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BY FAHAD KHALIL Hydrogen Cooling of Electrical Generators 1 Fahad Khalil

Hydrogen Cooling of Electrical Generators

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Page 1: Hydrogen Cooling of Electrical Generators

Fahad Khalil

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BY

FAHAD KHALIL

Hydrogen Cooling of Electrical Generators

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Fahad Khalil

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Generator Cooling Goals

1. Minimize drag - “windage”2. Keep generator internals clean3. Maximize generator output4. Minimize electrical, mechanical and corrosion problems5. Maintain low dewpoint for component durability

The cooling system for the generator needs to meet several goals, and recirculating closed loop hydrogen systems have proven to meet these challenging goals for nearly 60 years. There is every reason to expect that hydrogen cooling willcontinue to be the standard approach to base-load utility scale generator cooling.

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Why Hydrogen Cooling ?

1. Lowest density gas yields lowest drag2. Highest heat conductivity of any gas3. Controlled atmosphere to maintain Clean & Dry4. Inexpensive5. Easy to detect6. Excellent electrical properties7. Easy to manage – not readily miscible with CO28. purge gas9. Flammable

Hydrogen has a thermal conductivity of nearly seven (7) times thatof air, and its ability to transfer heat through forced convectionis about 50% better than air.

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Why H2 as Generator coolant?

Hydrogen has attractive characteristics as a fluid to both the windings of the generator, and to remove heat from the windings and deliver that heat to the cooling water. Hydrogen is nearly the perfect cooling gas, except for its one massive flaw (Flammability).

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Hydrogen Flammability Range

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Operating Above UFL is different

1. In nearly every case, we try to maintain flammable gases below LFL (lower flammability limit)

2. Operating below LFL, leaks don’t ignite3. Above UFL (lower flammability limit) is safe except

for leaks4. Above UFL, every leak is a potential fire5. The generator must be kept pressurized,

because air ingress can be catastrophic6. Procedures are important, and not obvious

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Generator Cooling According to Generator Rating

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H2 Detection

Hydrogen is a flammable gas – no more dangerous than other flammable gases. It is very buoyant, escapes readily and does not pool. Hydrogen has no natural odor, and is not odorized, so leaks require detection equipment. Hydrogen has no health effects other than potential flammability, and the ability to displace oxygen. The chief challenge with hydrogen is managing inventory and dealing with fast-leakinggas.

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Power Plant Hydrogen System Layout

This is a standard hydrogen-cooled generator equipment layout. There arebasically three hydrogen “systems” – the hydrogen supply side of the generator, the hydrogen recirculating cooling loop, and the hydrogen scavenge portion of the system. Hydrogen safety and quality can be affected by the choice, capabilities, and operation of the technologies in each of the systems.

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Generator cover gas density

Because air is 14 times as dense as hydrogen, the density of the fluid in thegenerator casing rises quickly with air impurity level. As can be seen from theequation, every 1 percent of air contamination is worth about 14% increase in fluiddensity. Increased fluid density means increased windage loss.

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H2 Purity & Generator Efficiency

The gas in the generator casing must be kept at the manufacturer recommended purity in order to achieve the manufacturer commitment on heat rate.

Note that the generator capability varies according to the gas pressure in the casing – if pressure is allowed to degrade – think Bleed & Feed – then the generatorcannot deliver full capacity.

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Payback from H2 purity enhancement

The payback from purity enhancement is immediate and impressive. An improvement from 95% to 99% purity, on a generator of 175 MW capacity, with a 60% CF%, is worth between $48,000 and $77,000 annually in additional sales dollars. Because there is no additional fuel burned to achieve these sales, the profitability is virtually 100%.

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Generator H2 Cooling

Hydrogen gas provides a remarkable solution for generator cooling by virtue of its low density and high thermal conductivity.A hydrogen cooled generator can achieve a higher rating and a greater efficiency compared to an air-cooled generator of an equal physical size.All hydrogen-cooled generators are supplied with auxiliary systems to accommodate hydrogen filling, purging, monitoring and shaft sealing.

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Moisture Effects on H2 Cooling System

Moisture contamination is measured in the hydrogen gas used to cool large stationary electric generators. Hydrogen is used to cool the bearings and other rotating parts of large stationary electric generators. Hydrogen is the best choice for a cooling gas because of its unique combination of high thermal conductivity and low viscosity.

