Stationary fuel cells are units that provide electricity (and sometimes heat) but are not designed to be moved. These include combined heat and power (CHP), uninterruptible power systems (UPS) and primary power units.
Stationary fuel cells can be used for decentralised power supply in off-grid areas. The market for backup power applications (BUP) is becoming increasingly important. Backup applications include firstly emergency power supply and secondly uninterruptible power supply (UPS).
Emergency generator sets are used for maintaining operation in the event of lengthy power outages. In such cases the switchover from the mains power supply is usually (briefly) interrupted.
Uninterruptible power supplies, on the other hand, are used to protect highly sensitive technical systems against mains supply fluctuations and short-term outages, so as to ensure continuous operation. Areas of use include in particular telecommunications and IT systems, such as radio towers or data processing centers.
In comparison to conventional thermal power plants, fuel cells have much higher electrical efficiencies of up to 60 %, even for small plants. This is advantageous from an exergetic perspective, since a lot of high-value electricity and little heat are produced.
In ongoing operation, fuel cell backups are characterized by the following advantages: long autonomous operation and service life, low maintenance costs due to the lack of moving parts, and quiet, (locally) emission-free electricity generation.
The backup capacity of stationary fuel cells varies from a few kW to over 1 GWe. Fuel cells with low-wattage electrical outputs are often portable fuel cells, which offer weight advantages over rechargeable batteries and generators. A variety of different fuel cell types are used in the stationary sector, in some cases also for cooling. In addition to hydrogen, methanol, natural gas and liquefied petroleum gas are used as fuels.
If, in addition to the generated electricity, the heat that is produced is also used, the process is referred to as combined heat and power (CHP). If such plants are used in the domestic heating sector, they are also described as micro-CHP or mini-CHP plants because of their smaller outputs.
CHP plants can be operated with two strategies: The plant covers either most of the electricity or of the heat demand. If electricity prices are high, an electricity-led mode of operation is appropriate. In this way, the purchase of electricity from the grid can be minimized, or the generated CHP electricity can be fed into the electricity grid and reimbursed.
The heat produced as a by-product of combined heat and power is used to cover part of the buildings heat demand. The mostly electricity-led mode of operation results in a low thermal output from fuel cell heating systems. The remaining heat requirement of the building is covered by an additional heating system, e.g. a condensing boiler. For that reason, fuel cells are particularly suitable for buildings with a low space heating requirement, such as low-energy or nearly zero-energy buildings. In buildings with a higher space heating requirement, hybrid fuel cell heating systems, comprising a fuel cell and a condensing boiler to cover peak heating requirements, are used.
Stationary fuel cells in the output range up to 10 kWe are usually PEM or SO fuel cells. The typical CHP output range for houses and apartment buildings is 0.7 to 5 kWe. If fuel cell systems are operated with natural gas as the fuel, an existing natural gas infrastructure can be used. However, the fuel must be reformed first. In the case of PEM fuel cells, reforming takes place externally. Owing to the higher temperatures, internal reforming is possible in SO fuel cells.
Probably the biggest advantage of fuel cells over thermal power processes is the direct electrochemical conversion during electricity and heat generation and the associated higher electrical efficiency. In combined mode, i.e. electrical and thermal, fuel cells can achieve efficiencies of up to 95 %. The electrical efficiency is up to 45 %. Furthermore, fuel cell systems are characterized by high efficiencies over all load points, they are quiet, have low maintenance costs and operate (locally) emission-free.