Stationary used of hydrogen via fuel cells   (NOT YET FULLY EDITED)

A hydrogen economy can only be claimed if almost every end user has combined heat and power generation from hydrogen. Fuel cells in the hydrogen network are therefore the centerpiece of a hydrogen economy. This real hydrogen economy has not yet started.

Initially the state of the art for combined heat and power generation is briefly presented.

The traditional path with engines

Combined heat and power units are currently implemented as a traditional engine. They may be scaled for units suitable for single family dwellings (a few kW) to large buildings with several engines (for 100kW). They are mostly powered by natural gas. For the purpose of comparison with fuel cells, an installation for a single family dwelling is described here.

Source: Dax

The pictures above show a 5 kWel unit with a gas engine, including a soundproof hood on the left, and on the right the complete installation with control console and hot water tank. The electrical efficiency is 27% at most. The water tank is necessary because the unit must be driven intermittently in a heat-led fashion. The maintenance needed twice each year gobbles up most of the combined heat and power state funding. In most cases it needs its own foundations for avoiding mechanical vibration. The unit costs around 25,000 € without foundations and hot water tank.

For larger combined heat and power gas engines, around 5000 € must be invested per dwelling for 10-100 dwellings.

Fuel cells in the natural gas network - a dead end

The introduction of fuel cells in the natural gas networks has begun. There are two approaches for providing domestic energy using them:

  • Introducing a miniature hydrogen factory (reformer) upstream of every membrane fuel cell (PEMFC)
  • Integration of a reformer in every high temperature fuel cell (SOFC)

Using the reformers, pure hydrogen is not produced, only a hydrogen-rich gas. This gas can only be roughly 70% used because the fuel cell would otherwise suffocate on CO2. Roughly 30% of the gases must then be burned. The reformers function at around 850°C. As the SOFC also functions at around 850°C, part of the gas can be reformed on the reverse side of the electrode - but this leads to an uneven temperature distribution at the electrode.

Source: Baxi

This picture shows a membrane fuel cell (red arrow) with reformer. Of the gas used by the fuel cell, it is currently possible for the unit to convert around 30% into electricity. From the natural gas inserted, that is around 21% (30 x 0.7). That is not enough. The domestic size is typically 0.8kW. As well as the waste gas burner already mentioned, a conventional boiler and hot water tank is required. As the reformer is difficult to control it must be run at constant power. Its connection to the electricity grid is absolutely necessary.

 

Fuel cell heating with an SOFC fuel cell has a similar capacity and a similar construction volume. However the electrical efficiency with respect to the input natural gas is at 42% twice as high as with a PEMFC unit. Power changes are only possible in a very limited range.

 

Both types are currently shipped for a price of 25,000 € not covering manufacturing costs. This sector quotes prices of 10,000 € each for mass production, in the longer term even 5,000 €. In Germany around 350 units were shipped. In Japan there were alreadz 10,000 (August 2014).

Decentralised hydrogen economy with fuel cells

Only when PEMFC fuel cells are used in a hydrogen network is it possible to fully exploit the advantages of a hydrogen economy. These fuel cells are quick, highly efficient (60%el), small and cheap (< 30 €/kW). A connection to the electricity grid is not necessary. Neither waste gas burner nor additional boiler is needed - no chimney and no chimney sweep. The excess electricity can be used for heating or to drive heat pumps if it is not fed into the electricity grid.

A fuel cell produces electricity, heat, and water - nothing else.

This picture shows the use of a simple PEMFC (80-110°C). The maximal capacity would be designed to be the maximum heat power required. If driven independently from the electricity grid, the design may also take into account the maximum electrical power required. For typical domestic use, 5 KWel should suffice. That is around 10 kW total output power, of which around 9.5 kW is available as heating capacity. The exact air- and water-inlets are not shown here. Whether the water circulation of the fuel cell should be hydraulically separated from the water circulation of the heating system, and whether the a hot water tank should be integrated should be found out by the heating system companies with the fuel cell producers. Using a hot water tank, up to 0.5% of the annual energy costs can be saved. The recovery of heat from the exhaust is also an important topic. In contrast to the vehicular sector, there is no ready-made system for a domestic energy supply with simple membrane fuel cells. It is easy to imagine that such a system would however be less costly than a vehicular system. In any case it is much less complex than the reformer-based fuel cells described above.

 

If the electricity cannot be used, it can be passed to the heating system via an immersion heater or be used by fan heaters or electric heaters. In this way rooms need only be electrically heated when they are actually used. That saves energy. The production of water should not be overvalued: as only around one bucket is produced daily. It is more relevant that the costs of purifying the poisonous condensation from conventional condensing boilers become irrelevant. That lightens the load on water treatment plants.

Experts unanimously agree that radiant heating systems raise physical comfort. Even if the room temperature is lowered, this physical comfort remains. This picture shows an electrically heated ceramic plate. Electrically heated works of art or wall paper heating are also conceivable.

Source: STIEBEL ELTRON

The main reason why combined heat and power systems have not taken over is mainly due to the high installation costs, and the missing hydrogen supply. It is disappointing that no independence from the electricity grid is achievable. That could change very quickly with hydrogen in the gas network. The possible ways to transition to a hydrogen economy are shown here. The additional uses of a hydrogen economy for the stabilisation of the national grid is described here.

updated: 06.08.2014