Geothermal energy is generated from the Earth's internal heat, primarily stemming from radioactive decay and pressure, with reservoirs of steam or hot water found beneath the surface. It is a renewable energy source accounting for a small percentage of global power generation, with significant potential in specific regions, including various geothermal fields and technologies applicable for direct heating and electricity generation. Despite its advantages, geothermal technology faces challenges such as seismic risks and environmental concerns that must be managed effectively.

1. Geothermal Energy and
Geothermal Energy Technologies
Dr. Akepati S. Reddy
School of Energy and Environment
Thapar University
Patiala (PUNJAB), India - 147001

2. Geothermal Energy
• The Earth’s core is a heat source
– Radioactive decay of long half-life nuclei generates heat
• Heat from the core is continuously transferred outwards by
convection (in the mantle) and conduction (in the crust)
– Estimated heat transfer from the core is 42000 GW
• Heat is continuously lost from the earth surface
– Mean heat flux of the earth surface is 1/16 W/m2 (16 kW/km2!)
• Earth shows a temperature gradient from core to surface, and the
gradient is steeper for the earth crust (crust appears insulating us
from the earth’s interior heat)
– Temperature drops from 930°C to 15°C through the 30-70 km thick
continental crust (oceanic crust is 5 to 10 km thick)
– Temperature of the earth increases with depth in the crust at 17-
30°C/km (1°C per 36 m !)
– The gradient is higher along the tectonic plate boundaries and in the
zones of high volcanic activities
• Beyond certain depth (>10 – 100 feet) the earth’s temperature is
constant throughout the year
– At shallower depths it varies both diurnally and seasonally

4. Geothermal Energy
• Geothermal energy is the heat energy stored beneath the earth
surface (heat generated by natural processes within the earth)
• Volcanic activity transports hot molten material (magma) closer to
the earth surface (5 to 20 km beneath the surface)
– Molten material flowing out from volcanic erruptions is lava
– Slow cooling of molten rock form very large crystals, and molten
magma heats up nearby rock (forms hot dry rock)
• In shallow depths (upto 500 to 3000 m), to which water can
penetrate, hydrological convection of heat occurs and geothermal
reservoirs are formed
• Geothermal reservoir: hot water or steam trapped in the
permeable porous rocks under a layer of impervious rock
• Geothermal field: thermal area of permeable rock formation
containing working fluid (water)
– Semi thermal fields: water with temp <100C can be obtained
– Wet hyper thermal field: product is pressurized water at >100C temp.
– Dry hyper thermal fields: product is dry saturated or slightly
superheated steam

6. Geothermal Energy
• Geothermal energy system requires heat, permeability and water –
if any of these are lacking/limiting they are provided
– Drilling to reach the geothermal heat source
– If no water, water is used for hydrothermal convection of heat
• Geothermal reservoirs (of hot water and/or steam under pressure)
– Hot geothermal water of a reservoir can manifest itself as hot springs
(hot water), geysers (water and steam) and fumerols (only steam)
– Hot water or steam are extracted from the reservoir and used either
for power generation or directly used
– Dry steam, flashed steam, and binary cycle for power generation
– Direct use of geothermal heat, and Combined-Heat Power generators
• Hot dry rock technology (crystalline rock formation, at 3-6 km depth
– Cold water injection into engineered geothermal reservoir and (high
pressure) hot water pumping out for power generation – Enhanced
Geothermal Systems (EGS)
– Geopressed resources and Super critical cycles
• Constant year round temperature below the ground surface
– Ground source heat pumps (geothermal heat pumps)
• Geothermal water a byproduct of oil and gas wells

7. How much geothermal energy?
• Geothermal energy is a clean, renewable energy resource
(inexhaustible, essentially limitless and constantly available)
• Geothermal energy accounts for 3% of the total renewable
energy based electricity
• In 2012, 11.224 GW geothermal power generation capacity
was online around the world (in 24 countries)
– US, Philippines, Indonesia, Mexico, Italy, New Zealand, Iceland,
Japan, El Salvador, Kenya, Costa Rica, Nicaragua are important
• US amounts to 28% of the world geothermal energy production
• Indonesia has 27150 MW potential
• In 2011, Italy has 50% of the Europe installed capacity (1600 MW)
• Iceland derives 25% of its electricity and 90% of its heating from
geothermal sources
• El Savador and Costa Rica, geothermal produces 24% and 12% of
the electricity respectively
– India is not in the list
• In 2010, the geothermal direct use in 78 countries amounted
to 51 GWt (includes GHPs also)

