EEE-488

Renewable and Alternate

Energy Systems
Dr. Rabiah Badar
Assistant Professor, Office no. 323,
Electrical & Computer Engineering Department,
COMSATS University Islamabad.
rabiah.badar@comsats.edu.pk
Objectives
 To get familiarized with relevant concepts and constituents of solar PV
power plant
 To design application specific solar PV power plants
 Design and develop solutions involving primary renewable energy technologies.
(C5-PLO3)
Solar Modules and Arrays
Modules come in a variety of sizes, types, and ratings. The performance of PV
modules are usually rated according to their maximum dc power output (watts)
under Standard Test Conditions (STC). The specific output depends on the size
and the internal wiring of the module.

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Solar Modules and Arrays
One type of module can blend in with roof shingles.
Each module is rated for 17 W.

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PV Cell Characteristics and Parameters
The I-V characteristic for a solar cell is essentially constant over a range of
output voltages for a specified incident light energy.

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Equivalent Electrical Circuit
 ‘IL’ is light generated current, ‘ID’ is diode current and ‘ISH’ is shunt leakage
current to ground
 ‘Rs’ is series resistance and depends on PN junction depth, impurities and
contact resistance
 ‘Rs’ and ‘RSH’ typically 0.05 – 0.10 Ω and 200 – 300 Ω resp.
 Small increase in ‘Rs’ leads to a large drop in output power but changes in
‘RSH’ doesn’t effect conversion efficiency
Equivalent Electrical Circuit (Contd…)
 Diode current given by
𝑄𝑉𝑜𝑐
𝐼𝑑 = 𝐼𝐷 𝑒 𝐴𝑘𝑇 −1
 Where, ‘ID’ is diode saturation current, ‘Q’ is charge on
electron (1.6022e-19 C), ‘A’ is the ideality factor, ‘k’ is
Boltzmann constant (1.38e-23 J/K) and ‘T’ is temperature on
absolute scale.
 Load current given by
𝑄𝑉𝑜𝑐 𝑉𝑜𝑐
𝐼= 𝐼𝐿 − 𝐼𝐷 𝑒 𝐴𝑘𝑇 −1 −
𝑅𝑆𝐻
𝑉𝑜𝑐
 Term ‘ ’ generally ignored in practical cells
𝑅𝑆𝐻

 ‘𝐼𝐷 ’ determined practically by applying ‘𝑉𝑜𝑐 ’ to cell in dark at

measuring current flowing into the cell
 Also termed dark current or reverse diode saturation current
𝑉𝑜𝑐 𝑎𝑛𝑑 𝐼𝑠𝑐
 Short circuit current measured by shorting output terminals under full
illumination conditions
 Open circuit voltage (at I = zero) ignoring ‘ISH’ given by

𝐴𝑘𝑇 𝐼𝐿
𝑉𝑜𝑐 = 𝑙𝑜𝑔𝑛 +1
𝑄 𝐼𝐷
𝑘𝑇
 ‘ 𝑄 ’ is 0.026 V at 300 K

 ‘𝐼𝐿 ’ is many times ‘𝐼𝐷 ’, therefore, ‘𝑉𝑜𝑐 ’ is many times 0.026 V

 Cell operated slightly to
the left of max. power
point and modelled as At knee point
constant current source in
electrical analysis
 PV Cell Characteristics and Parameters

Maximum power occurs on the “knee” of the I-V curve.

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 PV Cell Characteristics and Parameters

Current from a cell is proportional to the irradiance.

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I-V curve for PV
 PV Cell Characteristics and Parameters

 PV cells also have a temperature dependence. Increasing temperature

decreases the band gap and decreases the open-circuit voltage. Current changes
only slightly with temperature.

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 Energy Conversion Efficiency

𝑃𝑜𝑢𝑡(𝑚𝑎𝑥)
% 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = × 100
𝐸×𝐴

𝑃𝑜𝑢𝑡(𝑚𝑎𝑥) is the maximum electrical power output of cell, in watts (W)

𝐸 is the irradiance (light energy) at the surface of the cell, in watts/ meter2 (W/m2)

𝐴 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑒𝑙𝑙 , 𝑖𝑛 𝑚𝑒𝑡𝑒𝑟2 (𝑚2)

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 PV Cell Characteristics and Parameters

The fill factor is the ratio of the cell's actual maximum

power output (VMPP x IMPP) to its theoretical power output
(VOC x ISC).
FF = (VMPP)(IMPP) / (VOC)(ISC)

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Example 1

A certain PV cell is illuminated with as irradiance (E) of 1000 W/m2. If the cell is
100 mm x 100mm in size and produces 3 A at 0.5 V at the maximum power point,
what is the conversion efficiency?

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Example 2

Six series of 12 PV cells are connected in parallel. Assume each PV cell produces a
current of 1.5 A and 0.5 V at the maximum power point. Determine the output
current and power to a load under maximum power point conditions.

