U15EE1006R

RENEWABLE ENERGY SYSTEMS

 UNIT 2

SOLAR ENERGY SYSTEMS

Solar Energy Systems
 Solar radiation & its measurements

 Solar energy collectors

 Solar energy storage systems
 Solar pond & applications

 Applications of solar energy

 Solar pumping
 Solar cooking
 Solar Distillation
 Solar Greenhouse
 Solar Energy
• Energy from the sun
– Sun – Natural nuclear reactor
• Release packets of energy called Photons

• Photons: travel from sun to earth – 93 million miles in 8 minutes

– Energy Availability
• from sunrise to sunset

– Availability in India
• 5 – 7.5 kWh/m2 : common in most places
 Solar Energy
• Utilisation
– Energy from sun is converted to
• Electrical energy – indirect use: solar photovoltaic,

• Thermal energy – direct use : heating, concentrated solar power etc.

– Passive solar energy

• Harnessed in direct form – drying clothes

– Active solar energy

• Employs mechanical / electrical equipment to increase efficiency

• Water heating system: pumps to circulate, space heating

 CSP: to heat fluid, produce steam, run turbine, generate power

 Solar Radiation
• Introduction
– Solar radiation is the Electromagnetic rays emitted by the sun

– Solar radiation depends on

• Fusion of 4 hydrogen nuclei
• Produces one Helium nucleus – much lesser weight
• This difference of mass is compensated by huge release of energy
• Energy in the form of gamma ray photons
 Solar Radiation
• Introduction
– Solar radiation is the Electromagnetic rays emitted by the sun

– Solar radiation depends on

• Geographic location
• Time of day
• Season
• Local landscape
• Local weather

– Solar radiation that reaches earth’s surface

• Direct beam radiation
• Diffuse radiation
 Solar Radiation
• Solar radiation at the earth’s surface
– Beam of solar radiation
• Absorbed
– Ozone absorbs UV rays
– CO2 and water vapour absorb IR rays
• Scattered
– By Dust particles, pollutants, water vapour

– Direct Beam radiation

• Radiation that is neither scattered nor absorbed
– Directly reach the earth’s surface

– Diffuse radiation
• Scattered radiation that reaches earth’s surface from all directions
 Solar Radiation
• Solar radiation at the earth’s surface
 Solar Radiation
• Terminology
– Solar insolation (incident solar radiation)
• Solar radiation received on a flat horizontal surface on earth

– Global radiation
• Sum of beam and diffuse radiation
 Solar Radiation
• Terminology

– Solar constant
• Energy received from sun per unit time, on unit area of surface
perpendicular to direction of rays at the earth’s mean distance from
the sun.
• Mean distance varies
• World Radiation Centre (WRC) set universal standards
I SC  1367 W/m 2
360n
I ext  I sc [1  0.033 cos( )]
365

– Solar time (Local apparent time)

• Time used for calculating hour angle. It doesn’t coincide with clock
time
 Solar Radiation
• Hour angle (ω)
– Hour angle at any moment is the angle through which the earth must turn to
bring the median of the observer directly in line with sun’s rays
 Solar Radiation
• Geometry of Solar radiation
– Expressed by the following terms
• Angle of latitude
• Declination angle

• Hour angle
• Inclination angle (altitude)

• Zenith angle
• Solar Azimuth angle

• Slope (Tilt angle)

• Surface azimuth angle

• Angle of incidence
 Solar Radiation
• Angle of Latitude (ϕ)
– To indicate location of the considered point on earth
– Angle made by
• Line joining location (P) and centre of earth with
• Its projection on equator plane

– Northern hemisphere: positive

– Southern hemisphere: negative
 Solar Radiation
• Angle of Latitude (ϕ)
– Angle made by
• Line joining location (P) and centre of earth with
• Its projection on equator plane
 Solar Radiation
• Declination angle (δ)
– Angular displacement of the sun from the plane of earths equator
 Solar Radiation
• Declination angle (δ)
– Angular displacement of the sun from the plane of earths equator
Solar Radiation
Solar Radiation
 Solar Radiation
• Slope / Tilt angle (β)
– Angle between
• Collector’s inclined plane surface and
• The horizontal plane

