of heat carrier mode and radiation absorption through systems that are non-concentrating liquid flat plate collectors, non-concentrating air collectors, and concentrating collectors.
Non-Concentrating Liquid Flat Plate Collectors
This system contains absorber mat with the pipes for the purpose of heat, transparent cover, insulation material, and fluid passage tubes. It is generally used for an open air application heating systems. Material used for this type of system is ethylene propylene dienmonomers and helps to survive a temperature rate of 150°C and ultraviolet radiation. High efficiency is produced while using this method especially for a swimming pool application. It also called as absorber type collector. Non-concentrating liquid flat plate collectors are shown in Figure 1.2.
Non-Concentrating Air Collectors
Air collector design is comparatively simpler over liquid type collectors. There is much space area between absorber plate and air so that heat transfer coefficient is to be better. Figure 1.3 shows the non-concentrating air collectors. The different types of solar collectors are explained in Table 1.6.
Figure 1.2 Non-concentrating liquid flat plate collectors.
Figure 1.3 Non-concentrating air collectors.
Concentrating Collectors
Even higher range of temperatures is obtained by using this type of collectors. In these concentrating collectors, otherwise called as focusing collectors, solar energy is collected on the absorbing surface using a lenses or mirrors. Two main components are required in this collector device as concentrator and a receiver. The concentrator acts as a mirror reflector and focuses the sunlight radiation on its axis and then radiation is absorbed on the surface of absorber tube and transfers the liquid flows to it. The convective and radiative losses dissipated to the atmosphere are minimized when using a cover made up of a concentric glass which surrounds the absorber tube of the system. Tracking movement also involved in this method of collectors when focusing takes place from sunlight to the tube. Concentrating collectors are shown in Figure 1.4.
Table 1.6 Types of solar collectors [12].
| S.no | Based on movement of the systems | Types of collectors used | Absorber used |
| 1 | Motionless | Flat Plate Collector Evacuated Tube Collector Compound Parabolic Collector | Flat Flat Tubular |
| 2 | Single Track Axis System | Fresnel lens Collector Parabolic trough collector Cylindrical trough collector | Tubular Tubular Tubular |
| 3 | Double Track Axes System | Parabolic dish reflector Heliostat field collector | Point Point |
Merits:
✓ Absorbs both of diffuse and reflected radiation
✓ Less maintenance price
✓ Simple design
✓ Due to fixed tilt orientation, there is no need of tracking system
Figure 1.4 Concentrating collectors.
Demerits:
Due to the unavailability of optical concentration, resulted outcome efficiency will be lower and there is more chance for heat loss in a system.
1.7.2 Evacuated Solar Collector
One of the developed methods in place of conventional solar collector is the evacuated solar collector. The main aim of developing this type of design is to improve the efficiency rate, to reduce heat transfer loss rate, to improve the working conditional temperatures, and to minimize the constructional size of collector.
This method is designed by creating the vacuum among absorber plate and glass cover. A glass tube acts as a cover and plays important role to withstand pressure difference created in a system. There are numerous evacuated collector designs developed. Solar water heating systems used borosilicate glass cover as vacuum tubes to absorb radiation energy from the sunlight.
The main characteristic features of evacuated solar tube collector are as follows:
✓ Better quality glass tubes
✓ During winter and cloudy seasons, solar evacuated tube collector system gives a good heating performance.
✓ Insulation method is highly installed to maintain temperature even for a higher range of water.
✓ Compact design in size
✓ Maintenance is very easier and simple installation
✓ Less labor operating cost
✓ Eco-friendly system
1.8 Thermal Applications
The numerous solar thermal applications are categorized from the absorption of solar radiation either directly or indirectly. If the solar energy is used for generation of thermal energy resources of various industrial and domestic appliances [13] in order to avoid the lack of dependence on fossil fuel sources, then the solar energy also helps to eliminate the toxic gas production, enormously mainly like as carbon dioxide, sulfur dioxide, and nitrous oxide [14]. Even though, some practical difficulties are faced in coupling the solar thermal energy into various processes as dilute and some other natural solar radiation forms.
The consumption of energy distribution generally shows that approximately 13% of industrial thermal usage needs lesser amount of temperatures upto 100°C and then around 27% needs about 200°C [15] and rest of few more appliances requires a higher temperatures in the field of steel and glass processing industries. Many of the industries needs a heat in few components of their operational applications [12]. Heat utilization is involved in variety of industrial operations for a different ranges of temperature as low, medium, and high, approximately 20° to 240°C [16]. Table