Solar energy concentration techniques in flat plate collector

INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME
AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
IJMET
Volume 3, Issue 3, September - December (2012), pp. 450-458
© IAEME: www.iaeme.com/ijmet.asp ©IAEME
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SOLAR ENERGY CONCENTRATION TECHNIQUES IN FLAT PLATE
COLLECTOR
1
Pravin N. Gajbhiye 2 Rupesh S.Shelke
1
Student, III rd Semester, M. Tech. Heat Power Engineering
2
Assistance Professor Mechanical Engineering Department,
G.H. Raisoni College of Engineering , Nagpur-440016, India
Corresponding Author E-mail:- pravinpraving@rediffmail.com

ABSTRACT

The technology and thermal performance of flat plate solar collectors is summarized and
status of technology development in the field of concentrated solar power is reviewed.
Concentrated solar power (CSP) systems use mirrors or lenses to concentrate a large area of
sunlight, or solar thermal energy, onto a small area. Concentrating technologies exist in four
common forms, namely parabolic trough, dish starlings, concentrating linear Fresnel reflector,
and solar power tower. Flat-plate collectors are a very useful tool for low to medium temperature
heat collection from the sun. They can be used for many purposes including the various thermal
desalination methods from low to medium capacities. Flat-plate collectors have simple
characteristics: they are easily assembled, and easily operated. The developments are being
carried out continuously in the field of cover materials, absorber plate materials, absorber and
glazing coating etc. along with the changes in the design, fluid used for heat transfer. Numbers of
studies have been carried out on thermal performance of solar flat plate collector and found more
increase in the thermal efficiency in comparison to conventional solar flat plate collector. These
studies include use of double side absorber plate, honeycomb material, nano-material, more
efficient coatings and use of optical lenses. Analysis given in this paper will help to create the
best design and operational conditions with the best economic characteristics for solar flat plate
collectors.

KEYWORDS: Flat plate collector, solar concentration, Optical lenses, wire-coil inserts,
Translucent glazing, transparent conductive oxides, nanofluid.

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1. INTRODUCTION

Solar power is the flow of energy from the sun. The primary forms of solar energy are
heat and light. In recent years solar energy has been strongly promoted as a viable energy source.
One of the simplest and most direct applications of this energy is the conversion of solar
radiation into heat. The cost of these energy systems depend on the construction and
maintenance of the plant, the source of energy is free and unlimited. The environmental impact
of these systems is practically zero. Hence effective way that the domestic sector can lessen its
impact on the environment is by the installation of solar flat plate collectors for heating water.
Although it should be said that some of these collectors have been in service for the last 40-50
years without any real significant changes in their design and operational principles. The
importance of flat plate collectors are that their thermal performance can be predicted and treated
in considerable detail. Purpose of Flat plate collector is to convert the solar radiation into heat to
satisfy energy needs but with some limitations it is not being used on grid scale because of its
poor efficiency and higher initial cost. So there is a requirement of advancement in the flat plate
collector to overcome its limitations so that it can be used as a replacement of conventional
heaters and electric power consuming devices.
To match demand and production of energy, the thermal performance of the collector
must be evaluated. The instantaneous useful energy collected is the result of an energy balance
on the solar collector. The term ‘flat plate’ is slightly misleading in the sense that surface may
not be truly flat it may be a combination of flat, grooved or of other shapes as the absorbing
surface with some kind of heat removal device like tubes or channels. A flat-plate solar collector
consists of a water proof, metal or fiberglass insulated box containing a dark colored absorber
plate, with one or more translucent glazing. Absorber plates are typically made out of metal due
to its high thermal conductivity and painted with special selective surface coatings in order to
absorb and transfer heat better than regular black paint. The glazing covers reduce the convection
and radiation heat losses to the environment. A heat-conducting fluid, usually water, glycol, or
air, passes through pipes attached to the absorber plate. As the fluid flows through the pipes, its
temperature increases. This is the energy to be utilized for productive activities. The amount of
the energy taken by the working fluid corresponds to a fraction of the useful energy collected
after the heat losses. Objective of this paper is to presents extensive studies of the research
carried out in order to enhance the flat plate collector performance using solar thermal
concentration techniques in this paper both experimental and theoretical developments in the
field of solar water heater have been reviewed thoroughly.

