There are a large range of possible approaches and concepts for storing energy in electric utility systems. These are discussed in the following subsections.
The major barriers to widespread use of conventional pumped storage are siting, geological factors, environmental and space constraints because of the large size of commercially feasible installations, and long construction times. The uncertainty and potential high costs of underground construction costs are believed to be the reason that no underground projects have been pursued to actual construction.
In underground pumped storage, the lower reservoir and power plant are located in deep underground caverns and the upper reservoir is at the surface. By being free of surface topographical restrictions, the siting of these underground plants should be considerably easier than the siting of conventional pumped storage facilities. The underground reservoir and power plant could use naturally occurring caverns, abandoned mines, or a mined-out cavern consisting of a tunnel labyrinth excavated specifically for the pumped storage reservoir. In existing mine sites, firsthand knowledge of the subsurface rock formations is available, and existing shafts can be used. With the elevation difference (head) between the upper and lower reservoirs now a variable parameter not limited by surface topography, the major design restrictions are equipment capability and rock conditions. If very high heads are used, there may be cost penalties associated with very deep mining; nevertheless, mining costs per unit of energy storage capacity should decrease with depth because of proportional reductions in the volume of the reservoir.
Underground construction and mining technologies are available and can be adapted for this system. The largest uncertainty is construction of the underground reservoir: its cost, durability with pressure cycling, and the rate of water leakage into the lower reservoir. Costs are heavily dependent upon suitability of the site and local labor conditions. The economics of scale in pumped hydro dictates sizes in the range of 1000 to 2000 MW.
COMPRESSED AIR ENERGY STORAGE: Compressed air storage uses a modified combustion turbine (split Brayton cycle), uncoupling the compressor and turbine so that they can operate at different times and incorporating the intermediate storage of compressed air. During off-peak load periods, the turbine is disengaged and the compressor is driven by the generator, which is now used as a motor and takes its power from other generating units through the system’s interconnections. The stored compressed air is subsequently used during peak load periods when it is mixed with fuel in the combustion chamber, burned, and expanded through the turbine. During that period, the compressor is disengaged and the entire output of the turbine is used to drive the generator.
Since in normal operation the compressor consumes about two-thirds of the power output of the turbine, the rating of the combustion turbine operating from the stored compressed air is increased roughly by a factor of three. This permits redesign of the compressors, the combustion process, and the combustion turbine, free from the aerodynamic and thermodynamic restrictions inherent in designs of conventional combustion turbines. Current estimates for the heat rate of the combustion turbine operating from stored compressed air are in the range of 4000 Btu/kW-hr. A compression/generation energy ratio in the range of 0.65 to 0.75 should be readily available. The variable maintenance cost should not be any greater than that for a conventional combustion turbine.
The compressed air may be stored in naturally occurring reservoirs (caverns, porous ground reservoirs, and depleted gas or oil fields) or manmade caverns (dissolved-out salt caverns, abandoned mines, or mined hard-rock caverns). Air storage may be accomplished at variable pressure or, through the use of a hydrostatic leg, at constant pressure. Each approach has its advantages and all are applicable to different underground geologies and reservoir designs. Designs of plants in the 50 to 250 MW range and larger have been explored.
Research is required to investigate:
(a) Geological conditions for underground storage,
(b) New approaches to underground cavern construction,
(c) Energy losses storing and moving air,
(d) Alternative concepts of air storage, and
(e) Corrosion effects on turbines from air contamination.
The major uncertainties include the cost of the air storage facilities, the performance and durability of the storage facility with pressure and thermal cycling, and leakage from the storage reservoir. Also, additional geological survey work to identify the availability and number of possible sites is necessary before the future role of compressed air storage may be assessed. With the high costs of fuel and the needs of intermittent renewable resources, renewed interest in compressed air storage has developed and a concept that uses high-pressure storage in buried pipes for smaller installations is being explored.
