A 26 MW solar project in Nevada is
proving to be a good fit with an existing geothermal facility on the
same site, combining the strengths of two different renewable
technologies and creating a better whole.
By Tony Kryzanowski
The Stillwater renewable powergeneration plant near Fallon, Nevada, is
a remarkable sight at dusk, with the sun's rays reflecting off the
geothermal towers and array of solar panels stationed side by side in
the Nevada desert.
In May, Enel Green Power North America (EGP-NA), the American
subsidiary of Italian-based Enel Group's green power investment arm,
brought 26 MW of solar power production on line at the Stillwater site,
about 75 miles outside of Reno. The site is now a combined renewable
energy facility considering Enel Green Power began producing 33 MW of
geothermal power there in 2009.
EGP-NA describes this co-gen project as "the first and only hybrid
power plant in the world that combines the continuous generation
capacity of binary-cycle, medium-enthalphy, geothermal power with the
peak capacity of solar power."
The term ‘medium-enthalphy' refers to a geothermal resource with
moderate temperature, below 400 degrees Fahrenheit.
"Combining solar and geothermal energy is not a new concept," says
Francesco Venturini, country manager of Enel Green Power North America.
"Others have tried to develop it, but the only company that has been
able to implement it on an industrial scale is Enel Green Power."
The solar power component at Stillwater covers 240 acres. It consists
of more than 89,000 polycrystalline premium photovoltaic (PV) CNPV-295P
modules on fixed mounts. CNPV Solar Power SA is an integrated
manufacturer of solar photovoltaic products from the production of
ingots, wafers and cells to the assembly of PV modules. It also
designs, manufactures and supplies crystalline solar photovoltaic
Racking for the project was provided by PV Hardware. The company's
UtilityMax ground mount system was used on the project.
"We chose PV Hardware's UtilityMax system for this utility-scale
project because it saved us considerable time and money, with minimal
foundations, fast turnaround times, and bankable technology," said
Chris Brooks, director of Bombard Renewable Energy, the contractor on
the project. "PV Hardware exceeded our expectations in turnkey project
engineering and overall ease of installing the product."
As opposed to other solutions on the market, PV Hardware's
UtilityMaxground mount system incorporates a tilting feature that
enables horizontal module attachment. This tilting feature increases
installation rates in the field, says the company. It also eliminates
double-handling of modules, an additional job cost typically associated
with constructing pre-assembled module packs offsite, transporting
them, and the need for heavy machinery to hoist three or four modules
at once. Jobsite safety risks are also minimized with the
UtilityMax system as ladders and lifts are unnecessary.
Another distinctive feature that the company says separates
UtilityMaxfrom the competition is that panel rails are not needed as
modules are directly attached to structural Z-purlins per manufacturer
provided mounting holes. This direct mounting method eliminates the
thermal expansion and vibratory risk associated with compression clips.
The construction and operation
of solar power at the Stillwater project represents a significant
milestone for Enel Green Power North America. Its total installed green
power capacity in North America has now reached 1 gigawatt. It has
renewable energy plants operating, or under construction, in 21 U.S.
states and three Canadian provinces.
The inverters for the Stillwater project were provided by Siemens.
The power being produced at Stillwater is being sold by EGP-NA under a
20-year Power Purchase Agreement to NV Energy. Adding the solar
component was something that occurred to EGP-NA after building the
"The solar component of the Stillwater geothermal plant was not in the
original design plan for the project," says Venturini. "However, it is
clear that the first-ever geothermal-solar hybrid power plant
demonstrates that the concept works to combine the strengths of
different renewable technologies and create a better whole."
He adds that the Fallon site was particularly well suited for this
hybridized approach because of the abundance of both resources in the
area, noting that northern Nevada has seen significant geothermal
development and continues to be ripe for additional development.
The Stillwater project is rather unusual in two respects. First, it
combines two renewable sources of energy, one baseload and one
intermittent, into a co-generation scenario. A more common co-gen
approach is to attach a renewable component to a more traditional
source of power generation.
Second, EGP-NA is demonstrating how companies can save on installation
costs by sharing infrastructure when generating two types of renewable
energy on one site. The two facilities share electrical interconnection
lines, control systems, fire detection and protection schemes, and the
use of common operations and maintenance staff. The two power
installations also support each other's auxiliary power needs,
resulting in higher efficiency.
