PRIORITY CLAIM

[0001] The present application claims priority to United States provisional patent application Serial No. 62/585,048, filed November 13, 2017, with the same title and inventor as given above, and which is incorporated herein by reference in its entirety.

BACKGROUND

[0002] Thermal energy storage (TES) allows excess energy to be collected and stored for later use. Molten salt-based TES systems are commonly employed in a number of applications, notably concentrated solar energy systems. One primary design of such systems utilizes two storage tanks for molten salt— one "cold" and one "hot." With reference to Figure 1 below, molten salt is pumped from the cold tank through a solar energy heating system to fill the hot tank. This allows energy to be stored in the hot tank, and to be used to generate electricity when desired. The hot salt passes through a steam generator, and the cooled salt returns to the cold tank.

SUMMARY

[0003] In one general aspect, the present invention is directed a heater that heats molten salt with electrodes submerged in the molten salt, where the electrodes are energized at medium voltage (MV) levels, e.g., 1,000 to 69,000 volts. Such a heater could be used in a molten salt- based TES system, where the heater is used to heat the molten salt being transported from the cold tank to the hot tank. These and other benefits of the present invention will be apparent from the description that follows. DRAWINGS

[0004] Various embodiments of the present invention are described herein by way of example in conjunction with the following figures, wherein:

[0005] Figure 1 is a diagram of a TES system where molten salt is pumped from a cold tank through a solar energy heating system to fill a hot tank; and

[0006] Figure 2 is a diagram of a MV heater for heating molten salt according to various embodiments of the present invention.

DESCRIPTION

[0007] Figure 2 is a diagram of a heater 5 according to various embodiments of the present invention. Molten salt 8 from the cold tank 10, such as a cold tank of a TES system, is pumped, at a desired pressure level, by a pump 11 (such as a circulation pump) into and through a reservoir 12 inside a container 14. The heater 5 includes a number of electrically- conductive electrodes 16 that are submerged in the molten salt 8 in the reservoir 12. In various embodiments, there are three electrodes 16, as shown in the example of Figure 2, which may be individually energized with a three-phase MV supply 11. The MV supply 11 preferably provides three-phase, alternating current electric power, i.e., having a common frequency (e.g., 50 or 60 Hz) and a common voltage amplitude level relative to a common reference (e.g., ground), such as between 1,000 and 69,000 volts, but with a phase difference between output terminals of one third of a cycle (120 degrees or approx. 2.0944 radians). An output terminal of the MV supply 11 for each phase is respectively connected to one of the electrodes 16, to thereby energize that corresponding electrode 16. As such, the molten salt 8 in and flowing through the reservoir 12 completes the circuit and also acts as the resistor for the electrodes 16, thereby delivering energy into the system and heating the molten salt 8. In that sense, the electrodes could be considered immersion arc heaters.

[0008] The molten salt can be transported to the hot tank 20 by a pipe 15 or other type of conduit. A valve, e.g., the depicted throttle valve 13, can control the flow of the molten salt 8 to the hot tank 20; that is, the throttle valve 13 can be opened to allow the molten salt 8 to flow to the hot tank 20. The heater 5 may include one or more temperature sensors 21 or conductivity sensors 23 to sense the temperature and/or conductivity of the molten salt 8 in the reservoir 12 to control whether the molten salt 8 is being heated to the appropriate temperature level. Also, a pressure safety value 17 can allow over-pressurized molten salt 8 to escape the container 14. [0009] The heater 5 preferably includes a control system, which may comprise a Supervisory Control and Data Acquisition (SCAD A) system, which controls the operation of the heater based on the measurements received from sensors (e.g., temperature sensors, conductivity sensors, power meters, flow meters) of the system. For example, the SCADA could monitor conditions of the heater and, for example, control the current and/or voltage levels from the MV power supply 11 based on the temperature and/or conductivity of the molten salt 8 as detected by the temperature and conductivity sensors 21, 23. The SCADA could also control the pump 11 and/or the valve 15 to control the flow rate of the molten salt. Still further, in various embodiments, the SCADA could control the addition of materials to the reservoir 8 to affect the conductivity of the materials in the reservoir 8. For example, if the conductivity of the molten salt 8 is too low, the SCADA could add materials to increase the conductivity and, conversely, if the conductivity of the molten salt 8 is too high, the SCADA could add materials to decrease the conductivity. In that way, the SCADA could control the heating of the molten salt 8. The SCADA may comprise computers, programmable logic controllers, and/or discrete PID controllers. Also, in various embodiments, the AC voltage from the power supply 11 may be rectified into DC voltage.

