Solar PV Explained

Ministry of Energy and Mineral Resources

Solar PV Explained Solar PV on Grid Training, 29-31 October 2013

Todo Simarmata

10/22/2013

Contents

1 PV Cell ....................................................................................................................................... 2

1.1 Energy of photon ............................................................................................................... 2

1.1.1 The working principles of crystalline silicon PV cells.................................................... 2

1.1.2 Current Voltage (IV) characteristic of PV Cell .............................................................. 3

1.1.3 Irradiation and photo current ..................................................................................... 5

1.1.4 Output power and temperature ................................................................................. 6

2 PV Module................................................................................................................................. 8

2.1 PV Module Structure .......................................................................................................... 8

2.1.1 Low iron glass............................................................................................................. 8

2.1.2 Ethyl Vinyl Acetate (EVA) ............................................................................................ 8

2.1.3 Tedlar......................................................................................................................... 9

2.1.4 Frame......................................................................................................................... 9

2.1.5 PV Cells String............................................................................................................. 9

2.1.6 Bypass Diode............................................................................................................ 10

3 Battery .................................................................................................................................... 13

3.1 Vented Lead Acid ............................................................................................................. 13

3.1.1 Flat Pasted Plate Type .............................................................................................. 14

3.1.2 Tubular Type ............................................................................................................ 15

3.1.3 Valve Regulated Lead Acid........................................................................................ 16

3.1.4 Lithium Ion ............................................................................................................... 18

4 Optimum Energy Yield ............................................................................................................. 20

4.1 PV Module Tilt and Orientation ........................................................................................ 20

4.2 Performance Ratio ........................................................................................................... 20

4.2.1 Actual PV production................................................................................................ 21

4.2.2 Reference Yield ........................................................................................................ 21

4.2.3 Performance Ratio.................................................................................................... 21

5 Calculating Solar Energy........................................................................................................... 23

5.1 What is solar energy?....................................................................................................... 23

5.2 Calculating solar irradiance .............................................................................................. 23

6 Bibliography ............................................................................................................................ 27

1 PV Cell Author : Todo Simarmata

1.1 Energy of photon

PV cell conversion is a direct conversion from sunlight which contains of quasi particles namely

photon into electric energy. This photon energy formulated as (Creatore, 2010):

(1.1)

Where :

Eph = photon energy

h = Plancks constant = 6.626 10 -34 joules

v = the frequency of light

c = the velocity of light = 2.998 108 m/s

= the wavelength of light

Figure 1-1-1 Solar spectrum (Creatore, 2010)

As shown in above figure, sunlight has solar spectrum with wavelengths =0.39 m (violet) until

=0.77 m(red). So fill-in the constant in the equation 1.1, the largest energy of photons of visible

sunlight is 5.09e-19 Joules or 3.1790 eV (violet) and the smallest 2.58e-19 Joules or 1.6101 eV (red).

1.1.1 The working principles of crystalline silicon PV cells

In principle, a crystalline silicon solar cell is a semiconductor junction which has two regions. The p-

type region is created by doping with boron and the n-type with phosphorus. The difference of these

two region properties creates an electric field near the boundary (Creatore, 2010).

Figure 1-1-2 Solar cell structure (Creatore, 2010)

Typical dimension of crystalline wafer in the market is 125x125 mm for mono crystalline with 0.3

mm thickness and 156x156 mm for multi crystalline. There is an array of metal grid and crossways

the busbar. Usually consists of two or three busbars.

When a ray of light descent the surface of the n - type region, photons with energy Eph greater than

the semiconductor band gap Eg will absorbed and generate an electron- hole pair. The

semiconductor band gap is a minimum amount energy that is required to excite an electron from its

bound state to participate in the conduction.

An electron - hole pair created near the junction will experience strong electric field and be

separated, eventually the electrons reach the n-type area and collected by the contact grid. Current

generation occurs when electrons collected at the contact grid flow through busbar to the back

contact via the load as depicted in figure 1.2.

1.1.2 Current Voltage (IV) characteristic of PV Cell

When a PV cell exposed to sunlight radiation, a photo current Iph is generated. This photo current is

directly proportional to incident sunlight radiation to the PV cell surface. When there is no radiation

in dark condition, PV cells works as a P-N junction diode.

Thus, the most common model of solar PV consists of a current source to represent photo current

parallel with the diode. The complete model is adding series resistance Rs to represent the internal

losses and shunt resistance Rsh to represent leakage current to the ground. The simple model can be

seen in figure 1.3

Figure 1-1-3 Solar cell model (Creatore, 2010)

Current flow to the load I can be formulated as :

(1.2)

(1.3)

Where,

Id = diode current

Is = the reverse saturation current of the diode

q = electron charge

K = Boltzmans constant.

