solar drying of fruits , vegetables & herbs

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NKS solar dryer NKS solar dryer 1 Solar Drying of Solar Drying of fruits , vegetables fruits , vegetables & herbs & herbs Dr N K Srinivasan Dr N K Srinivasan Bangalore Bangalore

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A power point presentation with 22 slides on solar dryers-principle,simple design,applications for various food items and herbs all over the world,many examples,simple thumb rules for design,references,a do-it-yourself guide..

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Page 1: Solar Drying of Fruits , Vegetables & Herbs

NKS solar dryerNKS solar dryer 11

Solar Drying of Solar Drying of fruits , vegetables & fruits , vegetables & herbsherbs

Dr N K SrinivasanDr N K Srinivasan

BangaloreBangalore

Page 2: Solar Drying of Fruits , Vegetables & Herbs

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Why Solar Drying?Why Solar Drying?

Nearly 30 to 40% of fruits and vegetables get spoiled during post Nearly 30 to 40% of fruits and vegetables get spoiled during post harvesting in tropical countries….Solar drying is a great way of harvesting in tropical countries….Solar drying is a great way of food preservationfood preservation

Compared to freezing/canning, solar drying is simpler, less costly Compared to freezing/canning, solar drying is simpler, less costly with no fuel or energy needswith no fuel or energy needs

Solar dried fruits and vegetables could fetch much higher prices Solar dried fruits and vegetables could fetch much higher prices when stored and marketed.when stored and marketed.

Enables seasonal products available throughout the yearEnables seasonal products available throughout the year Provides better nutrition and serving pleasureProvides better nutrition and serving pleasure Besides fruits and veg, solar dryers are used to dry/roast seeds Besides fruits and veg, solar dryers are used to dry/roast seeds

like Coffee beans ,peanuts and so on.like Coffee beans ,peanuts and so on. Herbs ,dried, fetch a fantastic price and improves health..Herbs ,dried, fetch a fantastic price and improves health.. A practical solar dryer can be built for a few $ with house-hold-A practical solar dryer can be built for a few $ with house-hold-

garage items in a couple of hours—the technology is simplegarage items in a couple of hours—the technology is simple

Page 3: Solar Drying of Fruits , Vegetables & Herbs

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How solar drying works?How solar drying works?

The aim of any drying process is to The aim of any drying process is to remove moisture -80 to 90% from the food remove moisture -80 to 90% from the food item so that you can store for about a item so that you can store for about a year.year.

Solar dryer has two functional parts:Solar dryer has two functional parts: to absorb solar energy and heat the airto absorb solar energy and heat the airTo pass the hot air over the food itemTo pass the hot air over the food item• It is that simple!!It is that simple!!

Page 4: Solar Drying of Fruits , Vegetables & Herbs

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Solar energy absorberSolar energy absorber

Sun’s rays are let into a box through a glass or Sun’s rays are let into a box through a glass or plastic cover ,called ‘glazing’.plastic cover ,called ‘glazing’.

The light energy is absorbed by a black surface…The light energy is absorbed by a black surface…a black sheet metal or even black paper [craft a black sheet metal or even black paper [craft paper or black garbage bag] will do.paper or black garbage bag] will do.

The heat absorber emits infra-red or heat rays The heat absorber emits infra-red or heat rays which cannot pass through glass/plastic, acting which cannot pass through glass/plastic, acting as heat trap or green houseas heat trap or green house

So the temperature of the air inside the box rises.So the temperature of the air inside the box rises.

Page 5: Solar Drying of Fruits , Vegetables & Herbs

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Hot air dryerHot air dryer

The hot air surrounding the absorber is passed The hot air surrounding the absorber is passed over the tray or mesh containing food itemsover the tray or mesh containing food items

The hot air absorbs moisture from food itemThe hot air absorbs moisture from food item The hot air is let out through a vent.The hot air is let out through a vent. In most dryers a natural convection flow is In most dryers a natural convection flow is

used.used. Vents at the bottom let in cold air. The hot air Vents at the bottom let in cold air. The hot air

with moisture leaves at the top vents.with moisture leaves at the top vents. You can add a fan/blower to push the air faster.You can add a fan/blower to push the air faster.

