracted into the packed bed 20; while FIG. 1b shows the same boundary region 10 at its highest temperature within a cycle with the wall 30 most expanded out from the packed bed 20. Case (ii), larger coefficient, the packed bed granules have a larger coefficient of thermal expansion than the materials the wall is made out of. In this case FIG. 1a shows a section of a boundary region 10 at its highest temperature within a cycle with the packed bed 20 most expanded into the wall 30; while FIG. 1b shows the same boundary region 10 at its lowest temperature within a cycle with the packed bed 20 contracted away from the wall 30.

Features of the invention are protrusions 40 protruding from the wall 30 into the packed bed 20. These protrusions 40 act through relative movements in periods of cycles: Going from the period in a cycle of FIG. 1a to the period of the cycle of FIG. 1b, the basic thermal ratchet mechanism occurs of gap spaces opening up between the packed bed 20 and wall 30 and packed bed granules settling down next to the wall but the extent of downward granule settling is limited by the protrusions acting (like shelves) to support granules; while going oppositely from the period of FIG. 1b to the period of FIG. 1a, the invention's thermal ratchet stopping shovel mechanism occurs, that is, the combination of relative movement of the wall 10 toward the packed bed 20 and the protrusions 40 push fallen granules back up to close to their vertical position one thermal cycle earlier.

Referring to FIG. 1a, alternately FIG. 1b, in alternate embodiments the protrusions 40 have upper sides 41 which slope upward from the protrusion tips 42 back to the main body of the wall 30. This upward sloping of the upper sides 41, in combination with relative motion of the wall 30 into the packed bed 20, helps drive granules upwards, like a shovel pushed into a pile of pebbles tends to raise up pebbles close to the shovel face.

The protrusions extend horizontally in and/or out of the plane of the section FIG. 1a, alternately FIG. 1b. Protrusions extend horizontally (in and/or out of the plane of the section FIG. 1a, alternately FIG. 1b) so that the protrusions, either individually or in groups, are capable of limiting the falls of and supporting packed bed granules and also pushing fallen granules back up with the aid of thermally driven, relative movement (of the protrusions into the packed bed). The protrusions can extend horizontally (in and/or out of the plane of the section FIG. 1a, alternately FIG. lb) either individually by at least the average diameter of the packed bed granules or by combinations of two or more protrusions aligned vertically at the same, or close to the same, vertical height and with horizontal separations close enough that individual packed bed granules can be supported by two or more of the protrusions.

In alternate embodiments of the invention, the tips 42 (in FIG. 1a, alternately FIG. 1b) of protrusions are embedded inside the packed bed 20 at all times (that is, in all periods of thermal cycles). A protrusion 40 is embedded inside a packed bed 20 if one or more packed bed granules makes contact with the upper side 41 of the protrusion and, simultaneously, one or more packed bed granules makes contact with the lower side 43 of the protrusion. A protrusion 40 is also embedded inside a packed bed 20 if one or more packed bed granules makes contact with the tip 42 of the protrusion. The preferred condition that a protrusion is always embedded inside a heat storage unit's packed bed is to minimize, or stop altogether, movement of packed bed granules vertically past (from above to below) that protrusion. A protrusion can be made to stay embedded within a heat storage unit's packed bed by sizing its horizontal extent (measured out from the main body of the wall 30 to the protrusion's tip 42 in the same plane as FIG. 1a), so that this horizontal extent will be more than the distance between the extremes of relative movements (between the wall 30 and packed bed 20 in the boundary region 10 where the protrusion is located). The size of these relative movements (or distance between the extremes of relative movements) will depend on the materials the packed bed and wall are made of, as well as the size and shape of the packed bed and wall and also the highest and lowest operating temperatures of the packed bed.

In FIG. 1a (alternately FIG. 1b) the protrusions 40 are separated by equal vertical distances, this is not a requirement of the invention. The vertical distances between pairs of vertically neighboring protrusions 40 can differ between one pair and another pair.

The main body of the wall 30 may have a vertical interface 31 (as in FIG. 1a or FIG. 1b) or non-vertical interface 31 with the protrusions 40 and packed bed 20, as in, for example, FIG. 2 or FIG. 3. Gravity's down orientation is indicated for FIG. 2 and FIG. 3 by the arrow 100. FIG. 2 illustrates a section of the boundary region 11 of a heat storage unit 2 such that the packed bed 20 is wider higher up than lower down; while FIG. 3 illustrates a section of the boundary region 12 of a heat storage unit 3 such that the packed bed 20 is wider lower down than higher up. FIG. 2 and FIG. 3 each illustrate a detailed part of larger heat storage units 2 and 3 such that the right sides of the figures is the interior of heat storage units 2 and 3 and the packed bed 20 can be extended further to the right as well as up and down; likewise, the wall 30 is the containing boundary of the packed bed 20 and the wall 30 can be extended leftward to the outer wall surface (not shown) as well as up and down (so it contains extensions of the packed bed 20 and any other required enclosed part of the heat storage units 2 and 3). In other embodiments of the invention, in other heat storage units it is allowed that the interface between its protrusions and the main body of its wall could vary between vertical and non-vertical and such variations could be continuous.