In order to maintain these favorable properties, the hydrogen must be kept dry. Moisture is a contaminant that both reduces the thermal conductivity and increases the viscosity of the gas. The presence of moisture indicates that:a) either the system was not purged adequately after the last

maintenance cycle or b) that there is an ambient leak, which can be a very hazardous

condition.

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Moisture Detection Sensors

Any quartz-crystal based moisture analyzer is suitable for this application provided the sample pressure meets the analyzer’s minimum inlet requirement.Electrolytic sensors are not suitable due to the recombination errors that always occur in hydrogenatmospheres. Metal-oxide probes should also be avoided as the reducing hydrogen atmosphere may react with the probe’s oxide layer resulting in a loss of both sensitivity and calibration stability.

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Quartz-Crystal Moisture Analyzers Working

The cooling hydrogen is continuously circulated through the generator and then through a molecular sieve dryer. In order to be certain that the hydrogen is dry, the sample tap is installed in the return line from the generator to the dryer.A second measuring point, at the dryer outlet, will provide a control signal for dryer switching. This additional measurement will maximize dryer life, provide an indication of dryer efficiency, and minimize the possibility of a dryer upset effecting the generator’s operation.

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Advantages of Quartz-Crystal Moisture Analyzers

With a quartz-crystal based moisture analyzer the actual moisture concentration in the cooling gas is easily, and accurately measurable. These analyzers are highly reliable and require little maintenance.

In addition, the performance of this technology is very stable over long periods of time thereby increasing operator confidence in the system.

Moisture Range 5 ppmv to 500 ppmv

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Generator Heat Removal Capabilities of Various Fluids

Fluid Type Relative Specific Heat

Relative Density

Relative Practical Vol. Flow

Approx. Relative heat removal Ability

Air 1.0 1.0 1.0 1.0

H2 30 Psig (2.07 bar)

14.36 0.21 1.0 3.0

H2 40 Psig (3.10 bar)

14.36 0.26 1.0 4.0

Water 4.16 1000.0 0.012 50.0

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Issues with Water Cooling of Generators

Water cooling adds manufacturing complexity, as well as, requires the need for auxiliary water cooling and de-ionizing skid, plus associated piping, control and protection features.At higher ratings, the cost of this complexity is offset by the advantage of producing a generator of significantly smaller size than a comparable conventionally cooled generator.

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Newer Generators

Newer generators use H2 under pressure (from 15 to 75 Psig) for cooling. H2 is more effective than air in dissipating heat and the higher the h2 pressure, the more effective the hydrogen is in removing heat.

To contain the pressurized h2 gas, thick plate cylindrical construction is used. End shields are more rugged and contain a hydrogen seal system to minimize the leakage.

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Generator H2 Cooling System

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H2/Water Cooled Vs Air or H2 cooled Generators

The armature voltage and current of a H2/water cooled generator is significantly higher than those of air or H2 cooled units. As a result, the insulation voltage stress and forces on the armature windings can be several orders of magnitude larger than those experienced on lower rated units.

These present unique design requirements that must be addressed if high reliability and long life of the equipment is to be maintained.

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Why 4 pole generators are used in NPP’s

NPP’s typically use 4 pole generators. 4 pole rotors are used because a NPP:1. steam temperature & pressure are lower and 2. the lower the energy content of the steam, the

larger the turbine has to be. It would not be safe to rotate large steam turbines at 3600 RPM.

Gas turbine typically use 2 pole generators & rotate at a speed of 3600RPM and are provided with reduction gear to connect to the generator.

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Generator Frequency

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Rotor Design of H2 cooled Generators

All H2/water cooled generators use direct conductor cooling of the rotor winding of heat removal. Smaller 2 pole and all 4-pole generator use the radial flow design. At the end of the rotor body, H2 enters then windings through full length sub-slots and is discharged along the length of the rotor body through redial slots, machined or punched, in the copper conductors. The H2 passes from the conductors through the creepage blocks and wedges to the air gap, where the H2 is directed through the stator core to the H2 coolers.

