9.photovoltaic_systems_slides.pdf

Photovoltaic Systems ENR 320

1

Photovoltaic Systems - Outline • Principle of operation • The solar resource – PSH & Orientation • System overview, configurations & components • Design problem: system sizing • Installation issues 2

PV Systems – What are they? PV module

DC - Load

• Photo=‘light’, volt  electricity • Direct conversion : sunlight to electricity • DC power output. • Solid state semiconductor device; solar panels

3

Types of Solar Panel Works • Semiconductor made of Si or GaAs, CIGS and CdTe • Mostly Si- based • 90% are mono & multi-cystalline Si, panels

multi-crystalline Si Amorphous-Si

mono-crystalline Si

4

Photovoltaic panel – make up & Specs • Made-up of multiple cells • p-n junction with photovoltaic effect

Panel Specification Model

SUN-A-210-FA3

Power (Wp)

210 Watts

Open Circuit Voltage (V)

22.80 Voc

Short Circuit Current (A)

12.11 Isc

Maximum Power Voltage (V)

18.30 Vmp

Maximum Power Current (A)

11.48 Imp

Dimensions

Length X Width

5

Photovoltaic panel – Operation • Each cell = p-n junction • Requires minimum energy – band limited. • Extra energy causes heat. • Heat reduces efficiency: atom vibration. 6

Current-voltage curve

• Varying power output 7

Effect of sunlight intensity & temperature

8

• Temperatures above 25 are bad for panels!

The Sun as a Resource • Standard measure of resource = PSH • Perfect/Peak Sun Hours (PSH): the number of hours of a perfectly sunny day with no clouds. (sometimes equivalent sun hours -ESH) • A PSH is equivalent to having 1 kW/m2 (watts per meter squared) of sun for one continuous hour. • PSH is found by adding up all the amount of sun received for every hour of the day and then dividing the total by 1kW/m2. 9

Peak Sun Hours

10

PSH = area under the red curve

Calculating Peak Sun Hours

11

World map of PSH

12 SOURCE: www.campingworld.com

PV panel : Orientation Tilted towards the equator at an angle approximately equal to the latitude of the location

Optimize for winter: L + 15o Optimize for Summer L - 15o 13

PV system: Configurations PV Array

PV Array

Inverter

Utility Grid

DC - Load

2. Grid connected system

1. Direct coupled DC system

PV Array

Charge Controller

Inverter

Storage Batteries

Aux. Generator

AC - Load Hybrid system: AC with backup battery and generator

14

PV system: Configuration Direct coupled DC system PV Array

DC - Load Source: electrical-engineering-portal.com

15

PV system: Configuration Grid connected system PV Array

Inverter

Utility Grid Source: ecofriend.com

• Stringent power quality requirements on the inverter 16

PV system: Configuration Hybrid system: AC with backup battery and generator PV Array

Charge Controller

Inverter

Storage Batteries

Aux. Generator

AC - Load

Source: EXMORK

17

• Stringent power quality requirements on the inverter

PV system: Components • Solar Panel - Converts sunlight to electricity. • Charge Controller/regulator - Manages the flow of electricity between the solar panel battery and load. • Batteries - Store electricity. • Inverter - Converts DC power from the solar panel and battery to AC power. • Load - Application for electricity, e.g. lights, LED light, computer, radio. • Wires - Connect the other various components together.

18

Charge Controller: functions and specifications • Its central: The wiring from the solar panels, the batteries, and all of the loads goes through the charge controller • Manages electricity flow from panels, into and out of the batteries, and to the loads. It can: • Protects the battery from overcharging, by controlling how the PV panel charges the battery. • Protects the battery from over discharge: disconnects the loads when the battery voltage gets too low. • Gives information on the state of change of the charge controller.

• Important specs: Voltage & Max. Current Ratings

Rate Voltage

Max. Current

12V-48V

100A

19

Charge controller: examples

State of Charge LED Connection sockets

20

Wiring • The wiring is what carries the electricity from the panels through the charge controller to the batteries and from the batteries through the charge controller out to the loads. • if the wiring is not sized and installed correctly, the electricity will not flow properly to the loads.

Size Matters!

21

Wiring: Voltage drop •

22

Wiring: Loses vs. Size

23

Wiring: Sizing problem

• Example: Which wire size should be used? 24

Wiring: Sizing problem • Using 1.5mm2 copper wire, drops 15.02 v @ 100 meters. (so 8m drops: 0.08 * 15 = 1.2 V). Twice as much! • A 2.5 mm2 wire, drops: 0.08 * 9.46 = 0.8V. Okish?

