7. geothermal energy for power generation (pdf)

1

GEOTHERMAL SYSTEMS AND

TECHNOLOGIES

7. GEOTHERMAL ENERGY FOR POWER GENERATION

7. GEOTHERMAL ENERGY FOR POWER GENERATION2

Geothermal plant uses a heat source to expand a

liquid to vapor/ steam.

At a geother. plant - no burning of fuel is required.At a geother. plant - no burning of fuel is required.

A vapor dominated (dry steam) resource can be

used directly, a hot water resource needs to be

flashed by reducing the pressure to produce steam,

in absence of natural steam reservoirs, steam can

be also HDR or EGS engineered in the subsurface.


In the case of low temperature resource, generally below 150˚C, the use of asecondary low boiling point fluid (hydrocarbon) is required to generate thevapor, in a binary or organic Rankin cycle plant.

The so-called Kalina Cycle technology improves the efficiency of this process.The so-called Kalina Cycle technology improves the efficiency of this process.

The worldwide installed capacity (10717 MW in2010) has the following distribution: 29% drysteam, 37% single flash, 25% double flash, 8%binary/ combined cycle/hybrid, and 1%backpressure.


The first GPP - 1913 in Larderello, Italy - 250 kWe. Next at Wairakei, New Zealand-1958, an experimental plant at Pathe, Mexico-1959, and The Geysers in USA-1960.

One of the advantages of GPPs is that they can be built economically in muchsmaller units than e.g. hydropower stations.

GPP units range from less than 1 MWe up to 30 MWe.GPP units range from less than 1 MWe up to 30 MWe.

GPPs are very reliable: Both the annual load and availability factors are commonlyaround 90 %.Conversion Technology. Four options are available to developers:

� Dry steam plants.� Flash power plants.� Binary geothermal plants.� Flash/binary combined cycle.

7. GEOTHERMAL ENERGY FOR

POWER GENERATION5

Cooling System. Usually a wet or dry cooling tower is used to condense the vaporafter it leaves the turbine to maximize the temperature drop between the incomingand outgoing vapor and thus increase the efficiency of the operation.

Water cooled systems generally require less land than air cooled systems, and inWater cooled systems generally require less land than air cooled systems, and inoverall are considered to be effective and efficient cooling systems.

The evaporative cooling used in water cooled systems, however, requires acontinuous supply of cooling water and creates vapor plumes.

Air cooled systems, since no fluid needs to be evaporated for the cooling processare beneficial in areas where extremely low emissions are desired, or in arid regionswhere water resources are limited.

7.1. Dry steam power plant6

Dry Steam Power Plants were the first type ofgeothermal power plant (Italy, 1904). Also, theGeysers in northern California the world’s largest

Dry steam non-condensing geothermal power plant

Geysers in northern California the world’s largestsingle source of geothermal power, is dry steampower plant.

DSPP use dry saturated or superheated steam atpressures above atmospheric from vapordominated reservoirs, an excellent resource thatcan be fed directly into turbines for electricpower production.

Direct-intake, non-condensing singleflash GPP at Pico Vermelho (São MiguelIsland, Azores) exhausting steam to theatmosphere.

Dry steam non-condensing geothermal power plant7

The direct non-condensing cycle is thesimplest and cheapest option for generatinggeothermal electricity.

Steam from the geothermal well is simply

Dry steam non-condensing geothermal power plant

Steam from the geothermal well is simplypassed through a turbine and exhausted tothe atmosphere: there are no condensers atthe outlet of the turbine.

Direct non-condensing cycle plants requireabout 15 to 25 kg of steam per kWhel

generated electricity.

Dry steam condensing geothermal power plant8

Since almost all geothermal resources in the form of dry steam hasdissolved 2 to 10% non-condensing gases, the geothermal plant musthave built-in system for their removal.

Usually, for this purpose a two stage ejector is used, but in many casesvacuum pumps can be used, or turbochargers.

In a geothermal dry steam power plants with vapor condensation, vaporat the exit of the turbine is not discharged directly into the atmosphere,but passed in a condenser where constant temperature is maintained,usually 35 to 45oC.

7.1.2. Dry steam condensing geothermal power plant9

Dry steam condensing

geothermal power plantAdvantage of the SCPs in relation to

plants with non-condensing is very

efficient utilization of geothermal steam

and elimination of the risk forand elimination of the risk for

environmental noise pollution during

the steam discharge. But the larger

investments, more expensive main-

tenance, more complex performance

and the need for cooling of geothermal

steam, makes construction more

expensive and less favorable for

construction.

7.2. Flash steam power plant10

Flash Steam Power Plants, which are the most common, use water withtemperatures greater than 182°C.

A single flash condensing cycle is the most common energy conversion system forutilizing geothermal fluid due to its simple construction and to the resultant lowutilizing geothermal fluid due to its simple construction and to the resultant lowpossibility of silica precipitation.

A double flash cycle can produce 15-25% more power output than a single flashcondensing cycle for the same geothermal fluid conditions.

