NO20210187A1

NO20210187A1 - Element for a thermal energy storage, a thermal energy storage with the element and use of the element

Info

Publication number: NO20210187A1
Application number: NO20210187A
Authority: NO
Inventors: Pål G Bergan; Christopher Johan Greiner; Nils Høivik; Martin Skottene
Original assignee: Energynest As
Priority date: 2021-02-12
Filing date: 2021-02-12
Publication date: 2022-08-15
Also published as: WO2022173308A1

Description

ELEMENT FOR A THERMAL ENERGY STORAGE, A THERMAL ENERGY STORAGE WITH THE ELEMENT AND USE OF THE ELEMENT

Technical Field

The present invention relates to storage of energy. More specifically, the invention relates to an element for a thermal energy storage (TES), a TES with the element and use of the element. The element is based on the principle of a heat transfer fluid (HTF) flowing through a heat exchanger pipe to transfer heat from a source to a surrounding solid-state material in the element, wherein the thermal energy is stored in the solid-state material of the element until flowing HTF at a later moment in time transfer the thermal energy out from the solidstate material of the element to a user. The element and TES of the invention are particularly feasible for operating at high dynamic temperature and high absolute temperature and can provide significant reduction in cost for energy storage, all of which will be clearer from the description below.

Background of the Invention and Prior Art

A missing link for facilitating the shift towards sustainable energy production is energy storage at affordable cost. Wind energy and solar energy are variable, and reduced cost for energy storage would enhance the use of wind energy and solar energy. In addition, energy based on combustion, be it based on fossil fuel or biofuel, and nuclear energy, may also benefit from reduced cost for energy storage. Reduced waste of energy is considered a key factor for cost reduction for energy from all types of energy sources, including renewable energy sources, including solar thermal energy, and combustion-based energy sources and nuclear based energy.

With affordable energy storage, energy waste at shutdowns, startups and scaling production up or down can be reduced. For steam-based electricity production, used extensively and dominating the global market, the possibility of storing the thermal energy at affordable cost would represent a big advantage.

Connected turbine - electric generators can easier be operated at optimal operation points. The storages can be located locally, reducing the burden on the grid.

State of the art in thermal energy storage may be the existing storage technology of the Applicant. The most relevant of said technology is described and illustrated in the patent publications WO 2015/093980 A1, WO 2016/099289 A1, WO 2016/099290 A1, WO 2019/110655 A1 and WO 2020/251373 A1.

A demand exists for reduction of carbon emissions and climate gases from industrial installations and for facilitating the transition to electrification of the industry. Clearly, very large amounts of energy can be stored and, accordingly, such storage can be large and costly. Typical energy storage capacity of a relevant TES may range from about 5 MWh to several GWh. It is thus clear that improvements of existing technology with respect to enhancement of storage capacity, efficiency, ease of operation, and quality of heat will have great value. For specific applications one type of thermal element and TES may be preferable to another.

The objective of the invention is to provide technology for providing improvement with respect to one or more of the issues briefly discussed above.

Summary of invention

The invention provides an element for thermal energy storage, particularly feasible for operating at high dynamic temperature and high absolute temperature, comprising a solid state thermal energy storage medium and a pipe heat exchanger embedded into the solid state thermal energy storage medium, a pipe heat exchanger inlet and a pipe heat exchanger outlet, wherein each of said inlet and outlet are connectable to heat transfer fluid sources or users, and wherein said element preferably is cylindrical. The element is distinguished in that:

the pipe heat exchanger has the shape of a helical coil, embedded into said cylindrical element, with a helical coil axis coaxial to and/or parallel to a cylinder axis of the element,

the element further comprises longitudinal bars, embedded into the solidstate thermal energy storage medium, arranged along the pipe heat exchanger parallel to the cylinder axis and fastened to the pipe heat exchanger.

For most embodiments of the element, one pipe heat exchanger is embedded into the element, with a helical coil axis coaxial to and thus also parallel to a cylinder axis of the element, which elements are optimal for high temperature and high temperature variations.. Elements of the invention may also or instead comprise embodiments with two or more helical coil pipe heat exchangers arranged side by side or coaxial within the same element, said pipe heat exchangers have helical coil axes coinciding with each other, i.e. coaxial, or/and parallel to the element cylinder axis and with radius of the helical coils being the same or being of different size. The phrase helical coil axis coaxial to and/or parallel to a cylinder axis of the element, means coaxial and/or parallel within 10°, 8°, 6°, 4°, 2°, 1° or less, most preferable and optimal exactly coaxial for embodiments for demanding operating conditions, but preferably also exactly coaxial and/or parallel for other embodiments, i.e. larger or wider elements with several pipe heat exchangers for less demanding operating conditions. Practical achievable deviations from said optimal coaxial and/or parallel orientation, and/or the cylindrical cross section shape, with deviations small enough to still define elements with said embedded pipe heat exchanger(s), and still providing economical advantage over prior art elements and prior art thermal energy storages, are hereby addressed to be within the scope of protection of the claims and the invention.

Preferably the element is cylindrical and has length of at least 3 times, 10, 20, 30, or 40 times the external diameter thereof.

