ERC researchers are working with their counterparts in the Mexican state of Michoacan to model and test the use of supercritical CO2 as a media to extract geothermal energy from aquifers.Joe GoughThe ERC has signed a contract with the University of Michoacan San Nicolas de Hidalgo to study and test methods of using CO₂ to enhance the extraction of geothermal energy from underground aquifers and rock formations. The three-year project is receiving $1.67 million in total funding from Mexico’s National Council for Science and Technology (CONACYT).

The project will deliver several benefits, says ERC director Carlos Romero, who is co-principal investigator with Edward K. Levy, former ERC director and professor emeritus of mechanical engineering and mechanics.

Because CO₂ is a greenhouse gas, scientists are trying to develop ways of sequestering it, or storing it permanently, in underground mines and rock formations and at the bottom of the ocean. Combining sequestration and the reuse of CO₂ would be a less expensive way of preventing it from entering the atmosphere after it is emitted from coal- or oil-fired power plants.

In addition, says Romero, the physical properties of CO₂ promise to enable it to harvest geothermal energy more efficiently than water, the conventional geothermal heat extraction medium.

A continuous source of clean energy

Geothermal power, Romero notes, does not attract as much attention from the media or from funding agencies as wind, solar and other sources of renewable energy. But it generates electricity continuously—not just when the wind blows or the sun shines. And unlike fossil fuel-fired power plants, geothermal power plants produce electricity cleanly, emitting significantly less CO₂ and no sulfur dioxide or other toxic pollutants, such as mercury.

Mexico, with 8,000 megawatts (MWe), has the world’s second-highest proven reserves of geothermal power after Indonesia. It is also the globe’s 12th-leading emitter of CO₂. The country has set targets to cut national emissions of CO₂ 30 percent by 2020 and 50 percent by 2050.

Meanwhile, Mexico is hoping in the next four years to double its reliance on renewable energy sources—from 17 percent of total national consumption this year to 33 percent by 2018. In its drive to go green, the country is placing big hopes on geothermal power. The country has built, or is in the process of building, eight conventional geothermal power plants.

The typical sources of geothermal energy are volcanic rock formations, deep saline aquifers (DSAs) and hot dry rock formations (HDR), which lie further beneath the earth’s surface. If the temperature in a water-dominated reservoir is high enough, engineers can harvest geothermal energy by drilling one well, a production well, to the reservoir. The pressure from the heat of the water, combined with the difference in pressures on the aquifer and on the surface, causes the water in the reservoir to rise to the geothermal plant, where it turns the turbines that power the generators to make electricity.

In its project in Michoacan, the ERC will perform simulations and experiments using supercritical CO₂ (which has been heated and pressurized) as a medium to extract heat from all three types of geothermal energy sources.

A critical advantage

Because of its lower viscosity and larger density differences at different temperatures, says Levy, supercritical CO₂ is more mobile than water and should therefore percolate more readily into a geothermal reservoir. Heated CO₂ will rise through the production well, creating several options for generating power. The hot gas can turn the turbines to power the generator or it can be diverted into a heat exchanger working with an organic fluid or other medium. The heat would convert this fluid to steam to turn the turbines and power the generator. The heat can also be used in the process to capture CO₂ at a fossil fuel-fired power plant, or for district heating.

The superior properties of supercritical CO₂, says Levy, also give it the potential to mine geothermal energy more efficiently from a reservoir that lies closer to the earth’s surface. Water temperature is typically lower in these aquifers but their easier access reduces drilling costs.

The University of Michoacan San Nicolas de Hidalgo (UMSNH), says Romero, has two experimental geothermal units capable of generating 300 megawatts of electricity. The ERC, in collaboration with UMSNH, will convert one of these units into a pilot plant utilizing supercritical CO₂ and an organic heat-exchange fluid. In the second phase of the project, the researchers will install and test a pilot plant at UMSNH and then deploy the system at a geothermal site.

The long-term goal is to construct a geothermal plant in Mexico near or adjacent to an oil- or coal-fired power plant, where there will be a readily accessible supply of CO₂.

As part of the project, two students, including one from Mexico, will enroll through the ERC in Lehigh’s mechanical engineering Ph.D. program, and one visiting scientist from Mexico will work at the ERC during the upcoming academic year.

In the past four decades, the ERC has gained a global reputation for developing innovative technologies that enable coal- and oil-fired power plants to generate electricity more cleanly and efficiently. ERC technologies have greatly reduced emissions of harmful pollutants from the stack, as well as CO₂, mercury and other toxic substances and CO₂, while optimizing combustion and improving the thermal efficiency of the units.

The collaboration with CONACYT is the fourth major project the ERC has conducted in the past six years in Mexico. ERC researchers have also completed projects in the U.S., Canada, China and Europe.