Compressed air sustained pumped hydraulic storage

Posted on March 15, 2016
Posted By: Harry Valentine
 
The development of deep water compressed air energy storage (DW CAES) involves placing weight ballasted inflatable bags on the seafloor or submerged on a lake bed, with an air pipe connecting between ground level and submerged bags. It is a variation of water-displacement compressed air storage (WD CAES) that can deliver near constant air pressure throughout the power generation cycle. On a small scale, the line of submerged compressed air can be connected to a large underground water tank capable of holding substantial pressure. The compressed air can raise the equivalent `head' of the water.

Fluid Density and Mass:

While 100,000-cubic feet of compressed air at 100-psia pressure at 40-deg F would offer a density of 0.53-lb-per-cubic-foot, the total mass available to flow through an engine would be 53,000-lb of air. If compressed air at 100-psia were exerted on 100,000-cubic feet of water, a mass of 6,340,000-lb of water with an equivalent `head' of up to 230-ft could be available to flow through hydraulic turbines. A suitable cavern to which sealant may be applied to its interior surface inside a coastal mountain could offer the necessary volumetric storage capacity and hold the required air pressure above the water.

Compressed Air over Water:

A dome shaped cavern with 150-feet thickness of granite rock with specific gravity of 2.25 above it could contain an upward push of 14,400-lb-per-square foot, caused by 100-psia on internal air pressure. An internal cavern pressure of 1000-psia would require 1500-ft of granite rock above and around the cavern. The submerged bags of compressed air would be secured at a water depth of 2200 to 2500-ft below water surface to offer such pressure inside a cavern located above water level. A small volume of water under extreme pressure could sustain the operation of a venturi water pump.

Venturi Pumps:

Venturi water pumps involve an absence of moving parts and allow a small jet of water to pump a much larger volume to higher elevation. Such long-proven technology could greatly increase the output of a low-head pumped hydraulic storage installation, by producing a much greater equivalent `head'. It could also produce the equivalent of `head' in a river, lake or bay where a venturi pump could use a small volume of water under extreme pressure to push a large volume flow rate of water into a raised runner that leads to a low-head turbine.

While submerged kinetic turbines deliver 30% to 40% efficiency, some low-head turbines can deliver over 70% efficiency over a hydraulic head of 0.75-metres. The pressure behind the small volume of contained water would determine the height to which the small high-speed jet of water in a venturi could push a large volume of water at sufficient volume flow rate to sustain efficient turbine operation. Such a system would place turbines and electrical equipment above water, either along the coast or on a floating platform where equipment would be readily and easily accessible to maintenance personnel.

Heat of (Air) Compression:

The water-displacement, compressed air sustained pumped hydraulic storage could incur minimal loss of compressed air, in turn reducing the mass of `make-up' compressed air that would have to periodically be pumped into the system to assure sustained optimal operation. Reducing the volume mass of air to be compressed also reduces the amount of heat generated through compression. The compressed air that exerts pressure on an enclosed volume of water merely transfers equivalent hydraulic head from a lower elevation to a higher elevation. It is a terrain/geology/geography dependent energy storage system that could be developed at many locations internationally.

Terrain Requirements:

A high coastal mountain located next to a deep body of water would form the basis of a water-displaced-compressed air powered pumped hydraulic energy storage installation. The mountain would contain large dome caverns capable of sustaining high internal pressure, with dome base elevation close to water level of body of water. Air bags with ballast would be submerged to a depth of 200 to 600-metres, with an air pipe connecting between air bags and suitable dome caverns that may be filled to 90% capacity with water. The water under pressure could sustain venturi pumps that supply low-head turbines.

There is suitable terrain along the Pacific coast of South America, Central America and also Western Canada, though caverns may need to be developed in the coastal mountains. Suitable terrain occurs at the inlets and fiords of Norway, also around 2-African lakes, Lake Tanzania and Lake Malawi. Nations such as Zambia and Tanzania seek to increase future electrical generating capacity and energy storage installations would allow them to purchase off-peak electrical power from a nation such as South Africa, providing the energy to customers during their peak periods given that South Africa's generating capacity greatly exceeds their storage capacity.

