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Domestic Solar Water Heating


Solar water heating has the following advantages

  • Solar water heaters save electricity and thus money; electricity is becoming more and more expensive and its availability is becoming unrelaiable;
  • Solar water heaters are non-polluting.
  • Solar water heaters are safer than electric geysers as they are located on the roof


Yes. More than 20,000 domestic systems are being installed every year all over the country.


The working of solar water heaters is very simple to understand. The solar water heaters use two common principles for its functioning. They are

  • a black surface heats up when left in the sun, by absorption of solar radiation; The good absorption property of black surfaces is used to improve solar energy absorption in a solar heater
  • The inside of car/ bus parked in sun for a long time becomes hot. This is because solar radiation can pass through the glass windows of the bus but cannot come out. It is trapped inside and thus heats up the bus. Similarly water passing through insulated pipes kept in the sun becomes hot

These two phenomena are utilized in flat plate collectors of commonly available solar water heaters.

  • A typical domestic solar water heater consists of a hot water storage tank and one or more flat plate collectors.
  • The collectors are glazed on the sun facing side to allow solar radiation to come in.
  • A black absorbing surface (absorber) inside the flat plate collectors absorbs solar radiation and transfers the energy to water flowing through it.
  • Heated water is collected in the tank which is insulated to prevent heat loss.
  • Circulation of water from the tank through the collectors and back to the tank continues automatically due to density difference between hot and cold water (thermosyphon effect).

  • It is the heart of a solar water heating system.
  • It consists of an absorber plate which is coated on its sun facing surface with an absorbent coating, also called selective coating.
  • The absorber consists of a grid of metallic tubes and sheets. Water flows through the tubes. Sheet absorbs the solar radiation falling on it and transfers it to water.
  • The absorber plate is placed in a top open box to protect it from weather. The space between back and sides of the absorber and the box is filled with insulation to reduce heat losses. The front of the box is covered with a high transmittance glass plate.
  • Flat plate collectors are specified on the basis of their area and are of commonly 1x2 m size.


A typical solar water heating system

  • Flat plate collectors are the most common type used in the domestic solar water heating systems in India as they are relatively cheaper when compared to others.
  • Evacuated tube collectors have also been proposed for domestic solar water heating systems, but are not commonly available.
  • Concentrating collectors are likely to be more useful for higher temperature applications such as power generation and industrial use.


Evacuated tube collector type solar water heating system

  • Bureau of Indian standards has laid down specifications of flat plate collectors for use in solar water heating systems. Thus the ISI mark is an assurance on use of proper materials.
  • The important characteristics to be  noted are, the materials of construction of the absorber plate, the type of absorbent coating on it, the quality of the glass plate used, the material of the box, the insulation thickness, etc.

  • The hot water storage tank in domestic solar water heating systems is typically a double walled tank.
  • The space between the inner and the outer tanks is filled with insulation to prevent heat losses.
  • The inner tank is generally made of copper or stainless steel to ensure long life.
  • The outer tank could be made of stainless steel sheet, painted steel sheet or aluminum.
  • Electrical heating elements controlled by thermostats can be provided as an option in the tank itself to take care of those days when sun is not there or demand of water has gone up.
  • The capacity of the tank should be in proportion to the collector area used in the system. A commonly used thumb rule is to provide 50 liters of storage for every sq. m of collector area. Too large or too small tanks reduce the efficiency.

  • First and foremost requirement of a good solar heater is that it should have sufficient collector area for the capacity claimed. Collector area used in the system determines the capacity of water heating. Fox example, in typical north Indian weather conditions, on a sunny winter day, a sq. m. of collector area can be expected to heat around 50 liters of water by a temperature of 30-40° C.
  • Typical flat plate collectors made in the country have an area of around 2 sq. m and are thus capable of heating around 100 liters of water in a day. This proportion serves as a benchmark.
  • Further, the collectors should use good materials and the absorbers should carry a good quality coating (BIS approved collectors are being provided by large number of established manufacturers).
  • The system should be mounted on a rigid structure and should be firmly fixed with the roof to prevent damage in high winds.

  • The fundamental rule is that it is better to buy a system smaller than your requirement. When more water is required, other sources of water heating could be used. This will lead to better efficiency and cause lesser operational problems.
  • The best is to make an actual estimate of daily demand of hot water. While estimating, do remember that the solar system is capable of heating only an approximately fixed quantity of water and is designed for typical sunny days. Also remember that the temperature of water in the solar system is determined by the combination of collector area and the tank capacity. Typically it would be 50 - 60°C, which is much hotter than the bathing water temperature (around 40°C).
  • Estimation of your demand can also be made with the help of the table given below.
  • As a typical example on sizing of solar systems, it may be mentioned that a 100 liters system is considered generally optimum for family of 4 adult members.

