Blog Post Thursday 30 November 2017
Geothermal energy is a promising form of renewable energy. While it remains an emerging industry with virtually no commercial operations in Australia, we’re endowed with potential geothermal resources that could supply all our energy needs. Given this, it’s worth understanding how geothermal energy works and how it’s captured and delivered for use.
Geothermal energy captures the natural heat contained deep within the earth. It draws heat from deep in the earth and brings it close to the surface by pumping water down and back up. The water is pumped into underground cavities, which might be hot rock or even magma and molten lava. The heated water and/or steam is pumped back to the surface and used to run an electrical generator, thereby creating energy. The fluid is then returned to the cavities to be reused.
A few different types, or sources, of geothermal energy are used around the world. These include hot springs, hot sedimentary aquifers, and enhanced geothermal systems. Appropriateness of type depends on the geological features of the location.
Hot springs – This type of geothermal energy draws energy from hot springs associated with volcanic activity. This is the most common source of geothermal source.
Hot sedimentary aquifers – This source relies on naturally occurring reservoirs that are heated by proximate hot rocks or crustal heat flow.
Enhanced geothermal – Enhanced geothermal or ‘hot rock’ systems draw on the heat from rocks deep under the surface. The rocks are fractured (fracked, like oil fracking) to create permeable reservoirs into which water can be pumped.
Direct use – Direct use systems rely on shallow underground reservoirs where the groundwater is hotter than surface temperatures.
The potential for geothermal energy is virtually limitless because the Earth continues to retain a significant amount of heat generated during its formation. Additional heat is also being continuously generated due to the decay of radioactive elements in the Earth. While over time individual geothermal sites can lose heat as hot water and steam are produced and the energy captured, on the whole geothermal is highly renewable given the Earth affords an inexhaustible supply of heat.
Geothermal energy is considered a clean source of energy because it can be captured without using fossil fuels. Geothermal fields generate around one-sixth of the carbon dioxide natural-gas-powered plants create. Additionally, they release little if any nitrous oxides and sulphur-bearing gases. Some geothermal plants are binary plants, which means they’re close-cycle plants and release virtually no emissions.
While generating geothermal energy requires water inputs, this water can be reused repeatedly.
Suitable geothermal sites are those with plentiful hot geothermal fluid low in minerals and gases. Shallow aquifers are required for producing and reinjecting the hot water, and nearby transmission lines are ideal for transporting the captured energy.
Geothermal plants can have a lower environmental footprint than conventional power plants. Binary or closed-cycle plants have low emissions and discharge no liquids into the surrounding environment. Any dissolved minerals and salts in the geothermal fluid that’s pumped up is usually sent back down to the cavities, clear of water aquifers.
Geothermal plants can even assist with repurposing wastewater. For example, in Santa Rosa, California, treated wastewater is used in The Geysers geothermal plant as reinjection fluid.
Note some geothermal plants – non-closed-cycle ones – do produce sludge or solid by-products that need to be disposed of carefully. Some of these solids can be used to extract valuable substances like zinc, silica, and sulfur.
Geothermal power plants use a relatively small amount of space and there’s no need to store or transport flammable fuels. Emissions aren’t visible, and the plants have minimal visual impact.
Commercial geothermal is in its infancy in Australia, but geothermal plants are already prevalent in the US, where Western states like Hawaii, Alaska, and California have reservoirs of hot water that support the capture of geothermal power.
Iceland relies on geothermal for 25% of its total electricity generation, and the Philippines and Kenya both draw around 17% of their energy from geothermal sources Indonesia, Mexico, and Italy are also major users of geothermal power.
Geothermal energy is a promising clean, renewable, and relatively sustainable energy source. One of the top benefits of geothermal energy is it’s available at all times regardless of weather conditions, in addition to producing little or no by-products and emissions. With a closed-loop system in an ideal geological setting such as hot springs, a geothermal plant can be clean and cost-effective to set up.