CHAPTER-3 Solar Energy
CHAPTER-3 Solar Energy
CHAPTER-3 Solar Energy
3.1 INTRODUCTION
Solar power is the flow of energy from the sun. The primary forms of solar energy are heat
and light. Sunlight and heat are transformed and absorbed by the environment in a multitude
of ways. Some of these transformations result in renewable energy flows such as biomass,
wind and waves. Effects such as the jet stream, the Gulf Stream and the water cycle are also
the result of solar energy's absorption in the environment.
The Earth receives 174 petawatts (PW) of solar radiation at the upper atmosphere.
While traveling through the atmosphere 6% of the incoming solar radiation (insolation) is
reflected and 16% is absorbed. Average atmospheric conditions (clouds, dust, pollutants)
further reduce insolation by 20% through reflection and 3% through absorption. The
absorption of solar energy by atmospheric convection (sensible heat transport) and by the
evaporation and condesation of water vapor (latent heat transport) drive the winds and the
water cycle.
The Sun is absolutely pure and free energy source for the whole mankind. The solar
radiation can be used for obtaining electric power or for heating water, air or some other
materials. The sun is a star, after all, and it produces energy in many forms, from perceptible
heat, visible and invisible spectrums of light, radiation, and more. Autotrophy, organisms that
produce their own food from the sun (mainly plants), use solar energy along with carbon
dioxide and water to produce simple sugars in a process called photosynthesis. Heterotrophy,
organisms that eat other organisms (like animals and fungi), depend on autographs to form
the bottom level of the food chain. Heterotrophy could not exist without autographs , and
autographs could not exist without the sun, so life as we know it depends on electromagnetic
radiation.
The solar systems have a low environmental impact, and one of the most important benefits is
that it doesn’t have emissions like CO2 or other toxic gases or radioactive material, like the
ones that are produced by the current systems used to produce energy. The costs of these
energy systems consist only of the construction and maintenance of the plant, the source of
energy is free and in theory unlimited. The environmental impact of these systems is
practically zero. Some of the disadvantages are that these systems can only be installed in
areas in which the solar radiation is longer during the days and during the year. They are also
less efficient than the current energy systems
Mean distance from the earth : 149 600 000 km (the astronomic unit) (1.5 X
108 km)
Diameter : 1 390 000 km (109 × that of the earth) (1.39 X
106 km)
Volume : 1.41 X 1033 cm3 (1,300,000 times that of the
earth )
Mass : 1,993 × 1027 kg (332 000 times that of the earth)
Density (at its center) : >100 × 103 kg m-3 (over 100 times that of
water)
Pressure (at its center) : over 1 billion atmospheres
Temperature (at its center) : about 15 600 000 degrees Kelvin
Temperature (at the surface) : 5800 degrees Kelvin
Energy radiation : 380 × 1021 kW
The Earth receives : 170 × 1012 kW
Chemical Composition
Hydrogen 73.46
Helium 24.85
Oxygen 0.77
Carbon 0.29
Iron 0.16
Neon 0.12
Nitrogen 0.09
Silicon 0.07
Magnesium 0.05
Sulphur 0.10
Rotation (as seen from the Earth):
Of solar equator 26.8 days
At solar latitude 30 deg 28.2 days
At solar latitude 60 deg 30.8 days
At solar latitude 75 deg 31.8 days
3.2 Solar radiation
Radiation from the sun sustains life on earth and determines climate. The energy flow within
the sun results in a surface temperature of around 5800 K, so the spectrum of the radiation
from the sun is similar to that of a 5800 K blackbody with fine structure due to absorption in
the cool peripheral solar gas (Fraunhofer lines).
Part of the radiation from the sun is in the form of visible light which, when passed through a
prism, produces a spectrum containing all the pure colors from red at one end to violet at the
other.The total frequency spectrum covers visible light and near-visible radiation, (as shown
in figure below) such as x-rays, ultraviolet radiation, infrared radiation, and radio waves. The
visible light and heat of the sun makes life possible, and is called daylight or sunshine. The
earth’s atmosphere deflects or filters the majority of the sun’s harmful radiation, and our
near-perfect positioning in the solar system allows us to receive the benefits proximity to the
Sun without being baked or broiled like Venus or Mercury.
The electromagnetic radiation emitted by the sun shows a wide range of wavelengths. It can
be divided into two major regions with respect to the capability of ionizing atoms in
radiation-absorbing matter: ionizing radiation (X-rays and gamma-rays) and non-ionizing
radiation (UV rays, visible light, and infrared radiation). Fortunately, the highly injurious
ionizing radiation does not penetrate the earth's atmosphere.
Solar radiation is commonly divided into various regions or bands on the basis of
wavelength. Ultraviolet radiation is that part of the electromagnetic spectrum between 100
and 400 nm as shown in figure 3.1
Radiation Spectrum: The Radiation Spectrum is the distribution of radiation energy over
different wavelengths, or frequencies. Radiation in the earth-atmosphere system is shown in
Figure 3.2.
Figure…3.2….Solar radiation Spectrum
In solar energy engineering, we come across the term extraterrestrial solar radiation and
terrestrial solar radiation. In order to understand these terms, let us consider terrestrial and
extraterrestrial regions as shown in Fig. 1.4.4.
SUN
Atmosphere
Diffused Radiation
Terrestrial Region
Long Wavelength Radiation
Earth
The solar radiation at the entrance into the Earth atmosphere is known as extraterrestrial
radiation. The intensity of extraterrestrial solar radiation is changed because of the Sun
activity and the change in distance between the Earth and Sun. During the course of a year, the
value of this radiation changes in the range from 1307 (W/m2) to 1393 (W/m2).
According to the above equation…5 the apparent solar irradiance will be maximum when the earth is
nearest to the sun and minimum when earth is largest distance from the sun. This value varies by ±3%
as the earth orbits the sun. The relative motion of the earth and sun is shown in figure 3.3.
Figure 3.3 Relative motions of the earth and sun
Terrestrial solar radiation is generally broke down into two components- beam radiation
(also called direct radiation or direct beam radiation) and diffuse radiation.
(i). Beam radiation or Direct radiation(Ib): Beam radiation or Direct radiation is solar
irradiance that passes through the atmosphere in essentially a straight line without being
reflected, scattered or absorbed by particles or gases in the air as shown in figure 34.
(ii). Diffuse radiation(Id): Diffuse radiation is solar radiation, which is scattered, reflected or
absorbed by molecules of air, water vapour, aerosols and dust particles, but ultimately still
reaches the earth’s surface. The diffuse component of solar radiation striking a solar collector
also includes solar radiation reflected from the adjacent earth’s surface as shown in
figure…3.4.
