2. Solar Concentrating Collector
(Focusing Type)
Focusing collector is a device to collect solar energy with high
intensity of solar radiation on the energy absorbing surface.
It is a special form of flat-plate collector modified by
introducing a reflecting surface (concentrator) between
solar radiation and absorber.
It uses the optical system in the form of reflectors or
refractors for concentrating the incident solar radiation.
It results in an increased flux density on the absorber
surface area.
In these collectors, the radiation falling on a relatively large
area is focused onto a receiver of considerably smaller area.
In order to get a maximum concentration, an arrangement
for tracking the sun's virtual motion is required.
An accurate focusing device is also required.
3. Solar Concentrator component
A solar concentrator consists of a
1. focusing device,
2. receiver system and
3. tracking arrangement.
4. Solar Concentrator
As a result of the energy concentration, fluid can be heated
to the temperature of 7500 C or more.
Hence, they have the potential applications in both
thermal and power generation (electrical power) at high
delivery temperatures.
The main difference between flat plate collector and
concentrating collector is that the flat plate collector
concentrates only the direct radiation coming from a
specific direction since the diffuse radiation arrives from
all directions and only a very small proportion is from the
direction for which the focusing occurs whereas the
concentrating collector collects all type of radiation.
5. 1.Types of concentrating collectors
Solar concentrators may be classified on the basis of
whether tracking system is installed or not and type of
tracking system installed.
Tracking Type: Continuouy or Intermittent
One-axis design
Two-axis design
Non-tracking type
Solar concentrators may also be classified on the basis of
optical components.
Reflecting or refracting type collector
Imaging or non-imaging type collector
Line focusing or point focusing type collector
6. Based on the number of concentrating collector
geometrics, the solar concentrators may be classified
a) Parabolic trough collector
b) Minor strip reflector
c) Fresnel lens collector
d) Flat plate collector with adjustable mirrors
e) Compound Parabolic Concentrator (CPC).
7. Parabolic Trough Collector
It is the most preferable type of a concentrating collector.
The cross section of parabolic collector is shown in Figure1.
The solar radiation coming from the particular direction is
collected over the area of the reflecting surface and concentrated
at the focus of the parabola.
The solar radiation is focused along a line. It consists of a
cylindrical parabolic reflector and a metal tube receiver at a focal
plane as shown in Figure2.
The dimension of parabolic through collector can be varied over
a wide range.
The length of a reflector unit may be roughly 3 m to 5 m and
the width about 1.5 m to 2.4 m.
Ten or more such units are often connected end to end in a row.
Several rows may also be connected in parallel.
9. Parabolic Trough Collector
A parabolic trough collector with line focusing reflecting surface provides
the concentration ratio from 5 to 30.
Hence, higher temperature up to 3000C can be achieved. Parabolic through
reflectors have been made of highly polished aluminium of silvered glass
or of a thin film of aluminized plastic on a firm base.
The reflected light is focused on a central line of the parabolic trough
collector.
The tube located along the centre line absorbs the heat and the working
fluid is circulated through the pipe.
The absorber tube may be made of mild steel/copper.
A cylindrical parabolic trough may be oriented in any of three directions:
East—West, North—South or polar.
Trough type collectors are generally oriented in the east-west or north-south
directions.
The north south orientation permits more solar energy to be collected
than east-west arrangement.
10. Paraboloidal Dish Collector
A paraboloidal dish collector brings solar radiation to a focus at a
point actually a small central volume as shown in Figure1.
A dish 6.6 m diameter, as shown in Figure2 has been made from
about 200 curved mirror segments forming a paraboloidal
surface.
The absorber located at the focus is a cavity made of zirconium-
copper alloy with a black chrome selective coating.
The heat-transport fluid flows into and out of the absorber
cavity through pipes bonded to the interior.
The dish can be turned automatically about two axes so fhat the
sun is always kept in a line with the focus and the base of the
paraboloidal dish. Thus, the can be fully tracked at essentially all
times.
12. Mirror Strip Reflector
In this collector, a number of plane or slightly curved
(concave) mirror strips are mounted on a flat base.
The angle of the individual mirrors is arranged in such
a way that they reflect solar radiation from a specific
direction on to the same focal line as shown in Figure1.
