FIELD OF THE INVENTION

[0001] The present disclosure is directed to a thermal energy storage system.

BACKGROUND OF THE INVENTION

[0002] Thermal energy storage tanks for storing thermal energy material, such as molten salt, generated by concentrating solar power (CSP) plants are known. These cylindrical storage tanks typically have a large diameter (15-50 m) and are configured to hold thousands of tons of molten salt (30000 tons or more) at elevated temperatures up to 600°C.

[0003] Such storage tanks are typically constructed by welding stainless steel plates together to form the base, sidewall and roof of the storage tanks. A plurality of steel plates are disposed on a suitable thermally insulated and reinforced foundation and welded together to form the base of the storage tank and a plurality of steel plates are welded together to form the sidewall of the storage tank. The sidewall is typically connected to the base by welding the lowermost edge of the sidewall perpendicularly to the base.

[0004] Storage tanks of this type are subjected to significant thermal stress with changes in temperature and volume of the molten salt. Heating of the storage tank and adding heated molten salt to the empty storage tank will cause the steel plates to expand radially outwardly. Typically, once charged with molten salt, the storage tanks are maintained with at least a small amount of molten salt at the desired temperature to keep the base of the storage tank expanded. However, small increases and decreases in the molten salt temperature will cause the base to expand and contract respectively. Similarly, cyclical variations in the depth of molten salt in the storage tank will vary the hydrostatic pressure in the storage tank at the base, and this will cause circumferential stresses with the sidewall expanding and contracting as the depth of molten salt increases and decreases, respectively.

[0005] The ability of the base of the storage tank to expand and retract radially is impeded by friction between the base and the foundation and the vertical forces on the foundation due to gravity acting on the storage tank and its contents. The frictional restraint is particularly significant at the comer of the storage tank where the weight of the sidewall of the storage tank provides a constant substantial vertical load at the junction of the sidewall and base into the foundation. It is thought that it is this frictional restraint between the steel base and foundation, and particularly the additional frictional restraint due to the increased load under the sidewalls, acting against the thermal expansion of the base that causes buckling of the base plates, which can result in rupture and tank failure. Accordingly, attempts have been made to address this by using an appropriate foundation matrix of refractory material, as well as a dry lubricant between the base of the tank and the foundation to reduce the frictional forces.

SUMMARY OF THE INVENTION

[0006] According to a first aspect of the invention, there is provided a thermal energy storage system comprising: a thermal energy storage tank having a base and a sidewall, the thermal energy storage tank configured to store thermal energy storage material; a foundation on which the thermal energy storage tank is located; a fluid distribution system configured to introduce thermal energy storage material into the thermal energy storage tank; and a thermal regulator configured to increase a radial temperature gradient of the base that extends from a centre of the base toward a peripheral edge of the base.

[0007] In an embodiment, the thermal regulator may be configured to increase the radial temperature gradient of the base such that it is at least neutral, with the temperature either remaining constant or increasing from the centre of the base toward the peripheral edge of the base. The thermal regulator may also be configured to increase the radial temperature gradient of the base such that it is positive, with the temperature increasing from the centre of the base toward the peripheral edge of the base.

[0008] In another embodiment, the thermal regulator may comprise a duct located in the thermal energy storage tank and extending around the base, the duct having a duct inlet and a duct outlet; and the thermal regulator may be configured such that, during commissioning of the thermal energy storage system and prior to introducing thermal energy storage material into the thermal energy storage tank, a heated gas can be pumped into the duct via the duct inlet to increase the radial temperature gradient of the base.

[0009] In yet another embodiment, the duct may have a plurality of holes through which fluid is able to flow out of the duct and into the thermal energy storage tank and/or may be defined by a wall extending between the sidewall and the base.

[0010] In still another embodiment, the thermal regulator may be configured to control the fluid distribution system to increase the temperature gradient of the base. [0011] In an embodiment, the fluid distribution system may comprise: a first fluid distributor disposed in the thermal energy storage tank and configured to introduce thermal energy storage material into the thermal energy storage tank; and a second fluid distributor disposed in the thermal energy storage tank and configured to introduce thermal energy storage material into the thermal energy storage tank, wherein the first fluid distributor is configured to introduce thermal energy storage material into the thermal energy storage tank closer to the sidewall compared to the second fluid distributor.

[0012] In another embodiment, the first fluid distributor is a first fluid distribution ring; and the second fluid distributor is a second fluid distribution ring concentric with the first fluid distribution ring, wherein a diameter of the first fluid distribution ring is greater than a diameter of the second fluid distribution ring.

