Evaporative Cooling Increases Summer Capacity at Geothermal Plant

Issue 2 and Volume 106.

By Douglas J. Smith, IEng
Senior Editor

MAMMOTH PACIFIC’s geothermal plant in California’s Sierra Nevada Mountains uses geothermal energy (hot water) from the Casa Diablo Springs. The plant, constructed in 1984, sells 32 MW of power to Southern California Edison. After using the heat in the plant it is re-injected back into the reservoir where it is reheated. The site has 12 production wells and nine injection wells. Re-injecting the water helps to maintain the pressure in the reservoir.

Inspecting GLASdek media pads. Photo: Courtesy of Munters Corporation.
Click here to enlarge image

Water at 300-400 F is pumped from the reservoir to a heat exchanger. In the heat exchanger the hot water is used to vaporize a secondary “binary” liquid. The binary liquid, isobutene, boils at a lower temperature than water. The vaporized isobutene is used to drive a turbine generator. After passing through the turbine the isobutene is condensed back into a liquid and recycled. Unfortunately, because Mammoth’s condensers are air-cooled, the plant invariably looses condenser capacity during the hot summer months.

Improving Condenser Efficiency

In the summer of 2000, Mammoth needed to maximize plant output. After reviewing the different options, management decided to conduct a pilot program to determine the feasibility of adding evaporative cooling to the air intake of the condensers to increase cooling. Evaporative cooling can substantially reduce the ambient air temperature and aid in restoring lost capacity..

Because of the need for additional water for evaporative cooling, Mammoth installed a 2-1/2 mile long pipeline. The pipeline can supply up to 1.15 million gallons of water per day. All of the water is recycled wastewater supplied by the Mammoth Community Water District.

According to Bob Sullivan, general manager, Mammoth Pacific, the plant tested two different types of evaporative cooling:

  • A fogging system
  • Evaporative cooling

In the fogging system a water mist is sprayed onto “MI-T-Fog” media pads. The second system, a more conventional evaporative cooling system, uses GLASdek media pads. With evaporative cooling systems water is distributed over pads as the air passes through them. The fogging system, MI-T-Fog, and the evaporative cooling system, GLASdek, were both supplied by Munters Corporation.

After evaluating the two systems Mammoth Pacific chose the GLASdek system. In July 2001, the GLASdek media pads were installed in one section of the plant’s air-cooled condenser. According to Sullivan, the new media pads have increased the plant’s capacity during the summer months-June, July, August and part of September-by 20 percent. “Without Munters’ cooling system, we previously could not condense the isobutene to liquid as efficiently”, says Sullivan.

By using coolers in front of the fan fins of the condenser, the plant has dropped the ambient temperature by as much as 25 F. “This has been a very successful project and we are pleased with the results,” said Sullivan. Sullivan says Mammoth is considering adding additional cooling systems to the plant’s remaining fin fan condensers.