Category Concentrating solar power technology

Solar hydrogen from hydrocarbons

Natural gas steam reforming

The majority of the hydrogen produced globally derives from natural gas steam reforming, which is a well-established technology. Natural gas solar steam reforming (Eq. [20.4]) is based on the thermal decomposition of a mixture of methane and steam. The reaction also proceeds in parallel with the water gas shift reaction (Eq. [20.5]). The final amount of CO versus CO2 in the product mix depends on the operation conditions and the catalyst used. Usual temperatures for the reactions fall within the range from 800 to 1,000°C, while the process also involves the separation of CO2 from the product gases for pure hydrogen production. Reactors are typically tubular units with packed catalyst beds that use metal catalysts supported on ceramic pellets.

Reforming:

CH4 + H...

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Hydrogen production using solar energy

Solar hydrogen is a promising energy carrier (or solar fuel). Currently hydrogen (that is produced nearly entirely from fossil fuels) is used almost exclusively as a chemical in industrial processes (e. g. in ammonia synthesis and fertilizer production, in oil industries in refineries during hydrogenating processes and for the conversion of heavy and low crude oils into transport fuels) (Pregger et al., 2009; World Nuclear Association and Hore-Lacy, 2009). It is projected that hydrogen use is going to increase, due to the increase in the exploitation of heavy hydrocarbons (Pregger et al., 2009). Demand for hydrogen will increase rapidly, as it penetrates to other new sectors, such as transportation, generation of heat, electric power or mechan­ical energy (Pregger et al., 2009)...

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Applications of solar thermochemistry to fuel production

As was mentioned in the introductory paragraphs, the potential of the sun can be utilized for the production of solar fuels (i. e., solar hydrogen, solar hydrocarbons, alcohols and liquid fuels). Solar thermochemistry can be employed for the conversion of water and waste CO2 into H2 and CO, which are valuable building blocks for the production of synthetic fuels as well as other chemicals. The different pathways are summarized by Graves et al. (2011) in Fig. 20.2.

In the case of solar hydrogen, the technological maturity and the lack of necessary infrastructure do not allow its immediate large-scale application...

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Thermochemical and photochemical reactions

As mentioned above, the necessary energy for a reaction to take place can be provided thermally by an increase in temperature. Processes that are based on this kind of reactions are called thermochemical processes. An alternative way to provide the reactants with energy greater than the activa­tion energy of a reaction is via direct absorption of photons of sufficient energy. As the intensity of light increases, the number of molecules absorb­ing photons of sufficient energy is increased with subsequent increase in the rate of the reaction. These are called photochemical processes.

In the case of thermochemical reactions a catalyst may play a significant role in the realization of the reaction...

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Solar chemistry

Solar fuels are derived from ‘solar’ processes that exploit the energy of the sun, either in the form of heat or in the form of photons, to drive endother­mic (energy storing) chemical reactions. All chemical reactions can proceed in either direction and energy is either absorbed (in the endothermic direc­tion) or released (in the exothermic direction). The laws of thermodynamics dictate that at any given temperature or pressure, reactions will proceed until a state of equilibrium is reached where the rate of the forward reaction equals the rate of the reverse reaction:

A + B + AH « C + D [20.1]

According to the principles of Le Chatelier and van’t Hoff, a reaction in chemical equilibrium may be shifted, forward or reverse, by implementing a change in concentration, pressure or temp...

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Solar fuels and industrial solar chemistry

A. G. KONSTANDOPOULOS, Centre for Research and Technology Hellas, Greece and Aristotle University, Greece, C. PAGKOURA, Centre for Research and Technology Hellas, Greece and University of West Macedonia, Greece and S. LORENTZOU, Centre for Research

and Technology Hellas, Greece

Abstract: The main advantage of concentrated solar power (CSP) is the production of carbon free energy. However, the problem is that the energy produced must be directly consumed. This could be dealt with by the chemical storage of solar energy in the form of an energy carrier such as hydrogen (H2), which is transportable and can be used upon request. The available routes to produce solar hydrogen as well as different kinds of solar reactors known from the literature are presented...

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Sources of further information and advice

IEA SHC, International Energy Agency Solar Heating and Cooling Program.

www. iea-shc. org (accessed 02.01.2012).

Poship (2001), The Potential of Solar Heat for Industrial Processes. Available from http://www. solarpaces. org/Library/docs/POSHIP_Final_Report. pdf (accessed 02.01.2012).

PROCESOL II (2002), Solar Thermal Plants in Industrial Processes, Design Guidelines. Available from www. solarthermalworld. org (accessed 02.01.2012).

RHC-Platform, European Technology Platform on Renewable Heating and Cooling. www. rhc-platform. org (accessed 02.01.2012).

Weiss W and Biermeyer P (2009), Potential of Solar Thermal in Europe.

Available from www. estif. org (accessed 02.01.2012).

Weiss W and Mauthner F (2011), Solar Heat Worldwide. Available from www. iea-shc. org (accessed 02.01.2012).

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Future trends and conclusion

Low temperature solar thermal systems (water heaters) face increasing competition with photovoltaic (PV) driven heat pump systems. Also elec­tricity production in large CSP plants sees PV as serious competition as long as storage is not needed. On the other hand, it is presently not com­petitive to use PV for direct electric heating. At temperatures above 80-100°C, which is a technical limit for vapour compression heat pumps, solar thermal collectors will be the most cost effective way to generate solar heat for quite some time to come.

The integration of solar boilers into the steam networks of industry will have enormous potential in the future...

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