Unit : 2 Solar Collectors, Solar Energy Storage & Utilization.

Renewable Energy Technologies

Open Elective -II
6ME05

Unit - II Collectors, Solar Energy Utilization & Storage.

Prof. A. D. Bhandarkar
Department of Mechanical Engineering
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Sipna College of Engineering and Technology, Amravati.
 Unit : 2 Solar Collectors, Solar Energy Storage & Utilization.

Sipna College of Engineering and Technology, Amravati.

Department of Mechanical Engineering
6ME05 Renewable Energy Technologies (Open Elective –II)

Unit : 2 Solar Collectors, Solar Energy Storage & Utilization.

 Solar Collectors:
Collector is a device which absorbs the incoming solar radiation, converts it into heat, and transfers
this heat to a fluid (usually air, water, or oil) flowing through the collector.

 Classification of Solar Collectors:

solar thermal
collectors

None concentrated solar

concentrated solar thermal collectors
thermal collectors

Flat Plate Evacuated Plane fixed, circular

Tube receiver w ith concentrator
plane
concentrator

Fresnel lens
Compound
Parabolic
Parabolic
Dish
Cylindrical
Parabolic
Central
receiver with
heliostat
There are two main types of collectors:
1. Flat or non-concentric plate collector
2. Solar concentrating collectors

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 Unit : 2 Solar Collectors, Solar Energy Storage & Utilization.

 FLAT or NON-CONCENTRIC PLATE COLLECTOR

Flat-plate solar collector: It has no optical concentrator. Here, the collector area
and the absorber area are numerically the same, the efficiency is low, and temperatures of
the working fluid can be raised only up to 100oC.
A schematic cross-section of a flat-plate collector is shown in Figure A. It consists of
five major parts as mentioned below:
1. A metallic flat absorber plate of high thermal conductivity made of copper, steel, or
aluminum having black surface and thickness from 0.5mm to 1 mm.
2. Tubes or channels are soldered to the absorber plate. Water flowing through these
tubes takes away the heat from the absorber plate.

Figure A: Schematic cross section of a flat plate collector.

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 3. A transparent glass cover of 5 mm thickness. It reduces convection losses through
air layer between the absorber plate and the glass, and it reduces Radiation losses
because it is transparent to short wave radiation and nearly opaque to long wave
thermal radiation emitted byinterior collector walls and absorbing plate.
4. Fiber glass insulation of thickness 2.5 cm to 8 cm is provided at the bottom and on
the sides in order to minimize heat loss.
5. A container encloses the whole assembly in a box made of metallic sheet or fiber
glass.
 The commercially available collectors have a face area of 2 m2.
 The fixed flat-plate collector is installed in a tilted position at a suitable angle facing
south in the northern hemisphere.
 For the whole year, the optimum tilt angle of collectors is equal to the latitude of
its location.
 During winter, the tilt angle is kept 10-15o more than the latitude of the location
while in summer it should be 10-15o less than the latitude.
Working Principle:
🞂 Sunlight passes through the glazing and strikes the absorber plate, which heats up, and
solar energy isconverted into heat energy.
🞂 This the heat is transferred to the fluid passing through pipes attached to the absorber
plate by meansof convective heat transfer.

 EFFECT OF DESIGN PARAMETERS ON FLAT PLATE

COLLECTOR PERFORMANCE :
Important parameters are discussed below:

Selective Surface:

🞂 Absorber plate surface which exhibit characteristics of a high value of absorptivity for
incoming solar radiation and low value of emissivity for outgoing re-radiation are called as
selective surface.
🞂 Such surfaces are desirable because they maximize the net energy collection.
🞂 Some examples of selective surface layers are Copper oxides, Nickel black and Black
chrome.

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 Number of Covers:

🞂 With increase in the number of covers, the value of both (α)b and (α)d decrease and thus
flux absorbedby the absorber plate decrease.
🞂 The value of heat loss from the absorber plate also decreases.
🞂 However the amount of decrease is not the same in both cases. Maximum efficiency is
obtained withone or two covers.

Spacing:

🞂 Heat loss also varies with spacing between two covers and that between the absorber
plate and first cover.
🞂 The spacing at which minimum loss occurs varies with the temperature & also with tilt.
🞂 Since collectors are designed to operate at different location with varying tilt &
varying different condition, an optimum value of spacing is difficult to specify.
🞂 Spacing is in the range from 4cm to 8cm is normally suggested.

