On the revolutionary end of the spectrum of technologies for extracting heat from the ground are closed-loop geothermal systems, also called Advanced Geothermal Systems or AGS.
In AGS instead of drilling for water, or creating an artificial reservoir through stimulation, which is the case for EGS (Enhanced/Engineered Geothermal Systems), either a single circular loop, or multiple interconnected loops, collects the heat beneath the surface at depth.
A prominent proponent of this technology and perhaps the most important player in its development, from conception to testing to market opportunities and applications is the Canadian company Eavor Technologies. It is worth noting that this technology is in principle new and that to this date, its main technical aspects have proven its feasibility, thanks to the Eavor-Lite pilot project in Alberta – Canada which was built and commissioned in early 2019. The Eavor-Lite pilot project currently serves as a technology demonstration facility.
At the time of publishing this article, December 2021, a pipeline of Eavor-Loop™ projects in various locations worldwide are under evaluation, as communicated by Wouter Kool, Eavor’s Business Developer in Europe, in a Zoom interview carried out with me on December 22nd. In the same way, Eavor has structured a timeline of intended milestones for the next few years, from R&D to Field Testing to Commercialization and Operating Businesses for their various Eavor-Loop™ designs, and with plans for building their first power production facility in Gerestried – Germany, which in turn would become the first AGS heat to power plant in the world.
1. How does the AGS technology work? The basic concept:
A closed-loop design is different from conventional and enhanced methods in that a primary working fluid flows within a system of loop or loops that are essentially closed to the subsurface (closed buried-pipe system). In this method, a working fluid travels from the surface downwards through a wellbore until it reaches the bottom of the pipe system, absorbs heat from a hot-rock formation, and then the heated fluid returns to the surface through a second wellbore in an upward segment. A good analogy to this process is a vehicle radiator in the sense that much like a massive subsurface radiator, a Closed Loop system simply collects energy from the natural heat gradient of the Earth via a highly efficient conductive system.
It is worth noting that thanks to the thermosiphon effect as a product of the temperature differences at depth and at surface, the system does not require external pumping as it is demonstrated in the Eavor-Lite pilot, in which the working fluid is in circulation without pump support. Additionally, the working fluid does not leak into surrounding permeable formation as these are sealed with a “Rock-Pipe” sealant (Eavor’s proprietary product), and thus becoming impermeable.
The heat on the surface can be utilized for heating or power generation applications.
In the Power Generation case, at surface the heat is transferred to a secondary working fluid with a low boiling temperature and thus this fluid by acquiring the heat is capable of producing steam which in turn propels a turbine (or a set of turbines) and thus converting heat into electricity. The interaction of these two working fluids constitutes what is essentially a binary system and this form of converting heat to power corresponds to an organic Rankine cycle engine.
The primary working fluid then continues its circular course back underground and the cycle repeats itself over and over.
Fig. 1: Eavor-Loop™ – Heat to Power Overview. Source: Eavor
2. Technological Considerations:
When it comes to the technical viability of the technology, a major accomplishment of the Eavor-Lite pilot project was to prove the thermodynamic performance of the Eavor-Loop™, that means, the demonstration that the subsurface heat transfer and thermosiphon worked as expected. Thus, some of the most relevant technological considerations which are somehow fundamental to the Eavor-Loop™ are:
Thermosiphon Effect
An important feature which is also a big advantage is that the primary working fluid naturally circulates without requiring an external pump due to the thermosiphon effect. The Thermosiphon Effect consists on the working fluid becoming hotter as it collects heat during its passage in the loops underground, it also becomes lighter, which by density differential, produce it to rise in the outlet well to the surface; whilst at the same, the colder and heavier fluid from the surface flows down in the inlet well, thus creating a natural flow cycle.
When it comes to the technical viability of the technology, a major accomplishment of the Eavor-Lite was to prove the thermodynamic performance of the Eavor-Loop™, that means, the demonstration that the subsurface heat transfer and thermosiphon worked as expected.
Water is heated in the Earth, collecting the heat during the flow of the flow of the horizontal loops. The hot fluid has a lower temperature and rises up to the surface. At the same time, colder and heavier water flows into the loop on the cold end. This creates a natural flow cycle.
Fig.2: Thermosiphon diagram. Source: Eavor
Impermeability of the closed loops
The vertical part of the loop is isolated from the surrounding formation and groundwater with installed metal casing and concrete in the annulus, much like in O&G and conventional geothermal wells. For the lateral loops Eavor uses its own patented “Rock-Pipe” fluid, which isolated the bored holes from the formation with an environmental neutral chemical process.
Geological/geophysical aspects that determine the amount of energy (whether thermal or power) that the Eavor-Loop™ can produce
- Temperature gradient in relation to depth
- Thermal conductivity of the rock formations, which in turn is determined by the density of the rock and mineral composition. Igneous crystalline-granitic rocks are best.
Other aspects that determine the amount of energy that the system can produce
- The closed loop system is designed for meeting specific heat-energy requirements. Because the system design is based on data that is well identified and known, the collection of heat energy and therefore the amount of energy (thermal or power) that the system can produce is highly predictable.
- Once the system is built, the control of the flow rate in the loops determines the heat offtake from the subsurface.
