Patent Publication Number: US-2023163622-A1

Title: Hybrid energy storage systems

Description:
BACKGROUND 
     1. Technical Field 
     This disclosure relates to novel energy storage systems for long-term storage and delivery of electricity generated by any energy source including renewable energy sources such as solar energy and wind energy. 
     2. Background Information 
     Rapid growth of renewable energy sources such as solar energy and wind energy is a powerful driving force for cost effective and long-term storage of large amount of renewable energy. There are varieties of different energy storage systems (ESS) readily available in the markets. Major forms of ESS in the markets are Battery Energy Storage Systems, Flywheel Energy Storage Systems (FESS), Heat Energy Storage Systems (HESS), and Compressed Air Energy Storage (CAES). Molten Salt Energy Storage Systems (MSESS) are one type of HESS. Each of these energy storage systems have characteristic advantages and disadvantages. However, as this disclosure shows, a novel integration of such storage systems solves problems that are not solved by such stand-alone systems. 
     SUMMARY 
     This disclosure describes novel ways of integrating FESS and HESS to create hybrid energy storage systems (hereinafter referred to as “YKESS”). In some embodiments, but not all embodiments, the HESS can be a MSESS. However, the YKESS can otherwise include other general types of HESS that are integrated with FESS. 
     The YKESS described herein synergistically enhance the advantages of FESS and HESS technologies, as described further below. YKESS is a novel integration of FESS and HESS that enhances the strengths of FESS and HESS and eliminates or minimizes the weaknesses of FESS and HESS. For convenience, end users of electricity and network grids of electrical power will be referred to hereinafter as “users.” 
     In some embodiments, and as described further below, the HESS has a large mass (or volume) of heat reservoir material (e.g., with its temperature in the range of 500 to 1100° C.) contained in a thermally insulated container. The heat energy of the heat reservoir material is supplied by Ohmic heating device that is immersed in the heat reservoir material. The Ohmic heating device is heated by electricity from solar panels or wind turbines. In addition to the Ohmic heating device, a heat exchanger is immersed in the heat reservoir material. A heat engine is operatively attached to the container of heat reservoir material and a working fluid of the heat engine is heated as it flows through the heat exchanger that is immersed in the heat reservoir material. The heat engine has a compressor and pump for the work fluid, a turbine that is driven by hot and high-pressure work fluid emerging from the heat exchanger, and a turbine that drives an electric generator. There are a number of choices that can be made for each of the component parts of the HESS, thereby providing a flexible framework for heat energy that can be converted into electricity. Examples of such choices are disclosed in the various embodiments of the invention discussed below. In some embodiments, the working fluid of the heat engine may be water-steam or dry air. However, the working fluid of the heat engine is not restricted to water-steam or dry air. 
     In one aspect, this disclosure is directed to an energy storage and delivery system that includes one or more FESS and a HESS. The one or more FESS are configured to generate and deliver electricity to users. The HESS includes a container holding a heat reservoir material therein; a heater in thermal contact with the heat reservoir material; a heat exchanger in thermal contact with the heat reservoir material; a turbine fluidly coupled to an output of the heat exchanger; an electricity generator mechanically coupled to the turbine, wherein electricity output by the electricity generator is delivered to power flywheel rotations of the one or more FESS; and a compressor or pump fluidly coupled to an input of the heat exchanger, wherein the compressor or pump is powered by energy output by the one or more FESS. 
     Such a system may optionally include one or more of the following features. The heater may be configured to be electrically powered by electricity generated by solar panels or wind turbines. The system may also include the solar panels or the wind turbines. The system may be configured to send at least a first portion of the electricity generated by the solar panels or the wind turbines to users without powering the one or more FESS. The system may be configured to send at least a second portion the electricity generated by the solar panels or the wind turbines to the one or more FESS to power flywheel rotations of the one or more FESS. The heater may be configured to transfer heat to the heat reservoir material. The heat exchanger may be configured to transfer heat from the heat reservoir material to a fluid passing through the heat exchanger as the fluid flows toward the turbine. The system may also include a fluid storage tank positioned fluidly between an output of the turbine and an input of the compressor or pump. In some embodiments, the fluid storage tank is underground. In some embodiments, excess electricity from a renewable energy source is distributed to the one or more FESS and the HESS system such that energy stored in the HESS is at least two times greater than energy stored in the one or more FESS. The HESS may also be configured to deliver energy to power the compressor or pump. The system may be configured to automatically switch the energy to power the compressor or pump from being delivered by the one or more FESS to being delivered by the HESS in response to the energy contained by the one or more FESS reaching a lower limit. The electricity output by the electricity generator may also be delivered to the users. In some embodiments, each FESS of the one or more FESS stores 10 kWh or less of kinetic energy. Each FESS of the one or more FESS may weigh less than 1,000 kg. The one or more FESS comprises at least four FESS. 
     In another aspect, this disclosure is directed to an energy storage system for use during interruptions of electricity from a renewable energy source. The system includes a FESS for short-time delivery of electricity to users and an HESS for long-time delivery of electricity to the users. 
     Such a system may optionally include one or more of the following features. The HESS may include an air compressor, and the FESS may be configured to deliver energy to power the air compressor. In some embodiments, the HESS is also configured to deliver energy to power the air compressor. The energy storage system may be configured to automatically switch the energy to power the pump or compressor from being delivered by the FESS to being delivered by the generator of the HESS in response to the energy contained by the FESS system reaching a lower limit. 
     In another aspect, this disclosure is directed to a method of delivering electricity to users. The method includes: (i) delivering a first portion of electricity generated from a renewable energy source to the users; (ii) delivering a second portion of electricity generated from the renewable energy source to one or more FESS; and (iii) delivering a third portion of electricity generated from the renewable energy source to a HESS. 
     Such a method may optionally include one or more of the following features. The additional electricity may be delivered to the one or more FESS in response to a kinetic energy level of the one or more FESS being below a pre-determined lower threshold value. The method may also include powering, by energy delivered from the one or more FESS, a pump or compressor of the HESS. The method may also include, in response to a demand for the electricity from the users being greater than the first portion of electricity generated from the renewable energy source, delivering additional electricity to the users, wherein the additional electricity is generated by the one or more FESS. The method may also include, in response to a demand for the electricity from the users being greater than the first portion of electricity generated from the renewable energy source, delivering additional electricity to the users, wherein the additional electricity is generated by the HESS. 
     There are at least five major objectives and advantages that the YKESS described herein provides. The first objective and advantage is long-term and low-cost storage of renewable energy harvested from solar panels, wind turbines, and/or other forms of renewable energy devices. In this context, “long-term” means many days or many weeks of time. “Low-cost” means an initial capital cost of storage per kilowatt-hour that is significantly lower than cost of storage per kilowatt-hour by modern, commercially available FESS. 
