Geothermal energy could be used to make desalinated water without the carbon cost
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Scientists at Florida Gulf Coast University have proposed a carbon-free way to desalinate water using geothermal energy.
With water shortages increasing and rising global temperatures exacerbating drought conditions, countries such as Saudi Arabia are turning to desalination plants to provide drinking water to their residents.
But the method requires a high amount of energy, which can limit the use of the process in many parts of the world.
“We’re searching the world right now for better methods, low carbon methods, to create energy,” said Thomas Missimer from Florida Gulf Coast University. “We want electricity that’s generated at base load — in other words, generated 24 hours a day without interruption. Geothermal is one of those types of energies.”
Geothermal energy generation comes in two forms, wet rock and dry rock. Wet rock is uncommon and is used in places like Iceland. It takes hot water from deep in the earth to provide the energy to move turbines and produce energy.
Dry rock geothermal potential is more wide-spread globally. Missimer suggests using these areas—regions like southern California, North Africa, and the Red Sea region — to heat water that can be used to produce energy.
Dry rock geothermal methods are currently being used in parts of the world for energy production, but Missimer says that the heat can be used in more efficient ways, especially with desalination.
Usually, geothermally heated water from the ground is converted to steam, that steam powers a turbine to create electricity, then the heated water is vented to the atmosphere while it’s still hot — still over 100 degrees Celsius in most cases. Instead of venting, the new process uses that hot steam in the desalination processes.
The first desalination process is multiple effect distillation (MED), which requires hot water (above 100 degrees Celsius), but the second process, adsorption desalination (AD), can be run on cooler water. As the steam moves through the system and cools, it is still effective for powering desalination.
“Now you have an efficient system where you have conserved the latent heat that you’ve captured in the ground through three processes: turbine electricity generation, MED and AD,” Missimer said.
At the end of the desalination process, Missimer says that distilled water and chilled water (from the AD process) are the final products. While the distilled water can be consumed, even the chilled water is reused—the cool water can be recycled through the plant to help with air conditioning.
Lastly, the researchers propose to store excess water in aquifers for later use, which Missimer says is a future energy saver. “By storing some of that excess water, you can use that water seasonally when more water is required and sort of even out the changes in demand to the system.”
The benefits of a system like this are broad: there is no carbon dioxide being produced at the plant, as it is all self-contained and powered by geothermal energy, rather than fossil fuels. It also has an economic benefit.
“If you look at the benefit to the place like Saudi Arabia, saving 6 million barrels of oil a day, at $100 a barrel — that’s a bloody fortune,” Missimer added.
The process is more effective than a solar-powered plant he said, because it can be run 24 hours a day, compared to just daylight hours.
While the idea hasn’t been put into practice at this stage, the individual parts of the plant and the technology have been around for a while and the researchers are aiming to make a working system soon.
“We’ve learned that sometimes you don’t need new technology to make advances,” Missimer said. “Sometimes it’s piecing together old technology in a constructive way to provide more efficient operation.”
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