NL2028525B1 - System for storage of thermal energy - Google Patents

Abstract

The present invention relates to a thermal energy storage system, the system comprising a subsurface tank, comprising a modular framework wherein dimensions of the modular framework define a volume of said subsurface tank, wherein said modular framework comprises a lightweight rigid modular space frame structure; a foil, comprising elastomeric material, wherein said foil is wrapped around said modular framework for defining said subsurface tank such that said subsurface tank is arranged to comprise a liquid as a thermal energy buffer; a heat exchanger, comprising a thermal conduit arranged for being in heat-exchanging contact with said liquid in said subsurface tank for providing a thermal output for outputting thermal energy out of said system; characterized in that said thermal energy storage system further comprises a further heat exchanger, wherein said further heat exchanger is arranged on an outer surface of said subsurface tank and comprises a further thermal conduit for collecting thermal leakage of said subsurface tank for providing an auxiliary thermal output for outputting further thermal energy out of said system.

Description

Title: System for storage of thermal energy Description Background of the Invention The present invention relates in general to a system for storage of thermal energy, as well as the use of such a system in residential structures.

Climate change, concerns for global warming, the rapid decrease of oil reserves, and the increase in oil prices as a result thereof are just a few examples of incentives to invest in a transition towards higher adoption of renewable energy. A vast variety of renewable energy technologies can be distinguished. Well-known examples are renewable energy gained from wind energy, water (hydro) energy, and solar energy. Less known are other renewable energy sources like biomass energy, heat pumps, grid energy storage, and geothermal energy. Geothermal energy being thermal energy generated and stored in the earth.

An important aspect of renewable energy sources is the storage of energy. In for example seasonal thermal energy storage tanks, energy in the form of heat or cold can be stored for periods of up to several months. The thermal energy can be collected whenever it is available and be used whenever needed, for example, heat and cold can be stored in opposing seasons, e.g. heat stored during summer and used during winter, and cold stored during winter and used to cool during the summer season.

Such thermal energy storage can be fed by a plurality of (renewable) energy sources such as earth heat from heat pumps, solar energy from solar collectors, or waste heat from an air conditioning system. In these systems, the energy is transported towards, and stored in a storage tank in liquid form, i.e. in water.

Such thermal energy collection can take place on a household level, for example with air conditioning systems and solar collectors, or on an industrial level, by collecting heat from industrial processes, for example.

The way the energy is used from the storage tanks in for example these seasonal thermal energy storages can also vary, and examples thereof are heat exchangers, heat pumps, circular pumps, central heating systems, etc.

Another important aspect of renewable energy is the costs involved in investing in a renewable energy system or infrastructure and the costs per energy unit in US dollar or Euro per Joule. Since most current energy devices, systems, and infrastructure being highly dependent on conventional non-renewable natural resources such as oil, gas, and other fossil fuels, the costs for investing in non- renewable devices, systems, and infrastructure are relatively low. Moreover, the price of non-renewable natural resources in costs per energy unit is sometimes also lower when compared to renewable energy. For adoption of, or transition towards renewable energy it should be competitive to non-renewable energy, hence, not only price per energy unit should be low, the investment costs should be low as well, such that the period for return on investment is short enough.

In this respect, a renewable energy system is known EP3187661 from the same applicant as the present disclosure, which discloses a subsurface tank and a method of installing such a tank as a thermal seasonal energy storage system.

This subsurface tank, in use, will comprise a liquid, used as a thermal storage medium, which will have a temperature that is different from the ground surrounding the tank. For this reason, there will be thermal energy transfer between the liquid and the ground, due to the thermal leakage of the tank. Applying thermal insulation between the tank and the ground will lower the thermal leakage but will also add costs, thus increasing the costs per Joule of the system. Thus, reducing thermal energy loss will require reduction of the thermal leakage, and applying and improving thermal insulation, which in turn will increase costs reducing the overall effectiveness of the system. Accordingly, there is a need for an improved thermal energy storage system in which the thermal energy loss of the system is reduced. More, in particular, there is a need for an improved thermal energy storage system wherein at least some of the above-mentioned drawbacks have been obviated, which will result in a low price per energy unit.

