Chapter 5 pv systems feb 2011

Interfacing with the Utility• Net metering – customer only pays for the amount of energy that the PV system is unable to supply • In the event of an outage, the PV system must quickly an

Green Energy Renewable Energy Systems

Course-Biên sọan: Nguyễn Hữu Phúc Khoa Điện- Điện Tử- Đại Học Bách Khoa TPHCM

PV Systems

How Fast is Solar PV Growing?

The growth

in total solarenergy is slower(0.06 quad in 2001)versus 0.081 quad

in 2007) partiallydue to solar thermalretirements

PV Current-Voltage Variation with Insolation and Temperature

Pat Chapman Solar Example

2007 he added some solar PV

with 205 W each, for a

total of 2870W He has

a 3300 W inverter

• Total cost was about $27,000,

but tax credits reduced it

to $16,900

Source: www.patrickchapman.com/solar.htm

Lecture 24

PV Systems

Professor Tom Overbye

Department of Electrical and

Computer Engineering

ECE 333

Green Electric Energy

PV Systems – Three configurations

Grid-connected systems

PV Systems – Three configurations

Stand-alone systems which charge batteries

PV Systems – Three configurations

Stand-alone systems with directly-connected loads

Load I-V Curves

• PV panels have I-V curves and so do loads

• Use a combination of the two curves to tell where the system is actually operating

• Operating point – the intersection point at which the PV and the load I-V curves are satisfied

Resistive Load I-V Curve

• Straight line with slope 1/R

• As R increases, operating point moves to the right

• Can use a potentiometer

to plot the PV module’s

m

V R

I



Figure 9.5

Maximum power transfer

efficiency as the amount of insolation changes

DC Motor I-V Curve

• DC motors have an I-V curve similar to a resistor

• e = kω is back emf, R a is armature resistance

(9.3)

a

V  IR  k 

DC Motor I-V Curve

Linear Current Booster (LCB) helps the motor be able to start in low sunlight

Figure 9.9 Figure 9.10

Battery I-V Curves

• Energy is stored in batteries for most off-grid applications

• An ideal battery is a voltage source V B

• A real battery has internal resistance R i

(9.4)

B i

V V  R I

Battery I-V Curves

• Charging– I-V line tilts right with a slope of 1/R i,

applied voltage must be greater than VB

• Discharging battery- I-V line tilts to the left with slope

1/R i, terminal voltage is less than VB

Figure 9.12

Maximum Power Point Trackers

standard part of PV systems, especially grid-connected

• Idea is to keep the operating point near the knee of the

PV system’s I-V curve

• Buck-boost converter – DC to DC converter, can either

“buck” (lower) or “boost” (raise) the voltage

• Varying the duty cycle of a buck-boost converter can be done such that the PV system will deliver the maximum power to the load

MPPTs – Example 9.2

17 V and Im = 6A

is delivering power to a 10Ω resistance?

Hourly I-V Curves

• Can just adjust

the 1-sun I-V

curve by

shifting it up

or down

Grid-Connected Systems

• Can have a combiner box and a single inverter or small inverters for each panel

• Inverter sends AC power to utility service panel

– MPPT

– Ground-fault circuit interrupter (GFCI)

– Circuitry to disconnect from grid if utility loses power

– Battery bank to provide back-up power

Components of Grid-Connected PV

Individual Inverter Concept

• Connections to house distribution panel are simple

Interfacing with the Utility

• Net metering – customer only pays for the amount of energy that the PV system is unable to supply

• In the event of an outage, the PV system must quickly and automatically disconnect from the grid

• A battery backup system

can help provide power

to the system’s owners

DC and AC Rated Power

• P dc,STC - DC power of array from adding module ratings under standard test conditions (STC) (1-sun, AM 1.5,

25˚C)

• Conversion efficiency – includes losses from inverter, dirty collectors, mismatched modules, and differences in ambient conditions

in full sun

, (Conversion Efficiency) (9.10)

ac dc STC

Losses from Mismatched Modules

• Illustrates the impact of slight variations in module I-V

curves

Losses due to Cell Temperature

• As temperature increases, power decreases

• PVUSA test conditions (PTC) – 1-sun insolation in

plane of array, 20˚C ambient temperature, wind-speed

of 1 m/s

• P ac(PTC) AC output of an array under PTC test

conditions is a better indicator of actual power delivered

in full sun than the more commonly used P dc(STC)

• Describing a system based on P dc(STC) without

correcting for temperature and the inverter is

misleading

Ex 9.3 - PV Derating using PTC

• A PV array has rating of 1 kW under standard test

condtions (STC) Nominal operating temperature

(NOCT) from Chapter 8 is 47˚C

Ex 9.3 – “1 kW PV system” PTC

Rated AC Power

• The estimated cell temperature is

• Including inefficiencies, estimated AC rated power at PTC is

20

S (8.24) 0.8

“Peak-Hours” Approach

• 1-sun is 1 kW/m2

• We can say that 5.6 kWh/(m2-day) is 5.6 hours of “peak sun”

