CellForm asserts that its custom technology, however, can be easily scaled and confidently claims that: “When combined with a thickness down to 0.05 mm, we are able to create flow fields for the most efficient fuel cell systems.” Each part is produced using sophisticated machinery within a fully air-conditioned facility, it explains.

Passive hydroforming

Advertisements

The passive hydroforming process now means that presses which weren’t originally designed for high-pressure sheet metal forming can also be used for manufacturing bipolar plates (see fig. 2) – without the need for water hydraulic systems and pressure intensifiers. This method requires forming pressures of around 200 megapascals. The special feature of the process, which was developed by the Fraunhofer Institute for Machine Tools and Forming Technology IWU in Chemnitz, is its tool concept: The movement of the displacing piston as the press closes results in the compression of the active medium that is enclosed in the tool. This allows sufficient pressure to be created to form the BPP. A particularly challenging technical aspect is, according to IWU, the sealing of the active medium between the sheet being formed and the filling plate.

High-precision, stable, cost-optimized production

These challenges are also very familiar to the team at EKPO, a joint venture between ElringKlinger and the French company Plastic Omnium. “The BPP is one of the critical components that has a direct influence on stack performance and service life. The optimal distribution of media and cooling water uniformly across the stack and in the cells maximizes the performance of the membrane electrode assembly (MEA) and facilitates reliable control of the thermal load, even when operating at full power,” says Joachim Scherer.

According to the head of component product development at EKPO Fuel Cell Technologies, the high number of repeating units means that high-precision, stable and cost-optimized fabrication of BPPs is essential in order to achieve the necessary stack quality at a salable price. By integrating the sealing function as well as incorporating a robust design for metal BPPs, the time and effort needed for stack manufacture can be reduced which thereby lowers the inspection requirement and the costs.

EKPO refers to the flow simulation based on computational flow dynamics and a BPP design which is optimized with the help of finite element method. This, it says, makes for a dependable and error-free production process without affecting the properties of the product. In Scherer’s view this is necessary to reduce or avoid control processes while at the same time minimizing the rejection rate for each step in an up to 10-stage manufacturing process in the parts per million range.

“Robust processes for forming, joining, coating and sealing BPPs will decrease deviations and chances for error in production and thereby prevent defects, which in some cases could only be identified previously by carrying out laborious tests on the final product,” shares Scherer. This also particularly affects inspection costs, he continues, which can account for between 5 percent and 25 percent of the total cost depending on product readiness, in other words whether the product is a prototype or a mass-produced article.

At its site in Dettingen, Germany, EKPO has achieved an annual production capacity of 10,000 stack units in a setup which employs a number of different manufacturing stages. These include producing metal BPPs from coil with high-precision series tools in progressive die stamping operations and the joining of thin metal foils in a rapid laser process as well as testing interim products and ready-to-install BPPs through in-line process controls. In addition, spot checks are carried out to determine whether the dimensions and tightness comply with the usual principles and requirements for the automotive sector. In this case, the conductive coatings are either applied in advance to the coil or applied to the joined BPPs. There are also two different ways in which the company seals the cell, opting either for an integrated gasket on the BPP or a gasket that is integrally formed on the gas diffusion layer, otherwise known as the seal-on-GDL method.