Cooling Generators with Hydrogen

Interview conducted by Mychealla RiceOct 17 2017

Hydrogen gas is seven times more effective as a heat transfer medium than air. In this interview, Thomas Skoczylas from Nel Hydrogen talks to AZoM about cooling generators with hydrogen and why they are extremely efficient.

How is Hydrogen used in the Power Generation Industry?

Many power generators over 150 MW in capacity utilize hydrogen as a cooling method to transfer heat from the power generating winding enclosure to the heat exchanges known as H2 coolers.

Hydrogen cooled power generators are more efficient and have less mass of materials of construction than their air-cooled cousins.

Hydrogen gas is 7 times more effective as a heat transfer medium than air and 1/14^th the density, resulting in less friction losses and more fuel converted to electricity.

How is Hydrogen typically procured in the power generation industry?

Hydrogen is typically supplied to power plants by trucks in high pressure cylinders or bulk vessels known as tube trailers. The hydrogen is then transferred at the plant either by swapping the empty cylinders for full ones, swapping an empty tube trailer for a full one, or by transferring from a high pressure tube trailer truck to nearly empty cylinders at the power plant, or to nearly empty high pressure vessels or tube trailers installed at the site.

Most trucks have 50,000 to 100,000 scf of mobile inventory. This replenishing of hydrogen inventory at site can occur up to 4 times per month with volumes up to or beyond 50,000 ft³ per event. 50,000 ft³ of hydrogen is the chemical explosive equivalent energy to 5,800 lbs of TNT explosives. This hydrogen transfer activity is a manual one.

The frequency of these activities over time increases the statistical risk of an accidental release and subsequent potential accidental ignition. These accidents have occurred in the past, and can be expected in the future.

What is the typical hydrogen usage for generation facilities?

The amount of hydrogen used per day is generally a function of the power generator capacity and the condition of the generators hydrogen seals. The higher the capacity, the higher the hydrogen pressure required for sufficient heat transfer.

Higher pressure results with higher leakage rate, requiring a higher hydrogen flow to maintain the pressure required for optimal heat transfer.

Generators in the 150 to 250 MW range coupled to gas turbines or the steam turbines of combined cycle power plants typically consume 200 to 600 scf/day (1 to 3 cylinders equivalent), (5 to 16 Nm³/day).

For generators coupled to steam turbines at thermal plants (coal fired or thermonuclear) power generators of 400 to 1,200 MW can use up to 1,000 scf/day (4 or more cylinders per day), (25 Nm³/day).

Why does hydrogen purity matter for generator cooling?

Primarily purity matters because of safety. Hydrogen purity must be maintained above the UFL (upper flammability limit) of 75%.  Most generators have alarms set at levels greater than 90% with an automatic trip and purge if purity falls below 88%.

Additionally, hydrogen purity effects efficiency. Air is 14 times denser than hydrogen. When the purity of hydrogen in a power generator casing is less than 99% the contaminant consists mostly of air.

For every 1% of purity below 99% (the realistic achievable/maintainable purity in a power generator) the resulting hydrogen gas is 47% more dense.

Density directly affects viscosity which of course directly affects friction. Increased friction increases a phenomenon known as windage loss. Windage loss is the energy lost to heat imparted by the friction of the rotating rotor rather than producing electricity that could have been sold.

Depending on the capacity of the power generator and the design particulars of the generator OEM, this windage loss per percent purity can be in the range of 70 to 400 kW per every percent purity below 99.

This equates to as much as 600 to 3,650 MWhr/year depending on generator capacity per every percent purity maintained below 99%.  Hydrogen purities observed in power plants are in the 94 to 99% range.

What advantages does on-site hydrogen generation provide to power generation facilities?

On-site hydrogen generation nearly eliminates the frequency of hydrogen deliveries by truck, therefore reducing the statistical risk of an accident.

