Rural Applications of Solar Energy
Course: Rural Energy Systems
RDL – 722
1
Solar Drying
Importance of solar drying
•
The world population is more than 7 billion and about 800-900 million people
do not have enough food to eat.
•
There are three methods to solve hunger problem:
– Increase food production
– Reduce population growth
– Reduce loss of food during and after harvesting
•
It has been estimated that world as a whole more than 20-30 percent food
grains and 30-50 percent vegetables, fruits/fish etc. are lost before it reaches
to the consumers.
•
Drying is a traditional method for preserving food. Solar drying is an effective
method to preserve food.
•
Solar energy is diffuse in nature and thus suitable for crop drying, locally
available and thus saves transportation, solar dryers can be made locally of any
size and capacity and solar dryers are economical if cash crops are dried.
Advantages in Favour of Crop Drying
• It permits early harvesting and reduces the field losses of the
products,
• Reduces the risk of field losses caused by wild animals,
• It permits better planning of harvesting season,
• It reduces spoilage in storage drastically,
• It permits the farmer to sell his product at better price during early
period of harvesting season,
• Quality of the product gets enhanced significantly and hence
farmer gets more money for his product, and
• Transportation is easy with dried product.
OPEN SUN DRYING
•
•
It is the simplest method of drying used in most developing countries.
The food / crop is spread out in the open under the sun.
Advantages
(1) No technology involved
(2) Very low cost (cost of labour only)
Disadvantages
(1) Contamination of the product due to dirt and insects.
(2) Wastage by birds / mice.
(3) Spoilage due to sudden and unpredicted rain.
(4) There is no control of temperature over crop drying.
(5) Overdrying may cause loss of germination power, nutritional changes, sometimes
complete damage.
CLASSIFICATION OF SOLAR DRYERS
• DIRECT TYPE DRYERS : In direct or natural convection type dryers, the agricultural
product is placed in shallow layers in a blackened enclosure with a transparent
cover. The solar radiations are directly absorbed by the product itself. The food
product is heated up and the moisture from the product evaporates and goes out by
the natural convection/circulation.
• INDIRECT TYPE DRYERS : In these dryers the food product is placed in a drying
chamber. The air is heated in solar air heaters and then blown through the drying
chamber. In some of the designs, dryers receive direct solar radiations and also
heated air from solar air heaters. In these dryers manipulation of temperature,
humidity and drying rate is possible to some extent.
• FORCED CIRCULATION TYPE DRYERS : In these dryers, hot air is continuously blown
over the food product. The food product itself is loaded or unload continuously or
periodically. These kind of dryers are comparatively thermodynamically efficient,
faster and can be used for drying large agricultural product. These dryers can be of
Cross-flow type, concurrent flow type or counter-flow type.
TYPE OF SOLAR DRYERS
(a) Direct type solar
dryers
(b) Indirect type
solar dryers
(c) Forced circulation type solar dryers
Natural Convection or Direct type Solar Dryer
• These dryers appear to be more attractive for use in developing countries since these
do not use fan or blower to be operated by electrical energy.
• These dryers are low in cost and easy to operate.
• Some of the problems with these dryers are: slow drying, no control on temperature
and humidity, small quantity can be dried, and some products change colour and
flavour due to direct exposure to sun.
• Several direct type dryers are fabricated, tested, and analysed in many countries.
• The simplest direct type solar dryer is solar cabinet dryer.
Solar Cabinet Dryer
• The solar cabinet dryer in its simple form consists of a wooden (or of any other
material) box of certain width and length (length is generally kept as three times
its width), insulated at its base and preferably at the sides and covered with a
transparent roof.
• The inside surfaces of the box are coated with black paint and the product to be
dried is kept in the trays made of wire mesh bottom. These trays loaded with
product are kept through an openable door provided on the rear side of the drier.
• Ventilation holes are made in the bottom through which fresh outside air is sucked
automatically. Holes are also provided on the upper sides of the dryer through
which moist warm air escapes.
• This dryer has given encouraging results and reduced the drying time by one third
compared to open sun drying.
Details of solar cabinet dryer
Photograph of Solar Cabinet
Dryer
Mixed Mode Type Solar Dryer
• In the mixed mode type of solar dryers, the solar air heater with or without any
electric fan along with a drying bin is used.
• Such simple mixed mode type solar dryer was developed at AIT Bangkok for drying
paddy and therefore named as rice dryer.
• It consists of a solar air heater made of a frame of bamboo poles and wire covered
with 0.15 mm thick transparent PVC sheet. The ground is covered with burnt rice
husk which absorbs the solar radiation and heats the air in contact.
• The hot air in this air heater rises to the drying chamber which either consists of
transparent PVC sheets on bamboo frame absorbing directly the solar radiation or a
bamboo frame covered from all the four side with some opaque material.
• The drying material (rice etc.) is kept on the nylon net tray in thin layer through which
hot air heated from air heaters enters its bottom and goes up into the chimney.
