WO2019211786A1

WO2019211786A1 - Thermal energy storage facility

Info

Publication number: WO2019211786A1
Application number: PCT/IB2019/053591
Authority: WO
Inventors: Theodor Willem VON BACKSTRöM
Original assignee: Stellenbosch University
Current assignee: Stellenbosch University
Priority date: 2018-05-02
Filing date: 2019-05-02
Publication date: 2019-11-07
Anticipated expiration: 2020-11-02
Also published as: ZA202006649B

Abstract

There is provided a thermal energy storage facility comprising a packed bed formed by a pile of elements. The packed bed includes sides that slope from a top of the pile to a bottom of the pile at their natural angle of repose. A duct is provided and has a heat exchange end in fluid communication with the packed bed at a heat exchange zone and an opposite fluid supply end. The duct enables a working fluid at elevated temperature to be introduced into the packed bed during a charge cycle. The duct further enables the working fluid to be conveyed through a charged packed bed during a discharge cycle. A barrier extends across at least a major portion of the sloping sides of the packed bed to inhibit the movement of the working fluid therethrough.

Description

THERMAL ENERGY STORAGE FACILITY

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from South African provisional patent application number 2018/02861 filed on 2 May 2018, which is incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to thermal energy storage. More particularly, the invention relates to a thermal energy storage facility wherein energy absorbing material is provided by a packed bed.

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 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 cover. 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 as regards reduction of cost.

Packed rock beds are also used for thermal energy storage at high temperatures. In one such application disclosed in our PCT publication No. WO2014/174384, a pile of rocks is housed in a substantially sealed enclosure and used to absorb thermal energy from a heat transfer fluid in order to discharge the thermal energy at a later stage. The heat transfer fluid typically enters the sealed enclosure though a suitable inlet and heats the pile of rocks contained in the enclosure from the outside thereof during a charge cycle. During a discharge cycle, the cooling of the rock bed results in heat transfer fluid passing through the rock bed and exiting by way of the outside of the rock bed and then through the enclosure to an outlet therefrom. This system suffers from relatively high cost of construction of the sealed enclosure as well as damage that may result to the container through a phenomenon known as the ratcheting effect.

In another application disclosed in our PCT publication No. WO2015/173721 , a packed bed is provided with an essentially unconstrained outer region and a duct having a heat exchange end region located within a central lower region of the packed bed and a fluid supply end opposite the heat exchange end region. One drawback to this design is that buoyancy causes the hot air in the packed rock bed to rise and dissipate into the atmosphere over time, resulting in loss of heat in the packed rock bed through convection. This is particularly problematic if thermal energy is to be stored over periods in which the rock bed is not being charged or discharged. The applicant has found that a thermocline, which is the region between hot and cold areas in the packed bed where there is a large thermal gradient, is more stable within a rock bed if the hot region is vertically above the cold region. Constructing a packed bed with a duct extending through it may also involve complexities that should preferably be avoided, such as how to pour or stack the rock of the packed bed against or around the duct without damaging it and while ensuring the duct extends substantially centrally.

The present invention aims to address these and other problems, at least to some extent.

The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.

In this specification that follows, the term "packed bed" will be used to include not only elements made of naturally occurring rock, but also elements made of ceramic material, concrete, mining or industrial by-products and any other elements having appropriate heat capacity and other thermal properties.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a thermal energy storage facility comprising a packed bed formed by a pile of elements, the packed bed having sides that slope from a top of the pile to a bottom of the pile at their natural angle of repose, and a duct having a heat exchange end in fluid communication with the packed bed at a heat exchange zone and an opposite fluid supply end, the duct enabling a working fluid at elevated temperature to be introduced into the packed bed during a charge cycle and enabling the working fluid to be conveyed through a charged packed bed during a discharge cycle, wherein a barrier extends across at least a major portion of the sloping sides of the packed bed to inhibit the movement of the working fluid therethrough.

A further feature provides for the barrier to be a thermal insulating material that reduces heat loss to the environment through the sloping sides of the packed bed.

Still further features provide for the barrier to be substantially fluid impermeable; and for the barrier to act as a seal to substantially prevent fluid ingress through the sloping sides of the packed bed.

A yet further feature provides for the pile of elements to be a pile of rocks.

A further feature provides for the insulating material to include rock wool insulation, refractory material such as refractory bricks or other types of bricks, or any type of heat resistant insulating material.

Still further features provide for the packed bed to be covered by the barrier except at a top of the pile of elements where the heat exchange end is located to enable the heated fluid to be introduced to, and extracted from, the packed bed; for a vent to be provided near the bottom of the pile for colder fluid to exit the packed bed during the charge cycle and to enter the packed bed during the discharge cycle; and for the vent to be provided at or around at least a portion of the bottom of the pile. A valve that can be opened and closed may be situated in the duct near the heat exchange end, so as to restrict movement of working fluid through the duct when the thermal energy storage facility is idle and the packed bed is neither in a charge cycle nor a discharge cycle.

Yet further features provide for the heat exchange end of the duct to be in fluid communication with a top of the packed bed such that the packed bed is heated from top to bottom by the working fluid during the charge cycle; for the barrier to extend to the top of the pile and substantially seal around the duct; for the heat exchange end of the duct to include an open-ended pipe with a number of associated slots in a wall of the pipe adjacent its open end; for the open end of the pipe to be supported directly on the top of the pile; and for the top of the pile to be flattened to accommodate the open end of the pipe. Further features provide for the thermal energy storage facility to include a separate roof or sheet of material spaced from the sloping sides so as to form an outer wall of a flow passage between the roof or sheet of material and the sloping sides.

