Trainers Textbook - Solar Thermal

7/31/2019 Trainers Textbook - Solar Thermal

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Study materials in Renewable Energy Areasfor ITI students

Solar Thermal Systems Module Trainers Textbook

Ministry of New and Renewable EnergyGovernment of India


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Content Development, Editing, Design and Layout New Concept Information Systems Pvt. LtdE-mail: [email protected] www.newconceptinfo.com


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1Study Materials in

Renewable Energy Areas for ITI students

Study Materials in Renewable Energy Areasfor ITI students

Trainers TextbookSolar Thermal Systems Module

(Solar cookers and Solar water heaters)

Ministry of New and Renewable Energy

Government of India


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Unit 1. Introduction to Solar Energy 51.1 Solar energy 51.2 The need to use solar energy 51.3 Basic principle 51.4 Factors to consider for using solar energy 61.5 Measuring Solar Energy 71.6 Applications of solar energy 7

Unit 2. Solar Thermal Energy 92.1 Solar collectors 92.2 Working principle of a solar collector 92.3 What is heat? 10

2.3.1 Understanding Heat Movement 102.3.2 Conduction 102.3.3 Convection 102.3.4 Radiation 10

2.4 Solar thermal energy applications 11

2.4.1 Water heating 112.4.2 Solar cooking 112.4.3 Solar drying 122.4.4 Desalination / Distillation 122.4.5 Solar Pasteurisation 122.4.6 Solar thermal energy in architecture 13

Unit 3. Solar Cookers 143.1 Solar cooker 143.2 Basic principles of solar cooking 143.3 Advantages and limitations of using a solar cooker 143.4 Box solar cooker 15

3.4.1 Description 153.4.2 Precautions to take 173.4.3 Raw Materials Required For Fabrication 173.4.4 Standard testing procedure and quality control 173.4.5 Maintenance and servicing of box solar cookers 183.4.6 Customer education 18

Contents


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4Trainers textbook forSolar Thermal Systems module

3.5 Dish solar cooker 193.5.1 Description 203.5.2 Design 203.5.3 Material required for fabrication: 203.5.4 Maintenance and servicing of dish solar cookers 213.5.5 Customer education 21

3.6 Community Solar Cooker (for indoor cooking) 223.6.1 Description 223.6.2 Design 223.6.3 Material required for fabrication 233.6.4 How it Works 233.6.5 Customer education 24

Unit 4. Solar Water Heaters (SWH) 25

4.1 Solar Water Heating Systems 254.1.1 Basic Working Principle 25

4.2 Advantages and limitations 254.3 Details of Solar Water Heaters 26

4.3.1 Description 264.3.2 Design 27

4.4 Evacuated Tube Collector (ETC) based solar water heater 274.5 Domestic solar water heating system 28

4.5.1 Thermosyphon system 284.5.2 Forced circulation system 29

4.6 Site selection 314.7 Procedure for Operation and Maintenance 314.8 Operation of the Solar Water Heater 314.9 Maintenance of the SWH 324.10 Customer education 324.11 BIS Standards 334.12 Manufacturing of components of Solar Water Heater 33

Unit 5. Practicals 365.1 Engineering drawings: 36

5.2 Cross sections of solar water heater (SWH) 375.3 Installation Layouts 38

5.3.1 1500lpd Solar Water Heating System (SWHS) Layout at 60 O c 385.3.2 1500lpd SWHS layout 70 O c 395.3.3 3000lpd SWHS layou: 40

5.4 Calculations 41

Annexure 42


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Time: 1 hour

Methodology: Lecture

1.1 Solar energy

Solar energy is dened as the suns radiation that reaches the earth. It is the most readily available source of energy. The sun is the earths power station and the source of all energy on our planet. We use the solar energy every day in many different ways. When we hang washed clothes outside to dry in the sun, we are using the solar heat to do work, drying

our clothes. Plants use the solar light to make food. Similarly, solar panels absorb theenergy of the sun to provide heat for cooking and for heating water.

1.2 The need to use solar energy

With the growing population and fast pace of development, energy is becoming more expensive and our cities and towns face a major power crisis. The reality is thatresources like coal, oil and natural gas will not be around forever. We all realise today,that we need alternatives sources of energy that are renewable. Renewable energy isderived from natural resources such as sunlight, wind, tides and geothermal heat.Solar energy or energy powered by th ic Pri e sun is one of the most promising future

renewable energy sources.

1.3 Basic principle

Solar energy consists of light and heat emitted by the sun, in the form of electromagneticradiation. Technology today helps to capture this radiation and turn it into usable formsof solar energy - such as heating or electricity.

Unit 1. Introduction to Solar Energy

Learning Outcome At the end of the session, the students will have an overview of solar energy.


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Thus energy from the sun can be categorized in two ways:(1) In the form of heat (or thermal energy), and(2) In the form of light energy.

Solar energy can be converted to thermal (or heat) energy and used to: Heat water for use in homes, buildings, or swimming pools. Heat vessels for cooking food Heat spaces inside greenhouses, homes, and other buildings.

Solar thermal technologies are devices that use the solar heat energy to heat substances(such as water or air) for applications such as solar cooker, water heaters and poolheating. There are a variety of products available in the market that use solar thermalenergy. Often the products used for this application are called solar thermal collectors.

Solar energy can be converted to electricity in two ways:

Photovoltaic (PV devices) or solar cells change sunlight directly into electricity. Solar Power Plants - indirectly generate electricity when the heat from solar thermal

collectors is used to heat a uid which produces steam that is used to powergenerator.

1.4 Factors to consider for using solar energy

A few factors which need to be looked at when determining the viability of solar energy in any given location are the following: Geographic location Time of day Season Local landscape Weather conditions

Geographic locationGeographic location plays a vital role in harnessing solar energy.

India is located in the equatorial sun belt of the earth, thereby receiving abundant radiantenergy from the sun. In most parts of India, clear sunny weather is experienced 250 to300 days a year. The country has the capacity to receive 4500 trillion kWh of pure solarenergy each year. This allows solar energy in India to be a viable option.

The highest annual global radiation is received in Rajasthan and northern Gujarat. InRajasthan, large areas of land are barren and sparsely populated, making these areassuitable as locations for large central power stations based on solar energy.


