Geothermal Energy is the thermal energy generated by the radioactive decay of materials in the earth’s interior, it can be harnessed virtually anywhere on the Earth’s but viable methods are possible only at the surface or near-surface manifestations. These manifestations are only limited to some parts of the world. It is a very nascent renewable source of energy. Even though surface manifestation like hot springs have been used by humans since ancient times, the first geothermal pumps were used in the 1940’s and the first geothermal electric plant became operational only in 1960. Many countries since then have been investing in geothermal energy as a clean and renewable energy source, it is more relevant now that ever.
Several factors need to work together to make an area to be feasible for harnessing geothermal energy. There needs to be a geological heat source relatively close to the surface or at an approachable depth. There are two basic types of geothermal heat resources that can be harnessed. The first type is a Hydrothermal heat source, in this scenario, the heat is transferred by water to the surface, water is recharged into the ground by rain or surface bodies (like river, lakes or glaciers) and is then heated by underlying hot rock which is hot due to seismic or volcanic activity. The presence of hot springs in Iceland, The Himalayas, and The Alps is an example of this. The lithology of the area needs to permeable so that it can allow water to flow freely, it is necessary to recharge the system and also for the hot water to come up. The potential of a geothermal reservoir can be calculated by testing the temperature and flow rates. This is the more traditional source of geothermal energy as it is easy to locate and exploit.
Formation of a Geothermal Reservoir.
The second type of resource extraction can be done by Deep/Enhanced Geothermal systems in which a deep borehole is dug to reach the hot basal rock and then water is pumped into the holes to obtain steam, which is aimed to run a turbine. Accessing the deep basal rock bed is an expensive process and hence this method isn’t as popular.
The most common and the most ancient way to exploit geothermal energy is by direct usage of such near surface reservoirs. The highest capacity among the direct use methodologies is that of heating, mainly for residential and work spaces, the heated water from hot springs is directly pumped into boilers to heat the building. The other most common direct use methods is that of Geothermal Heat Pumps. These pumps exploit the relatively of the earth’s subsurface temperature at depths from 10ft- 300ft, hence they can be used almost everywhere in the world as they don’t require a strong heat source from the Earth, it uses the subtle temperature difference between the subsurface and the temperature above the surface. GHPs circulate water in horizontal or vertical loops under the surface. The loops absorb heat from the Earth and run it through a conventional duct system. For cooling, the system is reversed, the system absorbs heat from the building and transfers it to the loops underground. GHPs can reduce electricity use by 30-60%.
Even though the use of direct geothermal energy is more common and widespread, the majority of the capacity of geothermal resources is used in production of geothermal electricity, currently, the worldwide geothermal power capacity is around 12.8 gigawatts, and it is expected to rise to about 18 gigawatts by 2020. Because the source of energy is constant, power production is consistent for 24 hours much like coal or nuclear energy. This characteristic gives geothermal electric production a higher capacity factor over other renewable sources like solar or wind energy. Currently, only 24 odd countries produce geothermal electricity by using 3 main designs of power plants
The simplest and the most primitive type is the Direct Steam Power Plant, it directly uses steam from the geothermal system to rotate the turbine. It can only be used where a geothermal reservoir is generating a considerable amount of steam, like in the case of geysers and hot springs. The only emission is that of excess steam hence it is completely carbon neutral. The Geysers in northern California are home to complex of 22 geothermal plants, which draw steam form more than 350 wells, making the locality, the single largest source of geothermal electricity in the world.
The second type is the Flash Cycle Plant, fluid that is above 182oc is kept at a much lower temperature than the original pressure of the fluid, and this causes the fluid to vapourise or ‘flash’. The ‘flash’ is then used to rotate the turbine. If some fluid remains in the tank, it can be flashed again by transferring the fluid in a secondary tank at an even lower pressure.
Geothermal reservoirs which contain water below 200oc generally tend to use Binary Cycle Power Plants. In this plant, a secondary fluid (binary) which has a much lower boiling point is heated by moderately hot water to produce steam. The entire system works in closed loops, and hence there is no emission. This is the most common kind of plant available as extremely hot water isn’t needed for electric production. Most new geothermal plants use this system to generate electricity.
Philippines: The Philippines is located in the western part of the Pacific Rim (part of the larger Ring of Fire). The volcanism in the region occurs due to deep fractures in the earth, creating several geothermal centres. The islands also experience a healthy amount of rainfall which recharges the groundwater and makes for an ideal environment to develop geothermal power plants. The Philippines is the second largest producer of geothermal electricity, only after the United States. The Philippines is one of the few countries in the developing world that have invested heavily in geothermal power. It accounts for 25% of the island nation’s power consumption, and the government is investing in further projects.
Major Geothermal Sites in the Philippines. (Source: http://www.ngaphil.org/services)
Iceland: Iceland sits right on top of the mid-Atlantic ridge, providing an ideal setting for extensive volcanism. Due to its isolated location and the lack of other energy sources, Iceland has invested heavily in geothermal energy and other locally available geothermal resources. About 90% of Icelandic households directly use geothermal reservoirs for heating purposes and over one-fourth of the island’s electricity is produced by five major geothermal power plants.
Himalayan Geothermal Belt: The HGB is a large geothermally active area that stretches across the entire Himalayan orogeny spanning half a dozen countries. This region currently produces a negligible amount of electricity compared to its actual potential. Over 150 sites across India, China (Tibet), Nepal, Myanmar, and Thailand have been identified to be hot enough to produce electricity. The geothermal activity is the result of the collision which happened between India and Asia around
40 million years ago. The collision causes the Indian Plate to subduct under Asia, this caused the heating of large granitic batholiths under the Himalayas which heat the subsurface water. The water is recharged by large glaciers, creating an ideal environment for geothermal energy exploitation.
Since the countries around the region have ignored geothermal energy for such a long time and have only shown interest in the resource, the region has the potential to develop as a geothermal energy hotspot.
After the Paris Climate Summit, the need to invest in non-conventional, carbon-free sources of energy has become inevitable. Geothermal energy provides for a renewable and reliable source of energy. An average geothermal plant generates about 122 kg of CO2 per megawatt hour whereas an average coal-powered plant generates 1000 kg per kilowatt hour, just a fraction of the latter. Many developing countries are situated along these geothermal belts and they can use this resource, to balance the hegemony of nations rich in fossil fuels. It cannot be overlooked that the initial installation cost of geothermal plants is very high which makes it difficult for poorer countries to invest in, but with a combined global effort such technologies can be shared and developed around the world, to move towards a more sustainable planet.