     The second objective and advantage is that the YKESS described herein can use one or many low-cost, maintenance-free, and small-sized FESS e.g., with an energy storage capacity of about 10 kWh (kilowatt-hour). 
     Third, the YKESS described herein can provide essentially instant delivery of sufficient electric power to energy users to meet the widely and frequently fluctuating demand of electricity from the users. 
     Fourth, the YKESS described herein has the capability of sending sufficient electric power to one or many users continuously for one week or longer. 
     Fifth, the YKESS described herein can provide large-scale energy storage of renewable energy without using environmentally and biologically harmful materials. In addition, the YKESS only occupies a small area of land. 
     The YKESS described herein can be an indispensable source of energy for large data centers, large manufacturing plants, big hospitals, and the like. Moreover, the YKESS can provide a backup of the power grid (e.g., for large cities) in case an existing supply of electricity is disrupted by a brownout, or for a long period. 
     The YKESS described herein can also reduce the heavy dependence of power-hungry societies on energy generated by burning fossil fuels. 
     Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. In addition, the materials, methods, and examples of the embodiments described herein are illustrative only and not intended to be limiting. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description herein. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram illustrating an example YKESS that integrates FESS and MSESS in accordance with some embodiments described herein. 
         FIG.  2    is a schematic diagram illustrating another example YKESS that integrates FESS and MSESS in accordance with some embodiments described herein. 
         FIG.  3    is a schematic diagram illustrating an example YKESS that integrates FESS and HESS in accordance with some embodiments described herein. 
         FIG.  4    is a schematic diagram of another example YKESS in which many small low-cost units of FESS are connected to a single HESS having a large energy storage capacity. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure describes new and novel systems that integrate FESS and HESS into a single energy storage system that is referred to herein as the YKESS. The integrated/hybrid energy storage systems described herein (YKESS) enhance the individual advantages of FESS and HESS and avoid the individual disadvantages of FESS and HESS. In some cases, the YKESS described herein can provide a steady and uninterrupted supply of electricity to users for more than one week of time. 
     The Earth receives energy from the Sun that is more than 10,000 times the energy that all humans on the Earth consume. Wind energy is a derivative of the energy from the Sun. Yet, energy-hungry societies depend mostly on energy from burning fossil fuels. There is strong international pressure to reduce the consumption of fossil fuels and to switch to renewable energy sources such as solar or wind power. 
     The cost of renewable energy is now roughly equal to or lower than the cost of energy generated by fossil fuels. However, a serious problem of renewable energy is that electricity generated by solar panels and wind turbines cannot be stored economically for long period of time. The serious problem of renewable energy boils down to the problem of long-term economic methods of storing large amounts of the energy from the Sun. It is the problem of long-term economic storage of the energy from the Sun. 
     The capacity utilization factor (CUF) of renewable energy is low, averaging only about 17% to 20%. This is mainly due to day-and-night cycles and frequently changing weather. A typical solar power plant having a nominal capacity of 1 megawatt (1 MW) can generate about 990 kilowatts (99% of 1 MW) of electricity at a peak time (e.g., around noon on a clear sunny day for solar panels). However, a few hours around noon are not the best time for maximum user consumption of the electricity generated by solar power plants. Sometimes, renewable energy is not available for many days when foul weather or windless conditions continue that long. 
     In the YKESS described herein, the strengths of FESS and HESS are synergistically enhanced and weaknesses of FESS and HESS are eliminated or minimized by novel integrations of FESS and HESS. When applied widely in large scales, the YKESS described herein will eliminate or drastically reduce the dependence of energy-hungry societies on environmentally destructive and unhealthy burning of fossil fuels. 
     The YKESS described herein include novel integrations of two existing well-proven energy storage systems, namely, Flywheel Energy Storage Systems (FESS) and Heat Energy Storage Systems (HESS). The two technologies of FESS and HESS are entirely different from each other. Designers and developers of FESS and HESS have never worked together to achieve synergistic integration of the two energy storage systems. Therefore, no one in these two areas of energy storage systems technologies has uncovered incredible benefits of a novel and counter-intuitive integration of FESS and HESS as described in this disclosure. FESS and HESS have characteristic strengths and weaknesses as two separate stand-alone storage systems of renewable energy. 
     One of the inventive concepts of the YKESS described in this disclosure can be succinctly described as follows. The YKESS described herein integrates one or more low-cost FESS which stores only a relatively small amount of energy and a HESS that stores a large amount of energy as low-cost heat energy in such a way that the HESS becomes a large reservoir of energy and supplier of the energy to the FESS, when needed, and the FESS becomes a fast-responding provider of a large amount of electric power to users and power grids almost instantaneously in response to unpredictable and widely fluctuating needs of electricity during many days of foul weather for solar panels or windless weather for wind turbines. Strengths and weaknesses of FESS and HESS as two separate stand-alone systems, and novel integrations of FESS and HESS are described in the following sections. 
     Strengths and Weaknesses of Flywheel Energy Storage Systems (FESS) 
     A FESS can quickly generate and send electricity almost instantaneously to users when normal electric power is interrupted. The efficiency of modern FESS is high (e.g., in the range of 88% to 98%). A modern commercially available FESS is highly reliable and durable. It is virtually maintenance free for many years. Its service life is more than 20 years. However, the energy storage capacity of a FESS is rather limited. The capacity of energy storage of FESS increases as the square of the angular speed of its flywheel. For example, if the angular speed of a flywheel increases twice, the corresponding energy of the flywheel increases four times. Therefore, a designer of FESS tries to maximize angular speed of flywheel of a FESS. As of now, typical angular speeds of commercially viable FESSs are in the range of 5,000 rpm to 18,000 rpm (revolution per minute). When the angular speed of a flywheel of a FESS becomes significantly greater than 18,000 rpm, the cost of making the FESS and operation cost of the FESS becomes prohibitively expensive due to many difficult technical problems associated with a flywheel that rotates at such a high angular speed. A commercially available FESS that runs at an angular speed of 10,000 rpm can store only 32 kWh (kilowatt-hour). FESS is too expensive to store enough energy for long-term (more than one day) continuous delivery of a reasonable amount of electric power to users. 
     A fast-rotating flywheel is dynamically unstable. A fast-rotating flywheel is subject to gyroscopic precession and wobbling due to the rotation of the Earth and other minor causes. The gyroscopic precession and wobbling of the fast-rotating flywheel must be vigorously controlled (or suppressed) since the flywheel rotates at such a high angular speed in a tightly confined space of a vacuum chamber. Any failure of control of the dynamics of the fast-rotating flywheel will result in a catastrophic accident or destruction. A fast-rotating flywheel becomes ‘stiff’ in the sense that a change of direction of the flywheel’s axis of rotation requires a stronger torque when the angular speed increases. Varieties of permanent magnets and electromagnets (magnets powered by electricity) and sophisticated electronic control systems are used to control (or regulate) gyroscopic precession and wobbling of the fast-rotation flywheel in a FESS. These electromagnetic control systems consume a lot of energy. 
