Cultivating Heat: Innovations in Geothermal Energy (Part 1)

 

     Geothermal energy refers to converting the heat in the earth for direct power use and/or for direct heat use. It can also be used for cooling and to heat water in ground source heat pump applications. This post will focus most on the feasibility and niche uses of newer forms of geothermal innovations like Enhanced Geothermal Systems (sometimes also called Engineered Geothermal Systems), or EGS, Advanced Geothermal Systems, or AGS, waste-heat recovery improvements, mineral recovery through direct extraction, and closed loop design improvements. Some of the waste-heat recovery innovations also have applications for industrial and commercial projects and fossil power plants that recover heat from places other than the ground, such as from combustion flues. Industry has lots of waste heat that can be recovered. Heat recovery improvements lead to greater efficiency. Mineral recovery can improve project economics. Hydraulic fracturing in “dry rock” and new closed loop designs and configurations can make heat extraction more viable in cooler rocks which are available in more places.

 

Fig. 1 The Hottest of the Hot Spots Around the World.

      

     The figure above shows the hottest of the hot spots globally. As can be clearly seen by geologists and those familiar with plate tectonics the hot spots follow active plate margins, subduction zones, mid-ocean ridges, rift systems, and related tectonically active areas. This map also generally shows the areas with the most developed geothermal power production and the most future potential going forward for traditional geothermal power production. The Earth’s temperature is estimated to be 6000 deg C at its inner core. Temperature increases with depth. It is also estimated that 83% of geothermal heat is generated by the radioactive decay of elements in the upper crust (83%), with 17% generated from the formation of the planet.[1]

     Hydrothermal, or hot water reservoirs, are heated by magma, or molten rock. Magma temperatures vary from 700-1300 deg C (1300-2400 deg F). Sometimes magma reaches the surface as lava through volcanic activity. Sometimes magma heated water reaches the surface as geysers, hot springs, steam vents, underwater hydrothermal vents, and mud pots.[2]

     Geothermal resources are classified as high-enthalpy = above 250 deg C (482 deg F), medium-enthalpy = 150-250 deg C (302-482 deg F), or low-enthalpy = 100-150 deg C (212-302 deg F). Newer technologies can utilize heat as low as 80 deg C to run turbines. Ground heat resources that can be utilized for heat pumps can be as low as 10 deg C. High and medium enthalpy resources have been the most economic and are responsible for most of geothermal electricity generation. Low enthalpy resources are more recently being tested and developed in response to tech advancements in heat recovery, heat exchange, and optimization of working fluids.

     The chart below shows where the most geothermal energy is produced by country. The US leads by a big margin with California and Nevada being the biggest producers. Indonesia, the Philippines, and Turkey are a distant second. Add in New Zealand and that makes five countries producing a GW or more, with Mexico and Italy very near 1 GW. In terms of percentage of a country’s energy produced by geothermal, Kenya leads with about 50% of their energy being geothermal. In the US, although California produces by far the most geothermal energy, it is Nevada that produces the highest percentage of their energy with geothermal at 9.4%. About 0.4% of US electricity is produced by geothermal power plants and only about 0.24% of primary energy in BTUs (2% of renewable energy) is produced by geothermal according to the US Energy Information Administration.[3]

 

 

Fig. 1.2. Top 10 Geothermal Countries 2021. Data Source: ThinkGeoEnergy Research 2022 and California Energy Commission.

     We can see from the figure above that the US has about a quarter (23.5%) of total global geothermal capacity. We can also see that if California were a country, it would lead the world in installed capacity of geothermal electricity by a significant margin from number 2 country Indonesia. California has 17% of global geothermal installed capacity. The figure below shows that California and Nevada together make up nearly 95% of US geothermal capacity. Nevada also exports some geothermal electricity to California.

 

Fig. 1.3. Geothermal Electricity Production by US State: % of US Total and % of State Total. Data Source: US Energy Information Administration.[4]

 

     Geothermal development slowed in the 2010’s in the U.S. but is now reviving due to increased government incentives and subsidization, increased private investment, and new technologies including transfer of improved drilling and hydraulic fracturing technologies from the oil and gas industry.

