Solar Energy Course - The Fundamentals

WHAT IS ELECTRICITY

Electricity is all around us–powering technology like our cell phones, computers, lights, soldering irons, and air conditioners. Without using electricity, it is difficult to live even for a single day. Therefore, it is important for us to know some basics about electricity, its terminology, how to measure electricity, etc.

As an expert of the solar energy, it is very important that one is well aware of the basic concepts of electricity

The main terms associated with electricity are:

• Voltage

• Power

• Energy

• AC and DC power

WATER TANK ANALOGY

The flow of current in a circuit and the flow of water from a tank have several similarities.

When a tank is filled and a tap is opened, water flows out from the tank. Water flows out faster if a tank is fully filled as compared to the case when the tank is partially field. Similarly, in an electrical circuit, the current flows when there is a voltage.

• The greater amount of current flows when the voltage is higher.

• Therefore, the height of the water in a tank is similar to the voltage in the electrical circuit.

• This flow of water through a pipe per second is called water current.

• Similarly, the flow of charge (electrons) through a wire per second is called electric current.

The amount of water flowing through the pipe depends on the following two things:

• How hard the water is being pushed, i.e. how much pressure is being applied.

• It also depends on the diameter of the pipe which indicates the resistance to the flow of water.

In a similar way, the amount of current flows through a wire depends on the following two things :

• How much electrical pressure is being applied. This electrical pressure is called ‘Voltage’ in electrical terminology.

• It also depends on the diameter of the wire, which indicates the resistance to the flow of current. Higher diameter means low resistance to current flow.

ELECTRICAL TERMINOLOGIES

We deal with a wide range of voltage levels; from very small voltage level to very large voltage level.

• For instance, small pencil batteries provide voltage levels of 1.2V.

• Backup batteries for PC etc. provide voltage levels of 12V.

• In our homes electrical circuit, we get voltage level of 230V.

• In the industries, normally the voltage levels are about 11KV, 33KV or 66KV.

For small electrical circuits or for small electrical appliances,

• 1A current is very large current.

• Therefore, people use smaller unit of current like 0.001 A.

• The term 0.001 represents one thousand fraction of one and referred as ‘milli’ and represented by symbol ‘m’.

• Therefore, 0.001 A current is 1mA current.

The flow of current requires a medium. In the case of electrical current, the medium is conducting wires like copper and aluminum. Normally, the conducting wires are chosen to allow the smooth flow of current, but due to their material properties, all the conducting media possess some resistance to current flow is given by the term Resistance, which is represented by symbol R.

The resistance is measured in Ohm. Symbol of Ohm is a Greek alphabet Omega (Ω). The property of the resistance is to resist or impedes the current flow. Normally, in electrical circuit, we would like to have as small resistance as possible, because we do not want any resistance to current flow. One of the side effect of the resistance of a wire is the voltage drop across it when current flows. The voltage drop means drop of electrical pressure to drive the current.

The term “Power” incorporates both current and voltage in it. When electricity flows in an electrical circuit, it results in some work done. The term power (P), is a measure of the or speed of electrical work done. In this way, the more power means the electrical work is done at high speed and less power means the electrical work is done at low speed.

Power of electrical work done or power of electricity depends on electric pressure (electrical voltage) and electron flow rate (electrical current). So, in the case of electricity,

                                                                Electrical Power = Voltage X Current

                                                                                            Or

                                                    Power (watt) = Voltage (volt) X Current (ampere)

Therefore, the amount of electrical energy consumed by an appliance depends on two factors:

• Power of an appliance and

• Duration of usage.

The power of electrical appliances is given in terms of watt and duration of usage can be given in terms of hours. Therefore, the electrical energy can be given in following way :

                                                       Electrical Energy = Power X Duration of usage

                                                                                     Or

                                                         Energy (E) = Power (watt) X Time (hour)

                                                                          Or

                                                                      E (Wh) = P (W) X T (h)

UNIT CONSUMPTION CALCULATOR

Find the cost of heating water using a 2000 W water heater for 2 hours ? When the cost of a kilowatt-hour of electricity for residential customers is Rs 6.

Solution:

A kilowatt-hour is the unit that electricity utilities use when billing in our homes. we would change the watt-hours into kilowatt-hour, as shown below :

Electrical energy consumed   = 2000 X 2 Wh divided by 1,000 = 4 kWh = 4 units

                                                      = 2000 X 2 Wh divided by 1,000 = 4 kWh = 4 units

                                               Cost = 4 kWh X 6 = Rs 24

It would cost about 24 rupees to cook the food for 2 hours using a 2000 W water heater.

DC AND AC POWER

In electrical circuit, power flows in two forms; these forms are referred as follows:

• Direct current or DC power

• Alternating current or AC power

SERIES AND PARALLEL ELECTRICAL CIRCUITS

A DC circuit is a circuit in which current flows in only one direction. The direction of current does not change with time. A PV module produces DC current means that the current flows in only one direction in DC circuit in which DC loads are operating on DC power of PV modules. A DC load is a load that operates on DC power, or a DC circuit is a circuit which works on DC load.

Similarly, several other types of load like motor, refrigerator, fan, TV etc., are available that works on DC power.

In AC circuit, CURRENT flows in both the directions, clockwise and counter-clockwise. The variation of AC current with respect to time. The AC current changes its direction 50 times in one second (in this situation it is called that current has 50 Hertz frequency).

INTRODUCTION

To understand Solar radiation, we need to know some important details about the sun.

• The Sun is a nature born thermo-nuclear fusion reactor. The Sun’s core is about 15 million degrees Celsius.

• The Sun is 150 million kilometers from Earth.

• It takes 3 minutes for light to travel from the Sun to the Earth. It generates 3.6×1026 Watt power.

• The predicted life span of the sun is about 4 to 5 billion years from now.

• Radiation from the Sun, which is more popularly known as sunlight.

• It is a mixture of electromagnetic waves ranging from infrared (IR) to ultraviolet rays (UV).

• The mother Earth protects us from the harmful UV radiation by safety cover of its atmospheric air.

• The atmospheric air absorbs most of the UV radiation from the Sun light, retaining useful specific bands of light (visible and radio waves.