FLY WHEELS: Fly wheels store energy in the form of the kinetic energy of a rotating mass and have been used since the beginning of the industrial age. In recent years, the commercial application of flywheels to power quality and interruptible power supplies has become a commercial reality. Technological advances in rotating machinery and high-strength materials achieved since then hold promise for longer periods and greater capacity of energy storage, which raises the possibility of new applications.
Utility system applications have been restricted to special purpose uses for smoothing pulsed power needs or for short-duration power quality needs. Although advanced composite materials have been experimented with in test facilities and proposed for commercial application, the metal flywheel has been the one approach that is in general use. Proposed super flywheel designs deal primarily with the wheel itself, without treating the full energy storage system in sufficient detail. The large wheels once proposed for utility applications appear to be outside the size of current, state-of-the-art, cost-effective designs. Commercial applications have used steel wheels and various electromechanical machinery designs, including variable frequency converters.
(a) Hot water heating,
(b) Heating and air conditioning of buildings using off-peak (or solar) energy,
(c) low-temperature process steam storage,
(d) Central station thermal storage (especially for solar–thermal power plants), and
(e) Industrial process heat storage and district heating systems.
Depending largely on the temperature of the storage medium, these uses may be grouped into applications of low-grade (relatively low-temperature) and high-grade (high temperature) heat.
STORAGE OF LOW-GRADE HEAT: Storage of sensible heat in hot water reservoirs is an established commercial practice in off-peak water heating and in chilled water applications. Heat storage in the ceramic bricks of storage heaters gained commercial acceptance in
Europe several decades ago. Among the key tasks in making storage heaters practical were the design and refinement of control methods. Development resulted in improved ceramic materials of high specific heat capacity and good thermal cycling ability, new approaches to high quality thermal insulation, and extension of the concept from individual room units to central systems.
The operation of air conditioning systems with off-peak power requires coolness storage. Although coolness storage has found only limited applications, it would be a useful option if more widely applied in summer peaking electric utilities. Storage of relatively low-temperature heat will be a key requirement for the residential and commercial utilization of solar energy, and appropriate approaches have been explored experimentally. Thermal energy storage systems based on storage of latent (phase change) heat are attractive in principle because of their high specific storage capacity. However, despite their greater volume, sensible heat storage in liquids is almost certainly more practical and economical because they avoid the problems and costs associated with heat exchangers. Storage of waste heat from power plants or industrial processes for later use is another possible application of low-grade heat storage.
STORAGE OF HIGH-GRADE HEAT: Storage of high-temperature, high pressure steam/water mixtures is a prime example of thermal energy storage but is largely only of historical interest as a step in the development of the modern steam power plant. While the basic technologies associated with thermal storage via hot water and steam is straight forward, the large size and high costs of the storage vessels has largely ruled out this technology as part of conventional power plants. The benefit of energy storage is in conjunction with solar thermal power plants. The use of storage of sensible heat in a high-temperature oil or liquid molten salt can extend the operating hours of the power generation portion of the solar thermal power plant and result in a higher capacity factor for the plant than would be possible if the plant could only generate power when the sun was available.
The chemical energy storage methods and systems relevant to electric power (excluding conventional fuel storage) are largely secondary battery energy storage and hydrogen storage. In general, the reactant systems containing the stored energy must be reformed readily from their reacted (discharged) state upon addition of energy in a suitable form. Although many other schemes have been proposed, they remain only of research interest at the moment.
Many different electrochemical systems have been developed, or offer prospects for development, into practical storage batteries. For more than thirty years, efforts have been underway to develop and commercialize various battery systems for use by electric utilities at the scale of distributed energy storage in the size range of megawatts to tens of megawatts; several applications of lead acid batteries have actually been operated for extended periods and more advanced systems proposed and demonstrated.