The protrusions should be made of materials that can withstand the mechanical wear on the protrusions from contact with and relative movement with the granules of the packed occurring in the temperature cycling environment of a regenerative heat storage unit. Appropriate materials to make the protrusions out of are metals and ceramics which have a Mohs hardness of 6.0 or more; examples include high to ultra-high carbon (0.6-2.0 percent carbon, by mass) steel, cast iron (2.0+ percent carbon content, by mass), tungsten and high temperature porcelains (for example, alumina porcelain, with alumina mass content above 30 percent and alumina plus silica mass content above 80 percent, and zirconia porcelain, with zirconia mass content above 30 percent and zirconia plus alumina plus silica mass content above 80 percent) and even harder ceramics such as silicon nitride, tantalum carbide, silicon carbide, tungsten carbide, titanium carbide and boron carbide.

10. The apparatus of claim 9 wherein the protrusions can be made from a material selected from the group consisting of carbon steel with a carbon content between 0.6 and 2.0 percent, cast iron with a carbon content above 2.0 percent, tungsten, alumina porcelains with alumina content above 30 percent and alumina plus silica contents above 80 percent and zirconia porcelains with zirconia content above 30 percent and zirconia plus alumina plus silica contents above 80 percent, silicon nitride, tantalum carbide, silicon carbide, tungsten carbide, titanium carbide and boron carbide.

The protrusions should be firmly attached or integral to the walls.

For preventing thermal ratcheting, it is not necessary to have protrusions from horizontal bottoms of heat storage units and, again, it is not necessary to have protrusions above the packed bed of these storage units.

INDUSTRIAL APPLICABILITY

The invention is useful to regenerative heat storage units that might be used in electricity generating plants and industrial manufactories such as blast furnaces, chemical processing plants and any other plants using regenerative heat storage. The invention applies to heat storage units whose principal thermal media are packed beds made of granular solid material.

Claims

1. A cycling process in granular media, packed bed heat storage units comprising: providing a granular media, packed bed heat storage unit including a packed bed of granular media of granules, and a heat transfer fluid, wherein this fluid can flow through the interstitial passages between the granules in the packed bed, and one or more walls, wherein these walls contain and are in contact with both of the packed bed and the heat transfer fluid, and a plurality of protrusions that protrude from the walls into the packed bed, and one or more force applying devices for forcing the heat transfer fluid to flow through the interstitial passages between the granules;driving thermal charging and discharging cycling in the heat storage unit;responsive driven wall thermal cycling in the heat storage unit;during wall gap widening half cycles, with relative movement between the walls and the packed bed, widening of gap spaces between the walls and some packed bed granules in regions close to the walls, such that some of the granules fall into the widened wall gap spaces;during wall gap widening half cycles, limiting the falls of any of the granules to regions (a) bounded by the walls and (b) bounded above by either the top of the packed bed or one of the protrusions and (c) also bounded below by either the bottom of the packed bed or another of the protrusions, but bounded by less than the complete vertical distance between the top and bottom of the packed bed; andduring wall gap narrowing half cycles, with relative movement between the packed bed and the walls and narrowing of the gaps between granules of the packed bed and the walls, the pushing upwards by the protrusions of granules, so that, over repeating wall gap cycles, there is little or no net vertical movement of the granules.

2. The process of claim 1 wherein the protrusions have sloping upper sides.

3. The process of claim 1 wherein the tips of the protrusions are embedded in the packed bed at all times.

4. The process of claim 1 wherein the walls with protruding protrusions can have vertical sections or non-vertical sections or both.

5. The process of claim 1 wherein the protrusions are made of material that can withstand the mechanical wear on the protrusions due to contact with and relative movement with the granules of the packed bed.

6. The process of claim 5 wherein the protrusions can be made from a material selected from the group consisting of carbon steel with a carbon content between 0.6 and 2.0 percent, cast iron with a carbon content above 2.0 percent, tungsten, alumina porcelains with alumina content above 30 percent and alumina plus silica contents above 80 percent and zirconia porcelains with zirconia content above 30 percent and zirconia plus alumina plus silica contents above 80 percent, silicon nitride, tantalum carbide, silicon carbide, tungsten carbide, titanium carbide and boron carbide.

7. The process of claim 1 wherein the protrusions are either firmly attached to or integral to the remainder of the walls.

8. An apparatus for preventing thermal ratcheting in granular media, packed bed heat storage units comprising of at least: a packed bed of granular media of granules; heat transfer fluid wherein this fluid can flow through the interstitial passages between the granules; one or more walls wherein these walls contain and are in contact with both of the packed bed and the heat transfer fluid, one or more force applying devices for forcing the heat transfer fluid to flow through the interstitial passages between the granules, a plurality of protrusions that protrude from the walls into the packed bed, wherein the protrusions have sloping upper sides and the tips of the protrusions are embedded in the packed bed at all times.

9. The apparatus of claim 8 wherein the protrusions are made of material that can withstand the mechanical wear on the protrusions due to contact with and relative movement with the granules of the packed bed.

10. The apparatus of claim 9 wherein the protrusions can be made from a material selected from the group consisting of carbon steel with a carbon content between 0.6 and 2.0 percent, cast iron with a carbon content above 2.0 percent, tungsten, alumina porcelains with alumina content above 30 percent and alumina plus silica contents above 80 percent and zirconia porcelains with zirconia content above 30 percent and zirconia plus alumina plus silica contents above 80 percent, silicon nitride, tantalum carbide, silicon carbide, tungsten carbide, titanium carbide and boron carbide.

11. The apparatus of claim 8 wherein the protrusions are either firmly attached to or integral to the remainder of the walls.