A THERMAL ENERGY STORAGE

Field of the invention

The invention relates to a thermal energy storage for reversibly storing thermal energy. The thermal energy storage comprises a storage tank, at least one fluid inlet, a fluid outlet, and a phase change material arranged inside the storage tank. The invention also relates to a method for storing thermal energy in a thermal energy storage and use of a thermal energy storage.

Background of the invention

Storage of thermal energy (heating or cooling systems) is widely used for compensating between consumption and production of energy (e.g., solar thermal systems or district heating systems) or for peak shaving applications in order to reduce size of utility systems (e.g., compressors in cooling systems). For most residential, commercial, and industrial applications, thermal energy storages (TES) apply water as energy storage media, however the size of storage tanks as well as equipment costs easily become a barrier. Thus, it is known to use phase change materials (PCM) in relation to thermal energy storages as a way to reduce the size of TES by utilizing the phase change enthalpy for melting or solidifying the PCM.

However, PCM is more expensive than water and usually needs to be encapsulated in order to avoid mixing of the heating/cooling circuit fluid with the PCM and encapsulation compromises efficiency of the TES capacity and increases cost.

Thus, from the German patent application DE 10 2014 103 108 Al it is known to guide a thermal transport fluid in the form of oil down to the bottom of a TES from where a distributor plate distributes the oil before it flows upwards though a phase change material to exchange heat with the PCM before the oil is guided out of the TES at the top. However, since the PCM is in direct contact with the oil, there is a risk of the liquid PCM leaving the TES and the TES is therefore included in closed circuit in which the oil exchanges heat with another circuit through a heat exchanger. However, heat exchangers are expensive, reduces overall efficiency and the escaping PCM might damage or clog other components in the circuit.

An object of the invention is therefore to provide for an advantageous technique for storing thermal energy in a thermal energy storage.

The invention

The invention provides for a thermal energy storage for reversibly storing thermal energy. The thermal energy storage comprises a storage tank, at least one fluid inlet arranged at the top of the storage tank and a fluid outlet arranged at the bottom of the storage tank. The thermal energy storage further comprises a phase change material arranged inside the storage tank, so that a thermal transport fluid flowing through the storage tank from the at least one fluid inlet to the fluid outlet is in direct contact with the phase change material, and boundary layer detection means arranged to detect the lowest position of the phase change material inside the storage tank.

Forming the thermal energy storage so that the thermal transport fluid is in direct contact with the phase change material when it flows through the storage tank is advantageous in that direct contact ensures the fastest, most inexpensive, and most efficient heat exchange between the thermal transport fluid and the phase change material. Furthermore, providing the thermal energy storage with boundary layer detection means arranged to detect the lowest position of the phase change material inside the storage tank is advantageous in that it hereby is possible to stop the flow of the thermal transport fluid through the storage tank or regulate the flow rate of the thermal transport fluid to ensure that the phase change material does not move to close to the fluid outlet and thereby prevent that the phase change material escapes the storage tank. Arranging the at least fluid inlet at the top of the storage tank and the fluid outlet at the bottom of the storage tank is advantageous in that if the thermal energy storage is provided with only a single fluid inlet the flow of thermal transport fluid will cause the phase change material to solidify around the flow of thermal transport fluid to form a flow channel through the phase change material when the thermal transport fluid is colder than the phase change temperature between liquid and solid of the phase change material. And likewise, if the thermal energy storage is provided with several fluid inlets, the flow of thermal transport fluid will form several flow channels through the phase change material. And if the thermal energy storage is provided with a multitude of fluid inlets so that the thermal transport fluid is distributed across the top surface of the phase change material the flow will form a multitude of flow channels providing the solidified phase change material with a sponge-like or porous structure. Thus, when the process is reversed, and the the thermal transport fluid is hotter than the phase change temperature between liquid and solid of the phase change material, the thermal transport fluid will be cooled efficiently when flowing through this porous structure and/or flow channels formed in the solidified phase change material.