     The map below of US hot spots also shows some medium hot areas more suited to enhanced geothermal. The red (darker) areas mostly in the western states are hotter at comparable depths most favorable. The orange (medium dark) areas are still favorable for EGS development. The yellow and white (light) areas are least favorable but some of those areas can still be utilized for niche AGS systems, for direct use, and all areas can be used for ground-source heat pumps.

 

 

Fig. 1.4. Geothermal Resources of the United States. Identified Hydrothermal Sites and Favorability of Deep Enhanced Geothermal Systems (EGS). Source: US. Dept. of Energy. National Renewable Energy Laboratory. 2018.

 

     In order to determine geothermal energy favorability, the main factor is heat content at depth. This is determined from temperature probes in wells so that a geothermal gradient, which is typically the rate at which the temperature rises with deeper depths, can be determined, and mapped across regions. Oil and gas wells are typically logged with temperature probes that can provide good geothermal gradient data over an oil and gas sedimentary basin or field. Other methods such as gravity surveys, magnetic surveys, and active and passive seismic surveys can help unravel subsurface structure that can influence the configuration of hot spots and the characteristics of the hydrothermal system. Geothermal energy favorability is also enhanced when there is a hydrothermal brine system, usually a sealed aquifer, that can be circulated with high flow rates. High flow rates require permeable rock. Permeable rock can be created through hydraulic fracturing so that a hydrothermal brine system can be created as well. This is an Enhanced Geothermal System (EGS), often done in so-called “dry rock.” This can be done in hotspots without a hydrothermal brine system or in medium enthalpy, less hot rocks which correspond to the orange areas of the map, which significantly expands the favorable areas to more populated US regions.

Conventional Binary Cycle Geothermal Power Plants

 Geothermal power plants are of three main types: Dry steam plants, flash steam plants, and binary-cycle plants. The earliest geothermal plants were dry steam plants. The first one in Larderello, Italy was built in 1911 after successful tests in 1904. Dry steam plants are rare due to limited availability of naturally occurring dry steam resource. Dry steam plants directly use geothermal steam that is 150 deg C or greater to run turbines to generate electricity. Flash steam plants utilize hot water under high pressure which is converted to steam to run turbines. They require water temperature of 180 deg C or above. As the pressure drops when the hot water comes up the production well, some of it converts to steam which runs turbines. Left over water and condensed steam, or just condensed steam in a dry steam plant, is re-injected through an injection well back into the reservoir. This is the most common geothermal plant type in operation. Binary cycle plants utilize the hot water to heat a second fluid, the working fluid, to turn the turbines. The hot water, as low as 57 deg C, heats up and flash vaporizes the working fluid, which has a lower boiling point than water. The working fluid is enclosed in a closed loop. Binary cycle plants are the most commonly constructed plants currently, making up about 95% of US geothermal plants constructed since 2000. The working fluids are utilized in Organic Rankine cycles (ORCs) or Kalina cycles. ORCs often use pentanes or iso-butane as working fluids. Kalina cycles often use ammonia to increase condensation temperature of the working fluid which is a water and ammonia mixture. Kalina cycles offer better performance at medium-high temperatures (200-300 deg C). Binary cycle plants are typically smaller, in the 1-50 MW range. They have a thermal efficiency of 10-13%.

 

 Fig. 5.1. Typical Binary Cycle Geothermal Power Plant Configuration. Source: Wikipedia

References:

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[1] Geothermal 101, Geothermal Canada. Poster. Accessed 2022. GEOTHERMAL+101+V3.pdf (squarespace.com)

 

[2] Geothermal Energy. National Geographic. Geothermal Energy | National Geographic Society

 

[3] U.S. primary energy consumption by energy source, 2021. US Energy Information Administration. U.S. energy facts explained - consumption and production - U.S. Energy Information Administration (EIA)

 

[4] Geothermal explained. Use of geothermal energy. Energy Information Administration. 2022. Use of geothermal energy - U.S. Energy Information Administration (EIA)