• Surface Azimuth angle (γ)

– Angle in horizontal plane between
• Line along south direction and
• Horizontal projection of Normal to collector’s incline plane

• Angle of incidence (θi)

– Angle on horizontal plane between
• Sun’s ray incident on incline surface of collector and
• Normal (perpendicular) to that surface
 Solar Radiation
• Angle of incidence (θi)
– Angle on horizontal plane between
• Sun’s ray incident on incline surface of collector and
• Normal (perpendicular) to that surface
 Solar Radiation Measurement
• Actinometer
– Instrument used to measure intensity (heating power) of radiation

– Device that determines number of photons in a beam

– Used in meteorology to measure solar radiation

• Measurement of solar radiation

– Pyranometer

– Pyrheliometer

– Net radiometer
 Pyranometer
Solar Radiation Measurement
• Construction of Pyranometers
– Thermopile:
• Thermocouple: a temperature sensor that is measured thru’ voltage variation

• Thermopiles: Thermocouples connected in series

 Pyranometer
Solar Radiation Measurement
• Construction of Pyranometers
– Thermopile:
 Pyranometer
Solar Radiation Measurement
• Construction of Pyranometers
– Glass dome
• Hemispherical glass dome

• Shields thermopile from wind, rain and convection

 Pyranometer
Solar Radiation Measurement

• Working Principle
– based on measurement of temperature difference between
• dark surface: black coating absorbs the solar radiation
• clear surface: reflects the radiation; hence less heat is
absorbed.

– potential difference is created in the thermopile

• Due to temperature gradient between the two surfaces

– voltage measured using a potentiometer

 Pyranometer
Solar Radiation Measurement
• Advantages
– Very small temperature coefficient
– More accurate measurements
– Longer response time than a photovoltaic cell
– Integrated measurement of the total available short-wave solar
energy under all conditions.

• Applications
– Predicting insulation requirements for building structures
– Establishment of greenhouse locations
– Designing photovoltaic systems
– Meteorological and climatological studies
– Measurement of solar intensity data.
 Pyrheliometer
Solar Radiation Measurement
• Construction of Pyrheliometer

 Based on thermopile

 Measures beam of direct radiation

 Needs a tracker mechanism to measure all day

 Pyrheliometer
Solar Radiation Measurement
Construction of Pyrheliometer
– Shape: Collimator tube
• Collimator: device that narrows a beam or
particle; usually consists of a long narrow tube &
lens / other absorbing surface
– Ex : Telescope is an optical collimator (has a lens
and long tube to narrow visible light)

• Black coated insides: To absorb most of the

radiation (at various angles)

• Sealed with dry air: To avoid absorption of

radiation by water-vapour within tube
 Pyrheliometer
Solar Radiation Measurement

Construction of Pyrheliometer
– Measurement: Thermopile
• Wire wound thermopile

• Placed behind sensor plate

• Thermocouple: a temperature sensor that is
measured thru’ voltage variation
• Thermopiles: Thermocouples connected in
series
 Pyrheliometer
Solar Radiation Measurement

• Working Principle
– The direct radiation is channelized in the tube
• And absorbed due to the black coating

– based on measurement of temperature difference in thermopile

– potential difference is created in the wire-wound thermopile
• Due to temperature gradient

– Measured voltage is then converted to radiation data

 Pyrheliometer
Solar Radiation Measurement

• Angstrom type Pyrheliometer

– Pyrheliometer designed by Angstrom

– Simplest and most accurate

– Construction
• 2 similar strips: S1 and S2 of area A

• 1 end of thermocouple connected to S1 and other end to S2

• Sensitive galvanometer (G) is connected to thermocouple

• Strip S2 is connected to an external electric circuit

 Pyrheliometer
Solar Radiation Measurement

• Angstrom type Pyrheliometer

– Working Principle
• When both S1 and S2 are shielded from radiation
– Both junctions are at same temperature