[0013] In yet another embodiment, the thermal regulator comprises insulation located in the foundation under the base of the thermal energy storage tank; and a thermal resistance of the insulation decreases from under the peripheral edge of the base toward the centre of the base.

[0014] In still another embodiment, the thermal regulator comprises a cooling system configured to provide cooling to the base, and the thermal regulator is configured to control the cooling system to regulate the temperature gradient of the base.

[0015] In an embodiment, the cooling system comprises a plurality of concentric pipes located in the foundation; and the thermal regulator is configured to vary a flow rate of coolant through each of the concentric pipes to regulate the temperature gradient of the base.

[0016] In another embodiment, the cooling system comprises a spiral pipe located in the foundation, the spiral pipe having an inlet located under the base proximate the centre of the base and an outlet located under the base proximate the sidewall; and the thermal regulator is configured to vary a flow rate of coolant through the spiral pipe to regulate the temperature gradient of the base.

[0017] In yet another embodiment, the thermal regulator may comprise heating means provided on a lower, external surface of the sidewall, adjacent the base, said heating means preferably comprising one or more heat traces

[0018] In still another embodiment, the thermal energy storage material is molten salt. [0019] According to a second aspect of the invention, there is provided a method of commissioning a thermal energy storage system, wherein the thermal energy storage system comprises a thermal energy storage tank having a base and a sidewall, and a duct located in the thermal energy storage tank and extending around the base, the duct having a duct inlet and a duct outlet, and wherein the method comprises: pumping a heated gas into the duct via the duct inlet to generate the temperature gradient of the base, and introducing thermal energy storage material into the thermal energy storage tank.

[0020] In an embodiment, the duct may have a plurality of holes through which fluid is able to flow out of the duct and into the thermal energy storage tank, and a majority of the heated gas pumped into the duct flows out of the duct through the duct outlet and a minority of the heated gas pumped into the duct flows out of the duct through the plurality of holes.

[0021] According to a third aspect of the invention, there is provided a method of operating the thermal energy storage system in accordance with the embodiment of the first aspect in which the first and second fluid distributors are provided, the method comprising: determining a representative temperature of the thermal energy storage material stored in the thermal energy storage tank; introducing thermal energy storage material that is hotter than or equal to the representative temperature into the thermal energy storage tank via the first fluid distributor; and introducing thermal energy storage material that is cooler than the representative temperature into the thermal energy storage tank via the second fluid distributor.

[0022] In an embodiment, the representative temperature is an average temperature of the thermal energy storage tank determined based on outputs of a plurality of sensors, wherein each sensor is configured to monitor a temperature of the thermal energy storage tank.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Preferred embodiments of the invention will be described, by way of examples only, with reference to the accompanying figure;

[0024] Figure 1 shows a sectional view of a thermal energy storage system according to an embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] Surprisingly, the inventors have found that, in addition to the friction forces between the foundation and base, a significant contributing factor to the buckling of the base plates occurring is the base having a temperature gradient that decreases radially from the centre of the base, i.e. a negative radial temperature gradient. Specifically, when the temperature of a radially inner section of the base is greater than the temperature of a radially outer section of the base, the cooler radially outer section of the base prevents the hotter radially inner section from undergoing its full thermal expansion. This restriction on the thermal expansion of the hotter radially inner section of the base generates a radial, compressive stress, which can cause buckling in the base plates once it reaches a critical level.

[0026] It will be appreciated that there will be heat loss downwardly through the base of the storage tank and radially through the sidewall of the storage tank. Thermal energy storage material located closer to the centre of the storage tank will lose heat downwardly through the base, while thermal energy storage material located closer to the sidewall will lose heat downwardly through the base and radially through the sidewall. It will therefore be appreciated that thermal energy storage material located closer to the sidewall of the storage tank will lose more heat compared to thermal energy storage material located closer to the centre of the storage tank. This results in the thermal energy storage material within the storage tank having a temperature that decreases radially from the centre of the storage tank to the sidewall of the storage tank, i.e. a negative radial temperature gradient. This in turn results in the base also having a negative radial temperature gradient, i.e. a temperature that decreases from the centre of the base to the peripheral edge of the base. As the temperature of the base decreases from the centre of the base to the peripheral edge of the base, hotter radially inner sections of the base will be prevented/restricted from expanding radially outwardly by cooler radially outer sections of the base, thereby generating a radial, compressive stress, which may cause the base to buckle.

