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Simulation helps build a fuel-cell future

Simulation helps build a fuel-cell future

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

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.)

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.

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.

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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An adaptation of SOFC technology is what made the CHP design feasible. Conventional designs typically run at 800 to 1,000º C. The use of a new generation of ceramic material called cerium gandolinium oxide (CGO) instead of the industry standard yttria stabilized zirconia (YSZ) allows operation at 500 to 600º C. This, in turn, allows the use of conventional stainless steel as the cell substrate, separating the mechanical support from the electrochemical functions. This design makes it possible to use extremely thin electrochemical layers, which can be optimized for maximum performance, resulting in excellent power density levels and lower stack material costs. Fuel gets effectively converted into heat and electricity and the high efficiency is maintained across a wide load range.

The metal-supported CHP cells are much stronger than cells made totally from ceramic. In addition, they can be easily sealed via welding and have thermal expansion coefficients well-matched to their ceramic coatings. This provides greater resistance to thermal shock, which allows rapid start-up times and the potential for many on-off cycles.

The CHP fuel cell module is designed to generate all of the heating and hot water and the majority of the electricity needed by a typical U.K. home. A controller on the device matches output to daily and seasonal demands. The CHPs can operate on renewable biofuels as well as a range of hydrocarbon fuels.

Simulating the CHP module

Ceres engineers used Dymola software to model and simulate the complex, integrated fuel-module system. In general, models incorporate technologies that include mechanical, electrical, control, thermal, pneumatic, hydraulic, power train, and thermodynamic. Dymola helps develop and support component design through the simulated application of physics. In Cere’s control-systems development, the software lets engineers observe and program functions and monitor and develop their effects. For systems-integration, Dymola shows inter-relationships and ensures that electric and mechanical systems and the software that links them work well together.

The software is based on the non-proprietary, object-oriented, equation-based Modelica modeling language. Dymola uses symbolic manipulation to transform physical models into code that can be used for computer simulations, exported for real-time hardware-in-the-loop (HIL) simulations, or exported as C code for other applications.

At Ceres, engineers simulated different components and potential control system changes to digitally expose their effect in a physical CHP – before it is built. “This saved us from making physical prototypes until we had thoroughly explored and accurately calculated the different options,” says Mark Selby, Ceres Power senior engineer. “The software also simultaneously simulated the combined effects of changes to components and the control system, and it helped us measure the implication of decisions at different production volumes. Also, it let us introduce ‘faults’ to check system response and take appropriate design or control-programming decisions.”

Dymola also supports Ceres’s communication with its supply chain by allowing accurate target specifications to be issued and balances to be introduced on target cost-performance criteria. “There is always a tradeoff between price and performance in the supply chain,” says Selby. “The software helps us get maximum value at the best price while fully understanding the implications of commercial and technical cost-benefit choices.”

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As an aspect of its PLM system, Ceres Power also used Abaqus FEA software from Simulia, the DS arm for realistic simulation. “We use Abaqus alongside Dymola to simulate mechanical forces and resultant stresses arising through manufacturing and assembly,” says Matt Harrington, Ceres Power Lead Engineer. “Further, integration with thirdparty CFD software lets us clearly visualize component and assembly responses to thermal and pressure differentials. A common use is assembly simulation of gasket sealing stresses integrated with endplate designs. A new application is current field visualization around electrical contacts within the fuel cell. Abaqus helps us simulate our mechanical engineering, in great detail, to continually improve our understanding and the functional performance of our fuel cell stack.”

Under the hood of the fuel cell module

Besides it use in the CHP units, the Ceres Power Fuel Cell Module devices might go into portable units that provide prime or back-up power and are alternatives to generators and batteries, as well as in units that provide auxiliary power in automobiles, boats, and airplanes. The module comprises an “electrochemical engine” which generates electricity and heat from fuel and air.

The basic fuel cell which powers the module contains thin electrochemical layers deposited onto a stainless steel substrate. The cell’s operating temperature range lets it work well in combined heat and power applications.


Other kinds of fuel cells

Direct methanol fuel cells (DMFC) power devices such as laptops. System components are miniaturized and intended to deliver longer run times than existing batteries.

In automotive applications, polymer electrolyte membrane (PEM) fuel cells are being considered as potential replacements for internal combustion engines. The technology must first meet stringent cost targets before it can go mainstream. An additional barrier is the necessity of using pure hydrogen as a fuel source, and the consequent requirements for generating, distributing, and storing the gas.

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