FUEL CELL TECHNOLOGY

Power Generation,  How Fuel Cell Technology Works 

Fuel cells convert fuel into electricity through an electrochemical process instead of combustion of the fuel. William Grove invented the first fuel cell in Great Britain in 1839 and people have been trying to make them economical ever since. The technology went on to power all onboard electronics for the U.S. space program since the 1960s.

Since then, various fuel cell chemistries have been developed. This has resulted in products suitable for a wide range of applications. Interest in fuel cells has continued growing due to a world-focus on efficiency and pollution. Fuel cells pack high efficiency into small packages with almost zero sulfur oxide, nitrogen oxide and carbon monoxide pollution. In theory, fuel cells are better than piston engines, gas turbines and steam engines.

Power Generation

Engine-generator technology

A typical Internal Combustion Engine (ICE) generator set (genset) burns fuel in an engine to turn a shaft connected to a generator, spinning a magnet in a wire coil and "pushing" electrons out. This process uses combustion and results in significant greenhouse emissions. The fuel efficiency also varies greatly depending on the type of genset used. The theoretical maximum efficiency of an engine is called the Carnot limit. So, the larger the difference between the peak temperature in the engine and the exhaust temperature, the greater the efficiency. Unfortunately as peak temperatures climb to get more efficiency from an engine, so does the production of nitrogen oxides. Nitrogen oxides may be a minor greenhouse gas, but they are a major contributor to eye-stinging urban smog. With tough emissions restrictions, engine manufacturers go to great lengths and expense to reduce nitrogen oxide emissions. 

Best Gensets today:

  • Reach a maximum of 49% efficiency – for larger diesel-fueled piston engines ~ greater than 5 MW.
  • Reach a maximum of 42% efficiency - for medium-size diesels in the 1 MW range.  
  • Reach a maximum of 20% efficiency - for smaller gensets in the 6 - 10 kW range. 

Gas turbines

Gas turbines have historically achieved a maximum 38% efficiency. But the exhaust from a gas turbine is quite hot and can be used for making steam to drive a third kind of engine – a steam engine.  Steam engine-generators have achieved as much as 40% efficiency. Coupling a gas turbine – operating on the Brayton Cycle – to a boiler and steam engine using the Rankine Cycle, you get a combined cycle power generator.

Best Combined Cycle Power Generators:

  • Have spectacular efficiencies of 60%.
  • May achieve higher ratings in the future – but they are getting close to the theoretical Carnot limit.
  • Must be large to attain high efficiencies.

Smaller Gas Turbines:

  • Have efficiencies between 25% and 35% by themselves in the range of 25-300 kW.
  • Could be pushed possibly to 40+% in another version of a combined cycle system using their waste heat.

Fuel Cells

There are a variety of Fuel Cell types but in general they:

  • Are an alternative power generator.
  • Are not heat engines. 
  • Are not limited by Carnot efficiency.
  • Have theoretical efficiency limits well over 70%.
  • Can achieve high efficiencies in small-size units.
  • Operate at temperatures so low that nitrogen oxide pollutants are not formed.

Some 2 kW fuel cells have achieved 60% efficiency in converting the energy in natural gas to electricity. And some fuel cell providers have claimed efficiencies of 70%, but this is for fuel cells which require high purity hydrogen. The 70% efficiency claim does not include the energy required to split out hydrogen from natural gas or water. Including those losses, the net efficiency drops to between 25% and 30%.

At efficiencies of 60% in small units, fuel cells can provide power at the home, office or factory much more efficiently than can the utilities with all the losses in the power lines going from big power plants to the meter. Where there is no utility, fuel cells can provide power at double the efficiency of small engine-gensets. 

How fuel cell technology works

Fuel cells convert chemical energy in fuels to DC electric power using an electrochemical process – not through combustion. Similar to how a battery operates, a fuel cell is recharged by a fuel used to run a chemical "pump". The "pump" moves electrons from one side of a barrier (which becomes positively charged) to the other side (which becomes negatively charged). It does so continuously until the fuel supply is exhausted.  