It should be noted that in this context the term “phase change material” should be understood as any kind substance which releases/ab sorbs sufficient energy at phase transition between solid and liquid phase at a specific phase change temperature to provide useful heat or cooling. By melting and solidifying at the phase change temperature, the PCM is capable of storing and releasing large amounts of energy compared to water, brine, or other thermal transport fluids. Heat is absorbed or released when the material changes from solid to liquid and vice versa or when the internal structure of the material changes; PCMs are accordingly also referred to as latent heat storage (LHS) materials. PCMs includes any kind of organic (carbon-containing) materials derived from petroleum, from plants or from animals - such as any kind of paraffins, lipids or sugar alcohols; and inorganic materials such as salt hydrates - e.g., natural salts or are by-products of other processes; or other or any combination thereof. It should also be noted that in this context the term “boundary layer detection means” should be understood as any kind of boundary layer detector suited for detecting the lowest position of a phase change material inside a storage tank. I.e. the term includes any kind of radar system, X-ray system, vision system, floating switch system, any kind of system comprising a reflector device arranged in the boundary layer at the bottom of the phase change material, wherein the position of the reflector device can be detected by means of a sensor, any kind of sensor or sensor array arranged inside the tank, wherein the type of material, substance or fluid present at the sensor can be detected by the sensor(s) or deducted by means of output from the sensor(s).

Further, it should be noted that any reference to orientation - such as top, bottom, vertical, under, over, above, side etc. - throughout this document is made in relation to the thermal energy storage being orientated as it would be during normal use of the thermal energy storage - i.e. where the fluid inlet is at the top of the thermal energy storage and the fluid outlet is arranged at the bottom of the thermal energy storage.

In an aspect of the invention, the storage tank further comprises a fluid distributor arranged at the fluid inlet for distributing the thermal transport fluid across an upper surface of the phase change material.

Arranging a fluid distributor at the fluid inlet is advantageous in that when the thermal transport fluid is distributed across the upper surface of the phase change material the phase change material forms a sponge-like or porous structure during curing - i.e. during phase change from liquid to solid phase - which is permeable to the thermal transport fluid. Thus, by distributing the thermal transport fluid across an upper surface of the phase change material by means of a fluid distributor - such as one or more spray nozzles, spray arms, one or more jets, one or more droplet or drop generators or other - it is ensured that substantially all the phase change material is activated thereby increasing efficiency of the thermal energy storage.

In an aspect of the invention, the fluid distributor comprises distributing pattern changing means arranged for changing the distribution pattern of the fluid distributor continuously or occasionally. Providing the fluid distributor with distributing pattern changing means arranged for changing the distribution pattern of the fluid distributor is advantageous in that it hereby the chance of activating the entire volume of phase change material is increased and in that continuous or occasional changes in the distribution pattern of the thermal transport fluid will aid in forming the fluid permeable porous structure in the phase change material.

It should be noted that in this context the term “distributing pattern changing means” should be understood as any kind of distributing pattern changer suited for actively changing the distribution pattern of the fluid distributor continuously or occasionally. I.e., the term includes any kind of rotating, reciprocating or otherwise moving device in which the motion is driven by the flow through the fluid distributor, any kind of motorised or otherwise externally driven devices capable of changing the position, direction, or orientation of the fluid distributor or similar or any combination thereof.

In an aspect of the invention, the storage tank further comprises an outflow device having one or more device inflow openings and a device outflow opening, wherein the device outflow opening is connected to the fluid outlet and wherein a total area of the one or more device inflow openings is greater than the area of the device outflow opening.

If the flow rate of the thermal transport fluid out of the fluid outlet is too high the risk of phase change material being sucked out of the storage tank increases and it is therefore advantageous to provide the fluid outlet with an outflow device in which the total area of the device inflow opening or openings is greater than the area of the device outflow opening so that the flow rate through the device inflow openings is reduced.

