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the integration of solar collectors to an industrial thermal powered system. Processes which require water preheating have met higher efficiencies because of the nature of solar systems that input temperature is slightly low. The main reason is that in such systems simple collectors capture the sunlight at the temperature required for the load. Solar thermal is also used in textile industry for heating water at temperatures close to 100 ◦ C for bleaching, dyeing and washing purposes [15]. Currently, fossil fuels are used for fuel-run in textile industry. Therefore SWHs can significantly contribute to reduce the ecological problems associated with textile industry. Built-in- storage water heaters are introduced in Pakistani textile industry to further improve the performance and stability of the systems [1]. Another emerging SWH’s market which is already widespread and reached developmental stage is building industry. Statistics shows that SWHs and space heating and cooling is going to be gen- eralized and will achieve 20–30% of the full commercialization [20]. Most of the developing countries are located in warm climate and hence hot water is not as important as in developed countries which are situated in colder climate. However, according to [21], nearly 10 million SWHs are presently installed in developing countries. By 2000, the total area of 500,000 m 2 was covered by solar collector installations in India. By 2001, millions of Chinese households were equipped with SWH. Egypt and Turkey are using SWHs in hundreds of thousands of households. Botswana and Zimbabwe have installed 15,000 and 4000 SWHs, respectively. Thailand has captured 15% of SWH market worldwide. Furthermore, SWH market is available in Zimbabwe, Nambia, South Africa, Botswana, Morocco, Tunisia, Papua New Guinea, Kenya, Tanzania, Lesotho and Mauritius. In Africa, the payback period for a small scale SWH is as short as 3–5 years [21,23]. On the other hand, large scale SWHs has significant economical benefits. For example, in Nepal monthly electricity bill was reduced by 1200 Euro by installing SWHs in a school for 850 students. Even after 20 years 75% of collectors are still operating properly. Another advantage of installing such project is to encourage domestic sector to use the new technology for kitchen, bath and swimming pool with temperature between 45 ◦ C and 50 ◦ C. Designers, engineers, architectures, service engineers and material providers may play critical roles in sustainable development for the large scale production. Besides, various policies by governments and communities might have a great influence to encourage domestic and industrial sector to apply the new technology [22]. Low temperature steam is extensively used in sterilization processes and desalination evaporator supplies. Parabolic trough collectors (PTCs) are high efficient collectors commonly used in high temperature applications to generate steam. PTCs use 3 concepts to generate steam [24]; the steam-flash, the direct or in situ and the unfired-boiler. In the steam-flash method, pressurized hot water is flashed in a separate vessel to generate steam. In an in situ method, there are 2 phase flow in the collector receiver to generate steam. In an unfired-boiler system, steam is generated via heat-exchange in an unfired boiler. In this concept, a heat medium fluid goes through the collector. Fig. 6 is a schematic of a steam-flash system. The system pressurized the water to avoid boiling. The pressurized water goes through the solar collector and eventually flashed to a flash vessel. Water level in flash vessel is maintained at constant level through feed water supply. Fig. 7 shows the direct or in situ boiling concept. The only dif- ference is that flash-valve is removed in this configuration. Make up water is directly heated to generate the steam in the receiver. Fig. 8 illustrates the unfired boiler system. This system is rather simple than before mentioned systems. The pressure is quite low and control scheme is straightforward. Flash-steam and direct-steam systems require approximately the same initial cost [25]. However, in situ systems suffer from stability problems [26] and scaling of the receivers. To design an appropriate industrial application, the proper steam generation system with suitable decisive factors should be chosen. Solar drying and dehydration systems use solar irradiance either as the solely power supply to heat the air or as a supplementary energy source. Conventional drying systems burn fossil fuels for their performance while the solar dryers take advantage of sun irradiation for drying and dehydration processes in industries such as bricks, plants, fruits, coffee, wood, textiles, leather, green malt and sewage sludge [3]. They are categorized into 2 main groups: high and low temperature dryers. Almost all high temperature dryers are currently heated by fossil fuels or electricity but low temperature dryers can use either fossil fuels or solar energy. Low temperature solar thermal energy is ideal for use in preheating processes as well [27]. On the other hand, solar dryers are also classified based on the method of air flow generation into 2 major groups: natural- circulation (passive) and forced-convection (active) solar dryers. Generally, passive solar dryers use solar energy which is abun- dantly available in the environment. Therefore, this technique has been usually addressed as the only commercially available method in agriculture industry in developing countries. They are categorized into 2 main methods; open to sun and natural-circulation solar-energy crop drying method. Developing countries especially who are in tropical climate are widely taking advantage of open- to-sun passive drying systems. They dry the crops using 2 main approaches; in the field or in situ and by spreading it on the ground or any vertical or horizontal plate exposing to solar radiation. Open to sun passive dryers are very