A SPIRAL THERMAL ENERGY STORAGE SYSTEM

Field

The present application relates to energy storage systems and in particular to energy storage systems employing a phase change material.

Background of the Invention

Energy storage, and in particular thermal energy storage (TES), is becoming of increasing concern in modern technology due to the impeding shortage and increasing cost of energy resources. The concept behind a TES system is to aid the energy management by storing thermal energy at periods when it is abundantly available and then using it when and where it is required.

There are two modes by which heat energy may be stored, sensible and latent. Conventional thermal storage units (hot water tanks, thermal mass in storage heating systems etc.) only store sensible heat. A phase change material (PCM) allows for the storage of heat energy as latent heat and sensible heat. Latent heat is absorbed when the PCM changes from solid to liquid around its melting temperature. Using a PCM with melting temperatures in the range -20°C to 180°C, larger energy density storage can be achieved compared to conventional sensible storage units of similar size. An example of a PCM in this range is Wax: Melting temperature (65°C); Density:

880kg/m3 at 15°C; Specific heat capacity: 3100 J/kgK; Enthalpy of fusion: 173 kJ/kg. At around 65°C the PCM absorbs 173 kJ for every kilogram of PCM used when heating from solid to liquid. This is equivalent to the energy required to raise 40 kg of water by 1°C. The PCM thermal energy storage unit meets high volumetric energy density requirements and permits the heat release at an almost constant temperature or in a small range of temperatures. The PCM holds the temperature constant as it absorbs latent heat. This gives good temperature regulation at the solidification temperature when recovering heat from the PCM.

Whilst PCM materials offer improvements over conventional sensible storage, an identified problem is that the thermal conductivity of the PCM is lower when in the solid phase and as a result it is difficult to get heat to flow through the solid PCM material to cause the PCM material to melt. The present application is directed at this problem.

GB2484539 discloses an arrangement of a heat exchanger employing PCM material. The present application provides a number of improvements upon

GB2484539.

Summary

In one aspect, the present application provides a thermal energy storage system as detailed in claim 1. Advantageous embodiments are provided in the dependent claims.

In another aspect, the present application provides a thermal energy storage system comprising a cylindrical tank having a base. A layered arrangement is provided within the tank, with the layers arranged along the cylindrical axis of the tank. Suitably, at least one of the layers comprises:

a first fluid inlet for receiving a first fluid flow, a first fluid outlet in fluid communication by a first flow path to the first fluid inlet, wherein the first flow path is arranged in a spiral about the cylindrical axis from the first fluid inlet to the first fluid outlet.

A first manifold provides a fluid connection from the outlet of the layer to the inlet of a first flow path in an adjacent layer along the cylindrical axis and PCM material is provided within the tank about the layered arrangement.

The first fluid flow path may be formed from a single continuous section of pipe.

Suitably, the first fluid flow path is formed without using a heat exchange plate to join concentric sections of the spiral of the first fluid flow path. The first manifold is arranged generally in a direction parallel to the cylindrical axis.

The layers are suitable arranged to substantially parallel to one and other along the cylindrical axis. The interlayer separation distance is suitably the same between all adjacent layers of the arrangement.

The spiral may be substantially arranged as an Archimedean spiral.

Additionally, the at least one layer further comprises a second fluid inlet for receiving a second fluid flow; a second fluid outlet in fluid communication by a second flow path to the second fluid inlet; wherein the first and second fluid flow paths are arranged concentrically in a spiral. In this arrangement a second manifold is provided to form a fluid connection from the second fluid outlet to the inlet of second fluid flow path in an adjacent layer along the cylindrical axis.

In the top layer of the layered arrangement, connecting pipes are provided to each of the first and second fluid inlet and each of the first and second fluid outlet, and wherein the connecting pipes are arranged parallel to one in a direction transverse to the cylindrical axis.

To facilitate the transverse arrangement of the connecting pipes, an opening may be defined in the cylindrical wall to allow the connecting pipes to pass therethrough.