In an aspect of the invention, the density of the thermal transport fluid is higher than the density of the phase change material. Using phase change material having a lower density than the density of the thermal transport fluid is advantageous in that gravity hereby can be used for generating a flow of the thermal transport fluid down through the column of phase change material towards the fluid outlet at the bottom of the storage tank.

In an aspect of the invention, the boundary layer detection means comprise a distance sensor arranged at the bottom of the storage tank, wherein the distance sensor is arranged to determine a distance to a reflector device of the boundary layer detection means, wherein the reflector device is arranged inside the storage tank and wherein the reflector device has a density which is lower than the density of the thermal transport fluid and higher than the density of the phase change material.

Forming the reflector device so that it has a density lower than the density of the thermal transport fluid and higher than the density of the phase change material is advantageous in that the reflector device will then always stay at the bottom boundary layer between the phase change material and the thermal transport fluid - i.e., at the lowest position of the phase change material inside the storage tank - and it is thereby easy to detect the lowest position of the phase change material by means of a distance sensor - such as a ultrasonic sensor, a photo optic sensor arrangement, a vision sensor, a capacitive or inductive sensor or other - arranged to detect the vertical distance to the reflector device e.g. from the bottom of the storage tank or from the fluid outlet.

In an aspect of the invention, the distance sensor is an ultrasonic sensor.

An ultrasonic sensor is a simple and relatively inexpensive sensor particularly suited for detecting a vertical distance to a reflector device inside a storage tank in that an ultrasonic sensor can be arranged inside and/or outside the tank.

In an aspect of the invention, the distance sensor is arranged outside the storage tank. Arranging the distance sensor outside the storage tank is advantageous in that the sensor is hereby more easily accessible and protected from the moist environment inside the tank.

In an aspect of the invention, the boundary layer detection means comprises at least one temperature sensor arranged in close proximity of at least one heating element and wherein the at least one temperature sensor and the at least one heating element are arranged inside the storage tank above the fluid outlet.

By heating the substance surrounding the heating element by means of the heating element and detecting the temperature of said substance by means of a temperature sensor before and after each heating cycle, the thermal conductivity of the surrounding substance - or a change in the thermal conductivity of the surrounding substance - can be determined. Since heating elements and temperature sensors are simple, durable, and inexpensive devices they provide particularly suited means for detecting the type of substance surrounding the temperature sensors and by locating them inside the storage tank just above or a short distance above the fluid outlet it is possible to detect when the phase change material approaches the fluid outlet and then take action to ensure that the phase change material is not sucked out of the fluid outlet.

In an aspect of the invention, the thermal transport fluid comprises water or brine e.g., comprising an anti-freeze agent. Hereby is achieved an advantageous embodiment of the invention.

In an aspect of the invention, the thermal energy storage further comprises an overflow protection device forming a fluid channel between the top of the storage tank and the bottom of the storage tank through the phase change material inside the storage tank. If the phase change material inside the storage tank some reason becomes so solid that passage of the thermal transport fluid through the phase change material is severely hindered or completely blocked it is advantageous to arrange one or more pipes, tubes or channels inside the storage tank forming a fluid conduit between the top and the bottom of the storage tank so that the thermal transport fluid may still flow through the thermal energy storage even if passage directly through the phase change material is blocked or hindered. Furthermore, arranging the overflow protection device so that it is extending through the phase change material inside the storage tank is advantageous in that hot thermal transport fluid running through the overflow protection device hereby will heat up surrounding phase change material and make it change to liquid phase so that passage directly through the phase change material is restored.

The invention further provides for a method for storing thermal energy in a thermal energy storage. The method comprising the steps of:

• arranging a phase change material arranged inside a storage tank of the thermal energy storage,

• establishing a flow of thermal transport fluid through the storage tank from a fluid inlet arranged at the top of the storage tank to a fluid outlet arranged at the bottom of the storage tank so that the thermal transport fluid is in direct contact with the phase change material, and

• detecting the lowest position of the phase change material inside the storage tank by means of boundary layer detection means of the thermal energy storage.

