Photovoltaic panels to supply electricity for domestic consumers and storage battery system of 3kW

7/30/2019 Photovoltaic panels to supply electricity for domestic consumers and storage battery system of 3kW

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Project Title:

Photovoltaic panels to supply electricity for domestic

consumers and storage battery system of 3kW.

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Table of Contents:

List of Tables ...............................................................................................................................

List of Figures ................................................................................................................................CHAPTER I PHOTOVOLTAIC PANELS (PV) GENERALITIES.............................................

1.1. Introduction....................................................................................................1.2. The PV cell.....................................................................................................

1.3. Equivalent circuit for a simple solar cell .......................................................

1.4. Equivalent circuit for a more accurate model.................................................1.5. Crystalline silicon solar cells..........................................................................

1.6. Thin film solar cells........................................................................................1.7. Developing technologies.................................................................................

1.8. Module and Array...........................................................................................

1.9. Sun Tracking...................................................................................................

1.10. Maximum Power Point Tracker....................................................................1.11. MPPT Controller...........................................................................................1.12. Stand-alone photovoltaic system..................................................................

1.13. Grid connected photovoltaic system.............................................................

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CHAPTER I - PHOTOVOLTAIC PANELS (PV) GENERALITIES

1.1. Introduction

The photovoltaic effect is the electrical potential developed between two dissimilar

materials when their common junction is illuminated with radiation of photons. The photovoltaic

cell, converts light directly into electricity. The PV effect was discovered in 1839 by French

physicist Becquerel. The first silicon solar cell was produced by Bell Laboratories in 1954. Since

then it has been an important source of power for satellites. Having developed maturity in the

space applications, the PV technology is now spreading into the terrestrial applications. There

are different sizes of PV module commercially available (typically sized from 60W to 170W).

1.2. The PV cell

The physics of the PV cell is very similar to the classical p-n junction diode. When light

is absorbed by the junction, the energy of the absorbed photons is transferred to the electron

system of the material, resulting in the creation of charge carriers that are separated at the

junction. The charge carriers in the junction region create a potential gradient, get accelerated

under the electric field and circulate as the current through an external circuit. The current

squared times the resistance of the circuit is the power converted into electricity. The remaining

power of the photon elevates the temperature of the cell.

Fig. 1.1.Cell working principle

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Cells require protection from the environment and are usually packaged tightly behind a

glass sheet. When more power is required than a single cell can deliver, cells are electricallyconnected together to form photovoltaic modules, or solar panels. A single module is enough to

power an emergency telephone, but for a house or a power plant the modules must be arranged

in multiples as arrays.

Fig. 1.2. Basic construction of PV cell

Basic construction of PV cell with performance enhancing features (current collecting

mesh, anti-reflective coating and cover glass protection).

Three key elements in a solar cell form the basis of their manufacturing technology:

The first is the semiconductor, which absorbs light and converts it into electron-hole

pairs.

The second is the semiconductor junction, which separates the photo-generated carriers

(electrons and holes),

The third is the contacts on the front and back of the cell that allow the current to flow to

the external circuit.

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The two main categories of technology are defined by the choice of the semiconductor:

either crystalline silicon in a wafer form or thin films of other materials.

1.3. Equivalent circuit for a simple solar cell

A simple solar cell is usually represented by an electrical equivalent one-diode model which

is composed by:

Current source

A series resistance and a parallel one with the source

A diode

Fig 1.3. Equivalent circuit (PV)

The most important characteristics of this model are the I-V and P-V characteristics:

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Fig 1.4. I-V and P-V Characteristics

1.4. Equivalent circuit for a more accurate model

There are a few things that have not been taken into account in the simple model and that

will affect the performance of a PV cell in practice.

Series Resistance

In a practical PV cell, there is a series of resistance in a current path through the semiconductor

material, the metal grid, contacts, and current collecting bus. These resistive losses are lumped

together as a series resister (Rs). Its effect becomes very conspicuous in a PV module that

consists of many series-connected cells, and the value of resistance is multiplied by the number

of cells.

Parallel Resistance

It is a loss associated with a small leakage of current through a resistive path in parallel with the

intrinsic device. This can be 16 represented by a parallel resister (Rp). Its effect is much less

conspicuous in a PV module compared to the series resistance, and it will only become

noticeable when a number of PV modules are connected in parallel for a larger system.

