UNIT-1 SOLAR ENERGY

Questions & ANSWERS

Short Answer Questions

1. Define solar constant. What is its standard value?

Solar Constant: The solar energy reaching unit area at outer edge of the Earth’s atmosphere
exposed perpendicularly to the rays of the Sun at the average distance between the Sun and Earth
is known as the solar constant. Solar Constant = 1.4kW/m2.

2. Explain about solar pond

A solar pond is a large-scale thermal energy collector that captures and stores solar energy in a
body of saline water. It works by using salinity gradients to trap heat at the bottom of the pond.

Working Principle:

 The pond has three layers:

1. Upper Convective Zone (UCZ) – Low salt concentration, cooler, allows sunlight
to pass.
2. Non-Convective Zone (NCZ) – Salt concentration increases with depth, prevents
heat from rising.
3. Lower Convective Zone (LCZ) – High salt concentration, stores heat up to 85–
90°C.

Sunlight penetrates the top layers and heats the bottom. The salt gradient prevents convection, so
the heat remains trapped at the bottom.

Applications

Power generation using low-temperature turbines

Industrial process heating

Desalination

Greenhouse heating
3. Explain solar water heater.

A solar water heater is a device that uses solar energy to heat water for domestic, commercial, or
industrial use.

Working Principle:

 It uses solar collectors (usually flat-plate or evacuated tubes) to absorb sunlight and
transfer heat to water.
 The heated water is stored in an insulated storage tank for later use.
 It works on thermosiphon (natural circulation) or pumped circulation systems.

Main Components:

1. Solar Collector – Captures solar radiation and heats water.

2. Storage Tank – Stores hot water for use.
3. Pipes and Insulation – Transfer and retain heat in the system.

Applications:

 Domestic hot water supply

 Hotels, hospitals, hostels
 Industrial processes needing hot water

4. Explain solar green house

A solar greenhouse is a structure designed to use solar energy to grow plants. It traps sunlight
through transparent materials like glass or plastic, converting it into heat. This heat keeps the
interior warm, creating a suitable environment for plant growth even in cold weather. Solar
greenhouses often include thermal mass (like water barrels or stone) to store heat and maintain
temperature during night or cloudy days.

5. List any two solar energy storage systems

Two Solar Energy Storage Systems

1. Thermal Energy Storage: Stores solar heat in materials like water, salts, or rocks for
later use, especially in heating or solar thermal power plants.
 2. Battery Storage: Converts and stores solar energy as electricity using batteries (e.g.,
lithium-ion) for use when sunlight is not available.

6. Describe the requirements of solar energy storage systems

High Efficiency: The system should store and retrieve energy with minimal losses.

Capacity and Scalability: It should meet the energy demand and be expandable as needed.

Long Life and Reliability: It must function effectively over many charge-discharge cycles.

Low Cost: Installation, operation, and maintenance should be economically feasible.

Fast Response Time: It should respond quickly to energy supply and demand changes.

Safety: The system must be safe under all operating conditions.

Environmental Friendliness: Materials used should be non-toxic and sustainable.

7. Specifications of solar energy storage systems

Storage Capacity: Measured in kWh, indicates how much energy can be stored.

Power Rating: Measured in kW, defines how much energy can be delivered at once.

Round-Trip Efficiency: Percentage of energy retained after storage and retrieval (typically 70–
90%).

Depth of Discharge (DoD): Percentage of total capacity that can be used safely (usually 80–
90%).

Cycle Life: Number of charge-discharge cycles the system can handle (typically thousands).

Response Time: Time taken to supply stored energy (usually milliseconds to seconds).

Technology Type: Examples include lithium-ion batteries, flow batteries, or thermal storage.
 Long Answer Questions
1. Explain in detail various kinds of renewable energy sources and their potential and
discuss the solar energy option
 2. Draw and explain the P-V and I-V characteristics of the PV System for different
input quantities of irradiance and temperature

As solar is the most clean, abundant and easily available renewable energy source [1],
therefore, there is large interest for its use globally. Solar PV cell shows non-linear P-V and I-V
characteristics as shown in Fig.1. and it can be noticed that at one particular voltage (Vmp) PV
cell delivers maximum power (Pmax) and with change in irradiation and temperature P-V
(power-voltage) and I-V (current-voltage) characteristics change and hence, voltage at maximum
power point (Vmp), current at maximum power point (Imp), open circuit voltage (Voc) and short
circuit current (Isc) change with change in irradiation and temperature (as shown in Fig.1.).

Therefore, there is need to track maximum power point (MPP) and there are a lot of MPPT
techniques that have been reported and most commonly used MPPT techniques are incremental
conductance and perturb and observe MPPT [2]. In this paper perturb and observe (P&O)
technique is implemented.

Fig-1. I-V & P-V characteristics of PV Cell at different Irradiations and Temperatures

Fig. 1 shows the following characteristics

a) I-V characteristics of PV Cell at different Irradiations (in W/m2).

b) P-V characteristics of PV Cell at different Irradiations (in W/m2).
c) I-V characteristics of PV Cell at different Temperatures (in℃).
d) P-V characteristics of PV Cell at different Temperatures (in℃).
 3. What are the advantages and disadvantages of photo voltaic solar energy
conversion?