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Diagonal Cooling Method

As generator ratings and consequently rotor body length increase even further, the gap pick-up diagonal flow cooling method is employed.In this scheme, cold H2 is scooped up in the gas gap and driven diagonally through the rotor copper to directly remove the heat. At the bottom of the slot, the gas is turned and passes diagonally outward to the gas gap in a discharge stator core section. The stator core ventilation is coordinated with the rotor cooling gas flow, thus creating an in and out flow of H2 through the stator core, through the rotor and returning to the H2 cooler through the core.This cooling method results in a design which maintains the same average copper temperature, independent of rotor length.

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Diagonal Cooling Method

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H2 Gas coolers

Four (4) H2 gas coolers for a generator are located in a vertical position in the associated generator frame, two (2) at end of the associated frame and on both sides of the axial centerline. The H2 coolers are built and installed in the frame so that any one (1) of the coolers may be taken out of service for cleaning while the unit generator is still carrying load. This is accomplished by removing the water piping and cooler headers from the ends, without breaking the gasketed seal that maintains H2 pressure within the associated generator.

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Fabrication of Generator End Shields

The generator end shields are heavily reinforced fabrications, designed to support the weight of the rotor and to contain the H2 gas at maximum pressure without excessive distortion.The generator rotor bearings, the H2 shaft seals and oil passages supplying oil to these parts are contained and/or supported by the outer and shields. The end shields are split on the horizontal centerline, facilitating their removal. The joints between the shields halves and the joints between the shield and the stator frame are fitted and provided with grooves for the insertion of a sealing compound, to seal the H2 gas in the generator casing.

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Shaft Seal Attached To Each Outer End Shield

A shaft seal attached to each outer end shield, adjacent to the bearing inboard, prevents the escape of H2 gas from the generator along the rotor shaft penetration points. This arrangement permits the inspection of the generator bearings without removing gas from the machine. The Bearing ring and the shaft seal housing at the collector end of the machine are insulated from the generator frame to prevent the flow of shaft currents.

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Inner End Shields

The inner end shields are located between the ends of the armature windings and the outer end shields, to separate the gas discharge from the fans from the gas entering the fans. The inlet vanes and nozzle rings are attached to the inner end shields. This together with nozzle ring segments attached to the outer end shields provide optimum:a) gas entrance and b) discharge conditions for the fans

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H2 Cooling of Generator Lead

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Main Leads from Generator

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H2 Pressure & Purity

H2 pressure in the generator must be maintained above atmospheric pressure at all times to keep seal oil from being drawn into the generator.

H2 purity range indicator is 50% to 100%.

The H2 purity in the generator should always be maintained above the explosive limit of 75%.The low H2 purity alarm actuates when the purity decreases to 90% purity.

Normal purity is > 98%.

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High & Low Gas Pressure Alarms

Machine gas pressure high/low alarms are provided.

When a low pressure alarm actuates, H2 should be added to the machine.

The high pressure alarm actuates upon overfilling of the machine.

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Gas Analyzer Flow Meter

The flow meter for the gas analyzer has a range of zero (0) to 30 cub ft/hr.

Normal operating range for the gas analyzer is between one (1) and 1.5 cub ft/hr.

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Generator Seal Oil System

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Standard Generator Seal Oil

Since the unit’s rotor shaft ends of the H2 cooled generators for each unit must be brought out of the gas tight enclosure, means must be provided to prevent the escape of H2 gas from along the shaft.

The purpose of the generator seal oil system is to provide the means to prevent the escape of hydrogen from the generator enclosed.

Seal oil is lube oil, the flow of seal oil along the shaft inside the seal also provides lubrication.

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Generator Seal Oil System

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Vacuum Tank

During normal operation, oil from the lube oil system enters the vacuum tank through spray nozzles. The inlet spray nozzles are directed upward and separate some of the gases from the oil as it enters the tank.

Most of the gas remaining in the oil is separated by recirculation through a seconds set of spray nozzles directed downward.

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Seal Oil Pump

The main seal oil pump, pumps oil from the vacuum tank to the shaft seals. Excess oil over the seal requirements is returned to the vacuum tank by the recirculating spray nozzles.

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Seal Oil Vacuum Pump

The seal oil vacuum pump maintains an absolute pressure of about 0.5 inches of mercury in the vacuum tank.

