5. Solar PV Design

 System requirements 1. Charge controller 2. Battery bank 3. Inverter

4. Balance of System (BoS)

 Types Stand alone Grid connected

Why do we need batteries..

 Storing energy produced by the PV array during the day, and to supply it to electrical loads as needed

 To operate the PV array near its maximum power point  To power electrical loads at stable voltages

 To supply surge currents to electrical loads and inverters

Types of Battery  Lead Acid

 Lithium Ion

 Nickel Cadmium

Li-ion battery

Nickel Cadmium

battery Nickel metal

hydride battery Lead acid battery

Lead acid battery  Most economical

 For larger power applications  Used where weight is of little concern.

Li-ion battery

 Fastest growing battery technology

 Offering high energy density and low weight.

 Requires protection circuits to limit voltage and current for

Nickel metal hydride battery  Higher energy density compared to nickelcadmium

 Contain no toxic metals.  Reduced cycle life.

Nickel Cadmium battery  Long life

 High discharge rate

 Extended temperature range is important.

Dominant Energy Storage medium is Lead-Acid batteries

(Mostly used in off-grid systems)

Advantages

Simple and cheap to make

Low self discharge rate

Today, 98% of these batteries are recycled

Capable of high discharge rates

Disadvantages

Cannot be stored in discharge condition Low energy density makes

them bulky

Not environmental friendly

Thermal Runway can occur with improper charging

Battery Capacity: Amount of energy available in the battery. Depends on …

1. Quantity of active materials 2. Amount of electrolyte

3. Surface area of the plates

Rated capacity: Amount of charge available in ampere-hours (Ah) when battery discharged at specified rate.

Basic terminology

Depth of Discharge (DoD)

It is a measure of how much battery is discharged in a cycle before it is charged again. 60% DoD is equivalent to 40% state of charge (SoC).

Cycle life Cell

A cell is building blocks of battery it comprises a number of positive and negative charged plates immersed in an electrolyte that produces an electrical charge by means of an electrochemical reaction.

Cycle life The number of cycles (charge/discharge) a battery provides before it is no longer usable. A battery is considered non-usable if its nominal capacity falls below 60 to 80 percent.

Nominal voltage

The cell voltage that is accepted as an industrial standard. (Cell voltages of 1.20 and 1.25V are used for NiCd and NiMH batteries).

Comparison of batteries

Operational temperature -20 to 60°C -40 to 60°C -20 to 60°C

1. Load in Watt and type of load

(resistive, inductive etc.)

2. Availability of other source of electricity

3. Number of hours of operation per day

4. Autonomy in days

5. System Voltage

6. Charge controller, inverter efficiency

• Load - 1000 Watt

No. of hrs of operation per day – 24 Autonomy required – 1 days

Total power = 24000 Wh

• System Voltage required – 120V Total Ah/day – 200Ah

• Nominal voltage of cell = 2V No. of cells in series =

= system DC voltage/ Nominal voltage of cell = 120/2 (60 cells)

• Battery specification

k-factor for Back up time = 19.35 (from name plate of battery) Depth of Discharge - 0.6

Aging factor - 0.8

Efficiency - PCU (Charge controller) – 0.9 Inverter – 0.9

BATTERY SIZING: CASE STUDY

K factor –

Depending upon the End Cell Voltage & Back-up duration, these factors are manufacturer dependent.

Here, K-factor is 19.35

Aging factor

The effect of age of battery is reflected in its storage potential Here, Average aging factor is considered to be 0.8

Depth Of Discharge

It is a measure of how much battery is discharged in a cycle before it is charged again.

Typically consider Depth Of Discharge as 0.6 if autonomy required is 1 day and 0.8 if autonomy is 2 days and above

• Battery required =

(Load current) x (k-factor for Back up time)/DoD factor/Aging factor/efficiencies of PCU/Inverter.

• Battery required –

(1000/120) x (19.35)/0.6/0.8/0.9/0.9 = 415Ah

Something like a

charge controller

!!!

The

additional advantage

could be –

Increased battery life

A charge controller limits the rate at which electric current is added to or drawn from electric batteries.

Types

PWM (Pulse Width Modulation)

- Helps to remove buildup on the plates in a battery extending a battery’s life.

MPPT (Maximum Power Point Tracking)

Why solar charge controller is required !!!

Charge

controller

For battery

Increases

battery life

Regulate

power

Preventing

reverse current

Optimize

power output

Pulse Width Modulation or PWM technology is used in Inverters to

give a steady output voltage of 230 or 110 V AC irrespective of the load.

What do you think affects the battery

life..??

Pulse Width Modulation

Overcharging batteries

Fluctuating input

Helps to remove buildup on the plates in a battery extending a battery’s life. Pulse = Stop and start like with your heart beat

Width = Time the pulse is turned on and time is the pulse is turned off. Modulation = Controls more or less volume of the charge.

