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Geothermal Plant Uses Special Cooling Technology
Archeological evidence confirms that humans first used geothermal resources
more than 10,000 years ago with the settlement of Paleo-Indians at hot springs
locations across North America. These springs served as a resource for cooking,
warmth, cleansing and healing.
Today, the utilization of geo-thermal resources has expanded. While people
still bathe in shallow pools heated by the Earth, engineers have created technologies
that allow facilities to search miles below the Earth's surface to seek out
geothermal energy.
One such development is the Mammoth Pacific geothermal facilities located in
the California Sierra Nevada Mountains and fueled by geothermal fluid from the
Casa Diablo Hot Springs. Built in 1984, the plant generates 32 megawatts (net)
of renewable electricity that is sold to Southern California Edison. This is
enough power for approximately 32,000 homes. "Our earth's interior provides
heat energy from nature," said Bob Sullivan, general manager at Mammoth Pacific.
"This heat, called geothermal energy, yields warmth and power that we can use
to generate electricity without polluting the environment."
Geothermal power plants use steam, heat or hot water from geothermal reservoirs
to provide the force that spins the turbine generators and produces electricity.
The used geothermal water is then returned down an injection well into the reservoir
to be reheated, to maintain pressure and to sustain the reservoir.
"We pump water ranging in temperatures from 300 ? 400 degrees Fahrenheit to
the surface through the production well where it is passed into a heat exchanger,"
said Sullivan. "Our facilities consist of 12 production wells and 9 injection
wells. A total of 8 single-stage, radial-flow gas expanders are used in the
process."
The facilities pump the hot water from more than 300 feet underground. In
this binary system the water is passed through a heat exchanger, where its heat
is transferred into a second (binary) liquid, called isobutane, that boils at
a lower temperature than water.
When heated, the isobutane flashes to vapor, which, like steam, expands across
and spins the turbine blades, which in turn generates electricity. The vapor
is then recondensed to a liquid and is reused repeatedly. In this closed loop
cycle, there are no emissions to the air and no fuel is required. The isobutane
vapor runs through large fin fan condensers that reject heat from the vapor
and return it to a liquid state and the process continues through another cycle.
Fin fans are air-cooled heat exchangers that are similar to those found in
condensing units for commercial and residential air conditioners. They draw
large amounts of ambient air across their fins, thereby rejecting the heat from
the isobutane. Instead of dissipating heat into water and then transferring
that heat to the air, as with shell and tube heat exchangers and wet cooling
tower systems, an air-cooled fin fan dissipates heat from a fluid directly into
the air.
Geothermal power plants, like wind and solar power plants, are called "renewable"
technologies because they do not burn fossil fuels to generate power. As a result,
geothermal facilities are an environmentally sound way to produce electricity,
and they add to our nation's fuel diversity so that we are not too dependent
on fuels.
"Manufacturing electricity with geothermal energy helps to conserve precious
fossil fuels such as natural gas, oil and coal," said Sullivan. "In addition,
geothermal energy creates none of the emissions associated with combustion power
plants."
Geothermal power plants are designed to run 24 hours a day all year because
they sit right on top of their fuel sources. They are resistant to disruption
of power generation due to weather, natural disasters, and interruption in fuel
availability.
Last summer, Mammoth Pacific sought ways to improve efficiencies and generate
more power. Facility management decided to conduct a pilot program to improve
energy production using evaporative cooling systems to enhance the cooling effectiveness
of the large fin fan condensers.
"Our goal was to regain power at the plant by using materials to filter water
to cool the fin fans," said Sullivan. "During the research phase of our project,
we discovered that the Munters Systems Division offered evaporative cooling
technology that could meet our power increase needs."
The Mammoth Pacific geothermal facilities are air-cooled and the efficiency
changes depending on the ambient temperatures. As the air heats us during the
summer, the air coolers become less efficient. Evaporative cooling systems are
designed to substantially reduce the ambient air temperatures during these hot
summer days.
The principle of evaporative cooling is quite simple. Water is applied to
the top of the media and allowed to trickle down. It spreads out over the extensive
surface and mixes with the air, which is passed through the corrugations. When
the water evaporates, it requires energy to pass from the liquid to the gaseous
stage. The water vapor absorbs this heat from the air, thereby lowering the
temperature of the air as the relative humidity is increased.
However, in order to utilize evaporative cooling systems, Sullivan realized
that they would have to pump additional water to the plant. The water would
be necessary for the evaporative cooling schemes to lower the ambient air temperature
surrounding the condensers at the plant.
"To pump the tertiary treated water to the facilities, we built a 2-1/2 mile
long pipeline to provide us with up to 1.15 million gallons of water per day
for industrial cooling purposes," said Sullivan. "Up to 600 gallons of water
comes through the pipe every minute. In another effort to protect our natural
resources, the water we receive from the Mammoth Community Water District is
?recycled' wastewater."
Sullivan tested two different Munters evaporative cooling systems for use
at the Mammoth plant. One system featured a misting system that sprayed small
droplets of water onto Munters MI-T-Fog® media pads.
With MI-T-Fog, water droplets are collected onto the pads, creating the evaporative
cooling process. The proprietary paper used in MI-T-Fog product makes the media
pad efficient even on how summerdays.
The second system selected was a more conventional evaporative cooling system
using Munters GlaSdek® media pads. Using GlaSdek, water is distributed over
the pad as air is drawn through the media, hence cooling the air towards the
wet bulb temperature. GlasDek Evaporative Cooling Media were developed for applications
requiring UL900 Class II fire rating or compliance with NFPA codes, an important
factor when dealing with flammable liquids such as isobutane.
The Munters' evaporative cooling units were installed in July. With Munters
cooling the condensers, Sullivan hoped to increase the plant's productivity
by as much as 25 percent.
With the pilot program complete, Sullivan has been able to ascertain the benefits
received from the installation of the Munters evaporative cooling system.
"We improved our power output by as much as 20 percent and averaged 10-15
percent more power with the addition of the evaporative cooling units during
the test," said Sullivan. "Without Munters cooling systems, we previously couldn't
condense the isobutane to liquid as efficiently so power would drop. By using
coolers in front of the fin fans, the ambient temperature dropped by as much
as 25 degrees F.
"This has been a very successful project and we are pleased with the results,"
said Sullivan. "We started working on this project in March and we had to move
quickly with the energy shortage emergency in California. This was an opportunity
to get additional power on line this summer when the state really needed it."
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