BACKGROUND OF THE INVENTION

[0001] Solar electric power generation systems offer a source of clean, renewable energy.

However, the peak incidence of solar radiation does not necessarily occur when power is most needed. For example, in a typical installation in the desert southwest of the United States, incoming solar radiation may be at its peak near mid-day, but demand for electric power may peak in the late afternoon, and strong demand may continue into the evening and early nighttime hours when no solar radiation is being received.

[0002] Energy storage may be used to at least partially decouple the generation of electric power from the rate of incoming solar radiation, and greater efficiencies in storing thermal energy are desired.

BRIEF SUMMARY OF THE INVENTION

[0003] In one aspect, an energy storage system comprises a battery that further comprises a cathode medium and an anode medium. The system further comprises a connection for receiving electrical energy, and a heat transfer medium. At least some of the received electrical energy is stored electrochemically in the battery, and energy is stored thermally by heating the cathode medium and the anode medium with thermal energy received from the heat transfer medium. In some embodiments, the battery is a flow battery. In some embodiments, the system further comprises one or more heat exchangers for supplying heat to and extracting heat from the anode medium and the cathode medium. In some embodiments, the battery is a molten salt battery or a liquid metal battery. In some embodiments, the heat transfer fluid also serves as an electrolyte for the battery. In some embodiments, the system further comprises one or more heat exchangers for transferring heat between the heat transfer fluid and the cathode medium and between the heat transfer fluid and the anode medium. In some embodiments, heat exchange between the heat transfer fluid and the cathode medium and between the heat exchange fluid and the anode medium occurs through the walls of cells of the battery, such that a piping loop carrying the heat exchange fluid does not penetrate into cells. In some embodiments, the system further comprises a first tank for holding the anode medium in its hot state, a second tank for holding the anode medium in its cold state, a third tank for holding the cathode medium in its hot state, and a fourth tank for holding the cathode medium in its cold state. In some embodiments, thermal energy and electrical energy are extracted when the anode medium flows from the first tank to the second tank and the cathode medium flows from the third tank to the fourth tank. In some embodiments, electrical energy and thermal energy can be extracted from the system independently of one another. In some embodiments, the system further comprises a first tank for storing the anode medium, and a second tank for storing the cathode medium. In some embodiments, the system further comprises one or more heat exchangers for supplying thermal energy from the heat transfer fluid to the first and second tanks. In some embodiments, the heat transfer fluid can circulate through both the first and second tanks and through a power generation block for generating electricity. In some

embodiments, the heat transfer fluid can circulate through both the first and second tanks and through a concentrating solar collector. In some embodiments, the battery is a stationary battery. The stationary battery may be present in a stacked configuration.

[0004] According to another aspect, a power generation system comprises a photovoltaic solar collector that generates electricity from incoming solar radiation, a thermal solar collector that heats a heat transfer fluid using incoming solar radiation, and a battery comprising a cathode medium and an anode medium. At least some of the electrical energy is stored electrochemically in the battery, and energy is stored thermally by heating the cathode medium and the anode medium with thermal energy received from the heat transfer medium. In some embodiments, the battery is a flow battery. In some embodiments, the battery is a molten salt battery or a liquid metal battery. In some embodiments, the system further comprises a first tank for holding the anode medium in its hot state, a second tank for holding the anode medium in its cold state, a third tank for holding the cathode medium in its hot state, and a fourth tank for holding the cathode medium in its cold state. In some embodiments, thermal energy and electrical energy are extracted when the anode medium flows from the first tank to the second tank and the cathode medium flows from the third tank to the fourth tank. In some embodiments, electrical energy and thermal energy can be extracted from the system independently of one another. In some embodiments, the system further comprises a first tank for storing the anode medium, and a second tank for storing the cathode medium. In some embodiments, the system further comprises a power generation block that generates electrical power from thermal energy extracted from the cathode medium and the anode medium. In some embodiments, the system further comprises one or more heat exchangers for supplying heat to and extracting heat from the anode medium and the cathode medium. In some embodiments, the battery is a molten salt battery or a liquid metal battery. In some embodiments, the heat transfer fluid also serves as an electrolyte for the battery. In some embodiments, the system further comprises one or more heat exchangers for transferring heat between the heat transfer fluid and the cathode medium and between the heat transfer fluid and the anode medium. In some embodiments, heat exchange between the heat transfer fluid and the cathode medium and between the heat exchange fluid and the anode medium occurs through the walls of cells of the battery, such that a piping loop carrying the heat exchange fluid does not penetrate into cells. In some embodiments, the battery is a stationary battery. The stationary battery may be present in a stacked configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 illustrates a simplified schematic view of conventional concentrating solar thermal power plant.

