In a previous article titled “Geothermal Potential in the Caribbean,” I highlighted the vast geothermal energy potential of several islands in the Caribbean. The history of electricity production from geothermal energy dates back to the early 1900s, however, of the islands with geothermal potential only Guadeloupe to date has a 4.5MWe geothermal electricity plant. A conventional geothermal power plant taps into the earth’s hydrothermal circulation system via a production well to capture hot water or steam which is then used to drive steam turbines. The turbines intern drive electric generators to produce electricity, as shown the figure 1. The captured geothermal fluids are then returned to the earth via an injection well also shown in figures 1, 2, 3 and 4.
There are primarily three types of geothermal power plants namely Dry Steam, Flash Steam and Binary Cycle. All operate on the principle of pulling hot water and steam from the ground, using it, and then returning it to prolong the life of the heat source. A dry steam plant operates on hydrothermal fluid that is primarily steam, the steam is passed directly into the turbine to produce electricity. The steam then condenses (turns to water) and returns to the earth, as shown in Figure 2.
A flash steam plant, on the other hand, uses hydrothermal fluids above 360°F (182°C). The fluid is sprayed into a flash tank, which is at a much lower pressure than the fluid, causing some of the fluid to rapidly vaporize, or “flash. The vapour or steam then drives the turbine, as shown in Figure 3.
A binary cycle plant, unlike the dry steam and flash steam plants, uses a heat exchange system and a secondary fluid – such as isobutene – to capture the energy from the hydrothermal fluid of moderate temperature (below 400°F). This secondary fluid, which is used to drive the turbine as shown in figure 4, has a much lower boiling point causing it more easily convert into vapour.
The design choice is driven mainly by the nature of the available resources, however since moderate-temperature water is by far the most common geothermal resource, then future geothermal projects will mostly be binary cycle geothermal power plants. The 4.5MWe plant in Guadeloupe is a binary cycle power plant.