Geothermal Energy and Geothermal Power Plants

Geothermal Power Production:

There are two ways of electric power production from geothermal energy 

1. Heat energy is transferred to a working fluid which operates the power cycle. this may be particularly useful at places of fresh volcanic activity where the molten interior mass of earth vents to the surface through fissures and substantially high temperatures such as between 450 to 550 degree centigrade can be found. 

2. The hot geothermal water and/or steam is used to operate the turbines directly from the wellhead the steam is transmitted by pipelines up to 1 meter in diameter over distances up to about 3 km to the power station. water separators are usually employed to separate moisture and solid particles from steam.

Geothermal plants are of following types

1. Vapour dominated (or) dry steam fields

Steam extracted from the well is cleaned in centrifugal separator which removes solid matters. The cleaned steam is then supplied directly into the steam turbine and the exhaust steam from the steam turbine is wet steam (i.e a mixture of water and steam) which passes through the condenser. The condenser condenses wet steam into water (through a cooling tower)

2. Liquid dominated high temperature systems 

     a) the flash steam open system

     b) the binary cycle system

In flash steam open system hot brine from the reservoir reaches the wellhead at lower pressure by throttling process. the flash chambers brine separators which improves the quality of mixture. now steam is separated as a dry saturated steam and supplied to the steam turbine which produces electric power through a generator.

In binary cycle system the steam turbine is isolated from corrosive or non corrosive material and/or to accommodate higher concentration of non condensable gases. this is basically a rankine cycle with an organic working fluid.

3. Total flow concept system

in such a system both kinetic energy and heat energy of the steam fluid mixture, produced by flashing the geothermal brine are utilized. the overall efficiency for conversion in to electrical energy should be greater than other methods in which only the heat content of the brine is utilized. the hot brine from geothermal well is throttled where it becomes a two phase mixture of low quality. the two phases at this point are not separated and the full flow is expanded in the mixed phase expander which is coupled to the generator. the mixture from this expander/turbine is discharged in to condenser, then the brine condensate is re injected in to the well.

Geothermal Energy introduction

GEOTHERMAL ENERGY:

Geothermal comes from Greek words Geo meaning earth and thermal meaning heat. Geothermal energy is primarily heat energy from earth's own interior. 

INTRODUCTION: It is classified as renewable because it comes from the earth's interior which is almost inexhaustible as like solar and wind energy, so long as its sources are actively sought and economically tapped. As we travel down earth's surface radially, there exists a temperature gradient of 0.03 deg c per meter. Thus a 30 deg c increase in temperature can be obtained per km depth from the earth crust. There are many local hot spots just below the surface where the temperatures are much higher than expected. Ground water, when comes into contact with hot spots, either dry or wet steam is formed. By drilling holes to these locations, hot water and steam can be tapped and these can be used for power generation or space heating. 

Heat transfer from the earth's interior is by three means 1. Direct heat conduction 2. Rapid injection of ballistic magma along natural rifts penetrating deep into earth's mantles. 3. Bubble like magma that buoys upwards towards the surface.

IMPORTANT ASPECTS OF GEOTHERMAL ENERGY 

FORM OF ENERGY Thermal energy in the form of hot water, steam, geothermal brine, mixture of these fluids. 

AVAILABILITY Generally available deep inside the earth at a depth more than about 80 km. Hence, generally not possible to extract. In a few locations in the world, deposits are at depths of 300 to 3000 m. Such locations are called geothermal fields. 

METHOD OF EXTRACTION: Deep production wells are drilled in the geothermal fields. The hot steam/water/brine is extracted from the geothermal deposits by production wells, by pumping or by natural pressure. 

GEOTHERMAL FLUIDS Hot water Hot brine Wet steam Mixture of above 

RANGE OF GEOTHERMAL POWER PLANTS INSTALLED CAPACITY 5 MW to 400 MW 

AVERAGE GEOTHERMAL GRADIENT 30 deg c  per 1000 m length 

GEOTHERMAL ENERGY RELEASED THROUGH EARTH'S CRUST About 0.06 W/m 

TOTAL GEOTHERMAL RESERVES IN THE EARTH 4x(10^15) MJ (estimated) 

RATE AT WHICH THE RENEWABLE ENERGY CAN BE TAPPED FOR PRODUCTION OF ELECTRICITY 2x10^12 to 10x10^12 MJ/year (estimated) 

TYPES OF GEOTHERMAL ENERGY DEPOSITS 

Hydro thermal: hot water and steam, hot brine. 

Petro thermal: hot dry rock (HDR)