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Chapter-2 Solar Thermal Energy - A Ray of Hope 13
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
CHAPTER – 2
Solar Thermal Energy – A Ray of Hope
Chapter-2 Solar Thermal Energy - A Ray of Hope 14
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
2.1 INTRODUCTION
A solar thermal system has wide applications such as hot water
generation, operating house-hold electrical appliances, irrigation,
drying of fruits and vegetables, timber, coconut etc. A solar thermal
system can heat water or air. Usually, solar panels are installed on the
roof or on a frame at the ground level and a hot water cylinder is
installed near the panels.
Fig. 2.1 80 – 20 Principle of solar energy
2.2 PRINCIPLE
The solar panels (collectors) heat up when day light (the Sun's
radiation) falls onto them. This radiation heats up a liquid inside the
panel, usually anti-freeze. This heat is then transferred by the heat
transfer medium to the cylinder or heat exchanger to heat the water.
When cold water is pumped through the solar panel it will be heated
by solar energy [3]. This hot water can be used directly for industrial
purposes or can be supplied to the boiler to generate steam.
80 % AS HEATENERGY20 % AS LIGHT
ENERGY
Chapter-2 Solar Thermal Energy - A Ray of Hope 15
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
In fact, a solar thermal system needs daylight to work, not
direct sunlight. Any solar collector works on the amount of Sun's
radiation or light received on Earth. The energy can be collected on
clear summer days as well as on cloudy winter days. Current thermal
systems are much more efficient than ever before. In summer the
water in the cylinder can reach up to 800C within half a day and the
insulation on the cylinder will keep the water warm for the following
day [3].
Typically, the solar hot water system will reduce the firewood
consumption, deforestation and pollution.
2.3 SOLAR WATER HEATING SYSTEM [4]
A solar water heating system (SWHS) is made of several
important elements: one or more solar collectors, a pump, a heat
exchanger, a storage tank (or multiple tanks) and a back-up storage
tank. The Solar heating system can be classified as passive or
active.
For water heating purposes, the general practice is to use flat-
plate solar-energy collectors (FPC). However, although the Evacuated
Tube Collectors (ETC) and Evacuated Tube Heat Pipe Collectors
(ETHP) are more efficient, even the initial cost is comparatively
higher.
2.3.1 Solar Collectors [5]
Every installation is different and every collector brand is made
slightly different, hence, the annual energy output may vary from one
Chapter-2 Solar Thermal Energy - A Ray of Hope 16
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
to other. The output also depends on the aperture area of the
collector, which is the opening that collects the sun’s radiation, but
not on the gross surface area of the solar collector [3].
A solar collector is very efficient at turning sun light into heat.
There is a special coating called an “Absorber Surface Coating” that is
spluttered or fused at a very high temperature, with the metal sheet
inside the solar collector. This surface makes the collector efficient
and effective. It is all about the "aperture area or absorber surface"
which is part of the collector that collects light and transforms into
heat. All efficient panels are made of a strong aluminum body that is
non-corrosive.
The heat absorbed by the solar collector is protected by a
toughened solar glass that covers the entire solar panel. This
transparent glass cover prevents wind and breezes from carrying the
collected heat away (convection). Together with the frame, the glass
protects the absorber from adverse weather conditions. Typical frame
materials include aluminum and galvanized steel; sometimes
fiberglass-reinforced plastic is used.
As far as the water heating system is concerned, the panels may
be either Flat-Plate type or Vacuum Tube type. Both flat-plate and
vacuum tube collectors can work on cloudy days. If we compare flat-
plates and evacuated tubes by aperture area, vacuum tubes are more
efficient than the flat plates.
Chapter-2 Solar Thermal Energy - A Ray of Hope 17
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
Vacuum Tubes are more suitable for water heating systems,
where high temperatures are required even in cold climates. For a
similar output, a 3 m² vacuum tube system is sufficient in place of a 5
m² flat plate system. The average lifetime of a well-designed and built
solar thermal collector is over 25 years, and the maintenance on solar
thermal system is very little. Once the installation is completed, there
are no further fuel costs. Also, it will save tons of CO2 being released
through conventional energy sources, into the atmosphere.