(iii). Total or Global radiation (It): Total amount of solar radiation per unit area per day
reaching a part of the Earth is called the ‘insolation’, a short form of “incident solar
radiation”.
Solar insolation is defined as the amount of sunshine incident on the surface of the Earth per
unit area per day.
Total radiation may be given as:
It = Id + Ib cosθz …
Where, Id is the diffuse radiation, Ib is the direct radiation and θz is the zenith angle
Fig 3.4 .Solar Radiations on the surface on Earth
Ibi
Ib
Ibn Ibn
i
z
y
s
Let
Ibn = beam radiation at normal incidence
Is = beam radiation at horizontal surface
Ibi = beam radiation at tilted surface
From Fig. 1.5.1, we have
Ib = Ibn cos z
Ibi = Ibn cos i
The beam radiation tilt factor, Rb , is then given by
Rb = Ibi / Ib = cos i/ cos z
A tilted surface receives diffused radiation as well as reflected radiation form the ground and
surroundings in addition to the beam radiation.
The diffused sky radiation on the tilted surface at an angle s is given by I d (1+ cos s)/2, where
Id is the diffuse sky radiation on a horizontal surface.
If the ground and other surfaces seen by the tilted surface have a diffuse reflectivity τ g for
both beam and diffuse sky radiation, the amount of ground-reflected solar radiation on the
tilted surface is equal to τg (1- cos s)/2, of the total radiation (Ib + Id) on the horizontal
surface.
The beam, diffuse, and reflected components, can be added to give the total incident
radiation, It , on a tilted surface. The total solar radiation on a horizontal surface, I, is the sum
of horizontal beam and diffuse radiation.
3.4 ATMOSPHERIC INFLUENCES ON RADIATION
Global Shortwave Radiation Balance (overview):
• About 30 % of solar radiation is reflected by clouds, atmospheric gases and the surface
• About 25 % of solar radiation is absorbed by the atmosphere (clouds, atmospheric gases,
aerosol)
• About 45 % of solar radiation is absorbed by the surface (oceans, land surface)
The earth's atmosphere strongly absorbs radiation in the ultraviolet and infrared regions of the
spectrum.
Absorption in the Ultraviolet.-—The gases which absorb most of the ultraviolet energy are oxygen
and ozone. Oxygen is present to heights of several hundred miles above the earth's surface and as it
strongly absorbs radiation of wavelengths shorter than 1750 A. all such solar radiation has been
completely absorbed by the time it has penetrated to within 50 miles of the earth's surface. An oxygen
molecule can be dissociated into two oxygen atoms when it absorbs radiation of sufficiently short
wavelengths. The two resulting atoms can both receive additional energy in the form of an increase of
velocity, as a result of this absorption. Consequently at great altitudes; the few atoms and molecules
present are moving with great velocities which are normally associated with high temperatures.
Molecular oxygen is stable below 50 miles because no radiation is available to dissociate it; and the
atmosphere is cooler because it cannot absorb much solar radiation.
The air mass coefficient defines the direct optical path length through the Earth's atmosphere,
expressed as a ratio relative to the path length vertically upwards, i.e. at the zenith. The air mass
coefficient can be used to help characterize the solar spectrum after solar radiation has traveled
through the atmosphere. The air mass coefficient is commonly used to characterize the performance
of solar cells under standardized conditions, and is often referred to using the syntax "AM" followed
by a number. "AM1.5" is almost universal when characterizing terrestrial power-generating panels.
For a path length through the atmosphere, for solar radiation incident at angle relative to the normal
to the Earth's surface, the air mass coefficient is:
where is the zenith path length (i.e. normal to the Earth's surface) at sea level and is the zenith
angle in degrees.
The air mass number is thus dependent on the Sun's elevation path through the sky and therefore
varies with time of day and with the passing seasons of the year, and with the latitude of the observer.
Solar zenith angle (θz) which is simply the complement of the solar altitude angle.
Solar azimuth angle (γs) measured clockwise on the horizontal plane, from the north-pointing
pointing coordinate axis to the projection of the sun’s central ray.
The plane that includes includes the earth 's equator equator is called the equatorial equatorial plane. If
a line is drawn between the center of the earth and the sun, the angle between this line and the earth's
equatorial plane is called the declination angle (δ).
Solar angle can be calculated as a function of latitude (φ,north , positive),hour positive(ω) and solar
declination(δ) where δ is a function of day number:
The latitude angle (φ) is the angle between a line drawn from a point on the earth’s surface to the
center of the earth,, and the earth’s equatorial plane. The intersection of the equatorial plane with the
surface of the earth forms the equator and is designated as 0 degrees latitude.
Hour angle (ω) is the angular distance between the meridian of the observer and the meridian
whose plane contains the sun.
The hour angle is zero at solar noon (when the sun reaches its highest point in the sky).
Concept Concept of the hour angle is used for describing describing the earth's rotation about its
polar axis.
During the planning and execution of solar projects, extensive survey of the prescribed site analysis
has to be carried out. Measurement of solar radiation is the most important aspect of this study which
can give the accurate quantum of energy that can be derived from a perpendicular location. Three
types of instruments are mainly used in the solar radiation measurement, for three different aspects of
solar radiation and the duration of bright sun shine in a day is measured by means of a sunshine
recorder. These are:
(i). Pyrheliometer:
The pyrheliometer is a broadband instrument that measures the direct beam component of solar
radiation. Consequently, the instrument should be permanently pointed toward the Sun. Since they
need to be pointed directly at the sun, pyrheliometers are typically mounted on a tracking device that
follows the sun’s movements. A two-axis Sun tracking mechanism is most often used for this purpose.
The pyrheliometer aperture angle is 5 degree. Consequently, radiation is received from the Sun and a
limited circumsolar region, but all diffuse radiation from the rest of the sky is excluded. Sunlight
enters the instrument through a window and is directed onto a thermopile which converts heat to an
electrical signal that can be recorded. The signal voltage is converted via a formula to measure watts
per square metre. Pyrheliometers are used for scientific research and for placing solar panels.
Hukseflux DR01 First Class pyrheliometer (Hukseflux 2012) is presented in Fig. 3.17 (b).
• Principle of Working: A thermopile is a device that uses the Seebeck effect to create an
electrical voltage based on temperature differences. The Seebeck effect refers to the phenomenon
where two different metals at different temperatures will produce an electrical current when
connected. In a pyrheliometer, one end of the thermopile faces the sun while the other end remains
shaded. This arrangement will leave the two metals at different temperatures and, therefore, will lead
to a flow of current in the thermopile. When the solar irradiance is higher, a higher electrical voltage
will be measured. The schematic diagram of pyrheliometer is shown in Figure 3.17(a).