The element preferably comprises radial or transverse bars or spacers. Most preferably, the element comprises a plurality of longitudinal embedded bars or spacers, preferably 2-12, arranged along and around the helical coil shaped pipe heat exchanger on the inside or outside and fastened to the helical coil shaped pipe heat exchanger at crossing points. Further the element comprises a number of embedded spacers along the entire length of the helical coil(s) that ensure that the position of the coil is held at the wanted distance in relation to the outer cylindrical surface of the thermal element. The number of such spacers depend on the length of the element and the number of coil loops. The spacers attached to the helical coil may be reinforcement bars of length consistent with element and helical coil diameter or they may be structural elements of size and shape consistent with holding the coil in the correct distance to the outer surface.

At least 2, preferably 3-12, embedded radial or transverse bars or spacers are arranged and fastened to the helical coil shaped pipe heat exchanger or the longitudinal bars at crossing points, so as to form a single continuous structure. Said single continuous structure can be fabricated, transported and positioned accurately in an element casting form as one unit to provide cost saving.

The solid-state thermal energy storage medium is preferably concrete, preferably refractory concrete or specialized concrete for thermal energy storage, wherein the helical coil shaped pipe heat exchanger and longitudinal and radial or transverse bars have been embedded into the concrete by casting when fabricating the element, wherein said pipe heat exchanger and bars are the only structure embedded in the concrete. Concrete means material that can be cast and hardened to a solid material. Alternatively, the solid-state thermal energy storage medium is compacted particulate solid material. The casting form may be a reusable form that is applied to one or several elements for the casting process. Alternatively, each element may comprise a cylindrical outer shell that serves as casting form and remains with the element when put into use. Such outer shell must have sufficient thickness and strength to sustain the internal pressure from the fluid concrete during casting.

The element of the invention has a pipe heat exchanger with helical coil pitch d, wherein d is determined by numerical simulations for the actual element and the intended use thereof, including material properties and a chosen dynamic temperature response of the element, using the equation:

Vs/Lh = p (R<2 >H – r<2 >Lh) / Lh (4)

wherein Vs is the volume of solid-state storage material per unit length, Lh is the total pipe length of the helical coil shaped pipe heat exchanger, R is element radius, H is element height or length and r is pipe radius of the pipe heat exchanger. The length of the helical coil Lh is directly connected to the pitch d through the equation

Lh = H/d sqrt((2πRc)<2 >+ d<2>) (8)

where Rc is the radius of the helical coil. A suitable value of Rc, in relation to outer radius R, which ensures efficient heat transfer is close to 0.7 for the case of a single helical coil heat exchanger in the element.

Assuming that the heat storage material is some form of concrete a typical value of the volume to length ratio of equation (4) is 0.008 to 0.016 (m<3>/m), which implicitly defines the optimal pitch range for an element of the invention with one pipe heat exchanger embedded. A typical value for an element diameter of 0,4 m is 0.012 (m<3>/m). The optimal value will also to some extent depend on the diameter of the heat exchanger itself which in turn depends on heat exchanger fluid, material properties, pressure and the specific storage application. A larger diameter for the heat exchanger pipe will imply a somewhat larger optimal pitch and shorter overall heat exchanger length.

The helical coil design for the pipe heat exchanger is beneficial with respect to reduction of cracking due to shrinkage, which may occur in connection with chemical reactions and loss of water in the cast concrete. This means that the element of the invention, comprising an embedded helical coil shaped pipe heat exchanger, has a further surprising technical and economic advantage compared to a concrete block element with straight pipe heat exchanger sections embedded inside the concrete.

The element of the invention preferably has a ratio of volume of solid-state storage material of the element, Ve, to the volume of the helical coil shaped pipe heat exchanger (excluding the volume of bars or spacers) of the element, Vhe, as follows:

Ve/Vhe = 10, 40, 80 or 150 or higher.

The invention also provides a thermal energy storage (TES), comprising a thermally insulated housing comprising one or more walls, floor and roof, and further comprising at least one inlet for HTF and at least one outlet for HTF or a combined HTF inlet and outlet through the thermally insulated housing. The TES is distinctive by comprising a plurality of thermal energy storage elements of the invention, wherein said elements are arranged within the thermally insulating housing and coupled to receive and deliver HTF from and to said inlets and outlets, from sources and to users outside the TES.

Preferably, the TES is comprising vertical standing elements with inlet for HTF in one end and outlet for HTF in an opposite end or combined inlet and outlet for HTF in both ends. TES embodiments with vertically standing elements with combined inlet and outlet for HTF in both ends are considered optimal for use as part of the thermal battery according to WO 2020/251373 A1. Vertically standing elements are preferable when the heat exchanger fluid is water-steam since the helical coil geometry facilitates smooth flow and smooth separation between compressed water and steam in two-phase fluid conditions (liquid and gaseous steam).

In another embodiment, the TES preferably is comprising closely stacked horizontal laying elements with inlet for HTF in one end and outlet for HTF in an opposite end or inlet and outlet for HTF in the same end. These embodiments can be particularly feasible when the HTF is a one phase fluid, such as thermal oil.

All TES embodiments with two or more blocks of closely arranged element of the invention are feasible or particularly feasible for “mix and shift” operations according to the principles described and illustrated in WO 2019/110655 A1.