The angle of the mirrors must be adjusted to allow the
Change in the sun's elevation while the focal line
remains in a fixed position.
14. Mirror Strip Reflector
It has a refracting type focusing collector. It utilizes the
focusing effect of a Fresnel lens shown in Figure 2.23. To be
fully effective, the Fresnel lens must be continuously
aligned with the sun in two directions both along and
perpendicular to its length. In the Fresnel lens collector,
the solar radiation is focused into the absorber from the top
rather than from bottom as in a parabolic type. For a trough
type collector, the lens is rectangle about 4.7 m in overall
length and 0.95 m in width. It is made of acrylic plastic and
it can be probably produced tn quantity at low cost.
16. Flat Plate Collector with Booster
Mirrors
It is the simplest type of concentrating collector. It consists of a
flat plate facing south with mirrors attached to its north and
south edges shown in Figure.
Reflectors reflect total radiation in addition to beam radiation
incidence on the receiver. Mirrors are also called booster mirrors.
It has a maximum concentration value less than four.
The convection ratio of such solar concentrators is relatively low
and hence, it is not widely used. As the solar incidence angle
increases, the mirrors become less effective. For a single
collector, booster mirrors can be used on all the four sides.
If the mirrors are set at the proper angle, they reflect the solar
radiation onto the absorber plate.
The mirrors cut off part of the scattered radiation have reached
the absorber plate and only a part of the scattered radiation
falling ons mirrors will be reflected onto the absorber.
18. When a number of collectors are combined in two or
more rows, the rows must be set further apart in the
north south directions to allow for the additional sun
shading.
The efficiency of a boosted flat plate system can be
increased if the angle of the flat mirrors can be
changed several times during the year.
The advantage of such a system is that it makes use of
the diffuse radiation in addition to the beam radiation.
19. Compound Parabolic Concentrator
(CPC)
It is a non-focusing type but the solar radiation from many
directions is reflected towards the bottom of the trough.
Due to this characteristic, a large proportion of the solar
radiation including diffuse (scattered) radiation entering
the trough opening is collected on a small area.
A CPC with two facing parabolic mirrors are shown in
Figure.
In addition to collecting both direct and diffuse radiations,
an advantage of this collector is that it provides moderately
good concentration although less than a focusing collector
in an east-west direction without adjustment for sun
tracking.
21. Advantages and disadvantages of
concentrating collectors over flat-
plate type collectors
Advantages
l . The reflecting surface of the concentrating collector requires less material
and structurally simpler than flat-plate collectors.
The absorber area of a concentrator system is smaller than a flat-plate system.
The area from which the heat is lost to surroundings is less than flat-plate
collectors.
It can be used for electric power generation.
The total useful operating time per year is large for a concentrator system than
a flat-plate collector.
Initial installation cost of the collector is less.
The amount of heat which can be stored per unit volume is larger;
Heat storage costs are less for concentration systems than flat plate collectors.
Higher temperature of the working fluid is attained with a concentrating
system.
No anti-freeze is required to protect the absorber in a concentrator system.
22. Disadvantages of concentrating collectors
over flat-plate type collectors:
In concentrating collectors, only the beam component
is collected because the diffuse component cannot
reflect.
Costly orienting systems must be used to track the
sun.
Additional maintenance is required sto retain thc
quality of reflecting surface against dirt, weather and
oxidation.
It is non-uniform flux on the absorber whereas the
flux in flatplate collectors is uniform.
Optical losses and interrupt loss are in energy balance.
23. SOLAR DIRECT THERMAL
APPLICATIONS
There are many uses of solar energy. Direct thermal
applications involve the direct use of heat thereby
resulting the absorption of solar radiation for space
heating and cooling of residences, and other buildings,
to provide hot-water for such buildings, and to provide
heat for agricultural, industrial and other processes
that require only moderate temperatures.
The use of solar energy for types of applications is
unlimited.
Various solar direct thermal application systems are
discussed briefly here.
24. Solar Water Heater
It is a device to heat water by solar energy. Solar water heaters are one
of the best options to be adapted in the developing country. Solar water
heating systems are commercially produced in the country.
Most of the systems available in India are designed to give water
temperature from 60 to 900C. These are suitable for preheating of feed
water to boiler and processing industries and hot water application in
hotels, bakeries, industries etc.
