A photothermal system is the direct method where solar energy helps to transfer as incident radiation into heat. Some of the solar collection systems are designed based on this concept that are flat plate collector, evacuated collector, solar air collector, etc.
The basic principles involved in this energy conversion are absorption, reflection, emission, and transmission.
1.7.1 Flat Plate Collector
In any solar collection energy systems, flat plate collector plays an eminent role which absorbs the solar energy and then converted as heat. The heat source may be transferred into the line of a gas or a fluid. Generally, flat plate collector has the following components: absorber, transparent, cove, and frame. Heat passes through the inlet and outlet pipe lines.
Absorber plate: The plate is made up of aluminium, copper, polymeric materials, steel, and duct materials. Some of the basic materials are used with black painted on the radiation receiving sides. The function of absorber plate is to convert heat from short wavelength radiation to minimize the heat loss in a system.
Cover: Cover is made up of materials such as one or combination of more sheets, synthetic foils (i.e., Teflon and Polyethylene). It also helps to arrest the convective heat transfer loss to surroundings.
Collector box: It contains various parts which need for the radiating transmission, absorption, thermal insulation, and energy conversion from a source. The materials used for these elements are aluminium, galvanized steel, wood, and synthetic materials.
The collector designs are categorized into various types that depend upon the applications especially based on the conduct 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