Preferably, the TES of the invention has a thermal energy storage capacity of at least 5 MWh, more preferably at least 20 MWh, 40 MWh, 100 MWh, 500 MWh 1GWh or more, as the economic advantages of the TES of the invention increase with increasing size of the TES.

The invention also provides use of the element of the invention for building, scaling up or down and/or for maintaining a thermal energy storage. Adding or removal or replacing elements of the invention to or from a TES, are the steps required for the use.

With the element, TES and use of the invention, simplifications, combined with full utilization of all material in the element, flexibility with respect to size, the feasibility for high operating temperatures and high dynamic operating temperature and feasibility for prefabrication, all contribute in reducing the cost for energy storage, which reduces the cost for energy production and the grid, thereby facilitating the shift towards sustainable energy production. Estimates and modelling indicate that for a TES of the invention with energy storage capacity at and above 10 MWh, the improvements become significant. The TES can be built as large as required, up to several GWh, there is no inherent practical size limit and the improvements will normally increase with size and capacity.

The element and TES of the invention are designed for operation at high temperature, which in this context means above 100° C, such as from 100° C to 550° C, with typical element and TES operating temperature 200° C to 420° C, but also up to 550°C or above when the heat transfer fluid is water-steam and if specialized high temperature alloys are used, e.g. the so called nickel or cobalt based superalloys. The person skilled in the art should thereby exclude geothermal heat exchangers or probes from consideration as a thermal energy storage element, since boiling of water in the surrounding ground in addition to little or no continuous solid-state thermal energy storage medium prevents such probes from being feasible for the purpose of storing thermal energy within the probe.

The current invention can also be used for cold storage below zero degrees C. Clearly, in such case, the heat transfer fluid must be of a type which does not solidify during use.

The element and TES of the invention are designed for operation at high dynamic temperature, which in this context means that the temperature differences between heat transfer fluid inlet or outlet temperature and solid thermal energy storage material of the elements, can exceed 50°C, more preferably exceed 100° C, 150° C or even exceed 300° C. The person skilled in the art should thereby realize that the current invention is particularly suited for dealing with thermally induced stress and strain without resulting in damage and fracturing that could severely reduce the performance of the storage.

Operating the thermal energy storage and element of the invention at high dynamic temperature increases the overall storage capacity and thereby also size and cost of the storage. Further, a large temperature difference at the early stage of a heat charge or discharge process implies that the rate of heat transfer will be relatively high during this stage whereas this effect decreases as temperature in the solid state material becomes closer to the temperature of the incoming heat transfer fluid.

The term wherein said element is in substance cylindrical, means cylindrical within 10% or 5% of the largest cross-section dimension, more preferably cylindrical within 3, 2, 1, 0.5, 0.25, or 0,1 % or smaller of the largest cross section dimension. Cylindrical means having identical cross-section shape along a cylinder axis, preferably within very tight tolerances. A cylinder can have cross section shape that is triangular, square, rectangular, polygonal, elliptic or intermediate shapes or having in principle any fixed shape. However, most preferably the cylinder is circular with respect to cross section shape since maximum capability for high dynamic and absolute temperatures for operation thereby is achieved.

The objective of the invention is met by a particular thermal element design suitable as part of a thermal energy storage comprising

• A heat exchanger for transfer of heat to and from the thermal element having shape of a helical coil

• A solid-state heat storage material cast or densely placed around the helical coil heat exchanger

wherein the thermal element is characterized by

• Having a cylindrical shape where the length is more than 5 times the external diameter of the cylinder

• There is a heat exchanger inside consisting of pipe formed as a helical coil that spans from one end of the cylinder to the other

• The heat exchanger is positioned with coil axis coinciding with the central axis of the cylindrical element

• The diameter of the heat exchanger coil is between 0.6 and 0.8 of the external diameter of the cylindrical element

• The coil has a prescribed pitch, meaning distance between neighbouring coil loops, to be determined on the basis of ratio between volume of solid state material and heat exchanger length such that sufficient heat transfer rate can be obtained in relation to the operational requirements of the TES

• The heat exchanger has one inlet/outlet at one end and another inlet/outlet at the other end or, alternatively, both ends of the heat exchanger are at the same end by including a central pipe section connected with the helical coil going from one end to the other

• A solid-state material, for example cast concrete or other type of dense material, is placed within the thermal element filling the space between the helical coil and the outer surface of the thermal element ensuring direct contact between heat exchanger and solid-state material

• The solid-state material has properties suitable for sensible heat storage including good thermal conductivity, good thermal capacity, adequate structural strength and deformational properties, all of this within the entire thermal range of operation for the TES

• Transverse and longitudinal spacers are applied to ensure that the helical coil is correctly positioned within the thermal element

• The pipe heat exchanger is preferably a small pipe diameter heat exchanger designed for turbulent flow of HTF at a nominal design flow rate, as determined by a range of or minimum value of Reynold’s number, e.g. Re > 4000 or 5000.

In alternative versions of the invention a single element may include two or mor co-axial helical coil heat exchangers with same or different coil diameter. In such case the requirements listed above still apply.