[0006] FIG. 2 illustrates a simplified schematic view of conventional photovoltaic solar power plant.

[0007] FIG. 3 illustrates a simplified schematic view of a flow battery.

[0008] FIG. 4 illustrates a simplified schematic view of a molten salt battery.

[0009] FIGS. 5 A and 5B illustrate a system in accordance with a first embodiment of the invention.

[0010] FIG. 6 illustrates an energy storage system in accordance with a second embodiment of the invention.

[0011] FIG. 7 illustrates a system in accordance with a third embodiment of the invention.

[0012] FIG. 8 illustrates a system in accordance with a fourth embodiment of the invention.

[0013] FIG. 9 illustrates a system in accordance with a fifth embodiment of the invention.

[0014] FIG. 10 illustrates a system in accordance with a sixth embodiment of the invention.

[0015] FIG. 11 illustrates a system in accordance with a seventh embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0016] Solar energy may be harvested and utilized in various ways.

[0017] FIG. 1 illustrates a simplified schematic view of conventional concentrating solar thermal power plant 100. Example power plant 100 uses a central receiver or "power tower" 101 to heat a heat transfer fluid such as a molten salt circulating in a first piping loop 102. Incoming solar radiation 103 is directed to receiver 101 by a field of heliostats 104, and the heat transfer fluid can reach temperatures of hundreds of degrees Celsius. The heated fluid is passed through a steam generator 105, to generate steam in a second piping loop 106. The steam may be used to generate electricity, for example by turning a turbine 107, which in turn powers a generator 108, which supplies power to the grid 112. After passing through the turbine 107, the steam may be condensed 109 and reheated in steam generator 105. [0018] For the purposes of this disclosure, the components that convert thermal energy to electrical energy may be collectively referred to as a "power block". For example, in FIG. 5A, the power block comprises the second piping loop 106, the turbine 107, the generator 108, and the condenser 109.

[0019] During the day, the heat transfer fluid in its heated state may be accumulated in hot storage tank 110, for use at a later time. The system thus stores energy thermally by virtue of the specific heat of the heat transfer fluid and its elevated temperature. When no solar radiation is being received, the hot heat transfer fluid from the hot storage tank 110 can be passed through steam generator 105 to generate steam, and then accumulated in cold storage tank 111, to be heated again when sunlight is available. The thermal energy storage capacity of the system depends on many factors, including primarily the amount of heat transfer fluid that can be held in the hot storage tank 110, and the temperature differential between the cold storage tank 111 and the hot storage tank 110.

[0020] For the purposes of this disclosure, the terms "hot" and "cold" are to be understood in a relative sense. That is, the heat transfer fluid in the hot storage tank 110 is at a higher temperature than the heat transfer fluid in the cold storage tank 111, but even the "cold" heat transfer fluid may be considered very hot to human senses.

[0021] FIG. 2 illustrates a simplified schematic view of conventional photovoltaic solar power plant. Photovoltaic panels 201 convert solar radiation 103 directly to electricity, which can be directed to the power grid 112 through an inverter 202. Prior large scale photovoltaic solar power plants typically have not included any energy storage mechanism; the generated power is supplied immediately to the power grid 112. In smaller systems, storage may be provided, for example in the form of a battery 203. Various controls and switchgear (not shown) may be provided so that the generated electric power can be directed entirely to the inverter 202 or entirely to the battery 203, or so that battery 203 can be charged using a portion of the generated power while the remaining portion is directed to inverter 202.

[0022] While the illustrated embodiments show the use of an inverter such as the inverter 202 to deliver alternating current (AC) power to the grid 112, it will be recognized that other

embodiments may deliver direct current (DC) power to a DC system, without using an inverter.

