Solar thermal collectors and applications

Solar chemistry applications

Solar energy is essentially unlimited and its utilization is ecologically benign. However, solar radiation reaching the earth is intermittent and not distributed evenly. There is thus a need to store solar energy and transport it from the sunny uninhabited regions to the industrialized populated regions where energy is needed. The way to achieve this 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 are employed. Thus by concentrating solar radiation in receivers and reactors, energy can be supplied to high-temperature processes to drive endothermic reactions. Solar energy can also assist in the processing of energy­intensive and high-temperature materials.

Applications include the solar reforming of low hydrocarbon fuels such as LPG and natural gas and upgrade it into a synthesis 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 non-conventional fuels like biomass, oil shale and waste asphaltenes can also be fed into the solar upgrade process [175].

Other applications include the solar gasification of biomass and the production of solar aluminium the manufacture of which is one of the most energy intensive processes. Another interesting application is the solar zinc and syngas production which are both very valuable commodities. Zinc finds application in Zn/air fuel cells and batteries. Zinc can also react with water to form hydrogen which can be further processed for heat and electricity generation. Syngas can be used to fuel highly efficient combined cycles or can be used as the building block of a wide variety of synthetic fuels, including methanol, which is a very promising substitute of gasoline for fuelling cars [175].

A model for solar volumetric reactors for hydrocarbons reforming operation at high temperature and pressure is presented by Yehesket et al. [176]. The system is based on two achievements: the development of a volumetric receiver tested at 5000-10 000 suns, gas outlet temperature of 1200 °C and pressure at 20 atm and a laboratory scale chemical kinetics study of hydrocarbons reforming. Other related applications are a solar driven ammonia based thermochemical energy storage system [177] and an ammonia synthesis reactor for a solar thermochemical energy storage system [178].

Another field of solar chemistry applications is the solar photochemistry. Solar photochemical processes make use of the spectral characteristics of the incoming solar radiation to effect selective catalytic transformations which find application in the detoxification of air and water and in the processing of fine chemical commodities.

In solar detoxification photocatalytic treatment of non­biodegradable persistent chlorinated water contaminants typically found in chemical production processes is achieved. For this purpose PTC with glass absorbers are employed and the high intensity of solar radiation is used for the photocatalytic decomposition of organic contaminants. The process uses ultraviolet (UV) energy, available in sunlight, in conjunction with the photocatalyst, titanium dioxide, to decompose organic chemicals into non-toxic compounds [179]. Another application concerns the development of a prototype employing lower concentration CPC [175]. Recent developments in photocatalytic detox­ification and disinfection of water and air are presented by Goswami [180].

The development of a compound parabolic concentrator technology for commercial solar detoxification applications is given in Ref. [181]. The objective is to develop a simple, efficient and commercially competitive water treatment technology. A demonstration facility is planned to be erected by the project partners at PSA in Southern Spain.

Solar thermal collectors and applications

Collector thermal efficiency

In reality the heat loss coefficient UL in Eqs (2) and (42) is not constant but is a function of collector inlet and ambient temperatures. Therefore: TOC o "1-5" h …

Global climate change

The term greenhouse effect has generally been used for the role of the whole atmosphere (mainly water vapour and clouds) in keeping the surface of the earth warm. Recently however, …

Limitations of simulations

Simulations are powerful tools for process design offering a number of advantages as outlined in the previous sections. However, there are limits to their use. For example, it is easy …

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