Brief description of illustrations

Figure 1 illustrates two main versions of the helical coil thermal element

Figure 2 illustrates in further detail a section of a helical coil thermal element

Figure 3 illustrates equivalence models represented by slices and pieces of solid state material and heat exchanger pipe

Figure 4 shows a series of steps that may be taken to simplify numerical simulation of the transient heat transfer problem for helical coil thermal elements

Figure 5 illustrated a set-up for filling or casting thermal elements

Figure 6 shows an example of removable and reusable casting form

Figure 7 shows how once-through and one-sided couple elements may be serially configured

Figure 8 shows how once-through elements may be configured in parallel and combined into a completed module unit

Figure 9 shows an example of configuration of a large thermal energy storage consisting of many modules that can be suitable for time dependent shift strategy for charging and discharging of heat

Figure 10 shows examples of ways of stacking the elements in horizontal position

Detailed description of the invention

The thermal elements can be made by various manufacturing processes. The helical coil heat exchanger can readily be made by bending a straight pipe of prescribed length around a circular core with diameter corresponding to the prescribed internal diameter of the coil placed in the element after the bending process. The winding process also includes control that that the helical coil obtains the prescribed pitch. Other pipe bending methods for producing helical coils include bending of the straight pipe around three or more groove-type caster wheels positioned such that the coil obtains the prescribed diameter and pitch.

The placing of the solid-state material around the coil to form a thermal element may also be performed in different ways depending on the particular application. In case of using a castable material, such as various types of concretes, the element itself may include an outer container, such as metal ducts used for air ventilation systems or thin-walled steel cylinder with the prescribed diameter for the thermal element. This helical coil is placed in correct position with spacers in the container which also serves as casting form around the helical heat exchanger coil when placing the concrete. The outer container can also serve as extra protection and reinforcement for the thermal element, and leakage retention, when finally placed and used in the TES. Clearly, the casting is done in a vertical position, preferably with a cover lid to prevent leakage at the bottom with opening for exiting heat exchanger pipe.

Alternatively, a reusable cylindrical casting form may be used for the casting process. The helical coil is then placed in correct position with spacers within the casting form. The internal diameter of the form clearly determines the outer diameter of the element after casting. The casting form can be removed after the cast material has solidified with sufficient strength. The casting form can later be cleaned and repeatedly used for sequential casting.

As stated, the helical coil is placed inside the fixed or removable casting form before the casting begins. Exact positioning is ensured by spacers which ensure that the coil is correctly positioned in relation to the inner side of the casting form. Such spacers can be simple metal bars or fins attached to the helical coil with geometry fitting the specified distancing.

In addition to this, longitudinal spacers, such as steel reinforcement bars (rebars) shall preferably be attached to the helical coils. The reason for this is that a long, helical coil will act like a spring and deform or sag due to own weight even if the upper and lower coil ends are kept in place. It is thus necessary to ensure that each loop of the coil is kept in the right position by connecting the coil, at regular distance, with longitudinal spacers. The number of such spacing bars may vary from 2 to 12, and they can be attached to the heat exchanger coil by wire binding, which is a well know methods for rebar work, or by spot welding, or by other ways of connecting helical coil and longitudinal spacers. Application of rebar spacers also have the added advantage of serving as reinforcement for the thermal element which enhances its ability to withstand thermal stresses and reduce thermal cracking. Such reinforcement is also advantageous for providing bending strength when elements are precast, handled, and transported.

The thermal elements of the invention may be fully prefabricated in a factorylike manufacturing facility in which casting is performed. The hardened elements may thereafter be transported by truck, train, or ship to the installation site of the TES. In many cases it may be desirable to carry out such shipment within standard type, multimodal containers which provide protection as well as easy handling.

Alternatively, empty thermal elements with helical coil positioned inside, preferably inside a permanent casting form, can be transported to the TES site where they are positioned in their final, vertical orientation and thereafter filled with the solid-state casting material. Clearly, lateral structural support must be provided to prevent the vertically positioned elements from tipping over. Such support may either be provided by bundling of a group of thermal elements together or by providing a larger structural grid into which the thermal elements are inserted.

Final installation work includes connecting the thermal elements with the overall piping system for the TES. The invention will normally enable significant reduction of the number of piping connections to be made in relation to previous patents referred to herein. The reason for this is that each helical coil element will normally be larger in size and include a greater length of heat exchanger pipe than previous thermal element patents referred to herein. Cost savings can also be obtained in relation to patent NO 340371, corresponding to WO 2016/099289 A1 since there is no need for a strong steel cassette to carry the weight of the thermal elements.

The main parts of the invention are illustrated in Figure 1. The totality of the thermal element is denoted 100.101 is the formwork or casing or container of cylindrical form which defines the outer boundary of the thermal element. This part also serves as casting form. In case of casting in reusable casting form 101 will not be required. 102 is the helical coil pipe heat exchanger which contains the heat transfer fluid.103 is the solid-state material which is cast or otherwise filled into the container 101 with bottom lid or closure 104. The solid-state material 103 is closely in contact with the helical coil 102 and thereby ensures direct connection between the two parts for efficient heat transfer. The helical coil heat exchanger has a top pipe connection 105 to an external pipe system 110 and the bottom part of the coil has an equivalent pipe connection 106 to another part of the external pipe system 110. The figure also shows longitudinal bars or spacers 107 connected to the helical coil at multiple points and ensure that the helical coil retains its correct position in the longitudinal direction. These longitudinal spacers may be conventional rebars for concrete and can thereby also serve as reinforcement. Similarly, 108 are spacers that secure the positioning of the helical coil in relation to the outer side of the element. The thermal element shown in Figure 1a may be termed “once-through” because the heat transfer fluid is led once through the helical coil from one end to the other.