Collector Tilt:

🞂 The flat-plate collectors are normally used in a fixed position and do not track the sun.
🞂 Therefore, the tilt angle at which they are fixed is very important.
🞂 Optimum tilt depends on the nature of application.
🞂 The usual practice is to recommend a value of (ϕ+10°) or (ϕ+15°) for winter applications.
Eg:- waterheating, space heating.
🞂 (ϕ-10°) or (ϕ-15°) for summer applications. Eg:- absorption refrigeration plant.

Dust on the top of the cover:

🞂 When a collector is deployed in a practical system, dust get accumulated over it,
reducing thetransmitted flux through the cover.
🞂 This requires continuous cleaning of the covers, which is not possible in a practical
situation.
🞂 Cleaning generally done once in a few days.
🞂 For this reason, it is recommended that the incident flux be multiplied by a correction
factor whichaccounts for the reduction in intensity.
🞂 In general, a correction factor from 0.92 to 0.99 seems to be indicated.

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 Fluid Inlet Temperature:

🞂 The fluid inlet temperature is an operational parameter which strongly influence the
performance of a flat plate collector.
🞂 It is seen that the efficiency of collector decreases more or linearly with increasing
value of inlet temperature.
🞂 This is because the top loss co-efficient as well as temperature difference with surrounding
increases, the heat loss and the useful heat gain decreases.

 EVACUATED TUBE SOLAR COLLECTOR:

Fig B: Evacuated tube solar collector

Evacuated tube solar collector as shown in fig B. An evacuated tube solar collector is a
type of solar thermal collector that improve flat plate collectors. Solar collectors aim to
convert solar radiation into thermal energy reducing heat losses.
A conventional evacuated tube consists of concentric glass tubes, an aluminum fin,
and a copper heat pipe. The space between inner and outer glass walls is vacuumed for
preventing the heat loss. The inner glass wall is coated by the selective absorber coating
for absorbing the solar radiation. The solar radiation transforms to thermal energy to heat
the heat transfer fluid stored inside the copper heat pipe. The aluminum fin is placed
inside the space to make a contact between the concentric glass tube and heat pipe wall
and, therefore, increase thermal conduction surface area. The heat transfer fluid is

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 responsible for receiving thermal energy from the outside, then evaporating to the heat
pipe tip, exchanging heat with heating medium flowing through the manifold. After
transferring its latent heat to the heating medium, the heat transfer fluid vapor is
condensed and flows down to the bottom of copper heat pipe to receive heat again.
Eventually, the heating medium will be used for such a thermally applications.

 SOLAR CONCENTRATING COLLECTORS:

Concentrating-type solar collector: Here the area receiving the solar radiation is several
times greater than the absorber area and the efficiency is high. Mirrors and lenses are used
to concentrate the sun's rays on the absorber, and the fluid temperature can be raised up to
500 oC. For better performance, the collector is mounted on a tracking equipment to face
the sun always with its changing position. Some new terms that will be encountered in the
text hereinafter are defined now tor greater clarity. These are:
(i) 'Concentrator' is for the optical subsystem (Mirrors and lenses) that projects
solar radiation on to the absorber.
(ii) 'Receiver' shall be used to represent the sub-system that includes the absorber,
its cover and accessories.
(iii) 'Aperture' (W) is the opening of the concentrator through which solar
radiation passes.
(iv) 'Acceptance angle' (2θa) is the angle across which beam radiation may deviate
from the normal to the aperture plane and then reach the absorber.
(v) 'Concentration ratio' (CR) is the ratio of the effective area of the aperture to
the surface area of the absorber. The value of CR may change from unity (for
flat-plate collectors) to a thousand (for parabolic dish collectors). The CR is

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 used to classify collectors by their operating temperature range.
 Types of Concentric Collectors:
1. Flat plate collector with adjustable mirrors

2. Compounded parabolic collector

3. Cylindrical parabolic collector

4. Mirror strip reflector (or) Moving reflector with fixed concentrator

5. Fresnel lens collector

6. Parabolic trough collector

Flat plate collector with adjustable mirrors:

-By providing plane reflectors at the edges of a flat plate collector to reflect additional radiation
into the receiver.
-The concentration of solar radiation can be increased.
-These mirrors are also called as booster mirrors.
-The concentration ratio of these concentration has a maximum value of 4.
-Such a design is aligned in the east-west direction and requires periodic tilt adjustment.

Compounded parabolic collector:

-It consist of two parabolic mirrors segments, attached to a flat receiver.
-The segments are oriented such that the focus of one is located at the bottom end point of the
other in contact with the receiver.

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-It has large acceptance angle and need to be adjusted intermittently.
-Rays in the central region reaches absorber directly.
-Rays near the edges undergoes one or more reflections before reaching the absorber.
-Concentration ratio achieved is 3 to 7.

