Solar Thermal Technology

1. UNIT II
SOLAR THERMAL TECHNOLOGIES

2. Working Principle
Principle – To convert solar energy into heat energy
by absorbing it.
Principle components:
• Solar collector to covert energy efficiently
• Medium for energy transport
• Water/ air /others
• Storage system to overcome the mismatch between
energy available and demand
• Systems to transport and use energy/ medium
• Control systems

3. Types of Solar system
Thus energy from the sun can be categorized in two ways:
• In the form of heat (or thermal energy), and
• In the form of light energy.
Solar energy can be converted to thermal (or heat) energy
and used to:
• Heat water – for use in homes, buildings, or swimming
pools.
• Heat vessels – for cooking food
• Heat spaces – inside greenhouses, homes, and other
buildings.

4. Types…
Solar energy can be converted to electricity in
two ways:
• Photovoltaic (PV devices) or “solar cells” –
change sunlight directly into electricity.
• Solar Power Plants - indirectly generate
electricity when the heat from solar thermal
collectors is used to heat a fluid which produces
steam that is used to power
generator.

5. Design of solar water heating system
A solar water heating system consists of:
• Fat plate solar collector,
• Insulated storage tank kept at a height behind the
collector,
• Insulated hot water carrying pipes and
• Cold water carrying pipes. The system is
generally installed on the roof or on open ground,
with the collector facing the sun and connected to
a continuous water supply.

6. Designs…
The flat plate collector comprises
– front glass glazing
– blackened metallic (copper) absorber sheet with
built-in channels or riser tubes welded to it to
carry water.
– The entire assembly is placed in a flat metallic
box.

7. Procedure for Operation and Maintenance
• Check water in the cold water supply line and also storage
tank.
• Check the collector glazing. It should not be dirty. In case
dirt is deposited on the glazing it should be cleaned.
• Open all the control valves as suggested by the supplier
• Check electrical supply.
• Check the water pump. It should not be jammed.
• Check whether the Non Return Valve (NRV) on the cold
water inlet side is working.
• Ensure the lever of mixer in the bathroom is on either left or
right side AND NOT IN
• THE MIDDLE to avoid mixing of hot / cold water

8. Operation of the Solar Water Heater
• After ascertaining that the requirements indicated above are fulfilled,
start the circulating pump.
• The water will start circulating in the collector system and after
getting heated will be collected in the storage tank.
• The temperature of water in the storage tank will rise which can be
seen in the temperature indicators provided in the system.
• There is a possibility of air locking in collector riser tubes and
headers which can be released by increasing the flow of water by
opening control valves.
• As soon as the control valves are opened beyond the limits suggested,
water will start coming out from the air vents.
• In a THERMOSYPHONIC SYSTEM hot water will start collecting
in the storage tank as soon as sunlight falls on the collector and water
is available in the system

9. Classification:
Solar water heating systems are classified in 2
types based on the circulation of water.
• Thermosyphon system.
• Forced flow or forced circulation system

10. Thermosyphon system:
• In the Thermosyphon system, water comes from the over head tank
to bottom of solar collector by natural circulation and water
circulates from the collector to storage tank as long as the absorber
keeps absorbing heat from the sun and water gets heated in the
collector.
• The cold water at the bottom of storage tank runs into the collector
and replaces the hot water, which is then forced inside the insulated
hot water storage tank.
• The process of the circulation stops when there is no solar radiation
on the collector.
• Thermosyphon system is simple and requires less maintenance due
to absence of controls and instrumentation.
• Never use a Thermosyphon system for a large set-up.

11. Thermosyphon Heating system:

12. Thermosyphon Cooling system

13. Forced circulation system:
• In the forced flow system, a pump is used for
circulating water between the collectors and the
insulated hot water storage tank.
• The forced flow systems are more efficient as
compared to Thermosyphon systems due to
higher flow rate.
• Generally, the pumps are operated by differential
temperature control (DTC) system, which senses
the pre-setting temperature difference between
inlet and outlet of the collectors.