[0010] Examples of suitable molten salts for this invention include nitrate-, chloride- and fluoride-based salts. For example, an eutectic mixture of different nitrate salts, such as sodium nitrate (NaN03) and potassium nitrate (KN03) could be used, as could chloride- based mixtures of sodium chloride (NaCl), potassium chloride (KC1) and other salts, and fluoride-based salts such as FLiNaK and LiF-BeF ₂ . Such salts have varying conductance depending on temperature, i.e., their conductivity typically increases as temperature increases. The conductivity versus temperature characteristics of the chosen salt should be accounted for by the control system. Also, the materials for the electrodes 16 should be compatible with the specific salt media that is used. As an example, standard graphite electrodes are incompatible with strongly oxidizing heat transfer salts.

[0011] The control system should be designed to prevent "overheating" the molten salt so as to avoid causing irreversible degradation of the molten salt. The arcing between electrodes 16 should cause the salt temperature to rise with the highest temperatures in the heater 5 being between the electrodes. Heat will transfer to the rest of the localized solid salt and transform it into a liquid form. The control system preferably prevents the temperature of the salt from exceeding the maximum allowable temperature of the salt in order to prevent it from degrading or being transformed from a liquid to a gas. The electrodes 16 further should be positioned in the reservoir 12 so that the salt 8 is evenly heated and thereby reducing the occurrence of localized hotspots.

[0012] The chosen salt preferably has a high power density to maximize the power output of the heater 5.

[0013] As an example, the "cold" molten salt could have a temperature of the about 250°C; it could be heated to about 565°C; with a heater output of about 100 MW, and a flow rate of about 1500 to 2000 gallons per minute, such as 1900 gallons per minute (to prevent overheating the salt). That is, the electrodes 16 may increase the temperature of the molten salt by about 400 to 500 degrees Celsius (e.g., from about 250°C to about 565°C).

[0014] In one general aspect, therefore, the present invention is directed to a system comprising a reservoir 12 containing molten salt 8 and a plurality of electrodes 16 submerged in the molten salt 8 in the reservoir 12. The system also comprises a medium voltage (MV) power supply 11 connected to each of the plurality of electrodes 16 to energize the electrodes 16 such that the energized electrodes 16 heat the molten salt 8 in the reservoir 12.

[0015] In another general aspect, the present invention is directed to a method that comprises pumping molten salt 8 to the reservoir 12 and heating the molten salt 8 in the reservoir with the plurality of MV electrodes 16 that are submerged in the molten salt 8 in the reservoir.

[0016] According to various implementations of the system and method, the plurality of electrodes 16 comprises first, second and third electrodes and the MV power supply 11 comprises a 3 -phase MV power supply. As such, the 3 -phase MV power supply may comprises first, second and third output terminals. Each of the first, second and third output terminals may carry an alternating current of a common frequency and a common voltage amplitude relative to a common reference, and have a phase difference of one third of a cycle between each terminal. In such a configuration, the first output terminal may be connected to the first electrode; the second output terminal may be connected to the second electrode; and the third output terminal may be connected to the third electrode. The common voltage amplitude relative to the common reference may be between 1,000 and 69,000 volts, inclusive.

[0017] In various implementations, the molten salt may comprise a nitrate-based salt, a chloride-based salt and/or a fluoride-based salt.

[0018] In various implementations, the system may be part of a thermal energy system that comprises: a cold tank 10; a pump 11 for pumping molten salt 8 from the cold tank 10 to the reservoir 12; a hot tank 20; and a valve 13 for controlling flow of the molten salt 8 from the reservoir 12 to the hot tank 20 after heating of the molten salt 8 by the plurality of electrodes 16.

[0019] In various implementations, the electrodes increase the temperature of the molten salt by about 400 to 500 degrees Celsius (e.g., from about 250°C to about 565°C). Also, the molten salt may flow from the cold tank, through the reservoir, and to the hot tank at a rate of about 1500 to 2000 gallons per minute.

[0020] The examples presented herein are intended to illustrate potential and specific implementations of the present invention. It can be appreciated that the examples are intended primarily for purposes of illustration of the invention for those skilled in the art. No particular aspect or aspects of the examples are necessarily intended to limit the scope of the present invention. For example, although embodiments above were described in the context of a thermal energy system, the present invention is not so limited unless otherwise indicated, and can therefore be applied to other types of systems where molten salt is heated.

[0021] Further, it is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, other elements. While various embodiments have been described herein, it should be apparent that various modifications, alterations, and adaptations to those embodiments may occur to persons skilled in the art with attainment of at least some of the advantages. The disclosed embodiments are therefore intended to include all such modifications, alterations, and adaptations without departing from the scope of the embodiments as set forth herein.