Cylindrical parabolic collector:

-It consist of a cylindrical parabolic through reflector and a metal tube receiver at its focal line.
-The receiver tube is blackened at the outside surface to increase absorption.
-It is rotated about one axis to track the sun.
-The heat transfer fluid flows through the tube carrying thermal energy to next level of system.
-This type of collector may be oriented in any one of the direction: east-west, north-south.
-The concentration ratio achieved is 5 to 30.

Mirror strip reflector (or) Moving reflector with fixed concentrator:

- Due to practical difficulty in manufacturing a large mirror in a single piece in a cylindrical

parabolic shape, long narrow strips are used in this concentrator.
- The concentrator consist of fixed mirror strip arranged on a circular reference cylinder with a
tracking receiver tube.
-The receiver tube is made to rotate about the centre of curvature of the reflector module to
track the sun.
-Concentration ratio is approx. the same as the number of mirror strips.

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Fresnel lens collector:

-It is flat on one side and has fine longitudinal grooves on the other.
-The angles of these grooves are such that the radiation is brought to the focus.
-Lens are made of plastic sheets.
-Used upto 4000C.

Parabolic trough collector:

-When a parabola is rotated about its optical axis, a paraboloidal shape is produced.

-Solar radiation coming in a particular direction is collected and is collected on the focus of
the parabola.
-This requires two axis tracking.
-It can have a concentration ratio ranging from 10 to few thousand & can yield a temp. of
3000 degree Celsius.
-Dish collector has diameter of 6 to 7m.

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 Comparison between Flat plate type collector & Concentric type collector:

Flat Plate Collector Concentric Type Collector

Absorber area is large Absorber area is small
Concentration ratio is 1 Concentration ratio is high
It uses both beam and diffuse radiation It uses mainly beam radiation
Application limited to low temperature uses High temperature application such as power
suitable for all places as it can work in clear generation suitable where there are more
and cloudy days clear days in a year
Simple in maintenance Difficult in maintenance
It is less efficient solar collector It is the most powerful type of collector
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Maximum Temperature of fluid is 300 C Fluid temperatures up to around 5000oC can
be achieved
It can be used in water heating It can be used in solar furnaces and solar
power plants

Q. What are the different types of storage systems explain any one with neat sketch.

 Energy Storage Methods:

Energy can be stored in various forms and the storage methods are classified on the basis of the form
in whichit is stored. Some of the important energy storage methods are as follow.

1. Mechanical energy storage

 Pumped storage
 Compressed air storage
 Flywheel storage

2. Chemical energy storage

 Batteries storage
 Hydrogen storage
 Reversible chemical reactions storage

3. Electromagnetic energy storage

4. Electrostatic energy storage
5. Thermal energy storage
 Sensible heat storage
 Latent heat storage

6. Biological energy storage

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Mechanical energy storage:

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Flywheel storage

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 Applications of Solar Energy:
 Heating applications:

1. Solar Water Heater:

 The details of the most common type of solar water heater are as shown in fig.

 A tilted flat-plate solar collector with water as a heat-transfer fluid is used.

 A thermally insulated hot water storage tank is mounted above the collector.

 The heated water of the collector rises up to the hot water tank and replaces an equal
quantity of coldwater, which enters the collector.

 The cycle repeats, resulting in all the water of the hot water tank getting heated up. When
hot water is taken out from the hot water outlet, the same is replaced by cold water from a
cold water makeup tankfixed above the hot water tank.

 The scheme is known as passive heating scheme, as water is circulated in the loop
naturally due to thermo-siphon action.

 In avg. Indian climatic conditions, a solar water heater can be used for about 300 days in a
year.

 A typical 100 liters per day rooftop solar water heater costs approx. Rs. 18,000-22,000
(Year 2008) and delivers water at 60-80 0C.

 It has a life span of 10-12 years.

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2. Solar Space Heating:

 The passive space-heating system is as shown in fig. The south facing thick wall, called
Thrombe Wallis made of concrete, adobe, stone or composites of bricks blocks and sand,
design for thermal storage.

 In order to increase the absorption, the outer surface is painted black.

 The entire south wall is covered by one or two sheets of glass or plastic with some air gap
(usually 10-15 cm) between the wall and inner glazing.

 Solar radiation after penetration through the glazing is absorbed by the thermal storage wall.

 The air in the air gap between glazing and the wall thus gets heated, rises up and enters
the roomthrough the upper vent, while cool air from the room replaces it from the bottom
vent.

 Thus, the thermal wall collects, stores and transfer the heat to the room.

 Heating can be adjusted by controlling the air flow through the inlet and outlet vents by
shutters.

 Opening of dampers at the top of the glazing allows the excess heat to escape outside, when
heating isnot required.