An alternative representation of the invention is shown in Figure 1b. In this case both piping connections 105 and 106 for the heat exchanger are positioned at the same end. This is made possible by having a central return pipe 109 connected to the lower end of the heat exchanger and returning from the bottom end of the element to the top end. Clearly, this return pipe also contributes to increasing the heat transfer. A thermal element of this type may be termed “return” because the heat transfer fluid is led through the coil from one end to the other and thereafter returned with an internal return pipe back to the first end. Note also that use of a return pipe may also imply that the optimal radius for the helical coil 102 may be somewhat larger than in the once/through case.

Figure 1c shows a once-through arrangement being similar to Figure 1a with the difference that this case has two co-axial helical coil heat exchangers within the same element represented by 102 and 112. Clearly this representation of the invention requires an extra set of pipe connections 115 and 116, longitudinal bars 117 and spacers 118. The inner and outer coils may be coupled with the same manifold system or to separate manifolds. The figure shows that the inner helical coil has a radius that is different from the radius of the outer coil.

However, it is also possible to have multiple heat exchangers with same radius. For example, a double helical coil with same radius implies that the pitch for each coil is the double of the distance between adjacent pipes.

Further details of the invention are illustrated in Figure 2 showing geometric relationships. The cylindrical thermal element has external radius R, also termed Re, defining the position of the external shell or casting form 101. The position of the helical coil is similarly defined by radius Rc for the central axis of the coil pipe in relations to the central axis of the element. The coil pipe itself has radius r. The helical coil moves in the longitudinal direction with a distance d per full 360 degrees rotation; this distance is also denoted “pitch”. The full cross-sectional area A is divided in two parts, Ai is the area inside radius Rc and Ao is the remaining area outside radius Rc. Element radius R, coil position radius Rc, pitch distance d, and outer radius r and wall thickness t of the coil pipe are important geometric parameters that decide the overall heat transfer and heat storage properties of the element along with the thermal properties of the solid-state material. A preferred position Rc of the coil in relation to outer radius R is that Ai should be approximately equal to Ao which gives

Rc = R / sqrt(2) = 0.707 R (1)

Other coil positions can also be acceptable.

Figure 1b indicates that the thermal element may have a central return pipe 109 with connection 106 at the same end as pipe connection 105. In this case heat will be transferred directly to and from the central zone of the thermal element to the central pipe 109 and the position radius Rc of the coil can be increased somewhat in relation to equation (1) to provide increased heat transfer.

The pitch d along with the other geometric parameters determine the length of heat exchanger in relation to amount of solid-state storage material. The length of one loop of helical coil is given by

Lho = sqrt ((2pRc )<2 >+ d<2>) (2)

The total number of loops is given by the ratio between total thermal element height H and d; thus, the total length Lh of coil heat exchanger inside the element is given by

Lh = Lho H / d (3)

The volume of solid-state storage material Vs per unit length of heat exchanger is a key parameter for heat transfer given by

Vs/Lh = p (R<2 >H – r<2 >Lh) / Lh (4)

where r is the external radius of the heat exchanger. In practical terms this volume to length ratio depends on the unit of length, for example m<3>/m.

Figures 3 relates to thermo-mechanical analysis of the heat transfer element. It is to be noted that for the present innovation thermal heat transfer analysis can generally be decoupled from thermal stress analysis and these analyses may thus be carried out separately. In a most comprehensive heat transfer analysis the entire thermal element with heat exchanger and solid-state material may be analyzed by numerical simulation with discretization techniques such as the finite element method with full three-dimensional discretization. However, in most cases the pitch distance d will be chosen to be significantly smaller than outer radius R which means that a good approximation can be to model the heat transfer and thermal stress interaction between coil pipe and solid material as being an axisymmetric problem about the central element of the thermal element. This is illustrated in Figure 3a where one “disc” of the thermal element has thickness d’ with one loop of the heat exchanger positioned in the middle longitudinal distance within this slice. The equivalent thickness d’ is slightly smaller than d to ensure that the relationship between volume of solid-state material and length of heat exchanger is kept unchanged in relation to (4)

d’ = d 2pRc / Lho (5)

Figure 3b illustrates a “cake piece” model that can be analyzed as a simpler three-dimensional representation. Note that correct boundary conditions for the “cake piece” must be applied both for heat transfer and thermal stress analysis.

Figure 3c illustrates a further simplification step by analyzing the slice in Figure 3c as a two-dimensional, axisymmetric model, with or without a central pipe. Such model can be applied both for heat transfer and thermal stress analyses. Clearly this model is computationally more efficient at the same time as it has been shown that it gives very good accuracy.