The term solar water heater includes conventional flat plate collector
with either thermosyphon or forced circulation flow system. A
solar water heater normally consists of the following components.
A flat plate collector to absorb solar radiation and convert it into
thermal energy.
Storage tank to hold water for use and cold water feeding the flat plate
collector.
Connecting pipes inlet and outlet for feeding cold water from the
storage tank and taking hot water to the storage tank or point of use.
25. (i) Thermosyphon or natural
circulation system:
In this system, the circulation of heated water is accomplished by the natural
convection. A simple small capacity natural circulation system is suitable for
domestic purpose.
The storage tank is an insulated and contains. two inlets. One is for the hot
water from the collector and other one is to allow the cold water from the main
to reach the bottom of the tank without mixing with hot water. There are two
outlets.
One is for the withdrawal of hot water and other one is used to feed cold water
to the collector inlet. The tank is located above the level of the collector. The
entire length of the connecting pipes is covered with glass-wool insulation to
reduce the heat loss.
As water in the flat plate collector is heated by solar energy, it flows
automatically to the top of the water tank due to low density. The vacuum
created by this flow is filled up by the cold water from top of the storage tank.
Whenever this is done, cold water automatically enters at the bottom. An
auxiliary heating system is provided for use on cloudy or rainy days.
Typically, such systems have capacities ranging from 100 to 200 liters and
adequately supply the needs of a family of four or five persons.
27. (ii) Forced circulation system:
The schematic diagram of a solar water heater with
forced circulation mode is shown in Figure.
In this system, a small water pump is required for
the flow of water between flat plate collector and
storage tank. The collectors can also be connected in
series for higher water temperature if required.
29. Solar Cooker
In developing countries such as India, Pakistan, Bangladesh etc.
energy consumed for cooking shares a major portion of the total
energy consumed in a year. Varieties of fuel such as coal,
kerosene, cooking gas, firewood, dung cakes and agricultural
waste are used for cooking.
There is a rapid deterioration in the supply of these fossil fuels.
The solution for the above problem is the harnessing of solar
energy for cooking purpose.
The department of new conventional energy source has
calculated that a family using a solar cooker 275 days a year
would save 800kgs of fire wood of 65 liters of kerosene.
Similarly, an industrial canteen or a hostel mess using the larger
community solar cooker which can cook for 20 to 25 people
could save 400kgs of fire wood or 335 liters of kerosene per year.
30. Types of solar cooker
Basically there are three designs of solar cooker.
(i) Flat plate box type solar cooker with or without
reflector,
(ii) Multi reflector type solar cooker,
(iii) Parabolic disc concentrator type solar cooker.
31. Flat plate box type design
Flat plate box type design is the simplest of all the
designs.
This cooker allows solar radiation to enter through a
double walled glass cover placed inside a blackened
box which is well insulated and made airtight.
Maximum no load temperature with a single reflector
reaches up to 160°C. Flat plate box type cooker is
shown in Figure.
33. Multi reflector type
In multi reflector type, as shown in Figure 2.29, four
square or triangular or rectangular reflectors are
mounted on the oven body. They all reflect the solar
radiation into the cooking zone in which cooking
utensils are placed. Temperature obtained is the order
of 2000C. The maximum temperature can reach to
2500C if the compound cone reflector system is used.
35. Parabolic type cooker
In parabolic type cooker, parallel sun rays are made to
reflect on a parabolic surface and concentrated on a
focus on which the Utensils for cooking are placed.
The temperature of the order of 450°C can be obtained
in which solar radiation are concentrated on to a focal
point. The schematic diagram of parabolic disc solar
cookers is shown in Figure.
37. Construction and Working
Principles of box type solar cooker:
The solar rays penetrate through the glass covers and absorbed
by a blackened metal tray kept inside the solar box. The solar
radiations entering the box are of short -wave length. Two glass
covers are provided to minimize the heat •loss. fie loss due to
convection is minimized by making the box air tight by
providing a rubber strip all around between upper lid and box.
Insulating materials such as glasswool, saw dust or any other
material is filled in the space between blackened tray and outer
cover of the box. It minimizes the heat loss due to conduction.
