Module 2—Solar Thermal

Systems
By
Bharath.M
 FLAT AND CONCENTRATING
COLLECTORS
• Heliothermal Process-- the radiant solar energy falling on the
surface placed on the earth in the form of the visible light is
converted directly into thermal energy.
• The surfaces on which the solar rays fall are called collectors.
Basically, two types of collectors are used:
non-concentrating type– Known as flat plate collector
Incident solar rays are absorbed by the collector's surface
itself.
 concentrating type—Known as a focusing collector
the solar rays fall on a large curved reflecting surface
which reflects all the incident rays and focus them to form a
highly concentrated narrow beam which will be absorbed later.
The amount of solar radiation incident on a surface is called solar
insolation.
Flat plate collector:
 a blackened sheet of metal is used to absorb all the sunlight , direct,
diffuse and terrestrially reflected.
A sheet of metal coated in black has the property of absorbing the
sunlight falling on it
And convert it into heat.
 The heat generated in the sheet of metal is subsequently
transferred to the other fluids like, air, water, etc.
The heat energy will be continuously transferred to the fluid after
the temperature reach equilibrium state.
This method of solar energy conversion will have very high
conversion efficiencies, even as high as 100%.
conduction and convection losses can be minimised:
by placing the blackened sheet of metal in a closed
thermally insulated box having its top covered by a
transparent glass sheet
The use of a glass cover serves as a diathermanous
medium permitting the short wavelength solar radiation to
be transmitted through it
By coating the absorbing surface with selective coating
substances having high absorptiveness in
the short wavelength region and low emissivities in the long
wavelength region the radiation losses from the absorbing
surface may be reduced
• Flat plate collectors are used for a wide variety of low
temperature applications such as cooking, water
heating, drying of food grains and vegetables,
seasoning of wood, desalination of water,
Concentrating/focusing collectors
A mirror or a lens system is used to increase the
intensity of solar radiation.
• The focusing collectors intercept and concentrate
only the direct rays of the sun and they do not
perform satisfactorily when the sky is cloudy or hazy.
• The focusing collectors may be used for high
temperature heating applications for industrial
purposes.
Advantages and Disadvantages of concentrating
collectors over flat Plate type collectors:
Advantages:
1. Reflecting surfaces require less material and are structurally simpler
than flat plate collectors. (less cost )
2. The absorber area of a concentrating system is smaller than that of a
flat plate system for same solar energy collection.
3. Loss of energy after collecting is less than FPC.
4. Owing to the small area of absorber per unit of solar energy
collecting area, selective surface treatment and/or vacuum insulation
to reduce heat losses and improve collector efficiency are
economically feasible.
5. Can be used for electricity power generation.
6. Heat storage costs are less
7. Little or no anti-freeze is required to protect the absorber
Disadvantages:
1. Only beam components are collected.
2. Costly oriented systems
3. An additional requirement of maintenance is
required.
4. Non uniform flux on the absorber.
5. Additional optical losses such as reflectance loss
and the intercept loss, so they introduce
additional factors in energy balances.
6. High Initial cost.
Evacuated Tubular Collector
An evacuated tubular collector is a type of solar
thermal collector used to convert solar radiation into
thermal energy for heating purposes.
Design:An evacuated tubular collector consists of a
series of transparent glass tubes that are sealed at both
ends.
Each tube contains an absorber plate or tube
that absorbs solar radiation and converts it into heat.
Vacuum Insulation:The space between the outer glass
tube and the absorber is evacuated to create a vacuum.
This vacuum insulation minimizes heat loss through
convection and conduction,
Absorber Tube:The absorber tube inside each glass
tube is typically coated with a selective coating that