Still further features provide for the flow passage to be a cold fluid flow passage that extends between the vent and the atmosphere; for the bottom of the pile to have a portion at the vent where the insulating material is absent, such that air can exit the packed bed during the charge cycle, move through the passage and exit to the atmosphere through the vent; for the flow passage to enable air to be drawn or pushed in from the atmosphere through the vent to move through the passage and enter the packed bed through the vent during the discharge cycle; for a fan or pump to be provided in association with the vent to the atmosphere so as to blow atmospheric air into the flow passage and thus into the packed bed during the discharge cycle.

Yet further features provide for the pile to be shaped as a frustum or as an elongate mound with a frustum-shape in cross section in which case several ducts may be provided in horizontal spaced configuration; alternatively, for the pile to have a tetrahedron-like shape.

Further features provide for the sheet of material to be a tarpaulin anchored by cables at the bottom of the pile to the ground and held in place by a structure resting on the top of the pile; and for the structure to be a frame structure which resembles the shape of the pile.

A still further feature provides for the packed bed to rest directly on the ground.

Yet further features provide for the working fluid to be air; and for the elevated temperature of the air to be in excess of about 500 °C, and more preferably in excess of 600 °C.

Further features provide for the pile of elements to be packed in a generally undisturbed pile with the majority of the packed bed being essentially unconstrained; for the angle of repose to be in a range of between 25° and 65°, preferably between 34° and 42°; for the elements to be natural rocks selected from granite, gneiss or dolerite; for the elements to have sizes that are typically from 10mm to 70mm, and more typically from 30 to 50mm in diameter.

A still further feature provides for lower edges of the packed bed to be constrained by a wall.

The invention extends to a thermal energy storage system comprising a packed bed formed by a pile of elements, the packed bed having sides that slope from a top of the pile to a bottom of the pile at their natural angle of repose, and a duct having a heat exchange end in fluid communication with the packed bed at a heat exchange zone and an opposite fluid supply end, the duct enabling a working fluid at elevated temperature to be introduced into the packed bed during a charge cycle and enabling the working fluid to be conveyed through a charged packed bed during a discharge cycle, wherein a barrier extends across at least a major portion of the sloping sides of the packed bed to inhibit the movement of the working fluid therethrough, the system further including an energy source that heats the working fluid to the elevated temperature and a working fluid moving apparatus for moving the working fluid through the packed bed.

Further features provide for the energy source to be selected from a group including but not limited to: solar energy, combustible fuels including fossil fuels and biofuels, wind energy, geothermal energy etc.

Still further features provide for the fluid moving apparatus to be a pump, a fan, a compressor, a turbine, a reversible pump turbine, or the like.

The invention further extends to a method of constructing a thermal energy storage facility, the method comprising: providing a bottom surface for supporting a pile of elements; layering elements on the bottom surface to form a packed bed having sides that slope from a top of the pile to a bottom of the pile at their natural angle of repose; providing a duct having a heat exchange end in fluid communication with the packed bed at a heat exchange zone and an opposite fluid supply end; enabling the duct to introduce a working fluid at elevated temperature to the packed bed during a charge cycle; enabling the working fluid to be conveyed through a charged packed bed during a discharge cycle; and providing a barrier which extends across at least a major portion of the sloping sides of the packed bed to inhibit the movement of the working fluid therethrough.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Figure 1 is a front sectional view of a thermal energy storage facility including a packed bed formed by a pile of elements and illustrates a charge cycle of the facility; Figure 2 is a front sectional view similar to Figure 1 , however showing a discharge cycle of the facility;

Figure 3 is a sectional view taken along line Ill-Ill in Figure 1 and illustrates a duct in more detail;

Figure 4 is a sectional view of another embodiment of the energy storage facility wherein a roof is omitted;

Figure 5 is a diagrammatic representation of a plurality of thermocouples arranged in the facility for a test run thereof;

Figure 6 is a graph illustrating various positions of the thermocouples of Figure 5;

Figure 7 is a graph illustrating temperature distribution in the packed bed after a charge cycle of the test run of the facility;

Figure 8 is a graph illustrating temperature distribution in the packed bed after a discharge cycle of the test run of the facility;

Figure 9 is a diagrammatic representation showing a three-dimensional view of another embodiment of the facility wherein the packed bed has the general shape of an elongated mound with a cross-sectional shape of a frustum;

Figure 10 is a sectional view taken along line X-X in Figure 9 and illustrates a plurality of ducts used in this embodiment;

Figure 1 1 is a high-level block diagram of an exemplary thermal energy storage system which includes the packed bed;

Figure 12 is an exemplary block diagram illustrating a method of constructing a thermal energy storage facility; and

Figure 13 is an exemplary diagram illustrating an analytical model of the facility; and

Figure 14 is a graph representing experimental test results as compared to the analytical model of the facility. DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

Embodiments described herein provide an energy storage facility, system and method for storing thermal energy for later use. A heap of elements may be piled, packed or stacked on top of one another to form a bed or packed bed. The elements may for example be a rock bed of naturally occurring rock, but may also be elements made of ceramic material, metals, concrete, mining or industrial by-products and any other elements having appropriate heat capacity and other thermal properties. The packed bed may be insulated by insulative material and may have the general shape of a frustum, a pyramid, conical, dome shaped or an elongate shape such as an elongate frustum. A heat transfer fluid such as air is pumped or sucked into the packed bed during a charge cycle at an elevated or high temperature, to heat up the elements in the packed bed, to store thermal energy inside the elements. The heated fluid may generally be introduced at an upper or top region of the packed bed. After the fluid has passed through the packed bed and transferred heat to the elements, it exits the packed bed at a lower region of the packed bed. During a discharge cycle the inverse occurs, and non-heated or fluid such as ambient air is pumped or sucked through the elements. Stored heat in the elements is then transferred to the fluid and exits the packed bed at or near the upper region thereof, for later use.

