Friday, August 21, 2015

Fuel cells: a logical step towards a clean energy future

Is there a better, more efficient way to use natural gas as an energy source, today?  

Can this be a natural step to a fossil fuel free future? 

If we create electricity and heat from natural gas with fuel cells rather than by traditional combustion, this will be more efficient, create less or no pollution and lead to a fossil fuel free future.

Why hydrogen?

Hydrogen is a very efficient energy carrier and the most abundant element in the universe. It is assumed that around 75% of all mass of the universe is made out of hydrogen atoms, the simple chemical element with the atomic number 1, the first and the lightest element on the periodic table.  In addition to being the most abundant, it has one of the highest energy densities per weight; 1 kg of liquefied hydrogen, has 141.86 MJ/kg, 2.65 times more than LNG, 3.06 times more than gasoline and 3.11 times more than diesel.

How a hydrogen fuel cell works

Hydrogen atoms enter the fuel cell at the anode (A) coated in a catalyst that helps to separate hydrogen atoms into positive charged hydrogen protons and negatively charged electrons. The electrolyte (B) acts as a membrane that only allows hydrogen protons to pass through to the cathode on the other side of the fuel cell. As oxygen flows into the cathode (C) coated in another catalyst, oxygen atoms want to combine with hydrogen atoms into water and missing hydrogen electrons flow through the external circuit from the fuel cell anode to the fuel cell cathode thereby creating electricity. The process of combining hydrogen and water into electricity creates heat that can also be used in addition to the produced electricity.  

A single cell can create only very low voltage, usually below 1V, so many fuel cells must be stacked together (fuel cell stack) to produce useful DC voltage. 

There are various types of fuel cellsclassified by the type of electrolyte and type of electro-chemical reaction that takes place inside. 

There are various sources of hydrogen, of which the most abundant is water (H2O), a non-renewable resource, as well as methane (CH4), a fossil hydrocarbon created from biomass by various natural processes over millions of years.  

To create hydrogen from water we can use renewable electrolysis; using renewable sources of electricity to separate hydrogen and oxygen from water molecules. 

The cleanest way to create hydrogen from water is through photoelectochemical water splitting; using sunlight directly to split water into hydrogen and oxygen.

Solar thermal water splitting uses a solar reactor to create very high temperatures (500-2,000 deg. C) from concentrated solar power and a chemical reaction to produce hydrogen from water. 

Until we master the above processes, or have enough renewable energy to create hydrogen, we can produce it from methane which is the main component of natural gas. This can be done in an environmentally sensitive way using steam reforming, and in the process redirect natural gas from the combustion process that is very inefficient and provide a natural path towards the hydrogen economy. 

Steam reforming, the easiest way to make hydrogen from natural gas

Methane water in the form of steam reacts at high temperature (700-1,100 deg. C) over a nickel-alumina catalyst in reformer (A) producing carbon monoxide (CO) and hydrogen (3 H2). Then, produced carbon monoxide reacts with steam (B) in a shift converter over a catalyst producing more hydrogen (H2) and carbon dioxide (CO2)As a result of the reaction, we have pure hydrogen that can power a fuel cell and pure CO2, that is typically vented into the atmosphere but can be compressed and sequestered into or below the geological formation from which the natural gas was extracted.

You do not need to be alarmed, even if all CO2 is vented to the atmosphere, it is less than half what you would have if you used coal to generate electricity or 1/3 less than if you used gasoline, diesel or natural gas. There are no traditional pollutants in vented CO2 such as NOx, SO2, PM10 or PM2.5. Utilizing all available heat from fuel cells or CHP (Combined Heat and Power), will produce further reduction in emissions.  Fuel cells are not limited to the Carnot limit and with time their electrical efficiency will further increase, so this is a much better option than anything else currently used. 

Fuel cells are typically constructed as a standalone system that is capable to use utility quality natural gas and produce relatively clean electricity. They typically do not require outside source of water as water produced from the fuel cell reaction is utilized in a close loop system to produce steam for steam reforming.

How to generate power using a fuel cell

Clean natural gas enters the fuel cell (A). Stem reforming separates hydrogen from methane and water, and pure hydrogen (B) flows into the fuel cell stack. Heat and carbon dioxide are the byproduct of the reforming process. The fuel cell stack produces direct current electricity and when hydrogen combines with oxygen from air (C), water and heat are produced. Water is sent back to steam reforming. An inverter is used to convert direct current electricity to alternating current electricity that is fed into the utility grid or is used locally. Produced heat in the form of hot water is dissipated into the air at an integrated cooler or it is used to provide heat to external processes via heat exchanger.

Benefits of fuel cells
  • Fuel cells have higher efficiency of converting chemical energy to electricity and mechanical work than combustion engine, are not limited by the Carnot limit and are continually improving.
  • They can operate efficiently on hydrogen generated from traditional (fossil hydrocarbons) sources and from renewable sources. When used with hydrogen from renewable sources there is no greenhouse gas (GHG) emissions.
  • There are no air pollutants (NOx, SO2, PM10 and PM2.5) generated from fuel cells.
  • There are no moving parts, they operate silently, can be installed beside, on top or within any commercial or residential building.
  • Heat generated by fuel cells can be used for heating and for cooling (with absorption chiller). Can you imagine an office tower using incorporated fuel cells to provide local power, electricity, heat and cooling from locally available natural gas?
  • The absence of combustion and moving parts means that fuel cells are more reliable than combustion engines.
  • They are very responsive to fluctuating electrical loads.
  • Fuel cells are the ideal solution to use with natural gas and will allow a gradual move by society to renewable sources of hydrogen.
Disadvantages of fuel cells
  • It takes longer to startup a fuel cell than a combustion engine.
  • Fuel cells are more expensive than combustion technologies due to limited use and exotic and expensive materials used as catalysts. Some may argue that if full cost of power generated by combustion technologies were included, namely health and climate change costs, fuel cell generated power cost is already at par with combustion.
  • For mobile use, there is no hydrogen infrastructure to support a mobile application.

I believe that sooner rather than later we will embrace more fuel cells for distributed power generation with cogeneration (CHP) and trigeneration (CHRP). This will be an efficient use of natural gas, biogas and hydrogen generated using renewable sources of energy.

Fuel cells will also play a role in powering our future transportation in parallel to electric transportation. Toyota Mirai is the best example of what the future may look like, it features the Toyota Fuel Cell System, which combines fuel cell technology with hybrid technology. 

Hydrogen fuel cells are also evaluated for use in other modes of transportation.

China is implementing a hydrogen powered tram in Foshan, a city of 8 million people. In the next 5 years an investment of around $32 billion will allow Foshan to build infrastructure to move more people using clean energy powered with Ballard fuel cells from Canada.

Others experiment with fuel cell powered planes, such as Boeing Fuel Cell Demonstrator Airplane.

Marine transportation evaluates the use of fuel cells as range extenders, with potential future for full replacement of existing combustion engines and nuclear submarine propulsion.

The future looks exciting for fuel cell technologies!

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1 comment:

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