Summary In a first aspect of the present disclosure, there is provided a thermal energy storage system, comprising:

- a subsurface tank, comprising a modular framework wherein dimensions of the modular framework define a volume of the said subsurface tank, wherein said modular framework comprises a lightweight rigid modular space frame structure; - a foil, comprising elastomeric material, wherein said foil is wrapped around said modular framework for defining said subsurface tank such that said subsurface tank is arranged to comprise a liquid as a thermal energy buffer; - a heat exchanger, comprising a thermal conduit arranged for being in heat-exchanging contact with said liquid in said subsurface tank for providing a thermal output for outputting thermal energy out of said system.

The thermal energy storage system is characterized in that it further comprises a further heat exchanger, wherein said further heat exchanger is arranged on an outer surface of said subsurface tank and comprises a further thermal conduit for collecting thermal leakage of said subsurface tank for providing an auxiliary thermal output for outputting further thermal energy out of said system.

Thermal (seasonal) energy storage is a form of storing cold or heat for long periods, up to several months. The thermal energy can be collected in several manners whenever available. The preferred medium for the tank is water. The water is heated during the summer period, and the hot water is stored within the tank and used during winter to warm a building. Vice versa, during the winter, the water can be cooled and stored in the tank to be used during the summer period to cool the building. The water of such a system is stored in a large tank which is placed below the ground surface. From an input perspective, several systems can be coupled to the tank, or preferably to a heat exchanger provided in the tank, to heat or cool the water and hence, convert the thermal energy into the water stored in the tank. Examples thereof are solar collectors, heat pumps, geothermal pumps, industrial residual heat, or heat from air conditioning systems. From an output perspective, several systems can be coupled to the tank, or preferably to a heat exchanger provided in the thank, for use of the thermal energy. Examples thereof are heat exchangers, heat pumps, circular pumps, central heating systems, etc.

As indicated, thermal energy storage systems are known which are comprised of a subsurface tank for storing a liquid as a thermal medium to store (seasonal) thermal energy, which depending on the season could be heat or cold.

During summertime, the subsurface tank, which is filled with for example water as the thermal medium, is heated by solar collectors or indirectly by any other type of renewable energy source such as, but not limited to, geothermal heat, ground energy, solar heat, industrial waste heat, and wind turbines. Especially during (hot) summer months, the water from solar collectors may also be used directly by the subsurface tank.

Such a subsurface tank-based thermal energy storage is highly suitable for domestic or industrial self-sustainable energy supplies, especially when combined with an array of photovoltaic panels to supply electricity.

The known subsurface tank is characterized by lightweight, modular, rigid, and strong construction which not only has low installation costs and thus short return-on-investment but is also easy to install and transport, without compromising on the quality.

The framework is a space frame or space structure, which is made from a strong rigid lightweight structure constructed from interlocking struts in a geometric pattern, preferably a triangular pattern. The space frame is strong due to the inherent rigidity of the triangular shape. Preferably the form of the space frame is a horizontal slab of interlocking square pyramids and tetrahedral build from aluminum or tubular steel struts which are arranged to be submerged in water or any other liquid used in the tank for storage of the energy.

Using a space frame has several advantages over known structures used for forming a tank. Space frames are modular, hence, it is very easy to build any dimension of framework, corresponding to the required volume of the tank. Moreover, due to its modularity, the desired dimension can be determined on-site, even during the build of the framework. Furthermore, due to the modularity and construction of interlocking joints or nodes, and the bars or rods or struts between the nodes, the space frame can be stored with minimal volume, which enables cheap transport from a factory or warehouse towards the installation site. This significantly lowers costs and thus returns on investment. Moreover, space frames costs of joints and struts that are suitable for both low and high temperatures, without losing strength at low or high temperature. This makes a tank build from a space frame suitable for low-temperature water and high-temperature water, for example up to 90 degrees Celsius. More likely, the maximum temperature of the fluid in the tank will lie between 60 to 90 degrees Celsius, also defined as sensible heat, which in this form at these temperatures, can be used directly for heating purposes. This is in contrast to terrestrial heat, which is obtained at a temperature of about 12 degrees, and thus has to be compressed before it can be used for heating purposes.

Yet another advantage of using a space frame construction is that it 5 increases the strength of the subsurface tank to such an extent that the site on top of the tank can still be used functionally. Once the tank is covered, the site can be put back in use as a park, soccer field, garden meadow, lawn, or even (large, heavy) constructions can be built/placed on top of the tank. Accordingly, the tank can be used in a plurality of applications and locations.

