Monday, 23 July 2018

Far below the earth’s surface, hot rocks combine with water to create clean power  

Solar. Wind. Water. Nuclear. These are just a few forms of clean, renewable energy that the U.S. Department of Energy (DoE) is investing in to diversify the country’s energy portfolio. Add to that list a newer initiative to harness heat energy from the earth: geothermal energy. Sheng Dai, assistant professor in Georgia Tech’s School of Civil and Environmental Engineering, is working closely with the National Science Foundation (NSF) and DoE to drive the geothermal energy initiative forward.

As a geotechnical engineer, Dai’s research focuses on shallow geothermal energy for efficient heating and cooling of buildings, as well as deep geothermal energy from about three kilometers below the earth’s surface to generate clean, renewable energy on a larger scale.

“The rock temperature is very high at three to five kilometers below the earth’s surface – some may be above 300 degrees Celsius,” said Dai. “First, you drill down and fracture the rock bed, creating a permeable network. Then water, or any working fluid that produces vapor, flashes through the reservoir to create steam, which is used to generate electricity by powering a turbine.”

The water is procured from any nearby source, or it can be trucked in if the reservoir is in a desert region. This vital, clean energy resource supplies renewable power around the clock and emits little to no greenhouse gases -- all while requiring a small environmental footprint to develop.

NSF and DoE funds much of the research in Dai’s lab for geothermal energy discovery. It is in this lab that unprecedented tools exist to test geomaterials under elevated temperature and pressure conditions. The instruments in the lab are proprietary to Georgia Tech; they were built here and no other testing sites across the country have access to this type of technology. Much of Dai’s PhD work was spent building geomaterials characterization tools.

Harnessing the power of geothermal energy is one of the biggest issues that Dai is working to overcome. Deep geothermal energy has yet to be commercialized, and DoE is pushing hard to make that happen. A testing site has been selected in Utah for drilling to take place. The subsurface materials in that area of the country are untested, and Dai’s group will have to conduct material characterization in the lab to study the test site and its viability for producing geothermal energy.

“We are pretty far from commercializing geothermal energy,” said Dai. “There are still a lot of unknowns – we don’t have data on how geomaterials will behave three to five kilometers down. We also have to consider the long-term effects of running water into the ground and how the rocks will respond.”

Dai sees deep geothermal energy as an extremely promising energy alternative, and his lab hopes to work with DoE on its Utah testing site.

“I suspect that deep geothermal energy is going to make a big impact on the state of clean energy,” said Dai. “Long term, it’s more environmentally friendly and efficient, plus it doesn’t rely on other environmental factors. For instance, with solar energy, you’ll have low energy output if its overcast or dusty.”

Dai recognizes that renewable energy is one of the top challenges facing humanity, not just for the environment, but also for socioeconomic development.

“Fluctuations in energy supply greatly impact economic development,” said Dai. “When there is an energy shortage, cities must power down at certain times, restaurants and other areas of commerce must close. Newly developing countries will need more energy as the population grows and living conditions will be constrained if we don’t have enough energy production.”

The current global energy consumption rate is about 19 terawatts, and in next 20 years, an energy gap of 4.3 terawatt needs to be filled, that is about 10,000 times the power output of Hoover Dam. Solar and wind energy produce just a fraction of the needed energy. For the world to catch up with the energy needs, thousands of renewable energy plants will need to be built.  And Dai worries that infrastructure won't be able to keep pace.

“The magnitude of energy needed to power our daily lives is tremendous,” said Dai. “And there is no one energy source that can solve this problem. If we don’t look to other sources of energy, we won’t be able to keep pace with global energy needs.”