Patent Application
20120319410
With the increasing integration of renewable generation and the resulting stress on grid reliability, new, low cost, robust methods are needed to store power for buffering excess off-peak generation and balancing of electrical supply and demand. Presently, fossil fuel resources are ramped up and down to balance the variability of wind and solar generation, resulting in increased emissions and reduced reliability of equipment. Pumped hydroelectric storage is an example of the ability to charge a storage system during off-peak operation, and maintain availability to produce peak power on demand. Unfortunately, there are not sufficient pumped hydro sites available to relieve grid congestion and the ramp-up time required for such systems is substantial. Battery technologies are being demonstrated, however costs are high and reliability is untested over long periods.
Prior implementations of Thermal Energy Storage (TES) have seen limited success. For grid scale, TES has seen limited application, and primarily for direct application in concentrated solar power (CSP) utilizing very large storage vessels of molten salt. In this application, heat from the CSP collector is transferred to liquid phase salt and pumped to a large tank to store solar heat for later usage, where it is then pumped through a heat exchanger for energy extraction and to a low temperature second tank. To date, most commercial systems have imposed temperature limits below 350 degrees C. due to the desire to use low vapor pressure synthetic oils for heat transfer fluids. The salts used have typically been mixtures of sodium and potassium nitrates. These single-phase, molten salts have typically been designed to have a low melting point because they require pumping through closed loop systems and failure modes often involve solidification of the molten salt within the system, resulting in significant effort to re-liquefy the entire heat transfer circuit upon restart.
An embodiment of the present invention may therefore comprise: a system that stores thermal energy and provides electrical output comprising: a thermal energy storage module comprising: an insulated containment vessel; a source energy input in thermal communication with a thermal energy storage media and a thermal conductive endostructure, the thermal conductive endo structure that absorbs heat energy from the source energy and conducts the heat energy throughout the thermal conductive endostructure in a first time period, the thermal energy storage media that absorbs heat energy from the source energy and the thermal conductive endostructure to produce a phase change in the thermal energy storage media and store the heat energy as sensible heat and latent heat in a liquid form in the first time period; and, a heat exchanger in thermal communication with the thermal energy storage media and the thermal conductive endostructure that transfers stored thermal energy in the form of sensible heat from the thermal conductive endostructure to an energy transfer media in a second time period, the thermal energy storage media that transfers stored thermal energy in the form of latent heat from the phase change of the thermal energy storage media, and sensible heat from the thermal energy storage media, to the energy transfer media in the second time period; a stored energy output in thermal communication with the energy transfer media that facilitates transport of the energy transfer media from the insulated containment vessel to a turbine, the turbine that converts the heat energy from the energy transfer media into work and exhaust; a generator in mechanical communication with the turbine that utilizes the work to provide electricity in the second time period; and, a recuperator in thermal communication with the exhaust that extracts waste heat from the exhaust to preheat the energy transfer media before returning the energy transfer media to the source energy input.
An embodiment of the present invention may also comprise: a system that stores thermal energy and provides electrical output comprising: a primary thermal energy storage module comprising: a first insulated containment vessel; a first source energy input in thermal communication with a first thermal energy storage media and a first thermal conductive endostructure, the first thermal conductive endostructure that absorbs heat energy from the first source energy and conducts the heat energy throughout the first thermal conductive endostructure in a first time period, the first thermal energy storage media that absorbs heat energy from the first source energy and the first thermal conductive endostructure to produce a phase change in the first thermal energy storage media and store the heat energy as sensible heat and latent heat in a liquid form in the first time period; and, a first heat exchanger in thermal communication with the first thermal energy storage media and the first thermal conductive endostructure that transfers stored thermal energy in the form of sensible heat from the first thermal conductive endostructure to a first energy transfer media in a second time period, the first thermal energy storage media that transfers stored thermal energy in the form of latent heat from the phase change of the first thermal energy storage media, and sensible heat from the first thermal energy storage media, to the first energy transfer media in the second time period; a stored energy output in thermal communication with the energy transfer media that facilitates transport of the energy transfer media from the primary thermal energy storage module to a high pressure turbine, the high pressure turbine that converts the heat energy from the energy transfer media into primary work and primary exhaust; a secondary thermal energy storage module comprising: a second insulated containment vessel; a second source energy input in thermal communication with a second thermal energy storage media and a second thermal conductive endostructure, the second thermal conductive endostructure that absorbs heat energy from the primary exhaust and conducts the heat energy throughout the second thermal conductive endostructure in a first time period, the second thermal energy storage media that absorbs heat energy from the primary exhaust and the second thermal conductive endostructure to produce a phase change in the second thermal energy storage media and store the heat energy as sensible heat and latent heat in a liquid form in the first time period; and, a second heat exchanger in thermal communication with the second thermal energy storage media and the second thermal conductive endostructure that transfers stored thermal energy in the form of sensible heat from the second thermal conductive endostructure to a second energy transfer media in a second time period, the second thermal energy storage media that transfers stored thermal energy in the form of latent heat from the phase change of the second thermal energy storage media, and sensible heat from the second thermal energy storage media, to the second energy transfer media in the second time period; a second stored energy output in thermal communication with the energy transfer media that facilitates transport of the energy transfer media from the secondary thermal energy storage module to a low pressure turbine, the low pressure turbine that converts the heat energy from the energy transfer media into secondary work and secondary exhaust; a generator in mechanical communication with the high pressure turbine and low pressure turbine that utilizes the primary work and the secondary work to provide electricity in the second time period; and, a recuperator in thermal communication with the secondary exhaust that extracts waste heat from the secondary exhaust to preheat the energy transfer media before returning the energy transfer media to the source energy input.