In which case, a seal may be provided to seal the space between the connecting pipes and the opening to prevent leakage of phase change material.

The second fluid flow path may be formed from a single continuous section of pipe. The second fluid flow path may be formed without a heat exchange plate joining concentric sections of the spiral of the second fluid flow path or adjacent sections of the first flow path.

The second manifold may be arranged generally in a direction parallel to the cylindrical axis.

The interlayer separation distance may be substantially the same as the inter spiral distances between the first fluid flow path and a second fluid flow path within a layer. Suitably, the phase change material has a melting point in the range -120C to 220C and preferably the phase change material has a melting point in the range of 50 ^* C to 90¾.

The phase change material may be selected from the group consisting of waxes and salt hydrates.

An outlet may be provided toward the bottom of the cylindrical wall or in the base of the tank to allow for draining of PCM material from the tank.

Brief Description Of The Drawings

The present application will now be described with reference to the

accompanying drawings in which:

Figure 1 is a top view of a first coil assembly for use in a first aspect of the present application; Figure 2: is a top view of a second coil assembly for use in combination with the first aspect of Figure 1 ;

Figure 3 is a view of an inner unit assembly comprising a layered arrangement of the coils of Figure 1 and Figure 2 with connections and support frame;

Figure 4 is a view of the assembly of Figure 3 within an exterior tank housing and piping outlets for the first and second coils and base feet;

Figure 5 is a front side view of the inner unit assembly of Figure 3;

Figure 6 is a rear side view of the inner unit assembly of Figure 3;

Figure 7 is a top view of Figure 4;

Figure 8 illustrates exemplary temperature profiles obtained from a system; and Figure 9 illustrates the nature of the shape of the first and second coils shown in Figures 1 and 2.

Detailed Description Of The Drawings

There are many advantages to using PCM storage, including, for example smaller size comparable to conventional storage. Research by the present inventor has also identified a limitation of PCM storage systems, namely that latent heat storage depends on the thermal conductivity of the PCM, which is lower when in its solid phase. This means that it may be difficult to get the heat to flow through the solid PCM in order to melt it. Thick PCM storage blocks may only partially melt when exposed to thermal energy and overall latent heat storage capacity is reduced as a consequence. Whilst, it is known to employ webs (heat transfer plates) to improve heat transfer, the inclusion of webs in a design makes it difficult filling such heat storage device with PCM material and also adds cost and construction complexity. At the same time, conventional configurations lack flexibility in the manner of their construction to accommodate different uses. The present application addresses both aspects by providing a layered arrangement in which each layer has two separate fluid flow paths each arranged in a spiral in a tank with the PCM.

The pipework providing the two separate flow paths is arranged in spirals within the tank thus obviating the problems associated with PCM block storage units. More particularly, the thermal energy storage comprises a tank for holding a PCM material. The tank is most suitably substantially cylindrical in shape. In this cylindrical form, referring to Figure 4, the tank 40 comprises a base to which is joined a cylindrical wall 42 which extends upwards therefrom. The shape of the base may be planar or it may be concave or convex shaped. The base may be supported by a plurality of legs 46 to allow for insertion of the forks of a forklift or other means to lift/move the tank. Similarly, the legs may have castors to allow for movement of the tank. An outlet 44 may be provided in the base or in the region of the cylindrical wall adjoining the base to facilitate removal of PCM material from the tank. The outlet suitably comprises a valve (not shown). The valve may be manually operable to facilitate emptying of the tank of PCM material as required, for example during inspection or maintenance. The top of the cylindrical wall is suitably shaped for engagement with a lid (not shown). For example, a flange 48 positioned at the top of the cylindrical wall may provide an abutment surface extending about and transverse to the cylindrical wall. The abutment surface may have a plurality of bolt holes 72 provided therein for alignment with corresponding features on the lid to facilitate securing of the lid to the tank.

