Solar thermal energy harnesses heat from the sun to provide thermal energy and electricity. There are two main types of solar thermal systems - active systems with moving parts like pumps, and passive systems relying on design without mechanical components. Technologies are also grouped by temperature as low, medium, or high temperature systems. Low temperature applications mainly provide hot water or space heating using collectors like flat plate, evacuated tube, or evacuated flat plate designs. Medium and high temperature systems can be used for industrial processes or electricity generation using concentrated solar power technologies like parabolic troughs or towers.

1. Solar Thermal Energy
• Solar thermal energy (STE) is a form of energy
and a technology for harnessing solar energy
to generate thermal energy or electrical
energy for use in industry, and in the
residential and commercial sectors.
• Solar Thermal technologies capture the heat
energy from the sun and use it for heating
and/or the production of electricity

2. Types of Solar Thermal Systems
• There are two main types of solar thermal
systems for energy production – active and
passive.
– Active systems require moving parts like fans or
pumps to circulate heat-carrying fluids.
– Passive systems have no mechanical components and
rely on design features only to capture heat (e.g.
greenhouses, phase change materials etc).
• The technologies are also grouped by
temperature – low (<100°C) , medium (100-
250°C) or high (250°C >).

3. • Low-temperature (<100°C) applications typically use solar thermal
energy for hot water or space heating.
– Active systems often consist of a roof-mounted flat plate collector
through which liquid circulates. The collector absorbs heat from the
sun and the liquid carries it to the desired destination, for example a
swimming pool or home heating system.
– Passive heating systems involve intelligent building design practices,
which cut back on the need for heating or cooling systems by better
capturing or reflecting solar energy.
• Medium-temperature (100-250°C) applications are not common.
– An example would be a solar oven, which uses a specially-shaped
reflector to focus the sun’s rays on a central cooking pot.
– Similar systems could be used for industrial processes, but are not
widely used.
• High-temperature (250°C >) solar thermal systems use groups of mirrors
to concentrate solar energy onto a central collector.
– These concentrated solar power (CSP) systems can reach
temperatures high enough to produce steam, which then turns a
turbine, driving a generator to produce electricity.

4. Solar Thermal Collector
• A solar thermal collector collects heat by absorbing sunlight. The term "solar
collector" commonly refers to a device for solar hot water heating, but may
refer to large power generating installations such as solar parabolic troughs
and solar towers or non water heating devices such as solar air heaters.
• Solar thermal collectors are either non-concentrating or concentrating.
– In non-concentrating collectors, the aperture area (i.e., the area that receives the solar
radiation) is roughly the same as the absorber area (i.e., the area absorbing the
radiation). This type has no extra parts except the collector itself.
– Concentrating collectors have a much bigger aperture than absorber area (additional
mirrors focus sunlight on the absorber) and only harvest the direct component of
sunlight.
• Non-concentrating collectors are typically used in residential and commercial
buildings for space heating, while concentrating collectors in concentrated
solar power plants generate electricity by heating a heat-transfer fluid to drive
a turbine connected to an electrical generator.

5. • Commercially solar collectors are classified
according to its industrial/commercial and domestic
use. They are distributed into three categories –
collectors for Water Heating, Air Heating and
Electricity Generation
1. Solar thermal collectors heating water
– Flat plate collectors
– Evacuated tube collectors
– Evacuated flat plate collectors
2. Solar thermal collectors heating air
– Through-pass air collector
– Unglazed transpired solar collectors
3. Solar thermal collectors generating electricity
– Parabolic trough
– Parabolic dish
– Power tower

6. Types of Collectors
• Low temperature – to 320C
– Unglazed absorbers
• Mid temperature – to 700C
– Glazed flat plate collectors
– Integrated collector systems (ICS), thermosyphon,
antifreeze, drainback
• High temperature
– Evacuated tube – to 1750C
– Parabolic trough – to 3000C