14. Thermal Energy storage systems
• TES has attracted increasing interest for thermal
applications such as space heating, hot water, cooling,
and air-conditioning.
• TES systems have the potential for increasing the
effective use of thermal energy equipment and for
facilitating large-scale fuel switching.
• There are mainly two types of TES systems;
– sensible (e.g., water and rock)
– Latent (e.g., water/ice and salt hydrates)
• The selection of a TES system mainly depends on the
storage period required.
– seasonal, economic viability, operating conditions

15. TES System…
• TES is basically the temporary “holding” of
energy for later use.
• Application of TES
– solar energy for night and weekend use,
– of summer heat for winter space heating, and
– Of ice from winter for space cooling in summer,
– the heat or cool generated electrically during
– off-peak hours can be used during subsequent
peak demand hours

16. Principle (TES):
• A complete storage process involves at least
three steps: charging, storing, and discharging.

17. Design Considerations for Solar Energy-Based TES
• The energy from solar collectors tends to be at
low temperatures and requires a large storage
mass when stored as sensible heat.
• a space-conditioning system using solar
energy needs to be supplemented by an
electric- or fuel-powered auxiliary energy
source.
• the quantity of solar energy to be stored.

18. Solar cooker
Solar cooker is a device which cooks food
without any conventional cooking fuel/energy
like cow dung, wood, coal, gas, kerosene or
electricity. It converts sunlight into heat energy
and cooks the food.

19. Basic principles of solar cooking
• Solar cookers utilise the simple principles of reflection,
concentration, glazing, absorption and the greenhouse effect
to produce heat.
• Basically a solar cooker is an insulated container with a
multiple or single glass (or other transparent material)
cover.
• This kind of cooker depends on the “greenhouse effect” in
which the transparent glazing permits passage of shorter
wavelength solar radiation
• Mirrors may be used to reflect additional solar radiation into
the cooking chamber.
• A double-walled insulated box can also serve to hold the
heat inside the cooker.

20. • Box solar cooker
Description:
• The Outer Body
• The Inner Cooking Box/Tray
• Thermal Insulation
• The Double Glass Lid
• Mirror
• Cooking pots
• Side window

21. • Dish solar cooker

22. Design of dish cooker
• A typical dish solar cooker has a minimum aperture diameter
of 1.4 m and a focal length of 0.28 m.
• The parabolic dish, made of single reflector, or by joining
smaller pieces of reflector, is fixed firmly to a rigid frame.
• Size and shape of the dish should be such that when exposed
to sun in the normal direction, a point focus would be formed.
• The reflecting material used for fabrication of this cooker is
anodized aluminium sheet, which has a reflectivity of over
80%.

23. Material required for fabrication:
• Supporting Frame of the dish
• Stand for the dish
• Tracking Mechanism
• Cooking vessel (Optional)

24. Community Solar Cooker (for indoor
cooking)
• The community solar cooker too is a parabolic
reflector cooker.
• However, it is larger than the SK-14 type, and
is known as a Scheffler cooker.
• The unique feature of this cooker is that it
makes possible to cook using solar energy
within the kitchen itself.

25. Design of community solar cooker
• The community solar cooker consists of solar concentrator
with minimum 7.0 & 9.5 sq.m aperture area, which is
known as primary reflector.
• A mechanical clock type tracking arrangement is provided
to keep the dish facing the Sun.
• A secondary reflector further concentrates the rays on to
the bottom of the pot/frying pan (painted black to absorb
maximum heat).
• The dish is commonly known as “Scheffler” dish, which has
been found to be useful for generating heat to cook food
for 50 to 75 persons in community kitchens.
• The mechanical clockwork arrangement rotates the
primary reflector outside to automatically track the sun.

26. Solar Powered Desalination
World Water Resources
Salt Water
Fresh Water The Worlds Water
 97% Sea Water
 3% Fresh Water

27. Desalination
The separation and removal of ions, salts and
other dissolved solids from water.
Heat Based
Membrane Based
Two methods of desalination:
Direct
Indirect

28. Process Technology (1)

29. Process methodology
UNTREATED SOURCE WATER
MECHANICAL PRE-TREATMENT
SAND FILTERS
REVERSE OSMOSIS
DESINFECTION
REMINERALIZATION
TREATED DRINKING WATER

30. Solar pond
• Solar energy systems must incorporate storage
in order to take care of energy needs during
nights and on cloudy days.
• One way to overcome these problems is to use
a large body of water for the collection and
storage of solar energy. This concept is called a
solar pond.

