Renewable Energy Resources [Solar Thermal Energy, Solar Pond]

TOPIC NAME:
Solar Thermal
Energy, Solar Pond
Amar Preet Singh AJas Education
Amar Preet Singh
Academic Experience : 6+ years
Renewable Energy Resources
1
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Topic 1: Solar Thermal Energy
• Introduction
• Diffuse and Direct Solar Radiation
• Measurement of Solar Energy
• Geometry of Solar radiation
• Overview of Flat Plate Collectors
• Materials of Flat plate collectors
• Application of Flat Plate Collectors
• Water Heating
• Space Heating
• Power Generation
• Performance of Flat Plate Collectors
Amar Preet Singh AJas Education 2
Content

• Performance of Concentrating Collectors
• Application of Concentrating Collectors
• Solar Thermal Power Plants
• Parabolic Trough Power Plants
• Trough Power Plant Efficiencies
• Open Volumetric Air Receiver Concept
• Pressurized Air Receiver Concept
• Solar Cooling
Topic 2: Solar Pond
• Principle and Working of Solar Pond
• Salts For Solar Pond
• Limitation of Solar Thermal Energy
Content

TOPIC 1: SOLAR
THERMAL ENERGY
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• A solar radiation sensor measures solar energy from the sun.
• Solar radiation is radiant energy emitted by the sun from a nuclear
fusion reaction that creates electromagnetic energy.
• The spectrum of solar radiation is close to that of a black body with a
temperature of about 5800 K.
• About half of the radiation is in the visible short-wave part of the
electromagnetic spectrum.
• The other half is mostly in the near-infrared part, with some in the
ultraviolet part of the spectrum.
• The units of measure are Watts per square meter.
Introduction

• Every location on Earth receives sunlight at least part of the year. The
amount of solar radiation that reaches any one spot on the Earth's
surface varies according to:
• Geographic location
• Time of day
• Season
• Local landscape
• Local weather.
Introduction

• As sunlight passes through the atmosphere, some of it is absorbed,
scattered, and reflected by:
• Air molecules
• Water vapor
• Clouds
• Dust
• Pollutants
• Forest fires
• Volcanoes.
Diffuse and Direct Solar Radiation

• This is called diffuse solar radiation.
• The solar radiation that reaches the Earth's surface without being
diffused is called direct beam solar radiation.
• The sum of the diffuse and direct solar radiation is called global solar
radiation.
• Atmospheric conditions can reduce direct beam radiation by 10% on
clear, dry days and by 100% during thick, cloudy days.
Diffuse and Direct Solar Radiation

• Measurements of solar energy are typically expressed as total
radiation on a horizontal surface, or as total radiation on a surface
tracking the sun.
• Radiation data for solar electric (photovoltaic) systems are often
represented as kilowatt-hours per square meter (kWh/m2). Direct
estimates of solar energy may also be expressed as watts per square
meter (W/m2).
• Radiation data for solar water heating and space heating systems are
usually represented in British thermal units per square foot (Btu/ft2).
Measurement of Solar Energy

• Latitude (Φ). The latitude of a location is the angle made by the
radial line joining the location to the centre of the earth with the
projection of the line on the equatorial plane. It can vary from —90°
to +90°
• Hour Angle (ω). It is the angle through which the earth must be
rotated to bring the meridian of the plane directly under the sun. In
other words, it is the angular Displacement of the sun east or west of
the local meridian, due to the rotation of the earth on its axis at angle
of 15° per hour.
• ω = 15 (ST -12) where ST — local solar time.
Geometry of Solar radiation
for Numerical

• Declination (δ). The declination δ is the angle made by the line
joining the centres of the sun and the earth with its projection on the
equatorial plane. The declination angle varies from a maximum value
of +23.5° on June 21 to a minimum value of -23.5 ° on Dec. 21.
• 𝛿 = 23.45 sin 360 ×
284+𝑛
365
• Where n = no. of days
Figure 1: Curve of declination and month

• Altitude Angle (α). It is a vertical angle between the projection of the
sun's rays on the horizontal plane and the direction of the sun’s rays.
• Zenith Angle (θz). It is the complementary angle of sun’s altitude
angle. It is the vertical angle between the sun’s rays and a line
perpendicular to the horizontal plane through the point.
• cos θz = cos 𝜑 cos 𝛿 cos 𝜔 + sin 𝜑 sin 𝛿
• Surface Azimuth Angle (γ). It is the angle in the horizontal plane,
between the line due south and the projection of the normal to the
surface on the horizontal plane.
• cos 𝛾𝑠 =
cos 𝜃𝑧 sin 𝜑−sin 𝛿
sin 𝜃𝑧 cos 𝜑
for Numerical