Conclusions:

The technology to develop compressed air over pumped hydraulic energy storage is well proven over a period of many years. It can convert a small mass flow rate of working fluid that could drive a pneumatic engine to a greatly increased mass flow rate of working fluid that could drive low-head hydraulic turbines that could deliver power at competitive levels of efficiency. The system promises to minimize usage of air and the related heat lost by otherwise compressing large volumes of air.

 
 
Authored By:
Harry Valentine holds a degree in engineering and has a backround in free-market economics. He has undertaken extensive research into the field of transportation energy over a period of 20-years and has published numerous technical articles on the subject. His economics commentaries have included several articles on issues that pertain to electric power generation. He lives in Canada and can be reached by e-mail at harryc@ontarioeast.net .
 

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Comments

March, 15 2016

Richard Vesel says

First, I really like this concept from an engineering standpoint. I have seen it before, and would like to see it developed. "Constant pressure" is the holy grail of CAES.

Second, it seems to me that an ideal place to put such a station is in concert with a greenfield high-head hydroelectric project. Placing the bag storage system in the base of the to-be-flooded valley, when construction can still be performed in an open air environment should reduce construction costs, and allow easy pretesting of the storage containers under low-to-moderate pressures, to insure they are installed properly and of sound integity. Then, when the valley is flooded, and the CAES bags are under 100m or more of water, the system is ready to use.

Such an installation would increase the peaking capacity of the hydroelectric station without demanding higher water flow rates during peak periods, or would add the CAES generation capacity to the rated capacity of the hydro-tubine-generators.

RWV

March, 28 2016

Harry Valentine says

The present generation of gas turbines are designed for a maximum pressure ratio of 36:1, or 540-psia at sea level that corresponds to a 'head' of 1200-ft. Some hydraulic turbines (Voith) can operate with 1500-psia pressure upstream, allowing future installation of submerged air pressure chambers at the equivalent of over 2,000-ft below a reservoir of water. These pressure chambers may be excavated from salt caverns located deep in the earth's otherwise impervious bedrock. Natural gas is pumped to 2,000-psia and higher in such chambers. Using water to displace compressed air from such provides little variation in air pressure.

Using compressed air to exert pressure on water stored inside a cavern excavated into the base of a mountain allow for efficient hydroelectric power generation in the absence of natural gas to preheat the air prior to expansion in the turbine from a gas turbine engine. By restricting release of air from storage to the atmosphere, heat of compression is greatly reduced during system recharging when water displaces air from near sea level . . . and highly compressed air displaces water from a deep level storage chamber to a reservoir at higher elevation.

March, 29 2016

Harry Valentine says

Gas turbine engines achieve peak efficiency when rotating at maximum RPM with maximum possible turbine inlet temperature. In a carbon-free environment, cold air passing through a turbine would deliver comparatively low efficiency . . . . . . using compressed air over water in a cavern would deliver much greater efficiency

March, 30 2016

Harry Valentine says

The highest pressure ratio for a gas turbine engine is presently around 55:1 (800-psia or 5.5 MPa at sea level). In hydraulic terms using seawater, a 'head' of 1800-ft, except that Hydro Voith has developed Pelton Wheel technology capable of operating over a 'head' of up to 1800-metres or 6,000-ft. Air chambers secured at a depth of 6,000-ft under seawater could deliver an air pressure of over 2,500-psia . . . . . . natural gas is stored in deep level underground chambers pressurized to 2,000-psia and higher.

Water displaced compressed air from airbags under the sea or from deep level caverns in the earth's bedrock could deliver air pressure of over 2,000-psia to a series of caverns excavated at sea level, under a mountain of at least 3,000-ft elevation. Sealant applied to cavern walls should contain 2,000-psia air pressure exerted on water stored inside the caverns. A pair of caverns would allow water from one cavern to pass through turbines to generate power while water is simultaneously pumped into an empty, unpressurized companion cavern. Once near capacity, air pressure would exert force on the stored water as it begins to flow through the turbines to generate power.

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