WHAT IS SOLAR RADIATION?

Solar radiation, often called the solar resource, is a general term for the electromagnetic radiation emitted by the sun. Solar radiation can be captured & turned into useful forms of energy, such as heat & electricity

BASICS OF SOLAR RADIATION

Every location on Earth receives sunlight at least part of the year. The amount of solar radiation that reaches any one spot on the Earth’s surface varies according to:

• Geographic location

• Time of day

• Season

• Local landscape

• Local weather.

Because the Earth is round, the sun strikes the surface at different angles, ranging from 0° (just above the horizon) to 90° (directly overhead). When the sun’s rays are vertical, the Earth’s surface gets all the energy possible.

Because the Earth is round, the frigid polar regions never get a high sun, and because of the tilted axis of rotation, these areas receive no sun at all during part of the year.

The Earth revolves around the sun in an elliptical orbit and is closer to the sun during part of the year. When the sun is nearer the Earth, the Earth’s surface receives a little more solar energy.

As sunlight passes through the atmosphere, some of it is absorbed, scattered, and reflected by:

• Air molecules

• Water vapor

• Clouds

• Dust

• Pollutants

• Forest fires

SOLAR IRRADIANCE & IRRADIATION

Let us first understand Solar irradiance

SOLAR IRRADIANCE

• Solar irradiance is the sun’s radiant power, represented in units of W/m2 or kW/m2.

• The Solar constant is the average value of solar irradiance outside the earth’s atmosphere about 1366 W/m2.

• Typical peak value is 1000 W/m2 on a terrestrial surface facing the sun on a clear day around solar noon at sea level and used as a rating condition for PV modules and arrays.

SOLAR IRRADIATION

Solar irradiation is the sun’s radiant energy incident on a surface of unit area, expressed in units of kWh/m2.

• Typically expressed on an average daily basis for a given month

• Also referred to as solar insolation or peak sun hours

• Solar irradiation which can be termed as Energy is equal to the average solar irradiance multiplied by time.

Peak sun hours (PSH) is the average daily amount of solar energy received on a surface. PSH are equivalent to:

• The number of hours that the solar irradiance would be at a peak level of 1kW/m2

• Also the equivalent number of hours per day that a PV array will operate at peak rated output levels at rated temperature.

INTRO TO SOLAR ORIENTATION

Solar orientation is the positioning of a site, building, or space in relation to cardinal directions and, more importantly, the sun’s path. Whether it’s your site, your home, or even a specific room in your home, everything has a specific orientation and relationship to the sun’s movement across the sky. Solar orientation is very useful for designing the solar system and utilizing the maximum solar radiation from the Sun.

The azimuth angle is the horizontal angle measured from due south. The greatest angle would be on the horizon at sunrise/sunset. The altitude is the vertical angle above the horizon. The highest angle would be due south at midday.

How-To Determine the Sun Path and Sun Angles for Your Location

The quick and easy way to find the altitude of the sun at its highest point where you live, do this quick calculation:

• Step 1: Look up your city’s latitude

For example, let us consider the city of Columbus, Ohio state – whose latitude is 39.96°.

(You can get your city latitude by google searching the term “your city name latitude”

• Step 2: Subtract your city latitude from 90° to get your Equinox

In this case we get 90° – 39.96° = 49.04°

• Step 3: Sun angle at summer solstice = Equinox + 23.5°

Therefore, Summer solstice will be 49.04°+ 23.5° = 72.54°

• Step 4: Sun angle at winter solstice = Equinox — 23.5°

Therefore, Winter solstice will be 49.04°- 23.5° = 25.54°

Sun Angles for Columbus, OH

With your sun angles, you can start to visualize what the sun path looks like for your location. Here’s what the sun path and angles look like for Columbus, OH.

In the winter, notice that the sun does not rise due east but rather south of east at 58 degrees from due south. This is the azimuth angle. As it moves across the sky throughout the day it forms a low arc. At its highest point at noon, it only gets to 26 degrees above the horizon. This is the altitude angle. Like at sunrise, the sun does not set due west but rather south of west again at 58 degrees from due south.

In the summer, the sun rises 121 degrees from due south, moves across a higher arc at a max angle of 73 degrees above the horizon at midday, then sets again 121 degrees from due south.

Once you determine your azimuth and altitude angles, you’ll use this information to start to design your home to respond to the climate and movement of the sun, ultimately helping you take big strides to create a more sustainable home (and one with smaller utility bills!).

Solar PV Modules

A solar PV module is a collection of solar cells, mainly connected in series. These combinations of Solar Cell provide higher power than a single solar cell.

The PV modules are available in the power rating range from 3 watts to 300 watts.

They really form the basic building block of PV systems as power generating unit. With further connection of PV modules together, one can generate very large amount of power, in range of megawatt or MW.

In PV modules, many cells are connected together. The cells are connected in series fashion, wherein positive terminal of one cell is connected to the negative terminal of the cell and this is repeated to make a string of solar cells, or a solar PV module.

Cells strings connected in series

When we connect cells in series, we get a string of solar cells. The string of solar cells will also have two terminals.

When we connect cells in series the voltage of solar cells gets added, therefore, the terminal voltage of a PV string or PV module will be higher and equal to the sum of all the solar cells connected in series.

Suppose, terminal voltage of a solar cell is 0.5 V under operating conditions and two such identical cells are connected in series, so the terminal voltage of string of four solar cells will be 0.5 + 0.5 + 0.5 + 0.5 = 2.0 V.

When we connect cells in series

Voltage gets added & Current remains nearly the same as that of individual cell

Cells strings connected in Parallel

When we connect cell in parallel

The current gets added but the voltage remains nearly same as that of a single cell

Voltage of cells strings connected in series

If 6 cells are connected in series, than terminal voltage of series of 6 cells will be

                                                                                 = 0.5 X 6 = 3 V.

If 36 solar cells are connected in series, then terminal voltage of series of 36 cells, or PV string of 36 cells will be =

                                                                                    0.5 X 36 = 18 V.

Ratings of PV modules

The parameters of the solar PV modules (Voc, Isc, Wp), mentioned by the manufacturer are measured under some standard conditions of temperature (25°C) and solar radiation (1000 W/m2).