The Eavor-Loop™ itself
The proprietary designed Closed Loops by Eavor, are named as per the company, Eavor-Loops. Eavor describes the Eavor-Loop™, including its key elements as follow:
Fig. 3: Eavor-Loop™ definition and its Key Elements. Source: Eavor
The Eavor Loops have several patented designs and well geometry. From the most basic, the Eavor-Lite, which was designed and completed in order to test the technical viability of the technology; to more complex geometries which have been designed with the purpose of being capable of power generation (James Joyce 1.0); and moreover, to potentially becoming “massively scalable”, James Joyce 2.0.
Video: Eavor-Loop™ designs. Source: Eavor
Key features in the Eavor-Loop™ designs and systems
Figs. 4: Key features of Eavor-Loop™. Source: Eavor
3. Advantages of Closed Loop systems
When comparing Eavor-Loop™ to Traditional Geothermal, it boils down to disproportional advantages for the Eavor-Loop™, particularly when it comes to geographical and geological limitations, which are greatly suppressed with Eavor-Loop™; additionally, a significant advantage of the Eavor-Loop™ in comparison to Traditional Geothermal, is the ability of the Eavor-Loops to be scalable.
Fig. 5: Comparison between Traditional Geothermal and Eavor-Loop™. Source: Eavor
In fact, the Eavor-Loop™ technology, as a “technical proposition” –which at this stage is pending to be proved at a commercial scale-, meets all the most desirable attributes of any Energy Source, such as:
- It can produce continuous reliable 24/7 Baseload Power, which is a big advantage regarding intermittent sources of energy such as Solar and Wind.
- It is Scalable, its size and heat extraction depth can be designed at scale to meet specific energy requirements.
- Its energy output is highly Predictable, thanks to the fact that the system design is based on geological and geophysical data that is well identified and known, which in turn permits knowing the amount of heat energy and therefore the amount of energy (thermal or power) that the system would produce.
- It is Dispatchable “power on demand” and Flexible, It can be ramped up and down almost instantaneously to complement variable wind and solar energy. It does this by restricting or cutting off the flow of fluid. As the fluid remains trapped underground longer, it absorbs more and more heat. So, unlike with solar, ramping the plant down does not waste (curtail) the energy. The fluid simply charges up, like a battery, so that when it’s turned back on it produces at above nameplate capacity. This allows the plant to “shape” its output to match almost any demand curve.
- Unlike Traditional geothermal, it does not pose exploratory risks. Unlike oil and gas exploration, the geothermal resource, which for the case is hot-rock formations, will always be found deep underground.
Fig. 6: Eavor-Loop™ advantages. Source: Eavor
Environmental advantages:
- It is Clean, as it does not contaminate nor produce GHG emissions, since the circulating working fluid is isolated from the environment in the closed, impermeable loops.
- Mines heat directly at scale via conduction rather than using formation water from an aquifer, also because of this, it does not need to be treated for solids and minerals which could be the case when working with brines (mineral rich formation water).
- Likewise, it does not require water to be pumped from elsewhere, for being injected into an artificial reservoir, which would be the case in EGS (Enhanced/Engineered Geothermal Systems).
- Also, unlike EGS, it does not pose the threat of earthquake risks, since it does not require hydraulic fracking.
- Additionally, because they are placed in highly stable formation, by deeply evaluating geophysical subsurface data, and since the wellbores and loop system are sealed and therefore isolated, and no drilling is planned into structural faulted or fractured zones, there is no flow into the formation that might cause any instability. That also further eliminates the risk of induced seismicity.
- Typically a Closed-Loop plant in operation would have a smaller environmental footprint than other sources of renewable energy such as Wind or Solar.
4. Challenges for the implementation of AGS
- Unproven at a commercial scale. To grow as a solution, the Eavor-Loops must prove its economic viability.
- The Eavor-Loop™ also needs to overcome technical challenges if when scaling up to utility power production levels, as this will require to master steering horizontally deeper and hotter, in temperatures above 150°C, which at present remains difficult, with equipment prone to melting since these oil & gas technologies were not designed with that kind of high heat in mind. Also, as rock becomes harder, equipment must also be hardened to additional vibrations. And electronics need to be better insulated.
5. Final comments on AGS:
Proving Eavor’s closed-loop technological approach (or any other AGS proponent for that matter) at a commercial scale and more importantly at a cost that makes economic sense for its implementation has the potential for being truly disruptive and transformational.
Economies of scale that come with growth as more and more AGS projects would be built and become operational will see its costs greatly reduced as it has always been the case with any new technology. In O&G we know this well from two non-conventional technical approaches for harvesting oil, which actually have a lot in common with AGS, these being Shale Oil and Oil Sands.
Once all the technical and economic requirements that condition the commercial viability of AGS be achieved, its potential impact would be of global significance, moreover in the current context in which the world urges the decarbonization of its energy systems and transition to clean electricity.
Sources:
- http:eavor.com/technology/
- http:eavor.com/mediakit/
- How Eavor Works (Full): https://www.youtube.com/watch?v=8erbvqFZ9M8&t=188s
- Geothermal: Innovation & Opportunity in Alberta, COAA Best Practices Conference by Chris Cheng: https://www.youtube.com/watch?v=nBj1GCb70A0
- Zoom-Interview with Wouter Kool, Business Development Europe, December 22nd. 2021
Disclaimer: The AGS concepts and information provided in this article, including those of Eavor’s technology, comprise sources of public domain, as well as my own insights and perspectives.
Acknowledgements to Eavor’s Paul Cairns for approving the use of Eavor’s material utilized for this article.