[0023] Large scale battery technologies have been developed. For example, FIG. 3 illustrates a simplified schematic view of a flow battery 300. The flow battery 300 is an electrochemical energy storage (ECES) system in which liquid electrodes (one anode medium 301 and one cathode medium 302) flow through an electrochemical reactor 303 that reversibly converts chemical energy directly to electricity. The liquid electrodes are stored external to the reactor 303 in tanks 304 and 305, and are pumped through the cell (or cells) of the reactor. The reactor 303 contains a membrane 306 that separates the two electrodes, but allows for ion exchange. Large lead acid flow batteries have been in use for many decades for back-up power in remote locations. The energy storage capacity of the system depends on many factors, including primarily the amount of anode and cathode media and the electrochemical characteristics of the media.

[0024] FIG. 4 illustrates another battery technology called the molten salt battery 400. In example battery 400, two molten electrodes 401 and 402 are separated by an electrolyte 403. The electrodes 401 and 402 may be, for example molten sodium (Na) and sulfur (S), in which case battery 400 is a sodium-sulfur or NaS battery. During charging and discharging, sodium ions migrate through the electrolyte 403 between the electrodes 402 and 403. Many different cell chemistries are possible, and recently, batteries are being developed using liquid metals for both electrodes, and a liquid salt for the electrolyte. More information about liquid metal batteries may be found in Kim et al, "Liquid Metal Batteries: Past, Present, and Future", Chem. Rev., 2013, 113(3), pp. 2075-2099 (2012), the entire disclosure of which is incorporated herein by reference.

[0025] In batteries having molten electrodes, a concern is keeping the electrodes in a molten state, which requires that the battery operate at temperatures above the highest melting point of its constituents. Molten salt and liquid metal batteries may be expected to have very long service lives, because there are no solid transitions that can fatigue (the typical cycle failure of chemical batteries).

[0026] The selection of the battery electrode media will depend on the electrochemical properties of the two materials, cost, material compatibility considerations, and the working temperature of the materials. Some possible battery chemistries include, without limitation:

[0027] Coulombic efficiency and cell voltage are other properties of interest. Some molten metal batteries have achieved Coulombic efficiencies of 95%, so cell voltage is of greater concern. Na-Sn units have only achieved cell voltages of about .3 V, while the best performing Li-Te cells only produce 1.8 V per cell. In a plant, many cells would need to be put in series to achieve a useful voltage. Sodium-lead (Na-Pb) may be a particularly promising electrochemistry.

[0028] In some embodiments, that stationary batteries may be used. The electrochemical reaction that occurs inside the stationary batteries normally has ohmic losses that generate heat which is added to the storage system. This heat can be used in the power cycle, avoiding over heating of the cell and the need of a cooling system for reducing the temperature in the cell. The stationary battery can be present in a stacked configuration.

[0029] Many other battery types exist, and it is intended that unless specifically claimed, the invention is not limited to any particular battery type.

[0030] Typically solar thermal power plants and solar photovoltaic power plants have been designed and installed independently, and large scale photovoltaic plants have not included energy storage.

[0031] Embodiments of the invention combine thermal and electrochemical energy storage in a hybrid energy storage system. Net energy storage is increased, because the same materials that store energy electrochemically are also used to store energy thermally. Other advantages may be realized as well. For example, the cost of tanks, pumps, piping, media, or other system

components may be shared by the photovoltaic and thermal systems.

[0032] The potential energy stored electrochemically may be roughly one third that of the energy stored thermally. But since the conversion efficiency from chemical to electrical is much higher than the conversion of thermal energy to electrical energy (60-90% as opposed to 20-40%) the power that is delivered to the grid may be approximately the same from the two storage modes, depending on the electrochemistry used. Typically, chemical molten salt batteries operate at minimum temperatures of 400-650°C (with special chemistries operating as low as 273°C) so the required temperatures are compatible with current and future CSP plants.

[0033] FIG. 5A illustrates a system 500 in accordance with a first embodiment. System 500 includes photovoltaic panels 501 that convert incoming sunlight directly to electricity. The power generated by the photovoltaic panels 501 may be delivered to the power grid 112 through an inverter 502, or may be routed to a cell 503 that is part of a flow battery, or portions of the generated power may be delivered to both inverter 502 and cell 503. [0034] In cell 503, a membrane 504 separates an anode medium 505 from a cathode medium 506, but allows ion transfer between the two media. The anode medium 505 is stored in first and second tanks 507 and 508, and cathode medium 506 is stored in third and fourth thanks 509 and 510, such that the anode medium 505, cathode medium 506, and cell 503 form a flow battery. This embodiment may be described as a "four tank" system, because tanks are provided for storing the two different media in two different temperature states, but it will be recognized that any or all of the "tanks" of FIG. 5 A may actually be made up of multiple physical containers.

