US20160084587A1

US20160084587A1 - Packed rock bed thermal energy storage facility

Info

Publication number: US20160084587A1
Application number: US14/786,717
Authority: US
Inventors: Detlev Gustav Kröger
Original assignee: Stellenbosch University
Current assignee: Stellenbosch University
Priority date: 2013-04-26
Filing date: 2014-03-18
Publication date: 2016-03-24
Also published as: PT2989405T; ES2715405T3; EP2989405A1; WO2014174384A1; MA38384A1; AP2015008684A0; ZA201506145B; MX2015014953A; CN105339751A; AU2014259137A1; EP2989405B1

Abstract

A packed rock bed thermal energy storage facility is provided in the form of a pile of unconstrained rock that is free to expand and contract with changing temperature without creating significant stress. A hot fluid inlet and outlet space is provided above the bed and a cold fluid inlet and outlet space is formed by a grid and larger rocks, or by larger rocks only, that support the pile toward a bottom of the bed. Fluid flow can occur downward through the rock bed during heating of the bed and upward through the rock bed during cooling of the bed in a heat recovery cycle. The bed is completely covered by an insulated arched roof. The rock bed may be an enclosure having a roof, walls and a floor with a lower central region and somewhat inclined side regions extending at a gentle slope towards sidewalls supporting the roof.

Description

FIELD OF THE INVENTION

This invention relates to a packed rock bed thermal storage facility for use in the storage of thermal energy that has been derived from some suitable source.

BACKGROUND TO THE INVENTION

The generation of power from sources using conventional fossil fuels is increasingly being replaced by the use of renewable energy of one or other type. As far as the present patent application is concerned, the invention is especially appropriate for use in association with concentrating solar power plants or combined cycle power plants, although it is not limited to these applications.
The use of solar energy is associated with the need for storing the solar energy collected for use at a later time so that energy is available at night time or when the sun is obscured, typically by cloud. One practical way of storing energy is in the form of heat (thermal energy) that can be used subsequently for the generation of electricity, by way of a steam generating cycle and an associated turbine and generator.
Different thermal energy storage facilities have been proposed and are currently in use, at least to some extent. These include the storage of thermal energy in molten salts or alternatively, primarily as latent heat in the case of phase change materials. Although these are successful to a greater or lesser extent, there is considerable scope for improvement, particularly the reduction of cost.
Packed beds of ceramics have been used for thermal storage at high temperatures (>500° C.), for example regenerators in glass factories and cowper stoves in iron smelters. The use of packed rock beds has, on the other hand, received considerable attention from a theoretical point of view but, as far as applicant is aware, has not been implemented to any appreciable extent at high temperatures in practice although it is understood that a large high-temperature rock storage facility is currently under construction in Morocco. The general lack of use is most likely due to the fact that existing proposals for packed rock bed storage facilities are relatively costly or impractical, or both. The proposals include the formation of a packed rock bed in excavations in the ground utilizing primarily the ground as an insulating medium.