With on-site hydrogen generation, all piping is permanent and there are no manual connection and disconnection activities associated with the refilling of large volumes of hydrogen or transfer operations of truck deliveries. Furthermore, generating hydrogen on-site is less costly per unit volume over time.

A containerized C Series Hydrogen Generator used for generator cooling being prepared for to delivery to a powerplant customer. Containerized hydrogen generators offer powerplants a turnkey solution that can be installed outside in variety of climates.

Do users see improved flexibility in facility siting and safety during design and implementation of a new plant?

Of course. Many on-site hydrogen generators sized for use in power plants are designed and certified for installation into any nonhazardous, non-classified, ventilated space.

This means there are generally no special permitting or required minimum distances from other things in the plant such as any buildings, offices, electrical equipment, etc. as there is with high pressure tanks for hydrogen.  Secondly, power plants have two needs for hydrogen supply to a H₂ cooled power generator.

First is for daily supply to maintain pressure and purity inside the power generators. For this daily purpose, an on-site hydrogen gas generator automatically makes and delivers the exact amount.

The second purpose is to be needed during major maintenance actions of power generator. During such an event, the plant operators may need to purge all hydrogen from the power generator with CO₂ and then purge the CO₂ with Air in order to make it safe for maintenance personnel to open and work on the power generator.

Once maintenance on the power generator is completed, the operators will purge out the air with CO₂ and then purge out the CO₂ with hydrogen to bring the power generator back up to required hydrogen pressure and purity.

This refilling exercise, depending on make, model, and capacity can take up to 15,000 scf (395 m³) or more. Typically this refilling process is required to be completed in just a few hours so the plant can get the unit back up and making electricity.

Today most plants use a large number of cylinders and bulk high pressure storage for this activity.

Bulk high pressure tanks sometime permanently installed on the plant property take of a lot of square footage of area due to the free distances required for safety code.  An additional advantage of  on-site hydrogen gas generator means the plant can make ahead of time their own reserve for the maintenance activity by filling bulk low pressure tanks on site with excess capacity from the hydrogen generator.

Lower pressure tanks, although larger in physical size than high-pressure tanks can take up less square area in the plant.

This is due to the latest national safety code released for hydrogen gas system; The NFPA 2 Hydrogen Technologies Code 2011 Edition or newer. Due to the modern understanding on how hydrogen behaves if it were to leak from pressurized piping or tank, it is known that the higher the pressure the higher the hazard.

Thereby lower pressure storage systems holding the same amount found in high pressure system can take up less space due to the reduce hazard associated with the system. REF: NFPA 2:7.3.2.3 Outdoor Storage and NFPA 2: Section 7.3.2.3.1.1.

Can I bottle or store my hydrogen using my hydrogen generator?

Yes, hydrogen gas generators with capacity greater then the daily usage requirements can be selected. This excess capacity can be used to fill portable high-pressure cylinders or bulk high-pressure vessels with the use of a hydrogen compressor.

This can be done simultaneously while the exact quantity of low pressure hydrogen is safely delivered though piping to each power generator on the site. A better way to store hydrogen is to use lower pressure tanks, eliminating the procurement and O&M of a compressor(s).

Since hydrogen gas generators can deliver hydrogen at 225 or 435 psi, low pressure tanks have sufficient volume to meet the plant’s storage needs, and can easily be accomplished at a safer lower pressure.

Nel's Containerized Hydrogen Generators aren’t just used for makeup gas, the generators can be optioned with a compressor system to store hydrogen for regas operations.

Where can our readers go to find out more?

To find out more please visit https://nelhydrogen.com

About Thomas Skoczylas

Thomas joined Nel Hydrogen 1998 as a Chemical Engineer. Since that time he has served many functions at Nel Hydrogen as an engineer developing and validating Nel’s Technologies and Products.  He has been involved with Nel’s fundamental electrolyser development and system product validation for entry to market.

Thomas had also served as Manager of the Technical Service Department, Director of Applications Engineering, and most recently managing the sales of commercial products into different technological and geographic markets around the world as a Territory Sales Manager.