[0035] In addition, a concentrating solar collector 511 heats a heat transfer fluid circulating in piping loop 512. The heat transfer fluid in the piping loop 512 may be, for example, a molten salt. In this example, the solar collector 511 is a trough-type collector, and only a single collector module is shown, but it will be recognized that more collector modules may be used, or a different kind of collector may be used, such as the "power tower" collector shown in FIG. 1 or a hybrid collector such as is described in international patent application publication WO 2013/033200 published March 7, 2013 and titled "Hybrid Solar Field", the entire disclosure of which is incorporated by reference herein for all purposes.. The heat transfer fluid in the piping loop 512 is circulated to a power block 513, where electricity is generated and supplied to the power grid 112. The power block 513 may include, for example a steam generator, turbine, and generator similar to those shown in FIG. 1.

[0036] Using appropriate valves and controls (not shown), some or all of the heat transfer fluid in the piping loop 512 can be directed to a heat exchanger 514. FIG. 5A shows the system 500 in a "charging" mode, in which heat transfer fluid from the piping loop 512 is directed to the heat exchanger 514, where heat is transferred from the heat transfer fluid to the anode and cathode media 505 and 506. As the anode and cathode media 505 and 506 are pumped from the cold tanks 508 and 510 to the hot tanks 507 and 509, the anode and cathode media are heated, and thermal energy is stored in the anode and cathode media by virtue of their specific heat and the increase in their temperature.

[0037] In addition, the flow battery is charged in the cell 503 using electrical energy from the photovoltaic panels 501. Thus, the anode and cathode media store energy both thermally and electrochemically.

[0038] The flow battery in the embodiment of FIG. 5 A may be a lead-acid flow battery, or may use another suitable chemistry. In some embodiments, a membraneless flow battery may be used.

[0039] FIG. 5B shows the system of FIG. 5 A in a discharging mode. In FIG. 5B, no solar radiation is being received at the photovoltaic panels 501 or the concentrating solar collector 511. The flow of the anode and cathode media 505 and 506 is reversed as compared with FIG. 5A, so that the anode and cathode media flow from the hot tanks 507 and 509 to the cold tanks 508 and 510. In the process, electrical energy is extracted in cell 503 and provided to the inverter 502, and to the power grid 112.

[0040] At the heat exchanger 514, the still-hot anode and cathode media 505 and 506 pass through the heat exchanger 514 where thermal energy is transferred to the heat transfer fluid in a piping loop 517. The heat transfer fluid in the piping loop 517 is circulated to the power block 513, where power is generated and supplied to the power grid 112. When the anode and cathode media 505 and 506 reach the cold tanks 508 and 510, they have thus been depleted of both their thermally- and electrochemically-stored energy.

[0041] When sunlight is available again, the system can re-enter the charging mode of FIG. 5 A.

[0042] FIG. 6 illustrates a hybrid energy storage system 600 in accordance with a second embodiment. Similar to the energy storage system of the embodiment of FIGS. 5 A and 5B, system

600 is also a four tank system that uses a flow battery and combines thermal and electrical energy storage in the battery media. However, in system 600 additional components enable the thermal storage and electrochemical storage to be operated independently if desired.

[0043] In storage system 600, an anode medium 601 is stored in tanks 602 and 603 and can be moved between the two tanks 602 and 603 using reversible pumps 604 and 605. Similarly, a cathode medium 606 is stored in tanks 607 and 608, and can be moved between the two tanks 607 and 608 using pumps 609 and 610. A reactor 611 includes a membrane 612, and performs the same function as the cell 503 shown in FIGS. 5 A and 5B. That is, as the anode and cathode media

601 and 606 flow past the membrane 612, ion exchange can occur, charging or discharging the flow battery, depending on whether power is being supplied to or taken from the battery.

[0044] A heat exchanger 613 performs the same function as the heat exchanger 514 of FIGS. 5 A and 5B. That is, as the anode and cathode media 610 and 606 flow through the heat exchanger, heat transfer can occur to or from another heat transfer fluid, depending on whether the system is being thermally "charged" or thermal energy from the system is being used, for example to generate electricity. For ease of explanation, external components such as a solar photovoltaic collector field, a concentrating solar collector field, and the power grid are not shown in FIG. 6.