7. 1.
WATER HEATING Solar thermal
collectors

8. Flat plate collector

9. Flat plate collector
• Flat-plate collectors are the most common solar thermal technology . They consist of an
(1) enclosure containing
(2) a dark colored absorber plate with fluid circulation passageways, and
(3) a transparent cover to allow transmission of solar energy into the enclosure.
• The sides and back of the enclosure are typically insulated to reduce heat loss to the
ambient. A heat transfer fluid is circulated through the absorber's fluid passageways to
remove heat from the solar collector.
• The circulation fluid in tropical and sub-tropical climates is typically water. In climates
where freezing is likely, a heat transfer fluid similar to an automotive antifreeze solution
may be used instead of water, or in a mixture with water.

10. Evacuated tube collector
Direct Flow
Heat Pipe

11. Evacuated tube collector
• An evacuated-tube collector consists of parallel 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. Evacuated-tube collectors fall into two main groups.
• Direct-flow evacuated-tube collectors
– These consist of a group of glass tubes inside each of which is a flat or curved aluminium fin attached
to a metal (usually copper) or glass absorber pipe. The fin is covered with a selective coating that
absorbs solar radiation well but inhibits radiative heat loss. The heat transfer fluid is water and
circulates through the pipes, one for inlet fluid and the other for outlet fluid. Direct-flow evacuated
tube collectors come in several varieties distinguished by the arrangement of these pipes.
• Heat pipe evacuated-tube collectors
– These consist of a metal (copper) heat pipe, to which is attached a black copper absorber plate, inside a
vacuum-sealed solar tube. The heat pipe is hollow and the space inside, like that of the solar tube, is
evacuated. The reason for evacuating the heat pipe, however, is not insulation but to promote a change
of state of the liquid it contains. Inside the heat pipe is a small quantity of liquid, such as alcohol or
purified water plus special additives. The vacuum enables the liquid to boil (i.e. turn from liquid to
vapor) at a much lower temperature than it would at normal atmospheric pressure. When solar
radiation falls the surface of the absorber, the liquid within the heat tube quickly turns to hot vapor
rises to the top of the pipe. Water, or glycol, flows through a manifold and picks up the heat, while the
fluid in the heat pipe condenses and flows back down the tube for the process to be repeated.

12. Evacuated flat plate collector

13. Evacuated flat plate collector
• Evacuated flat plate solar collectors provide all the advantages of
both flat plate and evacuated tube collectors combined together.
They surround a large area metal sheet absorber with high vacuum
inside a flat envelope made of glass and metal. They offer the
highest energy conversion efficiency of any non-concentrating solar
thermal collector but require sophisticated technology for
manufacturing.
• Evacuated flat plate solar collectors require both a glass-metal seal
to join the glass plate to the rest of the metal envelope and an
internal structure to support such plate against atmospheric
pressure. The absorber has to be segmented or provided with
suitable holes to accommodate such structure. Joining of all parts
has to be high vacuum tight and only materials with low vapour
pressure can be used to prevent outgassing.

14. 2.
AIR HEATING Solar thermal
collectors

15. Concept of space (air) heating
• A simple solar air collector consists of an absorber material, sometimes having a
selective surface, to capture radiation from the sun and transfers this thermal energy
to air via conduction heat transfer. This heated air is then ducted to the building
space or to the process area where the heated air is used for space heating or
process heating needs.
• Functioning in a similar manner as a conventional forced air furnace, solar-thermal-
air systems provide heat by circulating air over an energy collecting surface,
absorbing the sun’s thermal energy, and ducting air coming in contact with it.
• Simple and effective collectors can be made for a variety of air conditioning and
process applications.
• Two important applications of “space heating air collectors” are
– Space heating and ventilating: This configuration operates by drawing air from the building envelope
or from the outdoor environment and passing it through the collector where the air warms via
conduction from the absorber and is then supplied to the living or working space by either passive
means or with the assistance of a fan. Ventilation, fresh air or makeup air is required in most
commercial, industrial and institutional buildings to meet code requirements. By drawing air through a
properly designed unglazed transpired air collector or an air heater, the solar heated fresh air can
reduce the heating load during daytime operation.
– Process heating: Solar air heat is also used in process applications such as drying laundry, crops (i.e.
tea, corn, coffee) and other drying applications. Air heated through a solar collector and then passed
over a medium to be dried can provide an efficient means by which to reduce the moisture content of
the material.

