A Brief History of Geothermal Energy
About 4,600 years ago, villagers of Mohenjo-Daro, now located near a city called Larkana, Sindh, Pakistan, found a mysterious puddle in the middle of the mountains, filled with warm water. The most interesting ability of this puddle was that it healed wounds of animals that dipped their bodies into the warm water. For the villagers of Mohenjo-Daro, this puddle would have been seen as magical or even sacred. The astonished villagers then used the warm water from the puddle to cleanse their bodies before participating in religious ceremonies. The warm puddle then became the first public bathhouse, and also the case was one of the earliest applications of Geothermal Energy.
Since the time of the Indus Valley Civilization, which included Mohenjo-Daro, numerous hot springs have been discovered and developed for various applications, mainly in the volcanically or geothermal active areas of the world (i.e. Japan, Indonesia, New Zealand, etc.). A few centuries later after the discovery of electricity, people began to study a way to generate electricity with the naturally-occurring hot springs. The first industrial geothermal power plant was built in 1911 in Larderello, Italy by an Italian businessman named Piero Ginori Conti. As head of a boric acid firm, he was looking for a way to decrease the production cost, and to improve the quality of the firm’s products. After spending a lot of time thinking about the problem, he developed a method to exploit natural dry steam geysers to power his boric acid plants. The first successful demonstration of the system happened on July 4, 1904, where he was able to power five light bulbs with a reciprocating steam engine using geothermal power. The production was increased to 20 kW, then to 2750 kW in 1916, providing electricity to the nearby cities and villages. The first geothermal power plant still exists today, and is being operated under name of “Larderello Energy Plant.” The geothermal site is now consisted of 34 plants operated and has a capacity of 800 MW (2% of Italy’s energy mix).
The second industrial geothermal power plant was built in 1958 In New Zealand. Since then, the world has been gradually adding geothermal energy production capacity. However, the pace of growth has been quite slow for the past few decades. In fact, according to the Renewables Global Status Report (GSR), released annually by the Renewable Energy Policy Network for the 21st Century (REN21, a think tank), the combined wind, solar, biomass, geothermal energy, and ocean power added a mere 2.0% to the estimated total final energy consumption. Of that 2.0%, 96% of the renewable electricity comes from wind power, solar PV, and bio-power plants. Geothermal energy can be also utilized as a heat source with help of geothermal heat pumps, but this application also accounted for less than 0.1% of global primary energy consumption, although the number has been increasing in recent years. As presented by the statistics, the representation of geothermal energy in the global energy mix is rather insignificant, still in 2020.
In the next few articles, the reasons behind the past slow growth, the recent growth and technological breakthroughs, and the potential of geothermal energy as one of the important players in the great energy transition will be addressed. In the second half of this first article, I would like to briefly explain the nature and the principles behind geothermal energy.
Fundamental Principles of Geothermal Energy
It is widely understood that the usable geothermal energy mainly has two origins; the continuous heat loss from Earth’s formation of the core, and the radioactive decay of isotopes in the crust of the Earth.
The Earth can be considered as a huge heavy ball that continuously radiates heat to the universe from its core. The heat from the core was generated from the collision of moving particles in the universe during the formation of Earth 4.56 billion years ago. As the size of the planet grew over a long time, pressures in the interior of the Earth were increased simultaneously which then resulted in the elevation of the internal temperature of the Earth.
Roughly estimated, a similar amount of the Earth’s geothermal energy is originated from the radioactive decay of isotopes. The contributing isotopes such as potassium (K), rubidium (R), thorium (Th) or uranium (U) are situated in different layers of the Earth. However, most of the heat produced by the radioactivity of the isotopes encountered in geothermal reservoirs are from the continental crust.
From these two sources, the heat is transferred towards the surface mainly based on the two heat transfer modes including convection and conduction. Convection is a combined process of heat conduction and advection (heat being transferred by mass movement). The convective heat flux is related to the phenomenon called density stratification. Because of gravity, materials with a lower density tend to rise above materials of a higher density. For instance, the planet’s density increases with depth. Without an energy source, the Earth would be in a stable configuration for the distribution of materials. However, the heat exchange continuously occurs between the hot, molten outer core and the base of the mantle. The heated minerals at the base of the mantle expand and their density and viscosity both are decreased. As they become more buoyant, they rise to the surface of the Earth. When they reach a point where the combined effect of density and viscosity on the materials is no longer sufficient to overcome the resistance to flow, they progress towards the core again.