Today, great interest and very substantial funding is being invested in lithium battery systems that operate at ambient temperatures and are targeted primarily for mobility and portable power, including especially the plug-in hybrid vehicle. If this current effort is successful and the promise of lower costs in large volume production is realized, practical battery energy storage may become a practical reality on a large scale, although each individual installation may be only the tens of kilowatt hours needed for the personal vehicle applications.
Closed-cycle thermochemical processes are being proposed for hydrogen production via water splitting, but current work is still in the conceptual and early laboratory stages. The incentive to develop such processes derives from the potential for efficiencies and economics that might be superior to those offered by electrolysis, particularly if sources of fairly high-temperature heat, such as high-temperature, gas-cooled reactors or, perhaps, focused solar heat, become available. Integration of these processes with nuclear heat sources and commercialization of the entire hydrogen production system are likely to require many years and large capital investments.
Hydrogen storage, the second major subsystem of hydrogen energy storage systems, can take several different forms. Storage of compressed hydrogen is technically feasible now, as is storing hydrogen in more concentrated forms as a cryogenic liquid or chemically bound in metal hydrides, and logistically attractive. However, cryogenic storage of hydrogen carries a significant efficiency penalty that is unacceptable for large-scale energy storage on utility systems, and capital cost, logistics, and safety are likely to present problems for mobile applications. The outlook is better for metal hydride storage, but development efforts are still required to establish the technical and economic characteristics of this method for hydrogen storage. Reconversion of hydrogen to electric energy can be done in fuel cells or in combustion devices (gas fired boilers or gas turbines). The fuel cell approach offers potential for high efficiency, with 60% as a target for pure hydrogen fuel.
Hydrogen has unique potential for utilization of primary energy sources and flexible use of the stored energy. However, unless major advances result from current research and development on hydrogen production, storage, and conversion technologies, relatively low efficiency and high capital costs will be major barriers to the introduction of hydrogen energy storage systems.
The charge separation creates an electrical potential or voltage between the two plates. Capacitors are routinely used in power systems to compensate for the inductance of the electrical wires or conductors. Properly designed capacitors can withstand high voltages and are well suited to serve as static devices in high-voltage applications.
In the last two decades, a new class of capacitors, super capacitors have been developed and commercialized that operate on a somewhat different principal; these are electrochemical double layer capacitors. They have significantly higher charge densities than ordinary capacitors. Their properties are between those of a conventional capacitor and a battery. These devices are finding uses in situations in which a larger capacitance is needed and fast response time is desirable, but the full storage capacity of a battery is not needed. They have also been proposed as devices to be combined with batteries in some applications. The material properties of the super capacitors are very different from those of conventional capacitors and they cannot withstand high voltages, but can be placed in series like batteries to operate at modest voltages.
1) MECHANICAL SYSTEMS
HYDRO PUMPED STORAGE: In hydro pumped storage, water is pumped from a lower to a higher elevation. The water at the higher elevation can be stored and used to generate electricity for later utility use when it flows down through a hydro turbine to drive an electric generator. Pumping and generation may also be accomplished with a reversible pump–turbine connected to a motor generator. The reservoirs needed for the pumped storage operation may be natural bodies of water, reservoirs of existing hydro plants or of water storage systems, especially constructed surface reservoirs, underground caverns, or a combination of these. Typical efficiency of this process is about 70%, with 30% used in the pumping generating cycle.The major barriers to widespread use of conventional pumped storage are siting, geological factors, environmental and space constraints because of the large size of commercially feasible installations, and long construction times. The uncertainty and potential high costs of underground construction costs are believed to be the reason that no underground projects have been pursued to actual construction.
