This invention is in the field of thermal energy and in particular systems for storing thermal energy, such as that generated by solar collection.

BACK GROUND

A significant problem with solar energy development is the cyclical nature of the energy collection due to day and night cycles, and the variability in the amount of energy collected due to cloud cover. For most practical uses it is necessary to have a steady supply of energy. Some uses, for example electrical power consumption, are also themselves cyclical in nature, with peak demand often twice the minimum demand.

It is therefore desirable to store thermal energy collected from the sun and draw the energy when needed. Present technology uses oil or molten salt as a thermal energy transfer medium. Molten salt is also used as a thermal energy storage medium. A molten salt presently being used is a mixture of 60 percent sodium nitrate and 40 percent potassium nitrate, and has certain desirable properties. It is liquid at atmosphere pressure, it provides an efficient, low-cost medium in which to store thermal energy, its operating temperatures are compatible with today's high-pressure and high-temperature steam turbines, and it is non-flammable and nontoxic.

The salt melts at 221°C and can be maintained in a liquid state in a "cold storage tank at about 280°C, then circulated through a solar collector apparatus where the temperature is increased to about 560^, then it flows into a heavily insulated "hot storage tank, where it can be stored for up to a week. When needed, hot molten salt is drawn from the hot storage tank and circulated through a conventional steam generator creating steam to operate a conventional steam turbine to generate electrical power. It is calculated that a 100-megawatt turbine would need tanks of about 30 feet (9.1 m) tall and 80 feet (24 m) in diameter to drive it for four hours by this design.

Conventional solar towers can increase the temperature of the molten salt to about 560°C, however the temperature of the molten salt drops in the steam generator such that the temperature of the generated steam is only about 280°C. Conventional steam turbines operating at this temperature have substantially reduced efficiency when compared to a higher temperature steam turbine operating at about 560°C. Solar collectors are also known which can generate thermal energy at increased temperatures of about 850°C. Such a collector is described for example in United States Published Patent Application Number 20080184990 of Tuchelt. Increasing the temperature of a storage medium to this increased temperature of 850°C in the system described above, or higher, would allow steam to be generated with a temperature of about 560°C, which is an ideal temperature for conventional steam turbines, and which would provide significantly improved efficiency, and therefore increased electrical production from collected solar energy and reduced cost of electricity.

SUMMARY OF T HE I VENT I ON

It is an object of the present invention to provide a thermal energy storage system that overcomes problems in the prior art.

The present invention provides a thermal energy storage system comprising an insulated storage container substantially filled with a particulate earth material. A heat input conduit circuit is buried in the earth material and is configured to transfer heat from an input liquid flowing in the heat input conduit circuit to the earth material. The heat input conduit circuit has an inlet port and an outlet port, each port defined in one of a top, bottom, and side wall of the storage container. A heat output system is operative to transfer heat from the earth material in the storage container to an external heat consumer. During operation the input liquid enters the inlet port of the heat input conduit circuit at an input operating temperature and leaves the outlet port at an output operating temperature, and the output operating temperature is above about 650°C. The input liquid remains liquid at the input and output operating temperatures under atmospheric pressure.

The high input operating temperature transfers heat energy to the earth material, which is generally a poor conductor, primarily by radiation. The high storage temperature allows heat to be removed at a higher temperature than conventional systems, which higher out put temperature provides greater efficiency for operating steam turbines and the like.

DESCRI P T I ON OF T HE DRAV\ ING S

While the invention is claimed in the concluding portions hereof, preferred embodiments are provided in the accompanying detailed description which may be best understood in conjunction with the accompanying diagrams where like parts in each of the several diagrams are labeled with like numbers, and where:

Fig. 1 is a schematic top view of an embodiment of a thermal storage system of the present invention, and also showing a heat output system that is provided by a heat output conduit circuit buried in the earth material of the thermal storage system;

Fig. 2 is a schematic side view of the thermal storage system and heat output system of Fig. 1;

Fig. 3 is a schematic top view of another embodiment of a thermal storage system of the present invention, where the storage container is divided horizontally and vertically into zones; Fig. 4 is a schematic side view of the thermal storage system of Fig. 3;

Fig. 5 a schematic sectional view of a portion of the heat input conduit circuit with an auxiliary conduit carrying an auxiliary liquid for melting the input liquid in the main conduit where the auxiliary conduit is adjacent to the main conduit;

Fig. 6 a schematic sectional view of a portion of the heat input conduit circuit with an auxiliary conduit carrying an auxiliary liquid for melting the input liquid in the main conduit where the auxiliary conduit is inside the main conduit;

