1.11 Open to solar drying system method.
Advantages:
✓ Needs small space for drying
✓ Efficiency is higher
✓ Prevention from micro-organisms and other foreign particles
✓ Easy available in market
✓ Maintenance is quite simpler
Figure 1.12 Direct solar energy crop drying method.
Figure 1.13 Forced circulation system method.
Disadvantages:
✓ Not suited for a larger capacity of drying foods
✓ Reduction of radiation transmission into cover due to moisture condensation occurs
1.9.2 Forced Circulation Systems
Active method is used for larger capacity of drying foods. As shown in Figure 1.13, solar air heater is utilized in a separate set and passed the heated air to food stored chamber. This type of dryer is applied for tea, spices, leather industries, etc.
1.10 Photovoltaic Conversion
The device used in the conversion purpose of photovoltaic is known as silicon solar cells. Due to photovoltaic effect, the solar radiation from sunlight incidents on the silicon solar cells where energy conversion occurs from solar energy into electricity. The benefit of this system is minimum labor cost [24]. There is no movable components and simple in construction. The output generated power is easily transferred to other operating source system such as solar home appliances, solar powered vehicle, water pumps, and other needed sources [25, 26].
1.10.1 Photovoltaic Effect
This is the conversion of light energy into electricity when the solar radiation falls on silicon cells or photovoltaic cells. Two types of p and n semiconductors are used to form p-n junction. When solar energy strikes on the photovoltaic cells, electron hole pairs are created based on internal fields. If the electric field exists over p-n junction which impacts the separation of electron hole pair and free electrons, then flow from n to p type semiconductor to current flow occurs. The working schematic is shown in Figure 1.14.
Absorbing Materials
All the photovoltaic cells need light absorbing materials to capture photons and free electrons, which are placed in a cell structure. Silicon cells play a dominant technology in photovoltaic cells [27]. The most commonly used materials are amorphous silicon, crystalline silicon, cadmium telluride, cadmium sulphide, gallium arsenide, and some other materials.
Silicon
The popularly named material for solar cells is crystalline silicon otherwise called solar grade silicon [28]. Crystalline material gives a maximum efficiency compared to other types of materials. Based on crystal size, the crystalline materials can be classified into single crystal silicon; poly-crystalline silicon has an average efficiency of about 14%–19% [29]. Antireflection coatings help to rise the capturing of light energy into solar silicon cells. Titanium dioxide is chosen as best suited antireflection coating materials due to its good surface quality. Crystalline silicon on glass technique increasing the efficiency rate as 12% to 13% from 8% to 9% of normal solar cells with lesser production price [30].
Figure 1.14 Schematic figure of photovoltaic module.
Amorphous Silicon
One of the most widely spread techniques is amorphous on thin film by plasma-enhanced chemical vapor deposition method, approximately 5% to 7%, and for double, triple junction raised upto 10% [31]. The following materials that are generated from amorphous silicon are amorphous silicon (a-Si), amorphous silicon germanium (a-SiGe), microcrystalline silicon (c-Si), amorphous silicon nitride (a-SiN), and nano-crystalline silicon (nc-Si) [32].
Cadmium Telluride
This material is simple for deposition and best appropriate to large scale industries. It produces high efficiency than other solar cells on thin film technique but its more toxic element [33].
1.10.2 Applications
Few applications are noted here as follows:
✓ Irrigation purposes
✓ In remote areas to supply electricity for street lights
✓ Refrigeration field
✓ In buildings
✓ Satellite communication
✓ Climate monitoring system, etc.
1.11 Photovoltaic Thermal Systems
When exposed to open atmosphere, the silicon solar cells may be exaggerated over a period of years due to change in climatic conditions such as humidity, temperature, and air [34]. Photovoltaic silicon modules produce lesser efficiency because of higher temperatures in summer season. Researchers stated that 5% percentage of efficiency would be fallen for every 10°C raise in temperature range since few amount of temperatures could not captured properly by the modules [35]. To overcome this type of problems, modern technology is established by scientists as photoelectric method, namely, photovoltaic thermal systems. This system is adopted by the combination of two models that are photovoltaic and solar thermal collectors, which results in better efficiency [36]. However, overall efficiency will be fallen during summer climatic conditions, so that fluid medium is used to improve the performance of a PV/T system. In the recent days, newly introduced mediums are nanofluids in order to increase performance even better than using water-based mediums [37, 38].
1.12 Conclusion
Applications and developing technologies of solar thermal energy resources on both domestic and industrial purposes are discussed in this chapter. The improvement of solar power energy in terms of quality products as well as utilization of various fields is studied. Even though there are many methods for conversion of solar energy into other forms of energy, suitable solar thermal collector is designed based on the applications related to systems arrangement. It was identified that solar thermal collector and photovoltaic thermal systems are more viable for different industrial and domestic usages.
Solar power systems compared with other energy conversion systems acquires a less initial cost with minimal emission generation. Possibility of integrated solar energy conversion systems over conventional method related to industrial applications like heating and cooling has more benefits than other surviving techniques.
References