• If we know Pac, computed for 1-sun, just multiply by

hours of peak sun to get kWh

• If we assume the average PV system efficiency over a day is the same as the efficiency at 1-sun, then

Energy (kWh/day)  P ac kW h/day of "peak sun"  (9.14)

Capacity Factor of PV

Energy kWh/yr  P ac kW CF 8760 h/yr   (9.15)

h/day of "peak sun"

US cities

Stand-Alone PV Systems

• When the grid isn’t nearby, the extra cost and

complexity of a stand-alone power system can be worth the benefits

• System may include batteries and a backup generator

Stand-Alone PV - Considerations

• PV System design begins with an estimate of the

loads that need to be served by the PV system

• Tradeoffs between more expensive, efficient

appliances and size of PVs and battery system needed

• Should you use more DC loads to avoid inverter

inefficiencies or use more AC loads for convenience?

• What fraction of the full load should the backup

generator supply?

• Inrush current used to start major appliances

Power Requirements of Typical

Loads

Table 9.10 – Power Requirements of some typical loads

Note that these tables are useful for getting an idea of the average values, but the best data comes from actual

measurements!

Consumer Electronics as Loads

• Consider the power when the device is actively used

standby

Table 9.10 – Power requirements of some consumer electronics

Batteries and PV Systems

• Batteries in PV systems provide storage, help meet

surge current requirements, and provide a constant

output voltage

• Lead-acid batteries are still the most

commonly-used batteries for PV systems

• The lead-acid battery is an electrical

storage device that uses a reversible

chemical reaction to store energy

• Lead-acid batteries date back to the

1860s

http://img.alibaba.com/photo/11244127/Lead_Acid_Batteries.jpg

Basics of Lead-Acid Batteries

Basics of Lead-Acid Batteries

• During discharge, voltage drops and specific gravity

drops

• Sulfate adheres to the plates during discharge and comes back off when charging, but some of it becomes

permanently attached

Stand-Alone PV Systems – Design

Summary

• Analysis of load

– Determine daily demands for power and energy

– What fraction of the worst month “design month” should you cover with the PV system? How much should you cover with

a backup generator?

– What PV system voltage should you have?

– Convert total DC load to amp hours @ system voltage

– Pick a PV module based on insolation data for the site for the design month

– Determine how many parallel strings of modules and how

many modules in each string

Stand-Alone PV Systems – Design Summary

http://www.ecosolarenergy.com.au/How_a_Standalone_System_Works-28.htm

ECE 333 Green Electric Energy

••Lecture 25

••PV Systems, Energy Storage

••Prof Tom Overbye

••Department of Electrical and

Computer Engineering

• Homework 12 is 8.3, 8.7, 8.9, 9.1, 9.7 It should be done before the final but need not be turned in

• Reading: Chapters 8 and 9

Room 106B8 Eng Hall and Ceramics Building 218

– If your last name begins with “A” through “K” come to 106B8 Eng Hall; otherwise go to Ceramics Building 218.

– Final is comprehensive, with more emphasis on solar (since it wasn’t on an earlier exam)

– Same procedure except you can bring in one new notesheet and your two previous notesheets

In the News

• On Monday US Environmental Protection

Agency (EPA) said that greenhouse gases are a danger to public health and welfare

• This is a necessary first step to allow the EPA to regulate greenhouse gas emissions

– Some industries are concerned these regulations

may be more restrictive than a legislative approach

• Some in Congress have called on EPA to

withdraw its proposal because of recent email

releases that question the underlying science

PV Systems – Four configurations

PV Systems – Four Configurations

DC Motor I-V Curve

• DC motors have an I-V curve similar to a resistor

• e = kω is back emf, R a is armature resistance

(9.3)

a

V  IR  k 

PV Systems – Four Configurations

PV Systems – Four Configurations

4 Microgrids

• As the name implies a microgrid can be thought of as a small electric grid with several generation sources

– The microgrid can be configured to operate either connected

to the main grid or standalone

• The military is a key proponent of microgrids, since

they would like the ability to operate bases independent

of any grid system for long periods of time

• Renewable generation by be quite attractive because it decreases the need to store large amounts of fossil fuel

– Time magazine reported in Nov 2009 that average US solider

in Afghanistan requires 22 gallons of fuel per day at an

average costs of $45 per gallon

47

Battery I-V Curves

Energy is stored in batteries for most off-grid applications

An ideal battery is a voltage source V B

A real battery has internal resistance R i

(9.4)