31. Solar ponds

32. Principle of a solar pond
• In a clear natural pond about 30% solar radiation reaches a
depth of 2 metres.
• This solar radiation is absorbed at the bottom of the pond.
• The hotter water at the bottom becomes lighter and hence
rises to the surface.
• Here it loses heat to the ambient air and, hence, a natural
pond does not attain temperatures much above the ambient.
• If some mechanism can be devised to prevent the mixing
between the upper and lower layers of a pond, then the
temperatures of the lower layers will be higher than of the
upper layers.
• The simplest method is to make the lower layer denser than
the upper layer by adding salt in the lower layers.

33. Solar drying
Solar dryers are devices that use solar energy to dry
substances, especially food.
There are two general types of solar dryers: Direct and
indirect
Direct solar dryers expose the substance to be dehydrated
to direct sunlight.
They have a black absorbing surface which collects the light
and converts it to heat; the substance to be dried is placed
directly on this surface.
These driers may have enclosures, glass covers and/or vents
to in order to increase efficiency.

34. • In indirect solar dryers, the black surface
heats incoming air, rather than directly heating
the substance to be dried.
• This heated air is then passed over the
substance and exits through a chimney, taking
moisture from the substance with it.

35. Classification of solar dryers:
• Passive solar dryers
– Tent dryers

36. Tent dryers….
• Tent dryers are cheap and simple to build and
consist of a frame wood poles covered with
plastic sheet.
• Black plastic should be used on the wall facing
away from the sun.
• The food to be dried is placed on a rack above
the ground.

37. Box dryers
• The box type solar dryer has widely used for
small food drying.
• It consists of a wooden box with a transparent
lid.
• The inside is painted is black and the food
supported on a mesh tray above the dryer
food.

38. Seesaw dryer
• The traditional seesaw dryer has a rigid, rectangular frame, the length of
which being 3 times the width resting on a support with an axis.
• This support is oriented north-south and is sufficiently high to allow the
frame to be tilted 300 .
• The material for drying is placed on a number of trays, that allow vertical
air circulation and maximum evaporation.

39. Cabinet dryer
• The crops is located in tray or shelves inside a drying
chamber.
• If the chamber is transparent, the dryer is termed an
integral type or direct solar dryer.
• If the opaque, the dryer is termed distributed type.
• Mixed mode dryers combine the feartures of the
integral type and distributed type.

40. Active solar cabinet dryers
Active ventilated cabinet solar dryers:
– If utility electricity is available it is cheaper to
connect the fans to the grid, compared to a
connection to the grid.
– In a PV-powered system, the fan is directly
coupled to the solar module, working without an
accumulator and load controller.
– Increasing solar radiation increases the modules
output, thus speeding up the fan.

41. Active ventilated cabinet solar dryers

42. Cabinet dryer with backup heating
• Natural convection dryer with additional
biomass back-up heater
– one exception is the dryer reported by which
used a sawdust burner to provide heat during
poor weather and at night.

43. Natural convection dryer with
additional biomass back-up heater

44. Greenhouse dryers
• The idea of a greenhouse dryer is to replace
the function of the solar collector by a green
house system.
• The roof and wall of this solar dryer can be
made od transparent materials such as glass,
fibre glass, UV stabilized plastic.

45. Natural convection greenhouse dryer:
The earliest form of practically-realised natural
circulation solar greenhouse dryer.
The consisted of two parallel rows of drying platforms
of galvanised iron wire mesh surface laid over wooden
beams.

46. Greenhouse dryer with forced ventilation

47. Tunnel dryers
Three major dryer components;
Solar collector,
food dryer compartment
Airflow system.

48. Drying behavior
Apart from weather conditions the drying behavior
of agricultural crops during drying depends on the:
– Product
– Size and shape
– Initial moisture content
– Final moisture content
– Bulk density
– Thickness of the layer
– Mechanical or chemical pre-treatment
– Turning intervals
– Temperature of grain