     The total energy that can be stored by a FESS is proportional to the square of its flywheel’s angular speed. However, increasing the angular speed beyond about 20,000 rpm becomes extremely technically difficult and extremely expensive to operate. This is a fundamental limit of FESS technology. There are R&amp;D teams who try to make a FESS that runs at 100,000 rpm and even higher. However, a FESS that runs at such a high angular speed is not technically and economically feasible for commercial ESS applications in foreseeable future. On the other hand, a commercially viable FESS that runs at a technologically feasible angular speed in the range of 5,000 rpm to 18,000 rpm has a rather limited total energy storage capacity. 
     One strength of FESS is that it can release its kinetic energy into large amounts of electric energy in a very short time to users (e.g., within about one second) when a fast release of a large amount of electricity is needed by users. A FESS that operates at a modest angular speed in the range of 5,000 rpm to 10,000 rpm is highly durable and almost maintenance free. Its service life is over 20 years. 
     Although the energy release time of a FESS is excellent, the total amount of energy a given FESS can deliver is quite limited. For instance, a commercially available FESS stores about 32 kWh and can release that energy at a rate of 8 kW. The total time that the FESS can supply energy is, therefore, only about 4 hours. For a multi-megawatt grid application, such a power boost is excellent for smoothing out power supply. However, the response to an electricity brownout (say, 10% drop in power in some regions) requires many units of FESS that are connected to a large MSESS, as described herein. 
     Strengths and Weaknesses of Molten Salt Energy Storage Systems (MSESS) 
     MSESS are one type of HESS. Many of the strengths and weaknesses of MSESS are also applicable to HESS generally. However, some of the weaknesses of the MSESS are specific to MSESS. In other words, not all types of HESS have all of the weaknesses of MSESS. 
     MSESS is a well-proven technology for large scale and low-cost storage of renewable energy. Salt is abundantly available, and its price is low. Its heat capacity is large for the storage of a large amount of energy per unit weight. It is not harmful to humans or to fauna or flora of ecosystems. 
     A molten salt heat reservoir has a high storage efficiency (e.g., above 90%). However, the efficiency of energy transformation from the heat energy of molten salt to electricity is lower than 50%. On the other hand, the cost of energy storage is about $30 /kWh. In comparison, the cost of energy storage in a FESS is about $300 /kWh and is trending downward. 
     The cost of Lithium-ion batteries has decreased dramatically, to about $137 /kWh (about 5 times the cost of MSESS), but there are other serious issues. Lithium-ion batteries are plagued with high temperature dependence that can limit their practicality in cold climates and cause them to overheat in warm weather; they have a limited lifetime that can be substantially less than 10 years; their use does not scale well in actual plant installations; and lithium-ion batteries depend on rare materials, the mining and disposal of which causes major ecological damage. The fact that lithium-ion batteries may need to be totally replaced several times in the lifetime of a FESS, or of a MSESS, is actually a multiple capital cost that may make long-term dependence on lithium-ion batteries for ESS unsuitable. In addition to repeated capital cost, there is inordinate, additional ecological damage. Accordingly, MSESS clearly shows advantages compared to lithium-ion batteries for long-term energy storage. 
     A major weakness of MSESS (and HESS generally) is that its response time is slow. The response time of a MSESS is defined as a characteristic time of a MSESS to transit from its idle state to its operational state. In the idle state, the MSESS does not generate electricity. In the operational state, the MSESS generates maximum electricity. A MSESS cannot quickly be jumpstarted to be in its operational state from its idle state by itself. External power is needed to quickly jumpstart the compressor and pump system of an idle MSESS. The jumpstart of a MSESS involves fluid compression, movement of a working fluid (e.g., steam or air) to a heat exchanger, and the slow process of heat exchange between the working fluid and molten salt. For these reasons, the response time of a MSESS is in the range of 5 minutes to 20 minutes. The response time of very large MSESS is in range of a few hours. In addition, the efficiency of MSESS decreases from is its normal efficiency when its generation of electricity changes in time. Therefore, a stand-alone MSESS cannot deliver electricity fast enough to users when fast delivery of a large amount of electricity is needed by users. This is a major weakness of MSESS. 
     There are variations of MSESS systems (e.g., CAES - Compressed Air Energy Storage) that improve the start-up times by using compressed air instead of water, but they have their own major problems. For example, to generate electricity by using only air instead of water one needs to use huge volumes of air that need to be stored in huge underground caverns under extreme pressure. There are relatively few geographical areas able to accommodate such installations. In addition, one loses the huge expansion associated with the vaporization of water into steam and the latent heat effects of the huge heat of vaporization effects of water. To maintain short response times, such units must be operated in a steady spinning state that uses energy 100% of the time just to be ready. That drastically reduces the overall efficiency of CAES. 
     The molten salt of a large MSESS can include thousands of tons of salt and occupies a large volume of space. Its ‘engine’ part, namely, compressor, pumps, turbine, and generator of electricity require frequent repairs and regular maintenance services. 
     Dry air is the preferred working fluid for the HESS, although water-steam would be a better working fluid for very large scale HESS. The response time of HESS with dry air is significantly shorter than the response time of heavy-duty large scale HESS with water-steam. 
     Synergistic Integrations of FESS and HESS (e.g., MSESS) 
     The YKESS described herein are synergistic, hybrid integrations of FESS and HESS that dramatically enhance the individual strengths of FESS and HESS, and eliminate or minimize the individual weaknesses of FESS and HESS. The YKESS inventions described herein are new types of economically-viable energy storage systems for the purpose of long-term energy storage (e.g., storage of large amounts of energy from renewable energy sources). The YKESS provide almost instant and non-interrupting delivery of electricity to users for many days or many weeks when renewable energy cannot be harvested from the sun and/or wind due to day-and-night cycles and/or many days or weeks of unfavorable weather conditions. 
     The FESS, as a part of the YKESS, functions to provide instantaneous delivery of large amounts of electricity to users. The HESS, as a part of the YKESS, functions to provide low-cost storage of large amounts of renewable energy in the form of heat (e.g., of molten salt in the case of MSESS), to convert the heat into electricity, and to send the electricity to the FESS when energy stored in the FESS becomes depleted below a certain lower threshold level, and/or to send electricity from the HESS to the users. 
     Since the energy of the YKESS is mostly stored in the molten salt of the MSESS, the FESS of the YKESS only needs to store a small amount of energy. For instance, if a stand-alone FESS can store 32 kWh and can instantly deliver 8 kW to users for 4 hours, a counterpart FESS in the YKESS may store a small fraction of 32 kWh and can still deliver 8 kW of immediately available power for much longer than 4 hours, since its energy will be frequently or continuously replenished by electricity generated by the MSESS. Once the MSESS system is fully up to speed and is operating at 100% of its power capacity, the majority of the power from the MSESS can bypass the FESS and supply the users directly. The FESS may then be used to smooth the power supply to eliminate fluctuations in power for which the MSESS is not suitable because of its dwell times. 