A layered arrangement 30 of pipework is provided within the tank. Referring to Figures 1 to 7, the individual layers 80 are concentrically arranged along the cylindrical axis of the tank. Each of the layers 80 has a first coil or flow path 12 starting at one opening at a manifold 15 and ending at an opening provided at a manifold 14 at the opposite end of the first flow path. The first flow path allows for liquid to flow from the first opening 15 to the second opening 14 or vice versa depending on the configuration as will be explained below. The first flow path is arranged in a spiral about the cylindrical axis. The first flow path is suitably constructed of a single piece of pipework which has been formed in a spiral shape. It will be appreciated that forming each spiral flow path from a single piece of pipework minimises the risk of failure in use and makes for a simpler and less expensive construction. The position of the inlet and outlet of the first flow path are reversed in each layer. Thus in the top layer, shown in Figure 1 , the inlet may be taken to be at the outside end of the spiral where the pipework joins at manifold 15 to an external connection 17 for connection to a source of heating fluid (e.g. water) and the outlet may be taken to be at the centre of the spiral where the pipework joins manifold 14. In the next (second) layer, the inlet for the first flow path will be at manifold 14 and the outlet will be at manifold 15. In manifold 14, the outlet connection from the first flow path of the top layer is connected to the inlet of the next (second layer) layer. Similarly, the connection at manifold 15 of the first layer will connect to the next (third layer) layer. Thus in the context of Figure 5 and 6 in which 12 layers are shown in total and in which the top layer may be taken to be generally indicated as Layer 1 (L1 ) and the bottom as Layer 12 (L12), then connections will be made at manifold 14 as follows: L1-L2, L3-L4, L5-L6, L7-L8, L9-L10, L1 1-L12.

Similarly, at manifold 15, the connections will be as follows inlet 17 - L1 , L2-L3, L4-L5, L6-L7, L8-L9, L10-L1 1 , L12 - outlet 16. Outlet 16 is connected to the manifold 15 by a vertical pipe connection 82 which extends from and connects to the outlet 16 at the top and to a connection 84 at the bottom of the manifold.

Suitably, each of the layers 80 further comprises a second flow path 22 having an opening to connect to a manifold 25 at one end and ending at an opening at the opposite end to connect to a further manifold 24. The second flow path in each layer allows for liquid to flow from the manifold 25 to the further manifold 24 or vice versa depending on the configuration as will be explained below. The second flow path is suitably the same shape and dimensions of spiral as the first flow path. To allow both flow paths to fit in a layer together, the spirals in each layer are rotationally offset at an angle of about 180° about the cylindrical axis with respect to one and other. In practise, the preferred angle is either above or below 180° so that the outlets of the first and second flow paths can be arranged to exit the tank together in parallel at the same point.

It will be appreciated that having an angle of about 180° allows for each of the first and second flow paths to be separately fabricated before being assembled together. This means that making connections between the manifolds and spiral pipework is significantly easier. As with the first flow path, the second flow path of each layer is suitably constructed of a single piece of pipework which has been formed in a spiral shape. It will be appreciated that forming each spiral flow path from a single piece of pipework minimises the risk of failure in use and makes for a simpler and less expensive construction. The position of the inlet and outlet of the second flow path are reversed in each layer. Thus in the top layer, shown in Figure 2, the inlet may be taken to be at the outside end of the spiral where the pipework joins at manifold 25 to an external connection 26 for connection to a source of heating fluid (e.g. water) and the outlet may be taken to be at the centre of the spiral where the pipework joins manifold 24. In the next (second) layer, the inlet for the first flow path will be at manifold 24 and the outlet will be at manifold 25. In manifold 24, the outlet connection from the second flow path of the top layer is connected to the inlet of the next (second layer) layer. Similarly, the connection at manifold 25 of the first layer will connect to the next (third layer) layer. Thus in the context of Figure 6 in which 12 layers are shown in total and in which the top layer may be taken to be generally indicated as Layer 1 (L1 ) and the bottom as Layer 12 (L12), then connections will be made at manifold 24 as follows: L1- L2, L3-L4, L5-L6, L7-L8, L9-L10, L1 1-L12. Similarly, at manifold 25, the connections will be as follows inlet 17 - L1 , L2-L3, L4-L5, L6-L7, L8-L9, L10-L1 1 , L12 - outlet 16. Outlet 26 is connected to the manifold 25 by a vertical pipe connection 86 which extends from and connects to the outlet 27 at the top and to a connection 88 at the bottom of the manifold 25. Each of the outlet/inlet pipes 26, 27 from the second fluid flow path are extend across the top so as to exit from the tank beside the outlet\inlets of the first flow path. In this respect, an opening is suitably provided in the abutment surface and a corresponding section of the upper part of the cylindrical wall to accommodate fluid outlet and inlet pipes. A sealing section may be provided which is shaped to allow the outlet and inlet pipes to be inserted and pass through and at the same time shaped to match with the opening in the abutment surface and top of the cylindrical wall so as to form a seal. This sealing section may be shaped or configured to provide a seal.

