In a continuation of the current alternative energy series, this week’s blog will examine the engineering ideas behind geothermal energy. As the name would suggest, geothermal energy is energy harvested from the natural heat of the Earth. Below the Earth’s crust is a layer of hot, molten rock called magma. Magma continually produces heat from the decay of naturally radioactive materials such as uranium and potassium. The available heat within 10,000 meters of the Earth’s surface contains 50,000 times more energy than all of the oil and natural gas resources in the world.i The highest underground temperatures are found in places with active or young volcanoes. These areas occur at plate boundaries with some of the hottest spots occurring along the Pacific Rim, or the Ring of Fire. The Pacific Rim includes regions in Alaska, California, Oregon and Nevada. Earthquakes and magma movement break up the rock below the surface of the Earth, which allows water to come to the surface. One such famous occurrence is the geyser known as Old Faithful in Yellowstone National Park. The water in geysers can reach temperatures of 430o F.ii
Methods of Energy Generation
There are three types of geothermal energy power plants. The first is known as a dry steam power plant. These types of plants draw steam from underground reserves and pipe it directly into the plant. After entering the plant, the turbine, which is the primary component in the energy generation process, receives the steam to begin generating power. Currently there are only 2 known dry steam reserves in the United States, one at The Geysers of northern California and one in Yellowstone National Park, Wyoming. However, due to a national park’s protection from development, the only dry steam power plant is in California. The second, and most common, type of geothermal energy plant is known as a flash steam plant.iii These plants use geothermal reservoirs of water that exist at 360o F, which is possible due to the immense amount of pressure the water is under. The water flows upward through pipes under its own pressure. As it rises, the pressure decreases and some of the water turns to steam. This steam separates from the water and powers a turbine. The leftover water that did not convert to steam is then injected back to the reservoir. The third and final type of geothermal plant is known as a binary steam plant. Binary plants utilize water at lower temperatures, normally in the range of 225o to 360o F.iv Instead of directly using the steam from the geothermal reserve, a binary plant uses the water to heat a working fluid. This working fluid is normally an organic compound with a low boiling point. As the working fluid vaporizes in a heat exchanger, the vapor spins a turbine. A pump then forces the water back into the original reserve so that it can be heated and the process can be repeated. This method has little to no emission because the water and the working fluid are kept separated the entire time.
Engineering a Turbine
For many years after the first turbine was designed in 1830, the design could not be perfected because metallurgy did not yield a strong enough product. It wasn’t until 1884 when Charles Parsons utilized new steel developments to design a turbine that could handle the stress from such rapid rotation. His design was able to spin at nearly 18,000 revolutions per minute.v These advances in materials science, particularly metallurgy, enabled the advance of turbine technology. In modern design, mechanical engineers also utilize computational fluid dynamics, CFD, to perform thermal analysis, heat transfer analysis, fluid analysis, and finite element analysis, FEA, to determine the stresses on the turbine. Technology advances have allowed today’s turbines to be designed with rocket technology in order to increase their electrical output and limit the level of CO2 emitted. One example of recent developments is cooling the inlet air before compressing it in the compressor. A lower temperature cases the air to have a higher density and therefore a higher mass flow rate through the turbine. This heat transfer processes, allowing for the higher mass flow rate, results in increased energy output. Modern steam turbines are made of a stationary set of blades,called nozzles, and an adjacent set of moving blades, called rotor blades. The stationary blades accelerate the steam to a higher velocity by expanding it to a lower pressure. The rotating blades change the direction of the steam flow, which creates a force on the blades, and because of the wheel geometry creates a torque on the shaft which the bladed wheel is mounted. This combination of torque and speed is what generates the output power of the turbine. While today’s turbines are tuned to specific gas densities, molecular weights, and viscosities, the main engineering ideas behind them are very similar to the turbine designs of 100 years ago.
i Union of Concerned Scientists CleanEnergy “How Geothermal Energy Works” December 2010
ii Union of Concerned Scientists CleanEnergy “How Geothermal Energy Works” December 2010
iii National Renewable Energy Laboratory Learning About Renewable Energy “Geothermal Energy Production” May 2012
iv Elliot Group “Steam Turbines” 2010
v Energy and Environmental Analysis “Technology Characterization: Steam Turbines” December 2008