– So, NO deflection in galvanometer

• S1 is exposed and S2 is shielded with cover M

– Temperature of S1 rises as it receives solar radiation

– Galvanometer shows deflection

• Current allowed to pass thru’ S2

– Until the galvanometer shows NO deflection

– Now S1 and S2 are at same temperatures

 Pyrheliometer
Solar Radiation Measurement

• Angstrom type Pyrheliometer

– Working Principle
• Heat absorbed by S1 is equal to Heat produced in S2
– Heat absorbed by S1 = Q.A.a

Q: heat radiation absorbed per unit area

A: area of strip

a: absorption coefficient

– Heat produced in S2 = V.I

Q.A.a = V.I

Q = V.I / A.a
 Sunshine Measurement
• Sunshine Recorder
– Instrument used to record amount of sunshine at a given location

– Also called heliograph

– Provides info about weather, climate and temperature of a location

– Used in meteorology, science, agriculture, tourism and other fields

 Sunshine Recorder
• Construction
– Glass sphere: Mounted on its axis parallel to that of earth

– Inside a Spherical bowl section (semi-circular brass bar)

– Sunshine card: special paper with timescale (placed inside groove of bowl)
 Sunshine Recorder
Working Principle
– Focussing sun’s rays:
• bowl and glass sphere are placed such that sun’s rays are focussed at a point
on the card

– Burnt path on the card:

• As sun moves,

• Focussed sunshine burns a path along the card

• Length od trace is duration of sunshine

– 3 overlapping pairs of grooves – for various seasons

Solar Energy Collectors
 Need for Solar Collectors

 Performance Indices of Solar Collectors

 Types of Solar Collectors
 Flat plate
 Concentrating type
 Need for Solar Collectors
• Introduction

Solar Energy Utilization

Solar Photovoltaic Solar Thermal

– Solar Photovoltaic
• directly converts solar energy into electricity thru’ PV cells

– Solar Thermal Systems

• thermal energy produced is used for various purposes
– Heating, cleaning, power generation plant etc.
 Need for Solar Collectors
• Need for Solar Collectors
– Solar Energy
• Low density per unit area due to diffusion

– Solar Collectors
• First unit of a solar thermal system
• For maximum utilization of solar energy
– Solar energy is absorbed as heat

– Transferred to heat up fluid

– Fluid delivers heat to next system for utilization
Performance Indices of Solar Collectors

• Collector Efficiency
– Ratio of useful energy to energy incident on collector
• Useful energy: actual energy absorbed and transferred to fluid

• Concentration Ratio
– Ratio of area of aperture to area of receiver
• Area of aperture: projected area of collector, facing the beam

• Temperature Range
– Range of temperature to which heat transfer fluid is heated up
Classification of Solar Collectors
Non-Concentrating type (Flat Plate) Collectors

• Characteristic
– Absorb radiation as received on the collector surface
• Absorb both beam and diffuse radiation

• Construction
– Simple and robust
• Back surface: to absorb the incident radiation
• Glazing cover: to prevent radiative & convective heat loss from surface
• Heat exchanger: to transfer heat absorbed
• Support structure & insulation: to increase the strength & efficiency
Non-Concentrating type (Flat Plate) Collectors
• Positioning of Flat plate collectors
Non-Concentrating type (Flat Plate) Collectors
• Advantages
– Easy to manufacture
– Low cost
– Collect both beam & diffuse radiation
– Permanently fixed
– Little maintenance

• Disadvantages
– High temperatures cannot be obtained
– Heat is lost from large area (due to lack of optical concentration)
 Liquid Flat Plate Collector
• Characteristic: Heat transport medium is a fluid (liquid)
• Construction
– Back surface: usually Cu, Al or steel absorber plate
– Transparent cover: 1 or 2 sheets of glass or plastic
– Heat exchanger: tubes, channels or passages in contact with absorber plate
– Support structure & insulation: weather tight, insulated container
 Liquid Flat Plate Collector
• Components
– Heat exchanger system
 Liquid Flat Plate Collector
• Components: Collector plates
– Pipe & Fin type
• Liquid flows in pipes only
• Low wetted area & liquid capacity
• Used for high temp. applications

