J A N U A R Y 2 0 0 5 : A T E C H N I C A L U P D AT E F R O M P R O T O N E N E R G Y
Continuous hydrogen replacement technique
offers improved power plant
electric generator efficiency and longevity
HOGEN hydrogen generators from Proton Energy offer dry, 99.999 percent pure hydrogen
BY JOHN SPERANZA
DIRECTOR OF APPLICATIONS AND TECHNICAL SERVICE, PROTON ENERGY
Water vapor and air: enemies of electric generators
In their quest to reduce costs and simplify operations, power generation
facilities face two formidable adversaries: water vapor and air. Both
substances increase friction drag (windage loss) on the rotor windings
within electric generators, reducing overall efficiency. And with today’s
fuel costs at an all time high, windage losses can mean the difference
between profits and losses.
In addition to increasing windage loss, water vapor contamination
inside a generator has been shown to reduce the life of its components,
and high humidity can induce stress corrosion cracking on retaining
rings. Water content of more than 500 ppmw in turbine oil creates
high dew points within generators that reduce generator efficiency and
may reduce a generator’s mean time between failures due to water
induced electrical shorts.
The costs to a power generation plant for unplanned shutdowns and
repairs are significant. In addition to hard costs of parts and labor, a
generator repair of this type usually means an extended plant outage
and significant lost revenues. The cost to a plant for stress corrosion
related repairs is estimated at more than $1 million.
Managing water vapor and air within hydrogen coolant
How can power plant operators reduce the destructive presence of water
vapor and air within their hydrogen-cooled generators? By displacing
these drag-inducing substances with a constant flow of dry hydrogen
gas, produced conveniently on the plant site from a hydrogen generator.
Hydrogen gas cooling is a primary tool in power generation plants’
efforts to reduce the friction (windage) losses within generators from
water vapor and air. Hydrogen’s high specific heat, high thermal conductivity and low density (14.4 times less dense than air and 8.9 times
less dense than water vapor) help keep wind resistance losses within
generator rotors to a minimum.
Hydrogen is the coolant of choice for high speed generators including
coal-fired, gas turbine and nuclear power plants, but it has to be pure
and dry. Hydrogen that has a high water content loses its advantageous
low density and high thermal conductivity and becomes less effective
as a coolant. For example, on an 800 MW generator, a two percent
decrease in hydrogen purity due to water contamination can cost a utility
more than $300,000 in power sales per year.
Hydrogen coolant that has a high oxygen content can spark corona
activity in a generator’s stator windings, which can in turn damage
a generator’s high-voltage insulation system.
Hydrogen that’s delivered in cylinders or tubes may start out pure and
dry, but it becomes increasingly impure and wet as it picks up moisture
from the turbine oil and from air ingress. In turn, this moisture may
be absorbed by the contaminants resident on the insulation, creating
a conductive bridge between coils in the generator’s ventilation circuit
that can cause coil-to-coil shorts.
A generator’s gas cooled stator windings have high voltage copper
exposed at each end of the stator bars. This design feature necessitates
long electrical creepage paths to prevent high voltage phase-to-phase or
phase-to-ground faults. Operators of hydrogen cooled generators have
found that moisture degrades the electrical creepage strength of a surface.
And when moisture migrates to the end turn area of a generator’s rotor
windings, it attacks the interturn insulation and results in shorted rotor
end winding turns.
Hydrogen dryers and purity analyzers: costs vs. results
Because moisture in cooling gas can cause significant problems with
a generator’s rotor winding insulation, plant operators must ensure
that dewpoints in their hydrogen coolant stays as low as practical.
To dry and ensure the purity of coolant hydrogen, power plants often
employ regenerative drying systems and purity analyzers. However,
these methods carry associated cost and logistics issues, with potential
negative side effects.
Drying systems are costly in terms of both capital expenditure and
maintenance. Furthermore, a dryer’s placement within the plant is
critical to ensure effectiveness. Condensation of water in the dryers’
purge exhaust drains should be closely monitored. Care must be taken
that desiccant materials from the dryer do not get into the generator.
Purity analyzers are good at detecting the percentage of air within the
hydrogen gas. However, if the impurity is something other than air,
the readings are less reliable. Readings can fluctuate based on ambient
humidity in the plant, the generator’s operational status, and where
the drying system is located.