• The most important parameter of a PV module is its peak output power. The solar PV modules are rated in terms of their peak power or watt peak (Wp) output.

• It is the most important parameter from a user point of view. The Wp is specified by the manufacturer under so-called standard test conditions (STC).

• The module rating under STC is widely accepted by the manufacturers and by the users.

Departure from STC conditions

The conditions specified in the STC do not occur for most of the time & locations. This happens mainly because of two reasons; the real solar irradiation is normally less than 1000W/m2 & the module temperature under real operation is more than the STC specified temp of 25 °C.

Both of these reasons, this result in lower module power output than the expected under the STC condition.

PV module parameters

A PV module is made up of many cells connected together, and the electrical behavior of PV module is similar to PV cells. Therefore, the PV module parameters are also similar to solar cell parameters. In the previous lessons, solar cell parameters have been discussed, which include;

• Open circuit voltage (Voc), &

• Short circuit current ( Isc)

• Maximum power point (Pm),

• Voltage at maximum power point (Vm),

• Current at maximum power point (Im,)

Terminal Voltages

The PV modules are designed for charging batteries of 12 V terminal voltages. Therefore, one of the modules requirements is to provide sufficient voltage to be able to charge 12 V batteries under typical daily solar radiation.

Generally, it is expected that the PV module must provide about 15 volts (or around this value) in all operating conditions, meaning low solar radiation (like in morning and evening) and high temperatures (like in summer).

Efficiency (ɳ )

For large power module, the design is done for

• 24 V battery level (two batteries in series),

• 36 V battery level (three batteries in series), etc. or,

We can say the PV modules are designed to provide voltages in a multiple of 12 V battery level, i.e 12 V, 24 V, 36 V, 48 V, etc

We must note here that for

• 12 V batteries voltage level, PV module (Vm) should be around 15 V, and

• For 24 V battery voltage level, PV module voltage should be around 30 V.

• Similarly, for 36 V battery voltage level, PV module voltage should be around 45 V.

Solar PV Array

• Interconnected solar PV modules provide power of 100W to several MW

• Schematic representation of PV module is shown here

• Interconnection of solar cells into modules and modules into solar PV arrays.

I V curve – Series Connection

• Let us consider a solar cell having Voc of 0.6V and Isc of 0.8A

• When two identical cells are connected in series, the Voc of the two cells will be added which will be 2V

• The Isc will be the same as that of a single cell that is 8A

I V curve – Series & Parallel Connection of Cells

Parallel connection

When the two cells are connected in Parallel

• The current Isc of the two cells will be added which will be 6A

• While Voltage Voc of the combination will remain same as that of single cell that is 6A

Series and parallel connection

When more than one series connected cells are connected in Parallel, more current and voltage will be obtained

Current Isc will be 1.6A, While Voltage Voc will be 1.2 A

Hot Spot in a Module

What is a Hot Spot in a module?

• In the module, there are many cells connected in series, if one of the cell is shaded, then under the short circuit condition of the string, the shaded cell will become reverse biased.

• Hot spot heating occurs when there is one low current solar cell in a string of at least several high short circuit solar cells.

• One shaded cell in a string reduces the current through the good cells, causing the good cells to produce higher voltages that can often reverse bias the bad cell.

• Power gets dissipated in the poor cell local overheating or “Hot-Spots”, leads to destructive effects.

• Cell or glass cracking, melting of solder or degradation of the solar cell.

Importance of Bypass Diode

• Bypass diode is a diode which is used to avoid the destructive effect of hot spots or local heating in series connected cells

• Bypass diode is connected in parallel with solar cells with opposite polarity

• In normal condition the bypass diode operates in reverse bias will appear across it.

• This reverse bias will act as a forward bias for the bypass diode.

Basic of Solar Charge Controller

In a solar PV system, there is a source which generates electricity and load that consumes electricity. For the reliable operation of solar PV systems, the power to the load should be made available whenever it is required by the user. In order to have reliable supply of the power from source to the load, there are several other components required in PV power systems. These components are referred as Balance of Systems (BoS).

The BoS components are mainly electronic components which are required to control the flow of power from source to load in controlled manner. For instance, in standalone solar PV system, where the batteries are used to store the electrical energy for night time applications.

The protection of battery is required to ensure that the batteries are not over-charged or over-discharge as both of these damage the batteries and reduces their lifetime. A device called charge controller is required in PV systems to protect the batteries.

A device called maximum power point tracking (MPPT) is used to ensure that PV module is supplying maximum possible power to the systems. In the absence of MPPT, the PV module may work in sub-optimal condition and may not generate power to its potential.

Since PV modules are expensive, it is important mainly for kilo-watt level system and above to ensure optimal performance of the PV modules and MPPT are two different functions, many manufacturers club these functions into a single electronic device called MPPT charge controller.

Function of Charge Controller

Charge controllers, as the name implies, control the flow of charge from the battery and to the battery. They protect the battery by preventing over-charge or deep discharge of batteries to preserve their life and performance.

When the battery gets overcharged by solar PV module, a charge controller will cut it off from the circuit so that no more charging is possible.

Similarly, if a battery goes into deep discharge (or over discharge) due to excessive use of batteries by the load, a charge controller detects and disconnects the battery from the circuit so that no current can be drawn from the battery.

Working of a Charge Controller

The status of over-charge and deep discharge is detected by measuring the voltage level of batteries. In overcharge condition, the battery voltage increases beyond a certain level and in deep discharge condition, the battery voltage decreases below a certain level.

In overcharge and deep discharge voltage conditions, the charge controller disconnects the battery. Also, when the battery voltage level reaches within the normal operating level, the battery gets connected to the circuit.

In case of over-charge, battery gets cut-off due to high voltage of battery. After high voltage cut-off, if the battery is used by load which means some discharge of battery, then its terminal voltage will fall. The charge controller will detect this and connect the battery again for charging.

Similarly, in the case of deep discharge, the battery gets cut-off due to low voltage of battery. Now, if the battery is under charging condition, its terminal voltage will increase after sometime due to charging.

