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  1. What is a solar water heater?
  2. What are the different types of solar water heater?
  3. How do these different types of solar water heating system works?
  4. How much energy can be saved using solar water heating system?
  5. What is the environmental impact of using solar water heating system?
  6. Advantages of evacuated tube?
  7. Why renewable energy?
  8. What will be the output temperature of hot water from evacuated system?
  9. What is the effect of hard water on the evacuated tube system?
  10. Comparison between evacuated tube and flat plate collector?
  11. What maintenance and servicing needed for evacuated tube systems?
  12. What is the life of evacuated tube system?

1. What is a solar water heater?
A water heater that depends on RADIANT ENERGY from the sun as its source of power. A Solar Water Heater absorbs the incident solar radiation and uses it to heat up water.

2. What are the different types of solar water heater?
The different types of solar water heater are:

i.  Evacuated Tube Solar Water Heater.
ii.  Flat Plate Solar Water Heater.
iii.  Concentrated Collector Solar Water Heater.
iv.  Parabolic Collector Solar Water Heater.

3. How do these different types of solar water heating system works?

Principle of operation Evacuated Tube Solar Water Heater

Sunlight is incident on the Vacuum Tube, this passes through the outer transparent glass tube and strikes the outer surface of the inner glass tube coated with selective coating. This coated glass tube acts like a black body and absorbs the radiation and gets heated up in the process. The reflecting sheet facilitates the process.

The presence of vacuum between the two tubes prevents heat loss to the surroundings. This heated inner tube then transfers the heat to the water with which it is directly in contact. This water gets heated up. Hot water has a lower density and hence has a tendency to rise up. Cool Water from the tank comes down to replace the hot water. This sets up a circulation by Thermo-siphon.

Following this process slowly all water in the storage tank gets heated up and is ready for use.

The Storage tank is adequately insulated using Polyurethane Insulation, this minimizes heat loss to an extent not greater than 5% during night time.

Principle of operation of Flat Plate Collector Solar Water Heater

Flat-plate collectors are the most common collectors for residential water heating. A typical flat-plate collector is an insulated metal box with a glass or plastic cover called the glazing and a dark-coloured absorber plate. The glazing can be transparent or translucent. The absorber plate is usually black because dark colours absorb more solar energy than light colours. Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar radiation into heat energy. The heat is transferred to the air or liquid passing through the flow tubes. Because most black paints still reflect approximately 10% of the incident radiation some absorber plates are covered with ďselective coatings,Ē which retain the absorbed sunlight better and are more durable than ordinary black paint. The selective coating used in the collector consists of a very precise thin layer of an amorphous semiconductor plated on to a metal substratum. Absorber plates are often made of metal usually copper or aluminium because they are both good heat conductors.

Principle of operation of Concentrated Collector Solar Water Heater

Concentrating collectors use mirrored surfaces to concentrate the sunís energy on an absorber called a receiver. They also achieve higher temperatures than flat-plate collectors; however concentrators can only focus direct solar radiation, with the result being that their performance is poor on hazy or cloudy days. The mirrored surface focuses sunlight collected over a large area onto a smaller absorber area to achieve high temperatures. Some designs concentrate solar energy onto a focal point, while others concentrate the sunís rays along a thin line called the focal line. The receiver is located at the focal point or along the focal line. A heat-transfer fluid flows through the receiver and absorbs heat. Concentrators are most practical in areas of high insolation, such as those close to the equator and in the desert areas.

Principle of operation of Parabolic Solar Water Heater

In parabolic system, parabolic mirror is used to collect the solar radiation and focus it on the tube. The water running through this tube acquires solar energy and gets heated. This heated water is collected in a tank for future use. This type of system is required to achieve very high temperature. Through this system we can achieve a hot water of 400 deg C. This is different from concentrating collector because it works on line heating where as concentrating use point heating.

4. How much energy can be saved using solar water heating system?

The energy saving can be calculated using the formula given below:

ENERGY SAVED = 4.2 * average increase in temperature of the system (deg. C) * size of the system (l) * 300

ELECTRICITY = Energy saved in a year in kJ & kWh / 3600 * cost of electricity per SAVED unit (Price per unit in kWh)

  • Thus by using a 110 litres system you can:
  • Save Energy - 6930 KJ
  • Save Energy - 1925 KWh
  • Reduction In Electricity Bill - INR 9600 (INR 5/KWh)

5. What is the environmental impact of using solar water heating system?

Solar water heating is an environmentally responsible technology that reduces fossil fuel consumption and helps mitigate global climate change. By displacing combustion of coal, oil, and natural gas, solar energy greatly reduces greenhouse gas emissions.

