Recursos energéticos e infraestructura.

Aenert. Research Laboratory news
Geothermal energy is a type of renewable power which is found deep underneath the Earth. Other than wind and solar power, geothermal power does not require large surface facilities or produce harmful CO₂ emissions.
Traditional geothermal plants use water from reservoirs located deep underground to power turbines which then generate electricity. However, this technology is very expensive. Also, geothermal sites need the right conditions so that they can be exploited effectively. By using advanced drilling techniques usually applied in the oil and gas industry, it is possible to tap a greater amount of heat in the ground and create geothermal energy almost anywhere.
Despite their proven benefits in the field of geothermal energy production, these technologies still need a certain amount of honing to fully adapt them to the conditions of a geothermal production site.



Aenert photos. Geothermal drilling rig. Italy

Now (2023), Sage Geosystems has developed a new form of energy storage system using the Earth as a giant bellows and pumping water into underground fractures, then letting it squirt back up at 70% efficiency – or 200% efficiency if you also harvest heat energy.

The so-called "huff & puff" method used in this process was modified from a similar technique used in oil production, where a fluid gets injected into a shale oil deposit and left there for several hours to heat the oil, which reduces its viscosity and thus makes it easier to pump out.

Sage further developed the technology and uses dense drilling mud to force it at high pressure into rock deep underground at disused oil wells, which widens slim fractures, then pumped water in, again at high pressure, to keep the fractures "inflated." This is done using excess renewable energy collected during daylight hours, and then a valve is closed to lock the water in.

In order to recover the energy, the valve is opened which makes the pressure from the earth all around squeezes the fracture it back together. This, in turn, forces the water back up through the pipes, where it can be run through a turbine to harvest electricity. It is the same electric motor and pump that forced it down there in the first place which can then be used as the turbine and generator that get the energy back out when the system runs in reverse.

This "EarthStore" system developed by Sage was tested using an old oil well in Texas, demonstrating a round trip efficiency of 70-75%, with measured fluid losses of just 1-2% and no detected induced seismic activity. A single well was found to be able to generate around a 3-megawatt maximum output if used as a load-following fast release system, or it can release energy in a more measured way to provide 18-odd hours of power through the night when solar isn't generating.

In this kind of array, the hot water from one underground reservoir could be released, cooled using a heat exchanger to harvest electricity, and immediately stored by pumping the cool water straight down the well next door. No above-ground fluid storage was required, and the process can run back and forth between these two cylinders.

The Texas pilot program has now wrapped up after six months. It made use of an exploratory oil well, using perforations at 2214 and 3395 m (2,400 and 3,400 m) deep to create a 1-km-high, 60-90-m-wide fracture. The walls of this fracture were only 2.5-5 mm apart, but by pumping water in and releasing it, and constantly maintaining enough pressure in the system to prevent the fracture from closing, the volume of the fracture expanded and contracted by a factor of two, fluctuating between 7,500-15,000 barrels (1.2-2.4 million liters, 315,000 to 630,000 gallons).

Scientists have long tried to harness the potential of geothermal energy storage. In 2020, scientists designed a geothermal-solar power plant which provided dispatchable power to the local electricity grid. The power plant was found to produce more power in the late afternoon and early evening hours of the summer. The unit could be run in two modes: a) as a binary geothermal power plant utilizing a subcritical Organic Rankine Cycle; and b) as a hybrid geothermal-solar power plant utilizing a supercritical cycle with solar-supplied superheat. Through thermal storage continuous power generation was enabled in the early evening hours. Switching to the second mode and adding solar energy into the cycle was found to increase the electric power generated 2 to 9 times during peak power demand at a higher efficiency (16.8%). Through constant supply of geothermal brine and heat storage in molten salts it was possible to produce dispatchable power in its two modes of operation with an exergetic efficiency higher than 30%.

Image: The development of the duck curve in the supply of electric power, when solar energy supplies a fraction of the total annual energy consumed



Source: Brady Bokelman, Efstathios E. Michaelides, Dimitrios N. Michaelides/ A Geothermal-Solar Hybrid Power Plant with Thermal Energy Storage/ A Geothermal-Solar Hybrid Power Plant with Thermal Energy Storage/ Energies 13(5):1018, February 2020/ DOI:10.3390/en13051018/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

In 2023, scientists developed a novel approach for the thermoeconomic evaluation of subcritical and supercritical isobutane cycles for geothermal temperatures of Tgeo = 100–200°C. The scientists were able to improve the isobutane cycles with regard to the maximum net power or minimum levelized cost of electricity (LCOE). Cycle optimisation was also achieved, using a minimum superheat temperature to avoid turbine erosion. Economic optimums were achieved in the far superheated region, while thermal optimums could be provided with dry saturated or with slightly superheated vapour at the turbine inlet (ΔTsup < 5°C). Supercritical cycles showed better thermal performance than subcritical cycles for Tgeo = 179–200°C. Internal heat recuperation was found to improve the cycle performance: the net power output increased and the levelised cost of energy decreased, but specific installation costs (SICs) increased due to the additional heat exchanger.

Image: The Rankine cycle configuration



Source: Andrea Arbula Blecich, Paolo Blecich/ Thermoeconomic Analysis of Subcritical and Supercritical Isobutane Cycles for Geothermal Power Generation/ Sustainability 15(11):8624, May 2023/ DOI:10.3390/su15118624/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

The new technology has several advantages: The EarthStore system pumps water in with hot rock, with lots of surface contact, and locks it down there. When it gets released, the heat increases the pressure with which the water rushes back up, driving the turbine harder. Also, there is the possibility to harvest the heat by running it through a heat exchanger, so that you end up getting more energy out of your "Earth battery" than you put in. Also if multiple EarthStore bores are sunk close to one another, money can be saved since the drilling equipment and other things don't need to be disassembled for transport. You can simply group them up to operate as multi-cylinder heat/pressure engines.

There are many opportunities for applying the new energy storage system – from remote mining operations to data centers to solving energy poverty in remote locations. It can interconnect with power grids or develop island/microgrids with a cleaner energy solution that is proven and ready to scale.

Investment has already procured from drilling specialists Nabors Industries and cleantech VC fund Virya, and a further US$30 million for its current Series A round will be provided to get on with proving, commercialising, optimising and scaling the system.

By the Editorial Board