"The solar plant not only sits next to the geothermal plant, it is
integrated with the geothermal plant and provides the energy to run the
geothermal plant's auxiliary loads," says Venturini. "Alternatively,
the geothermal plant provides auxiliary power to the solar plant when
there is no sunlight, thus eliminating the need for back feeding power
from the utility."
EGP-NA is using the binary cycle approach to generate geothermal power
at Stillwater. Hot geothermal fluid and a secondary fluid (with a much
lower boiling point than water) pass through a heat exchanger. Heat
from the geothermal fluid causes the secondary fluid to flash to vapor,
which then drives the turbines. Stillwater and its sister geothermal
plant (also located near Fallon) are the only plants in the world that
use large-scale submersible pumps for the extraction of the geothermal
fluid. This, along with the binary cycle process, results in no loss or
consumption of water and no air emissions.
"Because this is a closed-loop system, virtually nothing is emitted to
the atmosphere," Venturini says. "Moderate-temperature water is by far
the more common geothermal resource, and most geothermal power plants
in the future will be binary-cycle plants."
The submersible pumps also have other advantages over line shaft pumps
typically used to extract geothermal fluid. They are more
environmentally friendly because they are below ground, which reduces
noise and the visual impact.
They also do not require oil injection for lubrication of shaft
bearings. When combined with a variable frequency drive, the motor
speed can be changed, which compensates for various well field
conditions. EGP-NA says they are also much quicker to install, thus
reducing installation cost and resulting in less downtime. The pumps
can also provide geothermal production in deeper well fields and
require less surface ancillary support equipment.
The solar component of the Stillwater project boosts power production
from the site when geothermal power production is at its lowest, which
is typically in the high heat of summer. EGP-NA explains that the site
is already constrained by lower conversion efficiency due to the
reduced temperature differential between the geothermal brine and the
working fluid. A further loss is incurred during hot summer days when
the high ambient temperature saps the efficiency of the geothermal
This brings down the net-generation of the power plant precisely when
the utility and consumers need the power most, during the warmest times
of the day and year. The solar component compensates for some of that
Furthermore, by adding solar production capability to its geothermal
production, and given that peak solar power production typically occurs
at the same time as peak demand, EGP-NA describes the combined power
production capability of the site as "a more load-following production
profile." The company is able to market its solar power when there is
highest demand, thus earning the most potential income from this energy
Venturini says being the first to apply this hybridized approach has
had its challenges.
"It has to be said that since this
geothermal-solar project is a first of its kind, it met the regular
challenges of everything new in terms of combining two advanced
technologies, as well as challenges of a regulatory and administrative
nature," he says. However, all who were involved in the process, from
company engineers, to the power purchaser, to government
representatives, were eager to collaborate to overcome any hurdles that
had a direct bearing on the success of the project. He adds that
favorable policies at both the federal and state level were critical
for encouraging EGP-NA to make the investment in the first place.
In describing the business climate for pursuing renewable power
investments in Nevada, Venturinisays that "Nevada is certainly leading
the nation in the adoption of renewable energy policy and practices."
More specifically, he says that EGP-NA has found that the commitment to
lead the nation in renewable energy is a sincere effort and not just
lip service in Nevada.
"The state has enacted policies and has the governance to meet the
needs of the renewable energy business," he says. "We have been very
satisfied with the business climate and opportunities for growth. What
we have found most attractive is the honesty and open doors to meet
with elected officials and their offices."
Among the policies that the state has put in place is the goal to
achieve 25 percent renewable energy production by 2025.
Commissioning of solar power production at Stillwater represents a
significant milestone for EGP-NA, as its total installed green power
capacity on the continent has now reached 1 gigawatt. It has renewable
energy plants operating, or under construction, in 21 U.S. states and
three Canadian provinces, where it owns and operates 70 plants.
The Enel Group is no newcomer to the renewable energy industry. It is
Italy's largest power company and the second largest utility in Europe.
Overall, Enel Group has installed over 7 gigawatts of renewable power
production in more than 650 facilities worldwide, consisting of a mix
of wind, solar, hydro, geothermal, and biomass power production.
In addition to investing in the production of renewable power, the Enel
Group has entered into a thin-film photovoltaic factory venture in
Catania, Italy, with Sharp and ST-Microelectronics.
Venturini says EGP-NA considers the hybrid option for all its renewable
energy projects. The company is currently considering another hybrid, a
utility-sized project in California that combines wind and solar.