• 𝜔𝑠𝑡 = cos−1
− tan 𝜑 tan 𝛿
• 𝑁 =
2
15
. cos−1 − tan 𝜑 tan 𝛿
• Local Apparent Time (LAT)
• The time used calculating the hour angle is the local apparent time.
• Local apparent time is given by
• LAT = [Standard time + Equation of time correction ± (standard time
longitude – longitude of location)]
• In India standard time is based on 82.5° E longitude.
for Numerical

• The flat-plate solar collectors are probably the most fundamental and
most studied technology for solar-powered domestic hot water
systems.
• The Sun heats a dark flat surface, which collect as much energy as
possible, and then the energy is transferred to water, air, or other fluid
for further use.
Overview of Flat Plate Collectors

• These are the main components of a typical flat-plate solar collector:
• Black surface - absorbent of the incident solar energy
• Glazing cover - a transparent layer that transmits radiation to the
absorber, but prevents radiative and convective heat loss from the
surface
• Tubes containing heating fluid to transfer the heat from the
collector
• Support structure to protect the components and hold them in place
• Insulation covering sides and bottom of the collector to reduce heat
losses

Figure 2: Schematic diagram of flat plate collectors

• The flat-plate systems normally operate and reach the maximum
efficiency within the temperature range from 30 to 80 oC (Kalogirou,
2009).
• however, some new types of collectors that employ vacuum insulation
can achieve higher temperatures (up to 100 oC).
• Due to the introduction of selective coatings, the stagnant fluid
temperature in flat-plate collectors has been shown to reach 200 oC.

• The properties of the material used for collectors can be classified by
• Thermo physical properties such as thermal conductivity, heat
capacity etc.
• Physical properties like density, tensile strength, melting point etc.;
• Environmental properties like moisture penetration, corrosion
resistance and degradation due to pollutants in atmosphere.
• The material for absorber plate should have high thermal
conductivity, adequate tensile strength and good corrosion resistance.
The most common material used for absorber plate is copper
Materials of Flat plate collectors

• Other materials which are used for absorber plate are Aluminium,
Iron, Brass, Silver. Tin and Zinc etc.
• The material for insulation should have low thermal conductivity,
should be stable at high temperature. Some commonly used materials
are crown white wool, glass wool. calcium silicate. cellular foam etc.
• For cover plate tempered glass is most common material. Transparent
Plastic materials such as acrylic polycarbonate plastic. polyvinyl
fluoride are used for cover plate.
Materials of Flat plate collectors

• A flat plate collector is used for many applications such as :
• Water heating
• Space heating
• Power generation
Water Heating
• Solar radiation passes through the transparent cover of flat plate
collector and absorbed by collector plate.
• Water flowing in contact with the collector is heated and the heat
from the water is extracted. for use.
• The circulating pump keeps a continuous circulation of water through
the collector and storage tank.
Application of Flat Plate Collectors

Water Heating
Figure 3: Schematic diagram of water heating

• A space heating system is
shown in Figure. Water is
heated in flat plate collectors
and heated water is stored in
the tank.
• Energy is transferred to the air
circulating in the house by
water to air heat exchanger.
• Two pumps are provided for
forced circulation between the
collectors and the tank, and
between the tank and heat
exchanger.
Space Heating
Figure 4: Schematic diagram of
space heating

• For low temperature power generation using flat plate collectors is
shown in Figure.
• The solar radiations are received by flat plate collectors. The energy is
collected by water.
• The hot water is stored in insulated storage tank.
• From here, It flows through vapour generator through which working
fluid of the Rankine cycle is also passed.
• The working fluid has a low boiling point.
• Vapour at 90°C leaves the vapour generator.
• This vapour then executes a regular Rankine cycle by flowing through
prime mover, condenser and pump.
Power Generation