The top cover contains 3 mm thick two plain glasses fixed on
wooden or metal frame, keeping about 25mm distance between
these two. Neoprene rubber sealing is provided around the
contact surfaces of the glass cover and hinged on one side of the
glass frame.
38. Construction and Working
Principles of box type solar cooker:
When the cooker is placed in the sun the blackened surface
starts absorbing sun rays and temperature inside the box
starts rising.
The cooking pots which are also blackened are placed
inside with food material get heat energy and food will be
cooked in a certain period of time depending upon the
intensity of solar radiation and material of insulation
provided.
The amount of solar radiation intensity can be increased by
provided mirror or mirrors. The solar cooker is made up of
inner and outer metal or wooden box with double glass
sheet on it.
39. Advantages and Disadvantages of
solar cooker:
Advantages:
No attention is needed during cooking as in other devices.
No fuel is required.
Negligible maintenance cost.
There is no pollution,
Vitamins of the food are not destroyed and food cooked is nutrition
and delicious with natural taste.
There is no problem of over flowing of food.
Disadvantages:
Cook depends on sunshine, the menu has to preplanned according to
the same.
It is not possible to cook at short notice and food cannot be cooked
night or during cloudy days.
More time is required for cooking.
Chapattis cannot be cooked as it requires high temperature for baking.
40. Solar crop Dryer
The drying of food is necessary to store it for long time
i.e. between harvesting and consumption. High
moisture content in the food spoils it due to fungus
formation during the storage time of harvesting.
This problem can be solved by drying of a crop with
optimum moisture content. Dried crops can be used
for long period after storage.
41. (i) Open sun drying:
The working principle of open sun drying system is shown in Figure . It is the
simplest system still widely used in rural parts of our nation. In this system, the
crops are spread on the open surface area where there is direct sun light.
The short-wavelength solar radiation falls on the uneven crop surface. A part of
this energy is reflected back. The remaining part is absorbed by the crop
surface depending upon the colour of the crops. The absorbed radiation is
converted into thermal energy. Due to this the temperature of the crop starts
increasing.
It results' the long wavelength radiation and convective heat loss due to the
blowing wind losses from the surface of crop to ambient air. The crop is dried
due to evaporation of moisture takes place in the for-in of evaporative losses.
Further, a part of the absorbed thermal energy is conducted into the interior of
the product. It causes a rise in the temperature of the crop and further drying.
In this type of drying, there is a considerable loss due to various reasons such as
rodents, birds, insects and micro-organisms. The unexpected rain or storm
further worsens the situation.
Further, there may be a possibility of over drying, insufficient drying,
contamination of foreign materials such as dust, dirt, and insects.
43. (ii) Direct solar drying:
The working principle of direct solar drying system is shown in Figure. It is
referred to as a cabinet dryer.
In this system, the crops are spread inside the chamber base having metallic
net frame which is covered by a transparent glass cover. An air vent is provided
on side of the chamber to facilitate escape of hot air and moisture.
A part of the incident solar radiation on the glass cover is reflected back to the
atmosphere. Remaining part of solar radiation is transmitted through the glass
cover inside the cabinet dryer.
Further, a part of transmitted radiation is reflected back from the surface of the
crop.
The rest part is absorbed by the surface of the crop. Due to the absorption of
solar radiation, the crop temperature increases and the crop starts emitting
long-wavelength radiation that is not allowed to escape to atmosphere due to
the presence of the glass cover, unlike open sun drying.
Thus, the temperature above the crop inside chamber becomes higher.
45. (ii) Indirect solar drying:
The working principle of indirect solar drying system is
shown in Figure .
In this system, the crop is not directly exposed to solar
radiation to minimise the discolouration and cracking on
the surface of the crop.
A separate unit termed a solar air heater is used for crop
heating by allowing hot air into the drying chamber.
The hot air is allowed to flow through the wet crop. The
drying is basically achieved by the difference in moisture
concentrate on between drying air and air at the vicinity of
crop surface.
47. Solar Distillation or Solar Still
Solar still is a device used to convert saline water into pure
drinkable water by using solar energy. This process of
converting saline water into pure water is called
distillation.
The conventional distillation processes such as thin-film
distillation, reverse osmosis and electrodialysis arc energy
intensive techniques.
Therefore, solar distillation is an attractive alternative due
to its simple technology and non-requirement of highly
skilled labour for maintenance work and low energy
consumption.