Flash power plants typically require resource temperatures in the range of 177oC to260oC.

7.2.1. Single flash system11

In a single flash steam plant, thetwo-phase flow from the well isdirected to a steam separator;where, the steam is separated fromthe water phase and directed tothe water phase and directed tothe inlet of the turbine. The waterphase is either used for heat inputto a binary system in a direct-useapplication, or injected directlyback into the reservoir.

Steam exiting the turbine isdirected to a condenser operatingat vacuum pressure.

Simplified schematic diagram ofa single flash condensing system

Single flash condensing system12

The steam is usually condensed either in adirect contact condenser, or a heatexchanger type condenser.

Between 6000 kg and 9000 kg of steam

Temperature-entropy diagram of a single flash condensing system

Between 6000 kg and 9000 kg of steameach hour is required to produce each MWof electrical power.

Historically, flash has been employed whereresource temperatures are in excess ofapproximately 150oC.

Single flash back pressure system13

The term “back pressure” is used becausethe exhaust pressure of the turbine ismuch higher than the condensing system.The system does not use a condenser.

Simplified schematic diagram of a single flash

back pressure system

The system does not use a condenser.

The steam consumption per power outputis almost double that from the condensingtype at the same inlet pressure.

The back pressure units are very cheap andsimple to install, but inefficient (typically10-20 tone per hour of steam for everyMW of electricity) and can have higherenvironmental impacts.

7.2.2. Double flash system14

The double flash system uses

Simplified schematic diagram of a double flash

condensing system

The double flash system usesa two stage separation ofgeothermal fluid instead ofone, resulting in two steamadmission pressures at theturbine.

7.2.2. Double flash system15

Steam from the high pressureturbine is mixed with the steam fromthe low pressure separator and thendirected to the low pressure turbineto generate extra power.to generate extra power.

The brine from a low pressureseparator is piped to the reinjectionwells.

Temperature-entropy diagram of a double flash condensing system

16

From geothermal wells in theisland, with a depth between600 to 2500 m, geothermal fluidwith temperature 230 to 250oCis provided and steam in the

7.2.2. Double flash system

Schematic diagram of a double flashcondensing system of the Bouillantegeothermal power plant located at thecoast of the island Basse Terre, south ofthe Bouilante in Guadeloupe

is provided and steam in themixture of 20 to 80%.

7.2.3. Triple expansion system17

Developed to handle the caseswhen the EGS geofluid arrives atthe plant at super-criticalconditions, i.e. at a temperature> 374°C and a pressure > 22 MPa.

Triple-expansion power plant for supercritical EGS fluids.

> 374°C and a pressure > 22 MPa.

The triple-expansion system is avariation on the conventionaldouble-flash system, with theaddition of a “topping” dense-fluid, back-pressure turbine,shown as SPT.

7.2.3. Triple expansion system18

The turbine is designed to handlethe very high pressures.

Processes for triple expansion power plant

The utilization efficiency is about67%, and the thermal efficiency isabout 31%.

Given the high specific net power,it would take only about 15 kg/sof EGS fluid flow to produce 10MW in either case.

7.3. Binary cycle power plants19

Binary Cycle Power Plants operate with the lower-temperature waters, 74° to177°C.

These plants use the heat of the hot water to boil a “working fluid,” usually anorganic compound with a low boiling point.organic compound with a low boiling point.

This working fluid is then vaporized in a heat exchanger and used to turn aturbine.

The geothermal water and the working fluid are confined to separate closedloops, so there are no emissions in the air.

Because these lower-temperature waters are much more plentiful than hightemperature waters, binary cycle systems will be the dominant geothermalpower plants of the future.


In the binary process the geoth.water heats another liquid(“working fluid”), such asisobutene (e.g., isopentane,propane, freon or ammonia),that boils at a lower temperaturethan water.

The two liquids are kept

Simplified schematic diagram of a binary cycle power plant

The two liquids are keptcompletely separate through theuse of a heat exchanger used totransfer the heat energy from thegeoth. water to the “working-fluid" in a conventional RankineCycle, or alternatively KalinaCycle.


Approximately 15% of all geothermalpower plants utilize binary conversionpower plants utilize binary conversiontechnology. Binary cycle type plantsdepending on the temperature of theprimary fluid, usually have efficiencybetween 7 and 12%, and typically vary insize from 500 kW to 10 MW.

Ormat ORC binary cycle power plant


By selecting suitable secondary fluids,binary systems can be designed toutilize geothermal fluids in thetemperature range of 85-170°C.temperature range of 85-170°C.

The binary systems can also be utilizedwhere flashing of the geothermalfluids should preferably be avoided.

Correlation of binary plant cyclethermal efficiency with geofluidtemperature in °C


Binary plants are usually constructed in small modular units of a few hundredkWe to a few MWe capacity.

These units can then be linked up to create power-plants of a few tens ofmegawatts.megawatts.

Their cost depends on a number of factors, but particularly on the temperatureof the geothermal fluid produced, which influences the size of the turbine, heatexchangers and cooling system.

