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Micro Thermo-Photovoltaic
Power Generation
Submission for Term paperleading to Thesis
Supervised by
Prof. Ranjan Ganguly
and
Prof. Amitava dutta
Submitted by
Ankit Kumar Jain
001011502011
M.E., (Power Engg.)
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ACKNOWLEDGEMENT
My deepest thanks to Prof. Amitava Dutta and Prof. Ranjan Ganguly
(joint guides of the project) for guiding and correcting various documents of mine
with attention and care. They have taken pain to go through the paper and make
necessary correction as and when needed.
I heart fully hope to have the support of all the above
mentioned dignitaries for successful completion of this project
term paper.
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CONTENTS: PAGE NO.
INTRODUCTION 4
PRINCIPLE OF TPV SYSTEM 5
CONTENTS OT TPV 5-6
EFFICIENCY AND ADVANTAGE OF TPV 7-8
MICRO TPV DEVICE 8-9
DESIGN AND STUCTURE OF MICRO TPV 9-14
FABRICATION OF PV CELL ARRAY 14-17
SHORT-CIRCUIT CURRENT AND OPEN CIRCUIT VOLTAGE OF PV CELL 18-19
FILL FACTOR 20-21
DIFFERENT LOSSES 21-26
SYSTEM EFFICIENCY AND POWER OUTPUT AT DIFFERENT CONFIGUR. 27
APPLICATION 27-28
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Introduction
Traditional batteries have failed to satisfy the increased needs and demand for
smaller scale and higher densities power sources. A relatively new concept, micro
power has been developed to satisfy the growing trend of miniaturization of
electro-mechanical devices. A detailed review of such a technology can make
room for further improvements. There is a need to improve the efficiency of such
miniature devices , as not many but quite few of the micro power devices has been
developed. Practicing methods to improve such a technology can greatly enhancethe development of micro power devices. [1]
Potential Applications:
1. Portable electronics
a. Cellular phones, Notebook, Computer PDAs/Palmtops, various hand-held
devices.
2. Wireless equipments
a. Sensors and communication systems, remote control.
3. Miniature rocket for micro satellite, micro unmanned aerial vehicle, micro
rovers, micro air and space vehicle.
4. Military and security, portable cells for soldiers (signal sets, radio sets, etc),
Micro scouting vehicle, remote alarms.
5. Micro climate control.
6. Micro : - robots, motors, turbines, thrusters, automobiles.[2]
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Principle of TPV System:
The basic principle of thermo photo-voltaic (TPV) is the direct conversion of
thermal energy into electricity without involving any moving parts [3,4]. The TPV
system basically consists of four essential components. They are [1]
The heat source,
A selective emitter,
A filter system and
A low band gap photovoltaic converter.
When the heat source generates heat energy from either combustion, solar or
nuclear, this heat energy will be absorbed by the selective emitter. When the
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emitter is heated to a sufficiently high temperature, it will emit photons. Thus, the
selective emitter is used to convert heat from the source into radiation. The emitter
can be made from broadband materials such as,
1. Silicon carbide (SiC),
2. Selective emitting materials such as Er3Al5O12 (erbia),
3. Co/Ni (cobalt/nickel) doped MgO (magnesium oxide),
4. Yb2O3 (ytterbia).
The spectrums of broadband emitters operate at temperatures around 10001600
K. When the photons emitted from the emitter are impinging on the PV array, they
evoke free electrons and produce electrical power output. Thus, the photovoltaic
converter functions to convert heat radiation into electricity.
However, only those photons radiated by the emitter having energy greater than the
band gap (e.g. for GaSb cells, it is 0.72 eV, corresponding to a wavelength of1.7Um) of the photovoltaic cell can be converted into electricity. Those photons
with a wavelength longer than 1.7 Um cannot generate free electrons and produce
electricity when impinging on the photovoltaic cell. If these photons are not
stopped, they will be absorbed by the PV cells and subsequently result in a
destructive heat load on the system components, which will lower the conversion
efficiency of the system [5,6].
These photons should be sent back to the emitter in order
to improve the system. Thereby, a filter is often employed in the design of
conventional TPV systems. It serves to recycle and reflect back all the sub-band
gap photons with too low energy and transmits all convertible to the PV array.
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However, in the micro-TPV system, the existence of a filter will complicate the
fabrication and enlarge the volume of the system.
