Abstract: Geothermal, originating from Greek words geo(Earth) and there(heat), is the heat derived within the sub-surface of the earth. Continuously replenished under the Earth, it is a form of renewable energy and adopting which could solve the problem of providing cheap, safe, clean and abundant energy that could sustain human civilization for millions, if not billions, of years.
Energy is the universal currency. From photosynthesis that releases oxygen to the rocket that reaches outer space, energy is necessary for getting anything, and everything, done. Human civilization has progressed since the enlightenment only because of its ability to access more energy.
As discussed earlier, solving energy helps tackle both climate change and poverty. Geothermal energy could be the answer.
For a detailed analysis on geothermal energy, read Eli Dorado's The state of next-generational geothermal energy.
The Earth formed about 4.5 billion years ago and through various stages of evolution, it became differentiated into layers: crust, mantle and core. It's all heat down there and about 4000 miles down, the molten core of the Earth is about 6000°C which is roughly as hot as the surface of the sun.
Geothermal energy is the harnessing of this abundant heat available by mining them deep from the Earth and converting them as direct heat, generating electricity, or even both. Eli Dourado points out that the estimates from Stanford’s Global Climate and Energy Project show that there is 23,800 times as much geothermal energy(15 Million Zetajoules) in Earth’s crust as there is chemical energy in fossil fuels(630 Zetajoules) everywhere on the planet. That's more than enough to support human civilization for millions of years on Earth.
To add to that, it is clean and renewable as the heat is continuously replenished by the decay of naturally occurring radioactive elements. And this process is expected to last for billions of years. The rate of replenishment occurs at about 30 Tera-watts. For comparison, the total human energy consumption is 17.7 Terra-watts annually. double all human energy consumption. That process is expected to continue for billions of years.
Any level of geothermal heat could be directly harnessed for a variety of applications such as to run greenhouses or to dry cement. At about 177C, it could be used for hydrogen production and anywhere between 120 to 400C could be used for electricity production.
Despite having been considered a promising source of energy since the mid 20th century, geothermal has started to gain strong investor as well as public interest due to two major reasons:
- the growing demand for renewable energy sources to fight climate change combined with the ambitious goals set by nation-states
- an array of advances in technologies either developed exclusively for geothermal systems or borrowed from the oil and gas industry
However, there are few challenges in overcoming the barriers to realizing the potential of geothermal energy:
- A major hurdle is that of public perception and reputation from the fact that technologies and methodologies used in the gas and oil industry are replicated in the geothermal industry. The classic example is that of fracking i.e injecting fluids underground to fracture rocks.
- The biggest challenge may well be the requirement for iterative improvements in technologies to mine heat from at deeper levels and drier regions than what is possible today.
A geothermal system comprises two separate wells: the production well and the injection well. The production well extracts steam( at least 150°C and up to 370°C) from the hot water reservoir below and directs it to the turbine which turns a shaft connected to a generator that converts the energy to electricity. The injection well collects the steam in the form of water converted by a condenser and sends it back down to the reservoir to maintain pressure and continue the cycle.
Conventional Geothermal Systems
There are select areas across the globe where natural water or steam trapped through permeable rocks rise after being heated by Earth's core and manifest themselves on the surface as hot springs or fumaroles. These reservoirs of pressurized hot water are categorized under hydrothermal or conventional geothermal systems.
To extract the heat from these reservoirs, exploratory wells are drilled until a suitable location for production well is chosen and heat is extracted followed by which the fluids are cooled and returned to the field via an injection well. There are two major challenges with scaling conventional geothermal systems:
- exploration and characterization of new fields are expensive and uncertain. There are technical advances being made to tackle this.
- they are extremely geographically concentrated and limited to specific regions like Iceland and California where heat, water and porosity are at the right levels.
Enhanced Geothermal Systems
While it is true that conventional geothermal systems need a minimum amount of porosity in rocks for the heat to be harnessed, there is still an enormous amount of heat stored in nonporous rocks. Enhanced Geothermal Systems is about building reservoirs by drilling down into these non-porous rocks through fracking via the production well and then collecting the heated water through the injection well.
