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Fuel Cell Technology: Stepping stones

Siàƒ¢n Green

In December 2000, Siemens Westinghouse Power Corp. announced that it had been awarded two contracts that would enable it to move one step closer to the commercialization of its fuel cell technology. Two fuel cell-based power plants will be installed, one in Germany and the other in Italy, and will start operation in 2002.

Siemens Westinghouse will build and install two pre-commercial solid oxide fuel cell (SOFC) power systems, one for RWE Power AG of Essen, Germany, and one for Edison Spa of Milan, Italy. Siemens Westinghouse will supply each with a 300 kWe-class prototype hybrid system featuring tubular SOFC technology combined with a microturbine generator. These systems will be the world’s first grid connected SOFC-turbine hybrid power systems.

Both plants are designed for combined heat and power (CHP) applications, using heat from the exhaust for district heating applications. The RWE Power demonstration will be located at the Meteorit Technology Park in Essen, with delivery and startup scheduled for the summer of 2002. The Edison project will be located at Edison’s research and development facility in Sinetta Marengo in north west Italy. System startup will take place in the fall of 2002.

Figure 1. The 300 kW-class pressurized SOFC system will be installed at sites in Italy and Germany
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According to Siemens Westinghouse, these pressurized SOFC-microturbine hybrid power systems will reach electrical efficiencies of 58 per cent. Both will run on natural gas and will supply power to the electricity grid.

Siemens Westinghouse will draw on experience from a 100 kW SOFC power system demonstration project in Westervoort in the Netherlands, and also from a 220 kWe pressurized hybrid power system being tested by Southern California Edison in the USA.

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For RWE, one of the main objectives of this project is to examine the suitability of fuel cells as miniature power plants for supplying energy to the grid, residential households and small commercial customers. The utility intends to invest several million euros in fuel cell projects over the next five years.

These two projects are an integral part of Siemens Westinghouse’s plans to commercialize fuel cell technology for the power market. It is developing the technology under a cooperative agreement with the US Department of Energy’s National Energy Technology Laboratory. The company is planning to commercialize its SOFC systems – both atmospheric pressure and pressurized hybrid modules – by 2004, and is expecting to take commercial orders for atmospheric pressure systems by late 2001 or early 2002.

Siemens Westinghouse’s market entry products will be aimed at the distributed generation segment of the power and cogeneration markets in the range of 250 kW to 10 MW. For pure power applications, the plants will be pressurized SOFC modules coupled to one or more gas turbines. These units are expected to have a very high availability and according to Siemens Westinghouse, will be sold at less than $1500/kW once manufafturing has reached maturity.

A hybrid design

At over 69 000 hours, Siemens Westinghouse holds the world record for the operation of individual fuel cells. The company has concentrated its efforts on the development of solid oxide fuel cell technology, distinguished from other types of fuel cell designs by its all-solid state structure and its high operating temperature of around 1000à‚°C.

Figure 2. The two plants will each produce 300 kW of power
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Siemens Westinghouse’s SOFC technology is of a tubular design, with process-air cooled stacks with a natural gas reformer integrated thermally and hydraulically within its structure. As the stack operates at a high temperature, the high temperature exhaust gas can be used to drive a gas turbine generator set allowing the development of hybrid plant designs of this type to be delivered to Essen and Sinetta Marengo. Such hybrid designs lead to very high electrical efficiencies, predicted to reach 70 per cent for 2-3 MW systems.

Each fuel cell is an air electrode-supported tube with an axial interconnection that allows the cells to be connected together in series known as stacks. Each cell is 2.2 cm in diameter and 150 cm in length with one closed end.

The cell will operate at a temperature of 1000à‚°C; air is supplied to the cell interior and fuel to the cell exterior. When an external circuit is connected, a current will flow in the external circuit that is in direct proportion to the flow of oxygen ions through the electrolyte. At atmospheric pressure, a single fuel cell will generate up to 210 W dc, but additional power output is gained when the fuel cells are pressurized. At a pressure of 10 atmospheres, the power output is increased by ten per cent.

The cells are arranged vertically into bundles of 3 x 8 cells, and the bundles are arranged into bundle rows. Between each bundle is an in-stack gas reformer which is radiantly heated by the adjacent rows of cells. A standard stack will consist of 1152 cells and has a nominal rating of 100 kW ac, while a twin stack will contain 2304 cells.

In the pressurized system design, turbine work is extracted from the exhaust gas stream of the SOFC by an expander before the exhaust gas passes through a recuperator. The recuperator heats the process air and the natural gas fuel to an inlet temperature of 600à‚°C.

Siemens Westinghouse began operating its first 220 kW pressurized hybrid SOFC plant at the National Fuel Cell Research Center in California in conjunction with Southern California Edison in late 2000.

Analysis shows that where the turbine inlet temperature of the recuperated gas turbine is similar to the SOFC exhaust gas temperature of 850à‚°C, there are no benefits to exceeding a maximum process pressure of 6-10 atmospheres. The best configuration will have a SOFC:GT output ratio of between three and five.

At Sinetta Marengo and Essen, the power system will produce 300 kW of electricity, with 250 kW being produced by the SOFC and 50 kW by the microturbine. Siemens Westinghouse will draw extensively on its recent experience with the pressurized hybrid system operating in California for the projects in Germany and Italy, as well as on its experience with the 100 kW SOFC power system which operated for over 16 000 hours in Westervoort.

Scaling up

In December 2000, Siemens Westinghouse also announced a contract with a consortium of four European utilities to provide a 1 MW hybrid fuel cell power plant for demonstration in the European market. Funded by the European Commission’s Framework Five Programme and also by the US DOE, the plant will essentially be a scale-up of Siemens Westinghouse’s existing SOFC hybrid design.

Siemens Westinghouse SOFC technology is of a tubular design with an intergrated natural gas reformer
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German utility EnBW will be the host utility and programme manager for the demonstration project. French energy companies Electricité de France and Gaz de France will provide the microturbine and balance of the plant system along with Austrian power utility Tiroler Wasserkraftwerke Antiengesellschaft (Tiwag).

This 1 MW system will have electrical efficiencies approaching 60 per cent and will supply power to EnBW’s electric grid. It will operate for a demonstration period of at least 12 months. The SOFC generator module will be built at Siemens Westinghouse’s facility in Pittsburgh, Pennsylvania, and will be shipped to the EnBW site for integration with the microturbine in mid-2003. Operation will start in November 2003.

“In Europe, this system can be the forerunner of a new class of systems that can be used for a variety of distributed generation applications and can be located close to customer loads,” said Allan Casanova, Siemens Westinghouse director of business development for stationary fuel cells.

“When commercial, we expect these high efficiency systems that are environmentally friendly to be competitive with other technologies and products being applied today for similar distributed generation applications.”