maximizes solar absorption while minimizing heat loss.
This coating allows the absorber to efficiently convert
sunlight into thermal energy.
Heat Transfer Fluid: A heat transfer fluid, such as water or
antifreeze solution, flows through the absorber tubes to absorb
the thermal energy.
As the fluid passes through the collector, it absorbs heat from
the absorber tubes, which raises its temperature.
Heat Exchange: The heated fluid is then circulated through a
heat exchanger, where it transfers its thermal energy to a
separate water or space heating system.
Efficiency:The vacuum insulation and selective coating of
evacuated tubular collectors make them highly efficient, even in
low light conditions.
Applications: Evacuated tubular collectors are commonly used in
residential, commercial, and industrial applications for water
heating, space heating, and solar cooling systems.
Solar Air Collector:
A solar air collector is a type of solar thermal collector that
utilizes sunlight to heat air, which can then be used for space
heating, ventilation or other heating applications
Design:A solar air collector typically consists of a dark-colored
absorber plate or surface, enclosed in a glazed or transparent
cover.
Air Flow:Air is circulated through the collector by fans or natural
convection.
As the air passes over the absorber plate, it absorbs heat and
becomes heated.
The heated air is then circulated into the building or space where
it can be used for heating purposes.
Absorber Material: The absorber plate is often made of a
thermally conductive material with a high solar absorptance
and low emissivity to maximize heat absorption
Glazing:The glazing or cover of the collector is typically
made of glass or transparent plastic to allow sunlight to
pass through while trapping heat inside the collector.
Insulation:Solar air collectors are often insulated on the
back and sides to minimize heat loss and improve efficiency
Controls: Some solar air collectors may be equipped with
automatic controls to regulate airflow, temperature, and
operation based on weather conditions and heating
requirements.
 Solar Concentrators
A solar concentrator is a device
That focuses sunlight onto a small area to generate high
temperatures for various applications--power generation,
heating, and even cooking.
Here’s an overview of the key concepts and types:
Key Concepts
Concentration Ratio: The ratio of the area of the aperture
(where sunlight enters) to the area of the receiver (where the
sunlight is focused)
Higher ratios mean more concentrated sunlight and higher
temperatures.
Tracking: Some solar concentrators require mechanisms to track
the sun's movement across the sky to maintain optimal focus.
Tracking can be single-axis or dual-axis.
Reflectors and Lenses: Concentrators use reflective or refractive
surfaces to direct sunlight. Reflectors include mirrors and other
reflective materials, while lenses are usually made of glass or
plastic.
Types of Solar Concentrators
Parabolic Trough Concentrators
Design: These have a parabolic-shaped reflector
that focuses sunlight onto a linear receiver tube
along the focal line of the trough.
Tracking: Typically use single-axis tracking to
follow the sun from east to west.
Applications: Commonly used in large-scale
solar thermal power plants.
Parabolic Dish Concentrators
• Design: These use a parabolic-shaped dish to focus sunlight
onto a single focal point above the dish.
• Tracking: Require dual-axis tracking to follow the sun
accurately.
• Applications: Suitable for small to medium-scale power
generation and high-temperature industrial processes.
Linear Fresnel Reflectors
• Design: Use flat or slightly curved mirrors
arranged in rows to focus sunlight onto a
receiver tube positioned above the mirrors.
• Tracking: Use single-axis tracking.
• Applications: Can be used in solar power
plants and for industrial heating.
Heliostat Field Collectors
• Design: Consist of an array of mirrors
(heliostats) that track the sun and reflect
sunlight onto a central receiver on top of a
tower.
• Tracking: Require dual-axis tracking.
• Applications: Used in solar power towers for
large-scale power generation.
Applications of Solar Concentrators
Electric Power Generation:Solar concentrators are used in Concentrated Solar