Onsite generation = no guesswork
with hydrogen dryness and purity
A cost-effective approach to ensure hydrogen purity and dryness for
lowest windage loss is to generate it at your facility by a clean, safe
method such as Proton Exchange Membrane (PEM) electrolysis.
Proton Energy recently contracted with Atlantic Analytical Laboratory
to analyze hydrogen gas produced by one of its HOGEN® hydrogen
generators through PEM electrolysis. More than 50 trace elements were
quantified in the sample. Water vapor, oxygen, total hydrocarbons, helium,
argon, carbon dioxide and nitrogen each registered below the laboratory’s
detection limits. Hydrogen generated by the HOGEN hydrogen generator consistently reported a dewpoint of -85°F.
Proton Energy has solved these two obstacles with its onsite generators.
Proton’s HOGEN hydrogen generation systems not only make hydrogen
on demand, but also have the ability to monitor demand and alert
plant operators. For example, if a seal fails within the turbine generator,
hydrogen demand will increase rapidly and dramatically. Monitors
within the HOGEN hydrogen generator will sense the increased
demand, and display the increased usage to the operator giving them
time to take the appropriate actions to safeguard their facility.
TABLE B
How it works
TABLE A
Analysis Report – Atlantic Analytical Laboratory
Sample of H2 gas @ 225 psi
generated by Proton’s HOGEN® hydrogen generator
Test Description/Units
Result
Report Detection
Limit (D.L.)
Nitrogen (ppm vv/by MS)
nd
4
Oxygen (ppm v/v by MS)
nd
4
TABLE C
Carbon Dioxide
(ppm v/v by MS)
nd
4
Argon (ppm v/v by MS)
nd
4
Helium (ppm v/v by MS)
nd
10
Hydrogen (% v/v by MS)
99.9+
0.1
Total hydrocarbons
(ppm v/v as CH4)
—
0.1
Water Vapor
(ppm v/v by EDP)
nd
0.5
Power plants can increase operating efficiencies by producing hydrogen
on site to continually replenish hydrogen coolant within their generators.
D.L. = report detection limit. nd = indicates the concentration is less than the report
detection limit. — = test not performed. % = percent. ppm = parts per million. ppb
= parts per billion. v/v = volume analyte/volume sample. w/w = weight
analyte/weight sample. Unit conversions: 1 ppm v/v = 0.0001% v/v.
Delivered hydrogen vs. continuous hydrogen
replacement from a hydrogen generator
Many power generation industry veterans now advocate use of continuous
hydrogen replacement techniques to keep generators free of moisture,
oxygen, and other contaminants that can prematurely degrade equipment.
However, power plants have been reluctant to employ a continuous
hydrogen replacement method of maintaining purity for two reasons.
The high cost of delivered hydrogen is one barrier. An effective purge
method will require using a larger amount of hydrogen gas that a plant
is currently using. A second obstacle is plant operators’ safety concerns
about leaving a large volume of pressurized hydrogen from tanks
or cylinders “on line” and unattended.
Mirant Mid-Atlantic’s success story
The first HOGEN 40 hydrogen generator in the Mirant Corporation’s
Mid-Atlantic system was placed in service for testing on Unit 1 of its
Dickerson, Maryland plant on February 17. On February 17, the dew
point of the hydrogen coolant in the low-pressure generator measured
37°F. On March 4, the dew point was down to about 30.8°F. By May
18, the dew point was between 12 and 15°F. Presently the dew point
remains between 12°F and 15°F.
TABLE D
Dew point of hydrogen coolant in Mirant Mid-Atlantic’s Dickerson Unit 1
low-pressure generator after installation of Proton’s HOGEN 40 onsite
hydrogen generator.
changed positively from our pre-installation values. Operationally, they
have proven to meet the needs of our generators, enabling us to reduce
the storage, transport and manual operations required of cylinders.”
Mirant-Zeeland improves operational efficiency with
HOGEN H Series generator
The company’s Zeeland, Michigan plant is benefiting from their
HOGEN H Series’ hydrogen generator’s reliability along with savings
on delivered hydrogen and more productive use of plant personnel’s
time. Mirant-Zeeland’s generators require about 70 scf/hour of pure
hydrogen gas, about 4-6 cylinders per day. Before installing Proton’s H
Series hydrogen generator in March, the plant paid $30,000-$36,000
per year for hydrogen cylinder deliveries. After seven months’ testing of
a pre-production HOGEN H Series system to cool three of their electric
generators, Mirant’s Zeeland power plant has purchased a production
unit which will give them extra capacity for growth.