PWM is used as switching signal to control the charge flow to get steady output

The most basic charge controller simply monitors the battery voltage and opens the circuit, stopping the charging, when the battery voltage rises to a certain level.

Bulk:

• Is normally the first step.

• It will allow enough / max power through the controller to bring the voltage up to a set voltage.

Absorb stage:

• Is a tapered charge which the solar energy controller will slowly taper down the amps as the battery reaches full charge

• Does not allow the battery charge to go over the bulk voltage limits Float:

• The charge controller will auto switch to the float charge, which is a lower voltage closer to the nominal battery voltage.

• During this stage as you use more power from the batteries the unit will adjust the input amps up and down to hold this float voltage.

Pulse-width modulation (PWM), as it applies to motor control, is a way of delivering energy through a succession of pulses rather than a continuously varying (analog) signal. By increasing or decreasing pulse width, the

• MPPT is used to set the operating point at maximum power output • It is programmable controllers with computer interface.

What we want is to maximize power output

and charge the batteries faster ?

Remember PV panel gives Direct Current !

&

We need Alternating Current in many

applications !

Why do we need an inverter ?

Inverters are absolutely essential for operating solar power plants

Inverters are used for conversion of low voltage

Direct Current from the PV modules into high

voltage Alternating Current which is fed into the

Sine wave inverter Square wave Mechanism continually changes current direction

1.

Stand alone inverter

 Used in isolated systems where the inverter draws its DC energy from batteries charged by photovoltaic arrays.

 Unlike grid tie inverters, stand-alone inverters use batteries for storage.  These types of inverters are mostly used in residential buildings in remote

locations which are devoid of the utility grid and is powered by renewable energy sources.

2

. Grid tied inverter

 That converts direct current (DC) electricity into alternating current(AC) and feeds it into an existing electrical grid.

 During a period of overproduction from the generating source, power is routed into the power grid, thereby being sold to the local power

company.

 During insufficient power production, it allows for power to be purchased from the power company

SOLAR

POWER CONDITIONING UNIT (PCU) is used

as control system for grid tie inverter

The only power source is battery

In case of over production power is exported to grid In case of insufficient production power is imported from the grid

Sizing and system specification

 Typical Size - 1W to tens of kW

 Battery back up is essential for operation in monsoon and at night  Long life and low maintenance

 Upgradability is often required

 Loads are combination of DC and AC

Applications

 Remote housing  Water pumping  Telecom

Costing

 Module cost is 30-40% of system cost, battery cost is recurring and appliances cost is often included in system cost.

 System cost is in the range of Rs. 1.25-1.5 Lakhs/kW

Inverter Loss AC Loss Battery Loss DC Loss Charger Loss PV Losses

Loss Analysis

Off-grid System Loss Factors

Factors % Loss PV Response to Insolation 2-4 PV Mismatch 1-3 PV Soiling 1-3 PV Thermal Loss 5-10 DC Cable Loss 1-2 MPPT Charge Controller 1-2 Battery 10-15

Inverter including Transformer 4-6

AC Cable Loss 0.5-1

1. Customer requirement analysis and site survey and

feasibility assessment

2. Determine load, power and energy consumption

3. PV array and battery selections

4. Design PV array and system configuration

5. charge controller and inverter selection

6. Balance of Systems components (checklist)

1. Customer Requirement Analysis

Customer

concerns

Daily load requirements Constraints- Cost and space constraints Type of load 24x7power requirement for critical loads Seasonal load requirement Future load requirement

Site

survey

Latitude,

Longitude

of the site

Location,

shape, size and

level of

roof/terrace

Terrace/roof

orientation

and tilt

Height of

parapet

wall

Shadow

analysis

Static load

bearing

capacity of the

roof/terrace

2.1 Analyze Energy Requirements

 Day and night time loads, power and energy consumption.

 Load duty factor and use hours/day determine energy requirement in Watt-hour/day

 Consider use period during the year and energy requirements over a period of week

 List DC and AC appliances separately

 Consider name plate power ratings as first approximation.

 Monitor appliance currents using clamp current meter, apply 20% factor of safety and multiply by voltage to accurately determine power consumption. (W=1.2xVxA)

Type of Load Rated Power (Watts) No. of Loads Total Rated Power (Watts) Load Duty Factor (0-1) Total Avg. Power (Watts) Hour s of use Total Energy Consumption (Watt-hours/Wh)

Total Rated Power Consumption = Rated Power x No. of Loads Total Avg. Power = Total Rated Power x Load Duty Factor

Total Energy Consumption = Total Avg. Power x Hours of use

2.3 Assess Energy Saving Options

Use energy efficient appliances



CFL/LED, fan regulators



3-5 star ratings appliances

Use alternate energy sources for high power loads



Reduce electrical energy and battery storage requirements

eg. SHWS, gas cook top, gas heaters etc.