The charge controller will detect the increase in voltage and when voltage increases above the low voltage cut-off level, the charge controller will connect the battery to the circuit so that power can be extracted from the battery by the load.

Types of Charge Controller

• The following two types of charge controllers are mostly used:

• Pulse Width Modulation (PWM) charge controller or standard charge controller

• Maximum Power Point Tracking (MPPT) charge controller.

• PWM charge controllers have same nominal voltage across battery bank and PV array.

• PPT charge controllers can have different voltages across battery bank and PV array and operate at the maximum power point tracking (MPPT) of the PV panel.

• MPPT charge controller allows us to have a solar panel array with a much higher voltage than your battery bank voltage.

• For the same power flow, when voltage is high, current that will flow in the wires is small.

• Power is equal to current multiplied by voltage. Normally, it is desired to keep the current flow to small level.

• Therefore, a big advantage of having a higher voltage solar panel array is that we can use smaller gauge wiring to the charge controller, and the use of small gauge wire reduces the wiring cost.

Ratings of Charge Controller

• Typical ratings of 12V, 6A rated PWM charge

Maximum Power Point Tracking ( MPPT)

Power delivered by a module depends on the load connected to the module. Consider a module whose I-V characteristics and the corresponding power are shown.

We can see that at short circuit conditions that is when Voltage =0, current delivered by the module is maximum and is 5.1 A. As voltage across the load is increased by varying load up to 17.3 V, Power delivered to the load increases to 84 W that is V =17.3 V X I=4.86 A. Beyond this point, though the voltage is increased, power delivered decreases as the current decreases very sharply

Maximum power Point Tracking (MPPT)

So, the power delivered by the module has a point on I-V characteristics corresponding to maximum power and is called Maximum Power Point (MPP).

In order to extract the maximum power from PV modules, the load connected to the modules should work at maximum power pointer the operating point of PV module-load combination should be at maximum power point.

The operating point is the point of intersection of I-V characteristics of a source (PV modules) to any load like a fan, a TV, a resister, etc.

Power Output from a PV Module

Solar PV modules are rated for peak power output. The power of PV modules not only depends on input solar radiation but also on operating point (combination of current and voltage).

For instance even under very bright sun light condition also, if PV module is operating in open circuit mode or short circuit mode the power output will be zero. There is one operating point of a PV module at which the power output is maximum (maximum power point) and this operating point changes with change in intensity of solar radiation falling on PV modules.

There are electronic devices which ensure that at all light conditions, solar PV modules operate at maximum power point.

Need for Maximum Power Point Tracking?

In practice, because of the changing ambient solar radiation, the I-V characteristics of PV modules change throughout the day. Therefore, it is not possible to choose a load such that the operating point is always at maximum power point or close to maximum power point.

Intensity of solar radiation varies throughout the day. On a typical day, radiation is less intense at 9 a.m. and increases till noon. As the intensity changes, the I-V characteristics of the module also change. As a result, for a given load characteristics, the operating point also changes.

As an example, the operating point of a PV module and a resistive load for 1 p.m., 11 a.m. and 9 a.m. is schematically shown in Figure and is denoted by Z, Y and X respectively. But for 1 p.m., 11 a.m. and 9 a.m. the maximum power point is Z’, Y’ and X’ respectively. In order to deliver maximum power, the actual operating points X, Y and Z should be made as close as possible to X’, Y’ and Z’

MPPT Device

The MPPT device

• There is a device whose function is to bring the operating point of a load close to the maximum power point under different operating conditions.

• This device is called maximum power point tracking or MPPT. In this way, the function of MPPT is to extract maximum available power from PV modules under any given condition (less radiation, more radiation, high temperature etc.)

• The maximum power tracking mechanism makes use of an algorithm and an electronic circuitry.

• The mechanism is based on the principle of impedance matching between load and PV module which is necessary for maximum power transfer.

• Thus, in theory, whenever the impedance of load is matches with the impedance of source, maximum power transfer takes place between source and load.

In this way, the presence of MPPT between source (PV module) and load, ensures that maximum available power is extracted from PV modules.

MPPT Charge controller

Many manufacturers combine the function of charge controller and MPPT in one single device which is called MPPT charge controller.

MPPT & Sun tracking for more power

MPPT device is used to extract maximum power possible from a PV module in all operating conditions. The sun-tacking is referred as mechanical tracking of a solar PV module in such a way that is always perpendicular to the sunlight. In this way, the module should face eastward in the morning, southward in the noon time and westward in the evening to directly see the sun. When the PV modules are directly facing the sun all the time, they receive more solar radiation and therefore, they generate more power.

Here it should be noted that the MPPT is not the same as the sun-tracking of solar PV modules. In Sun-tracking, PV modules are rotated mechanically so that the radiation absorbed by a module is maximum, while in the case of MPPT, electronic circuitry is used to ensure that maximum amount of generated power is transferred to the load.

Specifications of MPPT Charge controller

• Maximum input power: This is the maximum power that the charge controller can handle from a PV array.

• Maximum open circuit voltage: This is the maximum open circuit voltage that the charge controller can handle.

• PV input

• MPPT tracking voltage range: These are the voltage levels that the charge controller can handle.

DC output to battery

• Nominal battery voltage: Voltage at battery operates in a system.

  
  

• Voltage regulation set point (VR) : It is the maximum voltage up to which a battery can be charged (without getting overcharged). If this threshold is reached, the controller either disconnects the battery from the source or starts regulating the current delivered to the battery.

  
  

• Low voltage disconnect (LVD) : It is the minimum voltage up to which the battery can be allowed to discharge without getting deep discharged. It is also defined as the maximum depth of discharge (DoD) of the battery. The charge controller disconnects the load from the battery terminals as soon as the battery voltage touches LVD, to prevent it from over discharging.

• Nominal PV array current or maximum charging current : This is the maximum PV array current that a charge controller should be able to handle. This is nothing but the array short circuit current. A safety factor 1.25 is used to account for variation in Short circuit at non STC.

DC load control

• Nominal voltage: This is the maximum load voltage that a charge controller should be able to handle.

• Maximum current: This is the maximum load current that a charge controller should be able to handle.