     Using the YKESS, brownouts may be eliminated for large data centers, large manufacturing plants, large hospitals, and the like. For example, if the YKESS adopts the same commercial FESS having a storage capacity of 32 kWh, it can deliver a power of 32 kW for 1 hour, or 64 kWh for 30 minutes. Therefore, the YKESS can deliver a large amount of rapidly changing electric power to its users for long period of time (e.g., as long as 30% of heat energy stored in molten salt of MSESS is not completely depleted). This means that YKESS can meet the needs for an instant large electric power of large hospitals, large data centers, etc., in the case of abrupt interruption of normal electric powers to such institutions. At present, most of such institutions use diesel-burning electric generators as their fast backup of electricity. YKESS can replace these air-polluting diesel generators in case major portions of energy of these organizations are renewable energy already. 
     This feature of the YKESS has far-reaching positive impacts in FESS industry. When the energy of the FESS is frequently or continuously replenished by its accompanying MSESS that can carry much a larger amount of energy in its molten salt, the amount of energy the FESS may store is only a small amount of energy while it can supply electricity to users for long time as long as the energy of the MSESS is not completely depleted. 
     The cost of energy storage in a large stand-alone FESS is much more expensive than cost of energy storage in a HESS. That is a major weakness of FESS. A strength of FESS is that it can reliably deliver large of amount electric power to users almost instantaneously in response to abrupt changes of electricity its users’ needs. The low cost of energy storage of the HESS is its major strength. The YKESS described herein are a synergistic integration of FESS and HESS for maximum exploitation of the strengths of FESS and the strengths of HESS. The bottom line of the YKESS described herein is that one can use much smaller and much cheaper (in price, not in its quality) FESS to provide instant and time-varying supply of large amounts of electric power to users for very long times. 
     The YKESS described herein power the compressor and pump of the MSESS (e.g., to compress air and push compressed air into heat exchangers that are immersed in the molten salt) by electricity generated by the FESS. Electric power that is generated by the MSESS and sent to the FESS varies because the demand of electricity from users is time varying. For this reason, the over-all efficiency of YKESS as an integrated system of the FESS and the MSESS is better when the compressor and pump of the MSESS is powered by electricity from the FESS to eliminate the dwell and synchronization times associated with MSESS alone. 
     The benefits of the YKESS described herein can be shown in the following actual example of the synergistic effects of combining FESS and MSESS (with computations pertaining to a realistic case) as follows. 
     An example commercially available FESS has the following specs: Maximum energy storage is 32 kWh, angular speed of its flywheel is 10,000 rpm, its weight is 5,000 kg (5 tons) and its housing dimension 132 cm x 137 cm (height x width). The 5-ton FESS can deliver 8 kW of electric power to users for 4 hours. 
     In the YKESS described herein, a FESS that is far smaller (e.g., far less weight than 5 ton) can deliver the same 8 kW to users many times longer than 4 hours. That is accomplished by the YKESS as follows in paragraphs (i) to (v): 
     (i) A MSESS has its characteristic transition time. The transition time of a MSESS is defined as the time the MSESS takes to transition from its idle state to its full operational state. It is typically up to 30 minutes for a MSESS that generates electricity with a turbine that is driven with hot and high-pressure air. For concreteness, it is assumed that the transition time of the MSESS adopted in this example is 10 minutes, and it can generate 20 kW to be sent to users and/or the FESS when the MSESS is fully operational. 
     (ii) The transition time of the MSESS and its maximum electric power output determines the optimum (or recommended) energy storage capacity of the FESS in the YKESS described herein. The optimum (or recommended) energy storage capability of the FESS should be slightly greater than the electric energy that the MSESS can generate at its full electric power during a time span of its transition time. Therefore, in this case, the electric power output of the MSESS is approximately 3.34 kWh during a time span of 10 minutes (equal to its transition time of 10 minutes). 
     (iii) Therefore, the optimal (or recommended) energy storage of the FESS in the example YKESS is just 4 kWh (which is slightly larger than 3.34 kWh). The energy storage capacity of the FESS is much less than the energy storage of 32 kWh of the ‘5-ton’ commercial FESS. If the MSESS can store 1000 kWh of energy in its hot molten salt, and the efficiency of the MSESS is 30%, 300 kWh of the energy stored in the MSESS can be supplied to the FESS for 37.5 hours at a rate of 8 kW. 
     (iv) A stand-alone FESS that can send 8 kW for 37.5 hours continuously to users must have an energy storage capacity of 300 kWh. The 5-ton commercial FESS has energy storage capacity of just 32 kWh. Therefore, one would need more than 9 units of the 5-ton FESS (9.375 units exactly) with 32 kWh of storage for continuous delivery of 8 kW for 37.5 hours. The 9.375 units of 5-ton FESS will weigh 47 tons. From these computations, it is clear that non-stop delivery of 8 kW to users with multiple units of FESS is simply economically not feasible. This is why many FESS are used for short-time delivery of large amount of electric power to users. 
     (v) There is another drastic difference between the commercial FESS and synergistic effects of the FESS and the MSESS in YKESS described herein. Since the FESS of YKESS may store just about 4 kWh of energy, it would weigh about 8 times less (32 kWh divided by 4 kWh) and its size will be significantly smaller than the commercial stand-alone FESS. The amount of energy the flywheel can store is linearly proportional to its weight. This means that the FESS with 4 kWh of capacity used in this invention would weigh only about 625 kg (5000 kg divided by 8), and its size should be significantly smaller than the 5-ton FESS. Obviously, the price of the FESS of YKESS would be significantly lower than price of the example 5-ton commercial FESS. 
     As shown in the above series of computations, the novel integration of FESS and MSESS brings in great benefits to the users of YKESS. The positive synergistic effects of the integration of FESS and MSES is not obvious at all. One must look at the two totally different energy storage systems critically and must make careful analysis of the synergistic effects as described herein. This is why no one has ever conceived of the integration of FESS and MSESS so far (to the best knowledge of the inventors). 