8. How much geothermal energy?
• As on 2013, globally 11.7 GW geothermal electricity
generation capacity was in operation
• In 2015, world’s geothermal electricity generation capacity
was 12.676 GW and geothermal direct application was 18 GW
• World’s geothermal resources potential was estimated (by
Geothermal Energy Association in 1999) as 35,448 to 72392
Kw (95% and 5% probability respectiely)
– Potential of the East African Rift System (in 2012): 10,000 to
20,000 MW
– Potential of the Central America: 3000 to 13000 MW at 50
identified geothermal sites
• Geothermal heat pumps potential is available everywhere on
the earth surface
• EGS (enhanced geothermal systems) technology can
immensely increase the geothermal energy potential

13. Geothermal energy potential: India
• India has about 400 thermal springs distributed in 7
geothermal provinces
• These springs are perennial and their
surface temperature range from 37 to 90oC and their
cumulative surface discharge is over 1000 l/m.
• Temperature of the water at Tattapani is 90oC; at Puga
(Himalaya) it is 98oC and at Tuwa (Gujarat) it is 98oC
• Estimated reservoir temperature are 120oC (west coast), 150oC
(Tattapani) and 200oC (Cambay)
• The geothermal systems are mostly liquid dominated (steam
dominated systems are seen in Himalayan & Sonata provinces)
• Depth of the geothermal reservoir is about 1 to 2 km
• The power generating capacity of the thermal springs is
estimated at about 10,000 MW
– Binary cycle method can be utilized to generate power
– Puga valley (Ladakh) has the most promising geothermal field

14. Geothermal provinces
Himalaya (a & b !)
– Puga and Manikaran
Sohana
- Delhi
Combay
- Tuwa
Sonata
– Tattapani, Jalgaon, Bakreswar
West coast
– Unai, Bombay
Godavari
Mahanadi
Heat flow values (mW/m2)
Thermal gradients (°C/km)

15. Technologies
• Traditional/ conventional hydrothermal power production systems
(geothermal power plants) types
– Dry steam
– Flash steam (2 types: single flash and double flash power plants
– Binary cycle
– Combined cycle and Hybrid
• Coproduction, Enhanced Geothermal Systems (EGS), Geo-pressured
and Supercritical systems
• Direct use of geothermal heat (without involving a power plant or a
heat pump)
– Space heating and cooling, food preparation, hot spring bathing
and spas, agriculture, aquaculture, green houses, snow melting
and industrial uses
– These are applied at aquifer temperatures 90-200C.
– The geothermal water/steam is accessed and brought to a plate
heat exchanger
• Ground Source Heat Pumps (GSHP)/ Geothermal Heat Pumps
(GHPs) – Geothermal Heating and Cooling Systems

16. Condensate must be good quality water
Mostly lost to atmosphere as vapours at
cooling towers (60-80%)
Water from some other source (treated
wastewater) can be injected
Geothermalsourceswith>150°Ctemp.(180-225°C)and4-8
MPapressureareused
Undergroundsteamisdirectlyusedintheturbines(steam
movesat200m/Sec.speed)
Opensystems-associatedwithairpollutionbyH2Sand
pollutionbytracesofarsenicandminerals

17. Hot geothermic fluid and steam
condensate can be put to direct uses
Steam condensate is lost into
atmosphere at cooling towers
Highpressurehotgeothermicfluidistransportedintoalow
pressurechamberforflashing-Flashedsteamdrivesturbine
Lowpressurefluidfromtheflashingchamberisinjectedback
Saltbuildupinpipelinescanbeaproblem

18. Liquidofthe1stflashtankisfurtherflashedina2ndflash
tankatstilllowerpressuretoproducelowerpressure
steamanddrivealevel-2turbine
Exhauststeamoflevel-1turbinemayalsobeusedas
lowerpressuresteaminlevel-2turbine

19. Geothermal resource of <150C
temperature can be used
Working/motive fluids:
isobutane, pentafluoropropane
A closed loop system – no
emissions – everything brought
to surface is returned
underground
Typical efficiency: 7-12%