By ensuring that the thermal transport fluid is in direct contact with the phase change material, when passing through the storage tank, efficient heat exchange between the thermal transport fluid and the phase change material is ensured. And by detecting the lowest position of the phase change material inside the storage tank it is possible to prevent phase change material from leaving the storage tank thereby ensuring a more efficient and durable process. And when leaking of phase change material from the storage tank is prevented, it is possible to exchange heat directly with the thermal transport fluid also flowing through the final cooling/heating device - such as chiller, air conditioning device, cooler or another device for which the thermal energy is stored in the thermal energy storage - i.e. when the phase change material is prevented from leaving the storage tank it is not necessary to include the thermal energy storage in an independent closed thermal transport fluid circuit arranged to exchange heat with another circuit comprising the final cooling/heating device through an expensive and efficiency reducing heat exchanger.

In an aspect of the invention, the method further comprises the step of controlling the flow of thermal transport fluid through the storage tank in response to output from the boundary layer detection means.

Controlling the flow of thermal transport fluid through the storage tank in response to output from the boundary layer detection means is advantageous in that it hereby is possible to reduce the flow rate of the thermal transport fluid or even stop the flow of the thermal transport fluid through the storage tank if the phase change material approaches the fluid outlet of the storage tank and thereby prevent that the phase change material escapes the storage tank.

In an aspect of the invention, the method further comprises the step of distributing the thermal transport fluid across an upper surface of the phase change material inside the storage tank by means of a fluid distributor arranged at the at least one fluid inlet.

Distributing the thermal transport fluid across the upper surface of the phase change material is advantageous in that this generates a plurality of flow channels through phase change material because such a distribution makes the PCM solidify with a with a sponge-like or porous structure which ensures a higher efficiency of the thermal energy storage due to more of the PCM being activated for heat or cold storage.

In an aspect of the invention, the method is performed by means of a thermal energy storage according to any of the previously discussed thermal energy storages. Hereby is achieved an advantageous embodiment of the invention.

The invention further provides for a method for storing thermal energy in a heat transfer circuit, wherein the heat transfer circuit comprises a thermal transport fluid flowing in a thermal transport fluid conduit of the heat transfer circuit. The heat transfer circuit further comprises a thermal energy storage according to any of the previously discussed thermal energy storages and a pump arranged for generating a flow of the thermal transport fluid through the thermal energy storage. Hereby is achieved an advantageous embodiment of the invention.

In an aspect of the invention, the heat transfer circuit is a closed circuit.

Making the heat transfer circuit a closed circuit is advantageous in that in case of an accident the phase change material will not escape the system.

The invention further provides for use of a thermal energy storage according to any of the previously discussed thermal energy storages for storing thermal energy in a heat transfer circuit. Hereby is achieved an advantageous embodiment of the invention.

Figures

An embodiment of the invention will be described, by way of non-limiting example, in the following with reference to the figures in which: fig. 1 illustrates a cross section through the middle of a thermal energy storage comprising a reflector device, as seen from the side, fig. 2 illustrates a cross section through the top of a thermal energy storage comprising a reflector device, as seen from the top, fig. 3 illustrates a cross section through the middle of a thermal energy storage comprising temperature sensors and heating element, as seen from the side, and fig. 4 illustrates a heat transfer circuit comprising a thermal energy storage, as seen from the side.

Detailed description

Fig. 1 illustrates a cross section through the middle of a thermal energy storage 1 comprising a reflector device 16, as seen from the side and fig. 2 illustrates a cross section through the top 4 of a thermal energy storage 1 comprising a reflector device 16, as seen from the top.

