ORNL Analyzes Use of 3D Printing for Geothermal Energy Production

As global industrial society struggles to transition from fossil fuels to other forms of energy, one source of power that is often overlooked comes from within the Earth itself. The most widespread form of geothermal power plants are located where molten rock is near the planet’s crust and produce hot water, though access to this energy can also be achieved by digging deep crevices. Hot water or steam from these locations are then used to spin a turbine to generate electricity.

Now, the U.S. Department of Energy’s Oak Ridge National Laboratory (ORNL) has used 3D printing to produce specialized tools for geothermal energy development. The use of additive manufacturing (AM) has rendered the tools cheaper and more efficient, translating those benefits to geothermal energy production.

To access geothermal energy, specialized tools are needed to drill through the surface of the earth. The ORNL researchers noted that “[m]any geothermal technologies, such as downhole tools and drilling equipment, have unusual material, design, and manufacturing considerations dictated by the harsh geothermal environment and extreme aspect ratios required for deployment in a borehole.”

These items are also made in very low quantities, due to the niche nature of the geothermal sector. The ORNL team explained, “Whereas tens of thousands of oil & gas wells are drilled and completed in the U.S. annually, there are typically only tens of geothermal wells that are drilled and completed. If a tool typically used in oil & gas applications cannot be directly used for geothermal, then the cost associated with making the tool suitable for geothermal is often prohibitive.”

Because these complex tools are made in low quantities at high costs, they may be prime candidates for production with 3D printing. To determine whether or not a specific tool might be more expensive to make with conventional or additive technologies, ORNL developed a techno-economic (TEA) calculator. This was then applied to a variety of production processes for a representative geothermal downhole toolset made up of 17 different parts.

The ORNL team determined that all of the parts could be made by combining AM and conventional manufacturing processes, but few where only AM would have been beneficial. However, they also noted that, because these parts were designed for traditional production technologies, they would have naturally been seen as better suited for conventional processes. Therefore, they concluded that design for AM should be used to better take advantage of the benefits of 3D printing. Additionally, because of the far smaller portfolio of metal powders available for AM, material substitutions would have to be made if 3D printing these items.

A rotor for a vane motor in a drilling tool, redesigned using topology optimization to increase rotational speed by 27 percent and maximum speed of the motor assembly by five percent. Image courtesy of ORNL.

Also worth noting was the fact that, if just one part were to be made for a given geothermal job, directed energy deposition (DED) AM would be less expensive than conventional manufacturing for nine out of 17 components. This makes a viable case for using 3D printing to produce replacement parts.

For the production of a larger number of parts at once, laser powder bed fusion (PBF) was seen as less expensive for 13 out of 17 cases. Manufacturing time could be further reduced if more than one component was produced in a single build with PBF.

Additionally, the study only looked at the ways that AM could optimize drilling tools briefly, examining a single rotor for a vane motor in a drilling tool. The part was redesigned using topology optimization to increase rotational speed by 27 percent and maximum speed of the motor assembly by five percent.

This information was determined based on the results of the TEA calculator, but not the physical manufacturing of parts. It could, therefore, inform further studies in which parts are actually made. What the study didn’t discuss was the fact that, though geothermal energy is considered renewable, it can still potentially have negative environmental effects.

For instance, open-loop systems may result in the emission of hydrogen sulfide, most commonly, as well as carbon dioxide, ammonia, methane, and boron. Hydrogen sulfide can result in the production of sulfur dioxide, which can cause acid rain, but SO2 emissions from geothermal plants are still 30 times lower per megawatt-hour than those from coal plants.10 percent of emissions for open loop systems is made up of carbon dioxide, with an additional smaller amount of the more potent methane also being released, thus contributing to global warming.

Mercury can also be produced from geothermal plants and toxic byproducts must be disposed of at hazardous waste sites. Of course, like most energy plants, it’s necessary to protect the ecology of the surrounding area during land development. Geothermal plants may also increase the risk of earthquakes in the area. For enhanced geothermal systems, in which deeper wells are required, this risk is increased, similar to the risks found in fracking.

In total, enhanced geothermal systems produce life-cycle global warming emissions of about 0.2 pounds of carbon dioxide equivalent per kilowatt-hour. This compares to natural gas, which produces between 0.6 and 2 pounds, and coal, which produces between 1.4 and 3.6.

There are more sustainable forms of localized geothermal energy that ordinary consumers could begin considering. That is the replacement of heating and cooling in your home with a series of underground pipes. In the winter, water that has been heated below the Earth’s surface is moved into your home, while, in the summer, heat from your home is moved back into the ground. This small, but burgeoning technology is becoming increasingly available through companies like Dandelion, which serves the New York area. Whether or not 3D printing can assist in the creation of local geothermal energy options has not yet been explored.

How small-scale geothermal heating and cooling works. Image courtesy of Dandelion.

What surely will be explored further is how AM can be used to produce tools for large-scale geothermal projects. This is just the beginning for ORNL’s research into the use of 3D printing for geothermal tools.