SUMMARY OF THE INVENTION

In accordance with a first aspect of this invention there is provided a packed rock bed thermal energy storage facility comprising an essentially unconstrained pile of rock that is free to expand and contract with changing temperature without creating significant stress; and with a hot fluid inlet and outlet space above the bed and a cold fluid inlet and outlet space that is supported by a grid and larger rocks, or by larger rocks only, below the bed, between which fluid flow can occur downward during heating of the bed and upward during cooling of the bed, the bed being completely covered by an insulated arched roof.
In accordance with a second aspect of this invention there is provided a packed rock bed thermal energy storage facility comprising first and second spaces separated by a rock bed of individual rock units such that working fluid may flow between the first and second spaces by way of interstices between rock units forming the rock bed in order to provide for the transfer of heat to or from the rock bed, in use, and at least one communication duct associated with each of the first and second spaces for a working fluid to flow into or out of the first and second spaces, the energy storage facility being characterised in that the rock bed is in the form of a pile of rocks with the first space being formed between an upper surface of the pile and an enclosure spaced upwardly from the upper surface of the pile, and the second space is formed generally towards the bottom of the pile of rocks.
Further features of the invention provide for the enclosure to have a roof that is preferably thermally insulated, walls and a floor; for the floor to have a lower central region and somewhat inclined side and end regions extending upwards at a gentle slope towards low sidewalls that support the roof, or end walls; for the first enclosed space to be defined by an upper surface of the rock pile and the roof; for the second enclosed space to be defined by a surface of an inner chamber that is preferably one or more passages or tunnels defined by a part of a bottom surface of the rock pile and a part of a floor of the enclosure wherein the bottom surface of the rock pile may include a support grid and larger rocks to support the rocks of the packed bed and maintain the second space; and for one or both of the first and second spaces to have associated with it at least two or more communication ducts that may be used selectively for a thermal energy absorption process and a thermal energy usage process.
Still further features of the invention provide for the rock bed to consist of rocks selected from rock types such as (although not limited to) granite, dolerite, gneiss and hornfels; for the rocks to be generally similar in size and, preferably from 10 to 50 mm in size; for the rocks to be either rounded or crushed; and for the rock pile to have a flat top and sides that slope downwards at a natural angle of repose. The rock pile height will typically be between 1-15 m.
The angle of repose can range between 20-50°. For most crushed rock it is usually 30-40°, although it can vary depending on the particle characteristics.
The size of the facility depends entirely upon the desired capacity (energy stored and maximum thermal power output) and the available source of thermal energy to charge the bed.
The facility could cover an area as small as a few square metres, or several thousand square metres or more. When the size is such that further scaling-up increases the cost non-linearly due to the requirements of the containment structure, a number of smaller beds may be constructed instead of a single large bed. In this way, if desired, a thermal power output of thousands of megawatts may be achieved, for thermal capacities of thousands of megawatt-hours.
In order that the above and other features of the invention may be more fully understood various proposed embodiments of the invention will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic cross-sectional elevation of a first embodiment of thermal energy storage facility in accordance with this invention.

FIG. 2 is a schematic longitudinal sectional elevation of the embodiment of the invention illustrated in FIG. 1;

FIG. 3 is a schematic cross-sectional elevation of a second but larger embodiment of thermal energy storage facility according to the invention;

FIG. 4 is a schematic longitudinal sectional elevation of the embodiment of the invention illustrated in FIG. 3;

FIG. 5 is a schematic cross-sectional elevation of a third embodiment of thermal energy storage facility according to the invention;

FIG. 6 is a schematic sectional plan view taken approximately along the line VI-VI in FIG. 5;

FIG. 7 is a schematic sectional plan view similar to FIG. 6 but showing a variation in the arrangement of and,

FIG. 8 is a schematic diagram illustrating one arrangement in which a thermal energy storage facility of the invention can be employed.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

