SHNELL, Jim, et al. “Energy from Ocean Floor Geothermal Resources.” Energy 19 (2015): 25.
Clean energy and resource management are hot topics when it comes to power. Currently the majority of power is from fossil fuels, like coal and oil, which have controversial impacts on the environment and are only available as reservoirs; eventually they could run out. Nuclear power is another method for power, but waste disposal is always a lingering concern and research and development still have a lot of work. There is also energy from wind, water, and solar power, but these methods have yet to be economically feasible or practical on a grand scale; they are speculated not powerful enough to replace the global reliance on fossil fuels. Another method to derive energy from the environment is to use heat generated from the inside of the earth. Currently on-land holes called enhanced geothermal systems (EGS) have been drilled that reach geothermal water and steam, at temperatures as high as 250°C, to generate power. Higher temperatures could be achieved by drilling deeper holes; however, deeper holes cost money and the seismic consequences of drilling have high uncertainty. A natural exception to this conundrum is Iceland, a unique geological hotspot that sits on a spreading center. This means that shallower holes will reach higher temperatures, as high as 500 or 600°C. It is suggested by the Iceland Deep Drilling Project (IDDP) that the supercritical temperatures and pressures of deep wells may produce ten times the amount of energy per hole (5 MW versus 50 MW of electric power). The drilling concepts and findings of the IDDP are a model for the future of clean energy harvested from the ocean floor.
An estimated 100 million quads of energy could be harvested annually from geothermal resources, compared to the 472 quads consumed globally in 2006. However, current methods of tapping this resource are not sufficient to replace global needs. So why not move it to the ocean floor? It is known that there is abundant geothermal energy on rift zones, they are relatively uniform, and they are plentiful. The ocean crust is also considerably thinner at rift zones (5km) than continental crust (30-100km).
Self-sustaining, submersible, remotely controlled turbine generators on the ocean floor would convert energy to electricity at mid ocean rift zones, which reduces the energy loss from transferring heat to the surface. The generator would be placed at a single injection well connected to four production wells, each 500 lateral meters from the injection well and 1600 meters below the sea floor. Direct current electricity generated from the turbines would be transferred to the continents by high voltages direct current transmission lines. This practice is already used between Norway and the Netherlands, as well as Oregon-Washington to Los Angeles. Although, these amounts (3.1 gigawatts over 846 miles) are less ambitious than what new technology would strive to achieve (10 gigawatts over 2000 miles).
The depth of the rift zone is not an issue because gas and oil industries have already drilled holes as deep as 8000 meters into the crust. Water depth is not an issue either because the oil and gas industries can drill in water as deep as 2800 meters. A potential difficulty is however, that the rock type is basalt, a much harder material than the shale and other sedimentary deposits that house gas and oil. The deep water also has potential advantages, like high pressure, cool bottom temperature (> 4°C), and low dissolved oxygen levels compared to the surface. There is also the potential to harvest the mineral deposits that will inevitably form. Operations could be limited to producing energy only during off peak hours, and if CO2 turbines were used there is potential for 50% efficiency. Sea floor geothermal harvesting has the potential for six times as much power generation than current EGS methods. A summary of the economics can be found in Table 1.
Harvesting energy from the seafloor to power the future seems like a potentially great idea. If it can be pulled off it will likely prove to be a cost comparable, but clean alternative, to the burning of fossil fuels.
|Capacity per station||Production per well||Plant availablilty||Duration||Capitiol cost||Operating cost||Cost with transmission per kWH||Potential offset|
|100 MW||50 MW||90%||30 years||1.365 billion||215 million for 30 years (7.16 million per year)||$.073||Harvested mineral resultant mineral deposits|
Table 1: Summary of ocean floor geothermal resource economics
Hello, welcome to Oceanbites! My name is Annie, I’m a marine research scientist who has been lucky to have had many roles in my neophyte career, including graduate student, laboratory technician, research associate, and adjunct faculty. Research topics I’ve been involved with are paleoceanographic nutrient cycling, lake and marine geochemistry, biological oceanography, and exploration. My favorite job as a scientist is working in the laboratory and the field because I love interacting with my research! Some of my favorite field memories are diving 3000-m in ALVIN in 2014, getting to drive Jason while he was on the seafloor in 2017, and learning how to generate high resolution bathymetric maps during a hydrographic field course in 2019!