Current-voltage characteristic can be developed by measuring solar PV response to the variable load

or current. Thus the response will be plotted from zero current which relevant to infinite resistance

or open circuit condition to the maximum current which means zero resistance or short circuit

condition.

Therefore two specific parameters can be formulated as below:

1. When the load R=0, the PV cell is in a short circuit condition, there is no current flow through

the diode ( Id=0 ), so I = Iph or

(1.4)

2. When the load increasing to infinite R = , PV cell in an open circuit condition, there is no

current flow through the load ( I=0) and all current flow through the diode, so Iph = Id and V

open circuit can be formulated as :

(1.5)

Figure 1-4 Current-Voltage characteristic

The current-voltage characteristic of solar PV is depicted in figure 3.4. There exists a maximum

power point which is characterized by Imp and Vmp. Maximum power point is the maximum power

that could be delivered through resistive load and can be formulated as:

(1.6)

The ratio between maximum power point and the product of Voc and Isc is defined as Fill Factor:

(1.7)

The fill factor is a value to quantify the real I-V characteristic. A typical fill factor value of a good cell

is above 0.7. Fill factor value is temperature dependent, as the temperature increased, the fill factor

will diminish.

Efficiency is defined as the ratio between the maximum power that PV cell could be harnessed and

the incident irradiation to the PV surface. Efficiency can be formulated as:

(1.8)

Ga is sunlight irradiation in W/m2 and A is PV cell area.

1.1.3 Irradiation and photo current

The open circuit voltage as in equation 1.5, is a function of the sunlight irradiation. It increases

logarithmically with photo current Iph which directly proportional to ambient irradiation. The short

circuit current as well as photo current is a linear function of the sunlight irradiation. Current voltage

characteristic for different irradiation and temperature is depicted in figure 1.5. The short circuit

current is slightly increased at higher temperature 50O C compare to 25O C at the same irradiation

1000 W/m2 while open circuit voltage decreases. However the decrease in open circuit eventually

influences overall efficiency and output power.

Figure 1-1-5 Current-Voltage characteristic for different irradiation and temperature (Longatt, 2005)

1.1.4 Output power and temperature

Output power voltage characteristic of PV module 60 Wp at temperature 0O C, 25O C, 50O C and 75O

C is depicted in figure 1.6. Output power at 25O C is 60 W, while at 50O C the output power decreases

10% become 54 W. This value is equal to 0.4% decreasing in output power for every OC increases in

temperature..

Figure 1-1-6 Power-Voltage characteristic for different temperature (Salmi, 2012)

There are two types that are mostly used in commercial PV modules for nowadays: Crystalline

Silicon usually abbreviate as c-Si and Thin Film. The first type is usually called the first generation; it

consists of monocrystalline (c-Si) and multicrystalline (mc-Si) PV. In 2010, crystalline or usually called

wafer technology contributed for about 86% of global PV production (Ardani & Margolis, 2011)

Figure 1-1-7 Multi crystalline PV cell, with 2 bus wires: front (left side) and rear (right side) surface (Boxwell, 2012)

The second type is called the second generation, this is a PV cell made from extremely thin

semiconductor material to reduce silicon material use. Material thickness is only few microns, which

is made from a-Si (amorphous Silicon), CIS (Copper-Indium-Diselenide), CdTe (Cadmium Telluride) or

CIGS. Other paths in second generation solar PV technology development are using multi junction

cells contain material from group III and V compounds includes Ga, In, Al, As, P and Sb.

The third generation goal is characterized by achieving a very low cost with moderate efficiency

10%-15% or relatively expensive with very high efficiency above 30%. Numerous new technologies

still in the development and could become viable on the commercial market in the future. These

third types include dye-sensitized, plastic PV cells and quantum dots (Creatore, 2010). However, the

first and the second generation have already achieved a mature market. The second generation as

depicted in figure 3.8 shows exponential growth following the first generation.

Figure 1-8 Production capacity Crystalline silicon and Thin films (Waldau, 2008)

2 PV Module Author : Todo Simarmata

PV module is designed for the outdoor environment, thus the design should be well protected to

withstand an outdoor environment, i.e. wind, exposure to UV, humidity, rain, and high temperature

exposure. PV module usually has the common structure that contains five layers as depicted in

figure 2.1.

2.1 PV Module Structure

The most common PV module layer consists of five layers and module frame. Each of the layer

structure is discussed in this part.

2.1.1 Low iron glass

The most common material chosen for the front layer is glass with low tempered or low iron

content. Low iron glass is chosen to fulfil those requirements because of several reasons including:

1. High transmission of sun irradiation, low absorption coefficient and leads to increased

conversion efficiency.