• A 4 mm2 wire, drops: 0.08 * 6 V = 0.48V. < 0.6 volts. Good! 25

Batteries: Specs • • • • •

Provide storage Lead-acid = most common Rated in Amp Hours (Ah) Self discharge rate(% per month) Life cycle for a given Depth of Discharge • (above 50% recommended). • 5-7 years life time

Lead-acid battery

Efficiency

Self discharge rate

(usually 12V)

83-90

3-10% per month 26

PV systems use deep-cycle batteries, Car batteries not recommended!

Batteries :State of Charge • Can be described by measuring the voltage with a digital multi meter (DMM). The voltage measurements must be made when • Battery is disconnected from the charge controller. • Battery has been at rest for 30 minutes.

27

Source: Green Empowerment , Photovoltaic System, Training Manual

Batteries: Role of the charge controller • Critical issues: • Charging : follows a charging cycle • Discharging: beyond 80% reduces life.

28 SOURCE: Handbook of Energy Efficiency and Renewable Energy

Inverters: About • Converts DC to AC • 3 types of Inverters: Square wave inverters, modified (quasi) square wave inverters and sine wave inverters. • Issues: Cost & power quality • Harmonics content  (e.g. interference on computer monitors / TV, slower heating on microwave). Some don’t work all together.

29 Increasing power quality & cost

Inverters: Pictures

Source: flexcell Source: flexcell

30

Inverters: Specification Important Specs: • Efficiency: varies with load (85% - 90% ). • Power rating: Higher than max. load. Recommended to (20-30% higher than load) Source: solar-energy-at-home.com

• So an AC system would need a larger panel and battery than a DC system. • Grid-integrated systems need: low harmonic distortion levels, low noise, Switch-off upon grid failure.

31

Designing a PV System

Client says: “Design a PV system for my granny in rural Limpopo. She needs two 60W lighting bulbs in the evening, a 230W iron on every morning. Do I need charge controller? What I need an Inverter? How big should my solar panel be? Battery?”

32

Designing a PV System: General Procedure 0. What is the nominal voltage of the system? 1. Determine load – daily energy consumption (Wh to Ah) 2. Consider system losses 3. Size panel, considering solar resource 4. Size battery, considering days without sun 5. Size inverter & charge controller 6. Install – remember the wire size Usually 12 V for cheap small system or 24 V for larger application!

33

Step 1: Load-Total Daily Consumption Calculate Total Daily Consumption in Watt-Hours: find total load to be powered by the PV System. For how long will each item be used?

34 Thus, load is: 236Wh/12V = 19.7Ah, for a 12Volts system

Step 2: Considering system efficiency • Consider efficiency of the PV System: The efficiency is not 100%, we need to size the panels and the batteries large enough to account for the losses, and still have enough power left over for the loads PV Array

Charge Controller

Inverter

Storage Batteries

Aux. Generator

AC - Load

• Recall: The inverter is at least about 85% efficient. • Thus for a 236Wh AC load on a 12V system, the load is 236Wh/(0.85*12V) = 23.1Ah higher than 19.7Ah for DC

35

Step 2 : Wire efficiency

• Wiring Efficiency: We wanted to keep the voltage drop to 5% or less. Properly designed wiring may have a combined efficiency of 97% or 0.97

36

Step 2: Panel efficiency loss due to Temperature • Accounting for non-standard temperature: Solar panels function the best at 25 ⁰C , For every degree that the solar panel temperature is above 25 ⁰ C the solar panel output 0.5% less.

37

Step 2: Panel efficiency loss due to Temperature • Formula: Solar Panel Temp = Air Temperature ⁰ C + 15 ⁰ C Efficiency drop = (Solar Panel Temp.)* 0.5 • Example: If the Air Temperature = 30 ⁰ C, then the temperature of solar panel is 30 ⁰ C + 15 ⁰ C = 45 ⁰ C . 45 ⁰ C-25 ⁰ C=20 ⁰ C (this is the amount that the solar panel is above the optimal temperature) . Efficiency is 20 ⁰ C * 0.5% per degree = 10% less, so the panel output is 90% at 30 ⁰ C 38

Step 2: System all losses - summary • Inefficiencies/losses are due to: • • • •

Inverter Charge controller functions Wiring losses Temperature of the panel

• Account for losses by: • Consider inverter efficiency for AC load @ 85% • Account for losses by adding 20% to the load.