(a) Schematic of a pyrheliometer. (b) Photo of Hukseflux DR01 first class pyrheliometer
Figure 3.17.
• Applications: Pyrheliometers are commonly used to study changes in solar radiation output,
such as the 11-year solar cycle. Another applications of the pyrheliometer is for the assessment of the
efficiency of solar collectors and photovoltaic devices. Different parts of the Earth receive different
amounts of solar irradiance, and this data can be collected by pyrheliometers and used to create maps
of average radiation levels. Polar latitudes generally have less incoming solar radiation, while other
areas have lower levels because of frequently cloudy skies. These maps can help inform where it
makes sense to place solar panels.
(ii). Pyranometer
A pyranometer is a type of actinometer used to measure broadband solar irradiance on a planar
surface and is a sensor that is designed to measure the solar radiation flux density (in watts per metre
square) from a field of view of 180 degrees. The name pyranometer stems from Greek, "pyro -"
meaning "fire" and "ano -meaning "above, sky". The real diagram of the pyrheliometer is shown in
Figure 3.18.
In order to attain the proper directional and spectral characteristics, a pyranometer’s main components
are:
• A thermopile sensor with a black coating. This sensor absorbs all solar radiation, has a flat
spectrum covering the 300 to 50,000 nanometer range, and has a near-perfect cosine response.
• A glass dome. This dome limits the spectral response from 300 to 2,800 nanometers (cutting off
the part above 2,800 nm), while preserving the 180 degrees field of view. Another function of the
dome is that it shields the thermopile sensor from convection.
The black coating on the thermopile sensor absorbs the solar radiation. This radiation is converted to
heat. The heat flows through the sensor to the pyranometer housing. The thermopile sensor generates
a voltage output signal that is proportional to the solar radiation.
• Principle: The black coating on the thermopile sensor absorbs the solar radiation. This
radiation is converted to heat. The heat flows through the sensor to the pyranometer housing. The
thermopile sensor generates a voltage output signal that is proportional to the solar radiation.
• Applications: A pyranometer can be also used to measure the diffuse solar irradiance Id,
provided that the contribution of the direct beam component is eliminated. For this, a small shading
disk can be mounted on an automated solar tracker to ensure that the pyranometer is continuously
shaded (as shown in figure 3.19 - b). Because the daily maximum sun elevation angle changes day by
day.
• Principle: A radiometer absorbs solar radiation at its sensor, transforms it into heat and
measures the resulting amount of heat to ascertain the level of solar radiation. Methods of measuring
heat include taking out heat flux as a temperature change (using a water flow pyrheliometer, a silver-
disk pyrheliometer) or as a thermo-electromotive force (using a thermoelectric pyrheliometer or a
thermoelectric pyranometer). In current operation, types using a thermopile are generally used. The
schematic diagram of radiometer is shown in Figure..3.20.
• Application: It is most commonly used in the field of eco-physiology. A multi-filter rotating
shadow band radiometer measures direct normal, diffuse horizontal and total horizontal solar
irradiance. A radiometer is an instrument designed to measure the radiated electromagnetic power.
When used in solar energy applications, it is usually desirable for radiometers to respond the same to
equal amounts of energy at all wavelengths over the wavelength range of the radiation to be
measured. Most radiometers therefore work by using a thermopile to measure the temperature rise of
a sensitive element whose receiving surface is painted dull black. Instruments for measuring solar
irradiance using a photovoltaic cell as the sensitive element have a non-uniform spectral response.
The duration of bright sun shine in a day is measured by means of a sunshine recorder shown in figure
3.21 the sun’s Rays are focused by a glass sphere to a point on a card strip held in a groove in a
spherical bowl mounted concentrically with the sphere. Whenever there is bright sunshine, the image
formed is intense enough to burn a spot on the cord strip. Though the day as the sun moves across the
sky, the image moves alone the strip. Thus, a burnt trace whose length is proportional to the duration
of sunshine is obtained on the strip. Sunshine recorders measure the number of hours in the day
during which the sunshine is above a certain level (typically 200 mW/cm2). Data collected in this way
are used to determine the solar insolation by comparing the measured number of sunshine hours to
those based on calculations and including several correction factors.
Solar energy is the most readily available source of energy. It does not belong to anybody and is,
therefore, free. It is also the most important of the non-conventional sources of energy because it is
non-polluting and, therefore, helps in reducing the greenhouse effect.
Solar energy has been used since ancient times, but in a most primitive manner. Before 1970,
some research and development was carried out in a few countries to exploit solar energy
more efficiently, but most of this work remained mainly academic. After the dramatic rise in
oil prices in the 1970s, several countries began to formulate extensive research and
development programmes to exploit solar energy.
When we hang out our clothes to dry in the sun, we use the energy of the sun. In the same
way, solar panels absorb the energy of the sun to provide heat for cooking and for heating
water. Such systems are available in the market and are being used in homes and factories.
In the next few years it is expected that millions of households in the world will be using
solar energy. India is one of the few countries with long days and plenty of sunshine. Solar
energy could be easily harnessed. Solar thermal energy is being used in India for heating
water for both industrial and domestic purposes.
Solar energy can be utilized through two different routes, as solar thermal route and solar electric
(solar photovoltaic) routes. Solar thermal route uses the sun's heat to produce hot water or air, cook
food, drying materials etc. Solar photovoltaic uses sun’s heat to produce electricity for lighting home
and building, running motors, pumps, electric appliances, and lighting (as discussed in chapter-2).
In solar thermal route, solar energy can be converted into thermal energy with the help of solar
collectors and receivers known as solar thermal devices. The Solar-Thermal devices can be classified
into three categories:
Low-grade solar thermal devices are used in solar water heaters, air-heaters, solar cookers and solar
dryers for domestic and industrial applications. Solar thermal power is a carbon-free, renewable, and
alternative to the power we generate with fossil fuels.
There are a large number of solar collector designs that have shown to be functional. These designs
are classified in two general types of solar collectors:
• Non concentrating collectors: In this the absorbing surface is approximately as large
as the overall collector area that intercepts the sun's rays. These collectors have same
area for intercepting and for absorbing solar radiation in these types the whole solar panel
absorbs the light.
• Concentrating collectors: Concentrating collectors use concave reflecting surfaces to
intercept and focus the sun’s beam radiation to a smaller receiving area, thereby increasing
the radiation flux. Concentrators are used mostly in commercial applications because they are
expensive and because tracker need frequent maintenance.
