Useful for B. Sc (Ag.) and B. Tech (Agril. Engg.) students.

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

2. Solar Water Heating
• Hot water is used for domestic as well as in
industrial application.
• In conventional method, water is heated by
burning fuel or by electrical heating.
• Solar energy can be utilized for water
heating and huge amount of energy can be
saved.
• The device in which solar energy is used to
heat the water is called solar water heater.

3. • Solar water heater (SWH) has been accepted
for commercial, industrial as well as domestic
application in towns and cities.
• It has very high potential for the use in rural
households, livestock production, and agro
industries activities.
• A large number of small and medium scale
manufacturers are producing and marketing
natural convection flat plate SWHs in the
country.

4. In general, a solar water heater consists of:
• Absorbers
• Glazing cover over the absorber
• Hot water tank
• Connecting pipes

5. Solar Water Heating
Types of solar water heating systems:
(i) Natural circulation (thermosyphon)
system (Passive)
(ii) Forced circulation system (Active)
• In both the cases, a flat-plate solar
collector or evacuated tube solar
collector is used to absorb the solar
radiation energy to heat the water.

6. Natural Circulation Water Heating System
• A natural circulation system consists of a
tilted collector, with transparent cover
plates, highly insulated water storage tank,
and well-insulated pipes connecting the
collector and storage tank.
• The bottom of the storage tank is kept at
least one foot higher than the top of the
collector.
• Circulation occurs through natural
convection currents, or thermo-siphoning.

7. • When water in the collector is heated by
the solar radiation, the density of water
decreases and rises up the collector,
through a pipe and into the top of the
storage tank.
• This forces cold water at the bottom of the
tank and flow out from storage tank by
gravity, enter into the bottom of the
collector through pipe provided at the
bottom of the storage tank.

8. • Thus water is heated and rises up into the
tank.
• The hot water remains at the top of the
insulated tank.
• This type of system does not involve any
pump, hence it is called passive.

9. Schematic diagram of natural circulation water heating system

10. Evacuated Tube Solar Water Heater

11. Forced Circulation Water Heating System
(Active System)
• When a large amount of hot water is
required for supplying process heat in an
industry or in a commercial establishment,
hotels or hostels etc, a natural circulation
system is not suitable.
• Large arrays of flat plate collectors are
then used and forced circulation is
maintained with a water pump.

12. • Water from a storage tank is pumped
through a collector array where it is heated
and then flows back into the storage tank.
• The pump for maintaining the forced
circulation is operated by an on-off
controller which senses the difference
between the temperature of the water at
the exit of the collectors and a suitable
location inside the storage tank.

13. • When the temperature in the storage tank is
reduced, the thermal controlling system
operates the pump and cold water is pumped
to the collectors.
• If the temperature of water in the storage tank
reaches to a predetermined value, the pump
automatically stop the pumping water from the
tank to collector.
• In the absence of solar energy, the auxiliary
heater operated by electrical power is used.

14. Forced Circulation Solar Water Heating System

15. A solar flat plate collector having collector
area of 2 m2 is used in 100 liters capacity
thermo- siphon solar water heating system.
The average solar radiation falling on
collector during a typical day is 5 kWh/m2.
Assume that water is not drawn during the
day. The initial temperature of water stored
in tank are 20 °C. The specific heat of water
may be assumed to be 4.2 kJ/kg K. The
collector efficiency of the solar water
heating system is 50 %. Determine the final
temperature of water.

16. • Solution: Solar radiation incident on
collector per unit area per day x collector
area x collector efficiency=mass of water
to be heated per day x specific heat x (T2-
T1)
• Or, 5 x 1000 x 3600 x 2 x 0.5=100 x 4.2 x
1000 x (T2-20)
• Or, T2=62.85 =63 °C
• Final temperature of water=63 °C Ans.

17. The hot water requirement for a hostel of a
college is 5000 l/day. The average solar
radiation available per day is 600
W/m2. The temperature of feed water is 30
°C and the final hot water temperature
should be 60 °C. The area of the solar
collector is 75 m2. The Specific heat of
water is 4.18 kJ/kg °C. Assuming 8 hours
of daily operation, compute the thermal
efficiency of solar water heater.