System load(Ah) =(AC load/0.85 + DC load)*1.2

39

Step 3: Size panel, considering solar resource 3.1 Calculate current requirement. 3.2 Find the optimal optimum module arrangement. Series

Parallel & series Source: Green Empowerment , Photovoltaic System, Training Manual

Parallel

40

Step 3: size panel, considering solar resource 3.1 Calculate current requirement. 3.2 Find the optimal optimum module arrangement. Consider a 12v DC system with 45.6Ah load, in Pretoria (5 PSH) [3.1.] Current = 45.6Ah/5h = 9.12 A

[3.2.] # Panels = 9.12 A/3.15A = 2.9 2.9 (rounded up) = 3 panels 15.9V panel output enough for 12V 9.12A requires 3 panels in parallel

Model

Shell SM50-H

Power (W)

50 Watts

Open Circuit Voltage (V)

19.80 Voc

Short Circuit Current (A)

3.40 Isc

Maximum Power Voltage (V)

15.9 Vmp

Maximum Power Current (A)

3.15 Imp

41

Step 4: Size battery, considering days without sun Rule of thumb: - efficiency of 85% - assume 3 days without sunshine - 50% max. discharge

• For a load of 230 Amp-hours. Then battery size is:

45.6 Amp-hrs / .85 for efficiency = 53.6 Amp-hrs • Considering dark days. 53.6 Amp-hrs needed per day x 3 days with no sun = 161 Amp-hrs

• Consider max. discharge:

161 x 2 = 322 Amp-hrs

42

Step 5: Charge controller sizing • Charge Controller: Max Current & Voltage rating. Voltage rating = Nominal system voltage. [12V] Max. Current > Short-circuit current *(# parallel panels) [>10.2A]

43

Step 5: Inverter sizing • Inverter: Efficiency & power rating. Efficiency = (As high as possible.) safely assume 85% Power rating = • (for grid connected systems: no general rule among country guidelines, location dependent) 70%-90% OR 80%-110% of solar array rating

• Stand alone systems: Definitely higher than the AC load! • so about 25% higher than load you wish to run simultaneously • For a 120kWh AC load, use 150kWh inverter!

44

Designing a PV System: Summary 1. 2. 3. 4. 5.

Determine load – daily energy consumption (Wh to Ah) Consider system losses Size panel, considering solar resource Size battery, considering days without sun Size inverter & charge controller

45

Installation: Panel Orientation • The panel should mounted on an angle approximately equal to the latitude of the area, and pointing to the equator. • If you are in the southern hemisphere, aim the panel to the north, and if you are in the northern hemisphere, aim the panel to the south. • If you are very close to the equator, then the latitude is close to 0 degrees. In these areas, the optimum angle for maximum power might be flat, but it is still good to tilt the panel 5 or 10 degrees to let the rain help to keep the panel free of dust and dirt.

46

Installation: Avoid shading

Source: Green Empowerment , Photovoltaic System, Training Manual

47

Installation: mounting The panel must be installed on sturdy mountings so its orientation will stay as originally designed, and so it will not be subject to being knocked.

48 Source: Green Empowerment , Photovoltaic System, Training Manual

Installation: Charge controller • The cabling from the Panel to the Controller should be sized large enough, and kept as short as possible, and located where it will not be a hazard or be pulled down.

• Controller needs to be mounted in a location where there is not a lot of activity, to avoid the possibility of being knocked into by carried items.

49

Installation: Battery • battery needs to be close to the controller to limit the length of the wire, and reduce the losses in this cable. • It should be in a non-metallic battery box (wood or plastic) vented to the air, and covered so nothing metal can be placed on top of the battery.

50

• Battery Safety Issues: • Chemicals - The acid in the battery is bad for people and for the environment • Gas - Batteries vent a gas that be extremely flammable • Electrocution- Batteries have a lot of electricity and can easily electrocute people

51

Installation: Wiring • Wiring should be neat, and fastened securely in all locations.

• This makes checking for trouble much easier, and avoids problems of things being hung on the wiring or cutting into the wiring.

52

Wiring: termination at the battery • Terminations: all wire connections need to be clean and tight

Source: Green Empowerment , Photovoltaic System, Training Manual

53

Installation: Lighting • First of all, try to install a separate light switch for every light. This way if only one light is needed, the others can remain off.

• It is good to install all of the light switches at the controller location, because this reduces the number of field terminations and connections, and points of potential failure. Then, all of the cables are direct runs from the controller to the lights.

54

Installation: Load Connections • All wiring to loads must be connected through the charge controller. • No wires should be terminated on the battery except for the wires going to the charge controller. • If loads are connected directly to the battery, then these loads are not disconnected when the charge controller decides to turn the loads off to protect the battery, and the battery will fail prematurely.

55

Photovoltaic systems • Principle of operation • The sun resource – PSH & Orientation • System overview, configurations & components • Design problem: system sizing • Installation issues

The End

56