Mixed Mode Type Solar Dryer (contd.)
• The chimney is a long cylinder made of bamboo frame covered with
black PVC to keep the inside air warm. There is a cap at the top of the
chimney, leaving some space in between chimney top and cap to
allow warm humid air to go out and protecting the product from rain
and other foreign materials.
• The height of the chimney and the hot air inside it creates a pressure
difference between its top and bottom thereby creating forced
movement of air through the rice bed to the top of the chimney.
• The drying rate will depend on the depth of the bed, initial moisture
content of the material, solar insolation, ambient temperature, and
the design of the dryer.
Cross section of chimney type solar dryer
Photograph of Solar Rice Dryer
Forced Circulation Type Solar Dryer
• As the name implies, such dryers use some kind of one or several electric operated
blower/exhaust fan to circulate air between air heater/storage bin/drying chamber.
• Such dryers are more efficient, faster, reliable, preferred and can be used for drying
large quantities of agricultural products.
• These dryters can be used at low as well as at high temperatures and used for drying
large quantities of product.
• These dryers are of bin type, tunnel type, belt type, column type, or rotary type.
• Some forced circulation type solar dryers use some kind of thermal storage unit, heat
recovery wheel and auxiliary heating arrangement.
• Auxiliary energy may be supplied either by electric heating or oil or gas burners and
used only when solar air heaters or the heat from the thermal storage device is not
sufficient to supply necessary energy for drying the product.
Forced Circulation Type Solar Dryer (contd.)
• Several storage systems are proposed but the most preferred one is the rock bed storage
system which stores the heat in the form of sensible heat and performs the dual function
of storing the heat and that of a heat exchanger.
• A hybrid solar dryer (solar assisted) was developed at Fresno, California for drying large
amount of fruits and vegetables.
• It consists of several solar air heaters with a total area of 1350 m2, a thermal storage (rock
bed type) of 350m3 volume, a rotary wheel type heat recovery whell and a tunnel
dehydration in which 14 trucks loaded with prepared food move at a rate of 24 hours per
truck in one direction and the heated air is sent from the other direction.
• The system is designed for a fixed air flow rate of 9.5m3/s to the dehydrator 24 hours a
day. The drying temperature varies from 60C in the beginning of June to 66C in August
to September.
• The solar contribution in this hybrid systems is 1582 MJ/hr which is about 60 per cent of
the total heat requirement of drying.
Photograph of Forced Circulation Type Solar Dryer
Leather Dryer with Roof mounted Solar Air Heaters
(4 x 167m2 area) at Ranipet, Chennai
Typical grain drying bin
SOLAR DESALINATION
Solar Desalination Systems
Water is one of the most abundant resources on earth, covering
three-fourths of the planet's surface.
About 97% of the earth's water is salt water in the oceans; 3% of all fresh
water is in ground water, lakes and rivers, which supply most of human and
animal needs.
The only nearly inexhaustible sources of water are the oceans  Their
main drawback, however, is their high salinity.
It would be attractive to tackle the water-shortage problem with
desalination of this water.
WATER DESALINATION TECHNOLOGY
• Nature is carrying out the process of water desalination since ages.
• Oceanic water due to solar heating converts into vapours and pours down as
precipitation on earth in the form of fresh water.
• Water is the most needed substance on the earth for sustenance of life.
• Due to rapid expansion of population, accelerated industrial growth and
enhanced agricultural production, there is ever increasing demand for fresh
water.
• Demand of fresh water (potable water) has increased from 15-20
litres/person/day to 75-100 litres/person/day.
• The ocean covers 71 recent of the earth's surface-140 million square miles with a
volume of 330 million cubic miles and has an average salt content of 35,000 ppm.
• Brackish/saline water is strictly defined as the water with less dissolved salts than
sea water but more than 500 ppm.
SOLAR DESALINATION TECHNIQUES
Potable Water
Less than 550 ppm
Requirement
Domestic, Industries and Agriculture
Sources of Potable Water
Rivers, Lakes, Ponds, Wells etc.
Demand of Potable Water
15-25 litres / person / day (OLD)
100-125 litres / person / day (NEW)
Underground Saline Water
2,000 – 2,500 ppm
Sea Water
30,000 – 50,000 ppm
WATER DESALINATION TECHNOLOGY
• Potable water (fresh water) suitable for human consumption should not contain
dissolved salts more than 500 ppm.
• For agricultural purposes, water containing salt content of 1000 ppm is considered
as the upper limit.
• Potable water is required for domestic, agriculture and industries.
• Some applications in industries like cooling purposes, sea water is feasible despite
the corrosion problems while other industries use higher quality water than is
acceptable for drinking water. Modern steam power generation plant need water
with less than 10 ppm.
• Potable/fresh water is available from rivers, lakes, ponds, wells, etc.
• Underground saline/brackish water contains dissolved salts of about 2,000-2,500
ppm.
