Distributed Generation Solar PV

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RENEWABLE ENERGY EDUCATION PROJECT

Grid-Connected Solar PV SystemDesign and Installation

Ahmed O BAGRE

June 2009

KNUST SHORT COURSES PROGRAMME

Part 1: Electricity fundamental

Part 1 Learning Objectives

Explain water analogy applied to electricity

Compare AC and DC electrical current andunderstand their important differences

Explain the relationship between volts, amps,amp-hours, watts, watt-hours, and kilowatt-hours

Hydraulic circuit

Electrical circuit

Pressure and Voltage

Q = Q1 = Q2P = PT1 + PT2

I = I1 = I2U=U1 + U2

Resistances in series

Resistances in parallel

T1 T2R1 R2

Q = Q1 + Q2PT1 = PT2 = P I = I1 + I2

UR1 = UR2 = U

Synthesis: Water –

Electricity analogy

Electricity Terminology Resistance (Ω)

The opposition of a material to the flow of anelectrical current

Depends on

Material

Cross sectional area

Length

Temperature

Electricity Terminology

• L: Length (m)

• : Resistivity depending oncable material ( /m/mm²)

- cu = 0.0183 /m/mm² for copper (Cu)

- alu =

•A:Cross section area (mm²)

Resistance

Electricity Terminology Voltage (E or V)

Unit of electromotive force

Can be thought of as electrical pressure

Ohm’s Law• V: Voltage (V)

• R: Resistance ( )

•I:Current flow (A)

Amps (I or A) Rate of electron flow

Electrical current

1 Amp = 1 coulomb/second = 6.3 x 1018

electrons/second

• V: Voltage (V)

• R: Resistance ( )

•I:Current flow (A)

Electricity Terminology Watt (W) are a measure of Power

Amps x Volts = Watts

1 Kilowatt (kW) = 1000 watts

Watt-hour is the unit of Energy

Amp x Volts x hours = Watt-hours

1 Kilowatt-hour (kWh) = 1000 watts-hour (wh)

Electricity Terminology Amp-hour (Ah)

Quantity of electron flow

Used for battery sizing (capacity) Amps x hours = Amp-hours

Amp-hours x Volts = Watt-hours

A 200 Ah Battery delivering 1A will last _____ hours

200 Ah Battery delivering10 A will last _____ hours 100 Ah Battery x 12 V = _____ Wh

Types of Electrical Current

DC = Direct Current

PV panels produce DC Batteries store DC

AC = Alternating Current

Utility power Most consumer appliances

use AC

Energy Conversion

Conversion efficiency

Energy conversion

Energy input

Solar irradiance1000 W/m²

Converter

Part 2: BASIC OF PHOTOVOLTAIC

Part 2 Learning Objectives

Learn how a PV cell produces electricity from

sunlight

Discuss the 3 basic types of PV cell technologies

Understand the effects of cell temperature and

solar insolation on PV performance Gain understanding of module specification

Identify the various parts of a module

Utilisations indirectes de l’énergie thermique solaire (production de vapeur pour

l’entraînement d’un alternateur grâce à une turbine à vapeur

UTILISATIONDIRECTE DES

RAYONNEMENTSSOLAIRES

CENTRALE

ELECTRIQUE PHOTOVOLTAIQUE

SOLAIREPHOTOVOLTAIQUE

SOLAIRETHERMIQUE

CHAUFFAGE DESBATIMENTS

CUISINIERESOLAIRE

SECHOIR SOLAIRE PRODUCTION DEVAPEUR D’EAU

CENTRALEELECTRIQUE

THERMODYNAMIQUE

CLIMATISATIONSOLAIRE

SOLAIRE PASSIF

CHAUFFE EAUSOLAIRE

1 Utilisations directes de l’énergie thermique solaire

SOLEIL

Got Sun?

Solar Energy Has Great Potential

80,000 Terawatts of Solar Power fall on the Earth constantly

Compare to 14.5 Terawatts current human power use

Every country has it

No one can embargo it or raise the price

As an alternative to fossil fuels, solar energy reduces air

and water pollution and global warming

It’s becoming more and more cost effective, especially

when you include these ‘external’ costs

Already the best value in remote areas

What is a solar cell? Solid state device that converts incident

solar energy directly into electrical energy

Efficiencies from a few percent up to 20-30%

No moving parts

No noise

Lifetimes of 20-30 years or more

Cross Section of Solar Cell

How Does It Work? The junction of dissimilar materials (n and p type

silicon) creates a voltage

Energy from sunlight knocks out electrons, creatinga electron and a hole in the junction

Connecting both sides to an external circuit causes

current to flow In essence, sunlight on a solar cell creates a small

battery with voltages typically 0.5 V. DC

Available Cell Technologies Single-crystal or Mono-crystalline Silicon

Polycrystalline or Multi-crystalline Silicon

Thin film

Amorphous silicon

Monocrystalline Silicon Modules

Most efficient

commercially available

module (11% - 15%)