Disadvantages

High Initial Costs

Sunlight Dependency
Weather Dependency
Space Requirements
Energy Storage Needs
Environmental Impact
4. Derive the expression for the diode current of a solar PV cell from its equivalent
circuit?
5. Write the advantages and disadvantages of concentrating
collectors over flat-plate types of solar collectors?
6. With suitable schematic, describe the construction and working of solar pond based
electric-power plant with cooling tower

7. Briefly discuss the following: i) solar irradiance ii) solar constant iii) extraterrestrial
radiations iv) terrestrial radiations

(i) Solar Irradiance : Solar irradiance is the power per unit area (surface power
density) received from the Sun in the form of electromagnetic radiation in
 the wavelength range of the measuring instrument. Solar irradiance is measured
in watts per square metre (W/m2) in SI units.

Solar irradiance is often integrated over a given time period in order to report
the radiant energy emitted into the surrounding environment (joule per square
metre, J/m2) during that time period. This integrated solar irradiance is
called solar irradiation, solar exposure, solar insolation, or insolation.

(ii) Solar Constant: The solar energy reaching unit area at outer edge of the Earth’s
atmosphere exposed perpendicularly to the rays of the Sun at the average distance
between the Sun and Earth is known as the solar constant. Solar Constant =
1.4kW/m2.
Always extra-terrestrial radiation is more than the terrestrial radiation.

8. Discuss the need of renewable energy systems in place of non-renewable energy

systems?

9 Discuss the construction and working of liquid flat plate

collector with a neat sketch. Explain the various parameters that affect the
performance of collector.
Design Aspects of a Flat Plate Collector

1. Absorber Plate

 Function: This is the main component that absorbs solar radiation and converts it into heat. It is
typically made of a metal such as copper or aluminum because these materials have high thermal
conductivity.
 Design: The absorber plate is often painted black to maximize its absorption of solar energy. Its
surface may have fins or other features to increase the surface area and improve heat transfer.

2. Glazing

 Function: The glazing is a transparent cover that reduces heat loss from the absorber plate by
trapping heat and allowing sunlight to pass through.
 Design: Common materials for glazing include tempered glass or acrylic. The glazing should be
designed to minimize reflections and maximize the amount of sunlight entering the collector.

3. Insulation

 Function: Insulation is placed behind and around the absorber plate to reduce heat loss to the
environment and improve the collector's efficiency.
 Design: Insulation materials might include fiberglass or foam, and they are usually placed on the
back and sides of the absorber plate to reduce thermal losses.

4. Collector Housing

 Function: The housing or frame holds all the components together and provides structural
support.
 Design: The frame is typically made from durable materials like aluminum or galvanized steel. It
should be designed to withstand environmental conditions and support the glazing, absorber plate,
and insulation.

5. Fluid Channels

 Function: These are the pathways through which the heat transfer fluid (often water or a glycol
mixture) circulates to absorb the heat from the absorber plate and transport it to where it is
needed.
 Design: Fluid channels are usually integrated into the absorber plate or located behind it. They
must be designed to ensure efficient heat transfer and minimize resistance to fluid flow.

6. Outlet and Inlet Ports

 Function: These ports are where the heat transfer fluid enters and exits the collector.
 Design: They need to be designed to ensure a secure connection to the piping system and
minimize any potential for leakage.
Basic Working Principle

1. Solar Radiation Absorption: Sunlight strikes the glazing and reaches the absorber plate,
where it is absorbed and converted into heat.
2. Heat Transfer: The heat from the absorber plate is transferred to the heat transfer fluid
circulating in the fluid channels.
3. Heat Transport: The heated fluid is then transported through the outlet port to a storage
tank or other heat exchange system, where the heat is utilized.
4. Heat Retention: Insulation minimizes heat loss from the collector to the surrounding

10 Draw and explain the design aspects of concentrating collector of solar energy?

A concentrating solar collector is a type of solar thermal collector that uses mirrors or lenses to
focus a large area of sunlight, or solar energy, onto a small area. This concentrated sunlight is
then used to produce heat, which can be used for power generation or other applications.
1. Reflector or Lens

 Function: The primary component responsible for concentrating sunlight. It can be made
of reflective materials (e.g., mirrors) or refractive materials (e.g., lenses).
 Types:
o Parabolic Reflector: Curved in a parabolic shape to focus sunlight onto a single
point. Often used in parabolic dish collectors.
o Cylindrical Parabolic Reflector: Curved in a parabolic shape along a single axis
to focus sunlight onto a line. Used in parabolic trough collectors.
o Fresnel Lens: A type of lens that can focus sunlight onto a small area, typically
used in compact systems.

2. Absorber

 Function: Captures the concentrated sunlight and converts it into heat.

 Design: Typically a pipe or plate placed at the focal point of the reflector or lens. It’s
often coated with a material that maximizes absorption and minimizes radiation loss.

3. Heat Transfer Fluid (HTF)

 Function: Transfers heat from the absorber to where it’s needed (e.g., a heat exchanger,
storage system, or power generation unit).
 Types: Common fluids include water, oil, or molten salts, depending on the temperature
range and specific application.

4. Tracking System

 Function: Keeps the collector oriented towards the sun to ensure maximum
concentration of sunlight throughout the day.
 Types:
o Single-Axis Tracker: Rotates around one axis, typically horizontal, allowing the
collector to follow the sun’s east-to-west path.
o Dual-Axis Tracker: Allows rotation around both horizontal and vertical axes,
optimizing solar exposure more precisely.

5. Support Structure

 Function: Provides structural support to hold the reflector or lens in place and ensure
proper alignment.
 Design Considerations: Needs to be sturdy and capable of withstanding environmental
conditions like wind and snow