MPPT charge controller is rated by the output current that they can handle and not by the input current from the solar panel array as it is done in standard charge controllers. We can end this module with a typical specifications of 12V, 6 A MPPT charge controller.

Basics of Solar Inverters

Let us now discuss about Basics of Solar Inverters

Solar Inverters are an important part of BoS in PV systems. The PV module generates direct current power or DC power & the battery also stores energy which is available in the form of DC power. But most of the loads that we use work on the alternating current power or AC power.

Therefore, before feeding the power from the PV module or the battery to the load, we need to convert DC power into AC power. The job of conversion of DC power into AC power is done by the device called inverter. In the absence of inverter, we will not be able to operate our AC loads using solar power.

• It is clear that in the solar PV power systems, several other components are required between the source of power (PV module) to the consumer of power (the load) for smooth & reliable operation of the system.

• Normally, electrical power generated as AC power in alternators or generators in remote locations and is transmitted to load centers in AC form. This is the major reason why we use AC loads rather than DC loads.

• We use AC power in our houses to power up AC loads like fan, light etc. We also use DC power to charge our cell phone batteries, laptop batteries etc.

Types of Power Converters

Many times in PV systems it is required to bring the current and voltage levels to the desired level. The desired current and voltage levels may be different than what the PV modules possibly can supply. Conversions are also required from one type of power to other type; that is, the conversion of DC power to AC power or vice versa. The conversion of voltage level and current level or type of power can be obtained by using power converters. These power converters can be classified by the following:

• DC to DC Converters

• AC to DC Converters also known as Rectifiers

• DC to AC Converters also known as Inverters

DC to DC Converters

DC to DC converters are used for converting one level of DC voltage usually raw, unregulated or un-controlled voltage level to regulated voltage level. Other than converting unregulated voltage level, the DC to DC converters are also used for converting one level of voltage to either higher or lower level of voltage.

AC to DC Converters

• We supply AC power to computers but the components inside the computer need DC power. Therefore, rectifiers are used inside the computer to convert AC power into DC power.

• Batteries are used to store the energy in the form of DC. When we get power from battery, we get DC power output but many appliances like fan, refrigerator, CFL Lamp use AC power. Therefore, we need to convert DC power from battery to AC power using inverters in order to use power.

DC to AC Converters.

The output from Solar Panels & Batteries are in the form of DC Power. But many appliances like fans, refrigerator, CFL Lamp use AC power. Therefore, we need to convert DC power to AC power using inverters in order to use this power. This conversion of DC power into AC power can be obtained using devices called inverters.

Types of Inverters

The solar inverters are an important interface between the solar PV module and the load. Depending on whether battery is used in the PV system or not, the solar inverters can be classified in three broad categories:

• Standalone Inverters or Off-grid Inverters

• Grid-tie Inverters or grid-interactive Inverter

• Battery back-up grid-tie inverters or Hybrid inverters:

Standalone Inverters or Off-grid Inverters:

These inverters are not connected to grid. They are normally used in standalone PV power systems. In standalone system, there is no back-up of power for energy storage, therefore, this type of inverters has battery back-up to supply the power to the load in case of non-sunshine hours.

Grid-tie Inverters or grid-interactive Inverter:

These inverters are connected to grid and do not have battery back-up. They have special circuitry to match inverter output voltage and frequency with that of grid. Grid is used as battery back-up when power generated by PV array is sufficient. These inverters also have in-built MPPT to extract maximum power from PV array.

Grid Connected Inverter Function

Following are the main function of grid tie inverters

• Maximum Power Point tracking

• Inversion

• Grid Synchronization & disconnection

• Safety & Protection

• Remote / Central Monitoring.

Battery back-up grid-tie inverters or Hybrid Inverters:

These inverters are grid-tied but also have battery back-up like standalone inverters.

A solar inverter’s main job is to convert DC power generated from the array into usable AC power. Hybrid inverters go a step further and work with batteries to store excess power as well. This type of system solves issues renewable energy variability and unreliable grid structures.

Inverters for grid-tied applications can only provide power based on what the array can immediately generate from the sun. Hybrid inverters can store power in batteries and then use it when needed during the non-sunshine hours for energy stabilization.

Inverter Topology – Comparison

Here we do a comparison between advantages and Disadvantages of

• Transformer Based Inverter

• High Frequency Inverter

• Transformer less Inverter

Transformer Based Inverter

Advantages are

• High Reliability

• Safety due to Galvanic isolation

Disadvantages are

• Low Efficiency

• High Weight and Volume

High Frequency Inverter

Advantages are

• Compact & Light

• High Efficiency

• Safety due to Galvanic isolation

Disadvantages are

• Complex

• Costly Technology

Transformer less Inverter

Advantages are

• Compact & Light

• High Efficiency

Disadvantages are

• Additional safety measures required.

Types of grid tied solar inverters:

There are 3 main different types of grid tied solar inverters:

• Micro

• String and

• Central

A micro-inverter is a very small inverter that is attached to the back of a solar panel. A micro-inverter only converts the power of one or two solar panels to AC so generally many micro-inverters are required in a single system. Micro-inverters have several advantages over traditional inverters including performance, safety and monitoring, however the upfront cost can be significantly higher.

Advantages

• Each panel is monitored and optimized individually to generate maximum power.

• Avoids shading, soiling & Panel mismatch

• Ideal for small roof

Disadvantages

• Expensive

• Maintenance cost is high.

String Inverter

• String of modules are connected in series.

Advantages

• Separate MPP tracking for each string.

Disadvantages

• Common scenario of partial shading

• MPP tracking may still not be sufficient

Reputed manufacturers of Tier One Solar string inverters are

• SMA

• ABB Solar

• Fronious

Central Inverter

Advantages

• PV Modules are connected in series called strings, generating sufficient high voltage to avoid amplification.

• All strings are then connected in parallel to support high power to output

• Generally, only one inverter is used to interface to grid

Disadvantages

• High Voltage DC cable

• MPP tracking and controlling mismatch between strings, resulting in efficiency loss.

