**5. Energetic applicarions and storage of H2**

Hydrogen can be used as energetic source in different systems and technologies, such as vehicular, stationary or portable devices.

In vehicular applications, hydrogen can be used as a supplemental fuel in conventional spark ignition engines without extensive engine modifications, reducing CO and hydrocarbons emissions, improving engine performance characteristics, such as thermal efficiency and specific fuel consumption. The researches show that when H2 is used as a sole fuel in spark ignition engine, it is more efficient (and cleaner, since its combustion produces only water) than fossil fuel [53].

Another option for vehicular transportation is use hydrogen in fuel cells (Figure 9). Fuel cells are high-efficiency power generation systems that convert hydrogen and oxygen directly into electricity using a low-temperature electrochemical process assisted by catalysts, emitting only water and virtually no pollutant. This system consists mainly of two electrodes (where electrochemical reactions takes place) separated by electrolyte or a membrane. Hydrogen and oxygen (i.e. from air) are fed into the fuel cell. The flow of ions between the electrodes occurs through electrolyte, while the excess electrons flow through an external circuit, providing electrical power [54].

**Figure 9.** Simplified schematic representation of a fuel cell

**Figure 8.** Schematic representation of CSTR reactor: (1) filling; (2) reaction; (3) sedimentation and (4) emptying (Adapt‐

When the pH is controlled, organic acids that favor microbial selection and the consequent production of hydrogen gas are formed. Other methods for elimination hydrogen consumers utilize ultra-sonication, acidification, sterilization and freezing/thawing [10]. Sá et al. [52] studied the biological hydrogen production using anaerobic sludge of the sewage treatment system of Rio de Janeiro city, Brazil. The authors (op cit.) applied heat treatment (120 °C for 1 h) upon the sewage to inhibit methanogenesis. Tests in anaerobic batch reactors using glycerin for H2 production were obtained of 0.80 mol-H2.mol glycerine-1. Therefore, all such methods need to be verified for crude glycerol fermentation and hydrogen production efficiency.

Hydrogen can be used as energetic source in different systems and technologies, such as

In vehicular applications, hydrogen can be used as a supplemental fuel in conventional spark ignition engines without extensive engine modifications, reducing CO and hydrocarbons emissions, improving engine performance characteristics, such as thermal efficiency and specific fuel consumption. The researches show that when H2 is used as a sole fuel in spark ignition engine, it is more efficient (and cleaner, since its combustion produces only water)

**5. Energetic applicarions and storage of H2**

vehicular, stationary or portable devices.

than fossil fuel [53].

ed from [50])

488 Biofuels - Status and Perspective

Fuel cells convert hydrogen into electricity at high efficiency since they are not subject to the Carnot cycle limitations [55]. In present-day vehicles, a petrol-driven car engine operates at 25% efficiency, in the other hand a hydrogen fuel cell engine can operate at more than 65% [54].

Several types of fuel cells have been developed at different scales and characteristics (Table 5). The Brazilian government (Ministry of Mines and Energy - MME) plans introduce H2 in the national energy matrix until 2025 not only as fuel for vehicles, but also as a clean source for stationary power generation for energy supply. According the MME plans, after 2020 the hydrogen produced in Brazil should be mostly provided from renewable sources [56].

Many global electronic companies such as Samsung, Sony, Toshiba, Motorola, Panasonic, Fujitsu, NEC, Hitachi and others have been developed patents and prototypes using fuel cells portable systems. Even though the market is not yet mature, the volume of investments of these companies indicates that technology should be consolidated in a few years [57]. How‐ ever, the widespread utilization of H2 as an energy source requires solutions to several problems: the gas must be able to be produced from a cheap and renewable source; safe storage and handling of H2 must be addressed and refueling infrastructure developed.

A significant challenge regarding the large scale use of hydrogen gas is its storage. A storage device is an important part of the hydrogen energy system and it is a serious problem due to high inflammability, adequate safety measures should be taken during the production, storage, and use of H2 fuel.


**Table 5.** Characteristics of various fuel cell types (adapted from [54])

Hydrogen is quite difficult to store or transport with current technology. There are many ways for storing hydrogen fuel; as a gas (hydrogen compressed), a liquid (liquid hydrogen) and chemicals (metal hydride) [53].

Hydrogen compressed in tanks (with similar technologies applied in natural gas compression) is the easiest and cheapest way to store it. These tanks can store hydrogen at a high pressure (about 25 MPa - 35 MPa), but even under these conditions the energy density by volume for hydrogen is lower than for gasoline or diesel as can be seen from Table 6. In liquid form (-253 ºC), the energy density has higher value than hydrogen in compressed form [58]. However, it is necessary to spend more energy to liquefy hydrogen than to compress it (up to 20% of the energy content of hydrogen is required to compress the gas and up to 40% to liquefy it), so the cryogenic process efficiency demands elevated costs [54].


**Table 6.** Energy density for some fuels (adapted from [58])

Metals and metal alloys can also be used as a storage medium for hydrogen (hydrides form), i.e. chemical hydrides, Ca2H, LiH, NaBH4, MgH2, LiAlH4 and H3NBH3 have been widely studied as storage materials [53]. The positive aspects of this storage technology are low risk of unwanted losses, low pressures and energy densities greater than liquid and compressed hydrogen. The greatest disadvantage is the weight of these storage systems, about three times heavier than compressed hydrogen tank [58]. Probably, for this reason Toyota chose to use high-pressure tanks to equip the "Mirai" - its first commercial car powered by hydrogen produced on industrial scale [59].

The massive use of energy from hydrogen is expected to be gradual, with production and utilization initially on-site, with the development of new strategies environmentally friendly without the necessity for storage and transportation. For ex, initialy the hydrogen produced in a treatment plant can be used as energy source for its production plant; the use of H2 for energetic necessity to maintaining a pump system would be appropriated.
