Underground Heat, Urban Cool: The Physics & Promise of Geothermal Cooling – CleanTechnica

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Cooling in the Persian Gulf is one of the hardest energy challenges anywhere on the planet. Air conditioning is not a luxury in the United Arab Emirates but a necessity, and it consumes as much as 70% of the country’s electricity. That reality has made Masdar City, the experimental urban district on the edge of Abu Dhabi, a proving ground for ideas that can reduce the strain of cooling while cutting emissions. ADNOC and Tabreed’s decision to build the first geothermal cooling plant in the Gulf fits neatly into that story, and it also ties into the larger global discussion about geothermal energy.

After publication of my assembled report on geothermal’s hype and value areas, a developer looking at geothermal opportunities in southeast Asia reached out to me to point out something I’d missed. I’ve added this as another chapter in my geothermal report now.

The UAE project, known as G2COOL, does not generate electricity. Instead, it produces chilled water for district cooling by drawing moderate temperature water from an underground aquifer, which took me a while to understand since it seemed so counter intuitive. The wells tap water in the 80° to 100°C range, hot enough to drive an absorption chiller but not nearly hot enough for a conventional steam turbine. This is geothermal heat used directly for thermal purposes, which in many cases is a better fit than forcing it into electricity generation. The chilled water produced at G2COOL already covers about 10% of Masdar City’s cooling needs.

To understand how hot water makes cold water, you have to follow the absorption cooling cycle. In this system the refrigerant is water and the absorbent is lithium bromide, a salt that strongly attracts water vapor. Hot geothermal water flows through the generator, where it heats a solution of lithium bromide and water. The heat drives water vapor out of the solution, leaving behind a more concentrated salt mixture. That vapor then passes into the condenser, where it releases heat to a cooling loop and becomes liquid water. The liquid is throttled down in pressure and enters the evaporator, where it boils at low pressure at about 5°C. When it boils it absorbs heat from another loop of water that is circulating through buildings. That chilled loop is what Masdar City uses to provide air conditioning.

The cycle does not stop there. The water vapor from the evaporator enters the absorber, where the concentrated lithium bromide solution soaks it up, releasing heat again to the cooling loop. The absorber heat and the condenser heat are both waste streams, and they are dumped into cooling towers. A small pump sends the weaker solution back toward the generator through a heat exchanger that improves efficiency by transferring energy between the strong and weak solutions. The process repeats continuously. The coefficient of performance for a single effect lithium bromide absorption system is usually between 0.6 and 0.8. That looks low compared to an electric compressor chiller with a COP of 3 or more, but in this case the geothermal heat is free and the electricity saved is valuable.

The underground source in this case is an aquifer, not a volcanic steam field. That means there is a different set of sustainability questions. If hot water is pulled out and dumped at the surface, the aquifer will cool and pressure will decline over time. The standard practice in responsible geothermal development is reinjection. After the water has given up its heat in the generator it is reinjected into the ground through another well, typically at some distance from the production well to allow time for reheating. The earth itself provides the recharge, and the system can run for decades if managed well. There is no reason to add heat back to the fluid before reinjection. The waste heat from the condenser and absorber is at about 30 to 40 °C, much lower than the geothermal water coming out at 100 °C. Trying to raise the reinjection temperature with this low grade heat would waste energy and reduce efficiency. Reinjection at a lower temperature maintains the gradient underground, which is what allows the reservoir to warm the fluid again.

This is a clear example of matching the right resource to the right need. The Gulf has immense cooling demand and a growing focus on lowering the carbon intensity of its energy mix. District cooling is already about 50% more efficient than building-level air conditioning, and coupling it with geothermal reduces grid electricity use even further. Every megawatt-hour of electricity avoided in the Emirates translates into less natural gas burned in turbines. In a country where per capita emissions are among the highest in the world, cutting cooling demand at the source is a rational strategy.

The ADNOC and Tabreed plant is also a reminder of where geothermal makes the most sense today. For all the attention given to enhanced geothermal stimulation, very large closed loop generation concepts and ultra deep drilling, the most bankable geothermal projects remain the ones that deliver heat directly to applications that need it. In China geothermal is being scaled up for district heating, replacing coal and gas in northern cities. In Europe geothermal has been used for decades in hot water networks. The Salton Sea in California is being looked at for combined power and lithium extraction, but even there the economics are challenging. The Masdar project belongs in the category of low to medium enthalpy heat being applied directly, and that is where the physics and economics line up most clearly.

There are still limits and risks. Absorption chillers only provide water at about 4 to 7 °C, which is fine for building air conditioning but not for deep refrigeration. Lithium bromide is corrosive, which means materials must be selected carefully and corrosion inhibitors added. If the solution becomes too concentrated at low temperature, lithium bromide can crystallize, shutting down the system. Reservoir chemistry and scaling can create issues for wells. All of these factors add cost and complexity. Yet they are known challenges with known solutions, not unknown unknowns.

Looking ahead, the G2COOL plant will not transform the energy landscape of the UAE on its own. Covering 10% of the cooling demand of one district is a small fraction of the national picture. But it proves that geothermal heat in the Gulf can be harnessed for a critical service. ADNOC’s $15 billion commitment to low carbon projects means this could be the first of several geothermal pilots. If reinjection and reservoir management are handled correctly, this kind of system could supply a steady slice of cooling for decades without drawing power from the grid.

The lesson is that geothermal is not a one size fits all solution. It is not destined to provide baseload electricity everywhere, nor is it a dead end. It has niches where it is both effective and economical. Cooling in hot climates is one of them. Using underground heat to take the edge off peak demand makes sense, and that is what this plant demonstrates. In the larger story of the energy transition, geothermal should be seen as a supporting actor, not the lead. The ADNOC and Tabreed project shows how, when cast in the right role, it can deliver real benefits.