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Fuel Cell Types and How Solar Fits In

Fuel cells are best known as the potential “green” mobilizer in hybrid automobiles. Toyota, Honda and Ford all have prototypes or expensive production models now available. But fuel cells’ potential may be greater outside the transportation sector, especially considering the likely future of EVs and battery technology, a potential and a promise that vary by situation and fuel cell type.

Fuel Cell Types

According to the Department of Energy, there are six types of fuel cells in production or development at this time. A seventh type of fuel cell (as of yet unproven) may revolutionize distributed solar energy systems - a prospect we’ll explore momentarily. But first to fuel cells:

Polymer Electrolyte Membrane (PEM) Fuel Cells

PEM fuel cells are best suited for the transportation industry. PEMs are what you’ll find in today’s very expensive models and likely in tomorrow’s more mainstream FCVs. PEM fuel cells are lightweight and low volume. They also have a quick start-up time and work at low temperatures compared to other fuel cells, making them ideal for automobiles where motion on-demand and safety are obviously important.

PEMs have two significant problems. The catalyst requires platinum, a very expensive noble metal. This increases costs significantly. Another issue is hydrogen storage. Because hydrogen is a low-density gas, it’s difficult to store enough hydrogen on-board the automobile to fuel it far enough to compete with today’s gasoline engines.

Direct Methanol Fuel Cells

Some hydrogen fuel cells will extract hydrogen from other fuels, such as methanol, ethanol and hydrocarbons, in a method known as reforming. Direct methanol fuel cells (DMFCs), however, are directly powered by pure methanol.

Methanol has a few advantages over hydrogen:

  • Methanol has a higher energy density and therefore avoids the storage problems inherent in many hydrogen fuel cells.
  • Given our current fuel infrastructure, methanol is easier to transport and supply to drivers because it’s a liquid like gasoline.

According to the Department of Energy, research on DMFCs is still three to fours years behind that of other fuel cells and is at somewhat of a disadvantage in that respect.

Alkaline Fuel Cells

AFCs were one of the first fuel cells ever developed (thanks to NASA), and are still a high-performing fuel cell compared to other technologies. They can operate at both high and low temperatures and have efficiencies reaching 60 percent in space applications. Unfortunately, AFCs are easily poisoned by carbon dioxide, which means that hydrogen and oxygen used in the cell must be purified first, increasing costs.

Phosphoric Acid Fuel Cells

PAFCs were the first fuel cells to be used on the commercial level and are typically used for stationary power generation. PAFCs can handle impurities in the fossil fuels reformed into hydrogen better than PEM cells (both use the same platinum catalyst), and are up to 85 percent efficient in a co-generation application. Alone, PAFCs are only slightly more efficient than fossil fuel power plants. PAFCs are also expensive because they rely on a platinum catalyst, greatly increasing the cost of production.

Molten Carbonate Fuel Cells

MCFCs operate at extremely high temperatures (about 1,200 degrees Fahrenheit), which enables the use of non-precious metals as catalysts. MCFCs are also relatively high in efficiency, reaching 60 percent standing alone, compared to the roughly 40 percent attainable by phosphoric acid fuel cells. The high operating temperatures also eliminate the need for reformers to purify fuels being converted to hydrogen. The high temps take care of that themselves within the fuel cell. They’re also resistant to poisoning by carbon dioxide or carbon monoxide, making them ideal for setup next to fossil fuel power plants.

All the benefits of high temperature operation also bring one major drawback: durability. Corrosion becomes a problem for MCFCs, accelerating breakdown and shortening the life of the fuel cell. Should corrosion-resistant materials be found and applied successfully, a big breakthrough for MCFCs may well occur.

Solid Oxide Fuel Cells

A hard, ceramic compound for its electrolyte makes a solid oxide fuel cell (SOFC) unique. Like MCFCs, solid oxide fuel cells operate at very high temperatures exceeding 1,800 degrees Fahrenheit. So again, there is no need for precious metals and we have a lower cost. SOFCs are very resistant to sulfur, carbon monoxide and carbon dioxide, making them ideal for co-generation applications at coal-fired power plants.

The same disadvantages of high operating temperatures apply for SOFCs, including material corrosion and a slow startup.

Regenerative Fuel Cells

As of yet, the above fuel cells have no real connection to solar power, except for the possibility that some could be used to convert waste heat or steam from solar thermal plants into electricity or - perhaps metaphorically only - as a Stirling engine.

However, regenerative fuel cells are now in production that can use solar power to create the electricity that splits water into oxygen and hydrogen fuel. Developed by a team of MIT researchers, this sort of fuel cell would take excess energy produced by a solar power system (or wind power, etc.) and use it to split water in a fuel cell. For solar applications at night, that energy could be used to power the home when the solar panels are not producing.

As much as solar power and fuel cells have little in common up to this point, the two together may end up revolutionizing residential solar power, eliminating the need for expensive battery backup systems, making off-the-grid and self-sustained living that much more affordable. The MIT discovery, however, was just announced last year and is still far from distribution itself. Researchers hope to have a workable model available within the next decade.

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