Power (CSP) plants, where they focus sunlight to produce steam that drives
turbines to generate electricity.
Industrial Heating: High-temperature industrial processes such as metal
smelting, chemical production, and food processing can use concentrated solar
energy.
Solar Cooking: Solar cookers use concentrators to focus sunlight to cook food
without the need for conventional fuel sources.
Solar Desalination: Concentrators can be used in desalination systems to produce
freshwater by evaporating seawater and condensing the steam.
Scientific Research: Concentrated solar energy is used in laboratories for high-
temperature experiments and materials testing.
Advantages:
• High Efficiency: By concentrating sunlight, solar concentrators can
achieve higher efficiencies than flat-plate solar collectors.
• Versatility: Can be used for various applications, from power
generation to heating and cooking.
• Scalability: Suitable for both small-scale and large-scale
applications.
Challenges:
• Cost: The initial cost of concentrators and tracking systems can be
high.
• Complexity: Systems require precise alignment and tracking
mechanisms, adding to maintenance requirements.
• Location Dependency: Best suited for regions with high direct solar
insolation and minimal cloud cover.
Solar Cooker:
A solar cooker is a device that uses solar energy to cook food.
It works by concentrating sunlight onto a cooking container,
which absorbs the heat and cooks the food.
Solar cookers are an eco-friendly alternative to traditional
cooking methods, as they do not require any fuel and produce
no emissions.
Types of Solar Cooker
• Box Cookers
• Parabolic Cookers
• Panel Cookers
• Hybrid Cookers
Box Cookers
• Design: A box cooker typically consists of an insulated
box with a transparent top (usually made of glass or
plastic) and reflective inner surfaces. The box traps heat
inside, creating an oven-like environment.
• Function: Sunlight enters through the transparent top
and is absorbed by the dark cooking pots, which convert
the light into heat. The insulated box retains the heat,
allowing temperatures to rise sufficiently to cook food.
• Applications: Suitable for baking, boiling, and slow
cooking. It can cook a variety of foods like rice,
vegetables, and bread.
Parabolic Cookers: Are a type of solar cooker that uses a
parabolic-shaped reflector to focus sunlight onto a small area to
generate high temperatures.
Reflective Surface: The parabolic shape of the reflector ensures
that all incoming sunlight is focused on a single focal point.
The surface of the reflector is usually made from polished metal,
reflective film, or a similar material that can efficiently reflect
sunlight.
Focal Point: The concentrated sunlight at the focal point can
reach very high temperatures. cooking pot or pan is placed at
this focal point to absorb the heat and cook the food.
Tracking the Sun: To maintain optimal performance,
parabolic cookers need to be periodically adjusted to track the
movement of the sun.
Some advanced models come with tracking mechanisms that
automatically follow the sun.
Advantages
• High Efficiency: Parabolic cookers can reach higher temperatures
compared to other types of solar cookers, making them suitable for
a wider range of cooking methods, including frying and baking.
• Fast Cooking: The high temperatures allow for faster cooking times.
• Eco-Friendly: They use renewable solar energy, reducing reliance on
conventional fuels and lowering carbon emissions.
Disadvantages
• Complexity: They are generally more complex to manufacture and
use compared to other types of solar cookers, such as box cookers.
• Sun Tracking: They require frequent adjustment to keep them
aligned with the sun, which can be inconvenient.
• Safety: The high temperatures and concentrated sunlight can
 Solar Distillation:
A Sustainable Way to Produce Fresh Water
In today’s world, access to clean drinking
water is a critical challenge.
Solar distillation offers a sustainable solution
by using the sun’s energy to purify water
We will learn the concept of solar
distillation, its benefits, and how it works.
• What is Solar Distillation?
• A method for purifying water using solar energy
• Works by evaporating water and condensing the clean vapor