An embodiment of the present invention may therefore comprise: a method of storing thermal energy and providing electrical energy output comprising the steps of: during a first time period; transferring thermal energy within an insulated containment vessel from a heat source to a thermal conductive endostructure and a thermal energy storage media; absorbing and conducting the thermal energy from the heat source throughout the thermal conductive endostructure; absorbing the thermal energy from the heat source and the thermal conductive endostructure with a thermal storage media; changing the phase of the thermal storage media from solid to liquid with the thermal energy; and, storing the thermal energy in the form of sensible heat in the thermal conductive endostructure, and storing the thermal energy in the form of sensible heat and latent heat in the liquefied thermal storage media; during a second time period; transferring the stored thermal energy in the form of sensible heat from the thermal conductive endostructure through a heat exchanger to an energy transfer media; transferring the stored thermal energy in the form of latent heat from the phase change of the thermal energy storage media, and sensible heat from the thermal energy storage media through the heat exchanger to the energy transfer media; transferring thermal energy with the energy transfer media from the insulated containment vessel to a turbine; converting the thermal energy from the energy transfer media into work and exhaust; utilizing the work to provide electricity with a generator in mechanical communication with the turbine; extracting waste heat from the exhaust with a recuperator; preheating the energy transfer media with the extracted waste heat in the recuperator; and, returning the energy transfer media to the insulated containment vessel.
An embodiment of the present invention may also comprise: a system for storing thermal energy and providing electrical energy output comprising: during a first time period; a means for transferring thermal energy within an insulated containment vessel from a heat source to a thermal conductive endostructure and a thermal energy storage media; a means for absorbing and conducting the thermal energy from the heat source throughout the thermal conductive endostructure; a means for absorbing the thermal energy from the heat source and the thermal conductive endostructure with a thermal storage media; a means for changing the phase of the thermal storage media from solid to liquid with the thermal energy; and, a means for storing the thermal energy in the form of sensible heat in the thermal conductive endostructure, and storing the thermal energy in the form of sensible heat and latent heat in the liquefied thermal storage media; during a second time period; a means for transferring the stored thermal energy in the form of sensible heat from the thermal conductive endostructure through a heat exchanger to an energy transfer media; a means for transferring the stored thermal energy in the form of latent heat from the phase change of the thermal energy storage media, and sensible heat from the thermal energy storage media through the heat exchanger to the energy transfer media; a means for transferring thermal energy with the energy transfer media from the insulated containment vessel to a turbine; a means for converting the thermal energy from the energy transfer media into work and exhaust; a means for utilizing the work to provide electricity with a generator in mechanical communication with the turbine; a means for extracting waste heat from the exhaust with a recuperator; a means for preheating the energy transfer media with the extracted waste heat in the recuperator; and, a means for returning the energy transfer media to the insulated containment vessel.
In the drawings,
While this invention is susceptible to embodiment in many different forms, it is shown in the drawings, and will be described herein in detail, specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not to be limited to the specific embodiments described.
