Homeowners Professionals

The fuel cell: function and savings potential

The picture shows an H2 symbol for hydrogen, which is needed to operate a fuel cell.

Topics at a glance

A fuel cell is a heating system that uses the functional principle of combined heat and power generation. This means that it generates both power and heat. For this power and heat production, the fuel cell needs oxygen and hydrogen. The latter is obtained from natural gas first and then converted or reformed in the fuel cell itself. Water is produced as a by-product.

The fuel cell used as standard

As a heating appliance, the fuel cell is tried and tested and operating reliably many times over. In Japan alone, more than 123,000 units have already been sold by various manufacturers for stationary applications since 2009 (as of 01/2015). The Viessmann fuel cell boilers Vitovalor PT2 and Vitovalor PA2, specially developed and optimised for detached and two-family houses, operate with a very high overall efficiency and are designed for power-optimised operation.

However, fuel cells also provide energy for powering vehicles and ships as well as for supplying electricity in the aerospace industry. Other areas of application include mobile phones (batteries), traffic management, security and surveillance, wind power and telecommunications. Furthermore, fuel cells can be found in the leisure sector for power supply (e.g. in motor homes, sailing boats, holiday homes and mountain huts).

Charging an electric car at home: How it works

Get the inside track! Practically every house (new build or modernisation project) can produce green electricity for the family plus an electric car; with a fuel cell, a power storage unit, solar cells and the right technology, electricity and petrol costs can be reduced to the maximum. Andreas Czylwick, our expert for power and heating systems, explains how this works and how it pays off.

Get the inside track! Practically every house (new build or modernisation project) can produce green electricity for the family plus an electric car; with solar cells, a fuel cell and the right technology, electricity and petrol costs can be reduced to the maximum. Andreas Czylwick, our expert for power and heating systems, explains how this works and how it pays off.

How does the fuel cell work?

The generation of heat and power in the fuel cell is based on an electrochemical reaction of the two elements oxygen and hydrogen. The type of combustion that takes place in conventional boilers does not occur, which is why the process is also referred to as cold combustion.

The picture shows the schematic of the chemical reaction in a fuel cell.

Although a plentiful supply of hydrogen is available in nature, it does not occur in the form required for cold combustion in the Vitovalor. It must therefore be obtained from natural gas in an earlier process. Depending on requirements, the Vitovalor PT2 can be operated with natural gas H, E, LL or bio natural gas. The Vitovalor PA2 auxiliary unit can be operated with E and LL natural gas.

The combustion gas supplied flows through a reformer built into the unit, where a catalytic converter splits it into hydrogen in a two-stage reaction. The first stage of the conversion process produces a mixture of hydrogen and carbon monoxide. In the second, downstream gas cleaning process, carbon monoxide is converted to carbon dioxide. This is followed by cold combustion, which produces electricity and heat simultaneously.  

The hydrogen obtained in this way is then supplied to the fuel cell module. Next, it is split at the anode by a catalytic converter into positive ions and negative electrons. The latter travel from the anode across an electrical conductor to the cathode, which produces direct current. The integral inverter converts this into alternating current before feeding it into the power circuit. At the same time, the positively charged ions reach the cathode where they react with oxygen. The heat released during this reaction is absorbed by the water-filled cooling channels of the fuel cell stack and transferred to a heat exchanger. The heating energy obtained in this way can now be used for central heating or DHW heating. Furthermore, the separation of positively charged ions and negatively charged electrons prevents an explosive oxyhydrogen reaction.

The following video provides information about the benefits of the Viessmann Vitovalor.

You can find more information on how fuel cell heating works on the advice portal  heizung.de.

How the fuel cell operates over the course of a day

For a large part of the day, the electricity produced by the fuel cell heating system is sufficient to cover demand. Power only has to be drawn from the public grid during peak times. In return, surplus power is exported to the grid in exchange for remuneration. This makes fuel cell heating system users less dependent on rising electricity prices.

The picture illustrates the operation of a Viessmann fuel cell heating system in the morning.