In underground pumped storage, the lower reservoir and power plant are located in deep underground caverns and the upper reservoir is at the surface. By being free of surface topographical restrictions, the siting of these underground plants should be considerably easier than the siting of conventional pumped storage facilities. The underground reservoir and power plant could use naturally occurring caverns, abandoned mines, or a mined-out cavern consisting of a tunnel labyrinth excavated specifically for the pumped storage reservoir. In existing mine sites, firsthand knowledge of the subsurface rock formations is available, and existing shafts can be used. With the elevation difference (head) between the upper and lower reservoirs now a variable parameter not limited by surface topography, the major design restrictions are equipment capability and rock conditions. If very high heads are used, there may be cost penalties associated with very deep mining; nevertheless, mining costs per unit of energy storage capacity should decrease with depth because of proportional reductions in the volume of the reservoir.
Underground construction and mining technologies are available and can be adapted for this system. The largest uncertainty is construction of the underground reservoir: its cost, durability with pressure cycling, and the rate of water leakage into the lower reservoir. Costs are heavily dependent upon suitability of the site and local labor conditions. The economics of scale in pumped hydro dictates sizes in the range of 1000 to 2000 MW.
COMPRESSED AIR ENERGY STORAGE: Compressed air storage uses a modified combustion turbine (split Brayton cycle), uncoupling the compressor and turbine so that they can operate at different times and incorporating the intermediate storage of compressed air. During off-peak load periods, the turbine is disengaged and the compressor is driven by the generator, which is now used as a motor and takes its power from other generating units through the system’s interconnections. The stored compressed air is subsequently used during peak load periods when it is mixed with fuel in the combustion chamber, burned, and expanded through the turbine. During that period, the compressor is disengaged and the entire output of the turbine is used to drive the generator.
Since in normal operation the compressor consumes about two-thirds of the power output of the turbine, the rating of the combustion turbine operating from the stored compressed air is increased roughly by a factor of three. This permits redesign of the compressors, the combustion process, and the combustion turbine, free from the aerodynamic and thermodynamic restrictions inherent in designs of conventional combustion turbines. Current estimates for the heat rate of the combustion turbine operating from stored compressed air are in the range of 4000 Btu/kW-hr. A compression/generation energy ratio in the range of 0.65 to 0.75 should be readily available. The variable maintenance cost should not be any greater than that for a conventional combustion turbine.
The compressed air may be stored in naturally occurring reservoirs (caverns, porous ground reservoirs, and depleted gas or oil fields) or manmade caverns (dissolved-out salt caverns, abandoned mines, or mined hard-rock caverns). Air storage may be accomplished at variable pressure or, through the use of a hydrostatic leg, at constant pressure. Each approach has its advantages and all are applicable to different underground geologies and reservoir designs. Designs of plants in the 50 to 250 MW range and larger have been explored.
Research is required to investigate:
(a) Geological conditions for underground storage,
(b) New approaches to underground cavern construction,
(c) Energy losses storing and moving air,
(d) Alternative concepts of air storage, and
(e) Corrosion effects on turbines from air contamination.
The major uncertainties include the cost of the air storage facilities, the performance and durability of the storage facility with pressure and thermal cycling, and leakage from the storage reservoir. Also, additional geological survey work to identify the availability and number of possible sites is necessary before the future role of compressed air storage may be assessed. With the high costs of fuel and the needs of intermittent renewable resources, renewed interest in compressed air storage has developed and a concept that uses high-pressure storage in buried pipes for smaller installations is being explored.
FLY WHEELS: Fly wheels store energy in the form of the kinetic energy of a rotating mass and have been used since the beginning of the industrial age. In recent years, the commercial application of flywheels to power quality and interruptible power supplies has become a commercial reality. Technological advances in rotating machinery and high-strength materials achieved since then hold promise for longer periods and greater capacity of energy storage, which raises the possibility of new applications.
Utility system applications have been restricted to special purpose uses for smoothing pulsed power needs or for short-duration power quality needs. Although advanced composite materials have been experimented with in test facilities and proposed for commercial application, the metal flywheel has been the one approach that is in general use. Proposed super flywheel designs deal primarily with the wheel itself, without treating the full energy storage system in sufficient detail. The large wheels once proposed for utility applications appear to be outside the size of current, state-of-the-art, cost-effective designs. Commercial applications have used steel wheels and various electromechanical machinery designs, including variable frequency converters.
