5. Solar PV Design

Solar PV system



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  Nickel Cadmium  Lead Acid  Nickel Metal Hydride  Lithium Ion

Li-ion battery

Lead acid battery

Nickel Cadmium battery

Nickel metal hydride 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 safety reasons.

Nickel Cadmium battery  Long life  High discharge rate  Extended temperature range is important. But it contain toxic metals

Nickel metal hydride battery  Higher energy density compared to nickelcadmium  Contain no toxic metals.  Reduced cycle life.

Dominant Energy Storage medium is Lead-Acid batteries (Mostly used in off-grid systems)

Advantages

Disadvantages

Simple and cheap to make

Cannot be stored in discharge condition

Low self discharge rate

Low energy density makes them bulky

Today, 98% of these batteries are recycled

Not environmental friendly

Capable of high discharge rates

Thermal Runway can occur with improper charging

How to define Battery capacity ? 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 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. 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). Cycle life 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.

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).

Comparison of batteries

Operational temperature

-20 to 60°C

-40 to 60°C

-20 to 60°C

BATTERY SIZING PARAMETERS 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

BATTERY SIZING: CASE STUDY • 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

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

Cont… BATTERY SIZING: CASE STUDY • 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 Preventing reverse current

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) - Adjusts the output voltage level to get maximum power output

Why solar charge controller is required !!!

Increases battery life For battery Regulate power

Charge controller

Preventing reverse current Optimize power output

What do you think affects the battery life..?? Overcharging batteries Fluctuating input

How can we overcome this problem..??

Pulse Width Modulation 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.

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.

Stages of PWM controller

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 controller regulates energy flow to the motor shaft

What we want is to maximize power output and charge the batteries faster ?

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

Basic algorithm used in MPPT

Use of MPPT in solar PV system

Why do we need an inverter ? Remember PV panel gives Direct Current ! & We need Alternating Current in many applications !

Inverters are used for conversion of low voltage Direct Current from the PV modules into high voltage Alternating Current which is fed into the power grid

Inverters are absolutely essential for operating solar power plants

Sine wave inverter

Mechanism continually changes current direction

Square wave

Induced to Sine wave

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

Inverter sizing

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

Off-Grid Systems: Scope, Applications and Costs 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

Loss Analysis

PV Losses

Battery Loss

DC Loss Charger Loss

Inverter Loss AC Loss

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

Total System Loss

25-40

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)

7. Impact Assessment

1. Customer Requirement Analysis Daily load requirements ConstraintsCost and space constraints

Future load requirement

Customer concerns Seasonal load requirement

Type of load

24x7power requirement for critical loads

Terrace/roof orientation and tilt

Height of parapet wall

Location, shape, size and level of roof/terrace

Latitude, Longitude of the site

Shadow analysis

Site survey

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 Watthour/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)

Prepare the load chart to analyze total energy consumption Type of Load

Rated Power (Watts)

No. of Loads

Total Rated Power (Watts)

Load Duty Factor (0-1)

Total Avg. Hour Power s of (Watts) use

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

Total Energy Consumption (Watthours/Wh)

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, eg. air conditioners, pump motors, etc.

3. System Concept Development

Decide system configuration Required Supply

DC / AC

Mode

Off-grid / Grid-support

Battery requirement

Charge controller

Size of battery

Type of Charge Controller

Inverter

PV Capacity

Capacity and type of PhotoVoltaic technology Inverter

Choosing PV technology and mounting structure  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.

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 (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.  Example: 24V/350Ah Battery Pack

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 9. Use nearest larger number of modules

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) •2xIsc (string) > Itrip (string fuse) > Isc (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.

4.

Summarize design.