Recombination

Recombination in the depletion region of PV cells provides non-ohmic current paths in parallel

with the intrinsic PV cell. As shown in Figure 2.4, this can be represented by the second diode

(D2) in the equivalent circuit.

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Fig 1.5. More accurate equivalent model (PV)

Summarizing these effects, the current-voltage relationship of PV cell is written as:

(1.1)

It is possible to combine the first diode (D1) and the second diode (D2) and rewrite the

equation in the following form.

(1.2)

Where:

- Isc is the short-circuit current;

- Io is the reverse saturation current;

- q is the electron charge (1.60210-19 C);

- kis the Boltzmanns constant (1.38110-23J/K);

- n is known as the ideality factor (n is sometimes denoted as A) and takes the

value between one and two;

- Tis the junction temperature inKelvin (K).

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1.5. Crystalline silicon solar cells

The usage of this kind of solar cells are 78 - 80% in the world.

Historically, crystalline silicon (c-Si) has been used as the light-absorbing semiconductor

in most solar cells, even though it is a relatively poor absorber of light and requires a

considerable thickness (several hundred microns) of material. Nevertheless, it has proved

convenient because it yields stable solar cells with good efficiencies (11-16%, half to two-thirds

of the theoretical maximum) and uses process technology developed from the huge knowledge

base of the microelectronics industry.

Two types of crystalline silicon are used in the industry:

monocrystalline, produced by slicing wafers (up to 150mm diameter and 350 microns

thick) from a high-purity single crystal bole.

multicrystalline silicon, made by sawing a cast block of silicon first into bars and then

wafers.

The main trend in crystalline silicon cell manufacture is toward multicrystalline

technology. For both mono- and multicrystalline Si, a semiconductor homojunction is formed by

diffusing phosphorus (an n-type dopant) into the top surface of the boron doped (p-type) Si

wafer. Screen-printed contacts are applied to the front and rear of the cell, with the front contact

pattern specially designed to allow maximum light exposure of the Si material with minimum

electrical (resistive) losses in the cell. The most efficient production cells use monocrystalline c-

Si with laser grooved, buried grid contacts for maximum light absorption and current collection.

Each c-Si cell generates about 0.5V, so 36 cells are usually soldered together in series to

produce a module with an output to charge a 12V battery. The cells are hermetically sealed under

high transmission glass to produce highly reliable, weather resistant modules that may be

warranted for up to 25 years.

1.6. Thin film solar cells

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The usage of this kind of solar cells are :18 - 20% in the world.

The high cost of crystalline silicon wafers (they make up 40-50% of the cost of a finished

module) has led the industry to look at cheaper materials to make solar cells. The selected

materials are all strong light absorbers and only need to be about 1micron thick, so materials

costs are significantly reduced. The most common materials are amorphous silicon (a-Si, still

silicon, but in a different form), or the polycrystalline materials: cadmium telluride (CdTe) and

copper indium (gallium) diselenide (CIS or CIGS).

The semiconductor junctions are formed in different ways, either as a p-i-n device in amorphous

silicon, or as a hetero-junction (e.g. with a thin cadmium sulphide layer) for CdTe and CIS. A

transparent conducting oxide layer (such as tin oxide) forms the front electrical contact of the

cell, and a metal layer forms the rear contact.

Thin film technologies are all complex. They have taken at least twenty years, supported in some

cases by major corporations, to get from the stage of promising research (about 8% efficiency at

1cm2 scale) to the first manufacturing plants producing early product.

Amorphous silicon is the most well developed of the thin film technologies. In its simplest form,

the cell structure has a single sequence of p-i-n layers. Such cells suffer from significant

degradation in their power output (in the range 15-35%) when exposed to the sun. Better stability

requires the use of a thinner layer in order to increase the electric field strength across the

material. However, this reduces light absorption and hence cell efficiency. This has led the

industry to develop tandem and even triple layer devices that contain p-i-n cells stacked one on

top of the other. In the cell at the base of the structure, the a-Si is sometimes alloyed withgermanium to reduce its band gap and further improve light absorption.

As before, thin film cells are laminated to produce a weather resistant and environmentally

robust module. Although they are less efficient (production modules range from 5 to 8%), thin

films are potentially cheaper than c-Si because of their lower materials costs and larger substrate

size.

However, conventional c-Si manufacturing technology has continued its steady improvement

year by year and its production costs are still falling too.