An example embodiment of a thermal energy storage facility (10) is shown in Figures 1 and 2. The thermal energy storage facility (10) includes a packed bed (12), in this embodiment a rock bed. The packed bed (12) may include a pile of elements which may be naturally occurring rocks for example selected from granite, gneiss or dolerite. The elements may be of approximately 10 mm to 70 mm in diameter and more typically from 30 mm to 50 mm in size or diameter. The elements or rocks may be able to withstand thermal cycling to high temperatures, for example about 700 °C. The packed bed (12) may have sides (18) that slope from a top (16) of the pile to a bottom (14) of the pile at their natural angle of repose (b). In other words, the elements or rocks may be piled on the ground (14) and they may come to rest at their natural angle of repose. Therefore, the packed bed (12) may rest directly on the ground (14). The natural angle of repose may be the steepest angle at which the rock surface (18) is stable. The angle of repose (b) may range between 20° and 65° measured from the ground (14). For most crushed rock the angle of repose (b) may be between 30° and 40° or between 34° and 42°, although the angle of repose (b) may vary depending on the shape of the elements or rocks. The pile of elements (12) may be packed in a generally undisturbed pile with the majority of the packed bed (12) being unconstrained. The top (16) may be at least partially flattened so that the packed bed (12) may have the general cross-sectional shape of a frustum. The thermal energy storage facility (10) may further include a duct (34) having a heat exchange end (32) in fluid communication with the packed bed (12) at a heat exchange zone and an opposite fluid supply end (33) which is shown in more detail in the sectional view in Figure 3. It will be appreciated that the heat exchange zone may be anywhere in the packed bed (12). The duct (34) may enable a working fluid at elevated temperature to be introduced into the packed bed (12) during a charge cycle (15) (shown in Figure 1). A valve (not shown) that can be opened and closed may be situated in the duct near the heat exchange end, so as to restrict movement of working fluid through the duct when the thermal energy storage facility is idle and the packed bed is neither in a charge cycle nor a discharge cycle. The valve may be controlled by an automatic actuator which opens the valve when the charge cycle or discharge cycle begins.

During the charge cycle (15), the working fluid may heat the elements in the packed bed (12) and transfer heat energy to the elements so that the heat energy is stored for later use. Forced convection may be used. The working fluid may be any suitable heat transfer fluid, for example air. The bottom (14) of the packed bed (12) may be constrained by a wall (20), which may be supported on both sides by sand or fine rocks. Embodiments are possible wherein the packed bed (12) is below ground level, but for exemplary purposes the bottom of the pile of elements is shown at ground level. The ground surface (14) may be flattened to support the pile of elements (12). One or more lower edges (24) of the packed bed may abut with, or be constrained by the wall (20). Embodiments may be possible wherein the wall (20) is not needed.

The heat exchange end (32) of the duct (34) is therefore in fluid communication with the top (16) of the packed bed (12) such that the packed bed may, during the charge cycle (15), be heated from the top (16) towards the bottom (14) by the working fluid. The barrier (22) may extend towards the top (16) of the pile and may substantially form a seal around the duct (34). The heat exchange end (32) of the duct (34) may include an open-ended pipe with a number of associated slots (37) in a wall of the pipe adjacent its open end. The open end of the pipe may be supported directly on the top (16) of the pile. The top (16) of the pile may be flattened to accommodate the open end of the pipe.

In the present embodiment, the wall (20) may be a sheet metal wall. At least a part of the side surface (18) of the packed bed (12) may be covered by the barrier (22). The barrier may be a layer of thermal insulation which may extend over at least part of the side surface (18) from the top (16) towards the bottom (14). The barrier (22) may reduce heat loss to the environment through the sloping sides (18) of the packed bed (12) and/or through the top (16). At or near a bottom edge (24) of the pile of elements (12) a gap, opening or vent (25) may be provided where the insulation (18) may be omitted, where the working fluid can flow through. In the present embodiment, the working fluid may be air. The thermal insulation (22) may reduce heat loss by convection, conduction, and radiation to the environment, and may also acts as a seal to inhibit fluid leakages through or along the side surface (18) of the packed bed (12). A frame structure may be provided for supporting the insulation or barrier (22). The frame structure for supporting the barrier (22) may for example be a rigid grid such as a stainless-steel grid. The barrier may for example be fastened to the steel grid. The barrier (22) may be substantially fluid impermeable and may act as a seal to substantially prevent fluid ingress for example through the sloping sides (18) of the packed bed (12), or through the top (16). The insulating material of the barrier (22) may include rock wool insulation, refractory material such as refractory bricks, or any type of heat resistant insulating material.