It is an important property of the current invention that, rather than using the complex three-dimensional geometry of the thermal element, the thermal performance may be analyzed in a simplified and highly efficient way. Figure 4 illustrates how the numerical simulation model for heat transfer (only) may be simplified one step further by assuming an equivalent axisymmetric simulation model about the center of the of the heat transfer pipe. This means that the axisymmetric model of Figure 3c is further simplified by assuming an axisymmetric representation about the pipe itself. Numerically speaking this is a one-dimensional model since discretization is only done in the radial direction, see dots indicated for discretization model. The figure illustrates further how the “cake slice” (a) is replaced by an axisymmetric disc (b) with same volume of solid-state material per unit length of heat exchanger pipe as for the actual helical coil element. This equivalence enables the numerical model and integration of the disc to be done by a one-dimensional model (c) rather than as a two-dimensional or three-dimensional model for the heat transfer simulation. The equivalent radius Re is determined on the basis of giving the same ratio of solid-state material to heat exchanger length as in equation (4)

Re = R sqrt(H/Lh) (6)

Thus, the full simulation of thermal response of the thermal element can be simulated as a one-dimensional, axisymmetric disk model (c) per unit length of the pipe along with a one-dimensional discretization in the length direction of the entire length of the helical coil within the thermal element (d) as shown in the lower part of the figure. This means that the proposed simplified, time dependent (transient) heat transfer analysis involves two one-dimensional spatial discretization combined with a time step integration of the full process of transfer of heat to and from the mass and heat flow of the heat transfer fluid in the pipe. Comparative, numerical simulations have confirmed that this approach gives satisfactory accuracy.

The current disposition describing simplification of the numerical simulation of the heat transfer process for thermal elements of the invention is highly relevant because it demonstrates that the invention is not only feasible in physical terms; it also demonstrates that the all-important numerical simulation of transient heat transfer to and from a large TES with up to many thousands of elements can be done using a numerical model which simplifies the overall complexity down to a simplified and manageable level.

The best heat transfer to and from the heat exchanger is obtained when the Reynolds number of the heat transfer fluid lies above the critical value and thereby ensures turbulent flow. The critical value is influenced by the radius of curvature of the helical coil. The coil radius Rc is defined in Figure 2 whereas the actual radius of curvature Rc’ is influenced by the pitch d through

Rc’ = sqrt (Rc<2 >+ (d/2p)<2>) (7)

Manufacturing of piping and placing of solid-state material within thermal elements of the invention can be done very efficiently. Figure 5 shows how the outer shell or casting form 101 can be positioned vertically for filling of the solidstate material which in typical cases may be some type of concrete. At this point the helical coil and spacers is correctly positioned inside the casting form. There is a lateral support system, such as lateral struts 120 or some type of frame structure that provide overall stability of one or several elements during the casting operation. In case the thermal element is of the type with pipe connections in both ends, see Figure 1a, the foundation plate 121 at the lower end has opening for the lower pipe end 106. Figure 5 also shows how a casting tube or hose 122 may be positioned on inserted from above with an arrow 123 indicating material inserted by way of pumping or other means. Vibration devices may also be applied to ensure good compaction during the casting process. The completion of the thermal elements 100 can be made in a prefabrication facility or in their final position at the site for the TES.

The casting form 101 can be a be a thin-walled steel cylinder or spiral duct that can be left permanently as an integral part of the thermal element. Alternatively, the casting form can be a reusable steel form that is removed from the thermal element after the solid-state material has hardened. Figure 6a illustrates a removable casting form comprised of a steel tube split in two halves 124. These parts may be joined together using bolts 125 or other types of clamping or locking mechanism, see Figure 6b and 6c. Figure 6c also shows helical coil 102, longitudinal spacers 107 and transverse spacers 108 placed in position before the filling process starts. Clearly, it is also easy to make forms for casting a multiple of elements at the same time.

The heat transferto and from the thermal elements is determined by the flow rate and heat transfer from the main heat delivery sources, alternatively by the the properties of main heat sinks, as well as the way the totality of heat transfer fluid is led through the various elements in the entire storage. This process depends on the configuration of the overall piping system and valves that control the flow. In principle, the flow through a group of thermal elements may be organized in two alternative ways: by serial flow or by parallel flow, or combinations thereof. Serial organization involving several elements increases the length of heat exchanger engaged through a continuous flow with increased associated solid-state material. A consequence of serial coupling consequence may will be increased flow rate (and Reynolds number) and higher internal friction and pressure loss through. Alternatively, parallel coupling of a group of elements implies that the mass flow to this group of elements is split into parallel flows smaller than the inflow to the group. Consequences of this are reduced flow rate (reduced Reynolds number) through each element and reduced overall friction and pressure loss. Choice of piping system organization depends on the characteristics of the specific application as well as on what type of heat exchanger fluid being used. Some applications may also benefit from using a time dependent shift strategy to achieve efficient heat transfer and acceptable mass flow. A best solution may be found by applying the principles of WO 2019/110655 A1.

