Abstract:
Grid energy storage is very challenging due to its large scale, required quick response, versatility, round trip efficiency, and new system infrastructure. However, the benefit is also huge because it would enable utilisation of intermittent renewable energy, leverage load, and allow energy management in hours/diurnal to seconds/minutes. A new invention is described to store up to 4-6% the entire grid energy by aluminum production, while returning its baseload back to the grid by idling aluminum smelter cells. The round trip efficiency will be close to 100%; switching between storage and load release can be instantaneous.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       FIELD OF THE INVENTION 
       [0002]    This disclosed technology relates to a grid-connected energy storage system that can make more efficient use of intermittent renewal energy sources, as well as a method of balancing the load of the power grid. 
       BACKGROUND 
       [0003]    The amount of power available from the grid is usually determined by the power generation capacity connected to the grid, which should at least meet peak demand. On the other hand, during off-peak hours, the energy demand may be very much smaller. Therefore, excess energy is available for storage. About 20% of the annual kilowatt hours (KWH) consumed in the US comes from peaking power plants, which costs substantially higher. Just store 3% electricity, and it would reduce 30% of the cost to generate peak electricity demand. 
         [0004]    Electrical demand varies considerably and often rapidly with the time of day, day of week, and season. Baseload generating plants, e.g. coal fired and nuclear, cannot meet rapidly shifting demands, because their time response is beyond hours while seconds to minutes are often desired. Recently, greater interest has been placed in utilizing renewable energy sources. However, the sun doesn&#39;t shine at night, the wind doesn&#39;t always blow, and tides, waves and currents fluctuate. To maximize usage of the intermittent renewable energy as well as to meet the varying demands, many studies have been carried out on how to store the power generated by these sources so that it can be used when it&#39;s needed. 
         [0005]    Pumped hydro is the most common and well-known technology but has siting restrictions and major environmental concerns for new construction. The total capacity is only 2% of grid electricity—and it&#39;s unlikely to get much larger. Also, the round trip energy efficiency is about 70%, so a substantial fraction of the input power is lost. The other energy storage options—flywheels, batteries, chemical energy storage, thermal, superconducting magnetic energy storage and mechanical lifting against gravity—fall into the categories of micro energy storage that is too limited in capacity, too expensive, or less energy efficient. The round trip energy efficiency, cost, scale up capability and quick response are among the major considerations of energy storage methods to be developed. 
         [0006]    Rechargeable battery technology has been actively investigated in Sodium-sulfur (NaS), flow batteries, lead acid/lead carbon, lithium ion, aluminum-ion, NIMH and NiCad but are limited to storage capacity. Due to the resulting electrolyte size limitations, multiple cells are assembled into large array of cells that require over 20,000 cells per MW capacity. There are two unconventional electricity storage technologies, which are incorporated herein by references: liquid metal battery by Sadoway et al in US patent Application 2008/0044725, entitled “High-Amperage Energy Storage Device and Method”, and energy storage in aluminum metal by Woodall et al in U.S. Pat. No. 8,080,233, entitled “Power generation from solid aluminum”. Both approaches solved the storage capacity issue by building large single cell using technology similar to the aluminum smelter process; however, Sadoway process suffers from typical battery round-trip efficiency minus expected thermal energy losses at 700° C. operating temperature, while Woodall process results in compounding degradation of energy efficiency in electrolysis, aluminum-water reaction for hydrogen and electricity production from hydrogen fuel cell or internal combustion engine. 
         [0007]    Energy storage is of primary importance in increasing the efficiency and scheduling of alternative sourced electrical power. While commercial utility power plants provide substantially continuous baseload energy during extended periods of weeks and months, most alternative energy systems that use solar or wind power are inherently intermittent because of variations in the input energy (night, clouds and wind speeds outside the operational window). At night, when energy requirements are relatively low, the produced but unused energy is effectively “wasted”. Although renewable energy sources are inherently intermittent and therefore somewhat inefficient, there is a vast market available for otherwise wasted energy, provided practical storage and conversion to electricity is available for use at alternative times. The present invention provides a means of taking advantage of this potential market and opening up opportunities for both suppliers and users of electrical energy. For suppliers, it provides a means for off-loading or decreasing demand during periods of normally high demand with the potential result of reducing the frequency to add or replace generating equipment. It also allows suppliers to sell more energy during traditionally low demand periods (night time) at competitive rates and allows for better balancing for the grid. For users, it provides a means of time-shifting the times they purchase electricity from the grid and saves a significant amount of money by purchasing at competitive rates during such low demand times, and then using the stored energy during times when rates and demand are highest. By combining the storage techniques of the present invention with auxiliary power sources, users can significantly reduce their electrical power costs. 