It will be appreciated that as each of the flow paths have separate connections that different configurations are possible. For example, the first and second fluid flow paths may be connected in parallel, i.e. where there is just a single supply of fluid. In this case, it will be appreciated that whilst the flow paths may be connected in parallel, the respective inlets and outlets may be reversed. In this way, the fluid may enter one flow path at the top of the tank and the other flow path at the bottom.

Alternatively, the tank may be configured as a heat exchanger with one of the fluid flows receiving a hot fluid and the other flow containing a fluid to be heated by the hot flow (i.e. having a lower temperature relative to the hot fluid). By having the direction of the flows reversed, i.e. hot fluid going from bottom to top and fluid being heated going from top to bottom, the relative difference in temperature between the flow paths is more consistent between layers.

The layered assembly 30 of first and second flow paths may be constrained by a frame or support 32 to allow for ease of lifting and installation into the tank 40.

The arrangement generally allows for the thermal storage unit to be constructed for a lower cost than alternative structures such as disclosed in GB2484539. It also may reliably be used at relatively high fluid pressures (10 bar) as the design allows for its construction with a minimum number of joints in fluid flow paths. Similarly, the arrangement allows access during manufacture for joints in the pipework to be readily formed by welding, which helps reduce costs. In this respect, it will be appreciated that as the structure may be formed one layer at a time and separately for the first flow path and second flow paths, with the two combined afterwards, that the construction process is relatively simple. At the same time, since the length of each spiral path is the same that the process for forming them is consistent with no customisation required. At the same time, the length of the spiral path may be chosen to ensure that an industry standard length of pipe may be used. The pipe material is suitably selected to provide for good heat transfer characteristics. Accordingly, metal may be preferred over plastics. At the same time, the material may need to be resistant to corrosion from the PCM materials present. Thus, the metal selected may be stainless steel.

It will be appreciated that the assembly of the present application does not employ webs (heat transfer plates) which simplifies the nature of construction and allows for example for the second coil assembly 10 simply to be lifted into place within the first coil assembly 20.

A further advantage of the present application is that the device is not limited to use as a heat exchanger. Instead, the two coils may be used by the same fluid to give faster charging and discharging rates of the PCM. Accordingly, in the present case, it is not necessary to have two coils in each layer. However having two coils allows a range of connections and configurations. For example, both coils may be used to charge the store and both coils may be used to discharge the store supplied by a single heat source/sink fluid circuit. In another configuration, one coil charges and discharges into a separate heat source/sink fluid circuit while the other coil charges and discharges into a different heat source/sink fluid circuit. In another configuration, both coils are connected in parallel for single charging/discharging to give a particular ΔΤ and transient time constant. In another configuration, both coils are connected in parallel for single charging/discharging particular ΔΤ and transient time constant.