     There is another important benefit of the synergistic integration of FESS and MSESS. A single large MSESS can supply its electricity to multiple units of FESS to many users. A large MSESS with a huge energy storage capacity is very slow when it comes to changing its electric output. In other words, its transition time is slow. It is in the range of 30 minutes to 3 hours. If the MSESS (solely) supplies its electricity directly to its users, it cannot change its output (power sent to the users) fast enough in case power needs of its users change abruptly and unpredictably. The synergistic integration of FESS and MSESS of YKESS can solve this problem nicely as follows. The MSESS with its large energy storage capacity generates electricity and sends it to users and to many separately operating units of FESS. Each of the FESS will have a small energy storage capacity. When multiple units of FESS receive electricity from a single MSESS, each of the FESS may have its own energy storage capacity that can be enough for the FESS to send 10 kW or 20 kW to its users for about 10 minutes or so. Since the large MSESS supplies its electricity to many units of FESS, the MSESS will run continuously. Therefore, each of the multiple units of FESS can ‘draw’ electricity from the MSESS to replenish depleting energy levels for continuous delivery of electricity to users. Very importantly, each of the FESS supported by the large MSESS advantageously has small energy storage capacity (e.g., for the reasons described above). 
     The raw materials used to manufacture the YKESS described herein are environmentally friendly and abundant materials. Most materials that will be used for manufacturing of the YKESS described herein will be non-toxic to humans and to flora and fauna around its site. The service life of the FESS will be more than 20 years without any maintenance and repairs. The service life of the MSESS will be well over 20 years provided that turbine, compressor, pumps, and other electrical and mechanical components of the MSESS are properly maintained and quickly repaired when needed. 
     The MSESS in the YKESS described herein is essentially a large energy reservoir that supplies its energy to one or more FESS that can deliver electric power fast and for a long time. However, the MSESS is not the only option for its role in the YKESS. Any system capable of storing a large amount of energy as heat energy (e.g., hot sands, hot gravels, or hot oil), and mechanical energy of compressed air can be adopted to YKESS as a large reservoir of energy to supply the FESS. However, MSESS is a preferred energy reservoir of YKESS since it is cheap for construction, it occupies relatively small and compact space, and it can be installed at almost at any place (e.g., on a flat roof of a large building, or underground in the backyard of a house). 
       FIG.  1    is a schematic diagram of an example YKESS  100  in accordance with some embodiments. Arrowheads on single solid lines indicate the flow of electric power from one unit to another of the YKESS  100 . Arrowheads on double solid lines indicate the flow of a fluid (e.g., air, steam, etc.) as a working fluid of the YKESS  100 . 
     The YKESS  100  is used to store energy that is produced by one or more energy generation sources. For example, in  FIG.  1   , the YKESS  100  is used in conjunction with a renewable energy source  200  (which can be solar panels  210  and/or wind turbines  220 , for example). 
     The YKESS  100  includes one or more FESS  110  (a single FESS  110  is depicted here, but multiple FESS  110  can be included in some embodiments) and a MSESS  120 . It should be understood that the MSESS  120  is just one example of an HESS and that other types of HESS can be substituted for the MSESS  120 . 
     The FESS  110 , as a part of the YKESS  100 , functions to provide instantaneous delivery large amounts of electricity to the users  300 . The MSESS  120 , as a part of the YKESS  100 , functions to provide low-cost storage of large amounts of renewable energy in the form of heat (of molten salt), to convert the heat into electricity, and to send the electricity to users  300  and/or to the FESS  110  (e.g., when energy stored in the FESS  110  depletes below a certain level). Once the MSESS  120  is operating at or near 100% capacity, the MSESS  120  can supply electricity directly to end users  300  to cover the bulk of the demand from the users  300 , while the FESS  110  can be used to provide just enough electricity to meet power demand surges/wrinkles and to ensure synchronization. The split between providing energy from the MSESS  120  to the users  300  and to keep the FESS  110  fully charged requires a control system that is not depicted in  FIG.  1   . 
     As depicted, at least a first portion of the energy generated by the renewable energy source  200  can be sent to users  300  without powering the one or more FESS  110 . Alternatively, or additionally, at least a second portion of the energy generated from the renewable energy source  200  can be sent to and stored in the FESS  110  and/or the MSESS  120 . 
     Energy received by the FESS  110  (from the renewable energy source  200  and/or the MSESS  120 ) is used to drive one or more flywheels that are used to generate electricity to be sent to users  300 , and/or that are used to power a compressor or pump  136  of the MSESS  120 . Energy received by the MSESS  120  (from the renewable energy source  200 ) is used to power a heater  230  that heats salt  122  (e.g., molten salt) within a container  121  of the MSESS  120 . In these ways, excess electrical power from the renewable energy source  200  can be stored in the FESS  110  and/or the MSESS  120 . 
     In the YKESS  100  described herein, the FESS  110  can provide electricity to users  300  on a fast basis in response to interruptions of power from the renewable energy source  200 . The FESS  110  can generate electricity quickly. In some cases, the response time of the FESS  110  is typically less than one second with high degree of efficiency. The FESS  110  can drive one or more generators to create electricity that is sent to users  300  when power from the renewable energy source  200  is interpreted or diminished by day-and-night cycles or by unpredictable weather. 
     In the YKESS  100  described herein, the MSESS  120  works as an economical and high-capacity energy storage system that outputs electricity to power the FESS  110   and/or to be delivered directly to users  300 . Molten salt  122  in MSESS  120  can store a large amount of energy at a low cost. 
     Examples of How the YKESS  100  Works 
     Assuming the FESS  110  has energy storage capacity available (i.e., the FESS  110  is/are not fully “charged”), electrical power from the renewable energy source  200  is sent to the FESS  110  when there is an excessive leftover amount of electricity after the electricity is sent to meet the demand of the users  300  at any given moment of a day. The power sent to the FESS  110  drives the high-speed rotations of the one or more flywheels of the FESS  110 . Accordingly, electricity from the renewable energy source  200  is stored by the FESS  110 . 
     Additionally, some of the excessive leftover amount of electricity from the renewable energy source  200  is sent directly to the heater  230  of the MSESS  120  to heat its molten salt  122  (e.g., whenever it is necessary to prevent the temperature of the molten salt  122  from cooling down below a predetermined or desired temperature of the molten salt  122 ). In some embodiments, in addition to the salt  122  in the container  121 , there is an inert gas  124  such as nitrogen or carbon dioxide in the container  121   
     The energy storage capacity of the FESS  110  is usually quite a lot smaller than the energy storage capacity of the MSESS  120 . In part, this tends to be the case since the cost of energy storage in the FESS  110  is significantly higher than the cost of energy storage in the MSESS  120 . Therefore, the energy storage capacity of the MSESS  120  is much larger than the energy storage capacity of the FESS  110  in the YKESS  100 . 
     When the energy-storing capacity of the FESS  110  is fully loaded, the YKESS  100  can automatically divert the remaining leftover amount of electricity from the renewable energy source  200  to the MSESS  120  (which can store a lot more energy than the FESS  110 ). In addition, the MSESS  120  receives electricity from the renewable energy source  200  when the temperature of its molten salt  122  goes below a certain pre-determined lower threshold value. 
