Simulation helps build a fuel-cell future

Sep 1, 2010 12:00 PM,

Nick Lerner
Dassault Systemes Technical Specialist
www.3DS.com

Fuel-cell modules that produce high power-density levels could fundamentally change the way homes are heated and powered.

From left to right, the components to the fuel cell technology are: raw materials of the fuel cell, a single Ceres Power Fuel Cell, Fuel Cell Stack, Fuel Cell Assembly, and a Fuel Cell Stack as the core in a residential combined heat and power product.

Consumers today are becoming more sensitive to the energy efficiency of their heating and cooling systems and major appliances. But they generally don’t realize that routing electricity to their homes has an efficiency cost. Estimates are that as much as 65% of the electricity supplied from central power stations is lost as it finds its way through the grid.

But there is a way to avoid this loss of electricity. A recent approach called “microgeneration” converts fuel such as natural or packaged gas into heat and electricity – at the point of use. This approach not only avoids centralized- power losses but also could potentially cut consumers’ total energy costs by an estimated 25%. Microgeneration isn’t just a way to generate electricity. It can do double duty as a home heating system, and an efficient one at that.

An example of a recent microgeneration device comes from the “combined inheat and power” (CHP) unit powered by a solid oxide fuel cell (SOFC) from Ceres Power in the U.K., (www.Ceres Power.com). The devices take the place of residential boilers, using the same connections. (Most homes in Europe use hot water heating rather than forced air as in the U.S.)

Abaqus FEA is analyzing the stress on a gasket developed by Ceres Power for its fuel cell stack design. The shading indicates moments of force and stress distribution. The lighter shades indicate a higher level of stress based on loading. The scale on the upper left is a key to stress levels.

In designing the CHPs, Ceres engineers used Dynamic Modeling Laboratory (Dymola) and Abaqus FEA software from Dassault Systemes (DS) to build as few prototypes as possible. In addition, the software helped optimize the design for the smallest practical size and the greatest output. This was important because in Western Europe alone, about 78% of all residential boilers mount on the wall. Therefore, the CHPs must be small enough to mount on the wall while supplying adequate heat and electricity. In addition, the software helped optimize costs against benefits, as well as structural integrity against thermal integrity.

The workings of a fuel cell

First, an understanding of how a fuel cell works is helpful. Fuel cells convert air and fuel, such as natural gas, directly into power and heat through a solid-state chemical reaction. A basic cell comprises a support structure, an external circuit, and an anode and cathode between which lies a thin, gas-tight, electrically insulating but ion-conducting electrolyte layer.

The control on the CHP module lets it meet daily and seasonal energy demands in the home by varying the heat and electrical output to maximize savings and avoid wasted energy.

When fuel passes over the anode side and air passes over the cathode, it causes negatively charged oxygen ions to flow from the cathode and across the electrolyte. At the anode, oxygen ions combine with positively charged hydrogen ions and release electrons which then flow around the external circuit to the cathode, generating direct current. The fuel-air reaction also generates heat, which can be captured and fed into a heat exchanger.

In general, fuel cells produce power significantly more efficiently than internal combustion engines because they do not use an efficiency-sapping mechanical phase. Fuel cells operate at maximum efficiency under load (whereas most ICE generators operate inefficiently) and their efficiency is largely unaffected by size. Further, fuel cells can be stacked to match the specific output power needed.

The Ceres Power fuel cell stack is a layered construction of metal-supported fuel cells separated by stainless steel interconnects that let fuel and air flow to each fuel cell. The special construction of welded sub-assemblies and gasket seals provides a durable design.

Stationary, power-generation devices based on fuel cells have served niche markets for many years, but the systems were mostly 100 kW or above. These systems work well with SOFCs because they can use conventional fuels such as natural gas, propane, or LPG rather than pure hydrogen. However, it has been impractical to make them in a size range for single homes, where around 1 kW is a ballpark.

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