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Time of Day Solar energy is at its peak at mid moon. A solar energy system works in the morning andin the late afternoon. It however reaches its maximum efciency around noon time.

Season A solar energy system is more efcient during summers than in winters when the daysare shorter.

Local landscape A solar energy system has to be installed in an open area unaffected by shade. If evena small section of the system is shadowed, the efciency reduces. This is the reason why solar energy systems are placed mostly on roofs. Even in this case, care needs tobe taken to ensure that no tall trees or taller neighbouring buildings cast a shadow onthe system.

Weather conditions Weather conditions have a large impact on the energy output of the solar energy system.Cloud cover, rain, snow, fog and smog all reduces the amount of sunlight that reaches thesystem. The system will work but its efciency will be greatly reduced.

1.5 Measuring solar energy

Radiation data (the amount of solar energy available at a given location) for solarelectric (photovoltaic) systems is often represented as kilowatt-hours per square meter(kWh/m 2). Direct estimates of solar energy may be expressed as watts per square meter(W/m 2).

1.6 Applications of solar energy

There are a variety of technologies that have been developed to take advantage of solarenergy. The technologies are classied as passive or active depending on the way they capture, convert and distribute sunlight. Active solar techniques use photovoltaic panels,pumps, and fans to convert sunlight into useful outputs. Passive solar techniques usessunlight for energy without active mechanical systems. Such technologies convertsunlight into usable heat (water, air, and thermal mass), cause air-movement forventilating, or store heat for future use, without the assistance of other energy sources. While costs associated with operating these products may be limited or nil, maintenancecosts are very low.

This module on Solar Thermal Systems focuses on two applications of passive solartechnology, namely solar cookers and solar water heaters. These systems are discussedin the next two units.


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The earth receives energy from the sun mostly in the form of visible light andnearby wavelengths. About 50% of the suns energy is absorbed at the earthssurface. Like all bodies with a temperature above absolute zero the earths surfaceradiates energy in the infrared range. Greenhouse gases in the atmosphere absorbmost of the infrared radiation emitted by the surface and pass the absorbed heat toother atmospheric gases through molecular collisions. The greenhouse gases alsoradiate in the infrared range. Radiation is emitted both upward, with part escaping to space, and downward toward earths surface. The surface and lower atmosphereare warmed by the part of the energy that is radiated downward, making our lifeon earth possible

Green House Effect:


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Methodology: Lecture

Time: 2 hours

2.1 Solar collectors

Solar thermal energy is the technology used for harnessing solar energy for thermalenergy (heat). A solar thermal collector captures the radiant energy from the Sun andconverts it into heat. The most basic type of collector is the solar cooker. And the mostcommon type of solar collector is the roof-mounted domestic hot water system. Solar

collectors are a cheap and effective means of converting sunlight into thermal heat. Thismodule discusses these two types of solar thermal collectors since they are commonly used in India.

2.2 Working principle of a solar collector

A solar collector works on the principle of converting solar energy into heat by taking advantage of a process known as the greenhouse effect .

The basic idea is that the solar energy passes through a layer of glazed glass where itis absorbed by the underlying material. The solar energy excites the molecules in the

underlying material resulting in heat. The glazing of the glass prevents the heat fromescaping, thereby effectively capturing the heat.

Once that heat is captured we can put it to good use. But in order to use it, we rst needto understand some of the basic principles of heat.

Unit 2. Solar Thermal Energy

Learning Outcome At the end of the session the students should have relevant knowledge andunderstanding of solar thermal systems


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2.3 What is heat?

Heat is simply a form of energy associated with the motion of molecules.

When the electromagnetic waves coming from the Sun hit an object, they excite themolecules of that object causing them to move. This molecular movement is heat .

Heat is always moving from higher to lower temperatures until the temperatures areequal. This is known as heat transfer . If you place two objects next to each other, the warmer object will cool down as its heat is transferred to the cooler object. The coolerobject in turn will warm up.

This heat transfer is driven by the difference in temperatures of the objects. The heat transfer rate is proportional to the difference in temperature. The larger the difference intemperature between the objects, the faster the heat moves.

2.3.1 Understanding heat movementTo really understand solar thermal energy , you need to understand about heat movement.This is especially important for passive solar energy applications.

There are three basic physical ways that heat moves: Conduction Convection Radiation

2.3.2 ConductionConduction is the transfer of heat through a solid material, or from one material toanother where their surfaces are touching. Heat is conducted more easily through a solidmaterial than through layers of material, even when the layers are held together tightly.This is important as the conduction of heat through building materials is a major sourceof heat loss.

2.3.3 ConvectionConvection is the transfer of heat by a moving uid, usually air or water. Natural convectionis caused by the heating and cooling of the air in a room as it contacts objects. As air is warmed, it expands resulting in it having a lower density than the cooler air around it.Since its density is lower than the cool air, it will rise. As the air cools, its density increasesand it will sink, starting the process all over again. This movement of air is known asconvection current.

2.3.4 RadiationRadiation is the transfer of energy via electromagnetic waves. The transfer of energy fromthe sun across nearly empty space is accomplished primarily by radiation. Radiation


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occurs without the involvement of a physical substance as the medium. The sun emitsmany forms of electromagnetic radiation in varying quantities.

Darker-coloured objects absorb more visible radiation, whereas lighter-colored objectsreect more visible radiation.

2.4 Solar thermal energy applications

2.4.1 Water heating The most common use for solar thermal technology is for domestic water heating.Hundreds of thousands of domestic hot water systems are in use throughout the world.

2.4.2 Solar cooking Solar cookers fall into two main categories solar box and direct solar concentrators.The basic design for a solar box is that of a box with a glass cover. The box is lined with

The gure given below summarises the different mechanismsof energy transfer.

Image adopted from The Atmosphere by F. Lutgens and E. Tarbuck, Prentice-Hall, Inc


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insulation and a reective surface is applied to concentrate the heat onto the pots. Theother approach is to reect the suns rays onto a pot, often with a parabolic dish. The potscan be painted black to help with heat absorption.

The main advantage to solar cookers is that wood does not need to be purchased orcollected, which is often a very time consuming activity for women.