     The MSESS  120  stores energy as follows. The MSESS  120  receives electricity directly from the renewable energy source  200 . The MSESS  120  converts the electricity into heat energy in its molten salt  122  using the Ohmic heater  230  that is in thermal contact the salt  122  of the MSESS  120 . In this way, the temperature of the molten salt  122  is maintained at or above a desired value. The molten salt  122  is contained in the container  121  (e.g., tank, vessel, etc.). The container  121  is well insulated against heat loss. Very good heat insulation can be achieved cost effectively by wrapping up the container  121  with a low cost and lightweight insulating materials such as fiberglass, polyurethane foam, or more expensive materials such as aerogel and Pyrogel™. 
     In some embodiments of traditional MSESS, molten solar salt serves as both a working fluid and as the energy storage medium. “Cold” salt (e.g., at about 350° C.) is pumped from a cold storage tank and then heated to about 565° C. from either solar energy or wind energy during good weather conditions. This “hot” salt is then sent to the hot storage tank. When needed for power production, the hot salt is then pumped to the molten salt electrical generation system. In this example, we will assume a conventional commercial off the shelf reheat steam turbine system operating in a Rankine cycle at input temperature of 550° C. The cold salt at about 350° C. is then returned to the cold tank to repeat the process. 
     In the YKESS  100  described herein, excellent heat insulation of the container  121  is beneficial since the MSESS  120  is used to store large amounts of heat energy for several weeks, or even for a few months. In one example, a targeted degree of thermal insulation of the container  121  is used to achieve a cooling rate of 10 C to 30 C (centigrade) or less per week, when temperature of the molten salt is in the range of 500 C to 900 C. The energy retention capabilities of the MSESS  120  will depend on factors such as the actual volume and surface area of the container  121  of molten salt  122 , and the degree of heat insulation of insulating materials that wrap around the container  121 . A thermally well-insulated MSESS  120  having a very large energy storage capacity as a source of electrical energy generation (for long-term delivery of electricity to users  300  in case of longtime interruptions of electricity generation from the renewable energy source  200 ) to power the FESS  110  is a novel feature of the YKESS  100  described herein. 
     The depicted MSESS  120  converts its heat energy into electricity as follows. As shown in  FIG.  1   , the MSESS  120  has its own compressed-air-driven-turbine (CADT) system  130 . Accordingly, the CADT system  130  can be considered as a subsystem of the MSESS  120 . The depicted CADT system  130  includes a turbine  132 , a generator  133 , an air storage tank  134 , a compressor pump  136 , and a heat exchanger  138 . 
     A working fluid (e.g., air or steam; air is used in this example) in the piping of the CADT system  130  is compressed by the compressor pump  136  (which is powered by the FESS  110 ) to push the air into the heat exchanger  138  (e.g., one or more heat exchanging pipes or tubing) that is in contact with (e.g., immersed) in the molten salt  122  of the MSESS  120 . As the air goes through the heat exchanger  138  (usually made of stainless steel), the air is heated by heat stored in the molten salt  122  to a high temperature and raised to a high pressure. This superheated air (air with high temperature and high pressure) is then sent to the turbine  132  of the CADT system  130 . The turbine  132  drives the generator  133  to create electricity that is transmitted to the FESS  110  (to drive rotations of one or more flywheels of the FESS  110 ). As the hot air drives the turbine  132 , the air expands and cools. If the air has a negligible amount of moisture, or has no moisture at all, it can be considered as an ideal gas in the temperature in range of 500 C to 900 C. If the air contains a non-negligible amount of moisture, the air may not be considered as an ideal gas, and efficiency of the CADT system  130  will be lower than the efficiency of the CADT system  130  with dry air. Therefore, for the YKESS  100  described herein, it is advantageous for the air in the CADT system  130  to be dry air, or to have a low level of moisture. 
     In some embodiments, after the air in the CADT system  130  is discharged from the turbine  132  the air is passed through an array of underground pipes to cool the air before its next use in the generator cycle. It is important that the air be dry to prevent condensation during this cooling step. 
     Since the air in the CADT system  130  can be considered an ideal gas as described above, the total energy contained in a unit mass of the air is proportional to its temperature (absolute temperature). Let T1 be the temperature of the air before it enters the heat exchanger  138  in the MSESS  120 . Let T2 be the temperature of the air as it expands and drives the turbine  132 . Clearly, a higher value of T2 and a lower value of T1 will result in higher efficiency of the CADT system  130 . Accordingly, the rate of energy delivered by the air to the turbine  132  is proportional to (T2 - T1) multiplied by flow rate of the air (e.g., the mass of the air that goes to the turbine blades per unit time), since the viscosity loss of the air is negligible as an ideal gas. A modern air compressor (such as the compressor pump  136 ) is highly efficient (e.g., 95% or higher). If we were to assume the power to drive the compressor pump  136  comes from the generator  133  of the CADT system  130  (contrary to the design of the YKESS  100 ), the total efficiency of the CADT system  130  would be about 70%. 
     However, in the YKESS  100  described herein, the energy used to run the air compressor pump  136  of the CADT system  130  will be typically supplied by electricity (or by direct mechanical power transmission) from the FESS  110 , not by the electricity generated from the CADT system  130  in conjunction with the MSESS  120 . This is very beneficial because the efficiency of the MSESS  120  is about 70%, while efficiency of the FESS  110  is about 95% (or even higher than 95%). For example, suppose the MSESS  120  must use 100 watts of power to run the compressor pump  136  of the CADT system  130 . In that case, the MSESS  120  in conjunction with the turbine  132  and the generator  133  must produce about 143 watts of power to deliver the 100 watts to run the compressor pump  136 . This is because the efficiency of the MSESS  120  is about 70%. However, the FESS  110  can deliver the same power of 100 watts to the compressor pump  136  by consuming about 105 watts (to produce the 100 watts). There is a difference of about 38 watts (143 watts minus 105 watts) between the MSESS  120  and the FESS  110  when each of the two forms of ESS delivers 100 watts to the compressor pump  136  independently. In other words, powering the air compressor pump  136  of the CADT system  130  using energy from the FESS  110  is much more efficient than by using energy from the MSESS  120 . 
     Using the YKESS  100  described herein, the MSESS  120  does not need to power the air compressor pump  136  of the CADT system  130  (although it can in some cases). Instead, the MSESS  120  gets the compressed air without spending its own energy, since it is the FESS  110  that provides the energy to compress the air. This means that MSESS  120  can generate electricity (via the turbine  132  and the generator  133 ) with significantly higher efficiency when the air compressor pump  136  is powered by the FESS  110 . This is a novel unique feature of the YKESS  100  described herein. 