Most expensive to

produce

Circular (square-round)

cell creates wasted

space on module

Polycrystalline Silicon Modules Less expensive to make

than single crystalline

modules

Cells slightly less

efficient than a single

crystalline (10% - 12%)

Square shape cells fitinto module efficiently

using the entire space

Amorphous Thin Film

Most inexpensivetechnology to produce

Metal grid replaced withtransparent oxides

Efficiency = 6 – 8 %

Can be deposited onflexible substrates

Less susceptible toshading problems

Better performance in lowlight conditions that withcrystalline modules

Photovoltaic terminology:

Cell to Array

Photovoltaic therminology

Solar cells(0.5V)

(0.3 - 2 watt) Modules(10-300 Watt)

Array (largest area > 5 MW)

Standard Test Conditions (STC)

Current-Voltage (I-V) CurveMaximum Power

(Vco: Open circuit voltage)

(Short circuit current)

Terminology Isc: Short circuit current, maximum current a module

can produce under given conditions (V= 0; P=0)

Voc: Open circuit voltage, maximum voltage under

given conditions (Isc = 0; P= 0) Imp: Current that results in maximum power under

given condition (V= Vmp; P = Pmp)

Vmp: Voltage that results in maximum power under

given condition (I= Imp; P = Pmp) Pmp = Imp x Vmp

Effects of irradiance As insolationdecreases:

• Current decreases

• Voltage dropsslowly

Effects of irradiance

Example: The module MSX-83 from BP Solar has a rated Isc of 5.27 A at STC. What will be the Isc at 800 W/m²?

- STC irradiance is 1000 W/m² = G1- G2 = 800 W/m² - ISC1 = 5.27A- ISC2 = ?

• Isc is directly proportional to irradiance

Effects of irradiance

Effects of Temperature

As the PV cell temperature

increases:

The current increases

slightly

The voltage decreases, the

change of voltage is

directly proportional to

temperature rise

Effects of temperature

Voltage change

Voltage change example 1 : The module SM55 from

Siemens has at STC Vmp of 17.3 V. If the module operates outdoor and heat up to 50°C, the Vmp at 50°C will be:

ΔV(50°C) = - 0.079 V/ °C x (50 - 25)

= - 1.975 V

Vmp (50°C) = 17.3 - 1.97

= 15.325V

15.325 V is still enough to fully charge a typical "12 volt" battery that actually needs up to 15 volts to reach full

charge. 43

Effects of temperatureVoltage change example 2 : The module POLY 175 from SCHOTT Solar has at STC Vmp of 44.3 V. Find out the Voc at 50°C.

Effect of temperaturePower change

Power change Example 1: The module SM55 from Siemens has at STC Pmax of 55 W. If the module operates outdoor and heat up to 50°C,the Pmax at 50°C will be:

Power change ΔP(50°C) = - 0.255 W/ °C x (50 - 25)

= - 6.375W

Pmax (50°C) = 55 - 6.375

= 48.625 W

Effects of temperaturePower change Example 1 : The module POLY 175 from SCHOTT Solar has at STC Pmax of 175 W. Find out the Pmax at 50°C.

Orientation and Tilt Angle

Site Selection –

Tilt AngleBest annual performance:

• Orientation: South

• Tilt angle : Latitude of thesite

Azimuth:

• South : 0°

• North : 180Max performance is

achieved when panels

are perpendicular to the

sun’s rays

Part 3: Modules connection

Part 3 Learning Objectives List the characteristics of series circuits

List the characteristics of parallel circuits

List the characteristics of series/parallel circuits (mix-

circuits)

Understand the shading effects on modules

Series Connections

Loads/sources wired in series

VOLTAGES ARE ADDITIVE

CURRENT IS EQUAL

One interconnection wire is usedbetween two components (negativeconnects with positive)

Combined modules make seriesstring

Leave the series string from aterminal not used in the seriesconnection

Principle

Series Connections:

Electrical circuit

Series Connections:Curve I(V)

Parallel Connections

Loads/sources wired in parallel: VOLTAGE REMAINS CONSTANT

CURRENTS ARE ADDITIVE

Two interconnection wires are used between twocomponents (positive to positive and negative tonegative)

Leave off of either terminal

Modules exiting to next

component can happenat any parallel terminal

Principle

Parallel ConnectionsElectrical circuit

Parallel Connections

Curve I(V)

Series/Parallel Connections

The module MSX-83 from BP solar is used in the following configuration to make a PV array:

4 modules in series 2 strings (of 4 modules in series) in parallel

The electric characteristics at STC are given below: Maximum power (Pmax) = 83W Short-circuit current (Isc) = 5.27A Voltage at Pmax (Vmp) = 17.1V

Open-circuit voltage (Voc) = 21.2V Current at Pmax (Imp) = 4.85A

What will be the electric characteristics of the PV array?

