A second option is bulk hydrogen refuelling. Network operators and fuel cell manufacturers have worked with major global hydrogen suppliers, initially in the United States, to establish a refuelling model similar to the diesel/propane model. In this model, the cylinders remain on site and are filled on site by the refuelling truck. This development has broadened the market for fuel cells to address higher capacity installations and sites requiring extended runtimes of several days. 

 A third option for providing hydrogen for fuel cells is the fuel reformer. The reformer takes a hydrogen-rich carbon-based fuel, such as methanol mixed with water and, using heat and a catalyst, separates the hydrogen from that fuel in order to deliver it to the fuel cell. Because these fuels tend to be liquid, energy density is better than with gaseous hydrogen, allowing for more runtime to be stored on site in a smaller space. However, reformers introduce additional cost and complexity to the fuel cell system and can reduce the reliability of the system as a whole. Hydrocarbon fuels, because they are not simple hydrogen, also emit some pollutants during the reforming process. In locations where hydrogen is not readily available or is priced too high, a reformer may be the fuelling option of choice. 

Integrating a fuel cell into a network

One of the attributes of a fuel cell that makes it attractive for deployment in TETRA environments is that a fuel cell produces DC power. This makes it akin to a standby rectifier source, as the power provided from the fuel cell can be directly connected to the site’s DC power bus. 

In an outage situation, the fuel cell turns on automatically, providing DC power formerly provided by the rectifiers. This means fuel cells can effectively function for long reserve times as a standby power source in customer applications. Fuel cell systems are intended to operate in parallel and augment the traditional DC power system components. 

Fuel cells may easily be added to an existing network or may be designed into a new network location. They may also be combined with solar and/or wind power to provide a clean hybrid power solution in locations where the grid is unavailable or unreliable. With hot and cold weather design features, many fuel cells are capable of serving loads in a wide variety of geographical locations. 

The hallmark of the TETRA network is its reliability. This is also true for fuel cells, with some products receiving field reliability ratings of 99.9 per cent.

A viable site-hardening plan would involve combining one or more backup power technologies in parallel, increasing the availability of the site by providing a highly reliable ‘backup’ to the primary backup power source. For instance, a site that would generally run off AC power and is already equipped with VRLA batteries connected to the DC bus, for response to an AC power failure, would benefit from a fuel cell also connected to the DC bus to carry the site load and charge batteries when the batteries dip below a certain voltage at loss of AC power or in the event of a rectifier failure. The fuel cell prevents the deep level of discharge on a battery string and allows the site to operate on backup power for much longer than on batteries alone.

The BOSNet TETRA network in Germany