In this embodiment, a typical Rankine steam power cycle is utilized to convert heat into work. Conditions given below might yield a power output of approximately 1 MW. The system as shown, utilizes a feed pump 124, which draws cool liquid-phase water from a buffer tank 122 and pumps it up to high pressure (i.e., 13.7 MPa at about 1 kg/sec flow), which is then fed to a recuperator 114 where it picks up waste heat from the outlet of the low-pressure steam turbine 110 and is fed into the primary thermal storage module 102. The primary thermal storage module 102 will be detailed below but has been pre-charged to retain heat from an external source and now exchanges this heat to the incoming water. This water is heated in the primary thermal storage module 102 to approximately 530 degrees C. thereby producing extremely high-temperature, high-pressure (13 MPa) steam that is delivered to the high-pressure steam turbine 106, which converts the heat and pressure of the superheated fluid into work, turning the drive shaft 111, which drives a generator 112 to produce electricity. This generator may produce AC or DC electricity for direct on-demand use or may be imparted back into an electric power grid. Lower pressure, lower temperature exhaust exits the high-pressure steam turbine 106 and flows to a three-way valve 108 where a portion of this high-pressure exhaust can be diverted to the feedwater heater 120, and exchange heat with the incoming feedwater to raise its temperature as it enters the primary thermal storage module 102. Upon heat exchange with the feedwater in the feedwater heater 120, the cooled exhaust stream (now in liquid phase) is transferred to the buffer tank 122 for storage and reuse.
The portion of the high-pressure exhaust that is not diverted by the three-way valve 106 to the feedwater heater 120, is reheated by the secondary thermal storage module 104, which, as was the case with the primary thermal storage module 102, has been pre-charged to retain heat from an external source and now exchanges this heat to the exhaust of the high-pressure steam turbine 106 to again form a high-temperature, medium-pressure (289 kPa) steam that is injected into the low-pressure steam turbine 110, which also converts the heat and pressure of the superheated fluid to work, turning the drive shaft 111. Lower pressure, lower temperature exhaust exits the low-pressure steam turbine 110 and flows into the recuperator 114, where waste heat is drawn from the low-temperature, low-pressure exhaust and used to preheat the incoming feedwater being pumped from the feed pump 124 to the feedwater heater 120. After this heat exchange with the feedwater, lower temperature exhaust is cooled and condensed in the condenser 116, where heat is typically rejected convectively to the ambient air. Cool, liquid-phase water exits the condenser and passes through a blowdown valve 118 (used for system charging and maintenance) and is returned to the buffer tank 122 for storage and reuse. Thus, a complete Rankine power cycle is realized and repeated, on-demand, throughout the thermal storage capacity of the system.
The embodiment shown in
The aforementioned embodiment facilitates the conversion of energy into stored energy (e.g., from kilowatt-hours to many megawatt-hours) at power rates from kilowatts to megawatts that may be input in a variety of forms, and from a wide variety of sources. For instance, the input energy may be in the form of electricity (AC or DC that drives electric resistance heaters) that produces heat, which is stored within the thermal storage module(s) 102,104 in one embodiment. In another embodiment, additional energy sources such as solar, wind, geothermal, hydro, fuel cells, nuclear, or the like, may be input into the thermal storage module(s) 102, 104 as the source of heat-in. Similarly, the output power of the system depicted in
In this embodiment, a typical Brayton thermodynamic cycle utilizes a gas turbine generator to convert heat into work. The system, under the conditions given below, and disclosed as shown in
6.8 kg/sec. The compressed air flows into a recuperator 214, where it picks up waste heat extracted from the exhaust of gas turbine 206 where the gas temperature is raised to about 564 degrees C. and fed into the thermal storage module 202. The thermal storage module 202 will be detailed below, but has been pre-charged to retain heat from an external source, and now exchanges this heat to the incoming preheated compressed air. This compressed air is heated in the thermal storage module 202 to approximately 927 degrees C., thereby producing extremely high-temperature, low-pressure air that is delivered to the gas turbine 206, which converts the heat and pressure of the heated air into work, turning the drive shaft 211, which is used to drive the compressor 224 and a generator 212 to produce electricity. This generator may produce AC or DC electricity for direct on-demand use or may be imparted back into an electric power grid.