The thermal energy storage facility (10) may further include a separate roof (26), cover, or sheet of material spaced from the sloping sides (18) so as to form an outer wall of a flow passage (28) between the roof (26) or sheet of material and the sloping sides (18) of the packed bed (12). In the present embodiment, the roof (26) may be a tent-like roof of tarpaulin or similar waterproof sheeting, or a rigid, free standing roof. The roof (26) may be configured to inhibit the ingress of moisture, dust or other foreign matter into the packed bed (12), and to keep the thermal insulation (22) substantially dry and dust free. The roof (26) may form an outer wall of the flow passage (28) which may extend from the bottom edge (24) of the packed bed (12) along the outside of the thermal insulation (22). The flow passage (28) may be in fluid communication with the packed bed (12) and with a cold fluid opening (30). Because the working fluid is cooled by transferring heat to the pile of elements (12) during the charge cycle (15), the working fluid that exits near the bottom edge (24) may be relatively cooler. This relatively cooler working fluid that may be conveyed through the flow passage (28) may enable the roof (26) to be made of a material that is not particularly heat resistant, such as a tarpaulin cover. The sheet of material or tarpaulin may be anchored by cables at the bottom (14) of the pile to the ground and held in place by a structure resting on the top of the pile. The structure may be a frame structure which resembles the shape of the pile (12).

In the present embodiment, the packed bed (12) may be covered by the barrier (22) except at the top (16) of the pile of elements where the heat exchange end (32) is located to enable the heated fluid to be introduced to, and extracted from, the packed bed (12). The vent (25) may be provided near the bottom (14) of the pile, for colder fluid to exit the packed bed (12) during the charge cycle (15) and to enter the packed bed during a discharge cycle (17) which is discussed in more detail below and shown in Figure 2. The vent (25) may be provided around at least a portion of the bottom of the pile (12), for example around a bottom periphery thereof.

During the charge cycle (15), heated working fluid (air in the present embodiment) may be introduced through the duct (34) into the packed bed (12) from the top (16). Heat of the working fluid may then be transferred to the pile of elements in the packed bed (12) and then the working fluid may exit the packed bed (12) near the bottom (14) for example at the opening or vent (25) near the bottom edge (24). Relatively cooler air may then be conveyed through the fluid passage (28) towards the cold fluid opening (30) as indicated by the directional arrows (A).

The flow passage (28) may therefore be a cold fluid flow passage that extends between the vent (25) and the atmosphere and the bottom (14) of the pile may have a portion at the vent (25) where the insulating material (22) is absent, such that air can exit the packed bed during the charge cycle (15), move through the passage (28) and exit to the atmosphere through the vent (25) and out of the cold fluid opening (30). The flow passage (28) may also enable air to be drawn or pushed in from the atmosphere (or from another upstream or downstream process) through the vent (25) to move through the passage (28) and enter the packed bed (12) through the vent (25) during the discharge cycle (17). A fan or pump may be provided in association with the vent (25) or in association with the cold fluid opening (30) and may be in fluid communication with the atmosphere so as to blow atmospheric air into the flow passage (28) and thus into the packed bed during the discharge cycle (17).

It will be appreciated that a pump, fan, turbine, compressor, reversable pump turbine, generator, or the like may be provided upstream of the duct (34) to move the working fluid at elevated temperature through the packed bed (12). A Brayton cycle (310) (also discussed below with reference to Figure 11) may for example be used and solar energy (and/or other energy or fuel sources) may be used to facilitate heating of the working fluid to the elevated temperature. Liquid petroleum gas may be used in the Brayton cycle (310) to heat the air to about 650° C, after which the air is conveyed through the duct (34) to charge the packed bed (12).

An enclosure (31) may be provided above the flat top surface (16) and a hot fluid opening (32) disposed therein may be connectable to the duct (34) which may be a hot fluid pipe (34) to provide hot working fluid to the packed bed (12) through the flat top surface (16) during the charge cycle (15). The duct (34) may include insulation (36) which may inhibit heat losses from the duct (34) to the flow passage (28), or to the environment. A top of the roof (26) may be supported by a supporting structure, such as an outside tripod not exposed to the hot fluid and may be suspended by the cables and therefore may not necessarily require pillars to form the fluid passage (28). The fluid passage (28) may be at an outer periphery of the packed bed and may have a generally conical shape, but may have any shape that for example corresponds to the shape of the packed bed (12). The cables anchoring the cover (26) may be flexible to allow for differential expansion between the packed bed (12) and the cover (26).

A discharge cycle (17) of the packed bed (12) is shown in Figure 2. The duct (34) may enable the working fluid to be conveyed through a charged (or heated) packed bed (12) during the discharge cycle (17). The barrier (22) may extend across at least a major portion of the sloping sides (18) of the packed bed (12) to inhibit the movement of the working fluid therethrough. It will be appreciated that a pump, compressor or fan may be provided at the cold fluid opening (30) for pushing ambient air through the charged packed bed (12) during a discharge cycle (17), or the air may be sucked through the charged packed bed (12) during the discharge cycle by a pump or fan at the duct (34).

During the discharge cycle (17) stored heat from the packed bed (12) may be recovered, and a direction of flow of the working fluid may be reversed, with cold air (e.g. ambient air) entering the packed bed (12) via the cold fluid opening (30), through the flow passage (28) as indicated by directional arrows (B). The cold air may then pass through the charged (hot) packed bed (12) and exit above the packed bed (12) into the enclosure or chamber (31) above the flat top surface (16). During this process, stored heat from the elements in the packed bed (12) may be transferred to the cold air and this heated air may then exit the packed bed through the hot fluid opening (32) towards the hot fluid pipe (34). Therefore, the stored heat energy may be transferred to the working fluid for later use. The flow passage (28) may be provided between an inside of the roof (26) and an outside of the thermal insulation (22) on the sides (18) of the packed bed (12). This may allow a single cold fluid opening (30) to supply cold air to the thermal energy storage facility (10) from where the air may be distributed as indicated by the directional arrows (B) towards the opening near the bottom edge (24). The flow passage (28) may also enable inspection of the thermal insulation (22) to check for leaks, to perform maintenance etc.