Fig. 7 a schematic sectional view of a portion of the heat input conduit circuit with an first and second auxiliary conduits carrying a first and second auxiliary liquids for melting the input liquid in the main conduit, where the first and second auxiliary conduits are adjacent to the main conduit;

Fig. 8 a schematic sectional view of a portion of the heat input conduit circuit with an first and second auxiliary conduits carrying a first and second auxiliary liquids for melting the input liquid in the main conduit, where the first and second auxiliary conduits are inside the main conduit,

Fig. 9 a schematic sectional view of a portion of the heat input conduit circuit with an first and second auxiliary conduits carrying a first and second auxiliary liquids for melting the input liquid in the main conduit, where the first auxiliary conduit is inside the main conduit and the second auxiliary conduit is inside the first auxiliary conduit;

Fig. 10 is a schematic view of a heat consumer for connection to the thermal storage system of Fig. 1, where the heat output conduit circuit is connected to an input loop of a heat exchanger, and the output loop of the heat exchanger is connected to a boiler Fig. 1 1 schematically illustrates a purge and makeup regulation system for use with a sealed storage container containing an inert gas atmosphere.

PET A LED DESC I P T I ON OF T HE I L L UST RAT ED B BODI ME T S

Figs. 1 and 2 schematically illustrate an embodiment of a thermal energy storage system 1 of the present invention. The system 1 comprises an insulated storage container 3 substantially filled with a particulate earth material 5. The earth material 5 will typically be a material such as sand, crushed lava rock, or the like that is available in the local area to reduce costs.

A heat input conduit circuit 7 is buried in the earth material 5 and is configured to transfer heat from an input liquid 9 flowing in the heat input conduit circuit 7 to the earth material 5. The heat input conduit circuit 7 has an inlet port 11 and an outlet port 13 defined in one of atop, bottom, and side wall of the storage container 3.

It is contemplated that the heat source 15 of the heat input liquid 9 will commonly be a solar energy collector capable of raising the temperature to the desired temperature above about 750°C to 900°C, however it is also contemplated that other energy sources could provide the input liquid 9 at the required temperature as well. During operation, the input liquid 9 enters the inlet port 11 of the heat input conduit circuit at an input operating temperature and leaves the outlet port 13 at a lower output operating temperature. The input operating temperature is above about 750°C to 900°C, and at this elevated temperature heat is transferred primarily by radiation. The earth material 5 in the storage container 3 is a poor conductor of heat, and so in order to effectively transfer energy to the earth material, the input liquid 9 must be at a relatively high temperature. The energy transferred from the input liquid 9 to the earth material is proportional to the fourth power of the absolute temperature of the input liquid 9. Thus it can be seen that at 800°C or 1073 kelvins (K), power output of the input liquid will be 1.325X, while at 150°C or 1023 K the power output of the input liquid 9 will be only 1.095X, or 83% of the power output at 800°C, and at 700°C or 973 K the power output of the input liquid 9 will be only 0.896X, or 67% of the power output at 800°C. At 650°C or 923 K the power output of the input liquid 9 will be only 0.726X, or 55% of the power output at 800°C.

Also it can be seen that increasing the input operating temperature will increase the rate of transfer of energy from the input liquid 9 to the earth material 5. For example at 900°C or 1173 K the power output of the input liquid 9 will be 1.893X, or 142% of the power output at 800°C.

As the input liquid 9 circulates from the inlet port 11 to the outlet port 13 energy moves from the input liquid 9 to the earth material 5 and the temperature of the input liquid 9 drops from the input operating temperature to the output operating temperature. The rate of transfer of energy to the earth material 5 is much reduced when the temperature drops and it is contemplated that the temperature of the input liquid 9 in the heat input conduit circuit 7 should not fall below about 650°C or insufficient heat transfer will occur to heat the earth material 5.

The temperature of the input liquid 9 falls generally proportional to the time it is in the heat input conduit circuit 7. The output temperature can thus be controlled by increasing or decreasing the rate of flow of the input liquid through the heat input conduit circuit 7. Thus for example where the input operating temperature is 800°C, the input liquid may be circulated at a rate of X gallons per minute to result in an output operating temperature of 650°C, but where the input operating temperature is only 750°C, the input liquid will need to be circulated at a higher rate of X+ gallons per minute to result in the desired output operating temperature of 650°C. A heat output system is operative to transfer heat from the earth material 5 in the storage container 3 to an external heat consumer 21, such as a boiler or like apparatus that will utilize the heat energy. Similarly to the input mechanism described above, and again because the earth material 5 is a poor conductor, the heat is drawn out of the earth material also primarily by radiation. While the earth material 5 is a poor conductor, it is also very cheaply available in the very large quantities contemplated as necessary for electric power generation or like large scale uses, and in the system 1 of the present invention using the high temperature input liquid 9, an economical storage system for heat energy is provided, and available to be drawn out for various uses.

