Photovoltaic Systems Engineering - Personal Datakarimpor.profcms.um.ac.ir/imagesm/354/stories/pv... · Photovoltaic systems engineering 2 0 1 6 Ali Karimpour & Reza Bakhshi 2016 24

Photovoltaic systems engineering 2

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Ali Karimpour & Reza Bakhshi 2016

Photovoltaic Systems

Engineering

Ali Karimpour

Associate Professor

Ferdowsi University of Mashhad

Reza Bakhshi

Ph.D. candidate

Ferdowsi University of Mashhad


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Lecture 8

Off-grid components (part 2)


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Off-grid photovoltaic systems

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Charge controller

+ - + - + -

Battery package

Off grid inverter

AC loads

PV array

DC loads

Charge controller: the charge and discharge of battery package.

Battery package: compensates the deficit/surplus generated power

Off grid inverter: converts DC power to the AC type for AC load consumption.

We want to know the detailed characteristics of mentioned elements.


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Energy Storage technologies

The Lead-Acid Storage Battery

The Nickel Cadmium Storage Battery

Other Battery Systems

Batteries

Hydrogen Storage

The Fuel Cell

Potential energy stored, Compressed air, flywheels,

Superconducting magnets, etc.


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The oldest rechargeable battery that was invented in 1859 by French

physicist Gaston Planté.

The input electrical energy can be stored as chemical energy and is then

converted to electrical energy as when required.

It has the ability to supply high surge currents; therefore, these cells have a

relatively large power-to-weight ratio.

Its low cost as well as providing high surge currents makes it attractive for

use in automobile starter motors.


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Chemistry

Charging process

Discharging process

Over charging Gassing

Deep discharging is not ok since cell

performance may be affected.

Sometimes it is ok.



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Lead-acid battery is still the most common for relatively economical storage of

electrical energy.



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Properties

Charging efficiency is around 95%, Discharging efficiency is around

95%, Overall efficiency is around 90%

High charging rate or discharging rate leads to lower efficiency.

The amount of energy in a battery is commonly measured in Ah and shownby C.

Charging rate or discharging rate is shown by C/x.



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Higher discharge rate (deeper discharge) less charge being available as

energy to a load.

Higher charge rate it takes more energy to fully charged.

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Properties



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Effects of discharge rates and temperature on the relative amount of charge

that a battery can deliver.

Properties


Higher capacity can be reached in high

temperatures in a given discharge rate

Lower capacity can be achieved in high

temperatures when energy is stored for future


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Properties


The number of battery's charge and discharge cycles (expected life) highly

depends to how much capacity of battery is discharged:

Trade off between: Battery numbers and deep of discharge

Deep discharge: discharging the battery fully before recharging

Shallow discharge: partially discharging then recharging


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The Nickel Cadmium Storage Battery

Nickel hydroxide for the cathode plates.

Cadmium oxide for the anode plates.

Potassium hydroxide for the electrolyte.

Charged cell has 1.29 Volts.

Was created by Waldemar Jungner of Sweden in 1899

Chemistry


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NiCd can survive freezing and high temperature.

NiCd can fully discharged.

NiCd has been affected less by overcharging.

NiCd Can be discharged at the rates up to C over a wide range of temperature

range, while still providing more than 90% of its capacity.

NiCd has very low internal resistance.

Elimination of charge controller

NiCd batteries last twice as long as its lead-acid counterpart (25 year good

condition to 8 years in bad).

Nickel Cadmium vs. Lead-acid

It is hard to determine state of charge in NiCd.

Both used toxic material.

Lead-acid is cheaper compared to NiCd.

Lead-acid can supply high discharge current and has low self discharge.


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Battery datasheet


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How a battery has been charged?

Different modes of battery charging

Bulk mode

The maximum available current descends to the battery package, hence its

voltage rises. This state happens usually in the morning and continues until

battery voltage reaches to the bulk voltage.


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Absorption/equalization mode

When the battery voltage reaches to the bulk value, the charge controller keeps

this voltage fixed; however, it lowers the current flowing to the battery.


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Float mode

The charge controller keeps battery charged in this mode. If the PV array

production was not enough, the battery will not be in float mode.


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Off-grid inverter

Off-grid inverter converts DC power to the AC type.

Different power sources such as solar, wind, etc. can be

connected to most off-grid inverters.

They should be able to produce reactive power for some loads.

Can be worked as charge controller (for some inverters)

Can work properly when battery voltage tolerates

High efficiency even in low loading


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Off-grid inverter performance

The off-grid inverter performance, as same as on grid inverters, depends on the

input voltage and its temperature.


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Off-grid inverter datasheet

Most inverters can work with different sources even AC types such as diesel

generator! So, both input DC and AC parts should be determined.

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Off-grid inverter datasheet (continue)

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Exercise

To the load

8.1. Based on the schematic of off-grid system,

find required energy that PV array should

produce for two following cases. Suppose

charge controller and inverter efficiencies are

97.5% and 95%. Also, the charge and

discharge battery efficiencies are 90%.

a) DC load power is 500 W for 5 hours. AC

load power 500 W for 5 hours.

b) DC load power is 500 W for 1 hours. AC

load power 500 W for 9 hours.


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Exercise

8.2. Suppose the peak sun hours of the system location is 4.25 h.

Find the PV array nominal power for two cases mentioned in

question 8.1?

8.3. Determine the capacity of 12 Volts batteries required to supply

load, mentioned in question 8.1 (for two cases)?

8.4. Consider the performance of the following battery vs. its

discharge time. Find the discharge energy of battery (VAh=Wh) in

terms of discharging time (Wh vs. discharge time)? Plot these data.


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References

1. Roger Messenger, Jerry Ventre, “Photovoltaic Systems

Engineering”, 3rd edition, CRC press, 2010.

2. Frank Jackson, “Planning and Installing Photovoltaic

Systems”, 2nd edition, Berlin, 2007.

3. www.sma.de

Documents

Photovoltaic Systems Engineering - Personal Datakarimpor.profcms.um.ac.ir/imagesm/354/stories/pv... · Photovoltaic systems engineering 2 0 1 6 Ali Karimpour & Reza Bakhshi 2016 24