[0027] Embodiments of the present invention are directed to providing the thermal energy storage system with a thermal regulator that increases the radial temperature gradient in the base that extends radially from the centre of the base to the peripheral edge of the base, i.e. producing a radial temperature gradient that is greater than the temperature gradient that would exist without the thermal regulator. It will be appreciated that, in some embodiments, this increase may result in a positive temperature gradient that places the base in tension. It will also be appreciated that, in other embodiments, this increase may result in a temperature gradient that is neutral, or even still negative, but sufficient to reduce the radial compressive loads the base is exposed to during operation of the thermal energy storage system below the critical buckling load of the base. [0028] Figure 1 shows a thermal energy storage system (TESS) 100 according to an embodiment. The TESS 100 is configured to store thermal energy storage material from a source of thermal energy. As an example, the TESS 100 may store thermal energy storage material (e.g. in the form of molten salt) generated from a concentrated solar thermal energy receiver of a CSP plant.

[0029] The TESS 100 has a thermal energy storage tank 101. Figure 1 shows a sectional view of the thermal energy storage tank 101. The thermal energy storage tank 101 has an interior volume 102, a base 110, a sidewall 120, a roof 130, a duct 140, and a fluid distribution system 150. The TESS 100 also has a foundation 170 on which the thermal energy storage tank 101 is constructed, a cooling system 180 installed in the foundation 170, and a control system 190.

[0030] The base 110 has a peripheral edge 112 and is formed from a plurality of base plates (not shown) welded together. The sidewall 120 has a lower edge 122, an upper edge 124, and is formed from a plurality of wall plates (not shown) welded together. The lower edge 122 of the sidewall 120 is welded to the peripheral edge 112 of the base 110. The roof 130 has a peripheral edge 132 and is formed from a plurality of roof plates (not shown) welded together. The peripheral edge 132 of the roof 130 is welded to the upper edge 124 of the sidewall 120. The base 110, the sidewall 120, and the roof 130 define the interior volume 102 of the thermal energy storage tank 100.

[0031] The duct 140 is located in the thermal energy storage tank 101 and extends around the outer circumference of the base 110. In this embodiment, the duct 140 has a single duct inlet 141 and a single, diametrically opposed duct outlet 142 extending out of the thermal energy storage tank 101. Fluid is able to flow into the duct 140 via the duct inlet 141 and fluid is able to flow out of the duct 140 via the duct outlet 142. It will appreciated that, in other embodiments, the single duct inlet and single duct outlet may not be diametrically opposed, and/or multiple duct inlets and duct outlets may be provided, e.g. two diametrically opposed duct inlets and two diametrically opposed duct outlets offset 90° from the duct inlets.

[0032] The duct 140 has an annular baffle wall 143 having an outer circumferential, upper edge 144 and an inner circumferential, lower edge 145. In this embodiment, baffle wall 143 is arcuate in cross section and has a plurality of upper holes 146 and a plurality of lower holes 147. The arcuate cross section of the baffle wall 143 provides the duct 140 with a substantially sector shaped cross section. It will be appreciated that, in other embodiments, the baffle wall may have a different cross section, e.g. right angled, giving the duct a different cross section, e.g. rectangular. In this embodiment, the plurality of upper holes 146 are located proximate the upper edge 144 of the baffle wall 143 and the plurality of lower holes 147 are located proximate the lower edge 145 of the baffle wall 143. It will be appreciated that, in other embodiments, the holes may be provided in different locations or may be omitted such that the baffle wall is completely sealed. The baffle wall 143 is formed from a plurality of baffle plates welded together.

[0033] The outer circumferential, upper edge 144 of the baffle wall 143 is welded to the sidewall 120 and the inner circumferential, lower edge 145 of the baffle wall 143 is welded to the base 110. The duct 140 is defined by an annular portion 114 of the base 110, a portion 126 of the sidewall 120, and the baffle wall 143. The duct 140 defines a duct volume 148.

[0034] The plurality of upper holes 146 and the plurality of lower holes 147 may be formed through the baffle wall 143. Alternatively, the plurality of upper holes 146 may be formed by leaving gaps between the sidewall 120 and the upper edge 144 of the baffle wall 143 when welding the upper edge 144 of the baffle wall 143 to the sidewall 120. Similarly, the plurality of lower holes 147 may be formed by leaving gaps between the base 110 and the lower edge 145 of the baffle wall 143 when welding the lower edge 145 of the baffle wall 143 to the base 110.

[0035] As the thermal energy storage tank 101 is for storing thermal energy storage material (e.g. molten salt), the base plates, wall plates, roof plates, and baffle plates must be constructed from a suitable steel alloy material capable of withstanding high temperatures and masses. Suitable material includes, but is not limited to 316 stainless steel, A588 carbon steel, and Inconel.

