The following points highlight the focusing and non-focusing types of solar collectors.
1. Focusing-Type Collector:
Focusing collector is a device to collect solar radiation with high intensity of solar radiation on the energy-absorbing surface. A focusing collector is a special form of flat plate collector by introducing a reflecting surface (collector) between the solar radiation and the absorber.
Thus, a focusing collector consists of a focusing device, a receiver, and a tracking arrangement. Theoretically, temperature as high as limited by the temperature of sun’s photosphere may be attained. However, in practice, these units are capable of heat delivery at many hundreds. The current technology includes single axis tracking concentrators.
A systematic diagram of a parabolic reflector is shown in Fig. 1.5.
The radiation received from the sun on the earth is almost parallel. They are reflected by parabolic reflector and focused on a vessel full of water. The water gets heated by utilizing solar radiation. Sometimes, the temperature becomes high; the water gets evaporated to form steam.
Focusing collectors may be of two types:
(a) Line focusing
(b) Point focusing.
In actual practice, the line is a collector pipe and the point is a small vessel, as shown in Fig. 1.6, through which the fluid flows to transport heat.
Another kind of focusing collector in which a large number of plain or slightly curved mirror strips are mounted on a flat base. The angle of individual mirrors are so adjusted that they reflect solar radiation on the same focal line as shown in Fig. 1.7.
The radiation received from the sun on the earth is almost parallel. This is shown in Fig. 1.8. In this system, the solar radiations falling on the earth are focused to a vessel (boiler) mounted on a high tower by using a large number of flat mirror reflectors which are steerable about to axis known as heliostat.
The mirrors are installed on the ground and are oriented such that to reflect the direct radiation beam on vessels. This produces high-temperature fluids. Radiation falling on the vessel is heated by black pipes in which working fluid is circulated. The working fluid is used to drive a turbine to produce mechanical energy.
2. Non-Focusing Type Collector:
The simplest concentrating collector is the mirror- boosted flat plate collector. It consists of a flat plate facing the mirror. It consists of a flat plate facing the mirror .If the mirrors are set at proper angle; they reflect solar radiation on the absorber plate.
The later receives reflected radiation in addition to the normal falling on it. The system employs pumps or blowers/fans to transport heat. It consists of a collector, heat transport fluid, thermal storage unit, heat distribution device, and their controls.
Flat Plate Collector:
Flat plate collectors, which are of the non-concentrating-type collectors, are convenient and effective, when temperature below 90°C is adequate for room heating or water heating.
They are made in rectangular panels of about 1.7-2.9 m2 area and relatively simple in construction and easy to install. Flat plate collectors absorb both direct and diffused solar radiations. They are effective even on cloudy days when there is no direct radiation. In a flat plate collector, a blackened sheet of metal is used to absorb all sunlight.
The metal sheet painted in black acts as a black body; the heat absorbed from the sunlight is transferred to another fluid such as air, water, etc. The metal sheet which acts as black body absorption plate should be in good thermal contact with the tubes holding water or any other fluid.
The thermal contact between the tubes and the plate is ensured by bonding tubes to the bottom of absorber plate as shown in Fig. 1.9. The bonding can be done by soldering or brazing by brass with thermal plate. Figure 1.9 shows the complete assembly of a solar plate collector.
The heat received is transferred continuously to the fluid. The fluid becomes heated at some elevated temperature. In a flat plate collector, the conduction and convection losses can be reduced by placing the blackened sheet of metal in a closed insulated box having its top covered by a transparent glass sheet to allow solar radiations and get entrapped. The collector box supports all the components and provides weather protection. An exploded view of collector assembly is given in Fig. 1.10.
The glass sheet on top allows the transmission of short wavelength radiations through it while blocking the long wave length radiations from the surface of blackened sheet. Flat plate collectors are used for wide range of low-temperature applications such as cooking of food, water heating, room heating, grain drying, etc.
A conventional solar system involves solar collectors and separate thermal storage. Energy is transported from collectors to storage and subsequently from storage to the utilization unit and control device. The flow of energy is regulated by the systems incorporated in the unit. These are controlled by using pumps and/or blower to transport heat.
The heat transfer from collector plate to storage unit may be achieved by natural convection or thermosiphon methods as shown in Fig. 1.11. Alternatively, it may be accomplished by the forced flow of fluid using an electrically operated pump as shown in Figs. 1.12(a) and 1.12(b).
In the flat plate collector heater, two modes are possible:
(a) Direct heating of storage tank [Fig. 1.12(a)] and
(b) Indirect heating of storage tank [Fig. 1.12(b)].
Figure 1.13(a) shows the arrangement of a flat plate collector utilizing solar radiation for heating water/liquid flowing through the tubes. The tubes are brazed with the metal plate surface forming heat-absorbing surface. Similarly, Fig. 1.13(b) shows the placement of collector for heating the air passing through the passage.