common since they have low initial and running cost and less maintenance required. However, open-to-sun drying method produces huge wastes and crop losses due to imperfect drying, fungus and insect infestation, birds and rodent encroachment. In addition, unpredictable changes in weather and climate changes such as rain and even cloudiness affect the efficiency of such systems. Natural-circulation dryers are another type of passive solar dryers which are favorable options for rural and isolated areas. In this type of dryer, the heated air flow toward the drying crops on the basis of buoyancy forces or using wind pressure or even a combination of both. They offer many advantages over open-to-sun drying systems: - Require smaller area of land for similar quantities of crops - High efficiency due to more protection against fungus, pets and rodents - Shorter time is needed - Protection against unpredictable rains - Low capital and maintenance cost - Commercially available Active solar drying systems use solar energy in combination with electricity or fossil-fuels to generate power for pumps and engines to provide air circulation. In this type of solar dryer, solar energy is the only source to generate heat. This method is used in large-scale commercial drying applications. Such a system can reduce the energy consumption along with controlling the drying conditions. High temperature solar heaters are used for direct drying process. However, for medium and low temperature systems, the fossil-fuel fired dehydrator is applied to boost the air flow temperature to the necessary point. The latter system is called “hybrid solar dryer”. It avoids the effects of fluctuating energy output from the solar collector at night or when the sun irradiation is low. Solar active dryers are widely used in high temperature drying processes where continuous air flow is required [28–30]. Based on system component arrangements and the way system uses solar heat; both active and passive solar dryers could be classified into 3 main groups: integral type, distributed type and mixed mode dryers [31]. Table 7 shows the working characteristics of integral and distributed methods of natural-circulation solar dryers. Industries which involve drying process usually use hot air or gas with a temperature range between 140 ◦ C and 220 ◦ C. Solar thermal systems can be integrated with conventional energy supplies in an appropriate way to meet the system requirements. Heat storage seems to be necessary when system is required to work in the periods of day when there is no irradiation [15]. Solar dryers can extensively be used in food and agriculture industry to improve both quality and quantity of production while reducing the wastes and minimize environmental problems. In spite of using large scale solar dryers in commercial food industries, lack of information is the main barrier to further improve the technology in developing countries. This type of dryer has high initial investment and installation costs. Therefore, only large farms can afford the monetary burdens [23,32]. Table 8 shows the classi- fication of solar energy drying and dryer systems. Increasing standards for living and working conditions, remarkable rate of urbanization, unpleasant outdoor pollutions and affordable price of air-conditioners have initiated increasing demand for air conditioning systems. The more request for air conditioning, the more need for electrical power. Hence, power stations meet their peak load demand in hot summer days leading to blown-out situations [33]. Statistics indicate a huge rise in the number of air conditioning installations within European countries since last 20 years where the cooling capacity has been five-folded. Energy consumption of air conditioners was 6 TJ and 40 TJ in 1990 and 1996, respectively and it is rising to reach 160 TJ in 2010 [34]. Fig. 9 is the block diagram of a typical solar cooling system with refrigerant storage. The peak demand in cooling loads is usually happening when the solar irradiation is high. Solar air conditioners are the type of solar energy application that fulfills this specific condition. They do not require Freon refrigerants or any other harmful substances that depletes ozone layer. Furthermore, operating costs are 15% less than conventional air conditioning systems. By installing solar assisted cooling systems in southern European and Mediterranean region, about 40–50% of primary energy was saved [36]. Fig. 10 illustrates the working principles of an absorption air conditioning system. Solar powered air conditioners are usually connected to the heat supply cooling devices. They require solar collectors, heat buffer storage, heat and cold distribution systems, heat supply cooling devices, cold storage and an auxiliary (backup) heater. The auxiliary heater is connected parallel to the collector or the collector/storage, or integrated as an auxiliary cooling device. It can even be combined to the system in both arrangements, simultaneously [36]. Indoor air conditioning seems to be a necessity in commercial and residential buildings such as hotels with growing market in building industry worldwide. Traditional methods to generate electricity can be replaced with solar energy in heat-driven cooling technologies. The most cost effective ventilation system in a building is a system which is capable to provide both heating and cooling requirements [35]. Generally there are two different solar powered air conditioning systems: closed (recirculating) and open cycle systems. The closed-cycle system uses heat-driven pump which is supplied by solar energy. It requires solar collectors for its performance which increases the initial investment required for the system. It rejects heat from condenser and supply desorbers. The operations are per- formed in two distinct pressure and three temperature levels.
reference;
https://researchgatefigure/233923685_fig9_Fig-9-Block-diagram-of-a-typical-solar-cooling-system-with-refrigerant-storage-35