[0036] The fluid distribution system 150 is configured to introduce thermal energy storage material into the interior volume 102 of the thermal energy storage tank 101. The fluid distribution system 150 has an outer distribution ring 152, an inner distribution ring 154, a switching valve 156, and a temperature sensor 158.

[0037] The outer distribution ring 152 is located in the interior volume 102 of the thermal energy storage tank 101 and may be concentric with the base 110 of the thermal energy storage tank 101. A plurality of outer suspension members 153 are coupled between the roof 130 and the outer distribution ring 152, thereby suspending the outer distribution ring 152 at a height above the base 110. Preferably, the outer distribution ring 152 is suspended as close to the base 110 as possible. However, the minimum height at which the outer distribution ring 152 may be suspended above the base 110 may be restricted by other components of the TESS 100 located in the thermal energy storage tank 101 (e.g. the duct 140 and/or other components of the thermal energy storage tank 101 not shown). Accordingly, as an example, the outer distribution ring 152 may be suspended about 1 metre above the base 110.

[0038] The outer distribution ring 152 has a plurality of holes 160. The plurality of holes 160 of the outer distribution ring 152 allow fluid to flow out of the outer distribution ring 152 and into the interior volume 102 of the thermal energy storage tank 101.

[0039] The diameter of the outer distribution ring 152 is less than the diameter of the base 110 of the thermal energy storage tank 101. The outer distribution ring 152 may have a diameter such that the outer distribution ring 152 is located as close as possible to the sidewall 120 of the thermal energy storage tank 101. For example, the maximum diameter of the outer distribution ring 152 may be restricted by other components of the TESS 100 located within the thermal energy storage tank 101 (e.g. the duct 140 and/or other components of the thermal energy storage tank 101 not shown).

[0040] The inner distribution ring 154 is located in the interior volume 102 of the thermal energy storage tank 101. A plurality of inner suspension members 155 are coupled between the roof 130 and the inner distribution ring 154, thereby suspending the inner distribution ring 155 at a height above the base 110. Preferably, the inner distribution ring 154 is suspended as close to the base 110 as possible. However, similar to the outer distribution ring 152, the minimum height at which the inner distribution ring 154 may be suspended above the base 110 may be restricted by other components of the TESS 100 located in the thermal energy storage tank 101. It is envisaged that the inner distribution ring 154 may be suspended at the same height above the base 110 as the outer distribution ring 152 or at a different height above the base 110 compared to the outer distribution ring 152.

[0041] The inner distribution ring 154 has a plurality of holes 162. The plurality of holes 162 of the inner distribution ring 154 allow fluid to flow out of the inner distribution ring 154 and into the interior volume 102 of the thermal energy storage tank 101.

[0042] The diameter of the inner distribution ring 154 is less than the diameter of the outer distribution ring 152. The inner distribution ring 154 may be concentric with the outer distribution ring 152. The inner distribution ring 154 is configured to introduce thermal energy storage material into the thermal energy storage tank 101 closer to the centre of the thermal energy storage tank 101 compared to the outer distribution ring 152, which is configured to introduce thermal energy storage material into the thermal energy storage tank 101 closer to the sidewall 120 of the thermal energy storage tank 101. Accordingly, as an example, the diameter of the inner distribution ring 154 may be several metres smaller than the diameter of the outer distribution ring 154.

[0043] The outer distribution ring 152 is coupled in fluid communication to the switching valve 156 via an outer distribution fluid line 164. The inner distribution ring 154 is coupled in fluid communication to the switching valve 156 via an inner distribution fluid line 166. The switching valve 156 is coupled in fluid communication to a source of thermal energy storage material (not shown). For example, the source of thermal energy storage material may be molten salt from a concentrated solar thermal energy receiver of a CSP plant.

[0044] The switching valve 156 is configured to selectively switch between the outer distribution ring 152 and the inner distribution ring 154 being in fluid communication with a source of thermal energy storage material. A pump (not shown) is configured to pump thermal energy storage material toward the switching valve 156 and the switching valve 156 is configured to direct the pumped thermal energy storage material to the outer distribution ring 152 and/or inner distribution ring 154. For example, a pump upstream of a concentrated solar thermal energy receiver of a CSP plant may pump molten salt from the concentrated solar thermal energy receiver to the switching valve 156.