Many variations of solar cooker have been developed from the very basic reectivecardboard sheet box to the very sophisticated large-scale institutional and commercialsolar cookers now being used in India.

2.4.3 Solar drying Controlled drying is required for various crops and products, such as grain, coffee,tobacco, fruits vegetables and sh. Solar thermal technology can be used for this

purpose. The main principle of operation is to raise the heat of the product, which isusually held within a compartment or box, while at the same time passing air throughthe compartment to remove moisture. The ow of air is often promoted using the stackeffect which takes advantage of the fact that hot air rises and can therefore be drawnupwards through a chimney, while drawing in cooler air from below. Alternatively a fancan be used.

Solar crop drying technologies can help reduce environmental degradation caused by the use of fuel wood or fossil fuels for crop drying and can also help to reduce the costsassociated with these fuels and hence the cost of the product.

2.4.4 Desalination / DistillationBasic solar stills can be used to purify water in remote regions where contaminated wateris present. They can be used to remove impurities such as uoride and salts to producedrinking water.

The basic still is made of a glass or transparent plastic cover and a shallow tray of water which has a black backing to trap energy. When the sun heats the water up within thestill water evaporates which then condenses on the underside of the covering glass. Theglass is at an angle so the water drains off and is captured in a trough separate to thecontaminated water.

Solar distillation can be combined with other useful functions so that a solar still may also be used for rainwater harvesting if modied slightly.

2.4.5 Solar PasteurisationIn pasteurization, water is heated to 65C for about six minutes, killing all the germs,viruses, and parasites that cause disease in humans, including cholera and hepatitis A andB. This is similar to what is done with milk and other beverages. It is not necessary to boil


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the water as many people believe. Pasteurization is not the only way to decontaminatedrinking water, but it is particularly easy to scale down so that the initial cost is low.

2.4.6 Solar thermal energy in architectureProviding comfortable buildings, while reducing the use of conventional fuels andelectricity, can be obtained through solar passive architecture. Solar Passive Architectureinvolves blending architectural principles and solar energy to design interiors whichremain warm in winter and cool in summer, thus providing year-round comfortableindoor environment. Solar designs can save up to 90% of the energy required to cool orheat a building.

The benet of solar energy is utilized through designing energy efcient buildings.Here specic attention is directed to the site and location of the dwelling, the prevailing climate, design and construction, solar orientation, placement of glazing-and-shading

elements, and incorporation of thermal mass.

As mentioned earlier we shall discuss two applications of solar energy which are solarcooker and solar water heater.


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Methodology: Lecture, AV Material, Practicals

Time: Theory 2 hours

Practicals: 4 hours

3.1 Solar cooker

Solar cooker is a device which cooks food without any conventional cooking fuel/energy like cow dung, wood, coal, gas, kerosene or electricity. It converts sunlight into heatenergy and cooks the food.

3.2 Basic principles of solar cooking

Solar cookers utilise the simple principles of reection, concentration, glazing, absorptionand the greenhouse effect to produce heat. Various types of solar cookers exist, harnessing one or more of these principles. Basically a solar cooker is an insulated container with amultiple or single glass (or other transparent material) cover. This kind of cooker dependson the greenhouse effect in which the transparent glazing permits passage of shorter

wavelength solar radiation, but is opaque to most of the longer wavelength radiationcoming from relatively low temperature heated objects. Mirrors may be used to reectadditional solar radiation into the cooking chamber. A double-walled insulated box canalso serve to hold the heat inside the cooker.

3.3 Advantages and limitations of using a solar cooker Advantages of solar cooker:1. Since the solar cooker does not consume any conventional fuel, it saves fuel.

Unit 3. Solar Cookers

Learning Outcome At the end of the session the students will be able to: develop an understanding of construction and working of different types

of solar cookers develop skills in installing, maintaining and repairing solar cookers


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2. It does not emit any smoke or smell and hence does not pollute the environment.There is thus no chance of causing cancer and affecting eyesight or lungs.

3. Since it does not require constant attention while cooking it helps saves time, whichcan be utilised for other productive work.

4. As it is a slow cooking device, it preserves the nutritional value of the food5. Simplicity of construction and operation with minimal attendance required during

the cooking process.6. It can be used for cooking 3 to 4 items at a time7. It keeps cooked food hot for 4 to 5 hours8. It produces no glare and presents no risk of res and burns. There is no possibility of

accidents like bursting of cylinders or catching re, electrical shocks, etc.9. It saves money.

Limitations of using a solar cooker:1. Slow cooking process due to low temperatures.2. It does not work in night or on a cloudy day 3. It cannot be used for frying or preparing chapattis

However, in spite of these limitations, if a solar cooker is used throughout the year it doeshelp to save substantial amount of cooking fuel and thereby contributes to savings.

3.4 Box solar cooker

3.4.1 Description:

The important parts of a box solar cooker are described below:

(Source: Solar Energy Centre, Faridabad, India)


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a.) The Outer Body The outer body of a solar cooker is made of a metal sheet or bre reinforced plastic having suitable dimensions.

b.) The Inner Cooking Box/Tray The inner cooking box is of aluminium sheet. All the ve surfaces of the inner cooking tray or the top surface of the cooking box which are exposed to the sun are coated withblack board paint which absorbs solar radiation. In the case of tray type cooker one may have to apply the black paint frequently as it comes out due to rubbing of the pots withthe coated surface during loading and unloading of the cooker.

In the case of box type cooking system, the coating is not touched and hence will not beaffected during such operations.

c.) Thermal Insulation

The space between the outer box and the inner cooking box/tray is lled with insulating material such as glass wool. This is done to prevent the heat loss from the cooker.

While selecting the insulating material, it should be remembered that the material shouldbe stable at least upto 200 C. Resin bonded insulating materials are not allowed in the solarcooker. The insulation should be in the form of a pad and should not be less than 5cm. thick.The performance will be better if the thickness of insulating material is more than 5 cm.

d.) The Double Glass Lid A double glass lid is provided on top of the cooking box/tray of the solar cooker. Thelid had length and breadth slightly greater than the cooking box/tray and is xed in

an aluminium frame maintaining a small spacing between the two glasses. This spacecontains air which acts as an insulator and prevents convective heat loss from inside.The frame is attached to the outer box by hinges. A strip of suitable insulating materialbetween the frame and the edges of the cooking box/tray prevents heat leakage when itrests on top of the cooking box/tray.