2. Low cost

3. Acceptable strength

4. Stable in outdoor exposure

5. Impenetrable with water (hermetic characteristic)

6. Electrical insulation from surrounding environment

2.1.2 Ethyl Vinyl Acetate (EVA)

The second and fourth layer performs as an encapsulation. It is used to provide bonding between PV

cells. The most common material that is used as an encapsulation is Ethyl Vinyl Acetate (EVA). The

EVA sheet is inserted on the top and rear surfaces of PV cells. Using PV laminated machines, this

sandwich is vacuumed and heated rapidly to 150OC (Creatore, 2010). The process then polymerizes

the EVA and bonded the PV cells together.

Figure 2-1 PV module layer configuration (Creatore, 2010)

2.1.3 Tedlar

The last layer is the back cover. The most common material used for this rear surface is Tedlar.

Tedlar is chosen for several reasons. The first reason is Tedlar can meet the performance required

including a low thermal resistivity and impenetrable by the water and vapour. The second reason is

the availability and the third tedlar is considered as a low cost material.

2.1.4 Frame

The most important feature required for the frame is toughness, light weight, and impenetrable by

the water which may damage the PV cells. Usually aluminium is used as the frame material.

2.1.5 PV Cells String

Typically a crystalline solar PV voltage is roughly equal to 0.5 Volt when it works on the maximum

power point. This voltage is not adequate for most applications. Most practical applications usually

need higher voltage, as for example lead acid battery available on the market uses nominal voltage

of 12 Volt. This is one reason to design a PV module that contains 36 cells in series.

PV module using 36 cells in series produces approximately the maximum power point voltage equal

to 17 to 18 Volt and open circuit voltage roughly equal to 21 to 22 Volt. Thus this voltage is adequate

to charge a lead acid battery with 12 Volt nominal voltages. Figure 2.2 shows how the connection is

made. The top contact is connected using tab wire to the back contact.

+

-

Figure 2-1 PV cells connected in series

However, series connection has the consequence that bad quality and performance in one cell yields

to the bad performance of the whole cell. The same consequences may occur when a shade

obscures one cell. This series connection is usually named as strings of cells.

2.1.6 Bypass Diode

Long string arrangements of crystalline silicon PV cells raise many problems. The most important

problem that should be addressed is when a single cell is exposed to a shade or obscured from the

sun irradiation. Shade may cause by a tree branch, building, dust or cloud. Shading on a single cell

may cause a big decrease in the photo current as it is proportional to the decrease of sun irradiation

on the shaded surface (Boxwell, 2012) . The photo current may decrease to 60% of the other cells.

According to standard IEC 61215 1, bypass diode and hot spot endurance are included in test

requirements to meet the pass criteria. Bypass diode and hot spot endurance test are specified in

IEC 61215-10-9. Usually bypass diodes are included as an integrated component in junction box as

can be seen in figure 2.3. This junction box placed at the rear surface of PV module.

Figure 2-2 Bypass diode integrated in junction box

2.1.6.1 Bypass diode as hotspot protection

Shadow in one cell result to decrease the sun irradiation and decrease the photo current. Decrease

in photo current is not only caused by partial or totally shaded but also by mismatch of short circuit

current, bad and damaged cells.

When the photo current of the bad cell decreases, then the overall current of the un-shaded cell is

restrained by the bad cell or cell with the lowest current.

1 shaded cell8 unshaded cell

+ -

Figure 2-4 Shaded cell in the string

1 IEC 612215 is an International Electrotechnical Commision type test and design qualification for Crystalline

Silicon PV module.

The un-shaded cell then produced more forward biased current. In short circuit condition or when

the cell string operated close to the short circuit current, these excess current then reverse biased

the shaded cell. Thus the shaded cell performs like an additional load dissipating heat for the other 8

un-shaded cells (Herrmann, Wiesner, & Vaaen, 1997).

The more un-shaded cell compares to shaded cell as depicted in figure 2.4, then the more heat

dissipated in the shaded cell. In the severe condition, hot spot phenomena could damage the PV

module, break the glass, crack the PV cells, failure in solder bonding, and even burn the whole

module.

Therefore bypass diode is a very important part in the field to ensure long term reliability. The

concept is to bypass the reverse current thus limit the dissipated energy in the form of heat.

shaded cell in substringunshaded sub string

+- D1 - reverse biased D2 - forward biased

- + -+

Figure 2-5 Bypass diode and shaded cell in the substring

Figure 2.5 depicts the concept of bypass diode. The eight PV cell string divided in two substrings.

