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 1.7 shows the industrial processing units and requirements of temperature.
Table 1.7 Utilization of heat in industries and their operating temperature ranges [17].
| S.no | Industries | Processes | Range of temperatures [°C] |
| 1 | Dairy Industry | Sterilization Pressurization Concentrates Boiler feed water Drying | 100 to 120 100 to 120 60 to 80 60 to 90 120 to 180 |
| 2 | Textile Industry | Bleaching and Dyeing Degreasing Fixing Press | 60 to 90 100 to 130 160 to 180 80 to 100 |
| 3 | Chemical Industry | Preheating water Soaps Synthetic rubber | 60 to 80 200 to 260 150 to 200 |
| 4 | Paper Industry | Drying Bleaching | 60 to 80 130 to 150 |
| 5 | Bricks and blocks | Curing | 60 to 140 |
| 6 | Meat | Washing Sterilization Cooking | 60 to 90 60 to 90 90 to 100 |
| 7 | Plastics | Preparation and Blending Distillation and Separation Separation | 120 to 140 140 to 160 200 to 220 |
The commonly used solar thermal radiations in domestic and industrial operations are as follows [18]:
✓ Solar water heating
✓ Steam generation
✓ Solar drying
✓ Space cooling and refrigeration
✓ Solar distillation
✓ Cooking
✓ Power generation
1.8.1 Solar Water Heating Systems
One of the most needed domestic and industrial thermal applications is solar water heating technology and is more efficient compared to other types of solar equipment that are presently in use. In many of countries, SWH technologies are become more commercial and there is a greater growth utilization rate of about 30% from last few decades [13, 18].
This type of systems constitutes the solar flat plate collectors and space for storage of water [13], which is insulated to avoid possibility of heat loss. Moreover, the heated liquid passed from the collector unit to storage tank occurs in two different methods as follows:
Figure 1.5 Natural circulation system [1].
✓ Natural mode of circulation
✓ Pumped mode of circulation
Natural Circulation
A small capacity unit is suited for home appliances in a natural mode of liquid transportation takes place as shown in Figure 1.5. The components used in this system are collectors and an insulated storage tank connecting two inlets: one is cold water that is traveled toward the collector from main pump line and another connected to tank from solar collector as heated fluid, where the tank is fixed above the height of the collector unit. The overall pipe line of the systems is to be covered with glass-wool material in order to minimize the heat loss [11]. Solar thermal radiation hits on the flat plate solar collectors from sunlight where it heats the fluid that passed inside the tube. Due to the density variation that occurs in the fluids between cold and heated, the heated water flows to the top of the storage tank through top header pipe line [13]. Cold fluid is entered into the tank from inlet of main pipe line when vacuum is generated.
Forced Circulation System
In an industrial process, a huge amount of heated fluid is needed for
