|Fig. 1: A diagram of a binary power plant. Source: EERE (Courtesy of the U.S. Dept. of Energy).|
Geothermal energy has been produced since 1913, and has been used to generate electricity since the 1960s. [1,2] Currently, 80 countries have been proven to be in possession of geothermal resources. 58 countries utilize geothermal energy for electricity or direct use. In fact, there are five countries that rely on geothermal energy for more than 10% of their electricity needs. Worldwide, the energy source accounts for 49 TWh of electricity and 53 TWh of direct use, per year. In the U.S. alone, 15.47 TWh of electricity have been produced from geothermal energy, accounting for 0.4% of the national energy requirement. 
While considering the efficacy of geothermal resources, one must consider costs. The price of geothermal energy is variable, but commonly hovers around 4 U.S. cents/kWh.  Plants can be constructed at any location possessing geothermal resources (down hole pumps can be drilled in places where wells do not flow spontaneously). 
There are three commonly used methods to extract geothermal energy: dry-steam plants, flash-steam plans, and binary-cycle plants. The following paper will examine the binary cycle, compare it to the other two, older methods of geothermal energy exploitation, and analyze its environmental effects and potential yields.
Binary plants, like dry-steam and flash-steam plants, make use of naturally sourced hot steam generated by activity from within the Earth's core. All geothermal plants convert thermal energy to mechanical energy, then finally to electrical energy.
Binary plants specifically use a second working fluid (hence, "binary") with a much lower boiling point than water. The binary fluid is operated through a conventional Rankine cycle. Generally, the working fluid is a hydrocarbon such as isopentane, or a refrigerant. The geothermal fluid (predominantly water vapor) and working fluid pass through a heat exchanger, where the working fluid flashes to vapor and drives the turbines. The cooled water vapor is then released back into the underground reservoirs, so the cycle can begin anew. No gas is emitted to the atmosphere, as the binary cycle is a closed system. 
The binary cycle can operate with geothermal fluid temperatures ranging from 85°C to 170°C. Depending on the temperatures, different working fluids are selected based on appropriate boiling points. The upper temperature limit is restricted by the working fluids well, as they are generally organic molecules that become thermally instable at higher temperatures. The low temperature limit is restricted by economic and engineering concerns. The heat exchanger size for a given capacity becomes impractical and costly at low temperatures. Parasitic loads that drive the plant also require larger percentages of the output energy. 
In the geothermal industry, "high temperatures" are characterized by vapor temperatures above 150°C. Low temperatures understandably restrict first-law efficiencies of geothermal plants, per 
Dry plants and flash plants use the geothermal brine to directly power the turbines. Therefore, they cannot be utilized for lower-temperature resources. Binary plants can exploit low temperature fluids, so can be used in more widespread applications. 
Environmentally, binary plants possess key advantages in that they do not release geothermal fluids into the environment. Earth's gases do not just include water vapor. They include nitrogen, carbon dioxide, hydrogen sulfide, ammonia, mercury, radon, and boron. Most of the environmental hazards are released through disposal water or into the environment. Although it is a matter of common practice for power stations to remove hydrogen sulfide from emitted geothermal steam, this toxic gas can still pose an environmental or health hazard. Also, the greenhouse (CO2) emissions are generally around 13-380 g/kWh, which is small compared to the 906 g/kWh from oil, 453 g/kWh from natural gas, or the 1042 g/kWh from coal, but still substantial.  Binary plants skirt these issues altogether by returning the cooled geothermal gas back to its underground reservoir.
In all likelihood, no single energy source can take over from the fossil fuels, which are too energy-rich to be easily replaced.  However, geothermal energy holds significant promise in the search for alternate energies. Potential usable energy estimates are at 2000 ± 140 TWh for electricity generation and 7000 TWh for direct heating, per year.  Most industry experts predict future geothermal plants will be binary-cycle plants that will exploit Earth's low-temperature resources. Looking further into the future, advancements in HDR (hot dry rock) technology would allow geothermal energy exploitation anywhere. 
© Zoe Yan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 I. B. Fridleifsson, "Geothermal Energy for the Benefit of the People," Renewable and Sustainable Energy Reviews 5, 299 (2001).
 M. Kanoglu , "Exergy Analysis of a Dual-Level Binary Geothermal Power Plant," Geothermics 31 709 (2002).
 T. Maghiar and C. Antal, "Power Generation from Low-Enthalpy Geothermal Resources," GeoHeat Center Quarterly Bulletin, 22, No.2, 35 (2001).