(a) (b)
Figure 3.23 Pictorial views of Evacuated heat pipe tubes collector
The manifold is wrapped in insulation and covered by a sheet metal or plastic case to protect it from
the elements as shown in Figure 3.23. The vacuum that surrounds the outside of the tube greatly
reduces convection and conduction heat loss to the outside, therefore achieving greater efficiency than
flat-plate collectors, especially in colder conditions.
3. Integral collector storage system
Integral collector-storage systems, also known as ICS or "batch" systems, are made of one or more
black tanks or tubes in an insulated glazed box. Cold water first passes through the solar collector,
which preheats the water, and then continues to the conventional backup water heater as shown in
Figure 3.24
ICS systems are simple, reliable solar water heaters. However, they should be installed only in
climates with mild freezing because the collector itself or the outdoor pipes could freeze in
severely cold weather.
Figure 3.24 Integral collector-storage systems
Solar air heat collector’s heat air directly, almost always for space heating. They are also used for pre-
heating make-up air in commercial and industrial systems. They fall into two categories: Glazed and
Unglazed.
Glazed systems have a transparent top sheet as well as insulated side and back panels to minimize
heat loss to ambient air. Air typically passes along the front or back of the absorber plate while
scrubbing heat directly from it. Heated air can then be distributed directly for applications such as
space heating and drying or may be stored for later use.
Unglazed systems, or transpired air systems, consist of an absorber plate which air passes across or
through as it scrubs heat from the absorber as shown in Figure 3.25. These systems are typically used
for pre-heating make-up air in commercial buildings.
5. Solar Bowl
A solar bowl is a type of solar thermal collector that operates similarly to a parabolic dish, but instead
of using a tracking parabolic mirror with a fixed receiver, it has a fixed spherical mirror with a
tracking receiver. This reduces its efficiency but makes it cheaper to build and operate. Designers call
it a fixed mirror distributed focus solar power system. The main reason for its development was to
eliminate the cost of moving a large mirror to track the sun as with parabolic dish systems. The
schematic diagram of solar bowl is shown in Figure 3.26.
Constructional Details:
The schematics of a flat plate collector are shown in Fig. 3.27. It usually consists of five main
components viz.
(a) Insulated Box: The rectangular box is made of thin G.I sheet and is insulated from sides and
bottom using glass wool, asbestos wool or some other insulating material of thickness 5 to 8 cm
to reduce losses from conduction to back and side wall. The box is tilted at due south and a tilt
angle depends on the latitude of location. The face area of the collector box is kept between 1 to 2
m2.
(b) Transparent Cover: This allows solar energy to pass through and reduces the convective heat
losses from the absorber plate through air space. The transparent tampered glass cover is placed
on top of rectangular box to trap the solar energy and sealed by rubber gaskets to prevent the
leakage of hot air. It is made of plastic/glass but glass is most favourable because of its
transmittance and low surface degradation.
However with development of improved quality of plastics, the degradation quality has been
improved. The plastics are available at low cost, light in weight and can be used to make tubes,
plates and cover but are suitable for low temperature application 70-120 0C with single cover
plate or up to 1500C using double cover plate. The thickness of glass cover 3 to 4 mm is
commonly used and 1 to 2 covers with spacing 1.5 to 3 cm are generally used between plates.
The temperature of glass cover is lower than the absorber plate and is a good absorber of thermal
energy and reduces convective and radiative losses of sky.
(c) Absorber Plate (metallic or plastic): It intercepts and absorbs the solar energy. The absorber
plate is made of copper, aluminium, steel or plastic and is in the thickness of 1 to 2 mm. It is the
most important part of collector along with the tubes or ducts passing the liquid or air to be
heated. The plate absorbs the maximum solar radiation incident on it through glazing (cover
plate) and transfers the heat to the tubes in contact with minimum heat losses to atmosphere. The
plate is black painted and provided with selective material coating to increase its absorption and
reduce the emission. The absorber plate has high absorption (80-95%) and low
transmission/reflection.
(d) Tubes or pipes: The plate is attached to a series of parallel tubes or one serpentine tube through
which water or other liquid passes. The tubes are made of copper, aluminium or steel in the
diameter 1 to 1.5 cm and are brazed, soldered on top/bottom of the absorber water equally in all
the tubes and collect it back from the other end. The header pipe is made of same material as tube
and of larger diameter. Now-a-days the tubes are made of plastic but they have low thermal
conductivity and higher coefficient of expansion than metals. Copper and aluminium are likely to
get corroded with saline liquids and steel tubes with inhibitors are used at such places.
(e) Casing which encloses the foregoing components and keeps them free of dust and moisture and
also reduces the thermal losses. The casing, enclosing all the components of the collector is either
made of wood or some light metal like aluminium.
Removal of Heat:
These systems are best suited to applications that require low temperatures. Once the heat is absorbed
on the absorber plate it must be removed fast and delivered to the place of storage for further use. As
the liquid circulates through the tubes, it absorbs the heat from absorber plate of the collectors. The
heated liquid moves slowly and the losses from collector will increase because of rise of high
temperature of collector and will lower the efficiency. Flat-plate solar collectors are less efficient in
cold weather than in warm weather.
It is box like structure. It consists of an absorber plate which receives beam as well as the diffuse
radiations through the transparent glass covers. The absorbed solar energy is partially transferred to
the liquid flowing through tube which is either fixed to the absorber plate or they form an integral part
of it.Fig. 3.28 shows a number of absorber plate designs for solar water and air heaters that have been
used with varying degrees of success. Fig. 3.28 (A) shows a bonded sheet design, in which the fluid
passages are integral with the plate to ensure good thermal conduct between the metal and the fluid.
Fig. 3.28 (B) and (C) shows fluid heaters with tubes soldered, brazed, or otherwise fastened to upper
or lower surfaces of sheets or strips of copper. Copper tubes are used most often because of their
superior resistance to corrosion. Thermal cement, clips, clamps, or twisted wires have been tried in the
search for low-cost bonding methods. Fig. 3.28 (D) shows the use of extruded rectangular tubing to
obtain a larger heat transfer area between tube and plate. Mechanical pressure, thermal cement, or
brazing may be used to make the assembly. Soft solder must be avoided because of the high plate
temperature encountered at stagnation conditions.
• Mechanism
When solar radiation passes through a transparent cover and impinges on the blackened absorber
surface of high absorptivity, a large portion of this energy is absorbed by the plate and then
transferred to the transport medium in the fluid tubes to be carried away for storage or use. The
underside of the absorber plate and the side of casing are well insulated to reduce conduction losses.
The liquid tubes can be welded to the absorbing plate, or they can be an integral part of the plate. The
liquid tubes are connected at both ends by large diameter header tubes.