[0045] When the outer distribution ring 152 is in fluid communication with a source of thermal energy storage material via the switching valve 156, thermal energy storage material is configured to flow through the switching valve 156, through the outer distribution fluid line 164, into the outer distribution ring 152, out of the plurality of holes 160 in the outer distribution ring 152, and into the interior volume 102 of the thermal energy storage tank 101. When the inner distribution ring 154 is in fluid communication with a source of thermal energy storage material via the switching valve 156, thermal energy storage material is configured to flow through the switching valve 156, through the inner distribution fluid line 166, into the inner distribution ring 154, out of the plurality of holes 162 in the inner distribution ring 154, and into the interior volume 102 of the thermal energy storage tank 101.

[0046] The temperature sensor 158 is configured to measure the temperature of the thermal energy storage material flowing toward the switching valve 156. The temperature sensor 158 may be a thermocouple, an infrared sensor, or any other suitable sensor known in the art that is capable of sensing, or generating an indication of, the temperature of thermal energy storage material flowing toward the switching valve 156.

[0047] The foundation 170 may include insulation to insulate the base 110 from the ground. The thermal resistance of the insulation may increase radially from the centre of the base 110 toward the peripheral edge 112 of the base 110 such that the thermal resistance of the foundation 170 under the centre of the base 110 is less than the thermal resistance of the foundation 170 under the peripheral edge 112 of the base 110. For example, the thermal resistance of the foundation 170 may increase constantly from under the centre of the base 110 toward the peripheral edge 112 of the base 110.

[0048] The thermal resistance of the foundation 170 may increase from under the centre of the base 110 toward the peripheral edge 112 of the base 110 by increasing the amount (i.e. the thickness) of the insulation from under the centre of the base 110 toward the peripheral edge 112 of the base 110. Alternatively, the thermal resistance of the foundation 170 may increase from under the centre of the base 110 toward the peripheral edge 112 of the base 110 by using insulation material with higher thermal resistance closer to the peripheral edge 112 of the base 110 compared to at the centre of the base 110.

[0049] The cooling system 180 includes a piping network 182 installed in the foundation 170 and a pump system 184. The pump system 184 is configured to pump coolant (e.g. air) through the piping network 182, which absorbs heat from the base 110, thereby providing cooling to the base 110.

[0050] The piping network 182 may comprise a plurality of concentric pipes. In this embodiment, the pump system 184 may be configured to vary the flow rate of coolant through each of the concentric pipes independent of the other concentric pipes.

[0051] Alternatively, the piping network 182 may comprise a single pipe having a spiral configuration. In this embodiment, the inlet to the piping network 182 may be located under the centre of the base 110 and the outlet of the piping network 182 may be located under the sidewall 120. It will be appreciated that, in this embodiment, as coolant flows through the piping network 182 it absorbs heat from the base 110. Therefore, in this embodiment, the coolant increases in temperature as it flows from the inlet to the outlet of the piping network.

Accordingly, it will be appreciated that, in this embodiment, the amount of cooling provided by the piping network 182 to the base 110 decreases from the centre of the base 110 to the peripheral edge 112 of the base 110.

[0052] The control system 190 is in data communication with the switching valve 156, the temperature sensor 158, and the cooling system 180. The control system 190 is configured to control the switching valve 156 to switch between the outer distribution ring 152 and the inner distribution ring 154 being in fluid communication with a source of thermal energy storage material. The control system 190 is also configured to detect the temperature of thermal energy storage material flowing to the switching valve 156 using the temperature sensor 158 and control operations of the cooling system 180.

[0053] The control system 190 may be configured to control the pump system 184 to vary the flow rate of coolant through the piping network 182 to vary the rate of cooling provided to the base 110. In the embodiment where the piping network 182 comprises a plurality of concentric pipes, the control system 190 may be able to control the pump system 184 of the cooling system 180 such that the flow rate of coolant is different in each of the concentric pipes. In the embodiment where the piping network 182 comprises a pipe having a spiral configuration, the control system 190 may be configured to vary the flow rate of coolant through the spiral pipe.

[0054] Operation of the TESS 100 will now be described.

[0055] After construction of the TESS 100, the thermal energy storage tank 101 will be at or close to ambient temperature. The thermal energy storage tank 101 may experience thermal shock if thermal energy storage material (e.g. molten salt) is introduced into the thermal energy storage tank 101 when it is at or close to ambient temperature. Exposing the thermal energy storage tank 101 to thermal shock may damage the thermal energy storage tank 101. During commissioning of the TESS 100, the thermal energy storage tank 101 is pre-heated before thermal energy storage material is first introduced into the thermal energy storage tank 101 to avoid/reduce thermal shock to the thermal energy storage tank 101.

[0056] During commissioning of the TESS 100, hot air (e.g. at approximately 350°C) is pumped from a source of hot air (not shown) into the duct 140 via the duct inlet 141. The source of hot air may be a temporary gas fired hot air blower, however, any other suitable sources of hot air known in the art may be used.

