The main barriers for applying fermentative hydrogen production as outlined in any economic analysis are the low yield of hydrogen, low production rates and the cost of the feedstock. In order to make the biohydrogen economy viable there is a major challenge to increase the yield and production rates through:

1. Overcoming the light saturation effect. In the light-driven processes the conversion efficiency of the solar energy to hydrogen is estimated to be 10% (Hallenbeck and Benemann, 2002). This estimation is considered to be optimistic since it is based on data obtained under low light conditions that favor the dark reactions, the rate of which is limiting. Under full sunshine, the mechanism for electron transfer in algae is ten times slower than that of the light capture. As a result 90% of the photons captured decay as heat or fluorescence. The so-called light-saturation effect also applies to the photosynthetic bacteria. To overcome the light-saturation effect, the efficiency of the process can be increased through application of suitable mixing patterns that would reduce the time of exposure to the intense light and increase the nutrients transfer, design of reactor configurations that would dilute the light fall on the algal surface, as well as development of algal mutants that would absorb and waste fewer photons (Hallenbeck and Benemann, 2002).

2. Improving the dark fermentation technology. Rapid gas removal and separation as well as bioreactor design enhance the yield and production rates of hydrogen. To keep CO2 and H2 at low concentration, rapid removal of these two gases is required and H2 purification to concentrate and remove CO traces that would contaminate PEMFCs is necessary. Techniques of removal of H2 and CO2 have already been presented in Section 13.3.6.

3. Improving the CO-water shift reaction. Levin et al. (2004) consider the CO­water shift reaction carried out by certain heterotrophic bacteria promising. However, in the case of the CO-water shift reaction, the supply of the CO gas in a large volume reactor may require new bioreactor design to facilitate the mass transfer and contact between bacteria and the gas.

4. Integration of bioprocesses. Integrated strategies consist of two steps, with the first one being the fermentative hydrogen production, and the second one being either photobiological hydrogen production or methane production or MEC for hydrogen production as already discussed in Section 13.3.7.

Apart from the limiting factors concerning the biohydrogen process technology, there are other important parameters that affect the economy of hydrogen. For example, the limited availability of infrastructure for the transport, distribution and storage of hydrogen. The traditional options for hydrogen storage are cylinders of pressurized or liquid gas which is very problematic in the case of hydrogen gas. Although hydrogen has a very good ratio of energy to weight, it has a poor ratio of energy to volume compared to other fuels (hydrocarbons), therefore large tanks are required. Application of pressure to reduce the volume or liquefaction may result in smaller tanks but these technologies are energy consuming. On the other hand, for transportation use, storage meets limitations of volume and weight, while sufficient fuels must be available to secure the vehicle autonomy over long distances compared to the gasoline. Another option of hydrogen storage is the physical (adsorption on metal hydrides) and chemical storage (formation of alkali metal hydrides). Nanostructured materials are another promising alternative since they ensure high capacity. Transfer through a pipeline grid is another option but there is a question about whether the existing gas pipeline systems can be used for hydrogen supply. The quality and condition of the material of the pipeline should be checked since any metallic components may be affected by the hydrogen. Parts of the pipeline as the welds, valves and flanges should also be checked for their ability to hold the hydrogen.

13.4 Future trends

Today, hydrogen is used mainly in the petrochemical industry or as a feedstock in the industry, but not as an energy carrier. For this, the development of the supply sector (production and distribution-transportation-storage) and the end-user application should evolve simultaneously. The high investment costs required for this venture would be counterbalanced by strong driving motivations such as (Groot, 2003):

• The use of hydrogen in an efficient and clean electricity process such as PEM fuel cells.

• The use of hydrogen as a fuel for vehicles (storage of hydrogen is an issue, especially for the small compact vehicles).

• The use of hydrogen as an energy carrier through conversion of the electrical energy generated by renewable resources (solar energy, wind power) to chemical energy in the form of hydrogen (via electrolysis). This will replace the need to store large amounts of electricity directly.