Referring again to Figure 1 , during the charge cycle (15), the packed bed (12) may be charged by introducing the hot working fluid at the top (16) of the packed bed (12) and the cold fluid may be removed at the bottom edge (24) thereof. This may inhibit heat losses to the environment. As is shown in Figure 2, cold fluid may be introduced at the bottom edge (24) of the rock bed (12) and heated fluid may be removed at the top (16) during the discharge cycle (17) which may also be referred to as a thermal energy recovery cycle. This may inhibit natural convection or heat losses, for example caused by de-stratification. In the embodiments of the invention, the pile of elements (12) may be made larger to inhibit losses which may be referred to as edge losses. Edge losses may occur when working fluid may tend to flow more towards the sides (18) of the packed bed (12). An example of such an embodiment is shown in Figure 11 which is described in more detail below. Other shapes of the pile may also be used to inhibit edge losses. Baffles may be provided to reduce preferential flow and resulting edge losses may be reduced.

It will be appreciated that the flat top surface (16) of the packed bed (12) may be unimpeded so that it may expand and contract as it is heated and cooled. This may prevent stress caused by thermal ratcheting, a process where the rocks expand and contract with repetitive heating and cooling, thereby packing together more tightly. This may further be facilitated by a differential expansion between the rock pile (12) and the roof (26). During heating of the packed bed (12), hot working fluid or combustion gases may enter the enclosure (31) above the flat top surface (16) via the hot fluid opening (32) before flowing through the packed bed (12). As the gasses flow through the packed bed (12) they are cooled before they exit the storage facility via the flow passage (28) and out via the cold fluid opening (30). One or more slots (37) may be provided in the duct (34) or hot fluid pipe, to facilitate the air to move therethrough during both the charge and discharge cycles (15, 17).

Another embodiment (100) of the facility is shown in Figure 4. In this embodiment, the roof is omitted, and a barrier (122) is provided on top of the side surface (1 18) of the pile of elements or packed bed (1 12). The packed bed (1 12), openings or vents (125) in this embodiment may be similar to the packed bed (12) in Figures 1 to 3. Likewise, the hot fluid opening (132) may be similar to the hot fluid opening (32) in Figures 1 to 3. Instead of having the roof, the air may in this embodiment simply exit from the opening or openings or vents (125) into the atmosphere during the charge cycle. Hot air is hence introduced from the hot fluid opening (132) and the packed elements in the packed bed may be charged in similar fashion as described above with reference to Figures 1 to 3. During the discharge cycle, ambient air may be drawn in through the openings or vents (125), through the charged bed (1 12) and through the hot fluid opening (132) for later use. A fan, pump or reversible pump turbine may be provided downstream of the hot fluid opening (132) (in other words, downstream in the discharge cycle), to provide suction for drawing the air through the charged packed bed (112) to recover stored thermal energy from the packed bed (112) when heat is transferred from the elements in the packed bed to the air. The fan, pump or reversible pump turbine may similarly be arranged (upstream in the charge cycle), to push heated air through the packed bed (112) during the charge cycle. In Figure 5 is shown a schematic diagram of thermocouples (40), for example a grid of thermocouples (40) that are provided in an exemplary test run of the facility (10) of Figures 1 to 3. A plurality of grids of thermocouples may also be used. Figure 6 is a corresponding graph (42) illustrating the relative positions of the thermocouples (40) used in the test run. Figure 7 is a graph illustrating exemplary temperature distribution in the packed bed (10) after the packed bed is charged by an exemplary charge cycle (15). Figure 8 is a graph illustrating exemplary temperature distribution in the packed bed (10) after an exemplary discharge cycle (17). The exemplary test run includes charge cycles of about 6 to 8 hours, however any number of hours or any amount of time may be used for the charge and discharge cycles, depending on the particular application. Average mass flow rates used during the test run were about 0.5 kg.s^-1 for the charge cycle (15) and about 0.45 kg.s^-1 for the discharge cycle (17). Discharge termination was performed at about 327°C. The roof or tarpaulin cover may have a temperature rating of about 90 °C. During the test run, the flow rate of the working fluid was monitored by measuring a pressure drop over a bell mouth situated at a pump in fluid communication with the duct (34). A centrifugal pump may for example be used. During discharge, ambient air may first fill the tarpaulin cover or roof and then flow through the packed bed (12) in an inverse direction to the charge cycle. It will be appreciated that if repetitive charging and discharging cycles are performed, the residual heat in the elements may result in less time being needed to re-heat the elements in successive cycles. Hence, a successive charge cycle may require only about 6.2 hours, whereas a first charge cycle may require about 8 hours.

Figures 9 and 10 illustrate another embodiment (200) of the facility. In this embodiment, the packed bed (212) may have a cross-sectional shape of a frustum, and may be an elongated heap, mound or pile. A roof (not shown in Figures 9 and 10) similar to that shown in Figures 1 to 3 may be provided, or it may be omitted, similarly to Figure 4. One or more vents (225) may be provided similarly to that in Figures 1 to 3. A plurality of ducts (234) or hot fluid openings may be provided, for example in horizontal spaced configuration, for distributing heated fluid to a top (216) of the facility (200). The working fluid (e.g. air) may follow a flow path similar to that indicated by the directional arrows (C). If a roof is provided, a similar flow path to the flow path of directional arrows (A) in Figure 1 may be used for this embodiment (200) (Figures 9 and 10).