2) THERMAL ENERGY STORAGE
Thermal energy storage may be defined as storage of energy in the form of (a) sensible heat and (b) the latent heat associated with phase changes such as the formation of ice. The major technical parameters for thermal energy storage include the storage medium, the operating temperature range, and the mode of heat exchange between the storage subsystem and the heat source/sink. Any practical system must include not only the thermal energy storage and transfer subsystems but also provisions for control and insulation. Thermal energy storage can be useful in a wide spectrum of applications, including(a) Hot water heating,
(b) Heating and air conditioning of buildings using off-peak (or solar) energy,
(c) low-temperature process steam storage,
(d) Central station thermal storage (especially for solar–thermal power plants), and
(e) Industrial process heat storage and district heating systems.
Depending largely on the temperature of the storage medium, these uses may be grouped into applications of low-grade (relatively low-temperature) and high-grade (high temperature) heat.
STORAGE OF LOW-GRADE HEAT: Storage of sensible heat in hot water reservoirs is an established commercial practice in off-peak water heating and in chilled water applications. Heat storage in the ceramic bricks of storage heaters gained commercial acceptance in
Europe several decades ago. Among the key tasks in making storage heaters practical were the design and refinement of control methods. Development resulted in improved ceramic materials of high specific heat capacity and good thermal cycling ability, new approaches to high quality thermal insulation, and extension of the concept from individual room units to central systems.
The operation of air conditioning systems with off-peak power requires coolness storage. Although coolness storage has found only limited applications, it would be a useful option if more widely applied in summer peaking electric utilities. Storage of relatively low-temperature heat will be a key requirement for the residential and commercial utilization of solar energy, and appropriate approaches have been explored experimentally. Thermal energy storage systems based on storage of latent (phase change) heat are attractive in principle because of their high specific storage capacity. However, despite their greater volume, sensible heat storage in liquids is almost certainly more practical and economical because they avoid the problems and costs associated with heat exchangers. Storage of waste heat from power plants or industrial processes for later use is another possible application of low-grade heat storage.
STORAGE OF HIGH-GRADE HEAT: Storage of high-temperature, high pressure steam/water mixtures is a prime example of thermal energy storage but is largely only of historical interest as a step in the development of the modern steam power plant. While the basic technologies associated with thermal storage via hot water and steam is straight forward, the large size and high costs of the storage vessels has largely ruled out this technology as part of conventional power plants. The benefit of energy storage is in conjunction with solar thermal power plants. The use of storage of sensible heat in a high-temperature oil or liquid molten salt can extend the operating hours of the power generation portion of the solar thermal power plant and result in a higher capacity factor for the plant than would be possible if the plant could only generate power when the sun was available.
3) CHEMICAL ENERGY STORAGE
Chemical energy storage is the storage of energy as chemicals (usually two different chemicals that can be gases, liquids, or solids) that can be made to react with a net release of energy. Storage of energy in chemical form has two inherent advantages. The high energy density of a chemical system results in compact, generally low-cost storage and ready transportability of energy, and chemical energy is readily converted into other useful energy forms by a variety of methods and devices. These advantages are responsible for the almost exclusive use of conventional fuels for energy storage and mobility applications.The chemical energy storage methods and systems relevant to electric power (excluding conventional fuel storage) are largely secondary battery energy storage and hydrogen storage. In general, the reactant systems containing the stored energy must be reformed readily from their reacted (discharged) state upon addition of energy in a suitable form. Although many other schemes have been proposed, they remain only of research interest at the moment.