Page 6: Solar Drying of Fruits , Vegetables & Herbs

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Simple Dryer designSimple Dryer design

In simple box type dryers, the design is pretty simple.In simple box type dryers, the design is pretty simple. Provide holes at the bottom or side near the bottomProvide holes at the bottom or side near the bottom Keep the black absorber close to the bottomKeep the black absorber close to the bottom Place a sheet with holes or netting [nylon] above the absorber to Place a sheet with holes or netting [nylon] above the absorber to

hold the food itemshold the food items Place the glass/plastic cover at the top.Place the glass/plastic cover at the top. Use insulation at the side and bottom to reduce heat lossesUse insulation at the side and bottom to reduce heat losses Keep the food tray or netting such that you can easily put them in Keep the food tray or netting such that you can easily put them in

or take them out. or take them out. Let the dryer face the Sun!!Let the dryer face the Sun!! You can make the dryer box out of a cardboard box!!You can make the dryer box out of a cardboard box!! Use black paper for absorber!!!Use black paper for absorber!!! Use newspaper or coir or rice hull or peanut shells for insulation!Use newspaper or coir or rice hull or peanut shells for insulation!

Page 7: Solar Drying of Fruits , Vegetables & Herbs

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‘‘Minimum’ Solar tray Minimum’ Solar tray dryerdryer

I built a “minimum ‘ solar dryer using a single ,shallow cardboard box----I built a “minimum ‘ solar dryer using a single ,shallow cardboard box---- The bottom tray is an aluminum tray, painted black or pasted with black construction paper;The bottom tray is an aluminum tray, painted black or pasted with black construction paper; A rectangular cut-out is made on the lid;A rectangular cut-out is made on the lid; A plastic sheet or two sheets pasted on the cut out to form the window or glazing.A plastic sheet or two sheets pasted on the cut out to form the window or glazing. Thin strips of Al foil is pasted on the inside side walls.Thin strips of Al foil is pasted on the inside side walls. Punch a few holes [4mm dia] on the two opposite sides as vent holesPunch a few holes [4mm dia] on the two opposite sides as vent holes Fill the space between the tray and the wall of the box with crumpled newspaper.Fill the space between the tray and the wall of the box with crumpled newspaper. Two panel reflectors are made with cardboard lined with Al foil, for the size of the cut-out window. Two panel reflectors are made with cardboard lined with Al foil, for the size of the cut-out window. This is This is

optional---on a bright day, you may not need them at all…make then easy to detach…optional---on a bright day, you may not need them at all…make then easy to detach… Fix the panels at the back and front, and adjust the angle with sealing tapes.Fix the panels at the back and front, and adjust the angle with sealing tapes. You may tilt the box towards the Sun with wooden or cardboard pieces ,typically 20 deg South.You may tilt the box towards the Sun with wooden or cardboard pieces ,typically 20 deg South. Measure the inner temperature with an oven [dial] thermometer.Measure the inner temperature with an oven [dial] thermometer. Total time to make this dryer after you have gathered the items: 1 to 2 hours only.Total time to make this dryer after you have gathered the items: 1 to 2 hours only. The operating temperatureThe operating temperature for drying is in the range 40 to 60 deg C. for drying is in the range 40 to 60 deg C. You may remove the reflector if the temp goes higher than the desired range.You may remove the reflector if the temp goes higher than the desired range. Adjust the vent holes [cover them with cardboard pieces to reduce the air flow rate.]Adjust the vent holes [cover them with cardboard pieces to reduce the air flow rate.] If moisture or caramel condenses on the glazing or elsewhere, increase the vent holes…If moisture or caramel condenses on the glazing or elsewhere, increase the vent holes… You need to make some trails on this score--- to adjust the air temperature and the air flow rate.You need to make some trails on this score--- to adjust the air temperature and the air flow rate. Maintenance: keep the glazing clean by wiping with cloth after each use; clean the tray for left over food Maintenance: keep the glazing clean by wiping with cloth after each use; clean the tray for left over food

items/pieces.items/pieces. If small insectsa re problem, cover the vent holes with nylon or steel mesh screens.If small insectsa re problem, cover the vent holes with nylon or steel mesh screens.