– Water sandwich type

• High wetted area & liquid capacity

– Roll bond type

• Also called semi-water sandwiched
• Intermediate between the two

Sandwich type: used for low temp

applications like swimming pool warming
 Flat Plate Air Heating Collector
• Characteristic: Heat transport medium is air

• Applications
– Drying for agricultural & industrial purposes
– Space heating

• Construction
– Similar to liquid flat plate collector: with change in absorber & tube
– Heat transfer coefficient b/w absorber & air is low
 Flat Plate Air Heating Collector

• Types: To improve heat exchange

• The surfaces are roughened

– Fin type
• Longitudinal fins in airflow passage

– Metal Matrix type

– Corrugated type
– Thermal trap type
 Flat Plate Air Heating Collector
• Advantages: Over liquid flat plate collector
– Compact, simple constructions & low maintenance
– Transfer of heat between two or more fluids is not required
• Air is directly used as working fluid

– Corrosion is completely eliminated

– Leakage of air from duct is less severe
– Possibility of freezing of working fluid is eliminated
– Pressure inside collector is always moderate

• Disadvantages
– Large amount of air is to be handled, due to low density
– Heat transfer is poor b/w absorber plate & air
– Less storage capacity due to low heat capacity
 Concentrating type Collectors
• Non-focus type
– Reflects additional radiation onto absorber plate
• Using reflectors to increase concentration of radiation

• Focus type
– Reflects and concentrates entire radiation onto
• A single line: Line focus
• A single point: Point focus
 Modified Flat Plate Collector
• Characteristic
– Reflectors: plane mirrors at the edges of flat plate collector
• Additional radiation is reflected into the collector
• Concentration of radiation is increased (normal & reflected)

– V-trough design: aligned in East-West direction

– Requires periodic tilt adjustment
– Relatively small increase in radiation concentration
• Thus not widely used
Compound Parabolic Concentrator (CPC)
• Characteristic
– Parabolic mirror segments: at the edges of flat plate collector
• Additional radiation is reflected into the collector (beam & diffuse)
• Concentration of radiation is increased

– Trough like arrangement: aligned in East-West direction

• Does NOT require tracking
Focus Type Collector
• Line Focus
 Cylindrical Parabolic Concentrator
• Characteristic
– Reflectors: cylindrical parabolic trough
– Receiver: tube blackened at outside surface
– Heat transfer: fluid in the receiver tube

– Rotated about one axis to track the sun

 Fixed Mirror Solar Concentrator
• Characteristic
– Concentrator: fixed mirror strips arranged on circular ref. cylinder
– Receiver: tube made to rotate at centre of curvature to track the sun
– Heat transfer: fluid in the receiver tube

– Overcomes difficulty in manufacturing large parabolic mirrors

 Linear Fresnel Lens Collector
• Characteristic
– Concentrator: Fresnel lens with linear grooves (optical quality plastic)
• behaves as a spherical lens by arranging the angle of grooves accordingly
– Receiver: tube
– Heat transfer: fluid in the receiver tube
Focus Type Collector
• Point Focus
Paraboloidal Dish Collector (Scheffler Solar Concentrator)
• Characteristic
– Concentrator: Parabolic dish with two axis tracking
– Receiver: at the focus point

– Requires 2 axis tracking

 Hemispherical Bowl Mirror Concentrator
• Characteristic
– Concentrator: hemispherical fixed mirror
– Receiver: moved along paraxial line through the centre of curvature

– Requires 2 axis tracking

 Circular Fresnel Lens Concentrator
• Characteristic
– Concentrator: Fresnel lens with several thin circular zones
• Tilt of each zone is adjusted so that the lens resembles a thin spherical lens
– Receiver: at the focal point of the lens
 Central Tower Receiver
• Characteristic
– Concentrator: several heliostats
• Heliostat: independently controlled flat mirrors
– Receiver: at the top of a central tower