METHODS OF CONVERTING BRACKISH WATER INTO
POTABLE WATER
• DESALINATION: The saline water is evaporated using thermal energy and the
resulting steam is collected and condensed as final product.
• VAPOR COMPRESSION: Here water vapour from boiling water is compressed
adiabatically and vapour gets superheated. The superheated vapor is first cooled to
saturation temperature and then condensed at constant pressure. This process is
derived by mechanical energy.
• REVERSE OSMOSIS: Here saline water is pushed at high pressure through special
membranes allowing water molecules pass selectively and not the dissolved salts.
• ELECTRODIALYSIS: Here a pair of special membranes, perpendicular to which there
is an electric field are used and water is passed through them. Water does not pass
through the membranes while dissolved salts pass selectively.
In distillation; thermal energy is used while in vapour compression, reverse osmosis,
electrodialysis, etc. some mechanical and electrical energy is used.
Schematic of basin-type solar still
COMPONENTS OF SINGLE EFFECT SOLAR STILL
1.
2.
3.
4.
5.
6.
7.
Basin
Black Liner
Transparent Cover
Condensate Channel
Sealant
Insulation
Supply and Delivery System
SOLAR STILL OUTPUT DEPENDS ON MANY
PARAMETERS
1. Climatic Parameters
I.
II.
III.
IV.
V.
Solar Radiation
Ambient Temperature
Wind Speed
Outside Humidity
Sky Conditions
2. Design Parameters
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
Single slope or double slope
Glazing material
Water depth in Basin
Bottom insulation
Orientation of still
Inclination of glazing
Spacing between water and glazing
Type of solar still
SOLAR STILL OUTPUT DEPENDS ON MANY
PARAMETERS
Contd
3.
Operational parameters
I.
II.
III.
IV.
V.
VI.
…
Water Depth
Preheating of Water
Colouring of Water
Salinity of Water
Rate of Algae Growth
Input Water supply arrangement (continuously or in
batches)
Double sloped experimental solar still
MULTIBASIN STEPPED SOLAR STILL
1. In this still small basins are put in steps like a stair case and named as multibasin
system.
2. The still consists of saline water trays in an airtight box with glass cover, charging
"pump, and distillate collecting tray.
3. The projected basin area of the still is 0.836 m2 and the glass cover is inclined at 45
degree. The depth of water in tray was 5.0 cm.
4. This still has not given higher output because of higher heat loss.
Multibasin stepped solar still
WICK TYPE SOLAR STILL
ADVANTAGES
1. Low thermal capacity resulting in higher output,
2. Receives and absorb more solar radiation because of optimum inclination.
PROBLEMS
1. Chocking of pores of wick with salt in due course of time,
2. Deterioration of the wick cloth,
3. Decolouration of wick cloth,
4. Difficult to maintain uniform flow of water.
Cross – section of titled multiple wick type
solar still
Regenerative inclined step solar still
Photograph of the multi effect H-D Solar Distillation unit at IIT Delhi
Multiple effect diffusion solar still
SOLAR COOKING
BOX SOLAR COOKER
Parabolic Domestic Solar Cooker
Principle of working of Scheffler Community Solar Cooker
World's Largest Solar Steam Cooking System at Tirupati, Andhra Pradesh
Location
• Installed at the temple town of Tirumala, Andhra Pradesh with nearly 50 percent funding from
MNES
System
• Employs automatic tracking solar dish concentrators to convert water In to high pressure
steam which Is used for cooking purpose* In the community kitchen.
Technical Details
• Solar dish concentrators (106 Nos) with total reflector area of about 1000m2.
• Modular in nature and consists of several units (parallel & series) connected to central pipeline system.
• Each dish consists of scheffler mirrors with an aperture area of 9.4 sq.m.
• Generates 4,000 kg of steam per day at 180°C and 10 Kg/cm2.
• Cook meals for around 15,000 persons per day.
• The cooker saves about 1,20,000 litres of diesel per year.
• The total cost of the system Is about Rs. 110 lakh.
Implementing Agency
• Ministry of Non-Conventional Energy Sources (MNRE).
World’s Largest Solar Steam Cooking System
WORLD’S LARGEST SOLAR STEAM
COOKING SYSTEM AT TIRUPATI
Solar Cookers
• Solar cookers are used and developed in
several countries including India.
• In India all the three types of solar cookers
like box type, reflector type (SK type) and
large size (Scheffler type) are in use.
• Typical box type solar cooker takes 2-2.5
hours for cooking of food and is 60x60x17
cm in size.
• Parabolic domestic solar cooker (SK-14)
with reflector diameter 140 cm gives about
200C temperature and cooks food for 810 persons at a time.
• Scheffler community solar cooker with
single or multiple dish (each of about 10
m2 reflector) is suitable for community use
and one such solar cooker for cooking
meal for 15000 people is in use at tirupati,
A.P with total reflector area of 1000 m2.
Thank You