• Removes impurities like salts and contaminants
• Solar distillation is a simple yet effective way to turn saltwater, brackish water, or
even contaminated water into clean drinking water.
• It Mimics the natural water cycle, where the sun heats water, causing it to
evaporate
How does Solar Distillation Works?
• Saline water is placed in a
basin
• A transparent cover (glass
or plastic) traps sunlight
• The basin’s dark lining
absorbs heat
• Sunlight heats the water,
causing it to evaporate
• Water vapor condenses on
the cool glass cover
• Condensed water drips
down into a collection
container
 Benefits of Solar Distillation
Uses renewable solar energy – sustainable and
environmentally friendly Requires minimal
maintenance
Simple and easy to operate Can be used in remote
areas
No need for electricity or infrastructure
Effective in purifying various water sources –
saltwater, brackish water, contaminated water
 Applications of Solar Distillation
Providing clean drinking water in developing
countries
Emergency water purification in disaster
situations
Desalination for coastal communities
Supplying water for agriculture and livestock
Solar Ponds:
• Solar ponds are a unique
technology that captures and
stores the sun's thermal energy.
• These large-scale ponds act like
natural heat batteries, offering a
sustainable and reliable way to
tap into solar power.
• We will learn on the workings of
solar ponds, their benefits
 How Solar Ponds Work
Three distinct layers:
• Top layer (non-convective): Relatively cool,
transparent zone
• Middle layer (convective): Gradient zone with
increasing salinity and density, preventing heat
transfer by convection
• Bottom layer (storage): Hot, dark zone where solar
energy is trapped
• Solar ponds function through a clever design that utilizes a
layered structure.
• top layer is a non-convective zone, acting as an insulator due
to its lower salinity.
• middle layer, with gradually increasing salinity, creates a
density gradient that prevents heat from rising to the surface
through convection
• The bottom layer, often lined with dark material, absorbs
solar radiation and becomes the thermal storage zone,
holding the captured heat for extended periods.
Importance of salinity gradient
As salt concentration increases with depth, the density of the water also
increases
This creates a stable stratification, where denser saltwater remains at the
bottom.
Process where warm water naturally rises and cool water sinks
This trapping mechanism allows the bottom layer to accumulate and retain
thermal energy from the sun.
Real-World Applications of Solar Ponds
Bhuj, India: Solar pond powering a desalination plant for
freshwater production
China: Ongoing research and development for various
industrial applications
Ein Bokek, Israel: Large-scale solar pond used for space
heating and tourism
 Solar Chimneys
The Power of Solar Chimneys:
The solar chimney is a fascinating concept that combines the
power of the sun and wind to generate electricity.
What is Solar Chimney
• A large-scale power plant that
utilizes solar and wind energy
Key components:
Collector: A vast, black-colored
surface that absorbs sunlight
Chimney: A tall, vertical
structure with a large diameter
Turbine: A wind turbine
positioned within the chimney
Power generation equipment:
Converts wind energy into
electricity
Massive collector: Its made of Black material to maximize solar
absorption.
This collector heats the air beneath it.
The heated air rises through a tall chimney, creating a strong
upward draft.
A wind turbine strategically placed within the chimney
captures the kinetic energy of the rising air, converting it into
electricity through a generator.
The Science Behind Solar Chimneys
The stack effect:
A principle governing air movement due to temperature
differences.
Warm air has lower density and rises
In a solar chimney, the heated air in the collector creates a
buoyancy force.
This force drives a continuous upward airflow through the
chimney
Benefits of Solar Chimneys
Sustainable and
renewable energy source:
Harnesses the power of the
sun and wind
Land-efficient design: The
collector can be built on
non-arable land
Minimal water usage:
Unlike conventional power
plants, requires minimal
water for operation
 Thermal Energy Storage Systems
