The Earth is constantly radiating gigantic amounts of heat. Earth’s total surface heat flux is estimated to be in the range of 47 terawatts, which is three times higher than the total energy consumption of the whole world population in 2019. This heat comes from the heat accumulated during the formation of the Earth (Primordial Heat), from radioactive decay in the mantle, and from the absorption of heat generated by incoming sunrays. Heat from the first two deep sources is slowly transferred towards the surface of the Earth through convection movements and conduction. At present, only a small part of this energy is used by geothermal plants to produce electricity.

Some of the challenges behind our difficulties harnessing this tremendous amount of energy are the relatively high set-up cost for individual plants, which include drilling several kilometers into the Earth’s crust, and the identification of relatively shallow geothermal systems including permeable rocks in an area with elevated geothermal gradient. A major issue towards capturing heat from deep sources lie in the location of the main sources of heat at the center of Earth’s oceans along so-called Mid-Ocean Ridges. There, magma is extruded under water and pushes tectonic plates and continents away from each other.

One of the only places in the world where a Mid-Ocean Ridge crops out above sea-level is Iceland, a divergent plate boundary that was formed ca. 64–58 million years ago in the Paleocene–Eocene epochs during the opening of the North Atlantic Ocean. There, magma pockets occur at relatively shallow depth and are ideal targets for geothermal plants. Recent advances have led scientists and the geothermal industry to drill through magma. Unfortunately, technical difficulties prevented connection of the magma-seated well to an active geothermal plant. Further work is needed but preliminary results are encouraging.

Mid-Ocean Ridges are not the only places in the world where shallow magma pockets occur. For example, convergent plate boundaries, like the Andes Mountains in South America, host numerous active volcanoes that are fed by such magma bodies. The occurrence of volcanoes in the Andes and around the whole Pacific Ocean in countries like New Zealand and Japan is actually related to the subduction of the Pacific oceanic plate under lighter continental plates. During subduction, water absorbed by sediments deposited on the ocean floor and by basalt extruded at Mid-Ocean Ridges is released into the overriding continental plate. This results into partial melting of both the subducting oceanic plate and overriding continental plate and, eventually, into the ascent of magma through the crust. Unfortunately, the contamination of magma (initially made up with heavy and dark minerals like olivine and pyroxene) by continental material (mostly consisting of light minerals like quartz and K-feldspar) increases the viscosity, and, thus, slows down the ascent of magma. This often leads to deep crystallization of magma bodies, i.e., out of reach for geothermal exploitation.

Other challenges in convergent plate boundaries include the relatively thicker nature of the continental crust (average of 35–70 kilometers thick) compared to oceanic crust like in Iceland (6–10 kilometers thick), the occurrence of frequent, high-magnitude earthquakes, which may disrupt geothermal wells, and the remoteness of potential geothermal systems in these areas.

By contrast to deep geothermal systems, heat absorbed from incoming sunrays is briefly stored in the uppermost layers of the ground before being dissipated. This heat can easily be used at relatively low cost to heat buildings and homes through geothermal heat pumps. Several models exist, of which closed-loop horizontal heat pumps are generally the most cost effective alternative, pending that sufficient land is available to host the heat pump (about 700–900 m² or the equivalent of a square with sides of 25–30 meters). Vertical heat pump require less land but may require drilling holes up to 100 meters and come therefore at a higher cost.

Geothermal heat pumps may also act as a cooling system in summer months and actually evacuate the excess heat in a house or building and transfer it into the ground. This system takes advantage of the relatively constant temperature of the Earth’s sub-surface throughout the year (i.e., warmer than the air at the surface in the winter months, and colder during summer).

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