18. Solution: [1 W=1 J/s] and [1 litre=1 kg]
Total available solar heat energy per day=Collector
area x Available solar radiation per day x operation
time
=75 x 600 x 8 x 3600= 1296000000 J=1296000 kJ
Total heat energy required to heat the water per
day=5000 x 4.18 x (60-30)
=627000 kJ
Thermal efficiency of solar water heater = [Total heat
energy required to heat the water per day x
100]/ Total available solar heat energy per day
=[627000 kJ x 100]/ 1296000 kJ =48.38 % Ans.

20. • A cylindrical parabolic solar collector is
designed to heat a fluid that enters the
absorber at 140 oC at a flow rate of 5
kg min-1. The specific heat capacity of the
fluid is 1.5 kJ kg-1 °C-1 and its outlet
temperature is 180°C. If the incident beam
radiation on the plane of aperture is 3000
kJ/h m2 and useful projected area of the
reflector is 2 m x 10 m, the efficiency of the
collector in percentage will be __________ .
[GATE (AG) 2017]

21. • Solution:
• Efficiency of collector = [rate of heat gain x
100]/rate of heat input
• Rate of heat gain = [(5 x 1.5 x (180-140))/60]
= 5 kJ/s
• Rate of heat input = [(3000 x 2 x 10)/3600] =
16.67 kJ/s
• Hence, Efficiency of collector=rate of heat
gain/rate of heat input=[(5 x 100)/16.67]
• =30% Ans.

22. Solar Distillation

24. Solar Distillation
• The demand for fresh water production is
growing day by day with the increase in
world population and with industrial growth.
• The process of distillation in which solar
energy is used for conversion of salt water
into potable water and the device used for
this purpose is called solar still.
• Solar stills are used across the world to
provide clean, drinkable water to the
masses.

25. Basin Type Solar Still
• Basin type solar stills are used for
distillation of water.
• A basin type solar still consists of a basin
coated with black paint.
• The basin stores brackish water, which is
used for distillation.
• The basin is closed with transparent cover.
• In the top of basin walls, condensate
channels are provided.

26. • The working of solar still is based on -
heating, evaporation, and condensation.
• The solar radiation incident on glass cover
of a solar still is absorbed by the salt water
and black surface.
• The absorbed solar radiation is converted
into heat energy and due to greenhouse
effect, temperature of water increases.

27. • After sometime, evaporation starts and
evaporated water gets condensed below the
transparent cover and water condensate travel
to the channel.
• The channels are provided gradient to one
side where the condensed water is collected.
• Thus pure potable water is obtained.
• The performance of solar still is the
amount of water distilled collected per m2
per day (liter/m2/day).
• A well designed still capacity is 3 lit/m2 on a
good sunny day.

28. Schematic diagram of a solar still

32. Schematic view of spherical solar still

33. Schematic diagram of tubular solar still

35. Solar Cooker

36. Solar Cooking
• Presently fossil fuels are commonly used for
cooking foods across the globe.
• The demand of energy for cooking is
continuously increasing due growth in
population.
• Cooking with solar energy is one of the
promising solutions for meeting energy
demands.
• The devices which are used for cooking food
utilizing solar energy is called solar cookers.

37. Solar Cooker
• Principle : Sunlight is converted to heat energy
that is use for cooking purpose.
• This is done as follows:
Concentrating sunlight.
Converting light to heat.
Trapping heat
• Employs greenhouse effect for cooking.

39. Types of Solar Cooker

40. Panel Cookers
• Panel cookers resemble an open, three sided box.
• The bottom and side panels of the box are covered
with shiny material creating reflectors.
• The sun rays hit the shiny panels and are redirected
into the center of the box.
• A “heat trap” is used in the center of the box to
capture the sun rays.
• Heat traps can be made from anything that is clear
and allows the UV rays to pass through.

41. • The black painted cooking container is placed inside of
the heat trap.
• The reflective panel directs sunlight onto a dark colored
pot.
• Due to greenhouse effect, solar radiation is converted
into heat energy and heat energy is trapped and heat the
container.
• The temperature can reach between 121° C to 149°C.
• This type of cooker is good for general cooking of rice,
pasta, soups, meats, vegetables, casseroles, etc.
• In bright sunlight the food is ready within 2-3 hours.

43. Box Type Solar Cooker
• Among different types of solar cookers,
box type solar cookers are the most
common, simplest, low cost and useful for
cooking meals of a family of 3-6 members.
• It is sometimes called as solar oven.
• Cost: Rs. 1400-3000 depending upon
make and model.

