Fuel cells may soon be available for the home, as the Technical University of Denmark (DTU) has entered into an agreement with a local manufacturer to market them.

The fuel cells will be called Topsoe PowerCores. They look like a small refrigerator, and use natural gas as their fuel, but they can also operate on biofuels or alcohol.

Andreas Richter, manager of business development at Topsoe Fuel Cell, the company that will be making the units, said he expects the first home versions to be available in 2015, and the goal is to get the price to the $10,000-$15,000 range.

The fuel cells would provide both heat and electricity to a home, and even allow for selling excess electricity back to the grid.  

A fuel cell is an electrochemical cell that converts fuel into electricity. They are similar to batteries in that they have a cathode (positive) and an anode (negative) terminal, connected by an electrolyte.

In a battery, the electrolyte is some chemical that isn't replenished, such as acid. When the metals in the cathode and anode react with the electrolyte, electrons are released and that produces the current.

Fuel cells are different in that they use a fuel that reacts with the anode, which strips off the electrons from the fuel, usually hydrogen. The electrolyte allows the positively charged atoms to pass through to the cathode, but not the electrons, which can then be used for current. When the positively charged ions reach the cathode, another catalyst turns them into the waste chemicals, usually water and carbon dioxide. But hydrogen is a difficult fuel to store and use.

Topsoe's design is called a solid oxide fuel cell, meaning that the anode, cathode, and electrolyte are all solids. They are ceramics, made from zirconium oxide doped with other elements.

Air comes in one side of the fuel cell and is exposed to the cathode, made of lanthanum manganite doped with strontium oxide. The cathode allows oxygen ions, which have picked up two electrons, to pass through the electrolyte, which is a zirconium oxide ceramic doped with yttrium oxide.

The electrolyte allows the oxygen ions to pass through, but not the electrons by themselves. When the negatively charged oxygen ions reach the anode, which is zirconium oxide and nickel oxide, it reacts with the fuel.

Ordinarily, natural gas would react with oxygen and burst into flame, releasing no current at all. But the oxide materials in the ceramic allow it to react without doing that. Instead, the natural gas reacts with both the oxygen and the anode, turning into carbon dioxide and water and releasing electrons, which are then used as current and pumped back into the cathode to ionize incoming oxygen.

Even though the natural gas doesn't burn, it does react at a high temperature, and that heat can be harnessed. In the Topsoe models it can be linked to a hot water system, for example.

Richter notes that the efficiency is much better with fuel cells than conventional sources of electricity because more of the chemical energy in natural gas - or most other fuels - is put to use. In a coal plant, he says, only about 25% of the energy in the coal is used to generate electricity. In a fuel cell up to 90% of the energy in the methane is turned into electricity or heat.

Soren Linderoth, a professor at DTU and head of the division of fuel cells and solid state chemistry, said this kind of fuel cell is actually an old idea, dating back to the 1930s. The big advantage over traditional fuel cell designs - besides the ability to use different fuels - is that the materials to make the fuel cells are much cheaper. Many fuel cell designs require using platinum or other rare (and sometimes toxic) metals.

Topsoe is partnering with Dantherm, another Danish company, to build the units. Thus far there are demonstration models but none have been mass-produced as yet. The company is looking at several markets, he says, such as Germany, the U.K. and Italy.

One factor in selling the fuel cells is how many homes use natural gas and whether the local utilities have a system in place for selling excess power back to the grid, Richter says. Areas where many people use natural gas for heat are most promising.