Enhanced geothermal systems, as the name suggests, tend to start with conventional geothermal systems and then branch into rocks near the existing reservoirs before targeting hotter rocks. And given that such nonporous rocks with enormous heat potential are present everywhere across the globe, EGS systems could be ubiquitous.
There are a couple of challenges besides technical advances and cost reductions that are required to make EGS systems a full-fledged reality:
- Although the supply is ubiquitous, the variables such as temperature, depth, well as well as reservoir permeability make it difficult to build standard EGS systems
- As Eli notes, "The rock between the injection and production wells needs to be permeable so that the water can flow through it and acquire heat energy. The rock above that layer needs to be impermeable so that steam doesn’t escape to the surface except through the production wells."
Advanced EGS/Closed Loop Geothermal Systems
Closed-Loop Geothermal System is a system in which no fluids are introduced to or extracted from the Earth and there is no need for fracking. They operate using a series of pipes in a loop formation through which fluids circulate. During the heating process, the fluid absorbs heat from the soil/water and carries it to the surface, where it is released for heating/repurposed into electricity. During the cooling process, the fluid captures heat from the building and releases it to the ground.
Such a system, where cool water sinks on one side and hot water rises on the other, does not require a pump as the loop is closed. A closed-loop system can offer highly efficient heating and cooling systems at a much more competitive price than a fuel-based or electrical system.
Eavor is a startup that is developing closed-loop geothermal systems. In addition to providing baseload power, the startup's loop solution can also complement wind or solar energy which are intermittent and variable by acting as a battery. The loop system can control the flow of fluid, ramping it up or down whenever required.
While the energy generated usually goes to waste in a solar or wind system when unused, the fluid in a closed-loop system absorbs more heat when it remains trapped underground. This functionality allows a geothermal plant to produce the desired energy based on demand.
The only additional resource requirement for a closed-loop geothermal system, to operate efficiently, is the presence of hot rocks. While current drilling can operate at 70C to 150C, operations at 300C require incremental engineering solutions.
Supercritical EGS is the argument for developing advanced drilling technologies at greater depths to harness heat at 500C while current technologies and immediate plans are between the 70 and 300C range. In addition to new drilling technologies, the use of supercritical fluid(a material that could be either gas or liquid above a critical temperature and pressure). Supercritical fluid provides high enthalpy (direct correlation with the amount of heat transferred) as well as high electrical output
Such fluid could also be used to retrofit existing coal plants by directing steam from a production well into a turbine at a coal plant is considered a viable step in repurposing orphaned non-renewable power plants for adopting geothermal solutions. Such a process would roughly produce the same amount of electricity but with no fuel costs or carbon emissions. While these drilling technologies haven’t been proven yet, the high-density geothermal energy output from such systems could be harnessed anywhere across the globe. Quaise is a company pursuing the supercritical EGS approach.
In addition to harnessing supercritical fluids, Sage Geosystems is developing a plethora of new configurations for geothermal systems to improve efficiency:
The geothermal market has picked up over recent years. Startups like Watinyoo and Terrapin are aiming to harness the existing reservoirs to provide geothermal resources.
Eli argues although there are certain hurdles in clearing the regulatory barriers for geothermal such as getting permits or the difficulty in leveraging tax subsidies, they are not the deal-breakers for unleashing geothermal projects at scale. The biggest challenge may be in accelerating the adoption of enhanced and closed-loop geothermal solutions are going to be necessary. Needless to say, there should be advances across drilling technologies, resource characterization, turbine technologies, etc.
Zanskar is a machine learning startup that is building a predictive platform to discover unknown geothermal resources around the globe. There are few startups and consultancy groups that are working towards repurposing orphaned wells that were constructed for the oil and gas industry into potential geothermal wells.
From a cost perspective, Dandelion Geothermal claims to offer heating and cooling services at $1,827 per year as compared to $3,408 as compared to conventional oil-based services. That's almost at 50% of the price reduction factoring in the installation costs. A reduction in costs in combination with a shift towards a renewable source of energy is certainly an appealing case for the adoption of geothermal energy.
You could purchase a database of geothermal startups among other FrontierTech startups here.