[0045] In some situations, it may be desirable to use the system to store energy thermally or electrochemically, but not both. For example, if the source of electrical energy is temporarily unavailable due to maintenance but the source of thermal energy is functioning, it may be desired to store energy thermally even though no electrical energy is available to be stored electrochemically.

[0046] In another possible scenario, an extended period of cloudy weather may be predicted so that neither photovoltaic or concentrating solar collectors will be productive. However, the system may be located at a site where electricity is available cheaply from the grid during off-peak hours. It may be economical to store energy from the grid electrochemically in the system 600 during the off-peak hours, and then re-sell the stored energy at peak rates at a later time.

[0047] To enable independent operation of the thermal and electrochemical storage capabilities of the system, a set of valves 614-625 is provided that can isolate either reactor 611 or heat exchanger 613 from the system, while permitting operation of the other. For example, for operation of only the electrochemical storage capability of the system, valves 615, 617, 619, 621, 623, and 625 would be closed, and valves 614, 616, 618, 620, 622, and 624 would be opened. To operate only the thermal storage capability of the system, the open and closed valves would be reversed. That is, valves 615, 617, 619, 621, 623, and 625 would be opened and valves 614, 616, 618, 620, 622, and 624 would be closed.

[0048] In another embodiment (not shown), selectable piping loops may be provided that enable circulation of the anode medium 601 from the tank 602 to the reactor 611 and back to the tank 602, and enable circulation of the cathode medium 606 from the tank 607 to the reactor 611 and back to the tank 607. This enables electrochemical storage using only two tanks. This capability may be useful, for example, during maintenance of one or both of the tanks 603 and 608.

[0049] FIG. 7 illustrates a system 700 in accordance with a third embodiment. Similar to the first embodiment shown in FIGS. 5A and 5B, photovoltaic panels 701 receive solar radiation and convert it directly to electricity. The electric energy can be provided to an inverter 702 for delivery to the power grid 112. At other times, some or all of the electrical energy can be delivered to a battery 703.

[0050] In this embodiment, battery 703 uses molten electrodes stored in cells 704 and separated by a molten electrolyte. For example, battery 703 may be a molten salt or liquid metal battery. Electrical energy may be stored electrochemically in the cells 704 when sunlight is available, and can be extracted directly from the tops and bottoms of the cells 704 at a later time for delivery to the grid 112 through the inverter 702. While embodiments having only a single cell or only a few cells may be envisioned, many cells 704 may be electrically connected in series to raise the voltage to a more useful level and allow better use of the thermal charging and discharging. [0051] One or more solar collectors 705 also heat a heat transfer fluid circulating in piping loop 706. The piping loop 706 can circulate the heat transfer fluid to a power block 707, where electricity is generated and can be supplied to the power grid 112. At other times, some or all of the heat transfer fluid in the piping loop 706 may be routed to a heat exchanger 708. The electrolyte from cells 704 is circulated by pump 709 through heat exchanger 708, so that the electrolyte is heated. As the electrolyte circulates through the cells, it also heats the molten electrodes, so that the electrodes and electrolyte store energy thermally.

[0052] FIG. 7 shows system 700 in a charging mode. In a discharging mode (not illustrated), the electrolyte is again circulated through the heat exchanger 708 by pump 709. The heat transfer fluid used in the piping loop 706 is heated in the heat exchanger 708 by the thermal energy being extracted from the cells 704, and is supplied to the power block 707. The piping loop 706 may be abbreviated during discharge, so that the heat transfer fluid is not unnecessarily circulated through the solar collector 705.

[0053] In other embodiments, the material of one of the molten electrodes could be circulated to the heat exchanger 708, rather than the electrolyte.

[0054] FIG. 8 illustrates a system 800 in accordance with a fourth embodiment. The system 800 is similar in some ways to the system 700 shown in FIG. 7, and similar components have been given the same reference numerals in FIG. 8 as in FIG. 7.