4) BATTERIES
In “storage” batteries, the conversion from electrical to chemical energy (charging) and the reverse process (discharging) are performed by electrochemical reactions. The electric form of input and output energy, compactness, and the modular characteristics common to electrochemical devices make batteries potentially the most useful among advanced energy storage methods.Many different electrochemical systems have been developed, or offer prospects for development, into practical storage batteries. For more than thirty years, efforts have been underway to develop and commercialize various battery systems for use by electric utilities at the scale of distributed energy storage in the size range of megawatts to tens of megawatts; several applications of lead acid batteries have actually been operated for extended periods and more advanced systems proposed and demonstrated.
Today, great interest and very substantial funding is being invested in lithium battery systems that operate at ambient temperatures and are targeted primarily for mobility and portable power, including especially the plug-in hybrid vehicle. If this current effort is successful and the promise of lower costs in large volume production is realized, practical battery energy storage may become a practical reality on a large scale, although each individual installation may be only the tens of kilowatt hours needed for the personal vehicle applications.
5) HYDROGEN ENERGY STORAGE SYSTEMS
Hydrogen energy storage represents the best known example of advanced chemical energy storage. Several approaches have been proposed and explored for each of the required sub-systems hydrogen generation, storage, and reconversion which can be combined in various ways into overall energy conversion and storage systems. For hydrogen generation from water, electrolysis is the only established industrial process. Current electrolysis technology is handicapped by high capital costs, but considerable potential appears to exist for development of more efficient, lower-cost electrolyzers. At present there is little commercial incentive to develop such technology.Closed-cycle thermochemical processes are being proposed for hydrogen production via water splitting, but current work is still in the conceptual and early laboratory stages. The incentive to develop such processes derives from the potential for efficiencies and economics that might be superior to those offered by electrolysis, particularly if sources of fairly high-temperature heat, such as high-temperature, gas-cooled reactors or, perhaps, focused solar heat, become available. Integration of these processes with nuclear heat sources and commercialization of the entire hydrogen production system are likely to require many years and large capital investments.
Hydrogen storage, the second major subsystem of hydrogen energy storage systems, can take several different forms. Storage of compressed hydrogen is technically feasible now, as is storing hydrogen in more concentrated forms as a cryogenic liquid or chemically bound in metal hydrides, and logistically attractive. However, cryogenic storage of hydrogen carries a significant efficiency penalty that is unacceptable for large-scale energy storage on utility systems, and capital cost, logistics, and safety are likely to present problems for mobile applications. The outlook is better for metal hydride storage, but development efforts are still required to establish the technical and economic characteristics of this method for hydrogen storage. Reconversion of hydrogen to electric energy can be done in fuel cells or in combustion devices (gas fired boilers or gas turbines). The fuel cell approach offers potential for high efficiency, with 60% as a target for pure hydrogen fuel.
Hydrogen has unique potential for utilization of primary energy sources and flexible use of the stored energy. However, unless major advances result from current research and development on hydrogen production, storage, and conversion technologies, relatively low efficiency and high capital costs will be major barriers to the introduction of hydrogen energy storage systems.
6) ELECTRICAL STORAGE
Capacitors and Super capacitors. The capacitor is an essential and elementary device in electrical circuits that stores electrical charge, typically between two metallic plates separated by dielectric, a material that is not particularly good at conducting current.The charge separation creates an electrical potential or voltage between the two plates. Capacitors are routinely used in power systems to compensate for the inductance of the electrical wires or conductors. Properly designed capacitors can withstand high voltages and are well suited to serve as static devices in high-voltage applications.
In the last two decades, a new class of capacitors, super capacitors have been developed and commercialized that operate on a somewhat different principal; these are electrochemical double layer capacitors. They have significantly higher charge densities than ordinary capacitors. Their properties are between those of a conventional capacitor and a battery. These devices are finding uses in situations in which a larger capacitance is needed and fast response time is desirable, but the full storage capacity of a battery is not needed. They have also been proposed as devices to be combined with batteries in some applications. The material properties of the super capacitors are very different from those of conventional capacitors and they cannot withstand high voltages, but can be placed in series like batteries to operate at modest voltages.