The systems from well experienced and reputed companies can
provide hot water for 25 years and are fully automated with no user
intervention. These collectors are also free from scales and frost
damage.
2.3.2 Advantages of Solar Water Heating System
Solar Water Heating System (SWHS) is more economical
compared to any other hot water generating systems that run on
conventional fuels. Generation of hot water is completed in a most
hygienic and eco-friendly way. SWHS has very low operation and
maintenance cost and last longer, typically 20 years to 30 years, with
minimum maintenance. The price for production of hot water does not
fluctuate. There will never be a shortage that will cause hot water to
become unaffordable. There are no expensive fuel transportation costs
for producing hot water as the sun shines everywhere.
Chapter-2 Solar Thermal Energy - A Ray of Hope 18
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
2.4 AVERAGE SOLAR RADIATION [6]
The average solar radiation in A.P. zone is 4.5 - 6 kWh/m²/day,
with an average of 300 clear days in a year. There is immense
potential solar radiation available to meet domestic and industrial
thermal energy demands. As per Ministry of Non-conventional Energy
Sources (MNES), the potential of solar water heating systems in the
country is around 30 million m² of collector area.
As a result of various promotional efforts, there has been a
steady growth in the cumulative collector area installed. The
cumulative collector area installed has increased from 119000 m² in
1989 to 525000 m² as of March 2001. As of 31 March 2005, in India,
the cumulative installed capacity of solar water heating systems was 1
million m² of collector area.
2.5 ENERGY CALCULATIONS
Thermal systems now are much more efficient than they were
years ago. For example, the maximum output from a domestic solar
hot water system (on a clear summer day) is approximately 700 W/m²
aperture areas [6].
A Flat-Plate (FP) system for 3 to 5 people, for example has an
aperture area of 4.4 m², then 4.4 m² x 700 W/m² = 3.08 kW is the
system output. So, a system of this size would have the same output
as a 3 kW immersion heater typically found in hot water cylinders.
The average ambient temperature in Andhra Pradesh would be around
Chapter-2 Solar Thermal Energy - A Ray of Hope 19
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
35°C. At maximum output of 3 kW, FPC based SWHS would take just
over 5 hours to heat 250 liters of water from 35°C to 65°C.
2.6 SWHS FOR SILK INDUSTRY [5]
The Solar Water Heating System (SWHS) is broadly of two types:
1) Flat-Plate Collector based SWHS (FPC-SWHS) 2) Evacuated Tube
Collector based SWHS (ETC-SWHS).
For water heating purposes, the general practice is to use Flat-
Plate Solar Collectors (FPC). But the Evacuated Tube Heat Pipe
Collectors or Evacuated Tube Collectors (ETHP or ETC) are more
efficient, even the initial cost is comparatively high.
2.6.1 Flat-Plate Collector based SWHS [5]
Fig. 2.2 Flat-plate collectors [7]
The FPC-SWHS, which is of older technology, can generate hot
water between 60°C and 70°C, depending on the size and quality of
the collectors. A flat-plate collector consists of an absorber, a
transparent cover, a frame, and insulation. Usually, a solar safety
Chapter-2 Solar Thermal Energy - A Ray of Hope 20
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
glass is used as a transparent cover, as it transmits a great amount
of short-wave light spectrum. Absorbers are usually black, as dark
surfaces will have high degree of light absorption.
As the absorber warms up to a temperature higher than the
ambient temperature, it gives off a great part of the accumulated solar
energy in the form of long-wave heat rays. The ratio of absorbed
energy to emitted heat is indicated by the degree of emission. The
most efficient absorbers have a selective surface coating, which
enables the conversion of a high proportion of solar radiation into
heat, simultaneously reducing the emission of heat.
The usual coatings provide a degree of absorption of over 90%.
Solar paints, which are painted or sprayed manually on the absorbers,
are not very selective, as they have a high level of emission. Some
selective coatings that include black chrome, black nickel and
aluminum oxide with nickel will absorb the heat more efficiently.
Relatively new is a titanium-nitride-oxide layer, which is applied via
steam in a vacuum process. This type of coating stands out because of
its low emission rates and efficiency.
2.6.1.i Cost of FP-SWHS
Assumptions: 1) Boiler consumes 2000 liters of water per day
2) Ambient temperature of cold water 300C – 350C 3) Sunshine days
as 300 per year 4) Cost of firewood Rs.2.00 per kg. (500 to 1000 liters
of water per day/boiler is the actual consumption by the reeling and
dyeing units.)
Chapter-2 Solar Thermal Energy - A Ray of Hope 21
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
FPC-SWHS of 2000 liters capacity, from a reputed company,
with 16 panels with 9 copper fins in each collector, to produce 65°C,
costs around Rs.3,50,000.00 to Rs.3,75,000.00, @ Rs.175.00 to
Rs.187.50 per liter (includes tank, freight and taxes). Nonetheless, the
plumbing charges are extra, which may be between 4% and 6% of the
total cost.
Table: 2.1 Returns-on-investment (Source: M/s Tata BP Solar India
Limited, Bangalore)
Customer: Silk Reeling and Dyeing Sectors
FP-SWHS Capacity 2000 Liters / day
FP-SWHS output temperature 65 Degree Celsius
Energy Calculations
Avg. Ambient temperature 30 Degree Celsius
Heat gained by SWHS
(2000 liters * (65-30))
70000 K.cals/day
Heat gained/year 21000000 K.cals/300 days
Energy Savings
Calorific Value of firewood 2800 K.cals/kg
Combustion Efficiency of fuel 55% (45% emission ofCO2, N2 and other gases)
Conversion efficiency 90% (10% for evaporation)
Equivalent fuel energy saved
through SWHS kg
15,152.00 21000000/(2800*0.55*0.9)
Cost saved / year through fuelRs.
30303.00 15,152.00 * Rs.2.00
Cost saved in fuel Inflation /year say @ 10% Rs.
3030.00
Cost saved in maintenancefuel cost inflation/ year say@ 10% Rs.
0.00
Total savings / year Rs. 33,333.00
Chapter-2 Solar Thermal Energy - A Ray of Hope 22
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
Table: 2.2 Payback calculations with FP-SWHS (Source: M/s Tata
BP Solar India Limited, Bangalore)
Estimated project cost (for 2000 liters FP-SWHS) - Rs.3,75,000.00
(@ Rs.187.5/- per liter )
Initial Investment
(Rs.)
Savings
(Rs.)
Balance at
year end (Rs.)
Yr.
No.
3,75,000.00 33,333.00 3,41,667.00 1
3,41,667.00 36,666.00 3,05,000.00 2
3,05,000.00 40,332.00 2,64,667.00 3
2,64,667.00 44,365.00 2,20,302.00 4
2,20,302.00 48,801.00 1,71,496.00 5
1,71,496.00 53,681.00 1,17,815.00 6
1,17,815.00 59,049.00 58,766.00 7
58,766.00 64,957.00 -(6,191.00) 8
The first common term is simple payback, which expresses the
number of years the investor has to wait before investment is paid
back out of fuel savings.
Here, the pay back period of FPC system is 7 years to 8 years.
2.6.1.ii Disadvantages
We are familiar with the flat-plate solar hot water systems seen
on roofs, collecting the radiant heat from the sun to heat the water. As
collection plates are flat, they have some limitations. They can only
operate at maximum efficiency when the sun is directly overhead at
midday. At other times, the sun rays that are striking the collector at
varying angles reduce efficiency.
Chapter-2 Solar Thermal Energy - A Ray of Hope 23
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
No sun tracking is possible due to flat surface collectors and the
maximum heat absorption is possible only at 12 noon. Further, each
FP panel weighs around 50 kg. Therefore, a 16 panel flat plate solar
water heating system alone weighs over 800 kg and weighs 2000-plus
kg load on a roof with water tank, which requires expensive
modification to bear around 3 tons weight. The hardness of water
should also be very less. If the hardness of water is more, then the
copper tubes will be choked due to scales that are very difficult to
remove.
These problems can be overcome through Evacuated Tube Solar
Hot Water Systems, as the collector consists of a series of tubes and
the sun rays are perpendicular to the tubes for most of the day,
allowing the system to operate at high efficiency for much longer than
flat-plate units.
Another advantage of the evacuated tube technology is that the
weight problems caused by standard flat plate systems are eliminated.
And the evacuated borosilicate tubes can work efficiently up to 600
ppm of water hardness [8].
The scales formed in the tubes can easily be removed due to
comparatively larger inner diameter of the tube. In ETC, circular glass
tubes allow auto sun tracking and ensure maximum heat absorption
through out the day.
Chapter-2 Solar Thermal Energy - A Ray of Hope 24
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
2.6.2 Evacuated Tube Collector based SWHS [9]
In order to reduce heat loss within the frame by convection, the
air can be pumped out of the collector tubes. Such collectors then can
be called as evacuated-tube collectors. They must be re-evacuated
once every one to three years.
Evacuated tube solar collectors are very efficient and can
achieve very high temperatures. They are well-suited to commercial
and industrial heating applications and can be an effective alternative
to flat-plate collectors for industrial purposes; especially in areas
where it is often cloudy. An evacuated-tube collector contains several
rows of glass tubes connected to a header pipe. Each tube has the air
removed from it to eliminate heat loss through convection and
radiation.
Fig. 2.3 Evacuated Tube Solar Collector (ETC)
Fig. 2.3 is a snapshot taken during demonstration of ETC
during Technology Up-gradation Programme organized at Silk
Chapter-2 Solar Thermal Energy - A Ray of Hope 25
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
Conditioning and Testing House (SCTH), Central Silk Technological
Research Institute (CSTRI), Central Silk Board (CSB), Dharmavaram.
2.6.2.i Types of Evacuated Tube Collectors [9]
There are two main types of evacuated tube collectors: i) Direct
Flow Evacuated Tube Collectors (ETC) and ii) Evacuated Tube Heat
Pipe (ETHP) collectors.
2.6.2.i.1 Direct-flow evacuated-tube collectors [9]
A direct flow evacuated tube collector consists of a glass tube,
with a flat or curved aluminum fin attached to a metal or glass pipe.
The fin is covered with a selective coating that transfers heat to the
fluid that is circulating through the pipes, one for inlet fluid and the
other for outlet fluid. In this type of vacuum collector, the absorber
strip is located in an evacuated and pressure proof glass tube. The
heat transfer fluid flows through the absorber directly in a U-tube or
in countercurrent in a tube-in-tube system. Several single tubes,
serially interconnected, or tubes connected to each other via manifold,
make up the solar collector.
2.6.2.i.1.a Cost of ETC-SWHS
ETC-SWHS of 2000 liters capacity, with 4 set collectors (1 set =
60 to 76 tubes) of 3-layer tubes, to produce 60°C to 80°C, from
reputed companies costs around Rs.3,00,000.00 to Rs.4,40,000.00
@ Rs.150.00 to Rs.220.00 per liter (which includes tank cost, freight,
Chapter-2 Solar Thermal Energy - A Ray of Hope 26
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
installation and taxes). Nevertheless, the plumbing charges are extra
that may vary 4% to 6% of the total cost.
The pay back period with ETC system could be 6 to 7 years.
2.6.2.i.1.b Requirement of Roof Area and Tank Height
For every 500 liters of ETC–SWHS, 3.5 m X 3.5 m shadow free
area is required. The system height from the roof top is 2.5 m and
minimum height of the over head tank from roof top should be 3 m.
2.6.2.ii.1 Evacuated Tube Heat Pipe Collectors
The ETHP based collectors do not heat the water directly within
the vacuum tubes. Instead, a sealed copper ‘heat pipe’ transfers the
heat via convection of its internal heat transfer fluid to a ‘hot bulb’
that indirectly heats a copper manifold within the header. The heat
pipes are inserted into curved absorbers forming an assembly which
in inserted into the glass tubes (Fig. 2.4). The tubes are made of
borosilicate glass, which is strong and has a high transmittance for
solar irradiation [10].
Heat pipes in the collector are inorganic, nontoxic heat transfer
compound and have continuous operating life of more than 1, 10,000
hours [10]. The heat pipe is hollow and the space inside is also
evacuated. The heat pipe contains a small quantity of liquid, such as
alcohol or purified water plus special additives. When sunlight falls on
the surface of the absorber, the liquid in the heat tube quickly turns
to hot vapor and rises to the top of the pipe [9].
Chapter-2 Solar Thermal Energy - A Ray of Hope 27
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
Fig. 2.4 Header pipe inserted in glass tube [10]
Glass evacuated tubes are the key component of the Evacuated
Tube Heat Pipe (ETHP) solar collectors. Each evacuated tube consists
of two glass tubes (Fig. 2.5). The outer tube is made of extremely
strong transparent borosilicate glass that is able to resist impact from
hail up to 38 mm in diameter [11].
Fig. 2.5 ETHP Glass Tube [12] Fig. 2.6 Array of ETHP solarcollectors [12]
Interestingly, the total weight of 20 Vacuum Tube Collectors
(say 1 set) would be 55 kg to 61 kg. Generally, the diameter of each
vacuum tube is 47 mm. The panel area of a 20 tube collectors would
be 2.25 m2 with an absorber or aperture area of 2.20 m2. Normally,
the angle of inclination is 150 to 900 [10].
Chapter-2 Solar Thermal Energy - A Ray of Hope 28
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
In good conditions it will take less than 5 minutes for the tip of
the heat pipe to get too hot to hold. The tube will continue to provide
heat even after the sun has set [10]. Evacuated tubes offer the
advantage that they work efficiently with high absorber temperatures
and with low radiation. Higher temperatures also may be obtained for
applications such as hot water heating, steam production, and air
conditioning [13].
The ETHP – SWHS can generate hot water even up to 95°C.
For example, if the entrepreneur desires to have hot water at
95°C, it may be very difficult to attain it with FP-SWHS, but it can be
attained through evacuated tube heat pipe collectors (ETHP).
Fig. 2.7 ETHP Vs Flat-plate collector efficiency diagram [12]
The efficiency diagram (Fig.2.7) depicted above illustrates the
effectiveness of both flat-plate and evacuated tube hot plate collectors.
The figure illustrates that at lower temperatures (40°C), the efficiency
of both flat-plate collectors and Evacuated Tube Heat Pipe collectors is
same; whereas at high temperatures (above 90°C) ETHP collectors
perform effectively.
ETHP v/s Flat Plate Collector
00.10.20.30.40.50.60.70.8
40 50 60 70 80 90 100 110 120 130
Heating Fluid Temperature
Colle
ctor
Effi
cienc
y
Flat Plate CollectorETHP Collector
Chapter-2 Solar Thermal Energy - A Ray of Hope 29
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
2.6.2.ii.1.a Cost of ETHP-SWHS
Assumptions: While solar energy is available for 10 hours a day, 300
days a year and 65 days in a year are considered as cloudy days.
We can expect shorter paybacks in areas with higher energy
costs. After the payback period, we accrue the savings over the life of
the system, which ranges from 15 years to 25 years or even more,
depending on the system and how well it is maintained.
ETHP based SWHS of 2000 liters capacity having 13 sets (1 set
=20 ETHP tubes), which produces 90°C hot water costs approximately
Rs.11,00,000.00 @ Rs.550.00 per liter. Tank (to store hot water) and
pump (to pump the hot water from solar tank to the boiler) costs
would be extra. The tank of 2000 liters capacity costs approximately
Rs.1,20,000.00 and the hot water pump of 0.5 HP costs around
Rs.10,000.00. The freight and taxes may be around 4% and plumbing
charges vary from 4% to 6% of the total cost.
The payback period with ETHP system would be 4 to 5 years.
(Courtesy: M/s Mamata Energy Private Limited, Ahmadabad, Gujarat).
2.6.2.ii.1.b Advantages of ETHP System [14]
Solar collectors can often be used in subzero temperatures
without the system sustaining damage. Flat plate systems often
require expensive and complicated "antifreeze" systems to be installed.
Evacuated tubes are strong, long lasting, inexpensive and easy to
replace. Their maintenance cost is quite negligible.
Chapter-2 Solar Thermal Energy - A Ray of Hope 30
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
If a flat plate collector panel is damaged the whole panel must
be replaced. Due to the high efficiency absorption of solar radiation
even during overcast conditions, combined with excellent insulation
properties of the solar tube, solar tube collectors can heat water all
year round.
Due to various advantages of evacuated tube collector over flat
plate collectors, a smaller collector can be used to provide the same
heating performance. When averaged over an entire year, evacuated
tube collector heat output per net m2 of absorber area, is much
greater than a flat plate collector. It is also very easy to install.
As per the list of Ministry of New and Renewable Energy, there
are two - ETHP-SWHS, seventy seven - ETC-SWHS and sixty three -
FP-SWHS manufacturers available in India, as approved by Bureau of
Indian Standards (BIS) [6].
2.7 SOLAR HOT AIR GENERATING SYSTEM
Apart from hot water requirement, it is also very much
essential to generate hot air for various applications like silk cocoon
stifling, silk yarn drying on reeling/re-reeling machines etc., which
are discussed in detail in Chapter 3. The conventional hot air drying
methods burden the entrepreneurs. This recurring financial burden
can be significantly minimized through free solar energy.
We can achieve the hot air of ~85°C temperature, through
Evacuated Tube Heat Pipe based Solar Hot Air Generating System
(ETHP-SHAGS). This is 74% of the maximum temperature i.e. 115°C
Chapter-2 Solar Thermal Energy - A Ray of Hope 31
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
that is required in conventional method of electrical hot air stifling.
Therefore, the silk industry can make use of the common solar energy
systems with electronic temperature controllers to minimize the
recurring expenditures on electricity and firewood. As SHAGS can
generate ~85°C hot air, it is easy to raise another ~30°C through
electrical heaters, which is discussed in Chapter 3.
Generating hot air from the sun's energy is the most efficient,
cost-effective way of transforming solar energy to domestic and
industrial energy use. The collectors which are used for generating
hot air are called “Solar Forced Air Heaters” because cool air is forced
by a fan through a specially designed solar heating panel that
absorbs heat energy generated by the sun. The solar energy for heat
conversion is instant and incessant as long as the sun shines,
including light cloudy days [15].
Drying is the removal of moisture from a product until its
moisture content is low for safe storage. The method of renewable
energy based drying is not a new concept, but the place of application
may be new. The process involves vaporization of water in the product
and carrying away the water vapor mixed with drying air. The factors
that affect the rate of removal of moisture from the product are
temperature, humidity and flow rate of air. Solar dryers are classified
as direct solar dryers, indirect solar dryers and mixed mode solar
dryers [16].
Direct solar dryers consist of an insulated chamber with a
transparent cover, and appropriately placed openings to facilitate
Chapter-2 Solar Thermal Energy - A Ray of Hope 32
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
airflow. Products to be dried are spread in a thin layer on wire mesh
trays and placed inside the chamber. In indirect solar dryers,
products are not exposed to direct sun, but hot air from a solar air
heater is passed through the drying chamber containing the products.
Airflow can be due to natural convection or forced circulation using
fans or blowers [17].