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Two-box designsTwo-box designs

The solar dryer can be made with two boxes: one absorber or The solar dryer can be made with two boxes: one absorber or collector boxcollector box and another and another dryer boxdryer box

The collector box can be big, with large surface area for glazing The collector box can be big, with large surface area for glazing and absorber sheet [typically 2feet by 4 feet area]and absorber sheet [typically 2feet by 4 feet area]

The dryer box is kept at higher level ,mainly to hold the food item The dryer box is kept at higher level ,mainly to hold the food item trays.trays.

It is easier to control the temperature in this design by covering It is easier to control the temperature in this design by covering the glazed area to reduce temp if required.the glazed area to reduce temp if required.

The two boxes are connected by a duct, well insulated.The two boxes are connected by a duct, well insulated. One square meter area of collector can dry 2 to 3 kg of food…this One square meter area of collector can dry 2 to 3 kg of food…this

is an approximate thumb-rule.is an approximate thumb-rule. See the diagram in the next slide.See the diagram in the next slide.

Page 9: Solar Drying of Fruits , Vegetables & Herbs

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A simple solar dryer designA simple solar dryer design

Page 10: Solar Drying of Fruits , Vegetables & Herbs

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Tent dryerTent dryer

A simple dryer for large quantities is a tent dyer.A simple dryer for large quantities is a tent dyer. The tent is made of bamboo or wooden sticks.The tent is made of bamboo or wooden sticks. The roof is made of plastic sheet.The roof is made of plastic sheet. The food item is kept on netting or mesh or just cloth.The food item is kept on netting or mesh or just cloth. A black cloth or sheet beneath the netting is enoughA black cloth or sheet beneath the netting is enough Such large tent dryers are used in Malawi to dry sweet Such large tent dryers are used in Malawi to dry sweet

potatoes…potatoes… Tent dryers are more economical and can be scaled up Tent dryers are more economical and can be scaled up

for larger quantities of food to be dried.for larger quantities of food to be dried.

Page 11: Solar Drying of Fruits , Vegetables & Herbs

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Roof dryersRoof dryers

This large scale drying is made of steel This large scale drying is made of steel [corrugated metal] roof, painted black.[corrugated metal] roof, painted black.

The food is kept in a loft close to the roof The food is kept in a loft close to the roof inside the building.inside the building.

You can add a glass glazing above the You can add a glass glazing above the steel roof but not necessarysteel roof but not necessary

Tea leaves and many other items have Tea leaves and many other items have been dried this way.been dried this way.

Page 12: Solar Drying of Fruits , Vegetables & Herbs

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Tunnel dryer designTunnel dryer design

For large scale application, a For large scale application, a tunnel is produced with tunnel is produced with polythene sheet covering the polythene sheet covering the tunneltunnel

The trays with the food items The trays with the food items is moved slowly through the is moved slowly through the tunnel, making it a continuous tunnel, making it a continuous systemsystem

Coffee Coffee drying/cocoa/copra/fruits/tea drying/cocoa/copra/fruits/tea leaves is often done by tunnel leaves is often done by tunnel dryersdryers

Tunnel dryers are suitable for Tunnel dryers are suitable for village/community level/small village/community level/small industry unitsindustry units

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Box cookers as dryersBox cookers as dryers

Box cookers modify as dryersBox cookers modify as dryers In cookers you seal the box…for drying, In cookers you seal the box…for drying,

allow vents or opening to pass the air’allow vents or opening to pass the air’ For drying you need only 40 to 70 deg For drying you need only 40 to 70 deg

C…may remove the reflectors…just the C…may remove the reflectors…just the hot box hot box

[cooking requires much higher [cooking requires much higher temperatures—70 to 100 deg C]temperatures—70 to 100 deg C]

Page 14: Solar Drying of Fruits , Vegetables & Herbs

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Using the dryerUsing the dryer

Clean the vegetables/fruits and wash them wellClean the vegetables/fruits and wash them well Cut in to small pieces as requiredCut in to small pieces as required Pretreatments like blanching, dipping in ascorbic/citric acid may Pretreatments like blanching, dipping in ascorbic/citric acid may

be neededbe needed Place in the dryerPlace in the dryer Adjust the vent holes to control the flow of airAdjust the vent holes to control the flow of air The temp inside may rise from 40 deg C to as high as 80 deg CThe temp inside may rise from 40 deg C to as high as 80 deg C You need 40 to 70 deg C for quick dryingYou need 40 to 70 deg C for quick drying It may take 1 to 3 days to completely dryIt may take 1 to 3 days to completely dry Veg will become brittle while fruits would be pliable and leatheryVeg will become brittle while fruits would be pliable and leathery Store the dried pieces after weighing in glass jars in a cool ,dry Store the dried pieces after weighing in glass jars in a cool ,dry

placeplace

Page 15: Solar Drying of Fruits , Vegetables & Herbs

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Box temperature-steady Box temperature-steady temperaturetemperature

While using ,you observe a steady temperature after some time…While using ,you observe a steady temperature after some time…the drying takes place at a constant temperature.the drying takes place at a constant temperature.