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Emergency Seal Oil Pump

The emergency seal oil pump is driven by DC motor to ensure maximum availability. The emergency pump is started by operation of pressure switch located in the main seal oil pump line. This pressure switch is adjusted to close on falling main seal oil pump pressure. The vacuum tank becomes inoperative when the emergency pump is in operation.

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Seal Housing

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Drain Pipes

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Seal Oil Cooler

Seal oil passes through the shell of the H2 side seal oil cooler, while bearing cooling water passes through the tubes. The cooler has inlet and outlet isolation valves and a bypass valve.

An in-line filter removes debris from the oil.

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Normal Operation

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Separator Tank

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Regulating Valve

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Vacuum Treatment

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Generator Side Seal Oil and H2 Content

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Secondary Seal Oil Supply

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Operation of Emergency Seal Oil Pump

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Generator Seal Oil System Simplified Flow Diagram

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Benefits of Onsite Hydrogen Generation

1. Reduced inventory (some inventory still req’d for regas)

2. Lower pressure3. Limited delivery rate4. Less operator interaction5. Facilitates continuous scavenge

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Replacement Instrumentation is not enough

New instrumentation should enable the plant to improve plant operation: Improved purity control Reduced dewpoint Tighter pressure control

Replacing old purity, pressure and dewpoint instruments with new versions without additional functionality doesn’t make the plant run better.

New instrumentation should enable the plant to modify procedures to improve plant operation:1. •Improved purity control2. •Reduced dewpoint3. •Tighter pressure control

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Lowest possible dewpoint is critical

The dewpoint of the hydrogen in the generator casing can affect the lifetime of the generator windings. Wet hydrogen will reduce winding life due to corrosion. Wet hydrogen can be catastrophic in the case of 18/5 rings, susceptible to stress crack corrosion. You must keep dewpoint within manufacturer specs.Either a dryer in the recycle loop, or an optimized scavenge system can control hydrogen dewpoint. This chart illustrates the performance of a Stable Flow hydrogen control system driving dewpoint to manufacturer compliant levels without a dryer. Note the pressure stability – the benefit of optimized constant scavenge.

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Effects of Cooling Gas Quality

The quality of the hydrogen cooling gas has an impact on the overall operation of an electric power generator in three principal ways.1. Hydrogen purity directly affects the operating efficiency of

the generator.2. Hydrogen's moisture content affects the longevity of the

generator's internal components.3. The stability of the hydrogen gas pressure within the

generator affects the maximum generating capacity of the electric power generator.

The density of the hydrogen gas within the generator casing has a physical affect on the windage loss of the generator and the thermal conductivity of the gas and its ability to remove heat.

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Advantages of Thermal Conductivity

The high thermal conductivity of hydrogen has proven tobe a key advantage in its use as a cooling fluid in electricpower generators. It permits a reduction of nearly 20% in theamount of active material required in the construction of agenerator of given output and for a given temperature rise ofthe windings. The density of hydrogen is also an advantageover that of air. Since hydrogen’s density is approximately one fourteenth (1/14) the density of air at a given temperature andpressure, the use of hydrogen reduces the windage frictionlosses within a generator to a small fraction of the lossesencountered when the generator is cooled by air.

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Proper Operation of H2 cooling System

A properly implemented continuous replenishment system will maintain a consistent:1. high level of purity, 2. low gas dew point, and 3. constant pressure within the generatorCritical to the proper implementation of such a system is the supply of a continuous stable flow of high purity hydrogen from a trusted source.

Hydrogen has a thermal conductivity of nearly seven times thatof air, and its ability to transfer heat through forced convectionis about 50% better than air.

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Advantages of High Thermal Conductivity of H2

The high thermal conductivity of hydrogen has proven tobe a key advantage in its use as a cooling fluid in electricpower generators. It permits a reduction of nearly 20% in theamount of active material required in the construction of agenerator of given output and for a given temperature rise ofthe windings. The density of hydrogen is also an advantage over that of air. Since hydrogen’s density is approximately one fourteenth (1/14) the density of air at a given temperature and pressure, the use of hydrogen reduces the windage friction losses within a generator to a small fraction of the losses encountered when the generator is cooled by air.

