Geothermal: A Bottomless Pit of Power?

The first commercial geothermal power plant was built in Italy in 1911, and in 1960, the first successful geothermal electric power plant began operation at The Geysers in California. The Geysers is actually still producing electricity, and the original turbine installed there by Pacific Gas and Electric operated more than 30 years.

When producing power from geothermal resources, the idea basically is to pump steam from the earth which turns turbines that generate electricity. Not only does this source of power not depend on outside fuel sources, it is also very clean. The global geothermal industry is expected to more than triple by 2020, driven largely by carbon policies and volatile energy prices. In 2010, The U.S. produced 3086 MW of geothermal electricity from 77 power plants.

While the technology is well-established, one of the challenges of geothermal is that the efficiency is low, only around 10 to 23% because geothermal fluid does not get as hot as the steam from boilers and exhaust heat is generally wasted. While that doesn’t affect cost of operation, it does affect the return on capital investments. Consequently, the task is always to utilize systems that use the available heat more efficiently, transfer the fluids with as little waste as possible, and replenish the source of the steam which runs the plants.

To find out how that is accomplished in the field, we spoke with Steve Burden, Project Manager at Calpine’s Geysers facility, which includes 15 power plants generating 700 MW of power in Lake and Sonoma Counties, California. We asked if there were special challenges for valves in his plant. He replied, “We have a huge network of pipelines, valve stations and pump stations, that distribute the steam to the plants and water back out to injection wells. A lot of it is extremely high pressure over challenging terrain. Some of the water systems are 500 to 1000 psi, so we have to use ASME codes instead of typical water codes.”

This plant is in Northern California, which has what is known as relatively “clean” steam. But not all geothermal plants pull clean steam. In the Imperial Valley of Southern California, the medium that comes from the geothermal source is brine, which is steam and water that is saturated with highly corrosive concentrations of salt and sodium chloride. According to Bill D’Andrea, Western Regional Manager of DFT Inc, at high temperatures, that mixture tends to wreak havoc on various metallurgies as well as packing.

We asked D’Andrea what that means when he is helping plant managers and specifiers choose valves. He said, “Well, the plant people just want longevity. To get that out of a valve, you want to be careful about the metallurgy. You have to know the process medium. If it is that steam with sulfuric acid, that could eat up seals. Packing is an elastomer. And how does that mechanical device, the valve, handle the medium? How is it going to be able to maintain its longevity? Whether it’s ball, check, any valve, you have to select the right material for that process. Even with Hastalloy, even with coatings, Mother Nature gives you challenges. And what mechanical device is going to last longest in that process medium? It’s very challenging.”

In addition to the challenges inherent in selecting and maintaining valves to transfer the steam out of the earth to the turbines, there are also many valves used in the process to pump water back into the earth. As Burden said, “As we take out steam, we’re basically depleting the water from the geothermal source. So we have injection wells. Out of all the steam we take, we replace about 80% of it with water now. Here we receive processed wastewater from Santa Rosa, up to 20 million gallons per day. And we can get up to 8 or 9 million from Lake County. There are two separate pipelines to bring in the water.”

We asked if there were special requirements for valves used in that process. “Yes,” Burden replied. “Here we have to use specialized valves because of the pressure drops and the fact that we inject into wells that pull a vacuum. We have a high delta P across the control valves in this application which is very critical when we have 500 to 800 psi upstream down to negative on the other side. So a big part of valve selection here is the pressure considerations.”

We wondered about the controls and actuation for these processes. Burden responded. “We use both pneumatic and electric actuators, but if there is instrument air, then we can use air actuation. We prefer that, because you have better control with pneumatic valves.”

“The other challenge is that every plant tries to get more power out of the system. That means they push things to the limit 24/7, and that can push the devices to a level at which they’re just not meant to operate. It’s important to choose a device that will meet the requirements you’ll expect it to meet.”

Kate Kunkel is senior editor for Valve Magazine. Reach her at kkunkel@vma.org.