Figure 7 illustrates two types of serial connections applied for a group of thermal elements by the invention. Figure 7a shows in principle serial coupling for a group of elements where each element is of type “once-through” as shown in Figure 1a. The figure indicates 12 “once-through” elements in a series whereas any other number of connected elements may apply.130 is the serial connection pipe on one side or end of the series and 131 is the connection pipe on the other side or end. There are connections 132 on the top side and 133 on the bottom side between neighboring elements. The elements stand on a foundation 134 with openings or cavities making space for the bottom connections 133. Figure 7b shows correspondingly serial connection for a group of elements of type “return”.135 is the serial connection pipe on one side and 136 is the connection pipe on the other. All pipe connections between neighboring elements 137 are on one (top) side. The foundation 138 on which the elements rest is indicated as a simple foundation slab. The serial configurations shown in Figure 7 may be preferable when the heat transfer fluid is thermal oil. A consequence of that each helical coil element normally will have a relatively long helical coil heat exchanger inside is that only a limited number of elements can normally be serially coupled because of internal friction and pressure loss.

A group of elements may alternatively be coupled in parallel. Figure 8 shows an example where a group of 25 elements according to the invention are configured in a 5 by 5 pattern for parallel flow with “once-through” elements, see also Figure 1a. In this case the combined fluid flow into the group of element system is split into 25 parallel flows by way of a manifold system at one end and recombined by a similar manifold to a single flow at the other end. A consequence is that the fluid mass flow through each element is much reduced compared with the inflow to the group resulting in relatively low flow rate (and Reynolds number) and low pressure loss. Parallel configuration may be preferable for the thermal elements of the present invention because of considerable length of heat exchanger coil in each element and the overall element size. Another advantage is the “smooth continuity” of a vertically oriented helical coil. This vertically oriented elements and the continuous heat exchanger geometry is particularly well suited for applications involving twophase heat transfer fluid, such as in the case of “direct steam”, because it facilitates smooth transition between the two phases with liquid phase retained in the power part and gaseous phase, such as steam, in the upper part. This property of the proposed design reduces instabilities and problems with formation of “plugs” and “slugs” and large bubbles in the piping system. This means that vertically standing thermal elements of current invention with oncethrough flow have advantages over previous thermal elements systems with horizontally lay-out of elements and piping. As mentioned more briefly above, this is in agreement with the principles of WO 2020/251373 A1, as combined with an additional pressure vessel coupled in parallel.

Specifically, In Figures 8a and 8b show a top view and side view of the piping system for a 5 by 5 group of elements where 141 and 142 are bottom and top connection pipes 141, 142 (headers) that that spread and assemble and the heat transport fluid through feed pipes 143, 144 for alignments of 5 elements. In case of a hot water and direct steam two-phase application the inflow and outflow of water is at the bottom and inflow and outflow of gas is at the top both for charging and discharging; the lighter steam phase will always be located in the upper parts of the helical coils. Figure 8b shows that the group of vertically standing elements rests on a support structure 145 which has openings for the lower pipe connection for each element. This structure may be a reinforced concrete slab or a steel grid with openings for the piping. Access space can be provided below the support structure to carry out the work to connect the pipes, as shown in the figure. Note also that there will be additional structures, not shown in the Figure 8b, to ensure correct positioning of the elements and to support the elements laterally.

A further development of this concept is shown in Figure 8c where a group of thermal elements are placed within a frame structure 150 to form a fully prefabricated cassette with completed elements and piping system for the element group. In the case shown cassette contains 5 by 5 elements, but any other number of elements may apply. The prefabricated cassette frame structure has sufficient strength to sustain forces during transport and handling as well as to provide sufficient stability after positioning and casting.

Accordingly, 151 denote members of the frame and 152 denote cross-bracings.

153 indicates a support structure onto which the elements will rest on during erected position. The figure further shows a reinforced concrete slab 154 onto the module is placed. As shown in the figure this concept allows for full prefabrication of the piping system 141, 142, 143 and 143 of the modules, and only connections to the bottom part of the main piping system need to be attached at site of the TES. Notably the cassettes can be efficiently prefabricated in a manufacturing facility and transported to site, erected to vertical position, and correctly placed on location within the total storage system for casting.

Figure 9 illustrates how the modular version of the invention may be implemented for use in a large TES; the figure may be seen as a combined representation of piping for both the bottom and top piping layers. The example shows a storage 160 consisting of three main sections 161, each section consisting of two arrays 162 with 5 modules 150 each. This storage thus contains 60 modules with 25 thermal elements each, and thereby a totality of 1500 elements. Each array has feed pipes 163, 164 at the bottom and the top coupled to the module manifold outlets 141, 142, respectively. The array feed pipes are assembled with common manifolds 165, 166 that connected with main pipes 167, 168 at the bottom and top of the entire TES assembly. Oncethrough TES layouts as generally illustrated in Figure 9 lend themselves to parallel shift strategy, in agreement with WO 2019/110655 A1. Time shifting of different parts or blocks of the TES require controllable valves to direct the flow of the heat transfer fluid into blocks as directed by a common control system coupled to a system of temperature sensors, mass flow meters etc. In this way very efficient heat transfer with turbulent flow can be obtained along with better temperature control for fluid outflow.