Reputed manufacturers of Tier One Solar Central inverters are

• SMA

• ABB Solar

• Fronious

Comparison between String inverters and Central inverters

Here we compare the string inverters and Central inverters on the following parameters

Costs:

Both the inverters are competitive in price depending upon the project size. Central inverters are less expensive than string overall for large utility-scale installations because fewer are required per site. But for smaller utility-scale projects, string inverters could win out for their easier serviceability

Cabling:

• More DC Cabling at higher voltage levels for central inverters

• More AC Cabling at lower voltage levels for string inverters

Monitoring:

• Monitoring at string level possible for central inverters

• Monitoring at sub-string level possible for string inverters

Maintenance:

• Cost for maintenance is very high for central inverters

• Easy replacements of string inverters

PV Plants Size:

• Ideal for plant size above 2 MW for central inverters

• Ideal for plant size below 2 MW for string inverters

Efficiency:

• Efficiency is very high in case of central inverters

• Efficiency is very low in case of string inverters

MPP tracking:

• Low MPP Tracking for central inverters

• High MPP Tracking for String Inverters

Basics of Solar batteries

• Batteries, as electrical energy storage medium, are very important and delicate part of standalone solar PV systems.

• They are important because without energy storage, a solar PV system will not be able to deliver the energy to the load when there is no sunlight.

• In the case of standalone systems, we need electrical energy for running our appliances in non-sunshine hours, while in the case of grid connected PV systems. We do not require any energy storage. Grid, if operational, provides the energy whenever it is required.

• To provide the power supply to these devices, a device having electrical energy stored in it is used.

• These devices having stored electrical energy or stored charge is known as battery.

• The charge stored in the batteries can be used to supply the energy to appliances when required.

• A battery stores electrical energy (charge) in the form of chemical energy.

• When a battery is used, the chemical energy stored is converted into electrical energy.

• A battery is a two terminal device. One terminal I called positive (+) and the other terminal is called negative (-).

• In the charged condition, there is a voltage difference between two terminals. This voltage difference drives the current in appliances when connected.

• For giving supply to a device from battery; the positive and negative terminals of a battery is connected to the corresponding terminals of the device.

Rechargeable Batteries

• Some batteries allow repeated charging-discharging cycles while others do not.

• The batteries which allow repeated charging-discharging cycle are known as ‘rechargeable batteries’.

• The rechargeable batteries are widely used in the solar PV systems.

• The batteries in PV system play an important role in the storage of electrical energy and providing continuous electric current supply irrespective of absence/ presence of the sun and the change in weather.

• Without batteries, it is difficult to think of ‘standalone solar PV systems.

How does a battery work?

• Generally, a battery is made of a combination of two or more units of electro-chemical cells (voltaic cells) connected together in series or parallel combination.

• It is called electro-chemical cell, because it deals with the electrical and chemical energy.

• The electro-chemical cell, in general, is also termed cell.

• A single unit of an electro-chemical cell consists of two half-cells. Each half cell consists of an electrode and an electrolyte.

• The two half-cells are electrically connected to each other by salt bridge.

• The electrodes in the two half-cells are of different metals. In each half-cell, a chemical reaction occurs at the metal electrode.

The operation of the cell involves two chemical reactions.

• One is oxidation reaction and the other is reduction reaction, commonly called the Redox reaction which converts the chemical energy into electrical energy.

• Oxidation is a process in which electrons are lost or released, and the reduction is a process in which electrons are accepted or gained.

Components of a Battery cell

It can be seen from the discussion in that operation of a battery requires anode (positive electrode), cathode (negative electrode), electrolyte, and salt bridge. The role of each of these components is briefly described below:

Anode : It is generally referred as positive terminal or positive node or positive lead. It is the electrode which gives up electrons to the external circuit, as a result the electrode is oxidized during the discharging reaction.

Cathode :It is generally referred as negative terminal or negative node or negative lead. It is the electrode which gains electrons from the external circuit, as a result of which the electrode is reduced during the discharging reaction.

Electrolyte: It is a medium which provides conductivity to ions between anode and cathode. One can say that an electrolyte is a medium through which current flows internally in a battery. An electrolyte is typically a liquid, such as water or other solvents with dissolved salts, acids or alkalis.

Salt bridge: It is a porous material used to keep the two electrodes connected but yet keep them separate from each other; otherwise the chemical reaction would stop. It is also referred as a separator.

Types of a Batteries

There are varieties of batteries that are available in the market for several types of applications. Each battery type is more suited for one particular application. The type of battery is identified by the chemistry of materials used in making it. The batteries are broadly divided into two categories:

• Non-rechargeable batteries or primary batteries, and

• Rechargeable batteries or secondary batteries.

In the non-rechargeable batteries, the electro-chemical reaction is not reversible. This type of batteries is used for one time and once discharged, they cannot be charged again.

The non-rechargeable batteries are the most convenient, simple, easy to use and require less maintenance. These types of battery are portable and are made in various sizes and shapes.

Non Rechargeable Batteries

These batteries have high shelf life, reasonable cost, energy & good power density. Generally, these high shelf life, reasonable cost , energy & good power density. Generally, these batteries are available in small capacities, typically below 20 Ah (Ampere-hour). These batteries can be operated in a wide range of temp; -400C to 700C.

The most common example of non-rechargeable battery is Zinc Chloride battery, commonly known as pencil cell.

Rechargeable Batteries ( Secondary batteries)

The batteries in which the conversion of chemical energy into electrical energy (discharging) and the reverse process, that is, conversion of electrical energy into chemical energy (charging) can take place are called the rechargeable battery or secondary battery.

The rechargeable batteries are the most widely used batteries in the world. These batteries are used for various applications, such as starting, lighting and ignition (SLI) in automotive, standby power supply, electronic appliances like DVD player, mobile phones, camera, camcorder, laptops etc. These batteries are available in a wide range of charge storage capacities in the market & can be easily procured.

Battery Storage Capacity

The capacity of a battery is the capacity to store the charge in the battery. It is the product of current (in amperes) it can deliver for a given time (in hours), i.e., Ampere X Hour (Ah). One ampere-hour (Ah) is the amount of charge delivered when constant current of one ampere (A) is used for one hour (h). In this chapter, we will express battery capacity in term of ampere-hour.