FPC is usually permanently fixed in position and requires no tracking of the sun. The collectors
should be oriented directly towards the equator, facing south in the northern hemisphere and north in
the southern. The optimum tilt angle of the collector is equal to the latitude of the location with angle
variations of 10–158 more or less depending on the application.
The collection efficiency of a FPC varies from 40 to 60 % for a temperature rise of about 15 °C but
drops to 30% or even less for a 50 °C temperature rise. However, the absorbing efficiency of a FPC
can be increased by applying a selecting coating to collected surface instead of a black paint. The
efficiency can also improved by applying anti-reflecting coating on the transparent cover.
(i). Lose more heat: Flat-plate collectors usually lose more heat to the environment than evacuated
tubes and this loss increases with temperature difference. So they are usually inappropriate choice of
solar collector for high temperature commercial applications such as process steam production.
(ii). Less efficient per square meter: Evacuated tube collectors have a lower absorber plate area to
gross area ratio (typically 60-80% of gross area) compared to flat plates. (In early designs the absorber
area only occupied about 50% of the collector panel. However this has changed as the technology has
advanced to maximize the absorption area.) Based on absorber plate area, most evacuated tube
systems are more efficient per square meter than equivalent flat plate systems. This makes them
suitable where roof space is limiting.
(iii). Deliver less energy when ambient temperature is low: In general, per installed square metre,
evacuated tubes deliver marginally more energy when the ambient temperature is low (e.g. during
winter) or when the sky is overcast for long periods.
(iv). Cost effective: In the areas where sunshine and temperature is less, some low cost flat plate
collectors can be more cost efficient than evacuated tube collectors. In most climates, flat-plate
collectors will generally be a more cost-effective solution than evacuated tubes.
(v). Efficient in low/ medium heat applications: Heating of water by a medium to low amount is
much more efficiently performed by flat plate collectors. Domestic hot water frequently falls into this
medium category. Glazed or unglazed flat collectors are the preferred devices for heating swimming
pool water. Unglazed collectors may be suitable in tropical or subtropical environments if domestic
hot water needs to be heated by less than 20°C.
The material for absorber plate should have high thermal conductivity, adequate tensile strength and
good corrosion strength and good corrosion resistance. The most common material used for absorber
plate is copper because of high conductivity and resistance to corrosion. Other materials that are used
for absorber plate are Aluminium, Iron, Brass, Silver, Tin and zinc.
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 etc.
For cover plate tempered glass is most common material. Transparent materials such as acrylic
polycarbonate plastic are used for cover plate.
Under steady state condition the useful heat (Qc) delivered by a solar collector is given by:
Qc Ic( A Uc A Tin Ta watt
….3.1
Where
A = collector surface area (m²)
Ic = Intensity of solar radiation incident on the collector surface (w/ m²)
= Fraction of the solar radiation that reaches the absorbing surface,
transmissivity
= 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)
The efficiency of a solar collector ( ηc ) is defined as the ratio of the useful heat output of the collector
to the solar energy flux incident on the collector.
if
(Tin – Ta)/ Ic)] = 0
Tin = Ta, then
ηc = Fr(
This is effective optical efficiency.
The outlet temperature f the collector heat transfer fluid is given by
The stagnation temperature Ts of the collector is defined as the temperature of the absorber which is
achieved when there is no flow of heat transfer fluid in the collector and therefore, its useful heat output
and efficiency both are equal to zero. Therefore, from the equation (3.4)
1. Water heating
Solar radiation passes through the transparent cover of flat plate collector and is 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.
2. Space heating
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
the heat exchanger.
fig. solar heating system.
3. Power generation
The solar radiations are received by flat plate collectors. The energy is collected by water.
The hot water is stored in insulating storage tank. From here, it flows through vapour
generation 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.
Solar collector efficiency is influenced by multiple factors such as surface area, heat gain, heat loss
through convection and conduction, and the conversion factor. Each collector type is ideally suited for
certain types of external conditions.
1. Life Time:
Provided that all components are of good quality, the life time of systems with flat plate collectors is
considerably longer. They have fewer components which are long lasting and very rare chances of
damage. In case of breakdown of a single component it can be easily replaced (collector, hot water
tank, etc). For high quality flat plate collectors a life time of 20 years and more is not a problem.
3. Efficiency:
Efficiency increase with low cold water temperatures and high air temperatures. With water
temperatures in the tank between 20 and 60 °C and not to low air temperatures, flat plate collectors
have always a higher efficiency than vacuum tube collectors.
These solar collectors use mirrored parabolic troughs to focus the sun's energy to a fluid-carrying receiver tube
located at the focal point of a parabolically curved trough reflector (see Fig.1 above). The energy from the sun
sent to the tube heats oil flowing through the tube, and the heat energy is then used to generate electricity in a
conventional steam generator.
Many troughs placed in parallel rows are called a "collector field." The troughs in the field are all aligned along
a north-south axis so they can track the sun from east to west during the day, ensuring that the sun is
continuously focused on the receiver pipes. Individual trough systems currently can generate about 80 MW of
electricity. Trough designs can incorporate thermal storage—setting aside the heat transfer fluid in its hot
phase—allowing for electricity generation several hours into the evening.
Currently, all parabolic trough plants are "hybrids," meaning they use fossil fuels to supplement the solar output
during periods of low solar radiation. Typically, a natural gas-fired heat or a gas steam boiler/re-heater is used.
Troughs also can be integrated with existing coal-fired plants.
Trough systems use linear concentrators of parabolic shape with highly reflective surfaces, which can
be turned in angular movements towards the sun position and concentrate the radiation onto a long-
line receiving absorber tube (see Figure 3.35-b). The absorbed solar energy is transferred by a
working fluid, which is then piped to a conventional power conversion system. The used power
conversion systems are based on the conventional Rankine-cycle/steam turbine generator or on the
combined cycle (gas turbine with bottoming steam turbine). Trough power plants are highly modular
and are already applied up to 80 MWe unit capacity using a thermal oil heat transfer system.
sah ure
(a)
(a) (b)
Fig 3.35 Parabolic Trough
Dish systems use dish-shaped parabolic mirrors as reflectors to concentrate and focus the sun's rays
onto a receiver, which is mounted above the dish at the dish center. A dish/engine system is a stand-
alone unit composed primarily of a collector, a receiver, and an engine. It works by collecting and
concentrating the sun's energy with a dish-shaped surface onto a receiver that absorbs the energy (as
shown in figure 3.36) and transfers it to the engine. The engine then converts that energy to heat. The
heat is then converted to mechanical power, in a manner similar to conventional engines, by
compressing the working fluid when it is cold, heating the compressed working fluid, and then
expanding it through a turbine or with a piston to produce mechanical power. An electric generator or
alternator converts the mechanical power into electrical power.