20. Combined cycle plants -
Hybrid power plants
Combined cycle power plant
• Steam from geothermal reservoir (after liquid separation) turns
level-1 turbine (dry steam or flashed steam power plant!)
• Exhaust steam is used in the working fluid vapourizer and vapours
of the working fluid turn level-2 turbine (binary cycle!)
• Exhaust of level-2 turbine is condensed and condensate of the
working fluid is preheated (prior to sending to the vapourizer) by
– separated geothermic liquid (brine)
– condensate of the vapourizer
• Efficiencies are significantly greater
Hybrid power plants
• Integration of numerous generating technologies
– Still water solar geothermal plant
– Geothermal pre-heated hybrid power plants

23. Hot dry rocks and EGS
• Deep Earth (3 to 6 km) geothermal resources (hot dry rocks)
– Lack of water and being at 3-6 km depth are the problems
– 1 km3 hot rock on cooling by 100C (70,000 ton coal equivalent
energy) will yield 30 MW years of power for 30 years
– Extraction of energy from deeper hot rock (about 20000 feet)
may prove uneconomical
• Enhanced geothermal systems (EGS) makes up for the deficiencies
of permiability and water
– Wells of 3 to 6 km depth drilled into the crust are used
– Fracture network in the rock is enhanced through introducing
working fluid (water) at higher pressure
– The geothermal system will have one injection well and two or
more production wells
– Pressurized cold water is injected into the fractured rock and
pressurized hot water is taken up for use
• EGS requires innovative methods to attain sustained, commercial
production rates while reducing the risk of seismic hazard

25. Direct use technologies
• Geothermal heat (hot water of <150C, 30-90C) is used directly,
without a power plant or a heat pump
• Direct uses may include
– For cooking and medicinal purposes (traditional uses of hot springs)
– In Geothermal heated spas, resorts and pools
– For heating buildings
– For heating industrial processes
– Using in Green houses, Aquaculture systems and Drying
– Melting of snow
• Geothermic fluid often contain chemical contaminants
– Use heat exchangers to transfer heat to cleaner water/air and use
• Typical geothermal direct use system configuration includes
– accessing the fluid
– bringing to plate heat exchanger
– returning the used fluid through recharge well
– a second loop of clean water or air for energy use
• Applications are usually within 10 km distance from source

26. Geothermal direct use system configuration

27. Air conditioning and heat pumps
• Heat pumps and their use for heating and cooling
– Heat pumps cause heat flow uphill against the temperature
gradient and bring about space heating and cooling
• Heat pumps (refrigeration cycles) two types
– vapour compression refrigeration cycle
– vapour absorption refrigeration cycle
• Heat pumps are two types: Air source heat pumps and water
source heat pumps
– Water source heat pumps are used in geothermal heat pumps
• Ground source heat pump systems includes 3 components
– Geothermal earth connection sub-system
– Geothermal heat pump sub-system
– Geothermal heat distribution subsystem
• Earth is a steady and incredibly large heat source (in winters)
and heat sink (in summers) of the geothermal heat pumps
– Beyond 6 feet depth earth temp. remains constant year round

28. Vapour compression air source heat pumps – warming cycle

29. Vapour compression air source heat pumps – cooling cycle

30. Vapour absorption refrigeration cycle

31. Water source heat pump

32. Geothermal heat pumps (GHPs)
• Also referred to as GeoExchange, earth-coupled, ground-source, or
water-source heat pumps
• These are able to heat, cool, and, if so equipped, supply hot water
• Earth’s constant temp. is used as exchange medium (instead of
variable temp. of outside air) – through a ground heat exchanger
– A few feet below, earth's temp. is constant (vary with latitude: 7-21°C)
• System efficiencies are high (300 - 600%) on coldest winter nights
– Air-source heat pumps are 175% to 250% efficient
– Some models use two-speed compressors and variable fans for more
comfort and energy savings.
• Installation costs are several times higher (compare with air source
units), but are quieter, last longer, and need little maintenance
– The additional cost is returned in 5 to 10 years as energy savings
– System life expectancy is 25 years for inside components and 50+
years for the ground loop
• Dual-source heat pump (water source and air source heat pumps)
– Efficiency ratings are higher than those of air-source units, but lesser
than those of water source units

34. Geothermal heat pumps (GHPs): Types
Closed loop systems
– Antifreeze solution circulates in a closed loop buried in the
ground or submerged in water
– heat transfer between the refrigerant in the heat pump and the
antifreeze solution in the closed loop occurs in a heat exchanger
– Direct exchange systems (a variant of closed loop system)
• Heat exchanger is not used – instead refrigerant is directly
pumped through copper tubing buried in ground of non-
corrosive soil
• Requires a larger compressor and works best in moist soils
• should avoid installing in soils corrosive to the copper tubing
– The loop can be in a horizontal, vertical, or pond/lake
configuration.
• Pond/lake
– Sites with a closeby water body (of minimum volume, depth and
quality) can go for this cheaper option
– Water supply pipeline is run underground from the building to
the water boy and coiled into circles at >8 feet depth

