difference exceeds a certain value and off when it falls below a certain value. Provision is also usually made for an auxiliary heater. In Fig. 3.9, the auxiliary heater is shown as an immersion heater located in the storage tank. Often it is located on the hot water line leading out of the storage tank. These types of solar water systems are well suited for factories, hospitals, hotels, offices, etc. Typically these systems cost about Rs 140 per litre/day of hot water.
24 Many variations are possible in the configuration. A popular variation is one with an open loop configuration shown in Fig. 3.10. In such a configuration, the water flows from a cold water tank through the collectors to an insulated hot water storage tank from where it is withdrawn for use. The control of such a system is usually designed so that hot water at the required temperature flows out of the collector array, the flow rate being adjusted to meet this temperature requirement. It is claimed that this system meets the design specification for 300 days in the year and helps to save about 200 000 liters of oil annually. Figure 3.11 shows a schematic diagram of one more forced circulation system for supplying process heat. In this case, a concentrating collector array is used to heat pressurized water or a thermic fluid to a temperature above 100°C and the heated liquid is kept in a storage tank. A heat exchanger is used to extract energy from the heated liquid and obtain cither hot water or low pressure steam for industrial or commercial use. An auxiliary heater is provided in the line in order to supply make-up energy in case the solar energy is inadequate.
25 3.13.2 Space Heating We now turn our attention to space-heating systems. Space heating is of particular relevance in colder countries where a significant amount of energy is required for this purpose. In India, it is of importance mainly in the northern and northeastern regions in winter. Active Methods We will first describe space-heating systems using active methods. An active method is one which utilizes a pump or a blower to circulate the fluids involved in the space heating system. One system is illustrated in Fig. 3.12. In this system, water is heated in solar flat-plate collectors (A) and stored in the tank (B). Energy is transferred to the air circulating in the space to be heated by means of the water-to-air heat exchanger (E). Two pumps (C) provide forced circulation between the collectors and the tank, and between the tank and the heat exchanger. Provision is also made for adding auxiliary heat (D). Since the solar energy is first being used to heat water, the system shown in Fig. 3.12 can be easily modified to be a two-in-one system supplying hot water as well as hot air for space heating. An alternative approach to space heating is to heat air directly in solar air heaters (A), as shown in Fig. 3.13. The heat is then stored in a porous bed storage (B) packed with rock, gravel or pebbles. Energy is extracted and transferred to the space to be heated by blowing cool air through the porous bed. Once again an auxiliary heater (D) is provided for supplying make-up heat.
- Fall '19
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