Inevitably, the thermal energy stored in the tank in the form of a liquid with raised or lowered temperature in respect of the ground surrounding the tank will exhibit a certain level of thermal leakage. The thermal leakage can be counteracted by installing thermal insulation partially or preferably fully as a shell around the entire tank. The application of thermal insulation lowers the thermal leakage and thus the loss of thermal energy of the system.

Not every type of thermal insulation is however suitable for (long- term) underground use and, in addition, some thermal insulating materials are not very durable and may be harmful to the environment. Thermal insulation that is suitable for use with such an underground tank is when expressed in terms of cost per standardized thermal insulation value, substantially cost-increasing.

To this end, the inventor came to the insight that an alternative should be found and that recovering the thermal loss is more cost-effective than reducing it. Therefore, according to the present description, the system is equipped with an additional heat exchanger which is installed on the outside of the underground tank. This additional or further heat exchanger comprises a thermal conduit for collecting thermal leakage of the subsurface tank by recovering the thermal loss. To this end, the further heat exchanger provides an auxiliary thermal output for outputting the thermal loss in the form of further thermal energy out of the system.

Recovering the thermal loss has a further advantage that the temperature of the ground surrounding the subsurface tank will not or only slightly increased such that micro-organisms are affected. In fact, it has been established that already a certain increase in the temperature of the ground/soil will have a negative effect on life and micro-organisms in the soil. Therefore, there are also regulations that stipulate that the temperature in the ground at such an installation as the underground tank should not exceed 25 degrees Celsius. By using the further heat exchanger the heat loss from the tank is absorbed and the ground will not or hardly warm up, so that these requirements can be met.

In an example said modular framework of said subsurface tank is made from a lightweight rigid modular space frame structure constructed from interlocking struts in a geometric triangular pattern.

The modular framework is a space frame or space structure, which is made from a strong rigid lightweight structure constructed from interlocking struts in a geometric pattern, preferably a triangular pattern. The space frame is strong due to the inherent rigidity of the triangular shape. Preferably the form of the space frame is a horizontal slab of interlocking square pyramids and tetrahedral build from aluminum or tubular steel struts which are arranged to be submerged in water or any other liquid used in the tank for storage of the energy. The ribs or struts are preferably steel, stainless steel, or aluminum struts and have a single, double or triple layer grid. In the single grid configuration, all elements of the framework are located on the surface, and in the double layer grid the elements are organized in two layers parallel to each other at a certain distance apart. Each of the layers forms a lattice of triangles, squares, or hexagons in which the projection of the nodes in a layer may overlap or be displaced relative to each other. Diagonal bars connect the nodes of both layers in different directions in space. In this type of meshes, the elements are associated into three groups: upper cordon, cordon, and cordon lower diagonal. In the triple-layer grid, the elements are placed in three parallel layers, linked by the diagonals. They are almost always flat.

For a tank to be suitable for a seasonal thermal energy storage system it requires a certain capacity. If for example water is used as the liquid for storing thermal energy, the volume of the tank needs to be plural cubic meter, e.g. up to 1000 cubic meter, to have sufficient water to store sufficient thermal energy for the application in which the tank is used, e.g. a capacity to keep a large building warm during winter and cold during summer.

In an example, the tank is arranged to resist corrosion, e.g. by adding an anti-corrosion substance, or by using only corrosion-resistant materials for the space frame, and/or arranged to resist to contamination of the water, e.g. for example by adding a water filter, or other means to extract or kill bacteria such as heating element. If the tank is arranged to resist corrosion and/or resist or at least control contamination, then the tank could be used as an open system wherein (fresh) water can be introduced and water can be drawn from the tank for example for showering or consumption of the water.

In an example, the tank may already be provided with a primary or first heat exchanger. This has the advantage that in such a way, a closed system can be obtained and the risk for contamination of the water, e.g. legionella and such, or the risk for corrosion is reduced to a minimum. Once the tank is filled with water, the tank can be sealed and thermal energy in the form of heat/cold can be stored and obtained through the heat exchanger inside the tank.

However, in yet another example, the tank is arranged to resist corrosion, e.g. by adding an anti-corrosion substance, or by using only corrosion- resistant materials for the space frame, and/or arranged to resist to contamination of the water, e.g. for example by adding a water filter, or other means to extract or kill bacteria such as heating element. If the tank is arranged to resist corrosion and/or resist or at least control contamination, then the tank could be used as an open system wherein (fresh) water can be introduced and water can be drawn from the tank for example for showering or consumption of the water.