       SUMMARY OF THE INVENTION 
       [0008]    High-density energy storage by battery seeks light elements in the Periodic Table with high electrochemical potential and multiple electron transfers. Because aluminum is trivalent, it can achieve 3-4 times the specific energy density of a lithium-ion battery. The element is abundant on Earth, and its various forms of compounds are environmentally benign comparing to other solutions. Processing various aluminum chemicals are already in large scale and the existing infrastructure can be maximum utilized. Electrolysis of aluminum oxide is the basic chemical reaction of aluminum smelter cell, the single largest electricity consumer that consumes about 4-6% of the entire grid energy. It&#39;s an electrochemical process that runs at high temperatures, and at a current of hundreds of thousands of amps over a sustained period of years. However, it is only half of a battery because the reaction is not reversible for electricity generation. 
         [0009]    The present invention stores energy from an electrical grid by making aluminum metal intermittently whenever excess electricity is available from the grid, and releases the electric load back into the grid during peak demand by holding aluminum smelter cells idling. The new baseload with energy storage is effectively higher than the existing baseload, as shown in  FIG. 1 , making aluminum smelter cells a “passive battery”. 
         [0010]    The fact that energy represents a large part (some 25%) of the costs associated with primary aluminum production means that producers have always had a vested interest in minimizing electricity consumption. It is estimated that, as much a 0.7 quadrillion BTUs/year are consumed in the process of maintaining grid balance while none of that energy is delivered to the end user. Because of the multiple benefits, improved economics for both the industrial and utility sectors warrants significant benefits from this approach. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A fully enabling disclosure of the present invention, including the best mode thereof to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying drawings. 
           [0012]      FIG. 1  illustrates baseload with and without storage (using example of BPA Balancing Authority Load &amp; Total Wind Generation, at 5-minute intervals in 7 days between Apr. 5-11, 2012). 
           [0013]      FIG. 2  shows design of an aluminum smelter cell with vacuum thermal insulation shell. 
           [0014]      FIG. 3  shows design of an aluminum smelter cell with external heating and insulation to maintain internal temperature. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Reference will now be made in detail to the embodiments of the invention, one or more examples of which are set forth below. Each example is provided by way of explanation of the invention, not the limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as comes within the scope of the appended claims and their equivalents. Other objects, features, and aspects of the present invention are disclosed in the following detailed description. It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary constructions. 
         [0016]    In describing the various figures herein, the same reference numbers are used throughout to describe the same material, apparatus, or process pathway. To avoid redundancy, detailed descriptions of much of the apparatus once described in relation to a figure is not repeated in the descriptions of subsequent figures, although such apparatus or process is labeled with the same reference numbers. 
         [0017]    In accordance with one or more embodiments, a solution for utility scale grid storage is disclosed herein that can provide power reliability to renewable energy sources that are inherently unreliable in nature, such as solar, thermal, photovoltaic (PV), wind, hydro, biomass and tidal. Existing aluminum smelter cells are incorporated in grid scale storage of low cost power and allow the load back to grid at high rates on demand by idling the cells. In this way, renewable utility operators can be provided with a grid storage solution that allows for 24-hour a day continuous power production without interruption when integrated in conjunction with conventional thermal power generation process (coal, nuclear, gas, oil), which also provides a storage component, whereby a portion of the energy can be stored and be available at any time on demand. Indeed, thermal power generation processes can respond to load changes within hours. The energy storage of only 4-6% grid energy by rapid altering aluminum production should be adequate to balance the entire grid in both renewable and nonrenewable power generation because load shifting beyond this storage capacity is at a slow pace that can be responded by thermal power plants. 
         [0018]    By many measures, aluminum remains one of the most energy-intensive materials to produce. Aluminum production is the largest consumer of energy on a per-weight basis and is the largest electric energy consumer of all manufactured products. Process heating accounts for 27 percent of the total energy consumed in U.S. manufacturing of aluminum. Process heating is required for holding, melting, purifying, alloying, and heat treating. 
         [0019]    Aluminum reduction cells are used to produce aluminum by electrolysis of aluminum oxide, a process known as the Hall-Héroult process. 
         [0000]      2Al 2 O 3 +3C→4Al+2CO 2  
 
         [0020]    Aluminum is formed at about 900° C., but once formed has a melting point of only 660° C. The smelting process required to produce aluminum from alumina is continuous; the potline is usually kept in production for 24 hours a day year around. A smelter cannot be easily stopped and restarted. If production is interrupted by a power supply failure of more than 3 hours, the metal in the pots will solidify, often requiring an expensive rebuilding process. Continuous bulk power supply is critical to the current design of aluminum smelters. 