The capacity of a battery is given by the expression shown below:

                                                                       Capacity (C) = Current (A) X Hour (h)

                                                                         Current (I) =            Capacity (Ah)

                                                                                                       Discharge duration (h)

The capacity of non-rechargeable batteries normally varies in few mAh (milli Ah) to several Ah range. The capacity of rechargeable batteries can vary from few Ah to thousands of Ah.

The capacity of batteries depends on temperature. The same battery will have different capacity

How much energy is stored in battery ?

If we know the terminal voltage of the battery and its charge storage capacity, we can obtain how much electrical energy is stored in the battery. The electrical energy is given in terms of the product of charge capacity and voltage. Thus, energy stored in a battery can be given by the following expression :

Energy (watt-hour) = Capacity (Ah) X Voltage (V)

The above expression indicates that large capacity battery and higher terminal voltage battery stores higher amount of electrical energy.

What is the power of the battery?

Power for any device is defined as product of voltage and current. In case of battery if we multiply the terminal voltage of battery with discharge current we will get the power of the battery. Thus, battery power can be written as:

Battery power (watt) = Terminal Voltage (V) X Current drawn (A)

Depth of discharge

In practical applications, all the charge stored in a battery cannot be used for running load. Only some percentage of total charge stored can be used. The percentage of total charge that can be used for running the load is referred as Depth of discharge (DoD).

50% DoD means that only 50% of the total stored charged can be used. 70% DoD means that only 70% of the total stored charged can be used. In general, we want higher DoD for the batteries which are used in solar PV systems. Therefore, normally, deep discharge batteries are preferred for which the allowable DoD is 100%.

Normally, the batteries used for SLI (starting, lighting & ignition, for instance, our car batteries) applications have small DoD, about 50%. The Li-ion batteries have DoD of 80% to 90%.

Manufacturers specify allowable DoD level for their batteries. The battery should not be discharged below manufacturers specified level in order to prevent damage to the battery.

If the batteries are discharged below their DoD rating, then the life of the batteries decreases very fast. It means that if the life of the battery in 3 years and it is continuously discharged below its DoD limit, then battery may stop functioning in 6 months only. For practical application.

It is better to take batteries to 50% of the DoD specified by manufacturer. The tolerable limit of DoD is determined from the charging / discharging efficiency of a battery.

State of charge (SoC)

• Rechargeable batteries have to be often charged for reuse. For such batteries, the time of charging is decided by the present values of charge level or present state of charge (SoC).

• The SoC indicates level of charge, i.e. percentage of total charge that is stored at this time in a battery.

• Thus, if 60% of the total charge storage capacity is still there in the battery then the battery’s SoC is 60%.

• The DoD is another way of showing SoC. The DoD is the inverse of Both DoD and SoC are expressed in percentage. Present SoC when subtracted from 100% gives the present value of DoD.

This can be written in the following way.   DoD (%) = 100 % – SoC (%)

                                                                                   SoC (%) = 100 % – DoD (%)

For example, suppose a battery after sometime of usage has SoC 70%.

This indicates that its present      DoD = 100% – 70% = 30%.

• As we keep using battery, its DoD percentage increases and the SoC decreases.

• It is discussed earlier that as the SoC decreases, the open circuit voltage & terminal voltage of the battery decreases.

• In other words, for higher SoC, the battery will have higher terminal voltage and for lower SoC, the battery will have lower terminal voltage.

Cycle of Battery

One cycle of battery means one charging plus one discharging cycle. Typically, the life cycle of a lead-acid battery is 500-800 cycles.

Generally, manufacturer gives the values of maximum charging-discharging current and voltage. The battery can be charged with the following three methods:

• Constant Voltage.

• Constant Current or

• Both

Time taken to reach the full tolerable DoD of the battery. The expression for C-rating is as follows:

C- rating (amphere) = Capacity ( C )

No of hours for full charge or discharge

Where, Capacity ( C ) is in ampere-hour (Ah), and time for full charge or full tolerable discharge (t) is in hours (h).

Let us Consider, a battery of capacity C and time for full charge or discharge is 1 hour, then C-rating will be C/1 or 1C. Similarly, If t=10 hours, then C-rating is C/10.

Battery efficiency

• The charging voltage of any rechargeable battery is greater than the discharging voltage.

• The charging voltage is the sum of battery e.m.f. and voltage drop due to the battery’s internal resistance.

• The discharging voltage is the difference of battery e.m.f. and voltage drop due to the battery’s internal resistance of the battery,

• The discharged energy is always less than the charging energy.

• Typically, a lead- acid battery is 80% to 90% efficient in doing charge transfer.

The expression for the charge transfer efficiency is given below :

Ampere –hour/ charge transfer of efficiency  = Discharged energy (Ah) X 100%

                                                                                                                   Charging energy

Another way to calculate energy efficiency for a battery, in terms of watt – hours, is shown in the expression below. It is called watt- hour efficiency.

Watt–hour/Energy efficiency = Discharged energy (Wh) X 100%

                                                                              Charging energy (Wh)

The energy efficiency is typically 65% to 70% for a lead-acid battery. Charge transfer efficiency / ampere-hours is usually used to calculate the panel array needed to charge the battery bank.

How to select a Battery?

The selection of a battery is done by seeing the battery parameters. The parameter values for voltage rating, current rating, capacity rating, number of charging-discharging cycles and shelf life differs from battery-to-battery.

These values also differ from one manufacturer to another. So, we must have good understanding of the battery parameters to identify a proper battery for an application before buying or using it.

In the market many types of batteries are available. While designing a solar PV system comprises batteries, the one problem often faced is a proper choice of battery from many available batteries. This problem can be simplified by making a list of minimum requirements, conditions and limitations

While selecting a battery following parameters are observed.

• Types of Battery

• Voltage and Current

• Temperature requirement

• Shelf Life

• Charge-Discharge Cycle (If Rechargeable)

• Cost

• Availability

The commonly available rechargeable batteries are listed below.

• Nickel Cadmium (NiCd)

• Lithium Ion (Li-ion), and

• Lithium Ion Polymer (Li-ion polymer)

• Lead Acid

Nickel Cadmium batteries

Nickel Cadmium (Ni-Cd) batteries are rechargeable batteries (also known as secondary batteries) that can be deeply discharged and have a high cycle life expectancy. Typically, Ni-Cd batteries are used when large capacities and high discharge rates are required.