There is one bypass diode for each string consists of 4 PV cells. Bypass diode placed in anti-parallel

with PV cells. In un-shaded substring, the bypass diode D1 reversed biased the current, thus takes no

effect on the flow of current.

If one cell in a substring is shaded, then the bypass diode D2 is forward biased. The excess current

generated from the other three cells instead of seven cells then circulated through the bypass diode

D2 and enable the other substring to produce the photo current. Therefore the use of bypass diode

has two reasons. The first reason is to minimize the hot spot phenomena that may damage the PV

module. The second reason is to maximize the energy yield of PV module.

The most common bypass diode configuration is placed in parallel for each 18 substring cells of 36

cells PV module. Therefore the excess current from un-shaded cells in the substring is dissipated and

circulated in bypass diode. PV module losses half of its output if one or more PV cells of one

substring is totally shaded.

2.1.6.2 Bypass diode to optimize output yield

The optimum number of bypass diodes can be designed to achieve optimum PV module output yield

in the shading condition. Using one bypass diode for one PV cell is likely the best solution but it is not

correct. In the real field, shade does not obstruct only one cell, but usually more PV cells are shaded.

In practice, one bypass diode is usually used for every 20 PV cells (Guo, Walsh, Aberle, & Peters,

2011). Therefore a normal 36 cells of PV module uses 2 bypass diodes.

2.1.6.3 Blocking Diode

In the PV system power generation with the battery bank, it is important to make sure there is no

reverse flow of current from the battery bank. In this way, the blocking diode performs as a one way

valve or a non-return valve. Basically a blocking diode has two main purposes:

1. To prevent reverse flow of current from the battery bank at the night or when there is no

photo current generated by the PV array.

2. In the PV array, blocking diode avoid current flow from un-shaded PV module string to

shaded PV module string in parallel PV module connection.

-

+

Bypass

Diode

Blocking

Diode

Junc tion

box

Bypass

Diode

Junc tion

box

CombinerBox

Figure 2-6 Typical bypass diode and blocking diode configuration in a PV array.

Typical bypass diode and blocking diode arrangement in PV arrays with series and parallel

connection is depicted in figure 2.6.

3 Battery Author : Todo Simarmata

There are three types of battery commonly used in PV system application, including Vented Lead

Acid (VLA), Valve Regulated Lead Acid and Lithium Ion.

3.1 Vented Lead Acid

Vented Lead Acid (VLA) or usually named as flooded lead acid batteries are the battery most used in

the automotive application. In the car and motorcycle, the VLA battery is used to start the engine.

The design is to provide short cycle and high current to drive the motor starter. The basic design has

not been changed since its invention in the mid 1800s (Exide Management and Technology

Company, 2002).

Therefore the VLA battery for automotive application is designed not to discharge more than 20% of

its nominal capacity. Thin plates and the relatively large surface area is used in order to decrease

material cost while achieving a high amount of current. The term Lead Acid refers to the material

used in the battery. Lead or Pb is used as the anode and cathode cell plate. The cathode plate is

composed of active material of PbO2 (Lead Dioxide) and the anode plate is made of Pb (sponge lead).

A mat separator which performs as an insulator is placed between anode and cathode plate. Sulfuric

acid H2SO4 with a 33% solution is used as an electrolyte liquid.

Figure 3-1 Electrochemistry process charge (left) and discharge (right) simplified (Woodbank Communications Ltd, 2005)

Electro chemistry process in lead acid battery is depicted in figure 3.1. Reaction at the cathode

electrode can be written as (D.Berndt, 2001):

Discharge

Charge [1.685 Volt] (3.1)

Reaction at anode can be written as :

Discharge


Overall reaction can be written as:

Discharge


Therefore a single cell of lead acid battery only generates a maximum 2.041 Volt at open circuit.

Instead of using light cycle battery, PV system application is designed to have the ability to discharge

between 50% until 80%. Usually this depth cycle performance can be achieved by using thicker

anode plate by adding more antimony (Sb) to the lead alloy and thicker separator.

Lead-acid batteries were chosen for PV system application due to their lower price (almost half price

compared to Li-ion). The most common design is Flat plate type and Tubular plate or OPzS type

(Exide Management and Technology Company, 2002).

3.1.1 Flat Pasted Plate Type

Back to 1881, Faure and Volckmar first invented the flat plate type (Rusch, Vassallo, & Hart, 2006).

Flat plate basically is made from pure lead. The positive plate or lead dioxide PbO2 and sponge lead

Pb depicted in figure 3.2.