     After leaving the turbine  132 , the now colder air enters the storage tank  134  before it goes back to the air compressor pump  136  of the CADT system  130 . In some embodiments, the temperature (T3) of the air in the storage tank  134  will be essentially at ambient temperature. When the air is compressed by the air compressor pump  136  of the CADT system  130  (before it enters the heat exchanger  138  in the MSESS  120 ), the air is thereby heated to the new temperature of T1 (as described above). However, the work to raise the air temperature from T3 to T1 is done by FESS  110 , not by the MSESS  120 . Therefore, the theoretical efficiency of the CADT system  130  will be (T2 - T3)/T2 rather than (T2 - T1 )/T2. Since T3 is lower than T1, the theoretical efficiency of the MSESS  120  without spending its own energy to run the air compressor pump  136  is significantly higher than its theoretical efficiency with spending its own energy to run the air compressor pump  136 . Since the dry air used in CADT system  130  can be considered as an ideal gas, the actual efficiency of the MSESS  120  will be nearly equal to its theoretical value. In this way, the MSESS  120  can generate its electricity with significantly higher efficiency than a conventional stand-alone MSES can generate. 
     In some cases, such as when an interruption of electricity generation by the renewable energy source  200  continues for many days and the energy storage of the FESS  110  becomes very low, the air compressor pump  136  will automatically switch from being powered by the FESS  110  to being powered by electricity generated by the MSESS  120  and the CADT system  130  (from the generator  133 ). This will usually be a rare event in the real world. In conventional MSESS, the energy for its air compressor pump is often supplied by an outside source such as an independent electric generator powered by burning fossil fuel. This runs contrary to the purpose of environmentally friendly renewable energy from the sunlight or wind. This is another benefit of the YKESS  100  described herein. 
     The generation of electricity by the renewable energy source  200  tends to fluctuate almost constantly due changing weather and day-and-night cycles. An interruption of the electricity generated by the renewable energy source  200  may last for minutes, a few hours, a few days, or a few weeks. In this embodiment of the YKESS  100 , the FESS  110  alone stores enough energy for a few hours of supply of electricity to the user  300 , and the MSESS  120  is used to store enough energy to supply electricity to drive the flywheel(s) of the FESS  110 , and/or to supply the users  300 , for several days or even for a few weeks. Accordingly, the YKESS  100  can provide electricity to users  300  from the FESS  110  (when powered by the MSESS  120 ) and/or from the MSESS  120  for several days or even for a few weeks when there are such interruptions in the generation of electricity by the renewable energy source  200 . 
     It should be noted that the YKESS  100  described herein is not limited by exact values of efficiency of the FESS  110  or the MSESS  120  (e.g., as described above) since exact values of efficiency of the FESS  110  and the MSESS  120  will depend on details of designs and work conditions of the FESS  110  and the MSESS  120 , and/or because it is almost impossible to measure exact values of efficiency of the FESS  110  and the MSESS  120 . However, the efficiency numbers mentioned in this document are reasonably estimated numbers. The novel features of the YKESS  100  described herein are independent of exact values of efficiency of the FESS  110  and the MSESS  120 , and material properties mentioned in this document. 
     The description of the YKESS  100  described herein is provided in the context of a solar or wind power plant. However, the energy from other energy sources can be stored in the same type or similar types of integrated YKESS  100 , as described herein. In the YKESS  100  described herein, the MSESS  120  may be replaced with large-scale compressed air storage (CAES) if it is more cost effective for the same role of the MSESS  120 . 
       FIG.  2    is a schematic diagram of another example embodiment of YKESS  100 ′. In this version, ambient air is sucked into a filter and dehumidifier  135 , compressed in the compressor or pump  136 , heated in the heat exchanger  138 , and then discarded into open ambient air after it passes the turbine  132  where it releases its energy to power the generator  133 . Again, electricity output from the generator  133  is sent to the one or more FESS  110  to drive the rotations of the one or more flywheels of the FESS  110 . The one or more FESS  110 , in turn, generate electricity that is sent to the users  300  and generate power that is used to drive the compressor or pump  136 . As in the case of YKESS  100 , energy from the generator  133  can be fed to the FESS  110  (to increase the kinetic energy of the FESS  110 ) and/or to the users  300 . 
       FIG.  3    is a schematic diagram of another example embodiment of YKESS  1000 . While the YKESS  100  and the YKESS  100 ′ are described above as including the MSESS  120 , it should be understood that the MSESS  120  is just one example of a type of HESS. Here, in the context of the YKESS  1000 , a general HESS  240  (instead of a MSESS specifically) is included as an integral part of the YKESS  1000 . It should be understood that the YKESS  100  and YKESS  100 ′ can comprise a general HESS as described below rather than the MSESS  120 . 
     The use of the molten salt in a MSESS as described above may present some operational challenges. For example, the pumping systems for molten salts are complicated systems consisting of a number of components that are prone to corrosion by common salt systems. Some MSESS include piping and valves, oil heat exchangers that heat the water in a boiler for the Rankine system used for electricity generation, a number of distribution ring headers, and so forth. There is even a complete Ullage gas system that displaces head-space air with nitrogen to ensure fire safety. Another disadvantage is that, to prevent corrosion, the choice of salts is generally limited to mixtures of nitrates and nitrites and the temperature range for the operation of the storage is limited to at most 200° C., hence the limitation to a Rankine type of electricity generation system. Such choices severely limit the efficiency of the system and increase the amount of molten salt that is required. Increased salt requirement both increases cost and volumetric space for installation. Accordingly, a MSESS may not be the preferred choice for a high-capacity heat-energy storage system portion of the YKESS in some cases. Instead, other types of high-capacity heat-energy storage system can be included in some embodiments of YKESS (such as in the YKESS  1000 ). 
     In some embodiments of the YKESS  1000 , the HESS  240  has a large mass (or volume) of heat reservoir material  242  (e.g., with its temperature in the range of  500  to 1100° C.) contained in a thermally insulated container  241 . The heat energy of the heat reservoir material  242  is supplied by an Ohmic heating device that is immersed in the heat reservoir material  242 . The Ohmic heating device is heated by electricity from solar panels  210  or wind turbines  220 . In addition to the Ohmic heating device, a heat exchanger is immersed in the heat reservoir material  242 . 
     A heat engine  410  is operatively attached to the container  241  of heat reservoir material  242 , and a working fluid of the heat engine  410  is heated as it flows through the heat exchanger that is immersed in the heat reservoir material  242 . The heat engine  410  has a compressor and pump for the working fluid, a turbine that is driven by hot and high-pressure work fluid emerging from the heat exchanger, and a turbine that drives an electric generator. There are a number of choices that can be made for each of the component parts of the HESS  240 , thereby providing a flexible framework for heat energy that can be converted into electricity. In some embodiments, the working fluid of the heat engine  410  may be water-steam or dry air. However, the working fluid of the heat engine  410  is not restricted to water-steam or dry air. 