Thermal energy storage systems, also known as TES.
Thermal energy refers to the total kinetic energy of
microscopic particles in a substance.
The higher the temperature, the faster the particles move and
the greater the thermal energy.
Coldness is simply the absence of heat, or heat at a lower
temperature
TES technologies allow us to capture and store thermal
energy, heat or coolness, for later use
Thermal energy storage is becoming increasingly
important as we look to integrate more renewable energy
sources into the grid
Renewable energy sources, such as solar and wind, are
intermittent, meaning they don’t produce energy all the
time.
Hence storing excess energy when it is produced and
using it when it is needed.
Types of Thermal Energy Storage:
There are three main types to store thermal
energy storage:
Sensible heat storage
Latent heat storage
Thermochemical heat storage
Sensible Heat Storage -- Sensible heat storage stores thermal
energy by heating or cooling a material, such as water, rocks, or
concrete.
Latent heat storage -Stores thermal energy by using materials
that change phase, such as melting or freezing.
Thermochemical heat storage: Stores thermal energy by using
chemical reactions.
Sensible Heat Storage:
 It’s the most Common type
 It works by heating or cooling a material such as rock ,
concrete,water
 The thermal energy is stored in the increased or decreased
temperature of the material.
 Storing coolness by lowering its temperature.
 Common materials used include water, rocks, and concrete.
 The amount of thermal energy stored depends on the mass of the
material and the temperature difference.
 Example: Chilling water in a tank during the night (when electricity
rates are lower) can provide cold storage for air conditioning during
the day.
 Water is the most common material used in sensible heat
storage systems
 Sensible heat storage is relatively simple and inexpensive to
implement
 Less efficient than other types of thermal energy storage
because some heat is always lost to the environment over
time.
Latent Heat Storage:
Latent heat storage uses materials that change phase to store
thermal energy.
 When a material changes phase, such as from solid to liquid
or liquid to gas
--> It absorbs or releases a large amount of heat without
significant change in temperature.
 This heat is called latent heat.
 Latent heat storage can be more efficient than sensible heat
storage because there is less heat loss to the environment.
The materials which are used for phase change are called phase
change materials (PCMs)
 When a PCM absorbs heat, it changes phase, typically from a
solid to a liquid.
 This process absorbs a large amount of heat without a
significant increase in temperature
 The stored heat can then be released later by allowing the
PCM to change phase back to its solid state.
 Common PCMs include paraffin waxes, salt hydrates, and
organic fatty acids.
 Latent heat storage can be more efficient—because ???
Thermochemical Heat Storage:
Thermochemical heat storage uses chemical reactions to store
thermal energy.
 Heat is used to drive a chemical reaction that stores energy in
the chemical bonds of the products.
 The stored energy can then be released later by reversing the
chemical reaction
 Is a promising technology for long-term energy storage, but it
is still under development.
Common Chemicals
Calcium hydroxide (Ca(OH)2): This is also known as slaked lime.
When heated, it releases water vapor and stores thermal
energy.
 Calcium carbonate (CaCO₃): This is the main component of
limestone.
Solar Photovoltaic Systems:
Solar photovoltaic (PV) systems offer a promising solution when it comes to
sustainable energy source.
A solar cell is the basic building block of a
solar PV system.
 A solar cell is a layered structure made
from a semiconductor material, typically
silicon.
 It has a p-n junction, where p stands for
positive and n for negative
When sunlight hits the cell, it excites
electrons, knocking them loose from their
atoms.
The p-n junction creates an electric field that
directs these free electrons, generating a flow
of electricity- the photovoltaic effect!
Solar Photovoltaic Systems
Substrate
The substrate is the base material upon which
the solar cell is built.