Figure 3-2 Plante Plate (Exide Management and Technology Company, 2002)

The grid is cast using pure lead material and applied with wet paste. Then the pasted plate is cured

and dried. A separator with micro porous usually made from PVC, polyethylene or synthetic material

inserted between positive and negative electrode. The separator performs as insulation between

electrodes to prevent short circuit. Complete configuration of the flat plate lead acid battery is

depicted in figure 3.3.

Figure 3-3 Cell construction (Woodbank Communications Ltd, 2005)

3.1.2 Tubular Type

Tubular plates were first invented in 1910. Basically they used the same active material with the flat

pasted type. The main difference with the flat plate is in the circular plate. The original design of the

tube was made of hard rubber.

As depicted in figure 3.4, the positive plate composed by lead alloy spines enriched with antimony or

calcium. Furthermore the alloy spines inserted into tubes made from resin treated gauntlet or

synthetic fibre. The tube is then filled with active material lead dioxide (Exide Management and

Technology Company, 2002).

Figure 3-4 Tubular Plate (Exide Management and Technology Company, 2002)

Typically, VLA with tubular plates is preferred than pasted plates for frequent cycle charge discharge

application as in PV systems. The tubular plate is preferred due to longer service life. An endurance

test experiment for both battery types showed that pasted plate type can withstand 1200 cycles

while tubular plate could reach 1800 cycles without loss of their capacity. The test was done

according to IEC 60 896-1 at 80% depth of discharge. The experiment also showed that accelerated

lifetime of tubular plate at 25O C is 27.5 years. This is longer than the life time of a pasted plate (21.3

years) (Rusch, Vassallo, & Hart, 2006).

Furthermore, the cycle and lifetime of battery depends on temperature, discharge rate and depth of

discharge (DoD) percentage2. Figure 3.5 shows the typical relation between the number of cycles

and depth of discharge percentage for the tubular plate battery. The number of cycles is higher for

the lower depth of discharge percentage as at 20% DoD the cycles is 8,000 while at 80% DoD the

cycles is about 1,700.

Figure 3-5 Typical VLA tubular plate (OPzS) number of cycles and depth of discharge (Hoppecke Batterien GmbH &

Co.KG, 2012)

3.1.3 Valve Regulated Lead Acid

Valve regulated lead Acid (VRLA) batteries were chosen for the PV system application because they

do not produce acid spillage, they have higher power density, lower maintenance requirements, no

water addition required and they have less self-discharge values compared to flooded lead-acid

batteries. Nevertheless cleaning and monitoring is needed to ensure any failure possibility.

The VRLA batteries perform the same electro chemical reaction with the same electrode and active

materials with the VLA. The main difference with the VLA batteries is that the electrolyte

immobilized. The immobilized process can be achieved by two methods (Exide Management and

Technology Company, 2002) :

1. Penetrated into an adsorbent fiberglass or mat (AGM type)

2. Transform into Gel by addition of Silica dioxide (Gel type)

Valve regulated batteries also known as Sealed Lead Acid, regarding the sealed construction instead

of vented. Included in this family are AGM and GEL Cell battery for the flat plate type and OPzV for

tubular gel type. AGM and Gel type most commonly used in a smaller PV application i.e. solar home

system and street lighting (Carolyn Roos, 2009).

A typical characteristic of AGM type battery cycle life versus depth of discharge from a manufacturer

in Indonesia is depicted in figure 3.6. The figure shows that the cycle life for 80% depth of discharge

is about 500 cycles and 1,750 cycles at 20% depth of discharge.

2 Depth of Discharge (DoD) is a method to describe the battery state of charge. 100% discharge stands for

empty battery and 0% discharge stands for fully charge condition.

Figure 3-6 Typical cycles number versus depth of discharge of AGM pasted plate (PT. Fokus Indo Lighting, 2011)

A series of tests on VRLA tubular OPzV was done following IEC 60896-2 3 procedure (Rusch, Vassallo,

& Hart, 2006). The cycle test was done with the battery capacity at 420 Ah, charge at 2.40 V (21h)

and discharge with loads of 84A (3h). The test duration was 55 months from year 1999 to 2004. Test

results showed that the capacity still remains 100% after 1,500 cycles. The test also revealed that the

OPzV battery could withstand on float test at 25O C with the lifetime as 22.5 years according IEEE

535-1986 4. Figure 3.7 depicted typical characteristic of cycle number versus depth of discharge

OPzV battery from a German manufacturer.

Figure 3-7 Typical cycles number versus depth of discharge of OPzV battery (Hoppecke Batterien GmbH & Co. KG, 2012)

The number of cycles at 80% is about 1,700 cycles, but in comparison with OPzS battery the cycles is

slightly higher at 20% DoD which is about 8,500 cycles.