Referring now to the drawings, one embodiment of packed rock bed thermal energy storage facility (1) comprises a pile (2) of individual rock units and first and second spaces, (3) and (4) respectively, separated by the rock bed such that working fluid may flow between the first and second spaces by way of interstices between rock units forming the rock bed. In so doing, heat may be either transferred from the working fluid to the rock bed in order to store heat in the rock bed or can be transferred from the rock bed to a working fluid to recover heat stored in the rock bed by absorbing it into the working fluid, as the case may be.
The first space (3) is formed between an upper surface (5) of the pile of rocks and an arched roof (6) of an enclosure spaced upwardly from the upper surface of the pile. The entire upper surface of the pile is thus exposed to the first space. In this embodiment of the invention, the rock pile has a flat top and surrounding sides and an end that slopes downwards at a natural angle of repose of the rock units.
The roof is thermally insulated by means of a suitable insulation layer (7).
The enclosure also has low side walls (8) along each side thereof that support the arched roof. A floor has a central flat lower region (11) and side regions (12) that are inclined upwardly at a gentle slope towards the low sidewalls that support the roof, and towards end walls (13).
The packed rock bed storage facility, in this embodiment of the invention, is of elongated generally rectangular shape with an arched shape in cross-section, as shown schematically most clearly in FIG. 1. The longitudinal section of the facility is illustrated schematically in FIG. 2.
The second space (4) in this embodiment of the invention is formed generally at the bottom of the pile of rocks by an inner chamber that is in the form of a passage or tunnel defined by a part of a floor of the enclosure and a support grid (16) supporting larger rocks (17) to retain the smaller rocks of the packed rock bed and maintain the second space.
The first space (3) has associated with it two communication ducts (21, 22) that may be used selectively. In this embodiment of the invention one communication duct (21) serves as an inlet for hot air for the purpose of heating the rock bed in a thermal energy absorption process. Depending on the application, communication duct (21) or (22) serves the purpose of an outlet for heated air flowing in a reverse direction through the rock bed during a thermal energy recovery cycle.
The second space (4) has a single communication duct (25) for conducting air out of the facility once it has passed through the rock bed during a thermal energy absorption process cycle or into the facility during a thermal energy recovery cycle.
As air flow requires large cross-sectional communication ducts and thermal losses from ductwork can be significant, even if insulated, the communication ducts carrying the air are as short as possible.
Natural convection can have a significant influence on packed rock beds. The rock bed is therefore charged by introducing the hotter air at the top of the rock bed and the cooler air is removed at the bottom thereof. Similarly, cooler air is introduced at the bottom of the rock bed and heated air is removed at the top during a thermal energy recovery cycle of heat stored in the rock bed. This at least partially prevents natural convection from causing de-stratification.
It should be noted that the top surface of the rock bed is unimpeded so that it can expand and contract as it is heated and cooled. The inclined unimpeded sides of the rock bed should prevent stress on the containment structure caused by ratcheting, a process where the particles expand and contract with heating and cooling, thereby packing together more tightly and exerting a force on the container.
During heating of the packed rock bed, hot air or combustion gases enter the space above the bed via the communication duct before they flow uniformly through the bed. As the gasses flow through the bed they are cooled before they exit the storage facility via the communication duct from the second space. During recovery of the stored heat from the bed, the flow is reversed, with cold air entering the bed via the communication duct from the second space to flow uniformly through the hot packed bed and exit above the bed into the first space and to exit in this instance through the communication duct (21 or 22).
A thermal energy storage facility according to the invention may be used in many different situations. The circuit of FIG. 8 is given to illustrate one practical application of the invention. In that instance the circuit may include a compressor (26) for supplying air to a central solar receiver (29) in a heliostat field (30). The compressor is driven by a first turbine (27) driven by the output from a combustor (28) in turn fed with heated air from the central solar receiver. Gases entering the first turbine drive a first generator (31) that generates electrical energy. The exhaust gas from the turbine at a temperature of about 500° C. or more is passed through the packed bed of rock described above to elevate the temperature of the rock and store thermal energy therein.
In order to recover the stored heat from the packed bed of rock, ambient air is passed through the packed bed in the opposite direction so that it enters the second space by way of the communication duct (25), passes through the heated packed rock bed, and thence to a boiler (32) of a Rankine cycle that includes a second turbine (33) driving a second electrical generator (34). The spent steam can be passed through a condenser (35) that could be of either a dry or hybrid type and the condensate can be recycled to the boiler. Both the first and second electrical generators (31) and (34) may feed electrical energy into a grid (36).
The rock used for thermal storage should not crumble and thereby tend to block air passages in the packed bed and increase the required pumping power. It should not decompose chemically or disintegrate at the maximum storage temperature, and it must withstand thermal cycling fatigue. Igneous rocks or metamorphic rocks formed at temperatures higher than the intended storage temperature should not decompose when heated, whereas sedimentary rock might contain compounds that thermally decompose, and will be more likely to be unsuitable.
A thermal storage facility according to this invention may thus consist of a packed bed in the form of a pile of well-rounded or crushed rock in a bed operating at high temperatures (≧500° C.). Since the cost of rock material is relatively low it can be readily replaced after some years if problems should arise. For the rock to be effective (have a relatively uniform temperature distribution within each unit), the Biot number based on the effective rock diameter should preferably be Bi=hd_r/(2k_r)≦0.1-0.2, where h is the air-side heat transfer coefficient, d_ris the effective rock diameter and k_ris the thermal conductivity of the rock.
Numerous variations may be made to the embodiment of the invention described above without departing from the script hereof. Simply by way of example, FIGS. 3 and 4 illustrate an arrangement that could be used for a much larger thermal energy storage facility and in this instance the facility is arranged very much as described above except for the fact that the second space is defined by multiple parallel tunnels (41) extending along the length of the facility.
Of course the packed rock bed could be circular in plan view as in the instance of the embodiment of the invention illustrated in FIGS. 5 and 6. In that instance the second space may be defined by a number of parallel tunnels (45) that extend on the floor (46) of the rock bed in the direction of a diameter thereof. As an alternative, and is illustrated in FIG. 7, the tunnels that form the second space may extend radially from a centre of the floor (51) of the rock bed. In either event, the first space (47), as shown in FIG. 5, could be a single duct (48) extending from the top of a dome shaped roof (49).
The rock bed material of this invention is constrained such that it is free to expand and contract with changing temperatures without creating significant stress and corresponding movement that may lead to deformation of the bed and containment, or erosion and breaking of the rock. The larger rock surrounding air passages prevents clogging; it may also be possible to construct self-supporting ducts by means of larger rock.
Numerous other variations may be made especially to the construction of the facility.

