Η Αποθήκευση Θερμότητας του Εδάφους Θα Μπορούσε να Λύσει το Πρόβλημα Αποθήκευσης Ανανεώσιμης Ενέργειας

Απαιτείται Αποθήκευση Ενέργειας Μεγάλων Διαστάσεων

There is a growing need for battery systems that can hold a lot of power for several hours or days. This is because solar and wind power are intermittent, fluctuating throughout the day and from day to day.

Making things even worse in cold climates, it is often when the temperatures are especially low that energy consumption goes upward in order to heat buildings. So, when the sky is cloudy or solar panels are covered with snow, this is when energy demand is the highest.

Using lithium-ion batteries like for EVs is unlikely to be a solution that works at scale. They are simply too expensive, not durable enough, and consume too much raw material.

This is why many different alternative battery chemistries have been considered for utility-scale energy storage. We look at most of these options in our article “Το Μέλλον της Αποθήκευσης Ενέργειας – Τεχνολογία Μπαταριών Κοινής Ωφέλειας“, including lithium-iron-phosphate, sodium-ion, redox-flow, iron-air, molten metal, nickel-hydrogen, sodium-sulfur, etc.

All of these batteries store energy in the form of electricity one way or another, usually through the oxidation and reduction of a metal.

The issue is that even very common metals like iron or aluminum would still require a lot of mining. So what if we store energy in another form?

Αποθήκευση Θερμότητας

There is a surprisingly large number of options for storing energy without using electricity as a means of storage. We looked at them in further detail in “Μη Χημικές Εναλλακτικές στις Μπαταρίες για τη Μετάβαση στην Ενέργεια“. These options include:

• Συμπιεσμένος αέρας.
• Μπαταρίες βαρύτητας (πέτρες, αντλούμενο υδροηλεκτρικό).
• Μπαταρίες θερμότητας.
• Περιστροφικά συστήματα (flywheels).
• Θερμική ηλιακή ενέργεια.

Each of these options, batteries and non-batteries, has advantages and disadvantages. The ideal storage energy solution would fit a few characteristics:

• Χωρίς ανάγκη για μέταλλα και σπάνια υλικά.
• Αποθήκευση ενέργειας μεγάλης διάρκειας, ιδανικά σε επίπεδο πολλών εβδομάδων ή μηνών.
• Χαμηλό κόστος.
• Εύκολη κλιμάκωση χωρίς περιορισμούς από διαθέσιμες τοποθεσίες ή πόρους.
• Μπορεί να μετατραπεί ξανά σε ηλεκτρισμό με ελάχιστες απώλειες.

For the first 4/5 criteria, heat storage would fit the bill. We explored some of the possible “heat battery” technology in our dedicated article “Μπαταρίες Θερμότητας που Αναπτύσσονται Γρήγορα Χωρίς την Ανάγκη Εξόρυξης Μετάλλων”.

However, even these solutions still require some manufacturing of the storage capacity, be it giant insulated silos full of sand. or blocks of pure carbon heated to 1500°C.

Researchers from the Kaunas University of Technology (Lithuania) are now exploring another possibility, storing the energy directly in the soil under buildings, without any extra manufacturing required.

They published their findings in a special issue of the journal Sustainability¹ under the title “Έρευνα για την Αύξηση της Ενεργειακής Αποδοτικότητας του Κτιρίου με τη Χρήση του Εδάφους Κάτω από Αυτό για Θερμο-Συσσώρευση”.

Θερμική Ικανότητα του Εδάφους

Because soils contain a mix of minerals and water, they tend to have a massive thermal mass, which is the quantity of energy in heat form it can contain for a given volume or mass. The nature of the soil, such as its physical structure, organic matter content, texture, and mineral composition, decides the soil’s thermal variations.

Due to this, soil tends to keep a more constant temperature below a certain depth (generally 2-3 meters / 6.5-10 feet), reflecting the annual average temperature of a given area.

Πηγή: MDPI

It is this principle that is leveraged by geothermal heat pumps, which take the heat stored by the soil in the summer to warm homes in the winter, instead of trying to pump heat from the much colder surface air. It can also work in reverse in hot climates, where the ground stays colder in the daytime and summer than the surface air. Οι ίδιες αρχές εφαρμόζονται επίσης σε γεωθερμικά θερμοκήπια,

Μέτρηση Θερμικής Διάχυσης

Πειραματική Διάταξη

Little testing has been conducted that analyzed the thermal diffusion in multiple layers of soil and measured it all year round, allowing for a proper understanding of soil’s potential as a practical thermal battery.