With a design life of 15 to 20 years, that system alone will reduce CO2 emissions by up to 1000 Kgs. It also saves approx. 3 full Solar water heating is also quiet, emission free, and it does not interfere with wildlife and other natural systems. Most of the hardware is made from simple, durable, and recyclable materials, meaning that the full life-cycle impact of solar water heating is relatively low compared to other energy sources.

6. Advantages of evacuated tube?

The various advantages of evacuated tube over flat plate collector are:

  • Sunrays remains always perpendicular to cylindrical absorber surface of vacuum tubes, so it can absorbs more energy .So efficiency of evacuated tubes is more.
  • In case of evacuated tubes, various capacity ranges can be designed by varying the number of vacuum tubes. Hence the evacuated tube systems are more economical and cost effective.
  • The flat plate collector use copper and aluminum which are costly. On the contrary evacuated system uses glass tube which makes the system cheaper.
  • The problem of scaling can be solved through proper cleaning of evacuated tube. Hence maintenance is easy in case of evacuated tube.

7. Why renewable energy?

Between 1970 and 1998, global energy use rose by about 70%, and demand for energy continues to rise at a rate of about 2% each year. While energy use and resource extraction fuel the global economy and development worldwide, they pose serious environmental hazards. Increases in conventional energy use mean parallel increases in emissions, including greenhouse gasses, which mean more smog, more global warming effects, and more risks to human and environmental health.

Renewable energy can reduce our dependence on fossil fuels, reduce the harmful pollution currently associated with energy production and consumption, and reduce our emissions of greenhouse gases. Alternative heating and cooling systems that run on non-traditional, renewable energy sources and produce fewer harmful emissions can substantially reduce consumersí energy costs.

8. What will be the output temperature of hot water from evacuated system?

The temperature of hot water from evacuated tubes varies from 45 deg. C to 95 deg. C depending on season and availability of sunshine.

9. What is the effect of hard water on the evacuated tube system?

In case of flat plate collector hard water cause scale deposition and its bonding with metal surface but in case of evacuated tube collector there is no bonding of scales possible on smooth surface of glass tube.

However there can be some settlement of salt & dirt particle at the bottom of vacuum. There is also a possibility of some stains or particle on inner wall of evacuated tube but these can be removed at the time of servicing.

10. Comparison between evacuated tube and flat plate collector?

The various comparisons between evacuated tube and flat plate collector are given below:

  Evacuated Tube Collector Flat Plate Collector
1. Quick heat generation Slow heat generation
2. Heat loss in the tubes during the daytime is negligible (evacuated tubes) Heat loss in the collector & tank during the day-time is high due to convection.
3. System hot water tank only is insulated using polyurethane insulation material which does not absorb water or moisture Collector & tank insulated with glasswool/ rockwool, absorbs moisture & gets wet during monsoons reducing the efficiency of the system
4. Negligible scaling of tubes which can be cleaned manually (inner tube dia. Is 37mm). Loss of efficiency consequently is minimal Heavy scaling of the copper/aluminum tubes which cannot be cleaned manually as the bore dia. is 12.50 mm. giving rise to substantial loss in efficiency of the system.
5. The collector glass tube absorbers being cylindrical the incident sunís rays on the tubes is at 90 degrees throughout the day. Hence peak heat absorption always. The collector fins & tubes being flat the incident sunís rays will be at 90 degrees at noon only for peak absorption
6. Satisfactory performance even in extreme cold condition (-18 deg. C) Freezing of water will take place at high altitude causing damage to the collector.
7. In locations with average availability of solar energy over-sizing of the system glass tube collectors is not required. Higher system sizing is required to get the desired result. Hence added cost.
8. Temperature range from 60deg. to above 95 deg. Temperature range from 60 deg. to 80 deg.
9. Advanced technology at competitive prices. Old technology at higher prices
10. Hot water availability for 350 days in a year. Hot water availability for 300 days in a year claimed.
11. No inter-connection hoses/pipes required between the collector & tank. Inter-connection hoses/pipes & fittings required resulting in heat loss & lowering the life of the system.
12. System life above 15 years. System life 20 years.

11. What maintenance and servicing needed for evacuated tube systems?

Generally the water supplied through local municipal authorities is always of good quality. If such water is used through evacuated tube, the maintenance and servicing required will be almost negligible.

When the water hardness is higher or when the suspended dirt & impurities are present in the water, there is a possibility of stains dirt accumulation in side the vacuum tube. This can be easily removed by removing the vacuum tube and cleaning them properly.

However for achieving consistent performance & better life of evacuated tube it is recommended to do servicing of whole system once in year, which includes cleaning of vacuum tubes & hot water tank.

12. What is the life of evacuated tube system?

The life of the evacuated tube depends on the good water quality and proper precaution. Generally the life of the evacuated tube system is 15 years.