Raising Efficiency and Productivity of Large-scale Energy Storage Systems (Part 1)
The deterrents to large-scale energy storage include environmental concerns regarding the destruction of valleys, the loss of efficiency when comparing output power against input power, location of energy storage systems and cost. Energy storage systems are best developed at locations where the price of power during peak demand periods greatly exceeds the off-peak prices. There are several factors that contribute to increasing the output/input efficiency and related productivity of large-scale energy storage systems. Part 1 of this article will explore alternatives that relate to compressed air storage while part 2 will explore alternatives that relate to pumped hydraulic storage.
Compressed Air Energy Storage (CAES):
Compressed air storage had successfully powered small railway locomotives that operated in mines during the early 20th century. The development of a range of pneumatic technologies plus the discovery of salt domes in the earth's crust allowed for further development of compressed air storage. The peaks of large salt domes may be located 2000-ft to 6000-ft below ground surface. When emptied of salt, large salt caverns in impervious rock could measure up to 5000-feet in diameter by up to 30,000-feet in vertical height.
While most salt domes are used to store compressed natural gas at pressures of some 1500-psia, a few salt domes are being used to store compressed air. A medium sized dome measuring 2500-feet in diameter by 10,000-feet in vertical height would provide a volume of 5.113269 x 1010-ft3. A dome top of 2000-feet below ground surface and an earth density of 133-lb/ft3 would provide a downward force of 1847-psi, allowing the salt cavern to hold an internal pressure of 1500-psia.
Direct Drive to Raise Efficiency:
The combination of a turbine driving an electrical alternator and an electric motor driving a compressor with both electrical items operating at 94% efficiency will provide 88% overall efficiency between power turbine and turbo-compressor. A direct-drive concept may be possible where a power station and an energy storage system are located in very close proximity to each other. It would be based on matching optimal rotational speed and output of a steam turbine with the rotational speed and power requirements of an air-turbo-compressor.
The direct-drive option would require a redesign of select steam-based power stations where the same steam turbines may either drive electrical alternators or air-turbo compressors at different times of a 24-hour day. The General Electric power station is rated at 1000MW while the Siemens SGT-300 gas turbine is rated at 300MW output, at up to 50% thermal efficiency. The efficiency level of the SGT-300 suggests that its air-turbo-compressors would require an input of 300MW. Companies such as Siemens and General Electric manufacture both steam turbine as well as gas turbine engines. It is within the scope of such companies to match steam turbines with air turbo-compressors.
To undertake a changeover from alternator to air compressors, electric motors would draw power from the grid or from storage to accelerate the turbo compressors to the steam turbine's rotational speed. The automotive industry perfected a power changeover system in manual-shifting truck transmission that were automated, that is, computers measure input speed and output speed and activate solenoids to shift the gears. Such a system could be adapted to the power generation industry to allow a clutch mechanism driven by a rotating steam turbine to engage an air turbo compressor rotating at identical speed, in a matter of a few seconds. During the changeover, the alternator may be taken "off-line" when its armature or stator is de-energized and its carbon brushes retracted from the slip rings.
A system where a steam turbine operating at 900MW directly drives air turbo compressors would save over 100MW in lost efficiency. Multiple steam-based thermal power stations located in close proximity to a compressed air storage installation could recharge a subterranean air chamber during the overnight hours at optimal efficiency. A group of power stations of 4500MW output could save 500MW through the direct drive option. The power stations would include nuclear, clean coal, biomass and possibly boron-fusion technology within the next decade.
Small-site Direct Drive Air Compressors:
The design of vertical-axis wind turbines allows for direct drive air compressor technology. The low rotational speed of the wind turbines would require the use of a rotary, positive displacement compressor technology. High-pressure pipelines would connect wind farms of large-scale, vertical-axis wind turbines driving air compressors to air storage chambers. Airborne wind energy technology involving kites can activate ground level compressors and pumped compressed air into storage via pipeline.
There is potential for ocean wave conversion technologies and kinetic turbines to directly drive air compressors and feed into a nearby air storage chamber via high-pressure pipelines. Wind energy technologies along with wave energy, tidal current and river current technologies can all directly drive air compressor systems and pump air into storage via pipeline. These technologies may also be designed to alternately drive air compressor technology during off-peak periods and electric generating technology during the peak demand periods.
Increasing Productivity of Oceanside CAES:
The heat of compression is a major energy loss for compressed air energy storage. A salt dome located near a body of seawater offers a unique opportunity to use the heat of compression productively, especially at geographic locations where there is a growing need for fresh potable water, district heating or district cooling. The heat of compression may be used to operate a thermal-desalination plant during the overnight hours. Pumping air from 14.7-psia to 1500-psia could be undertaken in 3-stages, using turbo-compressors with a 5 to 1 pressure ratio that operate at 93% isentropic efficiency.
Air at 40 F (5 C) being pumped to 5-times atmospheric pressure would rise to 354 F (179 C), warm enough to boil water at atmospheric pressure. Air at 80 F (27 C) would be compression heated to 419 F (215 C). The sheer volume of the salt dome would allow compressed air at 1500-psia at 152F to drop to 1400-psia at 140F and release 563,787-lbw/sec or 17,500-lbm/sec of air over a period of 8-hours. That mass flow rate of air being pumped into storage could transfer over 1,000,000-BTU/sec or over 1000MW via water-cooled intercoolers to a thermal desalination plant, a district heating plant or cooling plant that uses steam vacuum refrigeration.
