POWER April 2016 - 58

OPERATIONS & MAINTENANCE
3. Injecting new life. Two wastewater injection projects have ensured the continued effective
operation at The Geysers in Northern California, the world's largest geothermal field.
Courtesy: Calpine Corp.
of the geothermal resource became the first
challenge for the design. The requirement is
to quickly turn down generation and quickly
turn up generation within reasonable ranges.
The solution Ormat settled on was to maintain
geothermal fluid flows from the wellfield
at relatively steady rates and find ways
to provide bypass for fluid or heat around the
generation equipment as needed, governed
by the power demand from the AGC. "
Life-Extension Operation
Geothermal power uses a renewable, baseload
resource, but that resource can be subject
to lower capacity and efficiency over
time. The Geysers in Northern California,
which encompasses about 30 square miles,
is one example. (See the sidebar " Geothermal
and California: Mutually Supportive "
for details on geothermal's potential in
Southern California.) The first power produced
by the area's geothermal wells was
generated in 1921.
As the plant's current operator, Calpine
Corp., explains, " Throughout the 1970s and
1980s, vastly more steam was produced from
The Geysers reservoir than was replaced by
the injection of power plant steam condensate.
By 1989, accelerated development had
caused severe steam pressure decreases in the
reservoir, resulting in lower steam production
rates. This decline threatened the future sustainability
of Geysers power generation. "
To enable future operation of the faciltion
profile.
In addition, the company developed a
2-MW (17-MW thermal) concentrating solar
power system in 2015 to operate alongside
the existing Stillwater geothermal power station.
Using a 22-row parabolic trough system,
which concentrates the sun's energy by
75 times, the thermodynamic plant adds energy
to increase the temperature of geothermal
fluid entering the plant and increases the
amount of electricity that it can produce.
Flexible Control of Geothermal
Resources
In terms of plant operations, much progress has
been made since geothermal energy was first
harnessed in 1904. One recent breakthrough
is the ability for geothermal plants to operate
in fully flexible mode, which means they can
produce electricity at varying levels, according
to grid demands. This operating mode gives
" flexible " control to power utilities, in contrast
to the always-on baseload power traditionally
provided by geothermal plants.
GEA member Ormat Technologies Inc.
first accomplished this operating mode dur58
ing
2011 in Hawaii. Since then, fully flexible
operational capability has become the single
most important quality of a geothermal generator
on grids with large amounts of variable
renewables.
The site of Ormat's innovation, the Puna
Geothermal Venture (PGV) power plant
(Figures 1 and 2), won a GEA 2015 Honors
Award. Puna has a capacity of 38 MW, but
Hawaii Electric Light Co. can operate up to
16 MW of PGV's output on automatic generation
control (AGC). AGC allows remote
control operation via communication between
the Puna facility and the Hawaii Electric
Light Co.'s system to allow automatic
adjustments to plant generation in order to
match grid demand in coordination with other
facilities, including large volumes of variable
renewable generation. Ormat explains
that " The communication also allows the
plant SCADA system to update the system
operator on available capacity and spinning
reserves. "
According to Ormat, " This requirement
of quick response to changes in power generation
while maintaining stable operation
www.powermag.com
ity, in 1990, The Geysers, in collaboration
with Lake County and the California Energy
Commission, developed the 29-mile Southeast
Geysers Effluent Pipeline (subsequently
lengthened to 40 miles) to deliver roughly 9
million gallons per day of treated wastewater
to the Geysers (Figure 3).
Calpine explains that the success of the
initial recharging project in maintaining reservoir
pressure " provided momentum for a
similar project to bring tertiary treated effluent
from the Santa Rosa area to The Geysers. "
In 2003, the City of Santa Rosa and
Calpine partnered to construct a 42-mile
pipeline that became known at the Santa
Rosa Geysers Recharge Project (SRGRP).
" Since 2003, SRGRP has delivered approximately
11 million gallons per day of tertiary
treated wastewater to replenish The Geysers'
geothermal reservoir. "
Injection of this wastewater from area
communities has improved the sustainability
of the Geysers geothermal area while simultaneously
reducing the need to discharge the
wastewater into local surface waters. ■
-Allie Nelson is a geothermal news and
communications specialist for the Geothermal
Energy Association. POWER staff
contributed to this article.
POWER | April 2016

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POWER April 2016

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