Application

Typical Requirement of Hot Water at 60OC.

Household bathing using buckets

10-20 liters per person per bath.

Household bathing using shower with a mixing tap

20-30 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


Note: All the estimates are given for hot water at 600C. This hot water has to be mixed with cold water to bring down its temperature to endurance limits. Mixing will also increase quantity to actually required values.


A large solar water heating system

  • The total cost of a solar water heating system is dependent upon many things. These include, the capacity, the kind of back-up used, the materials used for the inner and outer tanks, the length of distribution piping required to take hot water to the bathrooms, and not insignificantly, the brand value.
  • Typically, for an Indian make system with single BIS approved flat plate collector of 2 sq. m area, the current market costs are reported to be in the range of Rs. 15,000-20,000, excluding the distribution piping. However this range is indicative, and could vary from manufacturer to manufacturer.

  • The basic requirement for functioning of a solar system is availability of unobstructed sunlight for the whole day.
  • Typically, domestic solar water heating systems are installed on the roof of the house.
  • The collectors of the system have to face the sun and hence should be oriented due south for maximum interception of sunlight. Thus there should be no obstruction to sunlight in the south, west and east directions in that order of preference (an arc of about 120 °, 60° both sides of the southern direction should ideally be shadow free).
  • As a thumb rule, the requirement of shadow free area is around 3 sq. m for each 1 x 2 m collector used.
  • It is best is the area is flat, away from rain water drains and as far as possible, close to the bathrooms where hot water is to be supplied.
  • Cold water should be available at the height of around 2.5 m from the base of the system.

  • The system can also be installed on a bracket on a south facing wall, close to the bathrooms. However, the installation is difficult and leads to extra cost.
  • The fixing of the system to the bracket should be proper.
  • Access to the system for repairs should also be ensured.
  • Cost of hot water distribution piping could be reduced in such installations as the system will be close to the use point.

  • A regular cold water supply, at a height of around 2.5 m is necessary for functioning of the solar water heater system.
  • In case cold water is not available continuously, a separate cold water tank may have to be installed, at least equal in capacity to the capacity of the solar water heater system.
  • In case cold water supply is interrupted during the day, the heating of water by the solar water heater system may not be affected significantly.  However, hot water cannot be drawn for use till the water supply is restored.

  • The heating of water by the solar system will obviously be affected.
  • If it is so cloudy that energy received from the sun is almost zero, the output of solar collectors also will be nil.
  • On partially cloudy days some output can be expected.
  • But, the system can be designed with a suitable electrical back up heater to take care of hot water demand on cloudy days.


The table below gives approximate likely electricity and money savings for typical 100 liters per day solar water heating systems located in different parts of the country.
Likely savings of electricity and money by use of a 100 liters domestic solar water heater (using 2.0 sq.m collector area)


Northern Region

Eastern Region

Southern Region*

Western Region*

Expected no. of days of use per year

200 days

200 days

250 days

250 days

Expected yearly electricity saving with use of full capacity, kwh

950

850

1200

1300

Monetary savings at different prices of electricity, Rs/year

Rs. 4/kwh

3800

3400

4800

5200

Rs. 5/kwh

4750

4250

6000

6500

Rs. 6/kwh

5700

5100

7200

7800


The use pattern and savings for southern region pertains to the typical climate of Bangalore, while those for western region relate typically to Pune climate.


Typical solar water heaters made using materials as per BIS specifications could last for 15 -20 years depending upon the general maintenance.


No electricity is required for any operation in solar water heating systems. However, in case a back up heater is provided to take care of hot water requirement during cloudy days, electricity will be required.

  • Hot water produced by the solar heating system during the day is stored in an insulated storage tank. The insulation of the tank is such that water should remain hot without significant drop in temperature for around 24 hrs.
  • Thus water heated during the previous day should be available for use in the next morning.


The Central Government through its Ministry of New and Renewable Energy provides soft loans for installation of domestic solar water heating systems. These loans are being provided through nationalized banks at minimal interest rates


There are more than 50 BIS approved suppliers of domestic solar water heating systems in the country.


Domestic solar systems do not require any special operational skills. However, if following are observed, the efficiency of the systems will be maintained at a high level:

  • Try to consume most of the heated water at one time - either in the morning or in the evening. Frequent on and off of the hot water tap would lead to reduced electricity savings.
  • If an electrical back up is provided in the tank, set the thermostat at the lowest acceptable temperature.
  • In the north Indian climate, hot water may not be used for bathing in summers. If the system is to be put totally out of use, it should be drained of water and the collector should be covered.
  • Alternatively, if the hot water requirement remains in summers also, though at a reduced level, cover the collector partially.
  • Dust deposition on the collector would reduce its efficiency. Try to clean it at least once in a week.

