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A Solar Water Heater is a device which provides hot water for bathing, washing, cleaning, etc. using solar energy. It is generally installed at the terrace or where sunlight is available and heats water during day time which is stored in an insulated storage tank for use when required including mornings.

A solar thermal collector collects heat by absorbing sunlight. A collector is a device for capturing solar radiation. Solar radiation is energy in the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short) wavelengths. The quantity of solar energy striking the Earth\\\'s surface (solar constant) averages about 1,000 watts per square meter under clear skies, depending upon weather conditions, location and orientation.

The term \"solar collector\" commonly refers to solar hot water panels, but may refer to installations such as solar parabolic troughs and solar towers; or basic installations such as solar air heaters. Solar power plants usually use the more complex collectors to generate electricity by heating a fluid to drive a turbine connected to an electrical generator.[1] Simple collectors are typically used in residential and commercial buildings for space heating.

Heat collectors

Solar collectors are either non-concentrating or concentrating. In the non-concentrating type, the collector area (i.e., the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation). In these types the whole solar panel absorbs light. Concentrating collectors have a bigger interceptor than absorber. Flat-plate and evacuated-tube solar collectors are used to collect heat for space heating, domestic hot water or cooling with an absorption chiller

Flat plate collectors

Flat plate thermal system for water heating deployed on a flat roof.

Flat-plate collectors, developed by Hottel and Whillier in the 1950s, are the most common type. They consist of (1) a dark flat-plate absorber, (2) a transparent cover that reduces heat losses, (3) a heat-transport fluid (air, antifreeze or water) to remove heat from the absorber, and (4) a heat insulating backing. The absorber consists of a thin absorber sheet (of thermally stable polymers, aluminum, steel or copper, to which a matte black or selective coating is applied) often backed by a grid or coil of fluid tubing placed in an insulated casing with a glass or polycarbonate cover. In water heat panels, fluid is usually circulated through tubing to transfer heat from the absorber to an insulated water tank. This may be achieved directly or through a heat exchanger.

Solar water heating systems are most likely to be cost effective for facilities with water heating systems that are expensive to operate, or with operations such as laundries or kitchens that require large quantities of hot water. Hot water solar systems require heat exchangers due to contamination possibilities and in the case of unglazed collectors, the pressure difference between the solar working fluid (water) and the load (pressurized cold city water). When heating hot water we are actually heating cold to warm and warm to hot. We can heat cold to warm as efficiently with unglazed collectors as we can heat warm to hot with high temperature collectors.


Q.No.1.  What is Solar Water Heater?

Ans.  A Solar Water Heater is a device which provides hot water for bathing, washing, cleaning, etc. using solar energy. It is generally installed at the terrace or where sunlight is available and heats water during day time which is stored in an insulated storage tank for use when required including mornings.

Q. No. 2. How does it work?

Ans. A Solar Water Heater comprises of an array of solar collectors to collect solar energy and an insulated tank to store hot water. Both are connected to each other. During the day time, water in solar collectors gets heated which is either pumped or flown automatically on thermosyphon principle to the storage tank. Hot water then stored in the tank can be used for various applications.

Q.No. 3. What are different types of Solar Water Heaters?

Ans. Two types of Solar Water Heaters are available; one based on flat plate collectors and the other based on evacuated tube collectors.   Flat plate collector (FPC) based systems are of metallic type and have longer life as compared to Evacuated tube collector (ETC) based system because ETCs are made of glass which are of fragile in nature. Both these systems are available with and without heat exchanger. They can also work with and without pump. Systems without pump are known as thermosyphon systems and those with pump are known as forced circulation systems.

Q. No. 4. Which type of solar water heater is suitable for different places/ category of users?

Ans. ETC based systems are cheaper than FPC based system. They perform better in colder regions and avoid freezing problem during sub-zero temperature. FPC based systems also perform good with anti-freeze solution at sub zero temperature but their cost increases.  In other regions, both perform equally good. Systems working on thermosyphon principle are simple and relatively inexpensive. They are suitable for domestic and small institutional  applications, provided water quality is good and it doesn’t have large chlorine contents.