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The emerging thin film technologies are starting to make significant in-roads in to grid connect

markets, particularly in Germany, but crystalline technologies still dominate the

market.eveloping Technologies: Electrochemical PV cells

1.7. Developing Technologies

Electrochemical PV

Unlike the crystalline and thin film solar cells that have solid-state light absorbing layers,

electrochemical solar cells have their active component in a liquid phase. They use a dye

sensitizer to absorb the light and create electron-hole pairs in a nanocrystalline titanium dioxide

semiconductor layer. This is sandwiched between a thin oxide coated glass sheet and a rear

carbon contact layer, with a glass or foil backing sheet. Some consider that these cells will offer

lower manufacturing costs in the future because of their simplicity and use of cheap materials.

The challenges of scaling up manufacturing and demonstrating reliable field operation of

products lie ahead. However, prototypes of small devices powered by dye-sensitized

nanocrystalline electrochemical PV cells are now appearing (120cm 2 cells with an efficiency of

7%).

Concentrators

Solar cells usually operate more efficiently under concentrated light. This has led to the

development of a range of approaches using mirrors or lenses to focus light on to specially

designed cells and use heat sinks, or active cooling of the cells, to dissipate the large amount of

heat that is generated. Unlike conventional flat plate PV arrays, concentrator systems require

direct sunlight and will not operate under cloudy conditions. They generally follow the sun's path

through the sky during the day using single-axis tracking. To adjust to the sun's varying height in

the sky through the seasons, two-axis tracking is sometimes used. Concentrators have not yet

achieved widespread application in photovoltaics, but solar concentration has been widely used

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in solar thermal electricity generation technology where the generated heat is used to power a

turbine.

1.8. Module and Array

The solar cell described above is the basic building block of the PV power system.

Typically, it is a few square inches in size and produces about one watt of power. For obtaining

high power, numerous such cells are connected in series and parallel circuits on a panel (module)

area of several square feet. The solar array or panel is defined as a group of several modules

electrically connected in series-parallel combinations to generate the required current and

voltage.

Fig. 1.6 Cell configuration

When the PV cells are wired together in series, the current output is the same as the single cell,

but the voltage output is the sum of each cell voltage.

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Fig. 1.7. Series configuration

After the array is created the PV needs to be mounted so it can start production of energy.

Here are some mounting ways:

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Fig.1.8. Mounting ways for PV panels

1.9. Sun Tracking

The sun tracking is necessary if you want the PV

to produce more energy by the end of the day. The PV is

installed on a sun tracker, with an actuator that followsthe sun like a sunflower.

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There are two types of sun trackers:

one-axis tracker, which follows the sun from east to west during the day.

two-axis tracker tracks the sun from east to west during the day, and from north to south during

the seasons of the year.

Fig. 1.9. Sun tracking principle

A sun tracking design can increase the energy yield up to 40 percent over the year

compared to the fixed-array design. The dual-axis tracking is done by two linear actuator motors,

which aim the sun within one degree of accuracy. During the day, it tracks the sun east to west.

At night it turns east to position itself for the next morning sun. Old trackers did this after the

sunset using a small nickel-cadmium battery. The new designs eliminate the battery requirement

by doing it in the weak light of the dusk and/or dawn.

Fig.1.10. Dual-axis sun tracker using tracking actuator principle.

Sun tracking actuator principle: The two differentially connected sensors at 45 generate signal

proportional to the pointing error.

When the sun is obscured by a dark cloud, the tracker may aim at the next brightest

object, which is generally the edge of the cloud. When the cloud is gone, the tracker aims at the

sun once again. Such sun-hunting is eliminated in newer suntracker design.

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Small pole-mounted panels can use one pole-mounted suntracker. Large array, on the

other hand, is divided into small modules, each mounted on its own single-axis or dual-axis

tracker. This simplifies the structure and eliminates the problems related with large motion.

1.10. Maximum Power Point Tracker

The maximum power point tracker (MPPT) is now prevalent in grid-tied PV power

systems and is becoming more popular in stand-alone systems. It should not be confused with

sun trackers, mechanical devices that rotate and/or tilt PV modules in the direction if sun. MPPT

is a power electronic device interconnecting a PV power source and a load, maximizes the power

output from a PV module or array with varying operating conditions, and therefore maximizes

the system efficiency. MPPT is made up with a switch-mode DCDC converter and a controller.