Thermophotovolatic schematic Author: [User:Fdu.uiuc],wikipedia[20]
Efficiency:
The overall efficiency of a conventional thermo photovoltaic (TPV) system is
given as follows [5,6]:
TPV = RS F pv
where the subsystem efficiencies are defined as follows:
RS is the net radiation power emitted by emitter,
F the radiation power absorbed in PV cell
pv is the electrical output power/radiation power absorbed in PV cell.
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The net radiated power is defined as the total emission minus the radiation returned
and absorbed in the emitter. The PV efficiency depends not only on the quality of
the cell itself, but also on the shape and power density of the spectrum absorbed in
it.
Advantages of thermo photovoltaic system: there are many advantages of
photovoltaic systems some of them are[5,6]:
1. Its a clean and quiet source of electrical power with no moving parts.
2. It has high power density.
3. We can use different type of energy source.4. In fabrication and manufacturing it is easy.
5. Reliable and low maintenance cost.
6. Inexpensive and convenience production.
So the thermo photovoltaic system has such a large advantages and has good
potential to replace the traditional batteries.
A micro thermo photovoltaic power device:
A micro power device is based on the principle of Thermo photovoltaic (TPV)
system. The concept and theory of the TPV method of power Generation has been
implemented into the design, development And fabrication of the micro-TPV
power generator. Each individual component of the device will be looked into.
This includes the[1]
1. Heat source (micro-combustor),
2. SiC emitter,
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3. Dielectric filter
4. GaSb photovoltaic (PV) cell array and
5. The cooling fins.
Other major issues concerning the design of the micro-TPV power generator such
as combustion in the micro-cylindrical combustor, effect of wall thickness of micro
combustors (on the performance of the system), and the effect of step height (of the
micro-combustor) on wall temperature is also considered.
The layout of TPV system
Energy band gap diagram
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Design and structure of the micro thermo photovoltaic device[1,7]:
The design of the micro-TPV power device comprises mainly of four main
components:
(1) A heat source;
(2) A micro-flame tube combustor (the wall of the micro-combustor will be a
broad band emitter);
(3) A simple dielectric filter;
(4) A PV cell array.
[6]
All these components are integrated together to form the general structure of the
micro power device. The schematic layout of the micro-TPV power device is
shown in Fig. Operating principle of micro TPV :The micro-thermo photovoltaic
(micro-TPV) system is a Micro power generator which uses photovoltaic (PV)
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cells to convert heat radiation (from the combustion of hydrocarbon fuel)into
electricity.
The general operating principle of the device is as follows: hydrocarbon fuel first
mixes with compressed air in the micro-mixer. The mixture then enters a
cylindrical micro combustor, where the combustion takes place. As the wall of the
micro-combustor is heated to a sufficiently high temperature, it emits a lot of
photos (as radiation) from a broadband emitter. The broadband emitter is on the
wall of the micro-combustor. The micro-combustor is peripherally enclosed by
arrays of photovoltaic (PV) cells, so that the Radiative heat from the wall of the
micro-combustor can be absorbed by the PV cells. The radiated photons interact
with the electrons in the PV cells by imparting their energy to the electron as
kinetic energy, which consequently produces an electric current. Hydrogen will be
used as the hydrocarbon fuel for combustion[8,9].
the cross-sectional schematic layout of micro thermo photovoltaic devic [21,1]
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Micro-combustor:
The most important components of the micro-TPV power system, the micro-
combustor. Compared to conventional combustors, a micro-combustor is more
highly constrained by inadequate residence time for complete combustion and high
rates of heat transfer from the combustor. Some of the major challenges and
problems with micro-scale combustion.. For micro-TPV applications, the desired
output is a high and uniform temperature along the wall of the micro combustor.
The major challenge in the micro-combustor design is to keep an optimum balance
between sustaining combustion and maximizing the heat output . A high surface-
to-volume ratio is very favorable to the output power density per unit volume.
However, a high heat output will affect the stable combustion in the micro
combustor[1,7]
Sic emitter:
The emitter is another most important component in the design of the micro-TPV
power device. The emitter is the wall of the micro combustor and it functions to
convert heat energy from the combustion into radiation by emitting photons. There
are basically two different types of emitters, namely broadband emitters and
selective emitters. Blackbody is a typical broadband radiation materials, its
emission behavior can be approximated by graphite or a soot-covered surface.