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THE EFFECT OF HYDROGEN ON GENERATOROPERATION AND PERFORMANC

The quality of hydrogen coolant gas has an impact on theoverall operation of an electric power generator in threeprincipal ways.1. ! Hydrogen purity directly affects the operating

efficiency of the generator.2. ! Hydrogen dew point within a generator affects the

condition and longevity of the generator's internal windings.

3. ! The stability of the hydrogen gas pressure within the generator affects the maximum generating capacity of the electric power generator.

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CO2 Purging

The purity of hydrogen within a generator casing is

important for several reasons. First and foremost is safety. An explosive atmosphere exists when the hydrogen over air concentration in a generator falls below 74%. The primary function of purity monitoring systems has been to avoid this disastrous condition. Most plants will initiate a shutdown and automatic CO2 purge of the generator if the concentration falls below 85%.

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Dew Point Disadvantages

When the impurity in hydrogen gas coolant is moisture, overall efficiency of the generator is compromised through increased windage friction loss. 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 its retaining rings and cause stator winding shorts. It is recommended that the hydrogen dew point be maintained below +32"F in most generators, but will vary depending on the generator's original equipment manufacturer (OEM), the size of the generator, and the hydrogen gas pressure. Studies have shown that generators that operate with high hydrogen gas dew points are much more susceptible to insulation degradation in windings that inevitably lead to disastrous shorts and major unplanned repair actions.

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H2 Pressure Effects on Generator

At increased pressures, hydrogen becomes denser, improving its capacity to absorb and remove heat. As a result, additional load may be carried with no increase in the temperature rise of the windings. An increase in kilovoltampere output of about 1 % may be obtained for every 1-psi increase in hydrogen pressure up to 15 psig, while for pressure between 15 and 30 psig, an increase in output of about ½ percent per psi of an increase in pressure may be obtained.Increasing hydrogen pressure also permits operation at normal load with the temperature of the water supplied to the gas cooler in excess of normal. An increase in cooling temperature of approximately 1"F may be obtained for every 1-psi increase in hydrogen pressure up to 15 psig, while for pressure between 15 and 30 psig, an increase in cooling water temperature of 0.5"F per psi of an increase in pressure may be obtained.

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H2 Pressure Effects on Generator

The improvement in the thermal capability of the generator is proportional to the square root of the absolute pressure increase.

Therefore, a pressure increase of 1 psi would improve the generator output by 0.6% towards the OEM rated generator capacity. The increased capacity is due to better heat removal from the copper windings

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Batch feed hydrogen supply methods

There is also the potential of pressure reducing regulatorfailures causing catastrophic generator casing or seal failuresdue to high pressure hydrogen supply systems as well.

To reduce the risk of a catastrophic leak in a plant, batch feedhydrogen supply methods are often employed to maintain thegenerator pressure manually. This manual batch feed system isalso used to track seal wear by tracking the hydrogen leak

rate.The hydrogen seal leak rate can be determined if the 1. Pressure drop in the generator, 2. temperature, and 3. duration between fills is known

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Disadvantages of Batch feed hydrogen supply methods

A batch feed method causes wide fluctuations in cooling gas purity and pressure. These fluctuations have a huge effect on plant operations and ultimately hit the plant’s bottom line. The development of a continuous hydrogen supply system that maintains generator cooling gas pressure and purity at optimum levels while assuring plant infrastructure and personnel are safe from the hazards of a major hydrogen leak has begun to change the way plants view their hydrogen demands.

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Vent Flow System

As the purity in the generator decreases the vent flow is increased until it reaches the desired purity set point. In most cases, just a small continuous “bleed” of the generators cooling gas can make a dramatic change in overall purity and dew point.

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Disadvantages of Vent Flow System

This method has certain disadvantages when implemented using delivered gas cylinders and tube trailers. The operating cost of such a system can be substantial if the price of delivered hydrogen is even a few dollars per 100 cubic feet. Also, the safety concerns associated with implementing a continuous feed gas supply method with bulk storage are certainly relevant with this method. Onsite hydrogen generation systems should be the primary hydrogen supply choice when implementing this method

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Delivered hydrogen versus onsite hydrogen generation

Delivered hydrogen is relatively expensive when compared to onsite generation. Delivered gas prices fluctuate with the volatility associated with the supply, transportation, and increased security concerns over bulk hydrogen.Onsite generation, especially when employed at a power plant, offers the plant operator a fixed cost of hydrogen supply. An electrolysis-based onsite hydrogen generator requires a small amount of de-mineralized water and electricity to operate. An on-site hydrogen electrolyzer sized for an average power plant requires less than 20 gallons of water a day and 17kWh of electricity per 100 cubic feet of hydrogen produced

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Delivered hydrogen versus onsite hydrogen generation (Cont.)