2. CONSTRUCTION ELEMENTS OF A FLAT PLATE SOLAR COLLECTOR

a. Absorber Plate or Selective Surface Is a metal, glass or plastic surface, mostly black in
color. It absorbs and converts radiation into thermal energy.
b. The Transparent Cover Is the upper part of the collector covering the tide absorber
plate. It is made from glass or transparent plastic sheet to permit penetration of solar
beams.
c. The Collector Insulation Consists of a material with very low thermal conductivity. It is
installed in the bottom and around the sides of the collector, in order to minimize heat
loss.

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d. The Heat Transfer Medium Flowing through the collector to transfer the heat from the
absorber to the utilization system. Can be either air or a liquid, usually water.

3. OPERATIONAL CHARACTERISTICS OF THE COLLECTOR
a. Collector efficiency (η) Is the ratio of useful gained thermal energy for period of time t
to the incident solar radiation onto the collector for the same time period.
b. Thermal Capacity of the Collector (C) Is the amount of heat that can be stored per
surface collector area and per unit of temperature change.
c. Pressure Drop (∆P) Is the difference in pressure between the inlet to the collector and
the outlet due to circulation friction.
d. Stagnant Conditions is characterized by no fluid circulation inside the collector during
the period in which the absorbing surface area receives a considerable incident radiation.

e. Incidence Angle Coefficient (kθ) The ratio of the optical efficiency of a solar collector
with a fixed beam angle of incidence to the optical efficiency of the collector at its
normal.
f. The cover reflectance (ρc)
g. Cover Transmittance (τc)
h. Cover Absorptance (αc)
i. Coefficient of cover Emissivity (εc)
j. Coefficient of Absorber Emissivity
k. Collector Efficiency Factor (F)Is the ratio of the real energy output of the collector to
the energy output in the case when the total absorber area was at the average fluid
temperature with the same fluid quantity of flowing water.

l. Collector Flow Factor (F″)Is the ratio of the energy that the collector can deliver at the
average temperature of the fluid to the energy that the collector can supply at the inlet
collector temperature.
m. Collector Heat Removal Factor (FR) Is the ratio of the energy collector output to the
energy output of the collector in temperature of the inlet fluid. It is temperature
dependent.
n. Collector Heat Loss Coefficient (UL) The coefficient of thermal loss of a collector is
defined as the ratio of the temperature difference per unit area of the cover to the ambient
temperature.
o. Incidence Angle Coefficient (kθ) The ratio of the optical efficiency of a solar collector
with a fixed beam angle of incidence to the optical efficiency of the collector at its
normal.

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3.1. Main Components of flat plate collector

Fig:- Advanced Flat Plate Collector

4. THE LIMIATIONS OF CONVENTIONAL FLAT PLATE COLLECTOR AND
TECHNICAL AVANCMENTS TO INCREASE ITS EFFECTIVENESS

The conventional Flat Plate collectors installed last 40-50 years are stationary devices
with limited solar radiation absorption area. Observed Practical Limitations of conventional flat
plate collector (FPC) are:-
1. Require large installation space hence difficult to install on small roof area.
2. As these are at constant angular inclination with surface it is difficult to utilize effective solar
radiations for long day hours.
3. Surface heating require more time to heat water.
4. Top front surface exposed to solar radiation hence only some part of solar heat is absorbed.
4. Operating temperature limits are inefficient.
5. Very low efficiency due to heat loss.
6. Installation cost is more as compared to performance
Hence unwillingness of costumers to handle such bulky and costly device.
In order to increase the efficiency and performance of flat plate collector various
technical advancements are proposed. To improve heat transfer in (FPC) metal wire coils of
helical structure inserted inside the water carrying and heating pipes. Nono fluids are used to
increase heat transfer rates. This increases the turbulence and heat transfer rate some extent
effectively. This advancement is not sufficient to improve overall performance. To reduce
radiation losses double glazing glass cover with more absorptive absorber plate is used which
improve thermal efficiency. Still effective utilization of solar energy is not achieved due
stationary FPC installation. All advancement provide surface heating of water carrying metal
tubes which increase required time to heat water. It need such arrangement to transfer heat
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directly into to the water inside tube instead of heating surface. To increase temperature solar
concentration techniques can be utilized along with flat plate collector. To increase solar
absorption side faces should be exposed to sun light. Heat loss can be reduced using green house
effect i.e. green transparent membrane along with transparent glazings. A small compact
effective solar flat plate collector can improve conventional solar utilization approach.