The first peak demand for power and heat is in the morning: for light, to prepare breakfast and for showering. The fuel cell generates heat and power for on site consumption. The peak load boiler automatically switches on for additional heat required (left). During the morning, the fuel cell continues to run and covers the base load –– the peak load boiler switches off.

The picture illustrates the operation of a Viessmann fuel cell heating system at midday.

More energy is also needed during the midday hours: for cooking or washing, for example. The peak load boiler in turn covers the additional heat demand. In the course of the afternoon, energy consumption drops again and the fuel cell continues to run on its own.

The picture illustrates the operation of a Viessmann fuel cell heating system in the evening.

In the evening hours, more electricity is often needed than the fuel cell generates. Then additional power is drawn from the public grid. As soon the house becomes quieter in the late evening, the energy demand also drops significantly. Surplus power from the fuel cell is fed into the grid and remunerated.

The picture illustrates the operation of a Viessmann fuel cell heating system at night.

The demand for heat and power is minimal. The fuel cell runs in basic mode. The system completely covers the energy needs of the house.

Vitovalor  PT2 and Vitovalor  PA2

With the Vitovalor  PT2, the successor model to the Vitovalor  300-P, and the Vitovalor  PA2, an auxiliary unit to supplement an existing heating system, Viessmann offers two efficient solutions for the production of power and heat in detached and two-family houses.

More information

The Vitovalor PT2 compact fuel cell heating appliance

The Vitovalor PT2 is the ideal energy centre for the modern detached house. The system combines heat and power generation on a very small footprint. The fuel cell heating system offers considerably higher electrical efficiency than that available through current combined heat and power (CHP) solutions. This reduces the amount of heat extracted and makes the fuel cell heating appliance particularly suitable for use in new build and renovation projects.

The diagram shows a system representation of the Viessmann Vitovalor PT2 fuel cell heating system.

System diagram of the Vitovalor PT2

[1] Standard unit with fuel cell module and gas condensing boiler
[2] Tower cylinder
[3] Communication interface
[4] Integrated export meter
[5] Router
[6] Domestic power circuit
[7] Internet
[8] ViCare app
[9] Public grid

The Viessmann fuel cell heating system consists of two units that can be transported separately. This allows for quick and easy installation even in narrow basement spaces. One unit contains the stainless steel DHW cylinder with a capacity of 220  litres, while the other unit houses the gas condensing boiler for covering peak loads, the weather-compensated control unit with a large colour touchscreen, and the fuel cell module with reformer, inverter and fuel cell stack (a series of many fuel cells). This visually coherent unit is compact and requires a footprint of only 0.72  square metres.

The picture shows a product cross-section of the Viessmann Vitovalor PT2.
Product cross-section of the Viessmann Vitovalor PT2

Vitovalor  PA2 as an extension

The Viessmann Vitovalor  PA2 is an ideal addition to an existing system. It is a compact system consisting of the fuel cell module, integrated reformer, control unit and hydraulic and sensor technology. Unlike the Vitovalor PT2, the gas condensing boiler is not integrated.  

The gas condensing boiler is primarily used to cover heat during peak load times. That is, when it is very cold outside or when a lot of hot water is required at short notice. The gas condensing boiler and fuel cell module are supplied via a shared gas line. They also share a common flue system, which makes installation as easy as for a wall mounted gas condensing boiler. This applies to the Vitovalor  PA2 in particular for the Vitodens  200-W from year of manufacture  2011.

Vitovalor  PA2 product cross-section

The picture shows a product cross-section of the Viessmann Vitovalor PA2.

How efficient is the Viessmann fuel cell heating system?

When electricity is generated, heat is also produced, which in large, conventional power stations is generally lost as unused waste heat. Fuel cell heating systems such as the Vitovalor on the other hand make use of this waste heat for central and DHW heating. They therefore have a very high level of overall efficiency. Furthermore, there are no losses during energy transfer as the energy is used directly on site. Even the conversion from combustion gas to hydrogen is very efficient due to the absence of intermediate thermomechanical steps. The constant electrical output of the fuel cell module is 0.75 kW. A large part of the electricity demand can thus be covered at any time.