The absence of a heat exchange plate or other heat transfer surface between the coils improves the ability to introduce PCM material into the system. Heat transfer between the two coils is limited to direct transfer through the PCM material (although notionally there may be some transfer through the support frame, this is not significant compared to the heat transfer through the PCM). Thus, it may be taken that at least 95% of the heat transfer between the coils takes place through the PCM material.

To facilitate construction, the primary and secondary coils are uniform in their curvature (no tight radii bends). In this respect, the coils are suitably shaped as Archimedean spirals as illustrated in Figure 9. Although for ease of construction, the spiral may not end at or close to the centre as shown in Figure 9, but at a distance out from the centre. Thus as shown in the figures of the coils, the internal ends of the spirals are at manifolds 14 and 24. Using an Archimedean spiral means that the distance is relatively constant between turns of the spirals and thus between the turns of the first and second coils in any layer.

At the same time, the distance between the layers is selected to match that between turns so that the same PCM thickness is between all pipes in the vertical direction and in the horizontal direction. This makes the PCM easier to melt.

Additionally as each of the primary and secondary coils is selected to be the same length, it allows for low cost manufacturing due to uniformity in construction.

Using a cylindrical housing reduces heat losses. It also reduces manufacturing costs and minimises materials used.

The PCM material used may be supplied to the tank as a powder, granular form, solid lumps or liquid. As the assembly is relatively open without the presence of webs, it readily facilitates adding the material in solid form, e.g. in powder or granular form or as lumps without having to melt it first.

Thus the completed system consists of a reservoir in the form of a cylindrical tank having a removable lid. The cylindrical tank in turn is employed to house a number of coils (spiral fluid flow paths in the form of layers of pipes with with each pipe arranged in a spiral). The tank is also employed to house PCM material. A wide variety of different phase change materials (PCM) are known and may be employed including for example, paraffin based materials. An example of a paraffin based phase change material is that of RT58 sold under the RUBITHERM ® trade mark available from Rubitherm Technologies GmbH of Berlin, Germany. An alternative material is a salt hydrate such as for example Barium Hydroxide Octahydrate [Ba(OH)2.8H20]. Indeed, it will be appreciated that depending on the intended application of the system that different phase change material may be employed. Thus generally, any phase change material with a melting point in the range -120C to 220C may be employed application dependent. However, where the system is intended for use in a situation where heat is being transferred to and from water, the melting point of the phase change material is preferably within the range of eO'C to 90 ^* C.

The tank operates at atmospheric pressure, i.e. the PCM materials housed within the tank are not intended to be pressurised. The tank operates at atmospheric pressure and may have a corrosion resistant liner, e.g. an internal Polypropylene liner for use with corrosive especially when salt hydrates are used.

PCM can be filled into the tank in powder, granular beads, solid chunks or liquid.

The coils can now be lifted in and out for inspection in the field as they are bolted to the side wall of the tank to form a seal. The tank may be generally constructed using a metal such as steel. However, an internal plastics liner (e.g. polypropylene) or other barrier coating may be employed to protect the metal from corrosion, for example when salt hydrates are used as the PCM material. It will be appreciated from the discussion of the structure which follows that the coils may readily be lifted in and out for inspection in the field. At the same time, the construction allows for PCM materials to be filled into the tank in powder, granular beads, solid chunks or liquid.

The arrangement of heat exchange coils within the tank will now be described in greater detail. The coils are arranged in a series of layers which are arranged along the cylindrical axis of the tank from the base to the lid. Each of the coils is a spiral which extends from a region close to the wall of the tank to a region close to the centre defined by the cylindrical axis of the tank. Each layer may comprise a single spiral coil. Alternatively, each layer may comprise two coils arranged in a concentric spiral configuration.