The inner glass of the double glass lid should be tough to avoid its being damaged due todaily heating and cooling effects. In box type cooker single glass cover of toughened glassis enough if the absorber is coated with selective coating material. The lid in such casesneed not have hinges and should permanently be xed since it does not have to be liftedduring loading and unloading of the cooker.

The insulating strip between the edges of the cooking box/tray and the glass lid shouldbe stable at least upto 200C and should not outgas, expand or contract between -30and 200C. Silicon rubber or EPDM is suggested for this strip. Gaskets used in pressurecookers can also be used after suitably modifying the design.

e.) MirrorMirror is used to increase the radiation input on the absorbing surface. The sunlightincident on the mirror gets reected from it and enters the box after passing through the


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glass lid. This radiation is in addition to the radiation entering the box directly and helpsto achieve better results during the cooking process by raising the inside temperature of the cooker. The reectivity of the mirror should be over 85%. The mirror could also be infour equal segments.

f.) Cooking potsThe cooking pots with cover are generally made of a suitable aluminium alloy or stainlesssteel. The pots are painted dull black on the outer surface so that they also absorbradiation directly. However, if the inner portion of the cooker is box type, the cooking pots need not be painted black.

g.) Side window A well insulated door is provided on one side of the cooker for loading and unloading when the inner cooking space is in the form of a metal box. The insulated box shouldhave no leakage when it is closed.

3.4.2 Precautions to take The surface of the cooking box exposed to solar radiation and the outer surfaces

of cooking pots should always be kept coated with black paint/selective coating material.

There should be no leakage of hot air through the joints or any other portion of thecooker.

The lid with double glass system should be perfectly sealed so that water vapour, donot enter into the space between the glass surfaces and get condensed reducing thetransmission of sunlight through the lid.

3.4.3 Raw Materials Required For FabricationG.I. sheet, Aluminium sheet, M.S. Channels, Glass, Mirror ,Asbestos bre Sheet, Glass wool,Caster wheel, Black board paint, Hinge, lock, Screws and other miscellaneous items.

Details of Machinery and Equipment Required For Fabrication

Hand saw, Hand shear, Portable drilling machine, Hammer, Screwdriver, Pliers, Measuring tape Painting brush etc.

3.4.4 Standard testing procedure and quality controlRoutine Test:i) Inner box leakage test (by lling with water, then joints are examined)ii) Cover gasket leakage test (By inserting pieces of papers in four positions in each

side of the cooker below the cover plate, after properly tightening the cover plate, thepaper pieces should exhibit a rm resistance at the time of withdrawal by hand)

iii) Leakage test of upper side of cover plate (by pouring a thin lm water on the coverplate and then by examining the cover plate for any sign of water entry between theglass sheets)


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iv) Leakage test of lower side of cover plate (after boiling of water in cooking pots by solar heating and then by keeping the cooker in shaded location to allow the vapourto condense, there should be no sign of vapour entry between inner and outer glasssheets of the cover plate)

V) Temperature test for paint on inner box, cooking tray and cooking pots at 160 degreeCelsius for one hour and temperature tests of cover gasket, cover glass and insulationat 200 degree Celsius in electric oven. There should be no sign of damage or crack.

Degradation test A minimum of 30 clear sunny days are required to complete the degradation test. Thecooker is kept in the sun with empty cooking pots inside and degradation, if any, of various components is studied carefully. There should not be any degassing from any insulating material, sealing gaskets, paints, etc. Collection of water vapours within thespace between the glasses of the lid, change in colour of the cooking pots, deposition of water vapours or any other material on the glass lid on exposing the cooker to outsideatmospheric conditions in the sun is not allowed.

3.4.5 Maintenance and servicing of box solar cookers Maintenance required is an occasional coat of black paint on the inside of the cooker

and on the outer surfaces of the cooking pots. Replacement of gasket Replacement of glass and mirror

3.4.6 Customer education

A. Box solar cookers available in the marketBox solar cookers normally measure about 60 60 cm, and each are provided with fourpots. Smaller models are also available. An electrical heater to serve as a back-up during non-sunshine hours is available in some models. These cookers can be procured frommanufacturers, suppliers, district and head ofces of state agencies, as well as from Adityasolar shops. National standards have been established for box type solar cookers by the BIS.

The cost of a box solar cooker varies from Rs 1200 to Rs 2500 depending on its size andfeatures. A normal-size box cooker is sufcient for a family of four or ve members. Ithas a life of 1012 years. Its payback period is 23 years depending upon the extent of use and place of use.

B. Cooking TimeCooking time is normally about 2 to 2 hours depending upon the kind of food and theseason of the year. It also depends on local conditions such as ambient temperature,time of the day, latitude of the place, etc.

C. Cooking tipsBlack pots work a lot better than silver pots. The pot needs to absorb as much light aspossible and silver tends to reect the light. Dull or matt nishes absorb more light thanshiny surfaces.


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Pots with close tting lids keep the heat in and help the cooking process. Placing thestove in a sheltered area stops the wind from cooling the outside of the pot

D. Trouble Shooting and Actions to be taken

Symptom Problem ActionFood notcooked even onclear sunny day

Black coating from thecooking box and from potshas faded or peeled off

Paint the cooking box andpots with jet black paint or therecommended paint.

Glass lid is broken Replace the broken glass lidMirror is broken or spoilt Replace the mirror or silver the

same mirrorHeat is leaking from the joints Find out the heat leakage points

and seal themDust has collected on the glasslid and mirror

Clean the glass lid and mirror

The absorber box has becomedirty

Clean the absorber properly

Lid is being opened frequently Minimise opening the lidDirection of the box is notproper

Keeps the cooker always facing thesun such that the reected rays fromthe mirror fall on to the cooking box and cover its whole area.

3.5 Dish solar cooker


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3.5.1 Description A dish solar cooker uses a parabolic dish to concentrate the incident solar radiation. Thissolar cooker is commonly known as an SK-14 type of cooker, and is useful for homesand small establishments.

