On March 21, 2003, the second Gulf War began with an amphibious assault on the Iraqi city of Umm Qasr, a small strategically important port on the Kuwaiti border. The battle lasted only a few days and was followed by a massive effort to remove underwater mines and clear a lane for delivery of ships to deliver humanitarian supplies.
Unmanned undersea vehicles (UUVs), or submarine drones, were instrumental in the demining effort, which is one of several reasons the U.S. Navy is funding research on advanced UUVs and subsea power systems to support them for long periods of time.
The “Persistent Renewable Energy for Undersea Systems (PREUS)” program is one of the more intriguing concepts begin considered by the Office of Naval Research. The PREUS program is developing generators that will be able to convert heat from geothermal vents on the ocean floor into electric power.
The objective is to extend the service life of UUVs by resupplying their energy in-situ. In other words, UUVs may soon be doing a lot more than detecting underwater mines.
The U.S. Navy’s key transformation plan, called Sea Power 21, suggests that advanced UUVs will not only shape the future battlespace but will enhance the entire “kill chain” from surveillance to time-critical strikes.
Subsea generators are considered to be critical enablers for achieving this ambitious vision. The preferred fuel is thermal energy from hydrothermal vents.
A hydrothermal vent is a fissure in the planet’s surface that emits huge amounts of heat. They are commonly found on ocean ridges near volcanically active places where localized temperatures regularly reach as high as 600 Fahrenheit. These high temperatures make the vents compelling candidates for geothermal energy.
There are 40,000 miles of ocean ridges that are constantly recharging their thermal activity by the uprising of magma, according to some studies. They represent 30% of all heat released by the earth. So far about 20% of the world’s ocean ridges have been studied. These studies have identified nearly 300 hydrothermal vents located at depths of 6,500 to 8,000 feet.
“The main conclusion is that the available (supposedly reasonable) geothermal power that could be generated from well known hydrothermal vent sites in some of the ocean ridges is of the same order of magnitude of the EGS geothermal power that eventually can be produced in the whole world,” according to a study by researchers from the Universidad Nacional Autonoma de Mexico.
The study evaluated the technical feasibility of tapping this energy source by siting submarine generators above the vent to run an organic Rankine cycle thermal plant. The economic and environmental barriers are huge, but so is the military’s budget.