In an example said modular framework having wall surfaces tapered towards the bottom of said subsurface tank, and wherein said modular framework, in particular, being any of an inverted pyramid shape, inverted truncated pyramid- shaped, inversed trapezoidal prism-shaped, and inversed truncated pyramid-shaped with chamfered ribs towards the base.

As indicated, the walls are preferably tapered towards the bottom. In a further example, only two of the wall surfaces are tapered. As such, the tank has the form of a reversed or inverted trapezoidal prism. Alternatively, all four walls are tapered, by which the tank has the form of an inverted pyramid, which is preferably truncated. Most preferably the framework is shaped as an inverted truncated pyramid with chamfered ribs towards the base, or put differently, as a truncated pyramid combined with a second inverted truncated pyramid in which the base surfaces of both pyramids are adjacent.

In an example the auxiliary thermal output is arranged for, in use, outputting further thermal energy out of said system at a temperature in between the temperature of the liquid of said subsurface tank, and an inground temperature surrounding said subsurface tank.

The temperature of the liquid in the subsurface tank may be defined according to a first temperature and the temperature of the ground surrounding the subsurface tank as a second temperature. Depending on the configuration and/or season the first temperature may be lower or higher than the second temperature. Due to the inevitable exchange of heat between the liquid and ground due to the thermal conduction of the subsurface tank, there will be a temperature gradient from the liquid to the ground. In this temperature gradient, the further heat exchanger will recover most or at least some of the thermal emission through the auxiliary thermal output which accordingly will have a third temperature which is in between the first and second temperature, during use of the system.

For the auxiliary thermal output to output thermal energy at a level in between the first and second temperature, the further heat exchanger may comprise a pump to transport a thermal medium such as water through the further heat exchanger.

In an alternative, the further heat exchanger may also be integrated into a heating, cooling, or other type of thermal system such as a domestic central heating system.

In an example said further heat exchanger comprises a ground collector comprising a conduit installed underground at least in the proximity of, and preferably in thermal contact with the outer surface of said subsurface tank.

In an example, the ground collector comprises a plurality of interconnected ground collector conduits arranged in a winding or meandering shape around the outer surface of said subsurface tank.

In an example, the ground collector is comprised of a single conduit arranged in a winding or meandering shape around the outer surface of said subsurface tank.

In an example said heat exchanger and said further heat exchanger is arranged in heat-exchanging contact with each other.

In an example, the further heat exchanger is arranged on at least one, but preferably two or more preferably all side surfaces of said subsurface tank.

In an example, the further heat exchanger is arranged on a top surface of said subsurface tank directed towards the ground level.

In an example, the further heat exchanger is arranged on a bottom surface of said subsurface tank directed away from the ground level.

The further heat exchanger may comprise one or preferably several heat exchanger elements or ground collectors which comprise one single or multiple interconnected conduits, pipes, or hoses which are arranged in preferably winding or meandering pattern.

In an example, the thermal energy storage system further comprises a layer of thermal insulation material arranged between said subsurface tank and said, further heat exchanger.

Although the use of the further heat exchanger in the system according to the present disclosure serves as an alternative to the use of (substantial) insulation, the further heat exchanger may also be used in combination with insulation. This way less insulation or insulation with a higher thermal transmittance may be used. Using a minimum level of insulation, expressed in for example thickness of the insulation or the thermal transmittance, may add significantly in reducing thermal loss, however, achieving a significant absolute low level of thermal loss will require substantial levels of insulation or thermal resistance. At a certain level, the costs due to the required materials will grow exponentially, thus making the use of the further heat exchanger increasingly interesting and comparatively cheaper.

In a further aspect, the use of the thermal energy storage system according to one or more of the above-mentioned aspects and examples is proposed, as a large capacity thermal energy storage tank, configured for providing thermal energy to a heating, ventilation, and air conditioning (HVAC) system of a residential structure.

The above-mentioned and other features and advantages of the invention are illustrated in the following description with reference to the enclosed drawings which are provided by way of illustration only and which are not limitative to the present invention.