By definition, heat conduction is the transfer of heat through direct physical contact. For instance, when two materials are in contact, energy is being transferred between atoms and electrons. After some time past, the materials will reach the same temperature and this state is called thermal equilibrium. Until then, the heat transfer continues, like how the heat from the inner Earth is continuously conveyed towards the surface.
The key metric of evaluating the potential or the method of exploration for geothermal energy is the Geothermal Gradient. Geothermal gradient is defined as ”the rate of increasing temperature with respect to increasing depth in the Earth’s interior.” A typical geothermal gradient is about 25 °C /km in the crust. However, the values of the geothermal gradient vary depending on the geological conditions.
The geological conditions that favor high geothermal gradient are associated with tectonic activities of the Earth. The movement of the tectonic plates is driven by the convection of the mantle. The tectonic activity forms regions where the geothermal gradient is significantly higher than the average (i.e. Japan, Indonesia, etc.). These regions include the mid-ocean ridge system, subduction zone, rift zone, and hot spots. For mid-ocean ridges, hot and upwelling mantle due to convection motion spread the planes away from each other. At the spreading center of the ridge, where the crust thickness is reduced, high heat flux and greater geothermal gradient are measured. In the subduction zone, where an oceanic crust is subducted below a continental crust by the down-welling convective movement of the mantle, volcanic activities are the highest. The high geothermal gradient of the subduction zone is governed by the water content of the oceanic crust. When wet rock containing hydrous minerals releases water as its temperature is increased to a sufficient level. The released water then causes the melting of the hot mantle immediately above the descending oceanic crust. This melt has a lower density than the solid rock and it moves upward by the buoyant force. Hotspots such as Hawaii and Iceland have significantly high geothermal gradient due to magma that has risen from the deep mantle. Mantle plume rises via a channel from the Earth’s core-mantle boundary. The plume migration is manifested as a volcano on the ground surface and the temperature of its surrounding subsurface is increased.
From the principles explained in this article, the most crucial limiting factor for geothermal energy development can be drawn. It is the fact that one must drill deep enough to exploit an immense amount of geothermal energy, and the drilling depth is based on the geothermal gradient of the interest area. Unless the area is volcanically or tectonically active (i.e. Hawaii, Japan, Indonesia, West Coast of USA, etc.), the drilling depth may have to be >1,000 m with the conventional approach, and it is very expensive to drill this deep. In the next article, I will further address some of the technical barriers for the development of geothermal energy, and some key technologies that are enabling the recent growth of geothermal energy share in the global energy mix.
References
Academic sources:
Fridleifsson, I.B., Bertani, R., Huenges, E., Lund, J.W., Ragnarsson, A., & Rybach, L. (2008). The possible role and contribution of geothermal energy to the mitigation of climate change.
Glassley, W. E. (2015). Geothermal energy: Renewable energy and the environment. Boca Raton, FL: CRC Press.
Lowell, R., Kolandaivelu, K., & Rona, P. (2014). Hydrothermal Activity. Reference Module in Earth Systems and Environmental Sciences. doi:10.1016/b978–0–12–409548–9.09132–6
Sleep, N. H. (1990). Hotspots and mantle plumes: Some phenomenology. Journal of Geophysical Research,95(B5), 6715. doi:10.1029/jb095ib05p06715
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https://www.power-technology.com/features/oldest-geothermal-plant-larderello/
Image sources:
https://ko.wikipedia.org/wiki/%EB%AA%A8%ED%97%A8%EC%A1%B0%EB%8B%A4%EB%A1%9C#/media/%ED%8C%8C%EC%9D%BC:Mohenjodaro_Sindh.jpeg
https://ko.wikipedia.org/wiki/%EB%AA%A8%ED%97%A8%EC%A1%B0%EB%8B%A4%EB%A1%9C#/media/%ED%8C%8C%EC%9D%BC:Civilt%C3%A0ValleIndoMappa.png
https://alchetron.com/Piero-Ginori-Conti
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https://www.vox.com/energy-and-environment/2019/6/18/18681591/renewable-energy-china-solar-pv-jobs
Boehler, R. (1996). MELTING TEMPERATURE OF THE EARTHS MANTLE AND CORE: Earths Thermal Structure. Annual Review of Earth and Planetary Sciences, 24(1), 15 – 40. doi: 10.1146/annurev.earth.24.1.15
Ficher, L. (2000). SYNOPSIS OF PLATE TECTONIC THEORY And Its Implications for Earth History. Retrieved October 23, 2018, from http://csmgeo.csm.jmu.edu/geollab/Fichter/PlateTect/plateboundry.html