Lower pressure, lower temperature exhaust exits the gas turbine 206 and flows into the recuperator 214, where waste heat is drawn from the lower temperature, lower pressure exhaust and used to preheat the incoming compressed air being pumped from the compressor 224 to the thermal storage module 202. After this heat exchange with the compressed air, lower temperature exhaust is ejected to the atmosphere at roughly 250 degrees C. This waste heat may also be used as an additional source of energy for any secondary heating demands. Thus, a complete Brayton power cycle is realized and repeated, on-demand, throughout the thermal storage capacity of the system. As with the embodiment of
As was similarly described in the embodiment of
Lower pressure, lower temperature exhaust exits the gas turbine 306 and flows into the recuperator 314, where waste heat is drawn from the low-temperature, low-pressure CO2 exhaust and used to preheat the incoming compressed CO2 being pumped from the compressor 324 to the thermal storage module 302. After this heat exchange with the compressed CO2, lower temperature exhaust CO2 exits the recuperator 314 at roughly 200 degrees C. and fed into the gas cooler 326, where it is cooled to approximately 31 degrees C. Thus, a complete Brayton power cycle is realized and repeated, on-demand, throughout the thermal storage capacity of the system. As with the embodiment of
The thermal storage module shown in
There is considerable flexibility with the number and geometry of heaters (electric heating elements 430) required to deliver the amount of thermal power needed to raise the temperature of the thermal storage media 432 to its fully charged state within a specified period. These heater segments may need to be controlled separately depending on the temperature distribution existing in the storage system after a thermal extraction has taken place. The combination of limited convection, due to the solid filler material in which the thermal storage media 432 is held, and lengthwise movement of the energy transfer media will tend to create a thermal wave that travels the length of the vessel. If the thermal extraction is halted prior to complete discharge of the storage vessel, a portion of the thermal storage module 402 nearer the energy transfer media output 440 may still be at an elevated temperature, still near its “full” thermal capacity. In this case, the heaters near this section will probably not be necessary for thermal recharge. However, the heaters nearer the energy transfer media input 438 will likely be necessary to recharge the now-cool thermal storage media 432 and thermal conductive endostructure 434.
Typically, a rectangular storage vessel geometry with horizontal movement of a heat extraction fluid would likely be unable to maintain a strong thermal boundary between an area of discharge and another area that is thermally charged if the liquid salt supported free convection cells. For a thermal storage media 432, whose density varied with temperature (such as a tank of molten salt without filler), the effects of thermal buoyancy would likely cause rapid mixing to negate such a thermocline. However, the addition of the thermal conductive endostructure 434 includes interstitial voids whose size limits the effects of convection. This thermal storage module 402 may be utilized in any of the aforementioned embodiments (
Because this embodiment is highly versatile, various types of thermal energy delivery (heat-in) into the above thermal storage module 402 may include, but are not limited by way of example to, resistance heating units, either on the outside of the storage vessel, or contained within the storage vessel (as embodied in
The thermal conductivity of the thermal storage media 432 and thermal conductive endostructure 434, when the salt is in its solid state, will be the limiting condition for rapid addition or removal of heat to and from the thermal storage module 402. For this reason, it may be unnecessary to rely on convection circulation cells being set up within the thermal storage media 432 and thermal conductive endostructure 434 when the salt has melted, although some level of natural convection could be beneficial. The thermal conductive endostructure 434, should have good thermal conduction, chemical compatibility with the thermal storage container and internal materials (e.g., heat exchanger, etc.), and be able to maintain its heat transfer capability and structure while undergoing thermal and mechanical stresses of rapid, high temperature swings while within a thermal storage media 432 that is undergoing phase changes. Examples of such materials might be silicon carbide (typically introduced as grains, chips, granules or flakes), or stainless steel (in the form of rods, pins, cones, cubes, brushes, bristles, wire, woven or non-woven fabric, spheres, or other small shapes), which may be sufficiently small to inhibit convection, while concurrently enhancing thermal conduction.
Accordingly, the geometry of the thermal storage module may be designed around the effective thermal conductivity of the thermal storage media 432 and thermal conductive endostructure 434. Changes in this thermal conductivity would lead to alternative optimization of distances between heaters, heat exchangers, pipe diameters, etc., the density and surface area of the thermal conductive endostructure 434, storage material, and the location and distribution of the heat removal circuit embedded within the storage vessel. One embodiment for such an optimized heat exchange design, with such a thermal storage media 432 mixture and thermal conductive endostructure 434 would result in a nearly complete thermal charge or discharge during the required time interval, a measure of the thermal “fuel gauge” of the system.
Another potential advantage of a convection suppressing endostructure would be the limits the small interstitial grain sizes would impose on phase separation of the different constituents of a salt mixture. Another desirable characteristic of convection suppression would be the ability to control a thermal wave through the storage vessel without having to impose a particular orientation to compensate for gravity. Thus, the outlet temperature from the thermal storage module 402, acting as a thermal battery, could be maintained at a nearly constant temperature for a particular period of time. Additionally, the described thermal storage module 402 eliminates the need to pump molten salt through plumbed networks of pipes and tanks. If the energy transfer media is a liquid, such as water under pressure, the liquid would travel some distance before sufficient heat transfer through the salt and mixture to the pipe wall would cause the liquid to vaporize. After vaporization, the fluid would continue its path through the piping of the heat exchanger 442, increasing its temperature until it likely becomes a “dry” vapor, whose temperature equaled that of the thermal storage media 432 and the thermal conductive endostructure 434 in proximity. This dry vapor would continue through the piping until it reached the energy transfer media outlet 440 of the thermal storage module 402. From there, the vapor would be delivered to the desired load.