Ns = 4 ⇒ VTmp =17.1V x 4 = 68.4 V and VTOC = 21.2Vx4= 84.8V

Np = 2 ⇒ ITmp = 4.85A x 2 = 9.70A and ITSC = 5.27A x 2 = 10.54A

PTmax = VTmp X ITmp = 68.4 x 9.70 = 663.48 W

PTmax = Pmax X 8 = 83 x 8 = 664 W

Series/Parallel Connections For PV array, the electric characteristics are:

Maximum power (Pmax) = 664W Short-circuit current(Isc) = 10.54A

Voltage at Pmax (Vmp) = 68.4V Open-circuit voltage(Voc) = 84.8V

Current at Pmax (Imp = 9.70A.

Quiz TimeIf you have 4 module of 12V / 3A in an array what

would the power output be if that array were

wired in series?

What if it were wired in parallel?

Is it possible to have a configuration that would

produce 24 V / 6 A? Why?

Dissimilar modules in series Voltage remains additive

If module A is 30V / 6A and module B is 15V / 3A the

resulting voltage will be?

Current taken on the lowest value

For modules A and B wired in series what would be

the current level of the array?

Dissimilar Modules in Parallel Amperage remains additive

For the same modules A and B what would the voltagebe?

Voltage takes on the lower value.

What would the voltage level of A and B wired in

parallel be?

Shading on Modules Depends on orientation of internal module

circuitry relative to the orientation of the shading.

SHADING can half

or even completely

eliminate the output

of a solar array!

Part 4: Photovoltaic System

Part 4: Learning Objectives

Understand the functions of PV components

Identify different system types

Direct coupled system

• No storage (batteries)

• Needs MPPT to maximize the energy feed par the PV generator

• Operates only during sunlight hours

• Better for water pumping, refrigeration system

Standalone system

Battery – stores DC energy Charger/Controller – senses battery

voltage and regulates charging Inverter – converts direct current (DC )

energy to alternating current (AC) energy Loads – anything that consumes energy

Hybrid System DC Bus

• Diesel generator cannot supplythe load directly

Hybrid System AC Bus

• Flexible system with modular components• PV and Diesel Generator can worksimultaneously• Battery to increase the availability ofenergy

Grid-Tied System(Without Batteries)

Complexity

Low: Easy to install(less components)

Grid Interaction

Grid cansupplement power

No power whengrid goes down

Part 5: BATTERIES

Part 4: Learning Objectives

Battery basics

Battery functions

Types of batteries

Charging/discharging

Depth of discharge

Battery safety

Batteries in Series and Parallel Series connections

Builds voltage

Parallel connections Builds amp-hour capacity

Battery Basics

Battery

A device that stores electrical energy (chemical energy to electrical

energy and vice-versa)

Capacity

Amount of electrical energy the battery will contain

State of Charge (SOC)

Available battery capacity

Depth of Discharge (DOD)

Energy taken out of the battery

Efficiency

Energy out/Energy in (typically 80-85%)

The Terms:

Functions of a Battery

Storage for the night

Storage during cloudy weather

Portable power

Surge for starting motors

**Due to the expense and inherit inefficiencies of batteries it isrecommended that they only be used when absolutely necessary (i.e.in remote locations or as battery backup for grid-tied applications ifpower failures are common/lengthy)

Batteries: The Details

Primary (single use)

Secondary (recharged)

Shallow Cycle (20% DOD)Deep Cycle (50-80% DOD)

Types:

Unless lead-acid batteries are charged up to 100%, they will

loose capacity over time

Batteries should be equalized on a regular basis

Charging/Discharging:

Rate of Charge or Discharge

Rate = C/T

C = Battery’s rated capacity (Amp-hours)T = The cycle time period (hours)

Maximum recommend charge/discharge rate = C/3to C/5

Cycle Life vs. Depth of Discharge

Depth Of Discharge (DOD) %

# of Cycles

Part 6: Controllers and inverters

Part 5: Learning Objectives Controller basics

Controller features

Inverter basics

Specifying an inverter

Controller Basics

To protect batteries from being overchargedFunction:

Maximum Power PointTracking

– Tracks the peak

power point of the

array (can improve

power production by

20%)!!

Features:

Additional Controller Features

Voltage Stepdown Controller: compensates for differingvoltages between array and batteries (ex. 48V array

charging 12V battery)

By using a higher voltage array, smaller wire can be

used from the array to the batteries Temperature Compensation: adjusts the charging of

batteries according to ambient temperature

Other Controller Considerations

When specifying a controller you must consider:

DC input and output voltage

Input and output current

Any optional features you need

Controller redundancy: On a stand-alone system it might

be desirable to have more then one controller per array in

the event of a failure

Inverter Basics

An electronic device used to convert direct current

(DC) electricity into alternating current (AC) electricity

Function:

Efficiency penalty

Complexity (read: a component which can fail)

Cost!!

Drawbacks:

Specifying an Inverter

What type of system are you designing? Stand-alone Stand-alone with back-up source (generator) Grid-Tied (without batteries) Grid-Tied (with battery back-up)

Specifics: AC Output (watts) Input voltage (based on modules and wiring) Output voltage (120V/240V residential) Input current (based on modules and wiring)

Surge Capacity Efficiency Weather protection Metering/programming

The sun is the primary energy source for almost all energy flows on the planet. It’s time

we started using it.

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