Solar energy is essentially unlimited and its utilization does not create ecological problems. However, solar radiation reaching the earth is intermittent and not distributed evenly. There is thus a need to collect and store solar energy and transport it from the sunny uninhabited regions, such as deserts, to industrialized populated regions, where great quantities of energy are needed. An effective way to achieve this process is by the thermochemical conversion of solar energy into chemical fuels. This method provides a thermochemically efficient path for storage and transportation. For this purpose, high-concentration-ratio collectors, similar to the ones used for power generation (see Chapter 10), are required. By concentrating solar radiation in receivers and reactors, one can supply energy to high-temperature processes to drive endothermic reactions.
Hydrogen is the main fuel (energy carrier) used in fuel cells (see next section). Today, however, no sources of hydrogen with a widespread delivery infrastructure are readily available. This issue can be solved by using fossil fuels to generate the hydrogen required. The transformation of a fossil fuel to hydrogen is generally called fuel reforming. Steam reforming is one example, in which steam is mixed with the fossil fuel at temperatures around 760°C. This high temperature can be obtained by burning conventional fuels or by high-concentration concentrating solar collectors. The chemical equation of this reforming reaction for natural gas composed primarily of methane (CH4) is
Fuel reforming can be done in facilities of different sizes. This can be done in a central facility such as a chemical plant at a large scale. Such a plant produces pure hydrogen, which can be a high-pressure gas or liquid. Fuel reforming can also be performed on an intermediate scale in various facilities such as a gasoline station. In this case, refined gasoline or diesel fuel would be required, which can be delivered to the station with its current infrastructure. On-site equipment would then reform the fossil fuel into a mixture composed primarily of hydrogen and other molecular components, such as CO2 and N2. This hydrogen would most probably be delivered to customers as a high-pressure gas.
The fuel-reforming process can also be performed on a small scale, according to the requirements, immediately before its introduction into the fuel cell. For example, a fuel cell-powered vehicle can have a gasoline tank, which would use the existing infrastructure of gasoline delivery, and an on-board fuel processor, which would reform the gasoline into a hydrogen-rich stream that would be fed directly to the fuel cell.
In the future, it is anticipated that most of the hydrogen required to power fuel cells could be generated from renewable sources, such as wind or solar energy. For example, the electricity generated at a wind farm or with photovolta-ics could be used to split water into hydrogen and oxygen through electrolysis. Electrolysis as a process could produce pure hydrogen and pure oxygen. The hydrogen thus produced could then be delivered by pipeline to the end users.
Chemistry applications include also the solar reforming of low-hydrocarbon fuels such as LPG and natural gas and upgrading them into a synthetic gas that can be used in gas turbines. Thus, weak gas resources diluted with carbon dioxide can be used directly as feed components for the conversion process. Therefore, natural gas fields currently not exploited due to high CO2 content might be opened to the market. Furthermore, gasification products of unconventional fuels, such as biomass, oil shale, and waste asphaltenes, can also be fed into the solar upgrade process (Grasse, 1998).
A model for solar volumetric reactors for hydrocarbon-reforming operations at high temperature and pressure is presented by Yehesket et al. (2000). The system is based on two achievements; the development of a volumetric receiver tested at 5,000-10,000 suns, gas outlet temperature of 1200°C, and pressure at 20 atm and a laboratory-scale chemical kinetics study of hydrocarbon reforming. Other related applications are a solar-driven ammonia-based thermochemical energy storage system (Lovegrove et al., 1999) and an ammonia synthesis reactor for a solar thermochemical energy storage system (Kreetz and Lovegrove, 1999).
Another interesting application is solar zinc and syngas production, both of which are very valuable commodities. Zinc finds application in zinc-air fuel cells and batteries. Zinc can also react with water to form hydrogen, which can be further processed to generate heat and electricity. Syngas can be used to fuel highly efficient combined cycles or as the building block of a wide variety of synthetic fuels, including methanol, which is a very promising substitute for gasoline to fuel cars (Grasse, 1998).
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