The dry temperature is the temperature of an empty dryer,without The dry temperature is the temperature of an empty dryer,without food itemd…this will be high –abou 50 to 60 deg C.food itemd…this will be high –abou 50 to 60 deg C.

When food items are placed,there will be a drop in temperature When food items are placed,there will be a drop in temperature due to moisture in the air…due to moisture in the air…

Again the temp will rise and oscillate Again the temp will rise and oscillate Finally a steady temp will be reached…Finally a steady temp will be reached… Performance of the dryer depends on this steady temperaturePerformance of the dryer depends on this steady temperature This temp depends on the food item and the solar energy at that This temp depends on the food item and the solar energy at that

time, for a given dryer designtime, for a given dryer design

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Back ups for dryersBack ups for dryers

To continue drying after sunshine or at night ,a thermal mass is To continue drying after sunshine or at night ,a thermal mass is needed.. rocks, stones even water bottles can be used inside the needed.. rocks, stones even water bottles can be used inside the drying chamber…to retain the heat…drying chamber…to retain the heat…

Some use hygroscopic calcium chloride to keep warm airSome use hygroscopic calcium chloride to keep warm air You need a fan or blower for larger systems to push hot air.You need a fan or blower for larger systems to push hot air. Forced circulation with fans increase the capacity of a dryer 6 Forced circulation with fans increase the capacity of a dryer 6

times; reduces the drying time by half, compared to natural times; reduces the drying time by half, compared to natural convectionconvection

Some use solar cells panel plus a small fan for air flow.Some use solar cells panel plus a small fan for air flow. Some use a thermal-ventilator which is like a small outside Some use a thermal-ventilator which is like a small outside

windmill.windmill. Additional reflectors can be added to reduce collector area.Additional reflectors can be added to reduce collector area.

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Dehydration reactionsDehydration reactions

While dehydrating food items, some chemical reactions may take While dehydrating food items, some chemical reactions may take place.place.

Formation of caramel,or ‘caramelisation’ is a common chemical Formation of caramel,or ‘caramelisation’ is a common chemical reaction while drying carrots, peanuts and so on…reaction while drying carrots, peanuts and so on…

The alcohols combine to form ethers and water which evaporates.The alcohols combine to form ethers and water which evaporates. 2 R-OH 2 R-OH R-O-R + H-H-O R-O-R + H-H-O Alcohol Alcohol ether + water ether + water The esters have fine aroma and also nice color [reddish orange] The esters have fine aroma and also nice color [reddish orange] The caramel may settle on glazing and should be cleaned with The caramel may settle on glazing and should be cleaned with

wet cloth.wet cloth. This process gives a nutty flavor to peanuts and brownish color.This process gives a nutty flavor to peanuts and brownish color. It could be oxidation of sugarsIt could be oxidation of sugars Caramelisation is complex and poorly understood set of chemical Caramelisation is complex and poorly understood set of chemical

reactions.reactions.

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RoastersRoasters

Roasting peanuts, almonds and coffee beans require Roasting peanuts, almonds and coffee beans require higher temperatures.higher temperatures.

Solar box cookers can be modified for roasting .Solar box cookers can be modified for roasting . Concentrating type collectors, like trough collectors are Concentrating type collectors, like trough collectors are

used for roasting.used for roasting. One company is USA is using a set of mirrors with 20 One company is USA is using a set of mirrors with 20

KW power to roast 30lb of coffee beans per batchKW power to roast 30lb of coffee beans per batch Peanut roasters with small parabolic reflectors have Peanut roasters with small parabolic reflectors have

been used in Africa and elsewhere. At the focal plane, been used in Africa and elsewhere. At the focal plane, keep a tube with peanuts which can be rocked for even keep a tube with peanuts which can be rocked for even roasting.roasting.