35. Geothermal heat pumps (GHPs): Types
• Horizontal
– Cost-effective for residential installations if sufficient land is
available
– Requires >4’ deep 2’ wide trenches, and two pipes burried in
the trench (one at 6’ and the other at 4’, or, both at 5’ side-by-
side) are used
– Use of more pipes can reduce the trench length required
• Vertical
– Large commercial buildings, schools, etc., the vertical systems
(avoids prohibitively large land area requirements of horizontal
loops)
– also used in places of too shallow soil for trenching
– ~4” holes are drilled 20’ apart upto 100 to 400 feet deep. – 2
pipes forming a loop by U-bend at the bottom are inserted into
the holes
– The loops are connected with a horizontal pipe or manifold
(placed in trenches) connected to heat pump at the other end

36. Geothermal heat pumps (GHPs): Types
• Open loop systems
– These use a well or surface body water as the heat exchange
fluid circulating directly through the GHP system
– The used water is either returned to back through a recharge
well, or discharged on the surface
– Practical only if adequate supply of clean water is there
• Hybrid systems
– A combination of one or more water source units and/or air
source units (i.e., a cooling tower) are used
– Effective if the cooling needs are larger than the heating needs
– standing column well system (variant of an open-loop system)
– one or more deep vertical wells are drilled
• Water is drawn from well bottom and returned to well top
• During periods of peak heating/cooling, part of the used water is
bleed out and not returned to the well
• Causes water inflow from aquifer and cools/heats well water
• Climate, soil conditions, available land, and local installation
costs will influence the selection of GHP type

37. Advances in geothermal technologies
• Expansion of usable resources, improvements to the economics of
generation, and new applications
• New and better working fluids for binary power systems to achieve
greater heat transfer efficiencies and produce power at lower temp.
– Kalina cycle and green machine: uses ammonia-water mixed working
fluid to produce upto 50% more power from the same heat source
• On-site small power generation (distributed generation facilities)
– Energy not being used could be sold back to the grid
• Use of hot water produced by oil wells
• Coproduction, enhanced geothermal systems (EGS), geopressured
and supercritical systems
– Coproduction of geothermal and oil/gas at oil and gas wells
– Geo-pressured resources (reservoirs of high-pressured hot water)
– Super critical cycles
• Pumping of supercritical fluid (CO2) into underground geological formation
(at 4-5 km depth and 400-600C temp.)
• Fluid is heated up and expanded, and fracture system is enhanced
• pumping out hot fluid to the surface power plant

38. Geothermal energy and Environment
• Geothermal wells
– Drilling of wells is noise polluting
– Land subsidence, land instability and induced seismicity
• Extraction of large quantities of geothermic fluid
• Greater concern specially for high temp. installations
• Induced seismicity - a potential problem for EGS technology
– Geothermal energy wells can blow up from the top
• Geothermal fluid can
– Be a brine solution and can be corrosive and scaling
– Trace amounts of toxic chemicals/ minerals (boron and arsenic)
– Have gases like H2S and CO2
• Solutions to problems
– Binary cycle or closed loop operation
– Chemical treatment or mineral recovery
• Zinc, lithium, manganese, strontium, rubidium, potassium,
manesium, lead, copper, boron, silver, tungston, gold, cecium,
barium, silica, and sulfur
– Monitoring and targeted injection can minimize the subsidence

39. Geothermal energy and Environment
• Geothermal energy is a renewable energy
• Geothermal power plants have capability to provide base load
power and there are no seasonal variations
– Immunity from weather effects and climate change impacts
– Unlike coal and nuclear plants geothermal plants can be online
for >90% time (75% for coal plants and 65% for nuclear plants)
• Geothermal power carbon generation is 13.38 g/kWh
– Natural gas plants produce 453 g/kWh, oil plants produce 906
g/kWh and coal plants produce 1042 g/kWh of CO2
• Land requirement is very less – 400 m2 per GW power
• Geothermal heat pumps produce 4 times the power they
consume
• Overall, geothermal energy is environmentally advantageous
– Environmental and social impacts of geothermal energy are site
and technology specific and largely manageable.