Green Technology – 10EG6036th Sem. 2013

Dr. Rajalakshmi MudbidreDepartment of Chemical EngineeringRV College of EngineeringBangalore, Karnataka, India

Green Technology – 10EG6036th Sem. 2012

CLASS 9- GEOTHERMAL

Geysers

http://en.wikipedia.org/wiki/Geyser

Clepsydra Geyser in Yellowstone

Hot Springs

Hot springs in Steamboat Springs area.

http://www.eia.doe.gov/cneaf/solar.renewables/page/geothermal/geothermal.html

FumarolesClay Diablo Fumarole (CA) White Island Fumarole

New Zealand

http://volcano.und.edu/vwdocs/volc_images/img_white_island_fumerole.htmlhttp://lvo.wr.usgs.gov/cdf_main.htm

Geothermal Energy - Introduction

Energy present as heat (i.e, thermal energy) in the earth’s crust

The more readily accessible heat in the upper most(10 km) or so, of the crust constitutes a potentially useful and almost inexhaustible source of energy

Apparent from the increase in temperature of the earth with increasing depth below the surface

On an average, temperature at a depth of 10 km is about 2000C

Geothermal Energy - Introduction US Geological survey define geothermal

source as “ all of the heat stored in the earth’s

crust above 150C to a depth of 10 km”

A typical geothermal field

A typical geothermal field

The hot magma(molten mass) near the surface solidifies into igneous rock

The heat of magma conducted upward to this igneous rock

Ground water that finds its way down to this rock through fissures in it, will be heated by the heat of the rock or by mixing with hot gases and steam emanating from the magma

Heated water will then rise convectively upward and into a porous and permeable reservoir above the igneous rock

A typical geothermal field

The reservoir is capped by a layer of impermeable solid rock that traps the hot water in the reservoir

the solid rock, has fissures that act as vents of the giant underground boiler

The vents show up at the surface as geysers, fumarols or hot spring

A well taps steam from the fissures for use in a geothermal power plant

Kinds of Geothermal sources

Four basic kinds i) Hydrothermal convective systems ii) Geopressure resources iii) Petro-thermal or Hot dry rocks iv) Magma resources

Hydrothermal convective systems

Those in which water is heated by contact with the hot-rock

Two kindsVapor-dominated systemsLiquid-dominated systems

Hydrothermal convective systems Vapor-dominated systems Water is vaporized into steam that reaches the

surface in a relatively dry condition at about 2000C and rarely above 7kg/cm3(8 bar)

Most suitable for use in turbo electric power plants, with the least cost

Problems: presence of corrosive gases and erosive material

Less in number : five known sites in the world Geysers plant in the US, largest in the world today & Larderello in Italy

Hydrothermal convective systems Liquid-dominated systems Hot water circulating and trapped underground is at a

temperature range of 175 to 3150C When tapped by wells drilled in the right places and to

the right depths, the water flows naturally to the surface or is pumped up to it

Contains relatively large concentration (3000 to 25,000 ppm) of dissolved solids

Power production is adversely affected by these solids- precipitate and cause scaling in pipes and heat exchanger surfaces, thus reducing flow and heat transfer

Much more plentiful than the previous one

Geopressured systems

Occur in large, deep sedimentary basins Reservoirs contain moderately high

temperature water(or brine) under very high pressure

They are of special interest because substantial amounts of methane(natural gas) are dissolved in the pressurized water and are released when the pressure is reduced

Geopressured water is tapped in much deeper underground acquifers at depths between about 2400 to 9000m

Geopressured systems

This water is thought to be at the relatively low temperature of about 1600C and is under very high pressure of about 1050 kg/cm2 (>1000 bar)

It has a relatively high salinity of 4 to 10 percent and is often referred to as brine

Geopressured systems are quite large: they could be used for the generation of electric power and the recovery of natural gas if suitable technology could be developed

Hot dry rocks or Petrothermal

These are very hot solid rocks occurring at moderate depths but to which water does not have access

Either because of the absence of ground water or the low permeability of the rock or both

Break-up impermeable rock at depth, introduce cold water, and recover the resulting hot water( or steam) for use at the surface

The known temperature of HDR vary between 150 to 2900C

Accounts for large percent of the geothermal resouce

Hot dry rocks

A way be found to render the impermeable rock into a permeable structure with a large heat-transfer surface

A large surface is necessary-low thermal conductivity of the rock

Rendering the rock permeable is to be done by fracturing it

Fracturing methods involve drilling wells into the rock and then fracturing it by

i) high pressure water or ii) Nuclear explosives

Magma resources

Consist of partially or completely molten rock, with temperatures in excess of 6500C, which may be encountered at moderate depths, especially in recently active volcanic regions