It is contemplated that the heat output system would typically be a heat output conduit circuit 17 with an output liquid 19 flowing therethrough, and arranged similar to the heat input conduit circuit 7 to absorb heat energy radiated from the heated earth material 5. The temperature of the output liquid 19 will be significantly lower than the input liquid 9, and so the output liquid 19 will typically be a different liquid than the input liquid 9 with a lower melting temperature. It is also contemplated that the heat output system could comprise heat pipes or other systems known in the art to move heat energy from the earth material 5 to a heat consuming process 21 such as a boiler.

The input liquid 9 is selected so that it will remain liquid at the input and output operating temperatures under atmospheric pressure. One possible choice that has several advantages is aluminum, with a melting point of 660°C and a boiling point above the operating temperature range. It is relatively economical and very light weight thereby reducing the energy needed to circulate it. It is also contemplated that in order to provide a significant portion of the world's energy from solar power, a great deal of this input liquid will be required, and aluminum also has the advantage of being very plentiful, as it is the third most abundant element in the earth's crust at about 8.1%. Another possible choice for the input liquid 9 is sodium, which has a melting point of just 98°C and an atmospheric boiling point 883°C which is above the contemplated operating temperatures. Sodium is also very light weight, inexpensive, and plentiful, but has the major drawback that it becomes explosive when mixed with water and poses a significant danger in the event of a failure. A further possible choice for the input liquid 9 is tin, which has a melting point of 232°C and a boiling point also above the operating temperature range, but tin is more costly, and less plentiful. It is contemplated other materials may be found to be suitable as well. Tin and sodium may be suitable for use as the output liquid 19, as both have a relatively low melting temperature.

Thus the only pressure in the heat input conduit circuit 7 is that exerted by the pumps circulating the input liquid. At the high operating temperatures of the present system 1, the metal of the pipes forming the heat input conduit circuit 7 is susceptible to failure, and by keeping the pressures inside low, the risk of failure, leakage, and the like is reduced. Operating at low pressure also allows for the use of less costly conduit materials than those required for both high temperature and high pressure operation.

The illustrated container 3 is formed by an inner wall and an outer wall with an insulation space 23 between the inner and outer walls that is filled with an insulating material.

Figs. 3 and 4 schematically illustrate a different embodiment of the thermal energy storage system 101 of the present invention where the storage container 103 is buried in the ground 102 such that the ground supports walls of the storage container 103. This arrangement significantly reduces the structural strength required of the container walls. Also the storage container 103 is a cube with equal dimensions for length, width, and height, and providing a maximum volume of earth material 105 with a minimum wall surface area, thus reducing heat loss through the walls. The storage container may also be cylindrical in shape as in the embodiment of Figs. 1 and 2. This cylindrical shape would be particularly applicable for an above ground installation in which the weight of the earth material 105 would want to naturally form this shape. It also may be possible in some areas to dig the hole required for the storage container 103 by removing suitable earth material 105. In the system 101, the heat input conduit circuit 107 from the source 115 is divided vertically and horizontally into eight substantially cubic input zones 129, as schematically illustrated by dotted lines 131. By manipulating valves 133, the input conduit circuit 107 can be configured such that the flow of input liquid 109 can be directed through selected input zones 129, or combinations of the input zones 129, or through all the input zones 129 at once to transfer heat to earth material in corresponding earth material zones 135.

The heat output system 117 may likewise be operative to transfer heat from selected earth material zones 135, or combinations of the earth material zones 135, or all the earth material zones 135 to an external heat consumer 121.

Thus with the system 101, the output system 117 would draw the temperature of the earth material in a zone 135 down by a desired amount, for example 50°C, and then the output system 117 would be changed to draw from a different zone 135. Similarly the heat input conduit circuit 107 could be configured to circulate input liquid 109 through each zone 129 separately or in combination, depending on the amount of heat available from the source 115 and the heat being drawn out by the heat output system 117.