In this embodiment the thermal energy storage 1 comprises a storage tank 2 having a fluid inlet 3 arranged at the top 4 of the storage tank 2 and a fluid outlet 5 arranged at the bottom 6 of the storage tank 2. In this embodiment the fluid inlet 3 is arranged at the highest point of the tank 2 but in another embodiment the fluid inlet 3 could be arranged in another location at the top 4 of the tank 2 - e.g., in the side of the tank 2. Likewise, in this embodiment the fluid outlet 5 is in this embodiment arranged in the side of the tank 2 at the bottom 6 of the storage tank 2 but in another embodiment the fluid outlet 5 could be arranged at or near the lowest point of the tank 2. In this embodiment a phase change material (PCM) 7 is arranged inside the storage tank 2 and in this embodiment the upper layer 19 of PCM 7 is in solid form and the lower layer 20 of PCM 7 is in a liquid state, in that in this embodiment a thermal transport fluid 8 having an entrance temperature higher than the temperature of the solid PCM 7 in the upper layer 19 is flowing through the storage tank 2 from the fluid inlet 3 to the fluid outlet 5 whereby it is in direct contact with the phase change material 7, so that some of the solid PCM 7 in the upper layer 19 changes phase to liquid and thereby aids in cooling the thermal transport fluid 8 as it travels down through the PCM 7, so that the thermal transport fluid 8 leaving the tank 7 through the fluid outlet 5 has an exit temperature which is lower than the entrance temperature. However, in another embodiment or at another time in the operation cycle the entrance temperature of the thermal transport fluid 8 could be lower than the temperature of the PCM 7 in the tank 2 and in this case the cold thermal transport fluid 8 will thereby cause a phase change from liquid to solid in any liquid PCM 7 in the tank 2. Thus, the thermal energy storage 1 can be used for reversibly storing thermal energy.

In another embodiment the PCM 7 could be chosen so that the upper layer 19 would be in liquid form and the lower layer 20 of PCM 7 would be in a solid state.

In this embodiment the PCM 7 in the tank 2 is paraffin formed by a mixture of n- hexadecane and tetradecane enabling that in this embodiment the PCM 7 will change phase between liquid and solid at a temperature around 10-18°C. However, in another embodiment the PCM 7 could be a mixture of hexadecane and pentadecane, it could be another type of paraffin mixture or another type of organic or inorganic PCM 7. And in another embodiment the PCM 7 could be chosen or mixed so that the phase change temperature between liquid and solid would be higher - such as between 15 and 23 °C, between 20 and 28°C or even higher or the PCM 7 could be chosen or mixed so that the phase change temperature between liquid and solid would be lower - such as between 7 and 15 °C, between 2 and 10°C or even lower, depending on the specific thermal energy storage 1, the specific use, available PCM 7 or other. In this embodiment the PCM 7 is also selected so that the PCM 7 is insoluble with the thermal transport fluid 8 and so that the density of the PCM 7 is different from the density of the thermal transport fluid 8. These qualities of the PCM 7 are inherent in substantially all direct contact PCMs but under some circumstances and in relation to some specific thermal transport fluid 8 the particular PCM 7 should be selected in relation to the specific density of the thermal transport fluid 8.

In this embodiment the storage tank 2 further comprises a fluid distributor 10 in the form of a spray nozzle arranged at the fluid inlet 3 for distributing the thermal transport fluid 8 across the upper surface of the phase change material 7 and in this embodiment the fluid distributor 10 comprises distributing pattern changing means 11 in the form of a rotating propeller-like device driven by the force generated by the thermal transport fluid 8 leaving the spray nozzle arranged for continuously changing the distribution pattern of the fluid distributor 10. However, in another embodiment the fluid distributor 10 and/or the distributing pattern changing means 11 could be formed by a rotating spray ball, moving nozzles, rotating spray wing/arm (as know in a dishwasher) or other either self-propelled or actively moved around by external drive means such as a motor. Also, in another embodiment the distributing pattern changing means 11 would not be present and the fluid distributor 10 could comprise a plurality of spay nozzles or jets distributed across the top of the inside of the tank 7. Distributing the thermal transport fluid 8 across the upper surface of the phase change material 7 (e.g., n-Hexadecane, n-tetradecane or Didecyl Ether) generates a porous structure of PCM during solidification of the PCM which results in a porous and permeable structure, allowing the thermal transport fluid to travel across the column of PCM in the storage tank while increasing the contact surface between the thermal transport fluid 8 and the phase change material 7.