Use ‘inverter-ready’ fridges and soft starters for large 3 ph.

appliances,

3. System Concept Development

Decide system configuration

Required Supply DC / AC

Mode

Off-grid / Grid-support Battery requirement Size of battery Charge controller Type of Charge Controller

Inverter

Capacity and type of Inverter PV Capacity PhotoVoltaic technology

 PV technology

c-Si/TF-governed by environment, space, cost etc.

 Mounting system

Rooftop/Terrace mounted/Ground mounted PV array

also determine orientation and tilt of PV array based on

latitude and usage pattern.

Choosing PV technology and

mounting structure

4.1 Battery Sizing

1. Required Supply Wh = Load Wh * (No.of day of storage + 1) 2. Include Efficiency factors from name plate of battery

Depth of Discharge (DOD) Battery efficiency factor (BEF) System AC efficiency (ACEF)

{ACEF = Inverter Efficiency x AC Cable Loss Factor} Battery Wh = Supply Wh / (DOD*BEF*ACEF) Select Battery Voltage (VBAT) based on system voltage

Small systems (<1kWh) are 12VDC

mid-range systems (1-3kWh) are 24VDC larger systems (>3kWh) are > =48 VDC. Battery Ah = Battery Wh/VBAT Select nearest larger rating available

To be on the safe side

4.2 Battery pack

Check available Battery unit voltage and Ah

Select Battery unit Ah such that Battery Pack Ah requirement is either met or exceeded with number of Battery units in parallel

No. of Battery units in parallel = Battery Pack Ah / Battery Unit Ah Battery Pack voltage is integral multiple of Battery unit voltage

No. of Battery units in series = Battery Pack Voltage/Battery unit voltage Example of battery selection –

 Standard deep cycle lead acid battery voltage rating available is 12V  Standard battery Ah available is 120Ah, 150Ah, 180Ah etc.

5.1 PV Sizing

1. Load Wh = Daily energy requirements

2. Average daily peak sun hours (PSH) in design month for selected tilt and orientation of PV array.

3. System Efficiency Factor (SYSEF) SYSEF = DCEF x BEF x ACEF

4. Where DCEF = PV Loss Factors x DC Cable Loss Factor x Charger Efficiency

5. PV Watts peak (Total Wp)= Load Wh/(PSH*SYSEF) 6. Select PV module voltage based on system voltage.

(System voltage is integral multiple of PV module voltage) 7. Select module Wp and size based on available space. 8. No. of PV Modules = Total Wp/Module Wp

5.2 Design of PV Array

1. Integral No. of modules in string = system voltage/module nominal voltage

2. No. of strings in array = Total No. of modules/No. of modules in string

3. Use nearest larger number of strings in an array.

4. List No. of modules in array and Standard Testing Condition Wp rating of array.

5. For use with MPPT charge controller, string voltage needs to match average MPP window of controller.

6.1 Charge Controller Selection

1. Match controller nominal system voltage to PV system voltage 2. Controller input voltage rating >= 1.2 x array open circuit voltage 3. Max. charge current >= 1.25 x array max. power point current. 4. Nom. load current = Max. DC Load Power/System Voltage. 5. Controller output current rating >= 1.5 x nom. load current.

6. Output current overload rating should exceed peak load current. 7. Output overload duration should exceed peak load current duration. 8. Standby power consumption should be minimum.

9. Consider charge profile, power conversion efficiency, environmental specifications, protections and monitoring functions.

6.2 Inverter Selection

•Match inverter DC input voltage to system voltage. •Match inverter AC output voltage to nom. load voltage.

•Inverter output power rating 1.5 to 2 times (min. 1.2 times) max. load power to allow for future expansion.

•Inverter nom. load current = Max. load power/Nom. output voltage

•Ensure max. inverter DC input current does not exceed C5 rate of battery. •Inverter output overload current 3-5 times nom. load current.

•Inverter output overload duration >= peak load current duration. •Select output AC waveform suitable for load.

•Check voltage and current protections. •Consider monitoring functions.

6.3 Select DC Wire and Fuse Ratings

•Imax = Isc (array) – Isc (string)

•I (string cable) >= Imax

•I (string cable) >= 1.25 times Isc (array) when string fuse is not used.

•Minimize total DC cable losses to typ. 2-3% of plant DC power rating.

•Consider twice the cable run from combiner box to Controller.

•String fuses/MCBs used with large size PV generators to avoid module

reverse currents under fault conditions.

•Fuses/MCBs must be rated for DC.

•I (string cable) >= Itrip (string fuse) when string fuse is used.

•Inom (string fuse) >= 1.25 x Inom (string)

Balance of system (checklist)

 DC Wire and Fuse Ratings

 Grounding structure  Mounting structure

 Switches and circuit breakers  Surge and lightning protector

Optimize and summaries Design..!!

1.

Optimize tilt of solar panel

2.

Estimate power and energy output of the plant based on

selected array and battery size and system efficiency

factors.

3.

Review system design, sizing and costs.