In such cases, the lower temperature fluid produced from thermo syphon method is not opted.
The number of solar collectors unit is connected in a series of array along with a external pumping system to circulate the fluid passage. Figure 1.6 is the schematic diagram of forced circulation solar water heating system especially for industrial purposes like factories and hospitals.
Figure 1.6 SWH by forced circulation-Industrial purposes [19].
Figure 1.7 Forced circulation system—Domestic purposes.
Table 1.8 Types of solar energy applications and technologies involved in a systems [20].
| S.no | Applications of solar energy | Technologies used |
| 1 | Solar water heating | Natural Circulation system Forced Circulation system Collection cum storage water heater Built-in storage water heater Shallow Solar Pond water heater Direct Circulation Indirect Circulation water heating |
| 2 | Refrigeration | Adsorption Absorption |
| 3 | Heating and Cooling systems | Air systems Water systems Mechanical systems Heat pump |
| 4 | Solar Distillation | Solar stills Vapor compression systems Multiple effect boiling |
| 5 | Solar power generation | Parabolic tower Parabolic dish Solar furnaces |
In this SWH operation, it contains the solar collectors, external pump to circulate the fluid, thermostat, and non-return valve. The controller is used to regulate the temperature during process. If the predetermined value of temperature exceeds, then valve becomes open to mix up heat water with supplying water system, and finally, it reaches the desired temperature as per required needs. Auxiliary heater provision is also made in this system [1]. There are two modes of technology developed in this system that are flows through a passage and recirculating water heaters [20]. The former technology is applied mostly in industrial operations and latter is same as used in the domestic heating purposes [20] and shown in Figure 1.7. The different types of solar energy applications and technologies involved in systems are explained in Table 1.8.
1.8.2 Steam Generation
In sterilization operations, the lower temperature vapor is widely applied. Most commonly, the higher temperature is obtained in parabolic trough collectors, which is more effective method also among other sources of solar collectors. There are three working theories established [21] in a PTCs to produce steam as follows: steam flash, direct, and unfired boiler.
Figure 1.8 Flash type [1].
Figure 1.9 Direct method [1].
In the first method, pressurized heated fluid is rushed in a separate vessel to obtain steam. The second method is an in situ; in this type, steam production is reached through the way of two phase flow type, and in the latter method, heat exchanger device helps to produce steam. In all these above working procedures, the heated water flows into the collectors.
Figure 1.8 shows the steam flash type where fluid is increased in the range of pressure to eliminate boiling, and then, it enters into collector and further moved into flash vessel. Fluid range is steadily maintained throughout the makeup water inlet.
Figure 1.9 denotes the direct type, and the change in this method compared with the former method is flash valve detached from the system. The feed water directly flows to the receiver end as heated water. Both these methods are quite lesser cost to installation [22].
Figure 1.10 describes the unfired boiler systems; this type of system is simpler than the previous described systems. However, pressure is quite less in this design.
1.9 Solar Drying
In our human life, food plays a predominant role followed by air and water. The main objective of introducing solar dryer arrangement is to prevent foods from high moisture content and also some insects, and dust may affects the products [1]. The moisture may be destroyed food products over a period of time without appropriate preservation made to it. In order to eliminate these challenges, solar energy is used to dry the food crops in a proper solar dryer at optimal moisture content and ensure the availability in it for the requirements [11]. Solar drying has the following benefits:
Figure 1.10 Unfired boiler [1].
✓ Longer period of storage
✓ Yield good profit
✓ Because of well dried product, no chance of forming fungus
✓ Easy handling and transportation
✓ Obtained better quality crops to market [1]
In a conventional system, fossil fuels are burned to generate heat [23], but in solar drying method, radiation comes into act. Solar dryers are classified into two different ways, i.e., depends on the air flow passage into systems that are natural circulation and forced circulation
1.9.1