However, in another embodiment the storage tank 2 would not comprise a fluid distributor 10 and the storage tank 2 would instead comprise one or more fluid inlets 3 each generating a flow of thermal transport fluid 8 down through the phase change material 7.

In this embodiment the density of the PCM 7 is around 900 kg/m3 and in this embodiment the thermal transport fluid 8 is water comprising an anti-freeze agent so in this embodiment the density of the thermal transport fluid 8 is around 997 kg/m ³ - i.e. higher than the density of the PCM 7 so that gravitation pull will make the thermal transport fluid 8 travel down through the PCM 7 when sprayed out on top of the PCM 8. However, in another embodiment the density of the PCM 7 could be higher - such as 920, 950, 980 kg/m3 or even higher - or lower - such as 880, 850, 800 kg/m3 or even lower - depending on the specific type of PCM. And/or in another embodiment the density of the thermal transport fluid 8 could also be higher - such as 1010, 1050, 1100 kg/m3 or even higher - or lower - such as 980, 950, 900 kg/m3 or even lower - depending on the specific type of thermal transport fluid 8 - such as water, brine, oil, glycol, steam, or a gas or other or any combination thereof. However, it is obviously advantageous that the density of the thermal transport fluid 8 is higher than the density of the PCM 7.

In this embodiment the storage tank 2 further comprises an outflow device 12 in the form of a T-pipe having two device inflow openings 13 and a device outflow opening 14 connected to the fluid outlet 5. I.e., in this embodiment the total area of the device inflow openings 13 are twice as big as the area of the device outflow opening 14 so that the flow speed at the device inflow openings 13 is severely reduced in relation to the flow speed at the device outflow opening 14. However, in another embodiment the outflow device 12 could comprise more than two device inflow openings 13 - such as four, seven, ten or even more - or the outflow device 12 could comprise only a single device inflow opening 13 if the outflow device 12 was funnel shaped so that the area of the device inflow openings 13 was larger than the area of the device outflow opening In this embodiment the thermal energy storage 1 also comprises boundary layer detection means 9 in the form of a distance sensor 15 arranged at the bottom 6 of the storage tank 2 on the outside of the storage tank 2 so that the distance sensor 15 may determine the vertical distance to a reflector device 16 arranged inside the storage tank 2. In this embodiment the reflector device 16 has a density of around 940 kg/m3 so that the density of the reflector device 16 is lower than a density of the thermal transport fluid 8 and higher than a density of the phase change material 7 thereby ensuring that the reflector device 16 always is located at the bottom of the PCM 7 and that the lowest position of the phase change material 7 inside said storage tank 2 can be detected by detecting the distance to the reflector device 16 by means of the distance sensor 15.

In this embodiment the distance sensor 15 is an ultrasonic sensor arranged to transmit signals through the column of substances and the signals are reflected by the reflector device 16 which is at all times positioned in the bottom boundary layer between the substances due to the differences in density of the two substances 7, 8 and the reflector device 16. The ultrasonic sensor signal can be used for processing and calculating the distance between the sensor 15 and the reflector device 16 and thus used in a control system for taking appropriate measures in case the distance between reflector device and sensor 15 reaches a critical value (e.g., reduce pump speed regarding the thermal transport fluid 8 and/or reduce flow of thermal transport fluid 8).

In this embodiment the reflector device 16 is designed as a hollow rod made of a plastic material and metal sticks are located inside the rod to reflect the ultrasonic signal from the ultrasonic sensor. However, in another embodiment the reflector device 16 could be shaped in numerous other ways - such as a board, a plate, a ring with an internal cross, a cross or other e.g., adapted to fit the internal shape of the storage tank 2 and the reflector device 16 could also or instead be made entirely from metal or another material - such as ceramic, wood, a composite material or other or any combination thereof.