[0057] Hot air will flow into the duct 140 via the duct inlet 141 and around the duct 140. A majority of hot air flowing into the duct 140 via the duct inlet 141 will flow out of the duct 140 via the duct outlet 142. A minority of hot air flowing into the duct 140 via the duct inlet 141 will flow out of the duct 140 through the plurality of upper holes 146 and the plurality of lower holes 147 in the baffle wall 143 of the duct 140.

[0058] As only a minority of the hot air flows out of the duct 140 through the plurality of upper holes 146 and the plurality of lower holes 147 in the baffle wall 143, it will be appreciated that the temperature of the annular portion 114 of the base 110 that partially defines the duct 140 will be greater than the temperature of radially inner sections of the base 110. Further, the temperature of portions of the base 110 that are proximate and radially inwards of the duct 140 will be greater than the temperature of radially inner sections of the base 110. Accordingly, heating the thermal energy storage tank 101 by pumping hot air into the thermal energy storage tank 101 via the duct 140 will create a positive temperature gradient in the base 110 such that the temperature increases from the centre of the base 110 to the peripheral edge 112 of the base 110.

[0059] In an embodiment, hot air flowing out of the duct 140 through the plurality of upper holes 146 and the plurality of lower holes 147 in the baffle wall 143 will ultimately flow out of the thermal energy storage tank 101 through an outlet (not shown).

[0060] The control system 190 may monitor the temperature of the annular portion 114 of the base 110 that partially defines the duct 140 using any suitable system, apparatus, and/or method known in the art. For example, the control system 190 may monitor the temperature of the annular portion 114 of the base 110 via temperature sensors (e.g. thermocouples, time of flight temperature sensors) installed on the annular portion 114 of the base 110 inside or outside of the thermal energy storage tank 101. The thermal energy storage tank 101 may be considered sufficiently pre-heated once the annular portion 114 of the base 110 reaches a predetermined temperature (e.g. 350°C), at which point pre-heating of the thermal energy storage tank 101 may be stopped.

[0061] After the thermal energy storage tank 101 has been pre-heated, thermal energy storage material may be introduced into the thermal energy storage tank 101 via the fluid distribution system 150. The control system 190 is configured to control the switching valve 156 to selectively introduce thermal energy storage material into the thermal energy storage tank 101 via the outer distribution ring 152 or the inner distribution ring 154 depending on the temperature of the thermal energy storage tank 101 and the temperature of the thermal energy storage material flowing to the switching valve 156. [0062] The control system 190 is configured to monitor the temperature gradient of the base 110. The control system 190 is also configured to detect the temperature of thermal energy storage material flowing to the switching valve 156 using the temperature sensor 158.

[0063] The control system 190 may monitor the temperature gradient of the base 110 using any suitable system, apparatus, and/or method known in the art. For example, the control system 190 may monitor the temperature gradient of the base 110 via temperature sensors (e.g. thermocouples, time of flight temperature sensors) installed on the base 110 inside or outside of the thermal energy storage tank 101 and/or via thermographic sensors. In an embodiment, thermographic sensors may be installed inside the thermal energy storage tank 101 to monitor the temperature of the top surface of the thermal energy storage material in the thermal energy storage tank 101. In this embodiment, the temperature gradient of the base 110 may be determined by extrapolation of the temperature of the top surface of the thermal energy storage material.

[0064] The control system 190 is also configured to determine an average temperature of the thermal energy storage tank 101. The control system 190 may determine an average temperature of the thermal energy storage tank 101 based on the output of several temperature sensors configured to monitor the temperature of the thermal energy storage tank 101 at different locations. In this example, as the temperature of the thermal energy storage tank 101 is measured at different locations, the control system 190 may determine a weighted average temperature of the thermal energy storage tank 101, where each temperature sensor may be assigned a different weighting based on its location. It will be appreciated that the average temperature of the thermal energy storage tank 101 is a representative temperature of the thermal energy storage tank 101. It is also envisaged that any other suitable method known in the art for determining a representative temperature of the thermal energy storage tank 101 may be used.

