Use of hydrogen by fuel cells   (NOT YET FULLY EDITED)

Hydrogen can be used just like natural gas. But the real advantages are only achievable with fuel cells. Fuel cells are:

  • cheap (30 €/kW),
  • have a very high efficiency (60% el),
  • extremely dynamic ( from 0 to 100% output power in 1 microsecond),
  • can stabilise the power grid almost losslessly for free,
  • have a long life and are low maintenance,
  • allow the use of condensing boiler technology,
  • can be integrated into existing heating systems.
  • are suitable for driving vehicles

The high dynamic makes a complete separation from the electricity supply possible, which is not possible with either engine-based heat power co-generation or with a steam reformer based fuel cell system. Fuel cells are operated as heat-led heat and power co-generators. Owing to their high efficiency no energy list lost - not even in summer, as the electricity consumption of a household is just enough to cover the need for hot water. Installation of a water tank of 40-1000 litres, this heat can also be used in the height of summer.

Standard fuel cells for domestic use have a synthetic membrane as electrolyte and have a working temperature between 80-200°C. If the heat is needed at a higher temperature, high temperature fuel cells should be employed. Although they have the same high efficiency they do not withstand any sudden changes in load. The working temperature is between 700-900°C.

 

Function and cost of fuel cells

Fuel cells are elctro-chemical energy converters. There is no internal burning. The combustion process occurs in a much more separated way at the electrodes. In contrast to electricity production, the fuel cell works silently. One can hear at most the sound of ancillary units like the fan. Here only the membrane fuel cell is described, because it will dominate around 90% of the market.

A fuel cell makes nothing other than water, heat and electricity out of hydrogen and oxygen. No smoke is produced, no environmentally damaging exhausts and no CO2. The outgoing air can be used in data centres as fire protection. By the reduction of oxygen to around 15% are rooms still accessible, but no fire can spread.

A fuel cell is similar to a battery. A battery must be charged from time to time with electricity. A fuel cell runs as long as there is hydrogen supplied from the pipe network or from a tank.

30 kW

Source: Proton Motor

A fuel cell is made up of many individual cells. At the anode hydrogen molecules (H2) are adsorbed by a catalyst and are split into electrons and protons (H+). The electron flows through the wire, and performs useful work. The proton migrates through the electrolyte membrane and with oxygen forms water (H2O). As the voltage of around 1V for a single cell is too small for technical applications, many such single cells are connected together in series to a package, to a stack. The individual cells are only a few millimetres thick. The power density for vehicle applications is achieved at around 3 kW per litre of Stack volume.

A fuel cell consists of a few separate parts, and is therefore very well suited to mass production.

  • The membrane is a plastic sheet and is moulded and wound onto rolls just like sheets for plastic bags.
  • Anodes and cathodes are imprinted onto this plastic sheet from a mixture of the substrate and catalyst (like pictures).
  • The bipolar plate is made from carbon containing plastic or metal, which is plated to be corrosion resistant. The bipolar plate is usually shaped as a hollow part, so that this chamber is used as a water cooler.

Source: BINE

Source: Ballard

The picture on the far left shows the disassembled stack. The component with the undulating edge is the membrane. It is coated with a grey synthetic powder on both sides, which are the electrodes. The plate lying on top is the bipolar plate. This supports grooves on both sides for distribution of hydrogen or air. All components have gaps. Through these gaps hydrogen and air are supplied and if needed also cooling water.

 

The picture to the right illustrates a fuel cell system for a car. At the back the stack can be seen, which is usually sized to between 80 and 160 KW. In the front part of the picture the provisioning system can be seen: controlling electronics, compressor, humidifier, fan, battery, blower and heating coil. The 700 bar pressurised tank is also part of the system costs (see below).

The provisioning system of a fuel cell for a car is so elaborate because it must function in a temperature region between -30°C and +50°C. The fuel cell is blown dry after use so that it does not freeze. Hot air is also blown through all the chambers of the fuel cell. That is why the car stands in a steam cloud in its parking space. In order to achieve a high power density, a compressor is used, just like in an internal combustion engine. This complex system is the basis for raising the costs of a fuel cell system as shown below.

For stationary operation most of the system components can be disposed of. Only a small fan is needed for air supply, and if necessary a humidifier for the hydrogen, as the proton (H+) travels preferentially with accompanying water molecules. A humidifier is needed if the back diffusion of the water from the cathode side is insufficient. The fuel cell in the home is always kept warm. That is why it has a similar service life to todays gas heating systems. For cars the service life is designed to be between 5000-10000 hours, which is sufficient for a cars lifetime. The fuel cell is then not broken, only the consumption is 12% higher. The service life in a car is mainly limited by the many cold starts, which do not occur in a home environment.

As with any technical development, the production costs fall following the learning curve. That is true for fuel cell systems too. Given the developmental status of 2010, these costs were 51 US$/kW. According to the estimates of the American department of energy, these costs will fall to 30 US$/kW by 2015. A study commissioned by the EU estimates costs of even 15 €/kW. The 700 bar pressurised hydrogen tank is also part of the system costs. The system costs are therefore considerably smaller than internal combustion engines today. It can be recognised that roughly half of the costs are for the provisioning system. A provisioning system for stationary fuel cells is much simpler, and will tend to be cheaper.

The diagram to the left shows the influence of the quantity on the production cost. For small scales the costs are many times higher than for a quantity of around 0.5 million per year. That is the dilemma of the car industry, in cases low volumes have to be brought to the market. Today in Germany alone 6 million cars are produced per year, worldwide 80 million are produced. To provide all German domestic households would require around 30 million fuel cells, but they would require lower rated output power.

Source: DOE Fuel Cell Market Report 2011

The diagram to the left shows the cell voltage of a pre-commercial individual cell when varying the supply of oxygen. The current strength lies between 0.3 A/cm2 and 3 A/cm2, depending on whether the cell is run  optimised for efficiency or power output. It can also be recognised why the cells in a car are run under pressure. On board oxygen would also increase the efficiency and power further and simplify the system. In a hydrogen economy, pure oxygen can also be made available at filling stations for no cost.

A voltage of 0.8 V corresponds to an electrical efficiency of around 68%. A stationary fuel cell, which for example is usually run at  0,2 A/cm2 can definitely temporarily be run at 2 A/cm2. However, the efficiency falls back from 68% to 43%. The electrical power is raised by a factor of 6.5. Fuel cells are therefore very capable of working in overload. Even then, they achieve efficiencies similar to those of large power stations. The term efficiency is misleading for stationary fuel cells, as a heat-led energy economy has no loss. A more appropriate term would be proportion of electricity.

Source: ZBT 2013

Applications of fuel cells

Fuel cells may be scaled to any size, without losing their principal properties. The range is from mobile phone to replacement power stations. As the energy providers were not amused by fuel cells in a hydrogen pipe network, the development was directed towards the most difficult application fields: vehicles. And then lok: where there is a will, there is a way. The first fuel cell vehicles will be on the market in a few years. If one talks of fuel cells today, one means cars. Despite that, the path to decentralised power generation with renewable energy carriers in the form of hydrogen cannot be stopped, because the lower costs make the energy system more efficient. In the following pages two applications are examined as examples.

updated: 05.07.2014