The Vitovalor works even more efficiently in combination with the Vitocharge power storage system. This can store surplus power for times of peak load which considerably increases independence from electricity suppliers. Alternatively, it is quite straightforward to export the surplus power to the public grid. The integral energy manager is self-learning and therefore optimises the level of on-site consumption.

Proven and reliable: Fuel cell technology from Viessmann and Panasonic

For Viessmann, innovating without giving top priority to reliability and durability would be unimaginable. Viessmann also relies on proven technology for fuel cell heating systems. That is why they were developed in cooperation with Panasonic. The Vitovalor fuel cell module comes from the Japanese company. Panasonic has manufactured more than 34,000  units in series production for the Japanese market.

The picture shows two boys playing in front of a Viessmann Vitovalor PT2.

Fuel cell technologies at a glance

The electrolyte in the PEM fuel cell consists of a plastic membrane that only allows protons to pass through. The PEM fuel cell is uncomplicated, as it makes do with the oxygen in the air. No complex filtering and cleaning processes are required. The PEM fuel cell can be used in stationary and mobile applications. Due to the low system temperature, this fuel cell can be operated very flexibly and switched on and off frequently.

The DMFC is a further development of the PEM. Instead of hydrogen, it runs on methanol. Since methanol can be stored and transported in a similar way to petrol, it is also suitable for use in vehicles as well as in portable power supplies and as a battery substitute.

The solid oxide fuel cell is made entirely of solids. A ceramic is used as the electrolyte. The SO fuel cell can be operated with natural gas without complex gas upgrading. Characteristic of SOFCs are long warm-up phases as well as longer running times, as they can only tolerate a few start-stop cycles over their service life due to their high temperature. Therefore, the SOFC is suitable for applications that allow almost continuous operation.

The AFC is one of the oldest types of fuel cell. Considerable effort is required to purify the reaction gases hydrogen and oxygen. Originally, it was mainly used in space travel –– but its production was already largely discontinued in the early  1970s.

The PAFC is a fuel cell developed for large combined heat and power units and power supply utilities. The fuel gas required for operation is obtained from natural gas. The oxygen comes directly from the air.

The carbonate fuel cell generates temperatures of 650  °C and enables optimum use of the waste heat. The MCFC is operated directly with natural gas and atmospheric oxygen. It is mainly used in the large power stations of power supply utilities.
The picture shows hydrogen molecules.

Basic knowledge: what is hydrogen?

Hydrogen…

  • is an energy source with the highest energy density by weight
  • is a chemical element with the symbol 
  • consists of a proton and an electron
  • has the atomic number 1 (it describes the number of protons in the atomic nucleus of a chemical element – hence also called proton number)
  • is the most common chemical element in the universe
  • does not produce CO₂, as H₂ does not contain carbon

The following explanatory video provides information about hydrogen as the energy storage medium of the future.

Hydrogen is hardly known in our everyday lives. On the contrary, H₂ is subject to preconceptions mostly based on ignorance or false information. However, it offers numerous benefits as a fuel.

Hydrogen…

  • does not self-ignite
  • does not decompose (unlike e.g. acetylene)
  • does not oxidise and is therefore not a fire accelerant
  • is not toxic, corrosive or radioactive
  • is odourless
  • does not contaminate water
  • harms neither nature nor the environment
  • is not carcinogenic
  • burns residue-free

Hydrogen will play an increasingly important role in the coming years as a fuel in road transport and as a storage medium in energy supply. Today, hydrogen is used to power fuel cells in vehicles already to a large extent. For example, in buses for local public transport. There have been no incidents here so far. After all, hydrogen is safe –– it won't explode on its own. This would require an oxidiser (e.g. air or pure oxygen) and an ignition source to be present (ignition limit in air: 4 to 75  percent by volume).

Comparison with other fuels

Unlike petrol or LPG, hydrogen, like methane, is lighter than air. It has the highest energy density of all fuels at 33.33  kWh/kg (based on mass; methane: 13.9  kWh/kg, petrol: 12  kWh/kg) and with 3.0  kWh/Nm3 one of the lowest energy densities (related to volume; methane: 9.97  kWh/Nm3, petrol: 8800  kWh/m3).