[0065] The control system 190 monitors the temperature of thermal energy storage material flowing to the switching valve 156 using the temperature sensor 158. If the temperature of the thermal energy storage material flowing to the switching valve 156 is greater than or equal to the average temperature of the thermal energy storage tank 101, the control system 190 controls the switching valve 156 so that the thermal energy storage material is introduced into the thermal energy storage tank 101 via the outer distribution ring 152. If the temperature of the thermal energy storage material flowing to the switching valve 156 is less than the average temperature of the thermal energy storage tank 101, the control system 190 controls the switching valve 156 so that the thermal energy storage material is introduced into the thermal energy storage tank 101 via the inner distribution ring 154. This is to assist with increasing the radial temperature gradient in the base 110, preferably such that it is positive, with the temperature increasing from the centre of the base 110 to the peripheral edge 112 of the base 110.

[0066] The control system 190 will continue to control the switching valve 156 to selectively introduce further thermal energy storage material into the thermal energy storage tank 101 via the outer distribution ring 152 or the inner distribution ring 154 using the method described above. This will continue to assist with maintaining and regulating an increased temperature gradient in the base 110, i.e. a temperature gradient that is greater than the temperature gradient that would exist without the use of the method, preferably such that it is positive, with the temperature increasing from the centre of the base 110 to the peripheral edge 112 of the base 110.

[0067] The control system 190 therefore controls the switching valve 156 to introduce thermal energy storage material that is hotter than or equal to the average temperature of the thermal energy storage tank 101 into the thermal energy storage tank 101 closer to the sidewall 120 (e.g. via the outer distribution ring 152). The control system 190 is also configured to introduce thermal energy storage material that is less than the average temperature of the thermal energy storage tank 101 into the thermal energy storage tank 101 closer to the centre of the thermal energy storage tank 101 (e.g. via the inner distribution ring 154). It will be appreciated that this will result in the temperature of thermal energy storage material located closer to the centre of the thermal energy storage tank 101 being less than, the same as or at least closer to, the temperature of thermal energy storage material located closer to the sidewall 120 of the thermal energy storage tank 101. This will increase the temperature gradient in the base, i.e. produce a temperature gradient that is greater than the temperature gradient that would exist otherwise, and may produce a positive temperature gradient in the base 110, i.e. a gradient where the temperature increases from the centre of the base 110 to the peripheral edge 112 of the base 110.

[0068] As discussed above, the thermal resistivity of the foundation 170 may increase radially from the centre of the base 110 to the peripheral edge 112 of the base 110. This will result in the amount of heat loss through the centre of the base 110 being greater than the amount of heat loss through the base 110 at positions closer to the peripheral edge 112 of the base 110. It will be appreciated that this may assist with maintaining and regulating an increased temperature gradient in the base 110, i.e. a temperature gradient that is greater than the temperature gradient that would exist without the use of the radially increasing thermal resistivity, preferably such that it is positive, with the temperature increasing from the centre of the base 110 to the peripheral edge 112 of the base 110.

[0069] The control system 190 may also control the cooling system 180 to maintain an increased temperature gradient in the base 110, i.e. a temperature gradient that is greater than the temperature gradient that would exist without the use of the radially increasing thermal resistivity, preferably such that it is positive, with the temperature increasing from the centre of the base 110 to the peripheral edge 112 of the base 110. As discussed above, the control system 190 is configured to monitor the temperature gradient of the base 110. If the control system 190 determines that the temperature of a radially inner section of the base 110 is approaching or is above the temperature of a radially outer section of the base 110, the control system 190 is configured to operate the cooling system 180 in order to reduce the temperature of that radially inner section of the base 110 to a temperature below that of the radially outer section of the base 110. Accordingly, the control system 190 is configured to operate the cooling system 180 to maintain and regulate an increased temperature gradient in the base 110, preferably such that it is positive, with the temperature increasing from the centre of the base 110 to the peripheral edge 112 of the base 110.

[0070] In the embodiment where the piping network 182 of the cooling system 180 is a plurality of concentric pipes, if the control system 190 determines that the temperature of a radially inner section of the base 110 is approaching or is above the temperature of a radially outer section of the base 110, the control system 190 may increase the flow rate of coolant through one or more of the concentric pipes underneath the radially inner section of the base 110. This may cool the radially inner section of the base 110 to a temperature below that of the radially outer section of the base 110. Accordingly, in this embodiment, the control system 190 may be able to vary the flow rate of coolant in each of the concentric pipes to vary an amount of cooling to different sections of the base 110 in order to maintain a positive radial temperature gradient in the base 110 such that the temperature increases from the centre of the base 110 to the peripheral edge 112 of the base 110.

