By John Denny
Observer Reporter
(Nevis) – There has been much discussion this year about the geothermal project being developed by West Indies Power. Most of the talk has been about who will profit and how much little attention has been given to explaining how it works.
A few miles under our feet, the Earth is liquid rock: magma. In certain hotspots such as Nevis, the magma is closer to the surface and heats water trapped in porous rock. This is what creates the hot springs below the Government House.
Several thousand feet down, the temperature of the water rises to well above boiling creating natural superheated steam. In traditional power plants, oil, coal or natural gas is the fuel used to create superheated steam.
The most common current way of capturing the energy from geothermal sources is to tap into naturally occurring “hydrothermal convection” systems where cooler water seeps into Earth’s crust, is heated up, and then rises to the surface. When heated water is forced to the surface, it is a relatively simple matter to capture that steam and use it to drive electric generators. Geothermal power plants drill their own holes into the rock to more effectively capture the steam.
There are three designs for geothermal power plants, all of which pull hot water and steam from the ground, use it, and then return it as warm water to prolong the life of the heat source. In the simplest design, the steam goes directly through the turbine, then into a condenser where the steam is condensed into water. In a second approach, very hot water is depressurized or “flashed” into steam which can then be used to drive the turbine.
In the third approach, called a binary system, the hot water is passed through a heat exchanger, where it heats a second liquid—such as isobutane—in a closed loop. The isobutane boils at a lower temperature than water, so it is more easily converted into steam to run the turbine
The choice of which design to use is determined by the resource. If the water comes out of the well as steam, it can be used directly, as in the first design. If it is hot water of a high enough temperature, a flash system can be used, otherwise it must go through a heat exchanger. Since there are more hot water resources than pure steam or high-temperature water sources, there is more growth potential in the heat exchanger design.
The largest geothermal system now in operation is a steam-driven plant in an area called the Geysers, north of San Francisco, California. Despite the name, there are actually no geysers there, and the heat that is used for energy is all steam, not hot water. Although the area was known for its hot springs as far back as the mid-1800s, the first well for power production was drilled in 1924. Deeper wells were drilled in the 1950s, but real development didn’t occur until the 1970s and 1980s. By 1990, 26 power plants had been built, for a capacity of more than 2,000 megawatts.
Because of the rapid development of the area in the 1980s, and the technology used, the steam resource has been declining since 1988. Today, the Geysers has a capacity of 850 MW, which still meets nearly 70 percent of the average electrical demand for California’s North Coast region. The plants at the Geysers use an evaporative water-cooling process to create a vacuum that pulls the steam through the turbine, producing power more efficiently. But this process loses 60 to 80 percent of the steam to the air, not reinjecting it underground. While the steam pressure may be declining, the rocks underground are still hot. Some efforts are under way to remedy the situation, including the Santa Rosa Geysers Recharge Project, which involves injecting treated wastewater from neighboring communities through a 40-mile pipeline. Recharging the existing reservoirs is estimated to increase output by 85 MW, providing enough electricity for approximately 85,000 homes.
One concern with open systems like the Geysers is that they emit some air pollutants. Hydrogen sulfide, a toxic gas with a highly recognizable “rotten egg” odor, along with trace amounts of arsenic and minerals, is released in the steam. In addition, at a power plant at the Salton Sea reservoir in Southern California, a significant amount of salt builds up in the pipes and must be removed. While the plant initially started to put the salts into a landfill, they now reinject the salt back into a different well. With closed-loop systems, such as the binary system, there are no emissions; everything brought to the surface is returned underground.