16. Heating of Living Spaces
• Best design of a building is for it to act as a solar collector
and storage unit.
• This is achieved through three elements:
– insulation, collection, and storage.
• Efficient heating starts with proper insulation on external
walls, roof, and the floors.
• The doors, windows, and vents must be designed to
minimize heat loss.
• Collection: south-facing windows and appropriate
landscaping.
• Storage: Thermal mass—holds heat.

17. Space Heating Through Passive Solar
• Passive solar design involves using specific
shapes, angles, and building materials to
maximize the amount of solar energy allowed
into the interior of a building during cold months.
• Overhangs are used to shield the home from the
high, summer sun, but are designed to allow the
low, winter sunlight to enter the windows.
• Materials, such as concrete and stone, are used
to absorb the sun’s energy and store it, releasing
it into the home at night after the sun has set.
This is called direct gain.
• Other features, such as sunspaces and trombe
walls, can be used. A sunspace uses glass to trap
heat like a greenhouse. The heated air within the
sunspace is allowed to circulate throughout the
building.
• A trombe wall is made of absorbing material and
is often painted black or covered with glass (or
both) to maximize the amount of energy
absorbed and keep it locked inside. The thermal
energy is released to the building slowly over
time, providing warmth throughout the night.

18. Daylighting
Using skylights, tube lights, and large windows on the sunward side of a building help
maximize the amount of light reaching the interior of a building and reduce the need for
artificial lights.

19. Drying Agricultural Products
Crops such as wheat and rice have been
allowed to dry in the fields for centuries. Hay
is mown and allowed to dry in the field before
being baled. A small solar collector is used to
dry herbs. Agricultural products can be dried
in a special tower that uses a solar collector to
heat air and channel it through perforated
trays of crops, which dries them and carries
the moisture out through the top.

20. Through-pass air collector
Front pass air collector

21. • Offering the highest efficiency of any solar technology the through-pass
configuration, air ducted onto one side of the absorber passes through a
perforated material and is heated from the conductive properties of the
material and the convective properties of the moving air. Through-pass
absorbers have the most surface area which enables relatively high
conductive heat transfer rates, but significant pressure drop can require
greater fan power, and deterioration of certain absorber material after
many years of solar radiation exposure can additionally create problems
with air quality and performance.
• Through pass air collectors may be a Back, front, and combination
passage air collector
– In back-pass, front-pass, and combination type configurations the air is
directed on either the back, the front, or on both sides of the absorber to be
heated from the return to the supply ducting headers.
– Although passing the air on both sides of the absorber will provide a greater
surface area for conductive heat transfer, issues with dust (fouling) can arise
from passing air on the front side of the absorber which reduces absorber
efficiency by limiting the amount of sunlight received.
Through-pass air collector

22. Unglazed transpired solar collector

23. Unglazed transpired solar collector
• The term "unglazed air collector" refers to a solar air heating system
that consists of a metal absorber without any glass or glazing over
top. The most common type of unglazed collector on the market is
the transpired solar collector.
• Unglazed air collectors heat ambient (outside) air instead of
recirculated building air. The exterior surface of a transpired solar
collector consists of thousands of tiny micro-perforations that allow
the boundary layer of heat to be captured and uniformly drawn into
an air cavity behind the exterior panels. This heated ventilation air
is drawn under negative pressure into the building’s ventilation
system where it is then distributed via conventional means or using
a solar ducting system.
• Transpired solar collectors act as a rainscreen and they also capture
heat loss escaping from the building envelope which is collected in
the collector air cavity and drawn back into the ventilation system.

24. 3.
ELECTRICITY GENERATING Solar
thermal collectors
To be continued……..