As the energy transfer media is being heated within the piping array of the heat exchanger 442, there would be a corresponding cooling process on the thermal storage media 432 and the thermal conductive endostructure 434 in external proximity to the piping. At the initial locations nearest the energy transfer media input 438, it is likely that the energy transfer media would start to freeze around the piping heat exchanger 442, as a crust, thinning out in the direction towards the energy transfer media outlet 440. As more heat is extracted from the thermal storage media 432 (in this example a salt mixture), the solid layer would thicken, extracting more heat from a larger distance from the heat exchanger 442. The thermal conductivity of the thermal conductive endostructure 434 would allow for larger distances between adjacent heat extraction pipes. If the thermal conductive endostructure 434, with thermal conductivity no higher than the thermal storage media 432 were utilized, then the pipes of the heat exchanger 442 might have to be so close together as to be economically and mechanically disadvantageous. For this reason, silicon carbide (SiC) is an attractive material, providing a significant enhancement in thermal conductivity, as well as being chemically inert to common salt mixtures. The material is also denser than typical salt mixtures, ensuring that it does not tend to float on top of molten salt. Thermally conductive materials with a density close to that storage media may also be used in order to promote an endostructure that is not tightly packed, and facilitates a higher proportion of media to structure (phase change to non-phase change material).
Other materials, such as stainless steel may be used, but their higher densities may benefit from shaping of the particles in order to avoid having a large mass of steel relative to the mass of the salt mixture. The addition of solid filler material in the thermal conductive endostructure 434 does act to reduce the overall thermal capacity significantly for a given volume, as the sensible heat energy capacity of such materials is significantly lower than the heat of fusion of the phase change salt mixtures. As the heat extraction process continues, a thermal wave develops, essentially depleting the initial section of the storage, and becoming a preheater for the energy transfer media progressing along the thermal storage module 402. This thermal wave continues until there is insufficient thermal energy left in the module, and the temperature of the energy transfer media at the energy transfer media outlet 440 starts to drop. At that point, the system should be considered near depletion, and a recharge cycle would be initiated.
Because the detailed embodiments utilize a stationary phase change material, the system of
The thermal storage module 402 depicted in
In charge mode, the heating element 430 draws energy from a variety of forms and transfers that heat to the thermal conductive endostructure 534 and to the thermal storage media 432, heating the endostructure and changing the phase and heating the liquid media. This sensible and latent heat is contained within the thermal storage module 402 by the insulated containment 436. In discharge mode, the energy transfer media 544 flows through the heat exchanger pipe 542 and absorbs heat from the thermal conductive endostructure 534 and the thermal storage media 432. Since the cooling of the thermal storage media 432 causes phase change (solidification) to media in the proximity of the heat exchanger pipe 542, and therefore acts to insulate the pipe, latent heat continues to be transferred from the thermal storage media 432 to the heat exchanger pipe 542, via the conductive path created by the thermal conductive endostructure 534 and the storage media 432. In this manner, heat is transferred in the initial portion of the heat exchanger removing both sensible and latent heat until the storage media in close vicinity is now at or near the temperature of the fluid entering the storage vessel. As this cooling begins in the initial portion of the heat exchanger pipe 542, the next downstream portion experiences a temperature differential and transfers latent and sensible heat to the energy transfer media 544. This “thermal wave”, which starts upstream and finished downstream on the heat exchanger pipe 542 allows full discharge of the media in a rapid, efficient and complete manner, which maintains a nearly constant output temperature of the energy transfer media as it leaves the thermal storage module 402.
In charge mode, the heating element 430 draws energy from a variety of forms and transfers that heat to the thermal conductive endostructure 634 and to the thermal storage media 432, heating the endostructure and changing the phase and heating the liquid media. This sensible and latent heat is contained within the thermal storage module 402 by the insulated containment 436. In discharge mode, the energy transfer media 644 flows through the heat exchanger pipe 642 and absorbs heat from the thermal conductive endostructure 634 and the thermal storage media 432 (in this instance, stainless steel pins 646). As was seen in the embodiment of
The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.