For grid-tied systems, a switch-mode inverter sometimes fills the role of MPPT. Otherwise, it is

combined with a DC-DC converter that performs the MPPT function.

1.11. MPPT Controller

Analog controllers have traditionally performed control of MPPT. However, the use of

digital controllers is rapidly increasing because they offer several advantages over analog

controllers. First, digital controllers are programmable thus capable of implementing advancedalgorithm with relative ease. It is far easier to code the equation, x = y z, than to design an

analog circuit to do the same. For the same reason, modification of the design is much easier

with digital controllers. They are immune to time and temperature drifts because they work in

discrete, outside the linear operation. As a result, they offer long-term stability. They allow

reduction of parts count since they can handle various tasks in a single chip. Many of them are

also equipped with multiple A/D converters and PWM generators, thus they can control multiple

devices with a single controller.

1.12. Stand-alone photovoltaic system

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AC loads


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Fig.1.11. Standalone model

Many photovoltaic systems operate in a stand-alone mode. Per definition, a stand-alone

system involves no interaction with a utility grid.

Such systems consist of:

PV generator;

energy storage (for example a battery);

AC and DC consumers ;

elements for power conditioning.

A PV generator can contain several arrays. Each array is composed of several modules,

while each module is composed of several solar cells.

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Pv generator

(arrays,modu

les,cells)

Power Conditioning

(Regulator, Converter,

Blocking Diodes,)

Battery

DC loads


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The battery bank stores energy when the power supplied by the PV modules exceeds

load demand and releases it back when the PV supply is insufficient.

The load for a stand-alone PV system can be of many types, both DC (television,lighting) and AC (electric motors, heaters, etc.).

The power conditioning system provides an interface between all the elements of the PV

system, giving protection and control. The most frequently encountered elements of the power

conditioning system are blocking diodes, charge regulators and DC-AC converters.

1.13. Grid connected photovoltaic system

The system design has the following components:

Fig. 1.12. Grid connected model

PV array or Generator. A number of PV panels connected in series and/or in parallel

giving a DC output out of the incident irradiance. Orientation and tilt of these panels are

important design parameters, as well as shading from surrounding obstructions.

Inverter. A power converter that transforms the DC power from the panels into AC

power. The characteristics of the output signal should match the voltage, frequency and power

quality limits in the supply network.

Load. Stands for the network connected appliances in the building that are fed from the

inverter, or, alternatively, from the grid.

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Meters. They account for the energy being drawn from or fed into the local supply

network.

Local supply network. A single or three-phase network managed by a public electricity

supplier. The supply network acts both as a sink for energy surplus in the building or as a backup

for low local generation periods.

REFERENCES

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[1] Ioan erban, Microreele hibride cu surse regenerabile de energie, Editura

Universitii Transilvania din Braov, 2008;

[2] C. Marinescu, M. Georgescu, L. Cloea, C.P. Ion, I. erban, L. Barote, D.M. Vlcan,

Surse regenerabile de energie. Abordri actuale, Editura Universitii Transilvania din

Braov, 2009;

[3] C. Marinescu, Energy Sources, lecture notes;

[4] C.P. Ion, Electronic Circuit Simulation, lecture notes;

[5] M. Georgescu, Power Plants, lecture notes;

[6] L. Iulian, Electrical Equipment, lecture notes;

[7] D. Ilea, Static Converters, lecture notes;

[8] J. H. R. Enslin, Integrated photovoltaic maximum power point tracking converter,

IEEE Transactions on Industrial Electronics,vol 49, 1997;

[9] A. Brambilla, New approach to photovoltaic arrays maximum power point tracking,

IEEE Power Electronics Specialists Conference, 1999;

[10] Masters, Gilbert M. Renewable and Efficient Electric Power Systems,

ISBN 0-471-28060-7, 2004;

[11] M. Faizal, Grid-connected photovoltaic system, University of Queensland, 2003;

[12] V. Quaschning and R. Hanitsch, Influence of shading on electrical parameters of solar

cells, Photovoltaic Specialists Conference, 1996;

[13] State of Charge (SOC) Determination-Performance Characteristics

http://www.mpoweruk.com/life.htm;

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[14] Battery Life (and Death) http://www.mpoweruk.com/life.htm;

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http://var/www/apps/conversion/tmp/scratch_1/%20http:/www.mpoweruk.com/life.htmhttp://var/www/apps/conversion/tmp/scratch_1/%20http:/www.mpoweruk.com/life.htm

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