However, a broadband radiating material of practical importance is silicon carbide
(SiC) with an emissivity SiC 0.9. While a selective emitter exhibits a high
emittance in the spectral range usable for the PV cells, and a low emittance
elsewhere. There are several methods have been developed to fabricate selective
emitters. One of them is to use oxides of rare earth materials such as[10].
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Er2O3 (erbia) and Yb2O3 (ytterbia)
Another way is micro-structuring the surface of the emitters. a new thermally
Excited Co/Ni (cobalt/nickel)-doped MgO (magnesium oxide) matched emitter.
This kind of emitters exhibits a better spectral efficiency .The prototype micro-
TPV power generator uses silicon carbide (SiC) as the material for the emitter .SiC
has been selected due to its good emissivity and high temperature reliability..
Furthermore, compared with other selective emitters such as micro-machining
tungsten and rare earth oxide, it is easier to fabricate into a cylindrical shape. The
SiC emitter is a typical broadband emitter and the spectrum of broadband emitters
generally operates at temperature of about 10001600 K .As part of an effort to
develop a suitable material for the emitter, carried out a series of experiments to
test the performance of prototype micro-TPV power generator with different
emitting materials. The efficiency of the micro-combustor can be improved by
replacing the SiC emitter with other selective emitters, which will consequently
lead to a higher overall efficiency for the micro-TPV power generator.
Dielectric filter:
The another component in the design of the micro-TPV power device is a simple
nine-layer dielectric filter. the SiC emitter is a typical broadband emitter. The
spectrum of broadband emitters operating at temperatures in the range of 1000
1600K contains a significant proportion of photons with energies not sufficient
enough to generate charge carriers in the PV cells. only those photons radiated bythe emitter having energy greater than the band gap (e.g. for GaSb cells, it is0.72
eV, corresponding to a wavelength of 1.7um) of the photo voltaic cell can be
converted into electricity. So we can say, those photons with a wavelength longer
than 1.7 micro m. cannot generate free electrons and produce electricity when
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impinging on the photovoltaic cell. These photos are called sub-band gap
photons .If this portion of energy is being absorbed by the PV cells, it will result in
a destructive heat load on the generator components, subsequently lowering the
conversion efficiency of the system.
To improve the overall efficiency of the micro-
TPV system, it is very important to recycle these photons. Therefore, a filter is
required in the micro-TPV system (to filter out photons with wavelength longer
than 1.7 um). The filter is able to recycle the energy emitted in the range of 1.83.5
_m mid-wavelength band, thereby improving the overall efficiency of the
system .The reflectance of the dielectric filter[9] . The reflectance is measured with
a customized in-house built optical test system done by the commercial fabricator
(with an uncertainty of less than 3%). Ideally, the filter should be able to reflect all
non-convertible photons back to the emitter and transmit all convertible photons to
the PV cell array. However, this is very difficult to achieve in actual practice.
Photovoltaic cell array:
The function of photovoltaic cell (converter) is to convert the heat radiation into
electricity (dc). The photons emitted from the emitter impinge on the photovoltaic
array, they stimulate (evoke) free electrons and produce electrical power output.
There are some typical low band gap PV cells developed for TPV applications are:
1. gallium antimonite( GaSb)
2. gallium indium arsenide(GaInAs)
3. InGaAsSb etc.
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The prototype micro-TPV power generator employs a GaSb PV cell array for the
micro-TPV system corresponding to the dielectric filter. The GaSb cell array is
able to respond the photons with wavelength 1.8 um
http://www.pveducation.org/pvcdrom/solar-cell-operation/short-circuit-current
"science.nasa.gov ... Science@NASA Headline News 2002[22]
http://www.google.com/url?url=http://science.nasa.gov/science-news/science-at-nasa/&rct=j&sa=X&ei=6svkTaGpLsG8rAe35-GjBg&ved=0CCYQ6QUoADAB&q=band+gap+of+pv+cell&usg=AFQjCNHp-tgV0B5CHhhsGDhpfU5eBYPlww&cad=rjahttp://www.google.com/url?url=http://science.nasa.gov/science-news/science-at-nasa/&rct=j&sa=X&ei=6svkTaGpLsG8rAe35-GjBg&ved=0CCYQ6QUoADAB&q=band+gap+of+pv+cell&usg=AFQjCNHp-tgV0B5CHhhsGDhpfU5eBYPlww&cad=rja -
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Fabrication of the GaSb photovoltaic cell array:
The technology used for to form the pn-junction is based on a Zn vapor diffusion
process into an N-doped GaSb substrate thus expensive epitaxial growth of this
A semiconductor layer is successfully avoided. The Zn vapor diffusion process is
Performed. The diffusion source is a mixture of Zn and antimony.