The simplest solar still is generally known as "'basin type
solar still" shown in Figure .
48. It consists of a blackened basin containing saline water at a shallow depth.
A transparent air tight cover encloses the space above the basin completely. It
has a roof-like shape.
The cover which is usually glass may be of plastic is sloped towards a collection
through
Solar radiation passes through the cover and it is absorbed and converted into
heat in the black surface.
Impure saline water in the basin or tray is heated by the solar radiation and the
vapor produced is condensed to purified water on the cooler interior of the
roof.
The transparent roof material transmits nearly all radiation falling on it and
absorbs very little. Hence, it remain cool enough to condense the water vapor.
The condensed water flows down the sloping roof and there is a provision to
collect the distillate at lower end of the glass cover. Saline water can be replaced
in the operation by either continuous operations or by batches. Solar still may
provide about 15 to 50 litres/day/10 m 2.
50. Advantages of solar distillation:
Low energy consumption.
No fuel is required.
Low maintenance cost.
No pollution.
Simple design.
Less skilled labour may be sufficient to operate the
plant.
51. Solar Pumping
Solar pumping uses the solar energy for water pumping which is useful
for irrigation. Solar pumping offers the following features that make its
utilization for irrigation pumping quite attractive.
The greatest need for pumping occurs during summer months when
the solar radiation is maximum.
The solar pumping can be intermittent to an extent during period of
low solar radiation when pumping decreases and the evaporation
losses from crops arc also Jow,
Relatively inexpensive pumped storage can be provided in the forms of
solar ponds.
The major obstacle to increase use of solar irrigation system is relatively
high capital cost.
In this system, water pump is driven directly by water heated by solar
energy which operates either a heat engine or turbine. Flat plate
collectors are used for low head pumping and for high head pumping
parabolic trough collector is used.
52. The basic system consists of the following
components.
(l) Solar collector
Heat transport system
Boiler or Heat exchanges
Heat engine
Condenser
Pump.
53. Solar pump
The solar pump is not much different from a solar heat engine working in a low
temperature cycle. The source of heat is the solar collector and sink is the water
to be pumped. A typical solar powered water pumping system is shown in
Figure.
The primary components of the system are an array of flat-plate collectors and
a Rankine heat engine with an organic fluid as the working substance.
During the flow of heat transfer fluid (Pressurized water) through the solar
collector arrays is heated to high temperature and thus, the solar energy is
converted to the thermal energy;
The fluid (water) flows into a heat exchanger (boiler) and it transfers its heat to
an intermediate organic fluid in the boiler.
This fluid evaporates and expands in the engine before reaching the condenser
where it is condensed at low pressure. The condenser is called by the water to
be pumped. The fluid is then reinjected in the boiler to close the cycle.
The Rankine heat engine is coupled to the pump and it could of course, be
coupled to an electric generation.
55. Solar Space Heating
Solar space heating systems capture the sun's energy to supplement the
existing heating system for a home or commercial building. The
heating system intensifies the sun's power to heat water or air that is
then used to heat the building.
Solar heating systems may be either active or passive systems. Active
solar water heating systems use pumps to circulate the liquid warmed
by collectors. Passive systems do not use pumps but instead, use gravity
and water pressure to circulate the liquid instead.
Direct circulation (open loop):
In this system, water is circulated directly through the solar collectors.
This system cannot be used in areas where the temperature drops
below freezing.
Indirect circulation (closed loop):
In this system, a non-freezing liquid is circulated through the collectors
and it heats the water in a tank afterwards through a heat exchanger.
56. Solar Space Heating
The solar collectors absorb sunlight to heat up the
water passing through it.
The heated water then transferred back to the water
storage tank. The temperature in the tank can reach
anywhere from 55°C to 80°C. An air heat exchange is
provided to transfer the heat energy from water to air
which is then passed through the building to ward up.
Pumps are provided for forced circulation between
collector and tank, and between tank and heat
exchanger.
58. Solar Space Cooling
Solar cooling is a technology which converts the heat
collected from the sun into useful cooling for
delivery to applications such as building air
conditioning.
In this process, solar heat is collected and it is used by
a thermally-driven cooling process which
generates chilled water or conditioned air for use
in the building.