     In some embodiments of the YKESS  1000  using the HESS  240 , solar energy  210  and/or the wind energy  220  is delivered to a converter, switching, and control unit  250  (hereinafter “control unit  250 ”) that distributes the electrical energy by means of electrical connections to three destinations: to the users  300 , to the FESS  110 , and to the HESS  240 . The control unit  250  distributes the load amongst those three destinations based on various factors. 
     The energy delivered to the HESS  240  and the FESS  110  is used to charge these systems (e.g., increase the levels of energy stored therein), whereas the energy delivered to the users  300  is used to power the user’s operations. The control unit  250  also allows the FESS  110  to provide small amounts of energy to the end users  300  as needed to smooth out ripples and to ensure synchronization caused by slightly varying supply conditions from the solar  210  and wind  220  sources. 
     The energy delivered to the HESS  240  is used to heat the heat reservoir material  242  that will store the heat energy until such time the energy is needed. The energy supplied to the FESS  110  (which may be multiple FESS) is used to bring the FESS  110  to full operating speed and to keep it at full operating speed until such time as energy from the FESS  110  is needed. 
     When the electrical power from the solar  210  and/or wind  220  sources is interrupted or sufficiently reduced, the control unit  250  will autonomously direct at least some energy from the FESS  110  to the end user  300 . In addition, the control unit  250  will in some cases direct energy from the FESS  110  to the compressor pump  136 . The energy to the end user  300  is used to power the end user’s operations. The energy directed to the compressor pump  136  is used to force the working fluid in the lines  411  through a heat exchanger in the HESS  240  then to the heat engine  410  to bring the heat engine  410  with its turbine and generator to 100% operating capacity while it is starting to draw energy from the HESS  240 . 
     Once the heat engine  410  is at full capacity, the heat engine  410  will provide the power to the users  300  and/or will supply the FESS  110  with sufficient energy to maintain the FESS  110  in a fully charged state. The FESS  110  will continue to participate in the supply of energy to the end users  300  to ensure ripple-free and synchronized energy to the end user. All of these operations will be controlled by the control unit  250 . 
     The heat engine  410  itself has several basic components. It uses a working fluid that is fed by the compressor pump  136  to a heat exchanger contained within the container  241  of the HESS  240 . The heat exchanger is in thermal contact with the heat reservoir material  242  in the container  241 . Accordingly, the working fluid can be heated by the reservoir material  242  in the container  241 . The heated working fluid is then used to power a turbine of the heat engine  410 . The turbine drives a generator of the heat engine  410  to create electrical energy. The electrical energy from the generator can be sent to the users  300 , the FESS  110 , and/or the compressor pump  136 . The distribution of the electrical energy from the generator to those three potential recipients can be determined and controlled by the control unit  250 . 
     In some embodiments, the working fluid is dry air that is heated up to about 1100° C. by the HESS  240  and is then used to drive the turbine and electrical generator system of the heat engine  410 . The anticipated 1100° C. operating temperature of the HESS  240  is much higher than the 500° C. temperature common among MSESS systems. Its thermodynamic efficiency is thus much higher. The use of dry air is not limiting. Other choices for the working fluid are anticipated being required for specific applications. 
     The YKESS  1000  as depicted herein anticipates that the HESS  240  has a single container  241  thus obviating the need for multiple containers and a complicated pumping system that connects them. Since the heat reservoir material  242  is isolated to a single container  241  and is not itself pumped through a complicated pumping system, the container  241  can be more easily protected from possible corrosion from the heat reservoir material  242 . The single container  241  can be more easily insulated than a system of several containers connected by a pumping system. 
     Many choices can be made for the type of heat reservoir material  242  in the HESS  240  (instead of the heat reservoir material  242  being the common Solar Salt that is a mixture of sodium and potassium nitrates and nitrites). For example, in some embodiments the YKESS  1000  uses advanced phase change materials as the heat reservoir material  242 . Such advanced phase change materials permit a higher temperature operating range and have higher thermal conductivities to enhance energy transfer to the heat exchanger. In some cases, the heat reservoir material  242  may comprise even simple materials such as concrete, sand, or gravel depending on the requirements of the final implementation. 
     The compressor pump  136  of the YKESS  1000  cannot self-start from its idle state. In the YKESS  1000 , the electricity from the FESS  110  jumpstarts the compressor pump  136  of the heat engine  410 , and the FESS  110  continues to power the compressor pump  136  until the heat engine  410  is operating at full capacity. 
     The energy capacity of the FESS  110  is designed to carry a bare minimum of kinetic energy by its one or more flywheels. For example, in some embodiments the FESS  110  may carry just enough energy to deliver its energy to end users  300  until its energy will be replenished by electricity from the generator of the heat engine  410 . If it takes 10 minutes for the heat engine  410  to jumpstart and become fully operation from its idle state, the FESS  110  energy capacity may be only enough energy to deliver its electricity to the users  300  for 20 minutes or 30 minutes. The short period of 20 minutes or 30 minutes is still significantly longer than the 10 minutes of time for the heat engine  410  to ‘wake up and run at full speed’. Effectively, this means that the FESS  110  that carries just enough energy to deliver its electricity only for 20 minutes or 30 minutes will actually support/trigger the delivery of electricity to users  300  continuously for one week or two weeks as long as the heat energy stored in the HESS  240  lasts. The cost of energy storage in mass of the heat reservoir material  242  is far less that cost of energy storage of FESS  110  alone. 
     The YKESS  1000  provides novel synergy by combining the flywheel of the FESS  110 , the heat reservoir material  242  of the HESS  240 , and the simple heat engine  410  operatively positioned between the heat reservoir and the flywheel of the FESS  110 . This unique feature of the YKESS  1000  enables designers of the YKESS  1000  to use a FESS  110  that has relatively small amount of kinetic energy of its flywheels. Therefore, the FESS  110  that is used for the YKESS  1000  can be a relatively low-cost and extremely reliable and long-lasting FESS  110 . 
       FIG.  4    is a block diagram that illustrates that a large scale YKESS  1000 ′ can include a HESS  240  that is operatively connected to a plurality of FESSs  110  in the same manner as described elsewhere herein (e.g., where electricity generated by the HESS  240  is used to power rotations of the flywheels of the FESSs  110 , and power from the FESSs  110  can be used to start up a pump or compressor of the HESS  240 ). 
     The multiple units of FESS  110  are connected to provide electricity to end users  300  of electricity or power grids  300 . While not shown for simplicity sake, the electricity generated by the renewable energy source  200  can also be delivered directly to each FESS  110  of the plurality of FESSs  110  to power rotations of the flywheels of the FESSs  110 . Also, the HESS  240  can deliver to the users  300  directly. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described herein as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various modules and components in the embodiments described herein should not be understood as requiring such separation in all embodiments, and it should be understood that the described components and systems can generally be integrated together in a single product or packaged into multiple products. 
     Particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. As one example, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous.