This document discusses various solar technologies including solar air heaters, solar dryers, and solar ponds. It provides details on the components, working principles, types (direct, indirect, mixed-mode), examples, advantages and limitations of each technology. Solar air heaters are used to heat air using solar energy by passing air through an absorber plate. Solar dryers utilize solar energy to dry agricultural crops and utilize direct, indirect or mixed-mode designs. Solar ponds store solar heat by creating salt concentration gradients that inhibit convection currents.

1. Md. I. A. Ansari
Department of Agricultural Engineering
(e-mail: irfan26200@yahoo.com)
Renewable Energy and Green Technology

2. Solar Air Heater
• Normally air is heated using fossil fuels,
electricity and steam for various purposes.
• Air can be heated using solar energy as it
is an ideal alternative clean, green,
inexhaustible and freely available source
of energy.
• The device which is used for heating air
using solar energy is called solar air
heater.

3. • A solar air heater consists of
transparent cover, insulation, pipe and
absorber plate. Air blowers or fans are
used in active system.
• The heat absorbed by the absorber plate
is transmitted to the air drawn into the
collector.
• If the size of collector is large, a blower
is used to draw air into the collector.

6. Application of Solar Air Heater
• Heating building
• Drying agricultural produces
• Heating greenhouse

7. Solar Drying
• Drying is an important unit operation in
which food materials are dried to safe
moisture level to store it for a longer time
period without spoilage.
• Drying of agricultural produces is an
energy-demanding process.
• For drying applications, conventional fuel
such as biomass, oil, or electricity is used
for heating air which is expensive and its
use causes pollution.

8. • Open sun drying is the most popular
method among farmers for crop drying in
which solar energy is utilized.
• Open sun drying is a slow process,
laborious, unhygienic, time consuming and
losses due to birds, rodents and insects are
more.

9. • The process of drying in which solar
energy is utilized for drying food
materials is called solar drying.
• The device which is used for drying food
materials using solar energy is called
solar dryer.
• With the rising costs of conventional
fuels and increasing awareness of the
dangers of pollution, solar dryers are
becoming a technically and
economically viable option in many
industrial and agricultural applications.

10. Types of solar dryers:
 Direct solar dryer
 Indirect solar dryer
 Mixed mode solar dryer

11. Direct Solar Dryer
Direct solar dryer is also called integrated
solar dryer in which solar energy collection
and drying take place in a single unit.
In direct solar dryer, food materials are
exposed to direct sunlight and solar radiation is
converted into heat energy due to greenhouse
effect.
The dryers also have a black absorbing surface
which collects the light and converts it to heat.

12. These driers have enclosure, glass
cover or plastic cover and vents in
order to increase efficiency.
Examples: Cabinet dryers, rack dryers,
tunnel dryers, greenhouse dryers, and
multi-rack dryers

13. Direct Solar Dryer

14. Polyethylene tent passive solar dryer

15. Cabinet Solar Dryer
• A cabinet type solar dryer is suitable for
small scale use.
• The dryer consists of an enclosure with
a transparent cover.
• The material to be dried is placed on the
perforated trays.

16. • The solar radiation entering the enclosure is
absorbed by the product itself and the
surrounding internal surfaces of the
enclosure.
• Due to greenhouse effect, temperature of air
increases and heat energy of air is utilized in
evaporating the moisture of food materials.
• Gradually the heated moist air goes up and
leaves the drying chamber through the air
outlet at the top of the drier.

17. • Suitable openings at the bottom and top
ensure a natural circulation.
• Temperature from 50-80ºC is attained
and drying time ranges from 2-4 days.
• Products like dates, apricots, grapes,
chillies, turmeric, vegetables, etc., can
be dried in a cabinet dryer.

20. Fig. Schematic diagram of cabinet solar dryer

21. Solar Cabinet Dryer

22. Solar Tunnel Dryer
• The solar tunnel dryer is a polyhouse dryer
suitable for drying of most of the food crops.
• It consists of a tunnel type semi-cylindrical
drying chamber provided with windows to allow
the ambient air to enter the dryer.
• The dryer is covered by UV stabilized semi
transparent polythene sheet of 200 micron
thickness.

23. • An exhaust fan is provided to
evacuate the moist air from the dryer.
• A sliding door is also provided to
facilitate easy handling of the produce.
• Trolleys and trays are provided inside
the dryer to hold food materials for
drying.

28. Indirect Solar Dryer
• Indirect solar dryer is also called distributed
solar dryer in which solar energy
collection and drying take place in
separate unit is called distributed solar
dryer.
• This type of solar dryer has two parts:
• A flat-plate air heater and
• A drying chamber

29. • In indirect type dryer, the solar radiation
does not fall on the products to be dried.
Air is heated separately in a solar air heater
and then forced into the drying chamber in
which the product to be dried is placed
and exits through a chimney.
• These dryers are suitable for food grains,
tea, tobacco, spices, fruits, vegetables,
etc.

30. Indirect Solar Dryer

35. Cross section of chimney type solar dryer

36. Mixed Mode Type Solar Dryer
• A solar dryer in which solar energy
collection takes place in both air heater
and drying unit, and drying takes place
in the drying unit only is called mixed-
mode solar dryer.
• In this dryer, solar energy is collected
through flat-plate solar collectors and
also by the roof of the drying chamber.

37. Mixed Mode Solar Dryer

38. Schematic layout of the hybrid indirect solar-electrical dryer
Hybrid Solar Dryer

41. Thermal Efficiency of Solar dryer
• The efficiency of a solar dryer is
defined as the ratio of the rate of heat
gain to the rate of heat input.
• Efficiency=[rate of heat gain x 100]/ rate
of heat input

44. A solar dryer is used to dry 100 kg of chillies
from initial moisture content of 70% (wb) to final
moisture content of 10 % (wb). The dimension
of one collector is 2m length and 1 m width.
The number of collector used for drying was 5.
The average solar insolation during the study
period was observed as 600 Wm-2. The
effective drying time was 16 hours. The initial
temperature of air and chillies was 27 ºC and
temperature of hot air was 50 ºC. The latent
heat of vaporization of water at 50 ºC is 2260
kJ/kg. The specific heat of chilli is 2.0 kJ/kg ºC.
Compute the thermal efficiency of solar dryer.

45. Solution: Total collector area=5 x 2x1=10 m2
1W=1J/s and 1 kWh=3600 kJ
Rate of input solar energy=A x I=10 x
600=6000 J/s=6 kJ/s
Total amount of chillies=100 kg
Moisture content on wet basis=[Weight of
water x 100]/Total weight of material
Or, 70/100= Weight of water/100
Or, Weight of water=0.70 x 100=70 kg
Weight of bone dry solid=100-70=30 kg

46. • Again, Moisture content on wet basis=[Weight
of water x 100]/Total weight of material
• Or, 0.1= Weight of water/[ Weight of water+
Weight of bone dry solid]
• Or, 0.1= Weight of water/[ Weight of water+ 30]
• Or, 0.1 X (Weight of water+ 30)= Weight of
water
• Or, 0.9 x Weight of water=3
• Or, Weight of water=3/0.9=3.33 kg
• Hence, amount of water evaporated during
drying=70-3.33=66.67 kg

47. • Now, rate of heat energy required for
drying=[100 x 2 x (50-27)+66.67 x
2260]/[16 x 3600]
• =2.69 kJ/s
• Thermal efficiency of solar dryer=[2.69 x
100]/6=44.83 % Ans.

48. Advantages of solar dryers:
• Much faster drying than open sun drying
• Less risk of spoilage because of the speed
of drying.
• The product is protected against flies, pests, rain
and dust.
• The quality of the product is better in terms of
nutrients, hygiene and colour.
• Solar dryers are more economical compared to
dryers that run on conventional fuel/electricity.
• Solar drying systems have low operation and
maintenance costs.

49. Limitations:
• Drying can be performed only during
sunny days.
• Solar drying process is slow in comparison
with dryers that use conventional fuels.

50. Solar Pond
• A solar pond is a water pond in which
significant temperature rises are
caused in the lower regions by
preventing the occurrence of
convection currents.
• The solar pond is a simple device used
for collecting and storing sensible solar
heat.

51. • A solar pond is mass of shallow water
about 1 – 3 m deep with a large
collection area, which acts as a heat
trap.
• A solar pond is also called salt-gradient
solar pond or non-convecting solar
pond.

53. • When solar radiation strikes the pond,
most of it is absorbed by the surface at
the bottom of the pond.
• If the pond contained no salt, the bottom
layer would be less dense than the top
layer as the heated water expands.
• The less dense layer would then rise up
and the layers would mix.

55. • The surface area of the pond affects the amount of
solar energy it can collect.
• The bottom of the pond is generally lined with a
durable plastic liner such as black polythene.
•
• This dark surface at the bottom of the pond increases
the absorption of solar radiation.
• Salts like magnesium chloride, sodium chloride or
sodium nitrate are dissolved in the water, the
concentration being densest at the bottom (20% to
30%) and gradually decreasing to almost zero at the
top.
• Typically, a salt gradient solar pond consists of three
zones or layers .

56. Layers of Salt Gradient Solar Pond

58. • An upper convective zone of clear fresh water
that acts as solar collector/receiver and which
is relatively the most shallow in depth and is
generally close to ambient temperature.
• A gradient which serves as the non-convective
zone which is much thicker and occupies more
than half the depth of the pond.
• Salt concentration and temperature increase
with depth.

59. • A lower convective zone with the
densest salt concentration, serving as
the heat storage zone.
• Almost as thick as the middle non-
convective zone, salt concentration and
temperatures are nearly constant in this
zone .

60. • But the salt density difference keeps
the ‘layers’ of the solar pond separate.
• The denser salt water at the bottom
prevents the heat being transferred to
the top layer of fresh water by natural
convection, due to which the
temperature of the lower layer may rise
to as much as 95°C .

61. • Pumping the brine through an external
heat exchanger or an evaporator extracts
the heat from this bottom layer.
• Another method of heat removal is to
extract heat with a heat transfer fluid as it
is pumped through a heat exchanger
placed on the bottom of the pond.

64. Examples of solar ponds:
• Bhuj solar pond in India
• Solar pond power plant of 150 kW in Israel.
• Salinity gradient solar pond in the United
States
• Demonstration solar pond in Melbourne,
Australia

65. Bhuj Solar Pond
• The Bhuj solar pond has been constructed by
the Tata Energy Research Institute (TERI).
• Area: 6000 square-metre
• The solar pond is 100 m long and 60 m wide
and has a depth of 3.5 m.
• Hot water is used for dairy at an average
temperature of 75°C .
• The pond contains 4000 tonnes of common
salt.

66. Bhuj solar pond, constructed by the Tata Energy Research
Institute (TERI)

67. Applıcatıons
 Boiler feed water heating
 Domestic hot water production
 Agricultural crop drying
 Desalination of salt water
 Heating and cooling of buildings
 Solar Refrigeration
 Power generation
 Process heat for production of chemicals,
foods, textiles, and other industrial
products.
 Heat for greenhouses

68. • A 2000 m2 solar pond has been constructed to
provide hot water for a swimming pool in
Miamisburg, Ohio, USA.
• A 3355 m2 solar pond at Texas has
demonstrated the use of a solar pond for food
processing, power generation and
desalination.
• The feasibility of grain drying using a solar
pond has been demonstrated at Montreal
(Canada), Ohio (USA) and for heating
greenhouses at Lisbon (Portugal).
• A 20,000 m2 solar pond in Italy has been used
for desalination of sea water and producing
120 tonnes of fresh water per day.

69. Advantages
• The heat storage is massive, so energy
can be extracted day and night
• Environment friendly energy – no
pollution
• Low maintenance costs

70. Limitations of solar ponds:
 Need large land area
 Require sunny climate
 Availability of brackish water
 Due to evaporation, saline water is
constantly required to maintain salinity
gradients.