Marshall hydrothermal recovery system
The Marshall Hydrothermal Recovery System is the first patented system designed to utilize underwater hydrothermal vents for energy, mining, and water desalination. It was designed by Bruce C. Marshall of California.
Hydrothermal vents typically spew out superheated fluid as high as 407 °C (765 °F) at velocities ranging from 1–5 m/s. The vent openings are anywhere from a few centimeters to several meters in diameter. The 1–5 m/s flow happens at depths of typically 2,300 m below sea level which is under pressure of more than 200 atmospheres. The vents are powered by the weight of seawater above forcing itself into fissures within the Earth's crust which is then returned to the ocean as a continuous, superheated geyser.
In one proposed embodiment of the system, the vent would be capped off and the hydrothermal fluid ducted to the sea surface through highly insulated pipes. A floating platform or ship on the surface would then extract the heat from the hydrothermal fluid and then use it for power generation.
In the closed loop system embodiment, a loop of insulated pipe would go from a floating platform or ship on the surface, down to the ocean floor, next to or in the hydrothermal vent, and would return to the ship or platform. A heat exchanger placed within the hydrothermal fluid flow would heat working fluid which would then be directed to the surface and used for power generation. The used working fluid would then be returned to the sea floor and be reheated.
A 3m opening and 3 m/s flow at 350 °C (662 °F) would create a flow of 21,205.73 L/sec, which is equivalent to 29,298.77 MW of raw energy, calculated as the reciprocal of the amount of energy needed to raise that volume of water to that temperature. In comparison, the largest fission nuclear reactor in the U.S., the Palo Verde Nuclear Generating Station, provides a maximum of less than 4,000 MW of power.
Computer modeling has shown the estimated producible energy density after generator losses to be about 1 MW/10 cm2 pipe area.
Hydrothermal energy is by far the densest and most highly concentrated natural source. Its producible energy content based on computer modeling is estimated to be about 3.3 x 106 more intense than raw solar radiation, and it is available 24 hours a day. By contrast, both wind and solar suffer from low density and intermittent operation.
Hydrothermal energy offers the potential of replacing existing power plants, something that other renewable sources can not promise.
When using the open-loop configuration, the Marshall Hydrothermal Recovery System proposes to capture the ores that are being ejected from the core of the Earth before they can settle to become hydrothermal veins. Hydrothermal veins (hydrothermal vents of the geologic past) are the natural source of virtually all surface mines in the world. The ores are known to be among the richest ever harvested, and because of the huge variety of metals and minerals in the fluid, mining promises to be as important as, or perhaps even more important than the energy that will be produced.
In the open-loop configuration, the Marshall Hydrothermal Recovery System allows the water component of the hydrothermal fluid to flash to steam, which can then be distilled and recovered as fresh water. The vast majority of the energy needed is provided by nature, but additional stages of purification may be needed.
The depth of the hydrothermal vents along the Juan de Fuca Ridge is about 1500m, well within reach of conventional oil rigs. But other technical challenges remain.
Subsea construction is always difficult, and those who would build the first Marshall Hydrothermal Recovery System will have to solve major primary engineering challenges on a scale of those faced when building the world’s first nuclear power plant, the Obninsk Nuclear Power Plant.
Materials and procedures must be utilized to deal with the highly acidic (and in some cases highly alkaline) vent fluid and extreme depth, and selecting the best method of exchanging the heat and the best working fluid will be an immediate concern.
Submarine power cable is costly and has never been laid at the depths in question. The main problem is not the depth, but the tensile strength of the cable. The weight of such a great length of cable is sufficient to tear it in half during the laying process if it is not properly designed.
The most economical means of recovering the ores from the mining products must also be determined.
Questions remain as to how much it would cost and whether it would be competitive with traditional methods of power generation such as nuclear.
The Peace River Nuclear Power Plant planned for Alberta, Canada is projected to cost about $2800 per kW, or a total of $6.2 billion for the 2,200 MW plant.
A semi-submersible oil platform for deep ocean use costs $500 million to $1 billion.
An undersea cable from Juan de Fuca Ridge would have to be 200 miles (320 km) long, which is around the same length of the longest submarine power cable. Unfortunately, transmission capacity for that cable is only 750 MW and it cost 550 million euros, about $800 million. In order to reach 2,200 MW equivalent capacity, 3 of these cables would be needed, for a cost of about $2.6 billion. The total so far is estimated at about $3.6 billion. The generators and control systems are the only parts of the two systems that would be roughly equivalent in cost, likely adding another $1 billion to the pricetag.
From these estimates, it appears that the cost of a hydrothermal generating plant would be about 28% less than the cost of a comparable nuclear plant, with no ongoing fuel or radioactive waste storage costs. There are many technological hurdles that need to be overcome, but even with the engineering challenges the system faces, its potential is great because of the extreme density and constancy of hydrothermal energy, its triple revenue streams of energy, mining products, and water, and the lack of external fuel needed for its operation.
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