Solar Hot Water System

Salient Features :

A solar water heater consists of a collector to collect solar energy and an insulated storage tank to store hot water. Based on the collector system, solar water heaters can be of two types:

Solar water heaters based on Flat plate Collectors (FPC based SWH):

Here the solar radiation is absorbed by flat plate collectors which consist of an insulated outer metallic box covered on the top with glass sheet. Inside there are blackened metallic absorber (selectively coated) sheets with built in channels or riser tubes to carry water. The absorber absorbs the solar radiation and transfers the heat to the flowing water.


Flat Plate Collector based Solar Water Heaters

Solar water heaters based on Evacuated Tube Collectors (ETC based SWH):

Here the collector is made of double layer borosilicate glass tubes evacuated for providing insulation. The outer wall of the inner tube is coated with selective absorbing material. This helps absorption of solar radiation and transfers the heat to the water which flows through the inner tube.

Evacuated Tube Collector based Solar Water Heater

Solar water heating is now a mature technology. Wide spread utilization of solar water heaters can reduce a significant portion of the conventional energy  being used for heating water in homes, factories  and other commercial & institutional establishments. Internationally the market for solar water heaters has expanded significantly during the last decade.  It is estimated that over 107 million sq.m. of collector area has so far been installed world wide for heating water.  In the country, the collector area so far installed for water heating is over 1.00 million sq.m. MNRE has plans to add another 1.00 million sq. m. in next two years.

Application of Solar Water Heating System:

  • Solar Water Heating System can heat the water up to 80˚C.
  • Solar water heaters (SWHS) of 100-500 liters capacity are suited for domestic application.
  • Larger systems (above 500 LPD) can be used in restaurants, canteens, guest houses, hotels, hospitals, dairies, industry etc.

ESTIMATES OF REQUIREMENTS OF HOT WATER (Thumb rules)

Application Typical Requirement of Hot Water at 60OC.
Household bathing using buckets 20-30 liters per person per bath.
Household bathing using shower with a mixing tap 30-40 liters for 10-15 minute bath
Shaving, while a tap runs 7-10 liters
Household bathing in bathtub (one filling) 50-75 liters
Wash basin with a mixing tap (hand wash, brushing of teeth, etc.) 3-5 liters per person per day.
Kitchen washing 2-3 liters per person per day.
Dishwasher 40-50 liters per wash cycle
Clothes washing machine 40-50 liters per cycle

Cost benefits of SWHS:

  • The most cost- effective way to install a SWHS is to integrate the collector assembly, cold-water supply and piping with the design of a new house under construction. SWHS can easily be installed in group houses and apartments, especially during construction, if adequate provisions are made for piping, collector assembly and cold-water supply. Proper load matching is required to ensure that the capacity of the system installed is optimized to meet the daily hot - water needs of the end-user.
  • Fuel Savings: A 100 liters capacity SWHS can replace an electric geyser for residential use and saves 1500 units of electricity annually.
  • Avoided utility cost on generation: The use of 1000 SWHS of 100 liters capacity each can contribute to a peak load shaving of 1 MW.
  • Environmental benefits: A SWHS of 100 liters capacity can prevent emission of 1.5 tones of carbon-dioxide per year.
  • Life : 15-20 years
  • Approximate cost: Around Rs. 22000 (SWHS using Flat Plat Collector) and Rs: 16500 (SWHS using Evacuated Tube Collector) for a 100 liters capacity SWHS. Rs.110-150 per Liter for higher capacity systems.
  • Payback period:
    • 3-4 years when electricity is replaced.
    • 4-5 years when furnace oil is replaced.
    • 6-7 years when coal is replaced.

Though the initial investment for a solar water heater is high compared to available conventional alternatives, the return on investment has become increasingly attractive with the increase in prices of conventional energy.   The pay back period depends on the site of installation, utilization pattern and fuel replaced.