Chilled water is not widely; used for cooling in
residential applications but it is used extensively in
commercial buildings.
59. Solar space cooling
For solar space cooling, either vapour absorption refrigeration
cycle or vapour compression refrigeration cycle may be used.
In vapour absorption system, a liquid or solid absorbent is used
to release heat during cooling. Commonly used absorbent are
Lithium bromide water and Ammonia-water.
These absorbent absorbs heat during evaporation and hence, it
produces a cooling effect. In vapour compression refrigeration
system, a heat pump is used to drive a compressor of the
refrigerator.
Vapour coolant such as chlorofluorocarbons (CFC) or hydro
chlorofluorocarbons (HCFC) is compressed initially and this
compressed vapour is then condensed to a liquid by passing it
through a throttle valve or expansion valve where it is evaporated
at low pressure. This process produces the cooling effect.
61. Vapour absorption solar cooling system
Vapour absorption system is more efficient than the vapour compression
system and it requires less solar heat to achieve a given amount of cooling.
This system consists of a generator, condenser, evaporator and absorber.
An absorber containing low temperature solution absorbs the low pressure
refrigerant vapour coming from the evaporator where it releases the latent heat
of condensation.
Due to this, the solution is heated up and becomes strong solution while the
absorber is cooled by the circulating water, absorbing the heat of solution and
maintaining a constant temperature.
The strong solution is pumped to the generator where the heat is transferred to
the solar collector and it becomes weak solution. This weak solution is returns
to the absorber through throttling valve.
The high pressure refrigerant vapour from the generator is condensed in the
condenser to a high pressure liquid refrigerant.
This liquid refrigerant is passed through the expansion or throttling valve
where it evaporates. This low temperature vapour is passed to the space to be
cooled where it absorbs the heat of evaporation from the warm air in the space.
62. Advantages of vapour absorption solar cooling system:
It is compact and less bulky. Hence, less space is
required for installation.
It has no moving part except the motor driven pump
and hence, it produces less wear.
It is quiet in operation and it has less moving parts.
Less maintenance is required.
63. Solar Chimney or Ventilator
A solar chimney employs convective currents to
draw air out of a building. By creating a warm or
hot zone with an exterior exhaust outlet, air can be
drawn into the house ventilating the structure.
The solar chimney consists of a heated black metal
absorber on the inside behind a glazed front that it
can reach high temperature and be insulated from
the house.
A rotating metal scoop at the top which opens
opposite the wind will allow heated air to exhaust
without being overcome by the prevailing wind.
66. Solar Furnace
A solar furnace is a structure that captures sunlight to produce
very high temperatures which is used for industrial
purposes.
It is done with a curved mirror (or an array of mirrors) that
acts as a parabolic reflector for concentrating light onto a
focal point.
The temperature at the focal point may reach 3,500 0 C and
this heat can be used to generate electricity, melt steel and
make hydrogen fuel or nanomaterials.
The term "solar furnace" has also expanded in its scope to refer
to solar concentrator heating systems using parabolic mirrors
or heliostats where 5380 C is now commonly achieved.
The largest solar furnace producing 1000 kW output is at
Odeillo in France, opened in 1970.
67. Solar furnace primarily consists of solar concentrator,
heliostat and sun tracking device.
A paraboloidal dish type concentrator collector or a
spherical reflector concentrator collector is commonly used for
receiving the solar energy and converting it into useful heat
energy.
Heliostats are used to orient solar radiation parallel to the
optical axis of the concentrator. If the size of the aperture of the
concentrator is taken as x, then the size of the heliostat should
be 1.4 x x 1.4 x.
Sun tracking system is used to continuously position the
heliostat in the direction of sun as it moves from morning to
evening so that the optimum output can be produced by the
concentrator.
69. Solar furnace
solar furnace at
Odeillo in the Pyrénées-
Orientales (Eastern
Pyrenees) in France can
reach temperatures of
3,500 °C (6,330 °F).
70. Advantages of solar furnace system:
Simple in design and working principle.
It uses no fossil fuels and hence, no polluted
greenhouse gases are formed.
It requires less operational cost.
It has no moving part except the motor sun tracking
system and hence, it produces less wear.
High heat flex is obtained.
Controlling of temperature is easy.
Less maintenance is required.
