SOLAR ENERGY APPLICATIONS IN DAILY LIFE

1. SOLAR AIR HEATER

Figure shows a schematic cross section of a conventional flat plate collector for heating air. It is called Solar Air Heater. The construction of solar air heater is similar to that of a liquid flat plate collector except for the passages through which are flows. In the diagram the air passage is shown simply parallel to the plate. Air passages should be sufficiently larger in order to keep the pressure drop across the collector within manageable limits.

Solar radiation is absorbed by black plate through transparent glass plate. Transparent cover reduces convection and radiation losses while insulation reduces conduction losses. It is not worthwhile to use more than two transparent plates because each plate reflects about 15% of sunlight.

• Material of absorber plate : Copper, Steel or Aluminium

•Material of tubes : Copper

•Material of covers : Glass or Plastic films

Transparent plates may be coated with anti-reflecting material.

Hot air, thus heated, may be used to dry the agricultural products or may be used in solar air conditioning system.

2. SOLAR GRAIN DRYER

Air is heated by solar radiation in an air heater separately. Hot air is ducted to a chamber in which the products to be dried are stored. Besides food grains (paddy, maize etc.), such dryers may be used to dry products like tea and tobacco. It may also be called a forced circulation type solar dryer because blower is used to sustain air circulation.

Compared with natural sun drying where the products are dried in open sun light, solar drying generates higher temperatures; reduces the risk of spoilage and provides protection against dust, insects and other animals.

3. SOLAR COOKER

One of the important domestic applications of solar energy is solar cooking. Quite a number of designs of solar cookers have been developed. Figure shows a simple Box Type Solar Cooker. It essentially consists of a rectangular enclosure insulated on the bottom and sides, having one or two glass covers on the top. Food to be cooked is placed in shallow vessels. Solar radiation enters through the top and heats up the vessels. Temperature around 100°C can be obtained in such cookers on sunny days. Pulse, Renewable Sources of Energy 7.9 rice, vegetables, dishes like upma, tomato bath, semya bath etc. can be readily cooked within 30 to 150 minutes. The cooking time is inversely proportional to the collector panel area.

The drawbacks with this type of cooker are :

• on cloudy days and nights food cannot be cooked

• they take relatively more time to cook

• dishes like chapathy and puri cannot be made because they require very high temperatures

• items like meat cannot be fried.

4. SOLAR WATER HEATER

1. Natural Circulation Type Solar Heater:

From an economic stand point of view, water heating is one of the most attractive applications of solar energy utilisation. Figure illustrates the small capacity solar water heater suitable for a domestic purpose.

The two main components of the system are the liquid flat plate collector and the storage tank. The tank is located above the level of collector. As the water in the collector is heated up by solar energy, its density reduces and flows automatically by natural circulation to the top of the tank. Meanwhile relatively cold water flows from the bottom of tank to the collector by gravity. Hot water can be periodically withdrawn from the top of the tank while fresh supply of cold water is given at the bottom.

Solar water heaters of natural circulation type have capacities ranging from 100 to 200 litres. They easily meet the demand of a small family of four to five persons. The temperature of hot water delivered ranges from 600 - 70°C.

2. Forced Circulation Type Solar Heater :

This type of heater is suitable when large quantity of hot water is required. Large arrays of flat plate collectors are used and forced circula-tion is maintained with a water pump. Storage tank need not be placed at a higher level. Whenever hot water is withdrawn cold water takes its place because of the ball-float control. The pump is operated by an on-off controller which senses the temperature difference AT between points A and B shown in figure. The pump is switched on whenever AT exceeds a certain value and switches off when AT falls below a certain value. Provision is also made for an auxiliary heater in the storage tank.

Applications :

Hospitals, Factories, Big hotels, Offices, Building Complexes etc. To maintain this rate of fluid flow we have to employ pumps. The pump power depends on pressure losses. Total pressure losses in the collector field depend on friction factor due to various curves, joints etc., in the pipe line. They can be read from charts.

5. SOLAR STILL

Solar stills are used to produce distilled water. Figure shows a simple single basin solar still.

The saline water is collected into a shallow tray of the still. This water is heated by the sun radiation and thus evaporated. The evaporated water condenses on a glass cover plate which is fitted to the casing slant wise. On account of slope, the condensed water flows down along the plate and is collected by the container as shown.

To evaporate 1 kg water at 30°C, about 2500 kJ of heat energy is required. Assuming an insolation of about 200 W/m2 averaged over 24 hours, the maximum amount of water which can be evaporated would 200 x 24 x 60 x 60/(2500 x 1000) = 6.88 litres /m2/ day. In practice, thermal losses do occur and efficiency will be 40% to 45%. And thus we can expect about 2.5 to 3.0 litres / m2 / day distilled water from such a simple still. This way solar energy can be utilised to desalinate sea water and convert it into potable water.