5. DEVELOPMENT IN FLAT PLAT COLLECTOR WITH SOLAR CONCENTRATION
TECHNIQUES
The developments are being carried out continuously in the field of cover materials,
absorber plate materials, absorber and glazing coating etc. along with the changes in the design,
fluid used for heat transfer.
R. Herrero Martín[1] proposed an enhancement technique applied to flat plate liquid
solar collectors for more compact and efficient design. The design consists of tube-side
enhancement passive techniques which are incorporated into a smooth round tube (twisted tapes,
wire coils). This type of inserted device provides better results in laminar, transitional and low
turbulence fluid flow regimes. To test the enhanced solar collector and compare with a standard
one, an experimental side-by-side solar collector test bed was designed and constructed. A
relevant improvement of the efficiency up to 5% has been reported and quantified through the
useful power ratio between enhanced and standard solar collectors.
Edward K. Summers, John H. Lienhard V,[2] provided collector using highly
transmissive polymer films or low iron glass with double glazing, and using a very absorptive
absorber, which is inexpensively accomplished by including a carbon black coating. .Double
glazing reduces radiative losses as glass is opaque to infrared radiation. Addition of rough
surface on the absorber plate provides 12% thermal efficiency increase. The collector was
designed for heating air. Madhukeshwara. N, E. S. Prakash[3] presented the performance of
flat plate collector with three different coatings for solar flat plate collectors where temperatures
up to 70°C are easily attained by flat plate collectors. With very careful engineering using special
surfaces, reflectors to increase the incident radiation and heat resistant materials, higher
operating temperatures are feasible. Otanicar et al. [4] proposed a direct absorption solar
collector operated on nanofluids. They demonstrate efficiency improvement up to 5% by
utilizing nanofluids as the absorption mechanism. Groenhout et al.[5] suggested a novel design
of a double-sided absorber with low emissivity selective surface coupled with high reflectance
stationary concentrators to reduce the radiative and conductive losses through the back of the
collector. This particular design reduce the net heat loss to be 30–70% lower than conventional
systems.
Martin et al. [6] presented heat transfer enhancement in a tube-on-sheet solar panel with
wire-coil inserts, using TRNSYS as the simulating tool and found thermal efficiency increases
up to 4.5% . N. Ehrmann and R. Reineke-Koch[7] used integration of double glazing with low
e- coating and Transparent conductive oxides (TCO) coating into a flat-plate collector.
Transparent conductive oxides (TCO) coatings are used as low e-coatings due to their optical

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selectivity with high solar transmittance. These coatings provide high efficiencies at
temperatures above 100 °C as well as at low solar irradiation.
Jason H. Karp*, Eric J. Tremblay and Joseph E. Ford[8] present a new approach to solar
concentration where sunlight collected by each lens in a two-dimensional lens array is coupled
into a shared, planar waveguide using localized features placed at each lens focus. This geometry
yields a thin, flat profile for moderate concentration systems which may be fabricated by low-
cost roll manufacture.
Jason H. Karp and Joseph E. Ford[9] provided micro-optic slab concentrator integrates
multiple, focusing apertures with a common, multimode waveguide equipped with optical lenses
array to direct solar energy to a single PV cell. Using the hybrid, imaging/non imaging approach,
the system becomes essentially planar while opening a new design space for flat plate
concentrating photo voltaic cell.
Otanicar et al.[11] proposed a direct absorption solar collector operated on nanofluids. They
demonstrate efficiency improvement up to 5% by utilizing nanofluids as the absorption
mechanism.
E. AlShamaileh[13] proposed a selective coating composed of a nickel–aluminium (NiAl)
alloy into the black paint having higher solar absorption efficiency compared to the commercial
black paint coating. Optimum composition was 6% NiAl alloy by mass.
E. Natarajan and R. Sathish [14] suggest the use of nano-materials in the solar devices to
increase the heat transfer and that can be useful in energy saving and compact designs.

6. SUMMARY OF PRACTICAL ADVANCEMENTS AND ITS RESULTS
Table: 5.1 Comparison between developed and conventional Flat plate collector (FPC)
(Experimental studies)