Indirect solar dryers are particularly useful in drying certain
high value products that are sensitive (like cocoons) to direct solar
radiation. In mixed mode dryers, where the features of both direct and
indirect solar dryers are combined, hot air from solar collectors is
passed through the product in a chamber with transparent cover and
absorber walls. There are many variations among the dryer designs.
Some solar dryers have heat storage elements to extend the drying
time into night periods [17].
In hybrid dryers, solar air heaters are supplemented with air
heated by a furnace operated with kerosene, Liquefied Petroleum Gas
(LPG), or biomass; therefore, drying can be continued over night or in
rainy seasons, resulting in considerably shorter drying times.
Successful drying depends on enough heat to draw out
moisture, dry air to absorb the released moisture; and adequate air
circulation to carry off the moisture. When drying cocoons, the key is
to remove moisture as quickly as possible at a temperature that does
not seriously affect the texture and color of them.
Chapter-2 Solar Thermal Energy - A Ray of Hope 33
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
Fig. 2.8 Solar hot air generating system with air storage tank [12]
Fig. 2.8 shows a solar hot air generating system with ETHP
solar panels and a storage tank, for storing the hot air that is
generated. Ambient air is heated by the solar collector, and the hot
air is passed through the drying chamber where products to be dried
are loaded. Airflow can be due to natural convection or forced
circulation using fans or blowers (Fig. 2.9).
Fig. 2.9 Schematic diagram of SHAGS [12]
Chapter-2 Solar Thermal Energy - A Ray of Hope 34
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
Effective drying is accomplished with a combination of heat and
air movement. Too much heat especially early in the process will
prevent complete drying. The hot air generated through the solar-
forced air heaters can be sent directly to the product to be dried or
can be stored in a puffed storage tank for later use.
The products are spread into thin layers in several trays inside
the drying cabinet to facilitate better contact of drying air with the
product. Warm moist air from inside the drying chamber exits through
the chimney at the top, assisted by draft created in the chimney.
There are many variations among the dryer designs. Some solar
dryers have heat storage elements to extend the drying time overnight.
The hot air dryers provide hot air continuously, which can be used for
stifling the cocoons in the present 5 kW to 7 kW, 3-Phase, 440 V
electric hot air dryers, available with the reelers, so that the electrical
charges can be slashed.
Also, with the same hot air, the yarn on reeling and re-reeling
machines too can be dried effectively, so that 30% to 40% of steam
consumption can be reduced.
2.7.1 Benefits of Solar Hot Air Drying System
The use of solar hot air drying system to dry the cocoons and
yarn is viable and economical. Solar dryer system improves the quality
of the product, while reducing waste and traditional fuels. SHAGS has
very low payback and optimal performance even at low ambient
temperatures. Another advantage is, seamless integration with
Chapter-2 Solar Thermal Energy - A Ray of Hope 35
Thesis Submitted to Jawaharlal Nehru Technological University, Anantapur
existing systems i.e. it is easily attachable with present conventional
drying systems [12].
It is most suitable for hot air requirements ranging start from
30°C to 85°C. It has a long life, low maintenance. Solar dried products
reduce fuel transportation and storage costs as well as associated
problems from climatic effects. Solar dryers are a cost-effective
solution. Implementing the use of solar drying systems will result in
significant savings to the reeling entrepreneurs.
Solar hot air is ideal for drying purposes in silk and other textile
industries.
2.8 CONCLUSION
Current thermal systems are much more efficient than ever
before. Depending upon the type of the SWHS installed the output hot
water temperatures can vary from 60°C to 95°C. Though the initial
cost for procuring the SWHS is high, the returns on investment can be
achieved with in a short period by means of saving the firewood. The
pay back period depends on the SWHS system that adopted for silk
industry. We can expect shorter paybacks in areas with higher energy
costs.
Similarly, SHAGS can solve several obstacles that involved in
the silk industry. Hot air that generated through it can slash the
electrical bill and saves 30% to 40% of steam. Typically, the solar
thermal systems will reduce the firewood consumption, deforestation
and pollution.
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