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Entrance in production: Assembly of a fuel cell heating appliance for the field test.
© Vaillant
Cogeneration with fuel cells
Projektinfo 10/2016

Design study of the devices used in CALLUX and ene.field, with auxiliary heater, heat recovery module and buffer storage tank
© Vaillant

The systems reach technical maturity in the Callux field test. Almost 500 FCHUs were installed at end customers during the project.
© Eon / Callux

Specifications of XellPower FCHU from Vaillant.
© Vaillant
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Fuel cell generates electricity and heat for homes

Highly efficient, low-emission and quiet: as small-scale combined-heat-and-power stations, new fuel cell heating units produce heat energy and electrical energy with an efficiency of over 90 per cent. They have proven themselves in extensive tests in single and two-family homes, and their operation and design have been improved incrementally. The first units are now on the market. They run on natural gas, and on hydrogen and methane produced using renewable energy or biomass. Decentralised fuel cells can generate demand-oriented electricity, and they can be used in a grid-friendly or grid-independent way.

Researchers at the heating manufacturer Vaillant developed a heater with solid-oxide fuel cells (SOFC) that is now ready for series production. The aim was a compact device that is easy to install and operate. With an electrical output of 0.7 kW and a thermal output of 1.3 kW, it meets the basic requirements of a single and two-family home. A gas-fired condensing boiler is integrated for higher heat demands. The aim of the developers was to make the overall system cheaper and easier to manufacture, as well as to improve the quality and durability of its components.

They gradually improved the fuel cell heating units (FCHU) and tested them in parallel in demonstration projects such as the Callux project. They used the data recorded from over 1 million hours of operation at the end users for further development. In the meantime, the technological maturity of the SOFC device concept has been proven for micro-CHP applications. They succeeded in halving production costs during its development. With the device generation that is now ready for mass production, the researchers achieved an electrical FCHU efficiency of over 33% and an overall efficiency at nominal load of more than 90%. The higher overall efficiency versus previous models can be explained by the more compact design of the new hot box enclosing the fuel cell stack.


Operation and performance of FCHUs

In fuel cell heating units, a reformer initially converts natural gas into a hydrogen-rich gas. In an electrochemical reaction that takes place in the fuel cell stack, the hydrogen-rich gas reacts with oxygen in the air. This produces steam; remaining residual gas is burned in the afterburner. FCHUs produce direct current and heat with very high level of efficiency. An inverter converts direct current into alternating current that can be used in households. Heat exchangers make waste heat produced by fuel cells and afterburners available for heating and domestic hot water heating. Compared to current condensing boiler technology, energy costs can be reduced by about 25% and climate-harming emissions by up to 50%. Decentralised production also relieves the distribution networks.

On average, a FCHU produces 3,500 kWh of electrical and 6,500 kWh of thermal energy per year. When the heat generated by the fuel cell is insufficient during the winter months, the auxiliary heater will guarantee a warm home. In order to ensure a long service life, FCHUs are operated as continuously as possible, without too many on-off cycles; in times of low heating demand in the summer months, they temporarily halt their operation.

Development from pre-production to maturity

For a mature system, the various functions had to be housed in a single casing. When installed as complete systems, the first units had large space requirements due to the separate assembly of the components. As development progressed, the researchers employed standard parts from other devices of the company to keep development effort and costs as low as possible.

In a first development step, the developers integrated the igniter and the afterburner in one component, and integrated all hot gas components including the stack module in a hot box. They combined the exhaust gas lines of the fuel cell and the auxiliary heater. Other components, such as the hot gas heat exchanger or the controller and safety board, were adapted to the specific requirements of fuel cell technology. For the heat recovery module, the developers took components from a single product line of the manufacturer. The integration of the entire hot gas assembly in a low-pressure housing is an important contribution to the safety of the unit. "We were able to realise a very simple and at the same time a very good security concept in our fuel cell heating units," says Jochen Paulus, Head of Technology Development Fuel Cell at Vaillant. "Since all system components operate in a low-pressure environment, an exhaust fan in conjunction with a few temperature sensors will suffice, we need no further protective measures or sensors."
Systems that operate under pressure, on the other hand, require significantly improved safeguarding against the undesired escape of gases.

The reduced system design and the use of standard components lowered material and production costs by about 60%. Compared to the predecessor model, installation costs and the footprint of the entire system have been halved.

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