A barrier (222) may be provided at the sides (218), and on the top (216) and on or around the ducts (234). To spread the flow of working fluid, one or more channels (not shown) may be formed in the top (216) around the vents (225), or a separate barrier may be used between vents (225) so as to direct the working fluid over a larger area of the top (216). As before, the sides (218) are sloped at their natural angle of repose. The barrier (222) may be spaced from the top (216) to form the one or more channels to spread the working fluid. The charge and discharge cycles of this embodiment may be similar to those described above with reference to Figures 1 and 2, however the time needed to charge and discharge may be different. The flow direction of the working fluid for a discharge cycle would be the opposite to that of directional arrows (C) in Figure 10.

In Figure 1 1 is shown an exemplary system (300) which includes the thermal energy storage facility (10). A combination of solar power and combustion may be used in a Brayton cycle (310) to provide the heated working fluid in excess of about 500 °C, and more preferably in excess of 600 °C, however hotter or colder temperatures may be used, depending on the particular application. During the charge cycle (15), the packed bed (12) may be heated by the heated working fluid as described above. Then, during the discharge cycle (17), ambient air may be heated by drawing or forcing the ambient air through the charged packed bed (12), and this heated air (including the recovered heat energy) may then be used in a further downstream process, for example in a Rankine cycle (320) (or in a part of a Rankine cycle or process, but other processes may be used). This energy may facilitate a steam turbine to drive a generator to generate power which may be introduced into the grid. One or more compressors, turbines, generators, boilers, heliostats, solar panels, feedwater pumps, fans, blowers and condensers may be used in the system (300), for example as illustrated in Figure 11.

The system may therefore include an energy source (330) that heats the working fluid to the elevated temperature and a working fluid moving apparatus (340) for moving the working fluid through the packed bed (12). It will be appreciated that embodiments are possible wherein the working fluid moving apparatus (340) may be provided upstream or downstream of the packed bed (12). The energy source may be a single energy source such as solar power, or a combination of energy sources may be used to heat the working fluid to the elevated temperature. For example, the energy source may be selected from a group including but not limited to: solar energy, combustible fuels including fossil fuels and biofuels, wind energy, geothermal energy etc. Any other energy source may be used.

Referring to Figures 1 and 12, there is further provided a method (400) of constructing a thermal energy storage facility (10). The method (400) may include providing (410) the bottom surface (14) for supporting the pile of elements (12). The ground may for example be flattened or prepared for receiving the packed bed of rocks or elements. Next layering (412) elements on the bottom surface may be performed to form the packed bed (12) having sides that slope from the top (16) of the pile to the bottom (14) of the pile at their natural angle of repose (b). The duct (34) may be provided (414) with a heat exchange end (32) in fluid communication with the packed bed (12) at a heat exchange zone and an opposite fluid supply end (33). The duct (34) may be provided to enable (416) a working fluid at elevated temperature to be introduced into the packed bed (12) during the charge cycle (15). The working fluid may be enabled (418) to be conveyed through a charged packed bed (12) during a discharge cycle (17). The method may include providing (420) a barrier (22) which extends across at least a major portion of the sloping sides (18) of the packed bed to inhibit the movement of the working fluid therethrough.

The method may further include cutting the slots (37) into the duct and providing the insulation around the duct and/or around the packed bed. The frame may also be constructed around the packed bed, for example the stainless-steel frame may be constructed over the packed bed (pile of rocks) to resemble its shape. The slots may include parts of the duct that is cut and bent open and these parts may be bent back later to decrease air flow if needed. The duct may be sealed to prevent air from flowing into areas that are not intended. During construction, the rocks or elements may be positioned closely to the frame to inhibit air or working fluid from flowing between the barrier (22) and the sides (18).

In Figure 13 is shown a diagram (500) representing an example analytical model of the facility (10). The packed bed may be represented by a conical shape. Figure 13 shows an approximation of the heat progression through the rock bed or packed bed (12). The development of a thermocline may be as result of a larger void fraction between a free surface or side surface (18) of the rock bed and the frame or mesh supporting the barrier (22) which may in turn result in a preferential flow path around the periphery of the rock bed. As the flow moves downward, a substantially constant progression angle of a = 22.25° may be observed. A cross sectional area of the rock bed increases from top to bottom as progression takes place. The progression may be represented by Figure 13, wherein L represents the direction of progression, while b represents the angle of the rock bed surface (18). The cross-sectional area may be a function of the radius and height of progression segments. The vertical progression may be determined by an approximate diameter of a typical rock in the rock bed with a horizontal progression being determined by a similar procedure. A ratio between the height and radius of the progression may be defined by a and from experimental results. The height and area of each progression segment may be determined by equations (1) and (2) below: h = rtar\(a) (1) where the variables h and r are shown in Figure 9. The following equations or formulae may furthermore be used in calculations:

Forced convection may be a dominant type of heat transfer used during the charging and discharging cycles (15, 17), whereas radiation and conduction may be negligible in comparison. Equations (3) and (4) may be used to calculate the rock and air temperatures at each of a series of heat progression steps through the rock bed. These equations may be solved iteratively in the flow direction for each progression segment as calculated by Acs. A representative model may be transient, with a number of steps being selected as appropriate. A more detailed explanation of the above equations is discussed in the following reference: K.G. Allen, T.W. von Backstrom and D.G. Kroger,“Rock Bed Pressure drop and heat transfer: Simple Design Correlations” In Solar Energy (Elsevier, Stellenbosch, 2015) pp. 525-536.