Have a large geothermal energy content but restricted to a relatively few locations

The high temperatures will make extraction of the energy a difficult technological problem

Advantages of Geothermal energy over other energy forms

Versatile in its use Cheaper, compared to energies obtained from

other sources Delivers greater amount of net energy from its

system Least polluting compared to some of the other

conventional energy sources Amenability for multiple used from a single

source Renewable resource that has practically no

intermittency, has the highest density

Disadvantages of Geothermal energy over other energy forms

Overall efficiency for power production is low, about

15 %, compared to 35-40% for fossil fuel plants Withdrawal of large amount of steam or water

from the hydrothermal reservoir may result in surface subsidence

Steam and hot water gushing out of the earth may contain H2S, CO2, NH3 and radon gas etc – air pollution

Drilling operation is noisy Large areas needed for exploitation

Resource identification and development

Development begins with exploration – to locate and confirm the existence of a reservoir with economically exploitable temperature, volume and accessibility

Most known reservoirs discovered from surface manifestations such as hot springs

But inadequate- provide meagre or misleading information as to reservoir capacity

Drilling- expensive; something prior to this to forecast geothermal reservoir performance

Resource identification and development

The procedure under study includei) rate of upward heat flow in the groundii) Chemical composition of surface and

ground wateriii) Electrical resistivity of the ground at

varying depthsiv)Seismic measurements

Resource identification and development

Exploratory drilling and production testing is then used to establish reservoir properties

Deep drilled survey wells commonly reach depths of 6 km, and the technology is available to drill to 15 km

Drilling technology derived from –petroleum industry Geothermal drilling- challenging and expensive Temperatures of upto 3500C encountered and higher

than those in oil well drilling hard volcanic rock needs to be penetrated – wear of

drill bit Drilling mud used to lubricate and cool the drill bit

deteriorates rapidly at temperature above 1750C

Resource identification and development

Staged development Modestly sized plant can be installed at

an early stage of field assessment Operation of this plant would provide

more information about the reservoir, which can lead to the installation of future stages

Resource identification and development

Recovery Geothermal energy- not as renewable as

solar and wind energy Tapping into the local sources of the earth’s

heat results in temporary decrease in the local amount of the heat available

Recovery period of geothermal resource depends on how it is used

Recent survey indicates a recovery period of 100-200 years

Geothermal power generation systems

Two types Dry Steam Power plants Wet steam Power plants

Four types i) Single flash steam system ii) Double flash steam system iii) Binary-cycle system iv) Combined flow system

Dry steam power plant

“Dry” steam extracted from natural reservoir 180-225 ºC ( 356-437 ºF) 4-8 MPa (580-1160 psi) 200+ km/hr (100+ mph)

Steam is used to drive a turbo-generator Steam is condensed and pumped back

into the ground Can achieve 1 kWh per 6.5 kg of steam

A 55 MW plant requires 100 kg/s of steam

Dry steam schematic

Single flash steam system

Steam with water extracted from ground Pressure of mixture drops at surface and

more water “flashes” to steam Steam separated from water Steam drives a turbine Turbine drives an electric generator Generate between 5 and 100 MW Use 6 to 9 tonnes of steam per hour

Single Flash Steam Schematic

Boyle, Renewable Energy, 2nd edition, 2004

Double flash steam system

Similar to single flash operation Unflashed liquid flows to low-pressure

tank – flashes to steam Steam drives a second-stage turbine

Also uses exhaust from first turbine Increases output 20-25% for 5% increase

in plant costs

Double Flash Steam Schematic

Binary-cycle system

Low temps – 100o and 150oC Use heat to vaporize organic liquid

E.g., iso-butane, iso-pentane Use vapor to drive turbine

Causes vapor to condense Recycle continuously

Typically 7 to 12 % efficient 0.1 – 40 MW units common

Binary Cycle system Schematic

Boyle, Renewable Energy, 2nd edition, 2004

Combined flow system

Combination of conventional steam turbine technology and binary cycle technology Steam drives primary turbine Remaining heat used to create organic vapor Organic vapor drives a second turbine

Plant sizes ranging between 10 to 100+ MW Significantly greater efficiencies

Higher overall utilization Extract more power (heat) from geothermal

resource

Geothermal Energy Plant

Geothermal energy plant in Icelandhttp://www.wateryear2003.org/en/

Geothermal Well Testing

http://www.geothermex.com/es_resen.html

Geothermal well testing, Zunil, Guatemala     

Heber Geothermal Power Station

http://www.ece.umr.edu/links/power/geotherm1.htm

52kW electrical generating capacity

Geysers Geothermal Plant

The Geysers is the largest producer of geothermal power in the world.

http://www.ece.umr.edu/links/power/geotherm1.htm