Geothermal energy is heat from the Earth. According to Law No. 21 of 2014 on Geothermal, geothermal is a source of heat energy contained in the hot water, steam, and rock along with the associated minerals and other gases that are genetically unable to be separated in a geothermal system.
Indonesia and the countries that are in the Pacific Fire Belt have geothermal resources with high temperature and characterized by manifestations such as hot springs, fumaroles and steaming ground. Geothermal potential can be determined by integrating applying geosciences.
Geothermal energy has many advantages when compared to other energy sources. These advantages include:
- Environmentally friendly due to pollutants produced is very small compared to fossil fuel power plants and does not generate waste because the fluid is injected back into the reservoir.
- Renewable because the waste fluid after being utilized is injected back into the reservoir. The fluid is also experiencing the natural cycle with the addition of meteoric water.
- Sustainable because geothermal sources have long-lived characteristic (hundreds of thousands of years) and can ensure the supply of natural heat energy that will not run out in human history if they are properly managed.
- Locally characteristic resources and can be used directly. This means that this energy can be used directly in place and only be moved to another place in the form of electrical energy so that they can contribute in supporting the prosperity of the region.
- Supply is stable because it is not depending on the season or the situation of the world market.
- The production technology and its utilization are relatively simple and safe.
- The total area of land for the production and utilization are relatively small.
Indonesia has been blessed with geothermal energy reserves abundantly. Geothermal potential is estimated to spread over 250 locations with a total potential of about 29,000 MW. Being renewable, sustainable, and local characteristic, geothermal energy becomes reliable, because the supply does not depend on the season and the overseas energy market conditions.
The heat source of the geothermal system is most likely an intrusion body of igneous rock (at a depth of 3-5 km) which is capable of transferring heat in a long time, for hundreds of thousands of years (Norton and Knight, 1977; Arehart et al, 2002). The heat source transferring the heat to the surrounding rocks and water that are collectively referred to as geothermal reservoir. Geothermal reservoir usually has the covering layer at the top in the form of impermeable rock.
The water which fills the reservoir is derived from meteoric water (rain water). It is heated and receives magmatic gases input and are called hydrothermal fluid. Due to the buoyancy effect, the hydrothermal fluid rises to a shallow depth, forming a flow structure known as the up-flow. Hydrothermal fluid also flows away from the center of heat, forming a structure called the outflow. There are also some fluid (usually a result of condensation of steam) flowing down to a greater depth, forming a structure called the down-flow.
When the geothermal fluid flows in the rocks, it interacts with the rocks resulting the change in the mineralogical composition and physical properties of rocks, while the composition of the fluid itself also changed either because of the process of interaction with the rock as well as the physical processes. That is why, in a geothermal system there are various kinds hydrothermal fluid. On its way to the surface, hydrothermal fluid can undergo boiling, condensation, mixing with the fluid coming from a shallow depth, and precipitate minerals whose components come from solutes. In contrast, the acidic condensate will dissolve rocks in its path. The process of changing the origin of rock due to interactions with hydrothermal fluids and mineral deposition process by hydrothermal fluid called geothermal alteration.
High-temperature geothermal systems in volcanic areas in steep terrain as it is common in Indonesia. It can be recognized hot structure, the types of fluid, as well as the deployment of the various geothermal manifestations (Utami, 2010)
Geothermal manifestations as an indication of the existence of geothermal prospects in the depths formed when the geothermal fluid from depth gets the way to reach the surface. The types of geothermal manifestations, among others:
- Fumaroles: the discharge of steam and gas from the reservoir through a hole that produce noise that can be heard.
- Geyser: hot springs that periodically ejects water vapor and gases, and less dense material that usually follows a certain cycle.
- The alkali chloride hot springs: hot springs that produce water with the composition of alkali chloride, are generally neutral or nearly neutral. Some of the alkali chloride hot springs precipitate silica mineral (called silica sinter) on the surface.
- Acidic hot springs: hot springs that emit sulfuric acid condensates are formed near the surface.
- The bicarbonate hot springs: the discharged fluid has the composition of bicarbonate acid, usually located around the border of the field.
- Warm springs: the discharged hydrothermal fluid is warm (not boiling); a relatively low temperature is likely due to hydrothermal fluid has traveled laterally with long distance and mixes with shallow water.
- Mud pool: mud puddles formed due to dissolution of rock by hydrothermal fluid condensate heat and acidic.
- Warm ground/ steaming ground: the phenomenon of heat transfer by conduction to the surface causes the ground in some parts of the geothermal area becomes warmer. If there is an appearance of geothermal steam through pores in the surface of the ground, then known as the steaming ground.
- Altered soil or rock: soil or rock on the surface which has undergone an alteration process by the geothermal fluid that change color, mineral composition and physical properties.
- Hydrothermal eruption: eruption which throws solid materials in which the energy released comes from heat loss and hydrothermal fluid phase change.
The utilization of geothermal resources can be generally classified into utilization for electricity generation and utilization of non-electric or direct use. This type of geothermal resource utilization will determine the method. Geothermal sources with high enthalpy (> 225oC) such as the source with dry steam or hot fluids commonly found in volcanic areas utilized for power generation.
The utilization for power generation is done by extracting geothermal energy from geothermal fluid obtained from the wells that penetrate the geothermal reservoir at a depth of about 1-3 km. The geothermal fluid in the reservoir that can be produced with existing technology is the neutral alkali chloride fluid. The heat energy is then converted into electrical energy through the process of electricity generation.
Geothermal energy can also be used directly for the industry, for example for drying agricultural and fishery products. Development of engineering sciences are needed to be able to produce and utilize geothermal energy.