  • Domestic solar water heating system does not need significant maintenance requirements. Occasional leakages in the plumbing could be easily repaired by common plumbers.
  • In case quality of water is hard, scale deposition in the collectors may result over the years. This may require descaling with acids for which it is best to contact the suppliers.
  • Broken glass may also have to be replaced by the suppliers.
  • If outside exposed surfaces are painted, the paint may have to be redone every 2-3 years to prevent corrosion of the surfaces.


Problem faced

Probable cause

No water in the hot water tap

  • No cold water supply
  • Valve at the outlet of system closed
  • Air lock in the pipes

Water not heated at all, although cold water flow is normal

  • Consumption of hot water may be too high; Check use points and use pattern
  • Collector may be shaded
  • No flow of  water through the Collector as it might be choked due to scaling; Get it checked from the manufacturer

Water not hot enough or sufficient quantity of hot water is not available

  • Cloudy weather
  • Consumption too high
  • Frequent on-off of hot water tap
  • Collector dirty
  • Vapour lock in the collector which can be removed by allowing it to cool and draining the system
  • Partial choking of the collector

Little quantity of boiling hot water is received

  • Vapour locking in the collector
  • Pinched inlet/outlet pipes

Frequently Asked Questions on Solar water pumping system

Solar cell is a primary device for solar photovoltaic systems since it converts solar radiation into electricity. Solar cells are made of fine sheets of silicon wafers, cut to the required shape. Number of such SPV cells are connected to design a SPV module of required power. These modules can be further coupled to form SPV array, to get desired power output to run the electrical appliances such as pumps.

The subsidized cost of installation and commissioning of a SPV water pumping system  will vary between Rs. 1,90,000 to Rs.2,70,000 depending upon the supplier and model.

  • Soft loans are available at an interest rate of 5% per annum through Indian Renewable Energy Development Agency Limited (IREDA).
  • The maximum amount of soft loan will be up to 90% of the unsubsidized part of the SPV water pumping system.
  • In case, the subsidy is not availed, the loan amount can be up to 90% of the sale price of the system.
  • The principal with the interest is repayable in 10 years (including one year moratorium) starting at the end of first year of sanction of the loan.

Agriculture and related uses such as horticulture, animal husbandry, poultry farming, high value crops, orchards, silviculture, fish culture, salt farming, drinking water etc. Battery charging is disallowed under this programme.

An individual farmer, non-governmental organization, cooperative society, corporate body, institution and govt. Dept. etc.

A SPV water pumping system is available with a photovoltaic array of capacity in the range of 200 watts to 3000 watts. (Capacity of motor pump set is from 0.5 hp to 2 hp)

  • A SPV water pumping system is expected to deliver a minimum of 65,000 liters per day for a 900 watts panel and 135,000 liters per day for a 1800 watts panel from a depth of 7 meters on a clear sunny day.
  • In case of deep well submersible pumps, the water output shall be a minimum of 45000 liters from 1200 watts panel.
  • The discharge from the pump would vary with the intensity of the sunrays from morning till evening. It would be maximum around noontime. The water output from the pumping would considerably drop with the increase in the depth from which water needs to be pumped.
  • The water output from a SPV pumping system must be confirmed by the supplier of the system, when he/she installs the system in the field.

The SPV water pumping system can be used to irrigate 0.5-6 hectares, when the water is to be pumped from a depth of 10 meters. However, water table, type of soil and water management are other factors that influence the areas that can be irrigated using the system.

Considering the average peak watt of SPV water pumping system to be 900 watts, the following table indicates critical irrigation area for different crops with the irrigation method used:


S. No.

Crops

Critical command area in ha.

Irrigation method

1

Year round vegetables cultivation

1.00

Surface

2

Chillies/ sorghum/ groundnut

1.41

Surface

3

Paddy nursery

0.70

Surface

4

Garlic

2.08

Micro sprinkler

5

Cucumber

1.82

Drip

6

Groundnut

1.97

Micro sprinkler

7

Grapes

2.14

Drip

8

Lime

4.89

Drip

9

Banana

2.36

Drip

10

Pomegranate

7.32

Drip

  • PV arrays are supplied with a performance guarantee for a period of ten years.
  • Motor pumps sets and accessories are supplied with a guarantee period of two year.
  • Further, minimum spare parts of the pump and other systems are also supplied with the SPV water pumping system for trouble free operation for a period of 3 years.