4.2.6. Geothermal

Geothermal energy is the type of renewable energy that uses heat derived from the sub-surface of the Earth, which can be transmitted in the form of hot steam, hot water, or a mixture between both forms. Nowadays, it has been one of the most important alternatives for energy sources with significant growth potential. It not only provides alternative energy but also helps to reduce the effects of global warming and the risks to public health due to the use of conventional energy sources, as well as our dependence on fossil fuels. Geothermal energy may be used for district heating purposes or harnessed to produce renewable electricity, depending on its characteristics. Lower enthalpy type of geothermal is mostly suitable for direct use (e.g., room heater, tourism, agriculture/agroindustry, and fisheries), while medium to high enthalpy type of geothermal can be used for generating electricity, which is typical for the regions with active tectonics [42]. Indonesia has varying types of geothermal energy that can be utilised for both direct heating and generating electricity [43].

Indonesia is one of the countries in the world that falls on the "ring of fire", which traversed around the edges of the Pacific Ocean and is responsible for most active volcanoes and earthquakes. Due to its tectonic setting, Indonesia is a host to most of these active volcanoes, which accounted for 117 active volcanoes in total [44,45]. These active volcanoes are distributed in Sumatra, Java, Nusa Tenggara, Sulawesi, and Maluku. Consequently, Indonesia has a considerable amount of high heat flow, which makes it one of the countries with a large potential for geothermal energy.

According to studies, Indonesia has the world's largest geothermal energy potential, accounting for about 40% of the world's potential or approximately at 28,617 MW [42]. Most of these potential energy resources and reserve are distributed in several regions in Indonesia. Sumatra and Java have currently the highest total potential energy, which accounts for 12,760 and 9717 MW, respectively [42]. The rest of the potential are distributed in many other regions, namely Bali, Nusa Tenggara, Sulawesi, Maluku, Kalimantan, and Papua [42,43].

According to FI, geothermal, when compared to other sources of renewable energy except nuclear, can guarantee the provision of electricity at a stable rate throughout the entire year without being affected by weather patterns and conditions. He further added:

*"The cost of geothermal technology in the future will be increasingly competitive and is expected to continue to fall. Thus, the optimisation of geothermal energy in Indonesia is*