[0055] In system 800, rather than circulating the electrolyte from the cells 704 to the heat exchanger 708, a secondary heat transfer fluid is circulated through another piping loop 801. Heat is exchanged between the electrodes and electrolyte and the secondary heat transfer fluid, but the secondary heat transfer fluid does not mix with the battery materials. Appropriate heat exchanger structures may be placed within the cells 704. Because the secondary heat transfer fluid is not involved in the electrochemical action of the cells 704, it may be selected based on its properties as a heat transfer fluid (e.g. freeze point, corrosiveness, specific heat, cost), with little or no regard to its electrochemical properties. This arrangement may also simplify the system design because it may be easier to electrically isolate, and the secondary heat transfer fluid may be less corrosive than the electrode or electrolyte materials.

[0056] While the piping loop 801 is shown as exchanging heat with all three battery materials (two electrodes and the electrolyte), other embodiments may exchange heat with only one or two of the battery materials, relying on thermal conduction to heat the remaining materials. [0057] In some embodiments, the heat exchanger 708 may transfer heat to a steam loop for direct use by the power block 707. In other embodiments, the intermediate piping loop 801 may not be used, and the heat transfer fluid in the piping loop 706 may be circulated through heat exchangers within the cells 704.

[0058] The heat exchangers are preferably electrically isolated so that the electrical energy is not wasted by unintended current flow. This allows the charging/discharging of the different systems to occur separately.

[0059] FIG. 9 illustrates a system 900 in accordance with a fifth embodiment. The system 900 is similar in some ways to the system 800 shown in FIG. 8, and similar components have been given the same reference numerals in FIG. 9 as in FIG. 8. In system 900, the piping loop 901 carrying the secondary heat transfer fluid passes through or near the walls of the cells 704, rather than through the cells 704. Heat exchange between the secondary heat transfer fluid occurs through the cell walls by conduction and convection. As compared with the system 800, the arrangement of the system 900 may simplify the design and construction of the battery 703, due to fewer penetrations into the cells 704.

[0060] FIG. 10 illustrates a system 1000 in accordance with a sixth embodiment. In the system 1000, some components are similar to components described above with respect to the systems 700, 800, and 900, and those components are again given the same reference numerals.

[0061] In the system 1000, an anode medium 1001 is held in a first tank 1002, and a cathode medium 1003 is held in a second tank 1004. As before, while only two tanks are shown for ease of illustration, it will be recognized that each "tank" may be made up of multiple containers. The anode and cathode media 1001 and 1003 are circulated through a cell or reactor 1005, which is separated by a membrane 1006. The tanks 1002 and 1004, the reactor 1005, and the anode and cathode media 1001 and 1003 thus are comprised in a flow battery as is described above.

Electrical energy produced by the photovoltaic panels 701 may be stored and recovered as the anode and cathode media are circulated through the reactor 1005 from their respective tanks.

[0062] The anode and cathode media 1001 and 1003 are also used as thermal storage media. One or more concentrating solar collectors 705 heat a heat transfer fluid circulating in a piping loop 706, which in turn heats a secondary heat transfer fluid circulating in secondary piping loop 1007 using heat exchanger 708. The secondary heat transfer fluid then heats the anode and cathode media in the tanks 1002 and 1004 by heat exchange from the secondary piping loop 1007, using pipes or other heat exchangers within the tanks 1002 and 1004. The thermal energy may be extracted and used to heat the heat transfer fluid in the piping loop 706, for delivery to the power block 707.

[0063] In some embodiments, the heat exchanger 708 may not be used, and the heat transfer fluid from the piping loop 706 may be circulated through the tanks 1002 and 1004, as well as to the power block 707.

[0064] FIG. 11 illustrates a system 1100 in accordance with a seventh embodiment of the invention. The system 1100 is similar to the system 1000 shown in FIG. 10, and like components have been given like reference numerals in FIG. 11. The system 1100 differs in the arrangement of heat exchange between the heat transfer fluid in the piping loop 1101 and the tanks 1002 and 1004, using appropriate thermal contact. In the system of FIG. 11, the piping loop 1101 does not penetrate the tanks 1002 and 1004. Rather, heat exchange occurs in or near the outer walls of the tanks 1002 and 1004. This arrangement may simplify the system, due to the reduced number of penetrations into the tanks.

[0065] While embodiments of the invention have been described in the context of co-located photovoltaic and thermal solar collector fields, it is to be understood that the invention is not so limited, and that a hybrid energy storage system embodying the invention may be used in other applications as well. It is further to be understood that any workable combination of the features and capabilities disclosed above in the various embodiments is also considered to be disclosed.