Power Generation
Figure 5: Schematic Diagram of power generation

• Under steady conditions, the useful heat delivered by a solar collector
is equal to the energy absorbed in the metal surface minus the heat
losses from the surface directly and: indirectly to the surroundings.
• The useful heat output of a flat plate collector is given by
• 𝑄𝑐 = A 𝐼𝑐 𝜏𝛼 𝑒 − 𝑈𝑐 𝑇𝑖𝑛 − 𝑇𝑎 watts (1)
• Where: 𝑄𝑐= Useful heat output of flat plate collector (W)
• A = Collector surface area (m2)
• 𝐼𝑐 = Intensity of solar radiation incident on the collector surface
(W/m2)
• 𝜏 = Fraction of the incoming solar radiation that reaches the absorbing
surface, transmissivity.
Performance of Flat Plate Collectors

• 𝜏𝛼 𝑒= Effective product of transmissivity of the transparent cover
and absorptivity of the absorber.
• 𝑈𝑐= Overall heat loss coefficient of collector (W/m2k)
• 𝑇𝑖𝑛= Collector fluid inlet temperature (°C)
• 𝑇𝑎 = Ambient air temperature (°C)
• Introducing heat removal factor FR in equation (4.26) we get
• 𝑄𝑐 = A 𝐼𝑐𝐹𝑅 𝜏𝛼 𝑒 − 𝑈𝑐𝐹𝑅 𝑇𝑖𝑛 − 𝑇𝑎 (2)
• The efficiency of a solar collector is defined as the ratio of the useful
heat output of the collector to the solar energy flux incident on the
collector.
• ɳ =
𝑄𝑐
𝐴𝐼𝑐
(3)

• Put the value of Qc from equation (2) in equation (3)
• ɳ𝑐 = 𝐹𝑅 𝜏𝛼 𝑒 − 𝑈𝑐𝐹𝑅
𝑇𝑖𝑛−𝑇𝑎
𝐼𝑐
(4)
• If
𝑇𝑖𝑛−𝑇𝑎
𝐼𝑐
=0
• Then ɳ𝑐 = 𝐹𝑅 𝜏𝛼 𝑒 (5)
• This is the efficient optical efficiency.
• The outlet temperature of the collector heat transfer fluid is given by
• 𝑇 𝑜𝑢𝑡 = 𝑇𝑖𝑛 +
𝑄𝑐
𝑚𝐶𝑃
(6)
• where. 𝑇𝑖𝑛 = Collector fluid inlet temperature (°C)
• Qc = Useful heat output of collector (W)

• m = Mass flow rate of collector fluid (kg/s)
• CP = Specific heat of collector fluid (J/kg K)
• 𝑇𝑆 = 𝑇𝑖𝑛 = 𝑇𝑎 + 𝐼𝑐
𝐹𝑅 𝜏𝛼 𝑒
𝑈𝑐𝐹𝑅
(7)

• The useful heat output Qc of a concentrating collector is given
• 𝑄𝑐 = 𝐹𝑅𝐴𝑎 𝐼𝑏𝑐ɳ𝑜𝑝𝑡 − ൗ
𝑈𝑐
𝐶 𝑇𝑖𝑛 − 𝑇𝑎 (8)
• where 𝐹𝑅 = Heat removal factor of collector
• 𝐴𝑎 = Unshaded aperture area (m’)
• 𝐼𝑏𝑐 = Intensity of beam solar radiation incident on the concentrator
aperture (W/m2)
• ɳ𝑜𝑝𝑡 = Optical efficiency of collector
• 𝑈𝑐= Total heat loss coefficient of collector (W/m" K) ,
• 𝑇𝑖𝑛 = Collector inlet temperature (°C)
• 𝑇𝑎 = Ambient temperature (°C).
Performance of Concentrating Collectors

• The optical efficiency of a concentrating collector is defined as the
ratio of the solar radiation absorbed by the absorber to the beam solar
radiation on the concentrator and is given by
• ɳ𝑜𝑝𝑡= ɳ𝑜𝑝𝑡 0°𝐶𝑜𝑝𝑡 = 𝜌𝛾𝜏𝛼𝑎𝐶𝑜𝑝𝑡 (9)
• Where ɳ𝑜𝑝𝑡 0° = Optical efheteney of collector at O°-tneidence angle
of beam radiation
• 𝐶𝑜𝑝𝑡 = Correction factor for deviation of incidence angle from 0°
• 𝜌 = Mirror reflectivity
• 𝛾 = Intercept factor
• 𝜏 = Transmissivity of transparent cover of the absorber
• 𝛼𝑎 = Absorptivity of absorber.