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Trough concentrator & Box Trough concentrator & Box cookers for roastingcookers for roasting

Trough concentratorTrough concentrator Box cookerBox cooker

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Herb dryersHerb dryers

Drying herbs require lower temperaturesDrying herbs require lower temperatures—typically35 to 40 deg C—typically35 to 40 deg C

Small dryers [DIY] can be made with Small dryers [DIY] can be made with photo frames, nylon netting or steel mesh photo frames, nylon netting or steel mesh –no reflector is needed.–no reflector is needed.

Large herb dryers are like the fruit dryers Large herb dryers are like the fruit dryers with higher air flow rate.with higher air flow rate.

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Use in tropical countriesUse in tropical countries

In tropical countries like India/parts of Africa ,fruits and In tropical countries like India/parts of Africa ,fruits and veg get spoiled in a few daysveg get spoiled in a few days

Drying helps to preserve for a year or so and then Drying helps to preserve for a year or so and then fetches higher prices too---especially fruits like grapes fetches higher prices too---especially fruits like grapes turned into raisins, mango leathers, gooseberry [Amla]turned into raisins, mango leathers, gooseberry [Amla]

Dried fruits are used to make sweet jellies and patties Dried fruits are used to make sweet jellies and patties & candies in India& candies in India

A tunnel dryer has been used for drying apples in A tunnel dryer has been used for drying apples in Nepal.Nepal.

Mango drying in parts of IndiaMango drying in parts of India One can save nearly 30 to 40% of precious food One can save nearly 30 to 40% of precious food

products this wayproducts this way

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Solar dryers world over!Solar dryers world over!

Latin America –Coffee beansLatin America –Coffee beans Dominican republic—CocoaDominican republic—Cocoa Trinidad-CopraTrinidad-Copra Uganda—fruits/veg including mangosUganda—fruits/veg including mangos Nepal—applesNepal—apples India—fruits, chillies, herbsIndia—fruits, chillies, herbs

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My experiencesMy experiences

I made box type dryers with card board box, plastic glazing and outside I made box type dryers with card board box, plastic glazing and outside insulation with corrugated paper sheets/thermocole; box size: 14 in x 12 in insulation with corrugated paper sheets/thermocole; box size: 14 in x 12 in x 8inx 8in

I use a reflector for additional heat inputI use a reflector for additional heat input Black construction paper as absorberBlack construction paper as absorber Use a thermometer for temp measurementUse a thermometer for temp measurement Have dried fruits/veg in wintry months too.Have dried fruits/veg in wintry months too. Temp difference between ambient and inside --_5 to 30 deg Centigrade in Temp difference between ambient and inside --_5 to 30 deg Centigrade in

December in Bangalore.December in Bangalore. Inside temp 40 to 50 deg C on most days.Inside temp 40 to 50 deg C on most days. Food items kept on trays with stainless steel mesh---about 100 to 200 Food items kept on trays with stainless steel mesh---about 100 to 200

grams in each batch…grams in each batch… Total cost : Rs 100 or $2 onlyTotal cost : Rs 100 or $2 only I also made an indirect dryer—two box design for larger amount of items I also made an indirect dryer—two box design for larger amount of items

in the dryerin the dryer

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A few referencesA few references

www.greenplants.orgwww.greenplants.org www.solar-alternatives.comwww.solar-alternatives.com www.backwoodshome.comwww.backwoodshome.com www.jrwhipple.comwww.jrwhipple.com www.iqsolarpower.comwww.iqsolarpower.com www.solarcookers.orgwww.solarcookers.org www.solardevicesforthirdworld.webs.comwww.solardevicesforthirdworld.webs.com Beth Halacy and Dan Halacy—Cooking with Beth Halacy and Dan Halacy—Cooking with

the sun [1992]the sun [1992] Eben Fodor---” Solar food dryer”Eben Fodor---” Solar food dryer”

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ContactContact

[email protected]@hotmail.com Dr N K Srinivasan Dr N K Srinivasan Location: Palo Alto [CA] orLocation: Palo Alto [CA] or Bangalore ,India [Camp]Bangalore ,India [Camp] Website: Website:

www.solardevicesforthirdworld.webs.comwww.solardevicesforthirdworld.webs.com