Frequent cylinder changes can introduce particulate impurities and introduce air gases if not purged. Purity varies from cylinder to cylinder depending upon the source of the gas and what the cylinder was used for in the past.

In contrast, onsite hydrogen generation systems monitor the hydrogen product purity continuously to insure the gas that is being introduced to the generator is of the desired quality. The ability to trend and provide alarms is also available. In addition to quality variances, delivered hydrogen presents siting, storage and occupational safety challenges. High-pressure cylinders require personnel to monitor supply, manage delivery schedules to prevent unnecessary downtime, monitor the cylinders’ condition and gas purity, and store cylinders in accordance with a facility’s safety protocols.

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Delivered hydrogen versus onsite hydrogen generation (Cont.)

The maximum flow rate from an onsite generator will never exceed its maximum capacity, which is very important when performing “worst case” safety analysis.An onsite hydrogen generator has the inherent ability to meter the flow rate of hydrogen used. This flow metering function can be used to alarm operators of a sudden demand for hydrogen in the case of a catastrophic component failure, while tracking seal wear over time to aid in preventative maintenance actions.

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Hydrogen replenishment

Power plant operators can reduce the destructive presence of water vapor and air within their hydrogen-cooled generators with a constant flow of dry hydrogen gas. Continuous hydrogen replenishment is a technique that offers a way to keep generators free of:1. moisture, 2. oxygen, and 3. other contaminants that can prematurely degrade equipment, maintain hydrogen pressure at a level that supports an increased load, and supply hydrogen on demand at the rate it is required by the electric generator. All of these benefits are in addition to providing seal leakage makeup gas at a fraction of the cost of delivered gas.

By using an onsite hydrogen generator combined with a continuous hydrogen replenishment technique, plants can minimize hydrogen inventory, while guaranteeing consistent purity, dryness and pressure.

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PEM technology for onsite hydrogen generation

The most widely used onsite hydrogen generation technology at power plants in the U.S. is PEM electrolysis.Onsite hydrogen generators that use Proton Exchange Membrane (PEM) technology offer a cost-effective approach to ensure hydrogen purity and low dew point. PEM electrolysis allows 99.999+% pure hydrogen gas at a dew point of -90ºF to be produced on demand at process pressure without mechanical compression and without caustic electrolytes.

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PEM electrolysis

PEM electrolysis works within a hydrogen generators’ cell stack, which is composed of individual membrane assemblies stacked into an embodiment that brings water in to produce hydrogen gas. As illustrated in Figure 2, de-ionized water flows into the positive side of the cell where it is dissociated by electrolysis into protons, electrons and oxygen. The oxygen is carried away by the water flow. The positively charged protons are attracted to the negative side of the cell. These protons use the sulfonic acid ion groups embedded within the membrane as the path to travel through the solid material. Meanwhile, the electrons flow through the power supply to the negative electrode where they link up with the emerging protons to form molecules of pure hydrogen gas.

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PEM electrolysis

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PEM Technique Inherent Advantage

A PEM electrolyzer has the inherent ability to allow for precise metering of gas as it is being produced.

This inherent ability allows the electrolyzer to become a gas supply that precisely matches the process demand, much like a bulk supply, but with a few distinct differences.

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PEM Technique Disadvantages

1. First, the electrolyzer has the ability to deliver an unlimited supply of hydrogen as long as water and electricity is supplied. A bulk supply will eventually empty and need to be replaced.

2. Second, the electrolyzer has the ability to limit the instantaneous flow rate of hydrogen gas due to its capacity limit. A bulk supply will deliver as much gas as can flow through the process infrastructure.

3. Third, the electrolyzer has a very low volume of stored hydrogen at any one time and only produces gas as it is demanded.