Lithium Ion batteries

Lithium ion battery uses liquid lithium ion as electrolyte while lithium polymer use solid or gelatin like polymers as electrolyte. Lithium ion batteries have high energy density and cost less than lithium polymer. Lithium polymer batteries are light weight and have improved safety.

Lithium Ion Polymer batteries

Lithium ion Polymer (LiPo) batteries employ Li-ion electrolytes and is packed with an aluminum plastic film. It has a high energy density and offers ideal cell sizes and shapes which can be The polymer electrolyte replaces the traditional porous separator, which is soaked with electrolyte.

Lead Acid batteries

The battery which uses sponge lead and lead peroxide for the conversion of the chemical energy into electrical power, such type of battery is called a lead acid battery. The lead acid battery is most commonly used in the power stations and substations because it has higher cell voltage and lower cost.

Lead-acid batteries are rechargeable, typically, they have capacities in the range of 1 to 12000 Ah, they have 500 to 800 charge-discharge cycles and life time is of about 2 to 3 years.

The lead acid-battery performs very well for a wide range of temperatures, from -150C to 600C. The replacement and maintenance of lead-acid batteries are simple.

All these factors make lead-acid batteries a good choice for PV applications.

Solar site assessment

• A professional solar site assessment is invaluable for helping you understand the benefits and challenges of installing a photovoltaic system.

• A site assessment provides a clear picture of how well a PV panel array will work for you.

• In the process of selection, design and installation of an appropriate solar PV system for your home or business, a professional site assessment is a crucial part.

• Cost and power output of your potential PV System significantly depend on it.

• The output of a PV module is directly proportional to the amount of sunlight strikes on it.

• The site assessment report will help you to bring your goals, budget and energy needs together with the unique solar opportunities at your location.

• Every site is different and needs evaluation specific to the site.

Why Site Survey?

• It’s customary for a PV system integrator to do a Site survey and collect information about local conditions and issues before any proposal is made to the customer.

• The information collected is then combined with the load patterns and the customer preferences to make a final proposal.

The site assessment depends upon following factors

• User requirements

• Location feasibility

• Tilt angle

• Load analysis

Now Let us discuss all this parameters in details

User requirements

User requirements are as below

Electricity Tariff

Electricity tariff is a very important aspect of solar site assessment as it impacts the

return of investment for the solar project

Available area

The customer may have a large power consumption requirement for his location.

But may not have sufficient space for the power plant. As a thumb rule we require

90 sq. Ft. approx. for a 1 Kw Solar power plant.

Installation timeline

During the site assessment it is important to analyse the site conditions like access

road to the site, soil condition, local govt permissions & licenses, shifting of the

material to the terrace etc to determine the time line of the project installation.

User demand

Power usage during the peak and off peak time should be calculated to understand the user demand

Load estimation

Load estimation is a continuous process which starts at the feasibility study stage of the project and goes on until almost the end of the project. For example, a factory wants to expand the existing power capacity in the near future should also be considered during the load estimation.

Aesthetics

Commercial and industrial markets have shown a rising demand for aesthetics-driven installations. Successful companies are finding ways to differentiate themselves from the pack and incorporating aesthetics as a powerful tool to stand out from the best.

Reliability of data

Site specific readings or information collected should come from reliable sources.

Scalability

While designing the power plant, it is important to consider the scalability

of the project for planned future expansion

Budget

Budget is the most important aspect to determine if the customer is ready to invest the required amount for the solar projects. Without this aspect everything else is meaningless.

Location feasibility

We get the location feasibility with the following parameters

Longitude and Latitude

In the earlier module, we learned how to find the longitude and latitude of a customer location. Latitude helps in determining the tilt angle of the solar panel array.

Available area

As we know that 1 Kw power plant requires roughly about 90 sq ft. For example if the customer power plant requirement is 100 Kw, then available area should be 9000 sq. ft.

Solar insolation

It basically means the amount of solar energy that strikes a square meter of the earth’s surface in a single day. Solar insolation or radiation can be found from using solar radiation meter

Temperature

Temperature at a customer location plays an important role in the performance of Solar panels, Inverters, batteries and other equipment.

Wind

Every location is classified into three broad categories. If your location has high wind possibilities, then special care should be given for the structural sizing.

Variability of weather

Extreme weather conditions or variability of weather will increase the installation cost of your projects.

Shading analysis

Shading analysis is one of the most essential steps in phase of solar plant design. It is important to analyse shading caused by surrounding trees, structures or buildings.

TILT Angle

The effect of an array’s tilt angle on solar PV energy output may be up to 20% compared to that of flat installations..

Whether you are installing a solar panel on a flat roof or a pitched roof, the output of the solar PV system would be increased by optimizing the tilt angle.

Load Analysis

Load analysis can be done with the following parameters.

We will start with

Detailed load requirements

This is total load requirement at the location. This can be deducted by analysing the electrical bills of your customer for the last 12 months. Customer inputs should be considered for the possibilities for addition of equipment’s like machineries, Air conditioners etc in the coming months.

Load profiling

An energy load profile, or consumption profile, is essential to determining the value that a solar installation can provide. An energy load profile shows how much energy a building uses at each time of day across each day of the year.

DC/ AC Loads

Load can be classified into DC/AC along with the power ratings. Separate power distribution line should be established for each type.

Surge load

Surge load are brief over voltage spikes or disturbances on a power waveform that can damage, degrade, or destroy electronic equipment within any home, commercial building, industrial, or manufacturing facility. The site assessor should look for surge capable equipment’s on the premises

Power quality

Power quality refers to the ability of electrical equipment to consume the energy being supplied to it. A number of power quality issues including electrical harmonics, poor power factor, voltage instability and imbalance impact on the efficiency of electrical equipment.

Critical loads

Critical loads are those loads to which power supply has to be maintained under any circumstances. Power supply to these loads should not ever be interrupted. For example ICU in hospitals, data centre’s servers etc.

Learn how to audit the electricity bill of your customer premises.