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Example 3 Configuration
A certain installation requires 36 V and at least 1 kW of
rated power at maximum output. Describe the
configuration using 36 cell module where each module
produces 18 V and 5A.
Solution

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Air Mass

 Represent conditions in which solar cell is operated

 AM0 represents condition of vacuum/space where max. insolation is present
(1350 W/m2)
 AM1 represents condition of sunlight normal to cell surface located in air with
no pollutants and at a dry afternoon
 AM1.5 represents condition of average quality air with average humidity and
pollution at an average inclination (1000 W/m2)
 AM4 in northern regions with solar irradiation at 15o from horizon
 Solar Module Data Sheet Parameters

Mechanical data includes physical characteristics of

the module. A sample of mechanical specifications
are:
Mechanical Data
Solar cells 72 monocrystalline
Front glass High transmission tempered
Junction box IP-65 with 3 bypass diodes
Dimentions 32 X 155 X 28 mm
Output cables 1000 mm length/ MC-4 connectors
Frame Anodized aluminum
Weight 33.1 lbs (15.0 kg)

In addition, a drawing of the module will be given

with dimensions.
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Example Specification of a 22 W Solar PV Cell
 Solar Module Data Sheet Parameters

Solar module data sheets are divided into several

sections. A sample of electrical specifications are:

Electrical Data
Peak power Pmax 215 W
Rated voltage Vmpp 39.8 V
Rated current Impp 5.40 A
Open circuit voltage VOC 48.3 V
Short circuit current ISC 5.80 A
Series fuse rating 15 A

In addition, temperature coefficients are included.

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 Example 4

Refer to the data sheet in slide #24 and determine the fill factor of the
solar cell.

Fill Factor = 0.767

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 Solar Module Data Sheet Parameters

The I-V curve as a function of irradiance and

temperature will be given. For example, for a module,
the I-V curve may look like the following:

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I-V curve for PV: Example
 Sun Intensity

Variation in Efficiency

Isc falls sharply with intensity

Voc relatively less affected by variations in intensity

Variation in Illumination
Temperature Effects
 Short circuit current and open circuit voltage given by
𝐼𝑠𝑐 = 𝐼𝑜 1 + 𝛼∆𝑇
𝑉𝑜𝑐 = 𝑉𝑜 1 − 𝛽∆𝑇
 Where, Io and Vo are values of short circuit current and open circuit voltage at
reference temperature
 ‘𝛼’ and ‘𝛽’ are resp. temperature coefficients
 New power, 𝑃 = 𝑉𝑜𝑐 𝐼𝑠𝑐 = 𝑉𝑜 𝐼𝑜 1 + 𝛼∆𝑇 1 − 𝛽∆𝑇

 Ignoring ‘α𝛽 ∆𝑇 2 ’ term, we get

𝑃 = 𝑃𝑜 [1 + 𝛼 − 𝛽 ∆𝑇]
 Decrease in Voc higher than
increase in Isc, therefore, cell
output power decreases with
temperature increase
 Module should be designed to adjust its
output voltage with changing temperature
to get max. power output at any given
temperature

V1 V2
Effect of Climate
 Cell can produce 80% of full sun power in partly cloudy conditions
 Can produce 30% of full sun power in extremely cloudy conditions
 Snow doesn’t collect on module as it is angled to receive sunlight
 Melts if it stays on surface for some reason
 Cell designed to withstand golf-ball sized hail
Sun Tracking
 One-axis tracker follows sun from east to west during the day
 Two axis tracker follows sun from east to west during the day and from north
to south during the seasons
 Can increase energy yield by 40% over entire year
 Sun hunting – in earlier designs, if sun was obscured by cloud the tracker
aimed at next brightest object which was cloud lining
 Tracks sun once again when cloud moves away
 Eliminated in newer designs
Peak Power Operation
 Array operating at any point at voltage ‘V’ and current ‘I’ in VI curve extracts
power given by, P=VI
 If the operating point is perturbed then new power given by:
𝑃𝑛𝑒𝑤 = 𝑉 + ∆𝑉 𝐼 + ∆𝐼 = 𝑉𝐼 + 𝑉. ∆𝐼 + ∆𝑉. 𝐼 + ∆𝑉. ∆𝐼 ≈ 𝑃 + ∆𝑃
𝑊ℎ𝑒𝑟𝑒, ∆𝑃 = 𝑉. ∆𝐼 + ∆𝑉. 𝐼
 ∆𝑃 is zero at max. power point, therefore, condition for max. power point is given
by setting ∆𝑃 = 0
∆𝑉 𝑉
⇒ 𝑉. ∆𝐼 + ∆𝑉. 𝐼 = 0 ⇒ =−
∆𝐼 𝐼
 In the limit when increment step approaches zero the above equation can be
𝑑𝑉 𝑉 𝑑𝑉 𝑉
written as 𝑑𝐼 = − 𝐼 , where, ‘ 𝑑𝐼 ’ is dynamic source impedance, ‘Zd’ and ‘ 𝐼 ’ is static
impedance, ‘Zs’.
Condition for Max. Power Extraction
Dynamic Impedance, Zd = – Static Impedance, Zs
MPPT Method 1
• Inject small signal current periodically in array bus and measure
dynamic and static impedances
• Increase or decrease operating voltage until dynamic impedance = –
static impedance
MPPT Method 2
• Increase operating voltage as long as dP/dV is positive
• Decrease operating voltage if dP/dV is negative
• When dP/dV is zero or in dead-band then the operating voltage is unchanged