VRLA and VLA tubular plate have the same relatively high cycle life. Both tubular VRLA gel (OPzV)

and VLA(OPzS) battery is the best choice for application which needs high cycle life as off grid PV

system. VLA is preferred for moderate cost preference while VRLA is preferred for less maintenance

but high cost considerations. VLA is also preferred for the application that requires safety as the first

3 IEC 60896-2 is a standard test of performance, durability and safety to characterize VRLA battery for

stationary use. 4 IEEE 535-1986 is a standard test for lead acid battery qualification for Nuclear Power Plant. The qualification

should be done in an artificial aging precondition.

priority. With regard the OPzS and OPzS have an expected lifetime of more than 10 years and high

number of cycles, they are the best choice for larger PV system application i.e. isolated grid PV Plant.

VRLA with flat plate AGM and Gel type is the preferred choice for the application that requires less

expected life time and less bridging time as Uninterruptible Power Supply (UPS) application. They

also provide a lower price for the same energy stored capacity.

The summary of applications and key parameter preferences for VRLA and VLA battery can be seen

in table 3.1

Table 3.1 Lead Acid Battery Key Parameter Selection and Application

Description VLA VRLA

Pasted Plate Tubular Pasted Plate Tubular Gel

OGi OPzS AGM Gel OPzV

Key Parameter

Cost Moderate Moderate Moderate High High

Float Life >20 years(*) >25 years(*) 5-10 years 10 years >20 years(*)

Cycle Life 600-1200 >1700 500 500 >1700

Application

Solar Power Fair Excellent Fair Fair Good

Traction Duty Fair Excellent Fair Fair Good

Standby Power Good Fair Good Good Good

Automotive Excellent N/A Good Good N/A

(*) at 25O C according to IEEE 535-1986 procedure (Rusch, Vassallo, & Hart, 2006)

3.1.4 Lithium Ion

Lithium ion battery has gained in popularity since is used in mobile phones, notebooks and electric

cars. It was back to 1991, when Sony and Asahi Kasei launched their first commercial lithium ion

battery. The lithium ion battery market was driven by portable electronic device as the mobile

phone and notebook computer at that time. The first lithium ion battery used lithiated cobalt

dioxide LiCoO2 as the positive electrode and carbon or graphite as the negative electrode.

Nowadays the use of LiCoO2 as the positive electrode still remains dominant for the portable battery

industry. It has higher energy density with specific energy as 518 Wh/kg and depth discharge cycles

500-700 cycles. (McDowall, 2008)

It has several advantages with the main superiority in energy density comparable to Lead Acid

battery. Lithium ion battery has a six times higher energy density compared to lead acid battery, as

for one kilogram lithium ion has the capability to store 150 Wh energy while for Lead Acid battery its

capability to store energy only 25 Wh.

Nevertheless lithium ion also has several disadvantages. Lithium ion cannot withstand high ambient

temperature and cannot be completely discharged so it has to be protected with electronic circuit as

well. The protective device has to be designed to switch off when the temperature so high and limit

the charge voltage.

Other disadvantages of lithium ion battery are safety issues. Overcharge voltage may lead to thermal

runaway and burst. In order to decrease the risk of failure of protective device, researchers have

developed another material for the positive electrode. One of them is lithium iron phosphate LiFeO4

that release smaller energy that prevents burst when overcharged.

Figure 3-8 Electro chemistry reaction of LiCoO2 (McDowall, 2008)

The electro chemistry reaction of lithium ion battery is depicted in figure 3.8. The lithium ion is

transferred from positive to negative electrode during charging voltage applied. The positive and

negative electrode is designed in a layered configuration as depicted in figure 3.8. The layered

configuration assists the insertion of the lithium ion process. The insertion of lithium ion into the

negative electrode can be done through the interface named as solid electrolyte interface (SEI) to

disable the immediate reaction due to too low of negative electrode potential. During discharging

condition, the lithium ion in negative electrode extracted and transferred back to the positive

electrode.

4 Optimum Energy Yield Author : Todo Simarmata

4.1 PV Module Tilt and Orientation

PV Module tilt and orientation have an impact to the annual energy yield. There are three methods

of PV module tilt angle and orientation to achieve maximum energy yield for the whole year due to

sun tilt angle of the earth that varies seasonally. These methods put the PV modules at a fixed

structure, a one axis tracking structure or a two axis tracking structure. The tracking structures are

not applied in Indonesia, therefore this part only discusses the PV module placed on a fixed

structure.

As the earth rotates and encircles the sun, the declination angle changes seasonally. This is due to

the tilting angle of earth rotation axis and vertical axis of the sun. The tilt angle is 23.45O, therefore

the declination angle is changing continuously due to 23.45O on the June 21 solstice,0O on the

September 23 solstice, -23.45O on the December 22 solstice until 0O on the March 22 equinox.