In Figure 14 is shown a graph (600) of experimental results from a series of test runs of the facility (10) when performing charge cycles and discharge cycles, as compared to the analytical model discussed above. The experimental testing may be used to verify the analytical model. For the test results, discharge temperature may be measured at an entrance of the hot fluid pipe. Three test runs were performed and compared to three experimental models shown in the graph (600). It will be appreciated that each model yields a higher discharge temperature than that of the experimental test runs. However, a curve of each model (as shown in Figure 14) has a sharper decrease than that of the corresponding experimental test. The curves for each model and its associated test results finish at about 20°C of one another for the same discharge duration, with the model giving the higher temperature. This may be indicative that the initial values of the model can be revised to achieve a more accurate representation of the experimental results. The model may not be configured to account for heat losses to the environment when discharge starts, which may contribute to a difference in results.

Two exemplary cycles are compared with their analytical models in the table below: Property Cycle 1 Analytical Cycle 2 Analytical Units

Heating capacity 335.8 313.2 337.9 313.2 kW_th

Total energy input 2.7 2.62 1 ,97 1.97 MWh_t

Useful energy

1.97 2.06 1.82 1.96

output MWh_jh

Thermal efficiency 72.75 78.6 92.4 99.2 %

Volumetric

61.8 57.5 60.3 57.5 % efficiency

It will be appreciated by those skilled in the art that there are many variations to the invention as herein defined and/or described with reference to the accompanying drawings, without departing from the spirit and scope of this disclosure.

For example, the packed bed may have a conical shaped with circular or oval base or bottom, alternatively the pile may have a tetrahedron-like shape or any other shape. The hot fluid opening need not be at the top surface of the packed bed and may also be provided on the side surface. A plurality of hot fluid openings may be provided for distributing heated working fluid to the packed bed. It will be appreciated that the hot fluid opening may function as an inlet during the charge cycle and as an outlet during the discharge cycle. The vent and/or the cold fluid opening may similarly function as an outlet during the charge cycle and as an outlet during the discharge cycle. Fluid flow may hence be reversed as required. In embodiments described herein the working fluid may be air, but any other working fluid may be used.

Because hot air enters from the top of the pile, the problem of an inverse thermocline (where the hot air is beneath) may be removed. The packed bed may be particularly simple to construct as the pile can first be constructed or poured before any other parts of the thermal energy storage facility are built, thus removing the danger of earth moving equipment or heavy machinery damaging more sensitive elements of the facility. The optional roof or sheet of material may be particularly simple to construct and is not required to be heat resistant because only the cold-air side of the system is in contact with the roof. The optional roof may also allow a single vent or duct to supply cold air to the packed bed and to allow even distribution during discharge. The roof can easily be removed to inspect or repair the insulating material.

The rock used for thermal storage may be selected so as not to crumble and thereby tend to block air passages in the packed bed and increase the required pumping power. The rocks 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. Dolerite may for example be used to form the packed bed of rocks. Dolerite may provide an inexpensive storage facility.

A thermal storage facility according to this invention may thus include 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 or even > 600 °C). Since the cost of rock material is relatively low it can be readily replaced after some years if problems should arise. The rock bed material of this invention may be 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. A region in the elements or rocks where the temperature gradient is high may be used to determine the shape of the pile of elements. The bottom of the pile may simply be supported on the ground and may not need vents or openings below the pile, which may, in turn, reduce the required construction costs. Moreover, a fan or pump may be provided at the cold fluid opening which fan or pump may be relatively inexpensive. In such an embodiment, the working fluid may be pushed through the packed bed from the cold fluid opening and a fan or pump need not necessarily be provided at the hot fluid opening, which may reduce manufacturing costs. An advantage of the separate roof may be that the facility may be enabled to operate at higher temperatures than if the roof was adjacent to the packed bed, because an operator of the facility would not have to limit the temperature to prevent damage to the roof. The relatively cooler working fluid in the fluid passage may protect the roof from the hotter temperatures in the packed bed.

Another advantage of embodiments described herein may be that the facility may be efficient at retrieving thermal energy from the packed bed and may not suffer from so-called dead zones within the packed bed. Pumping power may also be reduced by the embodiments described. A volumetric efficiency of the embodiments of the present invention may be increased compared to prior art systems and the present invention may be designed to utilise a relatively larger portion of the pile of elements or rocks. A cladding layer may be provided on the packed bed, for example externally to the side surfaces or even on top of the roof, to further protect the packed bed from the environment and to reduce heat losses. The fan or pump disclosed herein may provide both blowing and sucking, as required. The top of the pile may be substantially air tight or fluid tight, to enable working fluid to be pumped from a cold side of the facility through the packed bed. The barrier may also protect the packed bed from wind, rain and other environmental factors. This may also facilitate the charging and discharging cycles to be clean. Instrumentation such as sensors (e.g. thermocouples or temperature sensors, flow sensors etc.) may be provided externally of the barrier to protect these from heat. The plurality of hot fluid openings may be provided to enable heated working fluid to enter the packed bed at a variety of locations (i.e. not only at the top), which may be used to provide a required thermocline distribution. The wall may facilitate supporting the lower edge of the packed bed, but the wall may also facilitate attaching the roof thereto, to form a substantially fluid tight seal. The use of air as working fluid may provide the advantage of cost savings when compared to prior art systems that the applicant is aware of.

