Geothermal energy is classed as renewable energy. Renewable energy is generally described as energy obtained from sources that are essentially inexhaustible, in contrast to fossil fuels, of which there is a finite supply.
The term geothermal energy is often used to indicate that part of the earth's heat that may be recovered and utilised. Heat transferred from the earth's molten core to underground deposits of dry steam (steam with no water droplets), wet steam (a mixture of steam and water droplets), hot water, or rocks lying fairly close to the earth's surface. It is also generated locally within the earth's crust from the natural decay of the radiogenic elements that occur in rocks, and in certain granites they can be concentrated such that there is a marked elevation in the local surface heat flow.
The characteristics of geothermal systems vary widely, but three components are essential:
- a subsurface heat source
- fluid to transport the heat
- faults, fractures or permeability within sub-surface rocks that allow the heated fluid to flow from the heat source to the surface
The amount of heat being generated by the earth (heat flow) is one of the key factors that determine the temperature gradient at any location. The other major factor is the thermal conductivity of the crustal rocks, which controls how well they trap the generated heat. High heat flow will result in a higher temperature gradient, while an insulating blanket of sedimentary rocks over the heat source will trap that heat. Some rocks make better insulators than others, but in general, fine grained sedimentary rocks such as shale are better insulators than sandstone. The highest thermal gradients are therefore found in regions with both high heat flow and low thermal conductivity.
The benefits of geothermal energy
When properly developed and managed, geothermal systems are a clean, abundant, and reliable source of renewable energy. Use of geothermal energy for electricity generation or for direct use conserves non-renewable and more polluting resources. It is uniquely reliable, with conventional geothermal energy plants typically achieving much higher load factors compared to typical load factors for hydro and wind power plants. Geothermal energy is effectively a renewable resource that does not consume any fuel or produce significant carbon dioxide emissions.
Making geothermal energy viable
Notwithstanding the public policy imperative that is driving renewable energy opportunities, there are three key elements that must be satisfied before a viable geothermal prospect can move to commerciality. These are:
- Quantum of energy: temperature and flow rate
- Cost of production: exploration and development drilling
- Proximity to markets: transmission grid for geothermal energy, heat drying needs of domestic and industrial customers etc
Types of geothermal systems
There are two basic geothermal system types: Hydrothermal and Hot Rock or Enhanced Geothermal Systems.
Normally conventional hydrothermal reservoirs are found in fractured volcanic rocks where temperatures are relatively high near the surface such as in New Zealand. However, geothermal reservoirs are found in non volcanic areas where the crustal heat flow is sufficient to produce high temperatures and the rocks are permeable to allow the production of large volumes of fluid.
Greenearth Energy's domestic and international exploration and development focus is on conventional geothermal (Hydrothermal) systems. In Australia the focus is on Hot Sedimentary Aquifer (HSA) systems. In Indonesia the focus is on Volcanic Systems.
FIGURE 1: Schematic of Geothermal Systems
The extraction of heat from hot rocks is achieved by pumping water into the rocks at depth using an injection well, and subsequently withdrawing it from a production well at a much higher temperature after it has flowed under pressure through fractures in the hot rocks. Hot rocks are hot due to the heat which is generated at depth being trapped by the insulating effect of overlying rocks and sedimentary cover usually over 3000 metres in thickness.
Uses for geothermal energy
Direct uses for low and moderate temperature resources typically up to 150oC involves using the heat in the water directly (without a heat pump or power plant) for such things as heating of buildings, industrial processes, greenhouses, aquaculture (growing of fish) and resorts. Figure 2 provides an overview of such applications at varying temperatures.
FIGURE 2: Temperature ranges for Direct Heat Applications
Power generation using Geothermal Energy
Most geothermal power plants operating today are "flashed steam" power plants using high temperature water from production wells. Released from the pressure of the deep reservoir, part of the water flashes (explosively boils) to steam and the force of the steam is used to spin the turbine generator. In a binary power plant the working fluid (usually isobutane or isopentane) boils and flashes to a vapor at a lower temperature than water does, so electricity can be generated from reservoirs with lower temperatures.
Binary power plants have virtually no emissions but are relatively less efficient. Figure 3 provides a schematic of a typical binary power plant arrangement.
Figure 3 Schematic of Binary Cycle Power Plant Source: Oregon Institute of Technology (modified by Greenearth Energy) to take account of the change of the cooler to air cooling rather than vapour (evaporative) cooling.
Conventional hydrothermal systems have well-demonstrated economic and technological viability. Some geothermal fields have been supporting cost-effective, dependable electrical power generation for over 50 years. Ongoing advances in binary geothermal power-plant technology have lowered the temperature requirement for electrical power generation, and also improved the bottom-line economic viability of many conventional hydrothermal resources.