Domestic Solar Water Heating System

A domestic solar water heating system typically has the following components:

  • Solar collector(s)
  • Storage tank
  • Connecting pipes
  • Mounting

The principle of operation

Domestic Solar Water Heating System (DSWHS) works on the thermosyphon principle. The system is fed by a cold water tank (a separate cold water tank may be required depending on the location of main water storage tank). No pump is required for system operation. The storage tank of the system is above flat plate collectors. In the morning, as the sun heats the collectors, the hot water inside the collectors rises by natural convection and colder water from storage tank flows into the collectors by gravity. Thus, the water circulation is automatically established whenever there is sufficient sunshine. It stops automatically during insufficient sunlight. In order to prevent reverse circulation at night, the tank should be located above the collectors.

Since hot water is lighter than cold water, stratification tends to occur in the storage tank of DSWHS i.e. the water at the top of the tank would be hotter than at the bottom. To prevent mixing of hot and cold water inside the storage tank (thereby resulting in lukewarm water) while hot water is being withdrawn, rate of withdrawal normally should not exceed 10 liters per minute. Moreover, cold water supply must be available to the DSWHS storage tank for all the twenty-four hours of the day.

The water in DSWHS is warmed up to 60oC by the heat of the sun. Hence, on cloudy, overcast and rainy days, DSWHS will heat the water to lukewarm temperatures. As soon as the sky clears, it will continue to operate as efficiently as earlier. However on extremely cold or foggy days the water may be heated up to a temperature of 45oC or less, because of shorter sunshine hours and less solar radiation reaching the collectors. While the insulated storage tank and pipes ensure availability of this hot water even during the night as well as the early morning hours; hot water availability is also dependent on the capacity of DSWHS. Once the stored hot water is exhausted (especially on days when there are more users), one will either have to wait till the next day (for the heating cycle to begin again) or one can use electrical backup. To ensure adequate availability of hot water, the system’s capacity must be planned considering the family’s total hot water requirements and seasonal variations.

The tilt angle of solar collector (angle of the inclined solar collector with respect to horizontal) as well as its location also plays an important part in DSWHS’s output. Normally the tilt angle is kept equal to the latitude of the location (for Delhi it would be 280 North) whereas the collector should be facing south direction. However, depending upon house location, it may vary slightly towards east or west.

Tips for maximizing output

  1. Keep the collector glass clean. Layers of dust on the collector surface inhibit the sunrays from reaching the absorber plate, thus lowering the heating levels.
  2. Do not cover solar collectors.
  3. Never shut off cold water inlet. Since DSWHS works on its own, without the help of any pumps and controls, the only precaution required is the continuous supply of cold water. If the cold water supply to the system is stopped, the hot water cannot be withdrawn.
  4. Use the system as far as possible early in the morning. Do not use the system frequently over the day.
  5. If you require hot water in the afternoon, try to use the water before 1.00 pm.
  6. Use the lowest possible drain rate while using the system. This will ensure minimum mixing of the cold water in the system storage tank. (The maximum flow rate allowed is 10 liters/minute which means that a normal bucket (20 liters capacity) should fill up in about 2 minutes.
  7. As far as possible try and keep the vent pipe short (2–3 meters). It may be beneficial to have a separate cold water supply tank for the system, if your present cold water tank necessitates a long vent pipe.
  8. A 100 liters/day system should be connected to no more than two use points and should not have hot water line (½” in size) of more than 5 meters. The limits for a 200 – 250 liters/day system are 10 meters of hot water pipeline (½” in size) and three use points.
  9. Ensure that the hot water pipeline as well as the vent pipe is properly insulated and has leak proof cladding. Any leakage of rain water into the insulation will drastically affect the system output.

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