Forced circulation systems are generally preferred in industries or large establishments.

At places where water is hard and have larger chlorine content, if FPC based system is being installed, it must be with heat exchanger as it will avoid scale deposition in copper tubes of solar collectors which can block the flow of water as well reduce its thermal performance. ETC based systems will not block the flow of water but its performance may go down due to deposition of salt contents on inner surface of glass tubes, which could be cleaned easily once in a year or so.

Q. No. 5. What is the approximate cost of solar water heater?

Ans. Cost of solar water heater depends on size and type of system installed.

Smallest size of a system is 100 liter per day, which means that it can deliver 100 litres of hot water in a day at 60 C.  A 100 lpd capacity system is sufficient for a family of 3-4 members and it may cost Rs. 15,000 to Rs.22,000 in planes depending on the type of system. In hilly & N-E region, the cost may be 15 to 20% more.

The system cost does not include the cost of cold water tank, & its stand which is required if overhead tank is not installed in a house/ building.  Cost of hot water insulated pipe line also, may be extra if number of bathrooms is more than one. Additional cost towards all these components may increase by 5 to 10%. The cost, however, does not increase linearly with increase in capacity, rather it comes down proportionately as we go for higher capacity system.

Q. No. 6. Is there any subsidy available from Government?

Ans. Yes, the Government provides subsidy to the extent of 30 to 60% to different category of users and States subject to certain benchmarks as per below:

-> General category states for all types of beneficiaries: 30% capital subsidy or loan at 5% interest on 80% of the benchmark cost

-> Special category states for domestic & non commercial categories (not availing accelerated depreciation): 60% capital subsidy or loan at 5% interest on 80% of the benchmark cost.

-> Special category states for commercial users category (availing accelerated depreciation): 30% capital subsidy or loan at 5% interest on 80% of the benchmark cost

->  Benchmark Cost :

ETC based systems : Rs. 10,000/ sq. m.

FPC based systems : Rs. 11,000/ sq. m.

Q. No. 7. How can I avail this subsidy & get the system installed at my place?

Ans. The system can be installed at net of subsidy by following ways:

Domestic Systems 

-> Through State Nodal Agency - Contact respective state nodal agency in your state who will visit the site, provide information on cost, size & other details of system required and get it installed at net of government subsidy through some manufacturers.

-> Through Accredited Channel Partners of MNRE - Contact any of them and get the system directly installed from him at net of subsidy.

Institutional Systems Ø Through State Nodal Agency -In same fashion as above by inviting tenders and placing order to selected manufacturer 

-> Through DGS&D rates -

-> Through Accredited Channel Partners - In same fashion as above.

Q.No. 8. Why should I go for a Solar Water Heater? What do I save from it?

Ans. A 100 litre per day capacity system suitable for 3-4 people can save upto 1500 units of electricity in a year, depending on hot water used. It can also save around 140 litres of diesel in an establishment using oil fired boiler besides reducing green house gas emissions in the atmosphere. Higher capacity systems will save higher amount of electricity/fuel oil besides reducing higher amount of GHG emissions.

Electricity is expensive and is not available due to power cuts in many areas when required for heating water. Solar Water Heater, since it stores hot water in an insulated tank, provides water all the time when required.

Fuel oil is also expensive and creates pollution.

Storing the fuel oil for long term use in commercial establishments is another problem.

Q.No.9. What happens on cloudy/rainy day? Do I still get hot water?

Ans. On cloudy days also, if it is for a day or two, you still get warm water as water gets heated due to diffused radiation available in the atmosphere. The system, however, is either connected to an electric geyser in the house or an electrical back-up is provided in the storage tank of the system which is switched on when water is not sufficiently hot. So, you get hot water all the time even on rainy days.