• Then. the efficiency of a concentrating collector is
• ɳ𝑐 = 𝐹𝑅ɳ𝑜𝑝𝑡 −
𝐹𝑅𝑈𝑐
𝐶𝐼𝑏𝑐
𝑇𝑖𝑛 − 𝑇𝑎 (10)

• Concentrating collectors enable the generation of must higher
temperatures compare to flat plate collector.
• Hence they are used for application which need high amount of heat
such as power generation
• In solar power plant, solar energy is collected by large parabolic
collectors. This energy is transferred to water to convert into steam
this steam is used to run a turbine which drives and alternators for
generating electricity.
Application of Concentrating Collectors

• Most techniques for generating electricity from heat need high
temperatures to achieve reasonable efficiencies. The output
temperatures of non-concentrating solar collectors are limited to
temperatures below 200°C.
• Due to their high costs, lenses and burning glasses are not usually
used for large-scale power plants, and more cost-effective alternatives
are used, including reflecting concentrators.
• The reflector, which concentrates the sunlight to a focal line or focal
point, has a parabolic shape; such a reflector must always be tracked.
Solar Thermal Power Plants

Solar Thermal Power Plants
Figure 6: Schematic diagram of solar thermal power plants

• The parabolic trough collector consists of large curved mirrors, which
concentrate the sunlight by a factor of 80 or more to a focal line.
Parallel collectors build up a 300–600 metre long collector row, and a
multitude of parallel rows form the solar collector field.
• The collector field can also be formed from very long rows of parallel
Fresnel collectors. In the focal line of these is a metal absorber tube,
which is usually embedded in an evacuated glass tube that reduces
heat losses. A special high-temperature, resistive selective coating
additionally reduces radiation heat losses.
Parabolic Trough Power Plants

Parabolic Trough Power Plants
Figure 7: Schematic diagram of parabolic trough power plants

• The collector efficiency depends on the angle of incidence of
the sunlight and the temperature in the absorber tube, and can
reach values up to 75%.
• Field losses are usually below 10%. Altogether, solar thermal
trough power plants can reach annual efficiencies of about 15%;
the steam-cycle efficiency of about 35% has the most significant
influence.
Trough Power Plant Efficiencies

• The first type of solar tower is the open volumetric receiver concept.
A blower transports ambient air through the receiver, which is heated
up by the reflected sunlight.
• The receiver consists of wire mesh or ceramic or metallic materials in
a honeycomb structure, and air is drawn through this and heated up to
temperatures between 650°C and 850°C.
• the volumetric structure produces the highest temperatures inside the
receiver material, reducing the heat radiation losses on the receiver
surface.
Open Volumetric Air Receiver Concept

Open Volumetric Air Receiver Concept
Figure 8: Schematic diagram of open volumetric air receiver concept

• The volumetric pressurized receiver concept offers totally new
opportunities for solar thermal tower plants.
• A compressor pressurizes air to about 15 bar; a transparent glass dome
covers the receiver and separates the absorber from the environment.
• Inside the pressurized receiver, the air is heated to temperatures of up
to 1100°C, and the hot air drives a gas turbine.
• The waste heat of the gas turbine goes to a heat boiler and in addition
to this drives a steam-cycle process.
• The combined gas and steam turbine process can reach efficiencies of
over 50%, whereas the efficiency of a simple steam turbine cycle is
only 35%.
• Therefore, solar system efficiencies of over 20% are possible.
Pressurized Air Receiver Concept

Pressurized Air Receiver Concept
Figure 9: Schematic diagram of pressurized air receiver concept

• Solar energy can be used for cooling the buildings and preserving
food by refrigeration, if solar cooling of buildings is combined with
solar heating, then this combination becomes more economical.
• There are many
• Absorption cycle with liquid absorbents
• Absorption cycle with solid absorbents
• Vapour compression cycle etc.
• The liquid absorbents are LiBr — H2O, H2O – NH3, NH3 - LiNO3 etc.
• Solid absorbents are CaCl2 – NH3, Silicagel —- H,O, Zeolites — H2O
etc.
Solar Cooling

Solar Cooling
Figure 10: Schematic diagram of solar cooling

• The main elements of absorption cooling device are generator,
condenser, evaporator and absorber.
• In absorption cooling system refrigerants combine chemically with an
absorbent to release heat during absorption, and absorb the heat
during evaporation.
• Suppose absorbent-refrigerant solution is liquid. Hot water from a
flat-plate collector array is passed through a heat exchanger also
called generator, where it transfers heat to the mixture of absorbent
and refrigerant which is rich in refrigerant.
• The high pressure liquid now passes through the expansion valve and
evaporates in evaporator and extracting heat from surroundings, as a
result, cooling is observed in the space.
Solar Cooling

