EDUCATOR'S CORNER
and their wastes such as coal fines and fly ashes, mine tail- ings, waste streams, smelter slag, brines, seawater deposits, and end-of-life productions. The latter category is currently receiving much attention as this would also help clean up the massive mining wastes accumulated for over a century. In addition, deep sea mining of manganese nodules, cobalt-rich crusts on seamounts, and sulfides from hydrothermal vents are investigated as new frontiers.
Geothermal Energy
With geothermal gradients ranging from 15 to 75 °C/km (mostly 30 ±10 °C/km), Earth is a gigantic heat machine, far greater than we can utilize or even imagine. Iceland sitting in an active volcanic terrain is a geothermal success story. Geothermal energy occurs in underground hot rocks and fluids either as dry steam (vapor-dominated) or wet steam (liquid- dominated). A more detailed classification based on geologi- cal formation and temperature profile includes the following geothermal systems.
(1) Magmatic systems (between 700-1200 °C) at several km crustal depths, for which the exploitation technology has not been developed yet.
(2) Hot dry rock with temperatures of 180-240 °C in tight granitic rocks atop active magma chambers. Due to their impermeability, these rocks require fracture stimulation (with pressurized water or supercritical
CO2) in order to create conduits for fluid circulation; therefore, they are also known as enhanced/engineered geothermal systems (EGS).
(3) Geopressured systems are formations with high tem- perature brines (170-180°C) in deep sedimentary basins. These brines can also be mined for their critical minerals and metals – a good example of “coproduction.”
(4) Hydrothermal systems include both hot water and vapor trapped in porous or fractured rock formations. Those with higher temperatures (>130 °C) are used for
electric power generation, while lower water tempera- tures (<85°C) are directly used to heat buildings and greenhouses or provide hot spring resorts.
(5) Ground source heat pumps are the cheapest and easi- est of geothermal systems as they utilize the ambient ground temperatures (10-20 °C at 2-3 m depths, depend- ing on latitude) for winter-time space/water warming and summer-time space cooling through water circula- tion in high-density polyethylene tubes (closed or open loops).
Geothermal systems require heat and permeability; these rock properties should be mapped and quantified for field development. Geologists working on geothermal fields also construct 3D geologic models incorporating lithology, stra- tigraphy, structure, crustal depth and geothermal profile. Recently, attention has been given to abandoned oil and gas wells with high bottom-hole temperatures that can be retrofit- ted to geothermal power plants either as closed-loop (single well) or open-loop (injection and production wells) systems. In the UK, abandoned coal mines, which are located in many towns and are unaffected by seasonal temperature variations, are considered as a natural heat source.
Hydrogen
Hydrogen is a zero-carbon fuel burned with oxygen. It can be used in both fuel cells or internal combustion engines. Hydrogen vehicles are thus an attractive alternative to gasoline-powered vehicles. However, hydrogen does not occur naturally in large quantities on Earth; it should be “mined” from methane and coal (by reforming) or from water (by elec- trolysis). Depending on the processes used, various grades of hydrogen are identified3 (Figure 5), and these technologies need to be improved for better efficiency and minimal envi- ronmental impact.
Being the lightest element and a highly reactive gas, hydro- gen is not easy to be stored. There are no natural hydrogen
Figure 5. Various grades of hydrogen. (Source: International Renewable Energy Agency, 2020) 3. International Renewable Energy Agency (2020) Green Hydrogen: A Guide to Policy Making. Abu Dhabi, 57 p. 50 TPG •
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www.aipg.org
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