In either system 1 or 101, but referring for convenience to system 1 of Figs. 1 and 2, if the temperature of the input liquid falls below its melting point the liquid will solidify in the conduits of the heat input conduit circuit 7. For example where the input liquid is aluminum, when the temperature thereof drops to 660°C, the input liquid will turn to a solid. It is thus desirable to provide a system for reheating the input liquid to the melting point. Where the source 15 of the heat input liquid 9 is a solar collection system, it is contemplated that, particularly where the input liquid is aluminum with a higher melting point compared to tin or sodium, the input liquid will solidify at least periodically in some portions of the heat input conduit circuit 7 during the night or during cloudy periods. Pumps, valves, junctions, and like areas of the heat input conduit circuit 7 are typically heated to the melting point of the input liquid 9 by electrical heaters. The entire heat input conduit circuit 7 could also be heated by electricity however it is desirable to be able to heat lengthy portions of the heat input conduit circuit 7, such as those buried in the earth material 5 or that connect the storage container 3 to the heat source 15, directly with heat from the heat source 15.

Fig. 5 schematically illustrates a cross-section of a portion of the heat input conduit circuit 7 that comprises a main conduit 41 and an auxiliary conduit 43 arranged in proximity to the main conduit 41. In operation the input liquid 9 flows in the main conduit 41, and an auxiliary liquid 45 flows in the auxiliary conduit, such that heat transfers from the auxiliary liquid 45 to the input liquid 9. The auxiliary liquid is selected to have a melting temperature that is less than a melting temperature of the input liquid 9.

In operation then if the temperature of the portion of the heat input conduit circuit 7 drops below the melting temperature of the input liquid, the auxiliary will remain liquid until the temperature of the auxiliary liquid also drops below its melting temperature, which will be much lower, and so will not often be encountered unless the heat source goes cold for an extended period. While the auxiliary liquid 45 is liquid, it can be circulated through auxiliary conduit 43 to the heat source 15 to raise the temperature thereof well above the melting point of the input liquid 9 and the heat from the auxiliary liquid 45 circulating in the auxiliary conduit 43 will be transferred to the main conduit 41 to melt the input liquid 9. In order to avoid building pressure in the auxiliary conduit 43, the auxiliary liquid 45 can be selected to have a boiling temperature at atmospheric pressure that is greater than the input operating temperature. Fig. 5 shows a heat input conduit circuit portion 7 where the auxiliary conduit 43 is beside the main conduit 41, and Fig. 6 shows an optional arrangement where the auxiliary conduit 43 is inside the main conduit 41.

The auxiliary liquid could be a metal alloy with a low melting point, such as Field's metal with a melting temperature of 62°C or Woods metal with a melting temperature of 70°C. Field's metal may be more suitable as same contains no harmful lead or cadmium. The auxiliary liquid 45 may be relatively costly compared to the input liquid 9, but could be drained from the auxiliary conduit 43 and used in different heat input conduit circuits at different times as required, so it is contemplated that the cost will not be prohibitive.

While it is contemplated that the auxiliary liquid 45 will not often fall below its melting temperature, means should generally be provided to also melt the auxiliary liquid if it does solidify. Figs. 7 - 9 schematically illustrate a heat input conduit circuit comprising the main conduit 41 , and two auxiliary conduits 43A, 43B.

The first auxiliary conduit 43A is arranged in proximity to the main conduit 41 such that heat is transferred from a first auxiliary liquid 45A flowing in the first auxiliary conduit 43A to the input liquid 9 in the main conduit 41, and the second auxiliary conduit 43B is arranged in proximity to the first auxiliary conduit 43A such that heat is transferred from a second auxiliary liquid 45B flowing in the second auxiliary conduit 43B to the first auxiliary liquid 45A. The melting temperature of the first auxiliary liquid 45A is less than a melting temperature of the input liquid 9 and, in turn the melting temperature of the second auxiliary liquid 45B is less than a melting temperature of the first auxiliary liquid 45A.

The second auxiliary liquid 45B can conveniently be selected to also have a melting point lower than ambient temperature at the location of the system. Thus if the entire system goes cold, the second auxiliary liquid 45B will remain liquid and can be circulated through the heat source to raise the temperature thereof to a level above the melting point of the first auxiliary liquid 45A, which in turn is circulated through the heat source as described above to melt the input liquid 9.

The second auxiliary liquid 45B conveniently can be water. The melting temperature of the metal alloy of first auxiliary liquid 45A can be selected to be below the boiling temperature of the water at atmospheric pressure, such that the water of the second auxiliary liquid 45B in the second auxiliary conduit 43B is not under pressure.