Brief description of the Drawings Fig. 1 shows a thermal energy storage system with a subsurface tank;

Fig. 2 shows a thermal energy storage system with a subsurface tank and a further heat exchanger according to an example of the present disclosure. Detailed Description Fig. 1 shows a residential building 10 with an installed subsurface tank 20 below the building 10. In this example, the large construction may be built on top of the subsurface tank 20 once the tank is installed. The building 10 shown is merely an example of a building that may be built on top of the subsurface tank 20.

Other examples are industrial buildings or landscaping or other structures. Even very large and heavy constructions may be placed or build on top of the subsurface tank 20 but other buildings or constructions and landscape architectures may be placed on top of the subsurface tank, such as parks, hockey/soccer fields, garden meadows, lawns, etc.

In Fig. 1 the subsurface tank 20 is embedded into the ground 18. The tank 20 may not only be covered with a foil but preferably also an additional stabilization layer 17 which is provided on the wall surfaces or between the wall surfaces and the surrounding soil. Moreover, the bottom surface may also be provided with a stabilization layer 15. The stabilization layer provides additional protection for objects that could penetrate the foil, e.g. rocks or roots of trees, and furthermore provides additional thermal isolation. Preferably the top surface is provided with such a layer as well. The material used to cover the top surface and used on the bottom surface is preferably polystyrene foam and the material used for the walls polyurethane foam. However other materials could also be used or combined.

Preferably, the subsurface tank is thermally isolated with a certain layer of isolation material such as a polyurethane foam-based or polystyrene-based material. The isolation reduces the thermal leakage to the surrounding ground 18. Although adding a minimal layer of isolation material will already lower thermal leakage, the volume of extra isolation needed to decrease the thermal leakage to a desirable amount is significant. To this end, the present disclosure proposes to add a further or auxiliary heat exchanger or ground collector which is shown in more detail in Fig. 2. The ground collector will recover all or at least most of the thermal energy that leaks through instead of trying to further reduce the effect of thermal energy loss by installing more efficient or more volume of thermally insulating material.

Once the tank 20 is installed it can be filled with a liquid as a thermal medium, e.g. (tap) water, and optionally with a phase change material, for example, contained in units along which the water can flow 14. Once filled, the tank can be sealed in case of the tank being used as a closed thermal system, or can be connected to an inlet/outlet in case of an open thermal system wherein fresh-water can be introduced and water can be drawn from the tank.

The example shown in Fig. 1 is a closed/sealed tank 20 in which water 14 remains in the tank.

The tank is used as a thermal energy storage tank wherein the water temperature for example is raised during summer and the heat of the hot water can be used during winter to heat a building.

The way in which the thermal energy is stored in the tank 20 and drawn from the tank is for example and preferably by use of a heat exchanging module 13. A primary side of the heat exchanging module 13 is in thermal connection with the water 14 in the tank 20, and a secondary side of the heat exchanging module 13 is in connection with a medium in a duct 12 that is for example connected to the hot water tap in the building, a central heating system in the building or other device or system that could use the heat (old cold) from the tank 20. Vice versa, the heat exchanging module 13 is arranged not only to draw thermal energy from but also store thermal energy in the tank.

For example, a solar panel (not shown) could be connected to the heat exchanging module 13 to heat the water, or an air-conditioning unit could be connected to capture waste heat of the air-conditioning unit and store the heat in the tank by heating the water in the thank through the heat exchanging module 13. These are only examples, and the present disclosure is suitable and applicable for other types of applications as well, and the present disclosure is accordingly not limited to the examples described, but determined by the appending claims.

In Fig. 2 several steps 200 are shown from space frame components 211, 212, 213, 221, 222, 223, towards a fully build framework 271. The subsurface tank 20 is comprised of a modular space frame construction which can be defined as a three-dimensional framework known for architectures and structural engineering in above-ground surfaces.

A space frame is a lightweight (in comparison with for example conventional frameworks made from (solid) steel bars or concrete), rigid structure that is constructed from interlocking struts in a geometric pattern.

Although space frame structures are known for spanning large areas such as roof covering they are also very suitable to build a subsurface tank.

A structure formed by a space frame has a low overall weight in comparison with a structure having the same capacity but constructed from conventional materials, for example, cylindrical metal containers or subsurface storage formed by a concrete casting.

The framework can be built on-site, which is a huge advantage and simplifies transport and storage, e.g. low transport volume and weight hence low transportation costs.