So there are different process of fabrication which are as follows [10]:
1. Silicon nitrate diffusion (SiN) over an n-type GaSb wafer doped with Te.
2. Photolithography is used to open holes in the dielectric.
3. Front side pattern of the wafer is then protected with photo resist.
4. Etch nitrate and diffused Zn.
5. Backside nonselective etch and metallization.
6. Then front side metallization area is standard lift-off photolithography
7. Metallization and lift-off the front side area.
8. Matching of the emitter.
9. Depositions of an anti-reflection (AR) coating are performed to maximize
photocurrent.
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Fabrication process:
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The outline of the GaAb fabrication process[10]
Cooling fins:
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The operation temperature of the PV cells affects the overall power output of the
GaSb TPV cell on band gap and reverse the saturation current. As the operating
temperature increases, the conversion efficiency of the TPV cell will decrease.
Therefore, effective cooling would be necessary in a GaSb TPV system. Arrays of
cooling fins are attached on the back of the PV cells to remove the sink heat from
the PV cells. The cooling fins[1,21] can be made of highly conductive materials
such as aluminum or copper. The fin surfaces can be manufactured by extruding or
welding. The array of cooling fins will enhance the rate of heat transfer from the
surface of the GaSb PV cells by several folds, by exposing a larger surface area to
convection and radiation.
The array of cooling fins attached to a prototype micro-TPV power generator .
array of cooling fins attached to the back side of PV cell.[1,21]
Short-circuit current of photovoltaic cell:
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The short-circuit current is the current through the photovoltaic cell when
the voltage across the solar cell is zero (i.e., when the PV cell is short
circuited). It is written as ISC. The short-circuit current is due to the
generation and collection of light-generated carriers. For an ideal
photovoltaic cell at most moderate resistive loss mechanisms, the short-
circuit current and the light-generated current are identical. Therefore, the
short-circuit current is the largest current which may be drawn from the PV
cell.
http://www.pveducation.org/pvcdrom/solar-cell-operation/short-circuit-current
The short-circuit current depends on a number of factors which
are as follows:
the area of the solar cell. To remove the dependence of the PV cell area, it
is more common to list the short-circuit current density ( mA/cm2) rather
than the short-circuit current;
the number of photons (i.e., the power of the incident light source). Isc
from a solar cell is directly dependant on the light intensity.
http://www.pveducation.org/pvcdrom/solar-cell-operation/short-circuit-currenthttp://www.pveducation.org/pvcdrom/solar-cell-operation/short-circuit-current -
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the spectrum of the incident light.
the optical properties (absorption and reflection) of the photovoltaic cell .
the collection probability of the PV cell, which depends on the surface
passivation and the minority carrier lifetime in the base.
Open circuit voltage of photovoltaic cell:
The open-circuit circuit voltage, VOC, is the maximum voltage available from a
photovoltaic cell, and this occurs at zero current. The open-circuit voltage[11]
corresponds to the amount of forward bias on the PV cell due to the bias of the
PV cell junction with the light-generated current.