Claims

1. A packed rock bed thermal energy storage facility comprising a pile of unconstrained rock that is free to expand and contract with changing temperature without creating significant stress; and with a hot fluid inlet and outlet space above the bed and a cold fluid inlet and outlet space that is supported by a grid and larger rocks, or by larger rocks only, toward a bottom of the bed, between which fluid flow can occur downward during heating of the bed and upward during cooling of the bed, the bed being completely covered by an insulated arched roof.

2. A packed rock bed thermal energy storage facility comprising first and second spaces separated by a rock bed of individual rock units such that working fluid may flow between the first and second spaces by way of interstices between rock units forming the rock bed in order to provide for the transfer of heat to or from the rock bed, in use, and at least one communication duct associated with each of the first and second spaces for a working fluid to flow into or out of the first and second spaces, wherein the rock bed is in the form of a pile of rocks with the first space being formed between an upper surface of the pile and an enclosure spaced upwardly from the upper surface of the pile, and the second space is formed generally towards the bottom of the pile of rocks.

3. A packed rock bed thermal energy storage facility as claimed in claim 2 in which the enclosure has a roof, walls and a floor.

4. A packed rock bed thermal energy storage facility as claimed in claim 3 in which the floor has a lower central region and somewhat inclined side and end regions extending upwards at a gentle slope towards low sidewalls that support the roof, or towards end walls.

5. A packed rock bed thermal energy storage facility as claimed in claim 3 in which the first enclosed space is defined by an upper surface of the rock pile and the roof.

6. A packed rock bed thermal energy storage facility as claimed in claim 3 in which the second enclosed space is defined by a surface of an inner chamber in the form of one or more passages or tunnels defined by a part of a bottom surface of the rock pile and a part of a floor of the enclosure.

7. A packed rock bed thermal energy storage facility as claimed in claim 6 in which the bottom surface of the rock pile includes a support grid and larger rocks to support the rocks of the packed bed and maintain the second space.

8. A packed rock bed thermal energy storage facility as claimed in claim 2 in which one or both of the first and second spaces have associated therewith at least two or more communication ducts that may be used selectively for a thermal energy absorption process and a thermal energy usage process.

9. A packed rock bed thermal energy storage facility as claimed in claim 2 in which the rock bed consists of rocks selected from rock types being granite, dolerite, gneiss and hornfels.

10. A packed rock bed thermal energy storage facility as claimed in claim 1 in which the rocks are of generally similar size and within the range of from 10 to 50 mm in size.

11. A packed rock bed thermal energy storage facility as claimed in claim 1 in which the rocks are either rounded or crushed.

12. A packed rock bed thermal energy storage facility as claimed in claim 1 in which the rock pile has a flat top and sides that slope downwards at a natural angle of repose.