Introduction

Geothermal energy is the energy derived from the earth in the form of heat. It is a kind of renewable source of energy present in the pores and fractures of the rocks of the earth’s crust. An example of geothermal energy is the shallow groundwater and hot rock in the deep crust to the magma. Radioactive decay of material and geological processes generates heat that comes from Earth’s Interior.

The difference in the earth’s core and surface temperature always derives the continuous thermal energy conduction toward the surface from the center. The temperature in the earth’s interior reaches more than 4000oC, which causes the rock to melt and form magma that acts as plastically, and some portion of its moves upwards because of less dense than surrounding rock. The water and rock in the earth’s crust heat up and reach around 370 oC. The thermal energy contained in the fluids and rock is present at the shallow depth of the earth’s surface. Conversion of thermal energy  to electrical energy happens using various methods.

                         FORMATION OF GEOTHERMAL ENERGY RESOURCE

Capacity And Potential Of Geothermal Energy

This renewable energy is the most versatile form of energy used for various purposes like bathing, washing, cooking, and health. The direct use of hot water is long-lasting and has a wide range of applications. The first commercial electricity production plant by geothermal energy was installed in 1913  in Larderello, Italy. Geothermal energy use was increased since 1970.  The capacity of direct use of geothermal energy is tripled 15 to 50GWth in 2000-2010, globally. Whereas the electricity production is increased 10.7GWe from 8.0GWe. Currently, 78 countries use geothermal energy for heating purposes, and 24 countries use it for electricity production.

The total estimated capacity around the world is 12.8 GW. The major countries included the USA, Philippines, Indonesia, Mexico, and New Zealand. The total installed capacity of geothermal for direct use is about 70.3GW, and the leading countries are China, the USA, Sweden, Turkey, Germany, France, Japan, and Iceland.

Placement of India in the low and medium heat enthalpy zone with a temperature range (100-180 °C) and 340 regions are well known for geothermal energy as host springs in a different part of India with temperature ranges from min 35 °C to max 98 °C.  Manikaran, Tatapani, Cambay, Puga, and Tapovan are potential geothermal locations in India.

The hot spring in India is categorized into seven geothermal regions:

Himalayan – Puga Chhumthang Province,  Sohana Valley,  Cambay Basin (Son-Narmada-Tapi (SONATA) lineament belt, West Coast, Godavari basin and Mahanadi basin.

THERMAL CHARACTERISTIC OF POTENTIAL GEOTHERMAL PROVINCES

Resources of Geothermal Energy

Four kinds of geothermal resources are available magma, geo-pressured, hydrothermal, hot dry rock. Only hydrothermal geothermal energy resources are widely in use. The other three energy resources are harder to harness and still in developing states.

Hydrothermal resources have heated water to produce either electricity or for heating purposes. The hydrothermal reservoirs of hot water or steam are available naturally when magma reaches near the underground water, heats the porous or fractured rocks, and pass the heat to the water. Such resources are used based on their temperature.

Low temperature

When the temperature of the hydrothermal resource is 50F or up, it is directly used for heating purposes like heating the building, warm fish pond, in the spa, or some other use. In Iceland, people use such resources for heating the school, homes, and public buildings. 45 states of the US use geothermal heat pumps to heat and cool buildings and homes.  Lumber, vegetables, dry ceramics, and other products utilize Geothermal heat.

High temperature

The hydrothermal resources have a temperature of around 220F or more. Such resources are used to produce electricity. Generally, the electricity produced from hydrothermal resources has a temperature of about 300 to 700F.

Dry steam(vapor dominated) reservoirs and Hot water(liquid dominated) reservoirs

Methods Of harnessing Geothermal Energy

Geothermal energy can harness by producing electricity and can be used directly. To use geothermal energy brings to the surface for power generation from the resources. Some power generation system is used to harness the energy that depends on the characteristics of geothermal fluid (pressure, temperature, flow rate, etc), single, double, dry flash steam, and binary cycle.

Direct Utilization: In this geothermal energy, the temperature range low to intermediate is used directly for commercially heating, cooling the equipment, aqua pond, lumber drying, mineral ore extraction. There are no conversion efficiency losses. Geothermal energy can use 80-90% for annual heating and cooling.

Geothermal Heat Pump: This type of geothermal energy is used for heating and cooling and a small amount of electricity is used to run a compressor, such heat pumps use relative to the earth’s constant temperature. The closed-loop pipe is put in the ground, around 50-70m deep with water antifreeze solution in the circulating pipe for collecting ground heat in winters. The GHP efficiency is defined by the COP coefficient of performance, which is the ratio of output energy divided by input energy.

Electric Power Generation: Process of producing electricity by using hydrocarbon vapors or steam to run the turbines- generator and generate electricity. Geothermal energy dominates the vapor directly to produce electricity. While in hot water, resources required to generate steam that range is 15-20%. Some plants use double or triple flashes for efficiency improvement.

Geothermal Energy Power Plants

Geothermal power plants are those used to generate electricity. These plants used hydrothermal resources to produce electricity via magma as a heat resource in the earth’s crust. It is composed of water and heat, which is heated water and the temperature of hydrothermal resources can be 150 to 350 degrees Celsius that can be harnessed by drilling (hot water or dry stem well) and extracting hot water and stem via piping to the surface. The steam then directly turns the turbine and produces electricity.

Geothermal power plants are divided into three categories:

 1. Dry Steam Geothermal Plants

In this type of geothermal power plant, the underground generated natural steam through geothermal heat directly runs the turbine without the need for a boiler that turns the water into steam.

Conventional generators produce electricity by dry steam. It is also referred to as a vapor-dominated geothermal power plant. The pressure of the steam used from hydrothermal resources reaches 7atm and temperature 165 degrees Celcius for electricity production. This type of plant consists of a production well  (steam resources). The centrifuge separator separates the stem from the solid matter and the turbine is used to change the thermal to mechanical energy, which is connected with a generator for power production.

The last is a condenser, which condenses the wet steam from the turbine and cools it by direct contact with water.

     SCHEMATIC DIAGRAM OF DRY STEAM GEOTHERMAL PLANT

2. Flash Steam Geothermal Plants

In this type of geothermal power plant, the steam is produced by flashing the liquid, and steam is separated from liquid to run the generator turbine,  this flash steam technology is used when the liquid temperature of hydrothermal reaches the 177-degree Celsius. It is also referred to as a liquid-dominated or wet steam geothermal plant. In this dual cycle, the water separated from the steam at different pressure is sent back to the geothermal reservoir to manage the economy of flash hydrothermal plants, the dual flash generates 20% to 30% more power than a single flash system.

SCHEMATIC DIAGRAM OF FLASH STEAM GEOTHERMAL PLANT

3. Binary Power Plants

This hydrothermal power plant used hot water with low temperatures (100-300F). In this type, a heat exchange method is used, in which a secondary fluid is used as a heat exchanger with a low boiling point. Generally, isobutane or isopentane is used. The secondary fluid absorbs the heat of the hot water and converts it into steam, turning the turbines for the generator to produce electricity. The secondary fluid is continuously recycled and used again from the same process, while the geothermal fluid is condensed and sent back to the reservoir.

              SCHEMATIC DIAGRAM OF BINARY CYCLE BASED GEOTHERMAL ENERGY PLANT