3.5.2 Design A typical dish solar cooker has a minimum aperture diameter of 1.4 m and a focal lengthof 0.28 m. The paraboloid dish, made of single reector, or by joining smaller pieces of reector, is xed rmly to a rigid frame. Size and shape of the dish should be such that when exposed to sun in the normal direction, a point focus would be formed.

The reecting material used for fabrication of this cooker is anodized aluminium sheet, which has a reectivity of over 80%. The thickness of the Glass mirrors should be about3 mm, with suitable protective layer on back to minimize degradation of the reectivecoating due to weathering.

The cooker has to track the sun and has to be adjusted manually after every 1520 minutes.

The cooker can deliver power of about 0.6 kW, which can boil two to three litres of water inhalf-an hour. The temperature achieved at the bottom of the vessel could reach 350400C, which is sufcient for roasting, frying, and boiling. This cooker can meet the needs of about 15 people, and can be used for eight to nine hours during the day.

3.5.3 Material required for fabrication:

Supporting Frame of the dishIs made either of MS rings supported by MS strips, FRP material, or thick MS wire-meshstructure. It should be rigid enough to resist any deformation of the dish shape due to windpressure or manual handling. The MS structure should have epoxy/anti-rust coating.

Stand for the dishIt is made of MS with epoxy/powder coating. It is equipped with an arrangement to holdcooking pot of different sizes. It should have a suitable provision to secure the cooker tothe ground.

Tracking Mechanism

It is either manual or automatic, allowing unrestricted rotation of the dish along itshorizontal and vertical axes enabling its adjustment in the normal direction to the sunsrays. A provision of locking arrangement to hold/x the dish at a desired position ispresent. It is equipped with an arrangement (in the form of a vertical pointer) to enableusers to position the dish in a direction normal to the suns rays.

Cooking vessel (Optional)For dish cooker of about 1.4 m diameter, a pressure cooker of 5 l capacity with ISImark is required. For larger sizes, proportionately higher capacity of pressure cooker


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is required. High temperature resistant black powder coating is given to the bottom of the vessel.

Other requirements

The entire structure should be able to withstand wind pressure up to a speed of 60km per hour without damage. All parts/components should be weather resistant and able to withstand degradation

due to climatic conditions for a period of 15 years (except for reecting mirrors whichmay require replacement early).

Accessories, like cap, hand gloves, goggles, manual for its installation and proper use,and tool kit for installation are also supplied.

The life of this cooker is estimated at about 20 years for the metallic structure.

3.5.4 Maintenance and servicing of dish solar cookers

Maintenance required is an occasional coat of black paint on the outer surface of thepressure cooker. Replacement of reecting sheets once in 5 years Cleaning of the reective panels on parabolic solar cookers should be done after

every use. All that is needed is a soft cloth and clean water to remove dust and spilledfood.

The reectors should never be scrubbed or cleaned with soap or an abrasive materialbecause that will dull the shine and reduce the temperature at which it cooks.

Covers for parabolic solar cookers that are left outside at night will dramatically extend their life span. They can be sewn locally.

3.5.5 Customer education A. Dish solar cookers available in the marketThe cost of a dish solar cooker may vary from Rs 4300 to Rs 5000 depending on the typeof reector and supporting structure. The dish solar cooker can save up to 10 liqueedpetroleum gas (LPG) cylinders per year upon full use at small establishments. Its payback period ranges from 1.5 -2 years, depending on the extent of its use and the place of use.

B. Cooking TimeCooking time is as low as an hour for boiling water to about 2 to 2 hours depending upon the kind of food and the season of the year. It also depends on local conditions

such as ambient temperature, time of the day, latitude of the place, etc.C. Cooking tipsBlack pots work a lot better than silver pots. The pot needs to absorb as much light aspossible and silver tends to reect the light. Dull or matt nishes absorb more light thanshiny surfaces.

Pots with close tting lids keep the heat in and help the cooking process. Placing thestove in a sheltered area stops the wind from cooling the outside of the pot.


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3.6 Community Solar Cooker (for indoor cooking)

3.6.1 DescriptionLike the dish solar cooker, the community solar cooker too is a parabolic reector cooker.However, it is larger than the SK-14 type, and is known as a Schefer cooker. Mr. Wolfgang Schefer developed the large size concentrator cooker . The unique feature of this cookeris that it makes possible to cook using solar energy within the kitchen itself.

3.6.2 DesignThe community solar cooker consists of solar concentrator with minimum 7.0 & 9.5 sq.maperture area, which is known as primary reector. The shape of primary reector iselliptical dish shape. It is made up of multiple pieces of reecting mirrors supported witha rigid frame/ structure to from Schefer reector. A mechanical clock type tracking arrangement is provided to keep the dish facing the Sun. A secondary reector furtherconcentrates the rays on to the bottom of the pot/frying pan (painted black to absorbmaximum heat). The temperature attained is so high (up to 400 C) that the food can becooked quickly. Thus, the community solar cooker works like a conventional cooking device, with the difference that instead of using a conventional fuel like gas or rewood,it uses solar energy inside the kitchen.

The dish is commonly known as Schefer dish, which has been found to be usefulfor generating heat to cook food for 50 to 75 persons in community kitchens especially at religious places, hostels, ashramshalas, Govt. circuit houses and industrial canteens.Two dishes can be installed for 100 to 150 persons cooking needs. The community solarcooker is to be installed on proper civil structure and as per the actual requirements atsite. The secondary reector is to be installed inside the kitchen for indoor cooking.

Due to the high temperature attained with this cooker, it is suitable for making almost alltraditional dishes, including chapattis, purees, dosas, etc. The cooker can also be usedto season (that is, tadka) dishes. When not in use for cooking, the cooker can be used toheat/boil water.

The mechanical clockwork arrangement rotates the primary reector outside toautomatically track the sun. The cook has to set this reector in focus only once a day, inthe morning. By shifting two arms provided in the reector frame, it is possible to change

the curvature of the parabolic reector for seasonal adjustment, thus keeping it fully tracked on to the sun during all seasons.

This Cooker is technologically far superior to the box-type community cooker, having overcome a number of shortcomings of the box type cooker. The cooker is capable of achieving temperature upto 250 0 C as against 100-125 0 C in box type cooker. This helpscooking much faster. The conventional cooking arrangement within the kitchen does notrequire to be changed and the cooking can be done inside the kitchen. Additionally roasting & frying can be done with this cooker which is not possible in the box type solar cooker.