The tilt angle in a fixed structure is defined as an inclination angle of the PV module to the horizontal

line. Thus 0 degrees is equal to horizontal and 90 degrees is vertical.

The orientation of a fixed PV module is defined as the degree of the PV module to the vertical axis of

rotation. Therefore the default value of the azimuth angle is referring to angle clockwise of the true

north of the earth. In the northern hemisphere (facing south) the azimuth angle is 180O and 0O or

360O in the southern hemisphere (facing north).

The optimum tilt angle and orientation of a fixed structure PV module to achieve maximum solar

energy yield that can be harvested during a period of years is affected by several factors including

latitude position and clearness index. A study on determination of the optimum tilt angle (Bari,

2000) shows that for low latitude country ( 40O), the optimum tilt angle is equal to its latitude

angle and for the higher latitude country ( > 40O) the tilt angle should be added by 10O.

Other studies to determine optimum PV module orientation of the fixed structure shows that the

optimum orientation of the PV module is always facing true south for northern hemisphere and

facing true north for southern hemisphere region (Garg, 1982) (Duffie & Beckman, 1980). Thus for

regions with latitude 6O North e.g. Jakarta-Indonesia, the optimum tilt angle is 6O with orientation

facing true South.

However these studies do not include the cloudiness effect with the clear sky assumption (Lave &

Kleissl, 2011) . In the real condition, the result may slightly differ. In the case that there are no

meteorology data, these rules of thumbs are sufficient enough.

4.2 Performance Ratio

Performance ratio is a methodology to measure how much energy is actually produced by a PV Plant

in comparison with the possible solar energy yield available in the field. Performance ratio

determines the quality of a PV plant and do not depends on location since the reference energy yield

is location dependent.

Performance ratio is an important parameter for a PV plant since it gives the PV plant operator an

insight in overall system efficiency, performance degradation, permanent losses due to component

failures, shading and PV module mismatch. However, actual energy yield production not only

depends on overall system losses. PV module orientation and tilt have an important impact on

optimum energy yield. Thus the performance ratio is not only determined overall system losses but

also including incomplete solar irradiation utilization.

The common methodology that is used to quantify performance ratio is IEC 61724 5. It uses three

variables to determine performance ratio, actual PV production, reference yield and performance

ratio. The performance ratio is a dimensionless number that can be defined as a ratio between

actual PV energy productions to the reference yield. Those three variables can be elaborated as

below.

4.2.1 Actual PV production

Actual PV production expresses the amount of hours that PV produces the electricity in its nominal

capacity. Actual PV production is the net total energy produced divided by the nominal capacity of

PV array watt peak at standard test condition (STC)6. Actual PV production can be formulated as:

(4.1)

4.2.2 Reference Yield

Reference yield expresses in amount of hours available to produce maximum energy at the

reference location. Reference yield is a ratio between total solar irradiance per square meter to

irradiance at standard test condition. Reference yield can be formulated as:

(4.2)

4.2.3 Performance Ratio

Performance ratio (PR) is a dimensionless number then can be formulated as:

(4.3)

PR values are usually reported on an annual basis, but it might be useful to record PR on monthly

basis to enable monitoring of possibly component failures so that operator may fix the problem as

soon as possible.

5 IEC 61724 is a standard issued by International Electrotechnical Commision that describes general guidelines

to monitor and analyse PV system performance (International Electrotechnical Commission, 1998). 6 Standard Test Condition is defined as PV module test at 25

O C cell temperature, solar spectrum AM 1.5 and

standard irradiance 1,000 W/sqm.

A study report of 260 plants from IEA-PVPS database shows that on average, well maintained PV

plants could reach PR value of 72%. The study also found that 50% of the failures are caused by the

inverter. However the trend of inverter reliability improvements contributes to increasing PR value

(Jahn, Grimmig, & Nasse, 2000).

5 Calculating Solar Energy Boxwell, Michael (2012-03-02). Solar Electricity Handbook (Kindle Locations 1249-1445).

Greenstream Publishing. Kindle Edition.

Editor : Todo Simarmata

5.1 What is solar energy?

Solar energy is a combination of the hours of sunlight you get at your site and the strength of that

sunlight. This varies depending on the time of year and where you live. This combination of hours

and strength of sunlight is called solar insolation or solar irradiance, and the results can be expressed

as watts per square metre (W/ m ) or, more usefully, in kilowatt-hours per square metre spread

over the period of a day (kWh/ m / day). One square metre is equal to 9.9 square feet.