The internal thermal energy storage tank system consists of two coils optimally designed to reduce construction and manufacturing costs and having a uniformity of PCM material in the inter-space between the two coils as depicted in figure 1 to figure 8. The first coil 10 shown in figure 1 carries a fluid which may be referred to as the first fluid from inlet to outlet. The second coil 20 shown in figure 2 carries a second fluid. The system has been designed to operate in a range of modes where the first fluid and second fluid may both be used to charge the PCM material in a series or parallel flow path or separately depending on the heat sources used to melt the PCM. In a similar fashion, the discharging of the energy in the PCM may be through both coils or a single coil depending on the application. The coils are provided with a number of supports to maintain their structure when being lifted in and out of the tank. Vertically arranged manifolds are provided at each end of each coil to provide a fluid connection to each end of the coils. This optimal design based on a spiral coil makes the PCM material the only other internal part of the storage system, i.e. if the coils are lifted out all that remains within the liner is the PCM material. This removes the necessity for internal plates which are subject to corrosion over the life of the product. The inter-space between the coils in the vertical has been optimally designed to ensure minimum thermal stratification due to the ratio of the pipe surface area to the height of the tank. The high ratio of pipe surface area to tank volume has been optimised to make the vertical spacing of PCM of uniform thickness. The inter-space between the coils in the horizontal has been designed to be uniform between the coils. The choice between the first fluid and second fluid are liquid/liquid, gas/liquid, liquid/gas or gas/gas with this type of unit. This allows a range of heat sources to be used with this storage unit. The external connections of both coils are placed outside the external wall of the tank. The coils may readily take up to 10Bar pressure.

To reduce welds and improve assembly time, each of the spiral coils are formed from a single piece of pipe which is bent into a spiral.

To avoid expense and reduce problems associated with corrosion, the tank does not employ heat exchanger plates so there is no direct heat exchange between the first coil and second coil. Instead, heat is transferred to the PCM and from the PCM.

The PCM spiral thermal storage unit described herein is a low cost thermal storage solution. The simple geometry of the internal heat exchanger chambers doesn't require intricate pipe work associated with other types of heat exchangers. The materials used in the storage unit are standard manufacturing piping materials, and no specialised alloys and associated welding is required. The number of welded joints along the pipe work has been minimised and the construction of the rectangular manifold distribution pipes between the 12 vertical pipes of each coil unit is also minimised.

The PCM compact thermal storage unit according to the invention combines the advantages of a typical spiral heating coils without the need for internal heat transfer plates. In particular the spiral thermal energy storage unit described herein which comprises a spiral heat coils enables higher thermal energy storage density, that is, it can store significantly more energy for the same weight and space as conventional storage units. Phase Change Material thermal storage uses the latent heat required to turn the PCM from solid to liquid and results in a higher energy density to weight ratio.

A further advantage of the spiral thermal energy storage unit is low storage energy losses. The compact spiral geometry of the heat exchanger concentrates the high density energy at the centre of the unit. Exterior temperatures of the unit are lower than at the centre of the spiral, resulting in lower storage losses through the exterior of the unit. Any energy losses within the interior windings of the spiral are captured by the next outer winding of the spiral. The thermal performance of an exemplary tank is illustrated in figure 8 which demonstrates the ability of the PCM material to capture heat over a period of several hours. It will be appreciated that the PCM material need not reach the same temperature as the fluid being passed through the coils.

The design allows routine inspection and maintenance of the unit by removing the simple flat plate lid. The tank walls and lid form a hermetically sealed system when bolted together. The spirals are made from steel tubing turning rolled into a spiral and pass through a seal on the vertical wall of the tank.

The PCM fills the inter-space between the coils both horizontally and vertically. The PCM is placed into the storage unit in solid form, typically as a powder, in bead form or in a fluid state.

In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specifications and drawings are accordingly, to be regarded in an illustrative rather than in a restrictive sense.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word 'comprising' does not exclude the presence of other elements or steps than those listed in a claim. Furthermore, the terms "a" or "an," as used herein, are defined as one or more than one. Also, the use of introductory phrases such as "at least one" and "one or more" in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an." The same holds true for the use of definite articles. Unless stated otherwise, terms such as "first" and "second" are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. The words comprises/comprising when used in this specification are to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers , steps, components or groups thereof.