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3.6.3 Material required for fabricationFrame & support structure Rigid enough to resist deformation of the dish shape due to wind pressure or manual

handling. Made of anodized aluminium/mild steel with epoxy/powder coating

Tracking mechanism Automatic, mechanical clock work arrangement Designed for E-W tracking of the reector along its polar axis to align it in the direction

normal to the suns rays. Having a provision for seasonal adjustment manually for the tilt angle.

Secondary Reector Bright anodized aluminium sheet with minimum reectivity of 80% xed on a curved

metallic structure. Size & curvature of the structure is such that the reected rays get focused on to the

bottom of the cooking pot of about 300 mm diameter.

Other Requirements The entire structure should be able to withstand wind pressure up to a speed of 60

km per hour without any damage. All parts/components should be weather resistant and able to withstand degradation

due to climatic conditions for a period of 15 years (except for reecting mirrors whichmay require replacement early).

The cooker is installed and commissioned at the site by the supplier.

3.6.4 How it Works?

System Construction

Solar Concentrating Disc (Primary Reector) Part A


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The Community Solar Cooker (Rotating Disc Type) works on the principle of solar energy concentration using a Reecting Parabolic Solar Concentrator. A 7-squaremeter areaparabolic solar concentrator is used for concentrating solar radiation on a focal area where the cooking vessel (Part D) is placed.

Automatic Tracking System Part B With the help of a simple automatic mechanical tracking system the solar disc rotates inthe direction of the movement of the Sun to give continuous and accurate solar energy concentration. This mechanical device is made up of a simple clock mechanism withchain & gear arrangement to provide regulated tracking motion to follow the path of the sun through out the day from dawn to dusk and gives daily about 6 8 hours of operation time.

Secondary Reector Part CThis opening is provided in the north-facing wall of the kitchen or the cooking place just

below the cooking vessel. This reector receives the concentrated solar radiation andreects it on to the bottom of the cooking vessel as shown in the gure above.

Cooking Area Part DThis is where the cooking vessel is kept

Installation and Working of a Community Solar CookerThe Solar Cooker is installed in the open shadow-free area or on terrace tops facing theSouth. The reection of the disc falls on a secondary reector housed in an opening inthe North kitchen wall. The secondary reector further reects the radiation on to thecooking vessel place above in the kitchen.

3.6.5 Customer education A. Community solar cookers available in the marketThe cost of this solar cooker is about Rs. 50,000. The payback period is 4-5 years.

B. Cooking TimeCooking for about 40-50 persons is possible. One dish may take about 60-90 minutes.

C. Maintenance and caring for your solar stove. When not in use, keep the stove in the dry. To store the unit in winter, remove the grill andgrill support and turn upside down. A dirty reector will slow down the cooking times.

Clean the reector with a dry cloth or alcohol if available.


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Methodology: Lecture , AV Material, Practicals

Time: Theory 3 hours

Practicals: 6 hours

4.1 Solar Water Heating Systems

A 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 unitsof electricity every year, for every 100 litres per day of solar water heating capacity.

4.1.1 Basic Working PrincipleThe basic principle of a solar hot water heater is that the water passes through a solarcollector or solar panels that heat the water. The solar collectors or panels are in directcontact with the sun and this energy is used to heat the water. The water then passes toan insulated storage tank that conserves the heated water for later usage.

There are two variations on this principle: active and passive systems. The active systemuses a pump to move the water from the collectors or panels to the storage tank. The passivesystem has no power source to move the water but moves the water through gravity.

4.2 Advantages and limitations Advantages of SWH It does not require extra space as they can be mounted on roof tops, windows, etc. It is easy to install, operate and maintain

Unit 4. Solar Water Heaters (SWH)

Learning Outcome develop an understanding of construction and working of different solar

water heaters develop skills in installing, maintaining and repairing solar water heaters


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SWH of capacity up to 2500 litres per day (LPD) do not require circulating pump The initial investment although on the higher side is fully recovered through

savings in the recurring fuel costs. A solar water heater pays back its cost in34 years.

Hot water is available 24 hours a day depending on the use and system capacity. SWHs last a long time (1520 years) and require only simple maintenance. SWH is a pollution free system

Limitations of SWH The water may not heat enough on an overcast day

4.3 Details of Solar Water Heaters

4.3.1 Description

Solar water heaters based on Flat plate CollectorsHere the solar radiation is absorbed by at plate collectors which consist of an insulatedouter metallic box covered on the top with glass sheet. Inside there are blackenedmetallic absorber (selectively coated) sheets with built in channels or riser tubes tocarry water. The absorber absorbs the solar radiation and transfers the heat to theowing water.

Figure: Flat Plate Collector based Solar Water Heaters


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4.3.2 Design A solar water heating system consists of (1) at plate solar collector, (2) insulated storagetank kept at a height behind the collector, (3) insulated hot water carrying pipes and(4) cold water carrying pipes. The system is generally installed on the roof or on openground, with the collector facing the sun and connected to a continuous water supply.

The at plate collector comprises (1) front glass glazing (2) blackened metallic (copper)absorber sheet with built-in channels or riser tubes welded to it to carry water. The entireassembly is placed in a at metallic box. In certain models, evacuated glass tubes are usedinstead of copper; a separate cover sheet and insulating box are not required in this case.

Water ows through the tubes, absorbs solar heat, and is stored in a tank. The hot waterso stored can be used for various applications at homes, such as bathing, cleaning, and washing. It can also be used for a variety of industrial applications. The water stored inthe tank remains hot overnight as the storage tank is insulated and heat losses are small.Flat-plate solar collectors are manufactured in India; however, the tubes for evacuatedtube collectors are imported. So far, about 10 Lakh square metres of collector area hasbeen installed in the country.

4.4 Evacuated Tube Collector (ETC) based solar water heater

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 owsthrough the inner tube.


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4.5 Domestic solar water heating system

Most domestic solar water heaters are provided with electrical back-up. Electrical heating elements are usually placed in the storage tank and can be switched on during cloudy

days. In some cases, the solar-heated water is led into an existing electric geyser; thegeyser needs to be switched on only in cloudy conditions. Most domestic systems are inthe capacity range of 100 500 litres of hot water per day.