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 Cooling applications:

Solar Passive Cooling through Ventilation:

 Fig. shows the scheme for solar passive cooling through ventilation.

 This scheme utilizes a solar ‘Chimney effect’ and is effective where outside temperature are
moderate.
 Solar radiation is allowed to heat up the air between the glazing and the interior south wall.

 The heated air rises up, is ducted outside and the warm air from the room is drawn into this
space dueto the natural draught thus produced.
 As a result, cool outside air enters the room from the bottom air vent on the other side of the
room.

 Pumping Applications:
Solar Thermal Water Pump:
 A Schematic diagram of a typical Rankine cyle, solar thermal water pump is as shown in
fig. A solar- collector system may consist of flat plate-collectors, non-focusing type
(stationary) collector or sun- tracking concentrators.

 Water is used as a heat transport fluid, and yield its heat to a low-boiling point organic
working fluid (such as Freon, R113, R12, isobutene etc.) in a heat exchanger. Surplus heat
is stored in the thermal storage to be used later when the sun is not available

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  The high-pressure vapours of working fluid expand in the turbine, condense in the
condenser and return in the heat exchanger (boiler). A part of the irrigation-pumped water
is diverted through the condenser for cooling purpose.

 Power Production
SolarFurnaces:

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  It is basically an optical system in which solar radiations are concentrated over a small area.

 It has two main components: (i) a Concentrator, and (ii) a single piece of a large-sized
heliostat or a system of a large number of small heliostats. The large number of heliostats
direct solar radiation onto a paraboloidal surface.

 The heliostats are adjusted such that they direct the radiation parallel to the optical axis of
the paraboloid.

 Accurate sun tracking is required for this purpose.

 The concentrators focus the incoming rays at the target placed at their foci.

 There are few solar furnaces in countries like France, USA, Germany & Russia for
scientific study. One of the world’s largest solar furnace of 1000kW output was installed at
Odeillo, Font-Romeo, France in 1970. Temperature obtained are in the range of about 3500
0
C.

Advantages:
 Heating without contamination
 Easy control of temperature
 Simple working
 High heat flux obtainable
 Continuous observation possible
 Absence of electromagnetic field
Disadvantages:
 Use is limited to sunny days & that too for 4 to 5 hours only
 Cost is high
 Very high temp. are obtained only over a very small area

 Drying :
Solar Cabinet dryer:
 A simple cabinet-type solar dryer is shown in fig.

 It is an enclosure with a transparent cover, similar to green house.

 The material to be dried is placed on perforated trays.

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  Solar radiation enters the enclosure and is absorbed by the product as well as the
surrounding internalsurfaces of the enclosure, increasing its temperature.

 The inside air heats up to a temperature ranging from 50 0C to 80 0C and rises above.

 Natural circulation of air is ensured by providing suitable openings at the bottom & top. The
circulatingair removes the moisture from the product.

 Solar Distillation or Desalination of water:

Single Basin Single Slope Solar Still:

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  A simple basin-type solar still consist of a shallow basin filled with saline or brackish
water to bedistilled. The depth of water is kept about 5-10 cm.

 It is covered with a slopey transparent roof.

 Solar radiation after passing through the roof, is absorbed by the blackened surface of the
basin andthus increases the temperature of water.

 The evaporated water increases the moisture content, which gets condensed on the cooler
underneaththe glass.

 The condensed water slip down the slope and is collected through the condensate channel
attached tothe glass.

SolarPond:

 Solar pond is anatural or artificial body of water, which is about 1 to 2m deep, used for
collecting and absorbing solar energy as a heat. Thus, a solar combines solar energy
collection & sensible heat storage.

 Zone-I: Surface convective zone having Salinity < 5%

 Zone-II: Non-convective zone, Salinity increase with depth

 Zone-III: Storage zone having Salinity~20%

 A salty water near the bottom heats up, it expands but cannot rise because it is denser than
the less salty water above.

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  Thus the warm water trapped below.

 Heat loss due to conduction is very small as compared to that of convection.

 The lower water can even warm upto boiling point of pure water.

 It is cheap, inexpensive but its efficiency is low.

Solar Pond Electric Power Plant:

 Hot water from the bottom level of the pond is pumped to the heat exchanger, where
working fluid isvaporized.

 The working fluid is allow to pass through turbine blade where heat energy is
converted into mechanical energy.

 Vapour goes to condenser where cold water from upper layer of solar pond condenses the
vapour backto a liquid state.

 The liquid is pump back to the evaporator where the cycle is repeated.

 A 2000 Sq.m solar point equipped with a 20 KW Rankine cycle heat engine solar pond
power plant has been constructed at Australia.

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