B i

V  V  R I

Battery I-V Curves

•Charging– I-V line tilts right with a slope of 1/R i,

applied voltage must be greater than VB

•Discharging battery- I-V line tilts to the left with slope

1/R i, terminal voltage is less than VB

Stand-Alone PV Systems

•When the grid isn’t nearby, the extra cost and complexity of a stand-alone power system can be worth the benefits

•System may include batteries and a backup

generator

Stand-Alone PV - Considerations

• PV System design begins with an estimate of the

loads that need to be served by the PV system

• Tradeoffs between more expensive, efficient

appliances and size of PVs and battery system needed

• Should you use more DC loads to avoid inverter

inefficiencies or use more AC loads for convenience?

• What fraction of the full load should the backup

generator supply?

• Inrush current used to start major appliances

Batteries and PV Systems

• Batteries in PV systems provide storage, help meet

surge current requirements, and provide a constant

output voltage

• Lots of interest in battery research, primarily driven by the potential of pluggable hybrid electric vehicles

– $2.4 billion awarded in August 2009

• There are many different types of batteries, and which one is best is very much dependent on the situation

– Cost, weight, number and depth of discharges, efficiency, temperature performance, discharge rate, recharging rates

Lead Acid Batteries

• Most common battery for larger-scale storage

discharge, and 3) deep-cycle, allow much more

repeated charge/discharge such as in a solar

application

53

Basics of Lead-Acid Batteries

Basics of Lead-Acid Batteries

• During discharge, voltage drops and specific gravity

drops

• Sulfate adheres to the plates during discharge and comes back off when charging, but some of it becomes

permanently attached

Battery Storage

• Battery capacity has tended to be specified in

amp-hours (Ah) as opposed to an energy value; multiply by average voltage to get watt-hours

– Value tells how many amps battery can deliver over a

specified period of time.

– Amount of Ah a battery can delivery depends on its

discharge rate; slower is better

56

Figure showshow capacitydegrades withtemperatureand rate

Power W/kg

Estimating Storage Needs

Stand-Alone PV Systems – Design Summary

http://www.ecosolarenergy.com.au/How_a_Standalone_System_Works-28.htm

Photovoltaic Solar Systems

•Dr William J

Makofske

•August 2004

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%

• 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, creating a 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

Combining Solar Cells

• Solar cells can be electrically connected in series

(voltages add) or in parallel (currents add) to give any desired voltage and current (or power) output since P =

I x V

• Photovoltaic cells are typically sold in modules (or

panels) of 12 volts with power outputs of 50 to 100+ watts These are then combined into arrays to give the desired power or watts

Cells, Modules, Arrays

Rest of System Components

While a major component and cost of a PV system is the array, several other components are typically needed These include:

• The inverter – DC to AC electricity

• Batteries (optional depending on design)

• Monitor – (optional but a good idea)

• Ordinary electrical meters work as net meters

The Photovoltaic Array with its other electrical components

PV was developed for the space program in the 1960’s

PV Price and Quantity

Manufactured Relationship

Photovoltaic Array for Lighting

Telecommunications Tower

Remote Water Pumping in Utah

Recreation Vehicle Outfitted with Solar Panels

Solar Lanterns for Landscaping

A Solar Driven Band

The Market Expands

• As prices dropped, PV began to be used for stand-alone home power If you didn’t have an existing electrical line close to your property, it was cheaper to have a PV system (including batteries and a backup generator)

than to connect to the grid As technology advanced,

grid-connected PV with net metering became possible

received from the grid system and the meter turns

forwards Depending on PV size and electrical

consumption, you may produce more or less than you actually use Individual houses may become power producers

Net Metering can be done with or without a battery backup

• Batteries can be used to provide long-term or short-term electrical supply in case of grid failure Many grid-

connected houses choose to have a small electrical

battery system to provide loads with power for half a

day in case of outage Larger number of batteries are

typically used for remote grid-independent systems

Battery Sizing I

If your load is 10 kw-hr per day, and you want to battery

to provide 2.5 days of storage, then it needs to store 25 kw-hr of extractable electrical energy Since deep cycle batteries can be discharged up to 80% of capacity

without harm, you need a battery with a storage of

25/0.8 = 31.25 kw-hr A typical battery at 12 volts and

200 amp-hour capacity stores 2.4 kw-hr of electrical

energy

2 KW PV on Roof with battery storage Solar hot water collectors and tank

PV On Homes

• PV can be added to existing roofs While south tilted exposure is best, flat roofs do very well Even east or west facing roofs that do not have steep slopes can

work fairly well if you are doing net metering since the summer sun is so much higher and more intense than the winter sun The exact performance of any PV

system in any orientation is easily predictable