http://www.pveducation.org/pvcdrom/solar-cell-operation/short-circuit-current[11]
fill factor:
http://www.pveducation.org/pvcdrom/solar-cell-operation/short-circuit-currenthttp://www.pveducation.org/pvcdrom/solar-cell-operation/short-circuit-current -
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The short-circuit current and the open-circuit voltage are the maximum
current and voltage respectively from a photovoltaic cell. However, at both
of these operating points, the power from the solar cell is zero. The "fill
factor", more commonly known by its abbreviation "FF", is a parameter
which, in conjunction with Voc and Isc determines the maximum power
from a solar cell. The FF is defined as the ratio of the maximum power from
the pv cell to the product of Voc and Isc. So fill factor[12] is given by
FF= Vm x Im / Voc x Isc
http://www.pveducation.org/pvcdrom/solar-cell-operation/short-circuit-current[12]
Table for the description of FF, Voc, Isc:
http://www.pveducation.org/pvcdrom/solar-cell-operation/short-circuit-currenthttp://www.pveducation.org/pvcdrom/solar-cell-operation/short-circuit-current -
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Performance of GaSb PV circuits incorporated with filters
s.no. FF Voc(V) Isc(amp) Vmax(V) Imax(amp) Pmax(watt)
1. .750 2.84 2.25 2.36 2.03 4.80
2. .761 2.81 2.15 2.24 2.05 4.60
3. .738 2.79 2.15 2.32 1.91 4.42
4. .752 2.84 2.25 2.36 2.03 4.80
5. .757 2.82 2.23 2.36 2.01 4.75
Losses in photovoltaic cell:
1. Reflection losses
2. Thermal losses
3. Recombination losses
1. surface recombination loss
2. depletion region recombination
3. bulk recombination
4. recombination at metal semiconductor contacts
4. Series resistance losses
1. emitter resistance losses
2. metal semiconductor contact
3. metal fingers and bus-bars
1. Reflection losses:
The reflection losses occur from top surface of the photovoltaic cells which
Receives the light (photons). Reflection losses [13] affect the Isc short circuit
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current of the photovoltaic cell. Reflection reduces the absorbed carriers and hence
the Isc. It is necessary to improve the absorption and reduce reflection to improve
short circuit current. For a Si these losses account for more than 30%.
2. Thermal losses:
A major portion of loss in photovoltaic cell is a due to heat. Heat radiation
absorbed by the PV cells has excess energy than that required for generation of
electron hole pair (band-gap energy Eg). This excess energy is released in the form
heat .This thermal energy causes rise of temperature of cell. The parameters that
are affected by the temperature of the cell are band gap energy, diffusion length,minority carrier lifetime, intrinsic carrier density. The increases in diffusion length
and minority carrier concentration and intrinsic carrier concentration and decrease
in band gap energy causes the increases in the reverse saturation current Io . The
increase in Io reduces the open circuit voltage.
3. Recombination loss:
Photon incident on the PV cell generates electron hole pairs; these generated pairs
are called as carriers. Generated carriers need to be separated before they
recombine, with emission of energy. Recombination [14] causes loss of carrier and
affects the performance of the cell. Open circuit voltage Voc of the cell is affected
by recombination of carriers. As recombination increases the Voc reduces. Various
techniques are used to reduce the recombination in the PV cells and improve Voc.
Generation of carriers is in the entire volume of the PV cell material. The carriers
generated near depletion region are separated out very quickly as they get swept
away by the electric field present in the depletion region. Were as the carriers
which are generated away from the depletion region that is in the bulk region, on
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the surface, or at the back surface have less probability of getting separated. These
carriers will be lost and would not contribute to current flow if they recombine.
These losses account for major portion total input power. Different
recombination losses that occur at different regions of the PV cells are:
1. Surface recombination.
2. Bulk recombination.
3. Depletion region recombination.
4. Recombination at metal Semiconductor contact.
1. Surface recombination:
Surface recombination is high in Si due to the presence of incomplete
bonds. The incomplete bonds traps the generated carriers and get recombined [14].
2. depletion region combination:
Recombination occurring in the depletion region is less significant as compared
to the surface recombination due to the presence of electric field. Charge
carriers generated in depletion region are separated by electric field very
quickly avoiding any chance of recombination. Any recombination occurring in
the depletion region is mostly by the band to band recombination.
3. recombination at metal semiconductor contacts:
Metal semiconductor contacts regions provide very large recombination sites.
Semiconductor and metal contact junctions are formed at both front and back side
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of the solar cell. Back side contact contributes more to the recombination [15] as it
has more contact area with the semiconductor. Surface recombination velocity is
also present at the rear contact and needs to be reduced.
4. series resistance losses:
Series resistance losses contribute to around less than 20% of the total input
Power. But these losses increases tremendously when PV cell is operated at high
intensities. The high intensity is obtained when a large amount of photons is
focused on the PV cells. Series resistance of the cell is combination of [16],
1. Emitter layer resistance
2. Metal-semiconductor contact
3. Metal bus-bars and fingers
4. Bulk semiconductor resistance.
The metal bus-bars and fingers, emitter layer and metal-semiconductor contact
resistance contribute in large magnitude to series resistance. Bulk resistance is low
due to its high conductivity.