As previously noted, the invention also allows for placing the helical coil thermal elements of type once-through, see 1a, or return, see 1b, in horizontal position for use in a TES. Such position is highly feasible when the heat transfer fluid is a one-phase fluid such as thermal oil whereas it may not be suitable for twophase water-steam. Single elements may be fully prefabricated in an element factory and transported to site in, for example, standard containers. The elements may thus be stacked and configured as seen suitable for the application. Figure 10a shows simple stacking principle where elements are placed on top of each other with only supports 171 below. Figure 10b shows a similar case of staggered elements where lateral support frames 172 are supplied. Figure 10c has also lateral support frames 173 whereas the elements are configured in a aligned, vertical pattern rather than being in staggered position. The connecting pipes for the elements are not shown in these figures. Notably it is simple to connect external piping both for once-through and return type elements. The illustrations in Figure 10 do not show external insulation and protection which clearly is simple to implement.

The storage and the element of the invention are particularly feasible for operating at high dynamic temperature and high absolute temperature and can provide enhanced overall performance of thermal energy storages. The invention can be used for moderate and very high temperature absolute temperatures as well as for cold storage below zero. Said elements preferably comprise carbon or alloy steel pipes suitable for the actual thermal range of the storage whereas the solid-state material may be a special type of concrete or composite material likewise suited for the actual operational conditions of the TES. The present invention describes a design which can significantly simplify the TES using prefabricated or on site cast thermal elements with great flexibility in determining the size of the elements, including very large thermal elements, while still maintaining an optimal ratio between length of heat exchanger pipe and amount of solid-state storage material which is an essential parameter for efficient heat transfer. The elements can be arranged in horizontal or vertical position. The vertical arrangement seems preferential since it reduces requirements for supporting structure, it allows for simplified overall piping and valve systems and reduced foot-print of the TES, it has significant advantages with respect to use of two-phase water-steam HTF, and it allows for easy drainage of HTF. These improved functions enable more efficient operation and significantly reduced overall cost of the TES.

Claims

1.

Element for thermal energy storage, comprising a solid-state thermal energy storage medium and a pipe heat exchanger embedded into the solid state thermal energy storage medium, a pipe heat exchanger inlet and a pipe heat exchanger outlet, wherein each of said inlet and outlet are connectable to heat transfer fluid sources or users, and wherein said element is cylindrical, c h a r a c t e r i s e d i n that

the element further comprises longitudinal bars and/or spacers, embedded into the solid-state thermal energy storage medium, arranged along the pipe heat exchanger parallel to the cylinder axis and fastened to the pipe heat exchanger.

2.

Element according to claim 1, further comprising radial bars or spacers for positioning of the helical coil laterally.

3.

Element according to any one of claim 1-2, comprising a plurality of longitudinal embedded bars, preferably 2-12 in number, arranged along and around the helical coil shaped pipe heat exchanger on the inside or outside and fastened to the helical coil shaped pipe heat exchanger at crossing points, and further comprising at least 2, preferably 3-12 in number, embedded radial bars or spacers arranged and fastened to the helical coil shaped pipe heat exchanger and/or the longitudinal bars at crossing points, so as to form a single continuous structure that can be fabricated, transported and positioned accurately in an element casting form as one unit.

4.

Element according to any one of claim 1-3, wherein the solid-state thermal energy storage medium is concrete, preferably high temperature or refractory concrete or specialized concrete for thermal energy storage, wherein the helical coil shaped pipe heat exchanger and longitudinal and radial or transverse bars have been embedded into the concrete by casting when fabricating the element, wherein said pipe heat exchanger and bars are the only structure embedded in the concrete.

5.

Element according to any one of claim 1-4, wherein the helical coil pitch d is determined by numerical simulations for the actual element and the intended use thereof, in consideration of material properties and preferred dynamic temperature response of the element, using the volume to length relationship:

Vs/Lh = p (R<2 >H – r<2 >Lh) / Lh (4)

wherein Vs is the volume of solid-state storage material per unit length, Lh is the total pipe length of the helical coil shaped pipe heat exchanger, R is element radius, H is element height or length and r is pipe radius of the pipe heat exchanger.

6.

Thermal energy storage, comprising a thermally insulated housing comprising one or more walls, floor and roof, and further comprising at least one inlet for HTF and at least one outlet for HTF or a combined HTF inlet and outlet through the thermally insulated housing, c h a r a c t e r i s e d i n that the thermal energy storage comprises a plurality of thermal energy storage elements according to any one of claim 1-5, wherein said elements are arranged within the thermally insulating housing and coupled to receive and deliver HTF from and to said inlets and outlets, from sources and to users outside the TES.

7

Thermal energy storage according to claim 6, comprising vertical standing elements with inlet for HTF in one end and outlet for HTF in an opposite end or inlet and outlet for HTF only at one end or both ends, and preferably comprising groups of closely arranged elements and pipes and valves for controlling the flow of heat transfer fluid to and from said groups.

8.

Thermal energy storage according to claim 6, comprising closely stacked horizontal laying elements with inlet for HTF in one end and outlet for HTF in an opposite end or inlet and outlet for HTF at the same end where elements are kept in position by interlocking, staggered pattern or by use of laterally supporting frame structures.

9.

Use of the elements of any one of claim 1-5, for building, scaling up or down, and for maintaining a thermal energy storage, including closing off flow to or replacing damaged elements and including dismantling and reconfiguring elements for use in a new storage.