The thermal energy storage facility may hence be arranged to inhibit heat losses of the charged packed bed by way of the barrier. The packed bed or pile of elements may have the general shape of a truncated cone. The bottom of the pile may be square, rectangular, triangular, circular, oval or any combination of these shapes or any other shape. In embodiments disclosed herein the heated working fluid may enter from the top of the packed bed or pile, to benefit from a stabilising effect of buoyancy on a thermocline. The insulation may inhibit heat losses to the environment as result of convection, conduction and radiation. The roof may include layers and may for example include an inner insulative, heat resistant layer, and one or more further layers that are water resistant or substantially water impermeable. Embodiments are possible wherein the roof is directly adjacent the pile of elements. In embodiments wherein the roof is displaced from the barrier, the roof may be constructed of relatively inexpensive materials that do not necessarily need to be heat resistant. The cables that anchor the roof may allow for expansion and contraction, and some of the cables (e.g. at strategic locations) may have more slack compared to the others when the packed bed is relatively colder. The packed bed of elements may be relatively inexpensive because the elements may simply rest in their natural position at their natural angle of repose after they are deposited on the bottom surface.

Throughout the specification and claims unless the contents requires otherwise the word ‘comprise’ or variations such as ‘comprises’ or‘comprising’ will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

1. A thermal energy storage facility comprising a packed bed formed by a pile of elements, the packed bed having sides that slope from a top of the pile to a bottom of the pile at their natural angle of repose, and a duct having a heat exchange end in fluid communication with the packed bed at a heat exchange zone and an opposite fluid supply end, the duct enabling a working fluid at elevated temperature to be introduced into the packed bed during a charge cycle and enabling the working fluid to be conveyed through a charged packed bed during a discharge cycle, wherein a barrier extends across at least a major portion of the sloping sides of the packed bed to inhibit the movement of the working fluid therethrough.

2. A thermal energy storage facility as claimed in claim 1 , wherein the barrier is a thermal insulating material that reduces heat loss to the environment through the sloping sides of the packed bed.

3. A thermal energy storage facility as claimed in claim 1 or claim 2, wherein the barrier is substantially fluid impermeable to act as a seal to substantially prevent fluid ingress through the sloping sides of the packed bed.

4. A thermal energy storage facility as claimed in any one of the preceding claims, wherein the pile of elements is a pile of rocks.

5. A thermal energy storage facility as claimed in any one of the preceding claims, wherein the insulating material includes rock wool insulation.

6. A thermal energy storage facility as claimed in any one of the preceding claims, wherein the packed bed is covered by the barrier except at a top of the pile of elements where the heat exchange end is located to enable the heated fluid to be introduced to, and extracted from, the packed bed.

7. A thermal energy storage facility as claimed in any one of the preceding claims, wherein a vent is provided near the bottom of the pile for colder fluid to exit the packed bed during the charge cycle and to enter the packed bed during the discharge cycle.

8. A thermal energy storage facility as claimed in claim 7, wherein the thermal energy storage facility includes a separate roof spaced from the sloping sides so as to form an outer wall of a flow passage between the roof and the sloping sides.

9. A thermal energy storage facility as claimed in any one of the preceding claims, wherein the heat exchange end of the duct is in fluid communication with a top of the packed bed such that the packed bed is heated from top to bottom by the working fluid during the charge cycle.

10. A thermal energy storage facility as claimed in any one of the preceding claims, wherein the heat exchange end of the duct includes an open-ended pipe with a number of associated slots in a wall of the pipe adjacent its open end.

11. A thermal energy storage facility as claimed in claim 10, wherein the open end of the pipe is supported directly on the top of the pile.

13. A thermal energy storage facility as claimed in any one of the preceding claims, wherein the sheet of material is a tarpaulin anchored by cables at the bottom of the pile to the ground and held in place by a structure resting on the top of the pile.

14. A thermal energy storage system comprising a packed bed formed by a pile of elements, the packed bed having sides that slope from a top of the pile to a bottom of the pile at their natural angle of repose, and a duct having a heat exchange end in fluid communication with the packed bed at a heat exchange zone and an opposite fluid supply end, the duct enabling a working fluid at elevated temperature to be introduced into the packed bed during a charge cycle and enabling the working fluid to be conveyed through a charged packed bed during a discharge cycle, wherein a barrier extends across at least a major portion of the sloping sides of the packed bed to inhibit the movement of the working fluid therethrough, the system further including an energy source that heats the working fluid to the elevated temperature and a working fluid moving apparatus for moving the working fluid through the packed bed.

15. A method of constructing a thermal energy storage facility, the method comprising: providing a bottom surface for supporting a pile of elements; layering elements on the bottom surface to form a packed bed having sides that slope from a top of the pile to a bottom of the pile at their natural angle of repose; providing a duct having a heat exchange end in fluid communication with the packed bed at a heat exchange zone and an opposite fluid supply end; enabling the duct to introduce a working fluid at elevated temperature to the packed bed during a charge cycle; enabling the working fluid to be conveyed through a charged packed bed during a discharge cycle; and providing a barrier which extends across at least a major portion of the sloping sides of the packed bed to inhibit the movement of the working fluid therethrough.