So, the research created an experimental setup with multiple measurements at varying depths. They added an electrical resistance to later measure the diffusion of heat. The experimental research on heat dissipation in the soil started in December and was finished in July.

Πηγή: MDPI

The experimental measurements were compared to a digital simulation of the diffusion of heat in the soil, with both giving similar results, hence proving the validity of the simulation.

Πηγή: MDPI

“Οι ετήσιες μετρήσεις αποκάλυψαν φυσικά εποχικά μοτίβα στη θερμοκρασία του εδάφους και μας επέτρεψαν να εντοπίσουμε αρκετές τάσεις. Διαπιστώσαμε ότι ακόμη και η παθητική χρήση ενός απομονωμένου όγκου εδάφους κάτω από ένα κτίριο μπορεί να μειώσει τις απώλειες θερμότητας και να αυξήσει την ενεργειακή του αποδοτικότητα.”

Prof. Ždankus – Καθηγητής στο Πανεπιστήμιο Τεχνολογίας του Καούνας

Αποθήκευση Ενέργειας με Νερό

The researchers already knew that the water content of the soil, as well as air pockets, dramatically affected the speed of thermal diffusion in the soil and its thermal mass. As the depth increases, the accumulation time also increases, allowing the soil to store heat more slowly and for longer periods.

The activity of water was discovered to be a key factor in heat diffusion in the soil as well, as water can move the heat between soil layers as well as laterally.

In one test, the soil was heated to the point where moisture began to evaporate – triggering a phase change in which liquid water becomes vapor. This “phase change” from liquid water to vapor absorbs a significant amount of heat, increasing the soil’s total capacity for heat retention while also changing the distribution pattern.

“Η αλλαγή φάσης μπορεί να είναι ένας αποδοτικός τρόπος αποθήκευσης θερμότητας. Σημαντικά υψηλότερες ποσότητες ενέργειας μπορούν να φορτωθούν στο έδαφος.

Καθώς ο ατμός διασχίζει το έδαφος, διανέμει τη θερμότητα σε ευρύτερη περιοχή. «Παρατηρήσαμε μια απότομη αύξηση της θερμοκρασίας όπου έφτανε η ροή ατμού. Αυτό σημαίνει ότι η ενέργεια κινείται και μπορεί να ελεγχθεί.»

Prof. Ždankus – Καθηγητής στο Πανεπιστήμιο Τεχνολογίας του Καούνας

Πρακτικές Εφαρμογές

Μετατροπή Κάθε Οικοπέδου σε Μπαταρία

As every building has a significant footprint, utilizing the soil for both structural stability and heat storage would not require effort, especially on new buildings, with the heat exchange installed during the construction of the building’s foundations.

The same could be said of large built surfaces with concrete or asphalt.

“Ένα τέτοιο σύστημα θα μπορούσε να βοηθήσει στην εξισορρόπηση των δικτύων οικιστικής θέρμανσης ή να μειώσει το άγχος κατά τις υπερφορτώσεις του δικτύου ηλεκτρικής ενέργειας. Είναι επίσης δυνατόν να εγκατασταθούν θερμικοί συσσωρευτές για ατομική χρήση – κάτω από κατοικίες, δρόμους ή χώρους στάθμευσης.”

Prof. Ždankus – Καθηγητής στο Πανεπιστήμιο Τεχνολογίας του Καούνας

The idea would be to use the surplus of power during sunny hours and days for solar, or windy days, and warm the soil like a giant heat battery. The highest amount of energy was needed to heat the surrounding soil at the beginning.

In the experimental setup, it took around a week of continuous heat flux to reach a stable equilibrium, where no more heat could be added to the soil without it leaching out.

Αποκεντρωμένα Δίκτυα & Θερμότητα

A key advantage to this technology would be that it can easily capitalize on the decentralized nature of solar production, both in rural and urban environments.

It can cause issues, as the grid was not designed to carry widely fluctuating loads throughout the day in what used to be only a relatively stable point of consumption. In that respect, battery parks are of little help, as the electric grid still needs an upgrade to feed the batteries.