[0071] In the embodiment where the piping network 182 of the cooling system 180 is a pipe having a spiral configuration, if the control system 190 determines that the temperature of a radially inner section of the base 110 is approaching or is above the temperature of a radially outer section of the base 110, the control system 190 may start operation of the cooling system 180 if not currently operating or may increase the current flow rate of coolant through the spiral pipe. This will increase the rate of cooling provided to the radially inner section of the base 110, which may cool the radially inner section of the base 110 to a temperature below that of the radially outer section of the base 110. Accordingly, in this embodiment, the control system 190 is able to vary the flow rate of coolant through the spiral pipe in order to increase a radial temperature gradient in the base 110, preferably such that it is positive, with the temperature increasing from the centre of the base 110 to the peripheral edge 112 of the base 110.

[0072] It will therefore be appreciated that the duct 140, the fluid distribution system 150, the insulation in the foundation 170, the cooling system 180, and the control system 190 form a thermal regulator of the TESS 100 that increases a radial temperature gradient of the base 110, i.e. produces a radial temperature gradient that is greater than the radial temperature gradient that would exist otherwise. It will also be appreciated that each of the duct 140, the fluid distribution system 150, the insulation in the foundation 170, and the cooling system 180 may be used individually to increase the radial temperature gradient of the base 110 that extends from the centre of the base 110 to the peripheral edge 112 of the base 110. Accordingly, it is envisaged that, in some embodiments of the TESS 100, the thermal regulator may include one or more of the duct 140, the fluid distribution system 150, the insulation in the foundation 170, and the cooling system 180.

[0073] The TESS 100 may therefore be able to increase a radial temperature gradient in the base 110 of the thermal energy storage tank 101 such that it is positive, with the temperature increasing from the centre of the base 110 to the peripheral edge 112 of the base 110, during commissioning and operation of the TESS 100. As discussed above, this may reduce the likelihood of radially inner sections of the base 110 being prevented/restricted from expanding radially outwardly by radially outer sections of the base 110, thereby potentially reducing the likelihood of the base 110 buckling.

[0074] It will be appreciated that creating a positive temperature gradient in the base 110 such that the temperature increases from the centre of the base 110 to the peripheral edge 112 of the base 110 assists in placing the base 110 in tension. Placing the base 110 in tension reduces the possibility of a radially inner section of the base 110 expanding into a radially outer section of the base 110. A hotter radially inner section of the base 110 will be under compression as it expands into a cooler radially outer section of the base 110. Accordingly, instead of, or in addition to, monitoring the temperature gradient of the base 110 as described above, the control system 190 may monitor whether the base 110 is in tension or compression. For example, a plurality of strain gauges in data communication with the control system 190 may be disposed at different locations on the base 110. If the control system 190 determines that a section of the base 110 is in compression, the control system 190 may operate the cooling system 180 to place the base 110 back in tension (i.e. increasing the temperature gradient such that the temperature increases from the centre of the base 110 to the peripheral edge 112 of the base 110).

[0075] In known thermal energy storage tanks, the distribution rings are supported above the base by support members welded between the base and the distribution rings. These known support members may act against the base radially expanding and contracting, which may damage the base and/or the distribution rings. The thermal energy storage tank 101 addresses this problem by suspending the outer distribution ring 152 and the inner distribution ring 154 from the roof 130 of the thermal energy storage tank 101.

[0076] As discussed above, it may be preferable for a thermal regulator to increase the radial temperature gradient of the base 110 such that it is positive, with the temperature increasing from the centre of the base 110 to the peripheral edge 112 of the base 110. Such a positive radial temperature gradient may assist in placing the base 110 in radial tension. However, it will be appreciated that, in some embodiments, in may not be necessary for the base to be placed in radial tension by the thermal regulator increasing the radial temperature gradient to the point that it becomes a positive temperature gradient. Rather, in some embodiments, it may be sufficient for the thermal regulator to increase the radial temperature gradient such that, whilst it is neutral or even still negative, the radial compressive stresses the base is exposed to during operation of the thermal energy storage system are reduced below the critical buckling load of the base. This may be particularly the case for thermal energy storage tanks where additional means are used to reduce the likelihood of buckling occurring, e.g. thicker base plates, friction reduction means between the base plates and foundation, etc.

[0077] In the embodiments discussed above, it is envisaged that the thermal regulator may include one or more of the duct 140, the fluid distribution system 150, the insulation in the foundation 170, and the cooling system 180. It will be appreciated that other, non-illustrated, embodiments of the present invention may comprise a thermal regulator that includes other means, in addition or as an alternative to those already described, for increasing a temperature gradient of the base that extends from its centre toward its peripheral edge. For example, heating means, such as one or more heat traces, may be provided on a lower, external surface of the sidewall, adjacent the base.

[0078] It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

[0079] Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.

[0080] By way of clarification and for avoidance of doubt, as used herein and except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additions, components, integers or steps