According to Lawrence A. Dusold, senior consultant with Dickerson
contractor Cetrom Inc., the plant’s original intent was to reduce the
hydrogen dew point by upgrading the existing dryers. During their
investigation, Dickerson’s management learned that a HOGEN 40
hydrogen generator could provide dry hydrogen in excess of their rate
of consumption, allowing the plant to continually purge the generator
with pure, dry hydrogen. The installed cost of a single HOGEN 40
hydrogen generator, which can provide hydrogen to both the plant’s
high and low pressure generators, is a fraction of the cost of the two
driers that would have been installed.
Among the benefits that the plant is experiencing, Dusold said that
continually purging Dickerson’s generators with pure, dry hydrogen
produced by a HOGEN unit has reduced both the plant’s dependence
on hydrogen cylinders and the demurrage cost of keeping these cylinders
on site. Futhermore, hydrogen from the HOGEN generator is purer
than the hydrogen supply from cylinders.
The Dickerson plant employs General Electric hydrogen cooled
synchronous type ATB 4-pole, 3-phase 60-cycle generators, rated at
115,000 kilovolt-amperes at 1800 rpm and 13.8 kilovolts. They are
designed for a power factor of 0.85, 30 psig hydrogen cooling pressure
and armature amperage of 4811 amps. GE’s guidelines for this generator
project a 1/2 percent increase in kilovolt-amperes output for each 1 psi
increase in hydrogen pressure within the generator.
Dickerson’s generators typically operate on cycling load rather than continuous full load. Dusold projects that the stability increase in hydrogen
pressure within Dickerson’s electric generators since introducing the
HOGEN systems can produce an additional 900 kilowatts of additional
generation. As an example, based on an estimated 5000 operating hours
annually and an average electricity selling price of $.05kW/h, an additional
$225,000 in revenue can be realized from each of the three generators.
When when full load demands occur, Local Market Pricing (LMP)
policies allow Dickerson to further increase revenue from operating
the generators at maximum capacity.
“We are very happy with the performance of our HOGEN 40 hydrogen
generators,” said Michael Bennett, maintenance group leader at the
Dickerson plant. “With these units, both dew point and purity have
About Proton’s HOGEN hydrogen generators
Proton’s HOGEN H Series and HOGEN 20/40 hydrogen generators,
which are now installed at more than 20 power generation facilities in
the U.S. and Europe, offer reduction of windage loss, longevity, and
operational efficiency compared with delivered hydrogen. HOGEN H
Series and 20/40 generators produce 99.999 percent pure hydrogen gas
at up to 218 psig (15 bar) without use of a compressor. Seven capacities
are available, from 76 to 228 scfh. The H Series generator can be easily
upgraded in the field for additional capacity. The H Series generator's
enclosure and ventilation systems are weatherproofed for outdoor
installation. Built under Proton's ISO 9001:2000 quality system, the
units are Nationally Recognized Testing Laboratory (NRTL) approved
for use within U.S. and Canada, and CE approved for use within the
European Union.
About the author
John Speranza is Director of Applications and Technical Service at
Wallingford, Conn.-based Proton Energy, which manufactures HOGEN
on-site hydrogen generators for a diverse range of industrial and
energy applications. John has more than 15 years experience in the
design and development of Proton Exchange Membrane (PEM) based
products for industrial and energy uses. He leads a group of engineers
responsible for providing Proton’s commercial customers with hydrogen
supply solutions that are best suited for their specific application.
References
Albright, J.D. and Albright D.R., Generatortech, Inc. “Generator Field Winding
Shorted Turns: Moisture Effects”. Presented at EPRI™ Steam Turbine Generatortech
Workshop and Vendor Exposition, Nashville, Tenn., August 25-27, 2003.
Borkey, Ed (general manager, Fluid Energy); Reynolds, Tom (electrical engineer,
Progress Energy). “Water Contamination in Hydrogen-Cooled Generators Lurks
as Serious Operational Threat”. Power Engineering, August 2003.
Vandervort, Christian L. and Kudlacik, Edward L. GE Power Systems, Schenectady
NY: GE Generator Technology Update, April 2003
GE Power Systems’ Generator Products Overview, October 2003.