There are many types of legacy fuel cells such as proton exchange membrane (PEM), molten carbonate fuel cells (MCFC), direct methanol fuel cells (DMFC) and solid oxide fuel cell (SOFC). The table below lists characteristics of these various fuel cell approaches. These technologies use expensive precious metals, corrosive acids or hard-to-contain molten materials and have marginal performance. Most of these fuel cells generate electricity by moving fuel through the electrolyte. Solid oxide fuel cells differ in that they move oxygen from air through the electrolyte.

Fuel Cell Type Proton Exchange Membrane (PEM) Molten Carbonate Fuel Cell (MCFC) Direct Methanol Fuel Cells (DMFC) Phosphoric Acid Fuel Cells Solid Oxide Fuel Cells (SOFC)
Electrolyte Polymer Molten Membrane Acid Ceramic
Temperature Low High Low Low Highest
Precious Metals Yes No Yes Yes No
Fuel Flexible No Yes

No

 

No Yes

CO2 Emissions

(lbs/kWh)

None 1.0 0.730 1.050 0.750
Scalability No No No Yes Yes

Efficiency: Electrical

25-35% (including losses to create hydrogen)

50-60%

25-35%

35%

50-60%

Efficiency: CHP   85%   85% 90%
Durability Limited High Limited Medium High
Start-up Time Seconds (not counting the Reformer) Hours Seconds Hours

Tubular: Minutes

Planar: Hours/Days

Proton exchange membrane (PEM)

  • Most commonly known fuel cell design
  • Uses a thin polymer membrane and is lightweight
  • The membrane has a short life of 10,000 hours or less
  • Uses high purity hydrogen as a fuel and requires a catalyst (like platinum) to facilitate the electrochemical reaction due to low temperature of operation
  • Extracting and purifying hydrogen from other hydrocarbon fuels requires an external reforming process – adding significant cost, space requirements and inefficiency
  • Platinum catalyst is easily poisoned by the by-products of hydrogen extraction - e.g. by carbon monoxide
  • Some systems extract hydrogen from methanol 
  • High purity hydrogen requires specialized distribution and storage methods
  • Suitable primarily for occasional use and not intended for continuous use
  • The special requirements needed for PEM fuel cells offset most of their advantages

Molten carbonate fuel cell (MCFC)

  • Operates at high-temperature
  • Use a molten carbonate salt mixture which must be melted to work
  • Can operate on a variety of fuels
  • Are highly efficient
  • Better suited to larger scale (>250 kW) applications

Direct methanol fuel cell (DMFC)

  • Run directly on methanol without the need of a pre-reformer
  • Uses a thin membrane and are lightweight
  • Have a limited operating life
  • Mostly suited for smaller scale applications
  • Used as on-board charger for electric vehicles or as back-up power

Solid oxide fuel cell (SOFC)

Recognized as having the greatest potential out of all of the fuel cell technologies.

  • Use low cost, stable ceramics
  • Operate at elevated temperatures: 600oC to 1000oC
  • No need for special expensive catalysts
  • No external reformer required for light hydrocarbon fuels such as natural gas, methane, biofuels and liquid fuel
  • Acts as both the reformer and the power generator saving space and expense
  • Has demonstrated a life of over 75,000 hours
  • Is highly efficient at both small and large scales

Use of hydrocarbon fuels still produces carbon dioxide (a greenhouse gas). But SOFC power generation produces significantly low levels of carbon dioxide. Since fuel cells produce electricity through a chemical process versus combustion, there are virtually no nitrogen oxide (NOx), sulfur oxide (SOx) or carbon monoxide emissions. This adds up to unparalleled efficiency – less fuel to produce the same electricity.     

Learn more about Atrex Energy's Remote Power Generators using SOFC.

 

 

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