In another embodiment the boundary layer detection means 9 would not comprise a reflector device 16 and the distance sensor could instead be a camera, a radar, mechanical floating switches or other. Although mechanical level switches are less suitable when the PCM 7 changes to solid phase.

In this embodiment the thermal energy storage 1 stores thermal energy in the following way. First the phase change material 7 is arranged inside the storage tank 7 and a flow of thermal transport fluid 8 is established through the storage tank 7 from the fluid inlet to the fluid outlet 5 so that the thermal transport fluid 8 is in direct contact with the phase change material 7 as it flows down through the tank 2. The flow could be established by means of a pump being part of the thermal energy storage 1 or being part of a heat transfer circuit 23 which the thermal energy storage 1 forms part of. When the thermal transport fluid 8 is flowing through the tank 2, boundary layer detection means 9 will detect the lowest position of the phase change material 7 inside the storage tank 2 so that the lowest vertical position of the PCM 7 can be monitored to prevent the PCM 7 from being sucked out of the tank 2 through the fluid outlet 5. I.e. in this embodiment the method further comprises controlling the flow of thermal transport fluid 8 through the storage tank 2 in response to output from the boundary layer detection means 9 either by controlling a separate flow valve (not shown) being in fluid communication with the fluid inlet 3 of by controlling start/stop or speed of the pump generating the flow of thermal transport fluid 8 through the storage tank 2.

In this embodiment the storage tank 2 is a vertically cylindrical insulated stainless steel water storage tank capable of being pressurised. However, in another embodiment the storage tank 2 could be arranged differently, such as horizontally, tilting or other, and/or the storage tank 2 could be provided with another shape, such as a cube, a cuboid, a sphere, a polygonal prism or other. Furthermore, in another embodiment the storage tank 2 could also or instead be made from Polyethylene and/or other polymers - e.g., glass- or carbon fibre reinforced, another metal, such as aluminium, steel or other - e.g., comprising an internal water impermeable liner, glass, or any other material suitable for holding liquids, and/or the storage tank 2 would not be suited for being pressurised, e.g., if the storage tank 2 was used in an open system at atmospheric pressure. Or in another embodiment the storage tank 2 would not be encased in an insulating material. I.e., in another embodiment the storage tank 2 could be a standard domestic or industrial water storage tank or an industrial bulk container (IBC) enabling that inexpensive locally produced or purchased tanks can be used as storage tank in a thermal energy storage 1 according to the present invention whereby it can be avoided that large storage tanks 2 has to be shipped to the actual place where the thermal energy storage 1 has to be implemented. This means that the installation time, and the installation and manufacturing cost of a thermal energy storage 1 according to the present invention can be severely reduced.

Fig. 3 illustrates a cross section through the middle of a thermal energy storage comprising temperature sensors and heating element, as seen from the side.

In this embodiment the thermal energy storage 1 comprises boundary layer detection means 9 in the form of a vertical array of temperature sensors 17 each arranged in close proximity of a heating element 18 all arranged inside the storage tank 2. However, in another embodiment the boundary layer detection means 9 could comprise more or fewer temperature sensors 17 and heating element 18 - such as only a single temperature sensor 17 arranged in close proximity of at a single heating element 18, both arranged above the fluid outlet 5 to detect the nature of the substance above the fluid outlet 5.

