Solar Water Heating System

Introduction to solar water heaters

Solar water heating system is a device that uses solar energy to heat water for domestic, commercial, and industrial needs. Heating of water is the most common application of solar energy in the world. A typical solar water heating system can save up to 1500 units of electricity every year, for every 100 litres per day of solar water heating capacity.

Parts of the Solar Water Heating System

• A solar water heating system consists of a flat plate solar collector, a storage tank kept at a height behind the collector, and connecting pipes.
• The collector usually comprises copper tubes welded to copper sheets (both coated with a highly absorbing black coating) with a toughened glass sheet on top and insulating material at the back. The entire assembly is placed in a flat box.
• In certain models, evacuated glass tubes are used instead of copper; a separate cover sheet and insulating box are not required in this case.

Working of a solar water heater

• The system is generally installed on the roof or open ground, with the collector facing the sun and connected to a continuous water supply.
• Water flows through the tubes, absorbs solar heat and becomes hot.
• The heated water is stored in a tank for further use.
• The water stored in the tank remains hot overnight as the storage tank is insulated and heat losses are small.

Uses of solar water heater

• SWHs can be used at homes for producing hot water that can be used for bathing, cleaning, and washing. Solar water heaters (SWHs) of 100-300 litres capacity are suited for domestic application. Larger systems can also be used for a variety of industrial applications. Hot water at 60-80^oC could be obtained through use of solar water heaters.
• Fuel Savings: A 100 litres capacity SWH can replace an electric geyser for residential use and saves 1500 units of electricity annually.
• Saves cost on power generation - The use of 1000 SWHs of 100 litres capacity each can contribute to a peak load saving of 1 MW.
• Environmental benefits - A SWH of 100 litres capacity can prevent emission of 1.5 tonnes of carbon-dioxide per year.
• Pay back period - SWHs have a life span of 15-20 years. The pay back period is about 3-4 years when electricity is replaced, 4-5 years when furnace oil is replaced and 6-7 years when coal is replaced.

FAQs : Domestic Solar Water Heating

Why should we opt for solar water heating?

• 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

Is it being used in our country?

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

How do solar water heaters work?

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

Working of a solar water heater

• 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).

What is a flat plate collector?

• 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

What are the types of collectors used in solar water heating systems?

• 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

What should we look for while buying a flat plate collector?

• 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.

What are the desirable characteristics of hot water storage tank?

• 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.

How does one identify a good solar water heater?

• 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.

How big a system should one purchase?

• 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.
  
  
  

ESTIMATES OF REQUIREMENTS OF HOT WATER - SOME USEFUL THUMB RULES

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

What is the cost of a solar water heater?

• 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.

What are the site requirements for installation of a solar water heating system?

• 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.

Can the system be installed somewhere other than on the roof?

• 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.

What would happen if water supply is irregular ?

• 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.

What would happen on cloudy days?

• 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.

How much electricity and money can be saved?

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)

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.

What is the expected life of a solar water heater?

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

Is any electricity required for operation of the solar water heater?

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.

How long will the water heated by solar energy remain hot in the tank? Will one get hot water in early morning when sun is not there?

• 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.

Are there any financial incentives from the government on these systems?

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

Who are the suppliers of domestic solar water heating systems?

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

What are the operational requirements?

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.

What are the maintenance requirements?

• 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.

Trouble Shooting tips for Domestic Solar Water Heaters