In photovoltaic cells, the substrate provides

structural support and often serves as the
semiconductor material where the
photovoltaic effect occurs.
Thickness: 180 -300 μm
can be single/multi-crystalline
Emitter
The emitter is the layer of the solar cell that absorbs sunlight and generates
electron-hole pairs.

It's typically the top layer of the p-n junction and is doped to create an excess of
electrons (n-type)

Function:

Absorption of Photons: The emitter layer absorbs incoming photons from sunlight,
which excites electrons, creating electron-hole pairs

Charge Separation: The electric field at the p-n junction helps separate these charge
carriers, directing electrons toward the n-type layer and holes toward the p-type
layer.
Electrical Contacts
These are essential for collecting and transporting the generated electrical current.

Extracting the generated electrical current from the solar cell and connecting it
to an external circuit.

Front Contacts (Grid):

Collects electrons from the n-type layer and allows light to enter the cell with
minimal shading.

Back Contacts:

Collects holes from the p-type layer and completes the circuit.
Anti-reflective Coating

Anti-reflective coatings (ARC) are applied to the surface

of the solar cell
to reduce reflection losses
increase the amount of light entering the cell.
The ARC minimizes the reflection of sunlight from
the cell's surface,
allowing more photons to penetrate the
semiconductor material.
the coating increases the cell's efficiency by ensuring
that more light is absorbed.
 Characteristics of PV
I-V Characteristic Curve:
This graph depicts the relationship between voltage (V) and
current (I) output of a solar cell.
It helps identify the maximum power point (MPP)
 Factors like temperature and light intensity can influence the
shape of the curve.
Efficiency:
Ratio of converted sunlight energy to electrical energy output.
Higher efficiency cells are generally more expensive.
Open-Circuit Voltage (Voc)
 Maximum voltage across the cell when no current is flowing.
 Higher Voc is desirable for applications requiring high voltage.

Fill Factor (FF):

 This parameter reflects how "full" the I-V curve
 A higher FF signifies a more efficient cell that utilizes available
voltage and current effectively.
Classification of Solar Cells
can be categorized based on their material composition
and construction techniques
Crystalline Silicon (c-Si) Cells
These are the most widely used known for their good
efficiency and durability
They come in two main variants:
Monocrystalline silicon (mono-Si)
Polycrystalline silicon (poly-Si)
Monocrystalline silicon
(mono-Si):
Made from a single, pure
silicon crystal,
offering the highest
efficiency among
commercially available
options
Also being the most
expensive.
Polycrystalline silicon
(poly-Si):
Composed of multiple
silicon crystals, resulting in
slightly lower efficiency
compared to mono-Si
Offering a more cost-
effective option.
Thin-Film Cells:
These are gaining
attraction due to lightweight
and flexible nature.
They use a thin layer of
various materials like
cadmium telluride (CdTe)
copper indium gallium
selenide (CIGS)
amorphous silicon (a-Si)
Generally less efficient
than c-Si cells.
Concentrated Photovoltaic (CPV) Systems:
These utilize lenses or mirrors to focus sunlight onto smaller, high-
efficiency cells
CPV systems are ideal for areas with abundant sunlight
Requires tracking mechanism to maintain focus.

Organic Solar Cells:

These are still under development
Hold promise for low-cost, lightweight solar solutions.
They are made from organic materials like polymers and require
further research to improve their efficiency and durability.
How does solar cell work?

• Solar cell works on the principle of photovoltaic

effect.
• Solar cell consists of n type semi conductor
(emitter) and p type semiconductor (base).
• The two layers are sandwiched.
• Formation of PN junction.
• The surface is coated with anti-reflection coating
When sunlight hits the cell, it excites
electrons, knocking them loose from their
atoms.
The p-n junction creates an electric field that
directs these free electrons, generating a flow
of electricity
Photovoltaic Panels (Series & parallel arrays)

• Working voltage of single solar cell is 0.5 V and

current is about 50 mA.
• Solar cells are connected in series to get larger
voltage.
• Solar cells are connected in parallel to get
larger current.
Photovoltaic Panels (Series & parallel arrays)

Low power panel: 1.5 -6 V, a few milliwatts.

Uses in watches, clock, calculators , camera, etc.

Small panels: 1-10 W, and 3-12 V .

Area: 100 cm^2 to 1000 cm^2.
Uses in toys, battery charging circuit, small pumps and radios
Large panels: 10-60 W, and 6-12 V, Area:1000 cm^2 to 5000 cm^2.
Applications: Remote area power supplies, communications, power
supplies etc.

Assembly of series-parallel array

Calculation of number of cells required in series (Ns)

VB= Bus voltage

VD= Voltage drop across the blocking diode
VW=Voltage drop in the wiring system
VMP=Voltage at the maximum power
Solar PhotoVoltaic System
Photovoltaic cell: Generate voltage and current when
exposed to light.
•Module/Panel: Solar cells are connected together
•Array: combination of Module/panel
•Charge controller: Power conditioning equipment to
regulate the voltage.
•Battery storage: To store DC energy
•Inverter: Power electronic device which converts DC
to AC
•DC loads: Load operates in DC energy/DC supply
•AC load: Load operates in AC energy/AC supply