For building the framework in its most simplified version only two types of components are required, i.e. bars/trusses/struts/ribs as well as their connection pieces which interlock them.

These connecting pieces or units are structural nodes arranged to connect the bars in multiple directions.

The present disclosure is not limited to the type of nodes being used, e.g. being arranged to connect 3, 4,5,6,7, 8,9, 10, 11, 12, etc. bars in different directions, or in the way in which the bars can be connected to the node, for example by clamps, fasteners, screws, etc.

Preferably all bars have equal lengths, this gives the highest structural strength and simplifies the construction of the framework.

However, the present disclosure is not limited thereto, and also bars of different lengths can be used, as well as bars of different strengths or material, e.g. thick heavier bars for example for the horizontal connections and thinner lighter bars as struts in between the heavier bars.

Preferably all elements are made from stainless steel, but more preferably from aluminum, due to, amongst others, its strength (at all operating temperatures) and corrosion resistance and, alternatively materials having similar properties could be used as well.

The framework consists, as indicated, of several individual space frame elements.

These elements, when combined, can form a space frame element, e.g. square pyramid or a tetrahedron.

The outer dimensions of such an individual space frame element define the grid of the space frame.

For example, when looking from a top view, two parallel bars can extend between the walls of the framework.

These bars can thus have a length equal to the length or width of the framework or can be formed from several shorter bars.

The two parallel bars are interconnected by bars, for example forming a square in the case of a square pyramid. The dimensions of that square define the space frame grid. Accordingly, in an example the horizontal bars of the framework are longer than the remaining bars of the framework, preferably, the horizontal bars of the framework has a length corresponding to the length or width of the tank.

Since the space frame construction is very strong, often stronger than required, the framework can also be built from space frame elements with different, less dense grids. For example, the outer layers can consist of denser grids, i.e. double density, when compared to the grid density in or near the center of the framework. Preferably, one or more of the walls, bottom, and top surface are thus formed from space frame elements in a double density grid. This could be achieved by connecting every node with a bar near any one or more of the walls, bottom, and top surface, and skipping nodes for the remaining parts of the framework in or near the center. A lower density can preferably be realized by skipping nodes and using longer bars, for example, to interconnect a horizontal bar of a first layer with a horizontal bar of the third layer, obviating the nodes and bars that interconnect the first and second horizontal bars and the second and third horizontal bars.

The framework shown in Fig. 1 consists entirely of space frame structures, e.g. tetrahedrons. This is however only an example and provides a most rigid, strongest framework. The framework could however also consist of only walls, top and bottom elements formed by the space frame structures, which are interconnected to each other, hence, by which the center of the framework remains open. Accordingly, the time to build the framework and the number of components needed, and transported would also be lower, which lowers costs of installation, materials, and transport. This would have a positive effect on the return on investment.

The finished framework preferably has a shape of an inverted truncated pyramid or an inversed trapezoidal prism. Preferably the angle between the top surface and the wall surfaces is approximately 60 degrees. The advantage thereof is that the combined weight of the fluid and tank and the thus induced downward force is translated through the tapered shape of the framework to a horizontal force that neutralizes the force of the soil on the walls of the tank. In a more preferred example, framework 20 can have a shape as indicated in Fig. 1, on top of which a similar shape is placed in the opposite orientation. Thus a framework build-up from three horizontal layers of several interconnecting tetrahedrons (and/or square pyramids), as well as an additional (single) layer on top thereof but tapered towards the top surface. Hence, a framework is obtained which is, by way of example, build from four horizontal layers of interconnecting tetrahedrons of which the top layer is tapered towards the top surface and the remaining three layers tapered towards the bottom surface. This way the combined weight of the fluid and tank is even further neutralized and a further advantage is, that the top surface area is decreased, which enables the use of a larger tank for example under a conventional size construction area/building ground surface.

Fig. 2 shows in more detail the subsurface tank 20 with a plurality of further heat exchangers 21a-c, 22 or additional or auxiliary heat exchangers 21a-c,

22. The thermal energy storage system 10 as shown in Fig. 1 is thus further comprised of the single or a plurality of further heat exchanger(s) 21a-c, 22. The further heat exchanger 21a-c, 22 is arranged preferably on all sides on the outer surface of the subsurface tank 20, but is at least arranged on one of the sides, e.g. the top side 24, or on one or more of the sides 23 as shown in Fig. 2.