If the melting temperature of the metal alloy of first auxiliary liquid 45A is above the boiling temperature of the water at atmospheric pressure, some increased pressure could be maintained in the second auxiliary conduit 43B to increase the boiling temperature of the water in the second auxiliary conduit 43B. As schematically illustrated in Fig. 7, a valve 47 can be provided to selectively release pressure from the second auxiliary conduit 43B such that the water in the second auxiliary conduit 43B simply boils out of the secondary auxiliary conduit 43B as the temperature in the heat input conduit circuit 7 rises. Fig. 7 schematically illustrates a heat input conduit circuit 7 comprising the main conduit 41, and two auxiliary conduits 43A, 43B placed adjacent to the main conduit 41. In the embodiment of Fig. 8, the two auxiliary conduits 43A, 43B are placed inside the main conduit 41, and in Fig. 9 the second auxiliary conduit 43B is inside the first auxiliary conduit 43A which in turn is inside the main conduit 41. It is contemplated that placing the auxiliary conduits 43A, 43B inside the main conduit 41 may make a convenient package and facilitate installation and/or maintenance in the earth material filled storage container. Fig. 10 schematically illustrates a heat consumer 21 for connection to the heat output system of the energy storage system 1 of Figs. 1 and 2 that includes a heat output conduit circuit 17 with an output liquid 19 flowing therethrough. The heat output conduit circuit 17 is connected to a heat exchanger 51 which transfers heat from the output liquid 17 to a secondary liquid 53 from the heat exchanger 1 to a boiler 55. The heat exchanger 51 maintains separation between the output liquid 19 and the boiler 55 which contains water. The separation allows the output liquid to be more safely provided by sodium, which is relatively inexpensive, and has a low melting temperature of 98°C. With the sodium flowing as the output liquid in the input loop of the heat exchanger 51, and tin or some like non-hazardous as the secondary liquid 53 flowing in the heat output loop of the heat exchanger 51 to the boiler 55, there is little risk of contact between the sodium in the heat output conduit circuit 17 and the water in the boiler 55.

With the earth material 5 in the storage container 3 at a temperature of about 750°C, it is calculated that the boiler 55 could provide steam at a temperature of about 550°C which is an efficient temperature for operating a modem conventional steam turbine to produce electrical power. The temperature of the output liquid 19 flowing to the heat consumer 21 can be controlled to a desired temperature, for example by adjusting a bypass mixing valve 57, or by varying the rate of flow of output liquid 19 through the heat output conduit circuit 17 with a variable output pump 59.

Thus in a typical energy storage system 1 of the present invention, the input liquid 9 is aluminum with a melting temperature of 660°C, the first auxiliary liquid 45A is a metal allow such as Field's metal with a melting temperature of 62°C, and the second auxiliary liquid 45B is water. The first and second auxiliary conduit may remain empty until it is necessary to melt the aluminum input liquid. Initially at start up, the heat input conduit circuit 17 will be preheated with steam or the like to a temperature approaching 660°C and then the molten aluminum will be pumped through and substantially fill the heat input conduit circuit 7. From this point, depending on the operation of the heat source 15, the liquid aluminum 9 will circulate until the temperature thereof drops below 660°C. The Field's metal 45A will remain liquid if present until the temperature drops below 62°C. The water 45B will not usually be present in the auxiliary conduit 43B until it is needed to heat the Field's metal 45A.

It is calculated that a volume of about 14,700 cubic meters of earth material would provide sufficient thermal energy storage for a 20 megawatt electrical turbine. The storage container 3 would then be a cube about 11.4 meters on each side, with a heat input conduit circuit buried therein with conduits of about five centimeters (cm) in diameter spaced about 25 cm apart in a grid throughout the earth material 5 filing the storage container 3.

The container 3, and insulation space 23 if desired, can also be sealed and filled with a substantially inert gas atmosphere of nitrogen, carbon dioxide, helium, argon, or the like which will keep the earth material dry and reduce corrosion of the material of the container walls. A suitable wall material is stainless steel, which will resist corrosion. Where the storage container 3 and insulation space 23 is sealed, as schematically illustrated in Fig. 11, the pressure inside them will rise and fall as the temperature varies. To avoid excessive expanding and collapsing pressures being exerted on the container walls, a purge and makeup regulation system 61 is operative to selectively release inert gas from the storage container 3 and insulation space 23 through a vent 63 to the ambient atmosphere, and add inert gas from a pressurized gas container 65 to the storage container 3 and insulation space 23, to maintain atmospheric equilibrium therein during thermal expansion and contraction of the inert gas atmosphere as temperature changes

The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous changes and modifications will readily occur to those skilled in the art, it is not desired to limit the invention to the exact constmction and operation shown and described, and accordingly, all such suitable changes or modifications in structure or operation which may be resorted to are intended to fall within the scope of the claimed invention.