1. Emitter resistance[17]:
Emitter resistance of solar cell is one of the most dominating components of series
resistance of PV cell. Sheet resistance is measure of emitter resistance and it is
desired to have sheet resistance value in the range of 80-100 ohm/square .Factors
affecting the emitter resistance are, thickness of the emitter layer, current direction
in the emitter region, current collection by the metal fingers and bus bars.
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1. metal semiconductor contact[18]:
The other main component of the series resistance is the semiconductor to metal
contact resistance. Metal contacts are required in the PV cells for collection of the
carriers and transport them to the load to deliver power.
2. metal fingers and bus bars[19]:
Metal fingers and bus bar resistance causes considerable loss of power in the
PV cells. Metal contacts are placed on the front surface and back surface of the
PV cells to collect carriers and pass them to the load. Front contact metal is in
the form of fine grid lines were as the back contact is a metal plate covering
entire back surface. Metals used for the contact resistance are Al; Ti; Pd; Ag ,
etc. Back contact metal is Al were as for front contact the finer grid lines are of
high conductivity metal usually Ag or paste of Ag; Pd or Ti is used. These
contacts are deposited on semiconductor by using various techniques such as
Photo-lithography, Evaporation, Sputtering, Screen printing, Electroplating.
The possible measures that can help to reduce the series resistance are as
follows,
1. High conductivity base (substrate) material.
2. Optimizing the junction depth for reducing the emitter resistance .Increasing
the thickness to reduce the sheet resistance.
3. Increasing the number of fingers by reducing the width increases the current
collection and reduces the metal semiconductor contact resistance.
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4. Electroplated metal contacts to reduce the resistance of the metal contacts by
increasing the metal density.
5. Different metallization techniques..
6. Use of different techniques for depositing metal contacts at the front and
back surface. . This can reduce the cost of deposition of metals.
input and output with losses[16]
System efficiency and power output on different configuration:
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Micro TPV device
configuration
Power output
density(w/cm2)
Predicted
elec.
Poweroutput(w)
Predicted
efficiency of
emitter(micro-combustor)
Predicted
effic.of PV
cells
Ove
effi
Sic emitter, GaSb PV
cells
.48 .92 21.4 3.08 .
Sic emitter,
GaInAsSb PV cells[5]
.78 1.45 21.4 4.80 1.
Co/Ni-doped Mgo
matched emitter,GaSb PV cells[23]
2.47 4.71 21.4 15.72 3.
Sic emitter, GaSb PV
cells, length
increased to
22mm[10]
.55 1.45 29.4 3.52 1.0
Co/Ni-doped Mgo
matched emitter,
GaSb PV cells length
increased to
22mm[10]
2.10 5.5 29.4 13.3 3.9
APPLICATION:
In spite of the high initial cost, photovoltaic systems are being used increase to
supply electricity for many applications where small amount of power is needed.
Their cost-effectiveness increases with the distance of the location where they are
to be installed from the main power grid lines. For example it is more easy and
economical to install a alone PV system instead of a transmission line to a village
having a load of 10kw, if the village is more than 40km from the grid lines. So
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there are various applications for which PV system have been developed , so some
of them are:
1. Pumping water for irrigation and drinking.
2. Electrification for remote villages for providing street lighting and other
community services.
3. Telecommunication for the post and telegraph and railway communication
network.
[ prof. sukhatme IIT Mumbai]
Future work:
After some consideration we can say there are some work that has to do before it
can be established for commercial application. There are still some possibilities for
improvement in the micro TPV system.
Some of the future works that can be carried out are as follows:
1. More needs to optimization of the micro-combustor by numerical and
experimental studies. This will further develop or improve the system.
2. There are some other issues for predicting the efficiency of micro-TPV systems
such as, micro-combustor temperature, photon losses, non-uniformity of photon
flux. Apart from this some other issues like mismatched performance of individual
cells. So future work should work should be done in these areas.
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3. Development of new micro-combustors, so that we can achieve the stable
combustion and high wall temperature.
4. There should some work to investigate the possibilities of develop or integrate
the TPV system with a gas turbine to recover the exhaust heat, that could improve
the efficiency.
References:
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