44. Box Type Solar Cooker
• An insulated metal box painted black from
inside.
• Glass cover over box
• A plane mirror reflector
• Metal container painted black from outside

45. Working Principle
• When the box with glass cover is placed in
the sunlight, the glass cover allows the
short wave solar radiation to pass into the
box.
• Most of these radiations are absorbed by
black surface of the box and the box
becomes hot.
• But, after sometime when the black surface
becomes hot it starts radiating out heat in
the form of long wave infra red rays.

46. • But, the glass sheet cover placed over the
box does not allow the heat radiated by
the black surface to go out from the box.
• In this way, the glass cover enables the
cooker to trap the heat inside it by
exploiting the greenhouse effect.
• Usually, a plane mirror reflector is also
attached on the top of the box.
• The plane mirror reflector increases the
efficiency of solar cooker by reflecting
more and more sunlight inside the box.

49. Solar Box Cooker
• 500X 500 mm aperture
• Cooks rice, dal in 1- 2 hrs
• Cheap easy to use
• Max temp 1200C

50. Merits of solar cooker
 No orientation to sun is needed.
No attention is needed during cooking.
No fuel, maintenance and recurring cost.
Simple to use and fabricate.
No pollution.

51. Demerits of solar cooker
Cooking can be done only when there is
sunshine.
Quick cooking is not possible.
• It cannot be used during rainy season or
cloudy conditions.
All types of foods cannot be cooked.
Can not be used for baking and frying

52. • A solar cooker is used to cook 1 kg of rice
in which 2 kg water is used. Rice is cooked
in aluminium vessel weighing 0.5 kg. The
initial temperature is 30 °C and the required
cooking temperature is 100 °C. The specific
heat of rice, water and vessel are 3.8 kJ/kg
°C, 4.18 kJ/kg °C and 0.9 kJ/kg °C
respectively. The average solar radiation
during cooking is 700 W/m2. The total area
for solar radiation collection is 1.12 m2. The
time taken for cooking is 1.5 h. Compute
the thermal efficiency of solar cooker.

53. Solution:
Total heat energy required for cooking rice =
[mass of rice x specific heat of rice + mass
of water x specific heat of water + mass of
vessel x specific heat of vessel) x ∆T
=[(1 x 3.8) + (2 x 4.18) + (0.5 x 0.9)] x (100-
30)
= 882.7 kJ

54. • Assume that 30% of this energy is required
throughout the period of cooking.
• Then, total heat energy required for
cooking= 882.7+882.7 x 0.3= 1147.51 kJ
• Total input solar energy =available solar
radiation x area x time
• =700 x 1.12 x 1.5 x 3600 J= 4233600 J= 4233.6
kJ
• Thermal efficiency of solar cooker= [Heat
energy used for cooking x 100]/ Input solar
energy
• =[1147.51 kJ x 100]/ 4233.6 kJ=27 %

55. Parabolic Solar Cookers
• In comparison to solar box and panel cookers,
parabolic cookers can reach much higher
temperatures and can cook more quickly.
• Parabolic solar cookers are shaped like parabolic
satellite dishes.
• The dish is covered with a shiny material that makes
the entire surface reflective.
• Because of the parabolic shapes and with the aid of
reflecting material quite a lot of solar energy is
concentrated in the focal point.

56. • Consists of a large parabolic reflector and
cooking pot holder
• The cooking pot is placed at the focus of
the reflector.
• • The pot surfaces are blacked to improve
the absorption.
• A very high temperature of between 200
ºC and 300 ºC can be reached.
• It is suitable for baking, frying, roasting
and grilling.

57. Different Types of Solar Cooker in Use

58. Solar Parabolic Cooker

60. Community Solar Cooker
• The most popular version is the scheffler
community kitchen.
• The receiver of scheffler dish is placed at
the focus of the dish to capture the incident
solar radiation and transfer it to the thermal
medium.
• Tracking system enables the dish to be
focused towards the sun to capture
maximum possible direct radiation during
the day.

61. Scheffler Dish

62. • The Sai Baba temple complex at Shirdi,
Maharashtra’s Ahmednagar district, has
installed one of the world’s largest solar
cooking system based on schfeller dishes.
• The solar rays are used to heat up water to
generate steam which is directed through
pipes in to steam cookers to cook food.
• The system saves consumption of cooking
gas.

63. Sai Baba Temple Complex