         [0021]    This issue is solved by redesigning the smelter cell with much better thermal insulation. The best thermal insulation technology cuts off heat transfers, i.e., conduction, convection and radiation entirely. Refractory materials and vacuum jacketed insulation are combined in this application. As shown in  FIG. 2 , a typical aluminum smelter cell is modified with vacuum jacketed insulation at the wall, bottom and top. Alternatively, external heating and insulation can be applied in such a way that the heater&#39;s internal temperature matches the outer surface of smelter, e.g., bottom of Steel Supporting Cradle. As shown in  FIG. 3 , a typical aluminum smelter cell is added with an external heater and insulation at the wall, bottom and top. The heater is controlled to match the skin temperature of the smelter and therefore no heat transfer occurs. The heater can be electric, gas, oil, or any conventional heaters. The cell only loses heat in liquid aluminum product and gas scrubber during production, which is offset by joule heating and chemical reaction effects. If excess heat is generated, the smelter top can be partially opened to release heat. During idling, the gas scrubber and liquid aluminum flow are essentially stopped, therefore no significant heat losses are expected. A single aluminum smelter cell can be as large as 100 m 3  and can hold multiple tons of molten metal and electrolyte. The huge mass with a relatively small surface area allows molten temperature for days until cheap excess electricity is available to resume the production, however, intermittent production and idling at a frequency in seconds is also acceptable. 
         [0022]    The Hall-Héroult process has been improved from 23.3 to about 13 kWh/kg since World War II, owing in part to the computerization of smelting cells. The computer takes into account the various current operating variables, so that the voltage in the pot is always the best for prevailing conditions. By reprogramming the control, the energy storage and load shedding can be instantaneous within milliseconds. 
         [0023]    Diffusion of gas molecules into vacuum jacketed insulation increases significantly as the result of the high temperature operation (700-1000° C.) of aluminum smelter cells, therefore shortening the cycle length of the insulation effectiveness from typically 20 years in ambient and cryogenic application to weeks and months in aluminum smelters. The vacuum jacketed insulation has to be evacuated periodically and a getter material is re-applied/regenerated to extend the service cycle length. The getter material is a deposit of reactive material that is placed inside vacuum jacketed insulation, for the purpose of completing and maintaining the vacuum. When gas molecules strike the getter material, they combine with it chemically or by adsorption. Thus the getter removes trace amounts of gas from the evacuated space. 
         [0024]    The method can be used to store energy during periods when excess supply is available on the electrical grid, and then release the load during times of higher demand. Thus, the method can be utilized by a utility to reduce excess capacity requirements, and therefore reduce costs. Furthermore, the method can be utilized to supply high quality power by grid rebalancing and power problem correction as well as other ancillary services. The round trip efficiency is close to 100% since there is no discharge loss in the cell and all “would be” electricity load is available for the grid. 
       Example 1 
       [0025]    Example of an erratic electricity load at 5-minute intervals in 7 days is shown in  FIG. 1 . Wind power varies even more. Data was taken from the BPA Balancing Authority Load &amp; Total Wind Generation during Apr. 5-11, 2012 for illustration purposes. Aluminum production consumes about 5-6% grid power globally (4% in the US). By intermittent aluminum production, excess electricity can be stored as chemical energy in aluminum metal production. During high electricity demand from the grid, aluminum smelters can be set at idle, effectively increasing baseload by the amount that the aluminum smelter plant would consume. The aluminum smelter plant can work as a “passive battery” for energy storage and can return its load to the grid when it is needed. Both storage and load release can be instantaneous, and round trip efficiency can be just slightly lower than 100% only for heat loss during the idling of an aluminum smelter. Double capacity of an aluminum smelter plant will be needed for the same productivity in about 50% of production time. 
       Example 2 
       [0026]      FIG. 2  shows the new design of an aluminum smelter cell with vacuum thermal insulation at the cradle and top cover. The current aluminum smelter cell does not operate intermittently as required in Example 1 because the molten metal and electrolyte in the smelter would solidify after prolonged idling. By changing the design to  FIG. 2 , heat transfer by conduction, convection and radiation are reduced to minimum; therefore, the smelter cell can be left idling anytime as needed. This capability is important to enable its electricity load back to the grid. 
       Example 3 
       [0027]      FIG. 3  shows another design of an aluminum smelter cell with external heating and insulation to maintain the internal temperature of the smelter cell. Heat only transfers from high temperature to low temperature. When external heaters are set to match the skin temperature of cradle and top cover, temperature gradient becomes zero and heat transfer essentially stops. The smelter cell itself is therefore adiabatic.