Dish/engine systems use dual-axis collectors to track the sun. The ideal concentrator shape is
parabolic, created either by a single reflective surface or multiple reflectors, or facets. Many options
exist for receiver and engine type, including Stirling cycle, micro turbine, and concentrating
photovoltaic modules. Each dish produces 5 to 50 kW of electricity and can be used independently or
linked together to increase generating capacity. A 250-kW plant composed of ten 25-kW dish/engine
systems requires less than an acre of land.
Dish/engine systems are not commercially available yet, although ongoing demonstrations indicate
good potential. Individual dish/engine systems currently can generate about 25 kW of electricity.
More capacity is possible by connecting dishes together. These systems can be combined with natural
gas, and the resulting hybrid provides continuous power generation.
3. Fresnel reflector:
Fresnel reflectors are made of many thin, flat mirror strips to concentrate sunlight onto tubes
through which working fluid is pumped. Flat mirrors allow more reflective surface in the
same amount of space as a parabolic reflector, thus capturing more of the available sunlight,
and they are much cheaper than parabolic reflectors. Fresnel reflectors can be used in various
sizes of concentrated solar powers (CSPs). The Fresnel reflector is shown in Figure 3.37,
where sun radiations are concentrated to the two linear absorbers.
Central receivers (or power towers) use thousands of individual sun-tracking mirrors called
"heliostats" to reflect solar energy onto a receiver located on top of a tall tower. The receiver collects
the sun's heat in a heat-transfer fluid (molten salt) that flows through the receiver as shown in Figure
3.38(a).
The salt's heat energy is then used to make
steam to generate electricity in a
conventional steam generator, located at
the foot of the tower. The molten salt
storage system retains heat efficiently, so it
can be stored for hours or even days before
being used to generate electricity.
Therefore, a central receiver system is
composed of five main components:
heliostats, receiver, heat transport and
exchange, thermal storage, and controls.
The Electricity generation using central
receiver heliostats system is shown in Figure 3.38 (b)
Figure 3.38 (b) Electricity Generation Plant using Central Receiver Heliostats System
➢ Advantages:
(i) The size of the absorber can be reduced that gives high concentration ratio.
(ii) Thermal losses are less than FPC. However small losses occur in the concentrating collector
because of its optical system as well as by reflection, absorption by mirrors and lenses.
(iii) The efficiency increases at high temperatures.
(iv) In these collectors the area intercepting the solar radiation is greater than the absorber area.
(v) These collectors are used for high-temperature applications.
(vi) Reflectors can cost less per unit area than flat plate collectors.
(vii) Focusing or concentrating systems can be used for electric power generation when not used
for heating or cooling.
(viii) Little or no anti freeze is required to protect the absorber in a concentrator system whereas the
entire solar energy collection surface requires anti freeze protection in a flat plate collector
• Disadvantages
(i) Out of the beam and diffuse solar radiation components, only beam component is collected in case
of focusing collectors because diffuse component cannot be reflected and is thus lost.
(ii) In some stationary reflecting systems it is necessary to have a small absorber to track the sun
image; in others the reflector may have to be adjustable more than one position if year round
operation is desired; in other words costly orienting systems have to be used to track the sun.
(iii) Additional requirements of maintenance particular to retain the quality of reflecting surface against
dirt, weather, oxidation etc.
(iv) Non –uniform flux on the absorber whereas flux in flat-plate collectors in uniform.
(v) Additional optical losses such as reflectance loss and the intercept loss, so they introduce
additional factors in energy balances.
(vi) High initial cost.
The reflector should have high reflectivity. Therefore mirror glasses may be used. Glass is most
durable with low iron content and is used as a transmitting material. Now day’s plastics are also in
use. Acrylic is found to be a good material for Fresnel lenses. Aluminium and silver are very good
reflecting surfaces.
Glass and transparent plastic films are generally used as cover material for receivers. Coating is
required to have strong solar absorptivity, weather resistance, and stability at high temperature.
Insulation is required to reduce heat losses. Fibre glass with and without binder, urethane foams and
mineral fiber blankets are commonly used for insulation.
The optical efficiency of a concentrator collector is defined as the ratio of the solar radiation absorbed
by absorber to the beam solar radiation on the concentrator and is given by
Where,
ηopt 0° = Optical efficiency of a collector at 0°
Copt = Correction factor from deviation of incident angle from 0°
ρ = Mirror reflectivity
γ = Intercept factor
τ = Transmittivity of transparent cover
αa = Absorptivity of absorber.
ηc = Qc/ (AaIbc)
• The aperture area is that plane area through which the incident solar flux is accepted. It is
defined by the physical extremities of the concentrator.
• The acceptance angle defines the limit to which the incident ray path may deviate, from
the normal drawn to the aperture plane, and still reach the absorber.
• The absorber area is the total area that receives the concentrated radiation. It is the area
from which useful energy can be removed.
• Geometrical concentration ratio or the radiation balance concentration ratio is defined as
the ratio of the aperture area to the absorber area.
• The optical efficiency is defined as the ratio of the energy, absorbed by the absorber, to
the energy, incident on the aperture.
• The thermal efficiency is defined as the ratio of the useful energy delivered to the energy
incident on the aperture.
• Disadvantages
• Since concentrators can focus only direct solar radiation, their performance is
poor on cloudy days.
• Tracking mechanisms must be used to move the collectors during the day to
keep them focused on the sun. Maintenance and construction costs of the
system are therefore considerably increased.
• Concentrators are only practical in areas of high direct insolation, such as arid
and desert areas.
Since the average operating temperature of stationary non-concentrating collectors is low (max up to
0 0
120 C) as compared to the desirable input temperatures of heat engines (above 300 C), the
concentrating collectors are used for such applications.
In the basic process of conversion of solar into heat energy, an incident solar irradiance is collected
and concentrated by concentrating solar collectors or mirrors, and generated heat is used to heat the
thermic fluids such as heat transfer oils, air or water/steam, depending on the plant design, acts as heat
carrier and/or as storage media. The hot thermic fluid is used to generated steam or hot gases, which
are then used to operate a heat engine (as shown in Figure3.39).. In these systems, the efficiency of
the collector reduces marginally as its operating temperature increases, whereas the efficiency of the
heat engine increases with the increase in its operating temperature.
Central receivers (or power towers) use thousands of individual sun-tracking mirrors called
"heliostats" (A Heliostat is a device that tracks the movement of the sun which is used to orient a
mirror of field of mirrors, throughout the day, to reflect sunlight onto a target-receiver) to reflect solar
energy onto a receiver located on top of a tall tower (shown in Figure 3.41). The receiver collects the
sun's heat in a heat-transfer fluid (molten salt) that flows through the receiver.