Solar water heating systems are classied in 2 types based on the circulation of water.

a) Thermosyphon system.b) Forced ow or forced circulation system.

4.5.1 Thermosyphon systemIn the Thermosyphon system, water comes from the over head tank to bottom of solarcollector by natural circulation and water circulates from the collector to storage tank as long as the absorber keeps absorbing heat from the sun and water gets heated inthe collector. The cold water at the bottom of storage tank runs into the collector andreplaces the hot water, which is then forced inside the insulated hot water storage tank.The process of the circulation stops when there is no solar radiation on the collector.Thermosyphon system is simple and requires less maintenance due to absence of controls and instrumentation. Never use a Thermosyphon system for a large set-up.


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Limitations / Disadvantages: Limitation of the size / capacity of system. Beyond 2000LPD, the ow becomes

sluggish and efciency is reduced Rigid rules for the system construction. e.g. Overhead cold-water tank having

minimum head, hot water tank to be above the panels only etc Cold water is mixed in the hot water every time the hot water is drawn. This results in

less output than the capacity.

B) Fixed Temperature Forced Circulation (FTFC) System:In this, the hot water from the panels / collectors is pumped to the storage tank, onreaching the preset temperature only. The pump operation is automatic. When the hot water is pumped, the cold water pumped in lls the vacuum. The pump automatically stops on sensing the temperature at the outlet sensor point being lower than the presetvalue. This way the system operates during the day.

Advantages of this system are: No limit on the size of the system Flexibility in system installation Guaranteed system output Better efciency Reduced cost of superstructure / separate overhead tank

Limitations / Disadvantages: Needs continuous electricity for system operation The electronic controls may require periodical maintenance

Differential Temperature Forced Circulation (DTFC) System:This system works on the principle of Differential Temperature. Whenever thedifference between the temperature at the collector array outlet and the tank bottomis higher than the preset value, the pump circulates the water imparting the heat fromthe collectors ton the water in the tank. This way the total system capacity is heatedthrough out the day. The pump stops automatically once the temperature difference is within the preset value.

Advantages of this system are: No limit on the size of the system Flexibility in system installation Guaranteed system output Better efciency

Limitations / Disadvantages: Needs continuous electricity for system operation, which can be connected through

generator back up / inverter The electronic controls may require periodical maintenance


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4.6 Site selection

Site conditions are important. Collectors should face directly south. Turning a collectorsoutheast or southwest can affect its performance by about 20% or more. If hot water is

needed by noon, face the collector to the southeast; if hot water is more important in lateafternoon, face the collector to the southwest.

The site should be free from shade. Collectors should be placed so that they can be tiltedfrom the horizon to an angle equal to the latitude of the location. For better output in winter the angle of inclination should be latitude + 15 degree. If the latitude is unknown,the collector can be placed at a 45[degrees] angle, except in areas near the equator). Thelatitude for your area can be obtained from an atlas or globe.

4.7 Procedure for Operation and Maintenance

Before starting the system, do the following:1. Check water in the cold water supply line and also storage tank. In case there is

no water, ll the tank with water. The system should never be operated if water isnot available. Without water the pump will be damaged. A thermosiphonic system will not deliver hot water at the point of use if there is no continuous supply of cold water.

2. Check the collector glazing. It should not be dirty. In case dirt is deposited on theglazing it should be cleaned.

3. Open all the control valves as suggested by the supplier4. Check electrical supply.5. Check the water pump. It should not be jammed.6. Check whether the Non Return Valve (NRV) on the cold water inlet side is working.7. Ensure the lever of mixer in the bathroom is on either left or right side AND NOT IN

THE MIDDLE to avoid mixing of hot / cold water.

4.8 Operation of the Solar Water Heater

1. After ascertaining that the requirements indicated above are fullled, start thecirculating pump. The water will start circulating in the collector system and aftergetting heated will be collected in the storage tank. The temperature of water in thestorage tank will rise which can be seen in the temperature indicators provided inthe system. There is a possibility of air locking in collector riser tubes and headers which can be released by increasing the ow of water by opening control valves. Assoon as the control valves are opened beyond the limits suggested, water will startcoming out from the air vents. Keep the valves in this position for some time andthen bring them to their original position or as soon as water stops coming out of the air vents.

2. In a thermosiphonic system hot water will start collecting in the storage tank as soonas sunlight falls on the collector and water is available in the system.


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sufcient for a family of four or ve members. It costs Rs 15 000 to Rs 18 000, and can saveabout 1500 units of electricity per year.

C. Standards

National standards for at-plate collectors have been laid down by the BIS (IS 12933:1992) to ensure minimum quality and performance of the systems. The standards includespecications of components of collectors such as cover, absorber, insulation, box, andsealant 1.

4.11 BIS Standards:

This is used for the types and materials of constructions. Test procedures for solar atplate collector - please refer BIS 12933-2003. The BIS standard can be purchased fromany Bureau of India Standards ofce. The standards shall be supported by Scheme of Inspection and Testing (SIT) to plan and do the product testing.

4.12 Manufacturing of components of Solar Water Heater:

1) Solar Flat Plate Collector: The collector assembly involves the following activities-a) Making the outer frame by cutting the Aluminium extruded sections to required

length and chamfer for frame making. Making the slots for xing of the rubbergrommets and header pipe. Joining the 4 sections to make the frame.

b) Cutting the back sheet to required size. Fixing the back sheet to the frame by popup rivets.

c) Cutting of the copper header pipes to required length. Drilling of holes for brazing of the n tubes.

1 A sealant is a viscous material that changes state to become solid, once applied, and is used to prevent the penetration of air, gas, noise, dust, re, smoke or liquid from one location through a barrier into another.


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d) Flaring of the header ends after inserting the anges for connection.e) Brazing of n tubes with the headers with control on the tube penetration.f) Leak test of the absorber assembly as prescribed by BIS.g) Placing of insulation pads and Aluminium foil in the collector box.

h) Placing the absorber assembly in the box, sealing the joints with silicon sealant.i) Fixing the top glass cover with rubber beading, retainer angle with screws and

silicon sealant. j) Testing as per the BIS guidelines, xing of BIS number plate.