The temperature sensors 17 and heating elements 18 are mounted on a rod 21 as illustrated in figure 3. The number of sensors 17 and the distance between them determines the resolution of detection in the column. The rod 21 is installed at the bottom 6 of the tank 2 and reaches into the layer above the minimum acceptable position of the boundary layer between the substances 7, 8. With regular intervals, the heating element 18 positioned next to each sensor 17, heats the surrounding fluid/solid and the temperature sensors 17 are read by a simple algorithm. With regular intervals the substance 7, 8 surrounding each sensor 17 is heated (e.g., by applying the self-heating of the temperature sensors 17 and transmission of the heat into the surrounding substances 7, 8, or by individual heating elements 18). The temperature sensors 17 are read before and after each heating cycle, measuring the temperature of the fluid/solid surrounding each sensor 17. Depending on the thermal conductivity of the fluid/solid surrounding each sensor 18 representing different layers of the column, the temperature measurements of each sensor 17 will depend on the type of substance 7, 8 (substance 7, 8 below boundary layer, boundary layer, substance and phase of substance 7, 8 above boundary layer). The different substances 7, 8 in the tank 2 and the bottom PCM boundary layer can clearly be detected due to the difference in thermal conductivity of each substance 7, 8. By comparing the dissipation of heat (represented by temperature difference between heated and not heated cycle) between the different layers, the location of the boundary layer is detected. Temperature difference measured by the temperature sensors 17 located in the layers located in the substance 7 with lowest density will be significantly different from in the temperature difference measured by the temperature sensors 17 located in the layers containing the substance 8 with the highest density. By monitoring the temperature differences within a layer across the column, the location of the boundary layer may be monitored and managed.

In this embodiment the thermal energy storage 1 further comprises an overflow protection device 12 in the form of a vertical pipe forming a fluid channel between the top 4 of the storage tank 2 and the bottom 6 of the storage tank 2 through the phase change material 7 inside the storage tank 2, so that if the flow of thermal transport fluid 8 through the PCM 7 is hindered or blocked, the thermal transport fluid 8 may travel through the phase change material 7 anyway and heat up the solid PCM 7 surrounding the overflow protection device 12 to re-establish the normal flow. In another embodiment the overflow protection device 12 could comprise more than one vertical pipe - such as four, seven, ten or even more - e.g., distributed evenly across the cross- sectional area of the tank 7 and/or the pipe could be curving or comprise fins to increase the surface area. Thus, in case of hydraulic resistance of the thermal transport fluid 8 traveling through the column of PCM 7 is increased, the overflow protection device 12 is designed to prevent buildup of thermal transport fluid 8 above the PCM column.

Fig. 4 illustrates a heat transfer circuit 23 comprising a thermal energy storage 1, as seen from the side.

In this embodiment the heat transfer circuit 23 comprises a chiller 22 directing cooled thermal transport fluid 8 to a number of fan coil units 24 - e.g., arranged in a cold storage or the like - through fluid conduits indicated by the solid lines. In this embodiment the heat transfer circuit 23 comprises return fluid conduits indicated by the dotted lines through which the thermal transport fluid 8 flows from the fan coil units 24 to a thermal energy storage 1 arranged for storing thermal energy so that it may be used at another time. From the thermal energy storage 1 the thermal transport fluid 8 flows back to the chiller 22. I.e., in this embodiment the thermal transport fluid 8 is guided through the thermal energy storage 1, the chiller 22 and the fan coil units 24 directly in a closed circuit without the need of separate circuits exchanging heat with each other by means of heat exchangers.

In another embodiment the thermal energy storage 1 could be used for reversibly storing thermal energy in any kind of system such as any kind of domestic, commercial, or industrial heating or cooling systems, district heating/cooling systems, solar heating/cooling systems, central heating or cooling systems, process heating or cooling systems, geothermal heating or cooling systems, or other or any combination thereof. The invention has been exemplified above with reference to specific examples of storage tanks 2, phase change materials 7, boundary layer detection means 9 and other. However, it should be understood that the invention is not limited to the particular examples described above but may be designed and altered in a multitude of varieties within the scope of the invention as specified in the claims.

List

1. Thermal energy storage

2. Storage tank

3. Fluid inlet

4. Top of storage tank

5. Fluid outlet

6. Bottom of storage tank

7. Phase change material

8. Thermal transport fluid

9. Boundary layer detection means

10. Fluid distributor

11. Distributing pattern changing means

12. Outfl ow protect! on device

13. Device inflow opening

14. Device outflow opening

15. Distance sensor

16. Reflector device

17. Temperature sensor

18. Heating element

19. Upper layer of PCM

20. Lower layer of PCM

21. Rod

22. Chiller

23. Heat transfer circuit

24. Fan coil unit