Instead, the surplus of solar production in a given building could be sent locally as heat into the soil and reused later in the night or after several days to heat it up. This way, the local grid is not overstretched during the sunniest hours.

This can also work to some extent for cooling, with soil able to store cold as well as heat.

Περαιτέρω Έρευνα

A deeper assessment of the economics of the system needs to be done, integrating the prices of equipment used and construction costs, cost savings on grid upgrades, and how long the soil can keep the heat economically.

So far it seems this should be a better system for balancing energy demand in the day-night cycle, or over a few days, than multi-month and seasonal balancing.

Another thing needing investigation before a practical use of the technology is deployed is how it can use off-the-shelf existing solutions already in use by the geothermal industry.

“Ο άμεσος στόχος μας είναι να ενσωματώσουμε υπάρχουσες λύσεις, όπως γεώτρηση, πιλότοι και άλλες τεχνολογίες υπόγειας ανταλλαγής θερμότητας, σε ένα σύστημα που μπορεί να ωφελήσει τόσο τη βιομηχανία όσο και τον οικιακό τομέα.”

Prof. Ždankus – Καθηγητής στο Πανεπιστήμιο Τεχνολογίας του Καούνας

Επένδυση στην Γεωθερμική Ενέργεια

The sector is still relatively small compared to other renewables and is quickly evolving technologically. We discussed it in further detail in “Γεωθερμική Ενέργεια: Πράσινη Ενέργεια που είναι Καυτή“.

This means that many of the most advanced startups in the sector are still privately listed. For example, closed-loop geothermal energy Eavor, supercritical geothermy Quaise, or funds only accessible to accredited investors like Baseload Capital.

It also means that some advanced geothermal companies, like Iceland Drilling, might be just a small part of a much larger oil & gas drilling company (Archer Wells – ARCH.OL in this case).

Still, some companies are publicly listed and available to retail investors. You can invest in geothermal-related companies through many brokers, and you can find our recommendations for the best brokers on this website in the USA, Canada, Australia, the UK, and many other countries.

If you are not interested in picking specific companies, you can also look into ETFs like the Shares Global Clean Energy ETF (ICLN), the First Trust NASDAQ Clean Edge Green Energy Index Fund (QCLN), or the ALPS Clean Energy ETF (ACES) to capitalize on the growth of the geothermal energy sector.

Γεωθερμικές Εταιρείες

1. Ormat Technologies, Inc.

Ormat is the world’s largest geothermal owner and operator. The company has assets in the US, Kenya, Indonesia, and Central America + the Caribbean, with a capacity of 1.5 GW, or 10% of the global geothermal energy generation capacity, with 253 MW added in just 2024.

Πηγή: Ormat

The company is targeting a strong growth of power production capacity, with many new exploration wells being drilled, notably in Nevada, Utah, and California. In total, it should reach 2.6-2.8GW of power production capacity by 2028.

Πηγή: Ormat

Ormat is also entering the energy storage market, with 290 MW online. 385MW additional storage capacity is in development, with 2.9GW potential in the long term.

Πηγή: Ormat

It is also a provider of geothermal technology, bringing its expertise to 74% of binary plant projects, which transfer the heat from the ground to another liquid; binary plants represent 61% of the global geothermal energy market, meaning Ormat controls 45% of the global market.

Πηγή: Ormat

Geothermal energy is currently a quickly growing sector, but also one that is still very conservative due to the lack of familiarity with the technology for most utilities and industrial companies.

In that respect, this makes Ormat well-positioned to capitalize on the growing demand, while also being one of the most established players in the industry, and providing baseload renewable energy, something that other green technologies are still struggling to achieve.

Τελευταία για την Ormat Technologies

Αναφορά Μελέτης:

^{1. Τάντας Ζντάνκους, Σανντίπ Μπανταρβντάρκαρ, Γιουόζας Βαϊτσούνας, Γκεντμίνας Στελμοκάιτις και Άρνας Βαϊκάιτις (2025). Έρευνα για την Αύξηση της Ενεργειακής Αποδοτικότητας του Κτιρίου με τη Χρήση του Εδάφους Κάτω από Αυτό για Θερμο-Συσσώρευση. Ενσωμάτωση Ανανεώσιμης Ενέργειας και Εφαρμογή σε Κτίρια για την Καθαρτική Εκπομπή Άνθρακα, 2η Έκδοση https://doi.org/10.3390/su17010262}