During the generating phase, the turbines could be set to provide a cold exhaust (-40 F) or a hot exhaust (300 F). Minimal temperature of the air flowing from the cavern to the power turbines could provide up to 1300MWe over 8-hours (over 10,000 MW-hrs over 2-cycles of 4-hours duration each) and an exhaust temperature below the freezing point of water. The cold exhaust stream could sustain building air conditioning operations during peak periods. Heating the exhaust air from -40 F to 60 F could provide some 420,000-BTU/sec or the equivalent of 440MW of district cooling capacity to large buildings.
Conversely, the combustion of natural gas upstream of the power turbines could produce a hot exhaust stream that is well above the atmospheric boiling point of water. That thermal energy may operate a thermal-desalination plant, a district heating system a water-based, vacuum refrigeration-cooling system or preheat water for the thermal power station. The escalating demand for potable water will eventually include using seawater as a heat sink to condense potable water from the exhaust combustion gases.
Regenerators and Reheating:
The layout of a CAES installation replaces the common drive shaft between turbo-compressors and power turbines with separate drive shafts where a motor drives the turbo-compressor and the power turbine drives electrical generation equipment. It allows for easy installation of a regenerative heat exchanger that transfer heat from the low-pressure superheated exhaust stream to the cooler incoming high-pressure air stream. The layout also allows for a very large, high-temperature thermal storage system to be installed between the regenerative heat exchanger and the power turbines.
The exhaust stream that flows from the regenerative heat exchanger would still contain enough heat to sustain thermal desalination, district heating or steam vacuum district cooling. A natural gas fired combustion system that superheats air to over 1800 F may also be located between the regenerative heat exchanger and the power turbines. Future power output levels for natural gas fired large CAES systems could exceed 5,000MWe for up to 8-hours duration. If the storage chamber operates between 1500-psia and 1450-psia, power output would drop to 2470Mwe or 20,000Mwe-hrs.
Proven turbines for high-output applications already exist, courtesy of Siemens that makes the 300MW Siemens SGT 300. At 50% thermal efficiency, the compressor consumes 300MW while the turbine produces 600MW. A CAES installation of 10 x Siemens power turbines would have a maximum output capability of 6000MW. By comparison, hydroelectric power dams of equivalent power output use an equal number of hydraulic turbines and often more. A salt dome of 5000-feet in diameter by 25,000-feet in vertical height could generate over 50,000MWe-hrs of output with unheated air flowing through turbines as dome pressure drops from 1500psia to 1450psia. Power output could exceed 200,000MWe-hrs with superheated air activating the turbines.
A CAES installation may operate as a 2-stage turbine system with an inter-turbine reheating between the high-pressure and low-pressure turbines. Computer monitored and control would measure the oxygen content of the high-pressure exhaust and regulate flow rate of natural gas into both high-pressure and low-pressure combustion chambers. The layout of CAES systems allows multi-stage compressors to operate on a single drive shaft or operate independently of each other on separate drive shafts. Likewise, the 2-stage power turbine system may operate either on a single shaft driving a single alternator or on separate power shafts driving separate alternators.
Increasing Productivity of Inland CAES:
Salt domes and salt cavities can occur almost anywhere in the earth's crust, including under numerous inland locations, as is the case in Germany. Flushing salt from earth cavities can be problematic where water is scarce. The giant dry cooling technology developed in South Africa to condense steam at mega coal-fired power stations of over 4000Mwe may offer potential to develop CAES at inland locations in arid climates. To achieve such operation, a large steam-based power station would need to be located near a source of make-up water and right above a known salt dome. Such locations may be few and far between and would present a challenge to geologists and power station planners.
Exhaust heat from nearby steam-based thermal power stations plus the overnight condensing capability of giant dry cooling technology may be used to help flush salt from large salt domes. The dry cooling system may continually evaporate and condense a minimal amount of water in a repeating cycle as its operation can assists in flushing rock salt from the salt dome. That rock salt may be sold to salt markets or used as a solar-thermal storage medium in coastal salt ponds.
During the overnight off-peak hours the CAES system would undergo its recharging cycle. The heat of compression from its turbo-compressors may be transferred into the thermal power station to preheat pressurized water prior to its conversion into steam. The heat of compression from the turbo-compressors may be alternatively be transferred into the thermal storage medium of a solar-steam-electric power station, or sustain the overnight operation of a solar tower that could also serve as a dry cooler for a steam-based thermal power station. Such combined operation would require that all these technologies be located in close proximity to each other.
Most CAES installations are essentially extensions of natural gas fired gas turbine power plants. The CAES component offers opportunity to increase the overall efficiency of the power system. Direct drive between steam turbines and air compressors, wind turbines and air compressors will raise overall pumping efficiency between the energy source and the storage chamber. The overall efficiency from steam turbine, through CAES to electric power generation from the gas turbine will rise by some 12%.
Making productive use of the exhaust heat to preheat water to generate power, desalinate seawater or provide district heating could further raise the overall equivalent efficiency in terms of consumption of natural gas. The stand-alone CAES energy storage systems of today would likely evolve into combined-cycle energy systems in the future. Future designs of thermal power plants may likely be integrated into large-scale CAES installations involving 2-salt domes that are in close proximity; one dome will store compressed air while the other dome will store compressed natural gas.