In case of a plurality of further heat exchangers 21a-c, the plurality of heat exchangers may be interconnected by a central pipe 22 or other type of duct. The pipe 22 may comprise valves, etc. to control the flow in volume and/or direction.

The outer surface of the tank 20 may comprise a combination of thermal insulation and a further heat exchanger 21a-c, 22. The further heat exchanger 21a-c, 22 may also comprise a further thermal conduit for collecting thermal leakage of the subsurface tank 20 to provide for an auxiliary thermal output for outputting further thermal energy out of said system 10. This way a thermal conduit is provided for collecting thermal leakage of the subsurface tank 20 by recovering the thermal loss.

Although the use of the further heat exchanger 21a-c, 22 in the system 10 serves as an alternative to the use of (substantial) insulation, the further heat exchanger may also be used in combination with insulation. This way less insulation or insulation with a higher thermal transmittance may be used. Using a minimum level of insulation, expressed in for example thickness of the insulation or the thermal transmittance, may add significantly in reducing thermal loss, however, achieving a significant absolute low level of thermal loss will require substantial levels of insulation or thermal resistance. At a certain level, the costs due to the required materials will grow exponentially, thus making the use of the further heat exchanger increasingly interesting and comparatively cheaper.

Based on the above description, a skilled person can provide modifications and additions to the method and arrangement disclosed, which modifications and additions are all comprised by the scope of the appended claims.

Claims (13)

CONCLUSIONS CLAIMS 1. ATES system, comprising: - an underground tank, comprising a modular framework in which dimensions of the modular framework define a volume of the underground tank, wherein the modular framework comprises a lightweight rigid modular spaceframe structure; - a foil comprising elastomeric material, the foil being wrapped around the modular framework to define the underground tank, so that the underground tank is adapted to contain a liquid as a thermal energy buffer; - a heat exchanger comprising a thermal conduit adapted to be in heat-exchanging contact with the liquid in the underground tank for providing a thermal output for releasing thermal energy from the system; characterized in that the thermal energy storage system further comprises a further heat exchanger, the further heat exchanger being mounted on an outer surface of the subterranean tank and comprising a further thermal conduit for receiving thermal leakage from the subterranean tank to provide additional thermal output for releasing further thermal energy from the system. 2. ATES system as claimed in claim 1, characterized in that the modular framework of the underground tank is made of a lightweight rigid modular space frame construction constructed of interlocking struts in a geometric triangular pattern. ATES system according to any one of the preceding claims, wherein the modular framework has wall surfaces that taper towards the bottom of the underground tank, and in particular wherein the modular framework has an inverted pyramid shape, inverted truncated pyramidal shape, inverted trapezoidal prism shape and inverted truncated pyramidal with bevelled ribs to base. 4. ATES system according to any of the preceding claims, wherein the additional thermal output is arranged, in use, to discharge further thermal energy from the system at a temperature between the temperature of the liquid of the underground tank and a in-ground temperature surrounding the underground tank. 5. ATES system according to any one of the preceding claims, wherein the further heat exchanger comprises a ground collector comprising a conduit installed underground at least in the vicinity of and preferably in thermal contact with the outer surface of the underground tank. 6. ATES system according to claim 5, characterized in that the ground collector comprises a number of mutually connected ground collector pipes arranged in a meandering or meandering shape around the outer surface of the underground tank. 7. ATES system according to claim 5, characterized in that the ground collector consists of a single pipe arranged in a meandering or meandering shape around the outer surface of the underground tank. 8. ATES system as claimed in any of the foregoing claims, wherein the heat exchanger and the further heat exchanger are arranged in heat-exchanging contact with each other. 9. ATES system according to one of the preceding claims, wherein the further heat exchanger is arranged on at least one, but preferably two or more preferably all side surfaces of said underground tank. 10. ATES system according to one of the preceding claims, wherein the further heat exchanger is arranged on an upper surface of said underground tank facing the ground level. 11. ATES system according to one of the preceding claims, wherein the further heat exchanger is arranged on a bottom surface of said underground tank facing away from ground level. 12. ATES system as claimed in any of the foregoing claims, further comprising a layer of thermal insulation material arranged between the underground tank and the further heat exchanger. Use of the TES system according to any one of the preceding claims 1-12 as a large capacity thermal energy storage tank configured for supplying thermal energy to a heating, ventilation and air conditioning (HVAC) system of a home.