Q.No.10.How do I decide about the size/capacity of the system to be installed?

Ans. For a house with one bathroom and 3 to 4 members, 100 liter per day capacity system should be sufficient. For more numbers of bathrooms, the capacity will increase accordingly due to pipe losses & more number of family members. Generally the capacity is decided based on hot water required in mornings for bathing. If the usage is in evening & at other times also, the capacity is decided accordingly.

Q.No.11.How do I assure that a good quality system is installed at my house?

Ans. Ministry has laid down some minimum technical requirements for installation of

solar water heating systems in the field. These have been made mandatory for manufacturers and suppliers. These requirements are have been prepared in such a way that even a lay man can also check them and ensure that those are being adhered to by the manufacturers/suppliers. In case any manufacturer/supplier is found not sticking to these  requirements, he may be blacklisted if informed to the Ministry.

Q.No.12. Are there any maintenance requirements?

Ans Domestic solar water heating system do 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 de-scaling 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.

Solar Water Heating Systems We are blessed with Solar Energy in abundance at no cost. The solar radiation incident on the surface of the earth can be conveniently utilized for the benefit of human society. One of the popular devices that harness the solar energy is solar hot water system (SHWS).

A solar water heater consists of a collector to collect solar energy and an insulated storage tank to store hot water. The solar energy incident on the absorber panel coated with selected coating transfers the hat to the riser pipes underneath the absorber panel.

The water passing through the risers get heated up and is delivered the storage tank. The re-circulation of the same water through absorber panel in the collector raises the temperature to 80 C (Maximum) in a good sunny day. The total system with solar collector, storage tank and pipelines is called solar hot water system.

Broadly, the solar water heating systems are of two categories.

They are : closed loop system and open loop system. In the first one, heat exchange

rs are installed to protect the system from hard water obtained from borewells or from freezing temperatures in the cold regions. In the other type, either thermosyphon or forced circulation system, the water in the system is open to the atmosphere at one point or other. The thermosyphon systems are simple and relatively inexpensive. They are suitable for domestic and small institutional systems, provided the water is treated and potable in quality. The forced circulation systems employ electrical pumps to circulate the water through collectors and storage tanks.

The choice of system depends on heat requirement, weather conditions, heat transfer fluid quality, space availability, annual solar radiation, etc. The SHW systems are economical, pollution free and easy for operation in warm countries like ours.

Based on the collector system, solar water heaters can be of two types.

Flat Plate Collectors (FPC) based Solar Water Heaters

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. There are 60 BIS approved manufacturers of Solar Flat Plate Collectors.

Evacuated Tube Collectors (ETC) based Solar Water Heaters Evacuated Tube 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. There are 44 MNRE approved ETC based solar water heating suppliers.

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 and institutional establishments.  

Internationally the market for solar water heaters has expanded significantly during the last decade.

Salient Features of Solar Water Heating System

Solar Hot Water System turns cold water into hot water with the help of sun’s rays.

- Around 60 deg. – 80 deg. C temperature can be attained depending on solar radiation, weather conditions and solar collector system efficiency

- Hot water for homes, hostels, hotels, hospitals, restaurants, dairies, industries etc.

- Can be installed on roof-tops, building terrace and open ground where there is no

shading, south orientation of collectors and over-head tank above SWH system

- SWH system generates hot water on clear sunny days (maximum), partially clouded (moderate) but not in rainy or heavy overcast day

- Only soft and potable water can be used

- Stainless Steel is used for small tanks whereas Mild Steel tanks with anti-corrosion coating inside are used for large tanks.

- Solar water heaters (SWHs) of 100-300 litres capacity are suited for domestic application.

- Larger systems can be used in restaurants, guest houses, hotels, hospitals, industries etc.

Fuel Savings :

A 100 litres capacity SWH can replace an electric geyser for residential use and saves 1500 units of electricity annually.

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. Flat plate collector (FPC) based solar water heater Evacuated tube collector (ETC) based solar water heater.

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