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

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

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

3

5

6

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

Battery........................................................................................................................................... 13 3.1

4

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

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

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

Calculating Solar Energy ................................................................................................................ 22 5.1

What is solar energy?............................................................................................................ 22

5.2

Calculating solar irradiance ................................................................................................... 22

Bibliography .................................................................................................................................. 26

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

= Planck’s constant

v

= the frequency of light

c

= the velocity of light

= 6.626 × 10 -34 joule·s

= 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 µm (violet) until =0.77 µm(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 ptype 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

I (+) Iph

Id V

(-)

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

= Boltzman’s 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. 8 unshaded cell

1 shaded 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, & Vaaßen, 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. unshaded sub string

-

-

+

D1 - reverse biased

shaded cell in substring

+

-

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. +

Combiner Box

Bloc king Diode

Bypass Diode

Junc tion box

Bypass Diode

Junc tion box

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 1800’s (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 C ha rg e

[1.685 Volt] (3.1)

Reaction at anode can be written as : Discharge C ha rg e

[0.356 Volt] (3.2)

Overall reaction can be written as: Discharge C ha rg e

[2.041 Volt] (3.3)

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.

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 3-5 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.

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 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

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.

Table 3.1 Lead Acid Battery Key Parameter Selection and Application

Description Pasted Plate OGi Key Parameter Cost Float Life Cycle Life Application Solar Power Traction Duty Standby Power Automotive

VLA Tubular OPzS

VRLA Pasted Plate AGM Gel

Tubular Gel OPzV

Moderate >20 years(*) 600-1200

Moderate >25 years(*) >1700

Moderate 5-10 years 500

High 10 years 500

High >20 years(*) >1700

Fair Fair Good Excellent

Excellent Excellent Fair N/A

Fair Fair Good Good

Fair Fair Good Good

Good Good Good N/A

O

(*) at 25 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 (ϕ ≤ 40O), the optimum tilt angle is equal to its latitude angle and for the higher latitude country (ϕ > 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. 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

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 O Standard Test Condition is defined as PV module test at 25 C cell temperature, solar spectrum AM 1.5 and standard irradiance 1,000 W/sqm.

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 ²) or, more usefully, in kilowatt-hours per square metre spread over the period of a day (kWh/ m ²/ 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. NASA’s 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 o.75

Feb 1.37

Mar 2.31

Apr 3.57

May 4.59

Jun 4.86

Jul 4.82

Aug 4.20

Sep 2.81

Okt 1.69

Nov 0.92

Dec 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 sun’s 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 sun’s 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 ½ ° 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). Greenstream Publishing. Kindle Edition. Boxwell, Michael (2012-03-02). Solar Electricity Handbook (Kindle Locations 1435-1443). Greenstream Publishing. Kindle Edition.

Boxwell, Michael (2012-03-02). Solar Electricity Handbook (Kindle Locations 1295-1301). Greenstream Publishing. Kindle Edition.

Boxwell, Michael (2012-03-02). Solar Electricity Handbook (Kindle Locations 1292-1295). Greenstream Publishing. Kindle Edition.

Boxwell, Michael (2012-03-02). Solar Electricity Handbook (Kindle Locations 1291-1292). Greenstream Publishing. Kindle Edition. Boxwell, Michael (2012-03-02). Solar Electricity Handbook (Kindle Locations 1285-1290). Greenstream Publishing. Kindle Edition.

Boxwell, Michael (2012-03-02). Solar Electricity Handbook (Kindle Locations 1283-1285). Greenstream Publishing. Kindle Edition.

Boxwell, Michael (2012-03-02). Solar Electricity Handbook (Kindle Locations 1281-1283). Greenstream Publishing. Kindle Edition.

Boxwell, Michael (2012-03-02). Solar Electricity Handbook (Kindle Locations 1260-1261). Greenstream Publishing. Kindle Edition. Boxwell, Michael (2012-03-02). Solar Electricity Handbook (Kindle Locations 1249-1257). Greenstream Publishing. Kindle Edition.

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