Photovoltaic solar panels quote the expected number of watts of power they can generate, based on

a solar irradiance of 1,000 watts per square metre. This figure is often shown as a watts-peak (Wp)

figure and shows how much power the solar panel can produce in ideal conditions.

A solar irradiance of 1,000 watts per square metre is what you could expect to receive at solar noon

in the middle of summer at the equator. It is not an average reading that you could expect to

achieve on a daily basis. However, once you know the solar irradiance for your area, quoted as a

daily average (i.e. the number of kilowatt-hours per square metre per day), you can multiply this

figure by the wattage of the solar panel to give you an idea. of the daily amount of energy you can

expect your solar panels to provide.

5.2 Calculating solar irradiance

Solar irradiance varies significantly from one place to another and changes throughout the year. In

order to come up with some reasonable estimates, you need irradiance figures for each month of

the year for your specific location. Thanks to NASA, calculating your own solar irradiance is simple.

NASAs network of weather satellites has been monitoring the solar irradiance across the surface of

the earth for many decades. Their figures have taken into account the upper atmospheric

conditions, average cloud cover and surface temperature, and are based on sample readings every

three hours for the past quarter of a decade. They cover the entire globe.

For reference, here are the solar irradiance figures for London in the United Kingdom , shown on a

month-by-month basis. They show the average daily irradiance, based on mounting the solar array

flat on the ground:

Table 5-1

Jan Feb Mar Apr May Jun Jul Aug Sep Okt Nov Dec

o.75 1.37 2.31 3.57 4.59 4.86 4.82 4.20 2.81 1.69 0.92 0.60

These table show how many hours of equivalent midday sun we get over the period of an average

day of each month. In the chart above, you can see that in December we get the equivalent of 0.6 of

an hour of midday sun (36 minutes), whilst in June we get the equivalent of 4.86 hours of midday

sunlight (4 hours and 50 minutes).

5.3 Capturing more of the suns energy

The tilt of a solar panel has an impact on how much sunlight you capture: mount the solar panel flat

against a wall or flat on the ground and you will capture less sunlight throughout the day than if you

tilt the solar panels to face the sun. Table 5.1 show the solar irradiance in London, based on the

amount of sunlight shining on a single square metre of the ground.

If you mount your solar panel at an angle, tilted towards the sun, you can capture more sunlight and

therefore generate more power. This is especially true in the winter months, when the sun is low in

the sky. The reason for this is simple : when the sun is high in the sky the intensity of sunlight is high.

When the sun is low in the sky the sunlight is spread over a greater surface area:

Figure 5-1 Different position of sunlight.

Figure 5.1 shows the different intensity of light depending on the angle of sun in the sky. When the

sun is directly overhead, a 1m-wide shaft of sunlight will cover a 1m-wide area on the ground. When

the sun is low in the sky in this example, an angle of 30 towards the sun is used a 1m-wide shaft

of sunlight will cover a 2m-wide area on the ground. This means the intensity of the sunlight is half

as much when the sun is at an angle of 30 compared to the intensity of the sunlight when the sun is

directly overhead.

5.4 The impact of tilting solar panels on solar irradiance

If we tilt our solar panels towards the sun, it means we can capture more of the suns energy to

convert into electricity. Often the angle of this tilt is determined for you by the angle of an existing

roof . However, for every location there are optimal angles at which to mount your solar array, in

order to capture as much solar energy as possible. Using London as an example again, this chart

shows the difference in performance of solar panels, based on the angle at which they are mounted.

The angles I have shown are flat on the ground, upright against a wall, and mounted at different

angles designed to get the optimal amount of solar irradiance at different times of the year.

Figure 5-2 Impact of Tilting

As shown in figure 5.2, look at the difference in the performance based on the tilt of the solar panel.

In particular, look at the difference in performance in the depths of winter and in the height of

summer. It is easy to see that some angles provide better performance in winter; others provide

better performance in summer, whilst others provide a good compromise all-year-round solution.

5.5 Calculating the optimum tilt for solar panels

Because of the 23 tilt of the earth relative to the sun, the optimum tilt of your solar panels will

vary throughout the year, depending on the season. In some installations, it is feasible to adjust the

tilt of the solar panels each month, whilst in others it is necessary to have the array fixed in position.

To calculate the optimum tilt of your solar panels, you can use the following sum:

optimum fixed year-round setting =

90 your latitude = your latitude for horizontal tilting

This angle is the optimum tilt for fixed solar panels for all-year-round power generation. This does

not mean that you will get the maximum power output every single month: it means that across the

whole year, this tilt will give you the best compromise, generating electricity all the year round.

Boxwell, Michael (2012-03-02). Solar Electricity Handbook (Kindle Locations 1443-1445).




















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