• The low pressure absorbent refrigerant solution is now pumped to the
generator at high pressure to complete the cycle.
• The evaporator and absorber are in the low pressure side, while
generator and condenser are in the high pressure side of the system.
Solar Cooling

TOPIC 2: SOLAR POND
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• The sun is the largest source of renewable energy and this energy is
abundantly available in all parts of the earth. It is in fact one of the
best alternatives to the non-renewable sources of energy.
• Solar ponds are large-scale energy collectors with integral heat
storage for supplying thermal energy. It can be use for various
applications, such as process heating, water desalination,
refrigeration, drying and power generation.
Solar Pond

• In an ordinary pond, the sun’s rays heat the water and the heated
water from within the pond rises and reaches the top but loses the heat
into the atmosphere.
• The net result is that the pond water remains at the atmospheric
temperature. The solar pond restricts this tendency by dissolving salt
in the bottom layer of the pond making it too heavy to rise.
• A solar pond has three zones.
• The top zone is the surface zone, or UCZ (Upper Convective
Zone), which is at atmospheric temperature and has little salt
content.
• The bottom zone is very hot, 70°– 85° C, and is very salty. It is this
zone that collects and stores solar energy in the form of heat, and
is, therefore, known as the storage zone or LCZ (Lower
Convective Zone).
Principle and Working of Solar Pond

• Separating these two zones is the important gradient zone or NCZ
(Non-Convective Zone). Here the salt content increases as depth
increases, thereby creating a salinity or density gradient.
• If we consider a particular layer in this zone, water of that layer
cannot rise, as the layer of water above has less salt content and is,
therefore, lighter. Similarly, the water from this layer cannot fall as
the water layer below has a higher salt content and is, therefore,
heavier.
• This gradient zone acts as a transparent insulator permitting sunlight
to reach the bottom zone but also entrapping it there. The trapped
(solar) energy is then withdrawn from the pond in the form of hot
brine from the storage zone.

Figure 11: Schematic diagram of principle and working of solar pond

• Common salts which are utilized in the solar pond is sodium chloride
(NaCl) and magnesium chloride (MgCl2). Additional alternatives are
potassium chloride (KCl), Calcium chloride (CaCl2), Ammonium
nitrate (NH4NO3), Potassium nitrate (KNO3), Borax (Na2B4O7) and
Sodium sulphate (Na2SO4),
• The maximum temperature attained by storage zone in different solar
pond prepared with different salt is in order of NaCl > MgCl2 >
Na2SO4. The maximum temperature of the storage zone corresponds
to 39 °C, 38 °C and 33 °C for NaCl, MgCl2 and Na2SO4 in
comparison to 25 °C of upper zone temperature.
• The largest operating solar pond for electricity generation was
the Beit HaArava pond built in Israel and operated up until 1988. It
had an area of 210,000 m² and gave an electrical output of 5 MW.
Salts For Solar Pond

• Low energy density 0.1 to 1 kW/m2.
• Large area is required to collect solar thermal energy.
• Direction of rays changes continuously with time.
• Energy not available during night and during clouds.
• Energy storage is essential,
• High cost.
• Requires hybrid plant with storage facility for supplying energy
during night.
• Solar Central Power Plants in MW range are not economical.
Limitation of Solar Thermal Energy

• The sum of the diffuse and direct solar radiation is called global solar
radiation.
• Radiation data for solar water heating and space heating systems are
usually represented in British thermal units per square foot (Btu/ft2).
• Other materials which are used for absorber plate are Aluminium,
Iron, Brass, Silver. Tin and Zinc etc.
• The useful heat output of a flat plate collector is given by
• 𝑄𝑐 = A 𝐼𝑐 𝜏𝛼 𝑒 − 𝑈𝑐 𝑇𝑖𝑛 − 𝑇𝑎 watts
• Then ɳ𝑐 = 𝐹𝑅 𝜏𝛼 𝑒
• This is the efficient optical efficiency
• The maximum temperature of the storage zone corresponds to 39 °C,
38 °C and 33 °C for NaCl, MgCl2 and Na2SO4
Summary

Renewable Energy Resources [Solar Thermal Energy, Solar Pond]

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