Binary plant technology is a very cost-effective and reliable means of convertinginto electricity the energy available from water-dominated geothermal fields(below 170 °C).


The Kalina cycle, developed in the 1990s,utilizes a water-ammonia mixture asworking fluid (70% ammonia and 30%water). The working fluid is expanded, insuper-heated conditions, through the high-water). The working fluid is expanded, insuper-heated conditions, through the high-pressure turbine and then re-heated beforeit enters the low-pressure turbine. Afterthe second expansion the saturated vapormoves through a recuperative boiler beforebeing condensed in a water-cooledcondenser.

Simplified flow diagram for a Kalinabinary geothermal power plant

7.4. Combined cycle system25

In a combined single flash cycle and binary cycle, the heat from hot separatedbrine or exhaust steam from the back-pressure steam turbine is transferred to asecondary binary fluid.

Here, three configurations of combined cycles are considered.Here, three configurations of combined cycles are considered.

7.4.1. Brine bottoming binary (BBB) system

BBB system is a combination of a single flash cycle using a condensing turbineand a binary cycle as a bottoming unit.

7.4.1. Brine bottoming binary (BBB) system26

The dry steam from the separator is directed to a condensing steam turbine. Steam from the turbine exit is directed to a condenser operating at vacuum pressure.

The hot separated brinewhich still contains highenthalpy is utilized to

Simplified schematic diagram

of a BBB system

enthalpy is utilized tovaporize the working fluidin the binary cycle and thusproduce additional poweroutput.

7.4.1. Brine bottoming binary (BBB) system27

The working fluid absorbs heat from a heat source, inthis case the hot brine, via shell and tube heatexchangers. This heat causes the working fluid toevaporate, producing the high pressure vapor whichis then expanded through turbine connected tois then expanded through turbine connected togenerator. The exhaust vapor from the low pressureturbine is then condensed using either air-cooled orwater-cooled shell and tube heat exchangers. Fromthe condenser, the liquid working fluid is pumped to ahigh pressure and returned to the boiler to close thecycle.

Temperature-enthalpy diagram of a BBB system with n-pentane as the ORC fluid

7.4.2. Spent steam bottoming binary (SSBB) system28

A SSBB system is a combination of a singleflash cycle using a back pressure turbineand a binary cycle.

The dry steam from the separator isdirected to a back-pressure steam turbine.

Simplified schematic diagram of a SSBB system

directed to a back-pressure steam turbine.Steam exiting the turbine is thencondensed in the pre-heater and theevaporator of the binary cycle. Thus,condensation heat of the steam is used tovaporize the working fluid in the binarycycle.

7.4.2. Spent steam bottoming binary (SSBB) system29

The net power output of a SSBBsystem is calculated by summing upthe power output of the turbines(steam turbine and binary turbine)

Temperature-enthalpy diagram of a SSBB system

(steam turbine and binary turbine)and subtracting the auxiliary powerconsumption of binary fluid pumps,cooling-water pumps and coolingtower fans.

7.4.3. Hybrid system30

Hybrid system is a combination of aSSBB system and a BBB system.

Simplified schematic diagram of a hybrid

system

The plant configuration consists of asingle flash back pressure cycle, abinary cycle utilizing separated brineand a binary cycle utilizing the exhauststeam from the back pressure unit.

7.4.3. Hybrid system31

The net power output of a hybrid systemis calculated by summing up the poweris calculated by summing up the poweroutput of the steam and binary turbines,and subtracting the auxiliary powerconsumption for binary fluid pumps,cooling-water pumps and cooling towerfans.

Temperature-enthalpy diagram of a hybrid system

7.5. Cogeneration of electricity and thermal energy32

Heating capacity - 125 MWt

Power capacity - 16.4 MWe

8.4 MWe produced from two

Simplified schematic diagram of Svarsengy geothermal cogeneration power plants In Reykjanes peninsula in Iceland

8.4 MWe produced from two

turbines with organic working

fluid, and 8 MWe from steam

turbine with geothermal steam.


This system has a flash vessel for theproduction of steam, a compressordriven by an isobutene turbine, an

Basic system for upgrading geothermal fluid

driven by an isobutene turbine, anisobutene condenser, and a heatexchanger to heat and evaporate thecondensed isobutene by the geo-thermal fluid.


One of the possible uses of EGS-produced fluids is to provide both electricity and heatto residential, commercial, industrial, or institutional users.

The figure is a flow diagram in whichan EGS well field replaces the fossil

EGS system to supply MIT-COGEN energy requirements

an EGS well field replaces the fossilenergy input to the existing MIT-COGEN plant and supplies all of thecurrent energy requirements.


In the case of the MIT campus, theEGS system may be used inconjunction with GSHPs to provideall the heating and cooling needs.The EGS system shown still allows

EGS system to supply MIT-COGEN energy requirements using ground-source heat pumps

all the heating and cooling needs.The EGS system shown still allowsfor some direct heating using theback-pressure exhaust steam fromthe main turbine for those applica-tions where steam is essential.

7. geothermal energy for power generation (pdf)

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