Figure 3.41 Central receiver heliostats System
The salt's heat energy is then used to make steam to generate electricity in a conventional steam
generator, located at the foot of the tower. The molten salt storage system retains heat efficiently, so it
can be stored for hours or even days before being used to generate electricity. Therefore, a central
receiver system is composed of five main components: heliostats, receiver, heat transport and
exchange, thermal storage, and controls. The schematic diagram Solar - Central tower receiver power
plant is shown in Figure 3.42.
A Solar Chimney is much simpler but it works with much lower efficiency as compared to central
tower receiver power plant. The solar chimney has a tall chimney at the center of the field, which is
covered with glass. The solar heat generates hot air in the gap between the ground and the gall cover
which is then passed through the central tower to its upper end due to density difference between
relatively cooler air outside the upper end of the tower and hotter air inside tower. While travelling up
this air drives wind turbines located inside the tower (shown in Figure 3.43). These systems need
relatively less components and were supposed to be cheaper. However, low operating efficiency, and
need for a tall tower of height of the order of 1000m made this technology a challenging one. A pilot
solar chimney project was installed in Spain to test the concept. This 50kW capacity plant was
successfully operated between 1982 to 1989. Recently, Enviro Mission Limited, an Australian
company, has started work on setting up first of its five projects based on solar chimney concept in
Australia.
The dish/engine system is a concentrating solar power (CSP) technology that produces
relatively small amounts of electricity compared to other CSP technologies—typically in the
range of 3 to 25 kilowatts. Dish/engine systems use a parabolic dish of mirrors to direct and
concentrate sunlight onto a central engine that produces electricity. The two major parts of
the system are the (i). Solar concentrator and the (ii). Power conversion unit.
The solar concentrator, or dish, gathers the solar energy coming directly from the sun. The
resulting beam of concentrated sunlight is reflected onto a thermal receiver that collects the
solar heat. The dish is mounted on a structure that tracks the sun continuously throughout the
day to reflect the highest percentage of sunlight possible onto the thermal receiver (shown in
Figure 3.44).
The power conversion unit includes the thermal receiver and the engine/generator. The
thermal receiver is the interface between the dish and the engine/generator. It absorbs the
concentrated beams of solar energy, converts them to heat, and transfers the heat to the
engine/generator. A thermal receiver can be a bank of tubes with a cooling fluid—usually
hydrogen or helium—that typically is the heat-transfer medium and also the working fluid for
an engine. Alternate thermal receivers are heat pipes, where the boiling and condensing of an
intermediate fluid transfers the heat to the engine.
The engine/generator system is the subsystem that takes the heat from the thermal receiver
and uses it to produce electricity. The most common type of heat engine used in dish/engine
systems is the Stirling engine. A Stirling engine uses the heated fluid to move pistons and
create mechanical power. The mechanical work, in the form of the rotation of the engine's
crankshaft, drives a generator and produces electrical power. The Typical diagram of dish/
stirling engine system is shown in figure 3.44.
• Direct circulation systems: Pumps circulate household water through the collectors and into the
home as shown in Figure 3.46. They work well in climates where it rarely freezes.
• Indirect circulation systems: Pumps circulate a non-freezing, heat-transfer fluid through the
collectors and a heat exchanger. This heats the water that then flows into the home (shown in
Figure 3.47. They are popular in climates prone to freezing temperatures.
Figure 3.47 Indirect Circulation System for Heating Water
Passive solar water heating systems are typically less expensive and less efficient than active
systems. They do not use a pump to circulate water from the collector to storage or other locations. However,
passive systems can be more reliable and may last longer. There are two basic types of
passive systems:
• Flat-plate collector: Glazed flat-plate collectors are insulated, weatherproofed boxes that contain
a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate
collectors -- typically used for solar pool heating -- have a dark absorber plate, made of metal or
polymer, without a cover or enclosure.
• Integral collector-storage systems: Also known as ICS or batch systems, they feature one or
more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar
collector, which preheats the water. The water then continues on to the conventional backup water
heater, providing a reliable source of hot water. They should be installed only in mild-freeze
climates because the outdoor pipes could freeze in severe, cold weather.
A mechanism (guide for adjusting mirror) is provided to adjust the reflector at different angles with
the cooker box when the reflector is adjusted to shine in the cooker box, 115oC to 125oC. Temperature
is achieved inside the cooker box. Addition of the reflector is useful in cooking earlier particularly in
winter. The solar cooker is able to cook about 1.25kg dry food materials, which is enough for a
family of 5 to 7 persons. The total weight of the cooker is about 22kgs. Overall dimensions of a
typical model are 60x60x20cm height (shown in figure.3.50-a).
Figure 3.50 Solar Cooker
Solar cooker is a device, which uses solar energy for cooking, and thus saving fossil fuels, fuel wood and
electrical energy to a large extent. However, it can only supplement the cooking fuel, and not replace it
totally. It is a simple cooking unit, ideal for domestic cooking during most of the year except during the
monsoon season, cloudy days and winter months.
(i). Box type solar cookers: The simplest type of solar cooker is shown in Fig. 2.3.1. This is
called box type solar cooker. The solar energy is converted into heat which cooks the food
without use of cooking gas or kerosene, electricity, coal or wood. In any sunny day, a solar
cooker can cook two meals per day for four to five persons.
Fig. 2.3.1 Box Type Solar Cooker
A box solar cooker cooks food with the help of solar energy, and helps save conventional fuel. This
cooker can be used for the preparation of rice, dal, kadhi, vegetables, meat and fish dishes, snacks,
soups, sweets, desserts, sauces, jams, pickles, biscuits, cakes, puddings, and so on. However, it cannot
be used for frying or for baking chapattis as it can’t produce high temperatures required for such type
of cooking. It supplements cooking fuel but does not replace it totally. It is an ideal device for
domestic cooking during most of the year except on cloudy days. Typically, a box solar cooker
measures about 60 cm x 60 cm, and is provided with four pots. Smaller models are also available.
The cost of a box solar cooker varies from Rs 1800 to Rs 3000 depending on its size and features. The
Ministry of New and Renewable Energy (MNRE), Govt. of India and some State Govts. are providing
financial incentive for ISI-mark solar cookers. The incentive for non-ISI-mark solar cooker is less as
compared to ISI-mark solar cookers.
(ii).Parabolic concentrating solar cooker: A parabolic solar concentrator comprises of sturdy Fibre
Reinforced Plastic (FRP) shell lined with Stainless Steel (SS) reflector foil or aluminised polyester film. It
can accommodate a cooking vessel at its focal point. This cooker is designed to direct the solar heat to a
secondary reflector inside the kitchen, which focuses the heat to the bottom of a cooking pot. It is also
possible to actually fry, bake and roast food. This system generates 500 kg of steam, which is enough to
cook two meals for 500 people (see Figure 3.51). This cooker costs upward of Rs.50,000.