S.No Developed FPC Conventional FPC Effects of Development Reference
Design consists of tube-side Tubes without Efficiency optical factor R. Herrero
enhancement using a smooth round insertion device like increased by 15% Martín [1]
1
tube and twisted tapes, wire coils twisted wires
insertion inside carrying tubes.
Double glazing highly transmissive Single transmissive Provide 12% thermal Edward K.
polymer film and a very absorptive cover plate with efficiency increase Summers et. al.
2
absorber plate with rough surface conventional [2]
used. absorber plate used.
Coating of metallic particle composed Selective coating Increment of 5°C Madhukeshwar
3 of nickel-aluminium (NiAl) 6% alloy without embedding a. N ,E. S.
into the black paint metal Prakash [3]
Double sided absorber with low Single sided absorber Heat loss 30-70% lower N.K. Groenhout
4
emissivity and high reflectance plate. than conventional et.al. [5]
Honeycomb material inserted between Air between glass Reduces heat loss A.A. Ghoneim.
5
the glass cover and absorber. cover and absorber. effectively [18]
integration of double glazing with low Conventional black Provide high efficiency N. Ehrmann
6 e- coating and Transparent conductive paint coating above 100°C as well as and R. Reineke-
oxides (TCO) coating at low solar irradiation. Koch [7]
Two-dimensional lens array fitted on Single transmissive Provide high temperature Jason H. Karp
7 planar glass cover using localized cover plate used. concentration for photo et. al. [8]
features placed at each lens focus. voltaic cell

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Table: 5.2 Comparison between theoretical development and there result for flat plate
collector (FPC) (Theoretical studies)

S.No Proposed theoretical advancement Effect of advancement Reference
1 Tube-on-sheet solar panel with wire- Thermal efficiency increases up to 4.5% Martin et al.[6]
coil inserts, using TRNSYS as the
simulating (CFD) tool.
2 Direct absorption solar collector Efficiency improvement up to 5% by T.P. Otanicar et al.
operated on nanofluids. utilizing nanofluids as the absorption [4]
mechanism
3 Use of nano-materials in the solar Energy loss reduce and compact designs E. Natarajan and R.
devices obtaibed. Sathish [14]
4 Trapezoidal profile for absorber Give optimum efficiency B. Kundu [12]
plate
5 Indirect force circulation system Increase in hot water supply to demand in A. Hobbi and K.
using TRNSYS as the simulating winter conditions. Siddiqui [10]
tool.
6 1. If Teflon film used as second 1.Increase in performance estimated to B. Hellstrom et.al.
glazing 5.6%. [15]
2. If Teflon honeycomb used. 2. Increase in performance estimated to
3.Antireflection treatment of cover. 12.1%.
4. With external booster reflectors 3. Increase in efficiency estimated to 6.5%.
used. 4. Increase in efficiency estimated to 19.9
to 29.4%.

Table: 5.3 Practical advancements in flat plate collector and its effect summary

Practical Advancements :- Design consists of tube-side enhancement using a smooth round tube and twisted tapes,
wire coils insertion inside carrying tubes.

Fig 2:- Thermal Efficiency curves for
Fig. 1.:- The helical-wire-coil fitted in the raisers of the conventional and enhanced flat plate
modified solar collector. collector with insertion wire coil and smooth
tubes. [1]

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Practical Advancements :- Double glazing highly transmissive polymer film and a very absorptive absorber plate
with rough surface used.

Fig. 3. :- Double glazing highly transmissive polymer film
Fig.4. :- Comparison of baseline design
and a very absorptive absorber plate with rough surface
„double glazed, rough, nonselective absorber
used. [2] with existing air heaters

Practical Advancements: - Two-dimensional lens array fitted on planar glass cover using localized features placed
at each lens focus.

Fig.: Analysis of a 2mm diameter lens array and
Fig.5. :- The main components of the micro-optic slab 1mm slab waveguide symmetrically coupling
concentrator with focusing lens array for Photo voltaic C light to both edges.
cell Flat plate concentrator . [5]

7. CONCLUSIONS
This paper highlights the advancements in design configurations and component material
investigation to enhance efficiency and performance of flat plate collector. It has been found that flat plate
collector enhancement widely investigated both analytically and experimentally. Overall, improving the
transmissivity of the glazing by using highly transmissive polymer films or low iron glass, and using a
very absorptive absorber, which is inexpensively accomplished by including a carbon black coating,
would have the largest impact on performance of flat plate collector. Advancement like inserting devices,
double glazing polymer films, metal additives in absorber black coating, use of nano-material and fluids
provide improvement in flat plate collector performance lead to increase the solar flat plate collector

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application worldwide. Solar radiation concentration using optical lens arrays make possible to achieve
high temperature using conventional flat plate collector will provide cost effective performance.
Optimized study of various operating parameters of flat plate collector proved improved efficiency with
cost reduction. The information presented here will be beneficial for beginners in this area of research.

REFERENCE
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Solar energy concentration techniques in flat plate collector

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