Precautions to be taken: Following precautions should be taken during manufacturing and testing:1) Raw material / component quality checks should be carried out.2) The dimensional tolerance in the frame size should be followed strictly to avoid

breakage of glass while xing.3) The collector box should be air / water tight to avoid entry of water / moisture at

site.4) No stress should develop on the glass after xing to avoid breakage.5) Each n-tube assembly must be leak tested before brazing and also after the brazing

the absorber assembly should be tested @ the rated pressure.6) Flanges should be ared to allow adjustment at site while connecting the number of

collectors without stress.7) Finished collectors should always be stored in vertical position and not in at

position.

2) Hot water storage tank: The hot water storage tank can be made of Mild Steel,

Galvanised Iron or Stainless Steel.

Following are the activities involved in manufacturing of the solar hot water storagetank.a) Cutting of sheet to required size and rolling to form the cylinder.b) Welding the shell by ARC (Electric welding in case of M.S. or G.I.), TIG 9Tungsten

Inert Gas welding for S.S.) or MIG (Metal Inert Gas for M.S., G.I and also for S.S.)process as applicable.

c) Welding of side dish ends/ conical ends.d) Welding of connections.e) Hydraulic test of tank to check leakage as well as suitability to designed system

pressure.f) Insulation and cladding.

Precaution to be taken: Following precautions should be taken during manufacturing and testing:1) Raw material quality needs to be checked.2) The tank dimensions should be decided to ensure minimum surface area / material

weight and also wastage.


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3) Proper care should be taken while welding to minimise the heat affected zone /damage to the parent material.

4) Leak test should be carried @ 1.5 times the operating pressure held for 30 minutes.5) Insulation should be carried out after the leak test only.

6) Proper anti-rust coating should be provided for non S.S tanks with access for futuremaintenance.

3) Stands for collector and tank: This involves fabrication activity for the stand /supports for the collector and tank. We have to get the stands / supports designedto ensure that the same is suitable for the load and site conditions. The fabricatedstructure needs to be protected by either applying a primer coat followed by twocoats of paint or by Hot Dip Galvanising. The stands / supports should be suitable fortransport with nal assembly at site.

Precaution to be taken: Following precautions should be taken during manufacturing and testing:1) The stand support shall be balanced and steady to avoid stress on the tank after lling

of water.2) The materiel should be chosen after taking into consideration the corrosion

allowance.3) The tank should have complete resting on the stand along the circumference.4) Base plates should be provided for anchoring at site.5) The collector stand should have a provision for height / angle adjustment to suit site

conditions.6) The welding has to be complete, cleaned without any sharp edges and not just tack /

spot-welded.


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5.1 Engineering drawings:

Blue line: HOT WATER

Yellow Line: COLD WATER

Here is a typical drawing of a solar water heating system.

Unit 5. Practicals


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5.2 Cross sections of solar water heater (SWH):

1) Hot water to use2) Tank 3) Hot water to the Tank 4) Collector5) Cold water Inlet


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5.3 Installation Layouts:

5.3.1 1500lpd Solar Water Heating System (SWHS) Layout at 60 OC


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5.3.2 1500lpd SWHS layout 70 OC:


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5.3.3 3000lpd SWHS layout:


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5.4 Calculations: Variation in velocity with pressure difference:P1-P 2 = rho (V 2

2 - V 12)

So if P 1 = 1 bar

P2 = 2 bar

And V1= 0.5m/s

So, V2=0.248m/s

This shows variation in ow with variation of pressure.

So, Let us assume the diameter of the outlet pipe be 1inch.

Pressure(bar) 1 2 3 4 Velocity(m/s) 0.5 0.498 0..495 0.492Flow(Litres) 0.00988 0.00984 0.00978 0.009723


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List of Box Type Solar Cookers

a) BIS Certied Solar CookersSl. No. Name & Address of Manufacturer Contact Details1. M/S Universal Engineers Enterprises

Garg Bhavan, Prince RoadGandhi Nagar, Moradabad (U.P.)

0591-2493619(Telefax) : 0591-2499768

2. M/s Rural Engineering School ,Rojmal, Tal.: Gadhada (SN)District Bhavnagar-364750, Gujarat

Tel : 02847 294127Fax: 02847 253535e-mail: [email protected]

3. Khadi Gramodhyog Prayog SamitiGandhi Ashram, Ahmedabad-380 027

Telefax: 079-27552469Mobile : 9825484275, 9879784255

4. Sayala Taluka Khadi Gramodyog SevaMandal, Motiram Building , Below SBS Service Branch, PhulchhabChowk, Rajkot 360 001

Ph.: 0281-2477226Mobile : 09825074591e-mail:[email protected]

b) Other Known Manufacturers

Sl. No. Name & Address of Manufacturer Contact Details1. M/S J. N Enterprises,

F-12, Navin Shahdara, Delhi.Mobile : 2350859119

2. M/s. Vishvakarma Solar Energy Co.G.T. Road , Phillour, Distt.Jallandhar, Punjab

01826-2252301826-22217

3. M/s Fair Fabricators142, Tilak Nagar , Near Post Ofce,

Indore- 452 018

Telefax: 0731-2491488Mobile : 9425316707

[email protected]. M/s Rohtas Electronics,

15/268-B, Civil lines,Kanpur - 208001.

Ph. : 0512-2305564Telefax : 0512-2305390E-mail : [email protected]

5. M/s Rural Engineering School ,Rojmal, Tal.: Gadhada (SN)District Bhavnagar-364750, Gujarat

Tel : 02847 294127Fax: 02847 253535e-mail: [email protected]

Annexure


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6. M/S Usha Engineering Works40-A,Trunk Road, Madanur-635804 Vellore District, Tamil Nadu

Ph.: 04174-73613e-mail: [email protected]

7. M/S Geetanjali Solar EnterprisesP/14, Kasba Industrial Estate,Phase-I, E.M. Bye Pass,PO East Kolkata Township,Kolkata-700107

033-24420773/24424027( Fax ) 033-24420773e-mail : [email protected].


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Documents

Trainers Textbook - Solar Thermal