• One has to cook according to the sunshine, the menu has to preplanned.
Solar pumping consists in utilizing the power generated by solar energy for water pumping
useful for irrigation. Solar energy offers several features that make its utilization for irrigation
pumping quite attractive, first, the greatest need for pumping occurs during the summer
months when solar radiation is greatest second, pumping can be intermittent to an extent,
during periods of low solar radiation when pumping decreases, evaporation losses from crops
are also low. Finally relatively in expensive pumped storage can be provided in the forms of
bonds. A number of recently constructed solar irrigation pump installations are now
operational. The major obstacle to increase use of solar irrigation system at this time is their
Relatively high capital cost. If the costs of solar pumps can be substantially reduced and
assuming that conventional fuel costs continue to rise, solar; irrigation could become
economical, and increased use of such system might be anticipated in future.
The basic system consists of the following components:
(1) The solar collector.
(2) The heat transport system.
(3) Boiler or Heat exchanges.
(4) Heat engine.
(5) Condenser.
(6) Pump
The solar pump is not much different from a solar heat engine working in a low temperature
cycle. The sources of heat is the solar collector, and sink is the water to be pumped. The Solar
thermal water pump is shown in figure 3.52.
Figure 3.52 Solar Water Pump
The primary components of the system are an array of flat-plate collectors and an Rankine engine
with an organic fluid as the working substance. During operation a heat transfer fluid (Pressurized
water) flows through the collector arrays. Depending upon the collector configuration, solar flux
and the operating conditions of the engine, the fluid will be heated in the collector to a higher
temperature, the solar energy which is thus converted to the thermal energy. The fluid (water) flows
into a heat exchanger (boiler), due to temperature, gradient, and comes back to the collector. This
water yields its heat to an intermediate fluid in the boiler. This fluid evaporates and expands in the
engine before reaching the condenser, where is condenses at low pressure. The condenser is called
by the water to be pumped. The fluid is then re-injected in the boiler to close the cycle. The
expansion engine or Rankine engine is coupled to the pump and it could of course be coupled to an
electric generation.
Thermal energy storage can be stored as a change in internal energy of a material as sensible heat,
latent heat and thermo-chemical or combination of these. Thermal energy can be stored either can be
stored in well insulated fluids or solids. Thermal energy storage is essential for both domestic water
and space heating applications. In sensible heat storage the temperature of the medium changes during
charging and discharging of the storage (shown in Figure 3.53). In latent heat storage, the temperature
of the medium remains more or less constant, since it undergoes a phase transformation, i.e. the
transition from solid to liquid or from liquid to vapour as shown in figure 3.53.
In sensible heat storage (SHS), thermal energy is stored by raising the temperature of a solid or liquid.
SHS system utilizes the heat capacity and the change in temperature of the material during the process
of charging and discharging. The amount of heat stored depends on the specific heat of the medium,
the temperature change and the amount of storage material. The basic equation for energy storage is
given by:
Qs = mCp (T1- T2)
Where, Qs = Total thermal capacity, m = Mass of storage medium, Cp = Specific heat and (T 1-T2) is
the change in temperature.
Heat stored per unit volume is given by
Qs/Vs = mCp (T1-T2) / Vs
=ρs Cp (T1-T2)
Where, Vs = Volume of the given storage medium and ρs = Density of the storage medium
Water appears to be the best SHS liquid available because it is inexpensive and has a high specific
heat. However above 1000 C oils, molten salts and liquid metals, etc. are used. For air heating
applications rock bed type storage materials are used.
In a latent heat storage system, heat is stored in a material when it melts (after 0 degree celcius) and
extracted from the materials when it freezes as shown in Figure 3.53. The most suitable phase change
material is a paraffin wax. Other materials are calcium chloride hexahydrate, magnesium nitrate
hexahydrate, ice, sodium hydroxide etc.
For latent heat storage the phase transition solid-liquid (melting) for storage charging and liquid –
solid(solidification) for storage discharging are most suitable. The storage capacity of latent heat
storage is given by:
Q = m[Cs(tm-tmin)+hm+Cl(tmax-tm)] J
Where,
m= mass of phase change material storage medium
Cs= specific heat of PCM storage medium in solid state
Cl= specific heat of PCM storage medium in liquid state
hm = specific melting enthalpy of PCM storage medium
tmin and tmax = minimum and maximum storage temperature
tm= melting temperature of PCM storage medium
Table 3.1 Detailed State-wise solar power capacity installed under Phase-I of JNNSM
• In India the first Solar Thermal Power Plant of 50kW capacity has been installed by MNES
following the parabolic trough collector technology (line focussing) at Gwalpahari, Gurgaon,
which was commissioned in 1989 and operated till 1990, after which the plant was shut down
due to lack of spares. The plant is being revived with development of components such as
mirrors, tracking system etc.
• A Solar Thermal Power Plant of 140MW at Mathania in Rajasthan, has been proposed and
sanctioned by the Government in Rajasthan. The project configuration of 140MW Integrated
Solar Combined Cycle Power Plant involves a 35MW solar power generating system and a
105MW conventional power component and the GEF has approved a grant of US$ 40 million
for the project. The Government of Germany has agreed to provide a soft loan of DM 116.8
million and a commercial loan of DM 133.2 million for the project.
• In addition a commercial power plant based on Solar Chimney technology was also studied in
st
North-Western part of Rajasthan. The project was to be implemented in five stages. In the 1
stage the power output shall be 1.75MW, which shall be enhanced to 35MW, 70MW,
126.3MW and 200MW in subsequent stages. The height of the solar chimney, which would
initially be 300m, shall be increased gradually to 1000m. Cost of electricity through this plant is
expected to be Rs. 2.25 / kWh. However, due to security and other reasons the project was
dropped.
• BHEL limited, an Indian company in power equipments manufacturing, had built a solar dish
based power plant in 1990’s as a part of research and development program of then the
Ministry of Non-conventional Energy Sources. The project was partly funded by the US
Government. Six dishes were used in this plant.
• Few states like Andhra Pradesh, Gujarat had prepared feasibility studies for solar thermal
power plants in 1990’s. However, not much work was carried out later on.
BARRIERS:
Solar thermal power plants need detailed feasibility study and technology identification along with
proper solar radiation resource assessment. The current status of international technology and its
availability and financial and commercial feasibility in the context of India is not clear. The delays in
finalizing technology for Mathania plant have created a negative impression about the technology.